The "Cleanex" Process: A Versatile Solvent Extraction Process for

11 The "Cleanex" Process: A Versatile Solvent Extraction Process for Recovery and Purification of Lanthanides,

Downloaded by UNIV OF TEXAS AT DALLAS on April 25, 2017 | http://pubs.acs.org Publication Date: April 16, 1980 | doi: 10.1021/bk-1980-0117.ch011

Americium, and Curium JOHN E. BIGELOW, EMORY D. COLLINS, and LESTER J. KING Oak Ridge National Laboratory, Oak Ridge, TN 37830

The reagent di(2-ethylhexyl) phosphoric acid (HDEHP), a liquid cation exchanger, is used to extract trivalent actinide and lanthanide elements in the "Cleanex" solvent extraction process. The name "Cleanex" was coined because the process was initially developed to "clean up" several batches of rework solutions from a wide variety of contaminants. The process was so effective that i t was later incorporated into the mainline operations at the Transuranium Processing Plant (TRU) at the Oak Ridge National Laboratory (ORNL), where it has been in routine use for 12 years. At TRU, the process has been applied to a number of different batch solvent extraction operations at the 50-liter scale. These include purification of solutions of transplutonium elements, transfer of transplutonium elements from nitrate to chloride solutions, and recovery of transplutonium elements from rework and waste streams (1). The transplutonium elements involved are trivalent actinides. The Cleanex process was adapted from the Dapex process (2), which has been used for many years in the uranium milling industry. The c h e m i s t r y , r e a g e n t s , e q u i p m e n t , a n d b a s i c o p e r a t i o n s r e l a t i n g t o t h e C l e a n e x p r o c e s s and t y p i c a l r e s u l t s o b t a i n e d w i t h the Cleanex process are covered i n the s e c t i o n s below. Process

Chemistry

The p r o c e s s c o n s i s t s o f l i q u i d - l i q u i d s o l v e n t e x t r a c t i o n o f t r i v a l e n t a c t i n i d e s and/or l a n t h a n i d e s from a d i l u t e a c i d o r s a l t s o l u t i o n i n w h i c h t h e a n i o n c a n be e i t h e r n i t r a t e o r c h l o r i d e . The r e l a t i v e l y i n e x t r a c t a b l e i m p u r i t i e s c a n be s c r u b b e d f r o m t h e p r e g n a n t o r g a n i c p h a s e w i t h d i l u t e a c i d ( 0 . 0 3 M) a n d t h e s o l u t e s back-extracted, or s t r i p p e d , with more-concentrated acid (2-6 M). The p r o c e s s i s e q u a l l y e f f e c t i v e w i t h n i t r i c o r h y d r o c h l o r i c a c i d , or mixtures of the a c i d s . S i n c e t h e e x t r a c t e d s p e c i e s does n o t c o n t a i n c o m p l e x e d a c i d , t h e s o l u t e s may b e s t r i p p e d i n t o a n a c i d d i f f e r e n t t h a n t h e one f r o m w h i c h t h e y were i n i t i a l l y extracted.

American Chemical 0-8412-05i^^8û/47J17-l^$05.00/0

© 1980 Afte/fiSh efiîftfyal Society

1155 15th St. N. W.

Navratil and Schulz; Actinide Separations Washington, D. C. Society: 20036Washington, DC, 1980. ACS Symposium Series; American Chemical

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SEPARATIONS

The o r g a n i c e x t r a c t a n t i s t h e e s t e r 2 ? £ s - 2 - e t h y l h e x y l hydrogen phosphate, which has a c i d i c p r o p e r t i e s because o f the r e p l a c e a b l e h y d r o g e n i o n , and i s u s u a l l y c a l l e d i n t h e t r a d e d i ( 2 - e t h y l h e x y l ) p h o s p h o r i c a c i d o r HDEHP. Since t h i s reagent h a s a r e l a t i v e l y h i g h d e n s i t y and v i s c o s i t y , i t i s u s u a l l y d i l u t e d w i t h a nonpolar s o l v e n t t o a c o n c e n t r a t i o n of about 1 M t o p r o d u c e an o r g a n i c e x t r a c t a n t p h a s e w i t h s u i t a b l e o p e r a t i n g characteristics. In the Cleanex process, a l i p h a t i c d i l u e n t s are p r e f e r r e d over aromatic d i l u e n t s because of about a f i v e f o l d h i g h e r e x t r a c t i o n c o e f f i c i e n t f o r the t r i v a l e n t a c t i n i d e s . The HDEHP i s a c a t i o n e x c h a n g e r , and i t s e x t r a c t i o n o f t r i v a l e n t m e t a l i o n s ( M ) c a n be e x p r e s s e d by t h e f o l l o w i n g chem ical reaction:


