Solubility of Aluminum in Cryolite-Based Melts
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INTRODUCTION
ALUMINUM is produced by the electrolysis of alumina dissolved in cryolite-based melts. The primary cathodic reaction that occurs at the surface of the molten aluminum cathode is the reduction of Al(III)containing species. The next most-favored cathodic reaction is the deposition of sodium, which dissolves in the aluminum. Besides cryolite (Na3AlF6) and alumina, the electrolyte contains an excess of AlF3 with respect to Na3AlF6, and 2–5 mass pct CaF2. Other additions like LiF and/or MgF2 may be present as well. Both aluminum and sodium dissolve in the electrolyte. For aluminum, it is probably not physical dissolution but rather dissolution of species, which results from a reaction between the cryolite melt and the aluminum. When we speak about the solubility of aluminum, we understand that it is the sum of the dissolved aluminum and sodium. When the dissolved metal species are transported toward the anode, they are oxidized by CO2, which is the primary anode product when a carbon anode is used. This so-called ‘‘back reaction’’ lowers the current efficiency of the process.[1] A survey of the literature on the interaction between aluminum and the electrolyte as well as on the dissolution of aluminum in cryolite-based melts can be found in the monographs.[1,2] According to the cited literature, several authors have investigated the influence of temperature and composition on the aluminum solubility. However, the data were scattered, and in some cases, even the trends were conflicting. However, a principal agreement stated that the solubility of aluminum decreases with decreasing temperature. Most of the authors who investigated this topic have found that the concentration of aluminum dissolved in cryolitic melts V. DANIELIK, Associate Professor, P. FELLNER, Professor, and A. SY´KOROVA´, PhD Student, are with the Institute of Inorganic Chemistry, Technology and Materials, Slovak University of Technology, SK-812 37 Bratislava, Slovakia. Contact e-mail: vladimir.danielik@ stuba.sk. J. THONSTAD, Emeritus Professor, is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Manuscript submitted March 10, 2008. Article published online December 18, 2009. 430—VOLUME 41B, APRIL 2010
decreases with an increasing content of AlF3 in the melt. However, some Russian authors[3,4] have reported the opposite trend. To investigate aluminum solubility, Ødega˚rd[5] used an experimental technique that seemed to yield reliable results. The same technique also was used in the present work. Ødega˚rd et al.[6] published a consistent set of data for cryolite ratios (CR) ranging from 0.9 to 4 and between 960 C and 1060 C (CR denotes the ratio nðNaFÞ ; where n is the number of moles. The CR of nðAlF3 Þ Na3AlF6 equals 3.) In the present work, we investigated aluminum solubility in a wider temperature range than what had been tested so far.[3–10] A thermodynamic model of the molten system NaF-AlF3-Al2O3[11] made it possible to describe the aluminum solubility th
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