151
Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire NABIL SOROUR Maîtrise en génie des matériaux et de la métallurgie Maître ès sciences (M.Sc.) Québec, Canada © Nabil Sorour, 2016

Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

  • Upload
    others

  • View
    22

  • Download
    1

Embed Size (px)

Citation preview

Page 1: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as

Cathodes for Chlorate Production

Mémoire

NABIL SOROUR

Maîtrise en génie des matériaux et de la métallurgie Maître ès sciences (M.Sc.)

Québec, Canada

© Nabil Sorour, 2016

Page 2: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as

Cathodes for Chlorate Production

Mémoire

NABIL SOROUR

Sous la direction de :

Edward Ghali, directeur de recherche

Page 3: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

iii

Résumé

Ce projet de travail est divisé en deux études principales: (a) l'influence des certains additifs

organiques sur la consommation d'énergie et la pureté du métal de zinc déposé dans le

processus d'extraction électrolytique, et (b) l’électrodéposition des alliages binaires et

ternaires de Fe-Mo et Fe-Mo-P sur des substrats d’acier doux afin d’agir comme cathodes

pour la production de chlorate.

(a) Parmi les sept différents additifs organiques examinés, les sels des liquides ioniques

ont réussi à augmenter le rendement du courant jusqu'à 95,1% comparé à 88,7% qui a

obtenu à partir de l'électrolyte standard en présence des ions de Sb3+. La réduction

maximale de la consommation d'énergie de ~173 kWh tonne-1 a été obtenue en ajoutant de

3 mg dm-3 du chlorure de 1-butyl-3-méthylimidazolium dans le même électrolyte. La teneur

en plomb dans le dépôt de zinc est réduite de 26,5 ppm à 5,1-5,6 ppm en utilisant les sels

des liquides ioniques.

(b) Des différents binaires Fe-Mo et ternaires Fe-Mo-P alliages ont été électrodéposés

sur des substrats d’acier doux. Les alliages préparés ont une tenure en Mo entre 21-47 at.%

et une tenure en P de 0 à 16 at.%. L'activité électrocatalytique de ces alliages vers la

réaction de dégagement d'hydrogène (RDH) a été étudiée dans des solutions de chlorure de

sodium. La réduction maximale de la surtension de RDH de ~313 mV a été obtenue par

l’alliage ternaire préparé Fe54Mo30P16 par rapport à celle obtenue pour l'acier doux. La

rugosité de surface et l'activité intrinsèque des revêtements de Fe-Mo-P peuvent être

l'origine du comportement prometteur de ces électrocatalyseurs vers la RDH.

Page 4: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

iv

Abstract

This work project is divided into two main studies: (a) the influence of certain organic

additives on the power consumption and the purity of deposited zinc during electrowinning

process, and (b) the electrodeposition of binary and ternary alloys of Fe-Mo and Fe-Mo-P

on mild steel substrates to act as cathodes for chlorate production.

(a) Among seven different examined organic additives, the ionic liquid salts succeeded

to increase the current efficiency up to 95.1% compared to 88.7% obtained from standard

electrolyte in presence of Sb3+ ions. Maximum reduction of power consumption of ~173

kWh ton-1 was observed by addition of 3 mg dm-3 of 1-butyl-3-methylimidazolium chloride

to the same electrolyte. Lead content in the zinc deposit is reduced from 26.5 ppm to 5.1-

5.6 ppm by using the ionic liquid salts.

(b) Different binary Fe-Mo and ternary Fe-Mo-P alloys have been electrodeposited on

mild steel substrates. The prepared alloys have Mo content between 21-47 at.% and P

content from 0 to 16 at.%. The electrocatalytic activity of these alloys towards the hydrogen

evolution reaction (HER) was investigated in sodium chloride solutions. The maximum

reduction of HER overpotential of ~313 mV was achieved from the prepared ternary alloy

Fe54Mo30P16 compared to that obtained from mild steel. The surface roughness and intrinsic

activity of Fe-Mo-P coatings could be the origin of the promising behavior of these

electrocatalysts towards the HER.

Page 5: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

v

Table of Content

Résumé ............................................................................................................................................... iii

Abstract .............................................................................................................................................. iv

Table of Content .................................................................................................................................. v

List of Tables ...................................................................................................................................... ix

List of Figures .................................................................................................................................... xi

Acknowledgments ............................................................................................................................. xv

Forward ............................................................................................................................................ xvi

CHAPTER 1 ........................................................................................................................................ 1

INTRODUCTION ............................................................................................................................... 1

1.1. Background .............................................................................................................................. 2

1.2. Zinc Electrowinning ................................................................................................................. 2

1.2.1. Zinc Metal ......................................................................................................................... 2

1.2.2. Methods of Extraction of Zinc Metal ................................................................................ 3

1.2.3. Uses of Zinc ...................................................................................................................... 4

1.3. Electrodeposition of Alloys as Cathodes in Chlorate Production ............................................ 5

1.3.1. Chlorate Production ........................................................................................................... 5

1.3.2. Cathodes in Chlorate Production ....................................................................................... 6

1.4. Objectives and Detailed Approaches ....................................................................................... 7

1.4.1. Effect of Certain Organic Additives on Zinc Electrowinning Process .............................. 7

1.4.2. Performing the Electrodeposition of Fe-Mo & Fe-Mo-P Alloys as Cathodes .................. 8

CHAPTER 2 ........................................................................................................................................ 9

LITERATURE REVIEW .................................................................................................................... 9

2.1. Zinc Electrowinning Process .................................................................................................. 10

2.1.1. Lead-Based Anodes ......................................................................................................... 11

2.1.2. Corrosion of Lead-Based Anodes ................................................................................... 12

2.1.3. Oxygen Overpotential of Lead-Based Anodes ................................................................ 13

2.1.4. Role of Manganese Ions in the Electrolyte ..................................................................... 14

2.1.5. Surface Structure and Crystallographic Orientation........................................................ 16

2.1.6. Metallic Impurities in Zinc Electrowinning .................................................................... 17

2.1.6.1. Effect of Lead Impurity on Zinc Deposition ............................................................ 18

2.1.6.2. Effect of Antimony Impurity on Zinc Deposition .................................................... 19

Page 6: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

vi

2.1.6.3. Effect of Copper, Nickel and Cobalt Impurities on Zinc Deposition ....................... 21

2.1.7. Additives in Zinc Electrowinning ................................................................................... 23

2.1.7.1. Effect of Glue ........................................................................................................... 23

2.1.7.2. Effect of Natural Products and Surfactants .............................................................. 25

2.1.7.3. Effect of Synthetic Polymers .................................................................................... 26

2.1.7.4. Effect of Quaternary Ammonium Salts .................................................................... 27

2.1.7.5. Effect of Ionic Liquid Salts ...................................................................................... 28

2.2. Electrodeposition of Alloys as Cathodes for Chlorate Production ......................................... 31

2.2.1. Chlorate Production ......................................................................................................... 31

2.2.2. Mild Steel Cathodes ........................................................................................................ 33

2.2.3. Fe-Based Alloys Cathodes .............................................................................................. 34

2.2.4. Ni-Based Alloys Cathodes .............................................................................................. 35

2.2.5. Molybdenum Co-deposition ............................................................................................ 37

2.2.6. Phosphorous Co-deposition ............................................................................................. 38

2.3. Electrochemical Test Methods (Approach and Evaluation) ................................................... 40

2.3.1. Galvanostatic Polarization Technique ............................................................................. 41

2.3.2. Potentiodynamic Polarization Technique ........................................................................ 41

2.3.3. Cyclic Voltammetry Technique ...................................................................................... 43

2.3.4. Electrochemical Impedance Spectroscopy Technique .................................................... 44

2.4. Summary ................................................................................................................................ 46

CHAPTER 3 ...................................................................................................................................... 48

EXPERIMENTAL ............................................................................................................................ 48

3.1. Electrolyte and Set-up ............................................................................................................ 49

3.1.1. Zinc Electrolyte and Materials Preparation ..................................................................... 49

3.1.2. Fe-Mo & Fe-Mo-P Electrolytes and Materials Preparation ............................................ 50

3.1.3. Set-up .............................................................................................................................. 51

3.2. Electrochemical Techniques and Measurements ................................................................... 52

3.2.1. Galvanostatic Polarization ............................................................................................... 52

3.2.2. Current Efficiency Calculations ...................................................................................... 52

3.2.3. Power Consumption Calculations ................................................................................... 52

3.2.4. Potentiodynamic Polarization ......................................................................................... 53

3.2.5. Cyclic voltammetry ......................................................................................................... 54

3.2.6. Electrochemical Impedance Spectroscopy ...................................................................... 55

Page 7: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

vii

3.3. Deposit Examination Techniques ........................................................................................... 55

3.3.1. Scanning Electron Microscopy (SEM) ............................................................................ 55

3.3.2. Energy Dispersive Spectroscopy (EDS) .......................................................................... 55

3.3.3. X-ray Diffraction (XRD) ................................................................................................. 56

3.3.4. Inductively Coupled Plasma (ICP) .................................................................................. 56

CHAPTER 4 ...................................................................................................................................... 57

INFLUENCE OF DIFFERENT ORGANIC ADDITIVES IN ZINC ELECTROWINNING FROM ACIDIC SULPHATE ELECTROLYTE .......................................................................................... 57

Résumé .............................................................................................................................................. 58

Abstract ............................................................................................................................................. 59

4.1. Introduction ............................................................................................................................ 60

4.2. Experimental .......................................................................................................................... 61

4.2.1. Electrolyte and Experimental Setup ................................................................................ 61

4.2.2. Deposit Examination ....................................................................................................... 62

4.2.3. Potentiodynamic Polarization and Cyclic Voltammetry ................................................. 62

4.3. Results and Discussion ........................................................................................................... 63

4.3.1. Power Consumption and Current Efficiency ................................................................... 63

4.3.2. Characterization of Deposits ........................................................................................... 67

4.3.3. Potentiodynamic Polarization ......................................................................................... 70

4.3.4. Cyclic Voltammetry Measurements ................................................................................ 73

4.4. Conclusions ............................................................................................................................ 77

CHAPTER 5 ...................................................................................................................................... 79

ELECTROCHEMICAL STUDIES OF IONIC LIQUID ADDITIVES DURING THE ZINC ELECTROWINNING PROCESS ..................................................................................................... 79

Résumé .............................................................................................................................................. 80

Abstract ............................................................................................................................................. 81

5.1. Introduction ............................................................................................................................ 82

5.2. Experimental .......................................................................................................................... 84

5.2.1. Electrolysis ...................................................................................................................... 84

5.2.2. Deposit Examination ....................................................................................................... 85

5.2.3. Electrochemical Measurements ....................................................................................... 85

5.3. Results and Discussion ........................................................................................................... 86

5.3.1. Cell Voltage and Power Consumption ............................................................................ 86

5.3.2. Current Efficiency ........................................................................................................... 88

Page 8: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

viii

5.3.3. Deposit Examination ....................................................................................................... 89

5.3.4. Polarization Studies ......................................................................................................... 92

5.4. Conclusions ............................................................................................................................ 98

CHAPTER 6 ...................................................................................................................................... 99

ELECTRODEPOSITION AND STUDY OF THE ELECTROCATALYTIC ACTIVITY OF Fe-Mo-P ALLOYS FOR HYDROGEN EVOLUTION DURING CHLORATE PRODUCTION ....... 99

Résumé ............................................................................................................................................ 100

Abstract ........................................................................................................................................... 101

6.1. Introduction .......................................................................................................................... 102

6.2. Experimental ........................................................................................................................ 104

6.3. Results and Discussion ......................................................................................................... 105

6.3.1. Deposit Characterization ............................................................................................... 105

6.3.2. Steady-State Polarization Curves .................................................................................. 108

6.3.3. Electrochemical Impedance Spectroscopy .................................................................... 112

6.4. Conclusions .......................................................................................................................... 115

CHAPTER 7 .................................................................................................................................... 117

CONCLUSIONS AND OUTLOOK ............................................................................................... 117

7.1. Conclusions .......................................................................................................................... 118

7.2. Outlook ................................................................................................................................. 121

Bibliography .................................................................................................................................... 122

Page 9: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

ix

List of Tables Table 2.1. Electrode potential (V/SCE) vs. current density (A m-2) of anodes from lead and its

alloys in 1.8 M H2SO4 at 30oC [30] ................................................................................ 14

Table 2.2. Variation of current efficiency and preferred crystalline orientation of zinc deposit at

different concentrations of lead at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3

Zn2+ + 150 g dm-3 H2SO4 [37]......................................................................................... 18

Table 2.3. Variation of current efficiency and preferred crystalline orientation of zinc deposit at

different concentrations of antimony at 400 A m-2 and 35oC for zinc electrolyte of 55 g

dm-3 Zn2+ + 150 g dm-3 H2SO4 [37] ................................................................................ 20

Table 2.4. Effect of copper on current efficiency and crystal orientation of zinc deposit at different

concentrations and different current densities for electrolysis in 55 g dm-3 Zn2+, 150 g

dm-3 H2SO4 at 35oC [43] ................................................................................................. 21

Table 2.5. Variation of current efficiency and preferred crystalline orientation of zinc deposit at

different concentrations of nickel at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-

3 Zn2+ + 150 g dm-3 H2SO4 [37] ...................................................................................... 22

Table 2.6. Effect of [BMIM]HSO4 and Gelatin on current efficiency and power consumption during

zinc electrodepsotion [70] ............................................................................................... 29

Table 2.7. Effect of Sb3+ on current efficiency in absence and in presence of [BMIM]HSO4 during

zinc electrowinning [72] ................................................................................................. 31

Table 2.8. Kinetic parameters for the HER on the Ni-Cu-Fe electrode [88] .................................... 37

Table 3.1. Fe-Mo and Fe-Mo-P electrolytes compositions .............................................................. 50

Table 4.1. Effect of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on cell voltage, CE and PC in

absence and in presence of Sb3+ during zinc electrodeposition for 2 h at 50 mA cm-2 and

38оC ................................................................................................................................ 65

Table 4.2. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on surface morphology, crystal

orientation and lead contamination in absence and in presence of Sb3+ during zinc

electrodeposition for 2h at 50 mA cm-2 .......................................................................... 67

Table 4.3. Effects of additives on Tafel slopes, cathodic overpotential at 50 mA cm-2 obtained from

potentiodynamic polarization versus Ag,AgCl/KCl(sat) and NOP obtained from cyclic

voltammetry .................................................................................................................... 76

Table 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE and PC in absence and in

presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2 ....................... 87

Page 10: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

x

Table 5.2. Crystallographic orientations and lead concentration of zinc deposits obtained by adding

3mg of gelatin, [EMIM]MSO3 and [BMIM]Br in absence and in presence of Sb(III)

during zinc electrodeposition for 2h at 50 mA cm-2 ....................................................... 92

Table 5.3. Effect of [EMIM]MSO3, [BMIM]Br and gelatin on Tafel slopes, cathodic overpotential

at 50 mA cm-2, exchange current density and NOP ........................................................ 95

Table 6.1. The compositions of the coatings from four different electrolytes after 6 hours of

electrodeposition at 20 mA cm-2 and 30oC ................................................................... 106

Table 6.2. The measured kinetic parameters of HER for MS, Fe-Mo and Fe-Mo-P electrodes in

sodium chloride solution at 80oC and pH 6.4 ............................................................... 110

Table 6.3. The electrochemical data obtained by the Nyquist plots of MS, Fe-Mo and different Fe-

Mo-P alloys ................................................................................................................... 114

Page 11: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xi

List of Figures

Figure 1.1. Typical roast-leach-electrowinning processes for zinc [7] .............................................. 3

Figure 1.2. The major uses of zinc [13] ............................................................................................. 5

Figure 2.1. (a) Simple electrolysis cell for zinc (b) Aluminum cathodes deposited by zinc ......... 11

Figure 2.2. Potential-pH diagram obtained according to the ionic activities in an actual anodic film

for the Pb-H2O-H2SO4 system at 25oC (potential vs. SHE) [27] ................................... 12

Figure 2.3. Lead-based anode; (a) Before electrolysis and (b) After 5 hours of electrolysis ........... 16

Figure 2.4. Zinc deposit shows HCP Lattice among the three most important lattices .................... 17

Figure 2.5. SEM photomicrographs (X 385) showing the effect of current density on the

morphology of zinc deposits from addition-free electrolyte using unconditioned Pb-Ag

anodes. (a) 215 A m-2, 60 min, 0.125% Pb; (b) 323 A m-2. 60 min, 0.076% Pb; (c) 430

A m-2, 60 min, 0.04% Pb; (d) 538 A m-2, 60 min, 0.021% Pb; (e) 1076 A m-2, 30 min,

0.019% Pb; (f) 2152 A m-2, 15 min, 0.011% Pb [40] .................................................... 19

Figure 2.6. SE micrographs showing the morphology of 6h zinc deposits electrowon at 500 A m-2

and 38oC from electrolytes containing; (a) and (b) 0.02, (c) and (d) 0.04 mg dm-3 Sb

[42] ................................................................................................................................. 20

Figure 2.7. Quaternary ammonium salts; (a) Non-aromatic, (b) Aromatic ...................................... 27

Figure 2.8. Examples of ionic liquids salts; (a) Cationic, and (b) Anionic ...................................... 29

Figure 2.9. Schematic process of chlorate production (Chemetics Inc. B.C., Canada) .................... 32

Figure 2.10. Scanning micrographs of developed cathodes [86] ..................................................... 36

Figure 2.11. Potentiodynamic polarization plot [110] ...................................................................... 43

Figure 2.12. Theoretical cyclic voltammogram [102] ...................................................................... 44

Figure 2.13. (a) Simple electrified electrode/electrolyte interface, (b) Electronic components for the

same interface [112] ...................................................................................................... 45

Figure 2.14. Nyquist impedance plot for the showed circuit for Rs, Rct and Cdl [114] ..................... 46

Figure 3.1. Schematic experimental set-up for three-electrode cell [116] ....................................... 51

Figure 3.2. Electrolysis cell set-up ................................................................................................... 51

Figure 3.3. Polarization curve potential vs current density (log i) [119] .......................................... 54

Figure 4.1. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current efficiency: (a) in

absence of Sb3+ and (b) in presence of 0.0055 mg dm-3 of Sb3+ during zinc

electrodeposition for 2h at 50 mA cm-2 and 38оC .......................................................... 66

Figure 4.2. Scanning electron micrographs (x1000) of zinc deposits in absence of Sb3+; (a) SE, (b)

PAM, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f)

Chitin 3mg dm-3 ............................................................................................................. 68

Page 12: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xii

Figure 4.3. Scanning electron micrographs (x1000) of zinc deposits in presence of 0.0055 mg of

Sb3+; (a) SE, (b) PAM 3mg dm-3, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e)

BKCl 3mg dm-3 and (f) Chitin 3mg dm-3 ....................................................................... 69

Figure 4.4. Effects of the additives on the cathodic polarization during zinc electrodeposition with

different concentrations in absence and in presnce of antimony; (a) PAM, (b)

[BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin .......................................................... 73

Figure 4.5. Cyclic voltammograms during zinc electrowinning using aluminum cathode with

different concentrations of 0,1,5 and 40 mg dm-3 of: (a) PAM, (b) [BMIM]Cl, (c)

TBABr, (d) BKCl and (e) Chitin ................................................................................... 75

Figure 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE: (a) in absence of Sb(III) and (b)

in presence of 0.0055 mg of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2

....................................................................................................................................... 88

Figure 5.2. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in absence of

Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br ... 90

Figure 5.3. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in presence of

0.0055mg of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg

[BMIM]Br ..................................................................................................................... 91

Figure 5.4. XRD patterns of zinc deposit in absence of Sb(III); (a) 3mg [EMIM]MSO3, (b) 3mg

[BMIM]Br ..................................................................................................................... 91

Figure 5.5. Effect of [EMIM]MSO3 on the cathodic polarization during zinc electrodeposition

using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in

presence of Sb(III) ......................................................................................................... 94

Figure 5.6. Effect of [BMIM]Br on the cathodic polarization during zinc electrodeposition using

aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in

presence of Sb ................................................................................................................ 94

Figure 5.7. Cyclic voltammograms of [EMIM]MSO3 during zinc electrodeposition using aluminum

cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of

Sb(III) ............................................................................................................................ 96

Figure 5.8. Cyclic voltammograms of [BMIM]Br during zinc electrodeposition using aluminum

cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of

Sb(III) ............................................................................................................................ 97

Figure 6.1. Scanning electron micrographs (X500) of deposits; (a) Fe53Mo47, (b) Fe70Mo21P9, (c)

Fe61Mo26P13, (d) Fe54Mo30P16 after electrodeposition during 6 hours at 20 mA cm-2 at

30oC ............................................................................................................................. 107

Page 13: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xiii

Figure 6.2. XRD spectra of Fe-Mo and Fe-Mo-P deposits obtained from electrodeposition of 6

hours at 20 mA cm-2 and 30oC ..................................................................................... 108

Figure 6.3. Polarization curves of Fe-Mo and three Fe-Mo-P deposited electrodes compared to MS

in chlorate solution at 80oC and pH 6.4 ....................................................................... 109

Figure 6.4. The electrical equivalent circuit used for simulation of the impedance spectra for the

HER [171] .................................................................................................................... 112

Figure 6.5. The Nyquist plots for the HER process on a) MS, b) Fe53Mo47, c) Fe70Mo21P9, d)

Fe61Mo26P13 and e) Fe54Mo30P16 ................................................................................... 113

Page 14: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xiv

“To my kind mother and the memory of my great father.

To my adorable sister and beloved brothers who are my support in life.’’

Page 15: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xv

Acknowledgments

I would like to express my deepest appreciation to my supervisor Prof. Edward Ghali for

giving me the opportunity to work on such interesting subject also for his valuable advices

and guidance. His dedication and diligent work ethics are always my inspiration during this

study.

I wish to acknowledge Dr. Georges Gabra for his advices and participation during choosing

the working materials and their testing. Sharing his experience with me was really valuable.

Dr. Fariba Safizadeh and Dr. Wei Zhang are gratefully acknowledged for their support,

assistance, and contribution during this work. I’m really thankful to Mr. Georges Houlachi

for his insightful comments and valuable contribution.

Zinc Électrolytique du Canada (CEZinc) Limitée, Hydro-Québec, and Natural Sciences and

Engineering Research Council of Canada (NSERC) are gratefully acknowledged for their

financial support. I would like also to thank Mrs. Vicky Dodier, Mr. André Ferland, Mr.

Jean Frenette, and Mr. Alain Brousseau for their professional technical participation. Kind

help, friendships, and ideas of Deniz Bas, Chaoran Su, and Ramzi Ishak, are really

appreciated.

Page 16: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xvi

Forward

This thesis is composed of seven chapters and presented as articles insertion form. The first

chapter provides a brief introduction of two main electrometallurgical processes; the

electrowinning of zinc, and the electrodeposition of cathodes in chlorate production. In the

second chapter, a targeted literature review covers the previous studies concerning the

problems in zinc electrowinning process such as the metallic impurities also the beneficial

effect of organic additives on this process. Chapter two discusses as well the utilization of

different electrodeposited cathodes in the chlorate industry and their effect on hydrogen

evolution reaction overpotential. The experimental steps and conditions are explained in

chapter three. Chapters four, five, and six present the results of this work in the form of

three scientific papers as the following:

Chapter four

Influence of Different Organic Additives on Zinc Electrowinning from Acidic Sulphate Electrolyte

N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

This paper was presented in the 54th annual Conference of Metallurgists - COM, Toronto,

Canada (Aug. 23-26, 2015) and published in the proceeding by Canadian Institute of

Mining, Metallurgy and Petroleum. CIM-COM, paper #8986 pp 1-13, ISBN: 978-1-

926872-32-2.

In this work, different five additives from different organic groups have been studied in the

zinc electrowinning process. The experimental measurements and analysis along with paper

writing and presentation were performed by the first author. The scientific revision was

done by Dr. W. Zhang, Dr. G. Gabra, and Prof. E. Ghali. The project was supervised by

Prof. E. Ghali and Mr. G. Houlachi.

Page 17: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

xvii

Chapter five

Electrochemical Studies of Ionic Liquid Additives during the Zinc Electrowinning Process

N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

This paper is published in the journal of Hydrometallurgy, Vol. 157, 2015, pp 261-269.

In this work, the effect and importance of ionic liquids on zinc deposits and lead

contamination have been highlighted by using certain electrochemical techniques. The

experimental measurements and analysis along with paper writing were performed by the

first author. The scientific revision was done by Dr. W. Zhang, Dr. G. Gabra, and Prof. E.

Ghali. The project was supervised by Prof. E. Ghali and Mr. G. Houlachi.

Chapter six

Electrodeposition and Study of the Electrocatalytic Activity of Fe-Mo-P Alloys for Hydrogen Evolution during Chlorate Production

F. Safizadeh1,*, N. Sorour1, G. Houlachi2, and E. Ghali1

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

This paper is submitted to the International Journal of Hydrogen Energy, February, 2016.

Different alloys of Fe-Mo and Fe-Mo-P have been prepared to study their effects as

cathodes on hydrogen evolution reaction in similar conditions to chlorate production. The

experimental measurements were carried out by the second author, analysis was done by

the first and second authors while, paper was written by the first author. The scientific

revision and project supervision were done by Prof. E. Ghali and Mr. G. Houlachi.

Finally, chapter seven provides complete conclusions for this thesis as well as few

recommendations for future work plan.

Page 18: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

1

CHAPTER 1

INTRODUCTION

Page 19: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

2

1.1. Background

Electrolysis is a process by which an electric current is moved through a substance

to do a chemical change. This chemical change is occurred when the substance gains or

losses electrons (reduction or oxidation) and this is always preformed in an electrolytic cell.

Electrolysis is used enormously in many metallurgical processes, such as: extraction

(Electrowinning), deposition of metals or alloys (Electrodeposition), metal purification

(Electrorefining), and substrates plating (Electroplating) [1].

Electrowinning or Electrodeposition is an electrochemical process by which an adherent

film of desired metal or alloy can be deposited onto an electrode by electrolysis of a

solution containing the desired metals ions or their complexes [2].

Accordingly, the Electrowinning of zinc and electrodeposition of different metals or alloys

as cathodes in chlorate production are two major electrometallurgical processes which have

been always the concern of many studies and researches.

1.2. Zinc Electrowinning

1.2.1. Zinc Metal

Zinc is considered as the fourth most widely used metal following iron, aluminum

and copper [3]. Zinc is the 24th most abundant element in the earth’s crust, with an average

concentration of 65 g ton-1 (0.0065%) [4]. The discovery of pure metallic zinc was done by

the German chemist, Andreas Marggraf in 1764 [5]. The metal has silvery blue-gray color

with relatively low melting point of 419oC and boiling point of 907oC with density of 7.14

g cm-3 at 20oC. Zinc has medium strength and hardness properties which are greater than

those of tin and lead but less than those of aluminum and copper [6]. Global zinc

consumption grew from around five million tonnes per year in 1970 to over eight million

tonnes per year by the end of the 20th century and to 9.7 million tonnes per year in 2003 [7].

Canada is number seven globally in producing zinc with total production of 550,000 tons in

2013 according to The US Geological Survey.

Page 20: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

3

1.2.2. Methods of Extraction of Zinc Metal

The world primary zinc industry employs five different processes: Retorting

processes (I) Electrothermic, (II) Vertical, and (III) Horizontal which were the main

methods at beginning of last century but had been gradually declined by 1950. (IV)

Imperial Smelting ISF (blast furnace) which represents ≈15% of zinc metal production.

However, currently ≈85% of production of zinc metal is produced by (V) Electrolytic

process (Roast - Leach - Electrowinning) [7].

The production of zinc from sulphides is predominantly conducted through roast-leach-

electrowinning process which was used firstly in 1916 by Cominco - BC, Canada and

Zinifex - Hobart, Australia.

Figure 1.1. Typical roast-leach-electrowinning processes for zinc [7]

The preparation of the purified zinc solution for electrowinning plant is schematically

presented (Figure 1.1) and starts with the concentrate of the sulphide ore, in which

sphalerite (ZnS) is the predominant component, is roasted in a fluidized bed furnace

forming zinc oxide (ZnO) at 900-1000oC. The zinc oxide is then fed into the leaching tanks

together with sulphuric acid solution in the spent electrolyte from electrowinning. During

Zinc Sulphide Concentrate

Roasting

Leaching /Iron Purification

Purification

Electrowinning

Cathode Zinc

Acid Plant

Spent Electrolyte

SO2

Page 21: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

4

leaching, temperature and pH should be carefully adjusted and manipulated to encourage

the precipitation of ferric hydroxide which acts as metal ion collector and remove others

impurities such as; arsenic, antimony and germanium [7]. Further purification step is

carried out by zinc dust precipitation in 2-3 stages, in which the leached solution is mixed

with a fine dust of zinc which caused the reductive precipitation of the metal ions

electropositive to zinc, while the zinc metal is oxidized. Then, the purified zinc solution is

circulated through electrowinning plant.

The zinc metal is deposited on the cathode in solid form, while the anodic reaction is

the oxygen evolution. The metal deposition rate is always related to the available surface

area, maintaining properly working cathodes is important. Two cathode types exist, flat-

plate and reticulated cathodes, each with its own advantages. Flat-plate cathodes can be

cleaned and reused, and deposited metal is recovered. Reticulated cathodes have a much

higher deposition rate compared to flat-plate cathodes. These cathodes are not the best

choice as they are not reusable and must be recycled [8]. For zinc electrodeposition,

aluminum cathodes are usually used as they proved their high performance. During the

electrolysis the deposit is adherent to the aluminum cathodes while, it is separated easily by

mechanical methods after the electrodeposition [9].

However, actually one of the main challenges faced by the zinc electrowinning industry is

the presence of the metallic impurities in the electrolyte even after several purification

steps. These metallic impurities with very small concentrations of ppm or even ppb affect

negatively the current efficiency, power consumption, and the purity of the deposited zinc

metal [10].

1.2.3. Uses of Zinc

The electrochemical properties of zinc are very important in production as well as

applications of zinc. Electrowinning process in zinc refining, electroplating, zinc batteries

and zinc coating for corrosion protection of steel are all based on the electrochemical

properties [11].

The uses of zinc can be divided into six categories: (a) galvanic coatings for steel, (b) die

casting, (c) alloys, (d) zinc chemicals, (e) rolled zinc, and (f) miscellaneous including zinc

Page 22: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

5

oxides and others. As shown in Figure 1.2 the most important application of zinc is its

action as protective coatings against corrosion for steel structures due to its relative

corrosion resistance in atmospheric and other environments. Approximately, one-half of

zinc production is used for this purpose [12-13].

Figure 1.2. The major uses of zinc [13]

Galvanic protection is also called ‘’sacrificial protection’’ as the metal used is sacrificing

itself to protect the structure. This type of protection utilizes a galvanic cell consisting of an

anode made from more active metal than the structure [12]. So, zinc is the most common

metal used to protect steel due to its position in the electromotive series, [Zn/Zn2+:

-0.76V/SHE] VS [Fe/Fe2+: -0.44V/SHE] as zinc is more active than iron, accordingly it

starts sacrificing itself first before iron.

1.3. Electrodeposition of Alloys as Cathodes in Chlorate Production

1.3.1. Chlorate Production

Chlorate, chlor-alkali, and water electrolysis operations are among the largest

consumers of electricity in electrolytic industries. Sodium chlorate (NaClO3) is produced

industrially by an electrochemical process, where chloride ions (Cl-) are oxidized to

chlorine (Cl2) on the anode then dissolved in water forming chlorate ions and hydrogen gas

is evolved on the cathode. The selectivity of main reactions, as well as the energy required

by the chlorate process, depends on the electrode materials and surface state also on the

electrolyte composition [14].

