Rate Processes in Solvent Extraction of Metal Ion

Abstract

The mechanism and kinetics of metal extraction with several extractants, including chelating agents, acidic organophosphorous extractant and neutral extractants are discussed in this paper. Firstly the extraction rate of metal ion for the general extraction system is formulated by solving the diffusional equation with chemical reaction, in which the interfacial reaction is considered by the boundary conditions. The overall rate expressions of the complex formation with chelating agents are obtained both in aqueous phase and at the interface. The general formula of the extraction rate is simplified by considering the rate-determining steps. Next, the extraction rate of metal ion with chelating agents and acidic organophosphorous extractants is discussed. For the chelating agents, the most important properly to determine the locus of complex formation is the distribution constant between the organic and aqueous phases. The contribution of the interfacial reaction to the extraction rate increases with the increase in the distribution constant of chelating agents. The extraction rate of many metal ions with acidic organophosphorous extractant are determined by the diffusion process of final complex in the organic phase. Finally, the extraction rate of palladium ion with some neutral extractants is also discussed. The extraction rate is determined by the complex reaction in the aqueous phase and at the interface. The contribution of the interfacial reaction increases with the distribution constant of extractants.