Electrochemical Behavior of Chalcopyrite in Presence of Sodium Peroxodisulfate

Abstract

In this research assisted leaching of chalcopyrite using sodium peroxodisulfate was studied. Electrochemical techniques such as Tafel analysis and electrochemical impedance spectroscopy (EIS) were used to investigate the surface reactions of a chalcopyrite electrode in the presence and absence of Na₂S₂O₈. According to the results, addition of sodium peroxodisulfate increases the current exchange of the corrosion process and prevents the formation of a passive layer on the surface of the electrode. EIS tests show the trans-passive dissolution of the passive layer on the chalcopyrite electrode in presence of Na₂S₂O₈. Equivalent electrochemical circuits were modeled for Na₂S₂O₈-free and -containing solutions and the model parameters were compared to determine the effect of S₂O₈²⁻ on chalcopyrite dissolution. Copper dissolution from pure chalcopyrite mineral and chalcopyrite concentrate was carried out using 15 g/L sulfuric acid solution with and without Na₂S₂O₈. Simultaneous presence of 10 g/L ferric ion and 40 g/L peroxodisulfate salt results in about 40 and 18% of Cu extraction from chalcopyrite concentrate and pure chalcopyrite mineral, after 96 h, respectively.

Introduction

Chalcopyrite (CuFeS₂) is the most abundant copper-bearing mineral in nature, and is the primary mineral used for the commercial production of copper. It accounts for approximately 70% of the world’s copper reserves [1]. Various studies have considered the leaching and dissolution of CuFeS₂ in different aqueous media such as sulfate, chloride, nitrate and ammonia [2–5]. It is well understood that leaching of chalcopyrite is combined with electron exchange and is electrochemical in nature. In order to increase the leaching kinetics of chalcopyrite, the characterization of the layers formed during dissolution is essential and electrochemical methods are important to this end. In recent years, various electrochemical techniques such as potentiostatic polarization [6], potentiodynamic polarization [7–9], cyclic voltammetry [10–12] and electrochemical impedance spectroscopy (EIS) [1, 6, 7, 13] have been used.

Much effort has been made to improve the leaching rate of chalcopyrite. One way to improve the rate is to use leaching promoters [8, 14] such as silver ions [8, 15], pyrite [16], hydrogen peroxide–glycol [17] and sodium peroxodisulfate [18].The attractiveness of peroxodisulfate as a lixiviant for chalcopyrite dissolution is that it can be inexpensively produced from sulfuric acid using an electrochemical cell. Upon reaction with the chalcopyrite, each peroxodisulfate ion converts to two sulfate ions that can then be electrochemically recycled back to peroxodisulfate [18]. Turan and Altundogan [19] believe that the oxidizing leaching reaction of metal sulfurs by the proxodisulfate ion may be represented by following reaction:

$\rm{Me_{x} S_{y} + \, xS_{2} O_{8}^{{2{-}}} \to \, xMe^{2 + } + \, yS^{0} + \, 2xSO_{4}^{{2{-}}}}$

(1)

In this study a more detailed study of the role of peroxodisulfate ion in chalcopyrite leaching was carried out using electrochemical techniques.

Materials and Methods

Natural chalcopyrite was supplied by VWR International, Wards Natural Science. Chalcopyrite concentrate was obtained from Sarcheshmeh copper mine, Kerman, Iran. XRD analysis of the concentrate showed that chalcopyrite is the main copper mineral. The concentration of Cu was 26.31%. Sodium peroxodisulfate (Na₂S₂O₈), ferric sulfate, ferrous sulfate and sulfuric acid were prepared from analytical grade reagents supplied by Merck, Germany.

Chalcopyrite working electrodes were prepared from the mineral. The exposed surface area of the electrode was measured to be 0.3604 cm². The reference electrode was a Saturated Calomel reference Electrode (SCE) supplied by Metrohm. The counter electrode was platinum. The electrolyte used was 0.5 M H₂SO₄ containing determined amounts of sodium peroxodisulfate and all of the experiments were conducted at 25 °C. The electrode surface was subjected to a series of polishing stages. In all tests, the electrode was rotating at 1500 rpm.

