SOLVENT-EXTRACTION OF LANTHANIDES BY ACIDIC ORGANOPHOSPHORUS COMPOUNDS

Organophosphorus acids have been widely employed for extraction separation of lanthanides owing to their high extractability and large separation factor for lanthanide elements. The extraction behavior of heavier lanthanides, terbium, thulium and lutetium, was investigated by employing three kinds of organophosphorus acids, bis(2-ethylhexyl)hydrogen phosphate (BEHPA), 2-ethylhexyl hydrogen 2-ethylhexylphosphonate (EHEHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (BTMPPA). An organic extractant solution was equilibrated with an equal volume of aqueous solution containing lanthanide ions for 1 h at 25-degrees-C. After centrifugation, the concentration of lanthanide in the aqueous phase and in the acidic stripping solution from the organic phase was determined by ICP-AES. Lanthanides(M) were extracted as M(HA2)3(HA)2 is the dimer of extractant). The extraction constant (K(ex)) increased with atomic number owing to lanthanide contraction and decreased in the order, BEHPA> EHEHPA>BTMPPA (see Table 1) in accordance with the decreasing order of their acid dissociation constants. The K(ex) value greatly depended on the diluent used : hexane>carbon tetrachloride>1,2-dichloroethane>toluene>benzene for each lanthanide-EHEHPA complex. Organic phase,reactions such as dimerization of extractants and self adduct formation presumably play important roles in extraction equilibria. These regular trends would be useful guide for the selection of suitable diluents.