fundamental aspects of gold leaching in thiosulfate
TRANSCRIPT
F U N D A M E N T A L A S P E C T S OF G O L D L E A C H I N G
I N T H I O S U L F A T E S O L U T I O N S
by
C H E N G LI
M . E . , Kunming University of Science and Technology, P. R. China, 1988
A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F
T H E R E Q U I R E M E N T F O R T H E D E G R E E O F
M A S T E R OF A P P L I E D S C I E N C E
in
T H E F A C U L T Y OF G R A D U A T E S T U D I E S
Department of Metals and Materials Engineering
We accept this thesis as conforming
to the required standard
T H E U N I V E R S I T Y OF B R I T I S H C O L U M B I A
June 2003
© Cheng L i , 2003
In presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, I agree that the Library shall make it
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copying or publication of this thesis for financial gain shall not be allowed without
my written permission.
Department of Metals and Materials Engineering
The University of British Columbia
Vancouver, Canada
Date: June 18, 2003
j
ABSTRACT
The kinetics and mechanism of gold leaching in the ammonia-thiosulfate-copper ( A T S -
Cu) system have been studied using the rotating electrochemical quartz crystal
microbalance ( R E Q C M ) . Anodic polarization, cathodic polarization and leaching
experiments were included in this study. The effect of concentration of different reagents,
applied potential, p H , temperature, and electrode rotating velocity on the gold leaching
rate have been investigated.
The effect of solution copper species on the anodic reaction of gold dissolution in
thiosulfate solution was elucidated using the R E Q C M . With respect to the role of copper
on the anodic processes, two possible mechanisms were proposed. It was shown that, in
the absence of S2O3 2", the electrochemical reaction on the cathode is the reduction of
C u ( N H 3 ) 4
2 + to C u ( N H 3 ) 2
+ in the potential range of 0.2 to -0.3 V vs. S H E . Based on other
experimental results, it is believed that, in the presence of S2O3 2", the electrochemical
reaction on the cathode also is the same as the reaction in the absence of S2O3 2".
It was found that the anodic process and leaching process are under chemical reaction
control, but the cathodic process is under diffusion control. It may therefore be concluded
that the leaching process is under anodic control under the majority o f conditions tested
(cathodic control w i l l be important when the cupric ammine species is very dilute in
solution). Experiments show that increasing the ratio of NH3/S2O32" w i l l favor the
leaching rate. However, excess thiosulfate or excess ammonia may inhibit the dissolution
of gold. The effect of selected additives was also studied.
i i i
TABLE OF CONTENTS
ABSTRACT .ii
TABLE OF CONTENTS iv
LIST OF TABLES vi
LIST OF FIGURES vii
ACKNOWLEDGEMENTS . xvi
NOMENCLATURE xvii
1 INTRODUTION 1
2 LITERATURE REVIEW 4
2.1 Aqueous Chemistry 4
2.2 Stability of thiosulfate solution 16
2.3 Leaching of gold in thiosulfate system 19
2.4 Research on the kinetics and mechanism 25
2.4.1 Passivation phenomena 25
2.4.2 Kinetics study of gold thiosulfate leaching 27
2.4.3 Mechanism of gold thiosulfate leaching 37
2.5 Additives studies 44
3. EXPERIMENTAL APPROACH 48
3.1 Introduction 48
3.2 Basic principle of REQCM 49
3.3 REQCM 56
3.3.1 Design 56
3.3.2 Equipment set-up 58
3.3.3 Validation of the equipment 61
3.4 Experimental 64
3.4.1 Reagents 64
3.4.2 Experimental procedure and conditions 65
4. RESULTS AND DISCUSSION 69
4.1 Anodic polarization studies 69
iv
4.1.1 Preliminary experiments 69
4.1.2 Anodic polarization studies 74
4.1.2.1 Anodic polarization in the absence of copper 74
4.1.2.2 Anodic polarization in the presence of copper 79
4.1.3 Summary 90
4.2 Cathodic polarization studies 91
4.2.1 Preliminary experiments 91
4.2.2 Cathodic polarization studies 93
4.2.3 Summary 103
4.3 Leaching studies 104
4.3.1 Preliminary tests 104
4.3.2 Leaching studies under open potential 105
4.3.2.1 Leaching with higher concentrations of reagents 105
4.3.2.2 Leaching with lower concentrations of reagents 109
4.3.3 Leaching studies under applied potential I l l
4.3.3.1 Leaching with higher concentration of reagents I l l
4.3.3.2 Leaching with lower concentration of reagents 120
4.3.4 Summary 128
4.4 Additives studies 129
4.4.1 Anodic polarization studies : 129
4.4.2 Leaching studies 135
4.4.3 Summary 140
4.5 The mechanism 141
4.5.1 Anodic process 141
4.5.2 Cathodic process 143
4.5.3 The model of electrochemical mechanism 144
5 CONCLUSIONS AND RECOMMENDATIONS 146
5.1 Conclusions 146
5.2 Recommendations 149
6 REFERENCES 150
v
LIST OF TABLES
Table 2.1 A summary of various thiosulfate leaching conditions (Aylmore and Muir ,
2000b) 23
Table 3.1 The specifications and sources o f reagents 64
Table 4.1 Influencing factors for gold oxidation on anodic polarization( no copper) 90
Table 4.2 Influencing factors for gold oxidation on anodic polarization (with copper).. 90
Table 4.3 The influence o f variables on cathodic current response 103
Table 4.4 Calculated values o f Arrhenius activation energy values for gold leaching at
different applied potentials 120
Table 4.5 Influencing factors for gold oxidation on leaching tests 128
Table 4.6 Influencing factors for gold oxidation on leaching tests 128
Table 4.7 Influencing factors for gold oxidation on leaching tests 128
Table 4.8 Influencing factors for gold oxidation on leaching tests 129
Table 4.9 Effect of additives for gold oxidation on additives tests 140
Table 4.10 Effect of additives for gold oxidation on additives tests 140
v i
LIST OF FIGURES
Figure 2.1 A u - N H 3 - S 2 0 3
2 " system (conditions: 5 x l O ^ M A u ; 1 M S 2 0 3
2 " ; 1 M NH3/NP1/, 0.05M Cu 2 + ) (Aylmore et al, 2001a) 5
Figure 2.2 AU-NH3-S2O3 2 " system (conditions: S x l O ^ M A u ; 0 .1M S 2 0 3
2 ~; 0 .1M
N H 3 / N H 4
+ , 0 .05M C u 2 + ) (Aylmore et al, 2001a) 5
Figure 2.3 Effect of thiosulfate concentration on the rest potential of gold(Li et al,1996).
6
Figure 2.4 Eh-pH diagram of the gold-thiosulfate-ammonia-water system at 25°C. The
activities o f the species are 2.5 xlO" 5 M A u , 0.2 S 2 0 3
2 " and 0.4 M N H 3 . AG f ° (S 2 0 3
2 " )
= -518.8 kJ/mol (Mol leman et al, 2002) 7
Figure 2.5 C u - N H 3 - S 2 0 3
2 " system (conditions: 5 x l O " 4 M A u ; 1 M S 2 0 3
2 " ; 1 M N H 3 / N H 4
+ ,
0 .05M Cu)(Aylmore et al, 2001a) 8
Figure 2.6 C u - N H 3 - S 2 0 3
2 " system (conditions: S x l O ^ M A u ; 0 .1M S 2 0 3
2 " ; 0 .1M
N H 3 / N H 4
+ , 0 .05M Cu 2 + ) (Aylmore et al, 2001a) 9
Figure 2.7 Distribution of copper species at different E h conditions for high reagent
concentration (1 M S 2 0 3
2 \ 1 M N H 3 , 0.05 M C u , pH=10.0) (Aylmore et al, 2001a)
10
Figure 2.8 Distribution of copper species at different E h conditions for low reagent
concentration (0.1 M S 2 0 3
2 " , 0.1 M N H 3 , 5X10" 4 M C u , pH=10.0) (Aylmore et al,
2001a) 10
Figure 2.9 Distribution of copper species at different p H concentrations for high reagent
concentration (1 M S 2 0 3
2 \ 1 M N H 3 , 0.05 M C u , Eh=0.250 V ) (Aylmore et al,
2001a) 11
Figure 2.10 Distribution of copper species at different p H concentrations for low reagent
concentration (0.1 M S 2O 3
2",0.1 M N H 3 , 5x l0" 4 M C u , Eh=0.250 V ) (Aylmore et al,
2001a) 11
Figure 2.11 Distribution o f copper species at different N H 3 concentrations for high
reagent concentration^ M S 2 0 3
2 " , 0.05 M C u , p H 10.0, Eh=0.250 V ) (Aylmore et
al, 2001a) '. 12
V l l
Figure 2.12 Distribution of copper species at different NH3 concentrations for low
reagent concentration^.1 M S 2 0 3
2 \ 5X10" 4 M C u , p H 10.0, Eh=0.250 V ) (Aylmore
etal , 2001a) 12
Figure 2.13 Distribution of copper species at different S2O3 " concentrations for high
reagent concentration(l M N H 3 , 0.05 M C u , p H 10.0, Eh=0.250 V ) (Aylmore et al,
2001a) 13
Figure 2.14 Distribution of copper species at different S20 3
2 " concentrations for low
reagent concentration^. 1 M N H 3 , 5x l0" 4 M C u , p H 10.0, Eh=0.250 V ) (Aylmore
et al, 2001a) 13
Figure 2.15 Distribution of copper species at different C u 2 + concentrations for high
reagent concentration 1 M S 2 0 3
2 \ 1 M N H 3 , p H 10.0, Eh=0.250 V ) (Aylmore et al,
2001a) 14
Figure 2.16 Distribution o f copper species at different C u 2 + concentrations for low
reagent concentration^. 1 M S 2O 3
2",0.1 M N H 3 , p H 10.0 , Eh=0.250 V ) (Aylmore et
al, 2001a) 14
Figure 2.17 Effect of copper concentration and temperature on the dissolution of gold in
0.25 M S 2 0 3
2 " , 1.0 M N H 3 , 196 kPa 0 2 , stirring velocity 200 rpm (Tozawa et al,
1981) 26
Figure 2.18 Effect o f copper sulfate concentration on gold leaching rate(Li et al, 1996) 28
Figure 2.19 Effect of the concentration ratio of ammonia to thiosulfate on gold leaching
rate(Lietal , 1996) 29
Figure 2.20 Effect of ammonia on gold oxidation. Experimental conditions: 0.1 M
N a 2 S 2 0 3 , 30°C (Breuer et al, 2000a) 31
Figure 2.21 Kinetic plot showing the leaching of gold at fixed potential of 238 m V S H E
in a solution containing either thiosulfate, thiosulfate and ammonia, or thiosulfate,
ammonia and copper (Breuer et al, 2002) 34
Figure 2.22 Linear sweep voltammograms for the reduction of oxygen in ammonia-
thiosulfate solutions, and for the reduction of copper in ammonia and ammonia-
thiosulfate solution (Breuer et al, 2002) 35
Figure 2.23 Linear sweep voltammogram for a gold electrode in solutions containing
copper ( solid line). Also shown is the calculated partial current density for the
v i i i
oxidation of gold to gold thiosulfate derived from the mass change (dashed line)
measured using R E Q C M . Also shown as a square symbol is the measured mixed
potential and reaction rate(as current density) during leaching (Breuer et al, 2002 )36
Figure 2.24 The model of electrochemical-catalytical mechanism o f ammoniacal
thiosulfate leaching of gold (Jiang et al, 1993a) 38
Figure 2.25 Mechanism of gold leaching in A T S system in the presence of copper ion
(Ouyang, 2001) 42
Figure 2.26 The electrochemical model for the copper catalysis mechanism of leaching
gold with ammoniacal thiosulfate.( Aylmore et al, 2001a) 43
Figure 2.27 The effect of pyridine on gold leaching in A T S - C u system 45
Figure 3.1 A diagram of the R E Q C M electrode (Jeffrey et al, 2000a) 58
Figure 3.2 Assembly of cell and R E Q C M system 59
Figure 3.3 Schematic illustration of R E Q C M 60
Figure 3.4 A kinetic plot showing the deposition of silver from a solution containing
0.01 M A g ( C N ) 2 "and 0.02 M CN" . The experimental conditions were 0.1 m A , 700
rpm and 25° C 62
Figure 3.5 Validation of the Levich equation by measuring the hexacyanoferTate(IH)
reduction current density as a function of (om. The experimental conditions were
0.01 M FefCN) 6
3 " , 0.1 M NaC10 4 , -250 m V versus S H E and 25°C 63
Figure 4.1 Effect o f scan rate on the gold anodic polarization in 0.2 M (NH4)2S203
solution (pH 10, 25°C, 450 rpm) 69
Figure 4.2 Effect o f scan rate on the gold anodic polarization in 0.2 M (NH4)2S203
solution (pH 10, 25°C, 450 rpm) 70
Figure 4.3 The reproducibility tests of the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution (pH 10, 25°C, 450 rpm, lmV/s ) 71
Figure 4.4 The reproducibility tests o f the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution (pH 10, 25°C, 450 rpm, lmV/s ) . Test l :Solution prepared in
presence of air. Test 2: Solution prepared under nitrogen 71
ix
Figure 4.5 Effect of air in preparation of solutions on the gold anodic polarization in 0.2
M ( N H 4 ) 2 S 2 0 3 , 250 ppm [Cu] T solution (pH 10, 25°C, 450 rpm, lmV/s ) . Test 1:
Solution prepared in presence of air. Test 2: Solution prepared under nitrogen 72
Figure 4.6 Effect of adding copper (H) in solution preparation on the gold anodic
polarization in 0.2 M ( N H 4 ) 2 S 2 0 3 , 250 ppm [Cu] T solutions (pH 10, 25°C, 450 rpm,
1 mV/s).Test 1: Adding copper (II) after adjusting p H . Test 2: Adding copper (IT)
before adjusting p H 72
Figure 4.7 Effect of adding ammonia or adding ammonia ion on the gold anodic
polarization at p H 10 (25°C, 450 rpm, 1 mV/s) 74
Figure 4.8 Effect of temperature on the total current density on gold anodic polarization
in 0.2 M ( N H 4 ) 2 S 2 0 3 solution, no copper (pH 10, 450 rpm, 1 mV/s) 76
Figure 4.9 Effect o f temperature on the current density from gold mass change on gold
anodic polarization in 0.2 M ( N H 4 ) 2 S 2 0 3 solution, no copper (pH 10, 450 rpm, 1
mV/s) 76
Figure 4.10 Effect of rotating speed on the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution, no copper (25°C, p H 10, 1 mV/s) 77
Figure 4.11 Effect of rotating speed on the gold anodic polarization in 0.2 M ( N H 4 ) 2 S 2 0 3
solution, no copper (25°C, p H 10,1 mV/s) 77
Figure 4.12 Effect o f p H value on the total current density on the gold anodic
polarization in 0.2 M ( N H 4 ) 2 S 2 0 3 solution, no copper (25°C, 450 rpm, 1 mV/s) . . . . 78
Figure 4.13 Effect of p H on the current density from gold mass change on the gold
anodic polarization in 0.2 M ( N H 4 ) 2 S 2 0 3 solution, no copper (25°C, 450 rpm, 1
mV/s) 78
Figure 4.14 Effect o f [CU]T concentrations on the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution (25°C, p H 10,450 rpm, 1 mV/s) 80
Figure 4.15 Effect of [CU]T concentrations on the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution (25°C, p H 10, 450 rpm, 1 mV/s) 80
Figure 4.16. Effect of [CU]T concentrations on the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution (25°C, p H 10, 450 rpm, 1 mV/s) 81
Figure 4.17 [CU]T decrease the overpotential which is required for oxidizing the gold to
gold thiosulfate in 0.2 M ( N H 4 ) 2 S 2 0 3 solution ( 25°C, pHIO, 450rpm, lOmV/s) ... 82
Figure 4.18 Anodic polarization on a platinum and gold electrode in 0.2 M QSrH4)2S203
solution ( 2 5 ° C , p H 10, 450 rpm, 10 mV/s) 82
Figure 4.19 Gold anodic polarization test in 0.2 M ( N H 4 ) 2 S 2 0 3 solution ( 25°C, p H 10,
450 rpm, 1 mV/s) 84
Figure 4.20. Reproducibility test for adding 250 ppm copper (U) immediately before
starting the test in 0.2 M (NH 4 )2S20 3 solution (25°C, p H 10, 450 rpm, 1 mV/s) 84
Figure 4.21 Effects of thiosulfate, ammonia, and copper on gold anodic polarization (
25°C, p H 10, 450 rpm, 1 mV/s) 86
Figure 4.22 Effect of (NH 4 ) 2 S 2 03 concentrations on the gold anodic polarization in
0.2 M ( N H 4 ) 2 S 2 0 3 solutions (250 ppm copper, 25°C, p H 10, 450 rpm, 1 mV/s) . . . . 86
Figure 4.23. Effect of NH3 concentrations on the gold anodic polarization in 0.2 M
N a 2 S 2 0 3 solutions (250 ppm copper, 25°C, p H 10,450 rpm, 1 mV/s) 89
Figure 4.24. Effect of S2032" concentrations on the gold anodic polarization in 0.4 M
NH3 solutions (250 ppm copper, 25°C, p H 10, 450 rpm, 1 mV/s) 89
Figure 4.25. Reproducibility tests of gold cathodic polarization in 0.2 M (NH 4 ) 2 S 2 03
solution (250 ppm copper, p H 10, 25°C, 450 rpm, 1 mV/s) 91
Figure 4.26. Comparing the effects of using gold electrode or platinum electrode on the
cathodic polarization (0.8 M ( N H 4 ) 2 S 2 0 3 , 2 5 0 ppm copper,, p H 10, 25°C, 450 rpm, 1
mV/s) 92
Figure 4.27 Effect of adding ammonia or adding ammonia ion on the gold cathodic
polarization (pH 10, 25°C, 450 rpm, 1 mV/s) 92
Figure 4.28. Effect o f ( N H 4 ) 2 S 2 0 3 concentrations on the gold cathodic polarization
(copper 250 ppm, p H 10, 25°C, 450 rpm, 1 mV/s, air purge) 94
Figure 4.29. Effect o f copper concentration on the gold cathodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 (pH 10, 25°C, 450 rpm, 1 mV/s , air purge) 94
Figure 4.30. Cathodic polarization in 0.4 M N H 3 solution (pH 10, 25°C, 450 rpm, 1
mV/s , air purge or nitrogen purge) 95
Figure 4.31 Cathodic polarization in 0.4 M N H 3 , 2 5 0 ppm copper solution (pH 10,
25°C, 900 rpm, 1 mV/s , air purge). Thiosulfate was not added 96
Figure 4.32. Comparing cathodic polarization in 0.4 M N H ^ 250 ppm copper solution
between air purge and nitrogen purge. N o thiosulfate present 97
x i
Figure 4.33. Effect of rotating speed on the gold cathodic polarization in 0.4 M NH3, 250
ppm copper solution (pH 10, 25°C, l m V / s , air purge) 98
Figure 4.34. The relationship between the limiting current and the square root o f rotating
velocity for experiments with 0.4 M N H 3 , 250 ppm copper solution (pH 10, 25°C,
l m V / s , air purge) 99
Figure 4.35 Cathodic polarization in 0.4 M N H 3 , 250 ppm copper and 0.1 M S20 3
2 "
solution (compared with no S20 3
2 , air purge) 100
Figure 4.36. Effect of S20 3
2 " concentrations on the gold cathodic polarization in 0.4
M N H 3 > 250 ppm copper solution (pH 10, 25°C, 450 rpm, l m V / s , air purge) 102
Figure 4.37. Effect of concentration of N H 3 on the gold cathodic polarization in 0.2 M
N a 2 S 2 0 3 , 250 ppm copper (pH 10, 25°, 1 mV/s) 102
Figure 4.38. Leaching reproducibility tests in 0.2 M ( N H 4 ) 2 S 2 0 3 , 50 ppm copper
solution (pH 10, 25°C, 450 rpm, 0. 25 V vs. SHE) 105
Figure 4.39 Effect o f C u ( N H 3 ) 4
2 + o n gold leaching in A T S - C u system. (pH 10, 450 rpm,
25°C, air purge) 106
Figure 4.40. Comparing leaching tests in the presence o f S 2 0 3
2 " and absence of S 2 0 3
2 "
(pH 10, 450 rpm, 25°C, air purge) 107
Figure 4.41 Effect o f concentration of S 2 0 3
2 " on gold leaching in 0.4 M N H 3 , 250 ppm
copper solution (pH 10, 450 rpm, 25°C, air purge) 108
Figure 4.42 Effect of concentration o f N H 3 on gold leaching in 0.2 M N a 2 S 2 0 3 , 250
ppm copper solution (pH 10, 450 rpm, 25°C, air purge) 109
Figure 4.43 Effect of concentration of S 2 0 3
2 " on the gold leaching in 0.2 M N F f 3 ,
30ppm copper solution, open potential (pHIO, 450rpm, 25°C, air purge) 110
Figure 4.44. Effect of concentration of N H 3 on gold leaching in 0.1 M N a 2 S 2 0 3
solution, 30 ppm copper solution, open potential (pH 10, 450 rpm, 25°C, air purge).
I l l
Figure 4.45. Effect of potential on the gold leaching in 0.2 M (NFLt ) 2 S 2 0 3 solution, no
copper (pH 10, 450 rpm, 25°C, air purge) , 113
Figure 4.46 Effect of potential on the gold leaching in 0.2 M ( N H 4 ) 2 S 2 0 3 solution with
30 ppm copper (pH 10,450 rpm, 25°C, air purge) 113
x i i
Figure 4.47 Effect o f potential on the gold leaching in 0.2 M (NFL,)2S203 solution with
50 ppm copper (pH 10, 450 rpm, 25°C, air purge) 114
Figure 4.48 Effect o f potential on the gold leaching in 0.2 M (NH4)2S203 solution with
250 ppm copper (pH 10, 450 rpm, 25°C, air purge) 114
Figure 4.49. Effect o f concentration o f copper on gold leaching in 0.2 M (NFLt)2S203 solution with 0.20 V vs. S H E applied potential (pH 10, 450 rpm, 25°C, air purge).
116
Figure 4.50. Effect o f concentration of copper on gold leaching in 0.2 M (NH4)2S203 solution with 0.25 V vs. S H E applied potential (pH 10, 450 rpm, 25°C, air purge).
