Solvent extraction of metals with carboxylic acids — Theoretical analysis of extraction behaviour

Abstract

This paper describes a new, theoretical method for analysing the behaviour of metals during solvent extraction with carboxylic acids. A general solvent extraction equation, which allows for extraction of partially hydrolyzed species, solvation of the metal within the carboxylate complex and polymerization of the organic species, is used: M( OH) (n−q)+ q+ (n-q+m) p ( HR p= 1 x ( M( OH) q R n−q·m HR) x+(n−q) h + and a general expression for the distribution coefficient is derived, taking into account complexation of the metal in the aqueous phase by inorganic ligands. This general expression is then used to generate sets of theoretical log D/pH curves for different parameters in the extraction equation. For a given set of experimental log D/pH data, the most probable values of the different parameters are deduced by curve-matching. The equilibrium constant for extraction is then found from the position of the experimental curve. This direct approach readily reveals uncertainties arising from the interactive effect of some of the experimental parameters; in these cases alternative experimental techniques can be used to characterize the organic species, thereby improving the accuracy of the derived equilibrium constant. Examples of the technique are given for Fe(III), Cu(II) and Zn(II).