Influence of magnetite on the leaching of chalcopyrite in sulfuric acid

2.1 Minerals

Chalcopyrite and magnetite samples used in the experiments were obtained from Daye, Hubei province, China. The mineral samples were firstly hand-selected to remove the impurities and then crushed with a high-speed crusher and dry-screened to 38–74 μm for leaching experiments. XRD patterns of the samples are shown in Fig. 1, and the chemical elemental analysis showed that the chalcopyrite samples contained 34.24% Cu, 33.27% Fe, 30.86% S, and the magnetite samples contained 36.23% Fe²⁺, 34.35% Fe³⁺, 25.42% O. The purity of the minerals was more than 90% and thus could be used as pure minerals.

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Fig. 1

XRD patterns of chalcopyrite (a) and magnetite (b) samples.

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2.2 Leaching experiments

Leaching experiments were carried out in 250 mL Erlenmeyer flasks with 100 mL of sterilized sulfuric acid prepared from 100 mL sterilized distilled water and then adjusted to pH = 1.6 with 20% diluted sulfuric acid. The solution pH was not readjusted during the experiment. 1 g chalcopyrite (C) and different masses of magnetite (M) were added to the flasks respectively (the mass ratio of magnetite to chalcopyrite was represented as M/C). All flasks were placed in an air-bath rotary shaker with a constant temperature of 35 ℃, and the rotating speed was set at 170 r/min. pH and redox potential were monitored periodically. The concentrations of the copper ions, total iron, and ferrous ions in the leaching solution were measured by UV–VIS spectrophotometer. Sampling loss and evaporation loss were replenished with sterile sulfuric acid at pH = 1.6. To ensure the credibility of the results, all the leaching tests were performed in triplicate parallels.

2.3 Electrochemical measurements

The working electrode was a carbon paste electroactive electrode (CPEE) prepared by mixing 2.1 g of mineral (0.038 mm), 0.6 g of graphite, and 0.3 g of paraffin (Wang et al., 2016). The solid paraffin was heated and dissolved in a 50 mL beaker, and then graphite and mineral samples were added to the beaker. The mixture was stirred evenly and ground in an agate mortar, and then was poured into a cylindrical chrome steel mold and compressed into a cylinder with a diameter of 15 mm and a height of 3 mm by a pelleter for 10 min at a pressure of 500 kg/cm². The well-compacted cylinder was used as the working electrode. Before each electrochemical test, the working surface of the electrode was polished with 600#, 1000#, and 2000# silicon carbide paper successively, and rinsed with distilled water and anhydrous ethanol.

All tests were conducted in a conventional three-electrode system consisting of a working electrode, a graphite rod (counter electrode), and a saturated calomel electrode (reference electrode), and were performed on a Zahner Zennium E potentiostat (Germany) coupled to a computer. Sulfuric acid at pH = 1.6 were used as electrolytes. All potentials reported in this work were relative to the standard hydrogen electrode (SHE). Open circuit potential (OCP) measurements were performed with the freshly prepared chalcopyrite and magnetite electrodes for 600 s. Galvanic current between chalcopyrite and magnetite was measured by a Zero Resistance Ammeter (ZRA) (Deng et al., 2022). Chalcopyrite was connected to the working electrode, while magnetite was connected to the auxiliary/reference electrode. The potential between chalcopyrite and magnetite was set to 0 V, and at this time the potentiostat was functioned as a ZRA. The galvanic current was measured for 300 s. Cyclic voltammetry (CV) was carried out with the freshly prepared chalcopyrite electrodes at a sweep rate of 20 mV/s. The scanning range of voltage was OCP～990 mV～-890 mV～OCP.

