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THIOCYANATE LEACHING OF GOLD
by
Roselyn Sarpomah Yeboah
B.Sc., University of Mines and Technology-Tarkwa Ghana, 2015
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES
(Materials Engineering)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
September 2019
© Roselyn Sarpomah Yeboah, 2019
ii
The following individuals certify that they have read, and recommend to the Faculty of Graduate
and Postdoctoral Studies for acceptance, the thesis entitled:
Thiocyanate Leaching of Gold
Examining Committee:
Dr. David Dreisinger, Materials Engineering
Supervisor
Dr. David Dixon, Materials Engineering
Supervisory Committee Member
Dr Liu Wenying, Materials Engineering
Supervisory Committee Member
submitted by Roselyn Sarpomah Yeboah in partial fulfillment of the requirements for
the degree of Master of Applied Science
in Materials Engineering
iii
Abstract
This study focused on evaluating sodium thiocyanate as an alternative reagent to the conventional
cyanidation process for leaching of gold ores. Goldcorp’s Coffee project in Yukon-Canada
supplied three mineral samples namely, Supremo Oxide 68151, Supremo oxide 68151, Supremo
Composite 72142 for this study.
As a baseline for comparison with thiocyanate extraction results, cyanidation tests performed on
all the three samples showed that the samples are amenable to the conventional cyanidation
leaching, yielding gold extractions as high as 97% for Supremo composite 72142.
A series of leaching tests were performed on the 72142 B sample with SCN- solutions to determine
the feasible regions of gold dissolution and to maximize gold dissolution. The leaching tests were
conducted in the acidic regime (pH 1.5 -2) for these samples.
Notable results with SCN-, ferric sulphate and potassium iodide variation showed gold extractions
of 91 % in solutions containing 0.15 M thiocyanate; 92% with 0.15 M SCN- and 0.10 M Fe(III);
94 % with 0.10 M SCN- and 0.05 M KI; 94 % with 0.10 M SCN-, 0.05 M Fe(III) and 0.02 M KI
and 95 % with 0.15 M SCN- and 10 g/L H2O2.
The kinetic leach data were well fitted by the CIP/CIL leach model developed by Nicol et al, giving
estimates of the leach rate parameter of 72.1 hr-1 and leach tails grade after infinite leach time of
0.17 g/t, confirming fast leaching of the 72142 B sample.
The tests ended with gold adsorption from thiocyanate solution onto carbon. The average gold
loading onto carbon was 2100 g/t and 48 g/t for carbon concentrations of 0.25 g/L and 20 g/L,
respectively. Results obtained were excellent with greater than 98 % gold adsorbed in less than 0.5
hr when carbon concentration was above 5 g/L.
The results show that the gold ore from the Goldcorp’s Coffee project in Yukon-Canada is
amenable to extraction with acidified thiocyanate based lixiviant and subsequent adsorption of the
gold-thiocyanate complex onto activated carbon, giving gold extraction results that are comparable
iv
to cyanide-based gold extraction. The thiocyanate system is therefore a competitive and
alternative leaching reagent to conventional cyanidation.
v
Lay Summary
Sodium cyanide is a reagent used by the gold mining industry to dissolve gold into solution. It is
inexpensive and highly efficient. However, its toxicity has raised environmental concerns, leading
to strict regulatory scrutiny. Consequently, alternative reagents are being sought. Thiocyanate is
one the promising reagents that can replace cyanide because of its high efficiency, low toxicity
and fast leaching kinetics.
This work examined the possibility of using sodium thiocyanate as an alternative reagent to
dissolve gold into solution. Conditions that improved gold extraction such as reagent dosage,
increased addition of potassium iodide as a catalyst, use of different oxidants besides the
conventional iron salt oxidant, were tested to determine the amenability of gold dissolution in
thiocyanate solution. These tests were performed on oxidized mineral samples taken from the
Goldcorp Coffee Project.
Results showed that thiocyanate is a viable reagent and can dissolve gold in the acidic regime.
vi
Preface
This thesis is original, unpublished, and independent work by the author, Roselyn Sarpomah
Yeboah.
This thesis originated from the consultation of Dr. Marcus Tomlinson of Newmont Goldcorp with
the help of my thesis advisor Prof. David Dreisinger. The total supervision, guidance and editing
was done by Professor David Dreisinger.
All the experimental work reported were conducted by the author at the Materials Engineering
Laboratory, University of British Columbia (Vancouver, B.C.) with the help of Dr Be` Wassink.
Chemical analyses of samples were conducted by either SGS Canada, or the Department of Earth
and Ocean Sciences (EOS) University of British Columbia (Vancouver Campus).
This work was sponsored by Mitacs through the Mitacs-Accelerate Program and by Goldcorp
Incorporated now Newmont Goldcorp Incorporated, which also provided the mineral ore samples
for this project.
vii
Table of Contents
Abstract .......................................................................................................................................... iii
Lay Summary .................................................................................................................................. v
Preface............................................................................................................................................ vi
Table of Contents .......................................................................................................................... vii
List of Tables ............................................................................................................................... xiii
List of Figures ............................................................................................................................... xv
List of Symbols and Abbreviations............................................................................................ xviii
Acknowledgements ...................................................................................................................... xix
Dedication ..................................................................................................................................... xx
Chapter 1: Introduction ................................................................................................................... 1
1.1 Background and Thesis Objective ...................................................................................... 1
Chapter 2: Background and Literature Review ........................................................................ 3
2.1 Gold Production and Research ............................................................................................ 3
2.1.1 Gold in History ........................................................................................................... 3
2.2 Gold Mineralogy ................................................................................................................. 3
2.2.1 Classification of Gold Ores ......................................................................................... 3
2.3 Gold Cyanidation ................................................................................................................ 5
2.4 Why the Need for an Alternative Reagent for Gold Leaching? .......................................... 5
viii
2.5 Gold Leaching using Alternative Reagents ........................................................................ 6
2.5.1 Thiosulphate ................................................................................................................ 8
2.5.2 Thiourea ...................................................................................................................... 8
2.5.3 The Halide (Chlorine, Bromine & Iodine) .................................................................. 9
2.6 The Challenges to Developing Alternatives for Cyanide ................................................. 11
2.7 Thiocyanate Leaching of Gold (Chemistry and Thermodynamics) ................................. 11
2.7.1 Chemical Properties of Thiocyanate ......................................................................... 11
2.7.2 Gold Extraction with Thiocyanate ............................................................................ 12
2.7.3 Stability of Thiocyanate ............................................................................................ 13
2.7.4 Leaching of Gold in Thiocyanate Solutions ............................................................. 16
2.8 The Use of Different Oxidizing Agent ............................................................................. 21
2.8.1 Hydrogen Peroxide as an Alternative Oxidant ......................................................... 22
2.8.2 Potassium Iodide as an Additive to Thiocyanate Leaching ...................................... 23
2.9 Toxicity and Environmental Concerns with Thiocyanate ................................................ 26
2.10 Recovery of Gold from Thiocyanate Solutions ............................................................ 27
Chapter 3: Experimental Design and Methodology...................................................................... 29
3.1 Experimental Design ......................................................................................................... 29
3.1.1 Goldcorp Coffee Sample........................................................................................... 29
3.1.2 Sample Preparation ................................................................................................... 29
ix
3.1.3 Dry Grinding ............................................................................................................. 30
3.1.4 Sampling for Testwork and Analyses ....................................................................... 31
3.1.5 Solid SG Determination ............................................................................................ 31
3.2 Mineralogical and Chemical analyses – Head grade, XRD and ICP Analysis ................. 32
3.2.1 Head Grade Analysis ................................................................................................ 32
3.2.2 Mineralogical Analysis ............................................................................................. 33
3.2.3 ICP Analysis ............................................................................................................. 34
3.3 Experimental Setup ........................................................................................................... 34
3.4 Analysis of Results and Analytical Methods .................................................................... 35
3.5 Reagents used in Gold Leaching Tests ............................................................................. 36
Chapter 4: Results and Discussion ................................................................................................ 37
4.1 Introduction ....................................................................................................................... 37
4.2 Cyanide Leaching ............................................................................................................. 37
4.2.1 Cyanidation Test ....................................................................................................... 38
4.3 Thiocyanate and Ferric Sulphate Variation ...................................................................... 40
4.3.1 Effect of Thiocyanate Concentration on Gold Extraction ........................................ 41
4.3.2 Effect of the Concentration of Fe(III) on Gold Extraction ....................................... 44
4.3.3 Effect of Fe(III) and SCN Concentration on Thiocyanate Consumption ................. 46
4.4 Effect of the Concentration of Low and No Fe(III) Iron Addition on Gold Extraction ... 48
x
4.4.1 Effect of Low Iron Concentration ............................................................................. 49
4.4.2 Effect of No Fe(III) Iron Addition on Gold Dissolution........................................... 50
4.5 The pH Variation Test ...................................................................................................... 56
4.5.1 Effect of pH on Gold dissolution .............................................................................. 58
4.5.2 Effect of pH on the Leaching Potential ..................................................................... 60
4.6 Potassium Iodide Leaching Test ....................................................................................... 62
4.6.1 Gold Leaching with Potassium Iodide ...................................................................... 63
4.6.2 Gold Leaching by Thiocyanate with Addition of Iodide .......................................... 64
4.6.3 Gold Leaching by Iron(III)-Thiocyanate with Addition of Potassium Iodide .......... 66
4.6.4 Reagent Consumption ............................................................................................... 70
4.7 Hydrogen Peroxide Test ................................................................................................... 72
4.7.1 Gold Leaching with SCN Only ................................................................................. 73
4.7.2 Effect of Hydrogen Peroxide concentration on Gold Dissolution ............................ 73
4.7.3 Effect of Peroxide Concentration on Leaching Potential ......................................... 74
4.7.4 Effect of Hydrogen Peroxide Concentration on NaSCN Consumption.................... 75
4.8 Kinetic Leach Test ............................................................................................................ 77
4.8.1 Effect of Leaching Time on Gold Dissolution .......................................................... 77
4.8.2 Effect of Leaching Time on Thiocyanate Consumption ........................................... 80
4.9 Mixture of Reagents .......................................................................................................... 81
xi
4.9.1 Effect of Oxygen, Lead Nitrate and Hydrogen Peroxide on the Gold Extraction .... 83
4.10 Adsorption Test using Activated Carbon ...................................................................... 85
4.10.1 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 1 86
4.10.2 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 2 87
4.10.3 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 3 88
4.10.4 Comparison of the Three Conditions .................................................................... 90
Chapter 5: Summary, Conclusions and Recommendations Future Work ................................... 93
5.1 Summary and Conclusions ............................................................................................... 93
5.2 Recommendations for Future Work.................................................................................. 95
References ..................................................................................................................................... 97
Appendices .................................................................................................................................. 100
Appendix A: Analytical Methods ......................................................................................... 100
A1: Preparation of Au-Standards for AAS .................................................................... 100
A2: Free Cyanide Titration Procedure ........................................................................... 102
A3: Residual Thiocyanate Titration Procedure.............................................................. 103
A4: Solids Specific Gravity ........................................................................................... 104
Appendix B: Mineralogical Analysis Results ...................................................................... 105
B1: XRD Imaging Results for Untreated Samples ...................................................... 105
B2: Results of ICP Analysis .......................................................................................... 108
xii
Appendix C: Gold Extraction Calculations .......................................................................... 111
C1: Gold extraction........................................................................................................ 111
C2: Gold extraction from thiocyanate solutions ............................................................ 113
Appendix D: Iron Leaching .................................................................................................. 119
D1: Effect of pH on iron concentration ......................................................................... 119
D2: Effect of pH on Oxidation Potential ....................................................................... 119
Appendix E: Leaching Model .............................................................................................. 120
Appendix F: Grind Characterisation .................................................................................... 121
F1: Sieve analysis Results ............................................................................................ 121
xiii
List of Tables
Table 1 – Classification of gold ores [7] ......................................................................................... 4
Table 2 - Typical leaching conditions used in leaching gold with halides (Aylmore 2005) ........ 10
Table 3: Stability constants of thiocyanate complexes with iron and gold at 25oC [32] ............. 21
Table 4- Standard Potentials for Half-Reactions of Iodide and Thiocyanate Redox Couples in
Aqueous Solution at 25°C. .................................................................................................... 26
Table 5- Sample Identification ...................................................................................................... 29
Table 6 – Solid SG result .............................................................................................................. 32
Table 7 – Head grade analysis results ........................................................................................... 32
Table 8 – Results of XRD analysis for the three samples ............................................................. 33
Table 9 – Chemical analysis of three Coffee sample .................................................................... 34
Table 10 : Chemical Reagents Used for Leaching Tests .............................................................. 36
Table 11- Leaching conditions for cyanidation tests .................................................................... 37
Table 12 – Baseline cyanidation test results with their reagent consumptions ............................. 39
Table 13 – Leaching condition for the SCN and Fe(III) optimization ......................................... 40
Table 14 - Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant
Fe(III) concentration ............................................................................................................. 47
Table 15- Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at constant
SCN concentration ................................................................................................................ 47
Table 16 – Test condition for Low and No iron addition ............................................................. 48
xiv
Table 17 Comparison between the three conditions with 0.1 M SCN.......................................... 54
Table 18 - Thiocyanate Consumption for the No iron addition varied SCN concentration ......... 55
Table 19 – Leaching Condition for pH Variation Test ................................................................. 57
Table 20 - Leaching Conditions for potassium iodide tests .......................................................... 63
Table 21 – Results of the gold leaching with KI only .................................................................. 63
Table 25 – Leaching test result for thiocyanate leaching only at 0.1 M NaSCN ......................... 73
Table 26 – Leaching condition for kinetics test ............................................................................ 77
Table 27 – Leaching condition for mixture of reagents ................................................................ 81
Table 28 – Results of gold leaching experiments conducted under different leaching conditions82
Table 29 Initial composition for the three solution samples ......................................................... 86
xv
List of Figures
Figure 1 - Eh-pH diagram showing typical operating regions for alternative gold lixiviants [2] .. 7
Figure 2 - Eh–pH diagram for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C.
Short dashed lines show water stability limits [27]. ............................................................. 13
Figure 3 - Eh–pH diagrams for the SCN–H2O system at SCN concentrations of 1.0 M at 25 °C.
Short dashed lines show water stability limits ...................................................................... 15
Figure 4 - Eh–pH diagrams for the Au–H2O system at Au concentration of 0.0001 M at 25 °C.
Short dashed lines show water stability limits. ..................................................................... 17
Figure 5 - Eh–pH diagrams for the AuSCN–H2O system at SCN concentrations of 1.0 M, and Au
concentration of 0.00001 M at 25 °C. ................................................................................... 18
Figure 6 - Species distribution diagram for the Fe3+- SCN-H2O system at 25oC, pH 2 and Fe3+
activity of 10-1 [31] ................................................................................................................ 20
Figure 7 - Product of crushed sample ........................................................................................... 29
Figure 8 - Grinding mills used for the grinding process at UBC Mining Department ................. 30
Figure 9 – Particle size distribution of the 72142 B gold ore samples ........................................ 31
Figure 10 – A pictorial view of the experimental set-up .............................................................. 35
Figure 11 – Cyanidation of the three samples .............................................................................. 38
Figure 12 - Effect of the NaSCN concentration on the gold extraction, [Fe(III)] 0.10 M ............ 41
Figure 13 - Effect of the NaSCN concentration on the oxidation potential, [Fe(III)] 0.10 M ..... 43
Figure 14: Leach solution with ferric addition ............................................................................. 43
Figure 15 - Effect of the Fe(III) concentration on the gold extraction, [SCN] 0.1 M .................. 45
xvi
Figure 16 - Effect of the Fe(III) concentration on the oxidation potential, [SCN] 0.1 M. ........... 46
Figure 17 - Effect of thiocyanate concentration on gold extraction [Fe(III)] 0.01 M .................. 49
Figure 18 - Effect of Fe(III) (0.01 M and 0.1 M) concentration on gold SCN 0.1 M .................. 50
Figure 19 - Effect of the thiocyanate concentration on gold (No Iron addition) ......................... 51
Figure 20: Leach solution with and without ferric ........................................................................ 52
Figure 21 - Effect of the thiocyanate concentration on leaching potential ................................... 53
Figure 22– Effect of pH on Iron concentration at (a) 10 mins (b) 60 mins (c) 120 mins and (d) 180
mins ....................................................................................................................................... 58
Figure 23 - Effect of pH and the SCN concentration (0.025 M and 0.2 M) on gold extraction ... 59
Figure 24 - Effect of pH on the leaching oxidation potential @ SCN 0.025 M ........................... 60
Figure 25 - Effect of pH on leaching oxidation potential (ORP) @ SCN 0.2 M .......................... 61
Figure 26 - Effect of KI concentration on gold extraction, SCN 0.15 M ..................................... 64
Figure 27 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M ................... 65
Figure 28 - Effect of the KI concentration on gold extraction, SCN 0.15 M ............................... 66
Figure 29 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M ................... 67
Figure 30 - Effect of the KI concentration SCN concentration: 0.15 ........................................... 69
Figure 31 - Effect of the H2O2 concentration on gold extraction, SCN 0.15 M ........................... 73
Figure 32 - Effect of the H2O2 concentration on the potential readings, SCN concentration: 0.10
M ........................................................................................................................................... 75
Figure 33 - Effect of the H2O2 concentration on thiocyanate consumption, SCN 0.10 M ........... 76
xvii
Figure 34 - Kinetics of Gold dissolution in thiocyanate solution, (SCN 0.15 M and Fe(III) 0.1 M)
............................................................................................................................................... 78
Figure 35 – Kinetic leaching model fitting ................................................................................... 79
Figure 36 - Effect of leaching time on thiocyanate consumption ................................................. 80
Figure 37 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold extraction ............ 83
Figure 38 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold dissolution .......... 84
Figure 39 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution.
SCN 0.1 M, Fe (III) 0.05 M, pH 1.98, Temp 25 oC .............................................................. 87
Figure 40 – Adsorption of gold with variation in activated carbon in acidic thiocyanate solution.
SCN 0.1 M, Fe (III) 0.05 M, KI 0.02 M, pH 2, Temp 25 oC ................................................ 88
Figure 41 - Adsorption of gold with variation in activated carbon in acidic thiocyanate solution.
SCN 0.2 M, pH 2.06, Temp 25 oC ........................................................................................ 89
Figure 42 – Comparison of the carbon adsorption between the 3 conditions study. (a) runs at 0.25
g/L carbon at the 3 conditions, (b) runs at 0.5 g/L carbon at the 3 conditions, (c) runs at 5 g/L
carbon at the 3 conditions, (d) runs at 10 g/L carbon at the 3 conditions, (e) runs at 20 g/L
carbon at the 3 conditions ..................................................................................................... 91
xviii
List of Symbols and Abbreviations
AAS Atomic Adsorption Spectroscopy
DI De-Ionized water
DO Dissolved Oxygen in solution|
Eh Electrochemical potential, in Volts (V)
E° Standard electrochemical potential at 25°C, in Volts (V)
ΔGf°298K Standard Gibbs Free Energy (of formation at 298K or 25°C), in kJ/mol
Hr Hour
ICP-AES/OES LECO - Combustion analysis of total sulphur content using LECO® instruments
ICP-MS - Inductively coupled plasma mass spectrometry
LPM Litres Per Minute
Mins Minute denoting time
M Molarity (mol/L)
ORP Oxidation/Reduction Potential, in Volts (V)
pH Measure of acidity/alkalinity in solution
ppm Parts Per Million, also equivalent to grams per tonne
PLS Pregnant Leach Solution, the aqueous leachate solution at the end of leaching
P80 the diameter of screen hole that allows 80% of ore particles to pass through
RPM Rotations Per Minute
XRD X-Ray Diffraction analysis
xix
Acknowledgements
I would like to express my deep and sincere gratitude to the following people without whom this
project could not have been accomplished.
My uttermost appreciation goes to the Almighty God for His steadfast love and mercies which
have taken me through all the storms of academic life. My mouth will continually be filled with
your praise, declaring your splendor all day long.
I would like to express my most heartfelt appreciation to my supervisor, Professor David
Dreisinger for granting me the opportunity to be part of the hydrometallurgy team (UBC) and not
only that being a father; providing invaluable guidance throughout my study.
I am highly indebted to Dr Kodjo Afewu for the day to day guidance, encouragement and support
which helped me to successfully complete this program but also his mentoring in my life that went
far beyond helping me in my research work. I say Ayekoo and God bless you for all that you do.
Thank you to Dr, Marcus Tomlinson and Goldcorp Incorporated, for funding this project,
providing the samples for my analysis and most especially the technical advice and feedback given
at the start and during my research. My sincere thanks also go to the Mitacs Accelerate program
and their representative, Dr. Sherry Zhao, for the financial contribution for this work.
To Dr. Wassink Berend, thank you for your close supervision, guidance, technical support in the
set-up and analysis of the experiment which enabled me to successfully complete this research on
schedule. God bless you.
I would also like to extend my appreciation to the Materials Engineering group at University of
British Columbia (UBC) for the love. I also thank all the professors for the knowledge they
imparted to me throughout the period I spent on the program.
To my friends and colleagues, Chih Wei, Clara Asamoah, Patrick Aboagye, Prince Adu and
Richard Osae, I appreciate you.
xx
Dedication
I dedicate this work to God, who is the pillar of my life, without Him I am nothing. Thank you for
your grace that has kept me.
