Reaction of enargite (Cu3AsS4) in hot concentrated sulfuric acid under an inert atmosphere. Part III: reaction stoichiometry and kinetics

Abstract

Based on the extensive insights gained from the acid bake–leach reaction from our previous experiments, a rigorous experimental plan was designed to investigate the effects of time, temperature, particle size, nitrogen flow rate, sulfuric acid concentration and dosage, and reaction surface area on the extraction of Cu and As from a high-quality enargite sample through sulfuric acid baking and leaching. The baking experiments in the temperature range of 170–200°C over 1–4h revealed almost complete Cu and As extraction after 4h baking at 200°C, a sulfuric acid to enargite weight ratio (S/E) of 4, nitrogen flow rate ~100mL/min, and particle size P100=−45μm, and no arsenic was lost to the gas phase. The following reaction is suggested for the sulfuric acid baking of enargite:4Cu3AsS4+28H2SO4=3CuO•CuSO4+6CuSO4+2As2O3+28H2O+19.5SO2g+15.5SLack of a systematic dependence of reaction rate on particle size and the significant dependence of the rate on sulfuric acid concentration suggest that the rate of the baking reaction is controlled by the internal surface area of cracks at which surface the baking reaction occurs. It is expected that these cracks are formed as a result of a major volume increase during the baking reaction. High nitrogen flow rates encouraged arsenic loss to the gas phase. Decreasing the S/E ratio from 4 to 2 resulted in a decrease in the rate of reaction, while increasing sulfuric acid concentration enhanced the rate. Excess sulfuric acid was found to be necessary for both acceptable reaction rates and also for maximum As retention in the condensed phase. Doubling the reaction surface area did not affect the extraction of Cu and As.