Electrowinning of zirconium from zirconium tetrachloride
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S T E I N B E R G , S i e b e r t , and W a i n e r t'2 showed that z i r c o n i u m could be e l e c t r o w o n f r o m K2ZrF6 d i s s o l v e d in m o l t e n NaCI-KC1 m i x t u r e s . The e l e c t r o d e p o s i t s p r o duced were a p p r o x i m a t e l y two p a r t s salt and one p a r t m e t a l when r e m o v e d f r o m the cell. After c r u s h i n g and leaching, the m e t a l was 45 pct - 2 0 0 m e s h s i z e . Cathode c u r r e n t e f f i c i e n c i e s were 53 to 67 pct. R a y n e s , T h e l l m a n n , S t e i n b e r g , and W a i n e r 3 d e s c r i b e d the above p r o c e s s a f t e r it was s c a l e d - u p to p r o d u c e 4 to 6 lb. p e r hr of z i r c o n i u m u s i n g an NaC1-K2ZrF8 e l e c t r o l y t e and feeding K2ZrF6. As e l e c t r o l y s i s p r o ceeded, s o m e t i m e s K2ZrF6 and NaC1 w e r e added to the e l e c t r o l y t e . The K F c o n c e n t r a t i o n i n c r e a s e d as the z i r c o n i u m was depleted, r e s u l t i n g in a high m e l t ing point e l e c t r o l y t e . E l e c t r o l y s i s was t e r m i n a t e d when the e l e c t r o l y t e could not be m a i n t a i n e d in a m o l t e n s t a t e . The high c o n c e n t r a t i o n of K2ZrF6 and its low s o l u b i l i t y in w a t e r c a u s e d some leaching p r o b l e m s . B e c a u s e of the c h l o r i n e l i b e r a t e d at the a n o d e , a l l exposed p a r t s of the c e l l w e r e c o n s t r u c t e d of g r a p h i t e . R a y n e s , T h e l l m a n n , S t e i n b e r g , and W a i n e r 3 s p e c u l a t e d tlmt the r e d u c t i o n p r o g r e s s e d as follows: Z r ++++ ~ Z r +++ ~ Z r ++ ~ Z r and that ZrC12 m i g h t be p r e s e n t in the e l e c t r o l y t e . L a t e r detailed s t u d i e s by M e l l o r s and Senderoff 4 p r o v e d that in the f l u o r i d e s y s t e m the z i r c o n i u m is p r e s e n t only a s a 4 - v a l e n c e ion. They also proved that z i r c o n i u m m e t a l r e a c t e d with K F , producing p o t a s s i u m m e t a l that v o l a t i l i z e d f r o m the hot zone, l e a v i n g ZrF4 or ZrF7 - in the electrolyte. Potassium metal causes physical deterioration of g r a p h i t e when in contact with it, t h e r e f o r e , the p r e s e n t a u t h o r s decided a g a i n s t u s i n g p o t a s s i u m s a l t s in the e l e c t r o l y t e . I n i t i a l s t u d i e s with the a l l - c h l o r i d e s y s t e m s c o n f i r m e d the work of S t e i n b e r g , S i e b e r t , and W a i n e r , x'2 M e l l o r s and Senderoff, 4 and B a k e r , Nettle, and K n u d s e n . 5 The z i r c o n i u m e l e c t r o d e p o s i t e d a s powder m i x e d with a l a r g e a m o u n t of d a r k - c o l o r e d salt and r e c o v e r i e s w e r e v e r y low. Swaroop and F l e n g a s 6 p r o d u c e d ZrC13 in a s e a l e d s y s t e m by r e a c t i n g ZrC14 G. M. MARTINEZis Research Chemist, U.S. Bureau of Mines, Reno Metallurgy Research Center, Reno, Nev. D. E. COUCHis Supervisory Research Chemist, U.S. Bureau
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