Though the fig tree does not bud and there are no grapes on the vines, though the olive crop
fails and the fields produce no food, though there are no sheep in the pen and no cattle in the stalls,
yet I will rejoice in the Lord, I will be joyful in God my Savior. The Sovereign Lord is my strength;
he makes my feet like the feet of a deer; He enables me to tread on higher heights.
Habakkuk 3:17-19
I also dedicate it to my family, Benjamin Asante and Dr Kodjo Afewu. God bless you for being
there for me. I love you all.
1
Chapter 1: Introduction
1.1 Background and Thesis Objective
The most common leaching process for gold dissolution involves cyanidation, a process that has
been practiced for over a century and still remains dominant. Cyanide has been used successfully
in the processing of gold and silver ores, forming very stable cyano complexes in solution. It has
been the leach reagent of choice because of its high efficiency and relatively low cost. As a result,
over 85% of all gold extracted worldwide produced by hydrometallurgical extraction use cyanide.
In Canada, more than 90% of the gold mines use cyanide [1].
In recent years, one area of research activity for gold recovery has focused on alternative processes
involving non-cyanide lixiviants. Non-cyanide lixiviants for gold extraction have been considered
because of safety, environmental and permitting issues associated with the use of cyanide. Also,
while cyanide is very effective in leaching free milling ores, there are certain classes of ores that
are termed refractory or difficult-to–treat ores, that are mostly not amenable to the conventional
cyanidation process [2]. The search for an alternative, apart from the reasons mentioned above, is
also geared towards finding a highly economic reagent to replace cyanide since the processing
cost, (including CN destruction, waste water treatment, etc.) of cyanidation is increasingly
becoming expensive [3]. These reasons have fuelled the consideration of an alternative to cyanide
with effort devoted to finding an alternative that might compete with the conventional cyanidation.
Thiocyanate (SCN-) is amongst the many alternatives that has received much attention over the
years and stands as a competitor to replace cyanide. Goldcorp Incorporated (now known as
Newmont Goldcorp), has many mines around the Americas and a general interest in non-cyanide
lixiviants for gold. One deposit that is in the development pipeline is the Coffee deposit, a
potential high-grade open pit heap leach gold mine, located approximately 130 km of Dawson
City, Canada. This deposit is still in the exploration and development stage and it is an ideal
opportunity to examine new, improved and safer methods to process the ore. In this thesis, a series
of fundamental leaching tests with ferric thiocyanate solution were conducted on selected mineral
samples from the Coffee project. The primary objective was to ascertain the amenability of
extracting gold in thiocyanate solutions. This will help determine: a) the feasible conditions of
2
leaching gold in thiocyanate solutions b) the methods of improving the leach performance with
addition of leach enhancers/additives (such as potassium iodide) to increase the dissolution of gold
in thiocyanate systems c) the effect of alternative oxidants to ferric sulphate on the gold dissolution
rate. Finally, the adsorption of gold from thiocyanate solutions on carbon has also been studied.
This contributes to the development of an overall competitive process in the gold mining industry.
The thesis is divided into 5 chapters. Chapter 2 covers the relevant literature background
information about thiocyanate leaching; Chapter 3 discusses ore characterization and sample
preparation including the process of ore particle size reduction and methodology for reactor
leaching and carbon adsorption; Chapter 4 focuses on the results and discussions of the leaching
and adsorption tests conducted; Chapter 5 provides conclusions and recommendations for future
work.
3
Chapter 2: Background and Literature Review
2.1 Gold Production and Research
2.1.1 Gold in History
Gold has been known since pre-historic times and is one of the first noble metals to be recovered
because of its high dissemination in the earth crust [4]. It appeared as nuggets in streams and
flowing waters and was identified by virtue of its bright yellow color. Humans have valued gold
for its physical and chemical properties (lustrous color, ductility and durability) which make it
useful to the electronic, technological and jewellery industries. It is also used in design of religious
artefacts and is a form of wealth for individuals and countries serving as revenue when mined and
sold in the international market [5].
The high demand requires more gold to be sought and mined. Primitive methods (including gravity
separation and amalgamation) have been employed to recover and process gold. However, these
processes are only suitable for placer or free gold and are unsuitable for fine gold or gold associated
with sulphides. Many lixiviants were tested before the invention of the cyanidation process; for
example chlorination was used in the year 1887 and 1889 before cyanidation was patented which
really revolutionized the extractive metallurgy of gold and increased production [6].
2.2 Gold Mineralogy
2.2.1 Classification of Gold Ores
Generally gold ores can be classified as “free milling”, “complex” or “refractory”. Free milling
ores are mostly oxide and low sulphide containing gold ores. They are easy to treat and give good
gold recoveries > 90 % by direct cyanidation. Complex gold ores are those that contain cyanide
and oxygen consuming materials like tellurides, cyanicides, etc. These ores are mildly refractory.
Refractory gold ores are those that do not respond to simple cyanidation process and give gold
recoveries of less than 80 %, in some cases much less than 50 % [7]. The degree of refractoriness
is expressed in Table 1.
4
Table 1 – Classification of gold ores [7]
90%-100% recovery Free milling
80%-90% recovery Mildly refractory
80%-50% recovery Moderately refractory
< 50 % recovery Highly refractory
Generally, gold bearing materials can be grouped into 15 mineral-based processing categories [5].
This includes placers, oxidized, iron sulphides gold bearing materials, etc. Oxidized ores are
normally weathered materials mostly in a shallow zone that is typical of the primary sulfide
deposit. During oxidation and other hydrothermal alteration processes, the rock structure breaks
down due to exposure to the atmosphere and weather conditions resulting in increased permeability
and leachability. These ores are mostly free milling ores and are ideal for cyanidation processes as
high leaching extractions can be achieved. The treatment processes for these ores are simple and
are cost effective. However, the disadvantage of working with these kinds of ores is that the
weathering can produce hydrated, amorphous clay minerals that sometimes adversely affect
downstream processes (like slurry viscosity and such mass transfer processes as leaching and
carbon adsorption [5]).
Sulphide containing ores (refractory ores) are common throughout the world. Pyrite is a common
mineral associated with sulphide gold ores. Sulphide ores are mostly resistant to the conventional
cyanidation process. Leaching of fine gold grains contained within pyrite is a major difficulty in
gold ore treatment. Sulphide ores may require pre-treatment in order for cyanidation to be effective
in the recovery of gold. Fine gold inclusions in pyrite may require fine grinding and/or strongly
oxidizing conditions to liberate the gold for leaching. Moreover, for carbonaceous gold ores or
concentrates, gold extraction by direct cyanidation is usually low. A few alternatives like
thiosulphate has been successful in treating these ores. However, not much work has been reported
on the leaching of gold from these ores using thiocyanate solutions.
5
2.3 Gold Cyanidation
Cyanidation has been used as the typical gold leaching method for over 100 years. Cyanide is
known as a powerful lixiviant for gold and silver. Cyanide has simple reaction chemistry, lower
dosage requirements and better metallurgical performance than many other reagents. Cyanide,
complexes with gold after oxidation to form an anionic complex as Au(CN)2- [1, 5, 8]. The
reaction of gold in an aerated solution of sodium cyanide was demonstrated by Elsner in 1846 as:
Reaction 1: 4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH
The most common and effective oxidant used in the cyanide leaching system is oxygen, which can
be supplied from air making the process cheaper. The cyanidation process has been successfully
used in agitation leaching, heap leaching, and intensive cyanidation.
2.4 Why the Need for an Alternative Reagent for Gold Leaching?
The use of cyanide presents significant safety, environmental and production challenges. These
are summed up as follows:
1. Environmental concerns and high toxicity of cyanide compounds:
The high toxicity of cyanide is mainly due to its production of the toxic gas (HCN) when pH is
low. The release of cyanide into the environment represents potential environmental and health
hazards. Cyanide spills from mining wastes into the environment can cause damage to aquatic life,
loss of wildlife and plant life. For example, the cyanide spill in Summitville, Colorado, in the year
1992, resulted in the loss of aquatic species in the 22-mile Alamosa river in Colorado. Birds were
found dead in the immediate vicinity which was believed to have resulted from the birds drinking
the cyanide contaminated waters [1, 9].
6
2. Increasing Regulatory Scrutiny:
The environmental accidents that have occurred around the world have raised concerns about the
use of cyanide in many parts of the world. Some gold mining companies have left scars in some
communities due to improper handling and uncontrolled disposal of cyanide in mining wastes.
This has increased the regulatory scrutiny for new miners wanting to set up plants that use cyanide.
3. Nonadaptability of “stubborn” ores and concentrates:
Aside from the environmental concerns, one major reason for the search for the alternative is the
difficulty of cyanide to leach highly refractory gold ores. Cyanide is effective to leach free milling
ores. However, there are certain classes of gold and silver ores (i.e., carbonaceous, pyritic,
arsenical, manganiferous, cupriferous) that are considered refractory to conventional cyanidation
and record low gold dissolution with cyanide.
These together with the long leaching period with cyanidation have been the critical driver for the
industry to evaluate new and improved reagents with effort devoted to finding an alternative that
might compete with conventional cyanidation.
2.5 Gold Leaching using Alternative Reagents
More than 25 new alternative reagents have been seriously studied in order to find a reagent to
replace cyanide for gold leaching. These reagents include the halides (chlorine, bromine and
iodine), the thiosystem (thiosulfate, thiocyanate and thiourea), polysulfide, the ammonia system
and novel reagents and technologies. Researchers have made comparative studies on the
chemistry, thermodynamics and application of these reagents. Quite a few pilot plants and review
papers based on these research results have been reported [10]. A consideration of environmental
effects using these reagents have also been discussed [11],[ [6, 12, 13].
The operating regime, oxidation potential and pH of some of the alternatives are presented in
Figure 1. The Eh-pH diagram classifies these reagents into acidic, basic and neutral regime and
shows their corresponding potentials.
7
Figure 1 - Eh-pH diagram showing typical operating regions for alternative gold lixiviants
[2]
Various oxidants are used in these kinds of system. In the alkaline systems including cyanide,
ammonia-cyanide and ammonia, oxygen, air or peroxide are the main oxidants used. Gold
dissolution rates are controlled by oxygen solubility in solution. The neutral lixiviant systems
which includes the thiosulphates, halogens, etc., oxygen and iron sulphate are the main oxidants
used. In the acid leaching systems which includes, thiourea, thiocyanate, chlorine, etc., various
oxidants such as chlorine, ferric chloride, hydrogen peroxide, ferric sulphate, nitric acid are
reported. Gold dissolution in acidic solutions has been reported to be fast. However, major
disadvantages reported for some of these reagents which operate in the acidic regime include high
reagent consumptions and corrosion of materials of construction. Among all the lixiviants
examined, (thiosulfate [14, 15], thiourea, halogens, thiocyanate [16],) thiocyanate is one of the
few promising alternatives that can replace cyanide. Some of the alternatives have been discussed
briefly below.
8
2.5.1 Thiosulphate
Thiosulphate as an alternative to cyanide for the leaching of gold and silver ores has been
extensively studied [17, 18]. The dissolution of gold in thiosulphate solution is accomplished by
the formation of the gold-thiosulphate complex. Acceptable gold leaching rates using thiosulphate
have been achieved in the presence of ammonia with cupric ion acting as the oxidant [15].
However, reports show that the chemistry is quite complex (mainly due to the presence of
thiosulphate, ammonia and copper), and recovery of gold from thiosulphate solution has been the
main limitation to the process [15]. The relatively simple oxidation of metallic gold to the aurous
Au+ ion in ammoniacal thiosulphate in the presence of Cu(II) can be simply represented as follows:
Reaction 1: Au + 5 S2O32- + Cu(NH3)4
2+ = Au(S2O3)23- + 4 NH3 + Cu(S2O3)3
5-
Senanayake [19] showed that the leaching of gold in thiosulphate occurs at a potential between
0.1- 0.36 V and a slightly alkaline pH between 9-10. Li et al [6] reported that maintaining suitable
concentrations of thiosulphate, ammonia, copper and oxygen in the leach solution, and
consequently, suitable Eh and pH conditions, thiosulphate leaching can be made practical.
However, high thiosulphate consumption has been one of the limitations for the wide
industrialization of thiosulphate leaching. Barrick Gold Corporation [20] after recording low
recoveries, applied thiosulphate to the treatment of pre-oxidized carbonaceous gold ores that
exhibited preg-robbing characteristics and a gold recovery of 95% was recorded. Thiosulphate is
generally regarded as the most popular alternative to cyanide and more work has been completed
to ascertain the process.
2.5.2 Thiourea
Thiourea (H2NCSNH2) dissolves in water to yield an aqueous form which is also stable in acidic
solutions and forms strong gold complexes [21]. It has been known to have many advantages over
cyanide due to its high leaching rates. In gold dissolution, thiourea operates in the acidic regime.
The main oxidants used for gold dissolution are sodium peroxide, hydrogen peroxide, ferric iron,
oxygen (pure oxygen or air), ozone, manganese dioxide, manganate, dichromate, and others.
However, ferric ion is known to be the common and the most effective [21, 22]. The process can
be described by the following chemical equation:
9
Reaction 2: Au + 2CS(NH2)2 + Fe3+ = [Au(CS(NH2)2)2]+ + Fe2+
Thermodynamically, the potential for leaching gold with thiourea in the presence of ferric is
between 0.4 and 0.45 V (vs SHE) and at a pH between 1 - 3. It has been reported that thiourea is
intrinsically unstable and at a higher pH and potential, oxidizes rapidly to form formamidine
disulfide. Formamidine disulfide decomposes to a number of intermediate species where
elemental sulfur forms as the final product which may passivate the gold surface and stop the
leaching process [22]. Most successful applications with thiourea have been carried out on ores
that have high contents of cyanicides such as antimony, or sulfide ores that have undergone
pretreatment such as bacteria oxidation or pressure leaching. This makes the process more
advantageous since the pretreated ore can be directly leached with thiourea without a neutralisation
step as would be required for leaching with cyanide[2]. Thiourea has been demonstrated in a pilot-
scale agitation leaching, percolation leaching, in-stope leaching experiments at a Witwatersrand
mine, underground leaching at a Canadian mine.
However, in recent years, interest in this leaching approach has decreased due to the potential
carcinogenic properties of thiourea, higher reagent consumption during gold dissolution, serious
corrosion of equipment due to the acidic media in which it is operated, no selectivity of gold during
leaching and its complicated regeneration procedures [6].
2.5.3 The Halide (Chlorine, Bromine & Iodine)
The halogens are well known for leaching gold and silver. Chlorine dissolves gold in aqueous
solutions by forming soluble Au(I) and the more stable Au(III) chloride complexes. A big
advantage of chlorination is the high dissolution rate compared to alkaline cyanide leaching which
is due to higher solubility of chlorine and chlorides in water. The typical operating region lies at
pH values lower than 1.5 and temperature between 50oC - 70oC yielding a recovery of 99% in
small-medium scales [23].
The chlorination is capable for higher scale processing and has already been applied industrially
for pre-treating refractory and carbonaceous ores in several plants in the USA in the 1980’s [2]
[23]. Renewed interest in halides as a lixiviant for leaching gold occurred in the 1990’s after
10
several patents based on the bromine/bromide systems were lodged. A general equation for the
reaction with gold and chlorine or bromine is:
Reaction 3: 2Au + 3X2 + 2X- = 2(AuX4)-
where X is Cl, Br
These reagents are strongly oxidizing and show much higher dissolution rates than cyanide
leaching. Table 2 gives a summary of the leaching conditions used in leaching gold by the
halogens.
Table 2 - Typical leaching conditions used in leaching gold with halides (Aylmore 2005)
The disadvantage of these halogens is the difficult handling of the strongly corrosive chlorine
solution and the need for robust materials of construction and closed reaction conditions because
of the formation of chlorine gas [2]. Bromine and iodine have not been used industrially because
of difficult handling, high reagent costs and associated health issues.
Reagent Ligand Oxidant Gold Complex Typical Conditions pH
Chlorine Cl- Cl2 or HClO [AuCl4]- 5-10 g/L Cl2 or NaCl <3
Bromine Br- Br2 [AuBr4]- 2-5 g/L Br2 5-8
Iodine I- I2 [AuI2]- 1g/L I2, 9 g/L NaI 5-9
11
2.6 The Challenges to Developing Alternatives for Cyanide
For the past 30 years, there has been considerable effort to find an alternative to cyanide. However,
the main challenge lies in between finding an alternative that would match the characteristics of
the ore giving gold recoveries better or comparable to that in the cyanidation process. The general
challenges of alternatives appears to be high reagent consumption and difficult process control and
recovery of gold from the leach solution. For example, thiosulphate has received much attention
over the years and has been reported to have fast leaching kinetics, but it has complex solution
chemistry and is difficult to removing gold from leach solution. Thiourea has been reported also
to have a high reagent consumption and difficult management due to the potential carcinogenic
nature of thiourea. A suitable alternative should be selected to offset all these challenges. [2].
2.7 Thiocyanate Leaching of Gold (Chemistry and Thermodynamics)
2.7.1 Chemical Properties of Thiocyanate
Thiocyanate SCN- previously known as rhodanide (from a Greek word for rose) is a free ion of
thiocyanogen (SCN)2. It is a pseudo halide forming thiocyanogen, (SCN)2, and thiocyanic acid,
HSCN. This property is due to its similarities with that of the halogens [24]. Thiocyanate forms
insoluble salts or complexes with silver, mercury, lead and copper ions under certain conditions.
Thiocyanate forms stable soluble complexes with gold. It has a linear structure and shares its
negative charge almost equally between sulfur and nitrogen; thus, it is able to form covalent
compounds and complexes. SCN- is synthesized or manufactured as a salt of ammonium,
mercury, sodium, potassium and transition metals like copper. It is an ambidentate ligand
(meaning its unidentate ligand can bond through different atoms to form different coordination
compounds) and has a donor function either via the S or via the N atom. The SCN− species can
also bridge two (M−SCN−M) or even three metals (>SCN− or −SCN<). Experimentally it has
been reported that hard metals such as (Fe, Mn, Ni, Cu and Zn) tend to form N-bonded thiocyanate
complexes, whereas soft metals/acids such as second- and third-row transition elements, including
Au and Ag tend to form S-bonded thiocyanate complexes [25].
12
2.7.2 Gold Extraction with Thiocyanate
Thiocyanate as an alternative to cyanide for the leaching of gold ores was first discovered by White
in 1905. He demonstrated the effectiveness of thiocyanate by dissolving a gold leaf in a ferric
thiocyanate solution and also tested the effectiveness of various oxidizing agents [14].
In 1986, when safety concerns on the use of cyanide started becoming public, Fleming [20]
revisited the subject of thiocyanate leaching of gold. He investigated a South African pyritic ore,
extracting gold and uranium simultaneously by using the acid thiocyanate system with ferric as
oxidant. This was done to reduce production cost of uranium-gold ore. The uranium market was
in economic decline and thus the company wanted to cut down on the production cost by extracting
gold and uranium simultaneously. He reported after his findings that increasing thiocyanate
concentration does not affect uranium extraction and favours gold extraction, forming Au(SCN)2-
and Au(SCN)4- complexes.
Later in the 1990’s, Broadhurst and Du Preez [26] postulated the thermodynamics of the
thiocyanate leaching of gold in acidic ferric sulphate medium. Eh-pH diagrams and speciation
diagrams were constructed for all the gold/thiocyanate/iron/sulphate/water systems in
concentrations and at pH values that would be feasible in a typical leaching system. These
diagrams were used to predict the feasibility and the optimum leaching parameters of the
thiocyanate leaching of gold in the ferric medium. It was successful to also predict the factors
affecting the oxidative dissolution of gold.
More recent work has been done by Barbosa and Monhemius [16] on the thiocyanate leaching of
gold. This includes; detailed fundamental and thermodynamics study, kinetic and electrochemical
study and finally recovery of gold in thiocyanate solutions. As part of their findings they reported
that the mechanism of gold dissolution by iron (III) thiocyanate complex is linked to an
autoreduction process, in which SCN- is oxidized by the spontaneous reduction of ferric to ferrous.
13
2.7.3 Stability of Thiocyanate
Thiocyanate ions are metastable and can be oxidized in water to form carbonates, sulphates and
ammonia according to the following reactions 4, 5 and 6. Depending on the pH, reaction 4
produces cyanide or hydrogen cyanide which is considerably less stable and oxidizes quickly to
cyanate ion. This reaction is irreversible as cyanide and sulphate cannot be reduced back in any
form (chemically or electrochemically) to thiocyanate. Reaction 5 behaves like reaction 4 and is
irreversible. Reaction 6 is reported to have more stable products.
Reaction 4: SCN- + 4H2O = SO4-2 + CN- + 8 H+ + 6e-
Reaction 5: SCN- + 5H20 = SO4-2 + CNO- + 10 H+ + 8e-
Reaction 6: SCN- + 7H2O = NH3 + SO4-2 + CO3
-2 + 11 H+. + 8e-
Figure 2 - Eh–pH diagram for the SCN–H2O system at SCN concentrations of 1.0 M at 25
°C. Short dashed lines show water stability limits [27].
14
The Eh-pH diagram by Li et al [28] for two levels of thiocyanate concentrations (0.5 and 0.005
M) is shown in Figure 2. The concentration of all other species is 0.1 M. The Eh-pH diagram
suggests that thiocyanate oxidation to ammonia, sulfate and carbonic acid (Reaction 6) occur over
a pH range of 2 - 6.4. The Eh-pH diagram also suggests that the SCN-H2O system is not sensitive
to SCN- concentration in the absence of other ions.
