noncyanide lixiviant

Minerals Engineering 17 (2004) 785801This article is also available online at:

www.elsevier.com/locate/mineng

Gold leaching in non-cyanide lixiviant systems: critical issueson fundamentals and applications

G. Senanayake *

Department of Extractive Metallurgy and Mineral Science, A.J. Parker Cooperative Research Centre for Hydrometallurgy,

Murdoch University, Perth, WA 6150, Australia

Received 24 November 2003; accepted 20 January 2004

Abstract

Thermodynamics and speciation of the Au(0/I/III) system are reviewed on the basis of published data on disproportionation of

Au(I) and solubility of gold metal and Au(I/III) salts. Kinetics of gold oxidation from rotating gold discs, gold colloid, goldsilver

alloy and gold ores reported in the literature are reviewed to compare and contrast different lixiviant systems. Linear correlations of

stability constants of Au(I) complexes with respect to relevant complexes of Ag(I) and Cu(I) show the stability order of Au(I)

complexes with different ligands: CN >HS >S2O23 > SC(NH2)2 >OH

> I >SCN >SO23 >NH3 >Br >Cl >CH3CN. In the

case of Au(III) the stability order of complexes is CN >OH >SCN >Br >Cl. The EhpH and Ehlog[Cl] diagrams of Au(0/I/

III)OHNH3 and Fe(II/III)Cu(0/I/II)Au(0/I/III)Cl systems are revised to incorporate complex species such as AuOH0,

Au(OH)2 and AuCl2 . Solubility of gold salts follows the order NaAuCl4 >KAuCl4 >AuCl3 Au(NH3)4(NO3)2 >Au2S/

H2S>Au(OH)3/NaOH>Au(OH)3. Solubility of gold metal in the presence of oxidants decreases in the order MgS/H2S Fe(III)/Cl >Cu(II)/Cl > Cu(II)/NH3NaOH/H2O at a given temperature, but increases with increasing temperature. Results fromrotating disc studies in different lixiviant systems show that the rate of gold oxidation depends on the temperature and concentration of

oxidant and ligand. Silver dissolves faster than gold and thus Ag(I) catalyses the gold dissolution by redox-displacement. The surface

chemical reaction mechanism for gold dissolution is rationalized on the basis of electrochemical and adsorption theory. Rate of gold

leaching in different lixiviant systems is represented by a shrinking sphere/core model with an apparent rate constant of 105 s1.

2004 Elsevier Ltd. All rights reserved.

Keywords: Hydrometallurgy; Gold ores; Leaching; Reaction kinetics; Redox reactions

1. Introduction

Gold can be dissolved using a range of oxidants in the

presence of a range of ligands (L) to produce soluble

species Au(I)L2 and Au(III)L4 (charge ignored, Eqs. (1)

(6)). The cyanidation process (Eq. (1)) has been used to

leach gold over 100 years since it was patented in 1888by MacArthur and Forrest brothers (MacArthur et al.,

1888). Despite the long-term use of cyanidation there is

a growing interest in non-cyanide gold technology based

on the lixiviants noted in Eqs. (2)(6) and Table 1,

mainly due to the failure of cyanidation to extract gold

from the so-called difficult-to-treat ores and the envi-

ronmental and safety issues (Nicol et al., 1987; Hiskey

and Atluri, 1988; La Brooy et al., 1994; Sparrow andWoodcock, 1995; Ritchie et al., 2001).

* Tel.: +61-8-93602833; fax +61-8-93606343.

E-mail address: [email protected] (G. Senanayake).

0892-6875/$ - see front matter 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.mineng.2004.01.008

4AuO2 2H2O 8NaCN 4NaAuCN2 4NaOH 1

4AuO2 2H2O 8L 4AuIL2 4OH 22AuH2O2 4L

2AuIL2 2OHL Cl; S2O23 ; SCNH223

2Au L2 2Lor L3 L 2AuIL2L Cl;Br; I; SCN; SCNH22 4

Au 1:5L2 L AuIIIL4L Cl;Br; I 5

Au CuII or FeIII 2L AuIL2 CuI or FeII

L Cl; S2O23 ; SCN; SCNH22;NH2CH2COO

;

NH2CHCH3COO 6
mail to: [email protected]

Table 1

Oxidants and ligands for non-cyanide gold processing (selected from

Sparrow and Woodcock, 1995)

Oxidants Ligands

NH4 NH3H2S HS

, S2

H2O or OH OH

H2SO3 SO23

O2 or H2O2 Cl, OH, S2O

23 , HSO

3 , NH3

S or S2x HS, S2

X2, X3 or OX

X (X Cl. Br, I, SCN)Fe(III), Cu(II) Cl, S2O

23 , SCN

, SC(NH2)2,

NH2(CH2)COO, NH2CH(CH3)COO

786 G. Senanayake / Minerals Engineering 17 (2004) 785801

For example, coppergold ores consume cyanide whilst

carbonaceous (preg-robbing) material in gold ores

adsorb gold cyanide and prevent the solubilization of

gold. The silica encapsulation and the association of

gold with sulfide minerals make the refractory gold ores

resistant to gold cyanidation, even after fine grinding. In

such cases it is essential to pre-treat the ore by roasting,

pressure oxidation, and bio- or chemical oxidation todegrade the carbonaceous material or oxidize the sulfide

material. However, the neutralization of acidic residues

of pre-oxidation treatment, prior to cyanidation, is

uneconomical due to lime requirement and material

handling problems (Wan et al., 2003). Gold recovery in

cyanidation of washed acid-autoclave residue produced

at chloride levels of 100 g/t Cl or higher is lower,

and cannot be improved by extending the autoclaveresidence times (Ketcham et al., 1993). In addition to

these technical problems, several catastrophic accidents

caused by the failure of gold tailings dams have alarmed

the public and increased the concern about environ-

mental hazards of cyanide (Miller and Pritsos, 2001).

Despite long term interest in non-cyanide gold lixi-

viant systems, the validity of some published data for

equilibrium constants are questionable and have lead toerroneous EhpH diagrams. For example, recent studies

(Senanayake et al., 2003) on the copperammonia

thiosulphate system for gold leaching showed that the

measured equilibrium constant for the ligand displace-

ment reaction: Au(S2O3)32 +2NH3 Au(NH3)2 +2S2O23

is in the range 1091011 compared to the predicted

values in a much larger range of 100:41017, based on

the published thermodynamic data. Thus the use oferroneous thermodynamic data for the construction of

EhpH diagrams shows that Au(NH3)2 is a predomi-

nant complex at pH > 10 (Zipperian et al., 1988). Incontrast, no attempt has been made to incorporate

AuOH0 and Au(OH)2 in the EhpH diagrams, despite

the availability of reliable thermodynamic data for these

species. Gold solubility in weathering fluids containing

oxygenated-thiosulphate may be enhanced by the pres-ence of dissolved silver, generated by oxidation of co-

existing silver mineralization; thus the gold and silver

solubilities are interdependent due to the formation of a

mixed-metal complex of the type AuAg(S2O3)22 (Web-

ster, 1986). The lack of information on this type of

mixed-metalligand complexes hamper further investi-

gations in this area. Likewise, there is little agreement

amongst researchers regarding the nature of the actual

species present in a given Au(I) sulphide solution:

Au(HS)(H2S)0, Au(HS)2 , Au2S(HS)

22 and AuHSOH

(Belevantsev et al., 1981); AuHS0, Au(HS)2 , AuS22

(Renders and Seward (1989); AuS (Jiayong et al.,

1996). These facts highlight the importance of a reliable

thermodynamic database for Au(0/I/III) systems that

can be used to rationalize the complex chemistry in non-

cyanide leaching and recovery processes for gold.

Few attempts have been made to compare and con-

trast the rate data and reaction mechanisms for differentlixiviant systems (Pesic and Sergent, 1993; Dasgupta

et al., 1997; Ritchie et al., 2001; Jeffrey et al., 2001), and

the comparison of actual leaching data has been limited

(Chen et al., 1980; Navarro et al., 2002, Kholmogorov

et al., 2002). Since all the non-cyanide gold processes are

still at the developmental stages, a better understanding

of the thermodynamics and kinetics of the Au(0/I/III)

systems will (i) rationalize the complex chemistry, (ii)assist the selection and optimization of a suitable non-

cyanide leaching system with respect to different types of

material, and (iii) lead to investigation of novel gold

lixiviants. This paper intends to compare and contrast

the non-cyanide gold lixiviants of current interest on the

basis of (i) thermodynamics including stability constants

of Au(I/III) complexes, EhpH and Ehlog[Cl] dia-

grams, disproportionation of Au(I) to Au(III) and Au0,solubility of gold(I/III) salts and gold metal, and (ii) the

published rate data for the dissolution of gold from

rotating gold discs and from gold ores.

