The dissolution of aluminum oxide in cryolite melts
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F o r O - A l 2 O s - p r e p a r a t i o n B a y e r i t e was f i r e d 8 h at 850~ The b e s t r e s u l t s for 6-A12Os-preparation were get by h e a t i n g Boehmite at 900~ for 32 h. /3-Al~Os was p r e p a r e d b y m i x i n g a l u m i n u m hydroxide with NaOH, d r y i n g at 100~ for 12 h and f i r i n g this product for 24 h at 850~ All t h e s e products of heat t r e a t m e n t were e x a m i n e d by X - r a y methods and shown to be p u r e b y this d e t e r m i n a t i o n method. The a l u m i n u m oxides made in such a way were p r e s s e d to r o u n d t a b l e t s . The m a j o r i t y of the t e s t s was c a r r i e d out with t a b l e t s of a t h i c k n e s s of 10 m m thick and 21.4 m m diam. P r e l i m i n a r y t e s t s had shown that when the t a b l e t s w e r e d i s s o l v e d to about 60 pct of t h e i r o r i g i n a l weight, the d i s s o l u t i o n o c c u r r e d m a i n l y and e v e n l y on the round s u r f a c e facing the s t i r r e r . In e v a l u a t i n g the t e s t s , t h e r e f o r e , the r e s u l t s m a y be r e f e r r e d to this surface. The t a b l e t s w e r e put into the m e l t s c o n s i s t i n g of c r y o l i t e or of c r y o l i t e and a d d i t i v e s . The m e l t s were s t i r r e d at c o n s t a n t rotation speed. At i n t e r v a l s , a n a l y s i s s a m p l e s w e r e t a k e n f r o m the m e l t b y m e a n s of s t a i n l e s s s t e e l tubes. The AI~Os c o n t e n t s of t h e s e s a m ples were d e t e r m i n e d by c h e m i c a l a n a l y s i s . E X P E R I M E N T A L RESULTS At f i r s t s o m e e x p e r i m e n t s w e r e made with t a b l e t s of a-A120 s in c r y o l i t e m e l t s at a t e m p e r a t u r e of 1015~ using v a r i o u s s t i r r i n g speeds b e t w e e n 50 and 300 r p m . Except at low r o t a t i o n speeds the d i s s o l u t i o n r a t e was found to be independent of the speed of the s t i r r e r . All f u r t h e r t e s t s w e r e , t h e r e f o r e , c a r r i e d out at 200 r p m .
Thermocoup~ewith Sheath
Protective Lift
.~
---4 9~
Ceramic
Tube
External
lns~leliae
Resistar~e Furnace
.I Fig. 1--Test Apparatus.
Crucible Stirrer
9
Halt
J. GERLACH is a Member AIME, U. HENNIG and K. KERN are Professor of Metallurgy and Scientists respectively at the Institute of Metallurgy, Technical University of Berlin, West Germany. Manuscript submitted January 28, 1974. METALLURGICAL TRANSACTIONS B
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Aluminum Tablet Alumina
Oxide
Grit
VOLUME 6B, MARCH 1975-83
Fig. 2 shows the effect of the effective s u r f a c e a r e a of the AI~Os t a b l e t s . The a m o u n t of AI~Oa d i s s o l v e d a f t e r a t e s t p e r i o d of 5 m i n u t e s is t a k e n as a m e a s u r e for the d i s s o l u t i o n r a t e of the AI~O~. F o r t h e s e t e s t s , the d i a m e t e r s of the t a b l e t s were v a r i e d b e t w e e n 15.6 a n d 21.4 m m , and the effective s u r f a c e a r e a s w e r e c a l culated b a s e d on c i r c u l a r s u r f a c e s of the r e
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