Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolys
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= 1011 + 0.50[A1F31 - 0.13[AIF# 2 -
3.45[CAF2] 1 + 0.0173[CaF_~]
+ 0.124[CAF2] 9 [AIF3] - 0.00542 ([CaF2].[A1F3])'.5 7.93[A1203] 1 + 0.0936[A1203] - 0.0017[A1203] z - 0.0023[A1F3] 9 [A1203] 8.90[LiF] - 3.95[MgF2] - 3.95[KF1 1 + 0.0047[LiF] + 0.0010[A1F3] 2 where t is the temperature in degree Celsius and the square brackets denote the weight percent of components in the system Na3A1F6-AIF3-CaF2-A1203-LiF-MgF2-KF. The composition limitations are [A1F3] ~- [CaF2] ~- [LiF] < 20 wt pct, [MgF2] ~-- [KF] < 5 wt pct, and [A1203] up to saturation.
I.
INTRODUCTION
THE main component of the electrolyte (bath) in commercial aluminum electrolysis cells is cryolite (Na3A1F6), which has a melting point of 1011 ___ 1 ~ u.2~ In addition to cryolite, conventional electrolytes contain 5 to 13 wt pct A1F3, 3 to 7 wt pct CaF2, and 1 to 5 wt pct At,O 3 (alumina). Some companies have also introduced other additives such as LiF and MgF 2. Industrial cells with a conventional electrolyte usually operate in the temperature range 950 ~ to 980 ~ depending on bath composition, which is about 5 ~ to 15 ~ above the corresponding liquidus temperature for primary cryolite crystallization. Addition of LiF to this melt increases the electrical conductivityt3.41and lowers the liquidus temperature. However, the main purpose of using additives such as LiF and MgF2 is the potential of increasing the current efficiency with respect to aluminumy.61 By the introduction of low melting baths, one may also achieve lower carbon consumption and, possibly, prolonged cell life and easier adaption of inert electrode materials.t7.sl Bath composition and the corresponding liquidus temperature are key parameters for operation of commercial aluminum electrolysis cells. Several research groups have developed model equations ASBJORN SOLHEIM, SVERRE ROLSETH, and EGIL SKYBAI~MOEN, Research Scientists, and LISBET STOEN, Research Engineer, are with SINTEF Metallurgy, N-7034 Trondheim, Norway. ASMUND STERTEN, Professor, and TROND STORE, Postdoctoral Student, are with the Department of Electrochemistry, The Norwegian institute of Technology, N-7034 Trondheim, Norway. Manuscript submitted October 27, 1995. METALLURGICAL AND MATERIALSTRANSACTIONSB
describing liquidus surfaces for compositions corresponding roughly to the conventional electrolytes.tg-'31 Therefore, it should be emphasized that most of the equations are valid for a limited range of compositions. The present work describes a new equation based on our previous work,t'3~ as well as additional experimental data for both conventional electrolyte compositions, low melting bath with high concentrations of AIF 3, LiF, and CaF2 and with other additives like MgF 2 and KF. Phase equilibria in cryolite-based melts have been studied by thermal analysis (TA), by differential thermal analysis (DTA), by visual observations, and by quenching techniques.tg-'51 In the region of primary cryolite crystallization, TA has proven to give accurate freezing point depression data.t,.2.t3.~4] By using this method, it is essential to
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