Kinetics of palladium extraction with bis(2-ethylhexyl)monothiophosphoric acid in a hollow fiber membrane extractor

A kinetic study on palladium extraction from chloride media with bis(2-ethylhexyl)monothiophosphoric acid in EXXSOL D-80 was carried out at 303 K using a hollow fiber membrane extractor, together with equilibrium studies on the interfacial adsorption and aqueous distribution of the extractant as well as the loading test of palladium. It was found that bis(2-ethylhexyl)-monothiophosphoric acid exists as a dimeric species in organic diluent, which is hardly soluble in the aqueous phase but adsorbs at the interface. Palladium is extracted as a 1:2 metal/reagent complex. The order of the extraction rate was found to be 1, 0, and 1 with respect to the concentrations of palladium ion, hydrogen ion, and dimeric extractant, respectively, while it was found to be 0 and -n(n < 1) with respect to chloride ion in its low and high concentration region, respectively. The apparent reaction rate constants of the extraction reaction were determined experimentally. The extraction rates were reasonably explained in terms of a combination of a diffusion model and an interfacial reaction model.