Kinetics and Reaction Mechanism of Gold Cyanidation

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Hydrometallurgy 8

Short review

Kinetics and reaction mechanism of gold cyanidation:

Surface reaction model via Au(I)–OH–CN complexes

G. Senanayake *

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

Murdoch University, Perth, WA 6150, Australia

Received 25 February 2005; received in revised form 20 July 2005; accepted 3 August 2005

Available online 15 September 2005

Abstract

The current status of the mechanism of gold cyanidation based on diffusion and surface adsorption–reaction models are

reviewed. Published rate data based on chemical oxidation from flat gold surfaces in pure aerated cyanide solutions are

analysed to show a reaction order of 2.7 with respect to cyanide at low concentrations. At higher cyanide concentrations, the

reaction rate reaches a limiting value of RAu(lim)=7.3�10�6 mol m�2 s�1, independent of the cyanide concentration and

stirring rate. This chemically controlled dissolution of gold in pure cyanide solutions is considered to be different from the

widely reported cyanide or oxygen diffusion controlled dissolution of gold, depending on their relative concentrations. The

proposed reaction mechanism to rationalise this behaviour involves the formation of a heterogeneous redox transition state

(Au.H2O)2.(CN�)2–3.(O2) which produces the intermediate Au(I)(OH)(CN)� on the gold surface. Oxygen is reduced to

hydrogen peroxide which may degrade in three ways: (i) oxidize gold to produce the same gold(I) intermediates on surface,

(ii) oxidize cyanide to cyanate (iii) disproportionate to water and oxygen. The surface adsorbed Au(I) intermediate reacts with

cyanide to produce more stable Au(CN)2� in solution. The proposed surface chemical model rationalises the reaction order of

c3 at low cyanide concentrations and calculates an intrinsic rate constant of kAu=8.6�10�6 mol m�2 s�1 for gold

cyanidation by oxygen. This value is in reasonable agreement with the value of k =6.9�10�6 mol m�2 s�1 based on the

model proposed by Wadsworth et al. [Wadsworth, M.E., Zhu, X., Thompson, J.S., Pereira, C.J., 2000. Gold dissolution and

activation in cyanide solution: kinetics and mechanism. Hydrometallurgy, 57, 1–11.], which considered the mass transfer away

from the active crystalline gold surface site followed by fast charge transfer, combined with two-electron reduction of oxygen

on the gold surface.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Gold cyanidation; Reaction mechanism; Surface transition state; Kinetics; Rate constants; Gold(I) speciation

0304-386X/$ - s

doi:10.1016/j.hy

* Tel.: +61 8 9

E-mail addre

0 (2005) 1–12

ee front matter D 2005 Elsevier B.V. All rights reserved.

dromet.2005.08.002

3602833; fax: +61 8 93606343.

ss: [email protected].

G. Senanayake / Hydrometallurgy 80 (2005) 1–122

Contents

. . . . . . . 2

. . . . . . . 3

. . . . . . . 3

. . . . . . . 3

. . . . . . . 4

. . . . . . . 5

. . . . . . . 5

. . . . . . . 6

. . . . . . . 6

. . . . . . . 7

. . . . . . . 8

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Current status of reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Limitations of diffusion model due to effect of impurities . . . . . . . . . . . . . . . . . . .

2.2. Formation of surface films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3. Surface adsorption–reaction models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4. Need for further studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Gold(I) speciation: justification and importance . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Analysis of rate data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1. Levich equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2. Rate data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Reaction mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

Gold cyanidation has been reported to involve the

chemical reactions shown in Eq. (1) (Bodlander, 1896)

and Eq. (3) (Elsner, 1846), where Eq. (3) can be

treated as the sum of the two partial reactions shown

in Eqs. (1) and (2). Hydrogen peroxide produced at

the interface by reduction of oxygen can react with

gold (Eq. [(2)), or with cyanide ion (Eq. (4)), or

disproportionate to H2O+0.5O2; while cyanate pro-

duced in solution further degrades to other products.

2Auþ4CN�þO2þ2H2O¼2AuðCNÞ�2þ2OH�þH2O2

ð1Þ

2Au þ 4CN� þ H2O2 ¼ 2AuðCNÞ�2 þ 2OH� ð2Þ

4Au þ 8CN� þ O2 þ 2H2O ¼ 4AuðCNÞ�2 þ 4OH�

ð3Þ

CN� þ H2O2 ¼ CNO� þ H2O ð4Þ

Kudryk and Kellogg (1954) highlighted the impor-

tance of understanding the rate controlling factors of

gold cyanidation which would allow correct choice of

conditions such as agitation, temperature, and the

reagent concentrations. They showed the electroche-

mical nature of the gold cyanidation reaction and that

the rate is determined by the rate of diffusion of

cyanide or dissolved oxygen to the gold surface,

depending on their relative concentrations.