116
Figure 4.51. Effect o f concentration of copper on gold leaching in 0.2 M(NH4)2S2C>3
solution with 0.30 V vs. S H E applied potential (pH 10, 450 rpm, 25°C, air purge).
117
Figure 4.52. Effect o f temperature on gold leaching i n 0.2 M (NH4)2S203 solution, 250
ppm copper, 0. 20 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 118
Figure 4.53. Effect o f temperature on gold leaching in 0.2 M (NH4)2S203 solution, 250
ppm copper, 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 119
Figure 4.54 Effect o f temperature on gold leaching in 0.2 M (NH4)2S203 solution, 250
ppm copper, 0.30 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 119
Figure 4.55 Arrhenius plot for different potentials. . 120
Figure 4.56. Reproducibility tests for gold leaching in 0.1 M (NH4)2S203, 30 ppm [Cu]x
solution (pH 10, 25°C,450 rpm, 0.25 V vs. S H E , air purge) 121
Figure 4.57. Effect o f concentration of (NH4)S203 on gold leaching in ( N H O ^ C h
solution, 30 ppm copper, 0.25 V vs. S H E (pH 10,450 rpm, 25°C, air purge) 124
Figure 4.58. Effect o f concentration of [Cu]j on gold leaching in 0.1 M (NH4)2S2C>3
solution, 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 125
Figure 4.59. Effect o f concentration of NH3/S2O3 ratio on gold leaching in (NH4)2S203 solution, 30 ppm [Cu] T , 0.25V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 125
Figure 4.60. Effect of p H value on gold leaching in 0.1 M (NH4)2S203 solution, 30 ppm
[Cu] T , 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 126
x i i i
Figure 4.61. Effect o f temperature on gold leaching in 0.1 M (NH4)2S203 solution, 30
ppm [Cu] T , 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 126
Figure 4.62 Arrhenius plot for rate of gold leaching using a lower concentration of
reagents, 0.25 V vs. S H E 127
Figure 4.63. Effect of rotating speeds on gold leaching in 0.1 M (NH4)2S203 solution, 30
ppm [Cu] T , 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 127
Figure 4.64. Effect of 0.2 M A g + on the gold anodic polarization in 0.2 M (NH 4 ) 2 S 2 03
solution, no copper (pH 10, 450 rpm, 25°C, 1 mV/s) 131
Figure 4.65. Effect of 500 ppm N a C l on the gold anodic polarization in 0.2 M
( N H 4 ) 2 S 2 0 3 solution, no copper, (pH 10, 450 rpm, 25°C, 10 mV/s) 131
Figure 4.66. Effect of 1% N a C l on the gold anodic polarization in 0.2 M ( N H 4 ) 2 S 2 0 3
solution, no [Cu] T , (pH 10, 450 rpm, 25°C, 10 mV/s) 132
Figure 4.67 Effect of concentration of E D T A on the gold anodic polarization in 0.1 M
(NTl4)2S203 solution, 250 ppm copper (pHIO, 450rpm, 25°C, lmV/s ) 133
Figure 4.68. Effect o f E D T A on gold anodic polarization in a solution of 0.2 M
( N H 4 ) 2 S 2 0 3 , 250 ppm C u 2 + (pH 10, 450 rpm, 25°C, 1 mV/s) 133
Figure 4.69. Gold anodic polarization in a solution of 0.1 M Na 2S 203, 100 ppm copper,
0.01 M E D T A (pH 10, 450 rpm, 25°C, 1 mV/s) 134
Figure 4.70 Reproducibility tests of anodic polarization of gold in 0.1 M (NH 4 ) 2 S 2 03
solution, 250 ppm copper, 0.005M E D T A (pH 10, 450 rpm, 25°C, 1 mV/s) 134
Figure 4.71. Effect o f E D T A i n leaching test i n 0.1 M Na2S2C>3,100 ppm copper (pH 10,
450 rpm, 25°C) 135
Figure 4.72 Effect of E D T A in leaching test in 0 .2M ( N H 4 ) 2 S 2 0 3 , 250 ppm copper , 0.20
V vs. SHE(pH10, 450rpm, 25°C,air purge) 136
Figure 4.73. Effect o f E D T A in test in 0.2 M ( N H 4 ) 2 S 2 0 3 , 250 ppm copper, 0.25 V vs.
S H E (pH 10, 450 rpm, 25°C, air purge) 137
Figure 4.74. Effect of additives on gold leaching in 0.2 M (NH 4 ) 2 S 2 03, no copper (except
where added), 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge), 137
Figure 4.75 Effect of concentration of Ag+ on gold leaching in 0.2 M ( N H 4 ) 2 S 2 0 3 , no
copper, 0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 138
xiv
Figure 4.76. Effect of additives on gold leaching in 0.1 M (NH4) 2 S 2 03, 30 ppm copper,
0.25 V vs. S H E (pH 10, 450 rpm, 25°C, air purge) 139
Figure 4.77 The model A of electrochemical mechanism of gold leaching in A T S - C u
system 144
Figure 4.78 The model B o f electrochemical mechanism o f gold leaching in A T S - C u
system 145
xv
ACKNOWLEDGEMENTS
I. would like to express my sincere appreciation to Dr. David Dreisinger for his
supervision, and reviewing and editing this thesis. I am very grateful to Dr. Paul West-
Sells, my co-supervisor, for his constructive discussions, and reviewing and editing this
thesis. I would also like to acknowledge Dr. Des Tromans, Dr. Akram Alfantazi and Dr.
David Dixon for providing constructive ideas. Dr. Jianming Lu ' s kind help, especially in
debugging R E Q C M system, is very much appreciated.
Thanks to M s . Anita Lam and Dr. Berend Wassink for their contribution to my study.
Thanks are extended to my fellow graduate students and the staff o f the Hydrometallurgy
Group with whom I have enjoyed working.
Thanks to the sponsoring companies Anglogold, Barrick Gold, Newcrest, Newmont,
Placer Dome, and Teck Cominco for providing financial and intellectual support for this
work.
Finally, I would like to thank my wife, my parents, my brother and sister for their
encouragement and support.
x v i
NOMENCLATURE
A area (cm 2)
D diffusion coefficient (cm 2 s"1)
E potential of the electrode (V)
E° standard potential (V)
F Faraday constant, 96487 A s mol" 1
i current density (A cm"2)
J flux ( m o l m" 2 s"1)
M molarity (mol dm"3)
m mass(g)
n number of electrons transferred
p H negative logarithm to base of the activity o f hydrogen ion
ppm part per mil l ion
R gas constant ( 8.314 J K" 1 mol" 1)
r leaching rate ( m o l m" 2 s"1)
Re Reynolds number
S C E standard calomel electrode
S H E standard hydrogen electrode
T temperature (°C)
t time(s)
v kinematic viscosity ( c m 2 s"1)
co rotating velocity (revolutions per minutes )
x v i i
1 INTRODUTION
Cyanide has been used as a lixiviant for extracting gold for over one hundred years.
Conventional cyanidation is an economical, biodegradable process and it achieves
excellent recoveries from a wide range of ores. However, since cyanide is a very toxic
chemical, and because cyanide solution does not effectively leach carbonaceous or
complex ores, researchers have been interested in finding non-toxic chemicals and
environmentally safe substitutes. One area of current research is gold leaching in
thiosulfate solution.
Thiosulfate has the ability to complex gold. Thiosulfate leaching can be considered a
non-toxic process, the gold dissolution rates can be faster than cyanidation and, because
the gold thiosulfate complex does not adsorb on carbonaceous materials and the
interference o f foreign cations decreases in thiosulfate leaching, high gold recoveries can
be obtained from the thiosulfate leaching of complex and carbonaceous ores. In addition,
comparing reagent unit costs, ammonium thiosulfate is far cheaper than sodium
cyanide(US$0.13/kg vs. US$1.807kg). Consequently, with similar or even slightly higher
lixiviant consumption, the application o f thiosulfate for gold recovery can be economical
and compete directly with cyanidation (Molleman et al, 2002).
However, it was found that in the absence o f ammonia, gold dissolution in thiosulfate
solution stops due to gold surface passivation; in the absence o f the redox couple
C u 2 + / C u + as a catalyst, the leaching rate is very slow. Therefore, ammonia and the
1
C u / C u couple are needed in the leaching system. In addition to the oxidative
decomposition reactions of thiosulfate, many degradation species make the thiosulfate
leaching system (ATS-Cu) very complicated.
In recent years many researchers focused on the thiosulfate leaching o f gold, especially
on complex ores. However, both fundamental kinetic and electrochemical studies are still
limited, and consequently, the reaction kinetics and mechanisms are not fully understood.
So far this process has not been widely employed in the gold industry. Clearly, an
understanding of all o f the major variables and how they affect the leaching process is
necessary before achieving success on a commercial scale.
The rotating electrochemical quartz crystal microbanlance ( R E Q C M ) is a powerful
technique capable of detecting very small mass changes at the electrode surface that
accompany electrochemical processes (Jeffery et al 2000a). Use o f the R E Q C M for
studying gold leaching in A T S - C u system can minimize the interference of side reactions
on the gold leaching kinetics.
The objective of this work was to investigate the effect of different parameters
(concentrations of different reagents, potential, p H , temperature, rotating velocity, etc.)
on gold leaching in the A T S - C u system using the R E Q C M , and to try to gain an
understanding of the reaction mechanism. Also , the effect of selected additives on the
dissolution of gold in the thiosulfate system was studied. A n anodic polarization study,
2
cathodic polarization study and leaching test study were included in this work. In each
case, the R E Q C M was the key apparatus for completing the study.
This thesis consists o f five chapters. Following this introduction, chapter two gives a
review of existing literature on gold leaching in the A T S - C u system, especially on the
kinetics and mechanistic studies. The experimental methods are introduced and described
in chapter three, followed by the results and discussion in chapter four. The conclusions
and recommendations are presented in chapter five.
3
2 LITERATURE REVIEW
2.1 Aqueous Chemistry
T h e chemistry o f the a m m o n i a thiosulfate - copper ( A T S - C u ) system is c o m p l i c a t e d due
to the simultaneous process o f c o m p l e x i n g l igands such as a m m o n i a and thiosulfate, the
Cu(Il)-Cu(r) redox couple and the p o s s i b i l i t y o f oxidat ive d e c o m p o s i t i o n reactions o f
thiosulfate i n v o l v i n g the format ion o f tetrathionate and other addit ional sulfur
compounds .
Gold-thiosulfate-ammonia-water system
A n Eh-pH diagram can be used to show the predominant species under different
potentials and p H condit ions. A y l m o r e et a l (2001a) constructed the E n - p h d iagram for
gold-thiosul fate-ammonia - water system at h i g h reagent concentrations (Figure 2.1) and
at l o w reagent concentrations (Figure 2.2).
Those figures show that under certain condit ions g o l d c o u l d be present i n so lut ion as
A u ( N H 3 ) 2+ rather than Au(S203)23". T h e gold(I) thiosulfate c o m p l e x is the most stable
species i n the leaching system up to p H 8.5. A b o v e this p H , w h e n N r L ; + converts to N H 3 ,
the predominant g o l d c o m p o u n d is g o l d (I) d i a m m i n e c o m p l e x .
4
Eh (Volts) 2.0
1.5
1.0
03
0.0
-O.S
•1.0
•1.5
•2.0
1 1 1 r 1 , 1 i i
Au(NH3)4
t3(aq)
Au(Sa03)2-3(aq) ! {Au(NH3),+(aq)}
-
Au
1 1 1 1 1 1 1 _. i
10 12 14 pH
Figure 2. 1 Au-NH3-S203
2" system (conditions: SxlO^M Au; 1M S203
2"; 1M
NH 3/NH 4
+, 0.05M Cu2+)(Aylmore et al, 2001a)
Eh (Volte)
2.0 r 1 1 i . , , , , ^
1.5 Au(NH,)4tS(aq) A u 0 »
1.0
0.5 Au(S20,)2^(aq) j {Au(NH,y(aq»
0.0 •
•0.5 - -
-1.0 Au
•IS
-2.0 1 i ' ' 1 i i ' '
4 6 8 10 12 14
Figure 2. 2 Au-NH3-S203
2" system (conditions: SxlO^M Au; 0.1M S203
2"; 0.1M
NH 3/NH 4
+, 0.05M Cu2+) (Aylmore et al, 2001a)
5
Other researchers also constructed Eh-pH diagrams similar to Figures 2.1, 2.2 (L i et al.
1996, Molleman et al, 2002).
However, as L i et al (1996) remarked, it is generally accepted that the gold (I)-thiosulfate
species is the more stable species at p H 10 and this was confirmed by rest potential
measurements, since the gold rest potential varied with thiosulfate concentration instead
of ammonia concentration (Figure 2.3)
-o.io
W U -0.15 t/3 vi >
.15 | -0.20 o a.
3
1 -0.25
-0.30
-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 LogtWKM)
Figure 2. 3 Effect of thiosulfate concentration on the rest potential of gold(Li et
al,1996).
Aylmore et al (2001a) explained that the variation between thermodynamics and
electrochemical studies may be attributed to the high activation energy for A u ( N H 3 ) 2
+
formation (Meng and Han, 1993). Furthermore, i f a lower stability constant for
Au(NH 3 )2 + (10 1 3 otherwise 10 2 6) is used to calculate the diagram, a much higher
6
concentration o f ammonia is required to stabilize Au(NH3)2 + and Au(S203)23" exists over
the whole p H range under conditions examined in Figures 2.1 and 2.2.
Molleman et al (2002) constructed the Eh-pH diagram for gold-thiosulfate-ammonia -
water system. The gold concentration was 2.5xl0" 5 M (5ppm), the thiosulfate
concentration was 0.2 M and the ammonia concentration was 0.4 M . Two different values
of free energy of formation of the thiosulfate species were used in Molleman's work.
When a value of -532.2 kJ/mol was used, the Eh-pH diagram is similar to Alymore's
work. However, when a value of -518.8 kJ/mol was used, a totally different Eh-pH
diagram was developed as shown in Figure 2.4.
1.5 PLOT LfiBELS
T . - » • 2 9 8 . I S K I flu I . 2 . S E - 0 S IS203I • 0 . 2 INH31 - a.14
STH8LE flHEflS
B Bu S M Ro 03 <2• > (HOI C H2 flu 03 <•> (POl 0 Hu (0 H 13 E Bu [52 D3 >2 <3-> Ifffll
H20 STABILITY LIMITS 1 Q2/H20 2 H2/M20
Figure 2. 4 Eh-pH diagram of the gold-thiosulfate-ammonia-water system at 25°C.
The activities of the species are 2.5 xlO"5 M Au, 0.2 S203
2" and 0.4 M NH 3.
AGf
0(S2O32") = -518.8 kJ/mol (Molleman et al, 2002)
7
It can be seen that the gold(I)-thiosulfate complex is stable in the whole p H range shown
in Figure 2.3. This diagram is consistent with the results of rest potential measurements.
Copper-thiosulfate-ammonia-water system
Aylmore et al (2001a) constructed an Eh-pH diagram for the copper-thiosulfate-
ammonia-water system in high concentration and low concentration thiosulfate solutions
as shown in Figure 2.5 and Figure 2.6.
It can be seen that decreasing the ammonia thiosulfate and copper concentrations
significantly narrows the region of stability of Cu(NH3)4 2 + and Cu(S203)35" and expands
the stability region of CuO, CU2O and CU2S. It is clear that high p H values should be
avoided because copper w i l l precipitate from solution as copper oxides.
Eh (Volts)
Cu — 1 1 1
*<aq)
CuO
—1 1 1
Cu(NH3)4
+2(aq)
1 1 1
CuO
-Cu(S 20 3 )3"
—ZZiutir^^ Cu2G
: Cu 2S
-
1 1 L _ _
Cu
_ J • ' J 1 1
4 « 8 10 12 14 pH
Figure 2. 5 Cu-NH3-S203
2" system (conditions: 5x10"4 M Au; 1 M S203
2"; 1 M
NH 3/NH 4
+, 0.05M Cu)(Aylmore et al, 2001a).
8
Eh (VolU) 2.0 r
Cu(Nk)4*(aq)
Cu*a(ab) CuO
1.0 r CuO
-1.0
Cu
•2.0 4 6 8 10 12 14
p H
Figure 2. 6 Cu-NH3-S203
2" system (conditions: 5X10" 4 M Au; 0.1M S203
2"; 0.1M
NH 3/NH 4
+, 0.05M Cu2+)(Aylmore et al, 2001a).
Speciation diagrams can characterise the distribution of various C u - N H 3 - S 2 0 3
2 " species
co-existing in solution. Aylmore et al (2001) presented the speciation diagrams for
varying concentrations of Cu(IT), S 2 0 3
2 " and N H 3 , and E h and p H as shown in Figure
2.7-2.16. These speciation diagrams are useful for understanding and optimizing this
system.
Figure 2.7 and Figure 2.8 show that increasing the potential (e.g., with 0 2 ) but keeping all
reagent concentrations constant results in a decrease in C u ( S 2 0 3 ) 3
5 " and an increase in
C u ( N H 3 )4 2 + in the solution. A t low reagents concentrations in solution, only a small E h
range is available for maintaining the copper (IT) ammonia complex in solution.
9
)
5.00E-02
0 0.1 0.2 0.3 0.4 0.5 0.6 Eh(V)
Figure 2. 7 Distribution of copper species at different Eh conditions for high reagent
concentration (1 M S203
2", 1 M NH3, 0.05 M Cu , pH=10.0) (Aylmore et al, 2001a)
5.00E-O4
4.00E-O4
S e o I e 3.00E-O4 o
<5 2.00E-O4
1.00E-04
0.O0E+O0
Figure 2. 8 Distribution of copper species at different Eh conditions for low reagent
concentration (0.1 M S203
2-, 0.1 M NH 3, 5x10 M Cu , pH=10.0) (Aylmore et al,
2001a).
1 0
Figure 2. 9 Distribution of copper species at different pH concentrations for high
reagent concentration (1 M S203
2~, 1 M NH 3, 0.05 M Cu , Eh=0.250 V) (Aylmore et
al, 2001a).
Figure 2. 10 Distribution of copper species at different pH concentrations for low
reagent concentration (0.1 M S2O3
2~,0.1 M NH 3, 5x10"* M Cu , Eh=0.250 V)
(Aylmore et al, 2001a).
11
Figure 2.11 Distribution of copper species at different NH3 concentrations for high
reagent concentration^ M S203
2", 0.05 M Cu , pH 10.0, Eh=0.250 V) (Aylmore et al,
2001a).
6.0OE-O4
Total NH, cone (M)
Figure 2. 12 Distribution of copper species at different NH 3 concentrations for low
reagent concentration(0.1 M S203
2", 5X10" 4 M Cu , pH 10.0, Eh=0.250 V) (Aylmore
et al, 2001a)
12
Figure 2. 13 Distribution of copper species at different S2O3 2 " concentrations for
high reagent concentration^ M NH3, 0.05 M Cu , pH 10.0, Eh=0.250 V) (Aylmore
et al, 2001a).
Figure 2.14 Distribution of copper species at different S2O3 2 " concentrations for low
reagent concentration . 1 M NH3, 5x10"" M Cu , pH 10.0, Eh=0.250 V) (Aylmore et
al, 2001a).
13
0.4 0.6
Cu" Cone (M) as
Figure 2.15 Distribution of copper species at different Cu 2 + concentrations for high
reagent concentration^ M S203
2~,1 M NH 3, pH 10.0, Eb=0.250 V) (Aylmore et al,
2001a)
Tanorita
X )( )( X )( X )< X )( )( )( )< )<i )(. )( >E
C^l(82OA,• I I I I I I I I I I I I I I I
CufNHdT 3 0.002 0.0O4 0.006 0.008 0.01
Cu'*Conc(M)
Figure 2.16 Distribution of copper species at different Cu concentrations for low
reagent concentration(0.1 M S2O3
2",0.1 M NH3, pH 10.0 , Eh=0.250 V) (Aylmore et
al, 2001a).
14
From Figures 2.9 and 2.10, it can be seen that to leach gold under low reagent conditions
only a narrow p H region around 9.5-10 exists where the copper ammonia complex is
stable without the precipitation of copper (II) oxide, tenorite. A t high thiosulfate and
ammonia concentrations a broader p H range is available. A broader p H range would also
be available at very low concentrations of copper.
Figures 2.11, 2.12, 2.13 and 2.14 show that increasing total ammonia concentration but
keeping the other reagent concentrations, p H and E h constant, increases the stability
region o f the Cu(NH3 )4 2 + complex, whereas increasing the thiosulfate concentration
increases the stability region of the Cu(S203)35" complex. It would appear that a higher
ammonia to thiosulfate concentration would be required to achieve a higher C u ( N H 3 ) 4 2 +
concentration in solution over Cu(S203)35". However, a high C u ( N H 3 ) 4
2 + concentration in
solution may also result in higher losses of thiosulfate as Cu(NH3)4 2 + can oxidize
thiosulfate to tetrathionate.
Figures 2.15 and 2.16 show that, under the conditions used, the copper concentration has
a much more pronounced effect on the stability regions o f copper species where
Cu(S203)35", becomes more stable over the C u ( N H 3 )4 2 + complex. In addition,
precipitation of tenorite occurs with increased copper concentration in solution.
After thermodynamic analysis and the construction of Eh-pH and speciation diagrams,
Aylmore et al (2001a) suggested that the optimum E h and p H lies between 0.25 and
0.30V and from 9 to 10, respectively. A t low reagent concentrations, there is a smaller
15
window of E h and p H in which copper (I) and copper(U) species exists. Excess NH3 over
S2O32" favours the formation o f copper (II) amine species.
2.2 Stability of thiosulfate solution
Thiosulfate ions are metastable and tend to undergo chemical decomposition in aqueous
solutions. The degradation of thiosulfate not only results in loss of the lixiviant, but also
can lead to the formation of sulphides which in turn may passivate gold and limit the
leaching rate. How to maximize control of the stability of the leaching solution is a key
factor for the success of thiosulfate leaching.
When the p H of a thiosulfate solution is 5 or less, the following reaction occurs at an
appreciable rate (Skoog and West, 1975):
S 2 0 3
2 ' + H + o H S 2 0 3 " HSO3" + S(s) Equat ion 2.1
The rate of this reaction depends on the p H , i f in strong acidic solution, elemental sulfur
forms within a few seconds. This reaction explains why gold thiosulfate leaching
proceeds only in alkaline solution.