Galvanizing - 58%

Die Casting - 14%

Brass / Bronze - 10%

Compounds - 9%

Rolled Zinc - 6%

Miscellaneous - 3%

Page 23: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

6

The electrolytic hydrogen is quite pure and is acceptable for various applications.

Normally, hydrogen gas recovery process involves many steps: (a) cooling, (b) boosting,

(c) compression, and (d) purification. The obtained hydrogen is used as fuel or as raw

material for the production of HCl [15]. The hydrogen evolution reaction (HER) on

different metal cathodes in acidic or alkaline media is one of the most investigated reactions

in the electrochemistry field. The HER was always place of interest due to: (i) hydrogen is

an interesting candidate, energy carrier, for fuel cells applications, (ii) it is one of the main

reaction products during chlorine production and, (iii) the HER provides the high pure

hydrogen gas. This reaction is the main reaction produced in alkaline water electrolysis,

hydrogen-based fuel cells, and during some industrial practices such as chlorate cells. Due

to the high consumption of energy; reducing the hydrogen evolution overpotential is always

one of the challenges and purpose of many studies [15-16].

1.3.2. Cathodes in Chlorate Production

Cathode materials in the first years of chlorate manufacture were copper, nickel and

even platinum. Recently, mainly mild steel and in some plants titanium or a Ti-0.2% Pd

alloys are used. Chlorate electrolyte containing the oxidizing agents hypochlorite and

chlorate is extremely corrosive and oxidises most of those metals when they are not under

cathodic protection [17]. Mild steel is one of the most popular used cathodes in chlorate

production due to its low cost. However, these cathodes are not the best choice due to the

high overpotential values of HER reaching 850-950 mV, depending on the surface

roughness also due to low corrosion resistance in the aggressive chlorate electrolyte [18].

Therefore, efforts and attempts are made by many researchers in order to obtain binary and

ternary mild steel coated alloys with electrocatalysts exhibiting low hydrogen evolution

overpotential as well as improving the corrosion resistance and mechanical properties.

Molybdenum and phosphorus are among the different elements that employed to fabricate

new cathodes having a positive electrocatalytic behavior towards HER and corrosion

resistance. These two elements cannot be electrodeposited directly from the aqueous

solutions; therefore, they require another metal to stimulate its co-deposition [19].

Page 24: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

7

1.4. Objectives and Detailed Approaches

The aim of this work is divided into two main objectives:

1.4.1. Effect of Certain Organic Additives on Zinc Electrowinning Process

‐ The main challenge faced by zinc electrowinning industry is the presence of

metallic impurities; lead is one of the major impurities as far as lead-based anodes

are still used in this process. Pb2+ ions are usually reduced and co-deposited as

elemental lead with zinc metal on the cathode which reduces the purity of the

obtained zinc metal. Organic additives proved their good performance in improving

this process by reducing the detrimental effect on power consumption, current

efficiency and the purity of deposited zinc as well as modifying the surface

morphology.

‐ In this study, certain organic additives are chosen from different organic groups:

(1) Polyacrylamide [PAM] is one of the well known organic polymers used in

industry and has been tested previously as additive in copper electrowinning,

showing a good effect in improving morphology of the surface. (2) Tetra-

butylammonium bromide [TBABr] is one of the quaternary ammonium salts group

which have been examined also as additives. (3) Benzalkonium chloride [BKCl] is

a cationic surface-acting agent belonging to the quaternary ammonium salts with

aromatic group. (4) Chitin is also selected as it is one of the natural polymer

compounds which can be found in crabs, lobsters and shrimps.

‐ Ionic liquid salts are currently used in many chemical and hydrometallurgical

applications due to their chemical and physical properties. Ionic liquids are widely

used in liquid–liquid extraction and electrodeposition of some metals. Also, they are

considered as a medium in the electrodeposition of aluminum on a stainless steel

cathode. In this work also, different ionic liquid salts are selected in order to

examine their effects on zinc electrowinning process. (5) 1-butyl-3-

methylimidazolium chloride [BMIM]Cl, (6) 1-butyl-3-methylimidazolium bromide

[BMIM]Br, and (7) 1-Ethyl-3-methylimidazolium methanesulfonate [EMIM]MSO3,

are chosen to be examined as additives in zinc sulphate electrolyte with different

concentrations.

Page 25: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

8

‐ The combination between the selected organic additives and antimony as metallic

impurities is also considered in this work.

‐ Various electrochemical techniques and other techniques are employed to evaluate

the efficiency of these additives in zinc electrodeposition. Galvanostatic

polarization, potentiodynamic, cyclic voltammetry are conducted in order to

examine the electrochemical activity and the cathodic behavior. Scanning electron

microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma

spectroscopy (ICP) are used as well to determine the surface morphology,

crystallographic orientation and lead content in zinc deposit, respectively.

1.4.2. Performing the Electrodeposition of Fe-Mo & Fe-Mo-P Alloys as Cathodes

‐ Chlorate production process is one of the largest consumers of energy in electrolytic

industries. Due to this high consumption of energy; reducing the hydrogen evolution

overpotential is always one of the challenges and purposes of many studies.

Therefore, improving the cathodic materials exhibiting lower hydrogen evolution

overpotential is one of the targets of many studies in order to reduce the power

consumption.

‐ In this work, the electrodeposition of different Fe-Mo and Fe-Mo-P coatings on

mild steel substrates is carried out in order to study the effect of different

phosphorous and molybdenum contents on the HER. The electrocatalytic activities

of these cathodes are assessed in simulated conditions of chlorate industry.

‐ Potentiodynamic polarization and electrochemical impedance (EIS) techniques are

employed in this work to examine the electrocatalytic activity of the

electrodeposited coatings in alkaline solution. Also, (SEM) and (XRD) are used to

determine the surface morphology and state, respectively.

Page 26: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

9

CHAPTER 2

LITERATURE REVIEW

Page 27: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

10

Based on the previous reviews and research studies, this chapter is divided into

three main parts concerning: (a) additives and impurities in zinc electrowinning, (b) alloyed

metals and elements as cathodes used for chlorate production, and (c) recommended

electrochemical techniques for evaluation of additives and cathodes during electrolysis.

Also, at the end a short summary is given based on the reviewed literature and the

objectives of this project.

2.1. Zinc Electrowinning Process

Zinc ores are roasted, dissolved in sulphuric acid and highly purified by zinc dust as

explained in chapter 1. Then, metallic zinc is won from the purified zinc sulphate solution

by electrolysis using aluminum cathodes and lead-based anodes. Normally, many zinc

electrowinning plants operate with current densities of 400-500 A m-2 at temperature of

38±2oC [20].

It is important to understand and determine the electrochemical reactions occur in this

electrolysis process in order to measure the potentials and power consumed for

electrowinning of zinc. Figure 2.1a shows a simple cell of electrolysis by using aluminum

cathode and lead-silver anode in acidic zinc sulphate solution.

The cathodic reactions with standard potentials are:

2 → Eo = -0.763V (2.1)

2 2 → Eo = 0.00V (2.2)

The anodic reactions with standard potentials are:

→ 2 2 Eo = -1.229V (2.3)

The overall reaction is:

→ 2 Eo= -1.992V (2.4)

Page 28: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

11

As many plants add manganese ions to the sulphate solution due to its remarkable effect in

forming compact layers of MnO2 on the anode to reduce the lead contamination, so,

following reaction cannot be neglected.

2 → 4 2 Eo= -1.208V (2.5)

Approximately, 90% of the cathodic current is consumed to produce zinc metal as reaction

(2.1), while, 99% of the anodic current is consumed to produce oxygen gas as reaction

(2.3). There are several factors that affect these reactions and their potentials such as: Zn2+

concentration, pH, current density, and temperature [20]. Therefore, those variables must be

considered in the electrowinning process in any plant.

Figure 2.1. (a) Simple electrolysis cell for zinc (b) Aluminum cathodes deposited by zinc

2.1.1. Lead-Based Anodes

As far as lead anodes are used so, many problems occur due to the weakness and

ductility which cause buckling and sagging of the anodes resulting in current distortion in

the elctrowinning cell. Also, the corrosion of lead based anodes causes lead contamination

of the zinc deposit. When this contamination exceeds the normal level fixed by the plant,

maintenance or replacement of the anodes are required [21-22].

Instead, lead alloys containing 0.37 to 1% silver have been used as anodes in zinc

electrowinning industry since 1909 [23]. Silver is alloyed with lead anode to reduce the rate

of corrosion and improve conductivity of the anode. The addition of silver also reduces the

oxygen evolution overpotential by approximately 120 mV compared to pure lead [24]. A

Page 29: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

12

small amount of silver oxide maybe formed on the surface of the anode along with lead

oxides. The poor mechanical property is one of the disadvantages of Pb-Ag anodes

accordingly, they are relatively weak and bended quite easily when struck by aluminum

cathode sheets when they are removed or inserted to the cell. Therefore, calcium is

sometimes added to the alloy by percentage of 0.05 to 0.08% in order to improve the

mechanical properties [25].

2.1.2. Corrosion of Lead-Based Anodes

Lead metal can be dissolved by oxidizing acidic solutions with the formation of

divalent plumbous ions Pb2+. Further oxidation can result in conversion of divalent

plumbous ions into brown quadrivalent lead dioxide PbO2. In the absence of passivating

substances such as carbonates, and oxidizing action can cause lead to corrode, except at

high electrode potentials where PbO2 is stable [26].

Figure 2.2. Potential-pH diagram obtained according to the ionic activities in an actual anodic film for the Pb-H2O-H2SO4 system at 25oC (potential vs. SHE) [27]

Page 30: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

13

Guo Y. [27] determined the potential-pH relation of lead in sulphuric acid as Figure 2.2.

This diagram includes the basic lead sulphates PbO.PbSO4, 3PbO.PbSO4.H2O and the

tetragonal oxide, PbO (PbOt). When a lead electrode is immersed in sulphuric acid solution

and polarized anodically to potentials in the area of stability of PbO2.

It has also been observed by X-ray diffraction (XRD) analysis that two forms of PbO2 are

found with rhombic (α-form) being stable at lower potentials than the tetragonal (β-form).

α-PbO2 they found that they show more dense deposits, composed of large and closely

packed crystals. On the other hand, β-PbO2 deposits are less compact being composed of

poorly bonded, fine, needle shaped crystals [28]. However, the problem of lead

contamination of zinc deposit is still the solution by lead ions is the most critical problem in

zinc electrowinning.

2.1.3. Oxygen Overpotential of Lead-Based Anodes

Two main reactions occur on the anode which are; the evolution of oxygen gas O2

and the oxidation of PbSO4 to PbO2 [27]. The oxidation of water to oxygen is theoretically

possible at 1.23 V according to reaction (2.3), but production of oxygen is only observed at

potentials more positive than the equilibrium potential for the PbO2/PbSO4. Therefore, the

oxidation of lead sulphate to lead dioxide and the evolution of oxygen gas require

overpotentials.

During the electrowinning, lead alloys are immersed in the zinc electrolyte, the reaction

(2.6) takes place on the fresh anode surface at first;

→ 2 E= -0.356 V/SHE (2.6)

The anode surface is covered with time by non-conducting layer of PbSO4. The anodic

current density and the potential on that part of the anode surface of non-covering with

PbSO4 increase. The following reaction is expected on the Pb surface at atmospheric

temperature:

2 → 2 2 E= 1.685 V/SHE (2.7)

Page 31: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

14

This reaction takes place instead of reaction (2.6), thus, a well conducting PbO2 occurs

instead of PbSO4, so the current density and the anodic potential decrease. The reaction of

the oxygen evolution starts on the layer of PbO2 and sulphuric acid in the renewed

electrolyte. After the anodic film formation, ≈ 99.20% of the electricity goes to the oxygen

evolution, ≈ 0.67% for formation of PbO2 and ≈ 0.13% for the other reactions [29].

The overpotential of oxygen can be reduced if a good lead anode is used, or alloyed with

another metal. As discussed previously that silver is the major metal could be alloyed with

lead at certain limit due to its high cost to reduce oxygen overpotential. It has shown also

that anodic potential of Pb and Pb-Ag anodes becomes more positive with the increase of

electrolyte acidity. The overpotential of Pb-Ag (1% Ag) anodes is 80-120 mV less than that

of pure Pb [30]. Table 2.1 illustrates the relation of electrode potential by changing

different alloys of anode at different current densities.

Table 2.1. Electrode potential (V/SCE) vs. current density (A m-2) of anodes from lead and its alloys in 1.8 M H2SO4 at 30oC [30]

Current density (A m-2)

Electrode potential vs. V/SCE

Pb Pb-Sn 1%- Ca 0.07%

Pb-Ag 0.37% Ca 0.12% - Ti 0.99%

Pb-Ag 0.97% Sn 0.63%

Pb-Ag 0.76%

Pb-Ag 0.9% Ca 0.04%

250 1.915 1.925 1.880 1.850 1.835 1.810

500 1.940 1.955 1.915 1.885 1.870 1.850

1000 1.975 1.980 1.945 1.925 1.915 1.885

2.1.4. Role of Manganese Ions in the Electrolyte

Usually, manganese ions are added to electrolyte to reduce lead contamination as

mentioned previously. Mn2+ ions are active electrochemically at lead or lead alloyed anodes

and the manganese oxide may be formed after the formation of PbO2 on the anode before

extensive evolution of oxygen occurs. It has been shown that the electrochemical

deposition of manganese on the anode may act favorably to minimize disintegration of the

anode scale by decreasing the amount of lead dioxide PbO2 formed on the anode [31].

Usually, manganese ions are added continuously to the industrial electrolyte in the form of

MnSO4, and accordingly the followings reactions are expected on the anode [30]:

Page 32: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

15

2 → (2.8)

4 → 6 5 (2.9)

2 → 3 (2.10)

3 2 2 → 5 3 (2.11)

The lead anode is protected from corrosion by MnO2 and PbO2 layers, since the well

adherent oxide film of MnO2 increases the thickness and oxide layer PbO2-MnO2 which

acts as barrier on the anode surface. The presence of Mn2+ ions affects slightly the anodic

potential since the potential decreases as a result of depolarization effect during the

oxidation of Mn2+ on the anode. The potential decreases also as a result of the formation of

a protective layer of manganese oxides. Moreover, the presence of Cl- ions in the

electrolyte leads to a considerable decrease of the anodic potential for lead-based anodes.

Also, the increase of temperature results in a remarkable decrease of the anodic potential in

the presence of Cl- ions [30,32]. Figure 2.3 shows the lead-based anode before and after 5

hours of electrolysis as the corrosion of the anode is very remarkable with formed MnO2

layers.

However, studies proved that Mn2+ ions have also an effect on the cathodic reactions at

high concentrations. Zhang and Hua [33] revealed that adding Mn2+ ions in the

concentration range of 1-10 g dm-3 has no significant effect on the current efficiency (CE),

while a decrease in CE of more than 35% was happened at addition of high concentration

of 50 g dm-3. This decrease in CE was due to the strong depolarizing effect of MnO4- ions

and other oxidized products of manganese on hydrogen evolution reaction. The addition of

Mn2+ ions was also observed to change the surface morphology and deposit quality of the

electrodeposited zinc, affecting the crystallographic orientation.

Page 33: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

16

Figure 2.3. Lead-based anode; (a) Before electrolysis and (b) After 5 hours of electrolysis

2.1.5. Surface Structure and Crystallographic Orientation

Almost all electroplated or electrodeposited metals are crystalline, which means that

the atoms are arranged in a regular three dimensional pattern called ‘’Lattice’’. The most

known lattices are: (i) face centered cubic (FCC), (ii) body centered cubic (BCC), and (iii)

hexagonal close packed (HCP) (Figure 2.4). Normally, Zn atoms are arranged in lattice

type hexagonal close packed (HCP) [34]. The crystal structure resulting from an

electrodeposition process is strongly dependent on the relative rate of formation of crystal

nuclei and growth of existing crystals. Finer-grained deposits are the result of conditions

that favourite crystal nuclei formation, while larger crystals are obtained in those cases that

favourite growth of existing crystals. Generally, a decreasing crystal size is the result of

factors which increase the cathodic polarization such as: increasing current density,

different electrolytes, and addition of colloids or additives [35].

Texture, which is preferred distribution of grains (individual crystallites) having a particular

crystallographic orientation with respect to a fixed reference frame, is an important

structural parameters for bulk materials and coatings. It is important to be able to specify

certain planes in crystal lattices. Miller indices signify a single plane or set of parallel

planes which are always presented in parentheses such as (100).

Page 34: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

17

Figure 2.4. Zinc deposit shows HCP Lattice among the three most important lattices

Electrochemical parameters appear to be the only controlling factor. For example, texture

mainly depends on the cathodic potential and pH of the solution for a given electrolyte

composition, this also applies to current density if temperature is constant. It has been

revealed that electrodeposits have the (111) direction normal to the surface for BCC crystal

structures and the (110) direction for FCC substrates, independent of substrate orientation.

With hexagonal closed packed HCP metals such as zinc, the (101) direction is predominant

[36].

2.1.6. Metallic Impurities in Zinc Electrowinning

The presence of metallic impurities in zinc sulphate electrolyte is a critical problem

for zinc electrowinning industry. Low concentrations of metallic impurities influence

negatively the zinc deposition on the cathode; leading to a decrease in current efficiency, a

change in deposit morphology as well as an increase in cell voltage [37-38]. Actually, the

reduction of hydrogen ions in solution is affected in the presence of the impurities. Certain

impurities, e.g. Ge and Sb are hybrid formers may facilitate the hydrogen evolution reaction

HER, other impurities such as Ni and Co, more noble than zinc, cause re-dissolution of the

zinc deposit (low current efficiency) [39].

There have been many studies over the past decades dealing with the harmful effect of

impurities in zinc electrowinning. While, most of electrolytic zinc plants follow the same

general procedures in order to keep the optimal operating conditions which have usually

been arrived by experience and depend on the type of zinc ore treated and its impurity

content [37].

Page 35: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

18

2.1.6.1. Effect of Lead Impurity on Zinc Deposition

As mentioned previously that lead impurity is one of the challenges in zinc

electrowinning industry so, some studies were done to investigate the effect of lead ions in

the solution. Mackinnon et al. [40] conducted several investigations of the influence of lead

in an industrial zinc solution (55 g dm-3 Zn2+ + 150 g dm-3 H2SO4). They found that the

effect of lead (6 mg dm-3 at 35oC) on current efficiency was current density dependent,

producing an increase of ~0.7% at 400 A m-2 and a reduction of ~1.5% at 800 A m-2.

Increasing amounts of lead in the zinc deposits progressively changed the preferred

crystalline orientation from (112) to (101) to (100) to finally a poorly (002) crystalline

structure. The same results trend was also proved by Ault and Frazer [37]. The lead content

of the zinc deposits was dependent on the solution concentration of lead, the form in which

lead was added, the current density as well as presence of Sb and glue. Table 2.2 shows the

variation of current efficiency and preferred orientation of deposits in different

concentrations of lead. Also, Figure 2.5 shows the deposit morphology with different

additions of lead [37,40].

Table 2.2. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of lead at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]

Initial Pb (mg dm-3)

Final Pb (mg dm-3)

Pb removed (%)

Change in CE (%)

Preferred orientation

0 0.01 - - Random

1.0 0.25 75 0.3 (102) (103) (104)

2.0 0.6 70 0.8 (004) (002)

3.0 0.9 70 0.9 (004) (002)

4.0 1.4 65 1.0 (004) (002)

5.0 1.7 66 1.1 (004) (002)

Page 36: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

19

Figure 2.5. SEM photomicrographs (X 385) showing the effect of current density on the morphology of zinc deposits from addition-free electrolyte using unconditioned Pb-Ag anodes. (a) 215 A m-2, 60 min, 0.125% Pb; (b) 323 A m-2. 60 min, 0.076% Pb; (c) 430 A m-

2, 60 min, 0.04% Pb; (d) 538 A m-2, 60 min, 0.021% Pb; (e) 1076 A m-2, 30 min, 0.019% Pb; (f) 2152 A m-2, 15 min, 0.011% Pb [40]

2.1.6.2. Effect of Antimony Impurity on Zinc Deposition

Antimony has been known as one of the most toxic solution impurities with respect

to current efficiency (CE). Ault and Frazer [37] and Lafront et al. [41] studied the effect of

different concentrations of Sb3+ of 0.0055-19 mg dm-3 in high purity-solutions on CE and

morphology; they found that it has a dramatic effect on CE with decrease of ~5.2 to 62.3%.

With such small concentrations of antimony present in the solution, this decrease can be

assumed due to the catalytic production of hydrogen which inhibits the reduction of Zn2+.

The variation of current efficiencies and preferred orientation of deposit with addition of

antimony are shown in Table 2.3. Also, antimony had a dramatic grain-refining effect on

zinc deposit, reducing platelet size even at low concentrations of 0.02 - 0.04 mg dm-3

(Figure 2.6).

Page 37: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

20

Table 2.3. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of antimony at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]

Initial Sb (mg dm-3)

Final Sb (mg dm-3)

Sb removed (%)

Change in CE (%)

Crystal orientation

0 4.0 - - Random

4.0 4.0 0 -5.2 (112) (212)

7.0 5.0 29 -10.0 (112) (211)

10.0 9.0 10 -23.6 (112) (101)

14.0 10.0 29 -52.4 (104) (101)

19.0 13.0 32 -62.3 (004) (103)

Figure 2.6. SE micrographs showing the morphology of 6h zinc deposits electrowon at 500 A m-2 and 38oC from electrolytes containing; (a) and (b) 0.02, (c) and (d) 0.04 mg dm-3 Sb [42]

Although antimony has negative effect on zinc deposit, it showed very good effect when

combined with some organic additives such as glue and gelatin.

Page 38: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

21

2.1.6.3. Effect of Copper, Nickel and Cobalt Impurities on Zinc Deposition

Although copper readily removed from zinc electrolyte by zinc dust cementation

purification, it can re-enter the electrolyte via corrosion of the bus bars. It is known also

that copper co-deposit with zinc leading to a reduction of metal quality at certain

concentrations. Ault and Frazer [37] also Mackinnon [43] studied the effect of Cu on

current efficiency and crystal orientation at different concentrations and different current

densities (Table 2.4).

It is shown that content of electrodeposited zinc increased with increasing copper

concentration in the electrolyte and with decreasing current density. Although copper co-

deposited with zinc, it did not result in a dramatic decrease in the current efficiency but co-

deposited copper reduced the grain size of the zinc deposits [43].

Table 2.4. Effect of copper on current efficiency and crystal orientation of zinc deposit at different concentrations and different current densities for electrolysis in 55 g dm-3 Zn2+, 150 g dm-3 H2SO4 at 35oC [43]

Current density (A m-2)

Copper (mg dm-3)

CE (%)

Cathode Copper (%)

Crystal Orientation

430 0 93.6 - (112)(103)(102)

5 93.0 0.025 (112)(110)

10 95.3 0.060 (112)(110)

20 95.1 0.095 (112)

30 92.3 0.157 (114)(112)

50 92.5 0.254 (002)(101)

323 0 96.0 - (112)

10 94.0 0.041 (112)

20 92.4 0.129 (101)(103)

30 94.8 0.240 (002)(101)

50 93.0 0.399 (002)

215 0 94.3 - (112)

10 92.1 0.065 (101)(102)(103)

20 93.9 0.161 (101)(002)(103)

30 90.7 0.232 (101)(002)

50 88.5 0.393 (101)

Page 39: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

22

Nickel is one of the most injurious impurities in the electrolytes. During the electrowinning

of zinc from sulphate electrolytes in the presence of nickel, a re-dissolution process of the

deposited zinc takes place. Nickel co-deposits with zinc and forms numerous galvanic

micro-batteries. Hydrogen evolves on the nickel zone and surrounding zinc re-dissolves,

causing spongy and dark deposits [44]. After many formation and dissolution cycles of the

zinc deposits, the frequency of the cycle increases, since the cathode is polluted gradually

by nickel until zinc deposition can no longer occur. Ault and Frazer [37], Stefanov and

Ivanov [45], and Morrison et al. [46] studied the effect of Ni impurity on current efficiency

and deposit structure during zinc electrowinning (Table 2.5). Briefly, the more noble co-

deposited metals with Zn enhance hydrogen evolution in acidic medium and their effect is

expected to be in the following order: Ni, Co, and then Cu.

Table 2.5. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of nickel at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]

Initial Ni (mg dm-3)

Change in CE (%)

Crystal Orientation

0 - Random

0.25 -0.1 (114) (102)

0.5 -0.1 (114) (102)

1.0 -0.2 (114) (102)

1.5 -0.1 (211) (105)

2.0 -0.2 (114) (102)

5.0 -0.3 (204) (102)

The CE declined very slowly, with increasing nickel concentration, with most of the

decrease occurring in the 0-1 mg dm-3 range. The crystal orientation changed from a

relatively random pattern with (102), (104), (114), (204) as major plans, to an orientation

dominated by the (114), (102), (204), (203) plans, when the nickel concentration was in the

range 0.25-2 mg dm-3. At 5 mg dm-3 Ni the (114) plane was replaced by the (204) plane.

Cobalt combined with nickel is difficult to be removed from the electrolyte, can have

disastrous effect on zinc electrowinning under certain conditions [47]. Maja and Spineli

[48], and Maja et al. [49] have studied the effect of both impurities on induction time in

Page 40: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

23

zinc electrowinning. The term “induction time or period” is used in zinc electrowinning.

During this induction time, which coincides with the beginning of zinc electrodeposition,

the zinc deposits are uniform and adhere firmly to the cathode. The typical current

efficiency is 93‐95%. Following this induction time, zinc re‐dissolution occurs with

hydrogen evolution. After the zinc is completely dissolved, deposition restarts. The

induction time depends on several factors such as temperature, cathodic current density,

and the concentrations of sulphuric acid, zinc and impurities [50-51]. It has shown that an

induction period more than one hour exists before cobalt and nickel begin to have an effect

on CE and zinc deposit. After the induction period CE decreases rapidly with time. The

length of induction period decreases with increasing temperature, increasing acid

concentration and with decreasing current density [49].

2.1.7. Additives in Zinc Electrowinning

The presence of high concentrations of impurities in the industrial electrolytes

decrease the induction period associated with zinc electrowinning process resulting in

deterioration of zinc deposit quality and in decrease the current efficiency [51]. High

quality and high current efficiency of zinc deposit are always obtained from pure

electrolytes. However, various electrolyte purification steps are nonviable economically.

Accordingly, an alternative method to reduce the detrimental effect of metallic impurities is

to use suitable organic additives [52]. These additives may be classified into non-ionic,

anionic and cationic types. Most of used additives are organic materials with high

molecular weight which could be adsorbed on the cathode and act as a diaphragm

(hydrogen inhibitor and crystal growth modifier). In industrial zinc deposition the most

commonly used additives are the naturally occurring, gums, gelatins or glues which in

acidic solutions are cationic [53].

2.1.7.1. Effect of Glue

Animal glues are most known additives in the zinc electrowinning industry. Such

additives are often added to the electrolyte at low concentrations in order to have smooth

Page 41: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

24

and compact zinc deposit. Although glues have beneficial leveling effect on zinc deposit

but when increasing the addition alone to the purified electrolyte usually result in a

decrease in the current efficiency of zinc deposit. Also, addition of glues alone to the pure

electrolyte leads to cathodic overpotential this is due to the re-arrangement of

crystallographic orientations which consumes over voltage [39]. Moreover, glues also

interact in a beneficial way with certain impurities in the electrolyte; one of those famous

impurities is antimony. Addition of glue to an electrolyte contains low concentrations of Sb

(≤ 0.02 mg dm-3) optimizes zinc deposition CE and modify the zinc deposit morphology

and preferred crystallographic orientation. In spite of the detrimental effect of Sb on CE, a

small concentration of antimony is usually added to the electrolyte to reduce zinc deposit

adherence to the aluminum cathode and because its beneficial interaction with glue [37].

Glue in presence of antimony has also a significant effect on the zinc deposition

overpotential [54]. The addition of glue alone increases the overpotential; that is polarizes

zinc deposition while increasing the concentrations of antimony decrease the overpotential

due to the high hydrogen evolution. Accordingly, balanced additions of glue and antimony

produce zinc deposit potential or (nucleation overpotential) that results in an optimum

values for the CE with uniform deposit [22,55]. The term ‘’Nucleation Potential’’

corresponds to the commencement of the reduction of Zn2+ at the cathode. This potential

can be easily determined by cyclic voltammetry technique. While, the potential difference

between the crossover point and the point where the Zn2+ ions are started to be reduced on

the cathode is known as nucleation overpotential (NOP). NOP is used to elucidate the

extent of polarization of a cathode, and high NOP values indicate strong polarization. It is a

convenient parameter to show the effects of various additives on zinc electrowinning. The

number of nuclei can be calculated by the following equation:

. ɳ

Where; α and b are constants, ɳ is the nucleation overpotential. It indicates that the higher

nucleation overpotential, the much more fine-grained zinc deposits can be obtained with

good crystallographic orientation [56].

Page 42: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

25

It was found that, adding 0.02 mg dm-3 of Sb3+ ions to standard zinc electrolyte reduced CE

from 91% to 86.7% while adding 5 mg dm-3 of glue to this electrolyte in presence of same

quantity of antimony succeeded to increase CE from 86.7% to 92.4%. Presence of

antimony reduced NOP by 75 mV which indicates non-smooth and very small grain size as

well as distortion in crystallographic orientation while addition of 5 mg dm-3 of glue

restored the normal values of NOP and formed medium grain sizes [42].

2.1.7.2. Effect of Natural Products and Surfactants

Natural products and surfactants were always the concern of many studies as

additives in zinc electrowinning. Saponins, Licorice, Tennafroth 250, and Dowfroth 250

were studied as additives by different concentrations (0, 5, 10, and 15 ppm) also their

effects on acid mist suppression have been reported [57].

‐ Saponins: are found in various plants, they are amphipathic glycosides grouped

phenomenologically by the soap-like foaming.

‐ Licorice: is the root of Glycyrrhiza glabra from which a sweet flavour can be

extracted. The scent of licorice root comes from a complex and variable

combination of compounds, of which anethole is up to 3% of total volatiles. Much

of the sweetness in liquorice comes from glycyrrhizin, which has a sweet taste, 30–

50 times the sweetness of sugar.

‐ Tennafroth 250 and Dowfroth 250: are products of Dow Company which used as

foam sealants.

Studies reported that, none of the additives succeeded to increase the CE%. While,

Tennafroth 250 and Dowfroth 250 appeared to achieve high acid mist suppression

efficiency (66% and 62% respectively). The high suppression efficiency with low power

consumption for both additives was supported by surface tension results (67.6 and 67.9

mN/m respectively) and polarization behavior obtained by cyclic voltammetry (NOP at 66

and 48 mV [57].

Sodium lignin sulphonate had been studied by Alfantazi and Dreisinger [58]. The addition

of sodium lignin sulphonate up to 10 ppm had no negative effect on zinc electrowinning

Page 43: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

26

process, nor on the quality of the zinc deposits. CE% maintained constant with addition of

this surfactant.

Also, the extract of horse-chestnut tree (HCE) was tested as an additive in zinc

electrowinning. The additives increased the cathodic polarization and promoted leveling.