Thiocyanic acid will form at low pH with a pKa of 0.9 according to Equation 7:
Reaction 7: SCN- + H+ = HSCN
As mentioned in the introduction, thiocyanate is a pseudo halide and its related pseudohalogen is
thiocyanogen and related acid is thiocyanic acid. Thiocyanic acid exists in two tautomeric forms,
H-S-C=N and H-N-C=S. The formation of thiocyanic acid is known to lower the dissolution of
gold in a typical leaching process. This is due to the reduction in the activity of SCN- as a result
of its protonation to thiocyanic acid [27]. Also, the thiocyanic acid is less reactive and does not
participate directly in the dissolution gold.
It should be noted that several intermediate species are formed in the SCN- oxidative process. The
most important species are thiocyanogen (SCN)2 and trithiocyanate (SCN)3-, as suggested by
Itabashi [29] and later confirmed by Barbosa-Filho [16]. The formation of these metastable
intermediate products is indicated in Figure 3. The Eh-pH diagram corresponding to these
metastable species, thiocyanogen and trithiocyanate was constructed with data collected from
other sources [16, 27]. This provides insight of the mechanism of thiocyanate oxidation. The
diagram also confirms the predominance of trithiocyanate. The formation of HOSCN on the
diagram (one of the by-products of the H2O2/SCN mixture) occurs at a very high potential and
does not form in a Fe(III)/SCN complex solution.
15
Figure 3 - Eh–pH diagrams for the SCN–H2O system at SCN concentrations of 1.0 M at 25
°C. Short dashed lines show water stability limits
The standard potentials for these two intermediate products are as follows:
Reaction 8: (SCN)2 + 2e- = 2SCN- Eo = 0.770 V
Reaction 9: (SCN)3- + 2e- = 3SCN- Eo = 0.650 V
It is believed that under certain conditions trithiocyanate (SCN)3− is the most pre- dominant species
with respect to thiocyanogen (SCN)2. It decomposes to form thiocyanogen and thiocyanate as
shown in Reaction 10.
Reaction 10: (SCN)3- = (SCN)2 + SCN-
Thiocyanogen can also rapidly hydrolyse in acid to form SCN-, HCN and SO42- according to the
following reaction:
Reaction 11: 3(SCN)2 + 4H2O = 5SCN− + HCN + SO42− + 7H+
16
Li et al [27] reported that the formation of these metastable species enhances gold dissolution as
these species act as oxidant and complexant at the same time under certain oxidative leaching
conditions and is mostly catalysed by an auto reduction process of ferric to ferrous. Also, under
an actual leaching condition with ferric as the oxidant, thiocyanate can be oxidized by ferric
thermodynamically into carbonates, sulphates and ammonium as expressed in the reaction below:
Reaction 12: SCN-
+ 7H2O + 8Fe3+ = SO42- +CO3
2- + NH4+ + 10H+ 8Fe2+
△Go = -378.5 kJ/mol.
It should be noted that the formation of these metastable intermediate products depends on
leaching conditions such as concentrations of the oxidants, thiocyanate concentration, pH and
solution potential.
2.7.4 Leaching of Gold in Thiocyanate Solutions
In the Au-SCN-H2O system, gold can be dissolved in the acidic region, which allows the use of
several oxidizing agents that are suitable for acidic media. The Eh-pH diagram for the stability of
gold in water is presented in Figure 4. The short-dashed line indicates the stability region of water.
Activities of all dissolved species are regarded as 1 M. The predominant area is Au(OH)3(s). The
diagram was constructed with data collected from relevant sources [16, 30]. This diagram
indicates that metallic gold is more stable than gold oxide compounds and gold ions under the
conditions for stability of water. The gold metal has no coexistence boundaries with the gold
oxides within the water boundaries [10]. Therefore, the oxides would not be produced directly
from the metal by oxidation in water. The dissolution of gold by oxidation only can be achieved
at a higher oxidation potential and other complexing agent, such as cyanide ion, thiocyanate, etc.
to produce a gold complex which remains in solution.
17
Figure 4 - Eh–pH diagrams for the Au–H2O system at Au concentration of 0.0001 M at 25
°C. Short dashed lines show water stability limits.
Most of the earlier research focused on thiocyanate leaching of ‘metallic’ gold and silver with very
few studies on gold ores. There are no current research works on the leaching of an oxide gold
ore in thiocyanate solution. The chemical reactions for thiocyanate leaching of gold were
described by Barbosa [16]. In his postulation, he described that the two most important complexes
formed from the dissolution process are aurothiocyanate, Au(SCN)2-, and/or aurithiocyanate,
Au(SCN)4-. The formation of these complexes can be achieved over a wide range of pH and
potential according to Reaction 13 and 14.
Reaction 13: Au + 2 SCN- = Au(SCN)2- + e- Eo = 0.691 V
Reaction 14: Au+4SCN- =Au(SCN)4- +3e- Eo = 0.659 V
18
Figure 5 - Eh–pH diagrams for the AuSCN–H2O system at SCN concentrations of 1.0 M,
and Au concentration of 0.00001 M at 25 °C.
The Eh-pH diagram of the Au-SCN-H2O system presented in Figure 5 shows the predominance
of the two gold thiocyanate complexes. Gold may be dissolved at acidic pH values, which enables
the use of different oxidants such as iron(III), hydrogen peroxide and manganese dioxide. The
species are Au(SCN)2− which occurs at potentials around 0.630 V or higher and as Au(SCN)4
− at
potentials above 0.680 V. In this regard, the potential for an actual gold leaching system as
reported by Li et al [27] should be controlled in a range of 0.4 to 0.8 V versus SHE.
Barbosa [16] reported that the dissolution of gold as the auric complex Au(SCN)4- occurs at the
same potentials in which the predominant thiocyanate species is being oxidized to (SCN)3- whereas
the aurous complex, Au(SCN)2-, is formed at potentials at which the SCN- predominates. The
formation of the auric complex in the presence of (SCN)3- is shown in Reaction 15 as:
Reaction 15: AuSCN + (SCN)3- = Au(SCN)4
-
19
The oxidation potential is very important in this work as it determines the kind of complexes
formed. In a typical acid leach condition (pH 1 to 2), this potential can be controlled by such factors
as choice of oxidant, concentration of thiocyanate, pH, etc. As mentioned above, higher oxidation
potential is needed to make Au(SCN)4− predominant whereas lower potential signifies the
predominance of the Au(SCN)2− complex. The overall calculations also show that the redox
potentials required in a typical leaching process should be around 600-700 mV (vs. SHE).
Almost all the reports on thiocyanate leaching of gold refer to ferric ion as the most suitable
oxidizing agent. It should be noted that using ferric ion as an oxidant is very important in a typical
leaching system for most gold ores and also for gold leaching following oxidative pre-treatment
of sulfidic ores. Most oxide and refractory gold ores have a high amount of iron. Soluble iron as
an oxidant can be generated in-situ. Refractory ores are also associated with pyrite and other
sulfide minerals. High concentrations of ferric ion in an acidic media can be generated from these
ores after oxidative pre-treatment using roasting, bio-oxidation or pressure oxidation.
The overall gold dissolution reaction by thiocyanate in the presence of ferric can be written as:
Reaction 16: Au + 2SCN- + Fe3+ = Au(SCN)2- + Fe2+
It is well understood that SCN- forms stable complexes with ferric and that the presence of ferric
increases gold dissolution rates. The presence of ferric in the SCN- leaching system is also known
to release SCN- upon the reduction of ferric to ferrous which thereby increasing the thiocyanate
concentration [27]. The reduction of ferric to ferrous is shown in Reaction 17 as:
Reaction 17: Fe3+ + e
- = Fe2+
Eo = 0.771V
Since the oxidation potential of ferric is higher than that of SCN- it should be noted that the
formation of stable compounds could reduce the oxidizing potential of ferric ions and the
concentration of free thiocyanate required for gold dissolution.
The speciation diagram in Figure 6 gives an idea of complexes formed from the SCN/Fe(III)
mixture at different SCN- concentration and ferric fractions. One characteristic property of the
iron(lll)- thiocyanate complex is the formation of the red blood colour which occurs when solutions
20
containing ferric ions and thiocyanate are mixed together. The existence of any of these species
depend on the thiocyanate concentration and the molar ratio of the SCN/Fe3+ complex. For
example, at a molar ratio of 1 the predominant species Fe(SCN)2+.
Figure 6 - Species distribution diagram for the Fe3+- SCN-H2O system at 25oC, pH 2 and
Fe3+ activity of 10-1 [31]
As the formation of these complexes depend on the thiocyanate concentration, it should be noted
that lower SCN concentrations lead to the formation of cationic complexes such as Fe(SCN)2+ etc.,
whereas higher thiocyanate concentrations yield anionic complexes. However, these complexes
will coexist in a typical leaching process with SCN- concentration above 10-3 M [26].
The only complex reported so far for the complexation between ferrous and thiocyanate is the
FeSCN+. This is due to the weak stability constant between thiocyanate and ferrous ion. This
indicates that the ability of thiocyanate to complex with ferrous ions is much weaker than that with
ferric ions [16]. The complex formation between thiocyanate complexes with iron and gold is
presented in Table 3. The stability constants presented in Table 3 signifies that the iron-thiocyanate
complexes are less stable than gold- thiocyanate complexes.
21
Table 3: Stability constants of thiocyanate complexes with iron and gold at 25oC [32]
Metal Ion Complex Stability Constant
Fe2+ FeSCN+ 2.04 x101
Fe3+ FeSCN2+ 1.05 x103
Fe3+ Fe(SCN)2+ 2.00 x105
Fe3+ Fe(SCN)4- 3.31 x105
Fe3+ Fe(SCN)52- 1.58 x106
Fe3+ Fe(SCN)63- 1.26 x106
Au+ Au(SCN)2- 1.45 x1019
Au3+ Au(SCN)4- 4.57 x1043
Au3+ Au(SCN)52- 4.17 x1043
2.8 The Use of Different Oxidizing Agent
Ferric sulphate is the most researched oxidant in the thiocyanate system and almost all the reports
on thiocyanate leaching of gold refer to ferric ion as the most suitable oxidizing agent. The use of
ferric as an oxygen surrogate is to help overcome the slowness of oxygen reduction at room
temperature. The leaching of gold in thiocyanate solutions with ferric allows gold to be leached
at a pH of 1-3. The low pH of operation is due to the solubility of ferric sulphate as a function of
pH. Ferric is highly soluble in the acidic regime, and above pH > 3, the solubility of ferric
decreases with the formation of iron (III) hydroxide which precipitates at higher pH values [26].
Other potential oxidizing agents for the leaching of gold in acidic thiocyanate solution are
hydrogen peroxide, oxygen, air and MnO2. However as mentioned before, the major drawback of
the use of oxygen and air as an oxidizing agent is its low solubility in acidic solutions. Though it
is very effective for cyanide systems in the basic medium, it is expected not to be effective at any
pH above 3 in the thiocyanate system. This is due to the slow rates of oxygen reduction on gold
22
surfaces in non-cyanide solutions [2]. This explains why SCN- generated in cyanidation plants
does not participate in the gold dissolution process and simply reports as a cyanide loss.
Hydrogen peroxide and MnO2 have not received much attention. However, Barbosa [16] reported
that hydrogen peroxide can be used in both gold and silver leaching, the former requiring a weakly
acidic pH (pH < 2.5). Potentials for this kind of process are a bit higher as compared to that in the
Fe(III)/SCN systems. A few of these alternative oxidants have been discussed below:
2.8.1 Hydrogen Peroxide as an Alternative Oxidant
Hydrogen peroxide is known to be a good oxygen supplier. It has been used successfully to
increase dissolution of gold in alkaline cyanide solutions. Hydrogen peroxide decomposes to form
water and oxygen. Even though it has been used in most cases as an auxiliary oxidant, its
decomposition reaction enables it to serve as the main oxidant by increasing the oxygen to catalyse
a reaction [33].
Early studies with hydrogen peroxide in cyanidation showed that the dissolution rate of gold with
small amount of hydrogen peroxide is very slow under oxygen-free solutions. Other studies have
shown that an increase in the hydrogen peroxide concentration increases the dissolution rate of
gold significantly [34]. However, hydrogen peroxide has been used many times as an oxygen
supplement in many gold mining process plants to increase the concentration of dissolved oxygen
above that attainable with simple air sparging systems.
Wilson and Harris [35] investigated the reaction of H2O2 with SCN-. According to their
investigation, it was found that the oxidation reaction between thiocyanate and hydrogen peroxide
is pH dependent and catalysed by the production of H+ ion at pH below 2. The oxidative reaction
between the thiocyanate and H2O2 proceeds through an initial step of the reduction of H2O2 to H2O
and the production of an intermediate species; hypothiocyanite, (according to Reaction 18) which
undergoes a series of fast reactions leading to end products, which are dependent on the pH of the
medium. They observed that at a higher acid concentration (pH below 2), the reaction is acid-
catalyzed. This is due to the fast production of H+ ions and the formation of hypothiocyanite (as
seen in Reaction 19). Hypothiocyanite also undergoes fast reactions leading to end products of
cyanide and sulphate (according to Reaction 22).
23
Cyanide reacts further with HOSCN and forms sulphur dicyanide, S(CN)2. Also, at a lower pH of
4 -12, the reaction between thiocyanate and hydrogen peroxide was found to be pH independent
and the hypothiocyanite produced further oxidizes to sulphate, ammonia and bicarbonate (HCO3−).
Reaction 18: SCN- + H2O2 = HOSCN + OH-
Reaction 19: H3O+ + SCN- + H2O2 = HOSCN + 2H2O
Reaction 20: HOSCN + H2O2 = HOOSCN + H2O
Reaction 21: HOOSCN + H2O2 = H2SO3 + HOCN
Reaction 22: HOOSCN + H3O+ = H2SO3 + HCN + H+
Reaction 23: HOSCN + HCN- = (SCN)2 + H2O
Reaction 24: HOCN + 2H2O = HCO3-+ NH4
+
The rate determining reactions would be Reactions 18 and 19 for the pH-independent (pH>4) and
the acid catalyzed (pH<2) reactions, respectively.
2.8.2 Potassium Iodide as an Additive to Thiocyanate Leaching
Iodide/Iodine are halides that have been reported to leach gold, have high oxidizing potentials and
forms stable gold complexes in the aurous and auric states [36]. Chlorine, bromine and iodine are
halides and have been successfully used to dissolve gold. However, iodide solutions are known to
be the most stable amongst the halides [11]. It has been suggested for the extraction of precious
minerals such as platinum, gold and silver ores. Previous results have shown that gold leaching
rate in iodide solution is much higher than that of cyanides and other lixiviants such as,
thiosulphate, thiourea etc. Furthermore, iodide leaching can be carried out over a wide pH range
[30].
24
Gold dissolution in iodide solution takes place in the presence of a suitable oxidizing reagent (ferric
ions, hydrogen peroxide, iodine, hypochlorite, etc.) [36]. Among these, iodine seems to be the
most widely researched and suitable reagent due to its ability to perform at a wide range of pH
(usually below 11) yielding high gold dissolution rates.
It is understood that under general conditions, iodine dissolves in the presence of iodide to form
triiodide ion, which acts as the oxidant for the oxidation of elemental gold to gold(I)-iodide
complex. The oxidant reacts with the iodide which forms the tri-iodide and subsequently oxidizes
gold to form the stable complexes AuI2- and AuI4
-. The solubility products of the gold iodide (AuI)
and gold tri-iodide (AuI3) in water are 1.6×10-23 and 1×10-46, respectively [11, 36, 37].
Early studies on the use of the Iodine/Iodide system was first proposed by McGrew and Murphy
[38]. In their study, an electrolyte containing iodide ions was used for leaching sulphidic gold ores
(gold associated with marcasite, pyrite, galena, etc.). This electrolyte contained a mixture of iodide
ions and elemental iodine which served as the oxidant while the former acting as a complexant.
Gold dissolution was dependent on the concentrations of iodide ion and molecular iodine, which
recorded a redox potential sufficient for extraction [39]. This was made possible by the continued
addition of iodide which increased the iodide ion to achieve the desired concentration for leaching
gold. McGrew and Murphy [38] emphasized that due to the sparingly soluble nature of iodine in
low concentrations, to ensure sufficient iodide formation, the iodine lixiviant should be added to
an ore containing iodine reducing components. Gold was recovered on activated carbon and the
excess iodide formed during the process was re-oxidized electrochemically to iodine and reused
and thus requiring no addition of iodine. It should be noted that gold did not start to dissolve until
sufficient concentration of iodine species remained in solution.
A leaching test conducted by Morteza [30] on two different gold ores in the iodine/iodide solutions;
oxide and carbonaceous gold ores for 24 hr showed only 20 % extraction for the carbonaceous and
89 % for the oxide. The same test was repeated with iodate as the oxidant and no gold was leached
in 48 hr. He reported that the presence of sulphides and ferrous minerals slowly consumed iodine
and increased reagent consumption.
25
Thermodynamically, the dissolution of gold in iodide/iodine solutions is an electrochemical
process. Gold in its natural state is stable in water and can only go into solution when a mixed
potential is created by adding a complexant and an oxidant. The latter prepares the gold surface by
oxidizing it to Au+ and Au3+ and the former bonds with it to form a complex in the form, AuI2- or
AuI4- which brings the gold into solution. The gold dissolution chemical reactions in the
iodine/iodide solution are shown as follows [31]:
Reaction 25: Au+ + e- = Au Eo = 1.68 V
Reaction 26: Anodic Au + 2I- = AuI2- + e-
Reaction 27: Au + 4I- = AuI4- + 3e-
Reaction 28: Cathodic I3- + 2e- = 3I- Eo = 0.537 V
The oxidation of iodide is achieved by the cathodic half-cell reaction according to Reaction 28.
The overall reaction is written as:
Reaction 29: 2Au + I3- + I- = 2AuI2
- E = -0.042 V
Reaction 30: 2Au + 3I3- = 2AuI4
- + I- E = -0.024 V
It should be noted that I3- is the main oxidant of the iodine /iodide solution. The standard potential
of the I3-/I- is 0.536 V which is far lower than the standard potential of the gold couple Au+/Au
(1.68 V) or Au3+/Au (1.52 V).
Also, since the standard potentials for the overall reaction are negatives -0.042 V according to
Reaction (30) and -0.024 V for Reaction (31), it signifies that gold dissolution in iodine solutions
is not spontaneous under standard conditions. However, it has been reported that when triodide
and iodide concentrations are varied, it is possible to bring about a net spontaneous reaction [37].
Recently, experimental work done by Barbosa [39] on the effect of iodine/iodide additions on the
dissolution of gold in iron (III) thiocyanate solutions showed that small amounts of iodine/iodide
addition was successful in increasing the extraction rates. He stated that species such as I2- and I3
-
that form from the iodine/iodide system are very stable and form stable gold complexes as
compared to the intermediate species, (SCN)2 and (SCN)3- formed from the thiocyanate system.
26
It is believed that the production of these intermediate oxidation species ((SCN)2 and (SCN)3-)
caused by the autoreduction of ferric is the mechanism for gold dissolution in the thiocyanate
system. However, these species are not stable as their production is not continuous due to their fast
decomposition by hydrolysis. The instability of these intermediate species can be overcome by
additions of small amounts of iodide or iodine ions. It is revealed that the rate of extraction
increased with increase in thiocyanate and iodine or iodide concentrations. However, the addition
of iodide ions lowers the oxidation potential making the formation of the most active intermediate
species (SCN)3- thermodynamically not feasible. However, Barbosa [39] explained that ions such
as I2 and I3- are very stable in solution and complex with gold and thiocyanate forming stable
species I2SCN- and I(SCN)2- to cause gold dissolution. Thus, I2SCN- and I(SCN)2
- are believed to
cause faster gold dissolution by increasing leaching kinetics than (SCN)2 and (SCN)3- in ordinary
thiocyanate systems. The comparison of their half reactions is shown in Table 4.
Table 4- Standard Potentials for Half-Reactions of Iodide and Thiocyanate Redox Couples
in Aqueous Solution at 25°C.
Half Reaction Eo (V)
I2(aq) + 2e- + 2I-aq 0.621
I3- (aq) + 2e- = 3I-
aq 0.536
(SCN)2(aq) +2e- = 2SCN-aq 0.77
(SCN)3- (aq) + 2e- = 3SCN-
aq 0.68
2.9 Toxicity and Environmental Concerns with Thiocyanate
The use of sodium thiocyanate has been reported to be less harmful than cyanide. The reagent
shows some promise and will require further investigations on its toxicity. Li et al [27] highlighted
that thiocyanate is a naturally occurring compound which is present in many food products such
as cabbage, beets, cauliflower with its source being Cruciferae.
In animals, sudden death may occur from SCN- toxicity but usually the effects develop gradually.
Toxicity symptoms in animals may include vomiting, diarrhea, emaciation, general weakness, loss
27
of equilibrium and buoyancy, tremors, convulsion, coma and death. A person weighing 150
pounds will have to ingest 29 g of potassium thiocyanate as a potential single lethal dose [24].
In the gold leaching process, the stability of the thiocyanate species is strongly dependent on
oxidation potential and leaching environments. The oxidation of thiocyanate produces species
such as sulfate, ammonia, metastable cyanide, hydrocyanic acid, etc. Hydrocyanic acid is not
harmful and needs very high potential to form. However, its formation is unlikely and it is known
not to be toxic even if it does form [40]. However, further investigation is needed to determine
the correct methods for waste disposal.
