A New process for the separation of tantalum from niobium
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HIGHp u r i t y n i o b i u m m e t a l
is a n e c e s s a r y component for the p r o d u c t i o n of the z i r c o n i u m alloys which a r e used in n u c l e a r r e a c t o r s . N i o b i u m used for these p u r p o s e s m u s t c o n t a i n e x t r e m e l y s m a l l a m o u n t s of t a n t a l u m as the l a t t e r has a t h e r m a l n e u t r o n c a p t u r e c r o s s section of 21.3 b a r n s p e r atom, ~ which is twenty t i m e s that of n i o b i u m . However, n i o b i u m and t a n t a l u m a r e found together in n a t u r e , and t h e i r strong c h e m i c a l s i m i l a r i t y m a k e s it difficult to r e f i n e e i t h e r m e t a l free f r o m the o t h e r . The f i r s t i n d u s t r i a l p r o c e s s for s e p a r a t i n g n i o b i u m and t a n t a l u m f r o m each other was developed by M a r i g n a c 2 and involved the c r y s t a l l i z a t i o n of K2TaFv f r o m dilute h y d r o f l u o r i c acid s o l u t i o n s . However, in this method the n i o b i u m and all other i m p u r i t i e s a c c u mulate in the aqueous phase making the p r o d u c t i o n of high p u r i t y n i o b i u m difficult. This p r o c e d u r e has been s u p e r c e d e d by l i q u i d - l i q u i d e x t r a c t i o n . About 200 d i f f e r e n t c o m b i n a t i o n s of solvent and e x t r a c t a n t were tested by the U.S. B u r e a u of M i n e s with the p u r p o s e of developing an i n d u s t r i a l s e p a r a t i o n p r o c e s s . 3 Suitable aqueous p h a s e s were m i x t u r e s containing HF, HF-HC1, and HF-H2SO4. E x c e l l e n t s e p a r a tions were achieved with the o r g a n i c solvent methyl isobutyl ketone (MIBK). However, MIBK is a r e l a t i v e l y volatile s u b s t a n c e with a f l a s h point of 24~ and r e q u i r e s the use of flame proof e q u i p m e n t . In addition, MIBK is a c o n s u m a b l e r e a g e n t , d i s s o l v i n g in the a q u e ous phase (~2 pct), and c o n t r i b u t e s s u b s t a n t i a l l y to the cost. P e r h a p s the most s i g n i f i c a n t d i s a d v a n t a g e of the solvent e x t r a c t i o n s e p a r a t i o n method is that it i n t r o duces a s e p a r a t e h y d r o m e t a l l u r g i c a l step into the flowsheet for n i o b i u m p r o d u c t i o n which, o t h e r w i s e is e n tirely pyrometallurgical. Other methods include s e p a r a t i o n with m i c r o b i o logical c o l l e c t o r s , 4 e l e c t r o l y t i c s e p a r a t i o n , 5 and s e l e c tive r e a c t i o n of the p e n t a c h l o r i d e s of n i o b i u m and t a n t a l u m with v a r i o u s r e a g e n t s such as CaO and CaF2, 6 m o l t e n NaA1C14, 7 a l k a l i - m e t a l c h l o r i d e s in the p r e s e n c e of e x c e s s hydrogen, 8 and even n i o b i u m m e t a l itself. 9 D. R. SADOWAYis Professor, Department of Materials Science and Engineering,Massachusetts Institute of Technology, Cambridge, MA 02139. S. N. FLENGASis Professor, Department of Metallurgy and Materials Science, University
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