HCE had a beneficial influence on the zinc deposit quality, being a good leveling agent by

increasing the nucleation overpotential NOP and the deposition rate of zinc on the cathode

[59].

However, studies proved that gelatin acts more or less as glue in presence of small traces of

antimony in the zinc electrolyte. Small concentrations of antinomy (0.0055 mg dm-3)

reduced the CE by ≈7%, with depolarization by ≈40 mV, while addition 1 mg dm-3 of

gelatin to this electrolyte restored back the normal values of current efficiencies [41].

2.1.7.3. Effect of Synthetic Polymers

The behavior of zinc electrodeposition and Zn deposit morphology were studied in

electrolytes containing polymer additives such as polyethylene glycol (PEG) [60]. PEG

with molecular weight of 1.54x103 was added to the electrolyte with different

concentrations (0, 0.001, 0.01, 0.1, and 1 mg dm-3). Results showed that, PEG acts as a

polarizer to shift the deposition potential of zinc in a less noble direction. It was found that,

increasing the concentration of additive shifted the cathodic potential to more negative

values starting from ~-0.87 V at zero addition to ~-0.93 V at 1 mg of PEG. Another series

of experiment have been done in order to observe the effect of molecular weight of polymer

on the cathodic potential and polarization resistance. As the molecular weight of PEG

increases, the overpotential and the polarization resistance for Zn deposition first increased,

but then decreased when the molecular weight exceeded 1x104.

When the molecular weight of the polymer is less than 1x104, almost all the oxygen

radicals are utilized for adsorption to inhibit zinc deposition effectively. So, the electrolytic

solution contains a smaller number of longer chains as the molecular weight increases. As a

result, the Zn deposition potential is shifted in a less noble direction and the polarization

Page 44: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

27

resistance for Zn deposition increases. It can be concluded that, the degree of polarization

depends on the molecular weight of the additive.

The electrodeposits obtained from additive-free solution are composed of hexagonal

platelets with medium grain size, while the deposits obtained from solution containing PEG

are found to have grain size smaller than that obtained from free-addition electrolyte [60].

2.1.7.4. Effect of Quaternary Ammonium Salts

Quaternary ammonium salts are characterized by having positively charged nitrogen

(cation) covalently bonded to four alkyl group substituents (non-aromatic) (Fig. 2.7a)

and/or benzyl substituents (aromatic) (Fig. 2.7b). R = CnH2n+1, where n=8 to 18, with

mixture of carbon chain lengths, predominantly 12, 14 or 16. Quaternary ammonium salts

are known with their stability under neutral or acidic conditions up to 150oC, but

decomposition can occur with the quaternary ion acting as an alkylating agent in its

reaction with anion [61].

Figure 2.7. Quaternary ammonium salts; (a) Non-aromatic, (b) Aromatic

The effect of some quaternary ammonium salts represented in cetyltrimethyl ammonium

bromide (CTABr) and tetrabutyl ammonium bromide (TBABr) on zinc electrowinning

have been investigated [62]. Results indicated that CTABr has approximately similar

properties to glue the commonly used additive in industry. CTABr has been found to have

the same polarization behavior, crystallographic orientation and surface morphology like

glue while TBABr has less useful properties. This could be explained due to the higher

molecular weight. Addition of small concentrations of CTABr (1 mg dm-3) to the standard

electrolyte increased CE from 89.3% to 93% while adding same quantity of TBABr has no

Page 45: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

28

effect on CE in absence of antimony. In presence of antimony both additives succeeded to

increase CE up to 94.2%. Crystallographic orientation has not been changed for both

additives as the main orientation was (101) [62].

Another quaternary ammonium salt triethybenzylammonium chloride (TEBACl) has been

examined also in zinc electrowinning [52,63]. TEBACl was found to decrease the screening

effect of hydrogen bubbles responsible for the formation of local galvanic cells in presence

of some metallic impurities such as Ni2+ ions. Addition of small concentration of 0.2 mg

dm-3 alone to the standard electrolyte increased CE from 89.3% to 92.8% with total

reduction in power consumption of ≈178 kWh ton-1, while adding the same quantity in

electrolyte containing 0.01-0.02 mg dm-3 of Sb3+ increased the CE up to 95.6% with total

reduction in power consumption of ≈317 kWh ton-1.

2.1.7.5. Effect of Ionic Liquid Salts

Recently, ionic liquids have been used in many chemical and hydrometallurgical

applications due to their chemical and physical properties, as they are salts where the ions

are poorly coordinated, leading to being liquids below the boiling point and even at room

temperature [64]. Ionic liquids consist of an organic cation and inorganic or organic anion

(Figure 2.8); they have a wide range of solubility and miscibility. For example, some of

them are hydrophobic while others are hydrophilic; most of them are non-inflammable and

non-toxic [65-66]. Ionic liquids are widely used in liquid-liquid extraction and

electrodeposition of some metals due to their low melting point and the thermal degradation

properties which are important in the electrochemical media [67]. Also, as a medium in the

electrodeposition of aluminum on a stainless steel cathode [68]. They are used as organic

solvents in electroplating of a range of metals impossible to deposit in water due to

hydrolysis e.g. Al, Ti, Ta, Nb, Mo, and W [69].

Page 46: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

29

(a) Cationic ionic liquids

(a) Anionic ionic liquids

Figure 2.8. Examples of ionic liquids salts; (a) Cationic, and (b) Anionic

The ionic liquids salts in the form of 1-butyl-3-methylimidazolium hydrogen sulphate

[BMIM]HSO4 had been studied as an additive in zinc electrowinning from acidic sulphate

electrolyte [70]. Also, the effects of temperature and current density on zinc

electrodeposition in presence of [BMIM]HSO4 had been reported [71].

Table 2.6. Effect of [BMIM]HSO4 and Gelatin on current efficiency and power consumption during zinc electrodepsotion [70]

Additives mg dm-3

Current Efficiency Cell Voltage Power Consumption kWh ton-1 % V

Blank 89.3 2.89 2655 [BMIM]HSO4

1 90.5 2.78 2520 2 91.6 2.84 2543 5 92.7 2.84 2513 10 91.8 2.87 2564 50 87.8 2.90 2709

Gelatin 1 2

89.5 88.8

2.84 2.85

2603 2633

5 87.4 2.87 2694 10 86.1 2.92 2782 50 81.6 2.92 2935

Combined* 91.8 2.85 2547

* Combined addition of 5 mg dm-3 [BMIM]HSO4 and 1 mg dm-3 gelatin

Page 47: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

30

Studies showed that, addition of [BMIM]HSO4 to the electrolyte by concentrations of 0,1,2

and 5 mg dm-3 increased the CE from 89.3% to 92.7%, while, increasing the concentration

up to 50 mg dm-3 reduced the CE to 87.8% (Table 2.6) due to the excessive adsorption of

additive on the cathode surface which block the active sites.

It was found also, there is sharp decrease in cell voltage by addition low concentration of

additive (1 mg dm-3) from 2.89 V to 2.78 which affect positively the power consumption

from 2655 kWh ton-1 to 2520 kWh ton-1, while cell voltage is increased slightly by

increasing the concentration (50 mg dm-3) to reach 2.90 V which affect negatively the

power consumption.

It was reported that, ionic liquid [BMIM]HSO4 maintained the medium grain size of the

obtained zinc deposit as the standard electrolyte. It is increased the nucleation overpotential

(NOP) gradually by increasing the concentrations of additive. By addition of 0,1,2,5 and 10

gm dm-3 increased the NOP by 112, 115, 123,131 and 160 mV, respectively which

indicates smooth deposit could be obtained at high concentrations of additive [70].

The effect of antimony (III) on zinc electrodeposition in presence of [BMIM]HSO4 has

been examined in acidic sulphate electrolyte [72]. The presence of Sb3+ decreased the

current efficiency and decreased also the cell voltage due the hydrogen evolution reaction.

As shown in Table 2.7, current efficiency decreased from 90.8% to 68.2% by addition Sb3+

up to 0.08 mg dm-3. Also cell voltage decreased from 2.89 V to 2.86 V. The addition of

additive in the electrolyte containing antimony is found to inhibit the hydrogen evolution

reaction which favourites the zinc electrodeposition. Accordingly, CE values were

increased with increasing of the cathodic potentials. The presence of Sb3+ ions in the

electrolyte decreased the NOP values to 60 mV which is an indication of small grain size of

zinc deposit is obtained from, while the addition of additive restored back the normal

values of NOP [72].

Page 48: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

31

Table 2.7. Effect of Sb3+ on current efficiency in absence and in presence of [BMIM]HSO4 during zinc electrowinning [72]

Sb(III) mg dm-3

[BMIM]HSO4 mg dm-3

CE %

Cell Voltage V

PC kWh ton-1

0 0 90.8 2.89 2611 0.01 0 91.3 2.88 2588 0.02 0 90.5 2.87 2601 0.04 0 85.8 2.87 2744 0.08 0 68.2 2.86 3440 0.01 5 93.4 2.85 2503 0.02 5 94.7 2.84 2460 0.04 5 91.5 2.84 2546 0.01 10 92.6 2.86 2534 0.02 10 93.5 2.86 2509 0.04 10 90.4 2.86 2586

2.2. Electrodeposition of Alloys as Cathodes for Chlorate Production

2.2.1. Chlorate Production

Sodium chlorate is produced industrially by an electrochemical process in a typical

chlorate electrolyte consisting of 100-120 g dm-3 NaCl, 1-4 g dm-3 NaClO, Cr(VI)

corresponding to 1-6 g dm-3 Na2Cr2O7, at a bulk pH of 6.0-6.5 and a temperature of 70-

85oC. The electrolyte can also contain NaClO4 at concentrations that should not exceed 100

g dm-3. A high chlorate concentration (500-650 g dm-3 NaClO3) is essential for the

separation of NaClO3(s) by crystallization, and a high chloride concentration is important for

the anode operation [14,73]. The chloride ions are oxidized to chlorine on the anodes and

hydrogen gas is evolved on the cathodes (Figure 2.9). In this process, sodium chloride is

oxidized to sodium chlorate as global reaction explained in Eq. (2.12); at the cathode water

is reduced to hydrogen gas as explained in Eq. (2.13). While, at the anode chlorine is

formed and dissolved as Equations (2.14 & 2.15). Chlorate is formed by disproportionation

reaction (Eq. 2.17) [14].

3 → (Global Reaction) (2.12)

2 2 → 2 (Cathodic Reaction) (2.13)

2 → 2 (Anodic Reaction) (2.14)

Page 49: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

32

→ (2.15)

↔ (2.16)

2 → 2 2 (2.17)

In practice, the cell voltage of chlorate electrolysis is in the range of 2.75-3.5 V at

approximately 95% of average current efficiency (CE). Accordingly, for chlorate cell

operating at 3.3 V with CE of 95%, the power consumption will be equivalent to 5245 kWh

ton-1 according to following equation [14,16,73,74].

. /

Where, PC is power consumption (kWh ton-1), E is the cell voltage (V), and CE is the

current efficiency (%).The efficiency of the main reactions as well as the energy required

by the chlorate process depend on the electrodes materials and on the electrolyte

compositions [14]. Figure 2.9 illustrates the schematic chlorate production process.

However, this process consumes large amounts of energy due to the hydrogen evolution

reaction (HER) overpotential. Hence, the reduction of cathodic overpotential depends

mainly on the cathodic materials. Accordingly, various materials were studied to develop

durable cathodic electrocatalysts to reduce the overpotential of HER [16].

Figure 2.9. Schematic process of chlorate production (Chemetics Inc. B.C., Canada)

Page 50: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

33

2.2.2. Mild Steel Cathodes

One of the most used cathodes in chlorate production is mild steel. However, these

cathodes are not the preferable choice due to the following inconvenients: (i) when the

surface of mild steel is fresh, the overvoltage values of hydrogen evolution reaction at 250

mA cm-2 (η250) are between 850 to 950 mV depending on surface roughness [18]. During

electrolysis, the cathode surface is gradually covered by chromium oxide added in the

electrolyte in order to increase the current efficiency. This can lead to Ca and Mg

containing precipitation giving an increase of 1100 mV in overpotential, (ii) due to the

thermodynamic instability of iron, the steel cathodes are significantly corroded with time,

(iii) corrosion products create many problems during operation and the cathode life is

considerably shortened [16,18].

It has been found that a corroded steel surface requires higher chromate concentrations (>3

g dm-3 Na2Cr2O7) for a high current efficiency for hydrogen evolution. Then, chromate is

added to the chlorate electrolyte, mainly to hinder the side reactions of reduction of

hypochlorite and chlorate on the cathode as explained in Equations (2.18 & 2.19). During

electrolysis Cr(VI) is reduced and forms a thin film, less than 10 nm thick, of Cr(OH)3.H2O

on the cathode. This film hinders also some other cathodic reactions as oxygen reduction,

whereas hydrogen evolution can take place though with changed kinetics compared to that

on a bare electrode surface [15]. As Cr (VI) is harmful and not environmentally

recommended, a replacement of chromate addition is required. Therefore, efforts and

attempts are made by many researchers in order to have binary and ternary mild steel

coated alloys with electrocatalysts exhibiting low hydrogen evolution overpotential as well

as improving the corrosion resistance [75-76].

2 → 2 (2.18)

3 6 → 6 (2.19)

An ideal cathode as an electrocatalyst may include several properties as the following: (i)

low hydrogen overvoltage at industrial current density, (ii) no potential drift with time, (iii)

good chemical and electrochemical stability: long lifetime and no release of deleterious

products during electrolysis, (iv) low sensitivity towards presented impurities in the

Page 51: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

34

electrolyte, (v) low sensitivity to current shut down (short-circuit) or modulation, (vi) no

safety or environmental problems in the manufacturing process, and (vii) easy to prepare at

a low cost in comparison with its life time [16,77]. However, many attempts were

conducted on fabrication of the new cathodes for chlorate production based on only one

metal or alloys of several elements such as Ni, Cu, Co, Mo, Pd, Rh, Ti-Ru-O, Ni-Mo-P and

Ni-P in order to reduce the hydrogen reaction overpotential [78-79].

2.2.3. Fe-Based Alloys Cathodes

Fe metal alone or alloyed with other elements are always considered in certain

fields of research as enhanced catalysts. Mo is one of the most remarkable elements that is

alloyed with Fe for its high rate of hydrogen evolution due to the considerable real surface

area. Elezovic et al. [75] succeeded to deposit electrochemically Fe-Mo alloys which

showed a reduction in HER overpotential by 0.15-0.30 V as compared to mild steel in

chlorate electrolyte. They reported that with increasing the current density during the

electrodeposition, the Mo content is increased, while Fe content is reduced in the prepared

alloys. It was found that lowest HER overpotential was obtained for electrode containing

40.7 at.% and 59.3 at.% of Fe and Mo, respectively [16,75].

The electrocatalytic properties of some Fe–R (R = rare earth elements) crystalline alloys,

Fe90Ce10, Fe90Sm10, Fe90Y10, and Fe90MM10 (MM = mischmetal), have been studied for the

hydrogen evolution reaction (HER) in 1 M NaOH solution at 25oC. Those alloys were

compared with the G14 (Fe60Co20B10Si10) amorphous alloy, which is a good electrocatalyst

material for the HER. High catalytic efficiencies for the HER were achieved on the

Fe90Ce10 and Fe90MM10 electrodes, the latter being a better catalyst as compared with the

G14 alloy. The improvement of the electrocatalytic performances of the Fe90Ce10 and

Fe90MM10 electrodes as compared with the Fe90Y10 and Fe90Sm10 ones was attributed to

synergetic composition effects of these alloys [80].

The research team in Germany, Müller et al. [81] prepared several amorphous melt-spun

Fe-alloys such as Fe82B18, Fe80Si10B10, and Fe60Co20Si10B10. The electrolcatlystic activities

of the obtained amorphous structure of alloys were compared to the crystalline Ni and Fe in

Page 52: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

35

1M KOH solution at 25oC. The obtained overpotential values at 300 mA cm-2 (ɳ300) were

430, 430, and 360 mV for alloys Fe82B18, Fe80Si10B10, and Fe60Co20Si10B10, respectively,

compared to 480 mV for Fe or Ni. Maximum reduction of overpotential of 120 mV was

obtained for the alloy Fe60Co20Si10B10.

2.2.4. Ni-Based Alloys Cathodes

Ni and Ni-based alloys are the most interesting materials as electrodes for hydrogen

evolution reaction applications. In spite of the performance of Ni as a catalyst is not

remarkable as steel towards the electrocatalytic activity, but it has an excellent resistance to

corrosion in highly concentrated alkaline solutions and at elevated temperatures [15-16].

This is the reason why there are many studies to use nickel or its alloys as cathode for HER.

Ni is more stable than the other transition metals such as Fe or Co in alkaline media. Thus,

different binary and ternary Ni-metal electrodes with different configurations including

nanopowders, spinel or perovskite structures, and different preparation methods such as

electrodeposition or electroless plating have been reported in order to increase the surface

area of the electrodes and to improve electrocatalytic performance. Among these, Ni-Ti,

Ni-Zn, Ni-Co, Ni-W, Ni-P, Ni-Mo, Ni-Cu, Ni-Al, Ni-Fe, Ni-Mo-P, Ni-Mo-Cu, Ni-Mo-Zn

and Ni-Mo-Cr have been extensively considered [82-85].

Different binary Ni-based alloys such as: Ni-Co and Ni-W have been electrodeposited on

stainless steel substrates in order to investigate their activities towards HER [86].

Electrodeposition at very high current densities provided macro-porous materials due to the

fact that the metallic deposition takes place simultaneously during the gas bubbling at high

current densities (1000 A cm-2) (Figure 2.10).

Page 53: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

36

Figure 2.10. Scanning micrographs of developed cathodes [86] Hydrogen evolution on these electrodes was evaluated in 30 wt.% KOH solution by means

of steady-state polarization curves and electrochemical impedance spectroscopy (EIS) at

different temperatures 30, 50, and 80oC. At 80oC, the measured hydrogen evolution

overpotential values at 100 mA cm-2 of Ni-Co and Ni-W were compared to Ni catalyst, the

overpotential values were reduced by 52 and 33 mV, respectively. Values of exchange

current densities for these electrodes were in the range of 103 orders of magnitude higher

than those of commercial Ni electrode. This significant catalytic performance can be

attributed to the increase of real the surface area [86-87].

Giz et al. [88] carried out the electrodeposition of Ni-Cu-Fe film on mild steel substrates

from an acetate bath at current density of 25 mA cm-2, temperature of 45oC and at low pH

of 3.2. They succeeded to obtain crystalline surface composition of Ni = 49, Cu = 43 and

Fe = 8 at.%. They examined the catalytic activities toward HER in brine solution (160 g

dm-3 NaCl + 150 g dm-3 NaOH) at different temperatures through steady-state polarization

curves.

Analyzing the values of the overpotential measured at a current density of 210 mA cm-2 in

Table 2.8. An enhanced electrocatalytic activity to the HER is observed for the Ni-Cu-Fe

co-deposit. At 80oC (the operational temperature used in industrial electrolysers) the

overpotential for the Ni-Cu-Fe electrode is 249 mV lower than that of mild steel (404 mV),

and 19 mV lower than that of the previously obtained Ni-Fe material by Carvalho et al.

[89]. The values of the exchange current density (i0) are higher for Ni-Cu-Fe compared to

Ni-Fe and this indicates that the poor intrinsic catalytic activity of this material is

Page 54: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

37

compensated by a larger improvement due to its larger surface area. It is also found that the

larger active surface area is the main factor responsible for the enhanced activity of this

material.

Table 2.8. Kinetic parameters for the HER on the Ni-Cu-Fe electrode [88]

Temperature (oC)

Kinetic Parameters

(-bc) mV dec-1

(io) A cm-2

(ɳ(210)) mV

25 66 6.65 x 10-5 231

40 70 2.08 x 10-4 210

60 73 8.03 x 10-4 179

80 71 1.52 x 10-3 155

Also, it has been found that alloying Co with Ni has a significant effect on the oxygen

evolution reaction (OER) and that the actual effect depends on its content, with an optimum

value in the Ni-Co based materials could be easily prepared by means of electroless-plating

deposition leading to very active surfaces on any type of support, either conducting or non-

conducting. In addition, this synthetic approach has the advantage of covering substrates

with complex surface morphologies, resulting in strongly adherent metal deposits [90].

2.2.5. Molybdenum Co-deposition

Due to the superior properties of Mo towards HER, various investigations and

studies have been conducted during the past three decades about Mo containing alloys. Mo

containing alloys showed a high resistance to corrosion, and low hydrogen overpotential

[16,91]. The electrodeposition of molybdenum in the pure state from aqueous solutions has

not been performed yet, as Mo cannot be reduced alone in aqueous solutions. Accordingly,

Mo can be easily co-deposited with other elements such as: Fe, Ni and Co. Most of studies

suggested multi-steps reduction of Mo species for induced co-deposition. For example,

during the co-deposition with nickel, the first slow step molybdenum oxide (MoO2) reduces

into a mixed oxide with nickel via electrochemical reduction (Eq. 2.20).

4 8 ↔ (2.20)

Page 55: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

38

During this slow reaction, a surface compound that inhibits the hydrogen evolution is

produced by mixed oxides. This slow reaction could be coupled by fast global reaction

producing deposited alloy of Ni3Mo (Eq. 2.21) [92].

3 8 4 → 4 (2.21)

Podlaha and Landolt [93] showed that the deposition rate of Mo might be limited by mass

transport either Mo or Ni species depending on the relative concentrations. They observed

that in citrate electrolyte in the presence of low concentration of Mo ions and excess of Ni

ions, Mo co-deposition followed-up a mass control mechanism. In this way, Mo content

increases in the alloy with the increase of rotation rate of a disc electrode while it reduces

with the increase of current density [93].

Electrodeposition of Ni-Mo alloy coatings has been developed and their characterization as

cathode for HER has been studied as well [94]. K2P2O7 was used as complexing agent as it

was found that higher percentage of Mo could be co-deposited with Ni from such type of

electrolyte in comparison with the citrate-based electrolyte. It was confirmed that, the

percentage of Mo in the deposit increases with increasing deposition current density, from

about 28 at.% at 20 mA cm-2 to about 41 at.% at 100 mA cm-2. The electrodeposited

coatings of Ni-Mo exhibit porous surface morphology and much better activity toward the

HER than pure Ni electrode. The main contribution toward the apparent acitivity is a

consequence of the increase of the real surface area [94].

2.2.6. Phosphorous Co-deposition

The incorporation of phosphorous can give amorphous effect to alloys. Amorphous

alloys possess good mechanic and magnetic properties and high corrosion resistance

because of their special structure [95-96]. The sodium hypophosphite supplies the H2PO2-

anions that serve as the source of phosphorus during coating (Eq. 2.22) [97].

3 3 → 3 (2.22)

The amorphous characteristics can be observed for alloys with P content over than 8-10%

considering to different contributions [98]. The incorporation of very low contents of

Page 56: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

39

phosphorous (0.0003 to 0.5% by weight) showed also an improvement in corrosion

resistance [99]. Ordine et al. [100] reported also the presence of P in Zn-Ni or Zn-Fe alloy

deposits, even at very low contents would improve effectively the anti-corrosive properties

of deposits. They deduced that this phenomenon is due to amorphous characteristics of

these alloys. They observed fastest corrosion rate for crystalline structure of Zn-Fe-P alloy.

Other research group, Shibli and Dilimon [79] confirmed that the presence of phosphorus

enhances the corrosion resistance, hardness and wears resistance of Ni-based plates. They

studied the role of phosphorous content on physicochemical and electrocatalytic

characteristics of electroless Ni-P plates. They obtained amorphous structure in the

presence of phosphorous content (more than 13%) and crystalline structure in lower

amounts (less than 7%). They found optimum P content of 10% in the plates yielding high

electrocatalytic activity during HER. They also deduced that the plates containing higher

surface roughness produces large number of electrochemical active sites. The presence of

active sites facilitates maximum extent of hydrogen adsorption on the surface [79].

Shervedani and Lasia [101] electrodeposited adherent metallic films of Ni-Mo-P from

citrate-based electrolyte with pH 9 and temperature of 30oC at current density of 200 mA

cm-2. The obtained alloys were with phosphorous percentage between 2 to 10%. The

electrocatalytic activities of Ni-Mo-P alloy toward hydrogen evolution have been studied in

alkaline solution of 1M of NaOH by steady-state polarization. At 70oC, highest

overpotential value of 591 mV at 250 mA was obtained from pure Ni electrode. While, a

reduction of 252 mV was achieved from the electrode Ni74Mo16P10 which contains the

highest percentage of phosphorous. However, the highest reduction of overpotential of 436

mV was obtained from electrode Ni50Mo45P5 which means that HER depends on combined

percentage of Mo and P.

Different coatings of Co-Mo-P were electroplated from citrate baths containing ammonia

and hydrazine in order to study the relation between the composition, morphology and

corrosion resistance [76]. The Mo content in the alloy hardly changes with an increase in

the current density, while the P increases with increasing current density. The corrosion

resistance of Co-Mo-P coatings is found to be increased with increasing the molybdenum

content and decreased with increasing the phosphorous content in the alloy which means

Page 57: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

40

the phosphorous content does not produce a positive effect on the corrosion resistance of

Co-Mo-P alloy [76].

2.3. Electrochemical Test Methods (Approach and Evaluation)

Electrochemistry concerns the study of the chemical response of a system to an

electrical stimulation. Electrochemistry studies the loss of electrons (oxidation) or gain of

electrons (reduction) that a material undergoes during the electrical stimulation. These

reduction and oxidation reactions are commonly known as redox reactions and can provide

information about the concentration, kinetics, reaction mechanisms, chemical status and

other behaviors of species in the electrolyte. Similar information can be obtained

concerning the electrode surface or the electrode/electrolyte interface. In an electrochemical

experiment, many parameters such as: potential (E), current (i), charge (Q), resistances (Ω)

and time (t) can be measured. The response of a system depends on which parameter is

used as the excitation signal. Useful information can be obtained by plotting different

parameters in different ways [102].

In most electrochemical techniques, there are three electrodes: the working electrode, the

reference electrode and the counter (or auxiliary) electrode. The three electrodes are

connected to a potentiostat, an instrument which controls the potential of the working

electrode and measures the resulting current. In one typical electrochemical experiment, a

potential is applied to the working electrode and the resulting current measured, then

plotted versus time. In another, the potential is varied and the resulting current is plotted

versus the applied potential [103].

During electrolysis, there are certain useful electrochemical techniques such as: (1)

Galvanostatic Polarization, (2) Potentiodynamic Polarization, (3) Cyclic Voltammetry, and

(4) Electrochemical Impedance Spectroscopy (EIS) to study the kinetics in this process.

Kinetics is the study of the rate at which reactions occur. The oxidation and reduction

reactions on a metal each at a potential polarized from its equilibrium value. A general

definition of polarization is ‘’the deviation in potential of an electrode as a result of the

Page 58: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

41

passage of current’’ the amount of polarization is refereed to overpotential and generally is

assigned by the symbol (ɳ) [104].

2.3.1. Galvanostatic Polarization Technique

The galvanostatic polarization technique measures the polarization behavior of an

electrode by applying a constant current or controlling a constant current scan rate while

monitoring the potential response to the current. Galvanostatic polarization is used also to

carry out the electrodeposition of metals at constant current through their dissolved ions in

an electrolyte. The potential of a metal in an aqueous solution is a function of the inherent

reactivity of the metal and the oxidizing/reducing power of the solution. The goal of metal

potential measurements is to measure the potential without affection, in any way,

electrochemical reactions on the metal surface. Accordingly, the potential measurements

are necessary to be made with respect to a stable reference electrode so that any changes in

the measured potential can be attributed to changes at the metal/solution interface

[103,105].

2.3.2. Potentiodynamic Polarization Technique

Potentiodynamic polarization technique permits the measurement of polarization

behavior by continuously scanning the potential while monitoring the current response.

This experimental method permits the easy automation of curves and real time plots of the

experimental data [103,106]. Measurements of current-potential relations under carefully

controlled conditions can yield information on corrosion rates, coatings and films,

passivity, pitting tendencies as well as kinetics studies. When a metal specimen is

immersed in a solution, both reduction and oxidation processes occur on its surface.

Typically, the specimen oxidizes (corrodes) and the medium (solvent) is reduced. In acidic

media, hydrogen ions are reduced and hydrogen gas is evolved. The specimen must

function as both anode and cathode and both anodic and cathodic currents occur on the

specimen surface [106].

Page 59: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

42

Any corrosion processes that occur are usually a result of anodic currents. When a

specimen is in contact with a corrosive liquid and the specimen is not connected to any

instrumentation – as it would be “in service” – the specimen assumes a potential (relative to

a reference electrode) termed the corrosion potential, Ecorr. A specimen at Ecorr has both

anodic and cathodic currents present on its surface. However, these currents are exactly

equal in magnitude so there is no net current to be measured. The specimen is at

equilibrium with the environment even though it may be visibly corroding. Ecorr can be

defined as the potential at which the rate of oxidation is exactly equal to the rate of

reduction. If the specimen is polarized slightly more positive than Ecorr, then the anodic

current predominates at the expense of the cathodic current. As the specimen potential is

driven further positive, the cathodic current component becomes negligible with respect to

the anodic component. A mathematical relationship exists which relates anodic and

cathodic currents to the magnitude of the polarization. Obviously, if the specimen is

polarized in the negative direction, the cathodic current predominates and the anodic

component becomes negligible. Experimentally, one measures polarization characteristics

by plotting the current response as a function of the applied potential. Since the measured

current can vary over several orders of magnitude, usually the log current function is

plotted vs. potential on a semi-log chart. This plot is termed a potentiodynamic polarization

plot or curve (Figure 2.11) [107-110]. This plot can provide many kinetics parameters such

as overpotential (ɳ), Tafel Slope (b), and exchange current density (I0 or J0) which are

important parameters to understand the electrochemical reaction and electrode behavior in

known medium.

Page 60: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

43

Figure 2.11. Potentiodynamic polarization plot [110]

2.3.3. Cyclic Voltammetry Technique

Cyclic voltammetry (CV) technique is normally used to study qualitative

information about electrochemical processes at stationary non-agitated interface under

various conditions, such as the presence of intermediates in oxidation-reduction reactions,

the reversibility of a reaction through the determined peaks in the obtained E-I curve during

the polarization of the electrode [111]. A single CV experiment only hints at the events that

constitute the electrochemical reaction at the electrode. However, multiple CV experiments

can be used for a variety of applications, including:

The determination of reversible or irreversible behavior of a redox reaction.

The number of electrons transferred in an oxidation or reduction.

Nucleation overpotential potential for reduction reaction.

Formal potentials.

Rate constants.

Formation constants.

Reaction mechanism.

In a CV experiment, the potentiostat applies a potential ramp to the working electrode to

gradually change potential and then reverses the scan, returning to the initial potential

(Figure 2.12) [102].

Page 61: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

44

Figure 2.12. Theoretical cyclic voltammogram [102]

Single CV experiment could then reflect the influence of an additive or change in the

electrolyte composition on the electrochemical properties of the interface. This is

considered mainly in zinc electrowinning process to determine the formal reduction and

nucleation overpotential (NOP) [42].