The theoretical and practical aspects of gold cya-

nidation have been frequently reviewed (Cornejo and

Spottiswood, 1984; Nicol et al., 1987; Li et al., 1992;

Fleming, 1992), while the fundamental aspects of gold

cyanidation reaction and their relevance to leaching of

gold from ores have been advanced by Cathro (1963),

Cathro and Koch (1964), MacArthur (1972), Nicol

(1980), Kirk et al. (1980), Dorin and Woods (1991);

Osseo-Asare et al. (1984), Lorenzen and van Deven-

ter (1992); Zheng et al. (1995), Guan and Han

(1994), Crundwell and Godorr (1997), Wadsworth

et al. (2000), Jeffrey and Ritchie (2000a,b, 2001)

and Xue and Osseo-Asare (2001). Significant pro-

gress has been made in recent years on mixed

electrochemical-transport (diffusion) models that

are capable of explaining the effect of oxygen

pressure, cyanide concentration and agitation (Li

et al., 1992; Wadsworth et al., 2000).

Despite long-term interest and industrial applica-

tion, the reaction mechanism of gold cyanidation by

oxygen is still being debated and/or investigated—

especially in the following three areas:

(i) stoichiometry and chemical or diffusion con-

trolled nature of the reaction,

(ii) nature of the passivation layer on gold surface,

(iii) effect of host minerals and impurities in the

solid state or solution.

Although a number of different leaching models

have been presented, it is difficult to categorise them

due to the fact that each model considers a number

G. Senanayake / Hydrometallurgy 80 (2005) 1–12 3

of different factors such as diffusion, adsorption,

charge transfer, surface films etc. Nevertheless, it

is important to revise these models in order to

understand the current status of the reaction mechan-

ism of pure gold in aerated cyanide solutions in the

absence of impurities in solid and aqueous phases.

This is useful to develop a surface chemical model

that would rationalise the role of metal ions in

aqueous phase, alloyed metals, and the effect of

host minerals.

This work describes a surface chemical model on

the basis of surface adsorbed species such as Au(OH)0

and Au(OH)(CN)� to rationalise the results reported

by Jeffrey and Ritchie (2001) and to compare with the

adsorption-charge transfer model proposed by Wads-

worth et al. (2000).

2. Current status of reaction mechanism

2.1. Limitations of diffusion model due to effect of

impurities

Based on electrochemical studies, Kirk and

Foulkes (1978) described gold dissolution as a reac-

tion controlled by aqueous boundary layer diffusion

(mass transfer) of cyanide and oxygen to the gold

surface. Zheng et al. (1995) showed that the rates of

gold cyanidation based on a rotating disc electrode

quartz crystal microbalance (REQCM) were lower

than the predicted values based on the Levich equa-

tion. They related this to the effect of a boundary film

on the gold surface and the impurities in gold and/or

solution. Li et al. (1992) showed how the mixed

potential theory can be successfully used to model

the cyanidation kinetics reported by Kudryk and Kel-

logg (1954) by combining charge transfer with cya-

nide ion diffusion to the surface.

Jeffrey and Ritchie (2000b) found that the mea-

sured value for the rate of gold cyanidation by oxygen

in an air saturated solution of 20 mM cyanide on the

basis of oxygen transfer and stoichiometry in Eq. (3)

was smaller than the predicted value. Moreover, Jef-

frey and Ritchie (2001) showed that pure gold has a

very low rate of dissolution in aerated ultra-pure

cyanide solutions, while the reaction is chemically

controlled rather than diffusion controlled. Thus,

they attributed some of the conflicting results on

gold cyanidation reported by previous researchers to

the different methods of stirring and the presence of

impurities in cyanide or other electrolytes—as well as

impurities in the gold discs used in kinetic studies. For

example, alloyed silver and copper (Choi et al., 1991;

Sun et al., 1996; Xue and Osseo-Asare, 2001; Breuer

et al., 2005), and even the minor amounts of lead(II)

contaminated with analytical grade sodium cyanide

and sodium perchlorate (Jeffrey and Ritchie, 2001)

can affect the rate of gold cyanidation. Both silver(I)

and lead(II) cause a positive effect on gold dissolu-

tion; but excess dissolved lead(II) (Lorenzen and van

Deventer, 1992) and silver(I) (Wadsworth and Zhu,

2003) retards the leaching kinetics.

