Electrodeposition on pyrite from copper(I) cyanide electrolyte

Abstract

Cyanide and copper often co-exist in process water with complicated speciation. The deposition of copper on pyrite from cyanide bearing solutions is unknown and is the objective of this study. The electrochemical behaviour of pyrite in a solution with CN/Cu mole ratio of 3/1 has been investigated in different solution pH values. It was found that, at pH 7, cuprous cyanide can be oxidized to Cu(II) oxide/hydroxide which deposit on the pyrite surface and transform to Cu(I)-sulfide during the subsequent cathodic sweeping. However, this anodic deposition process is inhibited either at pH 10 due to the dissolution of solid Cu(II)/Cu(I)-species by Cu(CN)₃²⁻, or at pH 5 due to the high solubility of Cu(II) oxide/hydroxide. On the other hand, Cu(CN)₂⁻ can be deposited onto pyrite, forming a Cu(I)-sulfide layer at more reducing potentials. The understanding of the electrodeposition mechanisms provides opportunities for the recovery of copper from cyanide-bearing waste water, as well as the separation of pyrite from other raw materials by altering its surface properties.

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Introduction

Copper cyanide waste waters arise from industrial electroplating processes, metal finishing and gold mining. Of which cyanidation in gold extraction processes accounts for 80% of the total cyanide usage. Due to the significant quantity of gold processed in the 21st century that will be recovered from copper containing gold ores, copper(I) cyanide is commonly seen in process water after the dissolution of copper minerals in cyanide solution.¹ Depending on the total cyanide-to-copper ratio, pH and salinity of the solution, various copper cyanide species (Cu(CN)₂⁻, Cu(CN)₃²⁻ and Cu(CN)₄³⁻) can be formed.^2–4

Copper cyanide wastes must be adequately treated before being discharged from the process plant. Electrochemical destruction is capable of simultaneously recovering copper and destroying cyanide in waste solutions. One of the electrochemical methods is based on electrodeposition of copper at the cathode and oxidation of cyanide to cyanate, carbon dioxide and nitrogen gases at the anode.^5–7 Additionally, the electrochemical oxidation of cuprous cyanide leads to the cyanide decomposition accomplished with the film of copper oxides, deposited in situ on the anode such as platinum,^8–12 stainless steel,^8,12–14 graphite,¹⁵ glassy carbon^10,16 and Bi₂MoO₆ electrode.¹⁷

However, the electrochemical behavior of pyrite in cuprous cyanide bearing solutions is unknown. Pyrite is the most abundant sulfide minerals on earth, providing cheap anode substrate material for electrodeposition of copper and decomposition of cyanide. Moreover, the high reactivity and variation of pyrite surface lead to its high degree of compatibility with many anions and cations in the solution.^18,19 The adsorption of copper onto pyrite has been considered to be an electrochemical process involving the adsorption of Cu²⁺ onto pyrite surface and the reduction of Cu(II) to Cu(I) with simultaneous oxidation of pyrite resulting in the formation of Cu(I)-sulfide.^20–23 The activation of pyrite by copper enhances the adsorption of thiol collectors, which is essential to render pyrite surface hydrophobicity and separate pyrite from other raw minerals through froth flotation process.^24–26 On the other hand, the physicochemical mechanism of pyrite–cyanide interaction has been generally accepted as cyanide preferentially adsorbing on pyrite with iron cyanide compounds inhibiting electrochemical activities on the surface.^27–29 Unfortunately, the competitive deposition between copper and cyanide onto pyrite has not been studied yet, despite of their existence in the process water of many precious metal extraction plants.

In this study, the electrochemical behavior of pyrite in copper cyanide bearing solutions was investigated via an electrochemical approach by taking into account the electrode potential and pH of the solution.

Experimental

Materials

Copper cyanide solution was prepared by dissolving copper(I) cyanide powder (CuCN, 99.99%, Aldrich) in excess of sodium cyanide (NaCN, 99.9%, Aldrich) solution to give a final solution with CN/Cu = 3/1. The copper cyanide solution was prepared in deoxygenated solution under the protection of flowing nitrogen. A hand-picked natural massive cubic pyrite specimen with high purity originated from Spain was used as the working electrode. Energy Dispersive Spectroscopy (EDS) analysis on the pyrite specimen showed 66.35 at% sulfur and 33.17 at% iron with minor carbon contamination. XRD analysis also showed little impurities of the pyrite specimen.