2.3.4. Electrochemical Impedance Spectroscopy Technique

An electrochemical reaction at the electrode/electrolyte interface cannot be fully

understood by using traditional electrochemical measurements. A complete description

requires impedance measurements made over a broad frequency range at various potentials

and determination of all the electrical characteristics on the interface which can be thought

of as a thin capacitor that forms between the charged electrode and the counter ions lined

up parallel to it [112]. Electrochemical Impedance Spectroscopy (EIS) or ac impedance

methods have seen tremendous increase in popularity in recent years. Initially applied to the

determination of the double-layer capacitance and in ac polarography, they are now applied

to the characterization of electrode processes and complex interfaces. EIS studies the

system response to the application of a periodic small amplitude ac signal. These

measurements are carried out at different ac frequencies and, thus, the name impedance

spectroscopy was later adopted. Analysis of the system response contains information about

the interface, its structure and reactions taking place there. EIS is now described in the

general books on electrochemistry, and there are also numerous articles and reviews [113].

Page 62: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

45

EIS is an electrochemical method in which an ac signal is used. This signal is applied to an

electrode, and the response is measured. Usually a small voltage signal is applied and the

resulting current is measured. The measuring equipment processes the current-time and the

voltage-time measurements to provide the impedance at different frequencies, the

impedance spectrum [114].

Representations of the electrified interface have gradually evolved from repeated

modifications of the model first proposed by Helmholtz (Figure 2.13a) [115]. In a simple

case, the interface can be modeled by an equivalent circuit (Figure 2.13b). In Figure 2.13a,

the oxidants (red) with a positive charge diffuse toward the negatively charged electrode

(cathode) at the interface, the oxidants is also a counterion to the electrode. IHP and OHP

are the inner and outer Helmholtz planes, respectively. In Figure 2.13b, an equivalent

circuit representing each component at the interface and in the solution during an

electrochemical reaction is shown for comparison with the physical components; double

layer capacitor (Cd or Cdl), polarization resistor (Rp), Warburg resistor (W), and solution

resistor (Rs) [112].

Figure 2.13. (a) Simple electrified electrode/electrolyte interface, (b) Electronic components for the same interface [112]

Nyquist plot, sometimes known as a complex plane plot, this is a plot of imaginary part of

the impedance Z'' against the real part Z'. Since the majority of the responses of corroding

metals have negative Z'', it is conventional, for corrosion studies, the plot –Z'' against Z'.

Page 63: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

46

Consider the response of the resistor-capacitor circuit in the top part of Figure 2.14. The

expressions for the real and the imaginary parts of the impedance are given in equations

(2.23 & 2.24) and, when plotted, give Figure 2.14. Each point of the plots corresponds to

the impedance (or admittance) at one frequency [114]. The points trace out of a semicircle,

with center at Z' = Rs + Rp/2, Z''=0, and diameter Rp. As apparent from equations 2.23 &

2.24 the solution resistance can be obtained from the high frequency intercept with the real

axis, and the total resistance, Rs+Rp, from the low frequency intercept. Where, Rct,

represents the charge transfer resistance.

Z = R/1+ω2C2R2 – jωCR2 /1+ω2C2R2 = Z' + jZ'' (2.23)

Z' = Rs + R/1+ω2C2R2 – jωCR2 /1+ω2C2R2 (2.24)

This curve is easy to be obtained by potentiostat in order to determine all components of the circuit.

Figure 2.14. Nyquist impedance plot for the showed circuit for Rs, Rct and Cdl [114]

2.4. Summary

Many studies and directed researches projects have been done on the effect of

additives during zinc electrowinning. Since the effect of Mn2+ has been reported on the

cathode behavior, and barely of that on the cathode [30,31,33]. So, it is interesting then to

examine the effect of organic additives in presence of manganese ion on the cathode

behavior. A common average concentration of 8 g dm-3 of Mn2+ is chosen as currently

considered by the Canadian industrial electrolytes.

Page 64: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

47

Considering Sb3+ alone, it has a very harmful effect on current efficiency and morphology

of zinc deposit, however it shows a good effect once it is combined at low concentrations

with some additives such as gelatin and glue [37,41]. So, it is interesting also to study in

detail the combined effect of Sb3+ and Mn2+ in presence of the selected organic additives on

zinc electrodeposition process. Lead contamination of the zinc deposit should be considered

due to its importance on the zinc deposit quality.

Recently, ionic liquids have been used in many chemical and hydrometallurgical

applications due to their chemical and physical properties [64]. As reported here, only one

team studied the effect of ionic liquid on zinc electrodeposition in standard basic

electrolyte. So, the goal is to examine the effect of different cationic and anionic groups of

ionic liquids in presence of Pb2+, Sb3+, and Mn2+ during electrowinning and zinc deposit

contamination by lead.

Considering the HER during the sodium chlorate production by using Fe-Mo coated

steel cathode, a positive effect of phosphorous addition to the sodium chloride brine

electrolyte on HER was reported [75]. Also, Fe, Mo, and P were vastly doped with different

elements in the form of binary and ternary alloys as cathodes for chlorate and chlor-alkali

productions [97,101]. However, to our knowledge, the electrodeposition and

electrocatalytic activity of Fe-Mo-P alloys towards HER have not been reported yet.

During electrolysis, the electrochemical techniques such as: (1) galvanostatic

polarization, (2) potentiodynamic polarization, (3) cyclic voltammetry, and (4)

electrochemical impedance spectroscopy are found to be appropriate in studying the

kinetics in this process. Also other physical techniques such as SEM, XRD, and ICP are

found to be supporting techniques for evaluation and better understanding of the

electrometallurgical process.

Page 65: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

48

CHAPTER 3

EXPERIMENTAL

Page 66: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

49

This chapter discusses the assigned experimental protocol including all

experimental steps and conditions such as: electrolyte preparation, electrodes fabrication,

cell set-up as well as electrochemical and physical techniques.

3.1. Electrolyte and Set-up

3.1.1. Zinc Electrolyte and Materials Preparation

(a) Standard Electrolyte Preparation:

A standard acidic zinc sulphate electrolyte (SE) was prepared from the following content:

60 g dm-3 of Zn2+ (ZnSO4.7H2O), 180 g dm-3 of H2SO4 (Conc. 98%) and 8 g dm-3 of Mn2+

(MnSO4.H2O). All chemicals were dissolved in distilled water with continuous stirring at

room temperature till the solution became totally homogenous. The determined pH of

solution was very acidic (≤0.5) (H). The prepared electrolyte SE contained the following

impurities: Pb (0.003% = 30 ppm), Fe (0.001% = 10 ppm), and Na (0.05% = 500 ppm).

Reagents were supplied from Lab mat and VWR Canada.

(b) Introduced Additives to the Electrolyte:

The examined additives were dissolved separately in distilled water by using resonator in

order to have homogenous solutions. A quantity of 100 mg of each additive was dissolved

individually in 100 ml of distilled water giving then 1 mg dm-3 for each 1 ml. Additives

were added to the SE with concentrations (1,3,5,10 and 40 mg dm-3) individually. Also, the

effect of 1 and 3 mg of each additive was studied in combination with 0.0055 mg dm-3 of

Sb3+ (KSbC4H4O7.5H2O) as another impurity. The selected additives were supplied from

Sigma-Aldrich USA.

(c) Electrolysis Conditions and Electrodes Fabrication:

The electrolysis was performed in 1000 cm3 solution in double-glazed beaker. The solution

was heated by the flow of thermostated water in the double-glazed wall to reach the

working temperature 38 ± 1oC and solution was agitated by using magnetic agitator at 60

rpm. All used electrodes of pure Aluminum (>99.95%), Pb-Ag (Ag, 0.7%) and Platinum

Page 67: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

50

were connected to electric wires using a conductive two components silver epoxy adhesive

(MG Chemicals No. 8331) and were casted in polyester resin (SamplKwick 20-3566

Buehler) in order to have only the required exposed surface. The metallic electrodes were

manually polished by several grits of SiC papers (80, 320, 600 and 1200) to give uniform

shiny surface. Then, they were washed by distilled water, ethanol and dried few seconds

before immersion in the solution. Ag, AgCl/KCl(sat) (0.199 V/SHE) was used as reference

electrode.

3.1.2. Fe-Mo & Fe-Mo-P Electrolytes and Materials Preparation

FeSO4.7H2O, Na2MoO4.2H2O and NaH2PO2.H2O were employed as the sources of

Fe, Mo and P, respectively. Along with trisodium citrate dihydrate (Na3C6H5O7.2H2O) used

as a complexing agent and controller of the reduction rate. Four different electrolytes were

prepared according to concentrations in Table 3.1. All reagents were dissolved in double

distilled water with continuous magnetic stirring at room temperature. pH 6 was adjusted

by using citric acid. During the electrolysis, the electrolyte was heated at 30oC by the flow

of thermostated water in the double-glazed wall and magnetically agitated at 300 rpm.

Table 3.1. Fe-Mo and Fe-Mo-P electrolytes compositions

Electrolyte Electrolytes compositions (g dm-3)

Na3C6H5O. 2H2O

FeSO4. 7H2O

Na2MoO4. 2H2O

NaH2PO2.H2O

I 120 10 50 0

II 120 10 50 10

III 120 10 70 10

IV 120 10 70 30

Electrodes were prepared by the same method described in the previous section. Mild steel

and platinum foil with the surface of 1 cm2 were used as cathode and anode electrodes,

respectively. Both anode and cathode electrodes were mounted in epoxy resin and

assembled in a three-electrode cell. The reference electrode was a silver chloride electrode

with a saturated KCl double junction Ag, AgCl/KClsat (0.199 V vs. standard hydrogen

electrode (SHE).

Page 68: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

51

3.1.3. Set-up

The electrodes were mounted in Three-electrode cell as the schematic Figure 3.1. The

cathode and anode were adjusted with inter-electrode distance of 2 cm. The reference

electrode Ag, AgCl/KCl(sat) was adjusted in the cell to be far from the working electrode

few millimetres. The three-electrode cell was connected to a potentiostat, Gamry Reference

3000 – Gamry USA to carry out the electrodeposition and the other electrochemical tests.

The potentiostat was connected to Dell PC for data output (Figures 3.2).

Figure 3.1. Schematic experimental set-up for three-electrode cell [116]

Figure 3.2. Electrolysis cell set-up

Page 69: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

52

3.2. Electrochemical Techniques and Measurements

3.2.1. Galvanostatic Polarization

The galvanostatic polarization technique is typically used for special applications

such as measuring the potential of an electrode during the electrodepoistion process using

constant current and time [103]. During the current study of zinc electrowinning process,

galvanostatic polarization has been employed to carry out the zinc electrodeposition on Al

cathode at 50 mA cm-2 for 2 hours also to measure the corresponding cathodic and anodic

potentials during electrodeposition. For Fe-Mo and Fe-Mo-P alloys electrodeposition was

performed on mild steel (MS) substrates at 20 mA cm-2 for 6 hours. Each experiment is

done in duplicates and average was taken.

3.2.2. Current Efficiency Calculations

After electrolysis the cathode was dried and current efficiency was calculated by

weight using Faraday’s law:

% . .. .

%

Where W is the weight of deposit (g), F is Faraday’s constant (96500 C mol-1), n is the

number of electrons (2 electrons for Zn), I is the total cell current (A), t is the time of

electrodeposition in seconds and M is the atomic weight of metal [117].

3.2.3. Power Consumption Calculations

From the combination of Joule’s law and Faraday’s law the power consumption is

calculated by following relation:

/

Where, PC is power consumption (kWh ton-1), Vcell is total cell voltage (volt) obtained from

galvanostatic polarization, Ic the applied current (A), t is the deposition time (seconds) and

W is the weight of deposited metal (g) [117].

Page 70: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

53

3.2.4. Potentiodynamic Polarization

The potentiodynamic polarization for the deposit has been carried out by using 1

cm2 of deposited cathode as working electrode and 1 cm2 of platinum as auxiliary electrode,

also, Ag, AgCl/KCl(sat) was used as reference electrode. The three-cell electrode cell was

connected to the potentiostat Gamry Reference 3000 – Gamry USA. Based on the

experimental approach and previous studies, polarization for zinc deposit was carried out

from -1.05 to -1.25 V with scan rate of 5 mV s-1, while for MS, Fe-Mo and Fe-Mo-P was

carried out from -1.2 to 0.5 V with a scan rate of 1 mV s-1 in a solution containing 300 g

dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and magnetic agitation of 80 rpm [75]. The

pH 6.4 was always adjusted using NaOH. The parameters measured from this technique

are:

i. Cathodic Tafel slope (bc): This can be obtained by selecting two points on the

cathodic curve, first point is far by ~20-40 mV from the corrosion potential and the

other point is far by one decade of current density [118]. Also it is an important

parameter to measure the I0 (exchange current density) (Figure 3.3).

ii. Exchange current density (I0 or J0): is defined as the current flowing in both

directions per unit area when an electrode reaction is at equilibrium (and, hence, at

its equilibrium potential). If I0 is small, then little current flows and the reactions at

dynamic equilibrium are generally slow. Likewise, a high I0 gives a fast reaction.

The metal itself affects the value of I0, even if the reaction does not involve the

metal directly. I0 can be estimated by extrapolating the Tafel slopes to the

corresponding zero current-potentials (Figure 3.3).

iii. Cathodic overpotential (ɳc): overpotential is the potential difference between a half-

reaction's thermodynamically determined reduction potential and the potential at

which the redox event is experimentally observed in the same conditions of

electrolytes. It can be determined from following equations:

, 0.763 2⁄ / (3.1)

, 0.0 ⁄ / . (3.2)

ɳ , , (3.3)

ɳ , (3.4)

Page 71: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

54

Where; Ee is the equilibrium potential, R is the gas constant (equal to 8.314 mol-1 K-1), F is

the Faraday’s constant (equal to 96 500 C mol-1) and Em is the measured potential at 50 mA

in case of zinc electrodeposits or 250 mA in case of Fe-Mo and Fe-Mo-P deposits

[106,117].

Figure 3.3. Polarization curve potential vs current density (log i) [119]

3.2.5. Cyclic voltammetry

Cyclic voltammetry (CV) technique is normally used to study qualitative

information about electrochemical processes at stationary non-agitated interface under

various conditions, such as the presence of intermediates in oxidation-reduction reactions

[111]. CV could then reflect the influence of an additive or change in the electrolyte

composition on the electrochemical properties of the interface. This is considered mainly in

this work to determine the formal reduction and nucleation overpotential (NOP). The effect

of each additive alone or combined with antimony on the reduction of zinc ions on

aluminum cathode was studied by using cyclic voltammetry polarization [42].

Polarization for zinc deposit was carried from initial potential of -1.30 V to areversible

potential of -0.60 V at 38oC in presence of atmospheric air without agitation. NOP is the

Page 72: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

55

difference between the crossover potential and the start of the dissolution and the point at

which the Zn begins to deposit. This could be a useful parameter to identify the best

additive concentration ratio with antimony ions.

3.2.6. Electrochemical Impedance Spectroscopy

EIS measurements for MS, Fe-Mo and Fe-Mo-P deposits were preformed in a

solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and magnetic

agitation at 80 rpm. It was scanned over the frequency range from 100 kHz to 0.01 Hz, an

ac signal of 50 mA for galvanostatic mode at 250 mA cm-2.

3.3. Deposit Examination Techniques

3.3.1. Scanning Electron Microscopy (SEM)

A scanning electron microscopy (SEM) is a type of electron microscope that

produces images of a sample by scanning it with a focused beam of electrons. The electrons

interact with atoms in the sample, producing various signals that can be detected and that

contain information about the sample's surface morphology and composition. The electron

beam is generally scanned in a raster scan pattern, and the beam's position is combined with

the detected signal to produce an image. SEM can achieve resolution better than 1

nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet conditions

(in environmental SEM), and at a wide range of cryogenic or elevated temperatures [120].

In this study the deposits were washed by distilled water then dried after the

electrodeposition process, X500 and X1000 images have been taken by using JEOL JSM-

840a in order to examine the morphology of deposited metals.

3.3.2. Energy Dispersive Spectroscopy (EDS)

An energy-dispersive (EDS) detector is used to separate the characteristic x-rays of

different elements into an energy spectrum, and EDS system software is used to analyze the

energy spectrum in order to determine the abundance of specific elements. EDS can be

Page 73: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

56

used to find the chemical composition of materials down to a spot size of a few microns,

and to create element composition maps over a much broader raster area. Together, these

capabilities provide fundamental compositional information for a wide variety of materials

[121].

The Fe-Mo and Fe-Mo-P deposits were analyzed by using EDS detectors models JEOL

JSM-840a and FEI Quanta FEG 250 in order to determine the atomic composition

percentage of each element.

3.3.3. X-ray Diffraction (XRD)

X-ray diffraction (XRD) relies on the dual wave/particle nature of X-rays to obtain

information about the structure of crystalline materials. A primary use of the technique is

the identification and characterization of compounds based on their diffraction pattern

[122]. All deposited were analyzed using X-ray diffractor model Siemens - D5000 to

determine the crystallographic orientation and crystal/amorphous state.

3.3.4. Inductively Coupled Plasma (ICP)

Inductively coupled plasma (ICP) techniques can be very powerful tools for

detecting and analyzing trace and ultra-trace elements. Over the past years, ICP has become

the technique of choice in many analytical laboratories for providing the accurate and

precise measurements needed for today’s demanding applications and for providing

required lower limits of detection [123].

The zinc deposits were analyzed by using ICP model Optima 4300 Perkin-Elmer in order to

determine the lead concentrations in zinc deposits.

Page 74: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

57

CHAPTER 4

INFLUENCE OF DIFFERENT ORGANIC ADDITIVES IN ZINC ELECTROWINNING

FROM ACIDIC SULPHATE ELECTROLYTE

Page 75: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

58

Influence of Different Organic Additives in Zinc Electrowinning from Acidic Sulphate Electrolyte

N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

*Corresponding author: Tel: 418 656-2131 - Fax: 418 656-5343 ([email protected])

Published by Canadian Institute of Mining, Metallurgy and Petroleum. CIM-COM, paper

no. 8986, pp 1-13, ISBN: 978-1-926872-32-2.

Résumé

Les additifs polyacrylamide, chlorure de 1-butyl-3-méthylimidazolium, bromure de tétra-butylammonium, chlorure de benzalkonium et chitine ont été évalués durant l'électrolyse du zinc à partir de l’électrolyte synthétique acide de sulfate contenant des ions de Mn2+, en absence et en présence de 0,0055 mg dm-3 des ions de Sb3+. Des expériences de polarisation galvanostatique pendant 2 heures à 50 mA cm-2 et 38°C ont été effectuées afin de déterminer les potentiels cathodique et anodique, et le rendement du courant de zinc déposé. Les expériences de polarisation potentiodynamique et voltamétrie cyclique ont également été utilisées pour étudier le comportement électrochimique de chaque additif sur la déposition de zinc sur l'électrode d'aluminum. Les résultats montrent qu’en présence de Sb3+, la tension de la cellule augmente d’environ 0 à 7 mV en ajoutant de 1 à 3 mg de chlorure de 1-butyl-3-méthylimidazolium. Le rendement du courant a été augmenté d’environ 4,9-6,4%; aussi la consommation d'énergie a été réduite de ≈147-173 kWh tonne-

1 en ajoutant de 1 à 3 mg. Cependant, en absence de Sb3+, une diminution de la tension de la cellule d’environ 7-28 mV, une augmentation du rendement du courant d’environ 0,70-1,50%; une diminution de la consommation d'énergie de ≈41-47 kWh tonne-1 ont été réalisées en ajoutant de 1 à 3 mg de chlorure de 1-butyl-3-méthylimidazolium. L'effet des autres additifs sur la tension de la cellule, le rendement du courant, la teneur en plomb et la morphologie du dépôt de zinc a également été examiné. La morphologie de surface et l'orientation cristallographique du dépôt ont été étudiées en utilisant le microscope électronique à balayage (MEB) et la diffraction des rayons-X (DRX). La teneur en plomb dans le dépôt a également été mesurée en utilisant la spectroscopie de plasma à couplage inductif (PCI).

Page 76: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

59

Abstract

The additives Polyacrylamide, 1-Butyl-3-methylimidazolium chloride, Tetra-butylammonium bromide, Benzalkonium chloride and Chitin are evaluated during zinc electrolysis from synthetic acidic sulphate electrolyte containing Mn2+ ions, in absence and in presence of 0.0055 mg dm-3 of Sb3+ ions. Galvanostatic polarization tests for 2 hours at 50 mA cm-2 and 38oC were carried out to determine the cathodic and anodic potentials, and current efficiency of the deposited zinc. Potentiodynamic polarization and cyclic voltammetry tests have also been employed to study the electrochemical behavior of each additive on the zinc deposit on aluminum electrode. Results showed that, in presence of Sb3+, adding 1 to 3 mg of 1-butyl-3-methylimidazolium chloride increases the cell voltage by ≈0-7 mV. Current efficiency is increased by ≈4.9-6.4%; power consumption is reduced by ≈147-173 kWh ton-1 by adding 1-3 mg, respectively. However, in absence of Sb3+, cell voltage is decreased by ≈7-28 mV, current efficiency is increased by ≈0.70-1.50%; power consumption is reduced by ≈41-47 kWh ton-1 by adding 1-3 mg. The effect of the other additives on cell voltage, current efficiency, lead content and morphology of zinc deposit has also been examined. Surface morphology and crystallographic orientation of the deposit was studied using scanning electron microscopic (SEM) and X-ray diffraction (XRD), respectively. The content of lead in the deposit has also been measured using inductively coupled plasma spectroscopy (ICP).

Page 77: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

60

4.1. Introduction

Zinc is a common base metal with wide uses; it is used for fabrication of metal

components in the form of diecasting alloys and brasses. The major use of zinc which

cannot be neglected is the corrosion protection or galvanising of steel, this protection is

achieved by forming surface barrier as well as by corroding preferentially to the coated

steel [7,12]. Most of global pure zinc metal is produced via electrowinning process from

acidic sulphate electrolyte [124]. This process is very sensitive to the harmful effect of Pb

impurity coming from the used lead-based anodes and to the other presented metallic

impurities in the electrolyte such as: Sb, Fe, Cu, Co, Ni ...etc. [59]. Most of these metallic

impurities can reduce the zinc current efficiency, change in deposit’s morphology, and

change the cathodic and anodic polarizations. They can also assist the evolution of H2 gas

when sufficient amounts are presented in the electrolyte.

One of the considerable goals in zinc electrowinning is minimizing the power consumption

(PC). The two important factors which can determine the energy requirements are the

current efficiency (CE) and cell voltage which are affected negatively by the presence of

impurities [125]. Depending on the electrolysis conditions, one or several organic additives

may be added to the electrolyte in order to counteract the detrimental effects caused by

impurities [10]. The effect of organic additives in the electrolyte on the nature of the

crystallization presents one of the important aspects. Additives could be adsorbed

preferentially on the cathode to completely alter the growth of the deposit. Additives also

reduce the grain size by creation of more nucleation sites during the electrodeposition

[126]. They are also susceptible to decomposition by the presence of a large amount of

Mn2+ which is gradually oxidized to MnO4- or MnO2; this also can cause further alterations

in the electro-crystallization.

The most commonly used additives in industry are glues and gelatin which prompt the

deposit growth and minimize the negative effect of metallic impurities [54]. Gelatin

showed very good results in increasing current efficiency, reducing overpotentials,

producing better smooth and compact deposits in presence of traces of antimony [41].

Sodium lauryl sulphate [SLS] with low concentrations in presence of Sb (III) had been

examined as additive by Tripathy et al. [53]. It showed good results in increasing current

Page 78: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

61

efficiency, reducing power consumption and improving surface morphology. In addition,

Triethyl benzylammonium [TEBA] [127-128], 1-butyl-3-methylimidazolium hydrogen

sulfate [BMIM]HSO4 [70], and Perfluorinates surfactant [129] have been considered as

additives to investigate their effects on the electrodeposition characteristics of zinc from

acidic sulphate electrolytes.

In the present work, five additives have been chosen from different organic groups, in order

to examine their effects individually on zinc electrowinning process in absence and in

presence of antimony ions. (1) Polyacrylamide [PAM] is one of the well known organic

polymers used in industry and has been tested as additive in copper electrowinning,

showing a good effect in improving morphology of the surface [130]. (2) 1-Butyl-3-

methylimidazolium chloride [BMIM]Cl represents the ionic liquids group. Ionic liquids are

widely used in liquid-liquid extraction and electrodeposition of some metals due to their

low melting point and the thermal degradation properties which are important in the

electrochemical media [67]. (3) Tetra-butylammonium bromide [TBABr] is one of the

quaternary ammonium salts group which have been examined also as additives. (4)

Benzalkonium chloride [BKCl] is a cationic surface-acting agent belonging to

the quaternary ammonium salts with aromatic ring. Finally, (5) Chitin has been studied as it

is one of the natural polymer compounds which can be found in

crabs, lobsters and shrimps.

4.2. Experimental

4.2.1. Electrolyte and Experimental Setup

A standard electrolyte (SE) was prepared from the following content: 60 g dm-3 of

Zn2+ (ZnSO4.7H2O), 180 g dm-3 of H2SO4 (Conc. 98%) and 8 g dm-3 of Mn2+ (MnSO4.H2O)

dissolved in double distilled water. The effect of different concentrations of 0,1,3,5,10 and

40 mg dm-3 of each additive added to the SE was studied individually. Also, the effect of 1

and 3 mg of each additive was studied in combination with 0.0055 mg dm-3 of Sb3+

(KSbC4H4O7.5H2O). The following impurities were determined in the SE: Pb (0.003% = 30

ppm), Na (0.05% = 500 ppm), Mg (0.005% = 50 ppm) and Fe (0.002% = 20 ppm).

Reagents were supplied from Lab mat and VWR Canada, while, the selected additives were

Page 79: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

62

supplied from Sigma-Aldrich USA. An electrolysis cell was performed in 1000 ml solution

in double-glazed beaker. The solution was heated by the flow of thermostated water in the

double-glazed wall. The three-electrode cell used consisted of two plates of pure Al

(>99.95%) and one plate of Pb-Ag (Ag, 0.7%) as cathode and anode, respectively. The two

plates were cast in polyester resin with total exposed surface area of 1 cm2, they were

assembled in a Teflon holder. The inter-electrode distance was 2 cm. Ag, AgCl/KCl(sat) was

used as reference electrode. Both electrodes were manually polished by several grits of SiC

papers (80, 320, 600 and 1200) to give uniform shiny surface. Then, they were washed by

distilled water, ethanol and dried few seconds before immersion in solution.

The three-electrode cell was connected to a potentiostat, Gamry Reference 3000 – Gamry

USA. All elecrodeposition experiments were carried out for 2 hours at 50 mA cm-2 and 38

± 1oC with magnetic agitation of solution of 60 rpm. After electrolysis the cathode was

dried and current efficiency was calculated by weight using Farady’s law: CE% =

(W.F.n/I.t.M) x 100%; where W is the weight of deposit (g), F is Faraday’s constant, n is

the number of electrons, I is the total cell current (A), t is the time of electrodeposition and

M is the atomic weight of zinc.

4.2.2. Deposit Examination

Morphology of the surfaces of deposits was examined by scanning electron

microscopy (SEM) using JEOL JSM-840a. The crystal orientation of the zinc deposit was

determined using X-ray diffractometer model Siemens - D5000. Lead concentration in the

deposit has been measured using inductively coupled plasma (ICP) model Optima 4300

Perkin-Elmer.

4.2.3. Potentiodynamic Polarization and Cyclic Voltammetry

Electrochemical studies such as potentiodynamic polarization and cyclic

voltammetery measurements were preformed. The three-electrode cell consisted of 0.30

cm2 of Al (>99.95%) as working electrode and 1 cm2 of Pt as auxiliary electrode. Also a

Ag, AgCl/KCl(sat) was used as reference electrode. The cell was connected to the

Page 80: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

63

potentiostat Gamry Reference 3000 – Gamry USA. The cathodic potentiodaynamic

polarization was carried out from -1.00 to -1.25 V with a scan rate of 5 mV s-1 at agitation

of 60 rpm. Cyclic voltammetric scanning was conducted from an initial potential of -1.30 to

the final potential of -0.60 V at a constant scanning rate of 10 mV s-1 without agitation.

Both tests were done at 38oC under atmospheric conditions. Working electrodes were

manually polished with SiC abrasive paper (Leco Corporation) down to 1200 grit before

each experiment and washed by distilled water, ethanol and dried few seconds prior to the

experiment.

4.3. Results and Discussion

4.3.1. Power Consumption and Current Efficiency

The effects of addition of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current

efficiency, cell voltage and accordingly on power consumption have been studied in the

range of 0-40 mg dm-3. Table 4.1 shows the variance by changing the quantity of each

additive to the SE as it varies from one additive to another. Results showed that cell voltage

was increased gradually by low concentrations of 1-5 mg dm-3 of additives in absence of

antimony then started to be increased significantly by the quantity of additives from 10 to

40 mg dm-3. Addition of [BMIM]Cl showed the best results in reducing cell voltage by ≈28

and 7 mV by adding 1 and 3 mg dm-3, respectively, as compared to that obtained from the

standard electrolyte, SE. However, the other additives had negative effect on reducing cell

voltage, leading to an increase of overpotential from ≈2 to 127 mV. The Highest

overpotential was caused by adding 40 mg dm-3 of BKCl. In presence of antimony,

additives showed also an increase in cell voltage from range of ≈0-47 mV.

Effects of additives on power consumption (PC) were also investigated in absence and in

presence of antimony (Table 4.1). In absence of antimony, the maximum reduction of PC

was obtained from [BMIM]Cl as it was reduced by ≈41-47 kWh ton-1 due to the addition of

1-3 mg dm-3, respectively. On the other hand, the highest value of PC of 3303 kWh ton-1

was observed by adding 40 mg dm-3 of PAM, followed by 2952 kWh ton-1 with 40 mg dm-3

of BKCl as compared to that of SE (2560 kWh ton-1). Most of the additives showed good

results in decreasing PC in presence of 0.0055 mg dm-3 of antinomy. Results showed that

Page 81: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

64

the maximum reduction of ≈147-173 kWh ton-1 was obtained from adding 1-3 mg dm-3 of

[BMIM]Cl, respectively, followed by a reduction of ≈111-139 kWh ton-1 obtained by

adding 1-3 mg dm-3 of PAM. Although addition of PAM showed negative results in

reducing PC in absence of Sb3+, it showed good results in presence of Sb3+, acting more or

less as gelatin in removing the harmful effect of antimony on hydrogen evolution reaction

(HER). Antimony addition to the electrolyte decrease the HER overpotential as it catalyzes

this reaction leading to a decrease in the current efficiency and then a porous deposit is

obtained. Addition of the additive inhibits the HER and favorites the reduction of zinc ions

due to their adsorption on the cathode and smooth deposit can be obtained accordingly.

Addition of BKCl showed a minimum reduction of PC of ≈33-49 kWh ton-1, while addition

of TBABr and chitin has given a medium reduction values. Accordingly, the power

consumption values in SE with additives in presence of Sb3+ decreased in order of:

[BMIM]Cl > PAM > TBABr > Chitin > BKCl.

The values of current efficiencies are also plotted in Figure 4.1. Results showed that,

current efficiency (CE) values obtained from SE in absence and in presence of Sb3+ were

92.8% and 88.7%, respectively. Figure 4.1a shows that maximum values of current

efficiencies obtained in the absence of antimony were 94.3% and 94.0% by adding 3 and 5

mg dm-3 of [BMIM]Cl to SE solution, respectively. Addition of 10 mg dm-3 of TBABr

showed also an increase of 1.2% more than that obtained from SE. Addition of 1 mg dm-3

of [BMIM]Cl and 5 mg dm-3 of TBABr showed the same value of 93.5%. Generally,

increasing the concentrations of additives ≥10 mg dm-3 in the electrolyte leads to a decrease

in current efficiency values which could be explained by high adsorption of additives on the

cathode surface which could block the active sites and prevent further nucleation, such that

deposits start to be dissolved again in the acidic electrolyte.

