THE SOLVENT-EXTRACTION OF CADMIUM AND ZINC BY MIXTURES OF CARBOXYLIC-ACIDS ADN ALKANETHIOLS

The effect of some aliphatic sulphur-containing compounds on the solvent extraction of cadmium and zinc by representative carboxylic acids (HA) is described. Alkanethiols (RSH) were found to cause large synergistic shifts in the pH0.5 values for the extraction of cadmium (up to 2.85 pH units for the addition of 0.25 M thiol to 0.25 M carboxylic acid), with much smaller shifts for zinc (up to 0.84 pH unit). With the isomeric C8-thiols, the extent of the shifts for both metals is greater for the straight-chain compound (R = n-octyl) than for branched-chain analogues (R = 2-ethylhexyl or 2-octyl). Smaller synergistic shifts were obtained with dialkyl sulphides (RSR), whereas with dialkyl disulphides (RS.SR) slight antagonistic shifts were found. Slope analysis studies suggest that for mixtures of 3,5-diisopropylsalicylic acid (HA) and 1-octanethiol (RSH), the extracted cadmium species has the stoichiometry CdA(HA)Cd4(RS)10CdA(HA), for which two possible structures are suggested. With these extractant mixtures, the separation between the pH0.5 values for cadmium and zinc is about one pH unit, and about two pH units between cadmium and nickel for the extraction of 0.05 M metal sulphates from 0.20 M sodium sulphate. However, the distribution coefficient at a given pH value decreases markedly with decreasing cadmium concentration, making efficient recovery of metal values difficult.