The apparent solubility of aluminum in cryolite melts
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- 0.00192 (% CaFz)
0.00174 (% LisA1F6) - 0.00288 (% NaC1)
with a s t a n d a r d deviation of +0.017. R a n g e s c o v e r e d w e r e r a t i o 0.8 to 2.3, t e m p e r a t u r e s 969 ~ to 1054~ CaF2 -< 14 pct, LisA1F6 - 20 pct, and NaC1 -< 10 pct. T h e r e was no s i g n i f i c a n t effect of adding 0 to 38. pet K3A1F6 or 0 to 10 pet MgF2. It was found that s o l u b i l i t y was a p p r o x i m a t e l y p r o p o r t i o n a l to activity of a l u m i n u m when A1-Cu alloys w e r e u s e d . P o s s i b l e m e c h a n i s m s of solution a r e d i s c u s s e d . Monovalent a l u m i n u m is r u l e d out on the b a s i s of the v a r i a t i o n of s o l u b i l i t y with NaF/A1Fs r a t i o and aA1. The f a v o r e d , but not p r o v e n , m e c h a n i s m i n v o l v e s f o r m a t i o n of both s o d i u m a t o m s and a colloidal d i s p e r s i o n of a l u m i n u m .
IT is well e s t a b l i s h e d
that the chief c a u s e of l o s s of c u r r e n t e f f i c i e n c y in a l u m i n u m e l e c t r o l y s i s is d i s s o l u tion of a l u m i n u m in the e l e c t r o l y t e and s u b s e q u e n t r e oxidation by COz in the anode gas. ( F o r p r e s e n t p u r p o s e s the word " s o l u t i o n " is taken to include colloidal d i s p e r s i o n s or " f o g s . " ) S e v e r a l m e a s u r e m e n t s of the solubility of aluminum in cryolite have been made r e cently,~-4 but since the results vary by a factor of twenty it was apparent that more work was required. The results are very dependent on the experimental method used. Gerlach, Schmidt, and Schmidt~ and Yoshida, Ishihara, and Yokoi,4 measured the loss of weight of aluminum in cryolite and assumed that the loss represented metal in solution; they obtained solubilities of the order of 1 pct. Thonstad2 and Haupin~ made determinations of hydrogen evolved from the frozen cryolite and obtained values of the order of 0.I pct. The former method can give high results due to reaction of the aluminum with the crucible material or escape of vapor from the crucible; the latter suffers from uncertainties arising from possible oxidation of precipitated metal during handling of the samples, and if aluminum carbide is present some of the gas evolved will be methane and not hydrogen. To avoid some of these uncertainties it was decided to make analyses on the hot melts without freezing them or withdrawing them from the crucible, and titration with electrolytically generated oxygen seemed the most promising method. The alternative, coulometric determination by direct anodic oxidation, did not give reproducible results. Mass transfer was sufficiently slow that excessively long times were required for complete oxidation. With the oxygen generation method the evolved gas bubbles had a large surface area and generated t u r b u l e n c e , and r e a c t i o n was r a p i d .
K. YOSHIDA is with the Nippon Light Metal Research Laboratory Ltd., l(ambara, Shizuoka, Japan. E.W. DEWINGis with the Arvida Research Laboratory of the Aluminum Compan
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