3 +

M + 3 (HDEHP) Ζ M(DEHP) + 3H . aq org ^ 3, org aq 3 +

0

(1)

+

At e q u i l i b r i u m , the concentrations of the r e a c t i n g species are r e l a t e d as f o l l o w s ( n e g l e c t i n g a c t i v i t y c o e f f i c i e n t s ) : [M(DEHP) . ]

[H ]^ +

Κ =

(2)

51, [HDEHP]

o r g

[M

3 +

]

a q

where Κ i s t h e e q u i l i b r i u m c o n s t a n t f o r t h e r e a c t i o n . The e q u i l i b r i u m e x t r a c t i o n c o e f f i c i e n t (E) f o r t h e t r i v a l e n t m e t a l , d e f i n e d as t h e r a t i o o f t h e c o n c e n t r a t i o n i n t h e o r g a n i c phase t o t h e c o n c e n t r a t i o n i n t h e aqueous p h a s e a t e q u i l i b r i u m , c a n be o b t a i n e d by a rearrangement of E q . ( 2 ) : [M(DEHP) [M ] 3 +

aq

]

Κ [HDEHP];* [H ] +

3

aq

T h u s , t h e e x t r a c t i o n c o e f f i c i e n t , E, v a r i e s d i r e c t l y w i t h t h e t h i r d power o f t h e c o n c e n t r a t i o n o f u n c o m b i n e d HDEHP i n t h e o r g a n i c p h a s e ( n e g l e c t i n g a c t i v i t y c o e f f i c i e n t s ) and i n v e r s e l y w i t h t h e t h i r d power o f t h e aqueous p h a s e a c i d i t y (H ). The a c i d d e p e n d e n c y o b s e r v e d i n p r a c t i c e has* b e e n o n l y a p p r o x i m a t e l y i n v e r s e t h i r d power. Impurities i n Cleanex feed s o l u t i o n s o f t e n c a u s e a d e p a r t u r e f r o m i d e a l i t y ( e . g . , by commoni o n e f f e c t o r by c o n s u m p t i o n o f some o f t h e HDEHP), and we h a v e not been a b l e t o c o n t r o l the e x t r a c t i o n o f the a c t i n i d e elements s o l e l y by m o n i t o r i n g t h e aqueous-phase a c i d i t y . Fortunately, when p r o c e s s i n g t r a n s p l u t o n i u m e l e m e n t s , t h e h i g h s p e c i f i c a c t i v i t y of Cm f a c i l i t a t e s the d e t e c t i o n of t h a t isotope i n both phases, thus p e r m i t t i n g a r a p i d d e t e r m i n a t i o n of the degree of extraction. The e x t r a c t i o n c o e f f i c i e n t s o f t h e t r i v a l e n t a c t i n i d e s and l a n t h a n i d e s a r e a l l q u i t e s i m i l a r , so t h e Cm serves as an e x c e l l e n t marker f o r a l l the e x t r a c t e d i o n s . 2i+i+

2tfI+

Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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B i G E L O w E T AL.

