Semicontinuous determination of the concentration of alumina in the electrolyte of aluminum cells
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T h e c o n c e n t r a t i o n of a l u m i n a in a l u m i n u m cell bath was d e t e r m i n e d by a s e m i c o n t i n u o u s c h r o n o p o t e n t i o m e t r i c m e t h o d u s i n g a g r a p h i t e e l e c t r o d e i m m e r s e d in the bath of 140 kA p r e b a k e d c e l l s . T h e q u a n t i t y of liquid bath w a s a s s e s s e d by a d i l u t i o n t e c h n i q u e with LiC1 a s a d d i t i v e . A r a p i d i n c r e a s e in the a l u m i n a c o n c e n t r a t i o n w a s u s u a l l y o b s e r v e d by c r u s t b r e a k , but o c c a s i o n a l l y also a g r a d u a l i n c r e a s e o c c u r r e d . On a v e r a g e one t h i r d of the a l u m i n a a d d e d d i s s o l v e d v e r y r a p i d l y , w h i l e the r e s t w o u l d s e t t l e and d i s s o l v e s l o w l y in the i n t e r v a l s b e t w e e n c r u s t b r e a k s ( s e l f - f e e d i n g ) . P e r i o d s with l i t t l e o r no s e l f - f e e d i n g o c c u r r e d p a r t i c u l a r l y in c o l d - r u n n i n g c e l l s . High bath t e m p e r a t u r e s e e m e d to f a c i l i t a t e s e l f - f e e d i n g , and the c o n c e n t r a t i o n m i g h t even i n c r e a s e with t i m e . C e l l s with l a t e r a l c r u s t b r e a k and c e n t r a l c r u s t b r e a k b e h a v e d s i m i l a r l y .
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n o r m a l cell o p e r a t i o n the a l u m i n a is a d d e d b a t c h w i s e , c u s t o m a r i l y by b r e a k i n g the top c r u s t on a r e g u l a r s c h e d u l e e v e r y 2 to 4 h . T h e a l u m i n a may then e i t h e r d i s s o l v e a l m o s t i m m e d i a t e l y o r i t may s e t t l e , f o l l o w e d by a g r a d u a l d i s s o l u t i o n b e t w e e n c r u s t b r e a k s . It was s h o w n in a p r e v i o u s publication 1 that a l u m i n a w h i c h is e f f e c t i v e l y d i s p e r s e d in the bath d i s s o l v e s in the m a t t e r of 5 to 8 s , p r o v i d e d the a l u m i n a c o n c e n t r a t i o n is well b e l o w s a t u r a t i o n . Alumina w h i c h is a l l o w e d to s e t t l e , will d i s s o l v e much m o r e s l o w l y , d e p e n d i n g on the conditions. In o r d e r to a c h i e v e b e t t e r c o n t r o l of the a l u m i n a c o n t e n t in the b a t h , i t is d e s i r a b l e to know to what e x t e n t t h e s e two m o d e s of d i s s o l u t i o n o c c u r . T h e following i n f o r m a t i o n is n e e d e d : 1) the c o n c e n t r a t i o n of a l u m i n a in the bath a s a f u n c t i o n of t i m e , 2) the q u a n t i t y of l i q u i d b a t h , and 3) the a m o u n t of a l u m i n a b e i n g fed to the c e l l s . In the p r e s e n t w o r k the a l u m i n a c o n c e n t r a t i o n w a s d e t e r m i n e d by s e m i c o n t i n u o u s c h r o n o p o t e n t i o m e t r i c a n a l y s i s a p p l i e d d i r e c t l y in the bath of 140 kA p r e b a k e d c e l l s . T h e q u a n t i t y of liquid bath was d e t e r m i n e d by a dilution t e c h n i q u e . T h e a m o u n t
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