Stages of extraction processes: A nontraditional approach

The traditional approach to the study of an extraction process implies that the metal salt is complexed in the aqueous phase to which the extractant h as been passed. Then, the complexes go into the organic phase. Accordingly, the extraction constant K-ex is dependent not only on the stability of the complex beta (aq) but also on the constant of its distribution K-dc, which is unpredictable. This hampers data analysis and the prediction of K-ex an d selectivity. In particular, explanation of changes in the extraction cons tant K-ex with varying extractants by changes in complex stability becomes invalid. The approach to the study of the extraction process recommended by us considers a thermodynamic cycle involving ion association in the aqueou s phase, the transfer of association species into the diluent (K-d) and com plexation with the extractant in the aqueous phase (in the diluent). Here, the changes in K-ex with varying extractants reflect only the changes in th e complex stability (beta (org)) The values of the extraction constants by the diluent K-diI and of the association species distributions are small (K -d approximate to 10(-4)-10(-2) in the extraction of nitric acid) and rathe r sensitive to the nature of the diluent and the extent of its hydration. A correlation between K-d and the diluent solubility in water is observed.