Electrochemical measurements

The pyrite electrode was connected with a copper wire using a silver loaded conducting epoxy, and then mounted into non-conducting epoxy resin exposing only one side with a geometric surface of approximately 0.25 cm². A platinum plate with a surface area of 1 cm² was utilized as the auxiliary electrode (counter electrode). Potentials were measured and reported against an Ag/AgCl reference electrode filled with 3 M KCl which has a potential of +0.1915 V against the standard hydrogen electrode.

A fresh electrode surface was generated before each experimental run by wet abrading with silicon carbide abrasive paper (1200 grits). The 0.1 M potassium dihydrogen phosphate (KH₂PO₄, 99.9%, Aldrich) or 0.025 M sodium tetra-borate (Na₂B₄O₇·10H₂O, 99.9%, Aldrich) with a volume of 0.2 L was employed as supporting electrolyte at room temperature. The pH was adjusted to 5, 7 and 10, respectively with standardized 1 M KOH solution. A Radiometer PGZ100 potentiostat was used for cyclic voltammetry (CV) measurements in the presence or absence of cuprous cyanide. In some tests, after the pre-treatment of pyrite, copper cyanide containing solutions was removed from the electrochemical cell, which was then filled with supporting electrolyte for voltammetry measurements. The potential scan rate for CV was 0.02 V s⁻¹. All solutions were prepared with deoxygenated and deionized water. Flowing nitrogen gas was applied above the solution to expel the return of oxygen during the whole process.

Results and discussion

Anodic deposition of copper on pyrite

The voltammograms of an as-polished pyrite exposing to the supporting electrolyte at pH 7 are shown in Fig. 1. Anodic peak A1 commencing at −0.27 V is due to the oxidation of pyrite on a moderate level through eqn (1), (2) or (3).^30–32

FeS₂ + 3H₂O → Fe(OH)₃ + 2S⁰ + 3H⁺ + 3e⁻ (1)

nFeS₂ + 3(n − 2)OH⁻ → 2FeS_n + (n − 2)Fe(OH)₃ + 3(n − 2)e⁻ (2)

FeS₂ + 3xOH⁻ → xFe(OH)₃ + Fe_1−xS₂ + 3xe⁻ (3)

Fig. 1 The voltammograms of stationary pyrite electrode in supporting electrolyte of 0.1 M KH₂PO₄ at pH 7 and scan rate of 0.02 V s⁻¹.

Extensive oxidation of pyrite as shown in eqn (4) leads to a steady increase in current with the increasing of potential at A2.^30–32

FeS₂ + 11H₂O → Fe(OH)₃ + 2SO₄²⁻ + 19H⁺ + 15e⁻ (4)

A cathodic peak C1 at −0.37 V arises from the reduction of oxidation products formed during the prior anodic sweep. One of the possible reduction processes can be represented by eqn (5) with the formation of FeS-like species.³³

FeS_n + (n − 1)Fe(OH)₃ + 3(n − 1)e⁻ → nFeS + 3(n − 1)OH⁻ (5)

Some researchers disagreed with eqn (5) and proposed the mineral surface was similar to pyrite after reduction.³⁴ The cathodic peak C1 also contains contributions from the reduction of ferric hydroxide to ferrous hydroxide and the reduction of S⁰ to H₂S. The reductive decomposition of pyrite takes place at more negative potentials,^30,33 resulting in the further decrease of current with the decreasing of potential at C2. The reaction can be represented by:

FeS₂ + H₂O + 2e⁻ → FeS + HS⁻ + OH⁻ (6)

Fig. 1 also shows that varying anodic switching potential from 0.4 V to 0.6 V slightly raises the cathodic current of C1. This is due to the increased amount of ferric hydroxide formation on electrode surface with increasing anodic switching potential. However, the charge transfer involved in eqn (4) leads to the formation of soluble sulfate ions which is then not able to effectively participate into the reduction processes on electrode surface. On the other hand, the extent of anodic oxidation on the prior cycle has little influence on the anodic current at both A1 and A2. It is then deduced that, at pH 7, ferric hydroxide on electrode surface has been reduced to more soluble ferrous hydroxide which would not be involved in the reversal anodic reaction. Therefore, the currents at A1 and A2 reflect the oxidation of original pyrite.