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Materials


Feed S o l u t i o n s . The p r o c e s s i s a p p l i c a b l e t o s o l u t i o n s o f t r i v a l e n t a c t i n i d e s or lanthanides i n e i t h e r d i l u t e a c i d or a v a r i e t y of s a l t s o l u t i o n s i n e i t h e r n i t r a t e o r c h l o r i d e or mixed media. Sodium c o n c e n t r a t i o n s up t o 3 M c a n be t o l e r a t e d , b u t l i t h i u m b e g i n s t o have a d v e r s e e f f e c t s above 2 M and p o t a s s i u m i s o b j e c t i o n a b l e e v e n a t 0 . 5 M. C e r t a i n s p e c i a l c o n s t i t u e n t s c a n be t o l e r a t e d i n t h e f e e d i f s p e c i f i c p r e c a u t i o n s ( d e s c r i b e d l a t e r ) are taken. E x t r a c t i v e Reagent. The e x t r a c t i v e r e a g e n t i s c o m m e r c i a l g r a d e d i ( 2 - e t h y l h e x y l ) p h o s p h o r i c a c i d (HDEHP). Various l o t s of m a t e r i a l o b t a i n e d f r o m M o b i l , f r o m S t a u f f e r , and f r o m U n i o n C a r b i d e , have a l l appeared e q u a l l y u s a b l e . The c o m m e r c i a l m a t e r i a l t y p i c a l l y c o n t a i n s 1-1.5% o f the monoester (which i s d i a c i d i c ) and a s m a l l amount o f i n e r t m a t e r i a l , p r o b a b l y u n r e a c t e d 2 - e t h y l h e x a n o l and p y r o p h o s p h o r i c a c i d s . The HDEHP g e n e r a l l y t i t r a t e s a b o u t 98% ( 2 . 8 M) and i s u s e d w i t h o u t t r e a t m e n t o t h e r than d i l u t i o n . Diluents. An a l i p h a t i c d i l u e n t w i t h a h i g h f l a s h p o i n t i s preferred for safety reasons. A t TRU, b o t h Amsco 1 2 5 - 8 2 and NPH ( n o r m a l p a r a f f i n h y d r o c a r b o n , s u p p l i e d by The S o u t h Hampton C o . , S i l s b e e , T e x a s ) , h a v e g i v e n good r e s u l t s . C u r r e n t l y , we a r e u s i n g o d o r l e s s m i n e r a l s p i r i t s , w h i c h i s s u p p l i e d by Amsco and h a s a f l a s h p o i n t o f 5 2 ° C , a s p e c i f i c g r a v i t y o f 0 . 7 5 0 , and a b o i l i n g range o f 1 4 4 - 2 0 8 ° C . A l l o f t h e s e d i l u e n t s have been s a t i s f a c t o r y , and no e f f o r t h a s b e e n made t o s y s t e m a t i c a l l y e v a l u a t e them f o r o u r a p p l i c a t i o n s . We g e n e r a l l y p r e t r e a t t h e d i l u e n t by p a s s i n g i t s u c c e s s i v e l y t h r o u g h a p a c k e d c o l u m n o f s i l i c a g e l and a l u m i n a t o remove t r a c e s o f s u r f a c e - a c t i v e a g e n t s t h a t might l a t e r c o n t r i b u t e to the formation of s t a b l e emulsions. Equipment The e q u i p m e n t t y p i c a l l y u s e d a t TRU f o r c a r r y i n g o u t t h e C l e a n e x p r o c e s s b a t c h w i s e i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 1. The o p e r a t i n g t e c h n i q u e and d e s i g n c o n s i d e r a t i o n s f o r some o f t h e i t e m s i n F i g u r e 1 h a v e b e e n more f u l l y d e s c r i b e d by C h a t t i n (3). The f e e d i s a d j u s t e d , and t h e e x t r a c t i o n i s c a r r i e d o u t b a t c h w i s e i n a t a n t a l u m - l i n e d evaporator w i t h a c a p a c i t y of 60-70 l i t e r s . P h a s e c o n t a c t i s a c c o m p l i s h e d by a i r - s p a r g i n g . A vacuum t r a n s f e r s y s t e m i s u s e d t o r a i s e t h e aqueous p h a s e f r o m t h e b o t t o m o f t h e t a n t a l u m e v a p o r a t o r , through a phase s e p a r a t o r (or b u l l ' s - e y e ) , and i n t o a vacuum t r a n s f e r t a n k . R e p l a c i n g t h e vacuum s o u r c e w i t h a l o w - p r e s s u r e a i r l i n e , and o p e r a t i n g a v a l v e , p e r m i t s d i s c h a r g e o f t h e a c c u m u l a t e d aqueous p h a s e f r o m t h e vacuum t r a n s f e r tank i n t o e i t h e r a waste r e c e i v e r o r a product r e c e i v e r t a n k . The r a f f i n â t e s ( i n c l u d i n g s c r u b s ) a r e e v e n t u a l l y t r a n s f e r r e d t o


Navratil and Schulz; Actinide Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1980. FILTER

Figure 1.