In systems that contain bacteria, experiments indicate that the stability of thiosulfate
solution is at a maximum in the p H range between 9 and 10, because bacterial activity
appears to be at a minimum at this p H range, and solutions that are free of bacteria have
limited thiosulfate degradation (Skoog and West, 1975).
16
It was reported that decomposition of thiosulfate solution is catalyzed by copper (II) ions
as well as by the decomposition products themselves, and the decomposition rate is
greater in more dilute solutions (Skoog and West, 1975). Wan et al (1997) also reported
that dilute solutions of thiosulfate (0.01M or less) decompose more rapidly than
concentrated solutions (0.1M or higher).
Thiosulfate may also be oxidized to form tetrathionate under milder conditions in the
presence of cupric ion (Wasserlauf et al 1982):
2S 2 0 3
2 +2Cu 2 + -» S406
2"+2Cu+ (fast) Equation 2.2
2Cu++l/2Q2+2H+-> 2Cu2++H2Q (measurable) Equation 2.3
2S203
2+2H+ +l/202-> S 40 6
2+H 20 Equation 2.4
A i r oxidation of thiosulfate under normal pressures and temperatures is a very slow
process. Rola et al (1982) reported that a solution o f thiosulfate and polythionates, at p H
7, was aerated for 4 months under sterile laboratory conditions at Noranda research
centre with less than 10% change observed in the thio salt concentration.
Pryor (1960) showed that S 2 0 3
2 " is relatively stable in basic solution, especially in the
absence o f catalysts. The base hydrolysis was found to be very slow in basic solution,
even at 250°C.
Kerley et al (1981, 1983) suggested the addition of sulfite ions to thiosulfate solutions to
stabilize thiosulfate ions. Gong et al (1990) proposed this reaction as:
17
4S03
2"+2S2 + 3HzO o 3S203
2 +60H" Equation 2.5
Gong and H u (1993) described the substitution of sulfate for sulfite because SO42" ion
inhibits the oxidation and decomposition of S203
2" ion:
S O 4 2 +S2 +H20->S203
2 +20H Equation 2.6
However, this reaction is in question, because the SO42" species is very stable and
therefore unlikely to react with sulfide ion.
Copper(IT) and thiosulfate react homogeneously in the presence o f ammonia by the
following equation:
2Cu(NH3)4
2++8S203
2^2Cu(S203)3
5+8NH3 + S406
2- Equation 2.7
Breuer et al (2000b) studied the oxidation of thiosulphate by copper(II). It was found that
copper(II) can strongly catalyze the oxidation of thiosulfate, about 50% of the copper(IT)
reacted with thiosulfate after 3.8 hrs.
It can be concluded that degradation o f thiosulfate is very complex and that many
reactions between original and generated species in solution are possible. The exact
degradation pathways are not clear.
18
2.3 Leaching of gold in thiosulfate system
Laboratory studies
Work has been conducted on many types of raw materials: Oxide ores (Langhans et al.,
1992; Ji et al, 1991,Cai 1997 ), sulfide concentrates (Zhang et al.,1987; Cao et al,1992),
manganese ores (Zippering, 1998), carbonaceous ores (Hammeti et al, 1989; Wan et al,
1994), dissolution of pure gold (Tozawa et al, 1981). Most o f these experiments were
performed with finely ground materials.
Gold extraction and consumption of reagents are key factors to focus on in order to
compete with cyanide leaching. In recent years, low thiosulfate concentration and low
temperature have been widely used in tests for minimizing reagent losses.
The effects of reagent concentration, such as thiosulfate, copper (If), sulfite and ammonia
on the gold and silver extraction from gold concentrate, were studied by Zhang and L i
(1987). It was shown that maintaining proper [S 2 0 3
2 "] / [Cu 2 + ] and [S 203 2"]/[S0 3
2"] is very
critical. The gold leaching kinetics were fitted with the shrinking core model; the
activation energy was calculated to be less than 5 kcal/mol.
Hemmati et al (1989) investigated the thiosulfate leaching of gold from carbonaceous ore
that contained 2.5% organic carbon. The optimum conditions for gold extraction were
found to be 35 °C and 103 K P a oxygen over pressure, p H 10.5, 3 M N H 3 with a lixiviant
containing 0.71 M (NH 4)2S 20 3 , 0.15 M C u S 0 4 and 0.1 M ( N H 4 ) 2 S 0 3 . Extraction of gold
19
was 73% in 4 hours compared with only 10% observed using cyanide over 24 hours.
Hemmati et al concluded that the efficiency of thiosulfate leaching depends upon ore
type. For treating carbonaceous ores, thiosulfate was found to be chemically superior and
economically advantageous over cyanide.
Langhans et al (1992) investigated copper-catalyzed thiosulfate leaching for extracting
gold from low-grade oxidized gold ores. 83% gold extraction was achieved with 0.4 kg/t
ore S 2 0 3
2 ~ consumption. These results may be competitive with conventional cyanidation,
which showed 85% gold extraction and 0.21 kg C N " consumed/t ore.
Cai (1997) studied the effect of different parameters on gold extraction using ammonium
thiosulfate from oxide ores. It was found that only 9% of the gold was extracted in the
absence of ammonia. Gold extraction increased dramatically with an increase o f
ammonia from 0-2 mol /L ( N H 3 / S 2 0 3
2 + from 0 to 7), beyond which gold extraction was
not increased significantly. When the ammonia concentration was over 4 mol/L, a lower
extraction was obtained. Cai (1997) suggested that addition o f ammonia may lead to
dissolve some sulfide precipitates, such as CuS, C u 2 S and FeCuS 2 . These sulfides are
easily formed due to the nature of the ores and the decomposition of thiosulfate. The
formation of some precipitates such as ( N H 4 ) 5 C u ( S 2 0 3 ) 3 , which could cover the ore's
surface and hinder the attachment of thiosulfate to the ore, was suggested to explain the
detrimental effect of ammonia over 4 mol/L. Cai (1997) also found that the introduction
of ammonium sulfate does increase the gold extraction rate. It was also found gold
extraction was increased when increasing p H from 7.5 to 10.5, then levelled off.
20
Zippering and Raghavan (1998) identified the parameters of importance in the dissolution
of gold and silver values from a rhyolite ore with a high manganese content using
ammonium thiosulfate solution containing copper. The effects o f thiosulfate, ammonia
concentration, temperature, and copper sulfate addition were studied. Optimum
conditions were established at 2 M S 2 0 3
2 " , 4.1 M N H 3 , 6 g/L C u 2 + , 50 °C, and 2 hours
leaching in the absence o f oxygen. In the absence o f cupric ions only 14% of the gold
was extracted. Increasing cupric ion concentration enhanced the initial rate of gold
extraction, but the ultimate extraction was not influenced by the cupric ion concentration
in the range o f studies (up to 6 g/l Cu). Furthermore, it was concluded that maintaining
optimal p H and E h conditions (pH 10 and 200 m V ) were necessary to prevent
precipitation of copper as CU2S. Ha l f o f the thiosulfate in the lixiviant solution at p H 9.5-
10 was consumed during the dissolution process.
Nararro et al (2002) studied gold leaching from a flotation concentrate using ammonium
thiosulfate as leaching agent. The gold content i n the concentrate was 95 g/t, whereas the
main mineralogical species were chalcopyrite, pyrite, pyrrhotite, tennantite and
sphalerite. The highest level of gold dissolution (94%) was obtained at 0 .05M Cu(U), 0.3
M S2O32", p H 10 and 10% pulp density after 15 hours, whereas cyanidation gave the same
yield but required about 46 hours of reaction.
It was reported that gold is extracted without addition o f Cu(H) ions. The reason for this
behaviour is the presence of copper (chalcopyrite) in the concentrate which partly
dissolves as C u ( N H 3 )4 . It was found that the recovery o f gold is enhanced by increase
21
of the thiosulfate concentration (up to 0.3M) but that a negligible effect on gold
dissolution is obtained at higher thiosulfate concentration. Navarro et al (2002) observed
that gold dissolution is enhanced using p H values of at least 9.5. In fact, the recovery o f
gold did not exceed 10% at p H of 9.0.
Comparison of gold dissolution using thiosulfate or cyanide leaching was carried out.
Reasonable conditions were established as, cyanidation: 8.7 kg CNVt, air bubbling, 40%
pulp density and p H 11.0 (adjusted with Ca(OH) 2 ) ; thiosulfate: 117kg S 2 0 3
2 7 t , 4.7kg
Cu/t, pHIO (adjusted with N H 4 O H solution) and 40% pulp density.
Therefore, Nararro et al (2002) came to the conclusion that under reasonable conditions,
thiosulfate leaching compares very favourably with cyanidation. However, reagent
consumption data (especially thiosulfate) and evaluation (scaling-up) o f the ammoniacal
thiosulfate leaching process are still under consideration.
Aylmore and M u i r (2000b) have provided a summary review on thiosulfate leaching of
ores (Table 2.1)
22
Table 2. 1 A summary of various thiosulfate leaching conditions (Aylmore and
Muir, 2000b).
Ore type S 2 0 3
2
(M) N H 3
(M) Cu 2 +
(M) t °C PH A*
% B*
Tozawa et al 1981 Gold plate 0.5 1 0.04 65 Kerley,1981,1983 Sulfide 2%Mn 1.2 1.1 4g/L 7-9 95 4Kg/t Block-Bolten and Tama 1985,1986
Zn-Pb sulfide flotation
0.125-0.5
0.75 21-50
7-9 90 45 lb/t
Zipperian et al,1988
Rhyolite ore 7g/Kg MnO 2
2 4 0.1 35 10 90 50%
Hemmati et al,1989
Carbonaceous 2.5% org C
0.71 3 0.15 50 10.5 73 15-19%
Hu and Gong, 1991
0.048% Mn02,3.19%Cu
1 2 0.016 30-65
95.6
Muthy,1991 Pb-Zn sulfide 0.125-0.5
1 40 6.9-8.5
95
Cao etai, 1992 Sulfide cone. %Cu
0.2-0.3
2-4 0.047 21-70
10-10.5
95 4.8Kg/t
Lanhan etal, 1992 Oxidised ore 0.02% Cu
0.2 0.09 0.001 Room Temp.
11 90 2Kg/t
Wanet al,1994 Carbonaceous 1.4%C,1.0%S
0.1-0.2
0.1 60ppm Room Temp.
9.2-10
Abbruzzese et al,1995
Gold ore 2 4 0.1 25 8.5-10.5
80
Marchbank et al,1996
Pressure oxidized sulfide and carbonaceous ore
0.02-0.1
2000ppm 500ppm 55 7-8.7
70-85
Yan etal, 1996 Gold-copper ore 0.4 0.2 0.03 Room Temp.
11. 90
Wan and Brierley 1997
Carbonaceous sulfite
0.1 0.1 0.005 Room Temp.
9 50.7-65.7
Yan et al, 1998,1999
Gold copper ores ~0.36%Cu
0.5 6 0.1 Room Temp.
10 95-9.7 30Kg/t
Thomas et all998 Pressure oxidized sulfide ore
0.03-0.05
-500-lOOOppm
10-lOOppm
45-55
7.5-7.7
80-85
A * : Gold extraction
B * : Consumption o f thiosulfate
It can be seen that it is not easy to specify an optimum condition for thiosulfate leaching.
The leach conditions depend on the nature of the gold ores being tested.
23
Commercial effects
Kerley et al (1981, 1983) patented a process for the recovery of precious metals from
refractory ores, particularly those containing manganese and/or copper, using ammonium
thiosulfate leach solution. In this patent, Kerley claimed that the sulphite ions inhibit the
decomposition of thiosulfate and thus prevent the precipitation of metal sulphides. The
p H recommended was at least 7.5.
Perez and Galaviz (1987) modified Kerley's process to inhibit F e 3 + oxidation o f
thiosulfate by adjusting the minimum level of p H to 9.5 from 7.5. Trial runs at pilot scale
were carried out at L a Colorado, in the state of Sonora, Mexico. High gold recoveries
were obtained, but there was no report about reagent consumption.
Newmont Gold Company found that ammonium thiosulfate is effective for leaching gold
from bio-oxidized "preg-robbing" ore. This process has been successfully practiced both
in laboratory and pilot plant scale studies. Obtained gold recoveries were 50%-65% and
ammonium thiosulfate consumption was 5.2-8.9 kg/metric tonne. Typical leach solution
was p H 9.2-10.0, 0 .1M S 2 0 3
2 " , 0 .1M N H 3 and 30 ppm C u 2 + ( W a n et al, 1977).
Barrick Gold Corporation patented a combined pressure oxidation, thiosulfate and resin-
in-pulp process for treatment of refractory gold ores (Thomas et al,1998). In this process
ore is ground to 65-85% passing 200 mesh and thickened to about 40-50% solids.
Sodium carbonate is added to ensure that pressure oxidation is carried out under alkaline
conditions and CI" is added to improve the kinetics and to facilitate oxidation. The ore
24
which is pressure oxidised, leaves the autoclave at about 35% solids and is directed to a
leaching operation where it is contacted with ammonium thiosulfate (5 g/L) and copper
sulfate (25 ppm Cu).
Placer Dome Technical Services Limited have applied for a patent on a method for
thiosulfate leaching of precious metal-containing materials (Ji et al, 2001). In this
process, an oxygen partial pressure from about 4 to about 500 psia; A p H less than 9; A n
ammonia concentration less than 0.05M, a copper concentration between 0 to 15ppm; A
sulfite concentration no more than 0.01M; A n d a temperature from about 20 to about
80°C is used. A high level of precious metal extraction can be achieved - 70% and some
times at least about 80% under the above conditions.
2.4 Research on the kinetics and mechanism
2.4.1 Passivation phenomena
Tozawa et al (1981) demonstrated the formation of a copper sulfide coating on gold
preventing it from leaching at a temperature range of 65-100 °C in an ammoniacal
thiosulfate solution containing copper ions (Figure 2.17).
It was suggested that cupric sulfide formed by the reaction between cupric ions and
thiosulfate ions and/or the oxidation of cuprous thiosulfate complex ions led to
passivation:
C u 2 + + S 2 0 3
2 + H 2 0 = C u S + S O 4 2 " + 2 H + Equat ion 2.8
25
Cu(S203)2
3"+ l/202 + H zO = 2CuS + 2S306
2 + 20H Equation 2.9
- J — i — i — i .. i — i — i - — i — i — i — i i i i i i i_ 20 30 40 SO 60 70 80 30 100 110 120 130 140 ISO 160 170 180
T « o i p * r o t u r « (*C)
Figure 2. 17 Effect of copper concentration and temperature on the dissolution of
gold in 0.25 M S203
2", 1.0 M NH3,196 kPa 0 2, stirring velocity 200 rpm (Tozawa et
al,1981).
Ter-Arakelyan et al (1984) found that the use of sodium thiosulfate as a ligand to dissolve
gold was hampered by its oxidation during the leaching process with the formation of an
insulating layer on the gold surface generally consisting o f colloidal sulfur.
Bagdasaryan et al (1983) and Pedraza et al (1988) observed a sulfur layer as wel l as
copper sulfide in the thiosulfate-copper sulfate system. It was suggested that both
elemental and sulfide sulfur can be produced by the decomposition o f thiosulfate in
alkaline solution (Aylmore et al, 2000b).
26
Chen et al (1996) studied gold passivation by electrochemical impedance spectra (EIS) of
gold in 1 mol /L sodium thiosulfate. It was suggested that elemental sulfur may be formed
to prevent thiosulfate diffusion to the gold surface and hence, to inhibit gold dissolution.
The elemental sulfur coating is formed either by adsorption of elemental sulfur or by the
oxidation of sulfide ions on the gold surface:
Au°+S°=Au I S° Equation 2.10
or Au°+S2"=Au|s°+2e" Equation 2.11
Breuer et al (2000b) mentioned that the gold oxidation process is obviously hindered in
thiosulfate solution without ammonia and copper. Also some evidence showed that the
products of either the thiosulfate/copper(IT) or subsequent reactions bring about
passivation of the gold surface .
2.4.2 Kinetics study of gold thiosulfate leaching
Tafel current technique and leaching tests have been applied to study the kinetics o f gold
leaching with thiosulfate by Jiang et al (1993b). The exchange current density ( i c o r r ) was
utilized to identify the dissolution of gold. This investigation verified the catalytic effect
of copper ions and ammonia, it investigated the effect of thiosulfate concentration, p H ,
and C u ( N H 3 ) 4
2 + on icon. It was found that the effect of C u ( N H 3 ) 4
2 + was the most notable.
A s C u ( N H 3 ) 4
2 + was increased from 0.001 to 0.1 M , icon increased from 5.62 to 573.82
| j ,A/cm 2 . The activation energy for gold dissolution was reported to be 27.99 kJ/mol in
27
the absence of cupric ions and ammonia, and decreased to 15.54 kJ/mol in the thiosulfate
solution containing 0.01 M C u ( N H 3 ) 4
2 + and 0.5 M N H 3 .
0.00 0.01 0.02 0.03 0.04 0.05 [CuSOJ (M)
Figure 2. 18 Effect of copper sulfate concentration on gold leaching rate(Li et
al,1996)
L i et al (1996) found that at low copper levels, an increase in the copper concentration
results in a dramatic rise in the gold leaching rate, but, too high a copper concentration
significantly inhibits gold leaching (Figure 2.18).
L i et al (1996) pointed out that to enable the regeneration of the cupric ion, it is necessary
to keep the concentration ratio of ammonia to thiosulfate in a certain range. Increasing
the concentration of only one o f the ligands w i l l have a limited positive effect on the gold
leaching process. However this may have a negative effect, as an excess of either
ammonia or thiosulfate may make the regeneration of the cupric ion less attractive . This
effect is illustrated in Figure 2.19. A t lower ammonia/ammonium thiosulfate
28
concentrations, the gold leaching rate is reduced. There is also a good deal o f scatter in
the results, possibly indicating some passivation type process slowing the leach rate.
0.10 r
a •
8
| 0.08 -
1 0.06 -u
a
hing
0.04 -o o
• o 0.02 -
1 0 . 0 0 * — 1 1 1 — — — *
0.01 0.1 1 10 100
[NH.OHMCra^SA]
Figure 2. 19 Effect of the concentration ratio of ammonia to thiosulfate on gold
leaching ratefLi et al, 1996)
Ouyang (2001) investigated the effects of p H , temperature, rotating velocity and the
concentrations o f different reagents on the gold leaching. The R D E was used. For the
anodic process, it was found that the rotating velocity had no effect on current density.
Using the Levich equation, a limiting current density was calculated that was far higher
than the actual current density, so, it was suggested that the anodic process is under
surface reaction control. For the cathodic process, it was found that there exists a linear
relationship between the limiting current and the square root o f the rotating angular
velocity, which is in agreement with the Levich equation. So, it is suggested that the
cathodic process is under diffusion control or mixed control.
29
In this study, the baseline condition was: 0.2 M (NH4)2S203, 250 ppm [CU]T, p H 10, 25
°C, 450 rpm. The other results as follows (Ouyang ,2001):
ft 9 1
• The magnitude of gold leaching rate is around 1x10" mol .m" . s"
• The gold leaching rate increases with increasing rotating velocity
• The gold leaching rate increases with increasing p H
• The gold leaching rate increases with increasing ammonia concentration
• The gold leaching rate increases as the thiosulfate ion concentration increases from
0.1 to 0.2 M. Further increasing leads to low gold leaching rates
• A s the concentration of ammonium thiosulfate increases, the gold leaching rate
increases constantly
• The introduction of tetrathionate has a detrimental effect on the gold leaching rate • The gold leaching rate increases with increasing copper ion concentration
• A s temperature increases from 25 to 30°C, the gold leaching rate increases a little;
further increasing of temperature decreases the gold leaching rate significantly
• If the same parameter has contradictory effects on the anodic and cathodic process,
the gold leaching rate generally follows the pattern o f the cathodic process
The gold leaching rate can be correlated with different parameters (Ouyang, 2001):
roc[NH3]1-557x[S2032"]"0-63 x [ O H " f 4 4 7 x [ C u ] T
a 6 4 7 x[a>] 0 6 7 5 x T " 0 3 7 2
In recent years, Breuer and Jeffrey published several papers on fundamental studies on
gold leaching in A T S - C u system using the rotating electrochemical quartz crystal
microbalance (REQCM) (Breuer et al, 2000a; Breuer et al, 2000b; Jeffery 2001; Jeffrey
30
et al 2001; Breuer et al 2002). R E Q C M is a very new and powerful technique in
electrochemical studies. This technique w i l l also be used in this project, and w i l l be
discussed in Chapter 3.
The oxidation of gold in A T S - C u system was studied (Breuer et al, 2000a). Figure 2.20
shows the calculated current densities for gold oxidation at various ammonia
concentrations. It seems that passivation occurs with no ammonia, and addition of
ammonia can evidently result in increasing of anodic current densities. Thus, it was
suggested that the action of ammonia is to alter the surface of the gold electrode, reducing
the effect of the surface passivation.
20 -I
'in
Q 10 J
o
NONE 0.2M 0.4M 0.6M
/ //
/'• /'
/.'•
j /
/.''
f
•f /
/ J
/ /
S
s
*
*' -' _ * ^ - ^—
0 100 200 300 400
Potential ( m V v s S H E )
Figure 2. 20 Effect of ammonia on gold oxidation. Experimental conditions: 0.1 M
Na2S203, 30°C (Breuer et al, 2000a).
Also , it was found that the presence of copper (IT) enhances the gold oxidation process.
But there is no further increase in rate when the copper (U) concentration is greater than 2
m M (Breuer et al, 2000a).
31
The effect of various parameters on the gold thiosulfate leaching kinetics and the
undesirable homogeneous copper(U) reduction by thiosulfate have been investigated by
Breuer et al (2000b). The leaching kinetics were measured using a rotating
electrochemical quartz crystal microbalance ( R E Q C M ) . The cupric concentration in
solution was monitored by measuring the limiting reducing current for copper(U) on a
rotating platinum electrode at -150 m V vs. S H E . This study focused on homogeneous
reduction of copper(U) by thiosulfate according to the simplified overall reaction:
2 C u ( N H 3 ) 4
2 + + 8 S 2 O 3 2 " = 2Cu(S 2 0 3 )3 5 " + 8 N H 3 + S 4 0 6
2 " Equat ion 2.12
It was found that the Cu(U) concentration decreases continuously under the following
conditions: 10 m M C u S 0 4 , 0.4 M N H 3 , 0.1 M N a 2 S 2 0 3 , 30°C. This is because Cu(IT)
reacts with thiosulfate rapidly .
