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

Abstract 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 ZnCl 2 L 2 complexes. In strongly acidic chloride media this amide forms ion pairs (LH + ) 2 (ZnCl 4 2 − ) and (LH + )(ZnCl 3 − ), or (LH + ) 2 (ZnCl 4 2 − )(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 (ZnCl 2 ) 3 L 2 , ZnCl 2 L 2 or ZnCl 2 L 3 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).