FEED ADJUSTMENT AND EXTRACTION TANK

AIR-,

SOLUTION ADDITION-i

-VACUUM

PRODUCT COLLECTION AND EVAPORATION

X _ _ VACUUM TRANSFER TANK

ΙΓ

TO WASTE OR 2 n d . - S T A G E EXTRACTION

FLOW ADIVERTER VALVE

Equipment arrangement for Cleanex process

CONDENSATE COLLECTION TANK

PHASE SEPARATOR

AIR-


11.


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Process

a waste storage t a n k . The p r o d u c t r e c e i v e r i s u s u a l l y a n o t h e r t a n t a l u m - l i n e d e v a p o r a t o r so t h a t s u c c e s s i v e v o l u m e s o f s t r i p a c i d c a n be c o m b i n e d and c o n c e n t r a t e d p r i o r t o t h e i r i n t r o d u c t i o n i n t o the next step i n the o v e r a l l process.


Processing Steps Feed Adjustment. The p r o d u c t f r o m a s t e p p r i o r t o C l e a n e x i s t r a n s f e r r e d and f l u s h e d i n t o t h e t a n t a l u m - l i n e d e v a p o r a t o r . Most o f the excess a c i d c o n t a i n e d i n t h e composited C l e a n e x f e e d i s removed by e v a p o r a t i o n e i t h e r t o a f i n a l volume o f 2 l i t e r s o r , i f the s a l t content i s h i g h , to a f i n a l b o i l i n g temperature of 130°C, whichever occurs f i r s t . I f the feed i s a n i t r a t e s o l u t i o n , h y d r o l y s i s o f some e l e m e n t s ( e . g . , z i r c o n i u m ) may h a v e o c c u r r e d d u r i n g t h e e v a p o r a t i o n ; t h e r e f o r e , a b o u t 1 l i t e r o f 12 M HC1 i s added a n d t h e f e e d s o l u t i o n i s r e h e a t e d t o 1 2 0 ° C t o r e d i s s o l v e any h y d r o l y z e d m a t e r i a l s . The f e e d i s t h e n d i l u t e d w i t h w a t e r t o make a t o t a l volume o f 12 l i t e r s and i s m i x e d a n d sampled. An a d d i t i o n o f a c i d o r NaOH i s made ( i f n e c e s s a r y ) t o a d j u s t t h e s o l u t i o n a c i d i t y i n t o t h e r a n g e o f 0 . 1 t o 0 . 4 N. An o x i d a n t s o l u t i o n ( 1 . 5 M L i O C l ) i s a d d e d t o make t h e f e e d 0 . 1 M L i O C l , and t h e f e e d s o l u t i o n i s t h e n h e a t e d t o 80°C a n d d i g e s t e d f o r 20 m i n u t e s t o remove any g a s e o u s r e a c t i o n p r o d u c t s . This o x i d a t i o n p r o c e d u r e i s c a r r i e d o u t t o e n s u r e t h a t a l l molybdenum i n t h e f e e d i s o x i d i z e d t o t h e e x t r a c t a b l e Mo(VI) f o r m (£) . O t h e r w i s e , u n e x t r a c t a b l e molybdenum may h y d r o l y z e u n d e r t h e r e l a t i v e l y l o w - a c i d c o n d i t i o n s achieved l a t e r i n the e x t r a c t i o n step and r e s u l t i n s e v e r e e m u l s i o n p r o b l e m s . Extraction. A b o u t 30 l i t e r s o f t h e p r e v i o u s l y p r e p a r e d o r g a n i c e x t r a c t a n t i s added. Adequate m i x i n g o f the phases i s a c h i e v e d by a i r s p a r g i n g a t a s u p e r f i c i a l a i r v e l o c i t y o f 1 . 3 mm/sec ( 0 . 2 5 f t / m i n ) ; t h i s i s t h e same v e l o c i t y u s e d t o m i x a s i n g l e p h a s e p r i o r t o s a m p l i n g . A f t e r m i x i n g i s b e g u n , NaOH i s a d d e d t o f u r t h e r r e d u c e t h e aqueous p h a s e a c i d i t y and t h e r e b y i n c r e a s e the e x t r a c t a b i l i t y of the t r i v a l e n t a c t i n i d e s . The amount o f NaOH added i s c a l c u l a t e d t o r e d u c e t h e a c i d i t y t o 0 . 0 3 N , a l i m i t c h o s e n t o m i n i m i z e t h e l i k e l i h o o d o f h y d r o l y s i s and p r e c i p i t a t i o n o f u n e x t r a c t e d f e e d components, w h i c h might r e s u l t i n emulsification. The m i x i n g i s i n t e r r u p t e d a f t e r 3 0 - 6 0 m i n u t e s and t h e p h a s e s a l l o w e d t o s e t t l e . The aqueous p h a s e i s s a m p l e d and a n a l y z e d f o r f r e e a c i d and f o r g r o s s a l p h a ( p r i m a r i l y Cm) . The f r e e a c i d i s a l m o s t a l w a y s h i g h e r t h a n 0 . 0 3 Ν b e c a u s e a c i d i s t r a n s f e r r e d t o t h e aqueous p h a s e i n exchange f o r t h e m e t a l l i c c a t i o n s w h i c h were e x t r a c t e d i n t o t h e o r g a n i c p h a s e . A new NaOH a d d i t i o n i s c a l c u l a t e d , b a s e d on t h e a c i d r e s u l t s , and t h e c y c l e o f a c i d a d j u s t m e n t and s p a r g i n g i s r e p e a t e d u n t i l t h e amount o f u n e x t r a c t e d c u r i u m r e m a i n i n g i n t h e aqueous p h a s e i s l e s s t h a n the s p e c i f i e d l i m i t . On some o c c a s i o n s , i t may be n e c e s s a r y t o terminate the e x t r a c t i o n step short of the g o a l i f s u c c e s s i v e 21+lf


