1. Amphoteric Metal Hazardous Wastes and Hydrometallurgical Processes of Zinc and Lead

Various wastes containing zinc and lead are generated in industries such as galvanizing, casting, scrap recycling, smelting, and pyrometallurgical and hydrometallurgical process, as forms of dust, tailings, residues, sludge, and lower-grade lean ores. The disposals of these wastes are now becoming expensive due to the need for the treatment to render the wastes nonhazardous. The options currently available can be comprehensively classified as security landfilling and pyrometallurgical and hydrometallurgical processes. As the wastes containing zinc and lead have been classified as hazardous, conventional landfilling processes should be modified to security landfilling in order to meet the environmental constraints required for the hazardous. The pyrometallurgical processes to treat hazardous wastes containing zinc and lead include Inred, Wala Kiln, Plasma, etc., in which zinc and lead can be easily extracted due to the high temperature used, though a large amount of energy will be consumed and serious secondary pollutions arise. The hydrometallurgical treatment method for wastes containing zinc and lead is by dissolution in mineral acids and alkaline solutions. For acidic leaching process, though zinc and lead will be dissolved to an acceptable high level, the bulk materials, iron, calcium, etc., will also be dissolved completely, and the dissolved iron and other elements have to be precipitated from the leach solutions, possibly leading to the generation of new hazardous wastes and wastewaters. Moreover, a big fraction of zinc exists as zinc ferrites in the dust, which cannot be attacked effectively by acidic leaching processes. Therefore, the acidic leaching process seems not to be economically viable for the treatment of dust. The alkaline process is considered to be a cleaner technology for extracting zinc and lead from the hazardous waste bearing zinc and lead and will be fully introduced in this book.