Figure 4.1b shows the values of current efficiencies obtained by combination of 1 and 3 mg

dm-3 of each additive with 0.0055 mg of Sb3+. Results showed that the maximum CE

obtained was 95.1% from addition of 3 mg dm-3 of [BMIM]Cl to SE + Sb3+ as compared to

88.7% obtained from SE + Sb3+. This is followed by 94.8% from addition of 3 mg dm-3 of

PAM. This could explain since PAM has a very good effect on CE and accordingly on PC

in presence of antimony.

Page 82: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

65

Table 4.1. Effect of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on cell voltage, CE and PC in absence and in presence of Sb3+ during zinc electrodeposition for 2 h at 50 mA cm-2 and 38оC

Additive /mg dm-3

Sb (III) /mg dm-3

Cell voltage /V

CE /%

PC /kWh ton-1

SE 0 0 2.898 92.8 2560 0 0.0055 2.873 88.7 2655

PAM 1 0 2.900 92.0 2584 3 0 2.905 90.2 2640 5 0 2.920 87.8 2726 10 0 2.930 82.3 2918 40 0 2.970 73.7 3303 1 0.0055 2.892 93.2 2544 3 0.0055 2.910 94.8 2516

[BMIM]Cl 1 0 2.870 93.5 2516 3 0 2.891 94.3 2513 5 0 2.898 94.0 2527 10 0 2.908 93.4 2552 40 0 2.935 91.3 2635 1 0.0055 2.870 93.8 2508 3 0.0055 2.880 95.1 2482

TBABr 1 0 2.902 92.4 2575 3 0 2.912 93.0 2567 5 0 2.917 93.5 2557 10 0 2.965 94.0 2586 40 0 2.972 88.4 2756 1 0.0055 2.890 93.2 2542 3 0.0055 2.920 94.4 2536

BKCl 1 0 2.943 92.2 2617 3 0 2.982 91.0 2686 5 0 2.995 90.1 2725 10 0 3.010 89.3 2763 40 0 3.025 84.0 2952 1 0.0055 2.887 90.2 2606 3 0.0055 2.920 91.1 2622

Chitin 1 0 2.911 93.1 2563 3 0 2.933 93.9 2560 5 0 2.937 93.6 2572 10 0 2.944 92.9 2598 40 0 2.968 89.0 2734 1 0.0055 2.872 92.8 2537 3 0.0055 2.890 93.1 2545

Page 83: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

66

Addition of TBABr followed by chitin also showed an increase in CE in presence of

antimony while, addition of BKCl had a slight effect on the CE as compared to the other

additives. Comparing the results of Figures 4.1a and 4.1b, it could be deduced that

additives in presence of antimony can increase the current efficiencies, while their addition

to SE frequently decrease the current efficiencies. This indicates that these additives

counteract the harmful effect of Sb and have synergetic effects on current efficiencies.

Figure 4.1. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current efficiency: (a) in absence of Sb3+ and (b) in presence of 0.0055 mg dm-3 of Sb3+ during zinc electrodeposition for 2h at 50 mA cm-2 and 38оC

70

74

78

82

86

90

94

98

0 5 10 15 20 25 30 35 40 45

CE

%

Qty. of additives in (mg)

SE

PAM

[BMIM]Cl

TBABr

BKCl

Chitin

(a)

88

90

92

94

96

98

0 1 2 3 4

CE

%

Qty. of additives in (mg)

SE + Sb

PAM

[BMIM]Cl

TBABr

BKCl

Chitin

(b)

Page 84: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

67

4.3.2. Characterization of Deposits

The effect of 3 mg dm-3 of each additive in absence and in presence of Sb3+ on

deposit’s morphology and crystal orientation during 2 hours of zinc electrowinning was

examined using SEM and XRD, respectively. Results are listed in Table 4.2, and the

scanning electron micrographs are shown in Figures 4.2 and 4.3.

Results revealed that crystallographic orientation obtained from SE is (101) (102) (103)

(002) which changed to (101) (112) (102) (103) by adding antimony showing a decrease in

platelet size (Figures 4.2a and 4.3a). Addition of BKCl also has the same effect as antimony

alone as it showed a porous and small grain size. This could explain the decrease of current

efficiency by increasing the concentration of BKCl in the electrolyte, since hydrogen

evolution on cathode caused the porosity in the zinc deposit.

Table 4.2. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on surface morphology, crystal orientation and lead contamination in absence and in presence of Sb3+ during zinc electrodeposition for 2h at 50 mA cm-2

Additive /mg dm-3

Sb (III) /mg dm-3

Crystal orientation /hkl

SEM (Figure)

Pb Conc. /ppm

SE 0 0 (101) (102) (103) (002) 4.2a 26.49 0 0.0055 (101) (112) (102) (103) 4.3a 03.64 PAM 3 0 (100) (101) (110) (201) 4.2b 23.80 3 0.0055 (101) (110) (112) (100) 4.3b 11.60 [BMIM]Cl 3 0 (101) (102) (103) (002) 4.2c 10.40 3 0.0055 (101) (102) (110) (112) 4.3c 10.90 TBABr 3 0 (101) (102) (100) (201) 4.2d 23.00 3 0.0055 (101) (100) (102) (201) 4.3d 12.40 BKCl 3 0 (101) (112) (102) (103) 4.2e 11.80 3 0.0055 (101) (002) (102) (103) 4.3e 11.10 Chitin 3 0 (101) (102) (103) (112) 4.2f 11.40 3 0.0055 (101) (102) (103) (112) 4.3f 11.30

Page 85: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

68

Figure 4.2. Scanning electron micrographs (x1000) of zinc deposits in absence of Sb3+; (a) SE, (b) PAM, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f) Chitin 3mg dm-3

Page 86: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

69

Figure 4.3. Scanning electron micrographs (x1000) of zinc deposits in presence of 0.0055 mg of Sb3+; (a) SE, (b) PAM 3mg dm-3, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f) Chitin 3mg dm-3

The addition of [BMIM]Cl to the SE had no effect in changing the preferred crystal

orientation in absence of Sb3+, while it was changed to (101) (102) (110) (112) in presence

of Sb3+. The given deposit had a moderate platelet size with compact and smooth surface

(Figure 4.3c). Addition of TBABr in absence of antimony had very high peak intensity at

2θ = 43.247 with crystallographic orientation (101) (102) (100) (201) (Figure 4.2d) which

slightly modified in presence of antimony. Addition of PAM to the SE changed the most

Page 87: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

70

preferred orientation from (101) to (100) showing the highest peak intensity at 2θ = 39.013,

resulting in a needled deposit with low current efficiency (Figure 4.2b). Addition of PAM

in presence of Sb3+ restored the most preferred orientation to (101) giving smooth and

compact deposit which could explain the obtained high current efficiency of zinc in

presence of PAM combined with Sb3+ (Figure 4.3b). Addition of chitin showed a good

surface morphology in absence and in presence of Sb3+ with crystallographic orientation of

(101) (102) (103) (112) (Figures 4.2f and 4.3f).

The zinc deposits obtained were analyzed using inductively coupled plasma spectroscopy

(ICP) to determine the lead concentration, in order to examine the effect of each additive on

counteracting the lead contamination caused by the anode. Results in Table 4.2 show that

lead concentration found in zinc deposit obtained from SE was 26.49 ppm. Additions of

PAM and TBABr have approximately no effect in reducing this value, while other additives

succeeded in reducing it to the range of 10.40-11.80 ppm. The greatest reduction was in

presence of 3 mg of [BMIM]Cl. The lead concentration found in zinc deposit obtained from

SE with Sb was 3.64 ppm. This decrease in lead contamination in presence of Sb3+ could be

explained due to the effect of antimony in reducing the HER overpotential which cannot

reach to the required potential for the co-deposition of lead with zinc. Also, the effect of

high evolution of hydrogen bubbles could limit the access of reducible ions to the interface.

None of the additives succeeded in reducing this concentration in presence of antimony, but

rather they increased it in the range of 10.90-12.40 ppm.

4.3.3. Potentiodynamic Polarization

The effect of additives in the zinc electrolyte in absence and presence of antimony

ions on the cathodic polarization were examined by potentiodyamic polarization and cyclic

voltammetry scanning. Results of potentiodnamic polarization are plotted in Figure 4.4.

Addition of 1 mg dm-3 of [BMIM]Cl and chitin shifted slightly the polarization curves to

less negative potentials. The behavior of addition of 1 mg dm-3 of PAM and 3 mg dm-3 of

[BMIM]Cl gave more or less the same values as that of the standard electrolyte. With the

addition of TBABr and BKCl, even at low concentration of 1 mg dm-3, overpotentials were

increased during polarization. At high concentrations of the five additives, a remarkable

Page 88: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

71

polarization occurred and the polarization curves are shifted to more negative potentials,

which increased the overpotentials. However in presence of Sb3+, addition of all additives

except for 1 mg dm-3 of [BMIM]Cl into SE increased the overpotentials on zinc deposit

during cathodic polarization.

The effects of additives on Tafel slopes and cathodic overpotential have been also

investigated. Results are given in Table 4.3. Results revealed that by increasing the

concentrations of additives the Tafel slope values gradually increased, and the values of

cathodic overpotential at 50 mA cm-2 were varied depending on the concentration of the

additive used. It could be stated that the addition of 1 or 3 mg dm-3 of [BMIM]Cl

corresponds to the best concentration in the zinc electrolyte as cathodic overpotential is

decreased to 3-14 mV. However, the other additives showed negative effect in reducing the

cathodic overpotential. The higest overpotential value of 401 mV was obtained in presence

of 40 mg dm-3 of BKCl in absence of antimony ions compared to 360 mV that obtained

from SE.

Page 89: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

72

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SEPAM 1mgPAM 5mgPAM 40mgSE + SbPAM 1mg + SbPAM 3mg + Sb

(a)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SE

[BMIM]Cl 1mg

[BMIM]Cl 5mg

[BMIM]Cl 40mg

SE + Sb

[BMIM]Cl 1mg + Sb

[BMIM]Cl 3mg + Sb

(b)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SE

TBABr 1mg

TBABr 5mg

TBABr 40mg

SE + Sb

TBABr 1mg + Sb

TBABr 3mg + Sb

(c)

Page 90: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

73

Figure 4.4. Effects of the additives on the cathodic polarization during zinc electrodeposition with different concentrations in absence and in presnce of antimony; (a) PAM, (b) [BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin

4.3.4. Cyclic Voltammetry Measurements

The effect of each additive alone or combined with antimony on the reduction of

zinc ions on aluminum cathode was studied by using cyclic voltammetry polarization.

Polarizations were carried out from initial potential of -1.30 V to final potential of -0.60 V

without agitation at 38oC in presence of an electrolyte saturated with atmospheric air.

Figure 4.5 shows these results; also nucleation overpotential (NOP) values are listed in

Table 4.3.

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SE

BKCl 1mg

BKCl 5mg

BKCl 40mg

SE + Sb

BKCl 1mg + Sb

BKCl 3mg + Sb

(d)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05 -1.00

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SEChitin 1mgChitin 5mgChitin 40mgSE + Sbchitin 1mg + Sbchitin 3mg + Sb

(e)

Page 91: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

74

-400

-300

-200

-100

0

100

200

300

400

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SEPAM 1mgPAM 5mgPAM 40mg

A

BD C

(a)

-400

-300

-200

-100

0

100

200

300

400

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SE[BMIM]Cl 1mg[BMIM]Cl 5mg[BMIM]Cl 40mg

A

B CD

(b)

-400

-300

-200

-100

0

100

200

300

400

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SETBABr 1mgTBABr 5mTBABr 40mg

(c)

A

B CD

Page 92: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

75

Figure 4.5. Cyclic voltammograms during zinc electrowinning using aluminum cathode with different concentrations of 0,1,5 and 40 mg dm-3 of: (a) PAM, (b) [BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin

-400

-300

-200

-100

0

100

200

300

400

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SEBKCl 1mgBKCl 5mBKCl 40mg

(d)

A

B CD

-400

-300

-200

-100

0

100

200

300

400

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

SEChitin 1mgChitin 5mgChitin 40mg

(e)

A

BD C

Page 93: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

76

Table 4.3. Effects of additives on Tafel slopes, cathodic overpotential at 50 mA cm-2 obtained from potentiodynamic polarization versus Ag,AgCl/KCl(sat) and NOP obtained from cyclic voltammetry

Additive mg dm-3

Sb (III) mg dm-3

Tafel slope (bc) mV/decade

Cathodic Overpotential -ɳ(50) / (mV/Ref)

NOP mV

PAM

0 0 -123 360 62 1 0 -120 360 100 5 0 -126 364 108

40 0 -132 377 110 0 0.0055 -101 330 45 1 0.0055 -113 342 68 3 0.0055 -116 348 104

[BMIM]Cl 0 0 -123 360 62 1 0 -117 346 72 5 0 -121 357 70

40 0 -131 373 98 0 0.0055 -101 330 45 1 0.0055 -102 321 48 3 0.0055 -97 332 50

TBABr 0 1

0 0

-123 -125

360 362

62 76

5 0 -129 369 72 40 0 -141 390 79 0 0.0055 -101 330 45 1 0.0055 -120 369 60 3 0.0055 -124 374 68

BKCl 0 0 -123 360 62 1 0 -126 362 68 5 0 -131 377 70

40 0 -144 401 132 0 0.0055 -101 330 45 1 0.0055 -106 335 58 3 0.0055 -128 366 94

Chitin 0 0 -123 360 62 1 0 -119 352 60 5 0 -126 366 69

40 0 -138 380 98 0 0.0055 -101 330 45 1 0.0055 -106 337 56 3 0.0055 -118 350 82

Page 94: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

77

The voltammograms were initiated at point (A) at potential of -1.30 V vs Ag, AgCl/KCl(sat),

scanned in the positive direction, and then reversed at -0.60 V in the negative direction,

crossed-over at point (B). No significant current was observed until the potential reached

the point (B), corresponding to the reduction of Zn2+ ions. NOP is the difference between

the crossover potential (B), the start of the dissolution and the point at which Zn begins to

deposit (D). This could be a useful parameter to identify the best additive concentration or

ratio with antimony ions [42]. Results revealed that measured NOP in SE was 62 mV, this

value was found to be increased gradually by increasing the concentration of additive in the

electrolyte. The value range of 60 mV was observed at 1 mg of chitin to 110 mV at 40 mg

dm-3 of PAM. The best combinations with antimony ions are found at 1 and 3 mg of

[BMIM]Cl corresponding to NOP range of 48-50 mV, respectively.

4.4. Conclusions

Five different additives from different organic groups have been examined

individually during zinc electrowinning process in order to study their effects on power

consumption (PC), current efficiency (CE), surface morphology, lead impurity and

electrochemical behavior. The results are as follows:

- The presence of additives in the standard electrolyte containing antimony could

increase the current efficiencies and counteract the harmful effect of Sb, while additives

decreased the current efficiencies in most of the cases in absence of antimony. In

presence of 0.0055 mg of antimony ions, maximum reductions of PC of ≈147-173 kWh

ton-1 were obtained from adding 1-3 mg dm-3 of [BMIM]Cl, respectively, followed by a

reduction of ≈111-139 kWh ton-1 by adding 1-3 mg dm-3 of PAM.

- The addition of [BMIM]Cl to the SE did not change the preferred crystal orientation in

absence of Sb3+, but it changed the orientation to (101) (102) (110) (112) in presence of

antimony, giving compact and smooth deposit with moderate platelet size. Addition of

PAM to the SE changed the most preferred crystal orientation from (101) to (100)

direction, showing a needled deposit with weak current efficiency, while in presence of

antimony, it restored the (101) giving deposit with very small grain size.

Page 95: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

78

- The highest reduction of lead concentration in the deposit was obtained from 3 mg dm-3

of [BMIM]Cl in absence of antimony. Lead concentration found in zinc deposit

obtained from SE with Sb was 3.64 ppm. None of the additives succeed in reducing this

concentration in presence of antimony, but rather they increased it in the range of

10.90-12.40 ppm.

- Potentiodynamic technique showed that addition of 1 or 3 mg dm-3 of [BMIM]Cl

corresponds to an appropriate concentration in the zinc electrolyte as cathodic

overpotential at 50 mA cm-2 in presence of antimony was decreased by 3-14 mV,

respectively.

- Cyclic voltammetry technique revealed that the best combinations of additives with

antimony ions are found at 1 and 3 mg dm-3 of [BMIM]Cl, corresponding to nucleation

overpotential “NOP” range of 48-50 mV, respectively.

Acknowledgements

Zinc Électrolytique du Canada (CEZinc) and Natural Sciences and Engineering Research

Council of Canada (NSERC) are gratefully acknowledged for their financial support. The

authors would like to express their sincere thanks and appreciation to Mr. Gary Monteith

from CEZinc for his interest and fruitful discussions. Also, thanks to Mr. André Ferland for

SEM analysis, Mr. Jean Frenette for XRD analysis and Mr. Alain Brousseau for ICP

analysis.

Page 96: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

79

CHAPTER 5

ELECTROCHEMICAL STUDIES OF IONIC LIQUID ADDITIVES DURING THE ZINC

ELECTROWINNING PROCESS

Page 97: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

80

Electrochemical Studies of Ionic Liquid Additives during the Zing Electrowinning Process

N. Sorour1,*, W. Zhang1, G. Gabra1 and E. Ghali1, G. Houlachi2

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

*Corresponding author: Tel: 418 656-2131 - Fax: 418 656-5343 ([email protected])

Published in journal of Hydrometallurgy, Vol. 157, 2015, pp 261-269.

Résumé

1-éthyl-3-méthylimidazolium méthanesulfonate [EMIM]MSO3 et bromure de 1-butyl-3 imidazolium [BMIM] Br ont été évalués individuellement comme des additifs par rapport de la gélatine et de l'additif précédemment étudié [BMIM]Cl dans l’électrolyse du zinc à partir de l’électrolyte acide de sulfate contenant 8 g dm-3 des ions de Mn2+. Une impureté métallique de 0,0055 mg dm-3 des ions de Sb3+ a été examinée en combinaison avec de 1 à 3 mg de chaque additif. Des mesures galvanostatiques ont été utilisées dans un électrolyte acide de sulfate pour étudier les potentiels cathodique et anodique individuellement, aussi que le rendement du courant de métal de zinc déposé dans l'électrolyte sulfate acide pendant 2 heures à 50 mA cm-2 et 38°C. L’effet de chaque additif sur la morphologie de surface et l’orientation cristallographique a été étudié par la microscopie électronique à balayage (MEB) et la diffraction des rayons-X (DRX). Les impuretés de plomb dans le dépôt ont été analysées en utilisant le plasma à couplage inductif (ICP). Parmi les cinq différentes concentrations examinées de chaque additif (1,3,5,10 et 40 mg dm-3), les résultats ont révélé que l'addition de 1 et 3 mg dm-3 de [EMIM]MSO3 réduit la tension de la cellule d’environ 10-15 mV, respectivement, tandis que [BMIM]Br réduit la tension de la cellule par 5-10 mV en ajoutant de 1 et 3 mg dm-3, respectivement. Les efficacités de courant de 93,6-94,4% ont été obtenus en ajoutant 1-3 mg dm-3 de [EMIM]MSO3 ou 1-3 mg dm-3 de [BMIM]Br par rapport à 92,8% qui a obtenue à partir de l'électrolyte standard. La réduction maximale de la consommation d'énergie de ≈165 kWh tonne-1 a été obtenue en ajoutant de 3 mg dm-3 de [EMIM] MSO3 en présence des ions de Sb3+, suivie par une réduction de ≈154 kWh tonne-1 en ajoutant de 3 mg dm-3 de [BMIM] Br. La polarisation potentiodynamique et les études voltamétriques montrent que la polarization de l’électrodéposition de zinc a été diminuée en présence d'antimoine. Apparemment, les deux additifs ont un comportement similaire de la polarisation sur l'électrode d'aluminum dans l'électrolyte acide de sulfate.

Page 98: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

81

Abstract

1-ethyl-3-methylimidazolium methanesulfonate [EMIM]MSO3 and 1-butyl-3 methylimidazolium bromide [BMIM]Br were evaluated individually as additives compared to gelatin and previously studied additive [BMIM]Cl in zinc electrowinning from synthetic acidic sulphate electrolyte containing 8 g dm-3 of Mn2+ ions. A metallic impurity of 0.0055 mg dm-3 of Sb3+ ions was examined in combination with 1 and 3 mg of each additive. Galvanostatic measurements have been employed to investigate the cathodic and anodic potentials individually also current efficiency of the deposited zinc metal in acidic sulphate electrolyte for 2 hours at 50 mA cm-2 and 38oC. Effect of each additive on surface morphology and crystallographic orientation was studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Lead impurities in the deposit have been measured by using inductively coupled plasma (ICP). Among five different concentrations tested of each additive (1,3,5,10 and 40 mg dm-3), results revealed that addition of 1 and 3 mg dm-3 of [EMIM]MSO3 reduced the cell voltage by ≈15 and 10 mV, respectively; while [BMIM]Br reduced the cell voltage by ≈10 and 5 mV by adding 1 and 3 mg dm-3, respectively. Current efficiencies of 93.6% - 94.4% have been obtained by adding 1-3 mg dm-3 of [EMIM]MSO3 or 1-3 mg dm-3 of [BMIM]Br as compared to 92.8% obtained from the standard electrolyte. Maximum reduction of power consumption of ≈165 kWh ton-1 was obtained from adding 3 mg dm-3 of [EMIM]MSO3 in presence of Sb3+ ions followed by a reduction of ≈154 kWh ton-1 by adding 3 mg dm-3 of [BMIM]Br. Potentiodynamic polarization and voltammetric studies indicate that polarization for zinc electrodeposition decreased in presence of antimony. Apparently, the two additives have approximately similar polarization behavior on the aluminum electrode in the acidic sulphate electrolyte.

Page 99: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

82

5.1. Introduction

Zinc is considered as the fourth most widely used metal, following iron, aluminum

and copper [3]. Zinc is processed by many methods in order to obtain the metal in high pure

state, among these methods, electrowinning process which is the most commonly used

[131]. Electrowinning uses an electrolytic cell to reduce the zinc on an aluminum cathode

and electric current is run through a lead anode. During the electrolysis of zinc sulphate

electrolyte, two main reactions are competing on the cathode, one is zinc reduction, and the

other is hydrogen evolution reaction (HER) [132]. On the anode, oxygen gas is produced

through the overall electrochemical reaction; H2O → 2H+ + 2e- + ½O2(g) Eo= 1.229V.

Approximately, 99% of the anodic current is used for oxygen evolution reaction (OER),

consuming ≈40% of total cell voltage [20].

As far as lead anodes are used in electrowinning so this process is very sensitive to the

detrimental effect of Pb impurities and to the presented metallic impurities in the electrolyte

such as: Sb, Fe, Cu, Co, Ni ...etc. Low concentrations of these impurities substantially

affect negatively the zinc deposition process. This leads to a decrease of zinc current

efficiency (CE), change in deposit morphology, cathodic polarization and even anodic

polarization [59]. Although the costly steps used for purification, the zinc electrolyte is

usually contaminated with many metal ion impurities [134]. Zinc deposits contaminated

with lead were found to have characteristic morphologies and orientations as well as

negative effect on current efficiency. The overpotential which depends on the amount of

lead present in the zinc deposits and to the presence of other impurities such as antimony

and nickel in the deposits cannot be neglected [40]. One of the considerable goals in zinc

electrowinning is minimizing the power consumption. The two important factors which can

determine the energy requirements are the current efficiency and cell voltage which are

affected negatively by the presence of impurities [125]. Accordingly, additives are used to

reduce the negative effect on current efficiency, cell voltage and deposit morphology

through their adsorption on the surface of electrode [134].

Natural products and surfactants have always been the focus of attention as additives in

zinc electrowinning process. Among these additives; animal glues and Arabic gums which

showed a positive influence on the CE and deposit orientation in the presence of traces of

Page 100: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

83

Sb3+ ions in the industrial electrolyte [42,135]. The effects of saponin alone and in

combination with antimony and glue have been investigated; saponin alone decreased the

CE and was weakly polarized but in combination with glue + antimony at low

concentrations resulted in increase in CE [135]. Addition of sodium lignin sulphonate

alone into the industrial electrolyte at a range of 3-10 ppm had no negative impact on CE,

nor on the zinc electrowinning process [58]. Also many organic additives have been deeply

studied in zinc electrowinning process; Zhang et al. [63] have reported the beneficial effect

of triethyl benzyl ammonium chloride (TEBACl) and malonic acid on CE and cell voltage

in presence of Ni as impurity. Quaternary ammonium bromides in forms of

cetyltrimethylammonium bromide (CTABr) and tetrabutyl ammonium bromide

(TBABr) were studied by Tripathey et al. [62], as they increased the CE and reduced the

power consumption in presence of antimony. Mathieu et al. [136] have investigated the

effect of 2-butyene-1,4-diol and reported its significant effect on improving current

efficiency. Al2(SO4)3 and the horse-chestnut tree extract (HCE) showed their beneficial

effects on the deposit quality, being good levelling agents [64].

Recently, ionic liquids have been used in many chemical and hydrometallurgical

applications due to their chemical and physical properties, as they are salts where the ions

are poorly coordinated, leading to being liquids below boiling point and even at room

temperature [65]. Ionic liquids consist of an organic cation and inorganic or organic anion;

they have a wide range of solubility and miscibility. For example, some of them are

hydrophobic while others are hydrophilic; most of them are non-flammable and non-toxic

[65-66]. Ionic liquids are widely used in liquid-liquid extraction and electrodeposition of

some metals due to their low melting point and the thermal degradation properties which

are important in the electrochemical media [67]. Also, as a medium in the electrodeposition

of aluminum on stainless steel cathode [68]. They are used as an organic solvent in

electroplating of a range of metals impossible to deposit in water due to hydrolysis e.g. Al,

Ti, Ta, Nb, Mo, W [137]. Ionic liquids in the form of 1-butyl-3-methylimidazolium

hydrogen sulphate [BMIM]HSO4 showed their effects on the kinetics of oxygen evolution

as additive during zinc electrowinning process [138]. [BMIM]HSO4 is found to have good

influence in increasing current efficiency, reducing power consumption and producing

smooth and compact zinc deposits similar to that obtained from gelatin [70]. In other

Page 101: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

84

previous studies [139-140], the influence of the ionic liquid salt 1-butyl-3-

methylimidazolium chloride [BMIM]Cl as additive and its electrochemical activity has

been studied and showed its good ionic conductivity and its effect in reducing power

consumption.

This study investigates the effect of two different ionic liquid salts, 1-ethyl-3-

methylimidazolium methanesulfonate [EMIM]MSO3 and 1-butyl-3-methylimidazolium

bromide [BMIM]Br as additives compared to gelatin and [BMIM]Cl on CE, cell voltage,

morphology and electrochemical activity during electrowinning process. The current ionic

liquids additives are chosen in this research paper in order to study the effect of different

anion (Br-) compared to the previously studied one (Cl-) presented in [BMIM]Cl [139], and

also to examine the effect of different ionic liquid [EMIM]MSO3 which could show a good

performance among the examined additives.

5.2. Experimental

5.2.1. Electrolysis

A synthetic standard electrolyte (SE) similar to that used in the Canadian zinc

electrowinning industry. was prepared by dissolving 60 g dm-3 of Zn2+ (ZnSO4.7H2O), 180

g dm-3 H2SO4 and 8 g dm-3 of Mn2+ (MnSO4.H2O) in distilled water. Manganese ions are

added to the solution due to their remarkable effect in reducing the anodic potential and

forming compact layers of MnO2 on the anode. The supplied zinc sulphate containing the

following impurities: Pb (0.003%), Na (0.05%) and Fe (0.001%). The effect of additives

was studied individually with different concentrations of 0,1,3,5,10 and 40 mg dm-3 added

to the standard electrolyte. The influence of 1 mg and 3 mg of the selected additives on CE,

cell voltage and morphology was studied in presence of 0.0055 mg dm-3 of Sb3+ as

impurity. Reagents are supplied from Laboratoire MAT and VWR Canada while additives

are supplied from Sigma-Aldrich USA.

Small-scale galvanostatic electrolysis experiment was performed in an 800 cm3 solution in

double-glazed beaker heated by a flow of thermostated water in the double wall in order to

maintain the working temperature constant. Two plates of aluminum and Pb-Ag (Ag, 0.7%)

were used as cathode and anode, respectively. They were casted in polyester resin with total

Page 102: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

85

exposed surface area of 1 cm2 mounting in Teflon cell with an inner distance of 2 cm

between the two electrodes. Ag, AgCl/KCl(sat) (0.199 V/SHE) was used as reference

electrode. Electrodes were manually polished in several steps by several grits SiC papers

(80, 320, 800, 1000 and 3000) to give uniform surface. Then, they were washed by distilled

water, ethanol and dried few seconds before the experiment.

All electrowinning experiments were carried out for 2 hours at 50 mA cm-2 and 38 ± 1oC

with magnetic agitation at 60 rpm using magnetic bar (L=38 mm & D=10 mm). After

electrolysis the cathode was dried and current efficiency was calculated by weight using

Faraday’s law: CE% = (W.F.n/I.t.M) x 100%; where W is the weight of deposit (g), F is

Faraday’s constant, n is the number of electrons, I is the total cell current (A), t is the time

of electrodeposition and M is the atomic weight of zinc.

5.2.2. Deposit Examination

The surface morphology of the deposits was examined by scanning electron

microscopy (SEM) using JEOL JSM-840a. The crystal orientations of the zinc deposits

were determined using X-ay diffractometer model Siemens - D5000. Lead concentrations

in the deposits have been measured using inductively coupled plasma (ICP) model Optima

4300 Perkin-Elmer.

5.2.3. Electrochemical Measurements

Electrochemical studies were done on the base of potentiodynamic polarization and

cyclic voltammetery measurements. 1 cm2 of Aluminum as working electrode and 1 cm2 of

platinum as auxiliary electrode, Also, Ag, AgCl/KCl(sat) was used as reference electrode.

The three-electrode cell was connected to the potentiostat Gamry Reference 3000 – Gamry

USA. Potentiodaynamic polarization was carried out from -1.05 to -1.25 V with a scan rate

of 5 mV s-1. Cyclic voltammetric scanning was scanned from initial potential of -1.30 to a

reversible potential of -0.60 V at a constant scanning rate of 10 mV s-1. Both tests were

done at 38oC under atmospheric conditions. Working electrodes were manually polished

Page 103: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

86

before each experiment and washed by distilled water, ethanol and dried few seconds prior

the experiment.

