MG – 4111 HYDRO-ELECTROMETALLURGY
Semester I, 2010/2011
DR. M. Zaki Mubarok
Department of Metallurgical Engineering,
Faculty of Mining and Petroleum Engineering (FTTM)-ITB
LECTURE NOTES
Course Outline
I. Introduction to Hydrometallurgy
II. Thermodynamic and Kinetic Aspects in Hydrometallurgy
III. Leaching and Solid-Liquid Separation
IV. Solution Purification and Metals Recovery Methods from Pregnant Leach Solution
Course Outline
V. Leaching and Recovery of Metals and Oxides Ores (Au, Ag, Zn, Al, Cu, Ni)
VI. Leaching and Recovery of Sulphide Ores (Zn, Ni, Cu)
VII. Introduction to Electrometallurgy
VIII. Metals Production by Electrolysis in Aqueous Solution
IX. Fused Salt Electrolysis
Literatures
1. Havlik,T., ”Hydrometallurgy: Principles and Applications,” CRC publisher, 2008.
2. Habashi, F. ”A Textbook of Hydrometallurgy”, Metallurgie Extractive, Quebec,1993
3. Norman L. Weiss, “SME Mineral Processing Handbook“, Volume II, SME, 1985
4. Unit Processes in Extractive Metallurgy: Hydrometallurgy, A Modular Tutorial Course of Montana College of Mineral Science and Technology
5. Biswas, A.K. And Davenport, W.G., “Extractive Metallurgy of Copper”, Pergamon, Oxford, fourth edition, 2002
Literatures
6. Unit Processes in Extractive Metallurgy: Electrometallurgy, A modular tutorial course of Montana College of Mineral Science and Technology
7. Yannopoulus, J.C,”The Extractive Metallurgy of Gold”, Von Nostrand Reinhold, New York, 1991
Course Structure and Mark Distribution
• Course Structure– Lecture– Tutorial– Assignment and Lab Work
• Mark Distribution– 45% Midterm Exam– 45% Final Exam– 5% Assignment – 5% Lab Work
• Attendance: 70% minimum
CHAPTER I
INTRODUCTION TO HYDROMETALLURGY
Hydrometallurgy
Extraction, recovery and purification of metals,
through processes in aqueous solutions. Metals are
also recovered in the other forms such as oxides,
hydroxides.
Electrometallurgy
Recovery and purification of metals through
electrolytic processes by using electrical energy.
Hydrometallurgy Scope
• Traditionally, hydrometallurgy is emphasized for metals extraction from ores.
• Hydrometallurgical processing may be used for the following purposes:
Production of pure solutions from which high purity metals can be produced by electrolysis, e.g., copper, zinc, nickel, gold, and silver.
Production of pure compounds which can be subsequently used for producing the pure metals by other methods. For example, pure alumina to produce smelter grade aluminium.
• However, hydrometallurgy principles can be applied to a variety of areas such as metals recycling from scrap, slag, sludge, anode slime, waste processing, etc.
Unit Processes in Hydrometallurgy
• In general, hydrometallurgy involves 2 (two) main steps:
1. Leaching Selective dissolution of valuable metals from ore.
2. Recovery Selective precipitation of the desired metals from a pregnant-leach solution.
General outline of hydrometallurgical processes
Ore/concentrate
leaching
Solid-liquid separation
Solution purification
Precipitation
Pregnant Solution
Solid residu to waste
Leaching agentOxidant
Precipitant or electric current
Pure compound Metals
• Commonly, solution purification is conducted prior to metals recovery from the solution.
• Solution purification is aimed at obtaining a concentrated solution from which valuable metals can be precipitated in the next processes effectively
• Solution purification methods which are commonly used are as follows:
– Adsorption by activated carbon
– Adsorption by ion exchange resins
– Solvent extraction (using organic solvents)
– Precipitation with metals (cementation)
Solution purification
• Solution purifications by adsorption with activated carbon, ion exchange resins (IX) and solvent extraction (SX) have the same unit operations, namely:– Loading, and– Elution
• In the elution step, the adsorbers are usually regenerated for another process cycle.
