The effect of diluent in the liquid-liquid extraction of cobalt and nickel using acidic organophosphorus compounds.

The effect of the diluent in the extraction of Co, Ni and Na from nitrate media with di(2-ethylhexyl)phosphoric acid (HDEHP) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) was studied, using non-polar hydrocarbons and polar solvents with different electron-donor strength as diluents. The effect caused by the diluent-extractant interaction was found to be relatively weak, owing to the great tendency toward self-association of the extractant. With the higher alcohols as diluent, however, the diluent can coordinate with the extracted metal complex in the Co/HDEHP system. Furthermore, it substitutes for the neutral extractant molecule in the nickel complex. This substitution and the stabilization of the associated water molecules by the alcohol diluent enhances nickel extraction, achieving the same performance as in cobalt extraction. Successively larger aggregated metal complexes appear in all diluents tested, to differing extent depending on the nature of the diluent. Equilibrium formulations were determined for various combinations of metal and diluent.