5.3. Results and Discussion

5.3.1. Cell Voltage and Power Consumption

The effect of different concentrations of [EMIM]MSO3, [BMIM]Br and gelatin on

cell voltage and power consumption (PC) during zinc electrowinning from acidic zinc

sulphate solution were studied. Results are listed in Table 5.1. Results showed that in

absence of Sb3+ ions, a reduction of cell voltage of about 15 and 10 mV was obtained by

adding 1 and 3 mg of [EMIM]MSO3 to the standard solution, respectively, while the cell

voltage was reduced only by 10 and 5 mV by adding 1 and 3mg of [BMIM]Br,

respectively. Results of cell voltage are approximately compatible to that obtained from

adding [BMIM]Cl [139], as it reduced cell voltage by 7 mV for 3 mg dm-3 addition. Total

cell voltage was found to be increased gradually by increasing the concentrations of

additives reaching to overpotential of 32-40 mV at 40 mg dm-3. This could be explained

due to the strong adsorption of additives on the surface of electrode at high concentrations

which increases the potential of zinc reduction and decreases strongly the hydrogen

evolution reaction (HER). In presence of Sb3+ ions, the addition of [EMIM]MSO3 or

[BMIM]Br has no observed remarkable decrease in cell voltage compared to that obtained

from SE + Sb3+. However, according to the previous study [139], the maximum reduction

of 28 mV in cell voltage was reported by adding 1 mg dm-3 of [BMIM]Cl, this could be

explained due to its higher ionic conductivity and electrochemical stability [139-140].

Power consumption calculations revealed that the maximum reduction of PC of ≈52 kWh

ton-1 was obtained in zinc electrolyte without Sb(III) by addition of 3 mg of [EMIM]MSO3,

and reduction of ≈45 kWh ton-1 by addition of 3 mg of [BMIM]Br. The PC of zinc

electrolysis from zinc electrolyte in the presence of Sb(III) alone and combined with

gelatin, [EMIM]MSO3 and [BMIM]Br in the electrolytes during the electrodeposition of

zinc is also shown in Table 1. By the addition of very low concentration of Sb(III), the PC

is extremely increased due to the sharp decrease in current efficiency of obtained deposit,

while, the PC is found to decrease rapidly in the addition of tested two additives. Addition

of 1 mg and 3 mg of [EMIM]MSO3 to the zinc electrolyte in presence of antimony

Page 104: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

87

decreased the PC by ≈150 and 165 kWh ton-1, respectively, while, addition of 1 mg and 3

mg of [BMIM]Br decreased the PC by ≈141 and 154 kWh ton-1, respectively. However,

the maximum reduction of PC was obtained by addition of 3 mg of [EMIM]MSO3, as

compared to that obtained by addition of standard additive. Adding 1 mg and 3 mg of

gelatin to the zinc electrolyte in presence of Sb(III), the PC is reduced by ≈142 and only

122 kWh ton-1, respectively.

Table 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE and PC in absence and in presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2

Additive/ mg dm-3

Sb(III)/ mg dm-3

Cell voltage/ V

CE/ %

PC/ kWh ton-1

SE 0 0 2.898 92.8 2560 0 0.0055 2.873 88.7 2655

Gelatin 1 0 2.904 91.1 2613 3 0 2.909 90.6 2632 5 0 2.922 89.1 2688 10 0 2.928 85.3 2814 40 0 2.937 77.5 3107 1 0.0055 2.885 94.1 2513 3 0.0055 2.895 93.7 2533

[EMIM]MSO3 1 0 2.883 93.9 2518 3 0 2.888 94.4 2508 5 0 2.900 93.8 2534 10 0 2.904 93.1 2557 40 0 2.930 91.6 2622 1 0.0055 2.870 93.9 2505 3 0.0055 2.873 94.6 2490

[BMIM]Br 1 0 2.888 93.6 2529 3 0 2.893 94.3 2515 5 0 2.915 92.8 2575 10 0 2.928 92.4 2598 40 0 2.938 90.7 2655 1 0.0055 2.877 93.8 2514 3 0.0055 2.880 94.4 2501

Page 105: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

88

5.3.2. Current Efficiency

The effect of the two additives [EMIM]MSO3 and [BMIM]Br on current efficiency

compared to gelatin in absence and in presence of antimony are plotted in Figure 5.1. CE

was studied over range of concentrations of 0-40 mg dm-3 in absence of Sb(III) (Figure

5.1a). Results revealed that, current efficiency obtained from the standard solution is

92.8%. CE was increased gradually from 93.9% to 94.4% by adding 1 and 3 mg dm-3 of

[EMIM]MSO3, to standard zinc electrolyte respectively, while decreased consequently to

91.6% by increasing the concentration to 40 mg dm-3. Also, CE was increased from 93.6%

to 94.3% by adding 1 and 3 mg of [BMIM]Br to the standard zinc electrolyte, respectively.

Then, it was found that CE was decreased to 90.7% by increasing the concentration to 40

mg dm-3 of [EMIM]MSO3. This could be explained due to the excessive adsorption of

additive on the cathode surface which could block the active sites and forbids further

reduction of zinc ions.

Figure 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE: (a) in absence of Sb(III) and (b) in presence of 0.0055 mg of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2

The effect of Sb(III) on CE had been investigated for concentrations of 1 and 3 mg dm-3 of

additives as shown in Figure 5.1b. The presence of small quantity of antimony showed a

harmful effect on the quantity of obtained deposit from the standard electrolyte, which

could indicate that small traces of Sb3+ facilitate the hydrogen evolution reaction (HER) on

the cathode leading to produce weak and porous deposit with low current efficiency.

76

78

80

82

84

86

88

90

92

94

96

0 5 10 15 20 25 30 35 40 45

Cu

rren

t Eff

icie

ncy

(%)

Concentrations of additives (mg dm-3)

SE

Gelatin

[EMIM]MSO3

[BMIM]Br

(a)

88

89

90

91

92

93

94

95

96

0 1 2 3 4

Cu

rren

t Eff

icie

ncy

(%)

Concentrations of additives (mg dm-3)

SE

Gelatin

[EMIM]MSO3

[BMIM]Br

(b)(

Page 106: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

89

Positive effect on the current efficiency was obtained from the two examined additives. For

[EMIM]MSO3, adding 1 mg to standard zinc electrolyte increased the current efficiency to

93.9% while, addition of 3 mg to the zinc electrolyte showed the highest increase of 94.6%

compared to that obtained from the standard electrolyte (88.7%). Also, adding 1 and 3 mg

dm-3 of [BMIM]Br increased CE to 93.8% and 94.4%, respectively. The two studied

additives succeeded to counteract the harmful effect of antimony by their adsorption on the

cathode. The current and previous studied different salts of ionic liquids [EMIM]MSO3,

[BMIM]Br and [BMIM]Cl showed a good synergetic effect in presence of antimony in

reducing PC which complies to that reported by Zhang and Hua [70], from using

[BMIM]HSO4. The obtained reduction in cell voltage and increase of CE by using

[BMIM]HSO4 were higher than that obtained in this study, this could be explained due to

the different working conditions and parameters such as Mn2+ ions addition, current

density, agitation, temperature, lead-silver anode composition as well as different additive

function groups. However, gelatin is still one of the remarkable additives used to increase

the CE in presence of antimony.

5.3.3. Deposit Examination

The zinc deposits obtained were examined by using SEM and X-ray diffraction to

determine surface morphology and crystallographic orientations, respectively. SEM

photomicrographs are shown in Figures 5.2 and 5.3, also the crystallographic orientations

of zinc deposits from zinc electrolyte containing additives in presence and in absence of

Sb3+ are given in Table 5.2. Results revealed that crystallographic orientation obtained from

addition-free electrolyte is (101) (102) (103) (002) which changed to (101) (112) (102)

(103) by adding antimony or gelatin showing a decrease in platelet size (Figures 5.2b and

5.3a). The obtained deposit from SE + Sb3+ showed small platelet size and slightly porous

deposit, this could be explained due to the increase in hydrogen evolution leading to an

increase in agitation on the electrode’s surface. Adding [EMIM]MSO3 and [BMIM]Br to

the SE changed the crystal orientation to (101) (102) (110) (112) and (002) (101) (004)

(103), respectively, leading to an increase in the platelet size. [BMIM]Br changed the most

preferred orientation from (101) to (002) showing the highest peak intensity at 2θ = 36.30

Page 107: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

90

(Figure 5.4b). The obtained deposit from adding [BMIM]Br is found more smooth and

compact than that obtained from [EMIM]MSO3, this could be explained that the higher

molecular weight of [BMIM]+ cation helps the additive to be adsorbed and present more on

the cathodic sites. However, addition of 3 mg of [EMIM]MSO3 and [BMIM]Br to the SE in

presence of Sb3+ changed the preferred crystal orientation to (101) (112) (102) (103) and

(101) (102) (112) (103), respectively. The same effect approximately was given to have a

moderate platelet size producing a compact and smooth deposit by 3 mg of both additives

(Figures 5.3c and 5.3d).

Figure 5.2. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in absence of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br

Page 108: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

91

Figure 5.3. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in presence of 0.0055mg of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br

Figure 5.4. XRD patterns of zinc deposit in absence of Sb(III); (a) 3mg [EMIM]MSO3, (b) 3mg [BMIM]Br

The zinc deposits were analyzed by using inductively coupled plasma spectroscopy (ICP)

to determine the lead concentration, in order to study the effect of each additive on

counteracting the lead contamination. Analysis was done in duplicate and average was

calculated. Results in Table 5.2 shows that both additives of [EMIM]MSO3 and [BMIM]Br

have the same effect on reducing lead concentration, as adding 3 mg to the zinc electrolyte

reduced lead concentration from 26.5 ppm to 5.1-5.6 ppm in absence of antimony and to

4.0-5.1 ppm in presence of antimony. The reported concentrations of lead in zinc deposits

by addition of [EMIM]MSO3 and [BMIM]Br were less than that has been achieved by

adding [BMIM]Cl (10.4-10.9 ppm) [139]. This could be explained due to the presence of

Cl- ions which facilitates the dissolution rate of the lead-based anode. On the other hand,

adding 3 mg of gelatin alone to the standard solution reduced lead concentration to 17.4

ppm, showing very good effect on countering lead concentration to 4.1 ppm in presence of

antimony.

0

500

1000

1500

2000

2500

20 30 40 50 60 70 80 90

Inte

nsi

ty

2-Theta

(a)

(101)

(102)

(110)

(112)

0

500

1000

1500

2000

2500

3000

3500

4000

20 30 40 50 60 70 80 90

Inte

nsi

ty

2-Theta

(b)(002)

(101)

(004)(103)

Page 109: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

92

Table 5.2. Crystallographic orientations and lead concentration of zinc deposits obtained by adding 3mg of gelatin, [EMIM]MSO3 and [BMIM]Br in absence and in presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2

Additive/ Sb (III)/ Crystal orientation/ SEM/ Lead conc./ mg dm-3 mg dm-3 hkl Figure ppm

SE 0 0 (101) (102) (103) (002) 5.2a 26.5 0 0.0055 (101) (112) (102) (103) 5.3a 3.6

Gelatin 3 0 (101) (112) (102) (103) 5.2b 17.4 3 0.0055 (101) (102) (103) (112) 5.3b 4.1

[EMIM]MSO3 3 0 (101) (102) (110) (112) 5.2c 5.1 3 0.0055 (101) (112) (102) (103) 5.3c 5.1

[BMIM]Br 3 0 (002) (101) (004) (103) 5.2d 5.6 3 0.0055 (101) (102) (112) (103) 5.3d 4.0

5.3.4. Polarization Studies

The electrochemical behavior of [EMIM]MSO3, [BMIM]Br and gelatin in absence

and in presence of Sb(III) on the cathode have been investigated by potentiodynamic

polarization. This technique is very useful to understand the kinetics and electrochemical

behavior of the electrode during the electrowinning process. The cathodic potential –

current curves have been obtained by polarization of the electrode from -1.05 to -1.25 V

with a scan rate of 5 mV s-1 using different concentrations of additives. The kinetic

parameters represented in cathodic Tafel slopes (bc), overpotentials at 50 mA (ɳ(50)) and

exchange current densities (I0) were determined and listed in Table 5.3. The overpotentials

values were calculated from following equations:

, 0.763 2⁄ / (5.1)

, 0.0 ⁄ / . (5.2)

ɳ , , (5.3)

Where; Ee is the equilibrium potential, R is the gas constant (equal to 8.314 mol-1 K-1), F is

the Faraday constant (equal to 96 500 C mol-1) and Em is the measured potential at 50 mA

[117]. The apparent exchange current densities (I0) were estimated by extrapolating the

Tafel lines to the corresponding zero current potentials.

Results revealed that, in absence of Sb3+ Tafel slopes are varying from 117 to 132 mV/

decade by addition of [EMIM]MSO3, this small variance in Tafel slopes indicates that the

charge transfer reaction is not controlled by increasing the concentration of additive up to

Page 110: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

93

40 mg dm-3. While, Tafel slopes are increased from 118 to 139 mV/decade by increasing

the concentration of [BMIM]Br up to 40 mg dm-3. At small concentrations of additives the

polarization curves were slightly shifted to less negative potentials, the behavior of addition

of 3 mg of both additives acted more or less as that of the standard electrolyte (Figures 5.5a

& 5.6a). Approximately, the same trend had been obtained from the addition of [BMIM]Cl

to the standard solution [139]. In presence of Sb3+, the addition of [BMIM]Br restored the

regular values of Tafel slopes from 98 to 114 mV per decade compared to [EMIM]MSO3

which could be explained that the higher molecular weight of [BMIM]+ cation has been

adsorbed preferentially on the surface of cathode. This also could be confirmed by the

obtained values of overpotentials as it is increased from 347 to 366 mV and 347 to 372 mV

by adding [EMIM]MSO3 and [BMIM]Br, respectively. 3 mg of [BMIM]Br increased the

overpotential value from 347 to 356 mV which could confirm the resulting fine grain size

obtained [141]. In presence of antimony, 1-3 mg dm-3 of [EMIM]MSO3 increased the

overpotentials from 324 to 334 mV also same quantities of [BMIM]Br increased it up to

337 mV and shifted the polarization curves to more cathodic potentials (Figure 5.6a). This

indicates that the adsorption of low quantities of both additives on the cathode has the

ability to reduce the harmful effect of Sb3+ ions in reducing the hydrogen reaction

overpotential (HER), which leads also to higher current efficiencies of Zn deposition.

The determined exchange current densities (I0) for addition of additives in absence of Sb3+

indicate that at low concentrations of both additives (1-3 mg dm-3) values are close to what

determined for SE (0.079 mA cm-2). This could indicate that at low adsorbed quantities of

additives the active sites are still free and prompt further reduction of Zn2+ ions, while at

high concentrations I0 values are decreased to 0.050-0.060 mA cm-2 at 40 mg dm-3 that

could be related to the strong adsorption of additives on the electrode’s surface, blocking

certain active nucleation sites. In presence of antimony, I0 values are increased up to 0.104

mA cm-2 due to the high evolution of hydrogen. Addition of additives to the electrolyte (3

mg dm-3) decreased this value to 0.083-0.091 mA cm-2.

Page 111: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

94

Figure 5.5. Effect of [EMIM]MSO3 on the cathodic polarization during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)

Figure 5.6. Effect of [BMIM]Br on the cathodic polarization during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb Cyclic voltammetry technique is normally used to study qualitative information about

electrochemical processes at stationary non-agitated interface under various conditions,

such as the presence of intermediates in oxidation-reduction reactions, the reversibility of a

reaction through the determined peaks in the obtained E-I curve during the polarization of

the electrode [111]. CV could then reflect the influence of an additive or change in the

electrolyte composition on the electrochemical properties of the interface. This is

considered mainly in this work to determine the formal reduction and nucleation

overpotential (NOP). The effect of each additive alone or combined with antimony on the

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

0mg1mg3mg5mg10mg40mg

(a)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

(b)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

0mg1mg3mg5mg10mg40mg

(a)

0

20

40

60

80

100

-1.25 -1.20 -1.15 -1.10 -1.05

Cu

rren

t (m

A c

m-2

)

Potential vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

(b)

Page 112: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

95

reduction of zinc ions on aluminum cathode was studied by using cyclic voltammetry

polarization.

Table 5.3. Effect of [EMIM]MSO3, [BMIM]Br and gelatin on Tafel slopes, cathodic overpotential at 50 mA cm-2, exchange current density and NOP

Additive/ mg dm-3

Sb (III)/ mg dm-3

Tafel slope (-bc)/ Overpotential/ I0/ NOP/ mV/decade -ɳ(50) / mV vs Ref (mA cm-2) mV

[EMIM]MSO3

0 0 118 347 0.079 75 1 0 117

123 350 0.081 73

3 0 347 0.074 72 5 0 124 351 0.071 78

10 0 128 354 0.065 80 40 0 132 366 0.060 87 0 0.0055 98 324 0.104 62 1 0.0055 102 329 0.097 60 3 0.0055 104 330 0.091 62

[BMIM]Br 0 0 118 347 0.079 75 1 0 120 349 0.076 78 3 0 127 356 0.065 80 5 0 130 358 0.061 82

10 0 133 364 0.059 84 40 0 139 372 0.050 88 0 1

0.0055 0.0055

98 112

324 337

0.104 0.085

62 63

3 Gelatin

0.0055 114 334 0.083 65

0 0.0055 98 324 0.104 62 1 0.0055 118 344 0.079 66 3 0.0055 121 342 0.076 73

Page 113: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

96

Figure 5.7. Cyclic voltammograms of [EMIM]MSO3 during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)

Polarization was carried from initial potential of -1.30 V to a reversible potential of -0.60 V

at 38oC in presence of atmospheric air without agitation. Results are shown in Figures 5.7

and 5.8, and nucleation overpotential (NOP) values are listed in Table 5.3. The

voltammograms were initiated at point (A) at potential of -1.30 V, scanned in the positive

direction, and then reversed at -0.60 V in the negative direction, crossed-over at point (B).

No significant current was observed until the potential reached the point (B), corresponding

to the reduction of Zn2+ ions. NOP is the difference between the crossover potential (B), the

start of the dissolution and the point at which the Zn begins to deposit (D). This could be a

useful parameter to identify the best additive concentration ratio with antimony ions [42]. It

-400

-300

-200

-100

0

100

200

300

400

500

600

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential Vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

5mg

10mg

40mg

(a)Anodic

CathodicA

BD C

-400

-300

-200

-100

0

100

200

300

400

500

600

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential Vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

(b)Anodic

CathodicA

BD C

Page 114: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

97

was found that at small concentrations of additives, no changes happened on the cathodic

curves which were confirmed by potentiodynamic polarization, while increasing quantity of

additives shifted the cathodic curve to more negatives values leading to an increase in NOP

values. NOP obtained from standard electrolyte was 75 mV which gradually increased to

87-88 mV by adding 40 mg of [EMIM]MSO3 and [BMIM]Br. High NOP values could

indicate strong polarization and fine-grained zinc deposit can be obtained. However, at low

concentrations of additives the observed deposit has medium platelet size unlike the gelatin

which in presence of Sb3+ increased the NOP from 62 to 73 mV (3 mg dm-3) leading to

grain size reduction of the deposit.

Figure 5.8. Cyclic voltammograms of [BMIM]Br during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)

-400

-300

-200

-100

0

100

200

300

400

500

600

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential Vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

5mg

10mg

40mg

(a)Anodic

CathodicA

BD C

-400

-300

-200

-100

0

100

200

300

400

500

600

-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60

Cu

rren

t (m

A c

m-2

)

Potential Vs Ag, AgCl/KCl (V)

0mg

1mg

3mg

(b)Anodic

CathodicA

BD C

Page 115: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

98

5.4. Conclusions

The influence of the ionic liquids additives [EMIM]MSO3, [BMIM]Br as compared

to gelatin and previously studied additive [BMIM]Cl has been examined:

- Maximum power consumption reduction of ≈165 kWh ton-1 is obtained by adding 3 mg

dm-3 of [EMIM]MSO3 to the standard electrolyte containing 0.0055 mg of Sb followed

by a reduction of ≈154 kWh ton-1 from addition of 3 mg dm-3 of [BMIM]Br. However,

gelatin is still one of the best additives in reducing PC in presence of Sb3+ ions, showing

a reduction of ≈142 and 122 kWh ton-1 by adding 1 and 3 mg dm-3, respectively.

- 3 mg of [EMIM]MSO3 and [BMIM]Br added to the standard electrolyte in presence of

Sb3+ changed the preferred crystal orientation giving a moderate platelet size producing

a compact and smooth deposit.

- Both additives of [EMIM]MSO3 and [BMIM]Br showed better effect than [BMIM]Cl

in reducing lead contamination from 26.5 ppm to 5.1-5.6 ppm in the zinc deposits

while, they have almost no bad effect in presence of antimony.

- [EMIM]MSO3 and [BMIM]Br have approximately similar polarization behaviors,

slightly better than gelatin. Increasing concentration of additives shifted the polarization

curves to more negative values leading to an increase in Tafel slope values from -118

mV/decade to -132 & -139 mV/decade at 40 mg dm-3 and NOP from 75 mV to 87 & 88

mV at 40 mg dm-3 for both additives.

Acknowledgements

Zinc Électrolytique du Canada (CEZinc) Limitée and Natural Sciences & Engineering

Research Council of Canada (NSERC) are gratefully acknowledged for their financial

support. The authors would like to express their sincere thanks and appreciation to Mr.

André Ferland, Mr. Jean Frenette and Mr. Alain Brousseau for their professional technical

participations.

Page 116: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

99

CHAPTER 6

ELECTRODEPOSITION AND STUDY OF THE ELECTROCATALYTIC ACTIVITY OF Fe-Mo-P ALLOYS FOR HYDROGEN

EVOLUTION DURING CHLORATE PRODUCTION

Page 117: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

100

Electrodeposition and Study of the Electrocatalytic Activity of Fe-Mo-P Alloys for Hydrogen Evolution during Chlorate Production

F. Safizadeh1,*, N. Sorour1, G. Houlachi2, E. Ghali1

1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.

2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.

*Corresponding author: Tel: 418 6562131-Fax:418 6565343 ([email protected])

This paper is submitted to the International Journal of Hydrogen Energy.

Résumé

Des binaires Fe-Mo et ternaires Fe-Mo-P revêtements différents on été déposés par une méthode électrochimique à partir d’un électrolyte a base de citrate sur des substrats d’acier doux (MS). Des électrodépositions galvanostatiques ont été menées pendant 6 heures à 20 mA cm-2 et 30oC. L’activité électro-catalytique de ces alliages vers la réaction de dégagement d`hydrogène (RDH) a été étudiée en utilisant les techniques de polarisation à l’état-stable et la spectroscopie d'impédance électrochimique (SIE). Les expériences électrochimiques ont été réalisées dans des solutions de chlorure de sodium. Tous les alliages électrodéposés ont produit la structure amorphe, révélée par des diagrammes de diffraction de rayon-X. L’alliage ternaire préparé Fe54Mo30P16 a diminué la surtension de RDH par 30% par rapport à MS à la densité de courant de 250 mA cm-2. Cet électrocatalyseur a porté une amélioration de 16.5% à la surtension de la RDH en comparaison avec l’alliage binaire de Fe-Mo. Les résultats de la polarisation à l’état-stable et de SIE ont révélé que la rugosité de surface et l’activité intrinsèque des alliages Fe-Mo-P pourraient être l’origine du comportement prometteur de cet électrocatalyseur vers la RDH. l'alliage ternaire de Fe-Mo-P pourrait être un candidat considérable pour l'amélioration de la RDH.

Page 118: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

101

Abstract

Binary Fe-Mo and ternary Fe-Mo-P coatings have been electrochemically deposited from citrate-based electrolyte on mild steel (MS) substrate. Galvanostatic electrodepositions have been conducted for 6 hours at 20 mA cm-2 and 30oC. The electrocatalytic activity of these alloys towards hydrogen evolution reaction (HER) was assessed using steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques. Electrochemical tests were carried out in sodium chloride solutions. All electrodeposited alloys yielded the amorphous structure, revealed by X-ray diffraction patterns. At a current density of 250 mA cm-2, the Fe54Mo30P16 electrode reduced the HER overpotential by 30% in comparison with mild steel. This electrocatalyst also showed an enhancement of 16.5% for the HER overpotential as compared to the binary alloy of Fe53Mo47. The steady-state polarization and EIS results revealed that both the surface roughness and intrinsic activity could be the origin of the promising behavior of Fe-Mo-P electrocatalyst towards HER. The ternary alloy of Fe-Mo-P could be a considerable candidate in enhancing the HER.

Page 119: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

102

6.1. Introduction

Chlorine and sodium chlorate are between the most important chemical products,

being extensively applied in pulp and paper industry, water treatment, agriculture defoliant,

herbicide as well as fabrication of different polymers [14-16,142]. Chlorine production is

performed using chlor-alkali electrolysis. During this process, sodium chloride solution is

electrolysed to form chlorine at the anode and sodium hydroxide and hydrogen at the

cathode (Eq. 6.1).

2 2 → 2 (6.1)

During the electrochemical process of chlorate production, sodium chloride is oxidised to

sodium chlorate while water is reduced to hydrogen gas evolved at the cathode according to

the following reaction (Eq. 6.2) [14].

3 → 3 (6.2)

Mild steel (MS) is a popular cathode used for chlorate production and diaphragm chlorine

cells, owing to its low hydrogen overvoltage and high durability in sodium hydroxide, low

cost and capability of being shaped into different forms [18]. However, MS is not the best

choice as cathode due to some existing drawbacks [16,18]: (i) when the surface of mild

steel is fresh, the overvoltage values of HER are between 850 and 950 mV at 250 mA cm-2

(η250), depending on surface roughness [18]. Since Cr is present during electrolysis as well

as Ca and Mg as impurities in the electrolyte, their precipitations gradually cover the

cathode surface resulting in an increase of ~1100 mV as overpotential. In fact, the Cr(VI) is

reduced during cathodic polarization and formed a thin film of Cr(OH)3.xH2O on the

cathode so-called chromium diaphragm that contained a thickness less than 10 nm. This

film hinders also some other cathodic reactions such as oxygen reduction, whereas

hydrogen evolution can still take place on the surface though with changed kinetics

compared to that occurring on a bare electrode surface [15,18]. Thus, the presence of

chromium oxide is essential for enhancement of the overall cell efficiency. Moreover, the

presence of chromate inhibits the reduction of hypochlorite and chlorate ions on the

cathode (parasitic reactions), reducing the corrosion rate of the steel cathodes and finally

acting as a buffer in the pH range of 5-7 [14-15]. (ii) Due to the thermodynamic instability

Page 120: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

103

of iron, the steel cathodes are significantly corroded in hot concentrated caustic solution

with time especially during power shutoff (open circuit potential). (iii) The corrosion

products cause the shortening life of the cathodes. Therefore, replacement of steel cathodes

with new materials has a great economical interest for the industry.

The HER was extensively investigated on several elements such as Ni, Fe, Mo, Cu, P, Ti,

Pd, Mn, Ru, Co, W, Cr and graphite as well as on rare-earth elements for different

applications such as water electrolysis and hydrogen-based fuel cells [16,18,143].

Molybdenum and phosphorous are among new cathode materials exhibiting improved

properties for the HER. These two metals were doped with other elements to enhance the

surface roughness of the cathode or the intrinsic catalytic activity towards the HER [101,

142,144-151]. Molybdenum and phosphorous may yield an amorphous structure, creating

some modifications in the electronic structure and surface properties [145,152]. The

amorphous coating is usually achieved by co-deposition of an element from iron group (Fe,

Ni or Co) with a metal (Mo and W) or a metalloid such as P, B, Ge or Si that initiates the

defects in crystal lattice, leading to suppress the crystallization of the deposit [153-154]. In

general, transition metal phosphides offer interesting features such as enhanced electronic

conductivity and good stability in acidic and basic media, as compared to pure metals

[155].

Electrodeposition is a low cost technique for preparation of thin metal films. However,

preparation of alloys by electrodeposition is not always straightforward since different

experimental parameters such as current density, temperature and pH may influence the

chemical composition of the alloys. Furthermore, when two or more metals are

electrodeposited simultaneously, the elemental composition of the obtained coating does

not necessarily reflect the composition of the starting electrolyte [156]. Thus, the

experimental parameters and chemical composition of the electrolyte should be well

optimized in order to obtain a desired coating.

Krstajic et al. [157] reported the positive effect of phosphorous addition inside the sodium

chloride brine where Fe-Mo coated alloy on steel was used as cathodes during sodium

chlorate production. Although, Fe, Mo and P were vastly doped with different elements in

the form of binary and ternary alloys [95,97,101,154], to our knowledge, the

Page 121: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

104

electrodeposition and the electrocatalytic activity of Fe-Mo-P alloy towards HER has not

been reported yet. In this work, three Fe-Mo-P coatings on mild steel, comprising different

phosphorous contents, were prepared by electrodeposition in the presence of citrate ions.

Thereafter, the electrocatalytic activities of the coatings were assessed in the simulated

conditions of chlorate production. The results were compared to mild steel and Fe-Mo alloy

as references.

6.2. Experimental

Mild steel and platinum foil electrodes with a surface of 1 cm2 were used as cathode

and anode, respectively. Both electrodes were mounted in epoxy resin and assembled in a

three-electrode cell. The reference electrode was a silver chloride with a saturated KCl

double junction of Ag, AgCl/KClsat (0.202 V vs. standard hydrogen electrode (SHE)).

FeSO4.7H2O, Na2MoO4.2H2O and NaH2PO2.H2O were employed as the sources of Fe, Mo

and P, respectively. Trisodium citrate dehydrate (Na3C6H5O7.2H2O) was used as a

complexing agent and the reduction rate controller during deposition. All chemicals were

supplied from Lab Mat–Canada and Sigma-Aldrich – USA. The electrolytes were heated

by the flow of water in a double-glazed wall in order to maintain working temperature at

30oC during electrodeposition. The electrolyte was agitated magnetically at 300 rpm and

pH 6 was adjusted using citric acid. Galvanostatic tests were carried out for 6 hours of at

current density of 20 mA cm-2.

Electrochemical measurements such as potentiodynamic polarization and electrochemical

impedance spectroscopy (EIS) were carried out using potentiostat Gamry Reference 3000.

The potentiodaynamic polarizations were carried out from -1.2 to 0.5 V with a scan rate of

1 mV s-1. The triplicate tests were conducted for each experimental condition in this work.

Close duplicates are considered, however triplicate confirmed the same tendency. All tests

were performed in solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC

alongside magnetic agitation of 80 rpm. The pH 6.4 was always adjusted using NaOH. EIS

measurements were preformed over the frequency range of 0.01 Hz to 100 kHz. High

amplitude sinusoidal signals were used to overcome the interference of the hydrogen

Page 122: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

105

bubbles on the cathode surface. An AC signal of 50 mA was employed for galvanostatic

mode at 250 mA cm-2. Before the test, the electrode was held under cathodic current

density of 20 mA cm-2 for 10 minutes in order to remove the oxide layer on the surface. All

tests were performed under atmospheric conditions.

Surface morphology and elemental composition of the alloys was investigated by scanning

electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) (JEOL JSM-840a

and FEI Quanta FEG 250). X-ray diffraction patterns of the deposits were determined using

an X-ray diffractometer Siemens - D500.

6.3. Results and Discussion

6.3.1. Deposit Characterization

Four different binary and ternary coatings of Fe-Mo and Fe-Mo-P have been

electrochemically deposited using different electrolytes. All deposited films exhibited a

good adherence on the MS substrate and it was not possible to separate them mechanically.

The composition of each electrolyte and that of the produced coatings are listed in Table

6.1.

The electrodeposition of the Fe-Mo binary system is the reflect of “induced co-deposition

mechanism” which is referred to the condition of a metal that can not be deposited alone (in

this case Mo); instead, it will be co-deposited in the presence of another metal; called

inductor metal (in this case Fe) [158]. The chemical composition of the coating obtained by

the electrolyte I, shows that despite the higher concentration of Mo, as compared to Fe (5

times) in the solution, the final content of iron and molybdenum in the coating are close

together. This confirmed that in the competition between Fe and Mo for deposition, iron as

a less noble metal is preferentially adsorbed on the electrode surface, as reported by

Sanches et al. [159]. The comparison of the electrolytes I and II shows that the addition of

phosphorous in the bath resulted in an increase of the phosphorous content in the deposit.

