Convection effects during electrotransport of liquid metals
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r L L E C T R O T R A N S P O R T in a b i n a r y liquid m e t a l s y s t e m is a p r o c e s s in which a d i r e c t e l e c t r i c a l c u r r e n t c a u s e s a r e l a t i v e motion b e t w e e n the solute and s o l vent a t o m s . This r e l a t i v e motion i m p l i e s that s e p a r a tion of the c o m p o n e n t s is p o s s i b l e . V e r h o e v e n ~ has shown that one might e x p e c t e l e c t r o t r a n s p o r t to be u s e ful in r e m o v i n g s o m e s o l u t e s f r o m liquid m e t a l s p r o v i d e d that c o n v e c t i o n can be s u p p r e s s e d at f ie ld i n t e n s i t i e s of 0.05 V p e r c m and above. Although a c o l u m n has r e c e n t l y b e e n d e s i g n e d f o r the s e m i c o n t i n u o u s p u r i f i c a t i o n of low m e l t i n g a l l o y s using the e l e c t r o t r a n s p o r t p r o c e s s 2 e v e n in this c a s e the l a r g e p u r i f i c a t i o n f a c t o r s p r e d i c t e d a r e not e x p e r i m e n t a l l y attained. In all such a t t e m p t s at e l e c t r o t r a n s p o r t d r i v e n p u r i f i c a t i o n a c o n v e c t i o n m o t i o n has b e e n p r e s e n t . This c o n v e c t i o n is in s o m e way d i r e c t l y r e l a t e d to the p r e s e n c e of the e l e c t r o t r a n s p o r t c u r r e n t and is p r e s e n t e v e n if o t h e r t y p es of c o n v e c t i o n s a r e e l i m i n a t e d . S e v e r a l t h e o r i e s have been p r e s e n t e d in o r d e r to e x plain the m e c h a n i s m of this c u r r e n t dependent c o n v e c tion 4-8 and a n u m b e r of e x p e r i m e n t a l s t u d i e s 8-~2 have a l s o b e e n r e p o r t e d . None of the t h e o r e t i c a l m e c h a n i s m s has b e e n able to c o n s i s t e n t l y e x p l a i n the e x p e r i m e n t a l o b s e r v a t i o n s . H o w e v e r , the e x p e r i m e n t a l st u d i e s have u s u al l y been quite l i m i t e d in scope and in m o s t c a s e s p o o r e x p e r i m e n t a l t e c h n i q u e s w e r e used, thus clouding i n t e r p r e t a t i o n of the p h y s i c a l c a u s e of the c o n v e c t i o n . An e x p e r i m e n t a l study of c o n v e c t i o n d u r i n g e l e c t r o t r a n s p o r t in liquid m e t a l s was u n d e r taken b e c a u s e it was hoped that if the p h y s i c a l c a u s e of the c o n v e c t i o n could be d e t e r m i n e d , then one could d e s i g n e x p e r i m e n t s to a c h i e v e s e p a r a t i o n f a c t o r s ap p r o a c h i n g those t h e o r e t i c a l l y p r e d i c t e d . PREVIOUS WORK The p r e s e n c e of c o n v e c t i o n in an e l e c t r o t r a n s p o r t e x p e r i m e n t at s t e a d y s t a t e i n c r e a s e s the v a l u e of the m e a s u r e d diffusion c o e f f i c i e n t . The m e a s u r e d diffusion c o e f f i c i e n t is g e n e r a l l y c a l l e d the e f f e c t i v e diffuR. E. SHAW, formerly with Ames Laboratory, U.S.A.E.C., Iowa State University, Ames, Iowa 50010, is now Metallurgist, Hunt
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