A 250 mL thermostatted cell (supplied Metrohm) was used to carry out all the electrochemical measurements. Nitrogen was bubbled into the electrolyte for 10 min prior to starting the experiments. A Gamry Series G 300/750 Potentiostat was used for all the tests. In all tests the electrodes were stabilized in the solution for 30 min. Potentiodynamic polarization curves were obtained by changing the electrode potential automatically from −0.7 to +0.7 V versus OCP at scan rates of 10 mVs⁻¹. EIS tests were conducted at different potentials and in the frequency range of 10⁻²–10⁵ Hz with peak-to-peak amplitude of 10 mV.

Batch leaching tests were performed in a sealed, jacketed, 1 L glass reactor using 15 g/l sulfuric acid solution and the desired amount of ferric/ferrous sulfate salts and/or sodium peroxodisulfate. 25 g/l of mineral or concentrate was introduced to the reactor and the stirring speed was set at 900 rpm using overhead impeller.

Results and Discussion

Open Circuit Potential (OCP) Study

Figure 1 shows trends of the measured OCPs over a 15 min period at various pH in the presence and absence of Na₂S₂O₈. It was observed that the rate of potential increase is very gradual, more or less levelling out after a 15 min period, which means that steady-state is being reached between the mineral surface and solution.

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig1_HTML.gif — Fig. 1
The measured OCPs of chalcopyrite electrode in H₂SO₄ solution at various pH in the presence and absence of Na₂S₂O₈

In the presence of S₂O₈²⁻ ion, the curves do not level out entirely; rather the potential continues to increase with the time. This is indicative that either the surface of the mineral or solution conditions at the surface of the mineral are gradually changing.

Potentiodynamic Study

Figure 2 presents the potentiodynamic curves of chalcopyrite electrode in sulfuric acid solution with and without sodium peroxodisulfide addition. The oxidative polarization curves of chalcopyrite in sulfuric acid solution without sodium peroxodisulfide addition show negative slopes within the potential range of 0.2–0.7 V (Fig. 2a), indicating formation of passive layer on the surface of the electrode that hinder the dissolution of the electrode. The value of the current density was about 10⁻⁵ A cm⁻². The addition of sodium peroxodisulfate into the solution has increased the OCP of the electrode to 0.4 V as well as the current exchange to about 10⁻⁵ A cm⁻². Also it prevents formation of passive layer on the surface of the electrode, so that the negative slopes could not be seen (Fig. 2b).

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig2_HTML.gif — Fig. 2
Potentiodynamic curves of chalcopyrite electrode in sulfuric acid solution with and without sodium peroxodisulfide addition. Potential range −0.7 to +0.7 mV versus OCP, scan rates 10 mVs⁻¹

Figure 2c shows effect of sodium peroxodisulfate concentration on chalcopyrite dissolution at pH 1.5. As it could be seen while within the anodic potential of 0.4–0.8 V (0.4 V above OCP) the measured current is higher for 10 g/l concentration, at potentials higher than 0.8 V, the dissolution rate is higher at 30 g/l sodium peroxodisulfate.

Electrochemical Impedance Spectroscopy (EIS) Study

EIS studies on the chalcopyrite electrodes were conducted in three different potential values of OCP, 0.3 V above OCP (0.3 vs. OCP) and 0.6 V above OCP (0.6 vs. OCP), with and without sodium peroxodisulfate addition. The results are presented in Figs. 3, 4 and 5.

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig3_HTML.gif — Fig. 3
Measured and calculated Nyquist plots and the equivalent electrochemical circuit for the chalcopyrite/electrolyte interface in the presence and absence of peroxodisulfate at OCP

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig4_HTML.gif — Fig. 4
Measured and calculated Nyquist plots and the equivalent electrochemical circuit for the chalcopyrite/electrolyte interface in presence and absence of sodium peroxodisulfate at 0.3 V above OCP

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig5_HTML.gif — Fig. 5
Measured and calculated Nyquist plots and the equivalent electrochemical circuit for the chalcopyrite/electrolyte interface in presence and absence of peroxodisulfate ion at 0.6 V versus OCP

The Nyquist curves obtained at OCP potentials show flattened semicircles and such behavior is related to the existence of a thin surface layer on the electrode [7] (Fig. 3). Possibly, the following reaction may occur as considered by Hackl et al. [20]:

${\text{CuFeS}}_{ 2} \to {\text{ Cu}}_{{ 1 {\text{ - x}}}} {\text{Fe}}_{{ 1 {\text{ - y}}}} {\text{S}}_{ 2} {\text{ + xCu}}^{{ 2 { + }}} {\text{ + yFe}}^{{ 2 { + }}} { + 2}\left( {\text{x + y}} \right){\text{ e}}^{ - }$

(2)

To fit the experimental data, the equivalent electrical circuit shown in Fig. 3 was used. The modeled Nyquist plots and calculated values for different elements are presented in Fig. 3 and Table 1. The low chi-squared (χ²) values in Table 2 indicate a good fit to the experimental data.

Table 1

Model parameters for equivalent circuit of Fig. 3

Test	R_sol (Ω cm²)	R_SC (Ω cm²)	10⁻⁶ C_SC (F cm⁻²)	10³ R_P (Ω cm²)	10⁻⁶ C_P (F cm⁻²)	10³ R_CPE (Ω cm²)	10⁻⁶ Y₀	A	10⁻⁶ χ²
Free of Peroxodisulfte	37.84	869.8	74.38	5.9	28.65	2.77	53.69	0.76	35.93
Peroxodisulfate-Containing	22.60	1560	17.05	5.23	14.02	2.61	26.74	0.80	138.2

Table 2

Model parameters for equivalent circuit of Fig. 4

Test	R_sol (Ω cm²)	R_sc (Ω cm²)	10⁻³ C_sc (F cm⁻²)	C (F cm⁻²)	10⁻⁶ L2 (H)	10³ R_a (Ω cm²)	R_b (Ω cm²)	R_L (Ω cm²)	L1 (H)	10⁻³ R_CPE (Ω cm²)	Y0 10⁻⁶	A	Χ² 10⁻⁶
Peroxodisulfte-Free	36.66	1101	0.074	0.0002128	130.4	11.05	300.8	2.06	872.2	30.63	7.96	0.799	5584
Peroxodisulfate-Containing	21.69	17.06	2.20	4.49	23.03	11.85	361	66.5	11.52	80.95	26.59	0.774	270.5

The impedance curves at 0.3 V above OCP show a change in electrochemical behavior. In case of peroxodisulfate-free solution, the curve shows capacitive loop behavior at high frequencies, however, an inductive loop appears at low frequencies. This inductive loop is more distinctive for peroxodisulfate-containing solution. Probably, the inductive loop is related to the transpassive dissolution of the passive layer. According to Nava and Gonzales [21] the surface layer at these potentials is a CuS layer which forms as a product of reaction (5):

$2 {\text{CuFeS}}_{ 2} {\text{ + 13 H}}_{ 2} {\text{O }} \to { 0} . 7 5 {\text{ CuS}}_{{}} { + 1} . 2 5 {\text{ Cu}}^{{ 2 { + }}} {\text{ + Fe}}_{ 2} \left( {{\text{SO}}_{ 4} } \right)_{ 3}^{{}} { + 0} . 2 5 {\text{ SO}}_{ 4}^{{ 2 { - }}} {\text{ + 26 H}}^{ + } {\text{ + 28 e}}^{ - }$

(3)

Figure 4 shows the model applied for 0.3 V versus OCP potential and the calculated Nyquist plots and Table 2 shows the value of the elements. According to the data given in Table 2, addition of peroxodisulfate to the solution results in increasing of C from about 0 to 4.49 F cm⁻².

At the potential of 0.6 versus OCP, both electrodes show a capacitive time constant at high frequencies and an inductive loop at low frequencies. This loop is characteristic of anodic dissolution of semiconductor electrodes. At this potential, electrochemically active dissolution of the electrode occurs. Biegler and Swift [22] suggest that this dissolution reaction occurs according to:

${\text{CuFeS}}_{ 2} {\text{ + 8H}}_{ 2} {\text{O}} \to {\text{ Cu}}^{{ 2 { + }}} {\text{ + Fe}}^{{ 2 { + }}} {\text{ + 2SO}}_{ 4}^{{ 2 { - }}} {\text{ + 16H}}^{ + } {\text{ + 17e}}^{ - }$

(4)

Figure 5 shows the model applied for 0.6 V above OCP potential and the calculated Nyquist plots and Table 3 shows the value of the elements.