It was found that the gold leaching rate is not dependent on the initial Cu(H)
concentration (The Cu(H) concentrations of the tests were 5 m M , and 10 m M ) and further
electrochemical studies showed that copper(U)-thiosulfate reaction products hinder gold
oxidation.
It was found that as the ammonia concentration is increased (the ammonia concentrations
were 0 .2M, 0 .4M, 0.6M), the gold leaching rate decreased, and the copper (U) reductive
rate decreased significantly. This result is contradicted by the investigation of Ouyang
(2001), which showed that at ammonia concentrations of 0.1 M - 0.8 M , the gold
leaching rate w i l l increase with an increase of the ammonia concentration.
32
It was found that the gold leaching rate and the copper (U) reductive rate are increased
when temperature is increased.
It was found that as the thiosulfate concentration is increased (0.05 M , 0.1 M , 0.2 M ) the
gold leaching rate is increased, but the rate of copper (II) reduction is increased, too.
The effect o f the ammonium concentration on the rate of gold leaching and the rate of
reaction between Cu(II) and thiosulfate were investigated. It was found that both are
higher as the ammonium concentration increases.
After compromising between fast leach kinetics and high thiosulfate consumption, Breuer
et al (2000b) suggested the following conditions should be maintained: 0.4 M ammonia,
0.1 M thiosulfate, p H > 11.4 and a temperature of 30°C.
The electrochemistry of gold in thiosulfate solutions containing copper and ammonia was
studied using a combination of standard electrochemical techniques and R E Q C M (Breuer
et al, 2002).This work was different from the previous work (Breuer et al, 2000a; Breuer
et al, 2000b) due to the pure gold rather than a gold silver alloy was coated on electrode.
The baseline conditions were: 0 .1M sodium thiosulfate, 0.4 M ammonia, and 0.01 M
copper sulfate. Experiments were performed at 30°C, with a rotating rate of 300 rpm and
a scan rate of 1 mVs" 1 . The solutions were saturated with air for cathodic studies, and
deaerated using argon for anodic studies.
33
Initially, the R E Q C M was used to study the leaching of gold at a fixed potential o f 238
m V S H E (Figure 2.21). This is the mixed potential observed during leaching. In solutions
containing solely thiosulfate, very little mass change is observed. When ammonia is
added to the solution, the gold leaches more readily, but the reaction rate is substantially
lower than the measured gold leaching rate in the copper-ammonia-thiosulfate solution.
E - 1 5
Thiosulfate only
Thiosulfate + ammonia
Thiosulfate + ammonia *N%
+ copper(ll) V » N
' ' 1 ^ 1
100 200 300
t / S
Figure 2. 21 Kinetic plot showing the leaching of gold at fixed potential of 238 mV
SHE in a solution containing either thiosulfate, thiosulfate and ammonia, or
thiosulfate, ammonia and copper (Breuer et al, 2002).
In cathodic studies (Figure 2.22), oxygen reduction occurs at potentials more negative
than 50 m V S H E in the absence of copper; the reduction of the copper (Uj-amine
complex occurs at potentials more negative than 150 m V S H E in the absence of S2O3 2",
the current density reaches a limiting plateau at -150 m V S H E , at these potentials, the
34
reduction of copper (II) is limited by the mass transfer of cupric tetrammine to the
platinum surface. The most obvious change is the difference in the copper (H) reduction
potential in the presence and absence of thiosulfate which indicates that copper (IT) is
more readily reduced in a solution containing thiosulfate.
-5
"1 -15-|
-20
-25 H
-30
-35-1 -40 -200
Cu(ll). NH, / /
Cu(ll), NH,, SjO^'
-100 100 —I— 200 300
E/mV
Figure 2. 22 Linear sweep voltammograms for the reduction of oxygen in ammonia-
thiosulfate solutions, and for the reduction of copper in ammonia and ammonia-
thiosulfate solution (Breuer et al, 2002).
In anodic studies, it is clear from Figure 2.23 that in the presence of copper (U), the gold
oxidation reaction is rapid and occurs at low overpotentials. Brueuer et al (2002)
identified that copper (II) is dominant in promoting the gold oxidation half reaction. Even
though the solution containing copper (I) does not contain copper (U), it is believed that
there w i l l be copper (H) present at the electrode interface. This is the result of the
concurrent oxidation of copper (I) to copper (U), which occurs readily at potentials more
positive than 100 m V in these solutions.
35
Figure 2. 23 Linear sweep voltammogram for a gold electrode in solutions
containing copper ( solid line). Also shown is the calculated partial current density
for the oxidation of gold to gold thiosulfate derived from the mass change (dashed
line) measured using REQCM. Also shown as a square symbol is the measured
mixed potential and reaction rate(as current density) during leaching (Breuer et al,
2002)
It was found that copper (H) concentrations (0, 0.5, 2, 10 m M ) have a significant effect
on the gold oxidation half reaction. It is recommended that a copper (H) concentration
greater than 2 m M should be maintained in order to achieve appreciable gold oxidation
rates. Also , it was reported that an increase in the thiosulfate concentration (0.05, 0.1, 0.2
M ) results in an increase in the current due to gold oxidation.
36
The effect o f temperature on the gold oxidation half reaction was investigated. It was
shown that increasing the temperature also significantly enhances the reaction rate. A t a
potential of 200 m V S H E , an activation energy o f 55 kJ .mor 1 was obtained that is quite
high and indicative of the reaction being chemically controlled (Breuer et al, 2002).
2.4.3 Mechanism of gold thiosulfate leaching
Many researchers studied the electrochemical-catalytic mechanism of ammoniacal
thiosulfate leaching of gold (Jiang et al 1993, Zhu et al 1994, L i et al 1996, Breuer et al
2000a, Ouyang 2001, Almore et al 2001). However, there still are some conflicts in these
studies, thus, no model is generally accepted.
Jiang et al (1993) investigated systematically the electrochemistry of gold leaching with
thiosulfate. A stationary gold electrode was used. The results showed that the current
peak o f anodic dissolution of gold occurs at about 50 m V (SCE) and thiosulfate oxidation
occurs at 620 m V ( S C E ) . Ammonia remarkably improves the anodic dissolution rate of
gold and reduces the passivation as well as makes the current peak shift negatively. It was
found that cupric ions and cupric-ammonia ions have no evident effect on the anodic
process. A current step technique was used to investigate the role o f ammonia in the
anodic process and the adsorption mechanism of ammonia was excluded. After this
study, Jiang et al (1993a) proposed the anodic dissolution mechanism of gold as
following:
Au-»Au ++e" Equation 2.13
Au++2NH 3-»Au(NH 3) 2
+ Equation 2.14
37
Au(NH3)2
+ + 2S203
2"-> Au(S203)2
3" + 2NH3 Equation 2.15
The study of the cathodic process showed that the addition o f cupric ammine ions
actively changes the behaviour o f the cathodic process, ammonia and cupric ions have no
direct effect on the cathodic process. The reduction of C u ( N H 3 ) 4
2 + to C u ( N H 3 ) 2
+ was
supposed to be the real cathodic reaction. The regeneration of C u ( N H 3 ) 4 2 + was carried out
by oxidation by oxygen. It was suggested that thiosulfate is not involved in the cathodic
process. The mechanism of the cathodic process was put forward as follows:
Cu(NH3)4
2+ + e" = Cu(NH3)2
+ + 2NH3 Equation 2.16
Cu(NH3)2
+ + 1/2 O z + H 2 0 + 2NH3 = Cu(NH3)4
2+ +20H' Equation 2.17
The mechanism model proposed by Jiang et al (1993a) is showed in Figure 2.24.
Gold Surface
Anodic Area Au = Au%e / A u V 2 N H 3 = A u C N H 3 > 2 Sy.
Au
Cathodic A r e a .2* Cu(NH3>4 +e=Cu(NH 3 )2
Solution N M 3
t S a 0 | - A u ( S 2 0 3 > g -
Au(NH 3>2
C u ( N H 3 ) f * t
0 2 — O H "
+ Cu(NH 3 >2
Figure 2. 24 The model of electrochemical-catalytical mechanism of ammoniacal
thiosulfate leaching of gold (Jiang et al, 1993a)
38
Zhu et al. (1994) investigated the dissolution of gold in aqueous thiosulfate solution and
the effect of the presence of ammonia in the solution the methods of voltammetry and
electrochemical impedance spectroscopy (EIS). The electrochemical impedance spectra
of gold in the sodium thiosulfate solution in the absence of ammonia accords with the
active-passive electrochemical process on the metal surface. The authors attribute this to
the formation of elemental sulfur and thus the elemental sulfur at the gold surface
passivates the gold dissolution:
S2O3 2 ->S° + SO32" Equation 2.18
S 20 3
2 + 6 O H S O 3 2 " + 2S2' + 3H20 Equation 2.19
The elemental sulfur may absorb on the gold surface and anodic process may occur on
the gold electrode surface as:
S2" + Au^Au|S° +2e' Equation 2.20
The presence of ammonia eliminates the passivation phenomena even in the absence o f
copper ion. It was explained that ammonia prevents the gold electrode from passivation
by sulfur coating by being preferentially adsorbed on the gold surface to dissolve gold as
an amine complex, as was suggested by Jiang et al (1993a). This explanation was
supported by the fact that aqueous ammonia addition was superior to ammonium sulfate
addition.
Normally, gold dissolution in ammoniacal thiosulfate solution in the presence of oxygen
is described by the following reaction:
39
4Au + 8S203
2" + 2H20 + 0 2 = 4Au(S203)2
3" + 40H Equation 2.21
To reflect the influence of the copper catalyst in the presence o f ammonia during gold
dissolution, L i et al (1996) modified this reaction as:
Au + 5S203
2' + Cu(NH3)4
2+ Au(S203)2
3" + Cu(S203)3
5"+4NH3 Equation 2.22
Anodic reaction:
The reaction for C u ( N H 3 )4 regeneration:
Cu(S203)3
5' + 4NH3 + l/202 + H20-> Cu(NH3)4
2+ + 3S203
2' + 20H" Equation 2.25
This mechanism is quite different from Jiang's work (Jiang et al, 1993a). It shows that
ammonia does not directly take part in the anodic reaction. Also , it shows that both the
ammonia and thiosulfate ligands are involved in the cathodic reaction. To generate
C u ( N H 3 ) 4
2 + , it is critical to keep the concentration ratio o f ammonia to thiosulfate in a
certain range according to this study.
Breuer at al (2000a) studied the electrochemical aspects of gold oxidation in solution
containing thiosulfate, ammonia and copper using a rotating electrochemical quartz
Au + 2S203
2" -> Au(S203)2
3" + e Equation 2.23
Cathodic reaction:
Cu(NH3)4
2+ +3S203
2_+e" -> Cu(S203)3
5 + 4NH3 Equation 2.24
40
crystal microbalance ( R E Q C M ) . The quartz electrode was coated with a gold/silver alloy
that contained 2 wt% silver by electroplating. From the linear sweep voltammogram of
gold oxidation in thiosulfate (without copper and N H 3 / N H / ) the gold oxidation process
was found to be obviously hindered. It has been suggested that thiosulfate can
disproportionate in alkaline solutions to form sulfide on the gold surface.
3S 20 3
2" + 6OH" -> 4 S 0 3
2 " + 2S2" + 3HzO E q u a t i o n 2.26
Breuer et al (2000a) confirmed that ammonia/ammonium can reduce the passivation, but
disagreed that the oxidation of gold in ammonia thiosulfate solution occurs via
A u ( N H 3 ) 2
+ (Jiang et al, 1993a), because the standard reaction potential of A u ( N H 3 ) 2
+
(0.572 V vs. SHE) is more positive than the potential in which passivation has been
evidently reduced. It was suggested that the action o f ammonia is to alter the surface o f
the gold electrode, reducing the effect of surface passivation.
Breuer et al (2002) studied this process again but the quartz electrode was coated with
pure gold. This study shows that the dominant cathodic reaction appears to be the
reduction of copper(U) tetramine to a copper(I) thiosulfate complex, and this reaction
proceeds quite rapidly.
Ouyang (2001) studied the electrochemical polarization of gold using the R D E . It was
found that ammonia, thiosulfate and copper species all affect the anodic process. The
above species plus oxygen are involved in the cathodic process. The role o f copper
species are to facilitate oxygen reduction through the Cu(NH 3)4 2 + /Cu(S 2 0 3 )3 5 " couple.
41
The gold leaching rate in ammonium thiosulfate in the presence of copper species is
mainly controlled by the cathodic process, which is under diffusion or mixed kinetic
control. The results of this study support the mechanism proposed by L i et al (1996) as
shown in Figure 2.25. It is worth noting that the copper enhancing the gold anodic
oxidation rate was found in coulometric tests. Ouyang (2001) suggested that the copper
ion, like NH3, somehow activates the gold surface further to accelerate the gold
dissolution rate.
Anodic Zone
S A 2 -
Au(SAV-
J1NH3MV,
jCufSp,),5-
Passivation film
To cathode
C a t h o d i c Z o n e
•> CuCSjO, ) , 8 - • 4 N H , 5j +2H 2 G + 4«"-»
Surface accumulation
Cu<SiO^^Nfr,50H- Reactant concentration in
A u - Bulk solution
CU<NHJ 4*>O; ' 401*8,0,),* • O , +16NHj+ 2H,0 e , Surface depletion —>
4Cu(NHJ4'* • 12SJO,*- + 4 0 H "
F r o m a n o d e Dif fusion layer
Figure 2. 25 Mechanism of gold leaching in ATS system in the presence of copper
ion (Ouyang, 2001)
1
42
After combining the model by Jiang et al (1993a) and L i et al(1996), Aylmore et al
(2001a) suggested the electrochemical -catalytic mechanism o f gold leaching in A T S - C u
system as Figure 2.26.
Gold surface
Anodic i r a
Au=Au* • c A o * - f 2 N H , = Au(NH,) 1* Au* • 2SJO,1" = AuCSiO,)!*"
A u
Clthodic mrtM
Cu(NH,) /* + « = Cu(NH,),* + 2NH, Cu(NH,),* • 3S,0>^ = C I K S J O J , 5 " • 2NH,
Cu(NHJ)4 ,+S^), ,'
Cu(NHj),* + 2NH 3
Figure 2. 26 The electrochemical model for the copper catalysis mechanism of
leaching gold with ammoniacal thiosulfate.( Aylmore et al, 2001a)
In Figure 2.26, It is proposed that Cu(NH .3 )4 2 + species present in solution acquired
electrons on the cathodic portion of the gold surface and is directly reduced to
Cu(NH3)2 + .At the same time, either ammonia or thiosulfate ions react with A u + ions on
the anodic surface of gold and enter the solution to form either A u ( N H 3 ) 2
+ or Au(S203)23".
Depending on the concentration of S2O32", C u ( N H 3 ) 2 + converts to Cu(S203)35" ions, and
likewise for A u ( N H 3 ) 2
+ . Both the Cu(S203)35" species in solution are then oxidized to
43
C u ( N H 3 ) 4 with oxygen. The predominant cathodic reaction w i l l depend on the relative
concentrations o f the species in solution.
2.5 Additives studies
A s the consumption of the thiosulfate is a key issue which retards the commercial
application of the thiosulfate leaching process, part of the current research focus was on
the effect of additives in the A T S - C u system. The goal was to try to find suitable
additives that could enhance the gold leaching rate or hinder the degradation of
thiosulfate.
Kristjansdottir et al (1996) claimed that pyridine could enhance the gold leaching in A T S
- C u system (Figure 2.27). The solution was 0 .1M sodium thiosulfate, 0 .01M copper
sulfate, 0 .5M ammonium hydroxide. 10"3 m L pyridine was added into 50 m L solution.
Figure 2.27 shows that the sample with pyridine gives a much faster rate of gold
dissolution than the sample without the activator.
44
TIME (minutes)
• Thiosulfate + Pyridine —•—Thiosu l fa te
Figure 2. 27 The effect of pyridine on gold leaching in ATS-Cu system
L i and Kuang (1998) report that: in thiosulfate leaching of gold from both oxide and
sulfide ores, adding of N a C l instead of Cu(H) significantly increased the gold extraction.
It is suggested that the formation of intermediate [AuCl 2 ] " facilitates gold oxidation:
[AuCl2]~ + 2 S 20 3
2 -> [Au(S203)2]3" + 2C1" Equation 2.27
However it is questionable since the standard reduction potential o f Au/[AuCl 2 ]~ is as
high as 1.154 V vs. S H E (Molleman, 1998) and without a strong oxidant, gold is very
unlikely to form [AuCl 2 ] " in solution.
45
It was also reported that the addition of sodium dodecyl sulphonate (SDS) significantly
increased the gold extraction. This was reported to be due to the fact that SDS can
decrease the solution surface tension, and increase the leaching rate.
C. X i a et al (2002a, 2002b) investigated the effect of some additives on gold leaching in
the A T S - C u system. The main sample used in this investigation was a copper-bearing
mild-refractory low-grade gold ore. The grade was ranged from 2.80 to 3.12 grams per
ton. There was 10.4% pyrite, 5.1% iron oxide and 0.5% chalcopyrite in this ore. In tests,
the standard reagent composition was 0 .3M ( N H 4 ) 2 S 2 0 3 , 0 .03M C u S 0 4 and 3 M N H 3 .
Solution p H was 10.2. It was found that the thiosulfate consumption could be reduced
from 30kg/t to about 17kg/t by adding strong chelating agents ( E D T A , N T A ) at a proper
ratio ( E D T A : C u =1:1). The same improvement could be achieved by reducing copper
sulfate addition to a much lower level (0.00075M) or replacing copper sulfate with nickel
sulfate at 0.001M. It also found that the gold extraction was actually improved in these
three cases. The copper catalyst amount required for an acceptable gold extraction is
proved to be much smaller than 0.03M in this case. It was believed that the strong
chelating agents like E D T A and N T A are possible additives for optimizing the catalysis
condition and minimizing the thiosulfate consumption.
D . Feng et al (2002a) investigated the role o f heavy metal ions in gold dissolution in the
ammoniacal thiosulfate system. It was found that the effect of heavy metalllic ions on
gold dissolution strongly depended on the type of ion and concentrations, and reagent
concentrations. A t high reagent concentrations, Pb could only enhance gold dissolution
46
within a concentration of 50 mg/L, and reached the largest extent at 5 mg/L. Over 50
mg/L, Pb decreased gold dissolution. At low lead concentrations, the predominant species
for Pb in the leaching region could be Pb(OH) + in the Eh-pH diagram of the Pb-Au-NFL;-
S2O3 2 " system. AuPb 2 could also be present in the leaching region, enhancing the gold
dissolution. PbO could possibly be the predominant species at high Pb concentrations.
This could explain why Pb accelerated gold dissolution at low concentrations, and
retarded it at high concentrations. Zn resulted in a sharp decreased gold dissolution over
10 mg/L. Cd, Co, Cr and N i retarded gold dissolution at any ion concentration. The gold
dissolution rate in the C o 3 + / C o 2 + leaching system was much lower than that in the
Cu 2 + /Cu + system, while only very limited gold dissolution was observed in the C r 6 + / C r 3 +
system.
47
3. E X P E R I M E N T A L A P P R O A C H
3.1 Introduction
In the last twenty years, a new approach to examining electrodes and their interfaces, the
electrochemical quartz-crystal microbalance ( E Q C M ) , has emerged as a powerful
technique capable of detecting very small mass changes at the electrode surface that
accompany electrochemical processes.
This relatively simple technique only requires, in addition to conventional
electrochemical equipment, an inexpensive radio-frequency oscillator, a frequency
counter, and commercially available AT-cut quartz crystals. So far, the E Q C M has
evolved into a routine experimental method used in numerous electrochemical
laboratories.
However, the E Q C M just uses a stationary electrode by which it is not possible to obtain
reproducible and defined hydrodynamic conditions. Thus, in the last decade, a rotating
E Q C M ( R E Q C M ) was developed to solve this problem (Ritchie et al 1994; Zheng et al
1995; Mendez-Soares et al 1998: Shirtcliffe et al 1999; P. Kern et al 2000: Jeffrey et al
2000 a).
So far, no commercial product is available for the R E Q C M , Jeffrey et" al (2000)
developed a R E Q C M for the study of leaching and deposition of metals. A large number
of hydrometallurgical studies were carried out with this equipment.
48
In summary, the R E Q C M is a powerful technique for electrochemical studies in
hydrometallurgical process.
I) It can measure mass change in real time. So, it is the quickest, simplest and
most accurate method to study electrochemical or leaching kinetics.
II) It can distinguish the side reactions from:main electrochemical reactions.
III) It is useful to study the mechanism of electrochemical reactions.
IV) A R D E is used in the R E Q C M . Therefore, good hydrodynamic reproducibility
is available.
V) It is convenient to determine whether a reaction is chemically controlled or
diffusion controlled.
The electrochemical dissolution of gold in thiosulfate solution is complicated by the
oxidation of thiosulfate or other thiosalts at the electrode surface. Use of the R E Q C M for
the study of gold leaching in thiosulfate allows for direct measurement of gold mass
change, minimizing the interference of side reactions with the gold leaching kinetics.
Therefore, the R E Q C M was chosen as the main technique in this work.
3.2 Basic principle of R E Q C M
Piezoelectric Effect
In 1880, Jacques and Pierre Curie discovered that mechanical stress applied to the
surfaces of various crystals resulted in an electrical potential across the crystal whose
4 9
magnitude was proportional to the applied stress. This behavior is referred to as the
piezoelectric effect.
The piezoelectric effect exists only in materials that are acentric, that is, those that
crystallize into noncentrosymmetric space groups. A single crystal o f an acentric material
w i l l possess a polar axis due to dipoles associated with the arrangement of atoms in the
crystalline lattice. The charge generated in a quartz crystal under mechanical stress is a
manifestation of a charge in the net dipole moment because of the physical displacement
of the atoms and a corresponding change in the net dipole moment. This results in a net
change in electrical charge on the crystal faces, the magnitude and direction of which
depends on the relative orientation o f the dipoles and the crystal faces.
Curie also discovered the converse piezoelectric effect, in which the application o f a
potential across these crystals resulted in a corresponding mechanical strain. It is this
effect that is the operational basis of the E Q C M .