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ACTINIDE SEPARATIONS


a n a l y s e s show t h e amount o f u n e x t r a c t e d c u r i u m h a s begun t o i n crease. T h i s l a t t e r phenomenon i s b e l i e v e d t o be due t o d i s p l a c e m e n t o f t h e c u r i u m by a l u m i n u m r e s u l t i n g f r o m a s l o w , i r r e v e r s i b l e e x t r a c t i o n o f a l u m i n u m w h i c h consumes t h e r e a g e n t , HDEHP. The t o t a l t i m e t y p i c a l l y consumed i n t h e e x t r a c t i o n s t e p o f t h e p r o c e s s i s on t h e o r d e r o f 2 s h i f t s , o r 16 h o u r s . Follow i n g e x t r a c t i o n , t h e aqueous r a f f i n a t e i s t r a n s f e r r e d t o a s e p a r a t e t a n k a n d t h e p r e g n a n t o r g a n i c p h a s e i s s u b s e q u e n t l y s c r u b b e d and s t r i p p e d i n t h e same v e s s e l . Scrubbing. S i n c e a r e l a t i v e l y l a r g e amount o f NaOH i s added d u r i n g a Cleanex e x t r a c t i o n t o n e u t r a l i z e excess a c i d i t y , sodium i s u s u a l l y the predominant m e t a l l i c i m p u r i t y . A very small por t i o n o f t h e s o d i u m i s e x t r a c t e d by t h e HDEHP; t h i s amount c a n be reduced by s c r u b b i n g t h e p r e g n a n t o r g a n i c w i t h 0 . 0 3 Ν a c i d . A s e c o n d f u n c t i o n o f t h e s c r u b s , and p r o b a b l y t h e most i m p o r t a n t f u n c t i o n , i s t o f l u s h the r e s i d u a l , u n e x t r a c t e d i m p u r i t i e s from t h e aqueous h e e l u n d e r l y i n g t h e p r e g n a n t o r g a n i c l a y e r . The p r e g n a n t o r g a n i c p h a s e i s u s u a l l y s c r u b b e d t w i c e w i t h 0 . 0 3 Ν HC1 ( t h e f i r s t s c r u b i s 0 . 0 3 Ν H N 0 i f t h e f e e d s o l u t i o n had been i n t h e n i t r a t e f o r m ) . The v o l u m e o f e a c h s c r u b i s o n e t h i r d of the o r g a n i c - p h a s e volume. Each s c r u b i s mixed w i t h the o r g a n i c p h a s e b y a i r - s p a r g i n g a t l e a s t 30 m i n u t e s . The s c r u b r a f f i n â t e s a r e u s u a l l y combined w i t h the e x t r a c t i o n r a f f i n a t e t o provide a composite feed f o r a second-stage e x t r a c t i o n . 3