2. Non-cyanide lixiviants of current interest

A recent review by Ritchie et al. (2001) highlightedthe importance of a comparative study, using cyanide as

the bench mark, of the characteristics of alternative

leaching systems. Chloride, thiosulphate, sulphide,

ammonia, and sulfite have been identified as the low cost

systems; the first two are more beneficial in terms of

health and safety issues and low environmental impact.

For example, Pangum and Browner (1996) and Ferron

et al. (2003) demonstrated that the chloride catalysedacid-pressure leaching of refractory or sulphidic gold

ores and leach residues produced by preg-robbing con-

centrates has the potential for direct dissolution of gold

within the autoclave and thus avoiding the cyanidation

treatment. Hunter et al. (1996) summarized the advan-

tages of a sulphide leaching system in a bio-process

which makes use of the natural sulphur cycle to mitigate

the production of acidic solutions and tailings, and toenhance the recovery of gold from sulphide-hosted gold

G. Senanayake / Minerals Engineering 17 (2004) 785801 787

ores. The controlled pressure oxidation of gold bearing

sulphides with HNO3/O2 at relatively low temperature

and low oxygen pressure to produce sulphur allows the

alkaline sulphide lixiviation of gold based on Eqs. (7)

and (8) (Anderson, 2003).

4S0 6NaOH 2Na2SNa2S2O3 3H2O 72Aus S2x 2AuS

x 2S 8Wan et al. (2003) highlighted the continuing research

interest on the use of thiocyanate for gold recovery in an

acidic environment from a bio-oxidized low grade sul-

phidic ore. The use of the thiourea system with air, O2,

H2O2 or Fe(III) (Groenewald, 1976; Deschenes andGhali, 1988), the mixed thioureathiosulphate system

with ferricyanide (Hiskey, 1984), the mixed ammonia

thiosulphatesulphide system (Eqs. (7) and (8), Jiayong

et al., 1996) for gold leaching have been reported. Han

(2001) reviewed the use of ammonia with a range of

oxidants at temperatures of up to 200 C to leach goldfrom refractory ores. Aylmore and Muir (2001), Moll-

eman and Dreisinger (2002) and Grosse et al. (2003)highlighted the adaptability of the copperammonia

thiosulphate system to a range of technologies including

heap, dump and in situ leaching of gold.

Unlike Cl, OH and NH3, the sulphur containing

gold ligands are susceptible to degradation in an oxi-

dizing environment (Eqs. (9)(15)), which leads to (i)

high reagent consumption, (ii) lower rates of gold dis-

solution in leaching or diagnostic tests due to low con-centration of reagents and the formation of insoluble

products such as elemental sulphur, Cu2S, CuS, Au2S

and other gold salts (Zipperian et al., 1988, Lorenzen and

Van Deventer, 1993; Jiayong et al., 1996; Abbruzzese

et al., 1995; Muir and Aylmore, 2002), and (iii) interfer-

ence of degradation products in subsequent unit opera-

tions for separation and recovery of gold (Nicol and

OMalley, 2001; West-Sells et al., 2003). For the sake ofsimplicity the Cu(I/II) complexes with ammonia are not

shown in Eq. (10), but considered later in the discussion

related to the reaction mechanism for gold dissolution.

2Na2S2O3 O2 2H2O Na2S4O6 2NaOH 9Na2S2O33 or 4 CuSO4

0:5Na2S4O6 Na3 or 5CuS2O32 or 3 Na2SO410

Na2S4O6 2NaOH Na2S3O6 Na2S5O6 11Na2S5O6 3NaOH 2:5Na2S2O3 1:5H2O 122NaSCN Fe2SO43 SCN2 2FeSO4 Na2SO4

133SCN2 4H2O H2SO4 HCN 5HSCN 14

2CSNH22 Fe2SO43 CSNHNH2 2FeSO4 H2SO4 15
2
Whilst the oxidized forms of ligands such as

L2 (SCN)2 and (CS(NH2)2)2 in Eq. (4) are capable ofoxidizing gold (Groenewald, 1976; Li and Miller, 2002;

Wan et al., 2003), the ligand is partially reproduced in

some degradation reactions (Eqs. (12) and (14)). Oxi-

dation of ligands such as SC(NH2)2 (Chai et al., 1999)

and precipitation of metal sulphides can be avoided byadding Na2SO3 (Zipperian et al., 1988), whilst the

addition of amino acids (Michel and Frenay, 1999) and

ethylenediaminetetraacetic acid (EDTA) lowers the

thiosulfate degradation and improves the gold extrac-

tion with Cu(II)NH3S2O23 (Xia et al., 2003). In order

to avoid the thiosulphate degradation and toxic

ammonia, Ji et al. (2003) used O2/Na2S2O3/pH 1112 at

elevated temperatures of 6080 C and O2 pressures of10100 psig to leach gold from carbonaceous material.

3. Thermodynamics

3.1. Linear correlations of stability constants

The values of the equilibrium constant bn [AuLn]/[Auz][L]n for the reaction Auz+nLAuLn listed inTable 2 provide useful information on gold(I/III) spe-

ciation in process liquors; z 1 and 3 for Au(I) andAu(III) corresponds to the coordination numbers n 1,2 and 3, 4 respectively. The published values of bn aregenerally based on the measured reduction potentials

and/or saturated solubility of chlorides, cyanides, sulp-

hides and hydroxides. Unlike Ag(H2O)h and Cu(H2O)

2h

which are stable in pure water as hydrated ions of the

relevant salts (e.g. AgNO3, CuSO4), the mono-valent

ions Cu(H2O)h and Au(H2O)

h are unstable in the ab-

sence of strongly complexing ligands due to dispropor-

tionation e.g. 3Au(I) 2Au(s)+Au(III). A plot oflogfb2CuIL2 or AuIL2g against logfb2AgIL2gshown in Fig. 1, suggested by Finkelstein and Hancock

(1974), can be used for two purposes: (i) to examine thevalidity of the published data for Au(I) and Cu(I) sys-

tems, and (ii) to predict bn values for systems for whichreliable data are not available.

Linear functions (denoted by LF1 and LF2) used for

this purpose, based on Fig. 1, are shown in Table 3.

Surprisingly, the log b2 values of Au(NH3)2 and Cu(NH3)

2

show large deviations from LF1 and LF2 respectively.

The log b2 value of Au(SO3)32 also shows a significant

deviation from LF1, whilst that of Cu(I)L2 complexes of

CH3CN, SCN and CN show significant deviations

from LF2. Large deviations of Au(NH3)2 complexes

from LF1 have been related to (i) the error in the value of

E0 (Au/Au(0)) used in calculation by some authors, (ii)difficulty in the preparation of aqueous Au(NH3)

2 due to

disproportionation, and (iii) possibility of interaction of

other anions such as CN with Au(NH3)2 (Senanayake

et al., 2003). The substitution of published values of

Table 2

Stability constants of gold(I) and gold(III) complexes (log bn for n 1 4) at 25 CLigand Complex logbn Reference

Chloride AuCl2 9.71

AuCl4 25.3

Bromide AuBr2 12.7

AuBr4 32.8

Iodide AuI2 19.2

AuI4 47.7

Hydroxide AuOH0 10.2 Kissner et al. (1997)

20.6 Stefansson and Seward (2003)

Au(OH)2 24 Stefansson and Seward (2003)

Au(OH)03 44

Au(OH)4 45.6

Ammonia Au(NH3)2 18, 19.5, 21, 26

a Skibsted and Bjerrum (1974b, 1977)

Au(NH3)2 13

b Ritchie et al. (2001); Aylmore and Muir (2001)

Au(NH3)34 46, 30

b Skibsted and Bjerrum (1974b)

Thiocynate Au(SCN)2 17.2

Au(SCN)4 43.9

Cyanide Au(CN)2 38.3

Au(CN)4 85 Sharpe (1976)

Acetonitrile Au(CH3CN)2 1.6 Johnson et al. (1978)

Thiourea Au(SC(NH2)2) 23.3

Selenourea Au(SeC(NH2)2) 25.4

Glycinate Au(NH2CH2COO)2 18 Michel and Frenay (1999)