QCM and AT-cut quartz
The E Q C M is actually the electrochemical version of the Q C M , which has long been
used for frequency control and mass sensing in vacuum and air. The Q C M consists o f a
thin, AT-cut quartz crystal with a very thin metal electrode "pad" on opposite sides of the
crystal. The terminology " A T " simply refers to the orientation of the crystal with respect
to its large faces; this particular crystal is fabricated by slicing through a quartz rod at an
angle of approximately 35° with respect to the crystallographic x axis. The electrode pads
50
overlap in the center of the crystal with tabs extending from each pad to the edge of the
crystal where electrical contact is made. When an electrical potential is applied across the
crystal using these electrodes, the AT-cut quartz crystal experiences a mechanical strain
in the shear direction to that resulting from the opposite polarity. Therefore, an
alternating potential across the crystal causes vibrational motion o f the quartz crystal with
the vibrational amplitude parallel to the crystal surface and the x direction.
Sauerbrev Equation
The vibrational motion of the quartz crystal results in a transverse acoustic wave that
propagates back and forth across the thickness of the crystal between the crystal faces.
Accordingly, a standing-wave condition can be established in the quartz resonator when
the acoustic wavelength is equal to twice the combined thickness of the crystal and
electrodes. The frequency f0 o f the acoustic wave fundamental mode is given by the
equation:
f0=— Equat ion 3.1 2tg
Where Vtr is the transverse velocity o f sound in AT-cut quartz (3 .34xl0 4 m s"1) and tQ is
the resonator thickness.
The acoustic velocity is dependent on the modulus and density of the crystal. The quartz
surface is at an antinode of the acoustic wave, and therefore the acoustic wave propagates
across the interface between the crystal and a foreign layer on its surface. A n assumption
is made here that the velocity of sound in quartz and the electrodes is identical; while not
51
rigorously true, for a small electrode thicknesses the error introduced by this
approximation is negligible. In this case, a change in thickness o f the foreign layer is
tantamount to a change in the thickness of the quartz crystal. Under this condition, a
fractional change in the thickness results in a fractional change in the resonant frequency;
appropriate substitutions yielding the well-known Sauerbrey equation.
Sauerbrey demonstrated that the resonance frequency shift o f the quartz oscillator is
inversely proportional to a change in deposited mass for small mass changes: (Ward,
1995)
-A(MepQr5Af Am = —^ Equation 3.2
2 / o
Where: A : the piezoelectrically active area (m 2)
P Q : the density of quartz (kg/m 3)
U Q : the shear modulus of quartz (N/m 2 )
f0: the resonance frequency of vibration(l/s)
Am: the change of mass (kg)
Af: the shift in the resonance frequency(l/s)
This equation is the primary basis of most Q C M and E Q C M measurements wherein mass
changes occurring at the electrode interface are evaluated directly from the frequency
changes of the quartz resonator.
52
To achieve a good correlation between A m and A f (to obey the Sauerbrey equation), the
metal coatings need to be rigidly coupled to quartz and have constant surface roughness
(Jeffrey et al 2000).
Faraday law
The mass change can be converted into a calculated equivalent current density for gold
oxidation using Faraday's Law, as shown in the following equation:
nF dm AM dt
E q u a t i o n 3.3
Where n is the number of electrons transferred per atom of gold oxidized. F is the
Faraday constant (96484 C mol" 1), A is the surface area of the electrode m 2 , M is the
atomic mass o f the metal, m is the measured mass of the electrode (kg), and t is the
elapsed time of the experiment(s).
Rotating disk electrode
a) Rate- l imit ing-step
Heterogeneous chemical and electrochemical processes proceed in three main steps:
(i) transport o f reactants from the bulk solution to the solid-solution interface;
(ii) chemical/electrochemical reactions at the solid surface;
(iii) transport of the products away from the interface and into the bulk solution.
The rate at which the overall reaction takes place is determined by the slowest of the
three steps described above. This is referred to as the rate-limiting step. Thus, in order to
53
control the kinetics of an industrially important reaction, it is important to determine the
nature of the rate-limiting step for the process.
If step (ii) controls the overall speed of reaction, then the process is said to be chemically
controlled and marked increases in reaction rate can be achieved by raising the system
temperature, since chemically controlled reactions generally have high activation
energies. If any o f the other steps control the reaction rate, the process is said to be
diffusion controlled and increases in reaction rate can be simply achieved by increasing
the agitation of the solution.
b) RDE
The rate at which reactants are transferred to the so l id - solution interface, often called the
flux, can be critically dependent on a range of experimental variables, including the
sample geometry, solution agitation and the vessel design. Thus, it is difficult to get the
same results in different tests for the same experiments. However, the problem o f
irreproducibility can be overcome i f the solid sample is in the form o f a disc, which can
be rotated about its central axis, with one face exposed to the reactant solution. When the
disc is used as an electrode, it is known as a rotating disc electrode (RDE) . The fluid flow
to such a disc surface is laminar over a wide range of conditions and is very reproducible.
Therefore, when experiments using a given set of experimental variables are performed
with a rotating disc apparatus, the values of flux obtained from different experiments and
by various experimentalists are reproducible.
54
c) Levich equation
A R D E can continuously supply reactants to the electrode surface, the fluid flow to the
disc surface is laminar over a wide range o f conditions. Thus, the rates of mass transfer
can be controlled and good reproducibility is available. The rotating disc apparatus has
the added advantage that the flux of reactants to the disc surface can be calculated from a
theoretically obtained expression known as the Levich (1962) equation,
Ja = 0.62Do
2/ V W 2 [ o ] Equation 3.4
Where J 0 is the flux of species O (mol m ' V 1 ) , D 0 is the diffusion coefficient of species O
in solution (m 2 s"1), v is the kinematic viscosity of the solution ( m V 1 ) , co is the rotating
angular rates (s_ 1)and [O] is the bulk solution concentration of species O (mol m"3).
The Levich equation was based on the laminar flow condition, which is described by
Reynolds number Re:
coR2 . „ m Re = Equation 3.5 v
Where R was the radius of the electrode disk (m)
When the Re exceeds a critical value (1.8~3.1 x l O 5 ) , the fluid flow regime changes
qualitatively from laminar to turbulent. B y using the Levich equation, it is a simple
matter to compare the calculated flux of reactants with the measured reaction rate for a
55
diffusion controlled reaction. The dependence of the rate upon co is the simplest method
of determining whether a heterogeneous reaction is diffusion or chemically controlled.
3.3 REQCM
3.3.1 Design
The R E Q C M is based on harmonic oscillations and the piezoelectric effect. When an
alternating potential is applied to the quartz crystal, mechanical oscillations with an
amplitude parallel to the surface of the crystal occur. A t a sufficiently high frequency (of
the order of 10 M H z ) , resonant oscillations take place. In order to apply a potential to the
quartz crystal, there are thin metal coatings, which act as electrodes, on either side. If one
of these electrodes reacts with the solution in some way, the resultant change in mass,
Am, causes a shift in the resonance frequency, Af, of the quartz .
The circuitry
A n oscillation circuit is used in the construction of the E Q C M . The A C excitation for the
quartz crystal is generated by the T T L digital oscillation circuit; this oscillation is in the
form o f two out-of-phase 5 V squarewaves to either side of the crystal. When one side of
the crystal is at 5 V , the other side is at 0 V and vice versa. This causes the crystal to
oscillate at its resonance frequency (Jeffrey et al, 2000a).
56
REQCM electrodes
The R E Q C M electrodes were constructed using 10 M H z A T cut quartz crystals. Each 12
mm crystal was mounted onto a hollow P V C cylinder (11.95 mm inner diameter)
containing a 12.5 mm X 0.5 mm recess, as shown in figure 3.1. The quartz crystal
electrodes were prepared by sputtering a layer of either platinum or gold onto the
electrode. The platinum or gold was sputtered with a Balzers Union sputter coating unit
(model 020). The flag o f the crystal was covered with a thin layer o f insulating varnish to
leave a disc of metal. To study the chemical or electrochemical behaviour of the material
of interest, the material must be electrically connected to the surface o f the electrode.
This can be accomplished by a number of techniques, such as sputtering, evaporating,
electroplating, adsorption or self-assembly (Jeffrey et al, 2000a).
The diagram of R E Q C M electrode is shown in Figure 3.1.
57
Holder
(b)
Figure 3.1 A diagram of the REQCM electrode (Jeffrey et al, 2000a)
(a) An end view of the electrode
(b) An enlarged view of the end of the electrode
(c) The cross-section of the electrode
3.3.2 Equipment set-up
The frequency o f the crystal oscillation was measured with an FC-7150 frequency
counter, which was interfaced with an I B M compatible P C through the serial port. The
electrochemical experiments were performed using an E G & G P A R 273A potentiostat
and data acquisition was completed with software which was custom written in Q-Basic.
This software allowed the real-time measurement and analysis of potential, current and
frequency responses.
58
Mass changes were measured simultaneously with current measurement using the
R E Q C M . A platinum wire was used as the counter electrode. A l l potentials were
measured relative to the saturated calomel electrode, but are reported relative to the S H E .
The set-up of equipment is shown in Figure 3.2 and Figure 3.3.
• • ^ J Drive pulley
+5V
>y Support frame
E Q C M Circuit
Reference electrode
Electrolyte bridge
Figure 3. 2 Assembly of cell and REQCM system
59
Frequency counter
R E Q C M
Computer-
Prmtef
Cell
Work electrode
Reference electrode
Counter electrode
H i 1
RE
fCE,fl
f E G & G PARlyfodel273'; I* PbtehtiQstet/Gafyanostat
Figure 3. 3 Schematic il lustration of R E Q C M
3.3.3 Validation of the equipment
To make sure the reliability and accuracy of the R E Q C M , the validation of the Sauerbrey
equation and Levich equation by the method recommended by Jeffery et al (2000 a) was
accomplished.
Validation of the Saubrev equation
It was also necessary to establish that the circuitry of the R E Q C M was operating properly
and to determine whether the frequency response conformed to the Sauerbrey equation.
This was accomplished by measuring the rate of electrodeposition of silver from a
solution containing silver cyanide, a process which has been shown to occur at close to
100% current efficiency (Zheng et al 1995). Shown in figure 3.4 is the change in mass
(calculated using the Sauerbrey equation) versus the time response for the electroplating
of silver at 0.1 m A from a solution containing 0.01 M silver cyanide. It is clear that, as
the silver is deposited, the mass of the electrode increases linearly with time. The rate of
plating with silver can be simply estimated from the slope of the data, which corresponds
to 5.17 X I 0 " mol m" s" . Using the data in figure 3.4, the current efficiency is calculated
to be 99.0% (it was 97% in Jeffery's work (Jeffery et al 2000a)). It is thus clear that the
Sauerbrey equation gives a good correlation between changes in mass and the frequency
response of the quartz crystal during the electrodeposition of silver.
61
18
16 y=0.1115x-0.4687.
F^ = 0.9988,^*" 14H
2
« 1 2 i y cn c JS 8
a -1 4H
.2 0 20 40 60 80 100 120 140 160
Time(s)
Figure 3. 4 A kinetic plot showing the deposition of silver from a solution containing
0.01 M AgfCN)2 "and 0.02 M CN". The experimental conditions were 0.1 mA, 700
rpm and 25° C.
Validation of the Levich equation
In this study, hexacyanoferrate(IU) was used (with 0.1 M sodium perchlorate as
background electrolyte), because its reduction was found to be diffusion controlled at
sufficiently negative potentials. Shown in figure 3.5 is a plot of i against com at a fixed
potential of -250 m V . From this graph, it is clear that the experimental data obey a linear
relationship, which is consistent with the mass transfer to the surface o f the R E Q C M
conforming to the Levich equation. This point is further demonstrated by calculating the
diffusion coefficient of hexacyanoferrate(III) from the slope of the data. According to
Lide (1995), v for water at 25°C has a value of 0.89 X 10"6 m 2 s"1. The calculated value
of the diffusion coefficient is thus 0.732 X 10"9 m 2 s"1, which is in good agreement with
62
the reported value of 0.746 X 10 " 9 m 2 s"1 for hexacyanoferrate(m) in 0 . 1 M N a C l O 4
(Fogg and Gerrard 1991) and 0.74 X 10 " 9 m 2 s"1 in Jeffrey's work (Jeffrey et al, 2000). It
can therefore be concluded that the mass transfer to the surface of the R E Q C M electrode
does conform to the Levich equation
Figure 3.5 Validation of the Levich equation by measuring the
hexacyanoferrate(III) reduction current density as a function of com. The
experimental conditions were 0.01 M FefCN) 6
3", 0.1 M NaC104, -0.25 V vs. SHE
and 25°C
63
3.4 Experimental
3.4.1 Reagents
A wide variety of reagents were used in this study. Table 3.1 below gives a listing of
specifications and sources for most chemicals.
Table 3.1 The specifications and sources of reagents
Reagents Specification Category # Sources
(NH 4 )2S 2 03 >99% 33672-6 Aldr ich Chem. Com., Inc
C u S 0 4 . 5 H 2 0 Class I B C-489-500 Fisher Scientific
N a 2 S 2 0 3 >99 S-1648 Sigma Chem. Co.
N a O H 5 N solution SS256B-500 Fisher Scientific
H 2 S 0 4 I N solution SA212B-1 Fisher Scientific
Buffer solution p H 4,7,10 - Fisher Scientific
N H 4 O H 5 N Solution, 28-30% A-669-225 Fisher Scientific
A i r - - Praxair Tech. Inc
Nitrogen - - Praxair Tech. Inc.
Anthraquinone
-2-sulphonic
acid(AQ)
97% A9,000-4 Aldr ich Chem. Com., Inc
C o ( N H 3 ) 6 C l 3 99% 48.152-1 Aldr ich Chem. Com., Inc
A g N 0 3 0.1025N 31,943-0 Aldr ich Chem. Com., Inc
P b N 0 3 Crystals A . C . S M C I B
E D T A 99.88% Sigma Chem. Co.
N T A 99% N840-7 Aldr ich Chem. Com., Inc
64
3.4.2 Experimental procedure and conditions
Gold coating procedure
For all experiments, gold was electroplated onto the platinum electrode at 25Am" from a
solution containing 8 g/L K A u ( C N ) 2 , 25 g/L K C N , 25 g/L K 2 H P 0 4 and 25 g/L K 2 C 0 3
prior to each experiment.
To ensure the quality of the coating, there were 2 steps in the coating procedure. A t first,
a 5mA (250A/m 2 ) current was used for coating gold from 0 to -100 pg. Finally, a 0.5 m A
current was used from 100 pg to -300 pg. Rotating speed is controlled at 700 rpm.
After each test was completed, the remnant gold on electrode was dissolved in 0 .1M
N a C N solution.
A fresh gold coating was used for each test.
The solution preparation procedure
The procedure was to add the appropriate amount of water to the vessel, which was on
the top of a magnetic stirrer, and start the stirrer, then add a certain amount of ammonium
thiosulfate (or Sodium thiosulfate) into the vessel. If necessary, a certain amount of
ammonia was added into solution. A certain amount of cupric sulfate was added into
solution i f it was required. Also , a certain amount of additives were added in additive
65
studies. Adjust p H to desired value using N a O H solution, then transfer solution into
volumetric flask and add water i f necessary. Ful ly mix the solution.
Only fresh solution was used for each test.
Anodic polarization procedure
About 150 m L fresh solution was put into the cell, then the circulator was started to keep
a constant temperature. Solution was purged with nitrogen gas for 15 minutes, then the
electrode was put into solution, and rotation was started. After the purge time was up, the
purge was stopped, but the purge gas was kept in the cell above the solution. After
making sure that no bubbles were on the electrode surface and in the Luggin capillary,
the experiment was started.
Baseline conditions:
a) N o copper
0.2 M (NH 4)2S 203, 25°C, p H 10 ,450 rpm, nitrogen purge and scan rate o f 1 mV/s
b) Wi th copper
0.2 M (NHi)2S203, 250 ppm copper, 25°C, p H 10, 450rpm, nitrogen purge and scan rate
of l m V / s
Cathodic polarization procedure
The procedure is same as anodic polarization, but air was used instead o f nitrogen as
66
purge gas.
Baseline conditions:
0.2 M (NH 4)2S 203, 250 ppm copper, 25 °C, pH 10, 450 rpm, air purge and scan rate of
lmV/s
Leaching studies procedure
The procedure is same as before, and air was used for the purging gas.
Baseline conditions:
a) Open potential with higher reagents
0.2 M(NH 4)2S 203, 250 ppm copper, 25°C, pH 10, 450 rpm and air purge
b) Open potential with lower reagents
0.1 M(NH 4 ) 2 S 2 0 3 , 30 ppm copper, 25°C, pH 10, 450 rpm and air purge
c) Applied potential with higher reagents
0.2M(NH 4 ) 2 S 2 O 3 , 250ppm copper, 25°C, pHIO, 450rpm, potential 0.25 V vs. SHE and air
purge
d) Applied potential with lower reagents
0.2M(NH 4 ) 2 S 2 O 3 , 250ppm copper, 25°C, pHIO, 450rpm, potential 0.25 V vs. SHE and air
purge
67
Additives studies procedure
The procedure is same as before, and air was used for the purge gas.
Baseline conditions:
a) N o copper
0.2M(NH 4 )2S 2 O3, 25°C, pHIO, 450rpm, 0.25 V vs. S H E and air purge
b) Wi th copper
0 . 1 M ( N H 4 ) 2 S 2 O 3 , 30ppm copper, 25°C, pHIO, 450rpm, 0.25V vs. S H E and air purge
68
4. RESULTS AND DISCUSSION
4.1 Anodic polarization studies
4.1.1 Preliminary experiments
Preliminary experiments were performed to ensure that the experimental results were
reliable.
Scan rate
The effect of the scan rate on the gold anodic polarization was studied (Figures 4.1,4.2).
It was found that the current density curve is similar at the scan range of 0.5, 1 and 2
mV/s.
10 i
E 8H
« •o ** c 9
o 75 o
1mV/s
50 100 150 200 250 300 350 400 Potential vs SHE.mV
Figure 4. 1 Effect of scan rate on the gold anodic polarization in 0.2 M (NH 4 ) 2 S 2 03
solution (pH 10, 25°C, 450 rpm)
69
Figure 4. 2 Effect of scan rate on the gold anodic polarization in 0.2 M (NH 4) 2S203
solution (pH 10, 25°C, 450 rpm)
Reproducibility
It was found that the reproducibility is highly dependent on the sputtered metal substrate
placed on the electrode and the quality of the electrodeposited gold coating. However,
good reproducibility could be achieved under the experimental conditions (Figures 4.3,
4.4). To ensure the reliability of experimental data, the reproducibility was checked
frequently.
Effect of air in the preparation of solution
Thiosulfate is known to degrade quickly under oxidizing conditions (especially when
catalysts are present). About one half hour was needed for preparing the thiosulfate
solutions. Thus it was necessary to investigate the effect of air during preparation of
solutions.
70
Figure 4. 3 The reproducibility tests of the gold anodic polarization in 0.2 M
(NH4)2S2C>3 solution (pH 10, 25°C, 450 rpm, lmV/s).
5
0 100 200 300 400 Potential vs SHE mV
Figure 4. 4 The reproducibility tests of the gold anodic polarization in 0.2 M
(NH 4)2S 203 solution (pH 10, 25°C, 450 rpm, lmV/s). Test IrSolution prepared in
presence of air. Test 2: Solution prepared under nitrogen.
7 1
30
25
20 -\
E
'55
o 10 T3 £ 5^ 3
o
Total current density
Current density from mass change Test 1
100 200 300 400 500 600 0
-5
-10 Potential vs SHE.mV
Figure 4. 5 Effect of air in preparation of solutions on the gold anodic polarization in 0.2 M (NH4)2S2C>3 , 250 ppm [Cu]T solution (pH 10, 25°C, 450 rpm, lmV/s). Test 1: Solution prepared in presence of air. Test 2: Solution prepared under nitrogen.
35 -,
30 -
25 -
Aim
20 -
'35 15 -c <B
•o 10 -c 0) L_ 3 5 -O
0 -0
-5 -
-10 -
Total current density
Current density from mass change
Test 1
Test 2
100 200 300 400 500 600
Potential vs SHE.mV
Figure 4. 6 Effect of adding copper (II) in solution preparation on the gold anodic polarization in 0.2 M (NH 4)2S 203 , 250 ppm [Cu]T solutions (pH 10, 25°C, 450 rpm, 1 mV/s).Test 1: Adding copper (II) after adjusting pH. Test 2: Adding copper (II) before adjusting pH.
72
In Figure 4.5, test 1 was carried out in a solution prepared in the presence of air, test 2
was carried out in a solution prepared under nitrogen purging. Figure 4.5 shows that the
anodic polarizations are similar in both cases. Therefore, it can be concluded that a short
exposure to air has no effect on the preparation of solutions.
Effect of adding copper (TO in the preparation of solutions
A s the NH 3:NFf4 + ratio is dependent on the p H value, and copper (U) forms complexes
just with NH3, it is necessary to investigate the effect of adding copper (H) on
polarization tests (Figure 4.6). Test 1 is conducted in solution that is made by adding
copper (U) after adjusting p H . Test 2 is conducted in solution that is made by adding
copper (H) before adjusting p H .
Figure 4.6 shows that anodic polarizations are similar in both cases. Therefore, it can be
concluded that adding copper (U) has no effect on preparation o f solutions.
Comparing the effect of adding NEU or adding N H / at same pH
The effect of adding ammonia or adding ammonia ion at p H 10 were investigated (Figure
4.7). Figure 4.7 shows that both ammonia and ammonium have basically equal effects on
the oxidation of gold when the p H value is the same. This result is not surprising as it is
well know that an equilibrium, dependent on p H , exists between ammonia and
ammonium. Therefore, no matter what form o f ammonia is added, the effect w i l l only
depend on the p H value.
73
18 i
-4 J
Potential vs SHE.mV
Figure 4. 7 Effect of adding ammonia or adding ammonia ion on the gold anodic
polarization at pH 10 (25°C, 450 rpm, 1 mV/s)
4.1.2 Anodic polarization studies
4.1.2.1 Anodic polarization in the absence of copper
Effect of temperature
Figure 4.8 shows the effect of temperature on the total current density on the gold anodic
polarization in 0.2 M (NH4)2S203 solution. Figure 4.9 shows the effect of temperature on
the current density calculated from the rate of gold mass change on the gold anodic
polarization in 0.2 M (NH4)2S203 solution.
