LIME-CONCENTRATE PROCESS FOR ROASTING OF COPPER-BEARING SULFIDES .1. ANALYSIS OF OPTIMUM ROASTING CONDITIONS

An analysis has been made of the optimum conditions for the roasting of chalcopyrite with air in the presence of lime commensurate with conversion to CuO and minimum evolution of SO2. The analysis comprises assessments of the thermodynamics of the reactions of copper-iron sulphides with lime, the feasibility of copper ferrite formation and heat balances for the relevant roasting reactions. Thermodynamic considerations indicate that the roasting of CuFeS2 with CaO and air to produce Fe2O3, CaSO4 and CuO, CuSO4 or CuO.CuSO4 will occur readily. The exothermic nature of the reactions of copper-iron sulphides with lime results in favourable heats of reaction and they can proceed without any additional form of energy. Thermodynamics further indicate that CuFeO2 is stable relative to Cu2O and Fe2O3 at, effectively, all temperatures and that CuFe2O4 is unstable relative to CuO and Fe2O3 at low temperatures, becoming stable above 330-degrees-C. Heat-treatment of mixtures of CuO and Fe2O3 followed by X-ray diffraction analysis of the products indicates, however, that kinetic constraints prevent the formation of CuFe2O4 at temperatures below 700-degrees-C. As the formation of CuFe2O4 during roasting is undesirable, this sets an upper limit for roasting temperatures. If heat losses are assumed to be 25%, consideration of the heat balance for the roasting reactions indicates that excess air 2.5-4.5 times the stoichiometric requirement should be employed if the roasting temperature is to be kept below the limit for copper ferrite formation.