The chalcopyrite (CuFeS₂) is the most abundant copper sulfide in nature and belongs to the most explored group of copper minerals, accounting for about 70% of the total copper available [1]. The resistance to dissolution of chalcopyrite is generally attributed to the formation of passive layers on the mineral surface, which partially or totally blocked its surface, limiting its dissolution. Many species have been identified as able to "passivate" the surface of chalcopyrite as elemental sulfur, polysulfide and the iron hydroxides or hydrated oxides of iron and jarosites. This "passive" layer is considered to be formed after a certain period of contact of the surface with the solution, causing a blockage of mineral dissolution. Therefore, some leaching products have been pointed as responsible for this blockage which inhibits further mineral dissolution [2-4]. Other researchers have presented a different theory about what is called “passivation of chalcopyrite”, since they believed that passivation of chalcopyrite surface does not occur, but the slow leaching kinetics of chalcopyrite must be attributed to the semiconductor behavior of the mineral [5,6]. Recent results of our research group also suggest the absence of chalcopyrite passivation [7-8]. In the last two decades, several researchers have been investigated the formation of intermediates of the dissolution process of chalcopyrite, such as disulfides (S₂^2-), polysulfide (S_n^2-), metal-deficient sulfides (Cu_1-xFe_1-YS_2-z), CuS₂ that were identified as possible responsible for slowing the dissolution of chalcopyrite. Researchers showed that dissolution of chalcopyrite occurs at potentials higher than 0.615 V/SHE and the formed products on the chalcopyrite surface were metal-deficient sulfides, covellite and chalcocite [7-9].

The goal of this work was to study the different oxidation/reduction processes of chalcopyrite by means of different electrochemical techniques: cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). After the different electrochemical events, the surface morphology of the mineral was investigated by scanning electron microscopy (SEM) and the semi-quantitative analysis of the surface elements was performed by energy dispersive X-ray spectroscopy (EDS), and the total Cu and Fe ions concentration in the working solution were determined by atomic absorption spectrometry (AAS).

For all electrochemical studies, the electrolyte was composed by 0.5 gL^-1 of each following salts: MgSO₄.7H₂O, (NH₄)₂SO₄ and KH₂PO₄ with solution pH»1.8 and at 25^oC. Electrochemical tests were performed using carbon paste electrode (CPE) modified with chalcopyrite. The CPE electrodes used were prepared with the amount of 60 wt.% chalcopyrite and 40 wt.% carbon, one drop of binder and 0.6 mL of chloroform. Afterwards the mixture of carbon and chalcopyrite was placed cavity in the electrode to perform the electrochemical measurements. The CPE electrodes were sanded until a homogeneous and uniform surface was obtained. A three-electrode electrochemical cell being Ag|AgCl|KCl_{3 mol L}^-1 the reference, a platinum network the auxilary and the CPE the working electrode was used for all electrochemical studies.

The CV measurements were carried out at different potential intervals, started at the open circuit potential (E_OCP) and varied between ±1.5V/Ag|AgCl|KCl_{3mol L}^-1 at 20 mVs^-1. Some intermediate potential regions such as ±1.0V/Ag|AgCl|KCl_3molL^-1 was also explored in the CV tests to investigate the different redox pairs formed on the chalcopyrite surface. To investigate the chalcopyrite oxidation/reduction in saline solution, different step potentials were applied for approximately 14 h: ±0.40, ±0.60, ±0.80 and ±1.0V/Ag|AgCl|KCl_{3mol L}^-1. After these experiments, the EIS diagrams were acquired in sequence. Some potential values used in the CA tests were applied on SWV measurements for obtaining the best potential intervals for the dissolution of chalcopyrite.

The different CV experiments allowed describing qualitatively the electrochemical behavior of chalcopyrite powder in acidic salt solution. At least 10 current peaks were found in the potential range between ±1.5 V/Ag|AgCl|KCl_{3mol L}^-1. In the reduction studies, it was observed that the cathodic current peaks increased for potentials lower than -0.75V/Ag|AgCl|KCl_{3mol L}^-1 which was associated with the reduction of copper species to form new copper compounds as covellite and chalcocite and metallic copper on the chalcopyrite surface. The chronoamperometric tests confirmed that there is no blockage or passive layer formation on the mineral surface; on the contrary, they suggest dissolution of chalcopyrite to produce iron and copper ions in solution. By means of these electrochemical measurements and the morphological characterization, the partial conclusions are: for potential E ≥ +0.80V the chalcopyrite is dissolved to form mainly S⁰ on the electrode surface and Cu and Fe ions in solution. The application of potentials capable to overcome the band-gap of the chalcopyrite semiconductor does not generate passive layers, which was confirmed by chronoamperometry, electrochemical impedance and SEM/EDS analysis.