A Density Model for Multicomponent Liquids Based on the Modified Quasichemical Model: Application to the NaCl-KCl-MgCl 2
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IT is of great importance to be able to predict the density of liquid phases involved in various industrial processes—for example, the density of liquid alloys during casting or the density of the NaF-AlF3-CaF2Al2O3 base electrolyte used to produce aluminum by the Hall–He´roult process. The present article describes a theoretical model for the density of multicomponent inorganic liquids, based on the modified quasichemical model.[1–4] For a given liquid phase, the density can be derived from the molar volume, which is an intensive thermodynamic property defined as the pressure derivative of the molar Gibbs energy at fixed temperature and number of moles of components. Therefore, by introducing in the Gibbs energy of the phase temperaturedependent molar volume expressions for the pure components and pressure-dependent excess parameters for the binary atomic or ionic interactions, it is possible to reproduce the molar volume and the density of a given binary liquid. It is then possible to reproduce and eventually predict the molar volume and the density of the multicomponent phase from the binary (and, if necessary, higher-order) pressure-dependent excess parameters, using standard interpolation methods.[2,5] CHRISTIAN ROBELIN, Research Associate, and PATRICE CHARTRAND, Assistant Professor, are with the Centre for Research in Computational Thermochemistry (CRCT), Department of Chemical Engineering, Ecole Polytechnique, Montre´al, PQ, Canada H3C 3A7. Contact e-mail: [email protected]. GUNNAR ERIKSSON, Managing Director, is with GTT-Technologies, D-52134 Herzogenrath, Germany. Manuscript submitted May 1, 2007. Article published online October 9, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the present article, the density model is applied to the NaCl-KCl-MgCl2-CaCl2 liquid solution, which is used as an electrolyte for magnesium production. This liquid solution is also the reaction product during the removal of alkali and alkaline-earth elements in molten aluminum by argon and chlorine injection. Temperaturedependent expansivities and molar volumes of the pure salts and pressure-dependent model parameters were found for this system, through optimization of the experimental data published in the literature. A thermodynamic database has previously been developed for the NaCl-KCl-MgCl2-CaCl2 system.[6] A complete critical evaluation of all available phase-diagram and thermodynamic data was performed for all condensed phases of this system, and optimized model parameters were found.[6] The thermodynamic properties of the NaCl-KCl-MgCl2-CaCl2 liquid were modeled using the modified quasichemical model in the pair approximation,[1,2] evaluating second-nearest-neighbor cation-cation short-range order with Cl- as the only anion. In order to model the density of the NaCl-KCl-MgCl2-CaCl2 liquid, pressure-dependent excess Gibbs energy parameters are added to this thermodynamic model. The density model described in this article is suitable for multicomponent inorganic liquids. It has been applied successfully
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