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3AI +

[1]

A13++3Na~

[2]

and AI+6Na + ~

It h a s a l s o b e e n s u g g e s t e d that s o d i u m m a y o c c u r a s u n c h a r g e d m e t a l a t o m s . V e r y l i t t l e is known h o w e v e r about the o c c u r r e n c e of s u b v a l e n t s o d i u m , and in the p r e s e n c e of Na and Na + the s t a b i l i t y of Na~ m a y be questioned. Yoshida and Dewing 9 r u l e out the f o r m a t i o n of A1F o r A1§ a s a p r o d u c t of d i s s o l u t i o n and s u g g e s t that the s o l u t i o n m a y be p a r t l y a s s o d i u m in s o m e f o r m and p a r t l y c o l l o i d a l . The d i f f e r e n c e s of opinion m a y be due to the d i f f e r e n c e s in e x p e r i m e n t a l a p p r o a c h , and t h i s is e v i d e n t f r o m the g r e a t d i s c r e p a n c i e s in p u b l i s h e d d a t a ( i l l u s t r a t e d in T a b l e I). It is a p p a r e n t that there is a need f o r an i m p r o v e d e x p e r i m e n t a l and a n a l y t i c a l a p p r o a c h and t h i s h a s b e e n the p r i m e o b j e c t i v e of the p r e s e n t work. Two d i f f e r e n t m e c h a n i s m s of m e t a l l o s s have p r e v i o u s l y b e e n c o n s i d e r e d : in one CO2 d i s s o l v e s and o x i d i z e s the d i s s o l v e d m e t a l e i t h e r within the e l e c t r o l y t e o r n e a r the c a t h o d e ; in the o t h e r the d i s s o l v e d m e t a l d i f f u s e s to the anode and is o x i d i z e d b y CO, at the s u r f a c e of the g a s b u b b l e s . The f i r s t m e c h a n i s m is often d i s m i s s e d b e c a u s e the s o l u b i l i t y of CO2 in c r y o l i t e is v e r y low, of the o r d e r of 10 -s to 10 -6 m o l e s / c m Z , ls'~8 F o r the s e c o n d m e c h a n i s m f o u r s t a g e s have b e e n p r o posed:8,1~, 18 A. M. ARTHUR is with the British Aluminium Company Limited, Chalfont Technological Centre, Gerrards Cross, Bucks, England. Manuscript submitted October 31, 1973. METALLURGICAL TRANSACTIONS

1) I n t e r a c t i o n at the m e t a l - e l e c t r o l y t e i n t e r f a c e . 2) Diffusion of the r e a c t i o n p r o d u c t s t h r o u g h a thin b o u n d a r y l a y e r . 3) T r a n s p o r t of the p r o d u c t s t h r o u g h the bulk of the e l e c t r o l y t e . 4) Oxidation of the c a t h o d i c p r o d u c t s b y CO2 b u b b l e s n e a r the anode. T h e r e a r e d i f f e r e n c e s of opinion r e g a r d i n g which of the f o u r s t a g e s i s the r a t e - d e t e r m i n i n g step. Stages 1 and 4 a r e c h e m i c a l r e a c t i o n s and have high a c t i v a t i o n e n e r g i e s c o m p a r e d with the p u r e l y p h y s i c a l t r a n s p o r t p r o c e s s e s in s t a g e s 2 and 3. With i n c r e a s i n g t e m p e r a t u r e , the r a t e s of r e a c t i o n s 1 and 4 will b e g r e a t l y e n h a n c e d and a r e u n l i k e l y to be r a t e - d e t e r m i n i n g . Due to the v i g o r o u s a g i t a t i o n of the e l e c t r o l y t e in the c o m m e r c i a l c e l l ,