Hydrometallurgy development
Hydrometallurgy is developed after pyrometallurgy. Metals smelting has been practiced since thousands years ago.
Hydrometallurgy was developed after the people discovered acid and base solutions. However, modern hydrometallurgy development is commonly associated with the invention of Bayer Process for bauxite leaching and cyanidation for gold extraction at the end of 19th century (1887).
One of important highlights of hydrometallurgy development is uranium extraction (Manhattan Project) aimed at nuclear weapon production in second world war (1940‘s).
Important milestones in the development of
hydro-electrometallurgy
• Cementation & Aqua Regia Use - 8th Century• Cyanidation - 1887• Bayer Process - 1887• Hall-Heroult Process - 1886, 1888• Copper Electrowinning - 1912• Zinc Electrolytic Process - 1916• Manhattan Project (IX/SX) - 1940’s• Biooxidation of Sulphide Concentrates - 1960’s• Pressure Leaching– Sherrit Gordon Nickel Process - 1954– Pressure Acid Leaching of Ni Laterites - 1955• Large Scale Copper SX/EW - 1960’s
Important milestones in the development of hydro-electrometallurgy
• Carbon in Pulp (CIP)/Carbon in Leach (CIL)for Gold Recovery - 1980’s• Pressure Oxidation of Zinc Sulphides - 1981• Two-Stage Zinc Pressure Leach - 1993• Atmospheric Leaching of Zinc Sulphides– Albion (1993), Outokumpu (1999)Recent Developments:• Skorpion Project (Anglo American) – 2003 (Zn from ZnS)• Hydrozinc (TeckCominco) - 2004• Inco’s Goro and Voisey Bay Projects - 2007• Leaching of Chalcopyrite (CuFeS2) OresHydrocopper (Outokumpu) Cu from sulfidic ores•Atmospheric leaching of nickel laterite ore: 2008?
Hydrometallurgy vs. Pyrometallurgy
Hydrometalurgy Pyrometallurgy
Treat high grade ore?
Less economic More economic
Treat low grade ore? Possible withselective leaching
Unsuitable
Treat sulphide ore No SO2; otherwise So or SO4
2- are generated
SO2 generated (can be converted to H2SO4)
Separate similar metal, such as Ni and Co
Possible with certain method
Not possible
Pollutant Waste water, solid/slurry residues
Gases and dust
Reaction rates Slower Rapid
Hydrometallurgy vs. Pyrometallurgy
Hydrometalurgy Pyrometallurgy
Scale of operation? Possibly economic to be done at small scale operation and expansion is easier
Unconomic at smale scale operation
Capital cost Generally lower than pyrometallurgy
Higher
Energy cost Lower Higher
Materials Handling Slurry Easy to bePumped andTransported
Handle MoltenMetal, Slag,Matte
Residues Residues – Fineand Less Stable
Slags – Coarseand Stable
Thermodynamic and KineticAspects in Hydrometallurgy
CHAPTER II
Spontaneous Reaction, Equilibrium State
• As has been learned in basic engineering courses, chemical reaction will spontaneously occur when the Gibbs free (G) < 0.
G = Go + RT ln K
G = 0 process is in equilibrium state Go = standard Gibbs free energy– R = ideal gas constant = 8,314 J/K.mol– T = absolute temperature of the system (K)– K = equilibrium constant
• Standard Gibbs free energy is determined at:– Gaseous components partial pressure = 1 atm– Temperature = 25 oC (298 K)– Ions activity = 1
Equilibrium Constant
• For reaction:
aA + bB cC + dD
baaa
daca
BA
DCK ionconcentratA=]A[]A[γ=Aofactivity=Aa →
X of pressure parsial=X whichin
,Xγ=X→Xofcomponent gaseous Forp
pa
= activity coefficient of component A
Nernst Equation
• Hydro-electrometallurgical processes often involve electrochemical reactions.