Alanate Au(NH2CH(CH3)COO)2 17.7 Michel and Frenay (1999)

Bisulphide AuHS0 24.5 Renders and Seward (1989)

Au(HS)2 30.1 Renders and Seward (1989)

32.8 Belevantsev et al. (1981)

Sulphide AuS 38.9 Webster (1986)

Au2S22 41.1 Renders and Seward (1989)

Au2S(HS)22 72.9 Belevantsev et al. (1981)

Thiosulphate Au(S2O3)32 26, 24

b Pouradier and Gadet (1969)

28 Skibsted and Bjerrum (1977)

Sulphite Au(SO3)32 26.8, 16

b Webster (1986)

Based on the data from Finkelstein and Hancock (1974) and Bard (1973) unless stated otherwise.aA range of values are reported based on different values of E0 (Au/Au).b Value based on linear free energy correlations in Figs. 1 and 2 (Senanayake et al., 2003).

y = 1.85x

R2 = 0.99

y = 0.96x

R2 = 0.99

0

10

20

30

40

50

0 5 10 15 20 25

log{2(AgL2)}

log{

2(A

uL2

or C

uL2)

}

Au(I), LF1

Au(I) with large deviations

Cu(I), LF2

Cu(I) with large deviations

CH

3CN

Cl-

NH

3

SO

32- I- , S

C(N

H2)

2

S 2O

32-

SeC

(NH

2)2

HS

-

CN

-

H2O

SCN

-

Fig. 1. Loglog plot of b2 of Au(I) and Cu(I) complexes vs Ag(I)complexes at 25 C. Data: gold (Table 2), silver and copper (Hancocket al., 1974; Hogfeldt, 1982); Cu(I)HS (Young et al., 2003); Ag(I)

HS (Schwarzenbach and Widmer, 1966).


logfb2AgIL2g in LF1 and LF2 gives rise tologfb2AuNH3

2 g 13 and logfb2AuSO3

32 g 16

listed in Table 2.

Fig. 2 shows the variation of logfb4AuIIIL4g vslogfb2AuIL2g for the ligands Cl, Br, SCN, OHand CN and the approximately linear correlation is de-

noted by LF3 in Table 3. Likewise, the linear correlation

between logfb2CuIL2g vs logfb2AuIL2g for theligands HS, Cl, NH2CH2COO

and NH2CH-(CH3)COO

in Fig. 2 is denoted by LF4 in Table 3. The

substitution of logfb2AuCH3CN2 g 1:6 and logfb2

AuNH32 g 13 in LF3 predicts logfb4Au-CH3-CN34 g 4 and logfb4AuNH3

34 g 30 respec-

tively. Thus both Au(OH)2 and Au(OH)4 have higher

stability constants than their counterparts with LNH3,based on LF1 and LF3 (Table 2). Fig. 3 shows a good

linear correlation of the published values of log bn for thethree sulphide complexes MHS0, M(HS)2 and M2S-

(HS)22 of Cu(I) and Au(I) vs Ag(I), showing the validity

of these constants. In summary, the linear correlation of

stability constants of complex species proposed by Fin-kelstein and Hancock (1974) paves the way to establish

reliable thermodynamic data for Au(I/III) complexes in

non-cyanide lixiviant systems.

Table 3

Linear mathematical relationships between stability constants of Cu(I), Ag(I) and Au(I/III) complexes based on Figs. 1 and 2

No. Figure Linear relationship

LF1 Fig. 1 logfb2(AuL2)} 1.85 logfb2(AgL2)} (R2 0.99)LF2 Fig. 1 logfb2(CuL2)} 0.96 logfb2(AgL2)} (R2 0.99)LF3 Fig. 2 logfb4(AuL4)} 2.28 logfb2(AuL2)} (R2 0.98)LF4 Fig. 2 logfb2(CuL2)} 0.53 logfb2(AuL2)} (R2 1.00)

y = 2.28x

R2 = 0.98

y = 0.53x

R2 = 1.000

20

40

60

80

100

0 5 10 15 20 25 30 35 40

log{2(AuL2)}

log{

4(A

uL4)

} or

log{

2(C

uL2)

} Au(III), LF3Cu(I), LF4

Cl- B

r-

SCN

-

NH2CH2COO-

OH

-

HS

-

CN

-

H2O

Cl-

NH2CH(CH3)COO-

Fig. 2. Loglog plot of b4 of Au(III) and b2 of Cu(I) vs b2 of Au(I)complexes at 25 C. Data from Refs. shown in Fig. 1 caption.

y = 0.79x + 15.16

R2 = 1.00

y = 0.80x + 20.09

R2 = 1.00

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80

log { n( Ag(I) complexes)}

log

{n(

Au(

I) o

r C

u(I)

com

plex

es)} Cu(I)

Au(I)

MH

S0

M(H

S) 2

-

M2S

(HS

) 22-

Fig. 3. Loglog plot of b of MHS0, M(HS)2 and M2S(HS)22 for Au(I)

vs Ag(I) complexes at 25 C. Data from Table 2 and Schwarzenbachand Widmer, 1966.


3.2. Disproportionation of Au(I) and EhpH diagrams

Table 4 lists the disproportionation reactions denoted

by DR1-DR11 for Cu(I) and Au(I) to highlight the large

equilibrium constants (Kd) in the range 106:51013 com-pared to the small values for CuCl2 (10

5, DR1) and

Au(OH)2 (1011, DR4). Kissner et al. (1997) mixed a

solution of Au(CH3CN)2 ClO

4 prepared by anodic-

oxidation of gold in acetonitrile with water at 25 C andexamined the disproportionation at pH 2, 7 and 12 at 25C. Surprisingly, instead of disproportionation, 1 mMAu(I) was slowly reduced, yielding 99.4%, 98.4% and

98.1% Au(0) respectively with the formation of Au(III)

solutions of very low concentration: 0.6% (pH 2), 1.7%

(pH 7) and 1.9% (pH 12). The evolution of oxygen gas

was also observed at pH > 10. The two disproportion-ation reactions which explain these observations are

represented by Eqs. (16) and (17).

2AuCH3CNx H2O 2Aus 0:5O2g 2H

2xCH3CN predominant reaction 163AuCH3CNx 3H2O

2Aus AuOH03 3H 3xCH3CN minor reaction 17

Despite the large value of Kd 108 (DR9) for AuCl2 inTable 4, Diaz et al. (1993) reported that the rate of

disproprtionation of AuCl2 to Au(0) and AuCl4 is also

very slow, taking more than 2 months to reach equi-

librium. However, Pesic and Sergent (1993) showed that

the dissolution of gold from a rotating disc in Br2/NaBr

produces AuBr2 , which is subsequently disproportion-ated fairly rapidly to Au0 and AuBr4 . Skibsted and

Bjerrum (1974b) prepared a solution of Au(NH3)2

in situ by reducing Au(NH3)34 with hydrazine in a

background medium of 10 M NH4NO3 and observed

the disproportionation of Au(NH3)2 (Kd 1013, Table

4). They have also determined equilibrium constants for

the hydrolysis of Au(NH3)34 to Au(NH3)3OH

2 and

acid dissociation of Au(NH3)34 to Au(NH3)3NH

22

(Skibsted and Bjerrum, 1974a). Table 4 lists the reported

values of the relevant equilibrium constants for hydro-

lysis (Kh) and acid-dissociation (Ka). Due to the lowerstability of Au(NH3)

2 (Table 2) it may also hydrolyse to

AuOH0 and Au(OH)2 and subsequently dispropor-

tionate to Au(s) and O2, according to reactions HR3,

HR5, DR5 and DR7 listed in Table 4.