2.2. Formation of surface films

It has been widely reported that the anodic oxida-

tion of gold in cyanide media is initiated by the

adsorption of cyanide onto the gold surface, while

passivation and the peaks in polarization curves

have been attributed to the formation of adsorbed

gold(I) and gold(III) species such as Au(I)(CN)0,

Au(I)(OH)0, Au(I)(OH)(CNx), Au(III)(OH)(CN)3� and

Au(III)(OH)30 (Cathro and Koch, 1964; Kirk et al.,

1980; Nicol et al., 1987; Wadsworth et al., 2000; Xue

and Osseo-Asare, 2001). It is widely accepted that the

cyanidation of gold is a slow process due to passiva-

tion of the gold surface by a film of AuCN (Cathro

and Koch, 1964; MacArthur, 1972; Nicol, 1980;

Zheng et al., 1995; Jeffrey and Ritchie, 2001). The

anodic passivation of gold can also be a result of the

adsorbed hydroxide and the formation of AuOH on

the surface (Kirk et al., 1980). Supporting the views of

Nicol et al. (1987), Jeffrey and Ritchie (2001) pro-

posed the formation of a chain-like film of AuCN to

account for the low rates of cyanidation of gold in the

absence of impurities. While the dissolution of AuCN

only occurs at the chain ends, the presence of impu-

rities such as lead ions in the cyanide solutions accel-

erates this process.

Nicol et al. (1987) noted the difficulties in obser-

ving passivation of gold in aerated/oxygenated plant

liquors, despite electrochemical evidence for an

adsorbed layer of AuCN, because the liquors contain

trace heavy metal ions that disrupt the formation of

AuCN. Guan and Han (1994) failed to identify the

species that caused passivation, because it was an


unstable intermediate. Yet, Crundwell and Godorr

(1997) reported the dominance of a passivating

layer of AuCN on the gold surface at latter stages

of batch leaching experiments. They presented a

kinetic model using batch leaching data based on

half order reaction rates with respect to cyanide and

oxygen.

2.3. Surface adsorption–reaction models

According to the widely accepted combined diffu-

sion–adsorption–oxidation model, the cyanidation of

gold follows the five steps described by Eqs. (5)–(8),

which involve (i) the diffusion of cyanide from bulk

solution to interface denoted by b and i, (ii) surface

adsorption equilibrium, (iii) anodic oxidation, (iv)

stabilisation/desorption of surface products and (v)

diffusion of products into the bulk solution, leading

to the overall reaction given in Eq. (10).

CN�ðbÞYCN�

ðiÞ ð5Þ

AuðsÞ þ CN�ðiÞ ¼ AuðCNÞ�ðadsÞ ð6Þ

AuðCNÞ�ðadsÞ ¼ AuðCNÞ0ðadsÞ þ e� ð7Þ

AuðCNÞ0ðadsÞ þ CN�ðiÞYAuðCNÞ�2 ðiÞ ð8Þ

AuðCNÞ�2 ðiÞYAuðCNÞ�2 ðbÞ ð9Þ

AuðsÞ þ 2CN�ðbÞ ¼ AuðCNÞ�2 ðbÞ þ e� ð10Þ

A series of equations, similar to those in Eqs. (5)–(9),

can be written for the cathodic reduction of oxygen

(Hiskey and Sanchez, 1990). Juttner (1984) reported

that the two-electron reduction of O2 to H2O2 predo-

minates on Au substrate. Guan and Han (1994) con-

sidered the oxygen reduction as a two-stage process

described by Eqs. (11)–(12). More recent studies by

Wadsworth et al. (2000) also considered a two-elec-

tron reduction process and showed that dissolved

cyanide depresses the rate of oxygen reduction on a

gold surface.

O2 þ H2O þ 2e� ¼ HO�2 þ OH� ð11Þ

HO�2 þ H2O þ 2e� ¼ 3OH� ð12Þ

Wadsworth et al. (2000) reviewed the early literature

in support of various adsorption–reaction paths of

gold dissolution. They proposed the involvement of

two (or more) gold atoms as active sites shown by

Au2(s) in Eqs. (13)–(15), and bridging cyanide ions in

the rate-determining step, leading to higher reaction

orders up to 3 with respect to cyanide. Based on this

model, incorporating a two-electron reduction of oxy-

gen on gold surface, they derived the rate equation

given by Eq. (17).

