SEPARATION OF COBALT AND NICKEL USING SOLVENT EXTRACTION WITH ACIDIC ORGANOPHOSPHORUS COMPOUNDS

In the extraction of cobalt and nickel from an aqueous solution containing both metals and using di-(2-ethylhexyl)phosphoric acid (HDEHP) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA), interaction occurs which influences the amount of each of the metals extracted. For ideal conditions where only monomeric metal species is extracted, an extraction equilibrium expression for a mixed metal species has been obtained and the separation factor for cobalt and nickel can be reasonably expressed as a function of the concentrations of the species and the extraction constants involved. Under conditions typical of industrial operation, the separation factor cannot be correlated by such a simple function. The observed values, however, are found to be not greatly different from those obtained using the data for pure metal extraction, except in the limiting case of a highly loaded organic phase. The separation factor obtained with 100 mol/m3 EHPNA in xylene is around 1000, which is much greater than the values obtained using HDEHP.