2.10 Recovery of Gold from Thiocyanate Solutions
In gold processing, once gold is dissolved, the recovery of gold from the pregnant solution can be
carried out by different procedures e.g. adsorption, solvent extraction, etc. Adsorption of gold
from cyanide solutions using activated carbon is currently the most commonly used process in the
gold mining industry. The adsorption of gold with activated carbon in the cyanide system and
some of the alternatives to cyanide has been researched. However, not much has been done on the
carbon adsorption of gold from thiocyanate solution.
The strength/ability of activated carbon to adsorb gold in other solutions as discussed by Marsden
and House [5] has the following order:
AuCl2 > Au(CN)2 > Au(SCN)2 > AuSC(NH2)2)2 > Au((S2O)3)2
Activated carbon adsorption in thiocyanate solutions as reported by Li et al [40] gave a very high
gold recovery of about 98% and stands to be one of the ideal adsorbents for the adsorption of gold
in thiocyanate solutions. Carbon was observed to serve as a catalyst for the auto reduction process
of ferric to ferrous whiles oxidizing thiocyanate causing the formation of the two intermediate
species, namely (SCN)2 and (SCN)3-. This will become an advantage in CIL/CIP process systems
with SCN as the lixiviant, as the presence of these intermediate species will cause extra gold to
dissolve. The equilibrium of gold loading onto activated carbon were reported not to be affected
28
by temperature and pH. According to other studies [40], activated carbon has strong adsorption
properties for Au-SCN complexes for high gold recovery, and rapid kinetics, but lacks selectivity,
and needs stripping at high temperatures (about 150 °C).
Among the first alternative adsorbent tested was iron cementation for adsorbing gold in
thiocyanate solutions. The author concluded that the process was very effective, however, the gold
reaction was affected by solution pH, stirring speed, initial gold concentration, iron/gold mass
ratio, dissolved oxygen and presence of iron (III), while the reaction temperature and thiocyanate
concentration only slightly affected the cementation reaction. Also, the gold cementation reaction
was diffusion controlled and obeyed first-order kinetics with an activation energy of 9.3 kJ/mol.
It was observed that the de-aeration process of the solution enhanced the gold recovery and the
presence of ferric ions resulted in the low efficiency of gold precipitation as well as the low grade
of the cement [32].
In summary, thiocyanate SCN- is one of the most promising reagents among the several
alternatives to replace cyanide. With a careful control of leach conditions, the leaching of gold in
thiocyanate solutions can be controlled to attain maximum gold extraction. Thiocyanate was
evaluated for the extraction of gold samples (Supremo Oxide 68151 A, Supremo oxide 68151 B,
Supremo Composite 72142 B) from the Goldcorp Coffee project in Yukon, Canada with the
following objectives to:
1. Investigate the optimal conditions for dissolution of gold using thiocyanate
2. Improve gold extraction by the addition of additives (potassium iodide, hydrogen peroxide
and lead nitrate)
3. Recover gold from pregnant thiocyanate solution using activated carbon
29
Chapter 3: Experimental Design and Methodology
3.1 Experimental Design
3.1.1 Goldcorp Coffee Sample
Three different samples were received from Goldcorp’s Coffee Project, Yukon, Canada, for this
study. The samples were drawn from different zones of their mineralisation with the following
identity designations: 68151 A, 68151 B and 72142 Composite. The details of the as-received
samples are as shown in Table 5.
Table 5- Sample identification
KCA Sample Client ID Mass (Kg)
68151 A Supremo Oxide - 3.5 mm 5.76
68151 B Supremo Oxide - 1.70 mm 10.04
72142 B Supremo T2-T4 Comp - 62.5 mm 31.88
3.1.2 Sample Preparation
The purpose of this sample preparation was to have an evenly distributed representative sample
for each test.
Sample as received (72142 B) 1st Stage Crushing of the
same sample
2nd Stage crushing of the same
sample
Figure 7 - Product of crushed sample
30
The initial samples were received as rough rocks as seen in Figure 7. Upon receipt, the samples
were physically examined and weighed. Each of the three as received ore samples was crushed in
a two-stage crushing i.e. Jaw crusher following a cone crusher to reduce the particle size and
expose the gold surface for the subsequent processes. After the two-crushing stages, the samples
were dried at 60 oC to remove moisture, and dry-milled in a laboratory rod mill to P80 of 150 µm.
3.1.3 Dry Grinding
Grind characterisation was conducted under dry milling conditions and in batches (approximately
1 kg per sample) with the aim of attaining P80 of 150 µm. Each batch was ground (in Figure 8)
with 20 rods and subsequently sieved. The sieving times were varied at 5, 10, 15, and 18 minutes
to determine the time at which P80 of 150 µm was attained. The grind curve showed that P80 of
150 µm was attained after 15 minutes (see Appendix F for grind characterisation curve). The rest
of the samples were subsequently ground for 15 mins.
Figure 8 - Grinding mills used for the grinding process at UBC Mining Department
31
The product of the 72142 B material from the grinding stage was split using a riffle splitter to
obtain a representative sample for particle size analysis. The result of the particle size analysis
conducted 72142 B composite is shown in Figure 9.
Figure 9 – Particle size distribution of the 72142 B gold ore samples
3.1.4 Sampling for Testwork and Analyses
The samples were riffle split and sub-sampled into aliquots for the leaching testwork and
mineralogical and chemical analyses.
3.1.5 Solid SG Determination
Solid SG was determined for all the three gold ore samples. This was necessary to help in
metallurgical accounting and calculation purposes. Approximately 50 g of each of the sample was
placed in a known mass of 250 volumetric flask which was half filled with distilled water. The
flask containing the slurry was gently swirled and deaerated using a vacuum chamber to remove
32
any entrained air in the water. This was topped up with deaerated water and the new mass was
recorded. The solid SG is calculated as shown in equation in Appendix A5.
Table 6 – Solid SG result
Sample ID Supremo 68151 A Supremo 68151 B Supremo 72142 B
SG 2.60 2.62 2.66
3.2 Mineralogical and Chemical analyses – Head grade, XRD and ICP Analysis
Samples were submitted for mineralogical analysis by XRD, elemental chemical analysis by ICP
and head gold grade analysis by Fire Assay.
3.2.1 Head Grade Analysis
Samples were submitted to SGS Vancouver for head gold grade analysis by fire assay followed by
acid digestion. The results of this is presented in Table 7:
Table 7 – Head grade analysis results
Sample ID Supremo 68151 A Supremo 68151 B Supremo 72142 B
Head Grade (g/t) 1.40 1.42 1.98
33
3.2.2 Mineralogical Analysis
Table 8 is the results of an XRD analysis conducted on the three samples received from Goldcorp.
The samples were prepared and analysed according to the International Centre for Diffraction
Database PDF-4 using Search-Match software by Bruker. The analysis was conducted at the UBC
Earth and Ocean Sciences.
Table 8 – Results of XRD analysis for the three samples
Mineral Ideal Formula
Percentages of Supremo Ores
#1
68151 -
A
#2
68151 -
B
#3
72142 -
B
Quartz SiO2 56.0 57.4 55.3
Illite- Muscovite 2M1 K0.65Al2.0Al0.65Si3.35O10(OH)2/KAl2AlSi3O10 15.9 15.4 15.3
Illite-Muscovite 1M K0.65Al2.0Al0.65Si3.35O10(OH)2/KAl2AlSi3O10 15.2 15.6 10.6
Biotite K(Mg,Fe2+)3AlSi3O10(OH)2 1.2
Kaolinite Al2Si2O5(OH)4 4.4 4.3 8.5
K-feldspar
(microcline) KAlSi3O8 5.3 4.7 4.2
Plagioclase
(oligoclase) NaAlSi3O8 – CaAlSi2O8 2.1 1.5 4.9
Rutile TiO2 0.5 0.5
Chalcopyrite CuFeS2 0.2 0.3
Dolomite-Ankerite CaMg(CO3)2,Ca(Fe2+,Mg,Mn)(CO3)2 0.4 0.4
Total 100.0 100.0 100.0
Results of the XRD analysis on all the three samples showed high amount of quartz. The results
of quantitative phase analysis by Rietveld refinements are given in Table 8 and with the scans
shown in Appendix B1. The major phases of minerals for all three samples were quartz, clay
minerals (illite), kaolinite, plagioclase and feldspar. These amounts represent the relative amounts
of crystalline phases normalized to 100%.
34
3.2.3 ICP Analysis
SGS (Vancouver) conducted the chemical analysis of the samples by using the 56-element ICP,
and Carbon and Sulphur species by the Leco method. The results of the chemical analysis are
partly shown in Table 9, with the rest provided in Appendix B2. The elements with the highest
Table 9 – Chemical analysis of three Coffee sample
Cu (ppm) Fe (%) Ag (ppm) As (ppm) C (%) S (%) Al (%)
Supremo Oxide 68151 A 20 2.95 <1 1430 0.154 0.034 6.97
Supremo Oxide 68151 B 20 2.83 <1 1540 0.193 0.034 6.93
Supremo 72142 B 10 4.1 <1 1500 0.033 0.018 7.57
3.3 Experimental Setup
The gold dissolution experiments were carried out in a one-liter cylindrical baffled reactor obtained
from CANSCI. The presence of the baffles was to prevent vortex effect and to ensure that slurry
was well mixed. The reactor was then immersed in a thermostatically controlled water bath with a
suspended mixer over the reactor to mix the slurry, as shown in Figure 10.
The reactor was covered with a four-holed lid to prevent evaporation. Three of these holes were
covered and one was left open throughout the test for sampling and reading of ORP and pH’s. The
pH and oxidation-reduction potential (ORP) of the solution were also measured using a hand-held
Oakton pH pH/mV Meter.
Slurry samples were taken at time 2, 4, 8, 12 and 24 hr. After the leaching test was completed, the
final slurry was filtered using a vacuum pump attached to a buchner funnel/bottle and the gold
pregnant solution (filtrate) collected for gold assay and residual thiocyanate analyses.
The residue retained on the filter, was generously washed to remove any residual soluble gold
using deionized (DI) water to strip away all gold pregnant solution remaining in the cake. The cake
was placed in an oven and dried at 60 °C. The dried cake was crushed using a roller and a sample
35
was taken to SGS for tails grade analysis by fire assay followed by acid digestion. Solution grade
was analysed with AAS and/or ICP MS.
Figure 10 – A pictorial view of the experimental set-up
3.4 Analysis of Results and Analytical Methods
Gold in solution was analysed using Atomic Absorption Spectroscopy (on a Varian 240 AAS
instrument) with an air-acetylene flame and ICP MS (this was done in the UBC Geological
laboratory to confirm values from the AAS). Gold in solution was extracted into the organic phase
using DIBK containing 10g/L aliquat 336. Gold standards were prepared in the concentration of
0.1 – 5 ppm (see procedure in appendix) from a 1000 ppm gold standard stock solution. All the
solution gold grade analysis was done in the organic phase.
Residual thiocyanate was determined argentimetrically by the Volhard method titration [41]. This
was performed by pipetting a known volume of the titrand into a volumetric flask, acidified with
2-3 drops of concentrated HNO3, and titrated directly with AgNO3 in a glass burette. Ferric
sulphate solution was used as the indicator. The reaction of ferric and thiocyanate gives a deep red
colour. Silver nitrate precipitates thiocyanate quantitatively as the white solid AgSCN. Silver
thiocyanate is a stronger complex than ferric thiocyanate and the solution becomes colourless when
36
all of the thiocyanate is complexed with silver. (The Ksp of AgSCN is 1.0 x 10-12). The end point
was indicated by colour change of the precipitate from dark red to white/ colourless. Residual
thiocyanate was calculated as:
mg SCN/litre = ((Vol. titrant, ml) x (0.1mmoles Ag+/ml) x (58.08 mg SCN/mole))/litres
Residual cyanide was also determined argentimetrically. Samples were titrated directly with 0.1M
AgNO3. Rhodamine was used as the indicator. The reaction between the AgNO3 and the NaCN
is as: Cyanide reacts with Silver Nitrate (AgNO3) according to the following equation below:
AgNO3 +2NaCN NaAg(CN)2 +NaNO3
The end point was indicated by colour change of the precipitate from colourless to pink.
3.5 Reagents used in Gold Leaching Tests
All the reagents used in the leaching test are summarised in Table 10. These were used without
further purification or analysis to confirm their purity values. Deionised water was used for all
the gold leaching tests.
Table 10 : Chemical reagents used for leaching tests
Reagent Grade/Purity Source Form
CaO (ACS Reagent Grade) Alfa Aesar/Thermo
Fisher Scientific
Solid (Pellets)
NaOH (ACS Reagent Grade) BDH Chemicals/VWR
Analytical
Solution
NaCN 95% (ACS Reagent Grade) Anachemia (VWR) Solid (Powder)
AgNO3 Certified Analytical Grade Fisher Scientific Solution
KI (ACS Reagent Grade) ACROS Solid (Powder)
NaSCN 95% (ACS Reagent Grade) Anachemia (VWR) Solid (Powder)
H2O2 30% w/w (ACS Reagent Grade) Fisher Scientific Solution
37
Chapter 4: Results and Discussion
4.1 Introduction
This chapter discusses the results of the leaching and adsorption tests conducted in this study.
Preliminary leaching tests were conducted on all the three samples (68151 A, 68151 B and 72142
B composite material) to determine the responsiveness/amenability of the ores to gold extraction
by cyanide and thiocyanate solutions. This was followed by a series of leaching experiments aimed
at optimizing the thiocyanate leaching of gold. All the subsequent leaching tests excluding the
preliminary tests were conducted on the 72142 B composite material using the experimental setup
in Section 3.3 so as to eliminate sample variability from the interpretation of the results.
4.2 Cyanide Leaching
A cyanidation test was conducted in duplicate for each of the ores sample, the first one at a default
leaching time of 24 hr and a sodium cyanide concentration of 500 ppm and the duplicate at a
leaching time of 32 hr with the same cyanide concentration at 500 ppm. Two of the openings of
the reactor lid were left open to allow air into the reactor and thus, no air sparging equipment was
used. The purpose of the cyanidation test was to determine the maximum extraction attainable by
cyanide leaching and also to serve as a reference/baseline for the thiocyanate tests.
All the samples were subject to the same test procedure and sample preparation as detailed in
Section 3.3 and Section 3.1.2 respectively, with the leaching conditions summarized in Table 11.
Table 11- Leaching conditions for cyanidation tests
Volume of Solution ~ 0.6 L
Pulp density 40 % solid
Solution pH 10.5 – 10.8
Agitation Speed 700 rpm
Atmospheric condition Air /no air sparging
Temperature 20 -25 oC
Residence Time 24 - 32 hr
NaCN Concentration 500 ppm
38
4.2.1 Cyanidation Test
The result of the 32-hr cyanidation test performed on the three Coffee samples is shown in Figure
11.
Figure 11 – Cyanidation of the three samples
The result presented in Figure 11 indicates that the ore is free milling and amenable to conventional
cyanidation process. All three ores showed a recovery of ~90% after 2 hr of leaching indicative
of fast leaching kinetics. Supremo 72142 B exhibited the fastest kinetics and the highest gold
extraction rate after 2 hr relative to 68151 A and 68151 B.
The slight fluctuations seen in the extraction curves can be associated with the continuous washing
of measuring probes and the method of sampling employed in this test (i.e. by stopping the reaction
completely) as well as the gradual rise after time 8 hr could also be related to evaporation due to
continuous stirring resulting in an increase of the gold concentration. Overall gold extractions are
39
good and were calculated based on the final solids and solution grades at the end of the tests
(Appendix C1).
A slight increase in gold extraction of less than 1 % for two of the ore types was observed when
the leaching time was extended from 24 hr to 32 hr, as shown in Table 12.
Table 12 – Baseline cyanidation test results with their reagent consumptions
Sample ID
&
Composition
Residue
Au Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured NaCN CaO
Cyanide Leaching Tests ([NaCN] 500 ppm)
68151 A (24 hr) 0.070 1.29 1.28 94.6 0.62 1.87
68151 A (32 hr) 0.060 1.20 1.28 95.0 0.68 1.54
68151 B (24 hr) 0.080 1.47 1.45 94.6 0.60 1.95
68151 B (32 hr) 0.080 1.44 1.45 94.4 0.73 1.19
72142 C (24 hr) 0.075 2.00 1.98 96.3 0.77 1.33
72142 C (32 hr) 0.060 2.02 1.98 97.0 0.54 1.92
The sodium cyanide consumptions (as seen in Table 12) for all the ore samples were low and
within a similar range (0.5-0.7) kg/t ore and also indicating that a lower sodium cyanide
concentration can be used to achieve similar gold extraction results. The lower consumptions could
be attributed to the high pH range in which the test was conducted – as there was little or no cyanide
loss in the form HCN gas. Furthermore, due to the low content of base metals and sulphides in
the samples (as shown from the chemical and mineralogical analyses), the CN- losses from
reactions with the gangue matter would be minimal.
40
The calculated and measured head grade showed a very good correlation in terms of accountability
(within +/- 5%). Overall, the cyanidation test conducted provided much information on reaction
and leaching kinetics.
4.3 Thiocyanate and Ferric Sulphate Variation
In this section, sodium thiocyanate and ferric sulphate concentrations were varied over a wide
range of sodium thiocyanate and ferric sulphate concentration ratios. This was aimed at attaining
a high gold extraction for the SCN/Fe(III) system. The Fe(III)/SCN ratio was important to cause
higher gold dissolution and also increase leaching potential [28]. Thus, sodium thiocyanate was
varied in the range of 0.005 M - 0.2 M at a ferric sulphate concentration of 0.1 M while ferric
sulphate was varied from 0.05 M - 0.5 M at a thiocyanate concentration of 0.1 M in an initial series
of experiments.
The optimization test for the thiocyanate and ferric sulphate were conducted as per the
experimental procedure outlined in Section 3.3 with test condition outlined in Table 13.
Table 13 – Leaching condition for the SCN and Fe(III) optimization
Volume of Solution 0.6 L
Solid grade ~ 2 g/t gold ore
Pulp density 40 % solid
Solution pH 1.5 -2.0
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Residence Time 24 hr
41
4.3.1 Effect of Thiocyanate Concentration on Gold Extraction
The thiocyanate optimization test was conducted by maintaining all the leaching parameters
outlined in Table 13 and varying the thiocyanate concentration.
Freshly prepared 0.1 M ferric sulphate solution acidified with sulphuric acid to a pH of ~1.4 was
used. The initial pH and ORP’s were recorded for the acidified ferric sulphate solution. These
were in the range of 1.3 -1.4 and 650 - 700 mV (Ag/AgCl) respectively. The final pH and ORP’s
after adding ore sample and calculated mass of thiocyanate were also in the range of 1.6 -1.7 and
500 -560 mV respectively, and these became the initial pH’s and ORP’s for the leaching test. The
increase in pH from ~1.4 to ~ 1.7 was due to the basicity of the ore whereas the drop in ORP was
due to the addition of thiocyanate solid to the leaching solution as well as the reaction chemistry
between thiocyanate and gold and possibly other minerals.
Figure 12 - Effect of the NaSCN concentration on the gold extraction, [Fe(III)] 0.10 M
42
It is apparent from Figure 12 that increasing the thiocyanate concentration from 0.005 M to 0.15
M increases the gold extraction at a faster rate up to 0.05 M SCN and a further increase in
thiocyanate concentration insignificantly affects gold extraction. The increase in gold extraction
when the concentration of sodium thiocyanate was increased from 0.05 M to 0.15 M was marginal.
Barbosa [16] explained that the dissolution of gold in thiocyanate solution is dependent on the
Fe(III)/SCN- ratio and as the thiocyanate concentration is increased at a constant Fe(III)
concentration, there is the formation of various complexes that changes the speciation of the
system and affects the gold dissolution rate. Broadhurst [26] mentioned that thiocyanate ions have
the tendency of complexing with gold more quickly than iron; forming gold thiocyanate complexes
that are more stable than iron complexes and therefore increasing gold dissolution with an increase
in thiocyanate concentration.
Therefore, it can be concluded from Figure 12 that increasing thiocyanate concentration increases
the gold dissolution up to a point (around 0.05 to 0.1 M in this test) beyond which a further increase
becomes insignificant. Thus, for the subsequent leaching tests, the thiocyanate concentration was
maintained at 0.1 M. These results substantiate the other results [16, 26, 28]. Gold extraction
results obtained from these tests were reproducible (as some of the runs were replicated) as seen
in Table 14.
The potentials measured during the leaching tests are reported in Figure 13. It is observed that an
increase in the thiocyanate concentration decreases the leaching potential. This is due to the
increasing complexation of ferric by thiocyanate. The leaching potential also decreases with
leaching time signifying the reduction of ferric to ferrous.
43
Figure 13 - Effect of the NaSCN concentration on the oxidation potential, [Fe(III)] 0.10 M
It is believed that the drop in the leaching potential can be attributed to the reduction of Fe(III) and
the various thiocyanate species and ferric complexes that are formed when thiocyanate is mixed
with ferric sulphate [16]. A typical example is the formation of the two intermediates species
((SCN)2 and (SCN)3-) and possibly iron complexes such as Fe(II) and FeSCN2+. The formation of
FeSCN2+ complex was confirmed from the change in colour of the leach solution from colourless
to red-blood when the thiocyanate solid was immediately added to the ferric solution during the
leaching tests. The change in colour of the leaching solution after the leaching test is shown in
Figure 14.