Au2ðsÞ þ 2CN� ¼ Au2ðCN�Þ2ðadsÞ ð13Þ

Au2ðCN�Þ2ðadsÞ þ CN� ¼ Au2ðCN�Þ3ðadsÞ ð14Þ

Au2ðCN�Þ3ðadsÞ ¼ AuCN�ðadsÞ þ AuðCN�Þ2ðadsÞ

� ðrate determining stepÞ ð15Þ

AuCN�ðadsÞ þ CN�

ðadsÞ

¼ AuðCNÞ�2 þ e�ðfast charge transfer stepÞð16Þ

RAu ¼ kaUtot½CN��3 =f1 þ K½CN��3g ð17Þ

where ka= rate constant for the anodic reaction (mol

m�2 s�1),Utot = total number of adsorption sites shown

by Au2(s) (bare), Au2(CN�)2(s) and Au2(CN

�)3(s) in

Eqs. (13)–(14), and K is the product of equilibrium

constants of Eqs. (13) and (14).

Xue and Osseo-Asare (2001) used the mass trans-

fer law and the Butler–Volmer equation to show that

the reaction order with respect to cyanide concentra-

tion would depend on the rate-determining step. For

example, a slow rate controlling discharge step shown

by the forward reaction of Eq. (18) would give a

reaction order of 1, as observed by Xue and Osseo-

Asare (2001) and Guan and Han (1994). In contrast,

an equilibration of the discharge step in Eq. (18),

followed by the forward reaction of the adsorbed

species with cyanide shown in Eq. (8), will give rise

to an overall reaction order of 2 with respect to

cyanide concentration.

AuðsÞ þ CN�ðbÞ ¼ AuðCNÞ0ðadsÞ þ e� ð18Þ

Table 1

Stability constants of gold(I) and silver(I) complexes

Complex Ionic strength log ba

Ag(CN)2� 1(NaClO4) 20.1

Ag(OH)(CN)� 1(NaClO4) 12.8

Ag(OH)2� 0 3.6 (4.2 at 18 8C, 0.2 KNO3)

Ag(OH)0 0 (or dil) 2.3 (3.9)

Au(CN)2� 0.025(KCN) 36.6, 38.3

Au(OH)(CN)� 23.3b

Au(OH)2� 22

Au(OH)0 10.2, 20.6

Au(CH3CN)2+ 3.1

Au(CH3CN)(OH)0 10.7

a At 25 8C, Hogfeldt (1982); Sillen and Martell (1964); Kissner

et al. (1997); Stefansson and Seward, 2003; Nicol et al., 1987.b See text.


2.4. Need for further studies

It is important to extend these models, which have

been largely originated from electrochemical studies,

to develop a surface chemical reaction model for the

dissolution of gold in oxygenated cyanide solutions.

Wadsworth et al. (2000) showed how the calculated

rates based on anodic and cathodic currents of gold

oxidation and oxygen reduction combined with the

adsorption-charge transfer model described in Eqs.

(13)–(17) were in good agreement with the measured

data using rotating discs in aerated alkaline cyanide

solutions. A model that can be used to compare and

contrast the results reported by previous researchers,

and to rationalise the effect of solid state and solution

impurities, would also need to consider the following

issues, described in the present study.

(i) intermediate gold(I) species involved in the sur-

face reaction with oxygen and cyanide

(ii) a reaction mechanism involving the simulta-

neous reaction of gold and oxygen on gold

(iii) a rate constant for the intrinsic surface reaction.

3. Gold(I) speciation: justification and importance

The reported evidence for intermediate silver(I) and

gold(I) complexes such as Ag(OH)0, Ag(OH)2�,

Ag(CN)(OH)�, Au(OH)0, Au(CH3CN)(OH)0, Au(CH3

CN)2+ and Au(OH)2

� (Table 1) shows the possible for-

mation of intermediates such as Au(OH)0 and

Au(CN)(OH)� in addition to Au(CN)0 in surface

reactions during cyanide leaching of gold. Previous

studies (Finkelstein and Hancock, 1974; Senanayake

et al., 2003) highlighted problems associated with

measuring the stability constants of gold(I) com-

plexes and showed the importance of the following

relationship between the standard reduction poten-

tials of Au(I)/Au(0) and Ag(I)/Ag(0) redox couples:

EofAuðIÞ=Auð0Þg¼ 1:79E8fAgðIÞ=Agð0Þg þ 0:236

ð19Þ

This relationship can be used to predict the stability

constants of unstable gold(I) complexes by using the

published (Hogfeldt, 1982; Sillen and Martell, 1964)

values of stability constants of Ag(I) (Table 1). For

example, the stability constant b{Ag(CN)(OH)�}=1012.8 corresponds to E8{Ag(CN)(OH)�/Ag}=0.044V, based on E8{Ag+ /Ag}=0.799 V at 25 8C and

Eq. (20).

