Equilibrium characteristics for the extraction of cobalt and nickel into di(2‐ethylhexyl) phosphoric acid

The liquid‐liquid extraction of cobalt and nickel from aqueous sulphate solutions into di(2‐ethylhexyl) phosphoric acid, DZEHPA, has been investigated at 25°C and 60°C. The system studied was made up as follows: the aqueous phase‐cobalt and nickel dphates; the organic phase‐20% D2EHPA, 75% Esso solvent DX3641 and 5% tri‐butyl phosphate. Analogies to vapour‐liquid equilibria were made to develop three methods to correlate the binary equilibrium data, namely: 1) a delta Y method, 2) a method based on the correlation of solvent free mole fractions in both phases and 3) a “pseudo activity” coefficient or pseudo gamma method, γ AB . The methods were evaluated as to their suitability for the prediction of binary isotherms. The delta Y and pseudo gamma methods were found to give the best results.