Figure 14: Leach solution with ferric addition
44
During the leaching process, it was observed that the initial solution potentials were higher at
higher Fe(III)
concentration and lower SCN- concentration (usually around 600 mV as seen in
Figure 13) and lower when the thiocyanate concentration was increased. The latter could be
mainly due to the Fe(III)/Fe(II) redox couple and also the complex formation between thiocyanate
and ferric ligand. Also, since SCN- has a stronger affinity to complex with Fe(III) than with Fe(II),
the potentials decreased with increasing free SCN- concentrations resulting in an increase in the
concentration of Fe(II). However, in all the tests, the oxidation potentials were high enough to
oxidize both gold and thiocyanate.
The above mentioned corroborates the work of Barbosa and Li [16, 28] that the addition of sodium
thiocyanate to ferric sulphate solution, drops the leaching potential and turns leaching solution to
deep red blood colour (as shown in Figure 13).
4.3.2 Effect of the Concentration of Fe(III) on Gold Extraction
The leaching condition of the thiocyanate test was replicated for the Fe(III) iron variation test. The
only notable difference was the variable ferric sulphate concentration. The thiocyanate
concentration was maintained at 0.1 M. Broadhurst [26] mentioned that maintaining a high ferric
concentration relative to thiocyanate concentration improves the kinetics. This is due to the
production of the intermediate species (SCN)2 and SCN3-. Also, high concentration of Fe (III) is
needed to keep thiocyanate stable increasing the concentration of free thiocyanates in solution;
thus high concentration range (0.05M – 0.5 M) of the ferric sulphate concentration was chosen to
evaluate its effect on gold extraction and oxidation potential.
45
Figure 15 - Effect of the Fe(III) concentration on the gold extraction, [SCN] 0.1 M
There was a discernible increase in gold dissolution with increase in Fe (III) iron concentration
from 0.05 M to 0.2 M and thereafter a very slight fall (relatively flat across the range) in gold
dissolution for Fe (III) concentration between 0.2 and 0.5 M as shown in Figure 15. One important
thing to note is that maintaining a high concentration of Fe(III) iron relative to thiocyanate
significantly improves the gold dissolution. The improved gold extraction are believed to be what
is termed the auto reduction process in which SCN- ion is oxidized by the spontaneous reduction
of Fe(III) to Fe(II) releasing the two intermediate species which oxidize and complex gold to cause
dissolution to occur. Broadhurst [26] also reported that high ferric iron concentration is favorable
for thiocyanate stability, increasing concentration of free thiocyanate. However, very high Fe(III)
has been reported to insignificantly affect the dissolution of gold. Li et al [28], found that above
a certain limit of ferric iron concentration (0.022 M), the gold dissolution in their test, was
controlled by surface reaction making gold dissolution reaction independent of ferric
concentration. Thus, the concentration of Fe (III) may at least be 0.1 M or 0.2 M and further
increase beyond this has little or no effect on gold dissolution. Ferric ion was fixed at 0.05 M and
0.1 M for subsequent tests conducted in this research. The slight downtrend of gold dissolution for
46
Fe(III) iron concentrations at 0.5 M experienced in this work is not well understood and would
need further investigation.
Figure 16 - Effect of the Fe(III) concentration on the oxidation potential, [SCN] 0.1 M.
The potentials were continuously monitored throughout the leaching test to ascertain the potential
range in which the AuSCN2- and AuSCN4
- form. However, in all the tests, the initial potentials
were sufficiently high to oxidize both gold and thiocyanate. Solution potentials were also observed
to decrease with time and Fe (III) concentration as seen in Figure 16. This was mainly due to the
reduction of Fe (III) to Fe (II) and formation of SCN- complexes with Fe (III) and gold.
4.3.3 Effect of Fe(III) and SCN Concentration on Thiocyanate Consumption
The effect of Fe(III) and NaSCN concentration on thiocyanate consumption are shown in Tables
14 and 15. From Table 14, it can be seen that increasing SCN- concentration increases the SCN-
consumption. On the other hand (Table 15), increase in Fe(III) ion concentration decreases the
consumption of thiocyanate. This is due to the stability of SCN- that Fe(III) provides as explained
above.
47
Table 14 - Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at
constant Fe(III) concentration
Sample ID &
Composition
Residue
Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured NaSCN H2SO4
SCN (0.005 M) 1.67 1.89 1.98 11.7 0.39 14.30
SCN (0.01 M) 0.28 1.86 1.98 84.77 0.59 16.31
SCN (0.02 M) 0.22 1.82 1.98 87.90 0.63 14.30
SCN (0.02 M) 0.23 1.81 1.98 87.55 0.65 12.27
SCN (0.05 M) 0.17 1.86 1.98 90.86 1.25 14.21
SCN (0.05 M) 0.19 1.79 1.98 89.58 1.74 14.17
SCN (0.1 M) 0.17 1.86 1.98 91.09 1.94 12.19
Table 15- Gold Extraction by thiocyanate leaching and reagent (SCN) consumption at
constant SCN concentration
Sample ID
&
Composition
Residue
Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured NaSCN H2SO4
Fe3+ (0.05 M) 0.208 1.85 1.98 88.8 1.92 9.34
Fe3+ (0.1 M) 0.206 1.86 1.98 88.9 1.85 11.03
Fe3+ (0.2 M) 0.177 1.99 1.98 91.1 1.45 9.68
Fe3+ (0.5 M) 0.206 2.06 1.98 90.0 1.46 9.41
48
4.4 Effect of the Concentration of Low and No Fe(III) Iron Addition on Gold Extraction
Fe(III) iron has been reported and shown in this research to be necessary to keep thiocyanate stable,
thus increasing leaching potential which subsequently increases the gold dissolution and
concentration of free thiocyanate. However, beyond a certain concentration point, gold dissolution
is marginally enhanced by further increase in Fe(III) iron concentration in solution (as seen in
Section 4.3.2). This motivated testing of the following two conditions:
1. A lower iron concentration of 0.01 M
2. Leaching with no iron addition
Thiocyanate solution was prepared with 0.1 M sulphuric acid to a pH of 1.5. The concentrations
of thiocyanate in these tests were varied over a wide range to determine the effect iron addition on
gold dissolution.
Test condition 1 had 0.01 M Fe(III) with varying SCN- concentration (0.005 M – 0.2 M) while
condition 2 had no Fe(III) added with varying thiocyanate concentration (0.1 M – 0.6 M). The
general leaching test condition for both tests is summarised in Table 16.
Table 16 – Test condition for Low and No iron addition
Volume of Solution 0.6 L
Solid grade ~ 2 g/t gold ore
Pulp density 40 % solid
Solution pH 1.5 -1.9
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Residence Time 24 hrs
49
4.4.1 Effect of Low Iron Concentration
Figure 17 shows that; gold dissolution increases with increasing thiocyanate concentration at low
Fe(III) iron concentration (0.01 M). The increase in gold extraction with change in thiocyanate
concentration was sharp from a thiocyanate concentration of 0.005 M to 0. 1 M and a gradual
increase in gold extraction was seen thereafter. However, the gold extraction did not achieve a
plateau which signifies that a further increase in thiocyanate concentration could possibly increase
gold extraction.
Figure 17 - Effect of thiocyanate concentration on gold extraction [Fe(III)] 0.01 M
Comparatively, from Figure 18, the gold dissolution when the ferric concentration was increased
tenfold; that is from 0.01 M to 0.1 M showed a significant difference in gold extraction. The
increase in gold extraction is huge especially for low values of SCN- as seen in Figure 18. Gold
extraction with 0.1 M Fe(III) achieved a plateau at a lower SCN- concentration (0.05 M) whereas
Fe(III) at 0.01 M did not achieve any plateau. This confirms the importance of high Fe(III) to the
leach solution, however very high Fe(III) iron above certain concentration point (> 0.2 M) plays
insignificant role as seen in Section 4.3.2. It can be concluded from Figure 18 that; high gold
50
extractions can be obtained with either high Fe (III) (~ 0.1 M) and low thiocyanate concentration
( ~0.05 M) or low Fe (III) (~0.02 M or 0.05 M) and high SCN- concentration (>0.1 M).
Figure 18 - Effect of Fe(III) (0.01 M and 0.1 M) concentration on gold SCN 0.1 M
4.4.2 Effect of No Fe(III) Iron Addition on Gold Dissolution
In order to operate the process economically, the study was extended to determine the effects of
reducing reagent consumption for gold dissolution in the thiocyanate system. One such test was
studying the effect of gold dissolution in thiocyanate systems without addition of Fe(III). The main
aim of this was to compare this to the traditional cyanidation process where air is normally used
as the main oxidant.
The test was conducted on the 72142 B composite material. Thiocyanate solution acidified with
sulphuric acid to a pH of about 1.5 was used. One important thing to note here is that the leach
solution pH ranged from 1.5 -1.9; meaning there was no sequential addition of acid to maintain
51
the pH of the leaching solution to a particular pH value. The leaching test was conducted according
to the test procedure outlined in Section 3.3.1 and conditions in Table 16.
Figure 19 - Effect of the thiocyanate concentration on gold (No Iron addition)
Surprisingly, gold dissolution increased with increasing thiocyanate concentration without the
addition of Fe (III) as shown in Figure 19. A thiocyanate concentration of 0.1 M gave 81.5 % gold
extraction. When this was increased 6-fold with the same leaching condition; i.e. 0.6 M
thiocyanate, the gold extraction increased to 90.9 %.
Even though ferric was not added to the leach solution, it was visually observed that ferric was
generated in situ (leached from the ore sample as shown in Figure 20 b) mainly because the leached
solution turned red immediately when thiocyanate was added to the leach solution. The dissolution
of iron was due to the acidification of the leached solution which possibly extracted some soluble
iron into solution. The color intensity of the solution increased (red blood colour) within the first
52
30 minutes signifying the formation of the FeSCN2+ complex. A comparison of the leach solution
with and without iron is shown in Figure 20.
a. Leach solution with Ferric addition b. Leach solution without ferric addition
Figure 20: Leach solution with and without ferric
Though concentrations of dissolved oxygen were not measured during the test (without Fe
addition), the dissolution of gold may be attributed to the presence of dissolved oxygen acting as
oxidant thus possibly driving the iron in the ore to proceed through the auto reduction process
(Fe(III) to Fe(II)). Moreover, the low pH (1.5 - 2) of the leached solution could possibly have
dissolved part of the 4.1 % (from mineralogical analysis) iron from the ore. The dissolution of iron
into the leach solution could have caused the auto reduction process to proceed; producing the two
intermediate species which cause gold dissolution.
53
Figure 21 - Effect of the thiocyanate concentration on leaching potential
A quick look at the solution potentials in these tests showed a decrease with time and plateaued
after time 8 hr as seen in Figure 21. The reduction in the leaching potential signifies the auto
reduction of Fe(III) to Fe(II). The auto reduction process of Fe(III) to Fe(II), producing the two
intermediate species, is known as the mechanism that causes gold to dissolve in the thiocyanate
system. This supports the assertion that the gold extraction observed in this test could be due to
the oxidative dissolution of iron leached from the ore.
Moreover, higher concentrations of thiocyanate were seen to produce lower leaching potentials.
The trend seen here is similar to that in the Fe(III)/SCN system. Barbosa postulated that, “In the
runs with high free thiocyanate concentrations (0.05-0.5) M, the overall autoreduction process is
much faster in the initial moments and then slows down drastically, due to the marked Fe2+ and
FeNCS+ build up in solution. For lower free thiocyanate concentrations (<0.05 M), on the other
hand, the initial autoreduction rate is lower and, as the Fe2+ and FeNCS+ production is also
slower, the overall rate decrease is more gradual’’. Solution potentials were found to be in the
acceptable range (550 – 600 mV) for gold dissolution.
54
Table 17 compares the extraction of gold and NaSCN consumption among the three test conditions
outlined. The thiocyanate concentration used in these tests is 0.1 M.
Table 17 Comparison between the three conditions with 0.1 M SCN
Surprisingly, gold extraction obtained with 0.01 M Fe (III) is somewhat lower than that with no
Fe(III) as seen in Table 17. This could be due to unstable/inactive complexes formed between the
thiocyanate and the added Fe (III). Moreover, because a smaller amount of Fe(III) was used, the
slowness of the Fe (III) to auto reduce itself to Fe(II) causing the production of the two thiocyanate
intermediate species which are known to act as oxidant and complexant to cause the gold
dissolution [16].
It is not surprising that 0.1 M Fe(III) achieved an 88.9 % gold extraction. As previously mentioned
high Fe (III) is needed to keep thiocyanate stable. Comparing this to the low iron concentration
(0.01 M), the low gold extractions can be attributed to the Fe(III) concentration being too low to
keep thiocyanate stable exacerbating the formation of other complexes that affected the
concentration of thiocyanate. The increase seen in the test without Fe(III)/oxidant would need
further investigation. However, the red blood colour of the leaching solution (which is lighter than
tests with added Fe(III) iron) suggests that iron was leached from the ore sample which could serve
as the oxidant for the leaching process. Moreover, since the test was partly opened to allow air
into the slurry, the extraction could be attributed to the presence of dissolved oxygen which could
have aided in the production of the two intermediate species needed for gold dissolution in the
Test Condition
Residual
Gold
(g/t)
Gold
Extraction
(%)
Calculated
Head
Grade (g/t)
Measured
Head Grade
(g/t)
NaSCN
Consumption
(Kg/t)
Fe (0.1 M) 0.206 88.9 1.86 1.98 1.85
Fe (0.01 M) 0.391 79.9 1.95 1.98 1.13
Fe (0.00 M) 0.357 81.5 1.93 1.98 0.78
55
thiocyanate system by oxidizing Fe dissolved from the ore to Fe(III). Thus, for an optimal
performance of the acidic thiocyanate leach, Fe(III) concentration should be either 0.1 M or
preferably 0.2 M at 0.15 M or 0.1 M SCN-. Lower Fe(III) concentration like 0.05 M would need
higher amount of SCN 0.2 M.
Table 18 - Thiocyanate consumption for the no iron addition varied SCN concentration
Description SCN (0.1 M) SCN (0.2 M) SCN (0.6 M)
NaSCN Consumption
(Kg/t) 0.78 1.87 2.84
The thiocyanate consumption increased with increasing thiocyanate concentration as seen in Table
18. The lowest consumption occurred at 0.1 M and this increased when SCN concentration was
increased to 0.6 M.
56
4.5 The pH Variation Test
The traditional cyanidation process for most ores makes use of blower/compressed air/atmospheric
air as the main source of oxidizing agent. This process was tested with the leaching of gold in
thiocyanate solution.
The results obtained from Section 4.4.2 motivated the leaching of gold at varying pH in thiocyanate
solution without the addition of Fe(III) to be conducted. This is due to the red colour formation of
the leach solution observed in Section 4.4.2 which signified the presence of FeSCN2+ signifying
that iron was leached from the ore.
Thus, the main objective was to leach gold ore at low pH in thiocyanate solution in order to leach
iron from the ore. Moreover, since the solubility of iron in solution increases with reducing pH,
the pH was varied with the aim of ascertaining the pH which will cause sufficient iron in solution
for highest gold dissolution. Thus, the gold ore was leached with thiocyanate only; without the
addition of Fe(III) iron.
Prior to this, iron leaching from the ore using sulphuric acid was also conducted at varying pH to
determine the amount of iron that could be leached from the ore. The 72142 composite gold ore
which showed the highest percentage of iron in the ore (4.1%) was leached with sulphuric acid at
varying pH. The main aim was to determine the amount of iron that could be leached from the ore
at a varying pH. This if successful would mean that iron would be self-generated from the ore for
the leaching process. The parameters of the iron leaching test are summarised in Table 19 with
four variables pH, leaching time, oxidation potential and iron in concentration considered and
measured. The pH was kept constant and varied at 1.0, 1.5, 2.0, 2.5. The effect of pH on iron
concentration for times 10, 60, 120 and 180 minutes is shown in Figure 22 (and in Appendix D).
ORP’S were also monitored and recorded at these times.
57
Table 19 – Leaching condition for pH variation test
Volume of Solution 0.6 L
Pulp density 40 % solid
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
(a) (b)
58
Figure 22– Effect of pH on Iron concentration at (a) 10 mins (b) 60 mins (c) 120 mins and
(d) 180 mins
From Figure 22, the iron concentration increased with time and decreased with pH. The highest
iron concentration of 3135 ppm was recorded at time 180 mins and a pH of 1. From the mass of
solid used (~200 g), this signifies approximately 11 % of iron leached from the 4.1% iron reported
from the ICP analysis. Above pH of 2, the concentration of iron decreased approaching zero which
is due to Fe(III) iron hydrolysis and/or lack of extraction.
4.5.1 Effect of pH on Gold dissolution
The effect of pH on gold dissolution in 0.025 M and 0.2 M sodium thiocyanate solution is shown
in Figure 23. The results indicate an increase in gold extraction with increasing thiocyanate
concentration. Gold extractions recorded were low at lower thiocyanate concentrations. The gold
extractions at 0.025 M SCN were low and increased slowly with increasing pH. However, a
thiocyanate concentration of 0.2 M showed a higher gold extraction relative to a concentration of
(c) (d)
59
0.025 M. The highest gold extraction recorded for 0.2 M thiocyanate concentration was 87 % at
a pH of 1.5 with the lowest being 72.6% at a pH of 2.5.
Figure 23 - Effect of pH and the SCN concentration (0.025 M and 0.2 M) on gold extraction
However, a slightly lower gold extraction was recorded at pH of 1 which is supposed to have the
highest dissolved iron in solution. The reason for the low gold extractions recorded at pH of 1
even though it has the highest dissolved iron (as seen in Section 4.4 above) has been discussed.
Barbosa mentioned that in a very acidic solution, i.e. pH values below 1, gold dissolution is
expected to decrease due to the reduction in the activity of SCN-, which is caused by the
protonation of thiocyanate to thiocyanic acid (pKa for HSCN = 0.9). The slightly lower gold
extraction obtained at pH of 1 attest to this point. Also, another drop-in gold extraction was seen
at pH of 2.5. At pH of 2.5, iron in solution is expected to decrease due to hydrolyzation of the iron
which could probably slow down the autoreduction process (due to insufficient iron in solution)
causing lower gold extraction.
60
4.5.2 Effect of pH on the Leaching Potential
The leach solution potentials recorded for tests at 0.025 M and 0.2 M are shown in Figure 24 and
Figure 25 respectively. The solution potentials increased to a large extent with time, a reverse of
the observation SCN/Fe(III) systems. The leaching oxidation potentials measured by Ag/AgCl
electrode ranged from 450 mV to 500 mV and from 390 mV to 470 mV for tests with 0.025 M and
0.2 M thiocyanate, respectively. This also confirms that an increase in thiocyanate concentration
decreases the solution leaching potential.
For the 0.025 M thiocyanate, the initial potential at pH of 1 was low, however, it increased with
time. The XRD analysis on the gold ore used in this test showed high amount of iron in the form
of ferrous. This increase could be associated with the oxidation dissolution of iron from the ore to
ferric in the solution. Although the solution potentials were higher and fell within the range needed
for gold dissolution, the dissolution rates were still low for this concentration (0.025M).
Figure 24 - Effect of pH on the leaching oxidation potential @ SCN 0.025 M
61
At 0.2 M thiocyanate, though the potentials were relatively lower as compared to the 0.025 M,
they were still within the range for gold dissolution in thiocyanate solution. As seen in the lower
concentration 0.025 M SCN, initial solution potential at pH of 1 was lower. A similar pattern was
seen with the 0.2 M SCN, however, the potentials in this test did not take any particular trend
probably due to the sequential addition of sulphuric acid to keep the pH constant. The addition of
sulphuric acid to the leach solution was seen to increase the leaching potentials of the leach
solution.
Figure 25 - Effect of pH on leaching oxidation potential (ORP) @ SCN 0.2 M
62
4.6 Potassium Iodide Leaching Test
In the quest to increase the dissolution rate of gold in thiocyanate system, the effect of small
amounts of additives potassium iodide were explored. Potassium iodide (KI) is a halogen salt and
an alternate lixiviant to cyanide.
Barbosa postulated that adding small amounts of potassium iodide to the SCN/Fe(III) complex
would tend to stabilize the thiocyanate in solution and improve leaching performance. KI was
therefore used as an additive to improve gold leaching performance.
To ascertain the effects of potassium iodide on the gold dissolution in SCN systems, it was
expedient to know how it works separately, thus the leaching tests were conducted in 3 different
phases at SCN concentration of 0.1 M and Fe (III) concentration of 0.05 M;
(1) A baseline test with 0.1 M KI without SCN and Fe (III)
(2) Addition of potassium iodide to;
a) SCN, designated as SCN/KI
b) SCN and Fe (III) solution designated as Fe (III)/SCN/ KI
The leaching tests were carried out at KI concentration of 0.1 M for the phase 1 (leaching with
potassium iodide only). The test procedure same as described in Section 3.3. KI was varied over
a wide range in the subsequent tests with SCN only and SCN/Fe(III) mixture, to know its effect
on gold dissolution. The pH ranged from 1.5 - 1.9 and was continuously maintained by the addition
of sulphuric acid. The residence time was 24 hr and solution samples (of volume 7 mL) were
taken at selected time intervals (2, 4, 8 and 24) hrs and analyzed for gold by organic method by
AAS.
