Chalcopyrite leaching in ammonium chloride solutions under ambient conditions: Insight into the dissolution mechanism by XANES, Raman spectroscopy and electrochemical studies

Ammoniacal leaching solutions have been considered as an effective lixiviant for the oxidative dissolution of chalcopyrite. However, compared with the extensive studies of acidic chalcopyrite dissolution, the dissolution mechanism of chalcopyrite in ammoniacal solutions needs further understanding. In this study, the oxidative dissolution of chalcopyrite in ammonium chloride solutions under room temperature and pH 8.5 was studied by X-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy and electrochemical impedance spectroscopy (EIS). Both batch leaching experiments and electrochemical studies showed that Cu(II) ions were more efficient than dissolved oxygen for chalcopyrite oxidation in ammonium chloride solutions. Leaching kinetics analysis showed that without the initial addition of Cu(II), chalcopyrite dissolution rate was controlled by two different stoichiometric chemical reactions: the initial and slow leaching stage controlled by dissolved oxygen; then predominated by Cu(II) rather than dissolved oxygen with a fast leaching rate when copper concentration exceeded a certain level (>0.12 g/L). Thiosulfate (S2O32−) rather than sulfate (SO42−) was confirmed as the primary soluble sulfur species produced during chalcopyrite dissolution. Decomposition of S2O32− in the presence of both O2 and Cu(II) resulted in the final stable product SO42− accompanied by the formation of other sulfur intermediate species, including S4O62− and SO32−. No evidence was found for the formation of elemental sulfur. Amorphous hematite (Fe2O3) and six-line ferrihydrite (5Fe2O3·9H2O) were the major components residing on the chalcopyrite surface, which were responsible for the slow dissolution rate at late stage. According to the present work, a model for the chalcopyrite dissolution mechanism in alkaline ammonium chloride solutions under ambient conditions is provided.