A comparison between Au(NH3)2 (Kh 100:6 or 103

for HR3 and HR5) and AuCl2 (Kh 1015 or 103 forHR1 and HR2) in Table 4 clearly shows that AuCl2 is

more stable towards hydrolysis. Moreover, as shown by

DR4, DR5 and DR7, the calculated values of Kd for thedisproportionation of AuOH0 and Au(OH)2 to Au(s)

and O2 (100:2101) is much larger than Kd for the dis-

proportionation that produces Au(s) and Au(OH)4(1011). Figs. 4 and 5 make use of the informationin Table 4 to incorporate the Au(I/III) species such

as AuOH0, Au(OH)2 , Au(NH3)3OH2 and Au(NH3)3-

NH22 in the EhpH diagrams for Au(0/I/III)O2/

Table 4

Disproportionation (Kd), hydrolysis (Kh) and acid dissociation (Ka) equilibrium constants of Cu(I) and Au(I) in aqueous media at 25 C; calculatedusing bn from Table 2 and Hogfeldt (1982) and E

0 from Bard (1973) unless stated otherwise

No. Type of equilibrium K Reference

Disproportionation reaction (Kd)DR1 2CuCl2 (aq) Cu0 +CuCl(aq)+ 3Cl 105DR2 2Cu(aq) Cu0 +Cu2(aq) 106DR3 2Cu(NH3)

2 Cu0 +Cu(NH3)24 106:5

DR4 3Au(OH)2 2Au(0) +Au(OH)4 + 2OH 1011DR5 Au(OH)2 Au0 +OH +0.25O2 + 0.5H2O 100:2DR6 3Au(SCN)2 2Au+Au(SCN)2 + 2SCN 100:3 Skibsted and Bjerrum (1974a)DR7 AuOH0 Au+0.25O2 +0.5H2O 101:2DR8 3AuBr2 2Au0 +AuBr4 + 2Br 105 Skibsted and Bjerrum (1974a)DR9 3AuCl2 2Au0 +AuCl4 + 2Cl 108 Skibsted and Bjerrum (1974a)DR10 3Au 2Au0 +Au3 109DR11 3Au(NH3)

2 2Au0 +Au(NH3)34 +2NH3 1012 Skibsted and Bjerrum (1974a)

Hydrolysis reaction (Kh)HR1 AuCl2 + 2H2O Au(OH)2 +2H +2Cl 1015HR2 AuCl2 +H2O AuOH0 +H +2Cl 102:5HR3 Au(NH3)

2 + 2H2O Au(OH)2 + 2NH4 100:6

HR4 Au(NH3)34 +H2O Au(NH3)3OH2 +NH4 100:2 Skibsted and Bjerrum (1974a)

HR5 Au(NH3)2 +H2O AuOH0 +NH3 +NH4 102:8

Acid dissociation reaction (Ka)AD1 Au(NH3)

34 Au(NH3)3NH22 +H 107:5 Skibsted and Bjerrum (1974a)

AD2 NH4 NH3 +H 109:2 Hogfeldt (1982)

-0.5

0.0

0.5

1.0

1.5

2.0

-2 0 2 4 6 8 10 12 14

pH

Eh

/ V

Au3

+

Au(OH)3(s) or Au2O3(s)

AuOH0

Au(s)

Au(

OH

) 2-

Au(

OH

) 4-

(a)

(b)

Fig. 4. EhpH diagram for Au(0/I/III)H2O system at 25 C (lines aand b represent O2/H2O and H

/H2). [Au (I or III)] 105 M.

-0.5

0.0

0.5

1.0

1.5

2.0

-2 0 2 4 6 8 10 12 14pH

Eh

/ V

Au(NH3)3NH22+

Au(OH)4-AuOH0

AuOH0

Au(

OH

) 2-

NH

4+

NH

3

Au(s)

Au(s)

(a)

(b)

Au(NH3)42+Au3+

Fig. 5. EhpH diagram for Au(0/I/III)NH3 system at 25 C. [Au (I orIII)] 105 M.


NH3H2O systems. Unlike the previously reported Eh

pH diagram (Zipperian et al., 1988), Fig. 5 does not

show Au(NH3)2 , as a result of its tendency to hydrolyse

to AuOH0 and/or disproportionate to Au and O2.

3.3. Solubility of gold(I/III) salts

The saturated solubility of salts in hydrometallurgicalsystems provide useful information on chemical specia-

tion and plays an important role in selective leaching,

separation and recovery of metals or salts by precipita-

tion. For example, two recent reviews (Muir, 2002; Se-

nanayake and Muir, 2003) have highlighted the effect of

background chloride concentration, temperature and

pH on the solubility of base metal chlorides, depending

on the stability of chloro-complexes, which leads toimportant applications in chloride hydrometallurgical

processes. In general the solubility of complex salts such

as KAu(CN)2, Na3Au(S2O3)2, NaAuCl4 and KAuCl4are relatively high compared to Au2S and Au(OH)3(Table 5). The descending order of log b4 of Au(III)complexes with the three ligands: OH >NH3 >Cl

shown in Table 2 and Fig. 2 is not observed with

the solubility data shown in Table 5: NaAuCl4 >KAuCl4 >AuCl3 Au(NH3)4(NO3)2 > Au2S/H2S > Au(OH)3/NaOH>Au(OH)3. It is important to note that

despite the high value of logb3 44 for Au(OH)03 (Table2) the solubility of Au(OH)3 in water is as low as 6 mg/l

(Table 5) due to the precipitation of Au(OH)3 or Au2O3(Fig. 4). Moreover, the 1.9% Au(III) produced during

the disproportionation of 1 mM Au(I) at pH 12 (Kissner

Table 5

Solubility of gold salts

Salt T (C) Medium [S2] total (mol/kg) pH Au solubilitya

(mg/kg) (g/kg)

AuCN 25 0.4 m KCN 70

Au2S2O3.3BaS2O3 Water 6.6

Au2S 25 H2S(aq) 0.008 2.0 0.003

0.009 7.3 4.1

0.01 11.6 2.7

0.2 11.6 39

0.02 12.0 14

0.4 11.9 256

NaAuCl4 30 Water 967

KAuCl4 30 Water 495

AuCl3 Water 441

Au(NH3)4(NO3)3 25 Water 13

Au(OH)3 25 0.41 M NaOH 0.21

Au(OH)3 25 0.1 M NaOH 0.02

Au(OH)3 25 Water 0.006

Data: Linkey and Seidell (1958), Au2S data from Renders and Seward (1989).aGold (I/III) dissolved in 1 kg of water.


et al., 1997) according to Eq. (17) corresponds to 4 mg/l

Au(III); this value is in close agreement with the solu-

bility of Au(OH)3 in water (6 mg/l, Table 5).

Solubility of Au2S depends on the two parameters pHand total sulphide content [S2]t. For example, at a fixed

value of S2t 0:01 M, the concentration of Au(I)increases thousand-fold from 3 lg/kg at pH 2 to 4 mg/kgat pH 7 and drops to 3 mg/kg at pH 11.6. However, a

forty-fold increase in [S2]t from 0.01 to 0.4 M at

pH 12 causes an eighty-fold increase in the solubilityof Au(I) from 3 to 256 mg/kg. Fig. 6 shows loglog plots

of solubility of Au2S and Au(OH)3 against pH and[OH]/Kw (Kw ionic product of water) respectively tocompare and contrast the behaviour of the two systems.

An approximate slope of 12 for the initial increase in

log[Au(III)] against logfOH=Kwg shows the forma-tion of complex species according to Eqs. (18) and (19).

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

0 4 8 12 16

pH or log{[OH-]/Kw}

[Au(

I) o

r A

u(II

I)]

/ mol

/L

Au(III) / NaOH 0.8 - 8 mol / kg

Au(I) / hydrogen sulphide 0.004 - 0.01 M

Au(I) / hydrogen sulphide 0.02 - 0.1 M

Fig. 6. Effect of pH or [NaOH] or total sulphide [S2]t on solubility of

Au2S in H2S or Au(OH)3 in NaOH (25 C). Data: Au2S (squares,Renders and Seward, 1989); Au(OH)3/NaOH (circles, Linkey and

Seidell, 1958).

In the case of Au2S, the solubility dependence on both

pH and sulphide concentration has been related to the

formation of the three complex species shown in Eqs.

(20)(22) (Renders and Seward, 1989). Based on thegold(I) complex Au2S(HS)

22 shown in Fig. 3, Eq. (23)

can also be included in the Au2S solubilization equilib-

ria. It is clear from these equations that a decrease in

concentration of H2S in solution caused by a decrease in

partial pressure of H2S favours the precipitation of

Au2S.

AuOH3s NaOH NaAuOH4aq 18AuOH3s 2NaOH Na2HAuO3 2H2O 190:5Au2Ss 0:5H2Saq AuHS0 200:5Au2Ss 1:5H2Saq AuHS2 H 21Au2Ss Na2S Na2Au2S2 22Au2Ss 2H2Saq Au2SHS22 2H 23

3.4. Solubility of gold metal

Fig. 7 and Table 6 show the solubility of gold metal in

different oxidizing media O2/CN, Cu(II)/Cl, Fe(III)/

Cl, NaOH and MgS (McDonald et al., 1987; Liu and

Nicol, 2002; Stefansson and Seward, 2003; Fleet andKnipe, 2000), to highlight the effect of temperature,

nature of the dissolved oxidant and ligand in Eqs. (24)

(30). The descending order of Au(I/III) solubility in

various redox systems in Fig. 7: S2 (0.030.15 M)>

Fe(III)/Cl(0.010.41 mM)Cu(II)/Cl(0.060.6 mM)>Cu(II)/NH3(0.080.41 mM)>OH

/(0.022.5 lM) lar-gely reflects the composite effect of decreasing stability

of the relevant gold(I/III) complexes and the solubility

of salts listed in Tables 2 and 5.

