Alumina solubility in molten salt systems of interest for aluminum electrolysis and related phase diagram data
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[Al2O3]sat 5 A where
A 5 11.9 2 0.062[AlF3] 2 0.0031[AlF3]2 2 0.50[LiF] 2 0.20[CaF2] 2 0.30[MgF2] 1 B 5 4.8 2 0.048[AlF3] 1
42[LiF] z [AlF3] 2000 1 [LiF] z [AlF3]
2.2[LiF]1.5 10 1 [LiF] 1 0.001[AlF3]3
where the square brackets denote weight percent of components in the system Na3AlF6-Al2O3 (sat)AlF3-CaF2-MgF2-LiF and t is the temperature in degree Celsius. The standard deviation between the equation and the experimental points in the temperature range from 1050 7C to about 850 7C was found to be 0.29 wt pct Al2O3. A series of revised phase diagram data of interest for aluminum electrolysis was derived based on the present work and recently published data for primary crystallization of Na3AlF6 in the same systems.
I.
INTRODUCTION
SODIUM cryolite (Na3AlF6) is the main constituent of the electrolyte in cells for primary aluminum production due to its high alumina solubility. Previously, alumina was added batchwise to the electrolyte, and a high solubility was regarded as a prerequisite in order to minimize the accumulation of alumina sludge underneath the metal pad. In the last decades, a more sophisticated method with a nearly continuous feeding of alumina has been developed, and the restrictions concerning solubility have been somewhat lifted. In this situation, some interest in alternative electrolyte compositions, i.e., low melting baths, has emerged. However, the rate of solution of alumina and the solubility limit are still critical factors. The minimum limit is defined by the ‘‘state of the art’’ as far as the feeding technology is concerned. Reliable alumina saturation data are therefore crucial in the development of low melting electrolytes in aluminum electrolysis cells. The alumina solubility is relatively well established in the normal range of electrolyte compositions,[1,2,3] which also follows from the alumina liquidus equation derived by Dewing.[4] The data are more scarce at high contents of
EGIL SKYBAKMOEN and ASBJØRN SOLHEIM, Research Scientists, are with SINTEF Materials Technology, N-7034 Trondheim, Norway. ˚ SMUND STERTEN, Professor, is with the Department of Electrochemistry, A the Norwegian Institute of Technology, N-7034 Trondheim, Norway. Manuscript submitted December 12, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
additives and lower temperatures, although a number of Na3AlF6-AlF3-Al2O3-MaXb ternary and quaternary liquidus diagrams have been partly investigated.[5] In most cases, the alumina liquidus was established by visual methods[1] or by quenching techniques.[2,3] However, the nucleation of alumina crystals is sluggish,[1,6] and due to the steep alumina liquidus surface, a considerable supercooling and oversaturation will easily occur. The solubility limit as found by these methods would therefore tend to be too high, which is reflected in some of the works with cryolitic melts saturated with alumina.[5] The purpose of the present work was (1) to establish the alumina solubility in a wide range of temperatures and compositions in the cryolite-based system Na3AlF6-AlF3CaF2-
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