Fig. 2 shows the cyclic voltammetry of an as-polished pyrite electrode in 10⁻³ M cyanide solutions with CN/Cu = 3/1 at pH 7. The voltammograms were initiated from the open circuit potential on a negative-going direction, and then switched at −0.8 V to a positive-going direction. Fig. 2 shows the first four cycles of voltammograms sweeping from −0.8 V to 0.6 V. The shape of voltammogram on the first cycle in Fig. 2 is similar to that in supporting electrolyte. An additional anodic peak A3 arises from the second cycle for pyrite in cuprous cyanide solutions at pH 7. On the return scan, a well-defined cathodic peak C3 occurs at around 0 V.

Fig. 2 The first four cycle of voltammograms of stationary pyrite electrode in 10⁻³ M cyanide solutions: CN/Cu = 3/1, 0.1 M KH₂PO₄ at pH 7 and scan rate of 0.02 V s⁻¹.

A number of researchers have studied the adsorption of Cu²⁺ on pyrite in the absence of cyanide. Using Fourier Transform Infrared Spectroscopy (FTIR), it was found that the adsorption products resembled chalcocite (Cu₂S).²³ The Cu₂S can be oxidized with the formation of cupric oxide and Cu-deficient layer (non-stoichiometric copper sulfides) through eqn (7).^35,36

Cu₂S + xH₂O → xCuO + Cu_2−xS + 2xH⁺ + 2xe⁻ (7)

According to the E_h–pH diagram constructed by Woods,³⁷ this reaction occurs at potentials in the range of approximately 0.25 to 0.45 V (SHE) at pH 7, which is consistent with the potential at which A3 appears in Fig. 2. The peak C3 should be attributed to the reverse of eqn (7). In addition to Cu₂S, the formation of covellite (CuS) cannot be excluded during the voltammetric sweep of pyrite in cuprous cyanide solution. In fact, CuS has been transformed to Cu₂S during the cathodic sweeping through eqn (8):³⁶

2CuS + H⁺ + 2e⁻ → Cu₂S + HS⁻ (8)

whose equilibrium potential is −0.412 V (versus Ag/AgCl/3 M KCl) at pH 7 and [HS⁻] = 10⁻⁶ M. Then the current for eqn (8) may occur overlapping the current for eqn (5) at C2.

Fig. 2 also shows that the current densities at A3 and C3 increase from the second to the fourth cycle. The amount of copper on pyrite surface can be calculated from the integrated charge over the area of Cu₂S oxidation at A3 on the voltammograms. Assuming the true area of the polished electrode is twice of its geometric area, the copper mass density on pyrite electrode from the fourth cycle is calculated as 0.08 mg cm⁻² (x = 1 in eqn (7)). It was reported³⁸ that a monolayer of Cu on a (110) surface of pyrite is 0.07 mg cm⁻². Therefore, the cumulatively deposit copper during the first four cycles is approximately one molecular layer.

On the other hand, the interfacial processes of pyrite in cuprous cyanide bearing solution affect the electrochemical activity of original pyrite. The first cycle of voltammogram in Fig. 2 illustrates the current densities at the peaks of A1 and C1 are 150 and −470 μA cm⁻¹, respectively, and the current densities at the anodic end (0.6 V) and cathodic end (−0.8 V) correspond to 2100 and −400 μA cm⁻¹, respectively. On the first cycle of voltammogram in supporting electrolyte, the current densities at the peaks of A1, C1, the anodic end and cathodic end are 250, −700, 3700 and −875 μA cm⁻¹, respectively. It is worth noting that there is little amount of Cu(I)-sulfide formed on pyrite surface in the first cycle. Therefore, the suppressions of electrochemical activity of pyrite in cuprous cyanide bearing solution are due to the passivating effect by cyanide species. Furthermore, there is a steady decrease of anodic current at A2 with increasing number of cycling in Fig. 2. This could be attributed by the cumulative deposition of Cu(I)-sulfide, further passivating the underlying pyrite.

