LIME-CONCENTRATE PROCESS FOR ROASTING OF COPPER-BEARING SULFIDES .3. MECHANISMS OF ROASTING REACTIONS

A variety of experiments was conducted to provide an insight into the mechanisms of the reactions that occur when copper-bearing sulphide co ncentrates are roasted in air in the presence of lime. The effects of temperature and water content on the roasting reactions of chalcopyrit e were investigated and an apparent activation energy of 26.5 kJ mol-1 was determined. Measurements that were made during roasting indicate that temperatures at the centre of a pellet can be 80-100-degrees-C ab ove the bulk gas temperature owing to the exothermic nature of the roa sting reactions. The addition of water to pellets of copper-iron sulph ides and lime prior to roasting leads to the formation of CuO as the m ain roasting product, whereas in the absence of water CuSO4 and CuO.Cu SO4 are the principal products. During the roasting of pellets of copp er concentrate and lime, in which the main mineral was chalcopyrite, i t was found that after chalcopyrite decomposes FeS is oxidized to Fe2O 3 and the copper sulphide that is produced is converted to copper sulp hate according to the scheme CuS --> Cu1.8S --> Cu2O --> CuO.CuSO4 --> CuSO4 At the same time CuO is converted to CuSO4 by direct sulphation and through the intermediate formation of calcium sulphide. Roasting experiments were also performed with pellets containing covellite (CuS ), digenite (Cu1.8S) and pyrite (FeS2). X-ray diffraction analysis of the roasted products indicates that iron plays a significant role in t he oxidation-sulphation reaction. In the presence of iron both calcium sulphide and calcium sulphate were formed, whereas CaS was not detect ed in the products of roasting of CuS-CaO and Cu1.8S-CaO. The results of the study suggest that the overall rate of reaction in pellets of c opper-iron sulphides and lime during roasting in air is controlled by heat and mass transfer across the boundary layer and the layers of sol id product that are formed during roasting.