Mechanism of MgO dissolution in MgF₂–CaF₂–MF (M=Li or Na) melts: Kinetic analysis via in-situ high temperature confocal scanning laser microscopy (HT-CSLM)

The solid oxide membrane (SOM) process is a direct electrolysis method for refining magnesium and has become a popular and promising technology. In the electrolysis process of SOM, the metal oxide is dissociated into the metal cation and oxygen anion. Thus, it is important to investigate the dissolution reaction of metal oxides in molten fluoride flux, which contributes to the overall reaction mechanism and reaction rate. However, there are few fundamental studies on the reaction between oxide particles and fluoride flux. Notably, the dissolution behavior of magnesium oxide (MgO), which is a major source of magnesium production, into fluoride flux has not been reported. In addition, the dissolution behavior is mediated by the chemical and physical properties of the flux. Therefore, we investigated the dissolution reaction of MgO in fluoride flux 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 diffusion of O²⁻ ion, dissociated from MgO, through the boundary layer.