In copper and cyanide bearing solutions, the following reactions are considered:

CuCN ⇔ Cu⁺ + CN⁻ (9)

Cu(CN)₂⁻ ⇔ Cu⁺ + 2CN⁻ (10)

Cu(CN)₃²⁻ ⇔ Cu⁺ + 3CN⁻ (11)

Cu(CN)₄³⁻ ⇔ Cu⁺ + 4CN⁻ (12)

HCN ⇔ CN⁻ + H⁺ (13)

The copper cyanide speciation under the present experimental conditions was modelled using a computer program Visual MINTEQ (version 3.0).³⁹ In this modelling, copper(I) cyanide was specified as finite solid phase and was dissolved by NaCN at 25 °C. The solubility product (K_sp) of CuCN was 10^−19.5. The corresponding equilibrium constants for eqn (10)–(13) were 10^−23.9, 10^−29.2, 10^−30.7, and 10^−9.21, respectively. Parameters such as redox potential (E_h), pH and the molality of CuCN, CN⁻ and the balanced Na⁺ were considered to formulate the input data for the calculation. The Cu⁺/Cu²⁺ redox couple was specified with log K = 2.69. The E_h ranging from −0.4 V to 0.4 V (SHE) has little influence on the activity of cuprous cyanide species at CN/Cu = 3/1.

The speciation with initial [CN] = 1 × 10⁻³ M and CN/Cu = 3/1 as a function of pH is shown in Fig. 3. The predominant species is cuprous tri-cyanide (Cu(CN)₃²⁻) at alkaline pH, while both HCN and Cu(CN)₂⁻ become dominant in the solution when pH drops down due to the competing complexing effect of photon with cyanide. At pH 7, there are almost equal amount of Cu(CN)₃²⁻, Cu(CN)₂⁻ and HCN, dominating in the solution.

Fig. 3 Cyanide speciation as a function of pH: initial [CN] = 1 × 10⁻³ M; CN/Cu = 3/1; E_h = 0 V; 25 °C.

The anodic oxidation of copper cyanide on noble metal,^9–12 stainless steel,^8,12–14 and carbon^10,15,16 electrodes has been extensively investigated. These investigations applied highly alkaline solution (pH > 11) with Cu(CN)₃²⁻ being the dominant species. For example, with 1.7 mM copper, 6.8 mM cyanide and 1 mM NaOH in solution, an oxidation wave becomes visible starting at around 0.3 V (Hg/HgO).¹⁰ The deposition products after such a polarization sweep were identified to be black CuO film by XPS.¹⁰ It should be mentioned that in the work by Casella and Gatta,¹⁶ they were able to quickly transfer the film into an XPS and detected Cu(OH)₂ as well as CuO at the film surface.

The oxidation of cuprous cyanide is suggested in eqn (14).

Cu(CN)₃²⁻ + 8OH⁻ → 3CNO⁻ + Cu(OH)₂ + 3H₂O + 7e⁻ (14)

While others¹⁶ suggest that the electrodeposition process most likely involves a preliminary oxidation of Cu(I) to Cu(III) with the subsequent formation on the electrode surface of CuOOH. For low hydroxide concentrations, where the oxidation of cyanogen to cyanate will precede slowly, most of the cyanogen will diffuse out to the bulk solution and thus only one electron is transferred from each reacted cyanide.¹⁵ The reaction is described as eqn (15).

Cu(CN)₂⁻ + H₂O → (CN)₂ + CuO + 2H⁺ + 3e⁻ (15)

The anodic peak A4 at around 0.5 V in Fig. 2 should be attributed to the oxidation of cuprous cyanide with the formation of Cu(OH)₂ and/or CuO and the oxidation of cyanide to cyanogen. The shape and position of A4 seem to be stable during the cyclic voltammetry due to the surface reactions can not change the concentration of solution species significantly and this reaction is irreversible.

