Growing Mechanism of Polysulfides and Elemental Sulfur Formation: Implications to Hindered Chalcopyrite Dissolution

Metal-deficient polysulfides have been argued for a long time to be responsible for the low kinetics of chalcopyrite leaching to extract copper. It has been shown that chalcopyrite surfaces are the source of sulfur that is oxidized to form polysulfides and elemental sulfur. Electronic structure calculations were performed for HS (x = 0, 1, 2 and n = 1...20), aiming to understand the effect of the pH on the growing chains and the formation of elemental sulfur. The estimated pK_a1 of the H₂S polysulfides converges from 4.2 (n = 3) to 3.4 (n ≥ 8), and the estimated pK_a2 converges from 7.6 (n = 3) to 4.1 (n ≥ 8). The initial steps of the formation of polysulfide chains are more favored for protonated species. The elemental sulfur formation due to the decomposition of polysulfides to form smaller chains is mostly favored for protonated species with n smaller than 12. For larger chains, the decomposition is thermodynamically favored for polysulfides with any degree of protonation. The consequences of these results to the understanding of the mechanism of the chalcopyrite leaching process are discussed with the focus on the pH effect and the formation of elemental sulfur.