Purification of gadolinium by electrorefining
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m e t h o d s used at A m e s L a b o r a t o r y - D O E to p r e p a r e the r a r e e a r t h m e t a l s have b e e n r e p o r t e d p r e v i o u s l y . 1"~ T h e s e m e t h o d s r e s u l t in the p u r e s t r a r e e a r t h m e t a l s p r o d u c e d on a k i l o g r a m s c a l e . H o w e v e r , h i g h e r p u r i t y m e t a l can be obtained on a s m a l l s c a l e by s o l i d s t at e e l e c t r o l y s i s .4 In b r i e f , the method f o r p r e p a r i n g gadolinium c o n s i s t s of r e d u c i n g high p u r i t y g a d o l i n i u m f l u o r i d e in a t a n t a l u m c r u c i b l e u s i n g c a l c i u m as a r e d u c t a n t u n d e r i n e r t conditions. T h e m e t a l is then v a c u u m m e l t e d at 1800~ to r e m o v e the v o l a t i l e i m p u r i t i e s (Ca, CaF2, and H). Since g a d o l i n i u m d i s s o l v e s an a p p r e c i a b l e quantity of t a n t a l u m at i t s m e l t ing point (1313~ the f i n a l step is to d i s t i l l the gadol i n i u m l e a v i n g the t a n t a l u m as a r e s i d u e . Since gadol i n i u m has both a low m e l t i n g point and a low v a p o r p r e s s u r e t h e r e is a g r e a t d e a l of r e f l u x i n g d u r i n g t h e d i s t i l l a t i o n p r o c e s s and this c a u s e s c o n s i d e r a b l e at t a c k on the d i s t i l l a t i o n c r u c i b l e , w h e th e r it is t a n t a l u m o r tungsten. F o r this r e a s o n the d i s t i l l a t i o n is C a r r i e d out at as low a t e m p e r a t u r e as p r a c t i c a l (1725~ but t h e r a t e of d i s t i l l a t i o n is q u it e slow (1.5 g / h ) . B e c a u s e t h i s d i s t i l l a t i o n p r o c e s s is c o s t l y in t i m e and in m a t e r i a l s we have i n v e s t i g a t e d an e l e c t r o r e f i n i n g p r o c e s s as art a l t e r n a t i v e m e t h o d f o r p u r i f y i n g g a d o l i n i u m . T h e u s e of e l e c t r o d e p o s i t i o n f r o m a f u s e d s a l t h as b e e n studied e x t e n s i v e l y at the U.S. B u r e a u of Mi n es in Reno, Nev ad a, and t h e i r e f f o r t s up to 1968 w e r e r e v i e w e d by M o r r i c e , e t a l . s R e c e n t w o r k at the B u r e a u of Mi n es includes the study by F l e c k , e t a l 6 on the p u r i f i c a t i o n of y t t r i u m by e l e c t r o r e f i n i n g , w h e r e b y they w e r e able to r e d u c e the oxygen content of the y t t r i u m by e l e c t r o r e f i n i n g in f l u o r i d e and f l u o r i d e - c h l o r i d e e l e c t r o l y t e s . H o w e v e r , the y t t r i u m they obtained was not as p u r e as the m e t a l obtained by the c a l c i u m r e duction of the f l u o r i d e when it is followed by v a c u u m m e l t i n g and d i s t i l l a t i o n , a It was hoped that the e l e c t r o r e f i n i n g method could be f u r t h e r i m p r o v e d to y i e l d gadolinium of the s a m e o r h i g h e r p u r i t y as d i s t i l l e d metal.
EXPERIMENTAL Th e e l e c t r o l y t e studied was a L i C 1 - L i F
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