E8fMðCNÞðOHÞ�=Mg¼E8fMþ=Mg�0:059 logbfMðCNÞðOHÞ

�g

ð20Þ

For gold, this corresponds to E8{Au(CN)(OH)�/Au}=0.314 V and b{Au(CN)(OH)�}=1023.3 based

on Eqs. (19) and (20) and E8{Au+ /Au}=1.69 V at

25 8C. The value of 1023.3 for b{Au(CN)(OH)�} is ofthe same order as one of the values (1020.6) reported

for b{Au(OH)0}. However, both values are fifteen or-

ders of magnitude smaller than 1038 for b{Au(CN)2�}

(Table 1). Of the two values reported for b{Au(OH)0}in the literature (1010.2, 1020.6), the lower value shows

a better fit to the linear relationship with stability

constants of other gold(I) complexes (Senanayake,

2004). Thus, Au(OH)(CN)� would seem to be a

more plausible intermediate compared to Au(OH)0.

In the case of thiosulphate leaching of gold, the

intermediate complex Au(NH3)(S2O3)� has a stability

constant of 1020 compared to 1024 for the stable com-

plex Au(S2O3)23�. Yet, the rate of anodic oxidation of

gold in ammoniacal thiosulphate can be modelled on

the basis of the formation of Au(NH3)(S2O3)� as an

intermediate (Senanayake, 2005). Kissner et al. (1997)

noted that the high stability of Au(CN)2� is due to the

moderate basicity, minor hardness and k-acceptor cap-ability of cyanide ligand, while hydroxide, which is a


hard and strongly basic ligand, is a good donor for Au(I).

They also noted that OH� and NH3 are basic and strong

j-bond donors which should coordinate in the same

way with Au(I). These views are supported by the

stability constants reported in Table 1. Thus, it is rea-

sonable to analyse the rate of gold oxidation on the

basis of formation of reaction intermediates such as

Au(OH)0 and Au(CN)(OH)� adsorbed onto the gold

surface. However, they are eventually converted to the

more stable Au(CN)2� with a higher stability constant

of 1038.

Support for such adsorbed species also come from

electrochemical/nanobalance studies. Jeffrey and

Ritchie (2001) measured the anodic polarization

curve for pure gold in a pure 20 mM cyanide solution

maintained at 25 8C. The measured current density of

0.03 A m�2 in the potential region for leaching (�0.1

V) was independent of stirring rate. Assuming that the

measured current was due to the oxidation of gold to

AuCN, they calculated a mass increase of 650 ng in a

40 min scan. This was close to the measured mass

increase of 605 ng of the rotating gold disc. However,

Fig. 1 shows these two values as well as the predicted

mass increase on the basis of formation of other gold(I)

species shown in Table 1 and other intermediates such

as Au2O (Bard, 1973). It is clear that the measured

increase in mass is much closer to the predicted values

on the basis of solids such as Au(OH)0+Au(CN)0 or

Au(OH)0 than that based on Au(CN)0 alone. Although

500

600

700

800

Predicted Gold(I)

Pred

icte

d in

crea

se in

mas

s / n

g

Au(

OH

)(C

N)-

Au(

CN

O)0

Au(

OH

)0

+A

u(O

H)(

CN

)-

Fig. 1. Comparison between measured and predicted increase in mass of a

Au(OH)(CN)�may be formed as an ionic intermediate

on the surface, it would not be an insoluble film, but

would readily desorb/dissolve as the more stable com-

plex Au(CN)2�. These results support the formation of

an insoluble film such as Au(OH), in addition to AuCN,

in dilute cyanide solution and highlights the need to

consider surface reactions on the basis of such species.

4. Analysis of rate data

4.1. Levich equation

The Levich (1962) equation (Eqs. (21)) has been

widely used for the interpretation of rate data based

on the electrochemical or chemical dissolution of

metal from a rotating disk, under diffusion controlled

conditions:

JX ¼ 0:62D2=3X x1=2t� 1=6½X � ð21Þ

where, JX=flux of reactant (molm�2 s�1);x =rotation

rate of the disc=rpm. 2k / 60 (s�1); t =kinematic visco-

sity (0.89.10�6 m2 s�1 for water); DX =diffusion coef-

ficient of X(m2 s�1); [X]=concentration of X(mol m�3

or mM).

