Radiochim. Acta 95, 289–295 (2007) / DOI 10.1524/ract.2007.95.5.289  by Oldenbourg Wissenschaftsverlag, München Enhanced extraction and separation of zirconium(IV) and hafnium(IV) with 3-phenyl-4-benzoyl-5-isoxazolone in presence of various neutral organophosphorus extractants By K. J. Reddy 1 , A. V. Reddy 1 , B. S. Shaibu 2 and M. L. P. Reddy 2 , ∗ 1 Department of Chemistry, Sri Venkateswara University, Tirupati 517502, A.P., India 2 Chemical Sciences and Technology Division, Regional Research Laboratory, C.S.I.R, Thiruvananthapuram695019, India (Received August 4, 2006; accepted in revised form January 3, 2007) Synergistic extraction / Zirconium(IV) / Hafnium(IV) / 3-Phenyl-4-aroyl-5-isoxazolone / Neutral organophosphorus extractants / Enhanced separation Summary. Various 3-phenyl-4-aroyl-5-isoxazolones, name- ly, 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), 3-phenyl-4- (4-fluorobenzoyl)-5-isoxazolone (HFBPI) and 3-phenyl-4-(4- toluoyl)-5-isoxazolone (HTPI) were synthesized and examined with regard to the solvent extraction behavior of Zr(IV) and Hf(IV) from hydrochloric acid solutions. The results demonstrated that Zr(IV) and Hf(IV) extracted into chloro- form with 3-phenyl-4-aroyl-5-isoxazolones (HA), as ZrOA 2 and HfOA 2 , respectively. The equilibrium constants of the extracted complexes follow an order HFBPI > HPBI > HTPI, which is in accordance with their p K a values. The synergistic extraction of Zr(IV) and Hf(IV) was also investigated with mixtures of HPBI and tri-n-octylphosphine oxide (TOPO), trialkylphosphine oxide (TRPO), and tri-n-butylphosphate (TBP), where the complexation strength follows the order TOPO > TRPO > TBP. The equilibrium constants of the syn- ergistically extracted complexes have been correlated with the donor ability of phosphoryl oxygen of the neutral organophos- phorus extractants in terms of their 31 P NMR chemical shifts and their basicity values. The addition of neutral organophosphorus extractant to the metal chelate system not only enhances the extraction efficiency but also improves the selectivity between these metal ions. Introduction Zirconium and hafnium are two of the most important nu- clear materials. The use of Zircaloy in construction of fuel elements and other structural components in reactor cores meets many physical and technological requirements. Zir- conium metal should contain less than 100 ppm hafnium for use in nuclear reactors. The advantage of zirconium in nuclear applications is its low thermal neutron capture cross- section (0.18 b/a), whereas hafnium has 640 times higher neutron absorption [1]. As is well known, zirconium and hafnium have very similar chemical properties and are com- monly referred to as “chemical isotopes” [2]. The chemical similarity of zirconium and hafnium, both in their metallic *Author for correspondence (Email: mlpreddy@yahoo.co.uk). and compound states, is greater than that between any other homologous elements in the periodic table (Atomic radius: Zr = 1.45 Å, Hf = 1.44 Å). In recent years, the problem of separation of zirconium and hafnium has acquired significant importance. Solvent extraction processes are currently employed on commer- cial scale to separate hafnium from zirconium even though some alternative processes such as ion exchange, fractional crystallization and vapor phase dechlorination have been proposed by some researchers [2, 3]. Acidic organophospho- rus compounds, namely, di-2-ethylhexylphosphoric acid [4], 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester [5] and dithiophosphinic acids [6] have been employed in the solvent extraction studies of Zr(IV) and Hf(IV). Neu- tral organophosphorus reagents such as tributylphosphate (TBP) [7] and trialkylphosphine oxide [8] have also been widely employed. However, most of the existing commer- cial reagents exhibit poor selectivity for the separation of Zr(IV) and Hf(IV) (separation factor Zr/Hf = 10 with TBP; 7.0 with MIBK [9]). This prompted us to search for alternate liquid-liquid extraction reagents for the selective extraction and separation of Zr(IV) and Hf(IV) from acidic chloride solutions. Recently, various 4-acylbis(1-phenyl-3-methyl-5-pyrazo- lones), namely, 4-adipoylbis(1-phenyl-3-methyl-5-pyrazo- lone) (H 2 AdBP), 4-sebacoylbis(1-phenyl-3-methyl-5-pyr- azolone) (H 2 SbBP) and 4-dodecandioylbis(1-phenyl-3-me- thyl-5-pyrazolone) (H 2 DdBP) were synthesized in our lab- oratory and examined with regard to the solvent extraction behavior of Zr(IV) and Hf(IV) from hydrochloric acid so- lutions [10]. The extraction efficiency of these metal ions with various 4-acylbis(1-phenyl-3-methyl-5-pyrazolenes) follows the order H 2 SbBP > H 2 AdBP > H 2 DdBP. Fur- ther, the selectivity between these metal ions improves significantly with increasing polymethylene chain length from n = 4 to 10 (S.F. with H 2 AdBP = 1.23; S.F. with H 2 DdBP = 9.12). 4-Acyl-5-isoxazolones (1) are a class of fascinating chelating extractants capable of extracting metal ions from strong acid solutions due to its lower p K a values as compared to conventional β-diketones such as HTTA (p K a = 6.25). Among 4-acyl-5-isoxazolones, 3-phenyl-4- benzoyl-5-isoxazolone (HPBI) has come to occupy a special place in the solvent extraction of metal ions due to its lower