Ecological treatment of waste products from zinc hydrometallurgy

Abstract

The waste products of zinc hydrometallurgy fall into two categories: Ferric waste Toxic waste The first of these, produced by leaching the roasted zinc, is based on jarosite, goethite or haematite depending on the process followed. The second kind, toxic waste, arises from the purification of neutral leaching solutions, and consists of zinc powder and various cemented metals (copper, cadmium, cobalt, nickel, among others). In this paper, two laboratory-developed methods based on the technique of solvent extraction are presented and discussed. The ferric waste is treated by a conventional hot acid leaching process; the acid ferric leach is treated with an amine solution, and subsequently the extracted iron is precipitated directly in the organic phase. The ferric hydroxide is calcinated, and a high-quality iron oxide with 70% iron is obtained. On the other hand, in the case of the residue from purification, it undergoes mixed leaching, in the presence of an organic phase and another aqueous one in an oxidising atmosphere at room temperature. The zinc is recovered through stripping of the organic phase, and the metals remaining in the aqueous phase are recovered through selective extraction or through precipitation. Lead and silver are left in the leaching residue.