4. Leaching of Zinc and Lead Hazardous Wastes in Alkaline Solutions

ZnO, ZnCO₃, and Zn₂SiO₄ are the predominant mineralogical phases in solid wastes bearing zinc. ZnO, ZnCO₃, PbO, PbCO₃, and PbSO₄ can be easily dissolved directly by strong alkaline solutions. However, quite a big portion of zinc in the dust exists as zinc ferrites or zinc sulfide and lead as lead sulfide which cannot be dissolved completely in alkaline solution. Integrated hydrometallurgical processes for leaching the zinc and lead from zinc ferrites, zinc sulfide, and lead as lead sulfide in alkaline medium are developed. For zinc ferrites in EAF dust, the zinc is extracted by direct leaching-melting-leaching of melt or hydrolysis-melting-leaching processes. For the zinc sulfide in solid wastes, the leaching rate of zinc in alkaline solution is improved via chemical conversion with PbCO₃ so that the zinc in zinc sulfide can be extracted in NaOH solution with lead carbonate as additive, and the lead sulfide in the leach residues can be converted to lead carbonate by reacting with sodium carbonate solution using air as the oxidizing agent, and then the lead can be recycled in the whole process by dissolving the lead carbonate in NaOH solution. Over 90% of zinc can be extracted from the zinc sulfide, and over 95% of lead sulfide in leach residues can be converted. The Pb in leaded glass can be extracted in NaOH solution after mechanochemical reduction with metallic iron as additive. Over 90% of Pb can be extracted from the mechanochemical-reduced leaded glass, compared with less than 5% of Pb extraction for alkaline leaching of nonactivated samples, and 67% Zn extraction for alkaline leaching of activated samples for 2 h by planetary ball mill. Alkaline leaching process of smoke dust and lead oxide ore is conducted, which found that the leaching rate was over 90% after 30 min or longer. The resultant leaching residue contains lower than 1–2% of zinc, 0.5% of lead, 0.3% of copper, and 0.1% of cadmium, and over 35% of Fe, and may be classified as nonhazardous waste according to the results of leaching tests on the residues.