The leaching condition for all the tests is summarized in Table 20 for the phase 1 and phase 2 (i)
and 2(ii).
63
Table 20 - Leaching conditions for potassium iodide tests
Volume of Solution 0.5 L
Pulp density 40 % solid
Solution pH 10.5 – 10.8
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Residence Time 24 hr
NaSCN Concentration 0.1 M
4.6.1 Gold Leaching with Potassium Iodide
Leaching with 0.1 M KI was conducted to serve as a reference/baseline for the SCN- tests. The
reaction chemistry of gold iodide system has been discussed in the literature review. Gold
dissolution requires the use of an oxidant and a complexant. However, in this test, gold leaching
experiment was conducted with KI only without the addition of Iodine (oxidant). However, two
of the openings on the lid of the leach vessel was left opened to allow some air into the slurry. A
high concentration of KI (0.1 M) was chosen. This was necessary to know the operating window
in which KI would give comparable gold dissolution rate to that in Fe(III)/SCN mixture.
Table 21 – Results of the gold leaching with KI only
Sample ID
&
Composition
Residue Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured H2SO4
Iodide (0.1 M)
Only 0.1075 1.94 1.98 94.46 13.23
64
A 94.46 % gold extraction was obtained with the 0.1 M KI obtained under the conditions studied
(as seen Table 21). Although, iodine was not added as oxidant in this test, the high dissolution
rate signified the production of I3- which is both an oxidant and complexant for gold dissolution
[10]. Thus, for the subsequent leaching process for phase 2 i.e. SCN/KI and Fe(III)/SCN/KI, the
KI was varied in a lower range of (0.001 M – 0.1 M) and (0.001 M – 0.02) respectively
4.6.2 Gold Leaching by Thiocyanate with Addition of Iodide
The experimental results with 0.1 M SCN concentration achieved a 79.9% gold extraction. Six
experiments based on various KI concentrations were conducted. The KI concentration used were
0.001, 0.002, 0.005, 0.01, 0.05 and 0.1 M. SCN concentration was constant at 0.1 M with a
residence time for each test of 24 hrs. The result of the gold extraction for phase 2(i) is illustrated
in Figure 26 and its corresponding potentials in Figure 27.
Figure 26 - Effect of KI concentration on gold extraction, SCN 0.15 M
97% Au extraction attained for NaCN at 32 hr
65
From Figure 26, the addition of KI to SCN solution improved the gold extractions. The gold
dissolution rate obtained at a given iodide concentration above 0.01 M were higher than those
obtained in the Fe/SCN mixture. Gold extraction is seen to increase gradually with increasing KI
concentration and plateaued at a KI concentration of 0.05 M. This could possibly be the optimum
gold dissolution rate for this set of conditions. The addition of 0.1 M KI to the 0.1 M SCN yielded
the same gold extraction of 94 %, similar to that with 0.05 M KI. The gold extraction with the 0.1
M KI and 0.1 M SCN can be said to be an iodide leaching other than a thiocyanate leaching since
the same gold extraction was recorded when iodide was used alone.
Figure 27 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M
The leaching potentials as shown in Figure 27 in these tests with the addition of KI ions were lower
compared to Fe(III)/SCN tests conducted in Section 4.3. This making the formation of the most
stable intermediate species; SCN3- (which causes gold dissolution to occur) thermodynamically
not feasible. However, iodide species such as l2 and l3-, formed from the oxidation of iodide by
ferric are reported to be very stable species which complex with gold to form stable Aul2- complex
and thus causing high gold dissolution rate in this system.
66
4.6.3 Gold Leaching by Iron(III)-Thiocyanate with Addition of Potassium Iodide
The effect of KI to the ferric-thiocyanate mixture on gold dissolution rate is presented in Figure
28 and solution potential in Figure 29. The leaching tests were conducted with the condition stated
in Table 20. The concentration of SCN and Fe(III) used in this section are 0.1 M and 0.05 M,
respectively. KI concentration was varied at 0.001 M, 0.002 M, 0.005 M, 0.01 M and 0.02 M.
Figure 28 - Effect of the KI concentration on gold extraction, SCN 0.15 M
When the three reagents SCN/KI/Fe(III) were added together, the effect was significant (showing
an increase in gold dissolution with small amount of KI) as compared with the previous tests using
SCN/KI without ferric iron, suggesting a synergistic effect. The synergistic effect of iodide on the
gold dissolution rate can be attributed to the formation of relatively stable mixed iodide-
thiocyanate complexes which are I2SCN- and ISCN2-. These are the main intermediates species
formed and are known to participate in the gold dissolution process, as in the reactions 32 and 33
below [39] :
97% Au extraction attained for NaCN at 32 hr
67
Reaction 31 I- + 2SCN- = lSCN2- + 2e
Reaction 32 2I- + SCN- = l2SCN- + 2e
Gold extraction results obtained for the three mixed reagents were much higher than those obtained
in the KI/SCN without Fe(III) and the SCN/Fe(III) mixture. Gold extraction of 92.7% was obtained
at iodide concentration of 0.001 M, Fe(III) concentration of 0.05 M and SCN concentration of 0.1
M. This increased gradually with increasing iodide concentration up to about 94 % at iodide
concentration of 0.02 M.
Figure 29 - Effect of the KI concentration on the oxidation potential, SCN 0.15 M
68
The leaching potential of the KI/SCN/Fe(III) dropped drastically from 555 mV to ~ 480 mV. The
drastic drop is associated to the presence of ferric being reduced to ferrous. Though the interaction
between ferric and iodide is known to be slow, the reduction of ferric to ferrous is also known to
catalyse the formation of the two intermediates species produced during thiocyanate oxidation
(namely, SCN2 and SCN3-) which aid again in the dissolution rate of gold and probably causing
the higher reduction in the leaching potential [27] as seen in Figure 29. The reduction in the
leaching potential followed basically a similar pattern already described for KI/SCN in Section
4.6.3.
Comparatively, the potentials for the KI/SCN pair (in Section 4.6.3) were generally lower than
that of the KI/SCN/Fe(III) system. One interesting thing to note is the increase in the potentials in
both cases with leaching time. While there was a decrease in the potential in the ferric thiocyanate
solutions due to the autoreduction process, the reverse took place in the iodide system. The initial
autoreduction of iron in both cases were initially faster and decreased sharply and plateaued after
time 4 hr. A slight increase was seen after time 8 hr as shown in Figure 27 and Figure 29. The
potential rise as explained by Barbosa [16] is due to the production of the species l2 and l3-, which
are generated by the oxidation of I- by Fe(III). The behaviour observed in the experiments reported
in this section is similar to that reported both by Qi and Hiskey [10].
Moreover, the leaching solution (slurry) in the iodide tests did not produce a red blood colour as
seen in other leaching tests with and without the addition of Fe(III). This implies the absence or
slow formation of FeSCN2+ complex. A rather close to light orange colour was observed.
Although in other tests which were without the addition of ferric sulphate, it was evident that iron
was being leached from the ore forming FeSCN2+ complex and making the solution turn red the
reverse was seen in these tests. This might be due to the low concentration of iron in the form of
Fe(III) present in the solution and also the high affinity of iodide to complex with SCN more
quickly than with Fe(III) and Fe(II); making SCN unavailable for iron complexation.
69
Figure 30 - Effect of the KI concentration SCN concentration: 0.15
A comparison of the two results of iodide leaching gives a clear distinction between the Au
dissolution rate. A linear gold dissolution rate was seen with the SCN/KI/Fe(III) mixture with
increasing thiocyanate concentration (as shown in Figure 30). This means that increasing iodide
concentration from 0.002 M in the KI/SCN/Fe(III) mixture increased the gold dissolution
gradually (from 85% to 90%). This figure also indicates the need for ferric to be used in the
thiocyanate leaching process with respect to the intermediate species produced.
70
4.6.4 Reagent Consumption
Table 22 Reagent composition and the result of gold extraction and sodium thiocyanate
consumption on KI/SCN mixture
Sample ID
&
Composition
Residue
Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption (kg/t)
(g/t) Calculated Measured Measured NaSCN H2SO4
KI (0.1 M) Only 0.107 1.94 1.98 94.46
13.23
KI 0.1 M + 0.05 M SCN 0.114 1.95 1.98 94.15 3.69 12.04
KI (0.1 M) + 0.1 M SCN 0.108 1.91 1.98 94.36 8.66 15.39
0.001 KI + SCN (0.1 M) 0.500 2.23 1.98 77.61 2.55 27.32
0.002 KI + SCN (0.1 M) 0.285 1.96 1.98 85.45 5.46 24.17
0.005 KI + SCN (0.1 M) 0.241 1.92 1.98 87.43 5.89 25.15
0.01 KI + SCN (0.1 M) 0.181 1.93 1.98 90.62 4.88 29.77
71
Table 23 Reagent composition and the result of gold extraction and sodium thiocyanate
consumption on KI/Fe(III)/SCN mixture
Sample ID &
Composition
Residue
Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured NaSCN H2SO4
0.001 KI + SCN +
Fe3+ 0.141 1.93 1.98 92.71 2.83 22.09
0.002 KI + SCN +
Fe3+ 0.136 1.78 1.98 92.35 3.63 25.80
0.005 KI + SCN +
Fe3+ 0.125 1.88 1.98 93.34 3.74 25.15
0.01 KI + SCN + Fe3+ 0.121 2.06 1.98 94.11 3.82 28.16
0.02 KI + SCN + Fe3+ 0.115 2.05 1.98 94.40 4.60 28.84
In these two tests (SCN/KI and SCN/KI/Fe(III)), the consumption of NaSCN and H2SO4 were
almost similar for both test results as shown in Table 22 and Table 23. The presence of ferric in
the test with KI/SCN/Fe(III) tests did not affect the consumption of SCN. However, increasing
the KI concentration to 0.1 M increased the SCN consumption. The average consumption of
thiocyanate was about 6 times higher than cyanide. The average NaCN consumption was 0.5 kg/t,
while NaSCN consumption was 3 kg/t for the SCN/KI and 3.5 kg/t for the SCN/KI/Fe(III).
However, SCN consumptions in these tests were higher than those seen in the SCN/Fe(III) mixture.
Thiocyanate consumption could be due to the formation of various complexes other than the gold
complex.
72
4.7 Hydrogen Peroxide Test
Barbosa[16] postulated that the presence of Hydrogen Peroxide increases the leaching potential
and maintains it within the limits for the formation of the Au-SCN complexes. Therefore,
hydrogen peroxide was employed as an alternative oxidant to ferric sulphate to increase the
leaching potential and also serve as an alternative oxidant to ferric due to the constraint in the use
of iron. This is because iron hydrolyzes at higher pH’s (> 2.5) and thus will require robust
equipment in the acidic regime.
Gold leaching tests were conducted in the acidic (1.5-1.8) medium to ascertain its effectiveness.
Freshly prepared sodium thiocyanate solutions acidified with 0.1 M H2SO4 to pH of 1.5 was used
for all the tests. Sodium hydroxide was also used to adjust pH in the basic regime.
In order to determine the effect of hydrogen peroxide on the gold dissolution rate, the gold leaching
tests were carried out at a thiocyanate concentration of 0.1 M with varying amounts of hydrogen
peroxide ( 0, 4, 8, 10 and 14 g/L). All the experiments were conducted under atmospheric
condition (two of the holes on the lid were open to atmosphere). Gold leaching tests were
conducted on the Composite 72142 material. Sample was prepared as stated in Section 3.1.2. The
test condition is summarized in Table 24.
Table 24 - Leaching conditions of peroxide tests
Volume of Solution 0.6 L
Solid ~ 2 g/t
Pulp density 40 % solid
Solution pH 1.5 -2
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Residence Time 24 hr
NaSCN Concentration 0.1 M
H2O2 Concentration 0, 4, 8, 10, 14 (g/L)
73
4.7.1 Gold Leaching with SCN Only
The test conducted without peroxide was to serve as reference point for the H2O2/SCN system.
The SCN concentration used was 0.1 M. Result is shown in Table 25.
Table 22 – Leaching test result for thiocyanate leaching only at 0.1 M NaSCN
4.7.2 Effect of Hydrogen Peroxide concentration on Gold Dissolution
Figure 31 - Effect of the H2O2 concentration on gold extraction, SCN 0.15 M
Test Condition
Residual
Gold
(g/t)
Gold
Extraction
(%)
Calculated
Head
Grade (g/t)
Measured
Head Grade
(g/t)
NaSCN
Consumption
(Kg/t)
SCN Only 0.357 81.5 1.93 1.98 0.78
74
From Figure 31, it can be seen that gold dissolution rate increased appreciably with an increase in
peroxide concentration. The initial gold extraction without peroxide was 81.5% (as seen in Table
25). This increased to 90% with 4 g/L hydrogen peroxide and later peaked to 94% with 10 g/L
hydrogen peroxide. A fall in gold dissolution was observed when the hydrogen peroxide
concentration was increased further to 14 g/L.
The reason for the drop is not well understood, however, Barbosa [16] in their paper on using
hydrogen peroxide to increase the leaching potential by re-oxidizing ferrous back to ferric to cause
the production of the intermediate species SCN2 and SCN3- and also maintain the leaching
potential within the region of the formation of the gold thiocyanate complexes mentioned that new
intermediates species such as HOSCN, OSCN, HOSCN-, OSCN2-, HO2SCN, O2SCN-, are formed
when concentration of H2O2 is varied. These complexes are less active and may decrease the
dissolution of gold. Wilson also [35] reported HOSCN to be the main intermediate species formed
from the complexation of SCN and H2O2 and mentioned that the formation of this is dependent on
pH of the solution. Thus, the 14 g/L H2O2 used was too high and could possibly have a stronger
effect on the Au/SCN systems, producing more inactive intermediate species which possibly
slowed down the leaching rate (at 14g/L H2O2). On a more practical note, the leaching of gold in
thiocyanate solution with the addition of peroxide can be done sequentially aiming at keeping the
leaching potential within a limit where the two gold thiocyanate complexes form. In that way, a
lower amount of hydrogen peroxide may be used. More work should be done to expediate this
process.
4.7.3 Effect of Peroxide Concentration on Leaching Potential
The leaching potentials were continuously monitored and measured with Ag/AgCl electrode.
Higher concentrations of hydrogen peroxide recorded higher leaching potentials. From Figure 32,
it can be seen that the highest leaching potential was recorded for hydrogen peroxide concentration
of 14 g/L. This potential increased gradually with time. The potential recorded for the 14g/L was
higher than the other potentials and the potential region (500 mV-650mV) for the formation of the
two gold thiocyanate complexes. At such a high potential, the dominant species becomes HOSCN
75
which might have inhibited the dissolution process. This could have contributed to the lower gold
dissolution recorded for this test. The potentials of the other tests with H2O2 concentrations below
14 g/L decreased slowly with time. However, they were found to be within the formation region
(500 mV - 650 mV) of gold dissolution.
Figure 32 - Effect of the H2O2 concentration on the potential readings, SCN concentration:
0.10 M
4.7.4 Effect of Hydrogen Peroxide Concentration on NaSCN Consumption
The residual sodium thiocyanate was analysed after the gold leaching tests. The leaching solutions
were orange signifying the presence of little amount of Fe(III). This colour was enhanced with the
addition of few drops of 1 M ferric sulphate solution which served as an indicator for the sodium
thiocyanate titration. The sodium thiocyanate was determined argentimetrically with 2-3 drops of
HNO3 acid to increase intensity of the solution colour and titrated with 0.1 M AgNO3 solution. A
colour change from deep red to white precipitate was observed. Thiocyanate consumption was
calculated according to equation in Appendix A3.
76
The consumption of SCN increased with an increase in hydrogen peroxide concentration evident
in Figure 33. The consumption of SCN from the test without peroxide was pretty low, at 0.78 Kg/t.
The decrease in the peroxide consumption for the 14g/L H2O2 could possibly be the formation of
inactive complexes as mentioned above. The experimental results discussed in this section point
towards the need for further investigation.
Figure 33 - Effect of the H2O2 concentration on thiocyanate consumption, SCN 0.10 M
77
4.8 Kinetic Leach Test
A kinetic test was conducted to determine the leaching kinetics of the SCN/Fe(III) system. Gold
leaching experiments were conducted at selected times 1, 2, 4, 8, 12 and 24 hr in a fresh
SCN/Fe(III) solution. The leaching test was completely stopped after each of the times (1, 2, 4, 8,
12 and 24 hr) and analysed for solution and solid (leached residue) grade by AAS and fire assay,
respectively. The concentration of NaSCN and Fe(III) used were 0.15 M and 0.1 M respectively.
Test was conducted on the 72142 B composite sample. The leaching condition is summarized in
Table 26.
Table 23 – Leaching condition for kinetics test
Volume of Solution 0.6 L
Pulp density 40 % solid
Solution pH 1.5-1.8
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Fe(III) Concentration 0.1 M
NaSCN Concentration 0.15 M
4.8.1 Effect of Leaching Time on Gold Dissolution
Results of the leaching kinetics is plotted in Figure 34. In the kinetics plot, there was a sharp
increase in gold dissolution for the first 2 hr leading to a high gold extraction of close to 90%. The
kinetics tends to decrease after 2 hr and the plot starts to plateau toward 24 hr with minimum
increase in gold dissolution (91.6% at 24hr). The kinetics exhibited by the interrupted leaching
test indicated that sodium thiocyanate has the ability to dissolve gold quickly and requires just
about 2 hr – 4 hr to dissolve most of the available gold. This agrees with work done by Li et al
[28].
78
Figure 34 - Kinetics of Gold dissolution in thiocyanate solution, (SCN 0.15 M and Fe(III)
0.1 M)
Nicol et al (1984) said though the rate of gold leaching from ores by cyanidation was a complex
kinetic problem encompassing chemical, particle size distribution, mass-transport, and
mineralogical factors, it could be expressed by a relatively simple expression as given by
Equation 1:
−𝑑[𝐴𝑢]𝑝,𝑡
𝑑𝑡= 𝑘𝑝([𝐴𝑢]𝑝,𝑡 – [𝐴𝑢]𝑝,𝑒)
2
where
[Au]p,t is the concentration of gold in the ore at time t
[Au]p,e is the corresponding quantity at infinite time of leaching (the minimum
achievable residue grade)
kp is a rate constant.
The kinetic data obtained for this test was fitted to the model.
79
𝐴𝑢𝑡 = [𝐴𝑢0 + 𝑘𝑝𝑡 (𝐴𝑢𝑜 − 𝐴𝑢𝑝𝑒)]
[𝑘𝑝𝑡(𝐴𝑢0 − 𝐴𝑢𝑝𝑒) + 1]
Where Auo is the head grade of the ore (prior to leaching)
The curve was calculated from the integrated form of Equation 1, and best-fit values for the
parameters Aupe and kp were derived from a non-linear least squares treatment of the data as
provided in Appendix G. The agreement between the raw tails data (dots) and the predicted tails
data (curve) as expressed in Figure 35 can be seen to be good, giving kp and Aupe to be 72.1 hr-1
and 0.17 g/t, respectively. The profile shows that 89.7 % of the contained gold was leached within
1-2 hr and slowed down with little or no further leaching (no decrease in tails gold grade) between
4 and 24 hr.
Figure 35 – Kinetic leaching model fitting
80
4.8.2 Effect of Leaching Time on Thiocyanate Consumption
The thiocyanate consumption increased gradually with increased in leaching time and slowed
down after 12 hr as seen in Figure 36. The thiocyanate consumption increased from 0.71 kg/t at
time 1 hr to 1.58 kg/t at time 24 hr. The increase in thiocyanate consumption at time 24 hr is about
twice that at time 1 hr.
Figure 36 - Effect of leaching time on thiocyanate consumption
81
4.9 Mixture of Reagents
Finally, a combination of oxidants and reagents were tested. Different oxidants, complexants and
additives were combined in different proportions to determine their effects on gold dissolution.
Oxygen, hydrogen peroxide, lead nitrate and ferric sulphate were used together with thiocyanate.
The gold leaching experiments were conducted in the acidic regime to determine the behaviour of
thiocyanate in those conditions. Oxygen was sparged into the slurry using an oxygen sparger. This
was inserted into the slurry throughout the leaching test.
Pb(NO3)2 is known to increase gold dissolution in CN- systems by releasing its divalent cations
Pb2+ to speed up the leaching reaction. It was added in very small quantity to determine the effect
on the gold dissolution rate.
All other leaching parameters were maintained in the concentration outlined in Table 27. The tests
were conducted on the 72142 Composite oxide material. The gold extractions and reagent
consumptions recorded for these series of tests are shown in Table 28.