Thus, the pseudo first order dependence of the rate

of gold dissolution with respect to cyanide concentra-

tion and oxygen pressure could be the result of a

species formed on anode

Au(

CN

)0

Au(

OH

)0

Au(

CN

)0 +A

u(O

H)0

Au 2

O

Mea

sure

d in

crea

se in

mas

s

gold anode due to passivation by different gold(I) species (see text).


chemically or diffusion controlled surface reaction

(Sun et al., 1996; Guan and Han, 1994; Choi et al.,

1991). For example, at steady state, the rate of diffu-

sion is equal to the rate of surface reaction (Leven-

spiel, 1972). Thus, the Levich equation given in the

form of Eqs. (22) can be used to compare the rate data

obtained under diffusion controlled conditions, where

JAu(I) represents the flux in each case with m being the

relevant stoichiometric factor that should satisfy the

overall mass balance. For example, the relevant values

in the case of Eqs. (1) are m =2 for CN� and 0.5 for

O2 whereas for Eqs. (3), the relevant values are m =2

for CN� and 0.25 for O2.

logfRAuðIÞg ¼ logfJAuðIÞg¼ logf0:62mt� 1=6ðDX Þ2=3x1=2g þ log½X �

ð22Þ

4.2. Rate data

Jeffrey and Ritchie (2001) used a rotating electrode

quartz crystal microbalance to determine the progress

of cyanidation of freshly plated gold at 25 8C, in

aerated solutions of different cyanide concentrations

(pHc10) at a disc rotation speed of 300 rpm (x1/2=

5.61 s�1/2). They have also reported results obtained

using 4 different rotation rates, while the concentra-

y = 2.74x - 7.3

R2 = 1.00

-7.5

-6.5

-5.5

-4.5

0 0.4

log {[CN-

{log

RA

u /m

ol m

-2 s

-1}

Jeffrey and Ritchie, 2001Wadsworth et al., 2000

Fig. 2. Log–log plots of gold oxidation rate vs. [CN�]free at 25 8C. Data f(pH 10.5).

tion of cyanide was maintained constant at 20 mM.

The rate of gold dissolution was determined by mea-

suring the loss of mass with time. Wadsworth et al.

(2000) used a rotating gold disc in cyanide solutions

maintained at pH=10.5 and 300 rpm. The progress of

reaction was measured by determining dissolved

Au(I) in solution using ICP analysis. The results

were in excellent agreement with the results reported

by Jeffrey and Ritchie (2001). However, since the pH

used by the two groups were different, the concentra-

tion of free cyanide [CN�]free was calculated in the

present study using [CN�]total and pKa (HCN). A plot

of [CN�]free vs. [CN�]total gave a slope of 0.9 con-

firming that only a small fraction of cyanide was in

the form of HCN.

Fig. 2 shows a log–log plot of RAu vs. [CN�]free.

The slope at low cyanide concentration is 2.7. There-

fore, it is clear that rates at low cyanide concentrations

do not follow Eq. (22). This indicates that the rate is

controlled by a surface chemical reaction that involves

two to three cyanide ions. This behaviour of pure gold

in pure dilute cyanide solutions is different from

results reported previously which indicate that gold

cyanidation was generally first order with respect to

cyanide concentration, oxygen partial pressure and

square root of disc rotating speed with a low activa-

tion energy Ea=22–29 kJ mol�1 (Sun et al., 1996;

Guan and Han, 1994; Choi et al., 1991). By contrast,

7

0.8 1.2 1.6

]free / mol m-3}

rom Jeffrey and Ritchie, 2001 (pH=10) and Wadsworth et al., 2000


the higher activation energy of Ea=47–55 kJ mol�1

(Thurgood et al., 1981) for oxidation of a pure gold

anode in cyanide solutions, or Ea=47F2 kJ mol�1

for pure gold cyanidation by oxygen (Jeffrey and

Ritchie, 2001), as in the present example, supports a

chemically controlled reaction.

The value of RAu reaches a limiting value RAu(lim)

at higher concentrations of [CN�] (Fig. 2). According

to the mixed potential theory, the limiting rate at

higher cyanide concentrations is a result of the reac-

tion being controlled by oxygen diffusion to the inter-

face, so that the anodic dissolution rate of gold is

matched by the cathodic reduction rate of oxygen.

