Solvent extraction separation of zinc and cadmium from nickel and cobalt using Aliquat 336, a strong base anion exchanger, in the chloride and thiocyanate forms

Abstract

Zinc and cadmium solvent extraction separation from cobalt and nickel was studied using 30% Aliquat 336 in either the chloride (R 4NCl) or thiocyanate (R 4NSCN) forms, and in a mixed aromatic-aliphatic diluent. With NaCl solutions good separation was achieved using R 4NCl. Separation improved with decreasing NaCl concentration, but was still effective with 200 g/L NaCl. Zinc and cadmium loaded principally as [MCl 4] 2− complexes. The maximum loading was 18 g/L for zinc and 28 g/L for cadmium. There was a small selectivity for cadmium over zinc. The mole ratio of organic reagent to metal (R 4N:M) was about 2. Separation with R 4NSCN was studied with NaCl, NaNO 3 and Na 2SO 4 solutions. Zinc extraction was favored over cadmium in each case and cadmium extraction decreased markedly in the order NaCl>NaNO 3>Na 2SO 4. Maximum zinc loadings were, respectively, 20 g/L, 9 g/L and 6 g/L. The selectivity for zinc over cadmium was rationalized on the basis of the different types of thiocyanate complexes formed by zinc and cadmium. From chloride solution the loaded complexes contained chloride and thiocyanate. The R 4N:M ratio was about 2. From nitrate solution, [M(SCN) 4] 2− species loaded. With chloride or nitrate medium, separation of zinc from cobalt and nickel was good, but separation of cadmium from cobalt was only moderate at best. Thiocyanate losses from the organic phase in these two systems were generally low with zinc loading (<1%), but high for cadmium loading. In the case of sulfate medium, zinc loaded predominantly. However, nickel loaded up to 0.14 g/L even at the highest zinc loading. Ammonia stripping of zinc and cadmium loaded R 4NCl was also studied and was found to be effective.