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

10 100 1000

Temperature / C

[Au(

I) o

r A

u(II

I)]

/ mol

/L o

r m

ol/k

g

Au(s)/MgS

Au(s)/Cu(II)/NH3Au(s)/Cu(II)/HCl/NaCl

Au(s)/Fe(III)/NaCl/H2SO4Au(s)/NaOH

NaAuCl4Au(NH3)4(NO3)2

Au2S /0.2 M Na2S/pH 11.6

Au(s)/Au(III)/ClO4-

CH3CN/H2O/pH 2

Au(OH)3

Au(OH)3 /0.4 M NaOH

Fig. 7. Effect of temperature on gold solubility in different lixiviant

systems and comparison with gold salt solubility. Data: NaAuCl4(Linkey and Seidell, 1958); other gold salts (Table 4); Au/MgS (Fleet

and Knipe, 2000); Au/Fe(III)/H2SO4/NaCl (Liu and Nicol, 2002); Au/

Cu(II)/HCl/NaCl (McDonald et al., 1987) Au/Cu(II)/NH3 (Meng and

Han, 1996) Au/NaOH (Stefansson and Seward, 2003).


Aus FeCl3 NaCl NaAuCl2 FeCl2 24Aus CuCl2 2NaCl NaAuCl2 NaCuCl2 25NaAuCl2 2FeCl3 or CuCl2

NaAuCl4 2FeCl2 or CuCl 26

Aus H2O AuOH0 0:5H2g 27Aus NaOHH2O NaAuOH2 0:5H2g

28Aus H2S AuHS0 0:5H2g 29Aus 0:5MgS 1:5H2S

Mg0:5AuHS2 0:5H2g 30

The solubility of gold in 0.1 M FeCl3/2.4 M NaCl at 40

C (0.1 mM, Table 6) compares well with the value of

Table 6

Solubility of gold dissolved from metallic gold, goldsilver alloy (al), colloid

System [Oxidant] (mM) [Ligand] (mM) Acid

Au/O2/KCN 7 0.15

Au/Fe(III)/NaCl 100 2.4

Au/Cu(II)/NaCl 100 3.4 0.1

600 3.4 0.1

Au/H2O2/SC(NH2)2 0 8.6 Non

0 8.6 0.1

17.6 8.6 0.1

8.8 8.6 0.05

8.8 8.6 0.1

8.8 8.6 0.2

Au/O2/Na2S2O3 100b pH 8.6

Au(al) (64 mol% Ag)

Au(col)

Au(col) (12% Ag)

Au(col) (20% Ag)

Data from Linkey and Seidell (1958), if not stated otherwise.aAmbient.b Eh 0.3 V.

0.05 mM in 0.1 M FeCl3/1 M NaCl/1.1 M H2SO4 (Fig.

7). The two values from different sources represent the

saturated solubility associated with the redox-equilib-

rium between gold metal and Fe(III) in Eqs. (24) and

(26). McDonald et al. (1987) and Liu and Nicol (2002)

highlighted the importance of considering the thermo-

dynamic data at high temperatures to predict the rele-vant redox potentials and equilibrium data for gold

oxidation according to Eqs. (24) and (26). Table 7 lists

the equilibrium constant for selected reactions which

involve dissolution of gold by a range of oxidants to

highlight the effect of temperature. The increase in

temperature from 25 to 100150 C increases the equ-librium constant by 45 orders of magnitude in the case

of sulphide and chloride systems. The calculated equi-librium concentration of the gold dissolved in 0.6 M

CuCl2/3.42 M NaCl/0.1 M HCl at 102 C shows that thevalue of [AuCl2 ] is 10 times larger than that of [AuCl

4 ],

whilst the total concentration of these two species: 108

mg/l Au(I) and 12 mg/l Au(III) is in excellent agreement

with the measured value (115 mg/l, McDonald et al.,

1987).

For the purpose of comparison the solubility ofAu(III) compounds such as Au(OH)3, Au(NH3)4-

(NO3)3, Au2S and NaAuCl4(s) in water, NaOH(aq) or

Na2S(aq) solutions (Table 5) are also shown in Fig. 7.

Although the solubility of NaAuCl4 in water at 25 C isseveral orders of magnitude larger than that of Au(OH)3(105 mg/kg), Au(NH3)4(NO3)3 (102 mg/kg) andAu2S (104 mg/kg) it remains fairly constant at 3.85.9mol/kg in the temperature range 1060 C. In contrast,the solubility of gold metal in different lixiviant systems

shows large temperature dependence in chloride,

ammonia and hydroxide lixiviants. Thus Fig. 7 summa-

rizes the effect of temperature on gold solubility due to

the change in redox equilibrium constants and chemical

al gold (col) or mixed goldsilver colloid

(M) T (C) Time (h) Au solubility (mg/l)

18 1 4100

40 185 23

(HCl) 102 23 45

(HCl) 104 1 170 (McDonald et al., 1987)

e a 6 0

(H2SO4) 6 3

6 9.5

14 41

14 43

14 41

25 1512 1.5 (Webster, 1986)

24

0.7

10

11

Table 7

Effect of temperature on gold dissolution equilibria in selected non-cyanide media

Dissolution equilibrium K (25 C) K (150 C) Reference

Au(s) +H2S+HS Au(HS)2 + 0.5H2(g) 106 101:6 Benning and Seward (1996)

Au(s) +H2S AuHS0 + 0.5H2(g) 1011:5 107:1 Benning and Seward (1996)Au(s) +CuCl +3Cl AuCl2 +CuCl2 1012 108 (at 102 C) McDonald et al. (1987)Au(s) +CuCl02 + 2Cl

AuCl2 +CuCl2 1012 108 (at 102 C) McDonald et al. (1987)Au(s) +H +2Cl AuCl2 + 0.5H2(g) 1019 1012 Shenberger and Barnes (1989)Au(s) +H2O AuOH0 + 0.5H2(g) 1022 1013 Stefansson and Seward (2003)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-2.0 -1.0 0.0 1.0 2.0

log[Cl-]

Eh

/ V

Au(s)

AuOH0

AuCl2-

AuCl4-

Fe3+ FeCl2+

FeCl2+

FeCl3Fe2+

FeCl+ FeCl2

Cu(s)

Cu2+

CuC

l+

CuC

l 2

CuC

l 2-

CuC

l 32-

AuCl4-

Au(s)(a)

(b)

Cl2(g)Cl-

Fig. 8. Ehlog[Cl] diagram for Au(0/I/III)Fe(II/III)Cu(0/I/II)O2

Cl system at 25 C based on bn (Hogfeldt, 1982) and E0 (Bard, 1973),assuming unit activity coefficients, [Au(I)] [Au(III)] 105 M andothers at 0.1 M, pH 1.


speciation for the relevant reactions (Eqs. (24)(30)).

Some of the important features in Fig. 7 are noted below:

Solubility of Au(III) produced by disproportionationof Au(CH3CN)

2 in water at pH 2 (Eq. (17)) is as low

as 6 lmol/l at 25 C. In the presence of the oxidantFe(III) in mixed acidic chloride/sulfate media, the solu-

bility of Au(I and/or III) in equilibrium with Au(s) is

also of the same order of magnitude (9 lmol/l) at 20 C. However, the gold solubility increases to 0.41 mM

and 0.27 mM respectively in Fe(III) and Cu(II) chlo-

ride media, with an increase in temperature to 90 C. At 114 C the gold solubility in Cu(II)/Cl increases

to 0.7 mM. In comparison, the solubility of gold in

Cu(II)/NH3 is much lower, but increases from 0.08

mM at 140 C to 0.4 mM at 180 C. The highest solubility of gold metal is observed in

MgS/H2O/0.15 GPa pressure and increases from 62

to 150 mM with the increase in temperature from

116 to 200 C, and decreases to 32 mM at 400 C. In contrast, the solubility of gold in 0.5 mol/kg

NaOH is much lower even at high temperatures:

0.02 lM at 300 C and 2.7 lM at 500 C. This con-firms the lower stability of complex species such as

AuOH0 and Au(OH)2 and their tendency to dispro-

portionate to Au(s) and O2 (Table 4 and Fig. 4).

