한양대학교

Among the various technologies for magnesium extraction, the solid oxide membrane (SOM) process is environmentally friendly, has lower energy consumption, and zero carbon dioxide emission. Through direct electrolysis of metal oxides, magnesium is produced at a cathode with pure oxygen gas as the byproduct. In a previous study, we identified a low temperature SOM (LT-SOM) process utilizing a ceria-based oxygenion-conducting membrane anode that had energy- and cost-saving effects.

As global greenhouse gas emissions become more serious, the metal extraction industry is striving to adopt the direct electrolytic reduction process using halide-based flux. However, only a few studies investigated the dissolution reaction of oxide particles into the halide flux system. There is no approach to interpret the dissolution behavior of MgO in the halide flux system in terms of the viscosity of flux. As a result, this study will demonstrate the dissolution reaction of MgO in fluoride-based molten flux with the in-situ HT-CSLM measurement. Additionally, the dissolution rate and mechanism will be elucidated based on approximations of diffusion around the particle.

There are various types of metallurgical processes for magnesium extraction, among which the Pidgeon process and the electrolysis process are commonly employed in the magnesium industry. In the Pidgeon process, dolomite is calcined to MgO and CaO, and these oxides are reduced by silicon to yield magnesium gas. In the magnesium chloride electrolysis process, molten magnesium is produced by chlorination of magnesium oxide. These methods have several disadvantages, including being energy intensive and having lower efficiency for the Pidgeon process and toxic gas emissions and thus environmental risks for the electrolysis process.

In addition, lowering the operating temperature below 1273 K would bring various advantages, including energy- and cost-saving effects. Correspondingly, ceria-based membranes have become of interest for anode assembly because of their superior ionic conductivity at lower temperatures (around 1273 K) compared to yttria-stabilized zirconia (YSZ) membrane. Therefore, in the present study, we investigated potential additives to introduce into the MgF2-CaF2 flux to lower the melting temperature, and we proposed a new flux system for the modified low temperature SOM (LT-SOM) process using a ceria-based membrane. Moreover, the solubility of MgO in fluoride-based molten fluxes was quantitatively evaluated based on thermodynamic calculations and experimental confirmation.

In zirconium production, the raw material is zirconium tetrachloride (ZrCl4), which sublimates at about 603 K and melts at about 713 K (triple point). The rate of ZrCl4 sublimation may control the feed rate of reactants in a vessel in a way that affects production efficiency. According to Reshetnikov and Oblomeev, the overall processing rate is more influenced by the rate of ZrCl4 sublimation than the chemical reaction rate between ZrCl4 and liquid magnesium.

An experimental method for the measurement of the diffusion coefficient of organometallic compounds using thermogravimetric analysis (TGA) was reported by Siddiqi et al. This method has been very successful for the measurement of various diffusion coefficients. Thus, in the present study, we attempted to confirm the binary diffusion coefficient of ZrCl4 in an Ar gas atmosphere by using the TGA method.

Zirconium and its alloy are used extensively as nuclear fuel cladding materials in water-cooled nuclear power reactors. However, only a few countries in the world have access to commercial-scale zirconium industries and have the capability to manufacture reactor-grade zirconium. Metallic zirconium (sponge) can be produced from a magnesiothermic reduction known as Kroll’s process, which is similar to that used for titanium production except that the raw material (TiCl4) is in the liquid state.

In the present study, we attempted to confirm the sublimation mechanism of ZrCl4 based on a kinetic study of its sublimation using thermogravimetic analysis (TGA). The activation energy of ZrCl4 sublimation was measured under both isothermal and nonisothermal conditions.