MEDIUM EFFECT ON THE SEPARATION FACTOR IN LIQUID-LIQUID-EXTRACTION - APPLICATION TO THE SEPARATION OF TRIVALENT LANTHANIDE NITRATES BY TRI-N-BUTYLPHOSPHATE

A physicochemical expression of the separation factor derived from the Mikulin-Sergievskii-Dannus' model is proposed to predict the medium e ffect on the separation of lanthanide nitrates by liquid-liquid extrac tion from nitrate media, using tri-n-butyphosphate (TBP) as extractant . The factors exerting an influence on the separation factor have been highlighted and discussed more deeply in this paper, mainly the Plate r activity functions and the extraction equilibrium constant ratio of lanthanide nitrates. It has been shown that the water activity is a ce ntral variable to be controlled, particularly in concentrated aqueous solution. The present work is partly a review of the effects of aqueou s media on the lanthanide separations and partly an analytical approac h based on the physicochemical properties of aqueous solutions. Result s of qualitative and quantitative predictions of changes in the separa tion factor with the aqueous solution composition are discussed. The p roposed model provides a coherent explanation to the great number of e xperimental observations.