The oxidation of copper(I) to copper(II) takes place at approximately 0.1 V versus SCE at pH 7, leading to the formation of Cu(OH)₂.⁴⁰ Obviously, the current at A3 and C3 is not due to the oxidation and reduction of copper oxide/hydroxide. Actually, the structure of pyrite–CuO layer may undergo reconstruction with the formation of Cu(I)-sulfide. This process as described in eqn (16) is the well-known copper activation for sulfide minerals flotation.^24–26

CuO + FeS₂ + 2H⁺ ⇔ CuS + Fe²⁺ + S⁰ + H₂O (16)

As shown in eqn (16), the reduction of Cu(II) in CuO to Cu(I) in covellite (CuS) was coupled with the oxidation of surface sulfur on pyrite. During the process of activation of pyrite by Cu²⁺, the amount of Cu(I)-sulfide formation was promoted with reducing potential from 0.3 V to 0.1 V (Ag/AgCl/3 M KCl).^22,41 In the voltammetric process in Fig. 2, the formation of Cu(I)-sulfide could proceed simultaneously with the oxidation and deposition of cuprous cyanide, it could also proceed during the cathodic sweeping. According to the conducting character of the electrodeposited film, the oxidation and reduction waves comprised between −0.1 and 0.3 V, grow with the number of sweeps as new cuprous cyanide is oxidized, deposited on the electrode surface and transformed to Cu(I)-sulfide.

The potential range for which eqn (15) occurs is evaluated by employing different anodic switching potentials and the fourth cycle of the voltammograms are illustrated in Fig. 4 for each switching potential. It is observed that the characteristic redox peaks A3 and C3 for Cu(I)-sulfide is evident when the anodic switching potential of 0.3 V is employed, indicating the deposition of copper on pyrite from cuprous cyanide solution at 0.3 V. However, the current density at A3 and C3 is relatively small until the anodic switching potential increases up to 0.6 V. This is approximately in line with the maximum surface concentration of the Cu(III) electroactive sites obtained at an applied potential of 0.55 V from alkaline cuprous cyanide solution on either glassy carbon or platinum electrode substrate.¹⁶

Fig. 4 The fourth cycle of voltammograms of stationary pyrite electrode in 10⁻³ M cyanide solutions: CN/Cu = 3/1, 0.1 M KH₂PO₄ at pH 7 and scan rate of 0.02 V s⁻¹. Anodic switching potentials at −0.3 V, −0.4 V, −0.5 V and −0.6 V were employed for each voltammogram individually.

It was reported¹⁶ that the resulting active copper films, independently of the electrochemical procedure adopted (i.e. potentiostatic or potentiodynamic conditions), appear to be uniform, smooth and with a good degree of adhesion on the substrate such as glassy carbon or platinum electrode. In the present study, adhesion of the deposited copper film on pyrite surface was also examined. A voltammogram of the pyrite electrode previously treated in cuprous cyanide solution at pH 7 and potentiostated at 0.6 V for 60 seconds under stirring (50 rpm per min) is shown in Fig. 5. Here the copper cyanide bearing solution was removed and the electrochemical cell was re-filled with supporting electrolyte at pH 7, where the voltammetry was conducted. Stirring was applied for 2 min at 50 rpm per min for the supporting electrolyte before voltammetry measurement started. It is seen that the characteristic redox waves for Cu(I)-sulfide are evident after pyrite pre-treated at 0.6 V. However, the integrated charge under A3 is approximately 0.8 mC cm⁻², which is much smaller than charge (1.5 mC cm⁻²) under A3 of the second cycle of voltammogram in Fig. 2. Considering the longer exposure time for pyrite in Fig. 5 than that in Fig. 2 to cuprous cyanide solution at anodic potentials where the electrodeposition of copper is favorable, the less amount of copper deposition indicates some pieces of the anodic deposited copper layer may fall off the electrode. It is worth noting that the Cu(I)-sulfide formed on pyrite surface upon copper activation is in chemical bond with the pyrite substrate and a good adhesion is then expected. Therefore, the adhesion of the deposited copper layer potentiostated at 0.6 V is degraded to somewhat due to the formation copper oxide/hydroxide which is physically in contact with the pyrite substrate.