74
Figures 4.8 and 4.9 show that temperature significantly affects both the total current
density and current density from the rate of mass change, increasing temperature w i l l
result in increasing both current densities. This result is consistent with Ouyang (2001)
who measured the total current density using R D E under similar conditions.
Effect of rotating speed
Figure 4.10 and 4.11 show the effect of rotating speed on the gold anodic polarization in
0.2 M (NH 4)2S 203 solution.
Figure 4.10 and 4.11 show that rotating speed has almost no effect on both the total
current density and the current density calculated from the rate o f mass change. This
result is consistent with Ouyang (2001) who measured the total current density using a
R D E under similar conditions. This result once again confirms that the anodic process is
not under mass transfer control.
Effect of pH value
Figures 4.12 and 4.13 show the effect o f p H on the gold anodic polarization in 0.2 M
(NH 4)2S 203 solution. Figures 4.12 and 4.13 show that lower p H (<9) or higher p H (>11)
w i l l hinder the rate of anodic oxidation of gold, the best p H value appears to be around
10. This result does not agree with Ouyang (2001) who measured the total current density
using a R D E . Ouyang found that the current densities at p H 8 and 9 are significantly
higher than those at p H 10 and 11.
75
4 5 - i
4 0 -CN
E 35 -
30 -in c o
2 5 -u
2 5 -
"E £ 2 0 -
3 U 15 -75 o
1 - 10 -
5 -
0 -r
4 5 ° C
1 0 0 2 0 0 3 0 0 4 0 0
Poten t i a l v s S H E m V
5 0 0 6 0 0
Figure 4. 8 Effect of temperature on the total current density on gold anodic
polarization in 0.2 M(NH 4)2S 203 solution, no copper (pH 10,450 rpm, 1 mV/s).
100 200 300 400 Potent ia l v s S H E m V
500 600
Figure 4. 9 Effect of temperature on the current density from gold mass change
on gold anodic polarization in 0.2 M (NH 4) 2S 20 3 solution, no copper (pH 10, 450
rpm, 1 mV/s).
76
25
20
I 15
'5!
g 10 -| 4-*
c fc c 3 v
o
Total current density
Current density from mass change
200rpm
100 200 300 400
Potential vs S H E mV
500 600
Figure 4. 10 Effect of rotating speed on the gold anodic polarization in 0.2 M
(NH4)2S2C>3 solution, no copper (25°C, pH 10,1 mV/s).
25
20
~ E 15
c
I 0
-10
Total currnet density
Current density from mass change
1600rpm
100 200 300 400 500 600
Potential vs S H E mV
Figure 4. 11 Effect of rotating speed on the gold anodic polarization in 0.2 M
(NH 4)2S 20 3 solution, no copper (25°C, pH 10,1 mV/s).
77
2 5 n
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0
Potential vs SHE mV
Figure 4. 12 Effect of pH value on the total current density on the gold anodic
polarization in 0.2 M (NH4)2S203 solution, no copper (25°C, 450 rpm, 1 mV/s)
2 5 i
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0
Potential vs SHE mV
Figure 4. 13 Effect of pH on the current density from gold mass change on the gold
anodic polarization in 0.2 M (NH 4 ) 2 S 2 03 solution, no copper (25°C, 450 rpm, 1
mV/s).
78
4.1.2.2 Anodic polarization in the presence of copper
Effect of copper
a) Effect of concentration of copper
Figure 4.14 shows that the anodic current density of gold dissolution increases with
increasing [ C U ] T concentration from 0 ppm to 50 ppm.
Figure 4.15 shows that the anodic current density of gold increases with increasing [ C U ] T
concentration from 50 ppm to 250 ppm when the potential is less than 0.28V vs. SHE,
but does not change after 0.28V vs. SHE.
Figure 4.16 shows that the anodic current density of gold dissolution increases slightly
with increasing [ C U ] T concentration from 200 ppm to 500ppm.
In summary, these experiments show that copper can improve the oxidation of gold.
However, it is worth noting that copper (U) can consume thiosulfate at the same time.
There will therefore be a balance between fast leach kinetics and high thiosulfate
consumption under certain conditions.
79
30 !
25
120 >»
'</>
§15 T3 c
9> 10 H
Total current density
Current density from mass change
100 200 300 400 Potential vs SHE.mV
500 600
Figure 4. 14 Effect of [Cu]T concentrations on the gold anodic polarization in 0.2
M ( N H 4 ) 2 S 2 0 3 solution (25°C, pH 10,450 rpm, 1 mV/s).
30 i
120
Total current density
Current density from mass change
100 200 300 400 Potential vs SHE.mV
500 600
Figure 4. 15 Effect of [Cu]T concentrations on the gold anodic polarization in 0.2
M (NH4)2S203 solution (25°C, pH 10,450 rpm, 1 mV/s).
80
600 Potential vs SHE.mV
Figure 4. 16. Effect of [Cu]T concentrations on the gold anodic polarization in 0.2
M (NH 4)2S 20 3 solution (25°C, pH 10,450 rpm, 1 mV/s).
b) Scanning at wider potential range
Linear potential sweep voltammetry experiments over a wider range have been conducted
(Figure 4.17). It was found that the gold oxidation peak was around 0.55-0.7 V vs. S H E
without copper but dropped to around 0.25 V vs. S H E with 250 ppm added copper. This
result indicates that the overpotential, which is required to oxidize gold to gold
thiosulfate, can be decreased with added copper. To support this result, anodic
polarization experiments in the presence of copper on a platinum electrode (instead of
gold electrode) were conducted (Figure 4.18). It was found that there was no mass
change on the platinum electrode. This result confirms that only gold oxidation
contributes to the current peak calculated from the mass change, which was attributed to
the mass change measured in Figure 4.17.
j 81
Figure 4. 17 Effect of [Cu]T on the gold anodic polarization in 0.2 M (NH4)2S203
solution (25°C, pHIO, 450rpm, lOmV/s)
Potential vs SHE.mV
Figure 4. 18 Anodic polarization on a platinum and gold electrode in 0.2 M
(NH4)2S203 solution ( 25°C, pH 10,450 rpm, 10 mV/s).
82
c) Adding copper(II) immediately before starting the test
To understand the role o f Cu(NH3)4 2 +, the following test was designed and conducted
(results shown in Figure 4.19). Test A : 250 ppm copper (IT) added to solution 20-30
minutes before testing. Test B : 250 ppm copper (IT) added immediately before starting
the test.
It was observed in the experimental procedure that: Test A : The solution colour in the
cell was pale blue with an open circuit potential of about 0.04 V vs. S H E . Test B : The
solution colour in the cell was deep blue with an open circuit potential o f about 0.28 V
vs. S H E .
From the observation o f different colors and different open potentials o f solution, it is
obvious that the concentration of Cu(NH3)4 2 + in test B is higher than test A . So, a
reduction current appeared in test B (Figure 4.19), this reaction probably is:
Cu(NH3)4
2" + e" = Cu(NH3)2
+ + 2NH3 Equation 4.1
However, from Figure 4.19, it is interesting to find that the current densities from mass
change in both tests are basically the same. This result is not initially expected since a
higher concentration of copper tetrammine generally leads to a higher gold oxidation
current density. A reasonable explanation is that the 250ppm copper (IT) concentration is
too high under these experimental conditions, and excess copper (IT) has no catalyzing
role for gold oxidation but only oxidizes the thiosulfate. A repeat test was carried out and
reported in Figure 4.20 which shows very good reproducibility.
83
30 i
25 -
20 -CN
A/m
15 -
>. + J
"3 10 -c o
•a 5 -1 &. 3 0 -o 0
-5 -
-10 -
-15 -
Test A Total current density
Current density from mass change
600
Potential vs SHE.mV
Figure 4. 19 Gold anodic polarization test in 0.2 M ( N H 4 ) 2 S 2 0 3 solution ( 25°C, pH
10, 450 rpm, 1 mV/s). Test A: 250 ppm copper (II) added to solution 20-30 minutes
before testing. Test B: 250 ppm copper (II) added immediately before starting the
test.
30
25 H
20 CN
t 10 c a 2 5 c £ = 0^= o
-5
-10
-15
Total current density
Current density from
mass change
Test 1
Test 2
300 400 500 600
Potential vs SHE.mV
Figure 4. 20. Reproducibility test for adding 250 ppm copper (II) immediately
before starting the test in 0.2 M (NH4)2S203 solution (25°C, pH 10,450 rpm, 1 mV/s).
84
d) M o d i f i e d electrode
To understand the role of copper and ammonia on gold leaching in thiosulfate solutions,
a test using a modified gold electrode was designed and conducted.
The modified gold electrode was prepared by immersing the gold electrode into a 0.4 M
ammonia hydroxide solution containing 500 ppm copper (II). After 20 minutes, the
electrode was removed and rinsed. This modified electrode was then immersed into air-
saturated or nitrogen-purged 0.2 M sodium thiosulfate solution, free o f copper and
ammonia.
Disappointingly, it was found that there was no difference between using the modified
and unmodified electrodes.
Effect of S?(K [CUIT, FNEM
The effects of thiosulfate, ammonia, and copper on gold polarization have been studied.
A s can be seen from Figure 4.21, almost no current density of gold oxidation appeared in
0.2 M thiosulfate solution, and the current density of gold oxidation was very low in 0.2
M thiosulfate and 250 ppm copper solution. The gold oxidation also was reasonably slow
in 0.2 M thiosulfate and 0.4 M NH3 solution, especially at lower potentials (less than 0.4
V vs. SHE) . A reasonable gold oxidation current density only appeared in 0.2 M
thiosulfate, 0.4 M N H 3 and 250 ppm copper solution. This result indicates that the rate of
anodic gold dissolution increases to the maximum when each of thiosulfate, ammonia
and copper are present.
85
E
g , 6
u CO (0 A (0 H
E E p
(A C V
•o c 0
N a 2 S 2 0 3 0 .2M
[Cu] T 2 5 0 p p m
N H 3 0.4M
N a 2 S 2 0 3 0.2M
[Cu] T 250ppm
100 200 300 400 500 600
Potential vs SHE.mV
Figure 4. 21 Effects of thiosulfate, ammonia, and copper on gold anodic
polarization ( 25°C, pH 10,450 rpm, 1 mV/s).
100 200 300 Potential vs SHE.mV
400 500
Figure 4. 22 Effect of (NH 4)2S 20 3 concentrations on the gold anodic polarization
in 0.2 M (NH4)2S203 solutions ( 250 ppm copper, 25°C, pH 10, 450 rpm, 1 mV/s).
86
Effect of concentration of ( N H ^ S i O j
A s shown in Figure 4.22, the current density of gold oxidation increases significantly
with increasing concentration of (NH4)2S203 .This result can be explained by the equation
4.2, the regeneration of C u ( N H 3 ) 4
2 + , Which is believed to be the anodic electrochemical
reaction:
Cu(S203)3
5" + 4NH3 = Cu(NH3)4
2+ + 3S203
2_ + e" Equation 4.2
The anodic regeneration of cupric ammine from cuprous thiosulfate w i l l be favoured at
higher ammonia to thiosulfate ratios.
It is interesting to find that passivation occurs in high (NFLO2S2O3 concentration (0.4 M ,
0.8 M ) solutions at high potentials. This phenomenon may be due to S species (as
degradation products from thiosulfate) forming deposits on the gold electrode and
inhibiting further oxidation of gold.
Effect of concentration of N H i
A s shown in Figure 4.23, N H 3 has a positive effect on gold oxidation. Increasing the
concentration of NH3 w i l l increase the gold oxidation rate.
Many researchers believe that the role o f NH3 on the anode process is to alter the surface
of the gold electrode, reducing the effect of the surface passivation. This result is
consistent with this understanding. Furthermore, this result can be understand by equation
4.2,from which it can be seen that increasing the concentration of ammonia w i l l favour
87
the anodic reaction of cuprous to cupric oxidation when the concentration of S2O3 2" is
fixed.
It is worth noting that the current density in 1.6 M NH3 is similar, even slightly lower
than that measured at 0.8 M N H 3 in the potential range of 0-0.3 V vs. S H E . This result
may be due to an excess of NH3 blocking the gold surface, preventing thiosulfate access.
Thus, it can be concluded that the concentration of NH3 should be less than 1.6M under
experimental conditions.
Effect of concentration of S i O ^ ~
Figure 4.24 shows that the gold oxidation rate was similar between 0.1 and 0.2 M S2O3 2"
and also between 0.4 and 0.8 M . Also , it was found that the gold oxidation current at 0.1
and 0.2 M S2O32" was higher than the current at 0.4 or 0.8 M when the potential was
lower than 0.35 V vs. S H E . This result indicates that too much S2O3 2" w i l l hinder gold
oxidation when the concentration of N H 3 is fixed. Again, this result can be interpreted by
the equation 4.2.
Similar phenomenon was also reported by Ouyang (2001) who found that the gold
leaching rates were exactly the same at 0.1 and 0.2 M sodium thiosulfate concentration,
and then fell steeply from 0.2 to 0.4 M , further increasing the sodium thiosulfate
concentration has no significant influence on gold leaching.
88
600 Potential vs SHE .mV
Figure 4. 23. Effect o f NH 3 concentrations on the gold anodic polarization in 0.2
M Na2S203 solutions (250 ppm copper, 25°C, pH 10,450 rpm, 1 mV/s).
600 Potential vs SHE.mV
Figure 4. 24. Effect of S203
2" concentrations on the gold anodic polarization in 0.4
M NH 3 solutions (250 ppm copper, 25°C, pH 10,450 rpm, 1 mV/s).
89
4.1.3 Summary
In the absence of copper
Table 4. 1 Influencing factors for gold oxidation with anodic polarization( no copper)
Baseline conditions: 0 .2M (NHO2S2O3, no copper (pHIO, 450rpm, 25°C,nitrogen purge)
Factors Gold oxidation Comments p H value Positive
and negative
Optimum value is around 10
Temperature Positive Rotating speed N o effect Probably under chemical
control
In the presence of copper
Table 4. 2 Influencing factors for gold oxidation on anodic polarization (with copper).
Baseline conditions: 0.2 M (NH 4 )2S 2 03 , 250 ppm copper (pH 10,450 rpm, 25°C,nitrogen
purge).
Factors Eff ect Comments Factors Gold oxidation Consumption of thiosulfate
Comments
Copper (adding as C u 2 + )
Positive Negative Optimum concentration exits
N H 3 Positive 1.6M probably blocks the gold surface
S 2 0 3 ' - Negative and positive
Negative:0~0.35V vs. S H E Positive: over 0.35V vs. S H E
(NH4 )2S 2 0 3 Positive
90
4.2 Cathodic polarization studies
4.2.1 Preliminary experiments
Reproducibility
Figure 4.25 shows that cathodic polarization experiments have good reproducibility.
3 i
Figure 4. 25. Reproducibility tests of gold cathodic polarization in 0.2 M (NH 4) 2S20 3
solution (250 ppm copper, pH 10,25°C, 450 rpm, 1 mV/s).
Effect of gold electrode and platinum electrode
Figure 4.26 shows that the limiting reduction current densities on gold electrode and
platinum electrode are same. There was no mass change on platinum over the tested
potential range, but there was gold oxidation on the gold electrode at a higher potential
(>0 V vs. SHE).
300
-2
Potential vs SHE.mV
91
12
10
8
Total current density
Current density from mass change
Gold electrode
300
Potential vs SHE.mV
Figure 4. 26. Comparing the effects of using gold electrode or platinum electrode on
the cathodic polarization (0.8 M (NH 4 )2S 2 03 ,250 ppm copper,, pH 10, 25°C, 450
rpm, 1 mV/s).
E
•o
C
O
1.5
1 H
0.5
-0.5 A
NH 3 0.4M N a 2 S 2 0 3 0.2M Cu 250ppm
400 300
Potential vs SHE.mV
Figure 4. 27 Effect of adding ammonia or adding ammonia ion on the gold cathodic
polarization (pH 10,25°C, 450 rpm, 1 mV/s).
9 2
Comparing the effect of adding N H i or adding N H / at the same pH
Figure 4.27 also shows that both ammonia and ammonium have similar effects on the
gold cathodic polarization when the p H is the same. This result is consistent with the
results o f the anodic polarization studies (Figure 4.7).
4.2.2 Cathodic polarization studies
Effect of ( N H ^ S i O i concentration
From Figure 4.28, it can be seen that the concentration of (NH4)2S203 has a positive
effect on gold polarization, limiting current density increases with increasing the
concentration o f (NH4)2S203 under the experimental conditions. This result can be
explained by the impact of increasing ammonium thiosulfate on the the concentration of
Cu(NH3) 4 + in solution. The cathodic limiting current density is for the reduction of
cupric ammine. A s the limiting current density increases in Figure 4.28 with increasing
ammonium thiosulfate, this indicates that cupric ammine is increasing.
Effect of copper(II) concentration
Not surprisingly, as shown in Figure 4.29, the cathodic current limiting density increases
with increasing copper concentration. This is due to more copper addition giving more
2"i"
C u ( N H 3 ) 4 species in solution. From the equation 4.3, it can be seen that an increase in
Cu(NH3)42+ w i l l enhance the cathodic current density. C u ( N H 3 ) 4
2 + + e' = C u ( N H 3 ) 2
+ + 2 N H 3 Equat ion 4.3
93
12
10
8H E
'35 c a> •o c a> k. 3
o
Total current density
Current density from mass change
0.8M (NH 4 )2S 2 03
0.2M (NH 4 ) 2 S 2 0 3
300
Potential v s S H E . m V
Figure 4. 28. Effect of (NH^SjOs concentrations on the gold cathodic
polarization (copper 250 ppm, pH 10,25°C, 450 rpm, 1 mV/s, air purge).
300
Potent ia l v s S H E . m V
Figure 4. 29. Effect of copper concentration on the gold cathodic polarization in 0.2
M (NH 4)2S 203 (pH 10, 25°C, 450 rpm, 1 mV/s, air purge).
94
Cathodic polarization in 0.4M NH^solution
300
Potential v s S H E . m V
Figure 4. 30. Cathodic polarization in 0.4 M NH 3 solution (pH 10, 25°C, 450 rpm, 1
mV/s, air purge or nitrogen purge).
A s shown in Figure 4.30, reduction occurred below a potential of -0 .05 V vs. S H E with
N 2 purge, but occurred below -0.01 V vs. S H E with air purge, and the current density
with air purge was much higher than with N2 purge. Both currents did not reach the
limiting current in the experimental scan range. The currents may have resulted from the
reduction of oxygen in solution on the gold electrode surface.
95
Cathodic polarization in 0.4 M NHj_250 ppm copper solution
10
o
Mass change
F r o m a - b:
« -600 -500 -406 -300 -200 -100 0 1 0 0 / 2 0 0 ° 300 400
5 -10 - | / Current density from mass
change
(A § -20 13 C § -30 3
o
-40
-50 J
Total current density
C u ( N H 3 ) 4 ^ C u ( N H 3 ) 2
< |
F rom b - c:
C u ( N H 3 ) 4
2 ^ — • C u
or
C u ( N H 3 ) 2
+ — • C u
250
200
150 _ D)
100 "jf ro E
50 2 o 0
0
-50
-100 Potential v s S H E m V
Figure 4. 31 Cathodic polarization in 0.4 M NH 3 ,250 ppm copper solution (pH 10,
25°C, 900 rpm, 1 mV/s, air purge). Thiosulfate was not added.
A s Figure 4.31 shows, a reduction current appeared after the potential was lower than
0.22 V vs. S H E , and a high limiting current density (22 A / m 2 ) was observed in scan
range of -0.02 to -0.3 V vs. S H E . It was very interesting to find that the mass increased
dramatically after the potential was lower than -0.3 V vs. S H E (there was no mass
change above -0.3 V vs. S H E . This result indicated that something was deposited on the
electrode. Based on this system, it is obvious that elemental copper deposited on the
electrode. So, it is reasonable that at potential lower than - 0.3 V vs. SHE(b-»c) the
cathodic reaction should be:
9 6
Cu(NH3)2
+ + e" = Cu + 2NH3
Or Cu(NH3)4
2++ e" = Cu + 4NH3
E° =-0.1 IV
E°= -0.05 V
Equation 4.4
Equation 4.5
Due to the lack of mass change in the potential region of 0.22 to -0.3 V vs. S H E (a-»b) ,
the cathodic reaction that contributed a high limiting current density was probably:
Cu(NH3)4
2+ + e = Cu(NH3)2
+ + 2NH3 E°=0.1 V Equation 4.6
Comparing cathodic polarization in 0.4M NH^250ppm copper solution
between air purge and nitrogen purge
-30 J
Potential v s S H E , m V
Figure 4. 32. Comparing cathodic polarization in 0.4 M NH3, 250 ppm copper
solution between air purge and nitrogen purge. No thiosulfate present.
97
A s Figure 4.32 shows, when using nitrogen for purging, a limiting current still existed in
the scan range of 0 to - 0.29 V vs. S H E , but the value was 13 A / m 2 , which was lower
than the current density with an air purge (16 A / m ). This result indicates that oxygen
contributes to the cathodic process either by reoxidation o f cuprous ammine to cupric
ammine or by direct reduction of oxygen on the gold electrode surface. Also , after the
potential was lower than -0.29 V vs. S H E , it was observed that copper was produced and
precipitated onto the electrode.
Effect of rotating speed on the gold cathodic polarization in 0.4M,
250ppm copper solution
350
Potential vs S H E mV
Figure 4. 33. Effect of rotating speed on the gold cathodic polarization in 0.4 M NH3,
250 ppm copper solution (pH 10,25°C, lmV/s, air purge)
98
Figure 4.33 shows that the rotating speed significantly affect the cathodic reaction
(equation 4.3). The limiting current density was very low (less than l A / m 2 ) with no
rotation, but it was quite high (22A/m 2 ) with a rotating speed of 900 rpm. This result
suggests that the reaction (equation 4.2) is probably under mass transfer - diffusion
control under these experimental conditions.
A s shown in Figure 4.34, there exists a linear relationship between the limiting current
density and the square root of rotating velocity. This is in agreement with the Levich
equation, which confirms that the cathodic reaction is diffusion controlled.
0 10 20 30 40 50 60 70 80
Figure 4. 34. The relationship between the limiting current and the square root of
rotating velocity for experiments with 0.4 M NH3, 250 ppm copper solution (pH 10,
25°C, lmV/s, air purge).
