SOLVENT EXTRACTION OF COPPER AND CYANIDE FROM

WASTE CYANIDE SOLUTION

by

FENG XIE

B. Sc., The Northeastern University (PRC), 1992

M. Sc., The Northeastern University (PRC), 1995

M. A. Sc., The University of British Columbia, 2005

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

in

THE FACULTY OF GRADUATE STUDIES

(Materials Engineering)

THE UNIVERSITY OF BRITISH COLUMBIA

(Vancouver)

April 2010

© Feng Xie, 2010

ABSTRACT

The potential use of two commercial extractants, LIX 7950, a guanidine derivative, and LIX

7820, a solvent mixture of quaternary amine and nonylphenol, for recovery of copper and

cyanide from waste cyanide solution has been investigated. Low equilibrium pH favors copper

extraction while a high molar ratio of cyanide to copper depresses the copper loading. It is

confirmed that Cu(CN)32 is preferentially extracted over Cu(CN)43 and CN by the extractants.

Solvent extraction of the mixture of metal cyano complexes shows a selectivity order as follows:

Zn > Ni > Cu > Fe. The presence of S042 or S2032 shows an insignificant effect on copper

extraction while SCN ions may potentially compete for the available extractant with copper

cyanide species and thus depress copper extraction significantly. Both extractants exhibit an

affinity sequence as SCN > CNO > CN>S2032.The selectivity order of different anions with

the extractants can be explained by the interrelated factors including anion hydration, charge

density, compatibility of the formed complex with the organic phase and the geometry effect.

The extraction of Cu(CN)32with LIX 7950 is exothermic with an enthalpy change (AH°) of -191

kJ/mol. The copper extraction with LIX 7820 has little change when the temperature is varied

from 25 °C to 45 °C. For both extractants, the loaded copper and cyanide can be stripped

efficiently by a moderately strong NaOH solution. Further increase in NaOH concentration

results in the formation of a third phase. The presence of NaCN can facilitate stripping of the

loaded copper and cyanide by favoring the formation of Cu(CN)43 in the stripping solution.

The important findings suggest a possible solution to the separation of metal cyanide species and

free cyanide in the cyanide effluent. Both extractants can be used in a SX circuit for pre

concentrating copper into a small volume of strip solution which can be further treated by

electrowinning, AVR, SART or similar processes to recover copper products and cyanide. The

free cyanide will remain in the raffinate solution from solvent extraction circuit which allows for

the potential recycling of the barren solution to the gold cyanidation process.

11

TABLE OF CONTENTS

ABSTRACT.ii

TABLE OF CONTENTS iii

LIST OFTABLES vi

LISTOFFIGURES ix

ACKNOWLEDGEMENTS xiv

1 Introduction 1

1.1 Cyanide classification 2

1.2 Free cyanide 3

1.3 Cyanate and thiocyanate 5

1.4 Metal cyanide complexes 7

1.4.1 Cyanide complex equilibrium 7

1.4.2 Copper cyanides 9

1.4.3 Zinc cyanides 12

1.4.4 Nickel cyanides 14

1.4.5 Iron cyanides 16

1.4.6 Gold and silver cyanide complexes 17

1.4.7 Mixtures of metal cyanide species 18

2 Cyanide Destruction and Recovery 19

2.1 Chemical destruction process 19

2.1.1 Inco S02/Air process 19

2.1.2 Hydrogen peroxide 20

2.1.3 Caro’s acid 21

2.2 Cyanide and metal recovery process 21

2.2.1 AVR/MNR/SART process 22

2.2.2 Activated carbon 24

2.2.3 Ion exchange resin 25

2.2.4 Solvent extraction 29

V 111

2.2.5 Miscellaneous.33

2.3 Research objective 34

3 Experimental 36

3.1 Organic reagents and chemicals 36

3.2 Preparation of the extractant solvents 37

3.3 Preparation of aqueous solutions 37

3.4 Test procedure 39

3.5 Analysis 39

4 Extraction with LIX 7950 42

4.1 Terms and definitions 42

4.2 Equilibrium time 43

4.3 Organic formula 44

4.3.1 Effect of diluents 44

4.3.2 Effect of modifiers 46

4.3.3 Effect of the extractant concentration 49

4.4 Effect of temperature 51

4.5 Effect of CN/Cu ratio 55

4.5.1 Extraction results 55

4.5.2 FTIR analysis 57

4.5.3 Preferential extraction 58

4.6 Effect of phase ratio 63

4.7 Effect of other anions 67

4.7.1 Extraction of the mixture solution 67

4.7.2 Effect of S042 71

4.7.3 Effect of SCN, CNO and S2032 73

5 Extraction by LIX 7820 78

5.1 Equilibrium time 78

5.2 Organic formula 79

5.2.1 Effect of the molar ratio of nonyiphenol to Aliquat 336 79

5.2.2 Effect of diluents 84

5.2.3 Effect of the extractant concentration 85

iv

5.3 Effect of CN/Cu ratio.87

5.4 Effect of phase ratio 90

5.5 Effect of temperature 92

5.6 Co-extraction with other anions 94

5.6.1 Effect of non-metal anions 94

5.6.2 Extraction from a mixed solution of metal cyanides 96

6 Discussion 99

7 Stripping of the Loaded Copper and Cyanide 105

7.1 Effect of stripping reagents 105

7.1.1 Stripping with NaOH solution 105

7.1.2 Stripping with NaCN-NaOH solution 107

7.2 Effect of temperature 108

7.3 Effect of phase ratio 110

8 Conclusions and Recommendations 112

8.1 Conclusions 112

8.2 Recommendations 114

Bibliography 115

Appendix I Analysis Methods 126

Free Cyanide 126

Thiocyanate 127

V

LIST OF TABLES

Table 1-1 Simplified classification of cyanide compounds

(modified from Flynn and McGill, 1995) 3

Table 1-2 Solubility of common copper minerals in 0.1 % NaCN solutions

(after Hedley and Tabachnik, 1968) 10

Table 1-3 Some properties of copper(I) cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976) 12

Table 1-4 Some properties of zinc cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976) 14

Table 1-5 Some properties of nickel(II) cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976) 15

Table 1-6 Some properties of iron cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976) 16

Table 1-7 Some properties of gold and silver cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976) 18

Table 2-1 Some commercial base extractants for gold solvent extraction

(Rydberg, et a!., 2004) 30

Table 3-1 Some information of LIX® 79 and Aliquat® 336 36

Table 3-2 Some information of diluents and modifiers used in the research 37

Table 3-3 Some information of inorganic chemicals used in the research 38

Table 3-4 The major components in the mixture solution of metal cyanides 39

Table 4-1 Comparison of the effect of diluent types on extraction of metal cyano complexes . .46

Table 4-2 The effect of modifier types on copper extraction with LIX 7950

(Org: 10% v/v LIX 7950, aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5; A/O = 1; 25°C) 47

Table 4-3 The effect of temperature on copper extraction with LIX 7950

(Org: 10 % v/v 7950 and 50 g/L 1-dodecanol in n-dodecane; aq: [Cu] = i0 mol/L,

CN/Cu=3,A/O=1) 52

Table 4-4 Comparison of 1G° and uS0 for different solvent extraction systems 54

vi

Table 4-5 Copper extraction with LIX 7950 under pH uncontrolled conditions

(Org: 10% v/v LIX 7950 and 50 gIL 1-dodecanol in n-dodecane, aq: [Cu] = 500 mg/L,

CN/Cu = 5, initial pH 10.5; 20 °C) 65

Table 4-6 The separation factors for Zn, Ni, and Fe over Cu under different equilibrium pH

(Org: 30% v/v LIX 7950 and 100 g/L 1-dodecanol in n-dodecane,

initial aqueous solution as in Table 3-4; A/0=1; 25 °C) 70

Table 4-7 The effect of S042 on copper extraction with LIX 7950

(Org: 10%v/v 7950 and 50 g/L 1-dodecanol in n-dodecane,

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 3; AJO = 1; 20°C) 72

Table 4-8 The loaded anion content under different initial concentrations

(Org: 10%v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane;

aq: [Cu] = 3.93 x 10 mol/L, CN/Cu = 3; PHeq = 10.50 ± 0.05; A/0 1; 25°C) 77

Table 5-1 The effect of 1-octanol concentration on copper extraction with Aliquat 336

(Org: 9.4 x i0 mol/L Aliquat 336 in n-octane, aq: [Cu] = 3.93 x i0 mol/L,

CN/Cu=5;A/0=1;20°C) 83

Table 5-2 Copper extraction with LIX 7820 under pH uncontrolled conditions

(Org: 2% v/v LIX 7820 in n-octane, aq: [Cu] = 500 mg/L, CN/Cu = 5, pH 10.5; 20°C) 90

Table 5-3 The loaded anions under different initial anion concentrations

(Org: 2%v/v LIX 7820 in n-octane, aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 3;

PHeq = 10.50 ± 0.05; A!0 = 1; 20°C) 95

Table 6-1 Summary of the selectivity orders with various extractants 100

Table 6-2 The hydration properties of some anions 102

Table 7-1 Stripping of copper and cyanide from the loaded LIX 7950 by NaOH solutions

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane, loaded Cu

and CN are 3.68 x i0 mol/L and 1.11 x 102 mol/L, respectively; 0/A = 1; 20 °C) 106

Table 7-2 Stripping of copper and cyanide from the loaded LIX 7820 by NaOH solutions

(Org: 2% v/v 7820 in n-octane, loaded Cu and CN are 2.17 x i0 mol/L

and 6.50 x i0 mol/L, respectively; 0/A = 1; 20 °C) 106

Table 7-3 Stripping of copper and cyanide from LIX 7950 by NaOH-NaCN solutions

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane, Cu and CN are

3.68 x i0 mol/L and 1.11 x 102 mol/L, respectively; [NaOH] = 1 mol/L; 0/A = 1; 20°C) ....108

vii

Table 7-4 Stripping of copper and cyanide from LIX 7820 by NaOH-NaCN solutions

(Org: 2% v/v LIX 7820 in n-octane, loaded Cu and CN are 2.16 x 10-3 mol/L

and 6.50 x 10-3 mol/L, respectively, [NaOHJ = lmol/L; 0/A = 1; 20 °C) 108

viii

LIST OF FIGURES

Figure 1-1 Plot of equilibrium distribution diagram for free cyanide vs pH at 25 °C 4

Figure 1-2 Eh-pH diagram of CN-H20system ([CN] =102 mol/L, 25 °C) 6

Figure 1-3 Eh-pH diagram for S-CN-H20system

([CN] = i0 mol/L, [S] = i0 mol/L, 25 °C) 7

Figure 1-4 Eli-pH diagram for Cu-CN-H20system

([Cu] = 1 mol/L, [CN] = 1 mol/L, 25 °C) 11

Figure 1-5 Plot of mole fraction of copper cyanide species vs log [CN]

