Zinc extraction from chloride solutions with N-alkyl- and N,N-dialkylpyridinecarboxamides

N-alkyl- and N,N-dialkylpyridine-2(and -3)-carboxamides were used to extract zinc(II) from chloride solutions. N,N-dialkylpyridine-3-carboxamide is an effective extractant for zinc recovery from weak- and strong-acid chloride solutions. In weak-acid systems (pH > 3) zinc ions are extracted as neutral ZnCl2L2 complexes. In strongly acidic chloride media this amide forms ion pairs (LH+)2(ZnCl42−) and (LH+)(ZnCl3−), or (LH+)2(ZnCl42−)(LHCl) aggregates. The complexes composition depends on HCl concentration. N,N-dialkylpyridine-2-carboxamide extracts zinc(II) from chloride solutions only at [HCl] > 2 M. N-alkylpyridine-3-carboxamide removes larger amounts of zinc(II) from chloride solutions than N,N-dialkylpyridine-3-carboxamide. In weak-acid systems, it forms (ZnCl2)3L2, ZnCl2L2 or ZnCl2L3 complexes, slightly soluble in hydrocarbon diluents. N-alkylpyridine-2-carboxamide is an ineffective extractant of zinc(II). In strongly acidic media, hydrochlorides of pyridinecarboxamides can precipitate as a liquid third phase. The addition of hydrophobic alcohol to the extraction system eliminates the third phase formation. The obtained extraction data for Zn(II), Cd(II) and Cu(II) suggest that N,N-dialkylpyridine-3-carboxamide can be used for zinc separation from cadmium in concentrated chloride media at pH > 3, and for zinc and cadmium separation from copper in strongly acidic systems ([HCl] > 2 M).