• For electrochemical reaction
G = -nFE, Go = -nFEo, therefore
KlnnF
RTEE o
In which,
E = potential for reduction-oxidation reaction
Eo =standard potential for reduction-oxidation reaction
n = number of electron involved in the electrochemical reaction,
F = Faraday constant = 96485 Coulomb/mole of electron
Nernst Equation
• Spontaneous process E > 0 G < 0
Chemical reactions usually perform in leaching processes
• Dissolution by acid
– Example: ZnO(s) + 2H+ → Zn2+(aq) + H2O(l)
• Dissolution by base
– Example: Al2O3(s) + 2OH- → 2AlO2-(aq) + H2O(l)
• Dissolution by complex ion formation Example: CuO(s) + 2NH4
+(aq) + 2NH3(aq) →
Cu(NH3)42+
(aq) + H2O(l)
• Dissolution by oxidation
– Ex: CuS(s)+ 2Fe3+ → Cu2+(aq) + 2Fe2+ + So
(s)
Other oxidators: O2, ClO-, ClO3-, MnO4
-, HNO3, H2O2,
Cl2
• Dissolution by reduction mechanism
– Ex: MnO2(s) + SO2(aq) → Mn2+(aq) + SO4
2-(aq)
Chemical reactions usually perform in leaching processes
Correlation of free energy (G) and heat (enthalphy = H)
G = H - TS
Go = Ho - TSo
Cp = heat capacity at constant pressure (J/molK)
Where possible, processes are designed to be autothermal → maintain constant temperature by the heat given by the reaction
∆Ho = Standard enthalpy (kJ/mol)∆Go = Standard entropy (kJ/mol)
∆Go (reaction) = ∆Go (products) - ∆Go (reactants) ∆Ho (reaction) = ∆Ho (products) - ∆Ho (reactants)∆So (reaction) = ∆So (products) - ∆So (reactants)
∫T
298p298T dTC+H=H ΔΔ
Calc. example 1
• Find K for each reaction using
a) Standard free energy data
b) Standard electrode potential data
Calc. Example 2
a) What is the electrode potential of the
Ni2+/Ni reaction in sulphate solution at 25°C
at a Ni2+ concentration of 0.005 M (assumption: activity of Ni2+ is equal to its molar concentration)
b) At what pH is H2 at 10 atm at equilibrium
with this solution and pure nickel?
Ni2+ + 2 e = Ni E° = -0.26 V
2H+ + 2 e = H2 E° = 0.00 V
Pourbaix Diagram
• Pourbaix Diagram = Potensial (Eh) – pH Diagram.
• The diagram represents thermodynamic equilibrium of metal, ions, hydroxides (or, oxides) in aqueous solution at certain temperature (isothermal).
• The boundary of stability regions of metal, ion, hydroxides (or oxides) are equilibrium lines.
• Does not reflect reaction kinetics.
Pourbaix Diagram
• Three possible types of equilibrium lines:– Horizontal– Vertical– Slope
• Variations in ion activities are plotted as contours/dashed lines
• Horizontal Line: for equilibrium reactions that are independent of pH.
Horizontal Line
• Example:
Fe3+ + e = Fe2+ Eo = 0.77 V
R = 8.314 J/Kmol, T = 298 K, F = 96500 C/mol e-, n = 1 mol e-
+3a+2Feo
Fe
aln
nFRT
E=E -
If all ion concentrations are assumed to be equal to their molar concentrations 10-6 M.
[ ][ ]+2
+3o
Fe
Feln
nF
RTE=E -
[ ][ ]
[ ][ ]
77,0=EFeFe
log0592,077,0=E
FeFe
ln96500x2
298x314,877,0=E
+3
+2
+3
+2
-
-
Vertical Line• Reactions do not involve electron → n = 0, no
potensial , the equilibrium depends only on pH.
• Example:
Fe2O3 + 6H+ = 2Fe3+ + 3H2O
[ ][ ]--a
++a
OHlog)OH(glo=pOH
pOH14=HlogHglo=pH
-≈-
--≈-
For certain Fe3+ concentration we can determine the equilibrium pH for the above reaction.
K = [Fe3+]2/[H-]6
Slope Line• For reactions that depend both on potensial
(Eh) dan pH.
• Example:
If all ion concentrations are assumed to be equal to their molar concentrations 10-6 M.
