Electrochemical dissolution of chalcopyrite: Detection of bornite by synchrotron small angle X-ray diffraction and its correlation with the hindered dissolution process

The formation of bornite (Cu ₅FeS ₄), an iron-deficient sulfide, and its correlation with the slow oxidation rate of chalcopyrite (CuFeS ₂) in acidic media under atmospheric conditions is demonstrated. Chalcopyrite electrodes oxidized in 0.1 mol/L H ₂SO ₄ solutions at room temperature (25 ± 1 °C) were analyzed by micro Raman spectroscopy and synchrotron small angle X-ray diffraction (S-SAXRD), techniques indicated for thin films analysis. Anodic polarization curves of chalcopyrite electrodes showed two well-defined behaviors: quasi-potential-independent regime and potential-dependent regime. When the critical potential (Ec) is attained, which ranged from 0.75 to 0.90 V vs. Standard Hydrogen Electrode (SHE), the mineral oxidation rate becomes strongly dependent on potential. Potentiostatic current-time profiles at 0.60 and 0.70 V vs. SHE indicated a current decay, which suggests the formation of a progressively thickening protective layer. The profiles at 0.80 V vs. SHE showed a similar current decay, but also an active oxidation process for some samples. After chronoamperometry at 0.70 and 0.80 V vs. SHE, respectively, for 6 and 2 h, analyses by using S-SAXRD revealed peaks of bornite on oxidized chalcopyrite electrodes. Elemental sulfur (S ₈) was also detected by S-SAXRD at 0.80 and 1.00 V vs. SHE, respectively, for 2 and 0.5 h. An unidentified metal-deficient phase and covellite (CuS) were detected on chalcopyrite by micro Raman spectroscopy after chronoamperometry at 0.60 and 0.80 V vs. SHE, respectively. The formation or absence of these product phases under a constant applied potential correlated well with a hindered dissolution or active oxidation processes. The results of this work support the hypothesis that the formation of intermediate iron-deficient sulfides contributes to the slow oxidation rate of chalcopyrite under atmospheric conditions.