2.4 Leached residues analysis

Leached residues were characterized by Raman, XRD, and FTIR techniques. Raman spectra were measured in the range between 1200 cm⁻¹ and 100 cm⁻¹ with a step width of 0.1 cm⁻¹ on a HORIBA Scientific LabRAM HR Evolution Raman microscope, at an excitation wavelength of 633 nm. A very low laser power of 1 mW was used to protect the samples from damage. The laser spot of the Raman microscope was 2 μm in diameter. XRD patterns were obtained with a Bruker AXS D8 Advance Diffractometer with Cu Kα radiation. FTIR spectra were recorded by a Thermo Scientific Nicolet iS20 spectrometer in the transmittance mode (wavenumber range: 400–4000 cm⁻¹).

3.1 Leaching experiments

The leaching tests were carried out for 27 days, and the changes in copper extraction, total iron concentration, solution pH, and redox potential during the leaching of different mass ratios of chalcopyrite and magnetite are presented in Fig. 2. As shown in Fig. 2(a), no matter how much magnetite was added in the leaching system, the dissolution of chalcopyrite was promoted to a certain degree. The highest copper extraction of 8.03% was obtained in the presence of 0.5 g of magnetite. When the magnetite addition was more than 0.5 g, the copper extraction decreased and then had an increase with further addition of magnetite. Fig. 2(b) indicated that in the absence of magnetite, the total iron concentration was the lowest among the four mass ratios, which coincided with the lowest copper extraction in Fig. 2(a). As for the pure magnetite leaching system, the total iron concentration was rather high in the early stage, but then had a slow decrease with time, which might be related to the formation of some ferric precipitates. The M/C ratios of 1 and 2 exhibited a similar trend to the magnetite leaching system, but for the ratio of 0.5, the total iron concentration showed a slow rise in the whole leaching process. As shown in Fig. 2(c), when M/C was higher than 1, the pH increased in the first 3 days and then had a sharp decrease, while at the ratio of 0.5, the pH value increased continuously and reached to a plateau after 12 days.

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Fig. 2

Variation of copper extraction (a), total iron concentration (b), pH (c), and redox potential (d) during chalcopyrite leaching in the presence of different masses of magnetite.

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Redox potential plays a key role in the dissolution of chalcopyrite. As presented in Fig. 2(d), the redox potential in the absence of magnetite was maintained at 650–700 mV all the leaching time, which was well consistent with the pH variation. While adding 0.5 g magnetite, the redox potential did not show any obvious change and kept at the range of 650–700 mV. When more than 0.5 g magnetite was added, an evident difference was observed. After a slight decrease, the redox potential had sharply increased and finally stabilized at 850–900 mV. The redox potential in the leaching solution is related to c(Fe³⁺) / c(Fe²⁺) (Yang et al., 2018). Fig. 3 displays the change of c(Fe³⁺) and c(Fe²⁺). It can be seen that in the presence of magnetite, the initial c(Fe²⁺) was larger than 600 mg/L, indicating that there were a small number of soluble ferrous ions on the surface of magnetite. As the leaching proceeds, c(Fe²⁺) had a decline whereas c(Fe³⁺) had an increase, suggesting the oxidation of ferrous ions to ferric ions. When more than 0.5 g of magnetite was added, most ferrous ions were oxidized to ferric ions, raising the redox potential of the system to a higher value. As for the mass ratio of 0.5, less ferrous ions were oxidized, so the redox potential was maintained in a lower range. Due to different degrees of ferrous oxidation, the three systems with the addition of magnetite differed in their redox potentials.

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Fig. 3

Variation of c(Fe³⁺) (a) and c(Fe²⁺) (b) with time during chalcopyrite leaching in the presence of different masses of magnetite.

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As reported in a previous study, when the redox potential is in the range of 620–790 mV, chalcopyrite is reduced to chalcocite (Cu₂S), which is more liable to be oxidized to Cu²⁺, as shown in Eqs. (1, 2, 3) (Zhao et al., 2019). It is apparent that the redox potential with no addition of magnetite was in this range, but because the concentrations of Cu²⁺ and Fe²⁺ in the solution were rather low, the reduction of chalcopyrite to chalcocite (Eq. (1)) was difficult, thus contributing to the low copper extraction. For mass ratios of 1 and 2, most Fe²⁺ ions were oxidized to Fe³⁺ ions, so the redox potential was far beyond the range of 620–790 mV, which was not conducive to the reduction of chalcopyrite to chalcocite. In that case, chalcopyrite was directly oxidized by the high concentration of Fe³⁺ (Eq. (4)).