These results are in accordance with the findings in the case of the ternary system of Co-

Mo-P alloy, showing that the increase of hypophosphite anions (H2PO2-) in the electrolyte

increases the phosphorous content in the resulting coating [97]. However, the increase of

Page 123: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

106

sodium molybdate concentration from 50 to 70 g dm-3 (electrolytes II and III) not only

reduced the molybdenum and phosphorous content in the deposit but also led to a

significant increase of Fe content in the cathode. This phenomenon could be explained by

the enhancement of the hydrogen evolution reaction after addition of molybdenum to iron

[160].

Table 6.1. The compositions of the coatings from four different electrolytes after 6 hours of electrodeposition at 20 mA cm-2 and 30oC

Electrolyte

Electrolytes compositions (g dm-3)

Deposits compositions (at.%)

Na3C6H5O. 2H2O

FeSO4. 7H2O

Na2MoO4. 2H2O

NaH2PO2. H2O

Fe Mo P

I 120 10 50 0 53 47 0

II 120 10 50 10 54 30 16

III 120 10 70 10 70 21 9

IV 120 10 70 30 61 26 13

Apparently, the HER becomes major reaction in the presence of high concentration of Mo.

High hydrogen evolution inhibits the deposition of ions close to the electrode surface

however, since iron is deposited easier than Mo and probably P, the obtained coating

contained higher amount of Fe than Mo. The hydrogen evolution could also be the reason

for reduction of phosphorous content. It was already indicated that the higher hydrogen

evolution, during the electrolysis process of Ni-P alloy is an undesirable reaction

prohibiting the co-deposition of phosphorous [16]. This phenomenon yields a reduction in

the current efficiency and correspondingly in the deposition rate [160]. This could explain

the slight increase of phosphorous content (from 9 to 13 at.%) in the cathode (electrolytes

III and IV). In fact, despite an important increase of P concentration in the bath (three

times), phosphorous content was increased by only 4 at.%.

Scanning electron microscopy images showed the presence of nodules on all deposits. For

the Fe53Mo47 deposit (Figure 6.1a), the agglomeration of nodules could be clearly observed

in some points while the rest of deposit surface stays nodule-free, having fine cracks. The

presence of the cracks is ascribed to the relaxation of internal tensile stress in coating while

the existence of the cauliflower-shape grains on the surface could be explained by the

Page 124: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

107

growth of the secondary nuclei on top of the first layer that was already formed on the

substrate. Cracked morphology for some binary coatings such as Ni-Mo containing more

than 17 at.% Mo was already observed by several research groups [161-163]. The cracks

decrease the hardness of the coating and weaken its resistance to corrosion. The deposits

containing phosphorus, as illustrated in Figures 6.1b, c and d, present almost similar

morphologies. The most important difference between SEM images of the binary Fe-Mo

and ternary Fe-Mo-P electrodes is that the nodules are more homogeneously dispersed on

the electrode surface of the ternary alloys.

Figure 6.1. Scanning electron micrographs (X500) of deposits; (a) Fe53Mo47, (b) Fe70Mo21P9, (c) Fe61Mo26P13, (d) Fe54Mo30P16 after electrodeposition during 6 hours at 20 mA cm-2 at 30oC

The X-ray diffraction patterns of four prepared deposits show a broad peak around 2θ of

43°, indicating the amorphous structure of these alloys (Figure 6.2). As Stepanova et al.

[164-165] reported for the electrodeposition of Mo alloys with iron-group metals in citrate-

(b)

(c)

Page 125: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

108

ammonium baths, when the amount of Mo in the deposit exceeds 22-25 at.%, the alloy

contains the amorphous/nanostructured phases. Considering that all prepared Fe-Mo and

Fe-Mo-P coatings contained more than 21 at.% Mo, the observation of an amorphous

structure for all samples confirmed what have been reported previously. Furthermore, it

should be mentioned that the presence of more than 9 at.% phosphorous in ternary alloys

promotes formation of amorphous matrix.

Figure 6.2. XRD spectra of Fe-Mo and Fe-Mo-P deposits obtained from electrodeposition of 6 hours at 20 mA cm-2 and 30oC

6.3.2. Steady-State Polarization Curves

The activity of different coatings towards HER was studied using steady-state

polarization Tafel curves. The potential-current curves are presented in Figure 6.3 where

the current density is plotted against overpotential. As it was shown in Figure 6.3, the

curves follow a typical Tafel behavior. The kinetic parameters derived from the linear part

of the Tafel plots were calculated according to following equation (Eq. 6.3):

.log

.log log| | (6.3)

Inte

nsi

ty /

a.u

Fe54Mo30P16

Fe61Mo26P13

Fe70Mo21P9

Fe53Mo47

20 30 40 50 60 70 80

2θ /degree

MS

Page 126: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

109

Where, ɳ is the overpotential and j the current-density for equilibrium at ɳ=0. Using

equation 6.3, the parameters concerning the cathodic Tafel slopes (bc), hydrogen evolution

ovrepotentials at 250 mA cm-2 (ɳ250), charge-transfer coefficients (), exchange current

densities (j0) and the current densities at 200 and 300 mV were determined and listed in

Table 6.2. The apparent current density (j0) values were estimated by extrapolation of Tafel

plots to zero current potentials. The overpotential (ɳ) was corrected considering the ohmic

drop (iRs) where Rs (solution resistance) was obtained employing EIS method.

Figure 6.3. Polarization curves of Fe-Mo and three Fe-Mo-P deposited electrodes compared to MS in chlorate solution at 80oC and pH 6.4

It is well established that the HER in alkaline solutions proceeds via three following steps

[166]:

↔ Volmer step (6.4)

↔ Heyrovsky step (6.5)

2 ↔ 2 Tafel Step (6.6)

Where, M represents electrode material and MH the adsorbed hydrogen on the electrode surface.

-8

-7

-6

-5

-4

-3

-2

-1

0

0 200 400 600 800

Log

(j/

A c

m-2)

-ɳ /mVSHE

MS

Fe53Mo47

Fe70Mo21P9

Fe61Mo26P13

Fe54Mo30P16

Page 127: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

110

The first step (Eq. 6.4) is the electro-reduction of water with desorption of hydrogen. This

step is followed by either the electrochemical desorption of hydrogen (Heyrovsky step) or

chemical desorption (Tafel step). It has been stated that the rate-determining step (rds) is

the Volmer or the Volmer coupled with Herovsky or Tafel step if the charge transfer

coefficient is equal to 0.5 [85,167]. Since the charge-transfer coefficient of MS is close to

0.5, it can be concluded that the rds of HER on mild steel may be Volmer or Volmer in

conjunction with other two reactions of Heyrovsky or Tafel. Moreover, the Tafel plots for

all binary and ternary cathodes are linear at negative potentials. This is indicating that the

predominant mechanism of HER on these electrodes appears to be discharge of water

(Volmer reaction) followed by electrochemical desorption step (Heyrovsky)

[85,145,166,168].

The kinetic parameters in Table 6.2 revealed that the Tafel slope decreased significantly

from MS to the binary and ternary catalysts, indicating higher performance of these alloys

for the discharge of the hydrogen on the cathodic surface. Apparently, alloying binary Fe-

Mo catalyst with phosphorous influenced the hydrogen overpotential. The decrease of

overpotential could be observed with the increase of the phosphorous content in the alloys.

The lowest overpotential was obtained for the electrode containing the highest phosphorous

content (Fe54Mo30P16). This electrode showed a decrease of 313 mV in overpotential, an

improvement of 30% as compared to MS. The comparison between Fe53Mo47 and

Fe54Mo30P16 alloys revealed a decrease of 141 mV in overpotential (16.5%) caused by

phosphorous addition.

Table 6.2. The measured kinetic parameters of HER for MS, Fe-Mo and Fe-Mo-P electrodes in sodium chloride solution at 80oC and pH 6.4

Electrode -bc

(mV/dec) -ɳ(250)

(mV vs. SHE) J0

(mA cm-2) J200

(mA cm-2) J300

(mA cm-2)

MS 153 1029 0.46 13×10-3 0.16 0.72

Fe53Mo47 90 857 0.78 538×10-3 43 106

Fe70Mo21P9 79 776 0.88 237×10-3 33 134

Fe61Mo26P13 80 753 0.88 330×10-3 50 186

Fe54Mo30P16 80 716 0.88 429×10-3 84 583

Page 128: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

111

The charge-transfer coefficients, , were calculated using equation: = 2.303RT/bF; where

R is the ideal gas constant, T the temperature, bc the cathodic Tafel slope and F the Faraday

constant. It could be observed that the values were increased from 0.46 for MS to 0.78 for

binary Fe-Mo alloy and 0.88 for ternary Fe-Mo-P coatings. The increase of values is an

indication of an improvement of the charge transfer kinetics and a better electrocatalytic

activity of the HER. This parameter is used often as a comparative parameter instead of j0

[80,169].

The exchange current density measurements at the equilibrium potential (zero

overpotential) were also reported in Table 6.2. The apparent exchange current density

provides also the information about the catalytic activity of coatings. It is already known

that the hydrogen evolution reaction could not be occurred at open circuit potential without

certain overpotential [169-170]. Thus, the overpotential at a given current density is more

practical parameter to compare the activity of different electrodes and the current density

value at the equilibrium could not be considered as the solely criteria for evaluation of the

catalytic activity. Regarding the results presented in Table 6.2, an important improvement

in j0 and j could be seen for binary and ternary catalysts as compared to MS. The

comparison of j at two overpotentials, e.g. 200 and 300 mV clearly showed an increase in

current density with increase of the induced phosphorous content in the ternary system. A

direct comparison of two alloys, e.g. Fe53Mo47 and Fe54Mo30P16 containing similar iron

content shows that the substitution of Mo by P could be promising for decreasing the HER

overpotential. Although, the Mo is known for its promising catalytic properties, this

comparison induced that the replacement of the Mo by phosphorous was promising for the

catalytic behavior of the cathode. However, these results are not in accordance with the

results published on Ni-Mo-P ternary alloy. Regarding that both Ni and Fe are from iron

group elements, we may expect the similar behavior for two systems of Ni-Mo-P and Fe-

Mo-P. Nonetheless, it was shown by Shervedani and Lasia [101] that comparing two

electrodes of Ni74Mo16P10 and Ni71Mo27P2 tested in 1M NaOH at 70oC, the electrode

having more molybdenum and less phosphorous content presents better catalytic activity.

They also deduced that any treatment leading to remove of Mo deactivates the Ni-Mo-P

electrode.

Page 129: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

112

6.3.3. Electrochemical Impedance Spectroscopy

The electrochemical impedance data were modeled using modified Armstrong

equivalent-circuit along with a constant phase element (CPE) presented in Figure 6.4. This

electrical equivalent circuit diagram was used to model the solid/liquid interfaces and

thereafter the EIS experimental data were fitted using a CNLS program to this model. The

1-CPE (Constant Phase Element) model presented in Figure 6.4 predicts the appearance of

two depressed capacitive as well as two overlapped semicircle-shapes on Nyquist plots

(Figure 6.5).

Figure 6.4. The electrical equivalent circuit used for simulation of the impedance spectra for the HER [171]

0.00

0.04

0.08

0.12

0.16

0.20

0.5 0.6 0.7 0.8 0.9 1.0

Z''

/ Ω

cm

2

Z' / Ω cm2

(a)

0.00

0.04

0.08

0.12

0.16

0.5 0.6 0.7 0.8 0.9 1.0

Z'' /

Ω cm

2

Z' / Ω cm2

(b)

Page 130: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

113

Figure 6.5. The Nyquist plots for the HER process on a) MS, b) Fe53Mo47, c) Fe70Mo21P9, d) Fe61Mo26P13 and e) Fe54Mo30P16

This model was already used by several authors to study the behavior of Raney-nickel

composite coated electrodes [166,171-173]. Generally, the CPE is attributed to the

roughness or porosity of the real surface of solid electrodes, causing the depression of the

0.00

0.04

0.08

0.12

0.16

0.5 0.6 0.7 0.8 0.9 1.0

Z''

/ Ω

cm

2

Z' / Ω cm2

(c)

0.00

0.04

0.08

0.12

0.16

0.5 0.6 0.7 0.8 0.9

Z''

/ Ω

cm

2

Z' / Ω cm2

(d)

0.00

0.04

0.08

0.12

0.16

0.6 0.7 0.8 0.9 1.0

Z''

/ Ω c

m2

Z' / Ω cm2

(e)

Page 131: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

114

semicircles as a result of inhomogenieties present at a micro or nano (atomic/molecular)

scale [113,174-175]. In the case of MS, two complete semicircles, yet slightly depressed,

appear clearly while for the other electrodes, the overlapped flattened semicircles could be

observed on the complex plane plots. The scattering of the data, especially at low frequency

was observed that could be basically due to vigorous hydrogen evolution on the rough

electrode surface. The fitting parameters were listed in Table 6.3.

The Rct corresponding to the Faradic resistance for electrosorption reaction (charge transfer

process) of the catalyst was reduced from binary to the ternary coating. The lowest Rct was

obtained for Fe54Mo30P16 electrode, indicating the highest electrocatalytical activity for this

alloy. However, it should be noted that the values of the charge transfer resistance were

very close for all ternary catalysts. The highest CPE value was also determined for

Fe54Mo30P16 cathode, comprising lower overpotential according to the steady-state

polarization tests. Since the CPE could be related to the Faradic reaction of the HER, the

increase of CPE values could thus be indicating the increase of the electrocatalytic activity.

The lowest CPE was obtained for MS, presenting maximum overpotential to HER. The

overpotential is indicating the increase of hydrogen adsorption rate on the electrode surface.

In fact, the production of gas bubbles due to excessive hydrogen evolution could result in

blocking of the electrode surface, leading to the CPE reduction [169,176,177]. Adsorption

of the hydrogen gas bubbles on the surface could also reduce the effective surface area.

Table 6.3. The electrochemical data obtained by the Nyquist plots of MS, Fe-Mo and different Fe-Mo-P alloys

Electrode Rct

(Ω cm2) CPE (mF cm-2)

n Rf

MS 0.235 0.457 0.931 11

Fe53Mo47 0.180 261 0.652 2196

Fe70Mo21P9 0.172 271 0.709 3457

Fe61Mo26P13 0.174 361 0.708 5164

Fe54Mo30P16 0.167 437 0.590 3010

The parameter n is generally accepted to be a measure of surface inhomogeneity and

irregularities of the solid surface. The n values lower than 1 for all catalysts suggested a

Page 132: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

115

porous structure for coatings. The effective surface area, meaning the electrochemically

accessible surface area on which hydrogen is adsorbed, could be estimated from the double

layer capacitance (Cdl). The Cdl is calculated by following equation (Eq. 6.7) according to

EIS results [170,178].

(6.7)

Where, Rs is the solution resistance (Ω cm2). The roughness factor, that characterizes the

real-to-geometrical surface area, was calculated based on Rf = Cdl/20 µF cm-2 (20 µF cm-2 is

the value considered for the double layer capacitance of a smooth electrode) [179]. Table

6.3 shows that, All ternary catalysts showed higher roughness factor Rf (3010-5164) as

compared to binary catalyst (2196) and MS. Electrochemical impedance studies revealed

that the enhanced behavior of ternary alloys especially the Fe54Mo30P16 coating could be

attributed to both the higher surface roughness and the better intrinsic activity of the alloy

due to the synergetic effect of phosphorous with iron and molybdenum.

6.4. Conclusions

The binary Fe-Mo and ternary Fe-Mo-P electrodes were successfully

electrodeposited and their activities towards the hydrogen evolution reaction (HER) were

evaluated in simulated conditions of chlorate industry. The obtained results in agitated

solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and pH 6.4 can be

summarized as follows:

1. The overpotential of the HER determined by steady-state polarization for the

prepared coating Fe54Mo30P16 was decreased by 313 mV compared to mild steel,

and by 141 mV compared to Fe53Mo47 coating. The XRD analysis of the coating

confirmed the presence of amorphous structure for all electrodes.

2. The other kinetics parameters such as charge-transfer coefficient (), and cathodic

Tafel slope (bc) show an improvement in the catalytic activities of the ternary

coatings compared to that of the binary coating. is increased from 0.78 to 0.88

Page 133: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

116

while bc is decreased from 90 to 80 mV per decade for Fe-Mo and Fe-Mo-P

coatings, respectively.

3. Electrochemical impedance results revealed that, ternary alloys of Fe-Mo-P

exhibited better catalytic activity as compared to binary alloy of Fe-Mo. Moreover,

the phosphorous-containing electrodes exhibited higher roughness Rf (3010-5164)

as compared to the Fe-Mo (2196) and MS electrodes.

4. The beneficial effect of phosphorus addition in Fe-Mo alloy could be ascribed to the

increase of the effective surface area as well as the intrinsic activity of coatings.

These results revealed that the Fe-Mo-P coatings may be considered as a promising

cathode towards the HER. However, the corrosion performance of this alloy should

be considered in the future.

Acknowledgements

The Hydro-Québec Research Institute and the Natural Sciences and Engineering Research Council of Canada (NSERC) are gratefully acknowledged for their financial support.

Page 134: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

117

CHAPTER 7

CONCLUSIONS AND OUTLOOK

Page 135: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

118

7.1. Conclusions

Based on the obtained results in this research study, the following points can be

concluded:

(A) The effect of certain organic additives on the zinc electrowinning process;

Seven different organic additives from different groups were chosen to be examined during

the zinc electrodeposition. (1) Polyacrylamide [PAM], is one of the remarkable synthetic

polymers, (2) Tetra-butylammonium bromide [TBABr], is a quaternary ammonium salt, (3)

Benzalkonium chloride [BKCl], is a cationic surface-acting agent belonging to

the quaternary ammonium salts with aromatic group, (4) Chitin, is a natural compound.

Ionic liquid salts were well in this study due to their promising behavior in the

electrochemical media. Among these salts, (5) 1-butyl-3-methylimidazolium chloride

[BMIM]Cl, (6) 1-butyl-3-methylimidazolium bromide [BMIM]Br, and (7) 1-ethyl-3-

methylimidazolium methanosulfonate [EMIM]MSO3 were also chosen as additives

compared to gelatin. The effect of different concentrations of 1,3,5,10 and 40 mg dm-3 of

each additive have been examined in standard electrolyte, also 1 and 3 mg dm-3 have been

examined in electrolyte containing Sb3+ ions as a metallic impurity. The effect of each

additive on zinc electrowinning parameters such as: power consumption (PC), current

efficiency (CE), cell voltage (CV), lead contamination in the deposit, surface morphology,

crystallographic orientation, and polarization behavior has been studied. It was found that:

I. The obtained current efficiency and power consumption from the standard electrolyte

(free-addition) were 92.8% and 2560 kWh ton-1, respectively. The addition of additives

such as [PAM], [TBABr], and [BKCl] to the standard electrolyte (SE) decreased the CE

and increased the PC in most of the cases. The ionic liquids salts succeeded to increase the

CE by ~0.7-1.6% and decrease the PC by ~31-52 kWh ton-1 at low concentrations of 1-3

mg dm-3. Increasing the concentration of additives increased the overpotential and

decreased the CE due to the excessive adsorption which blocks the active sites on the

cathode.

II. The presence of small traces of Sb3+ reduced significantly the CE to 88.7%, also reduced

the cell voltage by 25 mV. This could be attributed to the acceleration of hydrogen

Page 136: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

119

evolution reaction (HER) which is catalyzed by the presence of Sb3+ ions and hinder the

Zn2+ ions reduction. All additives added to the SE + Sb3+ increased the CE and

counteracted the harmful effect of Sb3+ on the zinc reduction through their adsorption on

the cathode surface. The ionic liquid salts succeeded to increase the CE up to 95.1% from

standard electrolyte in presence of Sb3+ ions. Maximum reduction of PC of ~173 kWh ton-1

was observed by addition of 3 mg dm-3 of [BMIM]Cl to the same electrolyte. The PC

values in SE with additives in presence of Sb3+ was decreased in the order of: Ionic liquid

salts > PAM > TBABr > Chitin > BKCl.

III. Maximum reduction of lead contamination in zinc deposit from 26.5 ppm to 5.1-5.6

ppm was obtained from adding 3 mg dm-3 of [EMIM]MSO3 and [BMIM]Br individually in

absence of antimony. Both additives showed better effect than [BMIM]Cl in reducing lead

contamination (~10.7 ppm), this could be explained due to the presence of Cl- ions which

facilitate the dissolution of lead-based anode. All examined additives did not succeed in

reducing the Pb concentration in presence of antimony, since lead contamination is already

well reduced.

IV. X-ray diffraction and scanning electron microscope results revealed that, the

crystallographic orientation for zinc deposit obtained from SE was (101) (102) (103) (002)

showing moderate grain size. The addition of antimony showed a decrease in the grain size

with change in the morphology and crystal orientation. Most of additives restored the

crystallographic orientation of standard deposit and gave medium grain size with smooth

compact deposit which could explain the high current efficiency obtained in presence of

antimony.

V. The polarization studies showed that, the cathodic overpotential at 50 mA cm-2 increased

with increasing the additive concentration, this is explained due to the high adsorption of

additive on the cathode surface which blocks the active sites and lead to higher potential for

Zn2+ reduction. Addition of antimony to the electrolyte decreased the overpotential by ~23-

30 mV due to the hydrogen evolution reaction. Nucleation overpotential (NOP) values

obtained by cyclic voltammetry technique were found to be increased with increasing the

Page 137: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

120

additive concentration. For example, in SE containing Sb3+, [PAM] increased the NOP

from 68 mV to 104 mV by increasing the concentration from 1 to 3 mg dm-3, respectively.

High NOP values indicate that more fine-grained deposits can be obtained with good

crystallographic orientation.

(B) Fe-Mo and Fe-Mo-P coatings as cathode for chlorate production;

Different coatings of Fe-Mo and Fe-Mo-P were successfully electrodeposited from citrate-

based electrolyte at 20 mA cm-2 for 6 hours. The prepared alloys have atomic percentage of

Fe53Mo47, Fe70Mo21P9, Fe61Mo26P13, and Fe54Mo30P16 which analyzed by energy dispersive

spectroscopy.

I. All obtained deposits are found to have amorphous structure due to the presence of

phosphorous and high content of molybdenum. The hydrogen evolution reaction (HER)

overpotential for these cathodes was examined by potentiodynamic polarization in

simulated conditions as chlorate production electrolyte. The overpotential of HER for

Fe54Mo30P16 was decreased by 30%, compared to mild steel, and by 16.5%, compared to

Fe53Mo47.

II. The electocatalytic activities were also studied by electrochemical impedance

spectroscopy in the same electrolyte. It was found that, charge transfer resistance (Rct)

decreased gradually by increasing the P content in the alloy. The lowest value of Rct of

0.167 Ω cm2 was obtained by Fe54Mo30P16, compared to 0.235 Ω cm2 which obtained by

mild steel cathode. Also the roughness factor and the intrinsic electrolcatalytic activities

were found to be increased with increasing the P and Mo contents in the alloys and this

could explain the origin of this promising behavior of the prepared cathodes toward the

HER.

Page 138: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

121

7.2. Outlook

Based on the obtained results in this research study, the following perspectives and

future work could be considered:

Concerning the additives in zinc electrowinning;

- Different salts of ionic liquids could be considered with different anions (I- and F-) and

cations parts (C5H11+, C6H13

+, C7H15+, C8H17

+) in order to examine the effect of each

part individually on zinc electrodeposition.

- Different concentrations of Sb3+ ions (0.01, 0.015, 0.02 mg) could be used to study the

best combinations between the additives and antimony.

- Certain concentrations of Pb2+ ions (0.05, 0.1, 0.15, 0.2 mg) could be added to the

electrolyte to examine the difference between the contamination caused by the lead-

based anode and the soluble lead ions in the electrolyte.

- Ionic exchange membrane could be used to separate the cathodic and anodic

compartments to provide better understanding of the anodic and cathodic reactions also

ions movements towards the electrodes.

- Certain parameters such as current density, temperature, agitation, Mn2+ ion

concentration, Zn2+ ion concentrations and pH could be changed to study the effect of

each parameter individually on zinc electrowinning process.

- Electrochemical impedance spectroscopy (EIS) and electrochemical noise

measurements (ENM) could be conducted in the future work to give better

understanding of the electrochemical activities in presence of additives.

Concerning the deposited cathodes for chlorate industry;

- The corrosion resistance measurements for the prepared alloys could be examined in the

future in the same conditions by using electrochemical noise measurements (ENM) and

scanning reference electrode technique (SRET).

Page 139: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

122

Bibliography

[1] Encyclopædia Britannica Online, s.v. electrolysis, 2014. http://www.britannica.com /science/

electrolysis.

[2] Paunovic, M., and M. Schlesinger. Fundamentals of electrochemical deposition. Edited by

Paunovic and Schlesinger, pp 1, John Wiley & Sons. New Jersey, 2006.

[3] Porter, F. Zinc Handbook: properties, processing, and use in design. Edited by Frank Porter,

pp 1-15, CRC Press, New York, 1991.

[4] Abkhoshk, E., E. Jorjani, M. S. Al-Harahsheh, F. Rashchi, and M. Naazeri. Review of the

hydrometallurgical processing of non-sulfide zinc ores. Hydrometallurgy 149 (2014): 153-

167.

[5] Habashi, F. Handbook of extractive metallurgy. Vol: 2, pp 641-683. Wiley-Vch, Heidelberg,

Germany, 1997.

[6] Morgan, S.W.K. Zinc and Its alloys and compounds. pp 1-13, Wiley, New York, 1985.

[7] Sinclair, R.J. The extractive metallurgy of zinc. Edited by Roderick Sinclair, 1st edition, pp 3-

54, Australasian Institute of Mining and Metallurgy, 2005.

[8] Watt, A. Electro-Deposition a Practical Treatise. pp 395, Holmes Press, London, 2008.

[9] Han, J.S., and T.J. O’Keefe. Electrochemical evaluation of the adherence of zinc to

aluminum cathodes. Surf. Coatings. Tech. 53 (1992): 231-238.

[10] Gonzalez-Dominguez, J.A., and R.W. Lew. Evaluating additives and impurities in zinc

electrowinning. JOM 47 (1995): 34-37.

[11] Zhang, X.G. Corrosion and Electrochemistry of Zinc. Edited by Gregory Xiaoge Zhang, pp

1-32, Springer Science & Business Media, New York, 1996.

[12] Revie, R.W. Uhlig’s Corrosion Handbook. Edited by Robert Winston Revie, 3rd edition, pp

1001-1011, John Wiley & Sons, New Jersey, 2011.

[13] International Zinc Association India, http://www.zinc.org.in/zinc-oxide-applications/

[14] Cornell, A. Electrode reactions in the chlorate process. Doctoral thesis, pp 1-13, Royal

Institute of Technology, Stockholm, 2002.

[15] O'Brien, T. F., T.V. Bommaraju, and F. Hine. Handbook of Chlor-Alkali Technology, Vol. 1

pp 37-74, Springer science, 2005.

[16] Safizadeh, F., E. Ghali, and G. Houlachi. Electrocatalysis developments for hydrogen

evolution reaction in alkaline solutions – A Review. Int. J. Hydrogen Energy 40 (2015): 256-

274.

[17] Viswanathan, K., and B.V. Tilak. Chemical, electrochemical, and technological aspects of

sodium chlorate manufacture, J. Electrochem. Soc. 131 (1984): 1551-1559.

Page 140: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

123

[18] Jin, S., A.V. Nest., E. Ghali, S. Boily, and R. Schulz. New cathode materials for chlorate

electrolysis. J. Electrochem. Soc. 144 (1997): 4272-4279.

[19] Sanches, L.S., C.B. Marino, and L.H. Mascaro. Investigation of the codeposition of Fe and

Mo from sulphate-citrate acid solutions. J. Alloys Comp. 439 (2007): 342-345. [20] Scott, A. C., R. M. Pitblado, and G. W. Barton. A mathematical model of a zinc

electrowinning cell. In Proceedings of the Twentieth International Symposium on the

Application of Computers and Mathematics in the Mineral Industries, Metallurgy, 2 (1987):

51-62. [21] Weiner, F.S., G.T. Wever and R. J. Lapee. Metallurgical extraction, electrolytic zinc process,

the science and technology of the metal, its alloys and compounds. pp 174. New York, 1959. [22] Krauses, C.J., R.C. Kerby, R.D.H. Williams, and D. Ybena. Anodes for electrowinning

proceeding processes. pp 37, Metall. Soc. AIME, 1984. [23] Newnhan, R.H. Corrosion rates of lead based anodes for zinc electrowinning at high current

densities. J. Appl. Electrochem. 22 (1992): 116-124. [24] Stelter, M., H. Bombach, and P. Saltykov. Corrosion behavior of lead-alloy anodes in metal

winning. SISAPMM. 6 (2006): 451-461. [25] Prengaman, R.D., and C.E. Morgan. Electrowinning anodes which rapidly produce a

protective oxide coating. US Patent# 6224723B1, 2001. [26] Pourbaix, M. Atlas of electrochemical equilibria in aqueous solutions. pp 406-420, NACE

International, Houston, TX, 1974. [27] Guo, Y. A new potential-pH diagram for an anodic film on Pb in H2SO4. J. Electrochem. Soc.

139 (1992): 2114-2120. [28] Astakhov, I.I., E.S. Vaisberg, and B.N. Kabanov. Anodic corrosion of PbSO4 in H2SO4.

Doklady Akademii Nauk SSSR, 154 (1964): 1414-1416. [29] Kiryakov, G.Z., and I.A. Korchmarek. Role of lead dioxide film. J. Appl. Electrochem. 26

(1953):1263-1266. [30] Zhang, W. Performance of lead anode used for zinc electrowinning and their effects on

energy consumption and cathode impurities. Doctoral thesis, pp 20, Laval University, QC,

2010. [31] Yu, P. And T.J. O’Keefe. Evaluation of lead anode reactions in acid sulfate electrolytes II.

Manganese reactions. J. Electrochem. Soc. 149 (2002): A558-A569. [32] Ivanov, I., Y. Stefanov, Z. Noncheva, M. Petrova, T.S. Dobrev, L. Mirkova, R. Vermeersch,

and J.P. Demaerel. Insoluble anodes used in hydrometallurgy: Part II. Anodic behavior of

lead and lead-alloy anodes. Hydrometallurgy 57 (2000): 125-139.

Page 141: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

124

[33] Zhang, Q.B. and Y.X. Effect of Mn2+ ions on the electrodeposition of zinc from acidic

sulphate solutions. Hydrometallurgy 99 (2009): 249-254. [34] Weil, R. Structure, Brightness and Corrosion Resistance of Electrodeposits. Plating, 61

(1974): 654-661. [35] Dini, J.W. Deposit Structure. pp 11, 75, Plating & Surface Finishing, 1988. [36] Vlad, C.M. Texture and corrosion resistance of metallic coatings. Edited by H.J. Bunge, pp

199, DGM Informationsgesellschaft GmbH, Oberursel, Germany, 1989. [37] Ault, A.R., and E.J., Frazer. Effect of certain impurities on zinc electrowinning in high-purity

synthetic solutions. J. Appl. Electrochem. 18 (1988): 583-589. [38] Muresan, L., G. Maurin, L. Oniciu, and D. Gaga. Influence of metallic impurities on zinc

electrowinning from sulphate electrolyte. Hydrometallurgy 43 (1996): 345-354. [39] Mackinnon, D.J., J.M. Brannen, and P.L. Fenn. Characterization of impurity effect in zinc

electrowinning from industrial acid sulphate electrolyte. J. Appl. Electrochem. 17 (1987):

1129-1143. [40] Mackinnon, D.J., J.M. Brannen, and R.C. Kerby. The effect of lead on zinc deposit structures

obtained from high purity synthetic and industrial acid sulphate electrolytes. J. Appl.

