Recovery and separation of silver and mercury from hazardous zinc refinery residues produced by zinc oxygen pressure leaching

Abstract

Hazardous zinc refinery residues that contain Ag and Hg are a typical complex material obtained from zinc oxygen pressure leaching and currently there are few economically viable methods for disposal or reuse. The research presented here offers an effective approach for the comprehensive recovery of Zn, Fe, Cu, Ag and Hg from this highly toxic waste. During the initial hot-acid leaching stage, leaching efficiencies of 96.3% Zn, 96.0% Fe and 97.5% Cu were obtained, whereas the leaching of Ag and Hg were <0.2%. This resulted in the simultaneous separation of Zn, Fe and Cu and the enrichment of Ag and Hg ca. 300%. Subsequently, the leaching residues obtained from hot-acid leaching was further leached using acidic thiourea solution (pH = 1). The results of this second leaching step showed the almost 93% of Ag and 98% of Hg could be extracted using 20 g/L thiourea and 4 g/L Fe3+, with a L/S ratio of 8 and a temperature of 40 °C for 2 h. The residue after thiourea leaching could be used as raw material in lead smelting. In the next step based on zinc powder cementation, the recovery of Ag and Hg both reached 99.0% and the cementation residue comprised of 53.1 wt% Ag and 11.7 wt% Hg. The main phases present included Ag, Hg3Ag2 and Ag2S and these can be further treated by vacuum distillation in order to separate Hg from Ag. These findings demonstrate that high recoveries of Zn, Fe, Cu, Ag and Hg from the toxic waste could be achieved with stepwise leaching followed by zinc powder cementation. This treatment protocol for toxic zinc refinery residues not only avoids the potential harm to the environment but also significantly improves the economics of the process.