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 o f chalcopyrite with air in the presence of lime commensurate with conv ersion to CuO and minimum evolution of SO2. The analysis comprises ass essments of the thermodynamics of the reactions of copper-iron sulphid es with lime, the feasibility of copper ferrite formation and heat bal ances for the relevant roasting reactions. Thermodynamic consideration s indicate that the roasting of CuFeS2 with CaO and air to produce Fe2 O3, CaSO4 and CuO, CuSO4 or CuO.CuSO4 will occur readily. The exotherm ic nature of the reactions of copper-iron sulphides with lime results in favourable heats of reaction and they can proceed without any addit ional form of energy. Thermodynamics further indicate that CuFeO2 is s table relative to Cu2O and Fe2O3 at, effectively, all temperatures and that CuFe2O4 is unstable relative to CuO and Fe2O3 at low temperature s, becoming stable above 330-degrees-C. Heat-treatment of mixtures of CuO and Fe2O3 followed by X-ray diffraction analysis of the products i ndicates, however, that kinetic constraints prevent the formation of C uFe2O4 at temperatures below 700-degrees-C. As the formation of CuFe2O 4 during roasting is undesirable, this sets an upper limit for roastin g 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 th e roasting temperature is to be kept below the limit for copper ferrit e formation.