Cation-exchange separation of uranium from thorium in nitric acid medium

Transcription

1 DOI: /JRNC Journal of Radioanalytical and Nuclear Chemistry, Vol. 268, No.2 (2006) Cation-exchange separation of uranium from thorium in nitric acid medium A. Bhattacharyya, P. K. Mohapatra, P. N. Pathak, V. K. Manchanda* Radiochemistry Division, B.A.R.C., Trombay, Mumbai , India (Received November 6, 2005) Sorption behavior of Th and U on cation-exchange resins was investigated from nitric acid medium by both batch and column methods. The cationexchange studies involved the sorption of UO 2 2+ and Th 4+ and their cationic complexes onto Dowex 50Wx8 and Dowex 50Wx4 resins ( mesh). The batch data yielded a separation factor (K d,th /K d,u ) value of >100 for the cation-exchanger, Dowex 50Wx4 at 1 2M HNO 3. Separation of uranium from thorium was also carried out by column method in nitric acid medium using cation-exchangers, Dowex 50Wx4 as well as Dowex 50Wx8. While uranium elution was possible at 1M HNO 3, Th could be eluted only at higher concentration of nitric acid (>6M). The stripped solution emanating from a mixer settler employing di-2-ethyl hexyl isobutyramide as extractant and feed solution similar to THOREX process comprising 350 mg/l U and 380 mg/l Th in 0.75M HNO 3 was loaded on the column and the decontamination factor value for U in the product was >1000. Introduction Vast natural resources of Th can be exploited for power production only through the generation of manmade fissile isotope 233 U. However, separation of 233 U from bulk 232 Th, 231,232 Pa, fission products and structural elements is quite a challenge to the chemists working on the process development for the spent fuel reprocessing. Several versions of the THOREX process using tri-n-butyl phosphate (TBP) as the extractant have been developed to meet this challenge. Work carried out in our laboratory has demonstrated that a branched amide extractant di-2-ethyl hexyl iso-butyramide (D2EHiBA) is distinctly better than TBP as it preferentially extracts U(VI) over Th(IV) and Pa(V). 1 Mixer settler studies carried out in our laboratory using D2EHiBA 2 have shown that though co-extraction of Th from 4M HNO 3 is significantly reduced, the stripped solution contains nearly equal amount of Th and U ( mg/l). Better decontamination was possible by introducing a scrubbing cycle. Alternatively, the stripped solution could be purified by another separation method such as an ion-exchange method. The conventional method for the tail end purification of the product ( 233 U) is based on an anion-exchange method in nitric acid medium. 3 This involves sorption of the anionic nitrate complexes of Th(IV) onto the anion exchanger while 233 U is eluted from the column. This method, however, involves a large volume of washings to be passed through the column as U(IV) also forms anionic complexes, though to a relatively lesser extent. An alternative method involves the separation in the HCl medium wherein the anionic chlorocomplexes of U(VI) are held onto the column while Th (which does not form anionic chlorocomplexes) passes through. 4 This method requires, however, the cumbersome procedure of converting the nitrate complexes to the chloride form and also specially designed corrosion resistant equipments. However, Fe(III) is invariably found as a contaminant in the U product purified by this route. Search for alternative methods of purification have been reported earlier. 5 RASTOGI et al. 6 have suggested a method for ion-exchange separation of U and Th from nitric acid medium. The present work is a systematic study on the separation of uranium from thorium under various feed acidities and eluting conditions using cation-exchangers Dowex 50Wx4 and Dowex 50Wx8 in nitric acid medium. It also deals with the actual processing of a THOREX Exit Strip solution from mixer-settler runs using D2EHiBA as the extractant, which was also used as feed solution in the present work. Experimental Batch distribution experiments with cation-exchangers Batch experiments were carried out using about mg of the washed and dried cation-exchanger by equilibrating with 2 ml acid solution containing 233 U/ 234 Th tracer for 1 hour in a thermostat bath at 25±0.5 C. The K d values for Th(IV) and U(VI) as a function of acid concentrations were determined as: ( C C W K 0 )/ d = C / V where C 0 is the initial concentration of metal ion, C is the total concentration of metal ion after equilibration, W is the weight of resin taken in g, V is the volume of the aqueous phase in ml. * acsrcd@magnum.barc.ernet.in /USD Akadémiai Kiadó, Budapest 2006 Akadémiai Kiadó, Budapest Springer, Dordrecht

