5. Purification of Leach Solution of Zinc and Lead in Alkaline Solutions

Zinc and lead are usually concomitantly present in zinc and lead hazardous wastes such as electric arc furnace (EAF) dust. The selective and quantitative separation of lead from zinc is an important step in these alkaline hydrometallurgy processes. The separation of lead from zinc in alkaline zinc solution, in leaching solutions of zinc and lead hazardous wastes, can be achieved by sodium sulfide precipitation. When the weight ratio of sodium sulfide (average molecular weight 222) added to the lead present in alkaline solution is over 1.8–2.0 (molar ratio approximately 1.5–1.7), lead could be separated selectively and quantitatively, while the zinc remained in the solution without concomitant loss. The residues from the precipitation step are identified as mixtures of PbS and Na₂Pb(OH)₂S, with PbS being the predominant compound. Sn can be cemented quantitatively from alkaline zinc solution by the addition of zinc powder at 95 °C, 500 rpm with Zn/Sn molar ratio 6. Al in the leaching solution can be removed by Na₂SiO₃, As can be removed by Fe₂(SO₄)₃ at low NaOH concentration, Cl⁻ can be removed by increasing NaOH content, and Cu can be removed from alkaline lead leaching solution by Pb power replacement.