([Cu]= 0.1 mol/L, 25 °C) 11

Figure 1-6 Eh-pH diagram for Zn-CN-H20system

([CN] = mol/L, [Zn] = i0 mol/L; 25 °C) 13

Figure 1-7 Eh-pH diagram for Ni-CN-H20system

([CN] = 103mo1/L, [Ni] = lO4mol/L; 25 °C) 15

Figure 1-8 Eh-pH diagram for Fe-CN-H20system

([CN] = 103mo1/L, [Fe] 104mo1/L; 25 °C) 17

Figure 2-1 Use of solvent extraction in the recovery of copper and cyanide from solution

(after Dreisinger, et a!., 2001) 35

Figure 3-1 The schematic experimental set-up for pH and temperature-controlled tests 40

Figure 3-2 Picture of the shaking machine for pH-uncontrolled tests 41

Figure 3-3 Picture of the separation and filtration apparatus 41

Figure 4-1 Plot of variations of copper extraction and pH vs contact time

(Org.: 10% v/v LIX 7950 and 50 gIL 1-dodecanol in n-dodecane,

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu 5; A/O = 1; 25°C) 44

Figure 4-2 The effect of diluents types on copper extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 gIL 1-dodecanol in n-dodecane;

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5; A/O = 1; 25 °C) 45

Figure 4-3 The effect of the concentration of 1-dodecanol on Cu extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane,

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5; A/O = 1; 25°C) 47

ix

Figure 4-4 Plot of log Dcu[OW]2vs log [RG]org at different metal concentrations

(Org.: LIX 7950 and 50 gIL 1-dodécanol in n-dodecane; aq: CN/Cu = 3,

initial pH 10.00 ± 0.05; NO =1; 20°C) 51

Figure 4-5 Plot of the calculated Log Kex vs 1000/T (Org: 10 % v/v 7950

and 50 g/L 1-dodecanol in n-dodecane; aq: [Cu] = i0 mol/L, CN/Cu = 3; NO = 1) 54

Figure 4-6 The effect of CN/Cu ratio on copper extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 gIL 1-dodecanol in n-dodecane;

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5; NO = 1; 25°C) 56

Figure 4-7 The effect of CN/Cu ratio on cyanide extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane,

aq: [Cu] = 3.93 x 10 mol/L, CN/Cu = 5; NO = 1, 25 °C) 56

Figure 4-8 The calculated CN/Cu ratios in the organic phase under different pH

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane,

aq: [Cu] = 3.93 x 10 mol/L, CN/Cu = 5; NO = 1; 25 °C) 57

Figure 4-9 Plot of the calculated fraction of copper cyanide complexes vs pH

([Cu] =3.93 x i03 mol/L, CN/Cu = 5; 25 °C) 60

Figure 4-10 Plot of the calculated fraction of copper cyanide complexes vs CN/Cu ratio

([Cu] =3.93 x i0 mol/L, pH =11; 25 °C) 60

Figure 4-11 The schematic extraction of copper cyanide solution with LIX 7950 63

Figure 4-12 The distribution isotherms of copper extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane; aq: CN/Cu = 5;

pHeq = 10.50 ± 0.05; 20°C) 65

Figure 4-13 The distribution isotherms of cyanide extraction with LIX 7950

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane; aq: CN/Cu = 5;

pHeq 10.50 ± 0.05; 20°C) 66

Figure 4-14 The schematic McCabe-Thiele diagram for copper and cyanide extraction

(Org: 10% v/v LIX 7950 and 50 gIL 1-dodecanol in n-dodecane; aq: CN/Cu = 5;

PHeq = 10.50 ± 0.05; 20°C) 66

Figure 4-15 Plot of variations of metal extraction and solution pH vs contacting time

(Org: 30% v/v LIX 7950 and 100 g/L 1-dodecanol in n-dodecane;

initial aqueous solution as in Table 3-4; NO = 1; 25 °C) 68

x

Figure 4-16 The extraction of metals and cyanide with LIX 7950 under different pH

(Org: 30 % v/v LIX 7950 and 100 g/L 1-dodecano in n-dodecane;

initial aqueous solution as in Table 3-4; A/O = 1; 25 °C) 70

Figure 4-17 The extraction isotherms for CN, SCN, and CNO with LIX 7950

(Org: 10 % v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane; pH = 10.50 ± 0.05; 25 °C) ..74

Figure 4-18 The effect of SCN, CNO and S2032 ions on copper extraction by LIX 7950

(Org: 10 % v/v LIX 7950 and 50 g/ L 1-dodecanol in n-dodecane;

aq: [Cu] = 3.93 x i0 mol/L, CN/Cu =3; pHeq = 10.50 ± 0.05, AJO = 1; 25°C) 77

Figure 5-1 Plot of variations of copper extraction and solution pH vs contact time

(Org: 2% v/v LIX 7820 in n-octane; aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5;

A/O=1;20°C) 79

Figure 5-2 The effect of the molar ratio of nonylphenol to Aliquat 336 on copper extraction

(Org: 9.4 x i0 mol/L Aliquat 336 in n-octane, aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5;

A/O=1;20°C) 82

Figure 5-3 The effect of the 1-octanol concentration on copper extraction with LIX 7820

(Org: 2 % v/v LIX 7820 in n-octane; aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5;

A/O=1,20°C) 83

Figure 5-4 The effect of diluent types on copper extraction with LIX 7820

(Org: 20% v/v LIX 7820 in n-octane, aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 4;

A/O=1;20°C) 85

Figure 5-5 The effect of the extractant concentration on copper extraction with LIX 7820

(Org: LIX 7820 in n-octane; aq: [Cu] = 3.93 x i0 mol/L, CN/Cu = 5; A/O = 1; 20°C) 86

Figure 5-6 The effect of the initial copper concentration on Cu extraction with LIX 7820

(Org: 10% v/v LIX 7820 in n-octane; aq: CN/Cu = 5; A/O = 1; 20°C) 87

Figure 5-7 The effect of CN/Cu ratio on copper extraction with LIX 7820

(Org: 2% LIX 7820 in n-octane; aq: [Cu] = 3.93 x i0 mol/L, A/U = 1; 20°C) 88

Figure 5-8 The effect of CN/Cu ratio on cyanide extraction with LIX 7820

(Org: 2% LIX 7820 in n-octane, aq: [Cu] = 3.93 x i0 mol/L, A/U = 1; 20°C) 89

Figure 5-9 Plot of calculated CN/Cu ratios in organic phase vs equilibrium pH

(Org: 2% v/v LIX 7820 in n-octane; aq: [Cu] = 3.93 x i0 mol/L, NO = 1; 20°C) 89

xi

Figure 5-10 The distribution isotherms of copper extraction with LIX 7820

(Org.: 2% v/v LIX 7820 in n-octane, aq: CN/Cu = 5, pH = 10.50 ± 0.05; 20°C) 91

Figure 5-11 The distribution isotherms of cyanide extraction with LIX 7820

(Org: 2% v/v LIX 7820 in n-octane, aq: CN/Cu = 5; pH = 10.50 ± 0.05; 20°C) 91

Figure 5-12 The schematic McCabe-Thiele diagram for copper and cyanide extraction

(Org: 2% v/v LIX 7820 in n-octane, aq: CN/Cu = 5; pH = 10.50 ± 0.05; 20°C) 92

Figure 5-13 The effect of temperature on extraction of copper and cyanide with LIX 7820

(Org: 2% v/v LIX 7820 in n-octane, aq: [Cu] = 3.93 x10 mol/L, CN/Cu = 5;

P1eq = 11.00 ± 0.05; AJO = 1) 93

Figure 5-14 The effect of SCN, CNO and S2O32 on copper extraction with LIX 7820

(Org: 2% v/v LIX 7820 in n-octane, aq: [Cu] = 3.93 x10 mol/L, CN/Cu = 5;

pHeq= 10.50±0.05;AJO= 1;20°C) 95

Figure 5-15 The extraction isotherms of SCN, CN, CNO with LIX 7820

(Org: 2% v/v LIX 7820 in n-octane, aq: sodium salt; PHeq = 10.50 ± 0.05; 20°C) 96

Figure 5-16 The effect of contacting time on metal extraction and pH with LIX 7820

(Org: 5% v/v LIX 7820 in n-octane; aqueous solution as in Table 3-4; A/O =1; 20°C) 97

Figure 5-17 The extraction of metals and cyanide with LIX 7820 under different pH

(Org: 5% v/v LIX 7820 in n-octane, aqueous solution as in Table 3-4; AJO =1; 20 °C) 98

Figure 7-1 Stripping of loaded Cu and CN by NaOH solution under different temperatures

(Org: 10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane,

loaded Cu and CN are 3.68 x i0 mol/L and 1.11 x 102 mol/L, respectively;

aq: [NaOH] = lmol/L; 0/A = 1) 109

Figure 7-2 Stripping of loaded Cu and CN by NaOH solution under different temperatures

(Org: 2% v/v LIX 7820 in n-octane, loaded Cu and CN are 2.16 x i0 mol/L

and 6.50 x i0 mol/L, respectively; aq: [NaOH] = 1 mol/L; 0/A = 1) 109

Figure 7-3 The striping isotherms of copper from the extractant solvent of LIX 7950

(10% v/v LIX 7950 and 50 g/L 1-dodecanol in n-dodecane,

loaded Cu and CN are 3.68 x i0 mol/L and 1.11 x 102 mol/L, respectively;

aq: [NaOHJ = 1 mol/L; 20 °C) 111

xii

Figure 7-4 The striping isotherms of copper from the extractant solvent of LIX 7820

(2 % v/v LIX 7820 in n-octane, loaded Cu and CN are 2.16 x i0 mol/L

and 6.50 x i0 mol/L, respectively; aq: [NaOH] = lmol/L; 20 °C) 111

Figure 8-1 The schematic flowsheet for the potential application of SX circuit 114

xlii

ACKNOWLEDGEMENTS

The author would like to express his utmost gratitude to his supervisor Dr. David Dreisinger for

his kind guidance, support and encouragement during the whole thesis work. The contributions

and suggestions put forth by Dr. Berend Wassink are gratefully acknowledged. The author would

like to express sincere thanks to the people in UBC Hydro lab for their kind help and

suggestions.

The author also wishes to express gratitude to the Natural Science and Engineering Research

Council of Canada (NSERC), the Canadian Institute of Mining, Metallurgy and Petroleum (CIM)

- Hydrometallurgy Section of Metallurgy Society, CIM - Vancouver Branch, and the Cy and

Emerald Foundation for financial support. Cognis is thanked for supplying the solvent samples.

