Thermodynamic evaluation and optimization of the LiCl-NaCl-KCl-RbCl-CsCl-MgCl 2 -CaCl 2 system using the modified quasi-
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I. INTRODUCTION
MOLTEN alkali–alkaline earth halide solutions are of much technological importance in, for example, the production of Mg and Na. 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 LiCl-NaCl-KCl-RbCl-CsCl-MgCl2-CaCl2 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 the uncharted regions of temperature and composition. This article is the first in a series that will be published on alkali–alkaline earth halide systems. In an accompanying article in this journal,[1] we have presented a quasi-chemical model for liquid solutions which takes into account short-range ordering between nearest neighbors on a lattice or sublattice. Molten alkali chloride– MgCl2 solutions are well known for exhibiting extensive short-range ordering, increasing in importance from LiClMgCl2 to CsCl-MgCl2. This ordering gives rise to “Vshaped” enthalpy-of-mixing and “m-shaped” entropy-ofmixing curves, as in Figures 1 and 2. This has previously been modeled by introducing complex anions such as [2,3] However, until now, no model has been proMgCl2⫺ 4 . posed which permits a quantitative optimization of all subsystem data and which can satisfactorily be extrapolated to predict the properties of multicomponent solutions when appreciable short-range ordering is present. The modified quasi-chemical model[1] achieves this objective. An earlier version of the model has been used very successfully to model molten silicates.[4] ARTHUR D. PELTON, Professor, and PATRICE CHARTRAND, Research Fellow, are with the CRCT, Ecole Polytechnique, Montreal, PQ, Canada H3C 3A7. Manuscript submitted April 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
The model does not explicitely introduce complex anions. Instead, short-range ordering is treated by considering the relative numbers of second-nearest-neighbor cation-cation pairs. The parameters of the model are the Gibbs energy changes ⌬gAB/Cl for the pair-exchange reactions (A-Cl-A)pair ⫹ (B-Cl-B)pair ⫽ 2(A-Cl-B)pair ⌬gAB/Cl
[1]
As ⌬gAB/Cl becomes progressively more negative, reaction [1] is shifted progressively to the right, (A-Cl-B) pairs predominate, and the solution becomes progressively more ordered. In the previous article,[1] 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 Cl⫺ ions, the model can be used directly to treat cation-cation pairs on the cation sublattice. The parameter ⌬gAB/Cl is the parameter ⌬gmn of the previous article.[1] When ⌬gAB/Cl is sm
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