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F. NORDMO,

AND

J. B. PAULSEN

A l u m i n a powder d i s s o l v e s r a p i d l y in m o l t e n c r y o l i t e , i . e . in 4 to 8 s e c o n d s , p r o v i d e d that the p a r t i c l e s a r e effectively d i s p e r s e d in the m e l t . At a l u m i n a contents well below s a t u r a t i o n the t i m e of d i s s o l u t i o n i s i n d e p e n d e n t of the a l u m i n a c o n c e n t r a t i o n , and it i s not a p p r e c i a b l y affected by a d d i t i o n s of N a F , A1F3, o r CaF2. C o a r s e g r a i n s d i s s o l v e m o r e slowly than fine g r a i n s and a?A1203 (floury) d i s s o l v e s m o r e slowly than y-A1203 (sandy). The r a t e - c o n t r o l l i n g step a p p e a r s to be a slow c h e m i c a l r e a c t i o n . Bottom sludge in i n d u s t r i a l c e i l s c o n t a i n s about 40 pct A1203. T h i s m a t e r i a l has a h i g h e r a / y - r a t i o than the a l u m i n a being fed to the c e l l s , while the g r a i n size i s s m a l l e r due to d i s i n t e g r a t i o n of a g g r e g a t e s . The t i m e of d i s s o l u t i o n is i d e n t i c a l to that of any other a l u m i n a with the s a m e a / y - r a t i o .

IN c u r r e n t

p r a c t i c e the a l u m i n a is fed b a t c h - w i s e to the a l u m i n u m c e l l s by b r e a k i n g the f r o z e n c r u s t on top of the bath. A p p a r e n t l y the a l u m i n a does not d i s s o l v e completely in the bath i m m e d i a t e l y after' addition, and s o m e m a t e r i a l m a y even sink to the bottom of the cell to form a c r y o l i t e - a l u m i n a sludge u n d e r n e a t h the m e t a l pad. The p r e s e n c e of l a r g e a m o u n t s of sludge h a s an a d v e r s e effect on cell o p e r a t i o n . In the p r e s e n t work the d i s s o l u t i o n p r o c e s s of a l u m i n a was studied and the p r o p e r t i e s of bottom sludge w e r e e x a m i n e d . A) TIME OF DISSOLUTION O F ALUMINA

To a s s e s s the r a t e of d i s s o l u t i o n of a l u m i n a , the c o n c e n t r a t i o n of a l u m i n a in the m e l t should be d e t e r m i n e d as a function of t i m e during the d i s s o l u t i o n p r o c e s s . This p r o c e d u r e i s difficult e x p e r i m e n t a l l y , and i n s t e a d it i s c u d t o m a r y to d e t e r m i n e the t i m e of d i s s o l u t i o n (td) , i . e . , the t i m e needed to d i s s o l v e a s a m p l e of a l u m i n a powder in the m e l t . The e x p e r i m e n t a l m e t h o d s used in l a b o r a t o r y i n v e s t i g a t i o n s can be divided into two g r o u p s : 1) U s e of m o d e r a t e s t i r r i n g or no s t i r r i n g . With t h i s method, which h a s b e e n applied by m o s t i n v e s t i g a t o r s , 1-12 some of the a l u m i n a added will tend to clog and sink to the bottom of the c r u c i b l e . It will then d i s s o l v e r a t h e r slowly in the c o u r s e of s o m e m i n u t e s . As a whole, the p u b l i s h e d r e s u l t s show that t d i n c r e a s e s when the a l u