3.5. Ehlog[Cl] diagram for FeCuAuClO2 system

The Ehlog[Cl] relationships provide a convenient

way of summarizing the chloride complex species, such

as those shown in Tables 4 and 7, involved in redox

reactions in chloride media (Eqs. (24)(26)). Fig. 8 plots

the Ehlog[Cl] relationships for AuCuFeCl system

at 25 C in acidic media of pH 1 to incorporate AuOH0and AuCl2 and thus to update the recently reporteddiagram (Senanayake and Muir, 2003). The Eh of redox

couples at a given value of [Cl] 1 M follows the orderCl2/Cl

> O2/H2O >Au(III)/Au(I) >Au(I)/Au(0) >Fe-

(III)/Fe(II) >Cu(II)/Cu(I) at 25 C.McDonald et al. (1987) reported that (i) when copper

containing metal alloys are leached with CuCl2 solutions

in the absence of O2, the solubility of gold is considerably

reduced due to equilibration with Cu(I), (ii) when coppermetal is added to an equilibrium solution of gold dis-

solved in CuCl2, gold precipitates in the flask instead of

cementing onto coppermetal, (iii) the addition ofmetallic

copper lowers the Cu(II)/Cu(I) potential by lowering

[Cu(II)] according to the reaction Cu(s) +Cu(II)2Cu(I) and thus facilitates the reverse reaction of leaching

(Eq. (25)), (iv) the solubility of gold in CuCl2 increases in

the presence of oxygen, but decreases with the addition of

CuCl under nitrogen. All of these observations andinterpretations are consistent with the Ehlog[Cl] dia-

gram in Fig. 8. McDonald et al. (1987) and Liu and Nicol

(2002) confirmed that the dissolution of gold with Fe(III)

and Cu(II) in chloride systems at elevated temperatures

takes place according to Eqs. (24)(26). The role of O2 is

largely confined to the regeneration of Cu(II) or Fe(III)

(Eq. (31)) so that decrease in Cu(I) or Fe(II) activity

facilitates gold dissolution (Eqs. (24)(26)).

2CuI or FeII 0:5O2 2H

2CuII or FeIII H2O 31

4. Kinetics and reaction mechanism

4.1. Dissolution of metallic or colloidal gold

Table 6 compares the time taken for [Au(I)] or

[Au(III)] to reach different values of equilibrium con-centrations depending on the lixiviant system: cyanide,

0

2

4

6

8

10

0.25 M 0.01 M none 0.16 M 0.3 M 0.1 M

[Au

diss

olve

d]/ m

g/L

NaOH Ca(OH)2 HNO3 HCl H2SO4

Fig. 9. Effect of acids and bases on gold dissolution in 17.6 mM H2O2/

8.6 mM SC(NH2)2 in 6 h. Data from Linkey and Seidell (1958).


chloride, thiourea and thiosulphate. The general trend in

rate, based on the ratio of gold dissolved/time, follows

the order cyanide > chloride> thiourea > thiosulphate.

Fig. 9 shows the effect of adding different acids and

bases on the concentration of gold dissolved by H2O2/

thiourea in 6 h. The following points are worth noting:

Thiourea alone in the absence of oxidant or acid doesnot dissolve gold (Table 6).

The mixture 8.6 mM SC(NH2)2/0.1 M H2SO4 dis-solves 3 mg/l in 6 h, even in the absence of an oxidantsuch as H2O2.

At the same ligand/acid concentration a further in-crease in dissolved gold concentration to 9.5 mg/l is

observed in the presence of 17.6 mM H2O2 in 6 h.

The addition of bases is detrimental to gold dissolu-tion, whilst the addition of acids is beneficial

(HNO3 < HCl< H2SO4) (Fig. 9).

The dissolved gold concentration can reach highervalues of 4043 mg/l over a large period of time of

14 h, even with a lower concentration of H2O2 of

8.8 mM, irrespective of the H2SO4 concentration in

the range 0.050.2 M (Table 6).

In comparison to the thiourea system, the rate of golddissolution in O2/Na2S2O3 seems to be very slow, tak-

Table 8

Rate of dissolution gold, silver or goldsilver alloy in different lixiviant syste

Oxidant/ligand//pH/other conditions [Oxidant] (mM) [Ligand] (mM

Cl2/1 M HCl 2 1000

Cl2/Cl/pH 4 2 1000

Br2/Br/pH 4.5 25 97

Fe(III)/SC(NH2)2/0.1 M H2SO4 5 26.3

O2/CN/pH 10.5 77

Cu(NH3)24 /pH 9.7/0.5 M (NH4)2SO4 1

Co(NH3)36 1

O2 1

H2O2 1

O2/CN/100 g/l NaCl 0.15(air) 5

O2/CN 0.26(air) 5

O2/CN 1.28(O2) 5

Cu(II)/S2O23 /0.4 M NH3 10 100

HOCl/NaCl/pH 3.5 0.8 1700

Cu(II)/S2O23 /0.84 M NH3/pH 10

c 25 400

NaOCl/NaCl/pH 6 27 85



Cu(II)/NH3/pH 10/1.0 M (NH4)2SO4 1 2000

Cu(NH3)24 /0.32 M NaOCl 157

I2/I/pH 10/1 M NH3 1 10

Fe(III)/NaCl/0.5 M H2SO4 100 200

Fe(III)/NaCl/0.5 M H2SO4 100 400

O2/S2O23 sat 200

O2/S2O23 /0.2 M (NH4)2S2O3 sat 200

O2/SC(NH2)2/0.2 M (NH4)2S2O3 sat 5

O2/SC(NH2)2/0.2 M(NH4)2S2O3 sat 10

O2/SC(NH2)2/0.2 M Na2S2O3 sat 5

aGold dissolved from goldsilver alloy.b Pure silver.c Also contained 0.44 (NH4)2SO4.

ing 6 weeks to dissolve 1.5 mg/l. However, Table 6

shows that the presence of silver increases the solubil-

ity from 1.5 to 24 mg/l (gold metal) and from 0.7 to

11 mg/l (gold colloid).

Table 8 compiles the published rate data per unit

surface area (mol/m2/s) for the dissolution of gold, silver

and goldsilver alloy in different lixiviant systems, basedon rotating gold discs. The rate of dissolution of pure

ms

) T (C) 105 rate (mol/m2/s) Reference

12 4.6 Diaz et al. (1993)

12 2.3

25 24 Pesic and Sergent (1993)

25 2.4

25 1.2

75 0.06 Dasgupta et al. (1997)

75 0.05

75 0.04

75 0.01

20 )(2.5)a Jeffrey et al. (2001)20 0.1 (4.1)a

20 )(9.6)a

20 3.8 (3.8)

20 1.4 (1.6)a

20 )(3.9a, 26.6b) Jeffrey (2001)20 0.7 Tran et al. (2001)

20 14

20 7

135 0.06 Han (2001)

140 1.1

75 0.07

200 11.7 Liu and Nicol (2002)

200 19.3

30 0.01 Chandra and Jeffrey

(2003)

30 0.40

30 0.55

30 0.47

30 0.04

0

100

200

300

0 10 20 30 40 50 60

Time / weeks

[Ag(

I)]

/ mg

L-1

Ag metal

Ag / Au alloy (64% Ag)

Ag colloid

Ag / Au mixed colloid (20 mol% Ag)

Ag / Au mixed colloid (11mol% Ag)

Fig. 10. Concentration of silver dissolved from pure silver, silvergold

alloy or mixed silvergold colloid in oxygenated thiosulphate media

(see Table 6, Data: Webster, 1986).

0.0

0.1

0.2

0.3

0.4

0 0.1 0.2 0.3 0.4 0.5 0.6

[Ag(I)] / mmol L-1

[Au(

I)]

/ mm

ol L

-1

Ag-Au alloy

Ag-Au mixed colloid

Ag-Au oreSlope = 0.6

Slope 0.5

Fig. 11. Concentration of dissolved gold(I) against dissolved silver(I).

