The response of sulfur chemical state to different leaching conditions in chloride leaching of chalcocite

Heap chloride leaching of low-grade copper sulfides with the use of seawater has been proposed as a feasible alternative to address challenges associated with water scarcity and depletion of oxides. Our previous study of leaching kinetics showed that the leaching reaction slowed down in the second stage and the maximum copper extraction achieved at ambient temperature was around 70–80%. To uncover the reasons for the slowed-down leaching, we investigated the evolution of solid surface properties during leaching, particularly morphology and sulfur chemical state, and the responses of mineral surface properties to different leaching conditions using SEM-EDX and XPS techniques. The results showed that sulfur sequentially transformed from monosulfide S2− to disulfide S22−, and then to polysulfide Sn2− and elemental sulfur S0. Less elemental sulfur was present on the solid particle surfaces at high solution potentials, the removal of which by CS2 washing did not improve copper extraction. In contrast, at low solution potentials, a slight increase in copper extraction was associated with sulfur removal from the solid surfaces. These results led to the conclusion that the slow decomposition of polysulfide formed during leaching was responsible for the slow reaction at high solution potentials, whereas at low solution potentials, a combination of polysulfide decomposition and diffusion barrier by elemental sulfur layer was the reason for the slow dissolution. The effect of chloride was concluded to be important only at low solution potentials where the level of elemental sulfur crystallinity was lower. •Sulfur underwent a sequential transformation during chloride leaching of chalcocite.•Sn2- and S0 were responsible for a slowed leaching reaction at low potentials.•At high potentials, sulfur removal did not improve leaching performance.•Sn2- was deemed to be the cause of a slowed leaching reaction at high potential.•A progressive conversion model for chalcocite leaching in chloride media was proposed.