Transport and cathode deposition of reducible impurities during electrorefining with nonpolarizable electrodes
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METALr e f i n i n g is
p r i m a r i l y c o n c e r n e d with s e p a rating impurities. These separations are usually easier to m a k e when the c h e m i c a l potential of the i m p u r i t y is s i g n i f i c a n t l y g r e a t e r or s i g n i f i c a n t l y lower than that of the p r i m a r y m e t a l . E l e c t r o r e f i n i n g i s no exception. The " n o b l e " i m p u r i t y e l e m e n t s with much higher s t a n d a r d e l e c t r o d e p o t e n t i a l s (IUPAC sign convention) than that of the p r i m a r y m e t a l a r e r e t a i n e d in the anode or f o r m a r e s i d u e about the anode a s it d i s s o l v e s . The i m p u r i t y e l e m e n t s with much lower s t a n d a r d e l e c t r o d e p o t e n t i a l s r e a d i l y oxidize and e n t e r the e l e c t r o l y t e at the anode but a r e not e a s i l y r e d u c e d at the cathode. When the s t a n d a r d e l e c t r o d e p o t e n t i a l d i f f e r ence s e p a r a t i n g the p r i m a r y m e t a l and i m p u r i t y b e c o m e s s m a l l , the i m p u r i t y m a y be oxidized d u r i n g e l e c t r o r e f i n i n g , t r a n s p o r t e d to the cathode and r e d u c e d to c o n t a m i n a t e the cathode m e t a l . The i m p u r i t y c o n c e n t r a t i o n in the r e f i n e d m e t a l (cathode) will depend on the i m p u r i t y c o n c e n t r a t i o n in the e l e c t r o l y t e a d j a c e n t to the cathode and could be c a l c u l a t e d f r o m that c o n c e n t r a t i o n , if it w e r e known, and the e q u i l i b r i u m r e l a t i o n s h i p s , taking into a c c o u n t any n o n i d e a l i t y of both the e l e c t r o l y t e and cathode p h a s e s and any e l e c t r o d e p o l a r i z a t i o n p o t e n t i a l that may b e i m p o s e d at the cathode. S i m i l a r i m p u r i t y e q u i l i b r i u m and p o l a r i z a t i o n r e l a t i o n s h i p s would hold at the a n o d e / e l e c t r o l y t e i n t e r f a c e . The a b s o l u t e v a l u e s of the i m p u r i t y c o n c e n t r a t i o n in the anode, e l e c t r o l y t e and cathode at each of the two e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e s m a y a l s o depend on m a s s t r a n s p o r t p r o c e s s e s within the two e l e c t r o d e s and between the two e l e c t r o d e s within the e l e c t r o l y t e . A m a s s b a l a n c e of c o m p o n e n t s within the s y s t e m m u s t be m a i n t a i n e d and the electrolyte must remain electrically neutral. The p u r p o s e of the p r e s e n t p a p e r is to c o n s i d e r t h e s e r e l a t i o n s h i p s for the u s u a l cell a r r a n g e m e n t w h e r e the e l e c t r o d e s a r e opposite, p l a n e , p a r a l l e l s u r faces. T h i s allows t r e a t m e n t of the t r a n s p o r t p r o c e s s e s a s one d i m e n s i o n a l p r o b l e m s . The e l e c t r o r e -
R. W. BARTLETT is Associate Professor of Metallurgy, Department of Applied Earth Sciences, Stanford University, Stanford,
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