Oxidative pressure leaching of chalcocite in sulphuric acid

At INCO's Copper Cliff Copper Refinery, a copper sulphide residue containing precious metals is subjected to a pressure leach at 115°C. Copper occurs predominantly as Cu₂S and Cu₁.₉₆S. The leach produces a slurry of copper sulphate solution and basic copper sulphate and precious metals solids. The basic copper sulphate is dissolved in spent acid from the electrowinning tankhouse leaving a precious metals residue for further processing. The leach periodically develops a problem referred to as a "slow cook" where leaching times are greatly extended and/or incomplete leaching of copper is encountered. Based on earlier work, chalcocite leaching was proposed to occur sequentially via the following reactions: [formula omitted]. The exact reaction path is determined by the initial solution conditions (copper sulphate and sulphuric acid concentrations). Under normal batch makeup conditions, all of the Cu₂S is oxidized to cupric ions and sulphate via reactions 1,2 and 4. If the solution becomes depleted in copper and acid, reaction 3 could occur. Elemental sulphur can be produced via reaction 5. Experimental studies showed that the reactions were nearly sequential. Reactions 1 and 2 were found to be very fast relative to the rate of reaction 4. No slow cook conditions were observed in the laboratory under normal leaching conditions. There is evidence suggesting that the slow cooks are caused by oxygen mass transfer limitations under conditions where the slurry becomes highly viscous and pseudoplastic due to formation of finely divided basic copper sulphate.