Fig. 5 The voltammograms of stationary pyrite electrode after pretreatment in 10⁻³ M cyanide solutions with CN/Cu = 3/1 at pH 7 and potentiostated at 0.6 V for 1 min. Supporting electrolyte: 0.1 M KH₂PO₄ at pH 7 and scan rate of 0.02 V s⁻¹.

Influence of pH

The pH of the cuprous cyanide solution has a significant influence on the electrodeposition of copper on pyrite. The voltammograms of stationary pyrite electrode in 10⁻³ M cyanide solutions with CN/Cu = 3/1 at pH 10 is shown in Fig. 6. No evident redox wave of copper oxide or copper sulfide was observed although the oxidation of cuprous cyanide at anodic potential still occurs at A4. This might be due to the dissolution of surface copper layer of less than one molecule layer on pyrite by cyanide bearing species (Cu(CN)₃²⁻), which is dominant at pH 10, as shown in eqn (17) and (18).

Cu(OH)₂ + 4Cu(CN)₃²⁻ ⇔ 5Cu(CN)₂⁻ + [/]CN⁻ + ½CNO⁻ + ½H₂O + OH⁻ (17)

Cu(OH)₂ + 2Cu(CN)₃²⁻ + e⁻ ⇔ 3Cu(CN)₂⁻ + 2OH⁻ (18)

Fig. 6 The fourth cycle of voltammograms of stationary pyrite electrode in 10⁻³ M cyanide solutions with CN/Cu = 3/1, 0.1 M Na₂B₄O₇ at pH 10 and scan rate of 0.02 V s⁻¹.

The standard free energies of formation for the corresponding Fe–Cu–S–CN species are shown in Table 1. A negative Gibbs free energy change (−22.26 kJ mol⁻¹) is obtained for eqn (17), indicating this process is spontaneous. Additionally this process would be accelerated if a negative voltage is applied as electron donor for the reduction of Cu(II), as shown in eqn (18). It is worth noting that copper(II), if present as a hydrated complex or a dissolved ion, can be active for the chemical oxidation of cyanide, while the dry oxide does not appear to be active.¹⁰ Thus the freshly precipitated Cu(OH)₂ would dissolve when placed in a cyanide solution much faster than CuO.

Table 1 Fe–Cu–S–CN species and their standard free energies of formation

	ΔG298^0 kJ mol^−1	Ref.
H2O	−247.17	3
H^+	0	3
OH^−	−157.28	3
HS^−	12.05	3
S^2−	85.77	37
Cu(CN)2^−	258	3
Cu(CN)3^2−	400	3
CN^−	172.3	3
HCN	119.7	3
CNO^−	−98.7	3
(CN)2	296.3	3
Cu2S	−86.19	37
CuS	−53.56	37
Cu^+	50	37
Cu^2+	65.52	37
Cu(OH)2	−359.5	37
CuO	−134	37
Cu2O	−148	37
FeS2	−166.94	45
Fe^2+	−78.87	45

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The free cyanide anion CN⁻ is able to dissolve Cu(II)-oxide/hydroxide in a more efficient way than Cu(CN)₃²⁻. In fact, the concentration of these cyanide bearing species increases with the increasing of pH as shown in Fig. 3, accelerating the dissolution of surface copper layer. This leads to less amount of Cu(I)-sulfide formation during the subsequent cathodic sweep. The dissolution of Cu(I)-sulfide by Cu(CN)₃²⁻ has also been observed in leaching process through eqn (19).⁴²

Cu₂S + 4Cu(CN)₃²⁻ ⇔ 6Cu(CN)₂⁻ + S²⁻ (19)

The bulk solution speciation in Fig. 3 also shows that the Cu(CN)₃²⁻ concentration decreases and the concentration of Cu(CN)₂⁻ increases when pH decreases. This leads to the occurrences of eqn (17) and (18) impossible at pH 7 where nearly equal amount of Cu(CN)₂⁻ and Cu(CN)₃²⁻ existing in the solution. At pH < 4.5, copper precipitates as CuCN. It can be seen that at pH 5, the only dominant species is Cu(CN)₂⁻.

