The Application of Pressurized Ion Exchange to Separations of

12 The Application of Pressurized Ion Exchange to

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Separations of Transplutonium Elements DAVID O. CAMPBELL Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830

One of the first triumphs of ion exchange chromatography was the separation and identification of fission product rare earths in the Manhattan Project in the early 1940s. Initial publication of this work was withheld until 1947 when nine papers from the Oak Ridge National Laboratory and the Ames Laboratory at Iowa State University appeared in the Journal of the American Chemical Society (1-9). The science of rare earth separations was indeed revolutionized. Separations that had taken years could now be done in about a day. The transplutonium elements and the rare earths, or lanthanides, are so similar chemically that what is true for one group is generally true for the other. In practice, process development work is usually carried out with lanthanides, and frequently, a l l the solutions end up as analytical samples. Transplutonium elements, in contrast, are so valuable that the goal is the maximum yield of pure products. Accordingly, the methods and equipment developed with rare earth separations are applied directly to heavy actinide production separations.

These may be q u i t e s m a l l

i n s c a l e , but t h i s i s " p r o d u c t i o n " f o r some of these elements. Most o f the development data t h a t are s u i t a b l e f o r t h e o r e t i c a l i n t e r p r e t a t i o n , however, are acquired w i t h r a r e e a r t h s . F o r t u n a t e l y , such data can be t r a n s f e r r e d to a c t i n i d e s e p a r a t i o n s w i t h great confidence, as long as c e r t a i n precautions are taken. Two b a s i c approaches are used t o separate these elements, namely, e l u t i o n development and displacement development chromatography. Both were d e f i n e d i n the o r i g i n a l work; and i n both, the s e p a r a t i o n s are based p r i m a r i l y on d i f f e r e n c e s i n comp l e x i n g of the t r i v a l e n t i o n s by an organic reagent during e l u t i o n through a column o f strong ( s u l f o n i c ) a c i d i o n exchange r e s i n . Displacement development i s a p p r o p r i a t e t o l a r g e r - s c a l e separat i o n s because l a r g e r column l o a d i n g can be used and product concentrations are h i g h e r . E l u t i o n development i s p a r t i c u l a r l y s u i t e d t o s m a l l e r - s c a l e (even t r a c e r ) separations because the product bands can be completely separated from each other. The

0097-6156/81/0161-0189$05.00/0 © 1981 A m e r i c a n Chemical Society

Navratil and Schulz; Transplutonium Elements—Production and Recovery ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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d i v i s i o n between the two methods i s not sharp, but i t i s probably i n the v i c i n i t y of a few grams. Since World War I I there have been s i g n i f i c a n t advances i n three general areas. One i s the accumulation of data f o r the i n t e r a c t i o n of these elements w i t h a l a r g e number of d i v e r s e complexing agents, i n c l u d i n g d i s t r i b u t i o n c o e f f i c i e n t s , s e p a r a t i o n f a c t o r s , and complex s t a b i l i t y constants. The r e s u l t i s t h a t α-hydroxyisobutyrate i s g e n e r a l l y used f o r e l u t i o n development s e p a r a t i o n s , f o l l o w i n g the work of Choppin and S i l v a i n 1956 (10), and a b u f f e r of one of the p o l y a m i n o p o l y c a r b o x y l i c a c i d s (such as EDTA, DTPA, or NTA) i s used f o r displacement development. C i t r a t e , which was used i n the o r i g i n a l work f o r both approaches, i s now o n l y of h i s t o r i c a l i n t e r e s t . The second advance was the use of a metal " b a r r i e r " i o n such as Cu, Fe, or N i which was demonstrated by Spedding, P o w e l l , and Wheelwright (11, 12). The metal i o n forms a s t r o n g e r complex w i t h the e l u e n t than do the t r i v a l e n t i o n s of i n t e r e s t , and thereby holds back or " r e t a i n s " these i o n s . This a l l e v i a t e d s e v e r a l problems and c o n t r i b u t e d s u b s t a n t i a l l y to the usefulness of the process. The t h i r d advance, and the primary subject of t h i s paper, occurred i n a completely d i f f e r e n t d i s c i p l i n e , b i o c h e m i s t r y . This was the development of dependable systems f o r high-pressure l i q u i d chromatography d u r i n g the 1960s. The m o t i v a t i o n f o r t h i s development was the need to separate a number of v e r y s i m i l a r m a t e r i a l s i n b i o l o g i c a l and medical r e s e a r c h , such as n u c l e i c a c i d s . The u s u a l i o n exchange chromatographic methods were p a r t i a l l y s u c c e s s f u l , yet inadequate. I t was recognized t h a t g r e a t e r r e s o l u t i o n was r e q u i r e d to g a i n i n f o r m a t i o n about s e v e r a l key problems, and the o v e r r i d i n g g o a l was improved r e s o l u t i o n . Reviews of the b i o c h e m i c a l work g e n e r a l l y s t a r t w i t h M a r t i n and Synge i n 1941 (13) and then jump to the work of Cohn on n u c l e i c a c i d s e p a r a t i o n s by i o n exchange chromatography i n 1949 (14). I t so happens that Waldo Cohn was coauthor of one of those o r i g i n a l p u b l i c a t i o n s on r a r e e a r t h s e p a r a t i o n s i n 1947. I t was a l o g i c a l approach to apply t h i s new chromatographic method i n h i s o r i g i n a l f i e l d of i n t e r e s t , b i o c h e m i s t r y , once the wartime p r i o r i t i e s were suspended. Heftmann (15) has a t t r i b u t e d the a p p l i c a t i o n of i o n exchange chromatography i n the n u c l e i c a c i d f i e l d d i r e c t l y to the r a r e e a r t h s e p a r a t i o n s work of Tompkins, Khym, and Cohn (1) and the high-performance p r e s s u r i z e d i o n exchange systems evolved l a t e r to provide g r e a t e r r e s o l u t i o n . The f a s c i n a t i n g p o i n t i s that the technology t h a t grew out of t h i s work came f u l l c i r c l e a f t e r some twenty y e a r s , w i t h the a p p l i c a t i o n of the p r e s s u r i z e d systems to separations of the t r i v a l e n t a c t i n i d e s , the homologues of the r a r e e a r t h s . This technology, which was reviewed i n 1976 (16), c o n t r i b u t e s i m p o r t a n t l y to s e v e r a l papers at t h i s symposium. The road to b e t t e r r e s o l u t i o n was r e a l l y obvious. I t was to use ever s m a l l e r and more uniform i o n exchange p a r t i c l e s . The


