The selective solvent extraction of cadmium by mixtures of carboxylic acids and trialkylphosphine sulphides. Part 2. Practical applications in the separation of cadmium from zinc and nickel

Abstract

The addition of triisobutylphosphine sulphide (TIBPS) to 3,5-diisopropylsalicylic acid (DIPSA) causes a strong synergistic shift in the extraction of cadmium (1.9 pH units for the addition of 0.50 M TIBPS to 0.50 M DIPSA in xylene). The shifts are considerably smaller (up to 0.6 pH units) for other common divalent base metals; such as zinc, lead, manganese, iron, cobalt, nickel, calcium and magnesium, enabling the selective extraction of cadmium to be carried out in most cases. The influence on the separation between cadmium and zinc of variables such as: extractant concentrations, the identity of diluent, the presence of modifiers and the presence of sulphate ion was studied. In batch counter-current experiments using a synthetic solution containing cadmium (5 gl −1) and nickel (24 gl −1) as sulphates and 0.50 M TIBPS plus 0.50 M DIPSA in xylene at an organic to aqueous phase ratio of 2:3 in three extraction stages, quantitative recoveries of 99.8% pure cadmium and > 99.99% pure nickel were obtained. In a continuous counter-current mini-plant trial, a sulphate leach liquor derived from the cadmium cement cake from an electrolytic zinc plant was treated in four extraction stages, two scrubbing stages, and two stripping stages, using 0.40 M TIBPS plus 0.40 M DIPSA in Shellsol 2325. The feed solution, containing up to 16 gl −1 cadmium and 9 gl −1 zinc, as well as small amounts of lead, thallium and other impurities, was processed through to a strip liquor containing up to 110 gl −1 cadmium with a purity of about 99.95% (Zn 0.01, Pb 0.02, Fe 0.01 and Tl 0.005%).