General Model for Aggregation of Metal-extractant Complexes in Acidic Organophosphorus Solvent Extraction Systems

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Abstract
The physicochemical nature of the metal-extractant aggregates formed in the organic phase of acidic organophosphorus solvent extraction systems was investigated to further understand hydrometallurgical extraction. The experimental techniques of fluorescence, FT-IR, and NMR spectroscopy, static and dynamic light-scattering, tensiometry, as well as classical physicochemical methods and extraction measurements, were employed to clarify the nature of the molecular aggregates associated with the extraction of Ca2+, Co2+, Ni2+, and Zn2+ by di(2-ethylhexyl)phosphoric acid (HDEHP) in n-alkane diluents. Significantly, there appears to be three regimes of aggregation behavior in solvent extraction systems consistent with the mechanism suggested earlier by Neuman and co-workers [Colloids Surfaces, 46, 45 (1990)]. A simplified model is proposed herein for the aggregation of metal-extractant complexes in acidic organophosphorus solvent extraction systems: as the concentration of metal-extractant complexes increases, the initial metal-extractant complexes or nuclei grow in size via a stepwise polymerization mechanism to form linear aggregates that eventually undergo a structural reorganization to form cyclic aggregates or reversed micelles which, in turn, are capable of further growth. Furthermore, the role of reversed micelles and other association microstructures in solvent extraction is briefly addressed. Although the model seems to be fairly general and provides a unifying interpretation of the species formed in the organic phase of solvent extraction systems, complexities in practical extraction systems must be considered for a detailed understanding of the solvent extraction mechanism.

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