Jungshin Kang

A novel molten salt electrolysis process using an aluminum (Al) cathode was developed to directly produce Al–magnesium (Mg) alloys from magnesium oxide (MgO). The effects of the temperature of electrolysis, concentration of Mg in the Al–Mg alloy, and cathodic current density on the current efficiency were investigated. The electrolysis was conducted via chronopotentiometry at 1053–1173 K in a 33 mol% magnesium fluoride (MgF ₂ )–47 mol% lithium fluoride (LiF)–calcium fluoride (CaF ₂ ) molten salt using an Al cathode and a carbon (C) anode under an argon (Ar) atmosphere, with the cathodic current densities ranging from 0.159 A·cm ^–2 to 0.477 A·cm ^–2 for 160–480 min. During electrolysis, Al–Mg alloys were produced through the electrochemical reduction of MgO at an average cell voltage of 1.95–2.33 V. Under certain conditions, a current efficiency of 87.1–97.9% was achieved with 13.65–23.65 mass% Mg alloys, which consisted of Al ₃ Mg ₂ and Al (Mg) phases. Therefore, the novel electrolytic process using an Al cathode demonstrates the feasibility of the direct production of Al–Mg alloys from MgO.

Lithium sulfide (Li ₂ S) is a key precursor for sulfide-based solid electrolytes used in all-solid-state batteries. Hydrogen (H ₂ ) reduction of lithium sulfate (Li ₂ SO ₄ ) has attracted attention as an economic process for Li ₂ S production. However, a kinetic study of the H ₂ reduction of Li ₂ SO ₄ has not yet been reported. In this study, the H ₂ reduction of Li ₂ SO ₄ was investigated at 1023–1123 K using 10% H ₂ –argon (Ar) mixed gas. During the reaction, the Li ₂ SO ₄ –Li ₂ S mixtures formed a molten phase, indicating a phase transition from solid to liquid. Microstructural observations revealed the formation of sulfur-rich nuclei and their growth into clusters during the reaction. The reaction kinetics were analyzed based on the shapes of the conversion–time curves and interpreted using both the autocatalytic reaction and nucleation and growth models. The kinetic behavior exhibited strong temperature dependence at 1023–1123 K. At 1123 K, a significant acceleration in the reaction rate was observed as the product accumulated, indicating strong autocatalytic behavior. The initial addition of Li ₂ S seeds further increased the reaction rate. In contrast, the nucleation and growth mechanism dominated at 1023 K. These results indicate that the dominant reaction mechanism transitions from nucleation and growth to autocatalytic behavior with increasing reaction temperature.

To develop a CO₂-free process for the direct production of Fe from Fe ore, the molten oxide electrolysis (MOE) of Fe oxides and ores was investigated in a B₂O₃ – Na₂O molten oxide at 1173 K. Thermodynamic calculations and cyclic voltammetry measurements were performed to identify the electrochemical reduction behavior of the selected oxides. In addition, electrolysis was conducted by applying a cell voltage of 1.6 V in air or Ar gas atmosphere. When electrolysis was carried out in air, current efficiencies ranging from 65.50 – 68.72% were obtained, depending on the type of Fe oxide used, whereas electrolysis in an Ar gas atmosphere showed higher current efficiencies of 67.49 – 78.56% owing to the suppressed re-oxidation of reduced Fe under a low p_O2. Furthermore, electrolysis using hematite, goethite, and magnetite in air produced Fe metal with current efficiencies of 64.14%, 66.00%, and 67.58%, respectively. These results demonstrated the feasibility of the low-temperature MOE of Fe ores in a B₂O₃ – Na₂O molten oxide for direct and CO₂-free production of Fe.

For the efficient production of high-purity lithium sulfide (Li ₂ S), a crucial raw material for sulfide-based solid electrolytes in all-solid-state batteries, the reduction of lithium sulfate (Li ₂ SO ₄ ) with hydrogen sulfide (H ