Electrochemical deposition of silver and gold from cyanide leaching solutions.pdf

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Electrochemical deposition of silver and gold from cyanide

leaching solutions

V. Reyes-Cruz, C. Ponce-de-Leon, I. Gonzalez *, M.T. Oropeza

Departamento de Quı mica, Universidad Auto noma Metropolitana-Iztapalapa, Apdo. Postal 55-534, C.P. 09340 Me xico D.F., Mexico

Received 20 March 2001; received in revised form 14 November 2001; accepted 4 June 2002

Abstract

A systematic voltammetric study developed in this work allows the determination of the potential range at which the

selective deposition of gold and silver is carried out in the presence of a high content of copper. As a first approach, laboratory

solutions prepared with a high content of cyanide and copper and low values of gold and silver are used; later, the methodology

is applied to leaching solutions of industrial origin.

The chemical speciation and microelectrolysis studies showed that copper deposition occurs at more negative potentials than

deposition of gold and silver. Also, the voltammetric study of a cyanide solution containing low concentrations of Au(I) and

Ag(I), free of and with high concentration of Cu(I) was carried out. The study shows the potential range at which Au(I) and

Ag(I) are reduced despite the high concentration of the Cu(I) ions. The deposition of gold and silver was not interfered with by

the high concentration of Cu(I) ions when the leaching solution was electrolyzed in a laboratory electrochemical reactor FM01-LC with a reticulated vitreous carbon (RVC) cathode.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Silver cyanide; Gold cyanide; Copper cyanide; Electrochemical recovery; Cyanide leaching; RVC electrode; Electrodeposition

1. Introduction

Cyanide leaching processes have been used by the

mining industry for over 100 years in the extraction of

noble metals (Kordosky et al., 1992; Fleming, 1992).Minerals from which most of the noble metals are

obtained are increasingly poorer (gold and silver

concentrations in the order of 1– 5 ppm), so the

leaching solutions from these processes do contain

Au(I), Ag(I) and Cu(I), with copper in concentrations

typically over a hundred times higher than the other

two metals. This high concentration of Cu(I) might

interfere during the selective recovery of gold and

silver (Jha, 1984), and the use of ion exchange resins

or activated carbon to remove the Cu(I) ion is often

necessary to obtain a concentrated solution of Au(I)and Ag(I). The following stage after the removal of

interfering ions is the recovery of noble metals. Two

traditional methods have been used: cementation and

electrolysis (Fleming, 1992; Jha, 1984). The electrol-

ysis method has been used in concentrated solutions

in electrochemical reactors with plane electrodes (e.g.

electrodeposition of copper or zinc); however, with

dilute solutions, the efficiency is low (Jha, 1984;

Reyes, 1998). Works reported in the literature have

shown that the high surface area found in porous

0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 3 0 4 - 3 8 6 X ( 0 2 ) 0 0 0 8 3 - X

* Corresponding author. Fax: +52-5804-4666.

E-mail address: [email protected] (I. Gonzalez).

www.elsevier.com/locate/hydromet

Hydrometallurgy 65 (2002) 187–203



electrodes increases the efficiency when used with

diluted solutions (Bennion and Newman, 1972;

Coeuret, 1976; Coeuret et al., 1976; Trainham and

Newman, 1977; Olive and Lacoste, 1980; Coeuret andStorck, 1981). Most gold plants actually use stainless

steel wool as cathode. The recovery of Au(I) and

Ag(I) at concentrations between 11 and 30 ppm has

been recently studied (Waterman et al., 1984; Reyes et

al., 2002; Stavart et al., 1999). However, the recovery

of Au(I) and Ag(I) from solutions containing high

concentration of Cu(I) in cyanide solutions has

received less attention.

Since the metal with the highest concentration

would be expected to be the first deposited, the

presence of large amount s of Cu(I) is likely to

influence the selective recovery of gold or silver.

Preliminary studies performed on stainless steel 304,

vitreous carbon and Ti showed that vitreous carbon is

the most adequate material for performing Ag(I)

deposition from a diluted silver solution (11 ppm)

with a high content of copper (Reyes, 1998).

The solution used in this preliminary study (0.1

M Cu(I), 0.5 M CN and 10 4 M Ag(I)) differs

significantly from the solutions from industrial

leaching processes with cyanides; nevertheless, the

conditions of high copper and cyanide concentra-

tions allowed the realization of an electrochemicalstudy on a vitreous carbon electrode where the

recovery of silver was shown to be possible without

the deposition of copper, despite the fact that the

concentration of Cu(I) was 100 times higher than the

concentration of silver (Reyes et al., 2002). The

study showed that the Cu(I) deposited at potentials

more negative than Ag(I) and that the cyanide

species of Cu(I) modified the capacitive current

densities of the media (CN ), making favorable

the deposition of silver. On the other hand, in this

study, a systematic methodology is established todetermine the potential range at which the selective

silver deposition takes place. For that reason, it

confronts the problem of faradaic current density

response from the reduction process of a low silver

concentration (11 ppm) masked by the capacitive

current of cyanide. Thus, a part of information of a

previously performed study (Reyes et al., 2002) is

taken up again and the same methodology is applied

in order to study the model solutions containing (a)

Au(I) and (b) Au(I) with Ag(I) in the presence and

absence of Cu(I) (laboratory solutions). This allows

later analysis validation of the results obtained in the

study of an industrial leaching solution containing

low concentrations of copper and cyanide and muchlower concentrations of noble metals (of the order of

1–5 ppm).

Firstly, speciation and voltammetric studies of a

cyanide solution with a high concentration of Cu(I)

and low concentrations of Au(I) (laboratory solution)

were carried out. The purpose of this study was to find

out whether the high concentration of copper inter-

feres with the selective deposition of gold. After this

experiment, a voltammetric study of the cyanide

solution containing the three metallic ions (high

Cu(I) concentration and low concentrations of Au(I)

and Ag(I), laboratory solutions), was carried out in

order to determine if the interaction between the three

metal ions changes the potential range at which the

selective reduction of Au(I) and Ag(I) occurs. After-

wards, the same methodology was applied to the

industrial leaching solution.

Once the potential range of the Au(I) and Ag(I)

reduction was established, an electrolysis of the cya-

nide leaching solution containing the three metallic

ions was carried out in an electrochemical FM01-LC

laboratory reactor.

2. Experimental

A 100-ml Pyrex cell with a three-electrode system

and nitrogen inlet was used in the voltammetric study.

The working electrode was a vitreous carbon rod with

geometric area of 0.07 cm2. The electrode surface was

polished with 0.3-Am alumina powder/water and

rinsed with deionised water followed by ultrasonic

bath for 5 min to remove traces of alumina, grease and

a final rinse with deionised water. The reference andcounter electrodes were saturated calomel and graph-

ite, respectively. The solutions were:

Laboratory solutions for methodology validation

(a) Cyanide solution 0.5 M KCN

(b) Cyanide solution

of Cu(I)

0.1 M CuCN and

0.5 M KCN

(c) Cyanide solution

of Au(I)

10 4 M KAuCN2 and

0.5 M KCN

V. Reyes-Cruz et al. / Hydrometallurgy 65 (2002) 187–203188



All solutions except the industrial leaching solution

were prepared with deionised water 18 MV 1 cm 1

and analytical grade reagents. The pH was held

constant at 10 for laboratory solutions and the solu-

tions were deoxygenated with nitrogen for 10 min

prior to cyclic voltammetry experiments. A flux of nitrogen previously saturated in a CN solution was

held on the surface of the solution during the experi-

ments without disturbing the electrolyte.

The electrolysis of the selective reduction of Au(I)

and Ag(I) in the presence of high concentrations of

Cu(I) was carried out using an industrial leaching

solution on a filter press electrochemical reactor,

FM01-LC. A full description of this reactor has been

outlined in the literature (Brown and Plecher, 1992;

Brown and Plecher, 1993; Brown and Plecher, 1994).

A cationic NafionR

membrane NX550 divided theanodic and cathodic reactor compartments. The cath-

ode used was a reticulated vitreous carbon (RVC)

electrode with 60 pores per inch (ppi), filling a

volume of 31 cm3 (Electrosynthesis). Copper plates

glued by conductive carbon paste made the electrical

contact with the RVC electrode. The anode was an

A304 stainless steel plate.

The industrial solution was re-circulated through

the anolyte and catholyte compartments using two

centrifugal pumps obtained from Cole Palmer. A

potentiostat EG and G model PARC 273A with a

Booster KEPCO (0–20 A and 0–20 V) and the M270

software were used in the voltammetric and macro-

electrolysis experiments. The determination of totalAu(I), Ag(I) and Cu(I) concentrations was carried out

by atomic absorption spectroscopy (AAS) (Varian

model Spectr AA-20).

3. Results and discussion

3.1. Laboratory solutions

3.1.1. Chemical speciation study

A chemical speciation study was performed in order

to establish the nature of the electroactive species of

gold, silver and cop per in cyanide solutions. A Pour-

baix type diagram (Fig. 1) was constructed using the

thermodynamic data for soluble and insoluble species

of gold, silver and copper reported in the literature

(Ringbom, 1963; Zhang et al., 1997; Caroli and

Sharma, 1978), taking into account the laboratory

solution conditions ( pCN V=log[CN V] = 0.222 ,

pAu V= log[Au V(I)]= 4, pAg V= log[Ag V(I)]=4 and

pCu V(I)= log[Cu V(I)] = 1) using the method proposed

by Rojas et al. (1991, 1993). The M V(I) corresponds to

the gener alized chemical species of the metallic ion insolution (Rojas et al., 1991, 1993).

The full lines in Fig. 1 represent the conditional

potential change of the system M V(I)/M(0) with pH.

The dotted lines represent the limit of the pH range

where a particular metal ion species predominates

(high molar fraction) with respect to the other

cyanide species of the same metallic ion (generalized

chemical species), or in the other case, the solubility

limit for CuCN(s). Fig. 1 shows that under the

chemical conditions of leaching solutions, there are

four copper, two silver and one gold species of cyanide.

Fig. 1 shows that, at any pH, the redox potential

of the couple Cu V(I)/Cu(0) has more negative values

with respect to the potential of the couples Au V(I)/

Au(0) and Ag V(I)/Ag(0). These results indicate that

under thermodynamic considerations, the copper

species should not interfere with the selective reduc-

tion of Au(I) and Ag(I) under experimental condi-

tions of laboratory solution used to validate the

methodology.

(d) Cyanide solution

of Au(I) and Cu(I)

0.1 M CuCN,

10 4 M KAuCN2

and 0.5 M KCN

(e) Cyanide solution

of Au(I),Ag(I) and Cu(I)

0.1 M CuCN,

10

4 M KAuCN2,10 4 M AgNO3

and 0.5 M KCN

Industrial leaching solution

(f) Industrial leaching

solution

0.025 M (400 ppm)

free CN ,

8 10 3 M

(500 ppm) Cu(I),

5 10 6 M

(1 ppm) Au(I),

3 10 5 M

(3 ppm) Ag(I),

0.015 M (615 ppm) Ca,

0.025 M (575 ppm) Na

and 3 10 4 M

(17 ppm) Zn at pH 11

V. Reyes-Cruz et al. / Hydrometallurgy 65 (2002) 187–203 189



The calculated potentials associated with the elec-trochemical reactions Au V(I)/Au(0), Ag V(I)/Ag(0) and

Cu V(I)/Cu(0) at pH = 10, pCN V= 0.22, pAu V= 4,

pAg V=4 and pCu V =1 are:

AuðCNÞ2 þ 1e ! Au0 þ 2CN

E j ¼ 1060 mV vs: SCE ð1Þ

AgðCNÞ23 þ 1e ! Ag0 þ 3CN

E j¼ 1198 mV vs

:

SCE ð2Þ

CuðCNÞ34 þ 1e ! Cu0 þ 4CN


It is important to note that under experimental

conditions of the laboratory solution, the difference

in E j for gold and silver is very small; this would

indicate that the selective deposition of these twometals is not easy.

3.1.2. Voltammetric study of gold

A voltammetric study of cyanide solutions with

and without metal ions was carried out on a plane

vitreous carbon electrode within the range of 100 to

1800 mV vs. SCE. In this study, the influence of

Cu(I) and cyanide on the electrochemical deposition

of gold was determined.

Fig. 2 shows a comparison of voltammetric behav-

iors of the following solutions: cyanide solution freeof metallic ions (curve a), cyanide solution with Au(I)

(curve b), cyanide solution with Cu(I) (curve c) and

cyanide solution with both Cu(I) and Au(I) (curve d).

In Fig. 2a, the cyanide solution free of metallic ions

has a high capacitive current density at potentials

lower than or equal to 1400 mV vs. SCE. The

current density is attributed to the adsorption of

cyanide on the surface of the vitreous carbon electrode

(Reyes et al., 2000, 2001; Vilchis-Carbajal et al.,

2000) (see inset in Fig. 2). Towards more negative

Fig. 1. Pourbaix type diagrams of soluble and insoluble chemical species involved in the redox couples: Au V(I)/Au(0) (-n-), Ag V(I)/Ag(0) (-E-) and

Cu V(I)/Cu(0) (-y-). The diagrams were constructed with thermodynamic data from the literature considering as non-variable the following

concentrations: 10 4 M Au V(I), 10 4 M Ag V(I) and 0.1 M Cu V(I) in 0.5 M CN . The concentrations of these chemical species represents the

composition found in cyanide laboratory solutions.




potentials, the reduction of the medium increases due

to the water reduction.

The presence of Au(I) ions in the cyanide solution

shifts the current density associated with the reduction

of water towards more negative potentials with respect

to the cyanide solution free of metallic ions (Fig. 2b

and a, respectively). Since the reduction of water does

not have an associated peak, the influence of the

electroactive species in this process was measured

by the potential at which the current density was 85

AA cm 2. In each case, this potential ( E 85 AA cm 2)

was used as a parameter for comparison. In the metal-

Fig. 2. Typical cyclic voltammograms obtained from laboratory solutions containing 0.5 M CN at pH 10, on a vitreous carbon electrode, at 25

mV s

1

. The curves show the influence of different ions: (a) free of metallic ions, (b) 10

4

M Au(CN)2

, (c) 0.1 M Cu(CN)4

3

and (d) 10

4

MAu(CN)2 and 0.1 M Cu(CN)4

3. Theinsetshowsthe voltammetricresponsesat lowcurrent densities. Thecurrent density E 85 AA cm 2 chosen for

analysis of water reduction is indicated.




free solution, E 85 AA cm 2 = 1506 mV vs. SCE,

while in the solution containing Au(I), E 85 AA cm 2 =

1658 mV vs. SCE. The presence of gold reduces the

capacitive current density, which was attributed t o thecomplexes of gold with the adsorbed cyanide (Mac

Arthur, 1972; Kirk et al., 1980; Mughogho and

Crundwell, 1996; Chrzanowski et al., 1996; Bindra

et al., 1989). In the reverse potential scan for the

solution containing Au(I) and CN , the current den-

sity was higher than in the forward scan. This is a

typical behavior of an interface modified by the

presence of a gold deposit. In the forward scan, the

gold and water were reduced on the vitreous carbon,

while in the reverse scan, they were reduced on the

gold. Due to the low concentration of Au(I) in the

cyanide solution, the cathodic current density associ-

ated with the reverse scan is directly attributed to the

reduction of water which notably increases the inter-

facial pH. In the inset of Fig. 2b, it can be seen that

during the reverse scan, the curve has an associated

constant current density in the potential range of

1050 to 400 mV vs. SCE. This observation

suggests the formation of a passive film does not allow

the gold dissolution to be seen as it has been reported

in the literature (Mac Arthur, 1972; Kirk et al., 1980;

Mughogho and Crundwell, 1996). The said passive

film was enhanced by the pH modification at theinterface. This fact does not allow the gold deposition

to be clearly evidenced; however, the above descrip-

tion of voltammetric behavior suggests the presence of

deposited gold.

The influence of the Cu(I) ion in the cyanide

solution (Fig. 2c) can be seen in the voltammetric

response as a shift of the reduction potential of water

towards more negative potentials ( E 85 AA cm 2 =

1664 mV vs. SCE), and a modification of the

capacitive current density. The latter effect can be

attributed to the fact that Cu(I) is adsorbed on thevitreous carbon surface (Reyes et al., 2000, 2001)

and competes with the adsorbed cyanide species

(Reyes et al., 2000, 2001; Vilchis-Carbajal et al.,

2000). On the other hand, there are no peaks asso-

ciated with the reduction and oxidation of copper in

the range of the potential scan studied. In a previous

work, it was shown that the reduction of Cu(I) on a

vitreous carbon electrode from a cyanide solution

begins at a potential of 2200 mV vs. SCE (Reyes

et al., 2002).

From Fig. 2d, it can be seen that the presence of

Au(I) and Cu(I) in the cyanide solution causes a

more negative potential for the reduction of water

( E 85 AA cm

2 = 1728 mV vs. SCE) with respect to Fig. 2a, b and c. This indicates that Au(I) and Cu(I)

ions in the cyanide solution have an influence on the

capacitive current densities and the reduction of water

since both metals involve species adsorbed on the

vitreous carbon surface (Reyes et al., 2000, 2001; Mac

Arthur, 1972; Kirk et al., 1980; Mughogho and

Crundwell, 1996; Chrzanowski et al., 1996; Bindra

et al., 1989). The competition between different

adsorbates on the vitreous carbon causes a modifica-

tion of the voltammetric response. In this work, we

associated such behavior with the modification of the

capacitive current densities. A more detailed study

could consider the evaluation of these capacitive

currents, however, it is out of the scope of this work.

During the reverse scan in Fig. 2d, there are two

oxidation processes: process I at 900 mV vs. SCE,

and process II at 550 mV vs. SCE. These processes

can be attributed to the oxidation of gold as reported

in the literature (Mac Arthur, 1972; Kirk et al., 1980;

Mughogho and Crundwell, 1996) or to the dissolution

of the codeposit Cu/Au (as shown below). When

Cu(I) is in solution, it is possible to observe the

oxidation process of gold, which indicates that the passive process seen in alkaline media has been

modified by the Cu(I), because the pH modification

in the interface is notably diminished. This effect is

similar to that reported for the presence of lead in the

gold oxidation process.

From the above study, it was considered necessary

to identify the potential range at which the selective

deposition of gold occurs in the presence of high

concentration of Cu(I). This was done by a voltam-

metric study, shifting the switching potential E k

towards negative values.

3.1.3. Influence of the negative switching potential

(E k )

Fig. 3 shows the voltammograms obtained by

changing the switching potential E k during the

forward potential scan for cyanide solution with

Au(I) and Cu(I). It can be seen that at E k = 1500

mV vs. SCE, there is a dissolution shoulder II (see

inset) whereas at E kV – 1700 mV vs. SCE, there is a

dissolution peak I. The current density associated with




these two processes increases as the switching poten-

tial E k is changed toward more negative potentials.

These two processes could be associated with the

oxidation of metallic gold deposited during the for-

ward scan, since in a previous work, it was demon-strated that the reduction of Cu(I) in a cyanide solution

(Au(I) free) on a vitreous carbon electrode begins at

2200 mV vs. SCE (Reyes et al., 2002). However, the

potential of the peak I shifts towards negative potential

values as the E k turns more negative; at E

kV

2000 mV vs. SCE, the current density peak in-

creases, indicating the formation of the codeposit Cu/

Au or the massive deposition of copper.

The dissolution of both the codeposit Cu/Au and

the metallic Au occurs at very similar potentials and

therefore, the identification of each dissolution proc-

ess is complex. It was not possible to identify the

exact potential range of gold deposition free of

copper, so it was decided to quantitatively analyze

the voltammetric curves. The associated charge of theoxidation processes, Qa, was determined from the

area under the voltammetric curves of solutions con-

taining Au(I) and Cu(I), Fig. 3, and plotted as a

function of E k (Fig. 4a).

In Fig. 4a, the curve of the charge Qa as a function of

E k for the cyanide solution containing Au(I) and

Cu(I) ions shows a linear correlation in the switching

potential range of 1700 z E k, z 1900 mV vs.

SCE. This linear behavior can be attributed to the gold

deposit on the vitreous carbon electrode. On the other

Fig. 3. Partial view of typical cyclic voltammograms obtained from a laboratory solution containing 10 4 M Au(CN)2 and 0.1 M Cu(CN)4

3

in 0.5 M CN at pH 10, on a vitreous carbon electrode (0.07 cm 2) at 25 mV s 1. The voltammograms were obtained at different negative

switching potentials ( E k). The inset shows the voltammetric responses at low current densities.




hand, it can be seen that at more negative switching

potentials, the linear correlation of the curve Qa vs.

E k has two different slopes; the first one at potentials between 1900 and 2000 mV vs. SCE, due to the

Cu/Au codeposit, and the second at E k more negative

than 2000 mV vs. SCE, due to the massive deposi-

tion of copper.

Since the concentration of Au(I) is very low, the

faradaic current associated with its reduction process is

lower than or similar to the capacitive current, so it is

not possible to determine the potential range at which

the deposition of gold takes place; besides, it is not

possible to distinguish the gold deposit through its

process of electrochemical dissolution since whenthere are some, the reduction of water becomes more

important modifying the interfacial pH. However, the

presence of Cu(I) in cyanide solution allows the obser-

vation of the dissolution peak of gold through the

analysis of the area under oxidation peaks at different

E k. Therefore, it was possible to determine at which

potential the codeposit of Cu/Au begins. Up to now, the

influence of Ag(I) on this system is not known and it is

necessary to study the interaction between Au(I) and

Ag(I) in the same solution with and without Cu(I).

3.1.4. Vol tammetric study of silver and gold

Fig. 5 shows the voltammetric comparison of the

following solutions: cyanide free from metallic ions(curve a), cyanide with Au(I) and Ag(I) (curve b), and

cyanide with Cu(I), Au(I) and Ag(I) (curve c).

The metallic ions Au(I) and Ag(I) shift the current

density associated with the reduction of water towards

less negative potentials ( E 85 AA cm 2 = 1402 mV

vs. SCE) with respect to cyanide solution free from

metallic ions ( E 85 AA cm 2 = 1506 mV vs. SCE),

Fig. 5b and a, respectively, catalyzing the reduction

of water. This indicates that although the current

density associated with the reduction of Au(I) and

Ag(I) is not evident, either these ions modify thesurface of the vitreous carbon electrode or the

deposit of silver– gold increases the water reduct-

ion. The shift in the reduction potential of water is

more important when there is Au(I) with Ag(I) than

when there is only Au(I) (Figs. 5b and 2b, respec-

tively), showing that silver is responsible for such

behavior.

In the inset of Fig. 5, it can be seen that the

capacitive current density of cyanide solution free

from metallic ions is similar to the current of the

Fig. 4. Oxidation charge (Qa) associated with the dissolution peaks as a function of the negative switching potential ( E k) from the

voltammograms obtained on a vitreous carbon electrode (0.07 cm 2) at 25 mV s 1 from 0.5 M CN laboratory solutions at pH 10 containing:

(a) 10 4 M Au(CN)2 and 0.1 M Cu(CN)4

3, (b) 10 4 M Ag(CN)3 2 and 0.1 M Cu(CN)4

3, (c) 10 4 M Au(CN)2 with 10 4 M Ag(CN)3

2

without Cu(CN)4 3 and (d) 10 4 M Au(CN)2

with 10 4 M Ag(CN)3 2 in the presence of 0.1 M Cu(CN)4

3.




solution containing the three metallic ions, Fig. 5a

and c. The presence of Cu(I) in the cyanide solution

with Au(I) and Ag(I), shifts the reduction of water

towards more negative potentials ( E 85 AA cm 2 =

1494 mV vs. SCE). This indicates that the Cu(I)

ion has an important effect on the reduction of water and on the process of cyanide and gold adsorption.

On the other hand, the reduction of gold and

silver is masked by the capacitive current densities

(see inset of Fig. 5b and c). However, in the reverse

scan, there is only one oxidation peak at 600 mV

vs. SCE in both solutions (Fig. 5b and c). This

behavior could be attributed to the Ag/Au codeposit,

since this oxidation peak is present at more positive

potentials than the oxidation peak of silver deter-

mined in the previous paper (Reyes et al., 2002) and

at more negative potential than that corresponding to

the oxidation of gold (peak II). It is important to

note that the peak I, which appears when the gold

deposit takes place in the presence of copper (Fig.

2d), is not observed in this case; this is an additional

argument for the fact that a codeposit of Ag/Au istaking place. The oxidation peak in 600 mV vs.

SCE is slightly higher when Cu(I) ion is in solution,

indicating that there is either more deposit of Ag/Au

or codeposit of copper on this new surface (see

below).

In order to establish the potential at which the

deposition of gold and silver without copper takes

place, a voltammetric study was carried out chang-

ing the switching potential E k towards negative

values.

Fig. 5. Typical cyclic voltammograms obtained from laboratory solutions containing 0.5 M CN at pH 10, on a vitreous carbon electrode, at 25

mV s 1. The curves show the influence of different ions: (a) free of metallic ions, (b) 10 4 M Au(CN)2 , 10 4 Ag(CN)3

2 and (c) 10 4 M

Au(CN)2 , 10 4 Ag(CN)3

2 and 0.1 M Cu(CN)4 3. The inset shows the voltammetric responses at low current densities and the current density

E 85 AA cm 2 chosen for the analysis of water reduction.




3.1.5. Influence of the negative switching potential

(E k )

Fig. 6 shows the voltammograms obtained when

the negative switching potential E k was changedtowards negative values for the following solutions:

cyanide solution with Ag(I) and Cu(I) (curve a), from

a previous work (Reyes et al., 2002), cyanide solution

with Au(I) and Ag(I) without Cu(I) (curve b) and with

Cu(I) (curve c).

In Fig. 6a, from E k = 1400 mV vs. SCE, thereis an oxidation peak of Ag(0) to Ag(I) at 675 mV

vs. SCE; the current density associated with the

Fig. 6. Partial view of typical cyclic voltammograms obtained on a vitreous carbon electrode (0.07 cm 2) at 25 mV s 1 from 0.5 M CN

laboratory solutions at pH 10 containing: (a) 10 4 M Ag(CN)3 2 in the presence of 0.1 M Cu(CN)4

3, (b) 10 4 M Ag(CN)3 2 and 10 4 M

Au(CN)2 without Cu(CN)4

3 and (c) 10 4 M Ag(CN)3 2 and 10 4 M Au(CN)2

in the presence of 0.1 M Cu(CN)4 3. Voltammograms were

obtained at different negative switching potentials ( E k).




oxidation peak increases as the switching potential

E k turns more negative.

Fig. 6b shows that in the solutions with Au(I) and

Ag(I), there is an oxidation peak at 600 mV vs.SCE from E

k = 1500 mV vs. SCE, with a current

density that increases as E k becomes more negative.

This peak shifts towards less negative values with

respect to the oxidation peak of silver (Fig. 6a),

indicating the normal codeposit of Ag/Au although

the gold dissolution at more negative potential is not

observed in this case.

The voltammogram of cyanide solution containing

the three metallic ions (Fig. 6c) also shows an oxida-

tion peak at 600 mV vs. SCE from E k = 1500

mV vs. SCE. The current density increases, as the

E k turns more negative. This oxidation peak was

shifted towards less negative potentials within the

range of E k from 1500 to 1600 mV vs. SCE.

However, for E kV 1700 mV vs. SCE, the poten-

tial peak now changed to negative values. This change

of the oxidation peak potential is associated with the

dissolution of different deposits (see below).

In order to identify the potential range at which the

gold and silver deposition is free of copper, a quanti-

tative study of the voltammetric curves was carried

out. The associated charge, Qa, of the solutions withand without copper, was calculated and plotted as a

function of the switching potential E k. Fig. 4 shows

the curves Qa vs. E k of the following solutions:

cyanide with Ag(I) and Cu(I) (curve b) obtained from

a previous work (Reyes et al., 2002), cyanide with

Au(I) and Ag(I) (curve c) and cyanide solution with

Au(I), Ag(I) and Cu(I) (curve d).

In Fig. 4, cyanide solutions of Au(I) and Ag(I)

without and with Cu(I), Fig. 4c and d, show the same

slope during the first three switching potentials E k,

thus indicating the same reduction process in these

solutions, the codeposit of Ag/Au. At more negative

switching potentials, the slope of the curve Qa vs.

E k of these solutions was similar to the slope of the

solution containing Ag(I) and Cu(I) (Fig. 4c, d and b,

respectively), indicating the occurrence of the silver

deposition process. On the other hand, in the solution

containing the three metallic ions (Fig. 4d), there is a

Fig. 7. Pourbaix type diagrams of soluble and insoluble chemical species involved in the redox couples: Au V(I)/Au(0) (-n-), Ag V(I)/Ag(0) (-E-)

and Cu V(I)/Cu(0) (-y-). The diagrams were constructed with thermodynamic data from the literature considering as non-variable the following

concentrations: 5 10 6 M Au V(I), 3 10 5 M Ag V(I) and 8 10 3 M Cu V(I) in 0.025 M CN . The concentrations of these chemical species

represent the composition found in the industrial leaching solution.




change in the slope and an increase of the density

charge after E k = 1900 mV vs. SCE, which is

attributed to the codeposit Cu/Ag/Au and the massive

deposition of copper.From the comparative study of cyanide solutions

containing Au(I) and Ag(I) ions with and without

Cu(I), it is possible to determine the potential of the

codeposit Ag/Au free of copper. However, the poten-

tial deposition of silver and gold cannot be found

independently.

The methodology developed till now allows us to

establish the systematic electrochemical study that is

to be carried out in order to identify the codeposit of

Ag/Au without copper deposition from diluted gold

and silver solutions with a high content of copper and

cyanide. This methodology is then applied to an

industrial leaching bath.

3.2. Industrial leaching solution

3.2.1. Study of chemical speciation

The chemical speciation study of electroactive

species of Au, Ag and Cu present in the industrial

leaching solution ( pCN V= log[CN V]=1.6, pAu V=

log[Au V] = 5.3, pAg V= log[Ag V] = 4.55, an d

pCu V= log[Cu V]=2.1) is shown in Fig. 7. The

Pourbaix type diagram was constructed with thesame methodology used for the speciation study of

Fig. 1.

In Fig. 7, it can be observed that Cu presents four

cyanide species, whereas Ag presents two and Au

only one cyanide species. Under conditions of indus-

trial leaching (very dilute concentrations of gold,

silver and cyanide ions), it can be seen that the silver

cyanide species have a lower state of coordination

than that observed in the first study of speciation

carried out in this work (Fig. 1). On the other hand,

Fig. 7 shows that, again in all pH values, the potential

of the redox couple Cu(I)/Cu(0) appears at more

negative potentials with regard to the redox couples

Au(I)/Au(0) and Ag(I)/Ag(0), indicating that copper

does not interfere thermodynamically in the selective

deposition of gold and silver.

Potentials and thermodynamic reactions associated

with the Au(I)/Au(0), Ag(I)/Ag(0) and Cu(I)/Cu(0)

couples in an industrial leaching solution at pH = 11,

Fig. 8. Partial view of typical cyclic voltammograms obtained from an industrial leaching solution containing 5 10 6 M Au(CN)2 , 3 10 5

M Ag(CN)2 and 8 10 3 M Cu(CN)4

3 in 0.025 M CN at pH 10, on a vitreous carbon electrode (0.07 cm 2) at 25 mV s 1. The

voltammograms were obtained at different negative switching potentials ( E k).




pCN V = 1.6, pAu V= 5.3, pAg V= 4.55 and pCu V=2.1

are:

AuðCNÞ2 þ 1e

! Au0

þ 2CN


AgðCNÞ2 þ 1e ! Ag0 þ 2CN


CuðCNÞ34 þ 1e ! Cu0 þ 4CN

E W ¼ 1280 mV vs: SCEð6Þ

It is important to stress that, in principle, the above-

predicted potentials should be more negative than

those previously obtained in Eqs. (1)–(3) because of

having much lower concentrations of gold, silver and

copper in relation to the laboratory solution used.

However, the potentials are less negative, which is

attributed to the lower concentration of cyanide in the

industrial leaching solution.

In this case, the potential difference E j between Au

and Ag is even smaller with regard to that seen in the

laboratory solution; this predicts that the separation of

these two metals is more difficult.

3.2.2. Infl uence of the switching potential scan (E k )

Fig. 8 shows the voltammograms obtained by

modifying the switching potential in direct potential

scan ( E k) for the industrial leaching solution. This

figure shows that from E k 1200 mV vs. SCE,

there is an oxidation peak at 425 mV vs. SCE and

that the current density associated with this peak

increases as the switching potential is shifted towards

more negative values. The oxidation peak appears

at less negative potentials within the range of E k

from 1200 to 1400 mV vs. SCE whereas at

E kV 1500 mV vs. SCE, the oxidation peak ap-

pears at more negative potentials, indicating the dis-

solution of different codeposits of gold, silver and

copper on the VC surface. The oxidation peak dis-

placement observed at less negative potentials in

relation to the oxidation peak of the laboratory sol-

ution study is due to the difference in concentration of

Fig. 9. Oxidation charge (Qa) associated with dissolution peaks as a function of the negative switching potential ( E k) from the

voltammograms obtained on a vitreous carbon electrode (0.07 cm 2) at 25 mV s 1 at pH 10, from (a) industrial leaching solution containing:

5 10 6MAu(CN)2 ,3 10 5 MAg(CN)2

,8 10 3MCu(CN)4 3and0.025MCN and(b)laboratorysolutioncontaining:10 4 MAu(CN)2

Au(CN)2 with 10 4 M Ag(CN)3

2 in the presence of 0.1 M Cu(CN)4 3 and 0.5 M CN .




metallic ions of Au(I), Ag(I) and Cu(I) and a lower

concentration of cyanide in the solution.

On the other hand, in Fig. 8, it is observed that the

oxidation peaks of industrial leaching solution arewider with regard to laboratory solution and they

change their shape rapidly with increase in the switch-

ing potential. This indicates how complicated it would

be to develop the methodology to identify where the

Ag/Au codeposition takes place without the deposi-

tion of copper from the industrial leaching solution.

The statement above justifies the need to use higher

copper and cyanide concentrations to develop the

methodology used in this study.

With the purpose of determining the potential

range at which the deposition of Au(I) and Ag(I) free

of copper from the industrial leaching solution takes

place, the quantitative study of the voltammetric

curves was carried out through the charge associated

with E k-dependent oxidation peaks (Qa). Fig. 9

compares the Qa vs. E k curves of an industrial

leaching solution (curve a) in relation to that of the

previously obtained and discussed more concentrated

cyanide solution of Au(I), Ag(I), Cu(I) and CN

(curve b).

In Fig. 9, the behaviors of the slopes of the Qa vs.

E k curves of the cyanide solution, for the laboratory

solution prepared in the prior section, and the indus-

trial leaching solution are observed to be quite similar.On the other hand, it can be seen that for the industrial

leaching solution, there is a potential displacement

towards less negative values with regard to more

concentrated cyanide solution of Au(I), Ag(I), Cu(I)

and CN , which agrees with the speciation studies of

bot h solutions.

Fig. 9a shows that for the industrial leaching

solution, the codeposition attributed to Ag/Au takes

place within the potential range of 1200–1400 mV

vs. SCE (slope m = 0.13). Linear correlation in this

range does not show the same slope (m = 0.90) as

that obtained for more concentrated cyanide solution

of Au(I), Ag(I), Cu(I) and CN , because of the

nucleation processes of these metals on the vitreous

carbon electrode and the fact that, at lower over-

potentials, less gold and silver is deposited. However,

at E kV 1400 mV vs. SCE, the changes in the

slopes of the Qa vs. E k curves of the industrial

leaching solution are similar to the slope changes of

the more concentrated cyanide solution of Au(I),

Fig. 10. Change in the normalized concentration of cyanide metallic species in the catholyte compartment with time during the electrolysis of an

industrial leaching solution. Concentration profile of (a) Au(I), (b) Ag(I) and (c) Cu(I) at electrolysis potential of 1400 mV vs. SCE. The

electrolysis was carried out in an FM01-LC electrochemical laboratory reactor using an RVC of 60 ppi at a linear flow rate of 19 cm s 1.




Ag(I), Cu(I) and CN (Fig. 9a and b, respectively),

thus indicating the presence of: (i) the massive dep-

osition of Ag (slope m = 0.67), (ii) the codeposition

of Cu/Ag (slope m = 1.27) and (iii) the massivedeposition of Cu (slope m = 5.46) as the switching

potential turns more negative.

From the comparative study of the charges (Qa) of

industrial leaching solution in relation to the more

concentrated cyanide solution of Au(I) ions with

Ag(I), Cu(I) and CN , it is possible to determine

the potential at which the codeposition of Ag/Au takes

place without the deposition of copper. However, just

like in the study of the more concentrated cyanide

solution of Au(I), Ag(I), Cu(I) and CN , it is not

possible to determine the potential at which the gold

and silver deposit independently. In order to confirm

the results from the previous experiments, an electrol-

ysis study of an industrial leaching solution was

carried out.

3.2.3. Electrolysis study

The potential at which Cu(I) does not interfere

with the electrochemical deposition of gold and

silver has been established from the experiments

above. Taking this into account, an electrochemical

deposition of the species Au(I) and Ag(I) in a batch

re-circulation reactor using the industrial leachingsolution (solution f) was carried out.

The electrolysis was at constant potential on a

reticulated vitreous carbon electrode (RVC) for 25

min at a linear flow velocity of 19 cm s 1. The

concentration change of Au(I), Ag(I) and Cu(I) in

the catholyte compartment during the electrolysis

was analyzed by atomic absorption spectroscopy.

Fig. 10 shows the depletion of Au(I) (curve a),

Ag(I) (curve b) and Cu(I) (curve c) as C t / C 0 vs.

time, at a constant potential of 1400 mV vs. SCE,

where C t and C 0 are, respectively, the concentrations

of the metal ion at time t and at the beginning of the

electrolysis.

Fig. 10 shows the evolution in the concentration of

the three metal ions with time when the electrolysis

was carried out at 1400 mV vs. SCE. Depletions in

the concentration of Au(I) and Ag(I) were up to 48%

and 71%, respectively, from their original concentra-

tions (Fig. 10a and b). The concentration of Cu(I) was

Fig. 11. Scanning electron microscopy (SEM) image of 60 ppi of the RVC electrode after 25 min of the electrolysis of the industrial leaching

solution containing: 5 10 6 M Au(CN)2 , 3 10 5 M Ag(CN)2

, 8 10 3 M Cu(CN)4 3 and 0.025 M CN at pH 10 for an

electrolysis potential of 1400 mV vs. SCE.




constant during the first 10 min of electrolysis, how-

ever, in the following 25 min, the concentration of

Cu(I) fell 12% (Fig. 10c). This indicates that metallic

copper would be on the surface of the reticulated

vitreous carbon (RVC).

In order to analyze the morphology and composi-

tion of gold, silver and copper deposits on the RVC,

scanning electron microscopy (SEM) and X-ray dif-

fraction studies on the electrode surface were carried

out. Fig. 11 shows the SEM image of the RVCelectrode after the electrolysis at constant potential

of 1400 mV vs. SCE. In this figure, the metallic

nuclei on the RVC surface could be attributed to gold,

silver and copper deposits. Fig. 12 shows the X-ray

diffractogram obtained for the electrode surface after

the electrolysis at 1400 mV vs. SCE; this diffracto-

gram shows signals that correspond to gold, silver and

copper. The copper signal agrees with the depletion of

this ion observed after 25 min of electrolysis (Fig. 11).

It is important to stress that when the concentra-

tions of Au(I) and Ag(I) diminish in the solution, thedeposition of Cu(I) becomes energetically favorable.

Under these conditions, the electrolysis should be

stopped before the deposition of copper begins if a

quantitative separation of gold and silver is wanted.

The above analysis allows us to propose the recovery

of gold and silver at an applied potential of 1400

mV for the first 10 min of electrolysis in the electro-

chemical batch reactor with an RVC electrode.

The reduction of gold and silver in the electro-

chemical reactor FM01-LC is a preliminary study that

prov es that gold and silver can be sele ctively

removed. However, further studies, such as current

efficiency, mass transport and space–time yield are

necessary to design an efficient electrochemical reac-

tor for the recovery of gold and silver from leaching

process solutions.

4. Conclusions

The deposition of the three metallic ions follows

the sequence: Au, Ag and Cu, this agrees with the

results found in the speciation study. There is a potential range at which only Au(I) was

reduced even at high concentrations of Cu(I). The Cu(I) ion contributes to modify the capacitive

current densities of the cyanide media and helps to

enhance the oxidation process of gold since the

water reduction diminishes considerably. This work shows the influence of different

gold, silver and copper cyanide species on thesimultaneous process of water reduction occur-

ring during the deposition of these metals.

While the silver ions seem to favor this pro-

cess, the copper ions seem to diminish it con-

siderably. The voltammetric study shows that the simulta-

neous reduction of Au(I) and Ag(I) in a medium

with a high concentration of Cu(I) ions occurs

within the potential range of 1500 to 1900

mV vs. SCE without Cu(I) deposition for labo-

Fig. 12. XRD spectrum obtained from the metallic deposit on the RVC electrode surface shown in Fig. 11. The dark lines correspond to the

sample and the pale lines to the standards of gold, silver and copper.




ratory solution and 1200 to 1500 mV vs. SCE

for industrial leaching solution. The recovery of gold and silver was proposed at an

applied potential of 1400 mV vs. SCE for thefirst 10 min of the electrolysis in a batch reactor

with re-circulation on a RVC electrode. The reduction of gold and silver in the electro-

chemical reactor FM01-LC confirms the selective

deposition of both metals. However, these are

preliminary studies and further work is necessary

in order to optimize the reactor performance.

Acknowledgements

The authors express their acknowledgment to

Conacyt (project number 32539-T) and Servicios

Industriales Penoles (Monterrey, Mexico) for funding

this work. V. Reyes Cruz thanks Conacyt and

Servicios Industriales Penoles (Monterrey Mexico)

for his PhD scholarship.

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