Effect of Various Phenomena in the Organic Phase on Metal Extraction with Chelating Reagents in Countercurrent and Crosscurrent Extraction Systems

Abstract

The effect of the association phenomena (extractant association, modifier association, and extractant−modifier co-association) on the recovery of metal species in extraction−stripping multistage countercurrent and crosscurrent processes is discussed. The association phenomena change the optimum strength of extractant extraction ability needed to obtain the maximal metal transfer. The extraction−stripping isotherms depend mainly on the extraction constant, the extractant association constant, the ratio of extractant to modifier, and the acidity of the aqueous phase. The effect of modifier extractant co-association on extraction−stripping isotherms is observed for modifier/extractant mole ratios above 1.5. The countercurrent extraction−stripping process depends mainly on the extraction constant and the extractant association constant. Extractant association decreases the extraction ability and facilitates the stripping. Co-association of extractant with a modifier may play the same role as the extractant association. As a result, a maximum transfer of metal from the feed to the loaded electrolyte is observed for appropriate combinations of the equilibrium constants of extraction, association, and co-association. Extraction association and co-association have a negative effect for weak extractants and a positive one for strong extractants. Similar effects are observed for crosscurrent reagent flows. The association of a modifier and especially its co-association with extractant change the optimum strength of extractant (extraction equilibrium constant) for both the multistage countercurrent and crosscurrent processes.