The author is particularly indebted to his family (especially his wife and daughter) for their

tolerance, forbearance and encouragement throughout the period of the thesis.

xiv

1 Introduction

The cyanidation process has been practiced for treating gold ores by most of the gold processing

plants for more than 100 years. In 1846, Eisner (Eisner, 1846) first established the important

reaction for gold dissolution in oxygenated cyanide solution:

4Au + 8KCN + 02 + 2H20 = 4Au(CN)2+ 4KOH (1-1)

The modern cyanidation process was patented between 1887 and 1888 by McArthur and the

Forrest brothers, and was rapidly developed into a commercial process in the latter years of the19th century (Marsden and House, 1992). Though some alternative lixiviants have been

developed in recent years due to environmental pressure and process opportunity, none of them

has yet found its way to practical application. The cyanidation process is still the mainstream

technology for gold extraction from gold ore and it seems that this will continue. However, a big

challenge for the process is the treatment of the large amount of cyanide-contaminated effluents

since most of the cyanide consumed in the cyanidation process is actually wasted in the effluents,

some occurring as free cyanide, with the balance forming metal cyanide complexes (Botz. et al.,

2005; Fleming, 2005). For example, the initial reaction of pyrrhotite in the cyanide solution can

be expressed as follows:

Fe7S8 + NaCN = 7FeS + NaSCN (1-2)

FeS and SCN can be further oxidized to form iron cyanide and various aqueous sulfur species

such as sulphate:

FeS + 6NaCN + 202 = Na4Fe(CN)6+ Na2SO4 (1-3)

It is known that less than 2% of the cyanide consumed accounts for the dissolution of gold and

silver in many gold operations and the majority of the cyanide is consumed by the cyanide

soluble minerals found commonly in gold bearing ores (Marsden and House, 1992). The solution

chemistry of some cyanide species commonly encountered in the cyanidation process is

summarized as below.

1

1.1 Cyanide classification

The term cyanide refers to a large family of compounds each member of which contains a cyano

group (CN) as part of its molecular structure (Kunz, et al.. 1978). The cyano group consists of a

carbon atom and nitrogen atom joined by three electron pairs. The definition for cyanide covers

both organic and inorganic compounds. From the point of view of environmental effect, only

those compounds which are capable of releasing the cyanide ion, CN, are of concern due to the

high toxicity of CN. Although all cyanide compounds are capable, at least theoretically, of

dissociation in an aquatic environment to release cyanide ions, the extent of ionization of

different cyanide compounds varies in the range from infinitesimal to complete dissociation

(Flynn and McGill, 1995). For example, nitriles and cyanohydrins are both organic cyanide

compounds containing the cyano group. Since nitriles do not release cyanide ion in water, they

are not classified as cyanide species according to this criterion. On the other hand, cyanohydrins

are capable of releasing cyanide ions in water and therefore are considered as the cyanide

species. In practice, cyanide pollution is always associated with inorganic cyanides since

wastewaters from the principal sources of cyanide-contaminated effluent, e. g., metal

electroplating and finishing facilities and the mining industry (particularly gold mill operations)

always contain cyanides in the inorganic form.

Since the behavior of cyanide in an aqueous medium is affected by a number of factors,

including pH, temperature and the presence of other constituents in solution, the clear definition

and classification of cyanide species are difficult though traditionally, cyanides may be classified

into two general types of structures: free cyanide and complex cyanide, or more specifically,

WAD (weak acid dissociable) and non-WAD (or strong acid dissociable) cyanide (Table 1-1,

Flymi and McGill, 1995). In this chapter, special interest, particularly from the wastewater

treatment viewpoint, is focused on the chemistry and recovery process of free cyanide including

HCN and uncomplexed cyanide ion, CN, and some of the transitional metal cyanide complexes

including copper, zinc, nickel, and iron cyanide species. Some species derived from cyanide such

as CNO and SCN were also discussed.

2

Table 1-1 Simplified classification of cyanide compounds

(modified from Flynn and McGill, 1995)

Classification Compounds

Free cyanide CN, HCN

Weak-acid dissociable cyanide Zn(CN)42,Cd(CN)3, Cd(CN)42,Hg(CN)42,

Mn(CN)62,Mn(CN)63,Cr(CN)64

Strong-acid dissociable cyanide

Moderate strong complexes Cu(CN)j, Cu(CN)32,Ni(CN)42,Ag(CN)2

Strong complexes Fe(CN)64,Fe(CN)63,Au(CN)2,Co(CN)64

1.2 Free cyanide

Free cyanides are those cyanide compounds that are present in an aqueous solution in the form of

CN ion and hydrogen cyanide (HCN). The concentration of free cyanide is in fact the sum of

CN and HCN in the aqueous solution (Marsden and House, 1992, Flynn and McGill, 1995).

They are related by the acid dissociation of HCN,

HCN = CN ÷ H (1-4)

Hydrogen cyanide acid is a weak acid in aqueous solution. The pKa for HCN as a function of

temperature can be expressed as below:

2347.2pK =1.3440+ (1-5)a T+273.16

where T is temperature given in degrees Celsius (lzatt, et al., 1962). The diagram illustrating

equilibrium distribution for free cyanide as a function of pH at 25 °C is shown in Figure 1-1. At

pH above 9.21, CN predominates and at pH below 9.21, HCN predominates. As a result, a pH of

10.5 to 11.5 is usually maintained by most gold cyanidation operations to keep the predominant

species of cyanide as CN since HCN has a relative high vapor pressure and will volatilize from

the leachate and cause a significant loss of cyanide and a detrimental effect to the working

environment.

3

04-I

C.)(

0E

The electronic configuration of cyanide ion CN is

(Gis)2 (a* is)2(cy2s)2(cy*2s)2(a2p)2(7t2py)2(1t2p)2(Sharpe. 1976).

Thus the two atoms (C and N) are triply bonded by one bond and two ‘t bonds as

[:CN:]

Free cyanide (CN) in aqueous solution shows infrared maxima at 2080 cm1. Values of the

enthalpy of hydration of CN and the electron affinity of CN are close to those for the

corresponding quantities for Br (-306 KJ/mol and -33iKJImol respectively, Flynn and McGill,

1995). When forming complexes, the cyanide ion has the ability to stabilize transition metal ions

in a low oxidation state and acts as a monodentate ligand with carbon as the donor atom (Sharpe,

1976).

1

0.8

0.6

0.4

0.2

0

0 2 4 6 8 10 12 14

pH

Figure 1-1 Plot of equilibrium distribution diagram for free cyanide vs pH at 25 °C

4

1.3 Cyanate and thiocyanate

The most important cyanide compound of oxygen is cyanate (CNO), which is the major

oxidation product of cyanide (Flynn and McGill, 1995). The cyanate ion consists of one oxygen

atom, one carbon atom, and one nitrogen atom and possesses 1 unit of negative charge, borne

mainly by the nitrogen atom. The structure of cyanate can be considered to resonate between two

canonical forms:

eO_CN O=C=NeThe resonance hybrid resulting from these two contributory structures can be represented as

IOCEENIThe cyanate ion is isoelectronic with carbon dioxide, and so shares its linear shape (Greenwood

and Earnshaw, 1997).

In aqueous solution, cyanide is fairly easily oxidized to cyanate as shown by the following

potential calculated from the standard free energies of the species involved:

CNO + 2W + 2e = CN + H20 E° = -0.14 V (1-6)

Oxidation of CN to CNO occurs in the presence of oxidants such as S02/Air, hydrogen

peroxide (11202) and hypochiorite (dO). The oxidation of CN by 02 is observable and it was

reported that the process is catalyzed in the presence of copper and activated carbon (Adams,

1990),

2 CN +02= 2CN0 (1-7)

The equilibrium distribution diagram for cyanate as a function of pH at 25 °C is shown in Figure

1-2. Cyanate is much less toxic than cyanide. It may degrade in aqueous solution to form CO2

and NH4 and decomposes rapidly in acidic solutions. Strong oxidants such as ozone and

hypochiorite may further oxidize CNO to C032 and N2.

5

Eh (Volts)

2.0 —

1.5

1.0HCNO(a)

0.5CNO(-a)

-1.0

-1HCN(a) CN(-a)

2 4 6 8 10 12 14

pH

Figure 1-2 Eh-pH diagram of CN-H20system

([CN] 102 mol/L, 25 °C)

The most important sulfur cyanide species is the well-known thiocyanate ion (SCN).

Thiocyanate is analogous to the cyanate ion wherein oxygen is replaced by sulfur. The structure

and bonding of thiocyanate is as follows (Greenwood and Earnshaw. 1997):

°S—CNThe formation of SCN in the cyanidation process is attributed to the reaction of CN with solid

sulfur or dissolved polysulfide (S2), or thiosulfate (S2032)formed by the oxidation of sulfide

minerals. The simplified reactions are presented as below,

CN + HS + V2 02= CNS + OW (1-8)

8 CN + S8 =8 SCN (1-9)

CN + S2032 = SCN+ S032 (1-10)

6

The Eh-pH diagram of S-CN-H20system at 25 °C is shown in Figure 1-3. Those oxidants that

oxidize CN to CN0 can also oxidize SCN and oxidation of SCN by 02 is extremely slow at

ambient temperature (Wilson and Harris, 1960).

Eh (Volts)

2.0

1.5

1 0 HSO4(-

0.5 S04(-2a)

-1.0HCN(a) CN(-a)

H2S(a) HS(-a) (-2a-1.5

-2.0 —0 2 4 6 8 10 12 14

pH

Figure 1-3 Eh-pH diagram for S-CN-H20system

([CN] = i0 mol/L, [SI = i0 mol/L, 25 °C)

1.4 Metal cyanide complexes

1.4.1 Cyanide complex equilibrium

Cyanide bound to a metal ion is usually referred to as complexed cyanide which may occur as

solids or dissolved species.. When a metal ion forms more than one complex with cyanide the

corresponding series of equilibrium can be represented as follows (Flynn and McGill, 1995):

7

[MCN]M + CN = MCN K1 = [M][CN] (1-11)

[MCN21MCN1 + CN = M(CN)2h12 K2 = [MCN][CNj (1-12)

[MCIV’]M(cN)il1 + CN = M(CN)x K = [M(CN)111’j[CN] (1-13)

where M11 is a metal ion of charge n+, i is the number of cyanide that can be taken by the metal

and K is the stability constant of the complex. The equilibrium between the metal, the cyanide

ligands and the complex can also be represented by the following equation,

[M (CN) nj

+ i CN = M(CN) = [M][cN]1 (1-14)

where 3, is the formation constant of the i complex. The formation constant can be expressed in

terms of the stability constants by,

13=KoKiK2 K1 (1-15)

by definition K0 = fib = 1 (free ion) and K1 = fir. The fraction of the individual metal cyanidecomplex (M(CN)1)can be given as follows,

a= fi[CN]1

1

fi[CNjk (1-16)

The equilibrium constants for the formation of metal cyanides provide a measure of the strength

of the complex species. Many metal complexes with cyanide are stronger than those with other

ligands, such as chloride (Cr), ammonium (NH3), and even ethylenediaminetetraacetate (EDTA).

