Reaction of enargite (Cu3AsS4) in hot concentrated sulfuric acid under an inert atmosphere. Part II: High-quality enargite

Based upon the information obtained from the sulfuric acid baking of an enargite concentrate, a second series of experiments were performed using a high-quality enargite powder. The results obtained from the sulfuric acid baking of high-quality enargite provided significant insights towards a more rigorous plan of attack in identifying the meaningful range of parameters and perhaps the time and temperature range in which the rate of the baking reaction should be studied. Hence, the effects of baking time, baking temperature, sulfuric acid dosage, nitrogen flow rate, and particle size on the water extraction of copper and arsenic were studied, as well as the mineralogical transformations associated with sulfuric acid baking. It was found that baking of enargite for extended times at a temperature of 200 °C had no effect on the extraction of Cu and As, which leveled off at ~ 73.7% and ~ 59.4%, respectively. Arsenic loss to the gas phase corresponding to 7 h of baking was found to be ~ 10.4%. Increasing the baking temperature in the range of 200–260 °C at a constant time of 6 h had no beneficial effect, but it adversely increased the arsenic loss to ~ 54.7% at 260 °C. Increasing the nitrogen flow rate in the range of 163–813 mL/min at a constant baking temperature of 200 °C for extended times slightly increased the arsenic loss. Under identical baking conditions, sulfuric acid dosage had the greatest effect on the extraction of Cu and As, with effective extraction levels being achieved at a sulfuric acid to enargite weight ratio of about 3.7. No systematic effect of particle size was observed on the extraction of Cu and As from enargite. Upon sulfuric acid baking, enargite transformed into CuSO4, CuO·CuSO4, As2O3, and S8, all of which, except S8, are soluble in water. Based on the experimental results, the following reaction is suggested for the sulfuric acid baking of enargite:4Cu3AsS4 + 51H2SO4 = 3CuO·CuSO4 + 6CuSO4 + 2As2O3 + 51H2O + 54SO2 (g) + 4S