2 Column experiments Column experiments were carried out using a 1 cm 22 cm glass column containing 15 g of the cationexchange resin. The resin was conditioned beforehand by washing several times with 2M HCl followed by 1M HNO 3 to get rid of the metallic impurities. 7 The loading of the U and Th mixture was carried out under different feed conditions. The elution of U was done at 1M HNO 3 while Th was eluted using 6 8M HNO 3. Reagents Dowex 50Wx8 and Dowex 50Wx4 resins were procured from Fluka Chemie AG. Xylenol orange, dibenzoyl methane (DBM) and EDTA were obtained from Fluka, Switzerland and E. Merck, India respectively. THORON [2-(2-hydroxy-3,6-disulpho-1-naphthylazo)- benzene arsenic acid] was procured from Aldrich Chemicals, USA. All the other reagents used were of AR grade. Determination of uranium Uranium was determined by spectrophotometry using di-benzoyl methane (DBM) as the chromogenic agent. A solution of uranium was added into a cocktail containing pyridine-ethanol (1:1) buffer and 1% DBM (1 ml) in ethanol. Absorption spectra (monitored at 416 nm) were recorded using a JASCO V-530 double beam spectrophotometer. In samples containing a higher amount of U (1 5 mg/ml) the Davies-Gray s potentiometric method was employed. 8 Traces of U in the bulk of Th were determined by solid state nuclear track technique. 9 Determination of thorium The determination of Th at mg/ml level was performed by EDTA-complexometric titration. Smaller amounts of Th were determined by spectrophotometry using THORON as the chromogenic agent. 10 To the dried sample containing Th, 200 µl of conc. HCl and 600 µl of a THORON solution (1% in water) were added and the volume was made up to 10 ml with a solution of ph 1. The absorbance at 545 nm (ε max = 16,000 M 1. cm 1 ) was used to calculate the Th concentration. Batch distribution studies Results and discussion The batch distribution experiments using the cationexchanger Dowex 50Wx4 indicated that U has a relatively low K d value (Table 1) even at 1M HNO 3 (12.1) which decreased further to <0.001 at 6M HNO 3. On the other hand, Th showed very high sorption at lower acidities which decreased steeply at higher acidities. The data obtained using 234 Th tracer (<10 9 M) showed higher K d values than those obtained using macro amount of Th (~10 4 M). As shown in Table 1, the K d values of Th, obtained using 234 Th tracer at 1M HNO 3 was as high as 3160 which decreased to 27 at 6M HNO 3. Based on the reasonably high separation factor value (K d,th /K d,u ) obtained at 1M HNO 3, the column experiments with several simulated solutions were carried out at this acidity. Column studies Based on the batch results it is expected that if a mixture of U and Th is loaded onto a cation-exchange column, U can be conveniently washed out by 1 2M HNO 3 while Th is held in the column. As the K d,th values decrease sharply with increasing acidity, the column can be regenerated at 6 7M HNO 3. The elution of Th(IV) at higher acidity is a result of increased competition from hydrogen ion as well as due to the conversion of Th 4+ to the anionic Th(NO 3 ) 6 2 due to the increased nitrate concentration. The separation studies were carried out using cation-exchangers of different crosslinking viz. Dowex 50Wx8 and Dowex 50Wx4 and under simulated as well as actual conditions. Simulated studies In order to standardize the conditions for ionexchange separation, simulated studies were performed prior to the run involving the THOREX type strip solution. These studies were carried out using solutions of varying compositions with respect to U, Th and HNO 3. Usually, the concentrations of both U and Th were taken in the concentration range of few mg/ml. As expected for the THOREX strip solution, the feed acidity in the simulated studies was maintained at 0.5M HNO 3. Table 1. Uptake of Th(IV) and U(VI) by the cation-exchanger Dowex 50W 4 ( mesh) as a function of HNO 3 concentration (temperature = 25 C) [HNO 3 ], M K a d,u K b d,th K c d,th < a 233 U tracer only. b 234 Th tracer only. c 234 Th tracer and about 10 4 M thorium nitrate solution. 324

3 5 ml of the feed containing 3.23 mg/ml of Th and 2.8 mg/ml of U in 0.5M HNO 3 were loaded on a column made from cation-exchange resin Dowex-50Wx8 of bed volume 3.5 ml at a flow rate of 1 ml/min. Uranium was washed out with 45 ml of 2M HNO 3 and the elution profile is illustrated in Fig. 1. The Th which was held onto the column was partially eluted out with 6.7M HNO 3 (48% in about 40 ml, Fig. 2). 11 In view of the broad elution profile obtained with 6.7M HNO 3, the elution of Th was studied using a second eluent consisting of 1M ammonium acetate and acetic acid buffer at a ph of Using about 40 ml of the second eluent >99.5% of Th could be eluted out of the column. The percentage recovery of Th and U are listed in Table 2. It was also observed that the increased cross linking in the resin resulted in an increase of the elution volume of Th (Table 2). When Dowex 50Wx8 resin was used, the elution volume was nearly 2 times higher than that obtained employing Dowex 50Wx4 resin. 11 This was demonstrated in an experiment using Dowex 50Wx4 resin, in which 5 ml feed in 2M HNO 3 containing 1.69 mg/ml of U and 3.24 mg/ml of Th were loaded at a flow rate of 1 ml/min. 98% of the total U was eluted out in 40 ml of 2M HNO 3. Thorium recovery was completed in 30 ml of 6M HNO 3. However, 19.6% of the Th was eluted in the U fraction. The elution data for both U and Th are summarized in Tables 3 and 4. As this method yielded a significant decrease in the decontamination factor (~5), it was important to develop a method without the breakthrough of Th into the washings containing U. The U elution profile changed significantly depending on the acid strength used for washing (Fig. 3). When the washing was performed using 1M HNO 3, in 30 ml wash solution >99.2% of U was recovered without any Th breakthrough. Subsequently, ~97.2% of Th was recovered in 50 ml of 7.5M HNO 3 (Fig. 4). This condition of elution was employed for the separation of U and Th from a solution containing 444 µg/ml of U and 2.2 mg/ml of Th in 1M HNO ml of this solution was loaded on the column. Both U and Th were recovered completely in 125 ml of 1M HNO 3 and 230 ml of 7.5M HNO 3, respectively (Table 2). 140 ml of 1M HNO 3 and 125 ml of 7M HNO 3, respectively (Table 2). Product analysis was carried out by estimating the traces of Th in the bulk of U by a solvent extraction method 13 which yielded a value of <0.1 µg. On the other hand, estimation of traces of U in bulk Th was done by solid state nuclear track detector (SSNTD). The success of the separation procedure employed was evident from the high D.F. value (>10 3 ) obtained. A flow diagram of the separation method is indicated in Fig. 5. Fig. 1. Recovery of U from the cation-exchange column containing Dowex 50Wx8 resin as washings, load: U + Th in 1M HNO 3 ; wash: 1M HNO 3 Studies with THOREX type striped solution from mixer settler run In another experiment 500 ml of feed containing ~350 µg/ml of U and ~380 µg/ml of Th in 1M HNO 3 were loaded on the same column at a flow rate of 1 ml/min. The entire U and Th were eluted out with Fig. 2. Elution profile of Th from the cation-exchange column containing Dowex 50Wx8 resin, load: 2.8 mg/ml U mg/ml Th in 1M HNO 3 (5 ml); elution: 6.7M HNO 3 325