Water stability region (dotted lines)
• Upper boundary line
• Lower boundary line
At pO2 = 1 atm
At pH2 = 1 atm
Eh-pH diagram of Fe-H2O system at 25°C
Eh-pH Diagram of Zn-H2O System at 25 oC.
Eh-pH Diagram of Cu-H2O System at 25 oC.
Application of Eh – pH diagram in hydrometallurgy
• Predicting potential leaching behaviour for certain mineral system
• Predicting the possibility of metals ion precipitation at the purification of pregnant-leach solution
Application of Eh – pH diagram in hydrometallurgy
Fe(OH)3 or Fe2O3 can be precipitated from Fe3+ at lower pH than the precipitation of Zn2+ to Zn(OH)2 or ZnO.
Fe2+ have to be oxidized to Fe3+ to gain lower pH value for Fe(OH)3 precipitation.
Pourbaix Diagram can be constructed at various temperature for more than two systems
Eh-pH diagram of Zn-S-H2O system at 25oC
Diagram Pourbaix in Presence of Complex Ion
• Example: Au-H2O system with the presence of cyanide (CN-) ion (case of gold cyanidation leaching)
• Equilibrium of Au3+/Au
mol/KJ433
)Au(G)Au(GG
)1(Aue3Au3
f0
f00
3
Standard reduction potential for Reaction 1:
V5.1965003
10433
nF
GE
300
• Equilibrium reaction of O2/H2O
Eo = 1.23 V.
Therefore, Au3+ ions are not stable in water and readily reduced to
Au by oxidation of H2O to O2 (the opposite of Reaction 2). In the
other word, gold can not be oxidized (dissolved) in water only with
the presence O2.
OH2=e4+H4+O 2+
2
O2H/2OAu/+3Au22
+3
E>E23.1= OH/O
5.1= Au/Au
(2)
Potensial – pH diagram of Au–H2O system without the presence of complexing agent
With the presence of CN-, Au3+ forms STABLE COMPLEX of “aurocyanide“ (Au(CN)2
-) and the potential-pH diagram for Au changes significantly as follow:
Eh-pH Diagram of Au-CN-
H2O system at 25 oC for [Au]
= 10-4 M and [CN-] = 10-3 M
• By the presence of cyanide ions,
Au+ + e = Au E = 1.69 – 0.0591 log [Au+]Au+ + 2CN- = Au(CN)2
- (K = 2 x 1038)
Au(CN)2- + e = Au + 2CN- ........................... (3)
In comparison to the first reaction that has Eo of 1.69 V, Reaction (3) has much lower Eo at -0.57 V.
Dissolution of Au is limited by the following equilibrium of Reaction (3).
• During cyanidation leaching, dissolved oxygen is required to oxidize Au prior to the formation of stable complex of Au(CN)2
-.
2
2/log0591.0log0591.069.1 CNAuCNKE aa
Interactions in Electrolyte Solution
Two types of interactions in electrolyte: - Ion-ion interaction, and - ion-solvent interaction
Knowledge of interaction in electrolyte solution is important because the interactions affect solvation effects, diffusion, conductivity, ionic strength and activity coefficients of ions in solution.
Interactions in electrolyte solution influence the transport properties of ions in solution.
Ionic Strength and Activity Coefficient
- Ionic strength (I), expresses the ionic concentration that includes the effects of ionic charge.
- Ionic strength (I) is defined as follow:
- It is found that activity coefficient, electrical conductivity and the rates of ionic reactions are all the functions of ionic strength.
i
ii zcI 2
2
1
in which ci = concentration of ion i in molar (mol/L) and zi = the charge of ion i.
Ionic Strength for unit concentration in molal
- Remember, molality = moles of solute in 1 kg solvent. Molality can be converted to molality by the following correlation:
in which Mi = the molar mass of each solute in kg/mol (not in g/mol), ci = molarity of solute i, and is the density of the solution in kg/m3 (=g/L)
- In dilute solutions, ci 0.001mio (in which o = density of pure solvent).