3.2 Leached residues analysis

3.2.1

3.2.1 XRD analysis

In Fig. 4, the XRD results were shown of chalcopyrite and magnetite residues after leaching for 27 days. It can be seen from the Fig. 4 that no other chemical phases were observed in the four residues as compared with pristine mineral samples. But it does not mean that there are no solid products formed in the leaching process of chalcopyrite and magnetite. That might be due to the poor crystallization and low content, so they were not detected in the residues.

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Fig. 4

XRD patterns of residues after leaching for 27 days. (a) M/C = 0, (b) M/C = 0.5, (c) M/C = 2, (d) M = 1 g.

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3.2.2

3.2.2 Raman analysis

Fig. 5 displays the Raman spectra of the residues after the leaching of 27 days. When no magnetite was added to the chalcopyrite leaching system, three peaks were detected at 290, 320 and 365 cm⁻¹. As for the mass ratio of 0.5, three peaks at 290, 319 and 350 cm⁻¹ were found in the residue. As reported in earlier studies, the main band at 290 cm⁻¹ and the weak bands at 320, 350 and 365 cm⁻¹ are assigned to chalcopyrite (Majuste et al., 2012; Yang et al., 2018). While with the adding 2 g magnetite, another two peaks at 221 and 430 cm⁻¹ were observed in the spectra of the residue. The peak at 221 cm⁻¹ was the lattice vibration of jarosite, and another peak at 430 cm⁻¹ was identified as v₂ (SO₄^2-) in jarosite (Chio et al., 2010; Parker et al., 2008).

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Fig. 5

Raman patterns of residues after leaching for 27 days. (a) M/C = 0, (b) M/C = 0.5, (c) M/C = 2.

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3.2.3

3.2.3 FTIR analysis

As shown in Fig. 2(b), with the rise of M/C ratio, different tendencies in the total iron concentration were observed, and when the mass ratios were set higher than 1, the total iron concentration had a drop after leaching for a period, showing a similar trend with the magnetite leaching system. Therefore, the FTIR spectra of the magnetite was also tested before and after the leaching, and the results are presented in Fig. 6. The absorption peaks in the spectrum of the original magnetite samples appeared at 949.35, 873.94, 827.69, 570.46 and 472.35 cm⁻¹. The peak at 472.35 cm⁻¹ was related to the bending vibration of the Si-O-Si bond, and the peaks around 949.35, 873.94 and 827.69 cm⁻¹ were characteristic peaks of fayalite crystals (Wang et al., 2022). The bands at 570.46 and 513 cm⁻¹ were ascribed to stretching vibrations of Fe³⁺–O^2- in FeO₆ octahedra (magnetite) (Smith et al., 2006). In the spectrum of the residue, besides the three peaks of magnetite and Si-O-Si, three new peaks at 1185.38, 1086.19 and 1005.12 cm⁻¹ were detected. The peak at 1005.12 cm⁻¹ corresponded to the deformation of OH, and the peaks at 1185.38 and 1086.19 cm⁻¹ were derived from the v₃ vibration of SO₄^2- (Baron and Palmer, 1996; Gao et al., 2019).

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Fig. 6

FTIR spectra of pristine magnetite samples (a) and magnetite leaching residue (b) after leaching for 27 days.