Thus, the oxygen diffusion, which is enhanced at

higher rotation rates, should result in an increase in

RAu with increasing x1/2. This is not observed, as

noted by Jeffrey and Ritchie (2001), and the limiting

rate remains independent of rotation rate. Therefore, it

I II III IV V

OH2

Au OH2 O O

OH2 O OAu Au(OH)0

Au(OH)0

OH2

CN-

Au Au(OH)(CN)-

OH2 O O

OH2 O OAu Au(OH)0

OH2

CN-

Au Au(OH)(CN)-

OH2 O O

OH2 O OAu Au(OH)(CN)-

CN-

Fig. 3. Formation of Au(I) intermediates in surface reaction with O2 and (a

cyanide, (III) oxygen, (IV) adsorbed Au(I) species after reaction, (V) hyd

is important to consider a reaction mechanism that can

describe this behaviour of gold.

5. Reaction mechanism

It is possible to consider the chemical dissolution of

gold in oxygenated cyanide solutions as a reaction that

involves the simultaneous reduction of oxygen and

oxidation of gold as shown in Fig. 3. The three cases

shown in Fig. 3 consider the involvement of 2 gold

atoms per oxygen molecule and (a) reaction without

the involvement of cyanide producing Au(OH)0, (b)

reaction with only one cyanide ion producing

Au(OH)0+Au(OH)(CN)�, and (c) reaction with two

cyanide ions producing Au(OH)(CN)�. Eqs. (23)–(26)

consider only the case described in Fig 3b because it

is representative of the other two extreme cases.

H(a) cyanide not involved

H

H (b) cyanide involved in one siteH

H(c) cyanide involved in both sites

H

) 0; (b) 1; (c) 2 CN� ions (I) gold surface, (II) adsorbed water and/or

rogen peroxide.


Surface equilibration–redox reaction

2pAuðsÞ þ 2H2O þ CN� þ O2

¼ 2pAuðH2OÞ:ðCN�Þ:ðO2Þads ð23Þ

2pAuðH2OÞ:ðCN�Þ:ðO2Þads¼ pAuðOHÞads þ pAuðOHÞðCN�Þads þ H2O2

ð24Þ

Desorption/stabilisation

pAuðOHÞads þ CN� ¼ pAuðOHÞCNÞ�ads=aq þ OH�

ð25Þ

pAuðOHÞðCN�Þads=aq þ CN� ¼ p þ AuðCNÞ�2þ OH�: ð26Þ

The equilibration shown in Eq. (23) can be con-

sidered as the formation of a surface adsorbed transi-

tion state followed by the redox reaction which

produces the adsorbed gold(I) species and hydrogen

peroxide. The two unstable species Au(OH)0 and

Au(OH)(CN)� formed on the surface react with cya-

nide to produce more stable Au(CN)2� in solution as

shown in Eqs. (25)–(26), while hydrogen peroxide

can react in the manner described before. The reaction

model in Fig. 3 shows a maximum of 2CN� ions

involved in the surface reaction mechanism. It is

important to rationalise the reaction order of 2–3

demonstrated by the slope of linear relationship in

Fig. 2 at low cyanide concentrations. Thus, a general

form of the surface reaction is shown in Eqs. (27)–

(28), which involves nCN� ions. The equilibrium

constant for Eqs. (23), surface coverage (h), and the

rate expression are given by Eqs. (29)–(31).

General surface reaction

2Au þ 2H2O þ nCN� þ O2

¼ ðAu:H2OÞ2:ðCN�Þn:ðO2Þ ð27Þ

ðAu:H2OÞ2:ðCN�Þn:ðO2ÞYProducts ð28Þ

Kads ¼ h=fð1 � hÞ½O2�½CN��ng ðfor Eq: ð27ÞÞ ð29Þ

h ¼ Kads½O2�½CN��n=1 þ Kads½O2�CN��n ð30Þ

RAu ¼ kAu� ð31Þ

RAu ¼ kAuKads½O2�½CN��n=f1 þ Kads½O2�½CN��ng

ðfrom Eqs:ð30Þ; ð31ÞÞ: ð32Þ

At lower values of [O2][CN�]n

f1þ Kads½O2�½CN��ngc1 ð33Þ

Then,

RAu ¼ kAuKads½O2�½CN��n ðfrom Eq: ð32ÞÞ: ð34Þ

General rate equation

½CN��n ¼ kAuKads½O2�ðRAuÞ� 1 � Kads½O2�ðfrom Eq: ð32ÞÞ: ð35Þ

In the case of solutions of low cyanide concentra-

tions, Eq. (32) simplifies to Eq. (34). This explains the

value of n between 2 and 3 obtained as the reaction

order with respect to cyanide at low concentrations

(Fig. 2). Moreover, Eq. (32) can be rearranged to Eq.