∑ii
ii Mc-ρ001.0
c=m
Ionic Strength for unit concentration in molal
- Therefore for dilute solution,
If the solvent is water at 25oC (density 1000 kg/m3), then:
∑ zρm001.02
1=∑ zc
2
1=I 2
ioii
2ii
∑ 2ii
o zm2
ρ001.0=I
∑i
2iizm
2
1≈I Similar form with ionic
strength in molarity
- Molar activity coefficient can be converted to molal activity coefficient by the following correlation:)
mM
f
s
1
for salt, or
mM
f
s
ii
1
for single ion.
in which = total moles of ion formed during complete dissociation, m = ionic molality and Ms = molecular weight of solvent (kg/mol).
Activity and Activity Coefficient, DEBYE-HUCKEL LAW
- Debye Huckel Law correlates the activity coefficient (fi , i) with ionic strength (I).
- Forms of Debye-Huckel equations depend on concentration of solution and the unit concentration used.
- For dilute solution at 25 oC and I given in molar (M),
- The above equations are known as LIMITING DEBYE
HUCKEL LAW.
I-zz51159.0=flog +±
Izf ii251159.0log for single ion, and
for salt.
The limitation of LIMITING Debye-Huckel Equation
• The D-H Limiting Law is called a ”limiting” law
because it becomes increasingly accurate as the limit
of infinite dilution is approached.
• Up to concentrations of about 0.01m THE LIMITING
D-H LAW gives reasonable values, but at higher
concentrations the calculated activity coefficient
become inaccurate (high %error compared to the
values determined experimentally).
Debye-Huckel Law for Concentrated Solution
- For concentrated solution (> 0.01 molal), Limiting Law D-H is modified by considering the ionic size parameter:
IBa
IzzAf
1
log
- in which A and B are constants that depend on the kind of solvent and temperature, a = ion size parameter.
- For aqueous solution at 25 oC, A = 0.51159 and B = 3.2914 x 109 meter.
ACTIVITY AND MEAN ACTIVITY
- Molar activity and molar activity of a single ion i is determined as follow:
- For 1 mole of M+A- salt that dissociates to + mol of Mz+ and - mole of Az-
iii ma
M+A- + Mz+ + -
Az-
= + + -
iii Cfa and
Mean molal activity coefficient can be determined by the following correlation:
/1
Mean molality,
/1
mmm
Thus, mean molal activity,
ma
mm
/1
Note that m m
ACTIVITY AND MEAN ACTIVITY
Exercise: 11. Determine the molar activity coefficient of Ca2+ at 25oC using
relevant Debye Huckel Equation in the following solution:
a. 0.0004 mole of HCl and 0.0002 mole of CaCl2 in one liter solution
b. 0.004 mole of HCl and 0.002 mole of CaCl2 in one liter solution
c. 0.4 mole of HCl and 0.2 mole of CaCl2 in one liter solutionIon size parameter for Ca2+ = 0.4 nm.
Exercise 2:
2. The stoichiometric mean activity coefficient at 25 oC of the sulphuric acid in a mixture of 1.5 molal sodium sulphate (Na2SO4) + 2 molal H2SO4 is 0.1041. If the second dissociation constant, K2, for sulphuric acid is 0.0102 and the pH of the solution is –0.671, calculate:
a) the molal activity of H2SO4
b) the molal activity of SO42-
c) the molal activity of HSO4-
d) the mean activity of H2SO4
Exercise:31 gram FeCl2, 1 gram NiCl2 and 1 gram of HCl are added to 200 ml of deaerated water. Platinum electrodes are used to deliver electrical current so that the electrolysis performs. The anodic and cathodic current density are 1000 A/m2. The following are the reactions and Eo (in the reduction direction) that may occur:Fe2+ + 2e = Fe Eo = -0,277 VNi2+ + 2e = Ni Eo = -0,250 V
2H+ + 2e = H2 Eo = 0 V
Cl2 + 2e = 2Cl- Eo = 1,359 V
a) Calculate molar activity coefficients of the cations and anion contained in the solution (use the Finite Size of Debye Huckel Limiting Law)b) Calculate the activity of the cations and anion contained in the solutionc) Determine the half cell potential of the above reactionsd) Which pair of redox (reduction –oxidation reaction) that would occur (based on the calculation of c)
Exercise: 3 (cont.)
e) What would be the cell voltage of the reaction d
Data: Atomic weight Fe = 55.8, Ni = 58.7, Cl = 35.5, H =1
Ion size parameter in nm : Fe2+ = Ni2+ = 0.6, H+ = 0.9, Cl- = 0.3
H2 overpotential = 0.28 V
Cl2 overpotential = 0.03 VOhmic overpotential = 0.25 V.
