Selective separation and recovery of divalent Cd and Ni from sulphate solutions with mixtures of TOPS 99 and Cyanex 471 X

Studies on solvent extraction and selective recovery of Cd and Ni from sulphate solutions using TOPS 99 (Talcher organophosphorus solvent, an equivalent of di-2-ethylhexyl phosphoric acid) as extractant alone and in combination with Cyanex 471 X (triisobutyl phosphine sulphide) as synergist were carried out from an aqueous feed containing 0.541 g/L Cd and 1.182 g/L Ni. Variation of equilibrium pH of aqueous phase from 1 to 6 increased the percentage extraction of metals. Cd/Ni separation factor of 369 was obtained at an equilibrium pH of 4.5 with 0.05 M TOPS 99 and separation factor of 485 was obtained at an equilibrium pH of 4.0 with 0.1 M TOPS 99. Studies on extraction of Cd from Ni using mixed extractant systems containing 0.025, 0.05 and 0.075 M extractant and varying synergist concentrations at a total concentration of 0.1 M indicated that a combination of 0.05 M TOPS 99 and 0.05 M Cyanex 471 X concentrations is optimum to achieve ∼97% Cd extraction and 2.7% Ni co-extraction in single stage at an equilibrium pH of 4.2 with high separation factor (β) of 1085. McCabe–Thiele diagram for Cd extraction at different phase ratios with 0.05 M extractants each, indicated the necessity of two theoretical stages for >99% Cd extraction at an aqueous to organic phase ratio of 1.5. Accordingly, a two stage counter-current extraction simulation test was conducted at pH 4.2 with 0.05 M TOPS 99 and Cyanex 471 X each at an aqueous to organic (A/O) phase ratio of 1.5, which confirmed Cd extraction of 99.4% with co-extraction of 2.03% Ni. Under similar conditions of extraction at lower A/O phase ratio of 1.4 increased Cd extraction to 99.9% and decreased Ni co-extraction to 0.28%, which improved the Cd–Ni selectivity.