Table 24 – Leaching condition for mixture of reagents
Volume of Solution 0.6 L
Pulp density 40 % solid
Solution pH 1.5-1.8
Agitation Speed 700 rpm
Atmospheric condition Air
Temperature 20 -25 oC
Fe(III) Concentration 0.2 M & 0.5 M
NaSCN Concentration 0.05 M & 0.1 M
PbNO3 50 g/t
Oxygen 3 LPM
H2O2 3.5 g/L
82
Table 25 – Results of gold leaching experiments conducted under different leaching
conditions
Sample ID
&
Composition
Residue
Au
Grade Head Au Grade (g/t)
Au
Extraction
(%)
Reagent
Consumption
(kg/t)
(g/t) Calculated Measured Measured NaSCN H2SO4
Fe3+ 0.2 M + SCN 0.05 M + O2 0.21 1.94 1.98 89.24 1.88 18.60
Fe3+ 0.2 M + SCN 0.05 M + O2 +
H2O2 0.19 1.95 1.98 90.27 3.22 12.32
Fe3+ 0.5 M + SCN 0.1 M + O2 + H2O2 0.18 1.93 1.98 90.88 2.60 7.04
Fe3+ 0.2 M+ SCN 0.05 M+ H2O2 +
Pb(NO3)2 0.17 1.89 1.98 90.93 3.99 13.92
Fe3+ (0.5 M), SCN(0.1 M), H2O2,
Pb(NO3)2 0.15 1.90 1.98 92.20 6.91 13.66
Fe3+ (0.2 M), SCN(0.05 M), O2,
Pb(NO3)2 0.23 1.81 1.98 87.33 2.60 11.95
Fe3+ (0.2 M), SCN(0.05 M), O2,
H2O2, Pb(NO3)2 0.22 1.89 1.98 88.60 3.39 6.50
Fe3+ (0.5 M), SCN(0.1 M), O2, H2O2,
Pb(NO3)2 0.18 1.86 1.98 90.28 6.56 6.77
The results obtained in this table have been graphed in Figure 37 and Figure 38. The thiocyanate
and ferric concentrations were varied in each of them. The effect of the addition of the different
reagents to these have been grouped in the
83
4.9.1 Effect of Oxygen, Lead Nitrate and Hydrogen Peroxide on the Gold Extraction
Figure 37 Effect of Oxygen, lead nitrate and hydrogen peroxide on the Gold extraction
The gold extraction for SCN and Fe(III) at 0.05 M and 0.20 M, respectively, without any additive
was 91.1% (as seen Figure 37). In the Figure 37, the addition of oxygen, hydrogen peroxide and
lead nitrate to the SCN/Fe(III) solution insignificantly affected the gold extraction reaction.
Even though there was insignificant increase in the gold extraction in each of these tests, it was
observed that leaching potentials were a little higher (ORP ~50 mV higher) with tests with the
addition of 3.5 g/L hydrogen peroxide than those without peroxide addition. However, the little
increase in the leaching potential had no impact on the gold extraction. The highest gold extraction
being 92.2% was recorded for a combination of 0.5 M Fe(III), 0.1 M SCN, 3.5 g/L H2O2 and 50
g/t Pb(NO3)2 as seen in Figure 38.
91.1
89.2
90.3
90.9
87.3
88.6
85
86
87
88
89
90
91
92
- O2 O2 +H2O2 H2O2 +
Pb(NO3)2
O2 +
Pb(NO3)2
O2 + H2O2 +
Pb(NO3)2
Gold
Extr
act
ion
(%
)
Additives
SCN 0.05 M Fe(III) 0.2 M
84
Figure 38 Effect of oxygen, lead nitrate and hydrogen peroxide on the Gold dissolution
Moreover, the addition of Pb(NO3)2 to enhance the gold extraction did not seem to have any effect
on the gold extraction. This could be due to the mineralogy of the ore, since the treated ore is not
refractory, and the quantities of sulphide recorded from the analysis in Section 3.2.3 was not high.
Leaching potentials were observed not to be affected by the addition of Pb(NO3)2 and oxygen. The
leach solution in these tests were still in its deep red colour signifying the presence of the FeSCN2+.
The addition of H2O2 affected the colour of the leaching solution by changing it to light orange.
This shows that peroxide affected the thiocyanate and the ferric ions complexation by unknown
means.
90.9
92.2
90.3
85.0
86.0
87.0
88.0
89.0
90.0
91.0
92.0
93.0
O2 + H2O2 H2O2 + Pb(NO3)2 O2 + H2O2 + Pb(NO3)2
Go
ld E
xtr
act
ion
(%
)
Additives
SCN 0.10 M, Fe(III) 0.50 M
85
4.10 Adsorption Test using Activated Carbon
The adsorption test was carried out on filtered gold solution. Approximately, 1 L of pregnant gold
leach solution was generated using the 72142 composite gold ore sample with the leaching
conditions as listed below:
Condition 1
• NaSCN concentration – 0.1 M
• Ferric Sulphate concentration – 0.1 M
Condition 2
• NaSCN concentration 0.1
• Ferric Sulphate concentration 0.05
• Potassium Iodide 0.01
Condition 3
• NaSCN concentration – 0.2 M
The initial gold concentration, pH and thiocyanate concentration of the filtered solution were
measured for each of the tests (values recorded in Table 29). The effect of mass of carbon added
was investigated. These were conducted to determine the amenability of gold adsorption onto
carbon from thiocyanate liquors. Prior to the adsorption test, activated carbon was sieved through
a 1.19 mm sieve to remove all fines and soaked/washed with 0.1 M hydrochloric acid. This was
rinsed very well with distilled water and dried in an oven.
Five different tests with respect to carbon concentration (0.25, 0.5, 5, 10, 20) g/L were conducted.
For each test, 100 ml of solution were measured out into 250 mls plastic bottles and specified
masses of the treated virgin carbon, calculated from the carbon concentration was added to each
bottle. The contents of the flask were agitated at 500 rev per minute on a mechanical shaker for 4
hr. Samples (of volume of 5 mL) were taken at selected times 10, 30, 60, 120, 240, 360 and 420
mins for gold analysis. DIBK was immediately added to these samples to prevent further reaction.
86
The initial solution composition of the three samples before the carbon adsorption are summarised
in Table 29. NB: Because the initial gold concentrations were close to each other the gold in
concentration data was not normalised.
Table 26 Initial composition for the three solution samples
Condition 1 Condition 2 Condition 3
Gold in solution (ppm) 0.97 1.001 0.98
pH 1.98 2.0 2.06
NaSCN concentration (Kg/t) 9.16 9.4 18.6
4.10.1 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 1
The solution chemistry of test condition 1 was 0.1 M SCN, and 0.05 M Fe(III). The initial
concentration of gold before carbon addition for test condition 1 was 0.97 ppm.
Adsorption of gold in this solution was measured and the rate of adsorption measured with
variation in carbon concentration. The recovery of gold as seen in Figure 39 shows that increasing
carbon concentration increases the rate of adsorption. The adsorption profile for carbon
concentration at 0.25 g/L and 0.5 g/L showed very low adsorption rate. The highest recovery
obtained after 4 hrs for the 0.25 g/L and 0.5 g/L was about 43.3 % and 53 % respectively. However,
carbon concentrations of 5 g/L, 10 g/L and 20 g/L all gave greater than 98 % recovery at time 4
hr. The adsorption was very fast for 10 g/l and 20 g/L and almost close to completion within 0.5
hr.
87
Figure 39 – Adsorption of gold with variation in activated carbon in acidic thiocyanate
solution. SCN 0.1 M, Fe (III) 0.05 M, pH 1.98, Temp 25 oC
4.10.2 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 2
The solution chemistry of test condition 2 was 0.1 M SCN, 0.02 M K1 and 0.05 M Fe(III). The
initial concentration of gold before carbon addition was 1.001 ppm.
Condition 2 behaviour was similar to that in condition 1. Low carbon addition gave lower
adsorption rate. The recovery of gold as seen in Figure 40 showed that increasing carbon
concentration increases the rate of adsorption. The recovery for carbon concentration at 0.25 g/l
and 0.5 g/L after 4 hrs gave 44 % and 50 %, respectively, which was slightly higher than condition
1. However, gold adsorptions with carbon concentrations of 5 g/L, 10 g/L and 20 g/L approached
completion after 0.5 hr, yielding gold recovery of between 99 % and 100% within 4 hr.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Au
Co
nce
ntr
ati
on
(m
g/L
)
Time (h)
0.25 g/L Carbon
0.5 g/L Carbon
5 g/L Carbon
10 g/L Carbon
20 g/L Carbon
88
Figure 40 – Adsorption of gold with variation in activated carbon in acidic thiocyanate
solution. SCN 0.1 M, Fe (III) 0.05 M, KI 0.02 M, pH 2, Temp 25 oC
4.10.3 Activated Carbon Removal of Gold in Thiocyanate Solution – Test Condition 3
The solution chemistry of test condition 3 was 0.2 M SCN. The initial concentration of gold before
carbon addition was 0.98 ppm.
According to the results presented in Figure 41 for Condition 3, the adsorption of gold onto carbon
increased with an increase in carbon concentration. This behaviour is similar to that in conditions
1 and 2. Low carbon addition gave lower adsorption rate. The recovery for carbon concentration
at 0.25 g/l and 0.5 g/L after 4 hrs gave 53.67 % and 54.38 %, respectively, which was slightly
higher than from conditions 1 and 2. However, carbon concentrations of 5 g/l, 10 g/l and 20 g/L
were fast and approached completion after 0.5 hr yielding gold adsorption of between 99 % and
100% within 4 hr.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Au
Co
nce
ntr
ati
on
(m
g/L
)
Time (h)
0.25 g/L Carbon
0.5 g/L Carbon
5 g/L Carbon
10 g/L Carbon
20 g/L Carbon
89
Figure 41 - Adsorption of gold with variation in activated carbon in acidic thiocyanate
solution. SCN 0.2 M, pH 2.06, Temp 25 oC
90
4.10.4 Comparison of the Three Conditions
A comparison between the three conditions is presented in Figure 42. Apart from carbon
concentration at 0.25 and 0.5 g/L, the rest are overlapping , signifying that the adsorption of gold
from thiocyanate solutions may not be impacted by the presence of potassium iodide (at 0.02 M)
and Fe(III) (at 0.05 M).
(a) Carbon concentration 0.25 g/L (b) Carbon concentration 0.5 g/L
(c.) Carbon concentration 5 g/L (d) Carbon concentration 10 g/L
91
(e) Carbon concentration 20 g/L
Figure 42 – Comparison of the carbon adsorption between the 3 conditions study. (a) runs
at 0.25 g/L carbon at the 3 conditions, (b) runs at 0.5 g/L carbon at the 3 conditions, (c)
runs at 5 g/L carbon at the 3 conditions, (d) runs at 10 g/L carbon at the 3 conditions, (e)
runs at 20 g/L carbon at the 3 conditions
From Figure 42, the carbon concentrations of 0.25 g/l and 0.5 g/L loaded gold slowly and
incompletely. Increasing contacting time could probably increase the adsorption extent. However,
carbon concentrations above 5 g/L is very effective and showed very fast rate of adsorption.
One interesting thing to note is the slowness of condition 1 which had external Fe(III) added to the
solution. However, the test without external iron showed the best adsorption at lower carbon
concentration. The reason for the slight slowness of condition 1 has been discussed by Li et al.[40]
It is believed that activated carbon promotes the reduction of dissolved ferric ion in the thiocyanate
leach solution to ferrous iron, however this process in adsorption is not desirable as it has been
seen to slightly slow down adsorption rate. This assumption will need further investigation.
92
Moreover, an average gold loading onto carbon with all three conditions showed 2100 g/t and 48
g/t of gold adsorbed at time 4 hrs (results are shown in Appendix C2). The gold loading did not
reach equilibrium
Overall, the results obtained in these tests are similar to that reported by Li et al[40]. They reported
that activated carbon is very effective in adsorbing gold from thiocyanate solutions yielding a
recovery above 98% in 1 h for a carbon concentration of 10g/L. Also, from their studies they
reported that the recovery of gold is not dependent on the ratio of solution volume (mL)/amount
of AC (g), indicating that the system is suitable to a wide range of ratios under the experimental
conditions they studied (Au 10 mg/L, Fe (III) 1 g/L, SCN 0.05 M, 23 °C, activated carbon content
10 g/L, at pH 1.8).
93
Chapter 5: Summary, Conclusions and Recommendations Future Work
5.1 Summary and Conclusions
a. Gold dissolution in thiocyanate solutions has been evaluated. The study focused on
leaching an oxide gold ore from Goldcorp Coffee Project Yukon- Canada, in thiocyanate
solution. Three samples namely Supremo 68151 A, Supremo 68151 B and 72142
composites received from Goldcorp Coffee Project, Yukon, Canada were used for the
study. The solution chemistry of the gold-ferric-thiocyanate system was reviewed to
understand reaction chemistry and possible mechanisms for gold leaching in thiocyanate
solutions.
b. The study began with ore preparation of the rock samples that were received. This was
crushed in a laboratory jaw and cone crusher and further milled in a laboratory rod mill to
reduce the particle size and to expose the gold surface for the leaching process.
Representative samples were taken for chemical and mineralogical analysis. The head
grade analysis for the three samples; Supremo 68151 A, Supremo 68151 B and 72142
composite material were 1.28 g/t, 1.45 g/t and 1.98 g/t, respectively. Sub samples were
taken for the leaching tests.
c. A baseline cyanide leaching of the three samples were conducted with 500 ppm cyanide
with the leach vessel left half opened to allow air into the slurry. Calcium hydroxide was
used to control the pH of the solution to be above 10.5. The leaching test lasted for 24 hr
with periodical sampling at various times during the leaching process. Gold extraction
obtained for Supremo 68151 A, Supremo 68151 B and 72142 composites were 94.6 %,
94.6 % and 96.3 %, respectively for time 24 hr and 95%, 94.4 % and 97 % for 32 hr. Results
obtained showed the amenability of the ore to cyanidation and also indicated that the oxide
ore is free milling.
d. Ferric ion concentration was varied at 0.05 M – 0.5 M. The highest gold extraction
obtained at this concentration range was 91.1 % at a SCN concentration of 0.1 M and
Fe(III) of 0.2 M. Fe(III) concentration above 0.2 M was found to insignificantly improve
gold dissolution. However, from all the leaching tests conducted with Fe(III), it can be
concluded that high Fe(III) concentration ( between 0.1 M and 0.2 M) helps in keeping
94
thiocyanate stable and in the auto reduction of Fe(III) and thus reduces thiocyanate
consumption.
e. Increase in thiocyanate concentration was found to increase gold extraction. Gold
extraction increased from 11.7 % to 91 % as SCN concentration was increased from 0.005
M to 0.10 M. However above 0.05 M SCN, the increase of gold dissolution was found to
be marginal.
f. The addition of potassium iodide to the SCN and SCN/Fe(III) solution increased gold
extraction. When the two reagents (KI and SCN) were put together, the highest gold
extraction of 94.1 % was obtained at KI concentration of 0.05 M. On the other hand, when
the three reagents were put together, a synergistic effect was achieved yielding a 94.4 %
gold extraction with lower KI concentration of 0.02 M. Overall results obtained for
KI/SCN/Fe(III) is encouraging and would need further investigation.
g. The use of hydrogen peroxide was found to be effective in the dissolution of gold in the
thiocyanate system. Approximately 94 % gold was extracted at 0.1 M SCN and 10 g/L
H2O2. A slight drop of gold extraction was seen when H2O2 was further increased to 14
g/L.
h. Leaching with SCN only in an atmospheric condition without the addition of any oxidant
was found to be possible. However, it was observed that the gold dissolution was aided by
internal iron which was leached from the ore by acid added. When SCN concentration was
increased from 0.1 M to 0.6 M, gold extraction increased from 81.5 to 90.9 % gold
extraction.
i. The leaching kinetics of the SCN/Fe(III) system proved to be very fast yielding a rate
constant of 72.1 hr-1 when fitted to the leach model proposed for CIL/CIP systems by Nicol
et al. The gold extraction was very fast, obtaining 90 % after 2 hr of leaching and 91.6 %
at time 12 hr, at which it reached a plateau for the condition studied.
j. Carbon adsorption test performed on the three “best” leaching conditions (that obtained the
highest gold extraction) showed that adsorption of gold onto carbon in thiocyanate solution
is possible yielding > 98 % gold adsorption in less than 1 hr.
95
5.2 Recommendations for Future Work
The leaching of gold in thiocyanate is still in the development stage and further research is
recommended in the following areas:
a. It is important to investigate the leaching of gold in the KI/SCN/Fe(III) mixture. The results
obtained from this study show promise and further work should be continued to optimize
gold extraction.
b. Another area of research would be finding a suitable oxidant to leach gold in thiocyanate
solution over a wider range of pH which will make the process more convenient. Hydrogen
peroxide was very efficient in the acidic regime but less efficient above pH of 4. Further
investigation can be done to ascertain its effectiveness.
c. The safety issues pertaining to the use of hydrogen peroxide as an oxidant in the leaching
of gold in acidic thiocyanate should be investigated. By-products such as HCN, formed
from thiocyanate degradation in the acidic regime is a progenitor of sodium cyanide. This
HCN is a highly toxic and poisonous substance that the gold mining industry is finding
ways to get rid of. The formation of HCN as a by-product of SCN degradation occurs at a
very high oxidation potential making its formation impossible, however, this should be
thoroughly investigated.
d. The leaching of the different gold ore types (carbonaceous, sulphidic, etc.) in thiocyanate
solutions should be investigated. For example, most ore types are not amenable to
cyanidation and that has been one of the drivers for the pursuit of alternative reagents. Most
of these ores would need either an initial pretreatment option, addition of additives or
leaching at higher temperatures to achieve satisfactory results.
e. The current testwork on adsorption was done on filtered gold solutions. The testwork
should be extended to encompass in-pulp gold adsorption. Gold desorption should be
studied as well.
f. The current testwork as performed for only one leach feed particle size of P80 of 150
microns. Gold extraction and reagents consumptions should be studied with respect to
leach feed particle size.
g. This SCN leaching study should be extended to leaching gold from pre-treated refractory
ores and concentrates, for example from bacteria leaching, roasting and pressure oxidation.
96
The products of these pre-treatments are acidic and will sync with SCN gold leaching in
acidic media. Current industry-practice is to wash off the acid in counter-current
decantation (CCD) thickeners from the pre-treated material and then adjust the pH with
lime prior to cyanidation. Elimination of the acid washing and lime dosage will reduce
capital and operating costs. Furthermore, lime dosage to raise the pH of the acid solution
to slurry pH of 10.5 can result in high slurry viscosity that can impair all mass transfer
reactions like gold leaching and gold adsorption from the slurry.
h. No elution experiments have been undertaken on the loaded carbon. That should be
investigated along with any effects (if any) that leach additives may have on gold loading
on carbon and gold desorption from carbon.
97
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100
Appendices
Appendix A: Analytical Methods
A1: Preparation of Au-Standards for AAS
Organic Method from Stock solutions
a. Gold Stock Solution (100 ppm) (g/ml)
Pipette 25 ml of 1000 ppm standard gold solution into a 250 ml volumetric flask containing
about 150 ml of distilled water and 50 ml of concentrated hydrochloric acid which has been
allowed to cool. Make up to the mark and mix well.
b. Gold Stock Solution (50 ppm in DIBK)
Pipette 50 ml of 100 ppm gold stock solution into a 250 ml flask. With a pipette, add 100 ml
DIBK, containing 10g/L Aliquat 336, and shake vigorously for 60 seconds.
c. Gold Working Standard (3, 2, 1, 0.5, 0.1 etc. ppm Au)
Pipette the respectively amount of the 50 ppm Stock solution into a clean, dry 100 ml
volumetric flask. Dilute to the mark with DIBK/Aliquot 336 and shake well to mix.
Aqueous Method from stock solution
a. Gold Stock Solution
0.100 g/l (100 mg/L) Au - Into a 100 ml volumetric flask, pipette 30 ml of 1N NaOH and
10 ml of 10 g/l NaCN solution. Mix thoroughly, then pipet 10 ml of the g/l Au stock
solution into the flask and dilute to the mark with distilled water.
101
a. Working Solution
0.05 g/l (50 mg/l) Au - Into a 100 ml volumetric flask, pipet 15 ml of 1N NaOH and 10 ml
of 10 g/l NaCN solution. Mix thoroughly, then pipette 5ml of the 1 g/l Au stock solution
into the flask and dilute to the mark with distilled water
b. Gold Standard
g/l (10 mg/l) Au - Into a 100 ml volumetric flask, pipette 10 ml of the 0.100 g/l Au (Solution
I) stock solution. Bring to volume with NaOH (0.1 N) -NaCN (1 g/l) solution.
c. Gold Standard
g/l (1 mg/l) Au - Into a 100 ml volumetric flask, pipette 10 ml of the g/l Au (Solution III)
stock solution. Bring to volume with NaOH (0.1 N)- NaCN (1 g/L) solution.
102
A2: Free Cyanide Titration Procedure
Reagents:
• 0.2% (in Acetone) rhodamine as indicator
• Weigh out 3.2647 g of dried AgNO3, dissolve in distilled water, and dilute to 1000 mL (1
mL = 1 3 mg CN).
Titration Procedure:
a. Filter slurry sample to a clear solution and pipette 10ml into a 100ml conical flask.
b. Add about 2 to 3 drops of rhodamine indicator. This gives a pale-yellow coloration.
c. Fill the 50ml burette with Silver Nitrate solution to above the zero mark.
d. Titrate slowly with the Silver Nitrate solution, swirling at the same time until a pale pink
colour is seen. (This is best seen under a white background)
e. Note the volume of the Silver nitrate used: V(Ag+
) and calculate concentration as below:
Cyanide reacts with Silver Nitrate (AgNO3) according to the following equation:
AgNO3 +2NaCN Na[Ag(CN)2 ]+ NaNO3
Calculation: C(Ag+
) x V(Ag+
) = 2 x C(CN-) x V(CN
-)
Note: C(Ag+
) = Volume of AgNO3
V(Ag+
) = Concentration of AgNO3
C(CN-) = Concentration of NaCN
V(CN-) = Volume of NaCN sample taken
C(CN-) = 2 x C(Ag
+) x V(Ag
+)
V(CN-)
103
A3: Residual Thiocyanate Titration Procedure
Principle:
Thiocyanate is precipitated from the red ferric thiocyanate complex by adding silver ion (as silver
nitrate solution). Silver thiocyanate is a stronger complex than ferric thiocyanate and the solution
becomes colourless when all of the thiocyanate is complexed with silver. (The Ksp of AgSCN is
1.0 x 10-12).