Fig. 7 shows the cycle voltammetry of stationary pyrite electrode in 10⁻³ M cyanide solutions with CN/Cu = 3/1 at pH 5. The voltammograms were initiated from the open circuit potential on a negative-going direction to −0.8 V and then switched to a positive-going direction to 0.6 V. It is interesting to note that an anodic peak A3 appears around 0.4 V on the first positive-going cycle with a corresponding cathodic peak at C3. This pair of peaks can be easily assigned to the oxidation of Cu(I)-sulfide as shown in eqn (20) and its reformation during reduction processes.

Cu₂S + xH₂O → xCu²⁺ + Cu_2−xS + 2xH⁺ + 2xe⁻ (20)

Fig. 7 The first four cycle of voltammograms of stationary pyrite electrode in 10⁻³ M cyanide solutions with CN/Cu = 3/1, 0.1 M KH₂PO₄ at pH 5 and scan rate of 0.02 V s⁻¹.

This differs from the voltammograms of pyrite electrode in cuprous cyanide solution at pH 7. A relatively clean pyrite surface was maintained after subjecting to the cathodic sweeping to −0.8 V on the first cycle at pH 5. Copper can be coordinated to sulfur site on the surface of pyrite with the formation of Cu(I)-sulfide. The S dimer can accommodate Cu⁺ with the growth of covellite (CuS) on the surface without charge transfer involved. This process is described in eqn (21).

2Cu(CN)₂⁻ + FeS₂ + 4H₂O ⇔ 2CuS + 4HCN + Fe²⁺ + 4OH⁻ (21)

Using the thermodynamic parameters in Table 1, the Gibbs free energy change of eqn (21) is 303.31 kJ mol⁻¹, indicating this deposition process is non-spontaneous and the surface coordination would be thermodynamically impossible. Thus the reduction process occurs between the dominating cuprous species in the solution and pyrite surface is proposed in eqn (22).

4Cu(CN)₂⁻ + FeS₂ + 8H₂O + 2e⁻ ⇔ 2Cu₂S + 8HCN + 8OH⁻ + Fe²⁺ (22)

E (SHE)/V = −2.49 − 0.236 log[HCN] − 0.0295 log[Fe²⁺] − 0.236 log[OH⁻] + 0.118 log[Cu(CN)₂⁻]

In this reaction, the valence state of sulfur was reduced to S monomer. Eqn (22) has an equilibrium potential of 0.078 V (versus Ag/AgCl/3 M KCl) at pH 5 assuming [Fe²⁺] = 10⁻⁶ M. It is then confident to assign the cathodic current at C6 corresponds to the cathodic deposition of copper as Cu₂S in eqn (22). As the concentrations for all of solution species involved in eqn (22) are expected to be constant, the position and the current density of C6 are approximately consistent with increasing voltammetric cycling number as shown in Fig. 7.

At more negative potentials, the pyrite surface can be reconstructed with properties resembling those of pyrrhotite (FeS). The formation of FeS is suggested in eqn (5) at pH 7 in Fig. 1, which is significantly suppressed by the presence of cuprous cyanide as shown in Fig. 2. But the formation of FeS occurs much more extensively in cuprous cyanide bearing solution at pH 5, as can be seen from the larger current density at C2 in Fig. 7 when compare to that in Fig. 2. Apart from the growth of Cu₂S on the surface, chalcopyrite may form at the cathodic end of the voltammograms in Fig. 7. It is shown by Todd⁴³ that the most likely oxidation states of the chalcopyrite atoms are assumed to be Cu²⁺, Fe²⁺, and S²⁻. The oxidation of chalcopyrite involves two steps as shown in eqn (23) and (24), respectively⁴⁴

CuFeS₂ → CuS + Fe²⁺ + 2S + 2e⁻ (23)

E (SHE)/V = 0.293 + 0.0296 log[Fe²⁺]

CuS → Cu²⁺ + S + 2e⁻ (24)

E (SHE)/V = 0.59 + 0.0296 log[Cu²⁺]

