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4/27/04

20:08

Page 509

Modeling the Dependence of Alumina Solubility on Temperature and Melt Composition in CryoliteBased Melts YUNSHU ZHANG and ROBERT A. RAPP The solubility of alumina in NaF-AlF3 melts was calculated and modeled thermodynamically for the temperature range of 1240 to 1300 K (967 °C to 1027 °C). The solute complexes of alumina in the cryolite melts were identified to be Na2Al2OF6 (acidic solute), Na2Al2O2F4 (neutral solute), and Na4Al2O2F6 (basic solute). The assumption that the oxygen-free solute species in solution were Na3AlF6 and NaAlF4 was supported by the modeling results. The equilibrium constants for the formation reactions of the solutes were calculated and the corresponding
Gf0 values were evaluated as a function of temperature. The interaction derivatives ( ln aNaF/xadd,  ln aAlF3/xadd, and  ln aAl2O3/xadd) for small additions of LiF, CaF2, and MgF2 to the NaF-AlF3-Al2O3 ternary system were also estimated as a function of temperature and melt composition.

I.

INTRODUCTION

BECAUSE of its practical importance in primary aluminum production by the Hall–Heroult process, the solubility of alumina in cryolite melts has been extensively studied. In two previous articles,[1,2] the prior research in this field was reviewed, and the solubility of alumina in cryolite melts was thermodynamically modeled for 1300 K over the melt composition ranges of 1.5  cryolite ratio r  3 and 1.5  cryolite ratio r  12.5, respectively. The cryolite ratio r is defined as the ratio of moles NaF to moles AlF3 (r  mol NaF/mol AlF3). The oxyfluoride solutes responsible for the dissolution of alumina in cryolite melts were identified as Na2Al2OF6 (acidic solute), Na2Al2O2F4 (neutral solute), and Na4Al2O2F6 (basic solute). In those studies, the oxygen-free solute species were assumed to be Na3AlF6, Na2AlF5, and NaAlF4. The existence of the solute Na2AlF5 in cryolite melts is, in fact, questionable. From thermodynamic considerations, Raman spectroscopic studies, NMR studies, etc., some authors[3–9] suggested the existence of Na2AlF5 as a major or minor solute species in the molten NaF-AlF3 system, but others[10–14] found no evidence to support the presence of this five-fold coordinated aluminum complex in the system. In general, solute species existing in any system should possess the simplest geometric arrangements of large anions surrounding the small highly charged cation, e.g., in this case, AlF63 (octahedron) and AlF4 (tetrahedron). In the solute complexes AlF63 and AlF4, each F occupies an equivalent site as every other F ion. In the suggested solute AlF52, the five F ions would exist at least in two different sites, and they would not be symmetric in all directions. Therefore, the suggested higher energy solute AlF52 could not be as stable as solutes AlF63 and AlF4, and it would spontaneously dissociate: 2 AlF52 → AlF63  AlF4. As a result, since the publication of Reference 2, YUNSHU ZHANG, Research Scientist, and ROBERT A. RAPP, Professor Emeritus, are with the Department of Material