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Thermodynamics of Fluoride Molten Salts and Dissolution Kinetics of MgO for Extraction of Magnesium

Thermodynamics of Fluoride Molten Salts and Dissolution Kinetics of MgO for Extraction of Magnesium
Alternative Author(s)
Joo Hyun PARK
Issue Date
2019. 8
The solid oxide membrane (SOM) process has become popular in the direct production of magnesium from its oxide. The SOM process has great potential to replace current metals extraction processes because it provides various advantages, especially lower energy use and zero emissions. The currently developed SOM process for producing magnesium employs the MgF2-CaF2 eutectic flux including small amounts of YF3 in order to prevent the degradation of YSZ membrane at temperatures ranging from 1200 to 1300 oC. However, recently, the ceria-based membrane has been interested because the ionic conductivity of ceria-based membrane at temperatures lower than 1000 oC is superior to that of YSZ. If the operating temperature could be decreased, energy- and cost-saving effect will be higher. Nevertheless, there are no systematic studies on the design of low temperature flux (lower than 1000 oC) for the 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 salts. 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 salts at 1223 K. In addition, the 45MgF2-55CaF2 binary eutectic salt, 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 salts under MgO saturation, it was confirmed that the stability of MgO in the 7LiF salt is greater than that in the 10NaF salt. Hence, the driving force of MgO dissolution into the 7LiF salt is higher than that into the 10NaF salt. The newly developed molten salt for magnesium extraction using the LT-SOM process with an operating temperature lower than 1273 K is the 46.5MgF2-46.5CaF2-7LiF system. Furthermore, it is important to investigate the dissolution reaction of metal oxides in molten fluoride salt, which contributes to the overall reaction mechanism and reaction rate. However, there are few fundamental studies on the reaction between oxide particles and fluoride salt. Notably, the dissolution behavior of magnesium oxide (MgO), which is a major source of magnesium production, into fluoride salt has not been reported. In addition, the dissolution behavior is mediated by the chemical and physical properties of the salt. Therefore, we investigated the dissolution reaction of MgO in fluoride salt using high temperature confocal scanning laser microscopy (HT-CSLM) measurements to demonstrate the reaction mechanism governing the dissolution rate of MgO particles. Consequently, the rate-limiting mechanism is a boundary layer diffusion of O2- in the molten salt.
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