Metal cyanide species differ widely in their reactivity with acids (Flynn and McGill, 1995). As a

result, metal cyanide complexes have been traditionally classified into two groups in regard to

their reactivity with acids, namely, WAD, (weak-acid-dissociable), e.g., Zn, Cd and Ni cyanide

complexes and non-WAD, e.g., cobalt and iron cyanide complexes.

8

1.4.2 Copper cyan ides

Copper minerals dissolve to varying degrees in alkaline cyanide solutions depending on their

reactivity with cyanide and the leaching conditions. Many copper oxide and sulfide minerals are

very reactive in cyanide solutions (Hedley and Tabachnik, 1968). Except for chalcopyrite

(CuFeS2) and chrysocolla (CuSiO3), the reactivity of most common copper minerals such as

chalcocite (Cu2S), covellite (CuS), cuprite (Cu20), and malachite (CuCO3Cu(OH)2)with

cyanide is substantial (Sceresini, 2005).

Cu2S + 7NaCN + ½ 02 + H20 = Na2Cu(CN)3+ 2NaOH + NaCNS (1-17)

2CuS + 8NaCN + ‘/202 + H20 = 2NaCu(CN)3+ 2NaOH + 2NaCNS (1-18)

Cu20 + 6NaCN + H20 = Na2Cu(CN)3+ 2NaOH (1-19)

CuCO3 + 8NaCN + 2NaOH= 2Na2Cu(CN)3+ 2Na2CO3+ NaCNO + H20 (1-20)

The solubility of some typical copper minerals in cyanide solution is summarized in Table 1-2

(Hedley and Tabachnik, 1968). The fast dissolution kinetics of copper minerals in cyanide

solution requires the maintenance of a high level of free cyanide during gold leaching. This

results in a significant economical penalty in excess cyanide consumption and loss of valuable

copper in cyanide effluent. The presence of copper in the ore also causes serious problems during

cyanide effluent treatment as copper in the cyanide solution may occur as various copper cyanide

complexes. In the cyanide tailings, when the concentration of free cyanide decreases with time,

more cyanide can be liberated due to the equilibrium shift from Cu(CN)43 and Cu(CN)32 to

Cu(CN)2 and finally to CuCN precipitates. This results in a substantial level of free cyanide in

the tailing ponds. In contrast, when excess cyanide exists in the tailings, copper cyanide may

react with the cyanide ion to form stable complexes with a higher number of CN ligands.

Subsequently, the equilibrium among copper cyanide complexes buffer the free cyanide content

within the tailing waters and acts as a sink to store the free cyanide (Hedley and Tabachnik,

1968).

A typical Eh-pH diagram for Cu-CN-H20system is shown in Figure 1-4 (supposed CuO is a

stable species) and the plot of mole fraction of copper cyanide species as a function of free

cyanide concentration is shown in Figure 1-5 (based on data on Table 1-3). As shown in the

9

Figures, the formation of Cu(CN)2 only occurs at low pH. In alkaline cyanide solution, copper

mainly occur as Cu(CN)32 and Cu(CN)43.Higher concentration of free cyanide favors the

formation of Cu(CN)32 and Cu(CN)43.The infrared and Raman spectra for the copper(I)

complexes have been studied and the information on their geometry has been confirmed. The

schematic molecular structure of Cu(CN)32 and Cu(CN)43 complexes has been reported and the

analysis results indicated that Cu(CN)32 has planar triangular shape and Cu(CN)43 is tetrahedral

(Torre, et a!., 2006). Some properties of three copper cyanide complexes are summarized in

Table 1-3 (according to Flynn and McGill, 1995 and Sharpe, 1976).

Table 1-2 Solubility of common copper minerals in 0.1 % NaCN solutions

(after 1-Tedlev and Tahachnik 1 96X——---/

Minerals Formula Copper dissolved Copper dissolved

at23°C(%) at45°C(%)

Azurite

Malachite

Chalcocite

Metallic Copper

Cuprite

Bornite

Enargite

Tetrahedrite

Chrysocolla

Chalcopyrite

2CuCO3.Cu(OH)2

CuCO3.Cu(OH)2

Cu2S

Cu

Cu20

FeS2Cu2S.CuS

Cu3AsS4

4Cu2SSb2S3

CuSiO22H20

CuFeS2

94.5

90.2

90.2

90.0

85.5

70.0

65.8

21.9

11.8

5.6

100

100

100

100

100

100

75.1

43.7

15.7

8.2

10

1

0.8

0.1-iC.)

0.4

0.2

pH

Figure 1-4 Eli-pH diagram for Cu-CN-H20system

([Cu] = 1 mol/L, [CN] = 1 mol/L, 25 °C)

0

-8 -6 -4 -2 0 2 4

log [CN]/moIIL.

Figure 1-5 Plot of mole fraction of copper cyanide species vs log [CN]

([Cu] = 0.1 mol/L, 25 °C)

>.

w

1.5

1

0.5

0

-0.5

—1

-1.5

CuO

CuCN Cu(CN)32Cu(CN)43

CN

0 2 4 6 8 10 12 14

11

Table 1-3 Some properties of copper(I) cyanide species

(Flynn and McGillj995 and Sharpe, 1976)

Cu-C bond, nm Frequency, cm1 Molar absorptivity

(in aqueous) cm2 M4 x 103(in

Copper(II) cyanide complexes are unstable in the typical cyanide solutions with respect to the

reduction of Cu(II) by cyanide,

In acid solution, 2Cu2 + 4HCN 2CuCN(s) +C2N2(aq) + 4H

In basic solution, 2Cu2 + 7CN + 20W = 2Cu(CN)32+ CNO + H20

The log K values of these two reactions are so large that there is no known complexing agent for

Cu(II) that will prevent the reduction of Cu(II) by CN (Flynn and McGill, 1995).

1.4.3 Zinc cyanides

Zinc minerals occur infrequently and usually in small quantities in gold ores. Many of the zinc

minerals are moderately soluble in cyanide solution although sphalerite (ZnS) is inert under

normal cyanidation conditions (Hedley and Tabachnik, 1968). Metallic zinc has been commonly

used for the recovery of gold from cyanide leach solutions (Merrill-Crowe process) before the

invention of activated carbon adsorption process. The method is still adopted by some gold

plants. The displacement reaction is indicated as below,

Zn + 2Au(CN)2 = 2Au + Zn(CN)42 (1-23)

Species Geometry f3

aq)

CuCN(aq) Linear N N N 3.16x10’°

Cu(CN)2 linear 0.192 2125 0.16 5.01x102’

Cu(CN)32 triangular 0.190-0.193 2094 1.09 7.94x1027

planar

Cu(CN)43 tetrahedral 0.199 2076 1.66 3.16x1028

N: No data available

(1-21)

(1-22)

12

Reactions involving the cyanide complexes of metallic Zn are relatively fast. In the typical

cyanide leaching solutions (e.g. 0.01 M free CN), most zinc occurs as the soluble tetra

cyanozincate (Zn(CN)42).A typical Eh-pH diagram of Zn-CN-H20system at 25 °C is shown in

Figure 1-6. Zinc cyanide complexes are restricted to the oxidation state of +2 and five cyano

complexes have been identified. Zinc cyanide (Zn(CN)2)is a white powder which is sparingly

soluble in water (K = 3.16 x 1 0). Table 1-4 gives some information on the properties of these

zinc cyano complexes (Flynn and McGill, 1995 and Sharpe, 1976).

Eh (Volts)

2.0

1.5

1.0 . . .

Zn(CN)4(-2a)0.5 ....

Zn(+2a)

0.0

-0 5

______________

HCN(a) CN(-a)

: Zn2 4 6 8 10 12 14

pH

Figure 1-6 Eh-pH diagram for Zn-CN-H20system

([CNJ = 1 0 mol/L, [Zn] = 1 0 mol/L; 25 °C)

13

Table 1-4 Some properties of zinc cyanide species (Flynn and McGill, 1995 and Sharpe, 1976)

Species Geometry Zn-C Frequency, Molar absorptivity f3

bond, nm cm1 (in aq) cm2 M1 x 103(in

aq)

ZnCN linear N N N 2.OOx

Zn(CN)2(aq) linear N N N 1.17x1011

Zn(CN)3 trigonal N N N 1.12x1016

Zn(CN)42 tetrahedral 0.202 2149 0.11 4.17x1019

Zn(CN)53 bipyramidal N N N 1.47x102°

N: No data available

1.4.4 Nickel cyanides

Nickel minerals are sometimes occurring in gold ores. A typical Eh-pH diagram for the Ni-CN

H20 system (at 25 °C) is shown in Figure 1-7. Nickel cyanide (Ni(CN)2)is a green-blue solid

powder with a low solubility in water (K = 3.16 x 1020). In dilute cyanide solution, the only

important nickel species is the orange-yellow tetra-cyanonickelate ion (Ni(CN)4j.Intermediate

species such as Ni(CN)3 are evidently not stable and disproportionate to Ni2 and Ni(CN)42

(Person. 1976). According to the molar ratio of nickel to CN, Ni(CN)53 can also be formed. The

infrared and X-ray studies shows that the Ni(CN)42 ion is square planar or quite near so. This is

quite different compared with Zn(CN)42 and Cu(CN)43 which have a tetrahedral shape. Some

information on the properties of Ni(CN)53 and Ni(CN)42 is shown in Table 1-5 (Flynn and

McGill, 1995 and Sharpe, 1976). It should be pointed out that though overwhelming majority of

nickel cyano complexes has an oxidation state of +2, there are some well-defined complexes of

nickel (0) and nickel (I) (such asK4(Ni(CN)4andK4Ni2(CN)6)(Sharpe, 1976).

