Hydrothermal sulfidation of zinc-containing neutralization sludge for zinc recovery and stabilization

The conventional lime-neutralization process generates a huge volume of neutralization sludge (NS). The disposal of NS is difficult because it contains large amounts of heavy metals, moisture and complex components. The present study describes the application of hydrothermal sulfidation (HS) toward the conversion of certain heavy metals (mainly zinc) in NS to metal sulfide (MeS) for subsequent metal recovery through flotation. The effects of temperature, time, sulfur concentration and liquid-to-solid ratios were investigated under laboratory conditions. The extent of zinc sulfidation was up to 85% under optimal conditions. Flotation tests indicated that the ZnS produced could be separated from the NS, increasing the recovered Zn concentration from 19.9% to 38.8%. The toxicity characteristic leaching procedure (TCLP) revealed that stabilization and detoxification of heavy metals occurred during sulfidation. The formation of metal sulfide by HS enhanced the stabilization effect. Moreover, flotation technology provided a means to enhance metal recovery from the NS.