New approach in the modelling of the extraction equilibrium of zinc with bis-(2-ethylhexyl) phosphoric acid

Abstract

The extraction equilibrium of the possible test system for reactive extraction zinc sulphate/bis(2-ethylhexyl)-phosphoric acid (D2EHPA)/n-heptane was studied at 25°C. The results from slope analysis, computer modelling and quantitative FT-IR spectroscopy have been compared. The computer program SXLSQA was used for modelling the equilibrium taking into account the non-ideality of both phases, based on the Pitzer approach for the aqueous phase and the regular solution theory of Hildebrand/Scott for calculating organic phase activity coefficients. The three methods employed provide the same result: that the dominating zinc complex in the organic phase is ZnR2(RH). Some analytical improvements have been carried out, such as purification of D2EHPA, a purity determination method for D2EHPA, an analytical method for the determination of D2EHPA concentration in aqueous solutions. FT-IR has also been used to illustrate the role of water in the formation of the organic metal complexes. While in the extraction of nickel an appreciable amount of water is co-extracted, the extraction of zinc does not show the same behaviour.