Paper 14.4

A Thermodynamic Database for AlCl₃-based Molten Salt Systems

Robelin Christian, Chartrand Patrice and Pelton Arthur D.

Centre for Research in Computational Thermochemistry (CRCT), Ecole Polytechnique,

Montreal (Quebec), Canada, H3C 3A7

A thermodynamic database has been developed for the LiCl-NaCl-KCl-MgCl₂-CaCl₂- MnCl₂-FeCl₂-FeCl₃-CoCl₂-NiCl₂-AlCl₃ system. All binary subsystems as well as all higher order (mostly ternary) subsystems for which experimental data were available have been considered. In particular, a complete critical evaluation of all available phase diagram and thermodynamic data has been performed for the condensed phases (liquid, LiAlCl₄(s), NaAlCl₄(s), KAlCl₄(s), LiCl-NaCl-KCl solid solution) and gaseous species (LiAlCl₄(g), Li₂Al₂Cl₈(g), NaAlCl₄(g), Na₂Al₂Cl₈(g), KAlCl₄(g), K₂Al₂Cl₈(g)) of the LiCl-NaCl-KCl-AlCl₃ system, and optimized model parameters have been found. For the first time, the entire composition range (0 ££ 1) was considered. The binary systems ACl-AlCl₃ (where A = Li, Na and K) show strong negative deviations from ideality at the equimolar composition (due to short-range ordering in the liquid phase) and the binary mixtures exhibit a region of liquid-liquid immiscibility at high AlCl₃ content. The existence in such melts of the AlCl₄^- and Al₂Cl₇^- species was observed in particular by Raman spectroscopy. The liquid was modeled using the Modified Quasichemical Model which takes into account cation-cation short-range order. In order to introduce two different compositions of maximum short-range-ordering near the AAlCl₄ and AAl₂Cl₇ compositions (where A is an alkali), pure liquid aluminum chloride was modeled as a mixture of AlCl₃ and Al₂Cl₆ (with two paired aluminum cations). Particular attention was paid to the LiCl-NaCl-AlCl₃, LiCl-KCl-AlCl₃ and NaCl-KCl-AlCl₃ systems. The model predicts the existence of a reciprocal liquid-liquid miscibility gap in the basic region of the LiCl-NaCl-AlCl₃ and LiCl-KCl-AlCl₃ systems.