(35) that can be used to plot [CN�]�n vs. (RAu)�1 to

obtain values for Kads and kAu. Fig. 4a shows the

relevant plots for n =1 and 2 while Fig. 4b and c

represent n =3 and n =4 respectively. In all cases,

the dissolved oxygen concentration can be considered

as c0.25 mM, because the experiments were carried

out in air saturated solutions. The best linear relation-

ship is given by n =3 (Fig. 4b) with slope of 3�10�8

and intercept in the range �3.3�10�3 to �3.7�10�3 for the two data sets reported by Jeffrey and

Ritchie (2001) and Wadsworth et al. (2000). The value

of kAu=slope/(-intercept)=8.6�10�6 mol m�2 s�1

based on the average intercept is close to the limiting

rate of RAu(lim)=7.3�10�6 mol m�2 s�1 calculated

from the plateau in Fig. 2 described in Section 4.

Thus, the limiting rate of gold cyanidation in pure

impurity-free solutions at high cyanide concentra-

tions is approximately equal to the intrinsic rate

constant. The slope showing a reaction order of 2.7

indicates the predominant reactions represented by

Eqs. (36)–(38), which involve 2 or 3 cyanide ions in

0

0.2

0.4

0.6

0.E+00 2.E+06 4.E+06 6.E+06(RAu)

-1

{[C

N- ] fr

ee}-n

n = 1

n = 2

Solid lines: Jeffrey and Ritchie, 2001Dashed lines: Wadsworth et al., 2000

y = 3E-08x - 0.0033

R2 = 0.9999

y = 3E-08x - 0.0037

R2 = 0.9998

0

0.2

0.E+00 2.E+06 4.E+06 6.E+06(RAu)

-1

{[C

N- ] fr

ee}-n

Solid line: Jeffrey and Ritchie, 2001Dashed line: Wadsworth et al., 2000

n = 3

0

0.01

0.02

0.03

0.04

0.05

0.E+00 2.E+06 4.E+06 6.E+06(RAu)

-1

{[C

N- ] fr

ee}-n

Solid line: Jeffrey and Ritchie, 2001Dashed line: Wadsworth et al., 2000

n = 4

(a)

(b)

(c)

Fig. 4. Plot of {[CN�]free}�n vs. (RAu)

�1 using data from Fig. 2 to examine the validity of Eq. (35). (a) n =1 or 2, (b) n =3, (c) n =4.



the transition state. The intermediate Au(OH)(CN)�

formed at the interface will rapidly react with cya-

nide to produce the most stable Au(CN)2�.

ðAu:H2OÞ2:ðCN�Þ2:ðO2Þ ¼ 2AuðOHÞðCNÞ�þH2O2

ð36Þ

ðAu:H2OÞ2:ðCN�Þ3:ðO2Þ¼ AuðCNÞ�2 þ AuðOHÞðCNÞ� þ H2O2 þ OH�

ð37Þ

ðAu:H2OÞ2:ðCN�Þ3:ðO2Þ ¼ 2AuðOHÞðCNÞ�

þ CNO� þ H

2O ð38Þ

Wadsworth et al. (2000) reported the two values for

the terms kaUtot=6.9�10�6 and K =5.3�10�3 in

Eq. (17), based on an electrochemical model which

involved the anodic oxidation of gold and cathodic

reaction of oxygen via reactions in Eqs. (11)–(16).

The slight differences in rate constants determined in

this work (8.6�10�6 mol m�2 s�1) and from their

model (6.9�10�6 mol m�2 s�1) may be partly attrib-

uted to the use of free cyanide concentration in the

present analysis. Nevertheless, the good agreement in

rate constants supports the validity of both models and

highlights the possibility of extending the surface

chemical model to rationalise the cyanidation kinetics

of silver and gold alloys, which will be presented in

future communications.

6. Summary and conclusions

! The rate of chemical dissolution of gold in pure

cyanide solutions is controlled by the surface che-

mical reaction between gold, cyanide and oxygen

via a transition state (Au.H2O)2.(CN�)2–3.(O2).

! The simultaneous reaction produces the surface

intermediate Au(OH)(CN)� while oxygen is re-

duced to hydrogen peroxide.

! The intermediate Au(OH)(CN)� reacts with cyanide

ions to produce more stable Au(CN)2� in solution.

Hydrogen peroxide may react with gold or cyanide,

or disproportionate to oxygen and water.

! The reaction order for cyanidation at low cyanide

concentrations is close to 3.

! At higher cyanide concentrations, the reaction rate

approaches a limiting value of RAu(lim)=7.2�10�6

mol m�2 s�1 which is close to the intrinsic rate

constant kAu=8.6�10�6 mol m�2 s�1.

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Documents

Kinetics and Reaction Mechanism of Gold Cyanidation