Solvent extraction of cobalt and nickel by organophosphorus acids I. Comparison of phosphoric, phosphonic and phosphonic acid systems

Abstract

The solvent extraction of cobalt(II) and nickel(II) by xylene solutions of commercial dialkyl-phosphoric (D2EHPA), phosphonic (Shell RD 577) and phosphinic acids (Cyanex CNX) has been investigated. In each case, cobalt is extracted at lower pH values than nickel; under comparable conditions, the cobalt-nickel separation increases in the order phosphoric < phosphonic < phosphinic acids. Application of the slope analysis technique indicates stoichiometries of Co(HA 2) 2 and NiHA 2) 2(H 2 A 2) x (H 2O)2− x for the cobalt and nickel complexes, respectively, where H 2A 2 represents the dimerized extractant molecule and x = 0, 1, or2. The effect of a range of organic phase additives has been examined. Typical emulsion inhibiting reagents such as isodecanol and tributyl phosphate cause the cobalt—nickel separations to decrease significantly. A novel synergistic effect has been found with mixtures of D2EHPA and 2-ethylhexanal oxime (EHO); the synergism is so marked in the case of nickel as to reverse the selectivity for cobalt over nickel shown by D2EHPA alone. The electronic spectra of the organic extracts show that the organophosphorus acids examined from complexes of tetrahedral structure with cobalt and octahedral structure with nickel. In the D2EHPA—EHO system, however, the deep blue tetrahedral cobalt-D2EHPA complex is transformed into a pink octahedral complex, presumably of the type Co(HA 2) 2(EHO) 2, whilst the green octahedral nicke—D2EHPA complex is converted to a bright blue species of similar geometry, in which the neutral oxygen-donor ligands (H 2A 2 and/or H 2O) have been displaced by the nitrogen-donor oxime ligands.