Electrochem. 9 (1979): 55-70. [41] Lafront, A-M., W, Zhang, E. Ghali, and G. Houlachi. Effect of gelatin and antimony on zinc

electrowinning by electrochemical noise measurements. Can. Metall. Qaurt. 48 (2009): 337-

346. [42] Mackinnon, D. J., R.M. Morrison, J.E. Mouland, and P.E. Warren. The effects of antimony

and glue on zinc electrowinning from Kidd Creek electrolyte. J. Appl. Electrochem. 20

(1990): 728-736. [43] Mackinnon, D.J. The effect of copper on zinc electrowinning from industrial acid sulphate

electrolyte. J. Appl. Electrochem. 15 (1985): 953-960. [44] Zhang, H., Y. Li, J. Wang, and X. Hang. The influence of nickel ions on the long period

electrowinning of zinc sulfate electrolytes. Hydrometallurgy 99 (2009): 127-130. [45] Stefanov, Y., and I. Ivanov. The influence of nickel ions and triethylbenzyl ammonium

chloride on the electrowinning of zinc from sulphate electrolytes containing manganese ions.

Hydrometallurgy 64 (2002): 193-203. [46] Morrison, R.M., D.J. Mackinnon, and D.A. Uceda. The effect of some traces metal impurities

on the electrowinning of zinc from Kidd Creek electrolyte. Hydrometallurgy 29 (1992): 413-

430.

Page 142: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

125

[47] Mackinnon, D.J., R.M. Morrison, and N. Penazzi. The effect of nickel and cobalt and their

interaction with antimony on zinc electrowinning from industrial acid sulphate electrolyte. J.

Appl. Electrochem. 16 (1986): 53-61. [48] Maja, M., and P. Spineli. Detection of Metallic Impurities in Acid Zinc Plating Baths. J.

Electrochem. Soc. 118 (1971): 1538-1540. [49] Maja, M., N. Penazzi, R. Fratesi, and G. Roventi. Zinc electrocrystallization from impurity-

containing sulphate baths. J. Electrochem. Soc. 129 (1982): 2695-2700. [50] Jaksic, M. Impurity Effects on the Macromophology of Electrodeposited Zinc II: Causes,

Appearances and Consequences of Spongy Zinc Growth. Surface Technology 29 (1986):

113-127. [51] Wiart, R., C. Cachet, C. Bozhkov, and S. Rashkov. On the nature of the ‘induction period’

during the electrowinning of zinc from nickel containing sulphate electrolytes. J. Appl.

Electrochem. 20 (1990): 381-389. [52] Tripathy, B.C., S.C. Das, G.T. Hefter, and P. Singh. Zinc electrowinning from acidic sulphate

solutions Part II: Effects of triethylbenzylammonium chloride. J. Appl. Electrochem. 28

(1998): 915-920. [53] Tripathy, B.C., S.C. Das, G.T. Hefter, and P. Singh. Zinc electrowinning from acidic sulphate

solutions Part I: Effects of sodium lauryl sulphate. J. Appl. Electrochem. 27 (1997): 673-678. [54] Robinson, D.J., and T.J. O’Keefe. On the effects of antimony and glue on zinc electro

crystallization behaviour. J. Appl. Electrochem. 6 (1976): 1-7. [55] Zhang, Q.B., and Y.X. Stability of [BMIM]HSO4 for using as additive during zinc

electrowinning from acidic sulfate solution. J. Cent. South. Univ. 19 (2012): 2451-2457. [56] Wu X., Z. Liu, and X. Liu. The effects of additives on the electrowinning of zinc from

sulphate solutions with high fluoride concentration. Hydrometallurgy 141 (2014): 31-35. [57] Dhak, D., E. Asselin, S.D. Carlo, and A. Alfantazi. An investigation on the effects of organic

additives on zinc electrowinning from industrial electrolyte. J. Electrochem. Soc. 28 (2010):

267-280. [58] Alfantazi, A.M., and D.B. Dreisinger. An investigation on the effects of orthophenylene

diamine and sodium lignin sulphonate on zinc electrowinning from industrial electrolyte.

Hydrometallurgy 69 (2003): 99-107. [59] Muresan, L., G. Maurin, L. Oniciu, and S. Avram. Effects of additives on zinc electrowinning

from industrial products. Hydrometallurgy 40 (1996): 335-342. [60] Ohgai, T., H. Fukushima, N. Baba, and T. Akiyama. Effect of polymer on zinc

electrowinning. Shigen-to-sozai, 115 (1999): 700-704.

Page 143: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

126

[61] Jones, R.A. Quaternary ammonium salt. Ch. 4-6, edited by Richard Alain Jones, Academic

press, San Diego, 2001. [62] Tripathy, B.C., S.C. Das, P. Singh, and G.T. Hefter. Zinc electrowinning from acidic sulphate

solutions Part III: Effects of quaternary ammonium bromides. J. Appl. Electrochem. 29

(1999): 1229-1235. [63] Zhang, W., A-M Lafront, E. Ghali, and G. Houlachi. Influence of malonic acid and triethyl-

benzylammonium chloride on Zn electrowinning in zinc electrolyte. Hydrometallurgy 99

(2009): 181-188. [64] Wasserscheid, P., and W. Keim. Ionic liquids new solutions for transition metal catalysis.

Angew. Chem. Int. Ed. 39 (2000): 3772-3789. [65] Johnson, K.E. What’s an ionic liquid. Ch. 1, Electrochem. Soc. Interface, 2007. [66] Singh, G., and A. Kumar. Ionic liquids: physic-chemical, solvent properties and their

applications in chemical processes. Indian J. Chem. 47A (2008): 495-503. [67] Rout, A., Kotlarska, J., W. Dehean, and K. Binnemans. Liquid-liquid extraction of

neodymium (III) by dialkylphosphate ionic liquids from acidic medium: the importance of

the ionic liquid cation. J. Phys. Chem. 15 (2013): 16533-16541. [68] Yue, G., S. Zhang, Y. Zhu, X. Lu, S. Li, and Z. Xi. A promising method for electrodeposition

of aluminum on stainless steel in ionic liquid. AICHE J. 55 (2009): 783-796. [69] Enders, F., A.P. Abbott, and D.R. MacFarlane. Electrodeposition from ionic liquids. PP 1-14,

Wiley-VCH Velag GmbH & Co., Weinheim, 2008. [70] Zhang, Q.B. and Y.X. Effects of 1-butyl-3-methylimidazolium hydrogen sulfate

[BMIM]HSO4 on zinc electrodeposition from acidic sulfate electrolyte. J. Appl. Electrochem.

39 (2009): 261-267. [71] Zhang, Q.B., Hua, Y.X., T. G. Dong, and D.G. Zhou. Effects of temperature and current

density on zinc electrodeposition from acidic sulfate electrolyte containing [BMIM]HSO4 as

additive. J. Appl. Electochem. 39 (2009): 1207-1216. [72] Zhang, Q.B., Hua, Y.X., Y. Li and P. QiFei. Effects of antimony (III) on zinc

electrodeposition from acidic sulfate electrolyte containing [BMIM]HSO4. J. Appl.

Electochem. 39 (2009): 2329-2335. [73] Tilak B.V., and C.P. Chen. Electrolytic Sodium Chlorate Technology: Current Status. In:

Burney HS, Furuya N, Hine F and Ota KI (eds) Chlor-alkali and chlorate technology, The

electrochemical society proceedings series, pp 8, Pennington, 1999. [74] MacMullin R.B., and H.S. Burney. Chlor-alkali and chlorate technology. In: R.B. MacMullin

memorial symposium: proceedings of the symposium, vol. 9921; pp 8, 1999.

Page 144: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

127

[75] Elezovic, N., B.N. Grgur, N.V., Krstajic, and V.D. Jovic. Electrodeposition and

characterization of Fe-Mo alloys as cathodes for hydrogen evolution in the process of

chlorate production. J. Serb. Chem. Soc. 70 (2005): 879-889. [76] Kublanovsky, V., O. Bersirova, Y. Yapontseva, H. Cesiulis, and E. Podlaha-Murphy. Cobalt-

molybdenum-phosphorous alloys: electroplating and corrosion properties. Protect. Met.

Phys. Chem. of Surf. 45 (2009): 588-594. [77] Jaccaud, M., F. Leroux, and J.C. Millet. New chlor-alkali activated cathodes. Mat. Chem.

Phys. 22 (1989): 105-119. [78] Karimi S.R., and A. Lasia. Study of hydrogen evolution reaction on Ni-Mo-P electrodes in

alkaline solutions. J. Electrochem. Soc., 145 (1998): 2219-2225. [79] Shibli, S.M.A., and V.S. Dilimon. Effect of phosphorous content and TiO2-reinforcement on

Ni-P electroless plates for hydrogen evolution reaction. Int. J. Hydrogen Energy 32 (2007):

1694-1700. [80] Rosalbino F., D. Maccio, E. Angelini, A. Saccone, and S. Delfino. Electrocatalytic properties

of Fe-R (R= rare earth metal) crystalline alloys as hydrogen electrodes in alkaline water

electrolysis. J. Alloys Compd. 403 (2005): 275-282. [81] Müller C.I., T. Rauscher, A. Schmidt, T. Schubert, T. Weißgarber, and B. Kieback.

Electrochemical investigations on amorphous Fe-base alloys for alkaline water electrolysis.

Int. J. Hydrogen Energy 39 (2004): 8926-8937. [82] Mia H.J., and D.L. Piron. Composite-coating electrodes for hydrogen evolution reaction.

Electrochim. Acta 38 (1993):1079-1085. [83] Elumalai P., H.N. Vasan, N. Munichandraiah, and S.A. Shivashankar. Kinetics of hydrogen

evolution on submicron size Co, Ni, Pd and Co-Ni alloy powder electrodes by d.c.

polarization and a.c. impedance studies. J. Appl. Electrochem. 32 (2002): 1005-1010. [84] Dominguez-Crespo M.A., M. Plata-Torres, A.M. Torres-Huerta, E.M. Arce-Estrada, and J.K,

Hallen-Lopez JM. Kinetic study of hydrogen evolution reaction on Ni30Mo70, Co30Mo70,

Co30Ni70 and Co10Ni20Mo70 alloy electrodes. Mater. Charact. 55 (2005): 83-91. [85] Rosalbino F., S. Delsante, G. Borzone, and E. Angelini. Correlation of microstructure and

catalytic activity of crystalline Ni-Co-Y alloy electrode for the hydrogen evolution reaction in

alkaline solution. J. Alloys. Compd. 429 (2007): 270-275. [86] Gonzalez-Buch, C., I. Herraiz-Cardona, E.M. Ortega, J. García-Antón, and V. Pérez-Herranz.

Development of Ni-Mo, Ni-W and Ni-Co Macroporous Materials for Hydrogen Evolution

Reaction. Chem. Eng. Trans. 32 (2013): 865-870.

Page 145: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

128

[87] Navarro-Flores E, Z. Chong, S. Omanovic. Characterization of Ni, NiMo, NiW and NiFe

electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium. J. Mol.

Catal. A: Chemical 226 (2005): 179-197. [88] Giz, M.J., M.C. Marengo, E.A. Ticianelli, and E.R. Gonzalez. Electrochemical and physical

characterization of Ni-Cu-Fe alloy for chlor-alkali hydrogen cathodes. Eclet. Quim. 28

(2003): 21-28. [89] Carvalho, J. D., G. Tremiliosi-Filho, L. A. Avaca, E. R. Gonzalez. In S. Srinivasan, S.

Wagner, and H. Wroblowa. Electrode Materials and Processes for Energy Conversion and

Storage. The Electrochem. Soc. V.87-12, pp 356, Pennington, New Jersey, 1987. [90] Podesta J.J., R.C.V. Piatt, A.J. Arvia, P. Ekdunge, K. Jottner, and G. Kreysa. The behavior of

Ni-Co-P base amorphous alloys for water electrolysis in strongly alkaline solutions prepared

through electroless deposition. Int. J. Hydrogen Energy 17 (1992): 9-22. [91] Badway, W.A., H.E. Fekry, N.H. Helal, and H.H. Mohammed. Cathodic hydrogen evolution

on molybdenum in NaOH solutions. Int. J. Hydrogen Energy 38 (2013): 9625-9632. [92] Chassaing E., K.V. Quang, and R. Wiart. Mechanism of nickelmolybdenum alloy

electrodeposition in citrate electrolyte. J. Appl. Electrochem. 19 (1989): 839-844. [93] Podlaha E.J., and D. Landolt. Induced codeposition I. An experimental investigation of Ni-

Mo alloys. J. Electrochem. Soc. 143 (1996): 885-892. [94] Krstajic, N.V., V.D. Jovic, L.J. Gajic, B.M. Jovic, A.L. Antozzi, and G.N. Martelli.

Electrodeposition of Ni–Mo alloy coatings and their characterization as cathodes for

hydrogen evolution in sodium hydroxide solution. Int. J. Hydrogen Energy 33 (2008): 3673-

3687. [95] Armyanov, S., S. Vitkova, and O. Blajiev. Internal stress and magnetic properties of

electrodeposited amorphous Fe-P alloys. J. Appl. Electrochem. 27 (1997): 185-191. [96] Zhang, Y., M. Gong, and D. Xiong. Electrodeposition of amorphous Fe-Mo-P alloy films and

its corrosion resistance. Mat. Protect. 31 (1998): 7-8. [97] Siu, C.L., H.C. Man, and C.H. Yeung. Electrodeposition of Co–Mo–P barrier coatings for

Cu/Au coated systems. Surf. Coat. Tech. 200 (2005): 2223-2227. [98] Sridharan, K., and K. Sheppard. Electrochemical characterization of Fe-Ni-P alloy

electrodeposition. J. Appl. Electrochem. 27 (1997): 1198-1206. [99] Irie, T., K. Kyono, H. Kimura, and S. Kurokawa. Corrosion-resistant steel strip having Zn-

Fe-P alloy electroplated thereon. US Patent application #4.640.872, 1987.

Page 146: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

129

[100] Ordine, A.P., S.L. Díaz, I.C.P., Margarit, and O.R. Mattos. Zn–Ni and Zn–Fe alloy deposits

modified by P incorporation: anticorrosion properties. Electrochim. Acta 49 (2004): 2815-

2823. [101] Shervedani, R.K., and A. Lasia. Study of the Hydrogen Evolution Reaction on Ni-Mo-P

Electrodes in Alkaline Solutions. J. Electrochem. Soc. 145 (1998): 2219-2225. [102] Princeton Applied Research, Application Note E-4. Basics of corrosion measurements. Oak

Bridge, TN.

[103] Thompson, N.G., and J.H. Payer. DC Electrochemical Test Methods. Edited by Barry C.

Syrett, pp 1-54, NACE, Houston, TX, 1998. [104] Beavers, J.A, N.G Thompson, and D.C. Silverman. Corrosion Engineering Applications of

Electrochemical Techniques: Laboratory Testing. Corrosion/93, pp 348-352, NACE, Huston,

TX, 1993. [105] Yaro, A.S. A galvanostatic polarization investigation of steel corrosion in alkaline solutions.

J. Eng. 14 (2008): 2752-2761. [106] Baboian, R. Electrochemical Technique. pp 217-230, NACE, Houston, TX, 1986. [107] Fontana, M.G., and N. D. Greene. Corrosion engineering. Ch. 1, McGraw-Hill, New York,

1967. [108] Stern M., and A. L. Geary. Electrochemical Polarization, I. A. Theoretical Analysis of the

Shape of Polarization Curves. J. Electrochem. Soc. 104 (1957): 33-63. [109] Bhandari H., S. A. Kumar, and S.K. Dhawan, 2012. Conducting polymer nanocomposites for

anticorrosive and antistatic applications. Ch. 13, edited by Farzad Ebrahimi, INTECH Open

Access Publisher, 2012. [110] Princeton Applied Research, Application Note CORR-1. A review of techniques for

electrochemical analysis. Oak Bridge, TN. [111] Kissinger, P.T., and W.R. Heineman. Cyclic voltammetry. J. Chem. Educ. 60 (1983): 702-

706. [112] Park, S.M., and J.S. Yoo. Electrochemical impedance spectroscopy for better electrochemical

measurements. American Chem. Soc. 1 (2003): A455-A461. [113] Lasia, A. Electrochemical Impedance Spectroscopy and Its Applications, Modern aspects of

electrochemistry. Academic/Plenum publishers 32 (1999): 143-248. [114] MacDonald, J.R. 1990. Impedance Spectroscopy: Old Problems and New Developments.

Electrochimica Acta 35 (1990): 1483-1492. [115] Parson, R. Electrical double layer: Recent experimental and theoretical developments. Chem.

Rev. 90 (1990): 813-826.

Page 147: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

130

[116] Ghali, E. Electrochemistry, Corrosion and Protection. Course Notes, Laval University, 2015. [117] Barton, G.W., and A.C. Scott. A validated mathematical model for a zinc electrowinning cell.

J. Appl. Electrochem. 22 (1991): 104-115. [118] Kelly, R.B., J.R. Scually, D.V. Shoesmith, and R.G. Buchheit. Electochemical techniques in

corrosion science and engineering. pp 53, Marcel Dekke Inc., New York, 2003. [119] Shreir, L.L., R.A. Jarman, and G.T. Burstein. Corrosion, vol. 1, Metal/environment reactions.

pp 89, Butterworth Heinemann 4,1994. [120] McMullan, D. Scanning electron microscopy 1928-1965. Scanning 17 (2006): 175-185. [121] Argast, A., and C.F. Tennis. A web resource for the study of alkali feldspars and perthitic

textures using light microscopy, scanning electron microscopy and energy dispersive X-ray

spectroscopy. Journal of Geoscience Education 52 (2004): 213-217. [122] Harp, J.M., B.L. Hanson, D.E. Timm, and G.J. Bunick. Macromolecular crystal annealing:

evaluation of techniques and variables. Acta Crystall. 55 (1999): 1329-1334. [123] Montaser A., and D.W. Golightly. Inductively Coupled Plasmas in Analytical Atomic

Spectrometry. pp 1-24, VCH Publishers, Inc., New York, 1992. [124] Gupta, C.K., and T.K. Mukherjee. Hydrometallurgy in extraction processes. pp 1-26, CRC

Press, Florida, 1990. [125] Alfantazi, A.M., and D.B. Dreisinger. The role of zinc and sulfuric acid concentrations on

zinc electrowinning from industrial sulfate based electrolyte. J. Appl. Electrochem. 31

(2001): 641-646. [126] Sato, R. Crystal growth of electrodeposited zinc. J. Electrochem. Soc. 106 (1959): 206-211. [127] Ivanov, I. Increased current efficiency of zinc electrowinning in the presence of metal

impurities by addition of organic inhibitors. Hydrometallurgy 72 (2004): 73-78. [128] Ivanov, I., and Y. Stefabov. Electroextraction of zinc from sulphate electrolytes containing

antimony ions and hydroxyethylated-butyne-2-diol-1,4: Part 3.The influence of manganese

ions and a divided cell. Hydrometallurgy 64 (2002): 181-186. [129] Cachet, C., and R. Wiart. Influence of a perfluorinated surfactant on the mechanism of zinc

deposition in acidic electrolytes. Electrochim. Acta 44 (1999): 4743-4751. [130] Fabian, C.P., and T.W. Lancaster. Process for copper electrowinning and electrorefining.

U.S. Patent No. US8293093, 2012. [131] Bodsworth, C. The extraction and refining of metals. pp 148-161, CRC Press, New York,

1994.

Page 148: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

131

[132] Cachet, C., and R. Wiart. Zinc deposition and passivated hydrogen evolution in highly acidic

sulphate electrolytes: depassivation by nickel impurities. J. Appl. Electrochem. 22 (1990):

1009-1014. [133] Mackinnon, D.J., and J.M. Brannen. Zinc deposit structures obtained from high purity

synthetic and industrial acid sulphate electrolytes with and without antimony and glue

additions. J. Appl. Electrochem. 7 (1977): 451-459. [134] Kerby, R.C., H.E. Jackson, T.J. O’keefe, and W. Yar-Ming. Evaluation of organic additives

for use in zinc electrowinning. Metallurgical Transactions B. 8 (1977): 661-668. [135] Mackinnon, D.J., R.M. Morrison, J.E. Mouland, and P.E. Warren. The effects of saponin,

antimony and glue on zinc electrowinning from Kidd Creek electrolyte. J. Appl.

Electrochem. 20 (1990): 955-963. [136] Mathieu, D., D.L. Piron, and M. D’Ambroise. A chemical study of 2-butyne-1,4-diol. Talanta

32 (1988): 763-768. [137] Endres, F., A.P. Abbott, and D.R. MacFarlane. Electrodeposition from ionic liquids. pp 1-65,

Wiley-VCH Velag GmbH & Co. Weinheim, 2008. [138] Zhang, Q.B, and Y.X. Hua. Effect of the ionic liquid additive - with [BMIM]HSO4 on the

kinetic of oxygen evolution during zinc electrowinning. Acta Phys. Chim. Sin. 27 (2011):

149-155. [139] Sorour, N., W. Zhang, G. Gabra, E. Ghali, and G. Houlachi. Influence of different organic

additives on zinc electrowinning from acidic sulphate electrolyte. Canadian Institute of

Mining, Metallurgy and Petroleum. CIM-COM, proceeding, pp 1-13, paper #8986, Toronto,

2015. [140] Hunt, A.P., B. Kirchner, and T. Welton. Characterising the electronic structure of ionic

liquids: an examination of the 1-butyl-3-methylimidazolium chloride ion pair. Chemistry -

An Eur. J. 12 (2006): 6762-6775. [141] Winand, R. Electrocrystallization: Fundamental considerations and application to high

current density continuous steel sheet plating. J. Appl. Electrochem. 21 (1990): 337-385. [142] Moussallem, I., J. Jörissen, U. Kunz, S. Pinnow, and T. Turek. Chlor-alkali electrolysis with

oxygen depolarized cathodes: history, present status and future prospects, J. Appl.

Electrochem. 38 (2008): 1177-194. [143] Daly, P.B., and F.J. Barry. Electrochemical nickel-phosphorous alloy formation, International

Materials Reviews, 48 (2003): 326-338. [144] Lasia, A. Hydrogen evolution reaction. Handbook of fuel cells, 2010.

Page 149: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

132

[145] Karimi Shervedani, R., A.H. Alinoori, and A.R. Madram. Electrocatalytic activities of nickel-

phosphorous composite coating reinforced with codeposited graphite carbon for hydrogen

evolution reaction in alkaline solution. J. New Mater. Electrochem. Syst. 11 (2008): 259-265. [146] Shervedani, R. K., and A. Lasia. Kinetics of Hydrogen Evolution Reaction on Nickel-Zinc-

Phosphorous Electrodes. Journal of the Electrochemical Society, 144 (1997): 2652-2657. [147] Shervedani, R. K., and A. Lasia. Studies of the Hydrogen Evolution Reaction on Ni-P

Electrodes. Journal of the Electrochemical Society, 144 (1997): 511-519. [148] Kubisztal, J., A. Budniok, and A. Lasia. Study of the hydrogen evolution reaction on nickel-

based composite coatings containing molybdenum powder. International Journal of

Hydrogen Energy, 32 (2007): 1211-1218. [149] Krstajić, N.V., L.J. Gajić- Krstajić, U. Lačnjevac, R.M. Jović, S. Mora, and V.D. Jović. Non-

noble metal composite cathodes for hydrogen evolution. Part I: The Ni–MoOx coatings

electrodeposited from Watt’s type bath containing MoO3 powder particles. Int. J. Hydrogen

Energy 36 (2011): 6441-6449. [150] Elezović, N.R., V.D. Jovic, and N.V. Krstajic. Kinetics of the hydrogen evolution reaction on

Fe-Mo film deposited on mild steel support in alkaline solution. Electrochim. Acta 50 (2005):

5594-5601. [151] Jakšić, J.M., M.V. Vojnović, and N.V. Krstajic. Kinetic analysis of hydrogen evolution at Ni-

Mo alloy electrodes. Electrochim. Acta 45 (2000): 4151-4158. [152] Trasatti, S., H. Gerischer, and C.W. Tobias. Advances in electrochemical science and

engineering, vol. 2. pp 1-85, VCH New York, 1992. [153] Ratzker, M., D.S. Lashmore, and K.W. Pratt. Electrodeposition and corrosion performance of

nickel-phosphorus amorphous alloys. Plat. Surf. Finish. 73 (1986): 74-82. [154] Hassan, H.B., and Z. Abdel Hamid. The electrocatalytic behaviour of electrodeposited Ni-

Mo-P alloy films towards ethanol electrooxidation. Surf. Interface Anal. 45 (2013): 1135-

1143. [155] Kucernak, A. R., and V.N.N. Nickel phosphide: the effect of phosphorus content on

hydrogen evolution activity and corrosion resistance in acidic medium. Journal of Materials

Chemistry A, 2 (2014): 17435-17445. [156] Frey, A. A., N.R. Wozniak, T.B. Nagi, M.P. Keller, J.M. Lunderberg, G.F. Peaslee, P.A.

DeYoung, P.A. and J.R. Hampton. Analysis of Electrodeposited Nickel-Iron Alloy Film

Composition Using Particle-Induced X-Ray Emission. International Journal of

Electrochemistry, Sep 18, 2011. [157] Krstajic, N., V. Jovic, and G.N. Martelli. U.S. Patent Application No. 12/128,815, 2008.

Page 150: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

133

[158] Brenner, A. Electrodeposition of alloys. Principles and practices, vol. 2. pp 457-482. New

York: Academic Press, 1963. [159] Sanches, L. S., S.H. Domingues, A. Carubelli, and L.H. Mascaro. Electrodeposition of Ni-

Mo and Fe-Mo alloys from sulfate-citrate acid solutions. Journal of the Brazilian Chemical

Society, 14 (2003): 556-563. [160] Donten, M., H. Cesiulis, and Z. Stojek, Z. Electrodeposition of amorphous/nanocrystalline

and polycrystalline Ni–Mo alloys from pyrophosphate baths. Electrochimica Acta, 50 (2005):

1405-1412. [161] Melo, R.L., P.N. Casciano, A.N. Correia, and P.D. Lima-Neto. Characterisation of

electrodeposited and heat-treated Ni−Mo−P coatings. Journal of the Brazilian Chemical

Society, 23 (2012): 328-334. [162] Lima-Neto, P. D., A.N. Correia, G.L. Vaz, and P.N. Casciano. Morphological, structural,

microhardness and corrosion characterisations of electrodeposited Ni-Mo and Cr coatings.

Journal of the Brazilian Chemical Society, 21 (2010): 1968-1976. [163] Chassaing, E., N. Portail, A.F. Levy, G. Wang. Characterisation of electrodeposited

nanocrystalline Ni–Mo alloys. Journal of applied electrochemistry, 34 (2004): 1085-1091. [164] Stepanova, L. I., O.G. Purovskaya, V.N. Azarko, and V.V. Sviridov. Peculiarities of Ni-Mo

alloys electrodeposition from citrate electrolytes. pp 38-43, VESTSI-AKADEMIIA NAVUK

BELARUSI SERYIA KHIMICHNYKH NAVUK, 1997. [165] Stepanova, L.I., O.G. Purovskaya, and V.V. Sviridov. Influence of Ammonium Ions on

Chemical and Phase Composition of Ni-Mo Alloy Films Electrodeposited from Citrate

Electrolytes. RUSSIAN JOURNAL OF APPLIED CHEMISTRY C/C OF ZHURNAL

PRIKLADNOI KHIMII, 73 (2000): 66-70. [166] Lasia, A., and A. Rami. Kinetics of hydrogen evolution on nickel electrodes. Journal of

electroanalytical chemistry and interfacial electrochemistry, 294 (1990): 123-141. [167] Zheng, Z., N. Li, C.Q. Wang, D. Li, Y. Zhu, and G. Wu. Ni–CeO 2 composite cathode

material for hydrogen evolution reaction in alkaline electrolyte. Int. J. Hydrogen Energy, 37

(2012): 13921-13932. [168] Rami, A., and A. Lasia. Kinetics of hydrogen evolution on Ni-Al alloy electrodes. J. Appl.

Electrochem., 22 (1992): 376-382. [169] Solmaz, R., and G. Kardaş. Fabrication and characterization of NiCoZn–M (M: Ag, Pd and

Pt) electrocatalysts as cathode materials for electrochemical hydrogen production. Int. J.

Hydrogen Energy, 36 (2010): 12079-12087.

Page 151: Organic addittives in zinc electrowinning and ... · Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as Cathodes for Chlorate Production Mémoire

134

[170] Navarro-Flores, E., Z. Chong, and S. Omanovic. Characterization of Ni, NiMo, NiW and

NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium. J.

Molecular Catalysis A: Chemical, 226 (2005): 179-197. [171] Kellenberger, A., N. Vaszilcsin, W. Brandl, and N. Duteanu. Kinetics of hydrogen evolution

reaction on skeleton nickel and nickel–titanium electrodes obtained by thermal arc spraying

technique. Int. J. Hydrogen Energy, 32 (2007): 3258-3265. [172] Okido, M., J.K. Depo, and G.A. Capuano The Mechanism of Hydrogen Evolution Reaction

on a Modified Raney Nickel Composite-Coated Electrode by AC Impedance. J. the

Electrochem. Society, 140 (1993): 127-133. [173] Choquette, Y., L. Brossard, A. Lasia, and H. Menard. Investigation of hydrogen evolution on

Raney-Nickel composite-coated electrodes. Electrochim. Acta, 35 (1990): 1251-1256. [174] Kerner, Z., and T. Pajkossy. On the origin of capacitance dispersion of rough electrodes.

Electrochim. Acta, 46 (2000): 207-211. [175] Azizi, O., M. Jafarian, F. Gobal, H. Heli, and M. Mahjani. The investigation of the kinetics

and mechanism of hydrogen evolution reaction on tin. Int. J. Hydrogen Energy, 32 (2007):

1755-1761. [176] Jafarian, M., O. Azizi, F. Gobal, and M. Mahjani. Kinetics and electrocatalytic behavior of

nanocrystalline CoNiFe alloy in hydrogen evolution reaction. Int. J. Hydrogen Energy, 32

(2007): 1686-1693. [177] Shervedani, R.K., and A.R. Madram. Kinetics of hydrogen evolution reaction on

nanocrystalline electrodeposited Ni62Fe35C3 cathode in alkaline solution by electrochemical

impedance spectroscopy. Electrochim. Acta, 53 (2007): 426-433. [178] Wang, M., Z. Wang, Z. Guo, and Z. Li. The enhanced electrocatalytic activity and stability of

NiW films electrodeposited under super gravity field for hydrogen evolution reaction.

International Journal of Hydrogen Energy, 36 (2011): 3305-3312. [179] Chen, L., and A. Lasia. Study of the Kinetics of Hydrogen Evolution Reaction on Nickel-

Zinc Powder Electrodes. J. Electrochem. Soc., 139 (1992): 3214-3219.