Solvent extraction of zinc from chloride solutions using dialkyl derivatives of 2,2′-bibenzimidazole as extractants

Abstract

Four new 1,1′-dialkyl-2,2′-bibenzimidazoles were used to extract zinc(II) from acid chloride solutions at various concentrations of chloride and hydrogen ions. In low acidity media (pH > 2), the maximum zinc(II) extraction with the compounds is observed at chloride concentrations in the range 0.7–1.8 M. Zinc(II) ions are extracted as neutral (ZnCl2)mLn complexes (L = extractant) in which the m:n ratio varies from 1:2 to 3:3, depending on the metal-to-extractant molar ratio in the extraction media. In strongly acidic systems, anionic zinc(II) chloro-complexes may be extracted from aqueous solutions by protonated bibenzimidazoles as (LH+)2(ZnCl42−) or/and (LH+)(ZnCl3−) ion pairs. In weakly acidic media, at [Cl−] < 2 M, Zn(II) can be separated from Cd(II), Cu(II), and Ni(II) by 1,1′-didecyl-2,2′-bibenzimidazole, the best extractant from the tested dialkyl derivatives of 2,2′-bibenzimidazole. From the feed containing 26 g/L Zn(II), over 80% of zinc(II) ions is selectively separated after four extraction stages. 1,1′-Didecyl-2,2′-bibenzimidazole can be used for the selective separation of pure Fe(III) and Zn(II) from acidic chloride solutions. At a high chloride concentration (6 M) and at 0.5–0.8 M HCl in the aqueous feed, Fe(III) is efficiently transferred into the organic phase in the presence of Zn(II) and is then easily stripped off with water. Zn(II) can be completely removed from the iron-free raffinate after this aqueous solution is acidified to 1.5 M HCl.