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problem i s that the s m a l l p a r t i c l e s cause an extremely low f l o w r a t e under o r d i n a r y o p e r a t i n g c o n d i t i o n s . One way t o circumvent the problem i s t o apply a high pressure a t the column i n l e t . The f a c t o r s t h a t made a success of p r e s s u r i z e d i o n exchange were the development and commercial a v a i l a b i l i t y of dependable hardware such as pumps, v a l v e s , and f i t t i n g s , which occurred g e n e r a l l y i n the 1960s, and then the i n t e g r a t i o n of the components i n t o p r a c t i c a l systems and a p p l i c a t i o n of the systems t o a p p r o p r i a t e problems. Actinide Production

Considerations

The need f o r g r e a t l y increased production o f the heavier a c t i n i d e s became apparent i n the 1960s. There had been p r i o r separations of multigram q u a n t i t i e s of americium and curium, but only much s m a l l e r amounts of heavier a c t i n i d e s . Two programs were i n i t i a t e d . The High F l u x Isotope Reactor (HFIR) and the Transuranium Processing P l a n t (TRU) were b u i l t a t Oak Ridge N a t i o n a l Laboratory (ORNL) (17), and they have continued to supply elements up t o fermium, as d i s c u s s e d i n other papers here. The C a l i f o r n i u m Production Program (18) was e s t a b l i s h e d a t the Savannah R i v e r Laboratory (SRL) t o produce C f i n multigram q u a n t i t i e s f o r market development. Both these programs were designed o r i g i n a l l y to u t i l i z e s o l v e n t e x t r a c t i o n predominantly, although a t ORNL anion exchange was a l s o scheduled f o r enrichment of the transcurium elements. Cation exchange was used f o r the f i n a l p u r i f i c a t i o n of the i n d i v i d u a l transcurium elements. At both s i t e s v a r i o u s o p e r a t i o n a l problems developed w i t h s o l v e n t e x t r a c t i o n , whereas i o n exchange performance was unexpectedly good. I n a d d i t i o n , p r e s s u r i z e d i o n exchange, which was developed a t that time, permitted i o n exchange t o be a p p l i e d t o the highest r a d i a t i o n l e v e l s a n t i c i p a t e d i n these programs. P r e s e n t l y , i o n exchange i s used almost e x c l u s i v e l y f o r the a c t u a l transplutonium element s e p a r a t i o n s , and batch s o l v e n t e x t r a c t i o n i s u t i l i z e d f o r removal o f c o r r o s i o n , a c t i v a t i o n , and f i s s i o n products. I t was recognized t h a t a very h i g h r a d i a t i o n i n t e n s i t y would be encountered, w e l l beyond that i n r e a c t o r f u e l r e p r o c e s s i n g , f o r example. At the same time, the chemical s e p a r a t i o n s a r e among the most d i f f i c u l t t o accomplish. The s e p a r a t i o n f a c t o r s f o r success i v e p a i r s of transplutonium elements vary upward from about 1.3, and both the y i e l d and the extent of s e p a r a t i o n from adjacent elements a r e d e s i r e d t o be about 99.9%. These requirements t r a n s l a t e i n t o a s e p a r a t i o n system capable of a c h i e v i n g about 500 e q u i v a l e n t t h e o r e t i c a l p l a t e s . This i s not r e a l l y outstanding performance i n terms o f s m a l l - s c a l e , modern, h i g h - r e s o l u t i o n chromatography; but i t i s e x c e p t i o n a l performance when the r a d i a t i o n damage problems a r e taken i n t o account. The s c a l e o f work ranges from hundred-gram to k i l o g r a m q u a n t i t i e s o f combined f i s s i o n products, r a r e e a r t h s , americium, 2 5 2



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and curium, down t o 100-mg q u a n t i t i e s of c a l i f o r n i u m ,

The Application of Pressurized Ion Exchange to Separations of

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