The theoretical reversible potentials for eqn (23) with 10⁻⁶ M Fe²⁺, and eqn (24) with 10⁻⁶ M Cu²⁺ is −0.088 V and 0.208 V (Ag/AgCl/3 M KCl), respectively. It can be seen that the anodic waves A1 and A5 appear at potentials close to those expected for the two processes. In quiescent solution, the Cu²⁺ and Fe²⁺ ions may remain in the vicinity of the electrode surface and react with CuS and S in the reverse of eqn (23) and (24) for the reformation of chalcopyrite. The cathodic peaks at C5 and C6 are attributed by these reversal reactions. It is observed that the integrated charge for C5 is much smaller than A5 on the positive-going direction, providing further evidence in support of the proposed mechanism. However, others³⁴ hold different opinion for the oxidation of chalcopyrite, involving the formation of stable species of Fe(OH)₃ and metastable phases of CuFe_(1−x)S₂ and CuS₂ in the first step and the decomposition of CuS₂ take place in the second step. Though controversy proceeds for the electrochemical response of chalcopyrite in aqueous solution, its existence from the second voltammetric cycle was evident in Fig. 7. The current densities for all anodic peaks and cathodic peaks increase with increasing number of cycling, indicating increased amount of copper cathodic deposition in forms of either chalcocite or chalcopyrite.

The contribution from the anodic deposition of copper on pyrite at pH 5 is also of interest. However it is worthy reminding that the solubility of anodic deposition products copper oxide/hydroxide are high at pH 5, so the soluble Cu²⁺ may not be effectively involved into the activation process as shown in eqn (16) during cathodic sweeping. Similar with the method employed at pH 7, a pyrite electrode previously treated in cuprous cyanide solution at 0.6 V or −0.4 V for 60 seconds under stirring (50 rpm per min) and then the cuprous cyanide bearing solution was removed. Voltammograms of the pre-treated pyrite electrode were recorded in supporting electrolyte at pH 7 is shown in Fig. 8. It is seen that the characteristic redox waves for Cu(I)-sulfide is evident for pyrite electrode pre-treated at −0.4 V, but not at 0.6 V, indicating the cathodic deposited copper layer is stable, but the anodic deposited copper layer unstable.

Fig. 8 The voltammograms of stationary pyrite electrode after pretreatment in 10⁻³ M cyanide solutions with CN/Cu = 3/1 at pH 5 and potentiostated at 0.6 V and −0.4 V, respectively for 1 min. Supporting electrolyte: 0.1 M KH₂PO₄ at pH 7 and scan rate of 0.02 V s⁻¹.

Conclusion

The electrochemical behavior of pyrite in cuprous cyanide bearing solution with CN/Cu mole ratio of 3/1 is dependent on the pH of the solution. At pH 7, the reactions involved in the anodic process are the oxidation of cuprous cyanide to Cu(II) oxide/hydroxide which deposit on pyrite surface and transform to Cu(I)-sulfide during the subsequently cathodic sweeping. The potential for the anodic deposition of Cu(II) oxide/hydroxide has commenced from 0.3 V and became rapid at around 0.6 V. At pH 10, cuprous cyanide is oxidized to cupric cyanide complexes, producing cyanate. However, the precipitation of copper on pyrite is suppressed due to the dissolution of Cu(II) oxide/hydroxide and/or Cu(I)-sulfide by Cu(CN)₃²⁻ which becomes dominant at pH 10. The anodic deposition as Cu(I)-sulfide layer are suppressed at pH 5 due to the increased solubility of Cu(II) oxide/hydroxide in mildly acidic media. On the other hand, Cu(CN)₂⁻ dominant at pH 5 can be deposited on pyrite surface in a cathodic process which has started from around 0 V, resulting in a Cu(I)-sulfide layer. The results reveal that the copper from cuprous cyanide solution can be electrochemically deposited on pyrite surface by controlling the solution pH and the electrode potential. A method of using pyrite as a cheap substrate to recover copper and destruct cyanide from waste water is prospective, which ensures the future study of the reaction kinetics. The present work also suggests the optimized conditions for the activation of pyrite flotation using copper cyanide process water.

Acknowledgements

The authors acknowledge the financial support of this study from Newcrest and Morobe Joint Venture and the scholarship provided by the University of Queensland. Dr Yongjun Peng is acknowledged for the useful discussions.

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