Thermodynamics of fluoride-based molten fluxes for extraction of magnesium through the low temperature solid oxide membrane (LT-SOM) process

The melt quenching experiments and thermodynamic calculations of phase diagrams were carried out to investigate potential additives for the low temperature solid oxide membrane (LT-SOM) magnesium extraction process. The solubility of MgO, which is a major source of magnesium extraction, was also measured in the molten fluoride fluxes. The solubility of MgO in the 46.5MgF2-46.5CaF2-7LiF and 45MgF2-45CaF2-10NaF (wt%) systems reached 3.4 and 1.9 wt% at 1473 K, respectively, and 1.5 wt% MgO in both fluxes at 1223 K. In addition, the 45MgF2-55CaF2 binary eutectic flux, which has been widely used in SOM process, could dissolve up to 2.3 wt% MgO at 1473 K. This value is significantly lower than the literature value, i.e. 10 wt% MgO. From the evaluation of the activity coefficient of MgO in the 46.5MgF2-46.5CaF2-7LiF and 45MgF2-45CaF2-10NaF fluxes under MgO saturation, it was confirmed that the stability of MgO in the 7LiF flux is greater than that in the 10NaF flux. Hence, the driving force of MgO dissolution into the 7LiF flux is higher than that into the 10NaF flux. The newly developed molten flux for magnesium extraction using the LT-SOM process with an operating temperature lower than 1273 K is the 46.5MgF2-46.5CaF2-7LiF system.