Hydrometallurgy, 27 (1991) 169-177 169 Elsevier Science Publishers B.V., Amsterdam Liquid-liquid extraction of yttrium (III) with mixtures of organophosphorus extractants: theoretical analysis of extraction behaviour P.B. Santhi, M.L.P. Reddy*, T.R. Ramamohan and A.D. Damodaran Regional Research Laboratory (CSIR), Trivandrum-695019, India (Received September 10, 1990; revised version accepted February 23, 1991 ) ABSTRACT Santhi, P.B., Reddy, M.L.P., Ramamohan, T.R. and Damodaran, A.D., 199 h Liquid-liquid extrac- tion of yttrium (Ill) with mixtures oforganophosphorus extractants: theoretical analysis of exlrac- tion behaviour. Hydrometallurgy, 27:169-177. The extraction of yttrium (III) from chloride solutions with mixtures of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHEHPA) and di-2-ethylhexylphosphoric acid (DEHPA) or tri-n-octyl phosphine oxide (TOPO) in kerosene has been studied systematically over a wide range of condi- tions. A relatively small synergistic enhancement has been observed with mixtures of acidic organophosphorus extractants (EHEHPA+ DEHPA). On the other hand, antagonistic effects have been observed with mixtures of acidic and neutral organophosphorus compounds (EHEHPA+TOPO). These extraction data have been analysed theoretically taking into account complexation of the metal in the aqueous phase by inorganic ligands and plausible complexation in the organic phase. INTRODUCTION The synergistic extraction of rare earths by mixtures of a chelating agent and a neutral donor has been studied extensively by several authors and these data are covered in excellent reviews on synergistic extraction [ 1-3 ]. Tachi- mori and Nakumara [4 ] have studied the synergistic extraction of Nd (III) with mixtures of mono (2-ethylhexyl)phosphoric acid (MEHPA) and di-2- ethylhexylphosphoric acid (DEHPA). Nemoto and co-workers [ 5 ] have also successfully applied a mixed solvent (DEHPA + MEHPA) to the extraction of lanthanides. Sato and Ban [6 ] have studied the effect of the addition of TOPO on the extraction of rare earths from hydrochloric acid solutions by DEHPA and EHEHPA. However, no detailed studies relevant to the syner- gistic effects on the extraction of yttrium (III) with mixtures of 2-ethyl- *To whom correspondence should be addressed. 0304-386X/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.