Reaction of enargite (Cu3AsS4) in hot concentrated sulfuric acid under an inert atmosphere. Part I: Enargite concentrate

The sulfuric acid baking of an enargite concentrate was performed in a tube furnace under a flow of nitrogen gas. In this way the extent of reaction was measured in the absence of oxygen and a point of reference established. The effects of baking time, baking temperature, sulfuric acid dosage, and nitrogen flow rate on the water extraction of Cu, As, and Fe from the baked enargite concentrate were studied. Sulfuric acid baking of the concentrate for 3 h at 200 °C resulted in average Cu, As, and Fe extractions of ~ 75.7%, ~ 63.0%, and ~ 43.1%, respectively. The average As loss to the gas phase corresponding to the indicated operating conditions was ~ 5.9%. Characterization of the baked concentrate indicated the formation of S8, As2O3, HFe(SO4)2·4H2O, and Cu0.47Fe0.53SO4·H2O as a result of baking, identical to the phases formed upon sulfuric acid baking of the concentrate in a muffle furnace. The fact that no secondary copper sulfides were found to form around the enargite particles indicates that most probably enargite directly transformed to copper sulfate. The variation of temperature in the range 180–220 °C did not affect the extraction of Cu and Fe significantly, but As extraction decreased from ~ 65.2% to ~ 52.5%, owing to a systematic arsenic loss from ~ 3.7% at 190 °C to ~ 10.9% at 220 °C. Increasing the nitrogen flow rate from 340 mL/min to 1049 mL/min did not influence the extraction of Cu, As, and Fe upon water leaching. Sulfuric acid dosage had the greatest effect on the extraction of metals. Sulfuric acid to concentrate (S/C) weight ratios in the range 1.0–2.3 at a constant baking temperature of 200 °C over 6 h resulted in a steady increase of metals extraction from 52.6% to 82.2% for Cu, from 40.6% to 68.0% for As, and from 22.8% to 62.1% for Fe. No arsenic loss to the gas phase was observed when the highest S/C ratio (2.3) was used.