4 326 A. BHATTACHARYYA et al.: CATION-EXCHANGE SEPARATION OF URANIUM FROM THORIUM

5 Table 3. Elution data of U and Th from the cation-exchange column (Dowex 50Wx4, mesh). Load: 5 ml of feed containing 1.69 mg/ml of U and 3.24 mg/ml of Th in 2M HNO 3. Temperature: 25 C Volume of U eluate, a ml U concentration, mg/ml Volume of Th eluate, b ml Th concentration, mg/ml c c a U elution was carried out using 2M HNO 3. b Th elution was carried out using 6M HNO 3. c Part of Th loaded on to the column broke through in these two fractions. Table 4. Recovery of U and Th from a mixture (5 ml) containing 1.69 mg/ml of U and 3.24 mg/ml of Th in 2M HNO 3 using cation-exchange column (Dowex 50Wx4, mesh). Temperature: 25 C a Eluent: 2M HNO 3. b Eluent: 6M HNO 3. Element Recovery, % D.F. U a Th b 79.0 >640 Fig. 3. Recovery of U from the cation-exchange column containing Dowex 50Wx4 resin as washings; load and wash: 1M HNO 3 and 2M HNO 3 Fig. 4. Elution profile of Th from the cation-exchange column containing Dowex 50Wx4 resin; load: U + Th in 1M HNO 3 ; elution: 7.5M HNO 3 327

6 Fig. 5. Flow diagram of the cation-exchange separation scheme * The authors thank Dr. P C. KALSI for his help in the determination of trace amount of U in Th fraction by solid state nuclear track detection method. References 1. P. N. PATHAK, R. VEERARAGHAVAN, P. B. RUIKAR, V. K. MANCHANDA, Radiochim. Acta, 86 (1999) P. N. PATHAK, D. R. PRABHU, A. S. KANEKAR, P. B. RUIKAR, A. BHATTACHARYYA, P. K. MOHAPATRA, V. K. MANCHANDA, Ind. Eng. Chem. Res., 43 (2004) P. K. MOHAPATRA, Ph. D. Thesis, University of Bombay, J. J. KATZ, G. T. SEABORG, L. R. MORSS, The Chemistry of the Actinide Elements, 2nd ed., Chapman and Hall, New York, 1986, p A. MUKHERJI, R. T. KULKARNI, S. G. REGE, M. N. NADKARNI, BARC Report No. BARC-1532, R. K. RASTOGI, M. A. MAHAJAN, N. K. CHAUDHURY, BARC Report No. BARC/1992/E/018, M. MARHOL, Ion Exchange in Analytical Chemistry, Their Properties and Use in Inorganic Chemistry, Elsevier, Prague, W. DAVIES, W. GRAY, Talanta, 11 (1964) P. C. KALSI, Private communication. 10. K. S. CHUNG, J. P. RILEY, Anal. Chim. Acta, 28 (1963) P. K. MOHAPATRA, L. B. KUMBHARE, P. B. RUIKAR, V. K. MANCHANDA, Solvent Extr. Ion Exch., 22 (2004) R. T. CHITNIS, K. G. RAJAPPAN, S. V. KUMAR, M. N. NADKARNI, BARC Report No. BARC-1003, A. BHATTACHARYYA, P. K. MOHAPATRA, V. K. MANCHANDA, Separation of trace amount of Th from U using a substituted isoxazolone as the extractant, in: Proc. Intern. Symp. on Solvent Extraction, V. N. MISHRA, S. C. DAS, K. S. RAO (Eds), Allied Publishers, New Delhi, 2002, p