Purification of zinc sulfate solutions from chloride using extraction with mixtures of a trialkyl phosphine oxide and organophosphorus acids

Studies on the extractive purification of zinc sulfate solutions from chloride using mixtures of trialkyl phosphine oxide (TRPO, CYANEX 923) and organophosphorus acids (HR) as extractants have been carried out. The acids used were di(2-ethylhexyl)phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272). Chloride is extracted as the compound ZnCl2 center dot 2TRPO. The most efficient extraction system for chloride removal was shown to be the TRPO + CYANEX 272 + tributyl phosphate mixture in kerosene. The main process parameters that provided for high chloride removal (similar to 97%) were determined. Chloride stripping from the zinc extract is carried out with alkaline solutions (NaOH). The separation of chloride and zinc occurs at a final pH value of the strip liquors of 6-7 under which conditions chloride almost completely pass into the aqueous phase, while zinc remains in the organic phase forming zinc dialkylphosphinate. Thus the mixtures of TRPO with CYANEX 272 advantageously differ from the extraction systems containing TRPO alone wherefrom acceptable chloride stripping is not possible. Zinc stripping from the organic phase is carried out with a sulfuric acid solution.