Data: AuAg alloy and mixed goldsilver colloid with O2/Na2S2O3(Webster, 1986); AuAgCu ore with Fe(III)/SC(NH2)2 (Chen et al.,

1980).

y = 0.6x + 2.7

R2 = 1.0

y = 0.9x + 2.8

R2 = 1.0

-3

-2

-1

0

1

2

3

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0

log{[Oxidant] / mol L-1}

log

{105

R /

mol

m-2

s-1

}

Oxidant = Oxygen in cyanide

Oxidant = Cu(II) in ammonia-thiosulphate

Oxidant = Fe(III) in thiourea

Slope = 0.9

Fig. 12. Loglog plot of rate of gold dissolution from rotating discs vs

concentration of oxidant. Data: O2/cyanide (Table 7); Cu(II)NH3

S2O23 /30 C (Jeffrey, 2001); Fe(III)/SC(NH2)2/25 C (Chen et al.,

1980).


silver in Cu(II)NH3S2O23 (26.6 mol/m

2/s) is six times

faster than that of gold from AuAg alloy. Fig. 10

shows the concentration of silver dissolved from silvermetal, silver colloid, mixed silvergold colloid and sil-

vergold alloy in the presence of O2/Na2S2O3/pH 10/Eh

0.3 V. It is of interest to note that the rate of dissolution

of silver from both mixed silvergold colloid and alloy is

slower than that of pure silver. Moreover, the dissolved

silver seems to approach an equilibrium concentration

in the case of mixed silvergold colloid. Whilst pure

silver dissolves faster than pure gold (Table 7), golddissolves faster in the presence of silver (Table 6). Thus

silver(I) in solution seems to catalyse the gold dissolu-

tion according to Eqs. (32)(34) (c colloid).Aus or c AgI AuI Ags or c 32Aus or c S2O23 AgS2O

3

AuS2O332 Ags or c 33

Aus or c AgS2O332 AuS2O332 Ags or c 34

A plot of the concentration of Au(I) vs Ag(I) at differenttime intervals during the dissolution of alloy or mixed

silvergold colloid gives a slope 0.6 (Fig. 11). Thisvalue is in reasonable agreement with the equilibrium

constant for Eqs. (33) and (34) calculated using the

published values of DG0f of relevant species reported byZipperian et al. (1988) and Aylmore and Muir (2001). It

is of interest to note that the gold and silver leached

from a coppersilvergold ore using Fe(III)/SC(NH2)2/H2SO4 (Chen et al., 1980), also follow the same linear

trend (Fig. 11).

4.2. Comparison between different lixiviants

Table 8 shows the descending order of rate of gold

dissolution per unit surface area: Br2/Br (25 C)>

NaOCl/NaCl (20 C)>Fe(III)/Cl (200 C)>Cl2/HCl/12 C>Cu(II)/NH3/S2O23 (20 C) > Fe(III)/SC(NH2)2(25 C) > Cu(NH3)24 /NaOCl (140 C) > O2/SC(NH2)2(30 C)>O2/(NH4)2S2O3 (30 C). However, due to thefact that rate (R) is expressed by the general equationR k [Oxidant]a [Ligand]b, where a and b are thereaction orders with respect to oxidant and ligand

respectively, the rate constant (k) gives a better com-parison between different lixiviant systems. Thus, Fig. 12

compares the loglog plots of rate vs [Oxidant] in the

three different lixiviant systems O2/CN, Cu(II)/NH3/

S2O23 and Fe(III)/SC(NH2)2. Table 9 summarizes the

kinetic parameters based on Fig. 12. The slopes of linear

relationships in Fig. 12 indicates the reaction order 0.6

with respect to [O2] and 0.9 with respect to [Fe(III)] and

[Cu(II)]. In the case of Cu(II) the order decreases to 0 at

higher values of [Cu(II)]. Despite the lower concentra-

tion of 5 mM CN compared to 400 mM S2O23 and 140

mM SC(NH2)2, the value of k [Ligand]b i.e. the y

Table 9

Kinetic parameters in the rate equation rate k [Oxidant]a [Ligand]b, from Fig. 12Lixiviant [Oxidant] (mM) [Ligand] (mM) [Base] or [Acid] (mM) a (slope) k [Ligand]b (y intercept)

O2/CN 0.151.3 5 0.6 2.7

Cu(II)/S2O23 /NH3 125 400 840 (NH3) 0.90

Fe(III)/SC(NH2)2/H2SO4 7.243 140 5 (H2SO4) 0.9 2.8


intercept is comparable (2.72.8) in all three cases O2/

CN, Fe(III)/SC(NH2)2 and Cu(II)/S2O23 /NH3.

4.3. Surface reaction mechanism

It is well established that the oxidation of gold is

electrochemical in nature and thus the kinetics can be

modelled using corrosion theory for metals (Nicol,

1993). For example, the order close to 0.5 for the cy-anidation reaction with respect to [O2] shown in Table 9

is consistent with the previous results and the electro-

chemical model (Crundwell and Godorr, 1997). The

detailed analysis of reaction orders based on rotating

disc data which provide useful information on reaction

order with respect to concentration of reagents is well

documented (Jiang et al., 1993; Li and Miller, 2002).

The leaching results reported by Navarro et al. (2002)show that the initial % extraction of gold is independent

of the concentration of Cu(II) and S2O23 at pH 10, 25

C and higher reagent concentrations: [Cu(II)] > 0.05 Mand [(NH4)2S2O3] > 0.1 M. This result, consistent with

the zero order rate of gold dissolution with respect to

[Cu(II)] noted in Fig. 12, can be used to propose a

surface reaction mechanism for gold leaching with

Cu(II) by combining the electrochemical rate equationswith the well known adsorption theory.

Byerley et al. (1973) showed the importance of con-

sidering the mixed complex of Cu(II): Cu(NH3)nS2O3 in

order to rationalize the kinetics of oxidation of S2O23 by

ammoniacal CuSO4. Eq. (35) shows the equilibrium

which represents the adsorption of S2O23 and the mixed

complex Cu(NH3)nS2O3 onto the gold surface to form

s Au(S2O3)2Cu(NH3)2n ads. If the fraction of the goldsurface onto which S2O

23 and Cu(NH3)nS2O3 is ad-

sorbed is denoted by h, the adsorption equilibriumconstant Kads for Eq. (35) and h can be expressed by Eqs.(36) and (37) respectively, where [Cu(II)] is the concen-

tration of the mixed complex in solution.

s Au S2O23 aq CuNH3nS2O3aq s AuS2O32CuNH3

2n ads

surface adsorption equilibrium 35

Kads hf1 hCuIIS2O23 g1 36

h KadsCuIIS2O23 f1 KadsCuIIS2O23 g

1

37Due to the redox nature, the surface reaction for the

oxidation of gold by Cu(II) can be represented by the

simultaneous oxidation Au(0)fiAu(I) + e and reductionCu(II) + efiCu(I). For the sake of simplicity s Au-(S2O3)2Cu(NH3)

2n ads is represented by s Au(0)Cu(II)ads

and the electrode reactions by Eqs. (38) and (39). Thus

the overall redox reaction described by Eq. (42) i.e. thesimplified version of the sum of Eqs. (38) and (39),

represents the apparent surface reaction which is also

the rate determining step (RDS).

s Au0CuIIads s CuIIads AuIaq e anodic reaction

38s Au0CuIIads e

s Au0 CuIaq cathodic reaction

392s Au0CuIIads s CuIIads s Au0

AuIaq CuIaq overall 40s Au0CuIIAds s AuIaq CuIaq 41

s AuS2O32CuNH32n ads

s AuS2O332 aq CuNH3n aq rate determining step;RDS 42

According to the well known electrochemical kineticmodel (Nicol, 1993; Pesic and Sergent, 1993) the rate of

anodic (a) and cathodic (c) reactions are dependent on

the relevant electrochemical rate constants (ka and kc)and the concentration of the electroactive species. In the

present situation the concentration of the electroactive

species can be represented by h; this leads to Ra kahand Rc kch. However, the anodic and cathodic reac-tions take place at the same rate (Ra Rc R), leadingto Eq. (43). Thus the combination of the adsorption

theory with the rate equations reveals the relationship

between the rate (reader is referred to other references

e.g. Pesic and Sergent, 1993, for details on electro-

chemical equations related to this derivation) of gold

oxidation and the rate constants for the anodic, cathodic

and overall (kRDS) surface reaction described in Eq. (44).The substitution for h from Eq. (37) leads to Eq. (45)which shows the relationship between the rate and the

concentration of species in solution.

