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Abstract

The extraction of zinc, cadmium and mercury from chloride and sulphate media by solvating extractants, liquid anion and liquid cation exchangers has been reviewed both from the literature and by experiment. The results have been discussed in terms of both practical process possibilities and the use of slope analysis to identify the extracted species. These show that slope analysis cannot give positive unambiguous identification of extraction stoichiometry in systems in which the extracted metal species contains coordinating ligands from the aqueous phase. However, in the extraction of Zn and Cd from sulphate media by di(2-ethylhexyl)phosphoric acid (DEHPA) the extracted species is shown to be an MR2(HR)2 monomer by slope analysis demonstrating its use in simple systems. In the case of amines and solvating reagents such as TBPc and TOPO zinc appears to be extracted from Cl media as either ZnCl2 or sometimes HZnCl3 whereas Cd and Hg(II) appear to be always extracted as HCdCl3 and HHgCl3 species. The use of aqueous phase complexing by Cl to improve the separation of Zn and Cd by extraction is clear. Some results are presented for the recovery of zinc from sulphate media by oximes and mercury from sulphuric acid media by amines and DEHPA.