Thermodynamics of the system NaF-AlF 3 Part I: The equilibrium 6NaF( s ) + Al = Na 3 AlF 6 ( s ) + 3Na

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The p r e s e n t r e s u l t s a r e not c o n s i s t e n t with the suggestion that N a , F i s an i m p o r t a n t g a s e o u s s p e c i e s in this s y s t e m .

D E S P I T E the i m p o r t a n c e of the s y s t e m to the a l u m i n u m i n d u s t r y the t h e r m o d y n a m i c p a r a m e t e r s of m i x t u r e s and compounds in the s y s t e m NaF-A1Fs a r e not well defined. The o v e r a l l a i m of the p r e s e n t work is to r e c t i f y the s i t u a t i o n . As a f i r s t step the e q u i l i b r i a 6NaF(s) + Al(l or s) = NasA1Fs(s) + 3Na(g)

[1]

and 6NaF(s) + AI(/) = NasA1F6(s) + 3Na(A1)

[2]

had the a p p e a r a n c e of m e t a l l i c s o d i u m . It was washed out with w a t e r , f r e q u e n t l y giving a v i o l e n t r e a c t i o n , and the r e s u l t i n g solution of NaOH was t i t r a t e d to a m e t h y l p u r p l e e n d - p o i n t (pH4.8). Two s e r i e s of m e a s u r e m e n t s were made of the sodium c o n t e n t of a l u m i n u m in e q u i l i b r i u m with solid N a F and solid NasA1F~. The f i r s t was c a r r i e d out in graphite c r u c i b l e s , and some NaC1 was added to the m i x t u r e to g e n e r a t e a liquid phase (in the hope of

have b e e n studied. The f o r m e r gives a value for the f r e e e n e r g y of f o r m a t i o n of NasA1F6 (in t e r m s of the f r e e e n e r g y of N a F , which m a y itself be u n c e r t a i n ) , while the two together give v a l u e s for the a c t i v i t y coefficient of sodium in dilute solution in a l u m i n u m , a quantity which will be n e e d e d in s u b s e q u e n t work. EXPERIMENTAL Sodium p r e s s u r e s o v e r e q u i l i b r i u m [1] were m e a s u r e d with a gas t r a n s f e r e n c e method in an a l u m i n a app a r a t u s , Fig. 1. Argon p a s s e d diwnward through the o u t e r 1 in. ID by 18 in. long tube r through a p o r o u s a l u m i n a plug, through the c h a r g e of b r o k e n b r i q u e t t e s of N a F and a l u m i n u m p o w d e r s , and into the -~ in. exit tube v i a a s m a l l hole cut with a d i a m o n d saw into its b o t t o m . D u r i n g the h e a t i n g up p e r i o d the a r g o n flow was r e v e r s e d to p r e v e n t any s o d i u m vapor e n t e r i n g the c o n d e n s e r . T e m p e r a t u r e s were m e a s u r e d with a P t - 1 0 pet Rh t h e r m o c o u p l e located alongside the r e a c t i o n z o n e , the t h e r m o c o u p l e having been found to be c o r r e c t to within I~ at the m e l t i n g points of a l u m i n u m (660~ and NaC1 (800.5~ A r g o n v o l u m e s were m e a s u r e d with a w e t - t e s t gas m e t e r . At the end of a r u n the tube was withdrawn f r o m the f u r n a c e . When it had cooled a single r i n g of c o n d e n sate high up in the c o n d e n s e r tube could be s e e n ; it E. W. DEWING is Research Chemist, Alcan Research and Development, Ltd., Arvida, Quebec, Canada. Manuscript submitted October 2, 1969. METALLURGICAL TRANSACTIONS

--To gas meter Argon in Alumina

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