99
Comparing the cathodic polarization in 0.4 M NH^250 ppm copper
with and without 0.1 M S CW" solution
5 i
-400 b-300 -200 -100 0 100 / 200 300 400
E
c •o c -15 ©
O -20
-25
-30 J
S2O3^0.1M a y
From a - b:
C u ( N H 3 ) 4
2 ^ Cu(NH 3) 2
+
D S,03
2" 0 M
From b - c:
Cu(NH 3) 4
2-i-+ Cu C / or
Cu(NH3)2
+—». Cu
So, in presence of S 2 0 3 ,the cathodic reaction probably still is: Cu(NH 3 ) 4
2 + + e"= Cu(NH3)2
++ 2NH 3
Potential vs SHE .mV
Figure 4. 35 Cathodic polarization in 0.4 M NH3,250 ppm copper and 0.1 M S2O3 2 "
solution (compared with no S2O3 2 , air purge)
Figure 4.35 shows that the limiting current density of 16A/m 2 with no thiosulfate present
dramatically dropped to 1 A/m2. However, it is worth noting that when S2O3 2" was
present, the potential at which reduction occurred was almost the same as the potential (
starting at around 0.2 V vs. SHE) in the absence of S2O3 2 . Based on these results, it is
fair to conclude that about 1/16 th o f the copper in solution with thiosulfate present occurs
as cupric ammine. The balance of the copper has been reduced to a cuprous species
(either ammine or more likely thiosulfate complex) by reaction with thiosulfate.
100
Thus, in the presence o f S2O3 2 the cathodic reaction probably is the same as that
occurring in the absence of S2O3 2 , (equation 4.2) in Figure 4.32.
Cu(NH3)4
2++ e" = Cu(NH3)2
+ + 2NH3 E°=0.1V Equation 4.7
Therefore, equation 4.7 is the real cathodic reaction in A T S - C u leaching system.
Effect S?Oi 2" concentration on the gold cathodic polarization in 0.4M
NH^250ppm copper solution
Figure 4.36 shows that S2O3 2" has a significantly negative effect on the limiting current
of cathodic polarization. Increasing the concentration of S2O3 2" w i l l decrease the limiting
current under experimental conditions. This is in agreement with the result o f L i et al
(1996) and Ouyang (2001).
The reason for this behaviour is that excess thiosulfate increases the reduction of cupric
ammine (equation 4.8). The lower the concentration of cupric ammine, the lower the
limiting current density.
Reaction 4.8 is undesirable as S 2 03 2 " is consumed by reduction o f C u ( N H 3 ) 4
2 + . So, this
explains why the cathodic current decreases with the increasing of S2O3 2 " when the
concentration of NH3 is fixed.
2Cu(NH3)4
2+ + 8 S 2 O 3 2 " = 2Cu(S203)3
5" + 8 N H 3 + S4Ofi
: 2- Equation 4. 8
101
1-2 J S 2O 3
2-0.1M
Potential vs S H E . m V
Figure 4. 36. Effect of S203
2" concentrations on the gold cathodic polarization in
0.4 M NH3, 250 ppm copper solution (pH 10, 25°C, 450 rpm, lmV/s, air purge).
E -500 -4
w c 01 "D c S? •_ 3
o -6 A
-8
0.1 M N H , 0.4 M N H 3
I0 "250~ ^0i5 *~0 100 / / 2 0 0 300
0.8 M N H 3
1.6 M N H 3
Potent ia l v s S H E . m V
Figure 4. 37. Effect of concentration of NH 3 on the gold cathodic polarization in 0.2
M Na2S203, 250 ppm copper (pH 10, 25°C, 1 mV/s).
102
Effect of the concentration of ammonia
Figure 4.37 shows that NH3 has a significantly positive effect on the limiting current of
cathodic polarization. Increasing the concentration of NH3 w i l l dramatically increase the
limiting current density.
Ouyang (2001) also observed that the cathodic current density increased constantly with
increasing ammonia concentration. Ouyang pointed out that the increase of ammonia
concentration helps to stabilize C u (U) in the solution (higher concentration) and
therefore leads to a higher cathodic current.
4.2.3 Summary
a) Influencing factors on cathodic polarization
Table 4. 3 The influence of variables on cathodic current response. Baseline conditions: 0.2 M (NH4)2S203, 250 ppm copper (pH 10, 450 rpm, 25°C, air purge).
Factors Effect on cathodic current Copper Positive N H 3 Positive S2O3 2" Negative (NH4)2S203 Positive Rotating speed Positive Oxygen Positive
b) It can be concluded that the effective cathodic reaction is not the direct reduction of
oxygen in the A T S - C u system, but oxygen contributes indirectly to the cathodic reaction
by regenerating the cupric ammine species.
103
c) It was proved that the electrochemical reaction on the cathode in the potential range
0.2 to -0.3 V vs. S H E for the solution in the absence o f S2O3 "is:
Cu(NH3)4
2+ + e" = Cu(NH3)2
+ + 2NH3 Eo=0.1 V Equation 4.9
d) Based on the fact that the potential at which the cathodic current appeared in the
presence of S203
2"was close (around 0.2 V vs. S H E ) to that in the absence of S2O32",
it is reasonable to believe that the cathodic reaction in the A T S - C u system still is
equation 4.9.
4.3 Leaching studies
4.3.1 Preliminary tests
Reproducibility tests
To assure the reliability o f leaching rate data from the R E Q C M , many experiments
testing reproducibility were conducted. It was found that the reproducibility is highly
dependent on the platinum surface quality of the R E Q C M electrode. However, very good
reproducibility was achieved under the experimental conditions (Figure 4.40). This result
indicates that the leaching tests using the R E Q C M are repeatable.
104
200 i 180 -160 -
Test 1
3 140 -«r 120 -
| 100 -2 80 -O 60 -
40 -20 -
0 200 400 600 800 1000 1200 1400 1600 1800
T ime(Second)
Figure 4. 38. Leaching reproducibility tests in 0.2 M (NH4)2S203, 50 ppm copper
solution (pH 10, 25°C, 450 rpm, 0. 25 V vs. SHE).
4.3.2 Leaching studies under open potential
4.3.2.1 Leaching with higher concentrations of reagents
The baseline conditions for this part o f the study were NH3/NFJ_4+ = 0.4M, [S2O32"]=0.2
Mand [Cu] T = 250 ppm.
To understand the role o f Cu(NH3)4 , C u and NH3, a series o f leaching tests were
designed and conducted (Figure 4.39). Test 1: A leaching test was run in 0 .2M Na 2S20 3
solution which was purged by air for 15 minutes. After 280 seconds, 50 ppm Cu(NH3)4
2+
was added. It was found that almost no mass change before or after the C u ( N H 3 ) 4
2 + was
0
Effect ofCu(NH 2 )4-
105
added. Test 2: This test was the same as test 1 but using 0.2 M (NEL^SaOs instead o f
Na2S2C>3. It shows that initially, the gold dissolved at a very low rate, but, when
Cu(NH3)4 was added, the mass decreased dramatically. Test 3: This test was the same
as test 2 except copper was added as C U S O 4 instead of Cu(NH3)4 2 +. Also , it can be seen
that the mass decreased dramatically after the C u 2 + was added, indicating that gold was
leaching.
210 -1
Time(Second)
Figure 4. 39 Effect of Cu(NH3)4
2+on gold leaching in ATS-Cu system. (pH 10, 450 rpm, 25°C, air purge).
Based on these tests, it is clear that the Cu(NH3)4
2+ species catalyzes gold leaching in
A T S system. A s the cupric ammine species requires ammonia in solution to be stable,
these tests also confirmed that NH4 + /NH3 is necessary in this system.
106
Leaching tests in presence of S?Oi 2" and in absence of S?CM
Figure 4.40 shows that when there is no thiosulfate present, gold leaching does not occur
to any extent, this means that Cu-NH.3 system can not leach gold under these
experimental conditions. However, Han and Meng (1993) carried out a detailed
investigation on the dissolution kinetics of gold in ammoniacal media between 100°C and
200°C in an autoclave. The study revealed that the kinetics of gold dissolution in
ammoniacal solution under certain conditions of temperature, pressure and oxidant
addition is favorable.
107
Effect of concentration of S ? O i -
A s shown in Figure 4.41, the gold leaching rate increased with the increasing of
concentration of S2O3 " from 0.1 M , 0 .2M, 0 .4M while the concentration of NH3 was
fixed at 0 .4M. However, too high a concentration of S2O32" (0.8M) w i l l hinder gold
leaching. This result is in contrast with the anodic polarization studies (Figure 4.24),
which indicated that the anodic current densities were higher in 0 .1M and 0 .2M but lower
in 0.4 and 0 .8M S 2 0 3
2 \
212 n
Time(Second)
Figure 4. 41 Effect of concentration of S203
2" on gold leaching in 0.4 M NH 3, 250
ppm copper solution (pH 10, 450 rpm, 25°C, air purge).
Effect of concentration of NH^
108
A s shown in Figure 4.42, the gold leaching rate increased with the increasing o f
concentration o f ammonia from 0.2 M , 0 .4M, 0 .8M while the concentration of S2O3 " is
fixed at 0 .2M. However, too high ammonia (1.6 M ) hindered the gold leaching. This
result is consistent with the anodic polarization studies (Figure 4.23), which suggested
that the 1.6M NH3 may block the gold surface from other species.
190 n
0 200 400 600 800 1000 1200 1400 Time(Second)
Figure 4. 42 Effect of concentration of NH 3 on gold leaching in 0.2 M Na2S203, 250
ppm copper solution (pH 10,450 rpm, 25°C, air purge).
4.3.2.2 Leaching with lower concentrations of reagents
The baseline conditions for studying the behaviour of this system with lower
concentration o f reagents were: [NH 3 ]+[NH 4
+ ] = 0.2 M , [S 2 0 3
2 "] = 0.1 M and [Cu] T = 30
ppm.
109
Effect of concentration of S?Q ~
Figure 4.43 shows that the leaching rate decreased with the increasing of concentration o f
2 2
S2O3 at lower concentration. Again, the possible reason is due to excess S2O3 "
consuming the C u 2 + , which decreases the concentration of C u ( N H 3 ) 4
2 + .
0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 43 Effect of concentration of S203
2" on the gold leaching in 0.2 M NH 3 ,
30ppm copper solution, open potential (pHIO, 450rpm, 25°C, air purge)
Effect of concentration of N f t t
Figure 4.44 shows the effect of the concentration o f NH3 at lower concentrations of
S2O3 2" and [Cu] T . It was found that the gold leaching rate increased as the concentration
of NH3 increased from 0.2 M to 0.8 M .However, when the concentration of NH3 was 1.6
M , the gold leaching rate was less than that at 0.8 M NH3. This result indicated that NH3
has a positive effect on gold leaching but too much NH3 w i l l retard the leaching rate.
110
Also , this result is consistent with the polarization studies (Figure 4.23) and the results for
leaching with a higher concentration of reagents (Figure 4.42).
292 -
291 -
290 -I , , , , , , , , , 0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 44. Effect of concentration of NH 3 on gold leaching in 0.1 M Na2S203
solution, 30 ppm copper solution, open potential (pH 10,450 rpm, 25°C, air purge).
4.3.3 Leaching studies under applied potential
The in-pulp oxidation-reduction potentials o f 0.16 to 0.30 V vs. S H E are generally
found in conventional leaching experiments with air or oxygen addition. Therefore this
part o f the study w i l l focus on the leaching in the presence of an applied potential. The
baseline potential was 0.25 V vs. S H E .
4.3.3.1 Leaching with higher concentration of reagents
The baseline concentrations for this study were: [NH 3 ]+[NH 4
+ ] = 0.4 M , [ S 2 0 3
2 ] = 0.2 M
and [ C U ] T = 250 ppm.
Ill
Effect of potential with different concentrations of copper
a) No copper
Figure 4.45 shows that the leaching rate increases with the increasing of potential in the
absence of added copper. It also shows that almost no gold leaching when the potentials
are 0.15, 0.20, 0.25 V vs. S H E . This result is consistent with the anodic polarization
studies (Figure 4.14).
b) 30 ppm copper
Figure 4.46 also shows that the leaching rate increased with increasing potential in the
presence of 30 ppm copper.
c) 50ppm copper
Figure 4.47 shows that the leaching rate increased with increasing potential in the
presence of 50 ppm copper. This result is consistent with the anodic polarization studies
(Figures 4.13 and 4.14)
d) 250ppm copper
Figure 4.48 also shows that the leaching rate increased with increasing potential in the
presence of 250 ppm copper. The leaching rates are similar when the potentials are 0.20,
0.25 and 0.30 V vs. S H E . This point is consistent with the result in the anodic
polarization studies (Figure 4.15), in which a plateau appears when the potential is over
0.20 V vs. S H E .
From the Figures 4.45, 4.46, 4.47, 4.48, it is clear that, in general, a higher potential can
have higher leaching rate under experimental conditions.
112
150mVSHE 200mVSHE
160 -
140 -
120 -
100 -I 1 1 1 1 1 1 , 1 !
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 45. Effect of potential on the gold leaching in 0.2 M (NH4)2S203 solution,
no copper (pH 10,450 rpm, 25°C, air purge)
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 46 Effect of potential on the gold leaching in 0.2 M (NH 4 ) 2 S 2 03 solution
with 30 ppm copper (pH 10,450 rpm, 25°C, air purge).
113
140 -
120 -
100 ] 1 , 1 1 ; 1 , , , ,
0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 47 Effect of potential on the gold leaching in 0.2 M (NH 4) 2S20 3 solution
with 50 ppm copper (pH 10,450 rpm, 25°C, air purge).
120 -
100 \ , , , , , , , , ,
0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 48 Effect of potential on the gold leaching in 0.2 M (NH4)2S203 solution
with 250 ppm copper (pH 10,450 rpm, 25°C, air purge).
114
Effect of the concentration of copper at different potentials
a) 0.20 V vs. S H E
A s shown in Figure 4.49, the leach rate increased with the increasing concentration of
copper. It is worth noting that the concentration of copper has a significant effect on the
gold leaching rate when the potential was fixed at 0.20 V vs. S H E .
b) 0.25 V vs. S H E
A s shown in Figure 4.50, the leach rate also increased with increasing concentration o f
copper. However, the effect of the concentration of copper on the leaching rate is not as
great as that at 0.20 V vs. S H E (Figure 4.51).
c) 0.30 V vs. S H E
A s shown in Figure 4.51, the effect of the concentration o f copper is quite different
between 0.20 and 0.25 V vs. S H E (Figures 4.49, 4.50). The leaching rate was highest at
30 or 50 ppm copper. The leaching rate did not increase when the concentration of
copper was increased to 250 ppm. This result indicates that 30-50 ppm copper may be
sufficient for effective gold leaching in 0.2 M thiosulfate solution i f the potential is 0.30
V vs. S H E . 0.30 V vs. S H E is a higher potential than is normally achieved in thiosulfate
leaching of gold with air or oxygen oxidation.
Figures 4.49, 4.50 and 4.51 show that the potential has a greater effect at lower [Cu] T than
that at higher [ C U ] T .
115
Oppm copper
140 -
120 -
100 -I 1 1 1 1 1 1 1 1 1
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 49. Effect of concentration of copper on gold leaching in 0.2 M (NH4)2S203
solution with 0.20 V vs. SHE applied potential (pH 10,450 rpm, 25°C, air purge).
Oppm copper
140 -
120 -
100 -I , , , , 1 , , , ,
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 50. Effect of concentration of copper on gold leaching in 0.2 M (NH4)2S203
solution with 0.25 V vs. SHE applied potential (pH 10,450 rpm, 25°C, air purge).
116
120 -100 J , , , , , , , , ,
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 51. Effect of concentration of copper on gold leaching in 0.2 M(NH4)2S203
solution with 0.30 V vs. SHE applied potential (pH 10, 450 rpm, 25°C, air purge).
Effect of temperature
Chemically controlled reactions usually have high activation energy, and thus changes in
temperature dramatically influence the reaction rate. So an easy way o f enhancing the
kinetics of a chemically controlled reaction is to increase the temperature.
Figures 4.52, 4.53, 4.54 show the effect of temperature on the rate of gold leaching at
different applied potentials (0.20, 0.25, 0.30 V vs. SHE) . It can be see that temperature
has a significant positive effect on the rate of gold leaching at all potentials. This result is
consistent with the anodic polarization studies (Figures 4.8 and 4.9).
According to the Arrhenius equation, the relationship between activation energy and
leaching rate can be described as follows:
117
E 1 _ lnr = x — + C
R T Equation 4.10
Where r: leaching rate (mol/m .s)
E : activation energy (J/mol)
R: gas constant (8.314 J/K.mol)
T: temperature (K)
C: constant
Arrhenius plots for different potentials are shown in Figure 4.55. The apparent activation
energies are calculated from the slope o f the fitted straight line and are summarized in
Table 4.4. A l l o f the activation energies are higher than 40 kJ/mol. This result indicates
that the reaction is chemically controlled over the entire potential range.
200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 52. Effect of temperature on gold leaching in 0.2 M (NH4)2S203 solution,
250 ppm copper, 0. 20 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
118
o 100
200 400 600 Time(Second)
800 1000
Figure 4. 53. Effect of temperature on gold leaching in 0.2 M (NH4)2S2C>3 solution,
250 ppm copper, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
Figure 4. 54 Effect of temperature on gold leaching in 0.2 M (NH4)2S203 solution,
250 ppm copper, 0.30 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
1 1 9
Figure 4. 55 Arrhenius plot for different potentials
Table 4. 4 Calculated values of Arrhenius activation energy values for gold leaching
at different applied potentials.
Potential (V vs. SHE) Activation energy(kJ/moi) 0.20 51.14 0.25 76.07 0.30 68.47
4.3.3.2 Leaching with lower concentration of reagents
For the leaching of gold using lower concentrations of reagents, a baseline condition of
[NH3]+[NH4
+] = 0.2 M, [S203
2"] = 0.1 M and [Cu2+] = 30 ppm was chosen.
120
Reproducibility tests
Figure 4.56 shows good reproducibility for gold leaching tests with lower concentrations
of reagents.
260 -I . . , , , , 0 200 400 600 800 1000 1200
T i m e ( S e c o n d )
Figure 4. 56. Reproducibility tests for gold leaching in 0.1 M (NH4)2S203, 30 ppm
[Cu]T solution (pH 10, 25°C,450 rpm, 0.25 V vs. SHE, air purge).
Effect of concentration of (NIL^SiOi
Figure 4.57 shows that the leaching rate increases with the increase of concentration of
(NFLO2S2O3. This result is consistent with anodic and cathodic polarization studies
(Figures 4.22 and 4.28). Both of these figures indicate that (NH4)2S203 has a positive
effect on anodic current density and cathodic current density.
Effect of concentration of [CU IT
Figure 4.58 shows that the gold leaching rate increases with the increase of concentration
of [Cu]j. It can be found that the leaching rate with 50 ppm [CU]T is similar to that with
121
30 ppm[Cu] T . Also , this result is consistent with the anodic and cathodic polarization
studies. A l l o f the individual studies indicate that increasing [CU]T has a positive effect
on anodic current density and cathodic current density (Figures 4 . 1 4 , 4 . 1 5 , 4 . 1 6 and
4 . 2 9 ) .
Effect of NH,(totan/SzQ3 ratio
N H 3 ( t o t a i y S 2 0 3 = 0 : 0 M (NH 4)2S 20 3 , 0 M N H 3 , 0 . 0 5 M N a 2 S 2 0 3
NfL(total ) /S ? Ch = 1 : 0 . 0 5 M (NH 4)2S 20 3 , 0 M N H 3 , 0 . 0 5 M N a 2 S 2 0 3
N H 3 ( t o t a r j / S 2 0 3 = 2 : 0.1 M ( N H 4 ) 2 S 2 0 3 , 0 M N H 3 , 0 M N a 2 S 2 0 3
N H 3 ( t o t a l l / S 2 0 3 = 4 : 0.1 M ( N H 4 ) 2 S 2 0 3 , 0 .2 M N H 3 , 0 M N a 2 S 2 0 3
Figure 4 . 5 9 shows that the N H 3 / S 2 0 3 ratio has a significant positive effect on gold
leaching in thiosulfate solutions, thus highlighting the need for a sufficient high
N H 3 / S 2 0 3 ratio in the A T S - C u leaching system. Again, according to the equation 4 . 1 1 ,
4 . 1 2 :
Anodic electrochemical reaction:
C u ( S 2 0 3 ) 3
5 - + 4 N H 3 = C u ( N H 3 ) 4
2 + + 3 S 2 0 3
2 + e" Equat ion 4.11
Regeneration o f C u ( N H 3 ) 4
2 + on the cathode:
C u ( S 2 0 3 ) 3
5 ' + Vi 0 2 + H 2 0 + 4 N H 3 = C u ( N H 3 ) 4
2 + + 3 S 2 0 3
2 +20H" Equat ion 4.12
It can be seen that the N H 3 / S 2 0 3 ratio is a critical factor for regeneration of C u ( N H 3 ) 4
2 + .
Both the anodic and cathodic current w i l l increase with increasing N H 3 / S 2 0 3 ratio, thus a
higher N H 3 / S 2 0 3 ratio can enhance the leaching rate. L i et al ( 1 9 9 6 ) also suggests that it
1 2 2
is vital to keep the molar concentration ratio of ammonia to thiosulfate in a certain range
in order to regenerate the cupric species. The present work supports this suggestion.
Effect of pH value
Figure 4.60 shows that the leaching rate increases with the increasing of p H from 7 to 10,
but it decreases when the p H value is more than 10. In general, gold leaching w i l l benefit
from increasing p H (to the limit of p H -10). One reason is that thiosulfate is more stable
under alkaline conditions, the second reason is that increasing p H means having more
NH3, which can stabilize the cupric ammine species in solution. However, too high p H
w i l l retard the leaching rate. This result is consistent with anodic polarization studies
(Figures 4.11 and 4.12). Aylmore et al (2001) pointed out that a high p H value should be
avoided, because copper can precipitate from solution as copper oxides.
Effect of temperature
From Figure 4.61, it is clear that the leaching rate increases dramatically with increasing
temperature. The Arrhenius plot is shown in Figure 4.62. The apparent activation energy
was calculated from the slope of the curve as 70.1kJ/mol, which suggests that the
reaction is under chemical control. This result is consistent with leaching studies in
higher concentrations of reagents (Figures 4.55 and 4.56) and anodic polarization studies
(Figures 4.8 and 4.9).
