Oxidation of Copper Sulfides in Aqueous Ammonia. I. Formation of Sulfur

Abstract

Elemental sulfur, rather than sulfate or thiosulfate ions, is the major sulfur-containing product when CuS or Cu₂S are oxidized at temperatures below 40º with oxygen in a large excess of aqueous ammonia containing ammonium sulfate at pH 10-10.5. At temperatures above 60º, or in solutions containing low proportions of ammonia or ammonium sulfate, very little sulfur is formed. Higher partial pressures of oxygen lead to higher proportions of sulfur. These observations are explained in terms of concurrent and consecutive reactions involving copper polysulfides. The high proportion of ammonia to soluble copper ions which is required imposes some practical limitations on any potential leaching process aimed at producing sulfur. Proportions of sulfur from metal sulfide oxidation in aqueous ammonia decrease in the series of metal sulfides, Cu₂S > CuS > NiS > FeS > ZnS > CoS. Some aspects of the chemistry of sulfur in aqueous ammonia are discussed.