14

Table 1-5 Some properties of nickel(II) cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976)

Species Geometry Ni-C Frequency, Molar absorptivity

bond, nm cm1 (in aq) cm2 M’ x 103(in aq)

Ni(CN)42 square 0.186 2124 1.1 1.58x10°

planar

Ni(CN)53 bipyramidal N N N 1.58x 1029

N: No data available

4 6pH

Figure 1-7 Eh-pH diagram for Ni-CN-H20system

([CN] 103mo1/L, [Ni] = 10mo1/L; 25 °C)

Eh (Volts)

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

Ni(+2a)

Ni

Ni(C1

HCN(a)

[)4(-2a)

CN(-a)

2 8 10 12 14

15

1.4.5 Iron cyanides

Iron minerals are the most common minerals in gold ores. The formation of ferro- and ferric

cyanides is rarely a problem for gold cyanidation since the dominant iron sulfide mineral, pyrite

(FeS2), is inert in cyanide solution under normal gold leaching conditions. However, some iron

sulfides such as pyrrhotite (Fe1S) are quite reactive in alkaline cyanide solutions and may

decompose to form iron cyanide complexes and various sulfur species. Hexacyanoferrate(II),

Fe(CN)64,and hexacyanoferrate(III), Fe(CN)63,are among the best known of all iron cyanide

complexes. The Fe(CN)64 ion can be oxidized to Fe(CN)63 by oxygen in acid. However in

alkaline solution the reaction takes places at a very low rate. Some properties of these two

complexes are shown in Table 1-6 (Flym and McGill, 1995 and Shame, 1976). A typical Eh-pH

diagram for Fe-CN-H20system is shown in Figure 1-8. Though iron cyanides are extremely

stable to pH and chemical changes, they will be a source of concern if they should pass to the

tailings pond, since although the complexes themselves are of low toxicity, they are slowly

decomposed by UV radiation, with liberation of free hydrogen cyanide (Kunz, et al., 1978). The

primary irradiation reaction of Fe(CN)64 under ultraviolet or visible light is,

[Fe(CN)641hv/H20 [Fe(CN)5H20]3+ HCN + OH (1-24)

The Fe(CN)63 ion is rather more reactive than Fe(CN)64 even though thermodynamically more

stable with respect to dissociation into the constituent ions. The photochemistry of aqueous

solution of Fe(CN)63 showed that a variety of products including Fe(OH)3, [Fe(CN)5H20]2,

[Fe(CN)5H20]3,Fe(CN)64,and Prussian blue (a mixed oxidation state cyanide complex of iron

with an unknown structure) could be formed.

Table 1-6 Some properties of iron cyanide species (Flynn and McGill, 1995 and Shame, 1976)

Species Geometry Fe-C Frequency, Molar absorptivity f3

bond, nm cm1 (in aq) cm2 M1 x 103(in aq)

Fe(CN)64 octahedral 0.191 2044 N 2.51)4027

Fe(CN)63 octahedral 0.193 2118 N 2.51x1035

N: No data available

16

1.4.6 Gold and silver cyanide complexes

Since most of the gold and silver will be recovered in the cyanidation process, their content in

the cyanide effluent are usually insignificant. In this research work, no tests have been done on

studying their extraction behaviors by the extractants. Au(CN)2 is the only gold species found

under gold leaching conditions (pH above 10.5). Silver may form different complexes with

cyanide depending on the leaching conditions (pH, cyanide concentration and ionic strength,

etc.). Some of the properties of gold and silver cyanide complexes have been summarized in

Table 1-7 (Flynn and McGill, 1995, Sharpe, 1976).

Eh (Volts)2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

Figure 1-8 Eh-pH diagram for Fe-CN-H20system

([CN] = 1 0 mol/L, [Fe] = 1 0 mol/L; 25 °C)

2 4 6 8 10 12 14pH

17

Table 1-7 Some properties of gold and silver cyanide species

(Flynn and McGill, 1995 and Sharpe, 1976)

Species Geometry Fe-C Frequency, Molar absorptivity f3

bond, nm cm1 (in aq) cni2M1 x103(in aq)

Au(CN)j linear 0.212 2145 N 3.98x 1036

Ag(CN)2 linear 0.213 2135 0.26 3.02x102°

Ag(CN)32 trigonal N 2105 0.38 2.51x1021

Ag(CN)43 tetrahedral N 2092 0.56 6.31x102°

N: No data available

1.4.7 Mixtures of metal cyanide species

Usually the gold leaching solution contains a mixture of metal cyanide complexes in which

complex equilibria will be established. In the case of insufficient CN present in the solution to

convert all cyanide reactive metal species to their anionic cyanide complexes, metals may

precipitate as cyanides, oxide species, or mixtures of compounds. Of these, the heavy metal

hexacyanoferrates (II, III) are best known. These compounds nearly always form as colloids or

gelatinous precipitates that are not stoichiometric. They contain variable quantities of the alkali

ion from the hexacyanometallate salts and variable quantities of water of hydration. The detailed

information on the formation of these cyano complexes has been summarized by Flynn and

McGill (1995).

18

2 Cyanide Destruction and Recovery

2.1 Chemical destruction process

The current available methods to detoxify cyanide-containing effluents include destruction by

natural degradation, by biological processes or by chemical oxidants (Palmer, et al, 1988, Goode,

et al., 2001). Due to increasing environmental concerns, the natural degradation process is

seldom used as the sole detoxification step in cyanide effluent treatment. Biological processes

have been successfully practiced but are not extensively used in the gold mining industry,

partially because of its high cost and potential instability (Clark, et al., 2001). As the oldest and

most widely recognized process for cyanide destruction, the alkaline-chlorination process is

sometimes employed primarily in the plating industry though it is occasionally still used at a few

mining sites. As the most commonly adopted methods, INCO S02/Air and hydrogen peroxide

have been extensively utilized for treating cyanide effluents arising from the gold mining

industry. Caro’s acid has been quite popular especially in Europe in recent years. Some other

reagents and methods including ozonation (03), permanganate (MnO4), bromine compounds

(Br2 and Br02), and photo-catalytic processes were also investigated, but none of them has yet

been commercially practiced (Lanouette, 1977, Cooley, 1976, Domenech and Peral, 1988).

2.1.1 moo S02/Air process

Developed in the early 1980s, the sulfur dioxide (S02)-air oxidation process from INCO Ltd.

offers a reliable means of treating industrial cyanide effluents. By using a mixture of SO2 and air

at controlled pH (about 8-10) in the presence of dissolved copper as catalyst, the process can

oxidize free cyanide and most of the cyanide complexes with the exception of strong complexes,

such as iron cyanides (Borberly, et al., 1984, Devuyst, et al., 1989).

NaCN +02+ SO2 + H20 = NaCNO + H2S04 (2-1)

The WAD (weak acid dissociable) metal cyanides complexes (such as copper and zinc) can also

be removed from the stream,

19

Me(CN) + x SO2 (g) + x 02 (g) + x H20 = x CNO + x H2S04+ Me (2-2)

where Me represents a metal element. Iron cyanide in the wastewaters is removed by

precipitation as iron cyanide double salts with copper, zinc or nickel. Thiocyanate (SCN) can be

also oxidized to cyanate by SO2 and air, but the kinetics are much slower compared with cyanide

oxidation (Whittle, et a!., 1989). Hence SCN removal by S02/Air is incomplete.

SCN + 4S02 + 402 + 5H20 = CNO + 5 H2S0 (2-3)

The process can be applied to clear cyanide effluent and pulps. One of the benefits of the process

is the availability of the relative cheap reagents such as sulfur dioxide can be generated on site

from roaster gas or by burning elemental sulfur. During the past decades, the INCO SO2/Air

process has been successfully used in the treatment of cyanide effluent arising from both metal

finishing plants and gold cyanidation operations (Devuyst, et al., 1991).

2.1.2 Hydrogen peroxide

Hydrogen peroxide (H202)is a strong oxidant which is capable of oxidizing free cyanide as well

as the WAD cyanide species.

CN + H202 = CNO + H20 (2-4)

The cyanate then hydrolyzes to form carbon dioxide or carbonate depending on the pH of the

solution,

In acidic conditions, CNO + 2H + H20 = CO2 + NH4 (2-5)

In basic conditions, CNO + OH + HO = C032 + NH3 (2-6)

Thiocyanate can also be oxidized by hydrogen peroxide, but iron cyanides are not destroyed

(Wilson and Harris, 1960, Vickell, et al., 1989, Kunz, et al., 1978).

SCN + H2O2 = S + CNO + H20 (2-7)

The oxidation of cyanide with hydrogen peroxide proceeds fairly fast when treating wastewaters

with high cyanide concentration, but the reaction is quite slow in dilute waste cyanide solution.

A number of metals such as copper can act as the catalyst. The specific advantage of hydrogen

peroxide is that it is an ecologically desirable pollution control agent and yields only water

and/or oxygen upon decomposition. The process has been successfully practiced by more and

more gold mining operations though high consumption of the reagent is sometimes observed,

20

probably due to the vigorous catalytic decomposition by heavy metals and various organic

compounds (Goode, et al., 2001).

2.1.3 Caro’s acid

Caro’s acid (H2S05)has been quite popular for wastewater treatment in recent years. As a strong

acid and oxidant, it can react with cyanide to form cyanate which may further be decomposed by

hydrolysis.

CN + H2S205= CNO + H2S04 (2-8)

In the presence of excess Caro’s acid, cyanide is completely and rapidly destroyed,

2CN + 5H2S205+ 20ff = 2C02 + N2 + 5H2S04+ H20 (2-9)

As the addition of Caro’s acid causes a drop in pH, caustic soda is usually added simultaneously

to avoid the formation of volatile hydrogen cyanide (Clancy, et al., 1978, Goode, et a!., 2001).

Persulfuric acid (H2S208)or persulfate salts such as ammonium persulfate ((NH4)2S208)and

sodium persulfate (Na2S2O8)are also powerful oxidants and react with cyanide in a manner

similar to Caro’s acid. These agents are especially suitable for destroying cyanide in

concentrated solutions such as the spent cyanide wastewaters from electroplating industry and

can readily oxidize ferrocyanide to ferricyanide. Actually regeneration of the spent ferricyanide

bleach with persulfate is a widely used method in the photoprocessing industry (Cooley, 1976).

The regeneration reaction is expressed as below,

2Fe(CN)64 + S2082 = 2 Fe(CN)63 + 2S042 (2-10)

2.2 Cyanide and metal recovery process

Though the destruction processes (INCO S02/Air, Caro’s acid or hydrogen peroxide) can be very

efficient in destroying free cyanide in the cyanide effluents, the destruction of effluents

containing high cyanide concentration or valuable metals (such as copper) could severely

decrease the profitability of the gold plant operations. Sometimes this may even render the

cyanidation process ineffective if the copper and complexed cyanide are not recovered after gold

21

recovery (Goode, et al., 2001, Jay, 2001). At the same time, a number of accidents involving

cyanide contaminated effluents have resulted in the environmental constraints controlling the

discharge of cyanide from gold mining industry being tightened by local governments

worldwide. In the case of a cyanide spill at the Aural gold mine in Baja Mare, Romania in early

2000, the cyanide tailing dam broke which resulted in the release of cyanide effluent into the

Tisza River, one of the major waterways in Europe (DeVries, 2001). Stricter regulations to

reduce the effect of future accidents have been implemented as a result and the use of cyanide in

new mining operations has even been banned in some countries or districts (Greece, Turkey and

Montana of US). The concentration of free cyanide and the WAD cyanide species discharged

into the tailing is usually required to be controlled below a strict limit (in many sites it is 50 ppm

to tails and 0.1 ppm or less if there is any discharge to a receiving waterway). Subsequently,

there has been growing interest in technologies for the recovery of valuable metals and cyanide

from cyanide effluents arising from gold mining industry.

