Electrochemical evaluation of the surface of chalcopyrite during dissolution in sulfuric acid solution

The dissolution of a massive chalcopyrite electrode (98.1% chalcopyrite, 1.9% siderite) was studied in 0.5 M sulfuric acid solution. Different anodic potentials were applied and the behavior of the electrode was observed by means of EIS, potentiodynamic, and Mott-Schottky techniques. Electrochemical impedance spectroscopy studies at open circuit potential (around −235 mV vs. MSE) proved the existence of a thin surface layer on the electrode. This layer was stable up to 100 mV vs. MSE and was assumed to be Cu_1−xFe_1−yS₂ (y?x) based on reports from previous studies. By increasing the potential to the range of 100-300 mV vs. MSE, the previously formed layer partially dissolved and a second layer (Cu_1−x−zS₂) formed on the surface. Both of the layers showed the characteristics of passive layers at low potentiodynamic scan rate (0.05 mV s⁻¹) while at high scan rates they acted like pseudo-passive layers. However, in the potential range of 300-420 mV vs. MSE, both of these surface layers dissolved and active dissolution of the electrode started. Further increase in potential caused the formation of a CuS layer which hindered the dissolution rate of the electrode. The formation of CuS is concomitant with Fe₂(SO₄)₃ formation and the latter may act as a nucleation precursor for jarosite at higher potentials (around 750 mV vs. MSE). Jarosite precipitation on the electrode surface hindered the dissolution of chalcopyrite at higher potentials. Different equivalent electrochemical circuits were modeled for each potential range and the model regression results compared with the experimental results of EIS to determine the proposed sequence of chalcopyrite dissolution.