The Selectivity of Nickel Over Iron in the Lix63-HDNNS Solvent Extraction System

The effects of the aqueous phase pH and the organic phase HDNNS concentration on the distribution coefficients of Ni and Fe in pure HDNNS or mixed LIX63-HDNNS systems have been investigated. With the mixed extractants, there is antagonistic extraction of iron below pH 1.4 whereas synergistic extraction occurs above this pH. At pH 2.5 and a fixed LIX63 concentration, iron distribution increases markedly in comparison with the oxime-free system and goes through a maximum with increase in HDNNS concentration. A similar behavior is shown by nickel at both pH 1.0 and 2.5. The log DFeversus pH plot gives a curve with a slope of two for the LIX63-HDNNS mixed system, which suggests that iron is extracted in the form of a species with an ionic charge of 2, probably Fe(OH)2+. The extracted complexes are proposed as Ni(H2L)3D2where all three oxime molecules function as bidentate ligands, and Fe(OH) (H2L) 3D2where one of the oxime molecules functions as a monodentate ligand. The Ni/Fe selectivity is high at low pH and changes mainly as a result of variations in the nickel distribution coefficient, but it is low at high pH because of the synergistic extraction of iron.