Thermodynamics of the system NaF-AlF₃. Part III: Activities in liquid mixtures

Abstract

Measurements have been made of a) the sodium content of aluminum in contact with NaF-AlF₃ melts at 1020° and 1080°C, and in contact with liquid-solid mixtures along the cryolite liquidus, b) the position of the A1F₃ liquidus line, and c) the electromotive force of solid-electrolyte cells with one side in the NaF-Na₃AlF₆ two-phase region and the other on the cryolite liquidus. With previously determined data for the activity coefficient of sodium in aluminum and for ΔH°_298f for cryolite, activities of NaF and A1F₃ are calculated. As a limiting condition the melts conform to a regular solution model withRTlnγ’₁ = ∔12,200(1 − n’₁)² cal where γ’₁=a₁/n’₁ and n’₁ the molar fraction of either NaF or NaAlF₄ calculated with them as species. This model breaks down progressively as the NaAlF₄ composition is approached, the deviations starting earlier at higher temperatures. The most plausible explanation is the disproportionation equilibrium 2A1F-₄ ⇌ A1₂F-₇ + F^-, stoichiometric NaAlF₄ containing about 70 pct AlF^- ₄ at 1020° to 1080°C. The hypothetical undisproportionated NaAlF₄ has a free energy of formation from NaF(I) and AlF₃(s) of ΔG° = •101,235 + 32.085T + 5.929 × 10⁷/T. This equation, together with that above for γ’NaAlF₄, defines α AlF₃ in all regions where the regular solution model holds. Solutions of sodium in aluminum are found to conform to Henry’s law in the range 100 to 2 ppm, contrary to recent suggestions.