Kinetics in Hydrometallurgy
• Kinetics in hydrometallurgy deals with the kinetics of leaching, adsorption and precipitation
• Studying of leaching kinetics is done for the establishment of the rate expression that can be used in design, optimization and control of metallurgical operations.
• The parameters that need to be estabished:
– Numerical value of the rate constant
– Order of reaction
– Rate determining step
– Activation energy
Leaching Kinetics
• Consider the dissolution of a metal oxide, MO, with
an acid by the following reaction:
• The reaction rates for this leaching system can be given by
MO(s) + 2H+(aq) M2+
(aq) + H20(aq)
dt
dC=
dt
dC=r
ordt
dC
2
1=
dt
dC=r
O2H+2MP
+HMOR --
Leaching Kinetics
• For general example if a chemical reaction involves A and B as reactants and C and D as products, the stoichiometric reaction can be written as follows:
where
a, b, c, and d = stoichiometric coefficients of species A, B, C, and D, respectively
k1, k2 = reaction coefficients in the forward and reverse directions, respectively
dD+cCbB+aA1k
2k⇔ (1)
Leaching Kinetics
• The rate expression of this stoichiometric reaction can be written in a more general way:
where
CA, CB, Cc, and CD are concentrations of species A, B, C, and D, respectively and m, n, p, q are orders of reaction.
qD
pC2
mB
nA1
DCBA CCkCCk=dt
dC
d
1=
dt
dC
c
1=
dt
dC
b
1=
dt
dC
a
1--- (2)
Leaching Kinetics
• However, if the reaction given in Eq. 1 is irreversible, as in most leaching systems, Eq. 2 is reduced to the following form:
where k1’= k1 x a.
mB
nA
'1
A
mB
nA1
A
CCk=dt
dC
or
CCk=dt
dC
a
1
-
-
• For this system, the rate constant, k1', and the orders of reaction, n and m, should be determined with the aid of leaching experimental data.
• The rate expression given in the above equations can be further reduced if the reaction is carried out in such a way that the concentration of A is kept constant.
• For such situations, the rate expression is reduced to:
where k1”= k1
’ x CAn. It should be noted that the rate
constant and the order of reaction are constant aslong as the temperature of the system is maintained constant.
mB1
A Ck=dt
dC ″-
• Consider the dissolution of zinc in acidic medium:
Zn(s) + 2H+(aq) → Zn2+
(aq) + H2(g)
• For the above reaction, the rate of disappearance of H+ ion is directly related to the rate of appearance of Zn2+ ion; thus,
nH
nH
mZn
+H+2Zn kC=CCk′=dt
dC
2
1=
dt
dC-
• If concentration of zinc metals is assumed to be constant and CH is further
abbreviated generally as CA, then the equation can be written as follow:
• The order of reaction, n, can be any real number (0, 1, 2, 1.3, etc.).
• When n = 0, the reaction is referred to as “zero order” with respect to the concentration of A.
where CAo represents the concentration of A at t = 0, and XA represents
the fractional conversion, i.e., XA = [ (CAo — CA)/ CA
o].
nA
A kC=dt
dC-
0A
A kC=dt
dC-
tC
k=Xor
kt=dtk=CC=dC
oA
A
t
0
oAA
AC
oAC
A ∫∫ --
• If the plot of XA versus t gives a straight line, the zero-order assumption is consistent with experimental observations and the k value can be obtained from the slope of the plot.
