Thermodynamic evaluation and optimization of the LiF-NaF-KF-MgF 2 -CaF 2 system using the modified quasi-chemical model
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I. INTRODUCTION MOLTEN alkali-alkaline earth halide solutions are of much technological importance in, for example, the production and fluxing of Al. A large body of experimental thermodynamic and phase equilibrium data exists for these systems. In this article, all available data for binary and ternary subsystems of the LiF-NaF-KF-MgF2-CaF2 system are critically evaluated to obtain optimized parameters of models of all solution phases. These parameters form a computer database. The models are then used to predict the thermodynamic properties of the multicomponent system. When used in conjunction with currently available general software for calculating equilibria by Gibbs energy minimization, this database can be used to predict the thermodynamic properties and phase equilibria in uncharted regions of temperature and composition. This article is the second in a series that will be published on alkali-alkaline earth halide systems. Molten alkali fluoride-MgF2 solutions are known, from Raman spectroscopy,[1] to exhibit extensive short-range ordering (SRO), increasing in importance from LiF-MgF2 to CsFMgF2. Similar behavior in alkali chloride-MgCl2 melts has previously been modeled[2,3] by introducing complex-MgCl2⫺ 4 anions. However, until now, no model has been proposed that permits a quantitative optimization of all subsystem data and that can satisfactorily be extrapolated to predict the properties of multicomponent solutions when appreciable SRO is present. The modified quasi-chemical model[4] achieves this objective. The model has been used for the LiCl-NaCl-KCl-RbClCsCl-MgCl2-CaCl2[5] molten salt phase. The model does not explicitely introduce complex anions. Instead SRO is treated by considering the relative numbers of second-nearest-neighbor (SNN) cation-cation pairs. The parameters of the model are the Gibbs energy changes ⌬gAB/F for the pair exchange reactions: (A ⫺ F ⫺ A)pair ⫹ (B ⫺ F ⫺ B)pair ⫽ 2(A ⫺ F ⫺ B)pair ⌬gAB/F [1] PATRICE CHARTRAND, Research Fellow, and ARTHUR D. PELTON, Professor, are with CRCT, E´cole Polytechnique, Montreal, PQ, Canada H3C 3A7. Manuscript submitted April 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
As ⌬gAB/F becomes progressively more negative, reaction [1] is shifted progressively to the right, (A-F-B) pairs predominate, and the solution becomes progressively more ordered. In the previous article,[4] the model was developed in terms of nearest-neighbor pairs (A-B) for species mixing on one lattice. In the present case, since the anion sublattice is occupied only by F ions, the model can be used directly to treat cation-cation pairs on the cation sublattice. The parameter ⌬gAB/F is the parameter ⌬gmn of the previous article.[4] When ⌬gAB/F is small, the degree of SRO is small, and the solution approximates a random (Bragg–Williams) mixture of cations on the cation sublattice. Our aim is to develop a database for calculating multicomponent thermodynamic properties and phase equilibria in salt systems related to reactive metals extractive metallurgy. The LiF-LiCl-NaF-Na
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