O r g a n i c P h a s e M o d i f i c a t i o n and S t r i p p i n g . Iron i s frequently a s i g n i f i c a n t i m p u r i t y i n C l e a n e x feeds and, i f p r e s e n t , w i l l have b e e n o x i d i z e d t o F e ( I I I ) by r a d i o l y s i s a n d by t h e L i O C l t h a t was added t o t h e f e e d . The F e ( I I I ) w o u l d t e n d t o f o l l o w t h e t r a n s p l u t o n i u m e l e m e n t s t h r o u g h t h e e x t r a c t i o n and s t r i p p i n g s t e p s . In o r d e r t o r e t a i n t h e F e ( I I I ) i n the o r g a n i c phase w h i l e the t r a n s p l u t o n i u m e l e m e n t s a r e s t r i p p e d , a s o l u t i o n o f 1.6 M Adogen 364-HP (a s t r a i g h t - c h a i n t e r t i a r y amine o b t a i n e d f r o m A r c h e r D a n i e l s M i d l a n d C o . ) i n d i e t h y l b e n z e n e (DEB) i s added t o t h e p r e g n a n t Cleanex o r g a n i c phase j u s t b e f o r e s t r i p p i n g . The amount o f Adogen added i s enough t o make t h e o r g a n i c p h a s e 0 . 2 M i n a m i n e . F o l l o w i n g t h i s m o d i f i c a t i o n of the organic phase, the t r a n s plutonium elements are s t r i p p e d w i t h 6 M H C 1 — 0 . 5 M H 0 . The h y d r o g e n p e r o x i d e i n t h i s s t r i p s o l u t i o n overcomes t h e t r a n s i e n t o x i d i z i n g e f f e c t c a u s e d by i o n i z i n g r a d i a t i o n a c t i n g on t h e H C l medium, t h e r e b y p r e v e n t i n g o x i d a t i o n o f b e r k e l i u m t o t h e h i g h l y extractable tetravalent state. I t i s always necessary to s t r i p the o r g a n i c phase w i t h s e v e r a l batches o f s t r i p s o l u t i o n i n order to e f f e c t t r a n s f e r of a l l of the t r a n s p l u t o n i u m elements to the c o l l e c t i o n tank. S t r i p p i n g c o e f f i c i e n t s f o r the transcurium e l e m e n t s a r e l o w e r t h a n f o r c u r i u m ; t h u s , t h e amounts o f c a l i f o r n i u m r e m a i n i n g i n t h e o r g a n i c p h a s e and i n t h e aqueous h e e l s o l u t i o n (as i n d i c a t e d by g r o s s n e u t r o n c o u n t s ) a r e m o n i t o r e d t o confirm the completeness of the s t r i p p i n g o p e r a t i o n s . 2

2


11.


A d d i t i o n a l Operating

The "Cleanex" Process

153

Considerations

Exposure of the Organic E x t r a c t a n t to R a d i a t i o n . Radiolytic damage t o t h e o r g a n i c e x t r a c t a n t i s q u i t e l i k e l y t o o c c u r s i n c e Cm (2.83 watts/g) i s p r e s e n t i n the o r g a n i c phase f o r a s i g n i f i c a n t time. A t TRU we g e n e r a l l y t r y t o h o l d p l a n n e d e x p o s u r e s b e l o w 150 w a t t - h o u r s / l i t e r (6 χ 1 0 r a d ) . Thus, i n the case of a f e e d c o n t a i n i n g enough Cm to generate 5 w a t t s / l i t e r ( i n the o r g a n i c p h a s e ) , t h e e x t r a c t i o n and s c r u b b i n g must be c o m p l e t e d a n d s t r i p p i n g begun w i t h i n 30 h o u r s . We have n o t e x p e r i e n c e d any o p e r a t i n g d i f f i c u l t i e s w i t h C l e a n e x r u n s made u n d e r t h e s e c o n d i t i o n s , b u t s e v e r a l samples o f c u r i u m o x i d e p r o d u c t have been f o u n d t h a t c o n t a i n e d 5 t o 8% p h o s p h o r u s , p r e s u m a b l y f r o m r a d i o l y t i c d e c o m p o s i t i o n o f t h e HDEHP. P o t e n t i a l l y , the process of d e c o m p o s i t i o n can form e x t r a c t a n t s p e c i e s t h a t behave q u i t e d i f f e r e n t l y f r o m HDEHP and c a u s e a v a r i e t y o f d e l e t e r i o u s e f f e c t s . I n c r e a s i n g d i f f i c u l t y i n s t r i p p i n g h a s b e e n o b s e r v e d on o c c a s i o n s when t h e e x p o s u r e a p p r o a c h e d 200 w a t t - h o u r s / l i t e r . 2l+l+

