Solubility of alumina in cryolite melts: Measurements and modeling at 1300 K
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THE solubility of Al2O3 in cryolite melts and the melt structure of the NaF-AlF3-Al2O3 system have been active subjects of study for over a century because of the industrial importance of primary aluminum production by the Hall–Heroult process. Although many previous measurements of alumina solubility have been published, and significant progress has been made in understanding the melt structure, further work was necessary to achieve an improved knowledge and correlation in these two contexts. Skybakmoen et al.[1] accurately measured the solubility of Al2O3 in cryolite melts for melt compositions in the range of 1.5 ⱕ cryolite ratio r ⱕ 3 at 1300 K. The cryolite ratio r is defined as the ratio of moles NaF to moles AlF3 (r ⫽ mol NaF/mol AlF3). Solheim and Sterten[2] reported mathematical functions to describe the activities of NaF, AlF3, and Al2O3 in the ternary NaF-AlF3-Al2O3 system as a function of the r value for this range of cryolite ratio. As shown in Figure 1, the solubility of Al2O3 in basic cryolite melts (r ⬎ 3) has been studied several times,[3–6] but the data are not consistent. Therefore, in the present study, the solubility of Al2O3 in basic cryolite melts (r ⬎ 3) was measured experimentally, and (for the first time) described in terms of a quantitative measure of melt basicity. Likewise, the solute distributions for alumina undersaturated melts were modeled. The published literature for the melt structure of NaFAlF3-Al2O3 was reviewed in detail in a previous article.[7] It is generally accepted that in acidic (r ⬃ 1.5) cryolite melts, Na2Al2OF6 is the dominant solute; in less acidic melts, the neutral solute Na2Al2O2F4 becomes dominant. Some authors [8,9,10] included another neutral solute Na4Al2OF8 or Na3Al3O3F6 in their models. In basic cryoYUNSHU ZHANG, Research Scientist, XIAOXIA WU, Graduate Research Associate, and ROBERT A. RAPP, Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] Manuscript submitted June 4, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS B
lite melts (r ⱖ 3), some basic solute(s) gain in importance. There is controversy concerning the importance of basic solute entities, with Na4Al2O2F6,[7,11] Na6Al2OF10,[8,11] and Na3Al3O4F4[10,12] having been suggested. In the current study, the solubility of Al2O3 in basic cryolite melts was measured experimentally for 3 ⱕ cryolite ratio r ⱕ 12.5 at 1300 K. As noted in previous studies and solubility measurements, high vapor loses from the melts and attack of the containers are serious experimental problems with basic cryolite melts. Further, the solid phase transition of a-Al2 O3 to b-Al2O3 is expected to cause a change in cryolite ratio of the melt. Therefore, all previous measurements based on the original values of melt composition, rather than the equilibrium value of r, are subject to error. For this reason, thermodynamic activity probes for Na and Al were developed and used to monitor the melt basicity (or acidity) and thereby
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