Effect of the diluents on the synergistic solvent extraction of some lanthanides with thenoyltrifluoroacetone and quaternary ammonium salt Ivan L. Dukov * , Maria Atanassova Department of Inorganic Chemistry, University of Chemical Technology and Metallurgy, 8 Kliment Okhridski Boulevard, 1756 Sofia, Bulgaria Received 26 July 2002; received in revised form 24 October 2002; accepted 25 October 2002 Abstract The solvent extraction of La, Nd, Eu, Ho and Lu with thenoyltrifluoroacetone (HTTA) and the quaternary ammonium salt Aliquat 336 in chloride (QCl) or perchlorate (QClO 4 ) form in CHCl 3 ,C 6 H 6 , CCl 4 and C 6 H 12 has been studied. On the basis of the slope analysis data, the composition of the extracted species was determined as Q + [Ln(TTA) 4 ]  (Q + is the quaternary ammonium salt cation). The values of the equilibrium constant and the separation factors were calculated. The effect of the diluents on the metal extraction and separation has been discussed. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Synergistic extraction; Lanthanides; Thenoyltrifluoroacetone; Aliquat 336; Diluents 1. Introduction Thenoyltrifluoroacetone (HTTA) has been used for the solvent extraction of lanthanides alone (Poskanzer and Foreman, 1961; Alstad et al., 1974; Stary and Freiser, 1975) as well as in combination with various synergistic agents (Mathur, 1983; Hala, 1979; Nash and Choppin, 1997; Brunette, 1990; Bond et al., 2000). Mixtures of HTTA and high molecular weight amines (Newman and Klotz, 1972; Dukov and Jorda- nov, 1995, 1999; Saeed et al., 1989) or quaternary ammonium salts (Khopkar and Mathur, 1977; Genov et al., 1977; Dukov and Genov, 1980, 1981; Atanas- sova et al., 2002; Noro and Sekine, 1993) have also been used for the synergistic solvent extraction of these metals. The investigations have been carried out using a variety of organic diluents and different ionic media and ionic strengths in the aqueous phase. The interaction between the chelating extractant and the ammonium salts during the extraction process has been investigated by several authors. Some authors (Noro and Sekine, 1993; Brunette, 1990; Bond et al., 2000; Brunette et al., 1985) have found that in such systems the synergistic agent is the ion- pair obtained as a result of the interaction between the extractants but others (Dukov and Genov, 1981; Ke and Li, 1969; Sasaki and Freiser, 1983; Tochiyama and Freiser, 1981) have suggested that the synergistic agent is the ammonium salt. 0304-386X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII:S0304-386X(02)00171-8 * Corresponding author. E-mail address: dukov@uctm.edu (I.L. Dukov). www.elsevier.com/locate/hydromet Hydrometallurgy 68 (2003) 89 – 96