Thermodynamic evaluation and optimization of the Li, Na, K, Mg, Ca//F, Cl reciprocal system using the modified quasi-che
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I. INTRODUCTION
MOLTEN salts used in metallurgical applications are often based on alkali alkaline-earth chloride fluoride mixtures. For example, the electrolytes used for Mg production are based mainly on NaCl, CaCl2, and MgCl2 salts, which can also be used as fluxes for Mg casting; NaCl, KCl, MgCl2, NaF, and KF are used extensively as fluxing agents for liquid aluminum treatment; alkali chlorides and fluorides can be used as fluxes for the gas welding of light metals; electrodeposition of refractory metals (Nb, Ta) can be made from these solutions; and NaF, CaF2, LiF, and MgF2 are important bath components in the Hall–Heroult electrolysis cells for Al production. Developing a thermodynamic model for the LiF-LiCl-NaF-NaCl-KF-KCl-MgF 2-MgCl 2-CaF 2-CaCl 2 system will contribute to the understanding, calculation, and simulation of many present and future metallurgical applications. In reciprocal molten salt solution (i.e., solutions with more than one cation and more than one anion), large deviations from ideal mixing can occur due to the strong first-nearestneighbor (FNN) (cation-anion) interactions. Modeling such solutions is a challenge. If, simultaneously, there is also strong second-nearest-neighbor SNN (cation-cation) shortrange ordering (SRO), the solution becomes even more difficult to model. The binary common-ion systems KCl-MgCl2 and KFMgF2 exhibit extensive SRO, which has been associated 2⫺ with the formation of MgCl2⫺ 4 and MgF4 “complex ions.” To a lesser, but still appreciable, extent the NaCl-MgCl2, NaF-CaF2, and KF-CaF2 systems also exhibit such SRO. In a previous publication,[1] we presented a modified quasichemical model, in the pair approximation, in which this ordering is described in terms of the following equilibrium among SNN cation-cation pairs: (A-[X]-A) ⫹ (B-[X]-B) ⫽ 2(A-[X]-B) ⌬gAB/X
[1]
where A, B ⫽ Li, Na, K, Mg, or Ca and X ⫽ F or Cl. If PATRICE CHARTRAND, Research Fellow, and ARTHUR D. PELTON, Professor, are with the CRCT, E´cole Polytechnique, Montreal, PQ, Canada H3C 3A7. Manuscript submitted April 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
⌬gAB/X is negative, reaction [1] is shifted to the right, (A-[X]B) pairs predominate, and SNN SRO results. The amount of this ordering depends upon the magnitude of the Gibbs energy change, ⌬gAB/X, of reaction [1], which is a parameter of the model that can be expressed as a function of composition by an empirical polynomial expression. In earlier publications[2,3] all available phase diagram and thermodynamic data for the LiCl-NaCl-KCl-MgCl2-CaCl2 and LiF-NaF-KF-MgF2-CaF2 common-anion systems were critically evaluated to obtain optimized empirical coefficients, which reproduce all the data within the experimental error limits. At the same time, similar optimized model equations for all solid solution phases in these systems were obtained. When reciprocal salt solutions are formed containing both chlorides and fluorides, FNN SRO can occur, the extent of which is related to the Gibbs energy of the exchange reaction: exchange AF ⫹ BCl
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