Aqueous sulphur dioxide as a lixiviant for the extraction of cobalt, copper and nickel from discard smelter slag

SO2 was used for the hydrometallurgical treatment of discard smelter slag. Parameters studied included mixing strength, solid-liquid ratio, SO2(aq) concentration, temperature, particle size, pH and O 2. Without O2, the %extraction of Co, Fe and Zn (60 to 80%) was higher than Cu and Ni (35% for Ni), which was attributed to the existence of Cu and Ni as sulphides. Precipitation, likely as Chevreul's salt (Cu IISO3·Cu2ISO3·2H 2O) lowered Cu concentration with time. The shrinking core modelling of dissolution kinetics showed that product layer diffusion governed the leaching process. SO2(aq) was a more powerful lixiviant than H2SO 4, suggesting the contribution of sulphite complexation. SO2(aq) with dissolved O2, rapidly oxidised Fe(II) to Fe(III) and Cu(I) to Cu(II). Consequently, Cu precipitation was prevented, and Cu extraction reached 80%. The increase in acidity, due to the catalytic oxidation of SO 3-2 to SO4-2 by O2 did not improve Co and Ni extraction. The highest Co and Ni extraction (86% and 44% respectively) was achieved without O2 .