Circular 112—The dissolution of chalcocite in oxygenated sulfuric acid solution

By W. W. Fisher and R. J. Roman, 1971, 28 pp., 1 table, 13 figs.

Although the dissolution of chalcocite in a variety of lixivants has been studied by many investigators, none has completely defined the chemistry and mechanism of chalcocite dissolution, and in particular, the mechanism of dissolution in oxygenated acid solution has not been resolved. The importance of chalcocite as an ore mineral in copper leach dumps justifies a detailed study of its dissolution in oxygenated acid solution.

The dissolution of chalcocite in acidified ferric sulfate and in oxygenated sulfuric acid solution under ambient conditions at several acid and ferric ion concentrations has been studied. It was found that chalcocite is oxidized by acidified ferric sulfate in two distinct stages. Although the secondary product was not identified, it was proposed that chalcocite is oxidized to covellite and that the covellite oxidizes further to form free sulfur. Then the free sulfur as the final solid reaction product was identified. The dissolution of the intermediate product is slower than that of the original chalcocite. Work on chalcocite dissolution is much slower than the rate of dissolution in ferric sulfate.

The investigations of chalcocite dissolution show that chalcocite is oxidized in two steps, although the reaction sequence has not been established and CuS has not been positively identified. The intermediate product of the oxidation is proposed to be CuS. This study is to determine the reaction for the dissolution of chalcocite in oxygenated sulfuric acid solution, establish a general rate equation for this reaction, and describe a mechanism of chalcocite dissolution, based on a combination of thermodynamic, chemical, kinetic, and physical studies. The reaction rate is a function of stirring speed up to 700 rpm. Surface area has a first-order effect on the reaction rate. A proposed reaction mechanism consistent with the experimental results includes both physical and chemical processes, but the rate-controlling step is thought to be chemical.