Alumina solubility in molten salt systems of interest for aluminum electrolysis and related phase diagram data

The solubility of alumina in molten Na₃AlF₆ containing various amounts of AlF₃, CaF₂, and LiF was determined by measuring the weight loss of a rotating sintered corundum disc. The results were fitted to the following empirical expression: 1 $$ [Al_2 O_3 ]_{sat} = A\left( {\frac{t} {{1000}}} \right)^B $$ where 2 $$ \begin{gathered} A = 11.9 - 0.062[AlF_3 ] - 0.003[AlF_3 ]^2 - 0.50[LiF] \hfill \\ - 0.20[CaF_2 ] - 0.30[MgF_2 ] + \frac{{42[LiF] \cdot [AlF_3 ]}} {{2000 + [LiF] \cdot [AlF_3 ]}} \hfill \\ B = 4.8 - 0.048[AlF_3 ] + \frac{{2.2[LiF]^{1.5} }} {{10 + [LiF] + 0.001[AlF_3 ]^3 }} \hfill \\ \end{gathered} $$ where the square brackets denote weight percent of components in the system Na₃AlF₆-Al₂O₃ (sat)-AlF₃-CaF₂-MgF₂-LiF and t is the temperature in degree Celsius. The standard deviation between the equation and the experimental points in the temperature range from 1050 °C to about 850 °C was found to be 0.29 wt pct Al₂O₃. A series of revised phase diagram data of interest for aluminum electrolysis was derived based on the present work and recently published data for primary crystallization of Na₃AlF₆ in the same systems.