Table 3

Model parameters for equivalent circuit of Fig. 5

Test	R_sol (Ω cm²)	10⁻³ R_sc (Ω cm²)	10⁻³ C_sc (F cm⁻²)	C_P (F cm⁻²)	L (H)	RL (Ω cm²)	R_p (Ω cm²)	R_CPE (Ω cm²)	10⁻⁶ Y0	A	10⁻⁶ Χ²
Free Peroxodisulfte	20.98	0.00455	0.00896	5.570	1.296	11.47	81.15	30.18	4.822	0.70	167
Peroxodisulfate-Containing	20.76	0.00470	0.02554	0.431	0.107	2.349	5.355	321.8	17.65	0.79	204

Effect of sodium peroxodisulfate concentration on the impedance spectra of the chalcopyrite electrode was investigated. The results are shown in Fig. 6. At OCP, increasing the sodium peroxodisulfate concentration in the solution results in a semi-circle at high frequencies which is related to charge transfer process on the electrode. In other words, at higher sodium peroxodsulfate concentrations, the thickness of the passive layer increases because of the higher Cu dissolution rate. At 0.3 V above OCP, the electrochemical behavior of the electrode is similar. Increasing the sodium peroxodisulfate concentration causes an increase in a capacitive behavior of the chalcopyrite electrode. At 0.6 V above OCP, increasing the sodium peroxodsulfate concentration results in transferring the inductive loop to lower frequencies. The inductive loop at low frequencies might be related to a transpassive dissolution process.

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig6_HTML.gif — Fig. 6
Nyquist plots for the chalcopyrite/electrolyte interface in sulfuric acid solution with various sodium peroxodisulfate concentrations

Leaching Experiment

Effect of sodium peroxodisulfate on copper dissolution from chalcopyrite mineral and chalcopyrite concentrate was investigated at different conditions in presence and absence of ferric and peroxodisulfate ions. The results are shown in Fig. 7. According to the results, in the leaching of copper concentrate, while addition of 10 g/L ferric ion to the leaching solution results in a 20% increase in Cu extraction, simultaneous presence of 10 and 40 g/L ferric and peroxodisulfate ions has a great effect on copper dissolution, so that about 40% of Cu is extracted in 96 h. Higher concentration of 60 g/L sodium peroxodisulfate increases the Cu extraction to 50%. Positive effect of sodium peroxodisulfate on Cu dissolution from chalcopyrite mineral could also be seen in Fig. 7b. As shown, addition of 40 g/L sodium peroxodisulfate salt to the leaching solution results in a 10% increase in Cu extraction from chalcopyrite mineral.

../images/468727_1_En_101_Chapter/468727_1_En_101_Fig7_HTML.gif — Fig. 7
Effect of sodium peroxodisulfate addition on Cu extraction from a chalcopyrite concentrate and b chalcopyrite pure mineral

Conclusion

In this paper assisted leaching of chalcopyrite using sodium peroxodisulfate was studied. Electrochemical techniques were used to investigate the surface reactions of chalcopyrite electrode in the presence and absence of Na₂S₂O₈. Results showed that addition of sodium peroxodisulfate increases the current exchange density of the corrosion process and prevents formation of a passive layer on the surface of the electrode. EIS tests shows the transpassive dissolution of the passive layer on the chalcopyrite electrode in presence of Na₂S₂O₈. Equivalent electrochemical circuits were modeled for Na₂S₂O₈ free and containing solutions and the model parameters were compared to determine the effect of S₂O₈²⁻ on chalcopyrite dissolution. Copper dissolution from pure chalcopyrite mineral and chalcopyrite concentrate was carried out using 15 g/L sulfuric acid solution with and without Na₂S₂O₈. Simultaneous presence of 10 g/L ferric ion and 40 g/L peroxodisulfate salt results in about 40% and 18% of Cu extraction from chalcopyrite concentrate and pure chalcopyrite mineral, after 96 h respectively.

Acknowledgements

The authors acknowledge the support of Yazd University and University of Cape Town. We also wish to thank the staff of the Department of Chemical Engineering, University of Cape Town. Prof Petersen also wishes to acknowledge the support of the South African National Research Foundation (NRF) through their Incentive Funding for Rated Researchers (No 85864).

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