• When n = 1, the reaction is first order with respect to the concentration of A:
AA kC=
dt
dC-
k/CAo
XA
time
( ) ktAA
oA
A
t
0
AC
oAC A
A
e-1=Xorkt-=X-1ln
kt-=CC
ln
kt-=dtk=CdC
-
∫∫ -
k
time
ln (1 - XA)
For second order reaction,
ktC=X1
X
kt-=C
1
C
1
kt-=dtk=C
dC
oA
A
A
oAA
t
0
AC
oAC
2A
A
-
-
- ∫∫
k
time
XA
(1 - XA) CAok
If the second-order assumption is valid, we obtain a straight line from a plot of XA/(1 - XA) versus t, and the rate constant can be determined from the slope of the plot.
Temperature Effect on the Reaction Rate (Arrhenius Law)
Reaction rate increases markedly with increasing temperature. It has been found empirically that temperature affects the rate constant in the manner shown in the following equation:
RT/aEoek=k -
T
1
R303.2
Eklog=klog
T
1
R
Ekln=kln
ao
ao
-
-
where Ea is the activation energy and k° is a constant known as the frequency factor, frequently assumed to be independent of temperature.
Modeling of heterogenous reaction kinetics
• Heterogenous reaction between solid and fluid in hydrometallurgical processes is frequently modelled with “shrinking core“ model.
• If we select a model we must accept its rate equation, and vice versa.
• If a model corresponds closely to what really takes place, then its rate expression will closely predict and describe the actual kinetics;
• If a model differ widely from reality, then its kinetic expressions will be useless.
• Detailed of modeling and relevant kinetics equations for various rate determining steps can be found in previous course (Metallurgical Kinetics).
• For determination of Ea, number of experiments, at least at three or four different temperatures are needed, with all other variables being kept constant. The next step is to calculate the rate constant for each temperature as discussed previously.
• A plot of In k versus 1/T yields a straight line from which the activation energy, Ea, can be determined
• Activation energy value can be used to predict the rate determining step of the reaction:
• Ea = 40 – 80 kJ/mol: process is controlled by surface chemical reaction
• Ea = 8 – 20 kJ/mol: process is controlled by diffusion to and from the surface
Mass Transfer in Solution
• For hydrometallurgical system, mass transfer of component i in solution frequently consists of a molecular diffusion term, migration term, and convective diffusion term, as indicated in the following expression:
VC+FCμzCD=N iiiiiii Φ∇-∇-
where
Ni= flux of i, Ci = concentration of i, Di = diffusion coefficient of i
Ci = concentration gradient of i, zi = valence of the specified ion,
µi = ionic mobility, F = the Faraday constant, Ф = electrical
potential gradient, and V = net velocity of the fluid of the system
First and Second Fick’s Law of Diffusion
• If Ni consists of the molecular diffusion term only,
• Dimensionless Parameter for Convection Calculation
iii CD-=N ∇
)lawseconds'Fick(0=CD+t
Ci
2i
i ∇∂
∂
(Fick's first law)
ii D
LV
D
LV where µ = the viscosity of the fluid, ρ = the density of the fluid
Dimensionless Parameter for Convection Calculation
• The parameter LV/Di is known as the Peclet number and can be separated into two other parameters: Lvρ/µ that known as the Reynolds number, and µ/ρDi is the Schmidt number.
• Peclet number is regarded as a measure of the role of convection against diffusion,
• For most hydrometallurgical systems, the Schmidt number is on the order of 1,000 because the diffusivity of ions and kinematic viscosity of water are, respectively, on the order of 10-5 cm2/s and 10-2 cm2/s.
( ) diffusionmolecular
diffusionconvective=
L/C∇D
VC∇=
D
LV
ii
i
i
• Therefore, if the Reynolds number is greater than 10-3, the Peclet number is greater than 1, and consequently, convective diffusion is more dominating than molecular diffusion in such systems.
Mass Transfer Coefficients for Convective Diffusion
• For systems with large Peclet numbers, it is frequently assumed that there is a diffusion boundary layer at some distance from the solid surface. For such systems, it is quite common to write the mass flux from the bulk solution to the solid surface as follows:
Ni = km (Cb - Cs)
where
Ni = mass flux of species i
km = mass transfer coefficient, in cm/s
Cb = concentration of species i in the bulk solution, in mol/cm3
Cs = concentration of species i at the solid surface, in mol/cm3
• Because the units of measure of km are the same as those of (D/), where is the diffusion boundary layer thickness, km, is often substituted by this ratio. Therefore,
• The diffusion boundary layer thickness is often estimated by the relationship km = D/δ, provided km is known.