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3.3 Electrochemical measurements

Fig. 7(a&b) presents the influence of iron ions on the OCP values of chalcopyrite and magnetite and their galvanic current. The concentrations of iron ions were the same as that in the real leaching solution under the stable state of each system. In the sulfuric acid, the OCP of magnetite was rather higher than that of chalcopyrite, indicating that magnetite would react as the cathode while chalcopyrite would oxidize as the anode, leading to a positive current from chalcopyrite to magnetite and the increase of chalcopyrite oxidation. After adding iron ions, the OCP of chalcopyrite was rather higher than that of magnetite, contributing to a negative current from magnetite to chalcopyrite and the increase of magnetite oxidation. Based on the tests above, it is notable that the galvanic interaction between chalcopyrite and magnetite hinders the oxidation of chalcopyrite in the presence of iron ions. But as shown in Fig. 2, the addition of magnetite promoted the leaching of chalcopyrite, and Fe²⁺ and Fe³⁺ were produced in the leaching process. It is speculated that the influence of iron ions released from magnetite on the dissolution of chalcopyrite oxidation was much more significant than their galvanic interaction.

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Fig. 7

Open circuit potential (a), galvanic current density (b), cyclic voltammograms (c) of chalcopyrite (C), magnetite (M) and chalcopyrite-magnetite (C-M) electrodes in different electrolytes

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Fig. 7(c) shows the cyclic voltammograms of chalcopyrite in the sulfuric acid containing different concentrations of Fe³⁺ and Fe²⁺ ions. 5 anodic oxidation peaks A₁, A₂, A₃, A₄, A₅ and 4 cathodic reduction peaks C₁, C₂, C₃, C₄ can be found. Detailed reactions are as follows.

A₁ (Liang et al., 2011): CuFeS₂ → 2S⁰ + Cu²⁺+ Fe³⁺ + 5e^-. CuFeS₂ + 8H₂O → Cu²⁺+ Fe³⁺ +2SO₄^2- + 16H⁺ + 17e^- CuS + 4H₂O → Cu²⁺+ SO₄^2- +8H⁺ + 8e^- 2S⁰ + 2H₂O + 3O₂ → 4H⁺ +2SO₄^2-

C₁ (Elsherief, 2002): Cu²⁺+ CuS + 2e^- → Cu₂S. Cu²⁺+ S⁰ + 2e^- → CuS CuFeS₂ + 3Cu²⁺ +4e^- → 2Cu₂S + Fe²⁺

C₂ (Wang et al., 2016):9 CuFeS₂ + 4H⁺ + 2e^- → Cu₉Fe₉S₁₆ + 2H₂S + Fe²⁺. 5CuFeS₂ + 12H⁺ + 4e^- → Cu₅FeS₄ + 6H₂S + 4Fe²⁺

C₃ (Elsherief, 2002): 2Cu₅FeS₄ + 6H⁺ + 2e^- → 5Cu₂S + 3H₂S + 2Fe²⁺. 2CuFeS₂ + 6H⁺ + 2e^- → Cu₂S + 3H₂S + 2Fe²⁺

C₄ (Liang et al., 2011): CuFeS₂ + 4H⁺ + 2e^- →Cu⁰ + 2H₂S + Fe²⁺. Cu₂S + 2H⁺ + 2e^- → 2Cu⁰ + H₂S

A₂ (Liang et al., 2011): 2Cu⁰ + H₂S → Cu₂S + 2H⁺+ 2e^-.

A₃, A₄ (Rohwerder et al., 2003): H₂S → S₂^2- → S_n^2- → S⁰.

A₅ (Wang et al., 2016): Cu₂S → Cu_1.92S + 0.08 Cu²⁺ + 0.16e^-. Cu_1.92S → Cu_1.6S + 0.32Cu²⁺ + 0.64e^- Cu_1.6S → Cu_1.92S + 0.6 Cu²⁺ + 1.2e^-

It is apparent that the addition of Fe³⁺ and Fe²⁺ has an obvious impact on the degree of the redox reactions on chalcopyrite. Among the three systems, the current densities of the reaction peaks on the chalcopyrite electrode in presence of 50 mg/L Fe³⁺ +50 mg/L Fe²⁺ is the smallest, while that in the presence of 400 mg/L Fe³⁺ +300 mg/L Fe²⁺ is the largest, implying that the dissolution of chalcopyrite is associated not only with c(Fe³⁺)/c(Fe²⁺) but also with c(Fe³⁺), c(Fe²⁺).