7


2I+l+

O r g a n i c P h a s e E n t r a i n m e n t D u r i n g Aqueous P h a s e T r a n s f e r s . One a d v a n t a g e o f b a t c h s o l v e n t e x t r a c t i o n p r o c e s s e s i s t h a t a d e q u a t e t i m e c a n be a l l o w e d f o r t h e o r g a n i c and aqueous p h a s e s t o s e t t l e and s e p a r a t e . The aqueous (bottom) p h a s e c a n t h e n be t r a n s f e r r e d to a separate tank using a s p e c i a l phase-separator vessel. Although the p h a s e - s e p a r a t o r i s very e f f e c t i v e i n p r e v e n t i n g b u l k t r a n s f e r o f t h e wrong p h a s e , a s m a l l amount o f t h e o r g a n i c phase i s f r e q u e n t l y e n t r a i n e d i n t h e aqueous p h a s e . If the next p r o c e s s i n g step r e q u i r e s evaporation of the Cleanex p r o d u c t s o l u t i o n t o a s m a l l v o l u m e , t h e e n t r a i n e d HDEHP c o u l d be decomposed t o a t a r w h i c h c o u l d s o r b a s i g n i f i c a n t amount o f t h e a c t i n i d e elements. When t h i s p o s s i b i l i t y e x i s t s , t h e t r a n s f e r o f C l e a n e x p r o d u c t s o l u t i o n t o t h e e v a p o r a t o r i s made v i a a t a n k c o n t a i n i n g a few l i t e r s o f o r g a n i c d i l u e n t (DEB o r Amsco) so t h a t a s e c o n d , c l e a n e r p h a s e s e p a r a t i o n c a n be made. Possible entrain ment o f t h e p u r e d i l u e n t i n t o t h e p r o d u c t i s o f l i t t l e c o n s e q u e n c e s i n c e the d i l u e n t w i l l s t e a m - d i s t i l l out of the r e c e i v i n g tank d u r i n g e v a p o r a t i o n o f t h e aqueous p r o d u c t . Results T y p i c a l r e s u l t s o f t h e C l e a n e x p r o c e s s , as a p p l i e d t o a HFIR t a r g e t p r o c e s s i n g c a m p a i g n , a r e shown i n T a b l e I. P a r t A shows the q u a n t i t i e s o f the t r a n s p l u t o n i u m elements found i n a t y p i c a l t a r g e t c a m p a i g n , and t h e i r b e h a v i o r i n t h e p r o c e s s . P a r t Β shows t h e b e h a v i o r o f m a j o r f i s s i o n p r o d u c t s , w h i l e P a r t C shows t h e p u r i f i c a t i o n t h a t c a n be o b t a i n e d f r o m common m a c r o s c o p i c c o n taminant i o n s . Note t h a t the s o l u t e i o n s found i n the second s t a g e o r g a n i c phase a r e u s u a l l y s t r i p p e d and a c c u m u l a t e d as a r e w o r k f r a c t i o n t o be r e c y c l e d t o s u b s e q u e n t c a m p a i g n s . The f i r s t s t a g e o r g a n i c p h a s e and t h e s e c o n d s t a g e aqueous r a f f i n a t e are d i s c a r d e d .


154

ACTINIDE SEPARATIONS

T a b l e I . T y p i c a l C o m p o s i t i o n o f C l e a n e x F e e d S o l u t i o n s When P r o c e s s i n g T r a n s p l u t o n i u m E l e m e n t s D u r i n g a HFIR T a r g e t Campaign

% Distribution to Exit Amounts m Feed S o l u t i o n

Component

P a r t A; 243

Am


244

Cm 249, Bk 252 Cf 253. Es 257. Fm

( T o t a l Cm)

Transplutonium

The "Cleanex" Process: A Versatile Solvent Extraction Process for

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