R2 kakch2 43

R kakc0:5h kRDSh 44


R kRDSh kRDSKadsCuIIS2O23 =f1 KadsCuII

S2O23 g 45

R kRDSKadsCuIIS2O23 kpCuII 46R kRDS 47

At low reagent concentrations Kads[Cu(II)][S2O23 ] maybe assumed to be negligibly small compared to 1 andthus 1 Kads[Cu(II)][S2O23 ] 1. This simplifies Eq. (45)to Eq. (46) which shows that the dissolution reaction is

pseudo first order with respect to [Cu(II)] at constant

[S2O23 ] and kp is the pseudo first order rate constant.

This also agrees with Fig. 12 and the rate data based on

corrosion current reported by Jiang et al. (1993) for the

oxidation of a rotating gold disc, which led to a first

order reaction with respect to [Cu(II)]. In contrast, athigher concentrations of Cu(II) and/or S2O

23 where

1 Kads[Cu(II)][S2O23 ] 1, Eq. (45) simplifies to Eq.(47). This explains why the initial rate is independent of

the concentration of Cu(II) and S2O23 at higher reagent

concentrations (Fig. 12).

4.4. Leaching of gold ore

Tables 10 and 11 describe the mineralogical constit-

uents, gold and silver grade, particle size and pulp

density of different material used by previous research-

ers, with a range of non-cyanide lixiviants and condi-tions. For example, the material used in test J12 was a

highly preg-robbing oxide ore of gold and silver grades

910 and

y = 2E-05xR2 = 0.912

y = 2E-05xR2 = 0.9977

0.0

0.2

0.4

0.6

0.8

1.0

0 10000 20000 30000 40000

Time / s

X o

r 1-

(1-X

)1/3

J12 (X)J12 (shriking sphere model)N11 (X)N11 (shrinking sphere model)

Fig. 13. Gold extraction curves with Cu(II)/NH3/S2O23 and O2/S2O

23

and applicability of the shrinking sphere model for tests N11 and J12

described in Tables 10 and 11.

y = 3E-05x

R2 = 0.964

y = 4E-05x

R2 = 0.9893

0.0

0.2

0.4

0.6

0.8

1.0

0 5000 10000 15000

Time / s

X o

r 1-

(1-X

)1/3

K1 /thiourea (X)K2 / thiocyanate (X)K3 / thiosulphate (X)K1 / shrinking sphere modelK2 / shrinking sphere modelK3 / shrinking sphere model

Fig. 14. Gold extraction curves from arsenpyrite concentrate using

Fe(III) in thiourea, thiocynate or thiosulphate media and applicability

of shrinking sphere model for Tests K1K3 described in Tables 10 and

11.

Table 11

Leaching conditions and apparent rate constants (kss and kpl) for gold extraction from different types of gold ores using non-cyanide lixiviantsa

Lixiviant/Test/Material pH T (C) [Oxidant] (mM) [Ligand] (M) [NH3]t (M) 105 kssb (s1) 105 kplc (s1)

Cu(II)/S2O23 /NH3

B8/conc. 10 60 46.5d 0.34 0.67

Z4/Mn ore 60 94 1.96 4.12

J4/gold. conc. 10 60 63 0.71 4.41

A3/gold ore 8.510.5 60 100 2 4

N11/conc. 10 25 12 17 (pH 10) 2

O2/S2O23

J11/oxide ore 11 60 e 0.1

J12/oxide ore 12 40 0.1 2

Fe(III)/S2O23

K3/FeAsS conc. 6 20 15 0.5 0.8

Fe(III)/SCN

W1/bio-oxidized 2 200 0.05 0.8

K2/FeAsS conc. 2.2 20 15 0.4 3

Fe(III)/SC(NH2)2K1/FeAsS conc. f 20 15 0.2 4

U1/oxide ore 1 20 0.14 kg/t 100 g/kg

Cu(II)/O2/NH3H1/sulphidic/refractory 200 38 5.5 1

Information on mineralogical constituents shown in Table 10.aRefs.: A3 (Abbruzzese et al., 1995); B8 (Berezowsky and Sefton, 1979); J4 (Jiang et al., 1993); J11, J12 (Ji et al., 2003); K1, K2, K3 (Kholmogorov

et al., 2002); N11 (Navarro et al., 2002); U1 (Ubaldini et al., 1998), H1 (Han, 2001). PD 20%, pO2 600 kPa.b Slope of linear relationships in Figs. 13 and 14.c Slope of linear relationship in Fig. 16.dAir sparged.e pO2 100 psig.f 0.2 M H2SO4.


showing the effect of the high copper and/or silver con-

tent of the material used in test Z4 and B8. Jiang et al.

(1993) achieved a 90% gold extraction in 4 h at 50 C,compared to

0%

20%

40%

60%

80%

100%

0 1 2 3 4 5 6 7 8

Time / hours

Gol

d E

xtra

ctio

n

J11J4Z4A3B8

W1K3H1U1

J11

W1

U1

K3

A3

J4

Z4B8

H1

Fig. 15. Gold extraction curves from different materials described in

Tables 10 and 11 using different lixiviants at 60, 20 and 200 C.

0.0

0.1

0.1

0.2

0.2

0 5000 10000 15000 20000 25000

Time / s

1-3(

1-X

)2/3

+2(

1-X

)

W1

K3

Slope 8 x 10-6

R2 = 0.98

Fig. 16. Applicability of the shrinking sphere model with product layer

for gold extraction data in Tests W1 and K3 described in Tables 10 and

11.


(Table 11). These results highlight the importance of

considering the role of solid products on gold surface

that would retard the leaching reaction.

5. Summary and conclusions

Fundamental studies are vital for the rationalizationof the complexity of gold leaching in non-cyanide

media.

Linear free energy correlations of stability constantsof Au(I) complex species with respect to relevant spe-cies of Ag(I) and Cu(I) show the stability order of

Au(I) complexes with ligands: CN > HS >S2O23 >

SC(NH2)2 >OH> SCN >NH2CH2COO

>SO23 >

NH3 >Br >Cl >CH3CN. In the case of Au(III)

complexes the stability order is CN >OH >

SCN > Br >Cl.

Species such as AuOH0, Au(OH)2 and AuCl2 can beincorporated as intermediates in EhpH and Ehlog[Cl] diagrams.

Solubility of gold metal in the presence of oxidantsdecreases in the order MgS/H2S Fe(III)/Cl >Cu(II)/Cl > Cu(II)/NH3 NaOH/H2O at a given

temperature, but increases with increasing tempera-

ture.

Solubility of gold salts follow the order NaAuCl4 >KAuCl4 >AuCl3 Au(NH3)4(NO3)2 >Au2S/H2S>Au(OH)3/NaOH>Au(OH)3.

Results from rotating disc studies in different lixiviantsat different temperatures reveal the order of rate ofgold oxidation: Br2/Br

(25 C)>Fe(III)/Cl/H2SO4(200 C)>NaOCl/Cl (20 C)>Cl2/HCl/12 C>Cu-(II)/NH3/S2O

23 (20 C) > Fe(III)/SC(NH2)2 (25 C)

>Cu(NH3)24 /NaOCl (140 C)>O2/SC(NH2)2 (30

C) >O2/S2O23 (30 C), but do not reflect the rate con-stants of the surface reactions.

Silver dissolves faster than gold and thus Ag(I) catal-yses the dissolution of gold by redox-displacement.

Whilst the surface reaction can be rationalized on thebasis of a combined adsorptionelectrochemical

model, the analysis of gold leaching in different lixivi-

ant systems shows the validity of a shrinking particle

model with apparent rate constants (kss and kpl) of theorder 105 s1.

Acknowledgements

The author gratefully acknowledges the continuous

support from Professor M.J. Nicol (Murdoch Univer-

sity), Adjunct Professor D.M. Muir (CSIRO Minerals,

Perth) and the A.J. Parker Cooperative Research Centre

for Hydrometallurgy.

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Gold leaching in non-cyanide lixiviant systems: critical issues on fundamentals and applicationsIntroductionNon-cyanide lixiviants of current interestThermodynamicsLinear correlations of stability constantsDisproportionation of Au(I) and Eh-pH diagramsSolubility of gold(I/III) saltsSolubility of gold metalEh-log[Cl-] diagram for Fe-Cu-Au-Cl-O2 systemKinetics and reaction mechanismDissolution of metallic or colloidal goldComparison between different lixiviantsSurface reaction mechanismLeaching of gold oreSummary and conclusionsAcknowledgementsReferences

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noncyanide lixiviant