2.2.1 AVR/MNR/SART process

The AVR (Acidification-Volatilization-Regeneration) process was first developed in the early

part of the 20th century (Fleming, 2001). The process concept is relatively simple: the waste

cyanide solution is first acidified to weakly acidic pH (usually below 4-5 by addition of sulfuric

acid) and then contacted with high-pressure air. Most of the cyanide is converted to HCN which

is volatilized by air and then adsorbed in alkaline solutions to produce aqueous NaCN or

Ca(CN)2.The main reactions involved in this process are as following,

2CN ÷ H2S04= 2 HCN (aq) + 2 5Q42 (2-11)

HCN (aq) = HCN (g) (2-12)

HCN (g) + NaOH = NaCN + H20 (2-13)

The cyanide-free solution then passes through a neutralization step to precipitate the heavy

metals (Riveros, et al., 1997). The Cyanosorb process is a variation of the AVR process which

uses the same principle to treat waste cyanide puips instead of the clear solutions (Stevenson, et

al., 1996). However, during the acidification stage, copper is precipitated as copper cyanide

(CuCN) which is difficult to sell due to the presence of cyanide.

22

Cu(CN)32 + H2S04= Cu(CN)(s) + 2HCN + S042 (2-14)

In order to recover valuable copper, some modified AVR processes have been developed. The

MNR process was developed by Metallgesellschaft Natural Resources and involves a solid/liquid

separation process to obtain a clarified cyanide solution, to which water-soluble sulfide

compounds (NaSH or Na2S) are added to precipitate base metals (mainly subject to copper). The

solution is then acidified to pH < 5 by the addition of sulphuric acid. The copper sulfide

precipitate is recovered by filtration. The acidification and sulfidization reaction of copper

cyanides is presented below,

2Na3Cu(CN)4+ 3.5 HSO + NaSH = Cu2S + 3.5 Na2SO4+ 8HCN (aq) (2-15)

Other base metals present in the solution will also co-precipitate (Dreisinger. et al., 1995). The

neutralization step may be performed directly on the acidified solutions (after filtration of copper

sulfide) or may be linked to a volatilization step. The hydrogen cyanide gas generated in the

acidification process is volatilized and reabsorbed in an alkaline solution.

The SART process (Sulfidization/Acidification — Recycling — Thickening) is based on the same

chemistry as that of MNR, except that the copper sulfide precipitates are recovered first by

precipitation and thickening rather than direct filtration. The process has been successfully

practiced at Telfer Gold Mine in Australia (Dreisinger, et al., 2001, Barter, et al., 2001). The

main drawback associated with the AVR/MNR/SART process appears to be the high operational

cost - the reagents (acid and base) and the energy required by air sparging. The process can be

applied economically to effluent solutions containing > 150 mg/L total cyanide, but in case of

low cyanide concentration in the tailings, it is generally considered to be unsuitable for

producing a final solution for discharge because of the high cost of reducing the cyanide

concentration down to required control levels. The AVR process has been used for cyanide

recycle at Flin Flon (Canada) starting in the 1930s and was abandoned in 1975, partially due to

the reasons above (Marsden arid House, 1992).

23

2.2.2 Activated carbon

Activated carbon is a generic term for a broad range of amorphous, carbon-based materials,

prepared so as to exhibit a high degree of porosity and a large associated surface area. Due to the

particular affinity of gold and silver cyanide for adsorption, activated carbon has been

extensively used in the gold recovery process for the past decades. Since activated carbon can act

both as an adsorbent and as a catalyst for the oxidation of cyanide, it has been also suggested for

recovery of cyanide and metal-cyanide species from waste cyanide solution. An early technology

is the Calgon process which employed columns packed with granular activated carbon to recover

cyanide and metals from waste cyanide solution (Bernardin, 1976; Hoffman. 1973). In order to

increase the kinetics of cyanide oxidation on the carbon, cupric ions and oxygen were added to

the cyanide wastewaters before feeding them into the treatment system. The use of activated

carbon as a modification for the AVR process to remove metal and cyanide has also been

proposed (Batzias and Sidiras, 2001).

Since plain carbon adsorption is believed to be not very efficient at removing free cyanide from

the effluents, modification and impregnation technologies, such as Al, Cu, Ag and Ni —

impregnated activated carbons, have been developed (Manktelow, et al., 1984, Adams, 1994,

Williams, 1997, Adhoum and Monser, 2002). They suggested that the following reactions may

occur during adsorption on surface of the impregnated activated carbon (such as when Ag or

Ni was added),

Ag + -COOH = -COOAg + H (2-16)

Ni + -COOH = -COONi + H (2-17)

where —COOH represent the acidic carboxyl functional group present on the carbon surface.

Therefore, the adsorption capacity and the feasible removal rates of cyanides were substantially

boosted since they were not only removed by adsorption on the surface of the plain carbon, but

could be removed by those added chemicals (by forming Ag(CN)j and Ni(CN)42 on the

surface). For those gold cyanidiation plants where activated carbon is already used for extraction

process (CIP/CIL, carbon-in-pulp/carbon-in-leaching), the use of activated carbon for treating

cyanide effluent would be simple since it is convenient for installation on-site. However, due to

the low adsorption capability of activated carbon (even for those pre-impregnated carbons), it is

24

more suitable for use as a polishing process to remove cyanide to low levels when initial cyanide

concentration is already low (for example, 1-5 mg/L, Fleming, 2005). The process has not yet

been reported to be used in practice.

2.2.3 Ion exchange resin

Though the commercial scale application of ion exchange resins in the gold mining industry was

well established in the former Soviet Union in the 1970s, it was not until the late 1980s that such

processes had attracted the attention of the Western World with the commissioning of Golden

Jubilee resin-in-pulp plant in South Africa (Fleming and Cromberge, 1984 (A) and (). Moreresearch and investigations on the fundamental and practical aspects of ion exchange resin

technologies on gold cyanidation have been produced since then (Bolinski and Shirley, 1996,

Seymore and Fleming, 1986). Ion exchange resins also present a possible alternative for the

treatment of waste cyanide effluents. As early as the 1950s, Walker and Zabban (1953)

developed a bench scale ion-exchange resin process to concentrate cyanide from the aqueous

waste streams produced in electroplating operations. Bessent, et al. (1980) conducted a pilot test

on cyanide removal from coke plant wastewaters by selective ion exchange resins. FeSO4 was

added to the stream prior to introduction to the ion exchange column where Amberlite IRA-958

(Rohm & Haas) was loaded. The excess of iron is precipitated as Fe(OH)3 under alkaline

conditions. Though the cyanide concentration could be reduced below 2 mg/L, the disposal of

the sludge generated in the process caused another problem since the sludge contains

complicated hazardous materials (multi-metal cyanide precipitates, such as Fe2[Fe(CN)6]).

Goldblatt (1956, 1959) developed an ion exchange resin process to recover cyanide and gold

from the waste cyanide effluents arising from Stilfontein Gold Mine’ cyanidation operations.

The strong base ion exchange resin, Amberlite IRA-400 (Rohm & Haas), was applied to remove

cyanide and metals from recycled water containing CN, SCN, Zn, Ni, Co, and Cu. The system

was comprised of two adsorption columns. The metal cyano complexes were removed in the first

column. The effluent was then forwarded to the second column containing “CuCN-conditioned”

resin where the remaining free cyanide was removed as copper cyanide. The treated effluent was

returned to the leaching tanks. Both columns were then eluted with 1% 112S04 solution. CN was

25

converted to HCN and recovered as NaCN. The acid solution was then contacted with a strong

acid resin IR-120 (Rohm & Haas) to remove dissolved metals. After several adsorption/elution

cycles, copper cyanide (CuCN) was found to accumulate in the resins, causing the reduction of

its exchange capacity. This solid was further removed as a soluble complex by elution with a

ferric sulfate solution. Two typical resin technologies developed in recent years are AuGMENT

process and Vitrokele process in both of which strong base resin were used to recover metals and

cyanide from cyanide solutions. The use of guanidine-based resin to extract metals from gold

leachate is also suggested (Kordosky, et al., 1993, Jermakowicz and Kolarz, 2002).

2.2.3.1 AuGMENT process

The AuGMENT process was developed by SGS Lakefield research and the DuPont Corporation

in which strong base resin (quaternary amine functionality) was used to recover and pre

concentrate copper cyanide from gold-plant tailings (Fleming, 1998, Fleming, et al.. 1998). The

chemistry involved in the various unit operations is based on the formation of different copper

cyanide complexes as the cyanide to copper molar ratio is varied. CuCN precipitated resin was

used as the adsorbent for the adsorption of free cyanide and soluble copper cyanide. The

adsorption step is carried out with a barren solution containing a cyanide to copper molar ratio of

at least four. It was believed that during adsorption, Cu(CN)32 or a higher complex reacts with

CuCN producing Cu(CN)j which allowing the maximum copper loading (60-80 gIL resin) to be

achieved. The loading mechanism can be described by the following equation:

2R-S042 (CuCN()) + Cu(CN)32 + 2CN —* 3R-Cu(CN)2+ R-CN + S042 (2-18)

where R represents the resin matrix and functional group. Once loaded, the copper cyanide

species are eluted from the resin using a concentrated copper cyanide solution having a cyanide

to copper molar ratio of approximately four which can convert, the Cu(CN)i to Cu(CN)32.The

elution process can be described as,

2(R-Cu(CN)2)+ Cu(CN)32 + 2CN —* 2R-Cu(CN)32+ Cu(CN)32 (2-19)

Finally the resin is regenerated via conversion to the CuCN form with sulfuric acid. The eluate is

submitted to electrowinning to produce copper cathodes. Gold has to be recovered prior to

copper electrowinning and cyanide recovery. Cyanide can also be recovered via AVR circuit

where the copper cyanide is precipitated and re-dissolved in the loaded catholyte ahead of the

26

electrowinning circuit. One potential disadvantage of the processes is that the precipitated CuCN

may block the resin pores decreasing the opportunity for additional metal cyanide complexes to

be adsorbed into the resin. If cobalt is present in the effluent, the possible polymerization of

adsorbed cobalt cyanide complexes under strongly acidic conditions will poison the resins

(Goldblatt, 1959, Leao, et al., 1998, Jay, 2001).