Reagents:
• 0.1 N Silver Nitrate solution (keep in amber bottle, away from direct sunlight)
• HNO3, concentrated
• 0.1 M Ferric Sulphate solution: Dissolve 47.1 grams Fe2(SO4)3-5H2O in DI water, and
dilute to 1 litre in a volumetric flask. Transfer to a plastic container.
Titration procedure:
a. Pipet an aliquot of sample solution into an erlenmeyer flask. Add a stir bar.
b. For aliquots less than 100 ml, dilute to 125-150 ml with DI water.
c. Add 3 ml concentrated nitric acid.
d. For samples not already containing iron, add 1 ml 0.1 M Ferric Sulphate solution. If there
is thiocyanate in the sample, the solution will turn dark red.
e. Titrate with 0.1 N silver nitrate solution, just to the disappearance of the red colour. In
samples containing abundant thiocyanate, periodic settling of precipitate might become
necessary to see the red colour remaining in solution.
f. Calculation: mg SCN/litre = ((Vol. titrant, ml) x (0.1 moles Ag+/ml) x (58.08 mg
SCN/mole))/litres sample
104
A4: Solids Specific Gravity
1. Obtain a dry representative sample, minimum of 50g dry is required for testing. Use a rolling
pin to break up any larger clumps that may have developed from drying
2. If sample has any coarse particles it will need to be sieved at 2000-microns.
5. Weigh out 25 grams (+/- 0.5) of sample into a tarred calibrated volumetric flask. Record the
mass and repeat for second flask
6. Fill the flasks with distilled water halfway (leaving room in the flask for air to escape),
gently swirl in the palm of your hand until entire sample is wet.
7. Place both flasks into the vacuum chamber along with a beaker of distilled water covered
with a watch glass (to be used to top of the flasks after de-airing) and turn vacuum pump on.
10. Check for the production of air bubbles within the flask, this indicates that the vacuum seal
is good and working. Allow the samples to remain under vacuum for 2-4 hours, periodically giving
the chamber a “knock” to release the air bubbles forming.
11. When the sample is de-aired turn off the pump and allow the vacuum built up in the chamber
to gradually release
12. Using the water that was placed in the vacuum chamber with the sample, top the volumetric
flasks up to their 250mL line.
13. Weigh and record the mass of the volumetric flask and record in template.
14. Check that SG results are matching, if not double check volumes and re-weigh. Retest if
unable to get repeatable results.
15. The solids SG can be calculated with the following formula:
Solids SG = Solids / [(Flask Mass&Water + Solids) – Mass of Flask Water & Solids]
105
Appendix B: Mineralogical Analysis Results
B1: XRD Imaging Results for Untreated Samples
a. 68151 A Sample
1RY_68151-A.raw
2Th Degrees8075706560555045403530252015105
Sqrt
(Counts
)
150
100
50
0
Quartz low 56.00 %
Illite/Muscovite 2M1 15.92 %
Kaolinite 1A 4.40 %
Microcline (ordered) 5.32 %
Illite/Muscovite 1M 15.16 %
Rutile 0.50 %
Dolomite ? 0.42 %
Chalcopyrite ? 0.21 %
Albite low, calcian 2.07 %
Figure 1. Rietveld refinement plot of sample 68151 - A (blue line - observed intensity at each step;
red line - calculated pattern; solid grey line below - difference between observed and calculated
intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual
diffraction patterns of all phases.
106
b. 68151 B Sample
2RY_68151-B.raw_1
2Th Degrees8075706560555045403530252015105
Sq
rt(C
oun
ts)
200
150
100
50
0
Quartz low 57.36 %
Illite/Muscovite 2M1 15.44 %
Kaolinite 1A 4.35 %
Microcline (ordered) 4.69 %
Illite/Muscovite 1M 15.57 %
Rutile 0.46 %
Dolomite ? 0.40 %
Chalcopyrite ? 0.28 %
Albite low, calcian 1.45 %
Figure 2. Rietveld refinement plot of sample 68151 - B (blue line - observed intensity at each step;
red line - calculated pattern; solid grey line below - difference between observed and calculated
intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual
diffraction patterns of all phases.
107
c. 72142 C Composite Sample
3RY_72142-B.raw_1
2Th Degrees8075706560555045403530252015105
Sq
rt(C
oun
ts)
200
150
100
50
0
Quartz low 55.26 %
Illite/Muscovite 2M1 15.30 %
Kaolinite 1A 8.54 %
Microcline (ordered) 4.24 %
Illite/Muscovite 1M 10.55 %
Albite low, calcian 4.93 %
Biotite 1M 1.18 %
Figure 3. Rietveld refinement plot of sample 72142 - B (blue line - observed intensity at each step;
red line - calculated pattern; solid grey line below - difference between observed and calculated
intensities; vertical bars - positions of all Bragg reflections). Coloured lines are individual
diffraction patterns of all phases.
108
B2: Results of ICP Analysis
ANALYTE DETECTION UNITS
Supremo Oxide
68151 - A
Supremo Oxide
68151 - B
Supremo T2-T4
Composite 72142 - B
Al 0.01 % 6.97 6.93 7.57
Ba 10 ppm 760 640 960
Be 5 ppm <5 <5 <5
Ca 0.1 % 0.4 0.5 0.3
Cr 10 ppm 200 170 250
Cu 10 ppm 20 20 10
Fe 0.01 % 2.95 2.83 4.1
K 0.1 % 2.9 2.8 2.6
Li 10 ppm 20 20 20
Mg 0.01 % 0.31 0.31 0.53
Mn 10 ppm 540 520 900
Ni 5 ppm 20 18 91
P 0.01 % 0.04 0.04 0.05
Sc 5 ppm 11 10 17
Si 0.1 % >30 >30 >30
Sr 10 ppm 230 230 150
Ti 0.01 % 0.25 0.25 0.36
V 5 ppm 85 80 107
Zn 5 ppm 36 38 46
Ag 1 ppm <1 <1 <1
As 5 ppm 1430 1540 1500
Bi 0.1 ppm 0.2 0.3 0.4
Cd 0.2 ppm <0.2 <0.2 <0.2
Ce 0.1 ppm 61.6 62.4 61.8
Co 0.5 ppm 10.4 10.7 18.3
109
ANALYTE DETECTION UNITS
Supremo Oxide
68151 - A
Supremo Oxide
68151 - B
Supremo T2-T4
Composite 72142 - B
Dy 0.05 ppm 2.96 3.16 3.72
Er 0.05 ppm 1.65 1.89 2.23
Eu 0.05 ppm 0.87 0.93 1.16
Ga 1 ppm 15 15 18
Gd 0.05 ppm 3.25 3.35 4.14
Ge 1 ppm 1 1 1
Hf 1 ppm 3 4 4
Ho 0.05 ppm 0.62 0.63 0.73
In 0.2 ppm <0.2 <0.2 <0.2
La 0.1 ppm 35.9 35.8 33.4
Lu 0.05 ppm 0.31 0.32 0.35
Mo 2 ppm 7 6 5
Nb 1 ppm 11 10 11
Nd 0.1 ppm 22.6 23 25.9
Pb 5 ppm 18 19 19
Pr 0.05 ppm 7.08 7.02 7.51
Rb 0.2 ppm 142 143 125
Sb 0.1 ppm 82.9 67.5 71
Sm 0.1 ppm 3.8 3.9 4.9
Sn 1 ppm 4 4 4
Ta 0.5 ppm 1 1 0.7
Tb 0.05 ppm 0.47 0.5 0.63
Th 0.1 ppm 22.4 22.8 15.6
110
ANALYTE DETECTION UNITS
Supremo Oxide
68151 - A
Supremo Oxide
68151 - B
Supremo T2-T4
Composite 72142 - B
Tl 0.5 ppm 1.5 1.6 2.9
Tm 0.05 ppm 0.26 0.28 0.34
U 0.05 ppm 15.3 15.4 20.3
W 1 ppm 6 6 10
Y 0.5 ppm 17.5 18.1 21.1
Yb 0.1 ppm 1.9 2 2.3
Zr 0.5 ppm 118 133 168
C 0.005 % 0.154 0.193 0.033
S 0.005 % 0.034 0.034 0.018
Cs 0.1 ppm 4.8 4.6 6.2
111
Appendix C: Gold Extraction Calculations
C1: Gold extraction
The gold extraction calculations are explained below:
Example of the analytical spreadsheet used for the gold extraction is shown in the Table below:
Time Amount Assays Au Distribution Overall Soln Sample Soln Sample Residue Assay (g/t)
(h) (g or mL) (mg/L or g/t) (%) (mg Au) (mg Au) (mL) Sample A Sample B
Pregnant solution 0 365.2 0.000 0.0 0.000 5 0.285 0.285
2 365.2 0.939 75.6 0.343 0.0047
4 365.2 0.957 78.1 0.354 0.0048
8 337.3 0.964 73.7 0.335 0.0048 Extraction (%)
24 337.3 1.107 85.5 0.388 0.0055
Residue 231.6 0.285 14.5 0.066 Leach 85.5
Leach Head (Calc.) 231.6 1.959 14.5 0.454
Pregnant Solution 85.5 0.388
Residue 14.5 0.066
Test Head (Calc.) 1.96 100.0 0.454
Test Head (Assayed) 231.6 1.98
Accountability 99
Analtyical Data and Calculated Results
Product
The gold extraction was calculated
% 𝑇𝑜𝑡𝑎𝑙 𝐺𝑜𝑙𝑑 𝐸𝑥𝑡𝑟𝑎𝑐𝑡𝑖𝑜𝑛 =𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑜𝑙𝑑 𝑖𝑛 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑜𝑙𝑑 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒∗ 100%
Where
Overall gold in solution = ∑𝐴𝑢𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎𝑡𝑒 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 𝐴𝑢𝑝𝑟𝑒𝑔𝑛𝑎𝑛𝑡 𝑙𝑒𝑎𝑐ℎ 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Overall gold in Sample =
𝑇𝑜𝑡𝑎𝑙 𝐴𝑢𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 + 𝐴𝑢𝑟𝑒𝑠𝑖𝑑𝑢𝑒
112
Calculated Head grade
The calculated head grade was calculated based on the overall gold in the mass of solid tested by
the mass of the residue (which is the mass of solid after leaching) expressed in g/t. The calculation
is as follows :
𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐻𝑒𝑎𝑑 𝑔𝑟𝑎𝑑𝑒 = 𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑜𝑙𝑑 𝑖𝑛 𝑆𝑎𝑚𝑝𝑙𝑒
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑟𝑒𝑠𝑖𝑑𝑢𝑒
Where,
𝑂𝑣𝑒𝑟𝑎𝑙𝑙 𝑔𝑜𝑙𝑑 𝑖𝑛 𝑆𝑎𝑚𝑝𝑙𝑒 = 𝐺𝑜𝑙𝑑 𝑖𝑛 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 (𝑡𝑎𝑖𝑙𝑠) + ∑𝐺𝑜𝑙𝑑 𝑖𝑛 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Accountability
The accountability was calculated to determine the
𝐴𝑐𝑐𝑜𝑢𝑛𝑡𝑎𝑏𝑖𝑙𝑡𝑦 =𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐻𝑒𝑎𝑑 𝐺𝑟𝑎𝑑𝑒
𝐴𝑠𝑠𝑎𝑦𝑒𝑑 𝐻𝑒𝑎𝑑 𝐺𝑟𝑎𝑑𝑒∗ 100
113
C2: Gold extraction from thiocyanate solutions
The extraction of gold from pregnant thiocyanate were calculated as follows:
𝐴𝑢 𝑖𝑛 𝑠𝑎𝑚𝑝𝑙𝑒𝑑 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑔𝑜𝑙𝑑 𝑔𝑟𝑎𝑑𝑒 ∗ 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒
1000 (𝑚𝑔)
𝐶𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝐴𝑢 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = (𝑎 − 𝑏) ∗ 𝑐 + ∑𝑑
𝑎
Where
a. = Total Volume of Solution (L)
b. = Total Volume of sample taken at before the specific time (mL)
c.= Measured gold grade at specific time (mg/L)
d. = Sum of Au in sample before the specific time (mg)
The results of the carbon adsorption are tabulated in Tables I – VI
114
Table I: The raw data for Condition 1 (SCN = 0.1 M Fe(III)= 0.05 M)
Carbon Mass (g) 0.05 0.05 0.5 1 2
Solution Vol (mL) 100 200 100 100 100
Carbon Conc (g/L) 0.5 0.25 5 10 20
Solution Head Grade (mg/L
Au) Initial Gold Concentration = 0.97
Time (h)
0 0.970 0.970 0.970 0.970 0.970
0.17 0.840 0.890 0.289 0.107 0.091
0.5 0.700 0.821 0.138 0.031 0.003
1 0.595 0.812 0.014 0.000 0.000
2 0.541 0.799 0.003 0.000 0.000
3 0.527 0.750 0.000 0.000 0.000
4 0.508 0.710 0.000 0.000 0.000
Table II: The corrected data of Condition 1 based on the equation above
Carbon Conc (g/L) 0.25 0.5 5 10 20
Time (h)
0.25 g/L
Carbon
0.5 g/L
Carbon
5 g/L
Carbon
10 g/L
Carbon
20 g/L
Carbon
0 0.970 0.970 0.970 0.970 0.970
0.17 0.890 0.840 0.289 0.107 0.091
0.5 0.823 0.707 0.146 0.035 0.007
1 0.814 0.613 0.034 0.007 0.005
2 0.802 0.567 0.025 0.007 0.005
3 0.756 0.555 0.022 0.007 0.005
4 0.721 0.541 0.022 0.007 0.005
115
Table III: The gold loading onto carbon in g/t of Condition 1
Carbon Conc (g/L) 0.25 0.5 5 10 20
Condition 1
Time (h)
0.25 g/L
Carbon
0.5 g/L
Carbon
5 g/L
Carbon
10 g/L
Carbon
20 g/L
Carbon
0 0 0 0 0 0
0.17 320 260 136 86 44
0.5 589 526 165 94 48
1 624 715 187 96 48
2 674 807 189 96 48
3 855 829 190 96 48
4 995 858 190 96 48
Table IV: Raw data for Condition 2 SCN = 0.1 M KI = 0.01 M Fe(III) = 0.05 M
Carbon Mass (g) 0.05 0.05 0.5 1 2
Solution Vol (mL) 100 200 100 100 100
Carbon Conc (g/L) 0.5 0.25 5 10 20
Solution Head Grade (mg/L Au) Initial Gold Concentration = 1.001
Time (h)
0 1.001 1.001 1.001 1.001 1.001
0.17 0.920 0.964 0.095 0.073 0.041
0.5 0.789 0.862 0.039 0.013 0.006
1 0.696 0.799 0.014 0 0
2 0.604 0.745 0 0 0
3 0.556 0.712 0 0 0
4 0.508 0.693 0 0 0
116
Table V: The corrected data for Condition 2
Carbon Conc (g/L) 0.25 0.5 5 10 20
Time (h)
0.25 g/L
Carbon
0.5 g/L
Carbon
5 g/L
Carbon
10 g/L
Carbon
20 g/L
Carbon
0 1.001 1.001 1.001 1.001 1.001
0.17 0.964 0.920 0.095 0.073 0.041
0.5 0.865 0.796 0.042 0.016 0.008
1 0.803 0.712 0.019 0.004 0.002
2 0.752 0.634 0.007 0.004 0.002
3 0.721 0.595 0.007 0.004 0.002
4 0.705 0.559 0.007 0.004 0.002
Table VI: The gold loading onto carbon in g/t of Condition 2
Carbon Conc (g/L) 0.25 0.5 5 10 20
Condition 2
Time (h) 0.25 g/L Carbon
0.5 g/L Carbon
5 g/L Carbon
10 g/L Carbon
20 g/L Carbon
0 0 0 0 0 0
0.17 148 162 181 93 48
0.5 546 411 192 99 50
1 792 578 196 100 50
2 997 735 199 100 50
3 1119 812 199 100 50
4 1185 884 199 100 50
117
Table VII: Raw data for Condition 3, SCN = 0.2 M
Carbon Mass (g) 0.05 0.05 0.5 1 2
Solution Vol (mL) 100 200 100 100 100
Carbon Conc (g/L) 0.5 0.25 5 10 20
Solution Head Grade (mg/L Au) Initial Gold Concentration = 0.98
Time (h)
0 0.980 0.980 0.980 0.980 0.980
0.17 0.890 0.897 0.082 0.059 0.023
0.5 0.680 0.701 0.024 0.008 0.006
1 0.577 0.598 0.000 0.000 0.000
2 0.501 0.534 0.000 0.000 0.000
3 0.444 0.489 0.000 0.000 0.000
4 0.390 0.425 0.000 0.000 0.000
Table VIII: The corrected data for Condition 3
Carbon Conc (g/L) 0.25 0.5 5 10 20
Time (h)
0.25 g/L
Carbon
0.5 g/L
Carbon
5 g/L
Carbon
10 g/L
Carbon
20 g/L
Carbon
0 0.980 0.980 0.980 0.980 0.980
0.17 0.897 0.890 0.082 0.059 0.023
0.5 0.706 0.691 0.027 0.011 0.007
1 0.605 0.598 0.005 0.003 0.001
2 0.545 0.533 0.005 0.003 0.001
3 0.503 0.488 0.005 0.003 0.001
4 0.447 0.447 0.005 0.003 0.001
118
Table VI: The gold loading onto carbon in g/t of Condition 3
Carbon Conc (g/L) 0.25 0.5 5 10 20
Condition 3
Time (h)
0.25 g/L
Carbon
0.5 g/L
Carbon
5 g/L
Carbon
10 g/L
Carbon
20 g/L
Carbon
0 0 0 0 0 0
0.17 332 180 180 92 48
0.5 1096 579 191 97 49
1 1498 764 195 98 49
2 1741 894 195 98 49
3 1908 985 195 98 49
4 2132 1066 195 98 49
119
Appendix D: Iron Leaching
The results of the iron leaching of the ore in sulphuric acid are presented in Tables D1 and D2:
D1: Effect of pH on iron concentration
Fe Concentration (ppm)
Time pH 1 pH 1.5 pH 2 pH 2.5 pH 3 pH 3.5 pH 4
0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
10 1547.0 702.8 370.1 60.6 20.4 9.8 0.3
30 2104.2 1277.2 420.2 71.8 11.8 4.4 2.8
60 2306.9 1252.4 432.9 61.2 8.6 2.9 4.8
90 2282.5 1268.7 518.8 91.7 7.2 2.8 10.2
120 3033.9 1284.7 534.3 89.4 5.5 1.4 11.0
150 3061.3 1438.4 560.6 92.2 6.2 1.3 8.1
180 3137.5 1486.1 599.9 80.1 5.2 1.4 9.2
D2: Effect of pH on Oxidation Potential
ORP vs. Ag/AgCl
Time pH 1 pH 1.5 pH 2 pH 2.5 pH 3 pH 3.4 pH 4
0 0 0 0 0 0 0 0
10 512.0 467.80 468.0 494.0 489.9 444.0 444
30 504.8 471.80 470.3 497.0 492.1 445.0 444.9
60 506.3 478.40 476.8 508.9 493.2 446.0 444.0
90 474.4 475.80 479.1 519.7 499.9 448.0 446
120 475.9 479.30 497.5 539.9 509.5 454.1 446.4
150 478.8 482.80 496.3 543.8 514.7 461.7 449
180 480.7 486.80 494.8 544.7 514.7 465.0 449
120
Appendix E: Leaching Model
The results of the leaching model as presented by Nicol et al is presented below:
kp 72.13
Solid Tails Grade (g/t) Aupe 0.183236
Head Grade (g/t) Aui 1.980
Data Model
Time (h) Au (ppm) Aut Diff2 0 1.98 1.980 1 0.195 0.197 0.0000 2 0.197 0.190 0.0000 4 0.185 0.187 0.0000 8 0.181 0.185 0.0000 12 0.171 0.184 0.0002 24 0.198 0.184 0.0002
Sum 0.0005
121
Appendix F: Grind Characterisation
The sieve analysis result for Supremo 72142 B material is presented in Table F1 and Figure F1
F1: Sieve analysis Results
Figure F1: Results of the grind characterisation to determine optimum time for grinding the
Supremo 72142 B composite material
Sieve
(um)
Mass Retained (g) % Retained % Cum Passing
Time
(mins)
5 10 15 18 5 10 15 18 5 10 15 18
212 490.7 109.2 2.9 1.1 45.6 10.2 0.3 0.1 54.4 89.8 99.7 99.7
150 126.3 294.2 194.1 176 11.7 27.5 18.3 16.6 42.6 62.3 81.4 83.0
106 115 183.1 296.2 279.8 10.7 17.1 27.9 26.4 31.9 45.1 53.5 56.6
75 98.2 189.9 293.2 298 9.1 17.8 27.7 28.1 22.8 27.4 25.8 28.5
53 143.6 207.4 189.5 213.2 13.4 19.4 17.9 20.1 9.4 8.0 7.9 8.4
-53 101.3 85.3 83.9 89.1 9.4 8.0 7.9 8.4
Sum 1075.1 1069.1 1059.8 1057.2
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