Kinetics and mechanism of the conversion of covellite (CuS) to digenite (Cu1.8S)

Rates of conversion of covellite (CuS) to digenite (Cu_1.*S) in N₂ and various partial pressures of O₂ have been studied in the temperature range 260° to 400°C. The reaction is first order with respect to O₂ concentration in N₂?O₂ mixtures at 1 atm. The experimental activation energy for the conversion of CuS to Cu_1.8S is 23±3 kcal. In a nitrogen atmosphere, the conversion proceeds at a slower rate but with the same activation energy and leads to the formation of elemental sulfur instead of SO₂. An externally added small amount of iron powder (0.6 pct) increased the rate of conversion, but had no effect on the apparent activation energy. The rate-controlling reaction in the conversion mechanism is postulated to be Cu_1.8S→1.8Cu⁺+1.8e ^?+S at the Cu_1.8S/gas interface. Reduction of CuS by copper(I) ions and electrons occurs by a rapid reaction, 0.8 Cu⁺+0.8e ^?+CuS→Cu_1.8S at the CuS/Cu_1.8S interface, which steadily contracts with time.