Co-treatment of copper smelting slag and gypsum residue for valuable metals and sulfur recovery

Copper smelting slag and gypsum residue are by-products in copper making processes, and both contain valuable elements, such as copper, sulfur, etc. However, there is more than 0.25 wt% Cu in the smelting slag, which cannot be recovered with present technologies. A sulfidation process, which used the gypsum residue as the sulfur resource, was proposed and studied in treating copper smelting slag. Obtained results showed: 1) CaSO4 transformed to CaS under reductive decomposing conditions, and reactions between CaS and slag components occurred spontaneously during the slag cooling process; 2) the Fe/SiO2 mass ratio of the slag, the CaS excess coefficient (α), and the cooling rate (v) had an influence on the terminal phase composition, sulfide particle size, etc.; 3) under optimized conditions (Fe/SiO2=1.0: α=5, v = 1.00°C/min; Fe/SiO2=1.3: α=4, v = 0.25°C/min; Fe/SiO2=1.8: α=5, v = 0.25°C/min), the Cu contents in non-sulfide phases were lower than 0.08 wt%, and the mean particle size of sulfides were 37- 40 μm, which were feasible for flotation recoveries. TCLP (Toxicity Characteristic Leaching Procedure) tests indicated that the co-treatment process improved the stability of these starting materials. Based on samples characterization and thermodynamic analysis, a corresponding reaction mechanism was proposed.