Estudo morfológico e eletroquímico da oxidação de calcopirita em solução salina de biolixiviação

The dissolution of chalcopyrite is a complex process since some intermediate compounds (non-stoichiometric, polysulfides, bornite (Cu5FeS4), chalcocite (Cu2S) and covelite (CuS)) are detected during leaching. During the mineral dissolution sulfur, which can be oxidized in several stages due to its different oxidation states, and jarosite (XFe3(SO4)2(OH)6 where X = K+, NH4+, Na+ or H3O+) were obtained as products as result of the bioleaching process [1-2]. The objective of this work was to study the oxidation process of chalcopyrite by different electrochemical techniques and to analyze the morphology of the mineral after the different oxidation processes. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were carried out in salt solution containing 0.5 g L-1 of each salt: MgSO47H2O, (NH4)2SO4 and KH2PO4, pH ≈1.8. Electrochemical tests were performed using solid chalcopyrite. The CV and LSV measurements were obtained at different potential intervals, started in to the open circuit potential (EOCP) until +1.0 V/Ag|AgCl|KCl3 mol L-1 at 0.01 mV s-1 scan rate. In the CA tests, pulse potentials were applied from the electric double layer to chalcopyrite oxidation region in saline solution for 12 h. Afterward the CV, LSV and CA tests, the EIS diagrams were acquired. After different electrochemical tests, the total Cu and Fe ions concentration was determined in the solution by atomic absorption spectrometry (AAS), and the morphological surface characterization was performed by scanning electron microscopy (SEM). The SEM images, after different CV tests, showed a surfacerich in copper sulfide or a rich sulfur layer depending on the positive limiting potential. LSV showed the deposition of a film for potentials greater than 0.65 V and for potentials higher than 0.85 V high sulfur concentration was found on the chalcopyrite surface, and iron and copper ions in the electrolyte. CA measurements showed no film formation on chalcopyrite surface up to 0.45 V. At 0.65 V the current decreased with time, and at 0.85 V or 1.0 V, the current decreased for short times followed by an increase of the current until the end of the electrochemical test. There was no passive layer formation for potentials greater than 0.85 V. In the EIS tests, it was verified that the higher the potential, the smaller the Z real values, indicating a lower resistance to the chalcopyrite decomposition. (Part of the Master thesis of R.N. Peres, IQ-UNESP, 2016).