Dissolution mechanism of the oxidation process of covellite by ferric and ferrous ions

Abstract This study explores the influences of external iron ions on the dissolution behavior of covellite minerals at different temperatures. External iron ions can significantly accelerate covellite dissolution. The copper extraction of acid covellite leaching was less than 5%, while the addition of external iron ions (ferric or ferrous ions, Fe3+/Fe2+) effectively enhanced covellite dissolution. Covellite copper extraction with 0.1 mol/L Fe2+ was 25% at 303 K and 40% at 318 K after 18 days, and for leaching systems with 0.1 mol/L Fe3+, it was approximately 40% at 303 K and 60% at 318 K. The dissolution process of covellite with external iron ions can be divided into two stages. The dissolution kinetics at the first stage are controlled by the chemical reaction between iron ions and covellite, as the generated elemental sulfur has not yet blocked the covellite surface. Compared with ferrous ions, ferric ions further increase the dissolution rate of covellite, which is also suggested by the scanning electron microscopy analysis results. Additionally, Fe3+ accelerates the release of copper ions by effectively reducing the particle size of covellite. In the second stage (copper extraction >40%), the reaction kinetics shift to the mixed control of the surface chemical reaction with iron ions and diffusion due to the accumulation of an elemental sulfur layer on the surface. Although Fe2+ cannot directly oxidize covellite, the electron exchange between Fe2+ and oxygen is improved through the covellite mineral, which is attributed to its metallic properties. Then, the ferric ions generated in the above process gradually corrode covellite. In addition, the temperature rise can significantly accelerate the oxidation reaction process of iron ions with covellite by forming a more porous corrosive structure. Finally, leaching bacteria (Acidithiobacillus caldus and Leptospirillum ferriphilum) are proved to boost copper extraction from covellite by transforming Fe2+ into Fe3+ and eliminating surface elemental sulfur passivation. However, dense passivation layers inevitably form on the covellite surface at the end of bioleaching, which passivates the subsequent dissolution process.