Pressure Oxidation of Enargite Concentrates Containing Gold and Silver

Abstract

This project proposed a selective moderate temperature and pressure approach to the study of pressure oxidation leaching of enargite concentrates, which may be applied to copper sulfide orebodies and concentrates containing significant arsenic. Previous industrial methods have employed sulfuric acid-oxygen pressure leaching, alkaline sulfide leaching, and roasting. The approach includes evaluating the chemical reactions taking place and the effects of pressure, temperature, pH and redox potential on the fate of the minerals present in the concentrates. The main objective of this project was to develop and confirm an innovative, alternative approach to selectively upgrade enargite concentrates to recover the copper, gold and silver values while selectively leaching the arsenic. Enargite concentrate minerals were characterized before and after the experiments to determine any changes in mineralogy, composition and morphology. Optimized pressure oxidation resulted in arsenic extraction of up to 47%. Mineralogically, the leached residues showed higher pyrite content than the feed sample by 6.5–15 weight percent with a slight decrease in the enargite content. Further work with high purity enargite specimens under optimized conditions clearly showed the selective separation of arsenic to solution with retention of copper in the solid phase based on the experimental mass balances. Specifically, covellite is highlighted in the leach residue. This particle shows enargite mineralization on the outer edges of a covellite particle, which may be due to particle orientation or removal of the covellite outer product layer during sample preparation for MLA. However, more importantly, the image clearly shows that the covellite phase is occurring in conjunction and direct association with enargite, as was predicted by the thermodynamic Eh-pH analysis. Covellite appears to be a direct decomposition product of selective dissolution of arsenic from enargite. In summary, the propensity for moderate temperature selective pressure oxidation for separation of arsenic from enargite appears to be promising. Larger scale testing, mineralogical characterization and further optimization are suggested to confirm these results.