Thermodynamics of the system NaF-AlF 3 . Part III: Activities in liquid mixtures
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T H E p r e v i o u s p a p e r s in this s e r i e s l ' 2 h a v e e s t a b l i s h e d the t h e r m o d y n a m i c p r o p e r t i e s of the solid p h a s e s c r y olite (Na3 A1F6) and chiolite (NasAlaF~4), and the activity coefficient of sodium in liquid a l u m i n u m . The object of the p r e s e n t work is to u s e this i n f o r m a t i o n as a b a s e on which to build a t r e a t m e n t of the t h e r m o d y n a m i c s of liquid m i x t u r e s , and to i n t e r p r e t the l a t t e r as f a r as p o s s i b l e in s t r u c t u r a l t e r m s . T h r e e s e t s of e x p e r i m e n t a l m e a s u r e m e n t s have been m a d e - - d e t e r m i n a t i o n of the sodium c o n t e n t s of a l u m i n u m in e q u i l i b r i u m with liquid or t w o - p h a s e fluoride m i x t u r e s , d e t e r m i n a t i o n of the p o s i t i o n of the A1F3 l i q u i d u s in the s y s t e m , and m e a s u r e m e n t of a few c o n c e n t r a t i o n c e l l e l e c t r o m o t i v e f o r c e s with Ca 2ยง doped c r y o l i t e a s a solid Na+ e l e c t r o l y t e . 2 The sodium activity g e n e r a t e d by the e q u i l i b r i u m 3NaF + A1 ~- A1F3 + 3Na
d lnaNa -
1 + 2hAlF3 d In aNaF 3hAlF3
s i n c e nNaF + hAlF 3 = 1. Hence
In aNa F
f _13nA1F3
= g 1/3. (aNaF/a 1/3 A1F3)
[1]
s i n c e , in this case, CA1 = 1 and m a y be omitted. The t e r m in b r a c k e t s is a unique function of the m e l t c o m p o s i t i o n , but i n v o l v e s the a c t i v i t i e s of both c o m p o n e n t s . They can be s e p a r a t e d by i n t r o d u c t i o n of the G i b b s Duhem equation nNa F d l n a N a F + hAlF3 d In aA1F3 = 0
[2]
T a k i n g l o g a r i t h m s of Eq. [ 1] and d i f f e r e n t i a t i n g gives d InaNa
=d
1
In aNaF - - ~ d In aA1F3
[3]
E l i m i n a t i o p of aA1F3 gives E. W. DEWINGis Head of the Electrochemistry Section, Electrometallurgical Division,Alcan Research and Development, Arvida, Quebec, Canada. Manuscript submitted April 26, 1971. METALLURGICALTRANSACTIONS
+ 2"-~AIF3 d In aNn + const.
=
[5]
The value of the i n t e g r a t i o n c o n s t a n t can be fixed at the c r y o l i t e c o m p o s i t i o n where aNaF can be c a l c u l a t e d . With the a c t i v i t y coefficient 1 of s o d i u m in a l u m i n u m (RTlnYNa/n2A1 = 8,290 + 3.73 T) and the m e a s u r e d e q u i l i b r i u m sodium content, a n n is found; this is then u s e d with the m e a s u r e d 1 e q u i l i b r i u m c o n s t a n t for A1 + 6NaF ~ NasA1F6 + 3Na
[6]
to give a N a F . E l i m i n a t i o n of aNaF f r o m [2] and [3] gives
is aNa
[4]
In aA1F3 = --
f
3nNaF 1 + 2hAlF3 d In aNn + const.
[7]
The i n t e g r a t i o n c o n s t a n t can again be d e t e r m i n e d at the c r y o l i t e c o m p o s i t i o n s i n c e aA1F3 can be found f r o m aNa F and the f r e e e n e r g y 2 for 3NaF + A1Fs = NaaAIF6
[8]
EXPE RIMEN TAL
A l u m i n u m (~35 g) was m e l t e d with a c r y o l i t e m i x t u r e (~80 g) in a graphite c r u c i b l e (Grade ATJ) th
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