Formation of Elemental Sulfur during the Oxidation Leaching of Chalcocite

Abstract Nonstoichiometric minerals form during the leaching of chalcocite; they form the following sequential row between chalcocite and covellite: djurleite (Cu1.93–1.96S), roxbyite (Cu1.72–1.82S), digenite (Cu1.75–1.78S), anilite (Cu1.75S), geerite (Cu1.5–1.6S), spionkopite (Cu1.4S), and yarrowite (Cu1.1S). A hypothesis that the oxidation of chalcocite (Cu2S) ends in villamaninite (CuS2) rather than covellite (CuS) has been put forward and substantiated. The formation of various copper sulfides during the oxidation of chalcocite is related to the fact that copper has a variable valence and sulfur can be in them in both mono- and disulfide form. A scientific substantiation and chemical reactions are proposed for the formation of elemental orthorhombic sulfur at the last stage of chalcocite leaching, and the possibility of appearance of its other allotropes is shown. When chalcocite and other sulfides are leached below the melting temperature of sulfur (low-temperature regime), cyclic sulfur molecules, which are essentially liquid (molten) sulfur, form. During leaching, sulfate ions and elemental sulfur do not precipitate in two independent processes: they form in one way through the formation of polysulfides, which are transitional phases to molecular sulfur.