123
Effect of rotating speed
Figure 4.63 shows that the rotating speed has no effect on the gold leaching rate under
experimental conditions, which confirms that the reaction is under chemical control. This
result is consistent with the anodic process (Figure 4.10, 4.11). However, it is inconsistent
with the cathodic process (Figure 4.33, 4.34), which is under diffusion control. It may
therefore be concluded that the leaching process is under anodic control under the
conditions tested.
180 -I , 1 i • , , , , , 0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 57. Effect of concentration of (NH4)S203 on gold leaching in (NH4)2S203
solution, 30 ppm copper, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
124
o 220
200
180
250ppm
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 58. Effect of concentration of [Cu]T on gold leaching in 0.1 M (NH4)2S203
solution, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
NH3/S203 0
300
0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 59. Effect of concentration of NH3/S2O3 ratio on gold leaching in
(NH4)2S203 solution, 30 ppm [Cu]T, 0.25V vs. SHE (pH 10, 450 rpm, 25°C, air
purge).
125
Time(Second)
Figure 4. 60. Effect of pH value on gold leaching in 0.1 M (NH4)2S203 solution, 30
ppm [Cu]T, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
0 -I 1 1 1 1 1 1 1 1 1
0 200 400 600 800 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 61. Effect of temperature on gold leaching in 0.1M (NH4)2S2C>3 solution, 30
ppm [Cu]T, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
126
Figure 4. 62 Arrhenius plot for rate of gold leaching using a lower concentration of
reagents, 0.25 V vs. SHE.
0 2 0 0 4 0 0 600 8 0 0 1000 1200 1400 1600 1800
Time(Second)
Figure 4. 63. Effect of rotating speeds on gold leaching in 0.1 M (NH4)2S203
solution, 30 ppm [Cu]T, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
127
4.3.4 Summary
a) Open potential with higher reagent concentrations
Table 4. 5 Influencing factors on gold oxidation during leaching tests
Factor Effect of gold oxidation Comments N H 3 Positive or negative Positive: from 0.2, 0.4, 0.8 M
Negative at 1.6 M S 20 3
2- Positive or negative Positive: from 0.1, 0.2, 0.4 M Negative at 0.8 M
Baseline conditions: Open potential, 0.2 M (NFL;)2S203, 250 ppm copper (pH 10, 450
rpm, 25°C, air purge)
b) Open potential with lower reagent concentrations
Table 4. 6 Influencing factors on gold oxidation during leaching tests
Factor Effect of gold oxidation Comments N H 3 Positive or negative Positive: from 0.2-0.8 M
Negative at 1.6 M S2O3 2 " Negative 0.1,0.2, 0.4 M
Baseline conditions: Open potential, 0.1 M (NH4)2S203, 30 ppm copper (pH 10, 450 rpm,
25°C, air purge).
c) Applied potential with higher reagent concentrations
Table 4. 7 Influencing factors on gold oxidation during leaching tests
Factor Effect of gold oxidation Comments Copper Positive Effect is greater at lower potentials Temperature Positive Chemical control Potential Positive Effect is greater at lower copper
concentrations
128
Baseline conditions: 0.25 V vs. S H E , 0.2 M (NH4)2S203, 250 ppm copper (pH 10, 450
rpm, 25°C, air purge)
d) Applied potential with lower reagent concentrations
Table 4. 8 Influencing factors on gold oxidation during leaching tests
Factor Effect of gold oxidation Comments Copper concentration
Positive Effect o f 30 ppm and 50 ppm is similar
(NH4)2S203 Positive N H 3 / S 2 0 3
2 " is larger than 4/3 N H 3 / S 2 0 3
2 " ratio Positive Critical value is 4/3 p H valve Positive or negative Optimum value is around 10 Temperature Positive Chemical control Rotating speed N o effect Chemical control
Baseline: 0.25 V vs. S H E , 0.1 M ( N H 4 ) 2 S 2 0 3 , 30 ppm copper, (pH 10, 450 rpm, 25°C, air
purge)
4.4 Additives studies
A number of additives were tested for their impact on the rate o f gold leaching.
4.4.1 Anodic polarization studies
Effect of Ag +
Figure 4.64 shows that A g + addition to solution can improve the rate of gold leaching.
But, the reason is uncertain.
129
Effect of NaCl
a) 500pppmNaCl
From Figure 4.65, it was found that N a C l had no effect on gold anodic polarization in the
absence of copper.
b) l%NaCl
Figure 4.66 shows that the gold oxidation current decreased significantly when the
concentration of N a C l was increased to 1%. Also , it is very interesting to find that the
oxidation of S2O3 " was almost stopped.
c) Bench tests observation
Two kinds of solution were prepared and kept static for a period of time to compare the
changes in each of the solutions. Solution A : 0.2 M ( N H ^ S a O s , p H 10, 500 ppm copper,
no N a C l . Solution B : 0.2 M (NH4)2S203, p H 10, 500 ppm copper, 12.5 g/1 N a C l (1.25%).
After about 120 hours, there was a lot of black precipitation (some green) in solution A
but no colour change for solution B . After 3 weeks, it was found that there was more
black precipitation in solution A , but still almost no colour change in solution B . This
observation and Figure 4.66 indicates that C l " ion can reduce the degradation o f
thiosulfate.
130
Figure 4. 64. Effect of 0.2 M Ag+ on the gold anodic polarization in 0.2 M
(NH4)2S203 solution, no copper (pH 10,450 rpm, 25°C, 1 mV/s).
E 3
60
50
40
£ 30 w c V
T3 •£ 20
o 10
0
-10
Total current density Current density from mass change
800
Potential vs SHE,mV
Figure 4. 65. Effect of 500 ppm NaCl on the gold anodic polarization in 0.2 M
(NH4)2S203 solution, no copper, (pH 10,450 rpm, 25°C, 10 mV/s).
131
Figure 4. 66. Effect of 1% NaCl on the gold anodic polarization in 0.2 M (NH4)2S203
solution, no [Cu]T, (pH 10,450 rpm, 25°C, 10 mV/s).
Effect of EDTA
From Figure 4.67, it can be seen that E D T A does not improve gold leaching. In fact,
E D T A hindered gold leaching under standard experimental conditions, especially at high
E D T A concentrations (0.01 M ) .
Compared with no E D T A , Figure 4.68 also shows that E D T A hinders gold oxidation.
However, it is interesting to find that E D T A may be able to hinder the oxidation o f
thiosulfate at same time.
132
J
600 Potential vs SHE.mV
Figure 4. 67 Effect of concentration of EDTA on the gold anodic polarization in
0.1M (NH4)2S203 solution, 250 ppm copper (pHIO, 450rpm, 25°C, lmV/s)
30
25 ^
20 CM
E
S 10 •D
2 5 3 L)
0
-5
-10 J
Total current density
Current density from mass change
(NH^SjOs 0.2M [Cu] T 250ppm
( N H 4 ) 2 S 2 0 3 0.2M [Cu] T 250ppm EDTA 0.01 M
100 200 300 400
Potential vs SHE.mV
500 600
Figure 4. 68. Effect of EDTA on gold anodic polarization in a solution of 0.2 M
(NH4)2S203, 250 ppm Cu 2 + (pH 10,450 rpm, 25°C, 1 mV/s).
133
2
E 5
1.5
1
0.5 (A o 0 •o
o -0.5 H 5
-1.5
-2 J
Total Current densiti
Current density from gold mass change
100 200 300 400 500
Potential vs SHE mV
Figure 4. 69. Gold anodic polarization in a solution of 0.1 M Na2S2C>3, 100 ppm
copper, 0.01 M EDTA (pH 10, 450 rpm, 25°C, 1 mV/s).
20
E
c
0) u
-5 J
Total current density
Current density from
mass change
100 200 300 400 500
Potential vs SHE.mV
600
Figure 4. 70 Reproducibility tests of anodic polarization of gold in 0.1 M
(NH4)2S203 solution, 250 ppm copper, 0.005M EDTA (pH 10, 450 rpm, 25°C, 1
mV/s).
134
The volatile loss of ammonia is a serious design issue for commercial application of
ammonium thiosulfate leaching o f gold. It is desirable to identify an alternate catalyst
system that works well without volatile losses. Unfortunately, Figure 4.69 indicated that
Na2S203, C u 2 + , E D T A system does not leach gold, which means that E D T A can not be
substituted for NHs/NH/ to leach gold in the thiosulfate leaching system.
Figure 4.70 shows the reproducibility in presence of E D T A
4.4.2 Leaching studies
Various leaching tests were also performed in the presence of additives.
Effect of EDTA
207
206
_ 205 cn
I 204 ra | 203 o O
202
201 -I
200
Add 0.01 mM EDTA
100 200 300 400 Time(Second)
500 600
Figure 4. 71. Effect of EDTA in leaching test in 0.1 M Na2S203, 100 ppm copper
(pH 10, 450 rpm, 25°C).
1 3 5
In Figure 4.71, a leaching test was run in 0.1 M Na2S2O3,100 ppm copper solution that
was purged with air for 15 minutes. After 280 seconds, 0 .01M E D T A was added. It was
found that there was almost no gold mass change before or after E D T A was added. This
result indicates that E D T A is not an effective alternate catalyst to ammonia.
Effect of EDTA with a high concentration of copper
From Figures 4.72, 4.73, it can be seen that when the concentration o f copper is 250 ppm,
the effect o f E D T A was quite different from 30 ppm copper. It is interesting to find that
E D T A hinders gold leaching only in the early stage (for about 1200 seconds). After this
point, E D T A no longer hinders gold leaching.
300 -,
250 -
200 -"5f in ra E 150 -
T3 O O 100 -
50 -
0 -
No EDTA 0.001 M EDTA
200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 72 Effect of EDTA in leaching test in 0.2M (NH4)2S203, 250 ppm copper ,
0.20 V vs. SHE (pHIO, 450 rpm, 25°C, air purge)
136
I 150 -\
| 100
50 No EDTA 0.001 M EDTA
0 200 400 600 800 1000 1200 1400 1600 1800 Time(Second)
Figure 4. 73. Effect of EDTA in test in 0.2 M (NH4)2S203, 250 ppm copper, 0.25 V
vs. SHE (pH 10,450 rpm, 25°C, air purge)
Effect of additives in the absence of copper
No additives
Time(Second)
Figure 4. 74. Effect of additives on gold leaching in 0.2 M (NH4)2S203, no copper
(except where added), 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge),
137
Figure 4. 75 Effect of concentration of Ag+ on gold leaching in 0.2 M (NH 4 ) 2 S 2 03,
no copper, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge).
Figure 4.74 shows that Co(NH3)6 and anthraquinone(AQ) have almost no effect on gold
leaching. A g + has positive effect on gold leaching. It is clear that copper (U) has the
strongest effect among of all o f these additives.
Figure 4.75 shows that the effect o f 2 m M and 10 m M A g + is similar.
138
Effect of additives in the presence of copper
Figure 4. 76. Effect of additives on gold leaching in 0.1 M (NH 4 ) 2 S 2 03, 30 ppm
copper, 0.25 V vs. SHE (pH 10,450 rpm, 25°C, air purge)
Figure 4.76 shows that most additives hinder gold leaching under experimental
conditions. It seems that anthaquinone and pyridine probably have a small positive effect
on the rate o f the gold leaching process.
139
4.4.3 Summary
Table 4 . 9 Effect of additives for gold oxidation on additives tests
Species Gold leaching Degradation of thiosulfate Comments N a C l N o effect Positive at 1 %
no effect at 500 ppm A g N 0 3 Positive No effect Same effect at
2 m M and 10 m M E D T A Negative Positive A Q N o effect C o J + N o effect
Baseline: 0.25 V vs. S H E , 0.2 M (NH4)2S203, no copper, (pH 10, 450 rpm, 25°C, air
purge)
Table 4 . 1 0 Effect of additives for gold oxidation on additives tests
Species Effect of gold leaching P b ( N 0 3 ) 2
Negative N T A Negative C o ( N H 3 ) 6 C l 3 Negative A g N 0 3 Negative N a 2 S 0 4 Negative N a C l Negative H g C l Negative A Q N o effect Pyridine Probably positive
Baseline: 0.25 V vs. S H E , 0.1 M ( N H 4 ) 2 S 2 0 3 , 30 ppm copper, (pH 10, 450 rpm, 25°C, air
purge).
140
4.5 The mechanism
It is generally agreed that gold leaching in thiosulfate solution follows an electrochemical
mechanism and this system is very complicated. In prior studies, several different
mechanism models were proposed (Jiang et al, 1993, Zhu, 1993; L i , et al, 1996; Ouyang
2001; Muir , 2002). But, some controversy exists among these studies, and no mechanism
model was generally accepted.
4.5.1 Anodic process
Because of the limitation of the experimental techniques used, no model noticed the
effect of copper on the anodic process in prior studies, even though Ouyang (2001)
observed that copper enhanced the gold anodic oxidation rate in coulometric tests but he
simply supposed that the role of copper was to decrease the passivation on the gold
surface, like NH3.
A s the R E Q C M was used in this study, the effect of copper on the anodic process has
been distinguished and studied in detail. It was found that copper can reduce the
overpotential which is needed to oxidize gold in thiosulfate solution. Therefore, the role
o f copper can not be simply explained by only decreasing passivation, whereas the
copper should be involved in the anodic reaction and contributes a significant effect on
the anodic process. However, the exact role of copper on the anodic process is not clear.
141
Based on the effect o f copper, the following two possible mechanisms on the anode are
proposed:
Mechanism A
It was supposed that the regeneration of Cu(NH3)4 2 + was the electrochemical reaction on
anode.
On the anode:
Step 1: Regeneration o f Cu(NH3)4 2 +
Cu(S203)3
5" + 4NH3 = Cu(NH3)4
2+ + 3S203
2" + e" Equation 4.13
Then, C u ( N H 3 ) 4
2 + oxidizes A u to A u ( S 2 0 3 ) 2
3 "
Au + Cu(NH3)4
2+ + 2S203
2" = Au(S203)2
3" + Cu(NH3)2
+ + 2NH3 Equation 4.14
In the bulk solution:
Step 2: Cu(NH3) 2
+ enters solution then reacts with S 2 03 3 " to form the more stable
C u ( S 2 0 3 ) 3
5 -
Cu(NH3)2
+ + 2NH3 + 3S203
3" = Cu(S203)3
5" + 4NH3 Equation 4.15
Thus, the total anodic reaction is: Au + 2S203
2" = Au(S2C>3)2
3" + e" Equation 4.16
Mechanism B
It is believed that the dissolution of gold on the anode is composed of two parts: one part
is contributed by electrochemical reaction (equation 4.17), which produces anodic
current, the other part is contributed by chemical reaction (equation 4.18) and no currents
produced. Equation 4.17 and 4.18 occur on the anode independently.
142
On the anode
Electrochemical reaction:
Au + 2S203
2" = Au(S203)2
3" + e" Equation 4.17
Chemical reaction:
Au + Cu(NH3)4
2+ + 2S203
2- = Au(S203)2
3" + Cu(NH3)2
+ + 2NH3 Equation 4.18
4.5.2 Cathodic process
From the cathodic studies (4.2), the mechanism of the cathodic process is proposed :
On the cathode
Step 1 Cu(NH3)4
2+ + e" = Cu(NH3)2
+ + 2NH3 Equation 4.19
In the bulk solution
Step 2 C u ( N H 3 ) 2
+ enters the solution then reacts with S 20 3
3" to form the more stable
C u ( S 2 0 3 ) 3
5 -
Cu(NH3)4
+ + 2NH3 + 3S203
2 = Cu(S203)3
5- + 4NH3 Equation 4.20
Step 3 Regeneration of C u ( N H 3 ) 4
2 + in the presence of oxygen
Cu(S203)3
5' + y2 02+H20+4NH3 = Cu(NH3)4
2+ + 3S203
2 +20H Equation 4.21
Thus, the total cathodic reaction is:
V2 02+ H zO +e" = 20H" Equation 4.22
143
4.5.3 The model of electrochemical mechanism
Mode l A
According to the mechanism A on anodic process, the model A of the electrochemical
mechanism is proposed in Figure 4.77.
GOLD SURFACE
Anodic area A I Cu(S203)3 ! ,'+4NH3=Cu(NH3)42++3S2032 +e" / v Cu(NH3)2*+ 2NH 3
/j / ^ 3S203 :
Au + Cu(NH3)42++2S2032"= Au^Osfe3"* Cu(NH3)2
T+2NH3
Cathodic area
Cu(NH 3) 4
2 ++e" =Cu(NH3)2
++2NH3
SOLUTION
Cu(S203)3
5'+4NH3
AutS.Oj),3-
,Cu(NH 3) 4
2 + + 3S203
2" + 20H"
| H 2 0
A) t 0.5O;
CulSAh^+ANHj
3S 2 0 3
2
Cu(NH 3) 2
++2NH 3
Total reaction
Au + 2S203
2' +1/202 + H 2 0 = AutSzOjh3'* 20H"
Figure 4. 77 The model A of electrochemical mechanism of gold leaching in ATS-
Cu system
144
Model B
According to the mechanism B on anodic process, the model B o f electrochemical
mechanism is proposed in Figure 4.78.
G O L D S U R F A C E
Anodic area
SOLUTION
Au +2S20j2"= Au{S201)2
3'+ e
(Electrochemical dissolve)
Au + CutNHjJ^+aSzOj2^ Au(S203)2
3-+ Cu(NH3)2*+2NH3
(Chemical dissolve)
e Cathodic area
Cu(NH3)4
2 * + e" = Cu(NH3)2
++ 2NH3
^ Cu(NH3)4
2+
live) / \
S*../h.lLI \ + J . O M U ' Au^O,)^
Cu(NH3)2
+
2NH3
^CutNH^2** SSzOj 2* 20H"
0.5O 2^ H 20
CufSzO^+aNHj
Cu(NH3)2
++2NH3
t 3S203
2-
Total reaction
1) Au + 2S203
2" + 1/202+H20 = Au(S203)2
3 + 20H" (Electrochemical dissolve)
2) Au + C u f N H ^ ^ S A 2 - = AutSiOj)^ Cu(NH3)2
++2NH3 (Chemical dissolve)
Figure 4. 78 The model B of electrochemical mechanism of gold leaching in ATS-
Cu system
145
5 CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
This study investigated the kinetics and mechanism of gold leaching in the A T S - C u
system using the R E Q C M . The effect of some additives was also studied. Anodic
polarization, cathodic polarization and leaching experiments were carried out in this
study. On the basis o f the experimental results, the following conclusions were drawn.
Both electrochemical studies and leaching studies show that all o f thiosulfate, ammonia,
and copper are necessary in the A T S - C u system for gold leaching. In the absence of any
of these species, gold leaching can not proceed at acceptable rates under the experimental
conditions studied.
Because the R E Q C M was used in the anodic polarization study, the effect of copper on
the anodic process was distinguished. It seems that copper is involved in the anodic
reaction under electrochemical experimental conditions. Based on the role o f copper with
respect to the anode, two possible anodic mechanisms were proposed. In mechanism A , it
is supposed that copper directly catalyzes the electrochemical reaction (equation 4.13).
In mechanism B , it is believed that copper enhances the gold oxidation on anode just by
chemical reaction (equation 4.18). Leaching studies show that it was C u ( N H 3 ) 4
2 + that
catalyses gold leaching.
146
Using the R E Q C M in the cathodic polarization study, it was proved that, in the absence
of S 2 O 3 2 " , the electrochemical reaction on the cathode is the reduction of Cu(NH3)4 2 + to
C u ( N H 3 ) 2
+ in the potential range of 0.20 to -0.30 V vs. S H E (equation 4.19). Based on
the fact that the potential value at which the cathodic current appeared in the presence o f
S203
2" was close to that in the absence of S2O3 2". It is believed that the electrochemical
reaction on cathode also is the reduction of Cu(NH3)4 2 + to Cu(NH3)2 + (equation 4.19).
Anodic polarization studies show that the temperature has a significant effect but rotating
speed has no impact on gold oxidation. Also , leaching tests show similar results. The
activation energy values suggest that both anodic process and leaching process are under
chemically control. The rotating speed dramatically affects the cathodic current density.
The good relationship between cathodic current density and square root of angular
rotation speed indicates that the cathodic process is under diffusion control.
Experiments show that the ratio of NH3/S2O3 2" is an important parameter for gold
leaching in the A T S - C u system. From the mechanism studies, it seems that the critical
value is around 4/3, keeping the ratio larger than this critical value is needed to achieve a
reasonable leaching rate.
Copper(U) can catalyze gold leaching but also consumes thiosulfate. So, an optimum
copper concentration exists in a certain condition. Ammonia usually can enhance the gold
leaching. However, too much ammonia w i l l have a negative effect on the leaching rate,
the reason probably is that excess ammonia blocks the gold surface from other species ,
147
which need to reach the gold surface and become involved in the gold dissolution
reactions. The suitable concentration o f thiosulfate is dependent on many factors: the
concentration o f copper, the ratio value of NH3/S2O3 2 ", etc. Like ammonia, excess
thiosulfate also may passivate the gold surface.
Both anodic polarization studies and leaching studies indicate that the suitable p H value
for gold dissolution probably is around 10. One reason is that thiosulfate is stable under
this alkaline condition; Another reason is that lower p H w i l l decrease the concentration of
ammonia, which is needed to solubilize the copper as the copper (IT) ammonia complex.
However, too high p H w i l l lead to precipitation of copper as copper oxide.
Leaching tests at different applied potentials show that the impact of copper
concentration on leaching rate is greater at lower potentials than at higher potentials.
These tests also indicate that the impact of potential on leaching rate is greater at lower
copper concentrations than that at higher copper concentrations.
Additive studies show that, in the absence of copper , A g + enhances gold leaching,
although this effect is much less than that of copper(II). It was found that 1% N a C l can
inhibit oxidation o f thiosulfate, but it hinders gold leaching at same time. E D T A hinders
the oxidation of gold. However, it probably also can hinder the oxidation o f thiosulfate.
Finally, the studies show that EDTA-S2032"-Cu system is not effective for gold leaching.
148
5.2 Recommendations
In future studies, the passivation/surface layer formation should be investigated using A C
Impedance Spectroscopy method and other surface characteristic method such as X P S .
Because the high consumption of reagents is cited as a limitation for commercial
application, it is necessary to continue to seek an alternative catalyst to replace
ammonia/copper or to continue looking for an additive which can improve gold leaching
or hinder the degradation of thiosulfate.
It w i l l be interesting to further study the role o f A g + on the thiosulfate leaching system.
149
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