( )sbi CCδ
DN -=
Mass Transfer from or to a Flat Plate.
• The mass transfer coefficient for a flat plate where fluid is flowing over the plate at a velocity V0 has been well documented.
• The mass transfer coefficient for such a system can be estimated from first principles and has the following form:
where
D = the diffusivity of the diffusing species
v = the kinematic viscosity of the fluid
L = the length of the plate
62/10
2/1-6/1-3/2m 10<ReForVLD664.0=k ν
Rotating Disk.
• Although it is not a practical geometry, because the mathematical representation of the system is exact and follows very closely to the experimental data, a rotating disk is frequently used to determine the mass flux and the mass transfer coefficient.
• The mass transfer coefficient for this system is as follow
• The equation is valid for the Reynolds number, r2ω/ν is less than 105, where r and ω are, respectively, the radius and the angular velocity of the disk.
2/16/1-3/2m D62.0=k ων
Particulate System
• It has been demonstrated that the mass transfer coefficient for particulate systems can be given by the following equation:
where d = the diameter of the particle, Vt = the slip velocity, which is often assumed to be the terminal
velocity of the specified particle.
3/26/1-2/1-2/1tm DdV6.0+
d
D2=k ν
• The terminal velocity of a particle can be calculated using the following equation depending on the Reynolds number of the system, which is defined by dVtρ/μ, where ρ is the density of the fluid:
where ρs is the density of the particle. The preceding equation is often referred to as the Stokes' equation and is valid as long as the Reynolds number is less than 1.
( )μ
ρρ
9
g-r2=V s
2
t
When the Reynolds number is between 1 and 700, the following equations are used:
( )( )
2s
3
2/1
At
3
-gd4=K
55.5-Klog4.0+66.00.5=A
10d
=V
μ
ρρρ
ρ
μ
where g is the gravitational coefficient.
• A cementation reaction, Zn + Cu2+ → Cu + Zn2+, is taking place at the surface of a zinc plate of 10 cm x 10 cm area.
• Feed flowing parallel to the plate at a velocity of 1 m/s contains copper at 1 mo/dm3.
• Suppose we want to estimate the rate of deposition assuming that the mass transfer of Cu2+ to the zinc plate is rate determining step. The diffusivity of Cu2+ is 7.2x10-6 cm2/s, and the kinematic viscosity of water is 0.01 cm2/s.
Example 1:
whereS = the surface area of the plateNcu
2+ = the number of moles of Cu 2+ ionCub
2+ = the concentration of Cu2+ in the bulkCus
2+ = the concentration of CU2+ at the interface
km = 0.664 (7.2 x 10-6)2/3 (0.01)-1/6 (10)-1/2 (100)1/2
= 0.664 x 3.7 x 10-4 x 2.15 x 0.316 x 10= 1.7 x 10-3 cm/s.
( ) +2bm
+2s
+2bm
+2Cu Cuk=Cu-Cuk=dt
dN
S
1
Therefore,
Example 2:• Consider the situation from the previous example,
except that instead of a zinc plate, zinc particles 100 µm in diameter are suspended in a 1 mol/dm3 Cu2+ solution. Suppose we want to estimate the rate of deposition of Cu2+ (Note that the density of Zn is 7.14 g/cm3.)
5
23
1001.0
10100Re
./7.1000,1107.11 2
and
scmmoldt
dN
SCu
For particulate,
58.210/58.210Re
/58.210
,
412.055.53.80log4.066.05
3.80103
114.7101.09814
22
412.0
2/1
4
3
scmV
Therefore
A
K
t
scmmoldt
dN
Sk
Finally
scm
k
resultaAs
Cum
m
./19.9000,11019.91
,
/1019.9
1075.71044.1
102.701.001.058.26.001.0
102.72
,
23
3
33
3/266/12/12/16
2