Oxidation of Copper Sulfides in Aqueous Ammonia. III. Kinetic Characteristics

The kinetics of oxidation with oxygen of chalcocite, Cu2S, to CuS in buffered aqueous ammonia at pH 10.5 at 30º can be modeled approximately by a shrinking core of Cu2S within a thickening shell of CuxS (x = 1). The Cu2S core offers partial cathodic protection to the CuxS and diffusion of Cu+ through CuxS controls the rate of reaction. The kinetics of oxidation of covellite, CuS, to Cu2+, sulfur and sulfate ions in the same solvent can be modeled by a shrinking core of CuS surrounded by a shrinking sphere of CuyS (y < 1) which is much less effectively protected cathodically by the CuS core. Oxidation of CuS is subject to mixed chemical and diffusion control. Rates of oxidation of NiS and of CuS, in the presence and absence of tetrachloroethene and ammonium sulfate, show that, whether sulfur is a major oxidation product or not, the presence of sulfur has very little, if any, influence on the rate or mechanism of oxidation. This is contrary to current ideas on metal sulfide oxidation.