2.2.3.2 Vitrokele process

Ion exchange resins have also been considered as a modification to the AVR process for indirect

recovery of free cyanide and metals cyanide species (Silva, et al., 2003). A typical example is the

Vitrokele process which uses strong base resins for recovery of cyanide and metals from either a

clear solution or slurry. The resin is based on a highly cross-linked polystyrene structure

(VitrokeleTM 911 and 912, which probably have the quaternary amine functionality, Jay, 2001).

The loaded resins were eluted with the strong cyanide eluant to recover copper cyanide species.

Precious metals and other strongly bound metal cyanide complexes were “crowded” from the

resins with tetracyanozincate (Zn(CN)42).Sulphuric acid was used in the last elution cycle to

destroy most of the cyanide complexes to regenerate the resins (Whittle, 1992). The chemistry

involved in the process using strong base resins can be described as following,

Loading:

_____________________

{RN(CH3)}2[S04]2 + 2 [Au(CN)2f= 2 RN(CH3) [Au(CN)2f+ [S04]2 (2-20)

Stripping:

2 RN(CH3)[Au(CN)2f +[Zn(CN)4J2= {RN(CH3)}2[Zn(CN)4]2+ 2 [Au(CN)2]

(2-21)

Regeneration:

{RN(CH3)}2[Zn(CN)4]2 +H2S04= {RN(CH3)}2[SO4]2 + ZnSO4 + 4HCN(g) (2-22)

One disadvantage of the process is that significant amounts of non-WAD cyanide species such as

iron or cobalt cyanide complexes will poison the resins since only WAD cyanide complexes can

27

be removed by acid decomposition. This process has been successfully applied for treating the

heap leachate at the Connemara Mine in Zimbabwe (Satalic, et a!., 1996). It was also tested at

May Day Mines (Cobar, Australia) from July 1997 to June 1998 (DeVries, 2001). However,

some technical problems developed during the operation of the process resulted in the

abandonment of the Vitrokele process at the Mines. One of the major problems was that the

elution of copper from the resin was not effective. The reaction between the residual copper

cyanide in the resin with the eluant (H2S04)led to the formation of CuCN which blocked the

active sites on the resin surface.

2.2.3.3 Cognis AURIX resin

The AuRIX resin is a weak base ion exchange resin developed by Cognis for the recovery of

precious metals (gold and silver) from gold cyanide leachate (Kordosky, et a!. 1993). It is a

typical styrene-divinylbenzene resin bead functionalized with a guanidine functional group.

Guanidines are strong organic bases having an intermediate basicity between that of simple

amines and quaternary amines. Guanidines exhibit a pKa of approximately 12 and are capable of

being protonated to form a guanidinium cation at the operating pH of the gold leachate (usually

10 to 11). This guanidinium cation can form an ion-pair with aurocyanide resulting in gold

extraction from the cyanide solution. By increasing the basicity of the aqueous phase, the

guanidinium cation is converted to the neutral guanidine functionality. The neutral guanidine

functionality no longer forms an ion-pair with aurocyanide resulting in gold stripping from the

resin. Ideally, the extraction of Au(I) from cyanide solution by the resin can be described by the

following equations.

RG + H20 = RGW OH (2-23)

RGH OH + Au(CN)j RGH Au(CN)2 + OH (2-24)

The overall reaction is,

RG + H20 + Au(CN)2 = RGW Au(CN)j + OH (2-25)

where RG represents the function group of the resin and RGW represents its protonated form

(Virnig and Wolfe, et a!., 1996).

28

Ion exchange resins offer certain advantages over activated carbon. They are less easily poisoned

by organic matter and can be eluted at room temperature, and selectivity for particular metals can

be achieved by the choice of the functional group incorporated into the bead or by the selective

elution process. However, the high operational cost of the resins has severely hampered their

wide application in practice (Jay, 2001).

2.2.4 Solvent extraction

2.2.4.1 Recovery of gold cyanide complex

Solvent extraction has been successfully practiced in many metal recovery processes, such as

uranium, copper, and nickel. The use of solvents for purification and concentration of gold from

dilute cyanide solution has long been of interest, but has never been practiced commercially.

Some commercial base extractants which have been suggested for the recovery of gold and/or

silver from cyanide solutions are summarized in Table 2-1. A variety of amines were found to be

capable of extracting gold and silver from alkaline cyanide solutions. Both weak and strong base

amines can behave in a similar manner with that of weak and strong-base resins. For example, a

general equilibrium reaction for Au(I) extraction by the primary amine can be expressed as:

RNH2org + H + Au(CN)2org = RNH3Au(CN)2org (2-26)

where R represents the alkyl group associated with the amine (Caravaca, et al., 1996 (A);

Caravaca, et al., 1996 (B)). More technical papers emerged after the original findings that

showed the selective extraction of cyano-anions could be accomplished by addition of modifiers

(e.g. organophosphorous compounds) to weak base extractants (such as primary, secondary and

tertiary amines), which make it possible to effectively extract gold cyanide in alkaline conditions

(Miller and Mooiman, 1984, Mooiman and Miller, 1986). Quaternary amines can strongly

extract common anions with less selectivity but are more difficult to strip. Riveros, et al. (1990)

have even run a small pilot plant to demonstrate the potential application of quaternary amines

(Aliquat 336 dissolved in Solvesso 150) for recovery of gold from cyanide leachate. This process

seems difficult for further applicability in industry since acidic thiourea was recommended as the

stripping agent. This resulted in a more complicated process.

29

Table 2-1 Some commercial base extractants for gold solvent extraction (Rydberg, et al., 2004)

Reagent Class Structure Commercial Extractants

Primary amines RNH2 Primene JMT

R=(CH3)3C(CH2C(CH3)2)4

Secondary R’R2NH.

amines R1=C9H19CHCHCH2 Amberlite LA-i

R2=CH3C(CH3)2(CH2C(CH3)2)2

R1=CH3(CH2)11 Amberlite LA-2

R2=CH3C(CH3)2(CH2C(CH3)2)2

R1,R2=CH3(CH2)12 Adogen 283

R1,R2=CH3CH(CH3)CH2(CH2)6 HOE F2562

Tertiary amines R1R2R3N

R1,R2,R3=CH3(CH2)7 Alamine 300

R1,R2, R3 =C8-C10 mixture Alamine 336, Adogen 364,

Hostarex A 327

R1,R2, R3 = (CH3)2CH(CH2)5 Adogen 381, Alamine 308,

Hostarex A 324

R’, R2, R3 = (CH3)2CH(CH2)7 Alamine 310,

R1, R2, R3 = (CH3)(CH2)11 Adogen 363, Alamine 304,

R1, R2, R3 = CH3(CH2)12 Adogen 383

R1,R2, R3 = C28H57 Amberlite XE 204

R1=CH3(CH2)7,R2 = CH3(CH2)9,R3 = Adogen 368

CH3(CH2)11

Quaternary R1R2R3N(CH3) Cl . Aliquat 336, Adogen 464,

amines R1, R2, R3 = C8-C10 mixture HOES 2706

In recent years, solvent extraction systems such as guanidine derivatives were also developed in

order to find a suitable extractant for gold (and/or silver) recovery from dilute cyanide solutions.

After the introduction of this functionality as AURIXTM 100 ion-exchange resin by Cognis, the

solvent product with the similar functionality, LIX® 79, a tri-alkylguanidine extractant was also

30

developed (Kordosky, et al., 1992, Virriig and Wolfe, 1996). The extractant allows the extraction

of gold or silver up to an aqueous pH of 11 or more and the loaded gold can be stripped off with

strong basic solutions. Following to the same principle as AURIX resins the guanidine functional

group undergoes protonation to form guanidinium cation when contacting with an aqueous

solution. The guanidinium cation can form an organic soluble ion pair with the anions in the

aqueous phase which resulting in their extraction.

RGorg + H20 = RGH OH org (2-27)

RGHOHorg + Au(CN)2 = RGH Au(CN)2org + OH (228)

The overall reaction is,

RG org + H20 + Au(CN)i = RGH Au(CN)i org + OH (2-29)

where RGorg represents the extractant molecule and RGHorg its prononated form (Vimig and

Wolfe, et al, 1996).

2.2.4.2 Recovery of metals and cyanide

Due to the potential high selectivity and loading capacity, and the relatively fast extraction rate,

solvent extraction technology also offers an alternative method for recovery of metals and

cyanide from waste cyanide solution. In the early work of Moore (1975), and Moore and

Groenier (1975), solvent extraction of zinc and cadmium from alkaline cyanide solutions was

examined. They found that quaternary amines (Aliquat 336 and Adogen 464) have a good ability

to extract zinc and cadmium cyanide from highly alkaline solutions. Regeneration of the amine

solvent can be achieved by stripping off the loaded metals (Cd and Zn) with sodium hydroxide

(NaOH), sodium hypochiorite (NaC1O), or alkaline or acidic formaldehyde (HCHO). Villaverde

and Martin (1995) examined the feasibility of solvent extraction of gold and silver from the

Gossan barren dam waters originating from the gold cyanidation process. Different extraction

solvents including Primene (ATP, a primary commercial amine), TBP (tributylphosphate), and

Cyanex (general formulaC24H510P, not specified by the author), and their combinations were

investigated. They suggested that it was possible to separate and concentrate gold, and to a lesser

extent silver and copper, by means of solvent extraction with the synergistic extractant, Cyanex +

ATP. The waste dam water could be treated directly at pH 9 and overall 90 % gold recovery can

be achieved. The process using organophosphorous extractants Cyanex 923, di-butyl-butyl

31

phosphonate and tri-butyl-phosphate for the recovery of hydrogen cyanide (HCN) was also

developed. The process has been suggested to be used to replace the air sparging step in the AVR

process (after acidification of the cyanide solution) since the cost of air sparging is usually

substantially high (Larmour-Ship. K, et al., 2005).

Dreisinger, et al. (1995) investigated solvent extraction of copper from dilute cyanide solution by

LIX 79 and proposed the development of a copper SX/EW process for the treatment of waste

cyanide solutions or alternatively as a front end for any of the other final copper cyanide

recovery process. The process is carried out in four stages, solvent extraction of copper cyanide

complexes from clarified solution by LIX 79; stripping copper cyanide from loaded organic

phase using a high pH and copper cyanide rich spent electrolyte; electrolysis of the strip solution

in a membrane cell (NafionTM 417 membraneTM,Du Pont) to produce copper metal and liberate

free cyanide; and cyanide recovery from a bleed stream from the electrolysis cell (Dreisinger, et

a!., 2001).

