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Pressure leaching of copper sulphides in perchloric acid solutions Loewen, Fred

Abstract

The leaching of covellite (CuS), chalcocite (Cu₂S), chalcopyrite (CuFeS₂) and bornite (Cu₅FeS₄) was carried out in a shaking autoclave in perchloric acid solutions and using moderate pressures of oxygen. The temperature range of investigation was 105-140°C. It was found that covellite, chalcocite and bornite leach at approximately similar rates, with chalcopyrite being an order of magnitude slower. It was found that chalcocite leaching can be divided into two stages, first, the rapid transformation to covellite with an activation energy 1.8 Kcal/mole, followed by a slower oxidation stage identified as covellite-dissolution with an activation energy of 11.4 Kcal/ mole. These two stages of leaching were also observed in bornite with chalcocite (or digenite) appearing as an intermediate step. No such, transformations were observed in covellite or chalcopyrite. Two separate reactions were recognized as occuring simultaneously for all four minerals during the oxidation process: an electrochemical reaction yielding elemental sulphur and creating pits on the mineral surface, and a chemical reaction producing sulphate. The first reaction dominates in strongly acidic conditions, being responsible for about. 85% of the sulphur released from the mineral, but the sulphate to elemental sulphur ratio in solution increases with decreasing acidity. Above 120°C the general oxidation process is inhibited by molten sulphur coating the mineral particles; the sulphate producing reaction, however, is not reduced above this temperature. For chalcopyrite the activation energies for the sulphate producing reaction and mineral solubilized were found to be 16.0 Kcal/mole and 11.0 Kcal/mole, respectively. It is suggested that chalcopyrite may leach partially via the .formation of transient covellite on the surface but since its leaching rate is faster than that for chalcopyrite, no covellite is detected after leaching.

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