Selective separation of calcium from zinc-rich neutralization sludge by sulfidation roasting and HCl leaching

Abstract

Zinc-rich neutralization sludge is a hazardous waste which is from the process of zinc metallurgical industry. In this study, a combined process of sulfidation roasting and HCl leaching was developed to treat zinc-rich neutralization sludge. The effects of roasting parameters including carbon dosage, roasting temperature and time on sulfidation roasting were investigated by thermodynamic calculation and laboratory experiments. The optimal conditions have been established as: carbon powder dosage of 15%, temperature of 700 °C, and time of 60 min. Under the conditions, 95.48% of zinc contained in the residues was converted into sulfides and 98.36% of sulfur was fixed into the roasting product. Meanwhile, the calcium sulfate was converted into calcite and calcium sulfide. The roasted sample was then subjected to HCl leaching process for the separation of calcium and zinc. About 93.75% of calcium was extracted by the leaching under the optimal conditions: HCl concentration of 3 mol/L, liquid to solid ratio of 5 mL/g, leaching temperature of 25 °C, and leaching time of 10 min. After the HCl leaching, about 97.37% of zinc was concentrated into the leaching residue with zinc grade of 45.86%. XRD analysis indicated that the leaching residue mainly composed of sphalerite and wurtzite, which can be used as a raw material for zinc smelting.