Davis, et al. (1998) proposed the use of LIX 7800 series extractants (the mixture of the

quaternary amine Aliquat 336 and nonylphenol at different molar ratios) as the pre-concentration

step to recover copper from copper cyanide solutions. The extraction and stripping of copper

cyanide complexes are believed to occur via ion exchange mechanism.

(Q X)org + (HP)org + OH = (QP)org ÷ X + H20 (1-37)

where Q is the quaternary ammonium cation, HP is the protonated form of the nonylphenol, and

X is the extracted anion (Mattison and Vimig, 2001). Under low pH conditions, nonylphenol is

protonated and the quaternary ammonium compounds extract an anion from the aqueous phase.

Under highly alkaline conditions, nonylphenol starts to be significantly converted to phenoxide

anion (F) and forms an ion pair with the quaternary ammonium cation (QP). Consequently the

extracted anion will be gradually expelled to the aqueous phase with increasing equilibrium pH.

The economic aspects of the potential application of the process to recover copper from cyanide

solution have been developed based on the extraction results.

32

2.2.5 Miscellaneous

Lower and Spottiswood (1983) reported a process of cyanide removal from coke making and

blast furnace wastewaters by ion floatation of iron cyanides. The process was found reasonably

effective on ferricyanide but not on CN and ferrocyanide. Soto, et al. (1997) developed a method

by adjusting solution pH to recover cyanide and copper from cyanide solutions containing

copper and thiocyanate. Copper is first precipitated as copper thiocyanate (CuSCN) or as copper

cyanide (CuCN) depending on pH and the concentration of thiocyanate and cyanide in the

effluent. The precipitates then can be separated from the effluent by filtration after settling. The

decant solution and the filtrate containing the bulk of the cyanide were then oxidized with ozone

to transform the remaining thiocyanate into cyanide. Cyanide is not oxidized by ozone under the

adopted conditions. The regenerated solution rich in free cyanide can be recycled to the

cyanidation process. Recovery of over 96 % copper and cyanide were reported in their laboratory

tests. Lu, et al. (2002) developed a membrane-electrolysis cell with graphite felt to recover

copper and to recycle the cyanides. Copper recoveries up to 60% were achieved with an energy

consumption of 1-2 kWh/kg. The process has been suggested as a subsequent process for treating

the eluent or stripping solution from copper pre-concentration step in treating cyanide effluent by

either ion exchange resin or solvent extraction.

The study on the bio-sorption of heavy metals ions from cyanide solutions using a waste fungal

biomass containing killed cells of Aspergillus niger was conducted by Nataralan, et al. (1999).

The uptake of base metals from the industrial cyanide effluent was examined. It was found that

the biomaterials tested in the study have shown high value of metal uptake from cyanide effluent

particularly for gold and zinc. The bio-sorption process of metal-cyanide complexes,

tetracyanocuprate(II), Cu(CN)42,and tetracyanonickelate(II), Ni(CN)42,from waste cyanide

solutions by using different fungal cultures were studied by Patil and Paknikar (1997). They

observed that the fungi (C. Cladorporioides) showed maximum loading capacity (40 imo1/g

Cu(CN)42,and 34 jimol/g, Ni(CN)42)comparing to the other fungal absorbents studied and the

activated charcoal. It was found that 1 mol/L sodium hydroxide was effective to remove the

bound metal-cyanide species which could be concentrated to allow recycling in the plating

circuit in the user industry.

33

2.3 Research objective

To date the detoxification of cyanide effluent arising from the gold cyanidation process is still a

challenge to the gold mining industry, especially those gold plants dealing with gold ores

containing high concentrations of copper minerals. The current cyanide recovery technologies

including the AVR process and adsorption by ion-exchange resins or activated carbons are either

too expensive to construct and operate, or have relative low loading capability and hence are

impractical. Though still underdeveloped, solvent extraction technology offers an alternative

recovery system for treating those high tonnage wastewaters containing cyanide and valuable

metals like copper. Since the extraction kinetics of the solvent extraction systems are usually fast

and the process can operate through continuous stages, relative small organic inventory will be

required. An attractive application of this process is to incorporate a solvent extraction circuit as

a pre-concentration step to treat the high copper cyanide-containing waste solutions (shown in

Figure 2-1, Dreisinger, et al., 2001).

Ideally, the solvent extraction of metal species from waste cyanide solution should be

accomplished at around pH of 11 with stripping achieved at a moderate higher pH (for example,

pH of 12 -13). This requires that the extractant exhibits an appropriate basicity and has the

desired specificity for the targeted anions. The feasibility of the use of LIX 7820 (a mixture of

quaternary amine Aliquat 336 and nonyiphenol) and LIX 7950 to recover copper from cyanide

solution has been proven. However, the fundamental aspects on these extraction systems for

treating cyanide effluents, especially the knowledge of the extraction behavior of cyanide and the

potential effect of other anions on the recovery of copper and cyanide are still lacking. Much

research work needs to be conducted to elucidate the characteristics involved in the extraction of

the common anions occurring in the cyanide solution by the extractants to predict their potential

application. In this work, the extraction chemistry of common anions occurring in cyanide

effluents arising from gold cyanidation process by two commercial extractants (LIX 7820 and

LIX 7950) has been examined. The possible solution to recovery of valuable metal (subjected to

copper) and cyanide was examined. More specifically, the research was designed to determine:

34

> The extraction chemistry of metal cyanide complexes (principally copper cyanides) with

two extractants under different experimental conditions, including the effect of the

organic formula, the molar ratio of cyanide to copper and the temperature;

> The co-extraction behavior of mixed metal cyanide solution and the selectivity of the

extractants for the specific metal (copper);

The potential effect of non-metal cyanides anions including SCN, CNO, S2032 on the

extraction of copper and cyanide with the extractants;

> The appropriate reagents for stripping of loaded copper and cyanide;

> The extraction and stripping isotherms of copper and cyanide with the two extractants

> The potential solution to recovery of copper and cyanide from waste cyanide solution

Reagents

Copper — Cyanide

_________

Cu-CN Barren SolutionSolution Solvent Ext. To Recycle or Disposal

ConcentratedCu — CN Solution

‘1

__

JrReagents and Power

‘+ Cu EW SART AVR

‘1r 1,Copper Metal Cu2S and CuCN and+ Recycle CN Recycle CN Recycle CN

Figure 2-1 Use of solvent extraction in the recovery of copper and cyanide from solution

(After Dreisinger, et a!., 2001)

35

3 Experimental

3.1 Organic reagents and chemicals

Two extractants, LIX 7950 and LIX 7820 which are commercial extractants produced by Cognis

were used in the research. Similar to the guanidine extractant LIX 79, LIX 7950 is also based on

formulation of an alkylguanidine but has a higher concentration of guanidine and shows a higher

basicity than LIX 79 (which has a pKa of 12 or higher, Kordosky, et a!., 1992). LIX 7820 is a

solvent mixture of Aliquat 336 (a commercial quaternary amine produced by Cognis) and 4-

nonylphenol at a molar ratio of 1: 2. Some information of the extractants, LIX 79 and Aliquat

336 is summarized in Table 3-1 (Kordosky, et a!., 1992, Mattison and Virnig, 2001). Information

regarding the diluents and modifiers used in this research is summarized in Table 3-2.

Table 3-1 Some information on LIX 79 and Aliquat 336

LIX 79 Aliquat 336

Molecular Formula N C25H54C1N

Molecular weight, g/mol N 404.16

Specific Gravity (25° C) 0.80-0.85 0.884

Schematic structureR1 (CH )7CH3

N

)NcN( CH3/4 R3 (CH2)7CH3

CH3(CH2)7R = H or alkyl

N: No information available

36

3.2 Preparation of the extractant solvents

The extractant solvent containing LIX 7950 was prepared by dissolving a desired volume of the

extractant and a designated mass of the modifier (1-dodecanol) into the diluent. The extractant

solvent was then used directly for solvent extraction tests without any pre-treatment. The

extractant solvent for LIX 7820 was prepared by dissolving a desired volume of the extractant

into the diluent. The solvent mixture was first washed with 1 mol/L sodium hydroxide (NaOH)

solution three times and then washed with de-ionized water twice. Through this procedure, the

extractant LIX 7820 was converted from chloride form to hydroxide form. The extractant

solvents with different molar ratios of Aliquat 336 and 4-nonylphenol were also prepared

following the same procedure before use in the extraction tests.

Table 3-2 Some information on diluents and modifiers used in the research

Chemicals Formula Molecular Specific Purity Supplier

weight, g/mol Gravity(25° C)

4-nonyiphenol C15H240 220.35 0.940 95% Cognis

1-octanol C8H180 130.23 0.824 98% Acros

1-dodecanol C12H260 186.34 0.83 1 98% Acros

n-octane C8H18 114.23 0.707 97% Acros

Decane C10H22 142.29 0.730 99% Acros

n-dodecane C12H26 170.34 0.750 97% Acros

Toluene C7H8 92.14 0.867 99.9% Fisher

3.3 Preparation of aqueous solutions

Information regarding the inorganic chemicals used in this research is summarized in Table 3-3.

De-ionized water was used in the preparation of aqueous solutions. Synthetic metal cyanide

solution was prepared by dissolving the corresponding metal cyanide salt in an aqueous NaCN

solution. A mixture solution of metal cyanides was prepared and the content of the major

components is shown in Table 3-4. Copper content in the waste cyanide solution arising from

gold mining industry may vary from several mg/L to 100 gIL depending on copper minerals and

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their content in gold ores. The copper content of 250 mg/L in the mixture solution was chosen in

this research. For comparison, the concentration of other metals was established at the equivalent

molarity to that of copper. Cyanide, cyanate (CNO), thiocyanate (SCN), and thiosulfate (S2032)

solutions were made up from their sodium salts, respectively.

Table 3-3 Some information of inorganic chemicals used in the research

Chemicals Formula Molecular Purity Supplier

weight, g/mol

Copper(I) cyanide CuCN 89.56 99 % Aldrich

Zinc cyanide Zn(CN)2 117.42 99 % Fisher

Nickel(II) cyanide tetrahydrate Ni(CN)2 4H20 182.73 99 % Alfa Aesar

Potassium hexacyanoferrate(II) K4Fe(CN)63H20 422.41 99.5 % Anachemia

Potassium hexacyanoferrate(III) K3Fe(CN)6 329.26 99.2 % Fisher

Sodium cyanide NaCN 49.01 95 % Anachemia

Sodium thiocyanate NaSCN 82.07 99.6 % Fisher

Sodium thiosulfate NaS2O3 158.11 99.9 % Fisher

Sodium hydroxide NaOH 40.00 99.2 % Fisher

Sodium sulfate Na2SO4 142.04 99.8 % Fisher

Sodium chloride NaCl 58.44 99 % Fisher

Sodium nitrate NaNO3 84.99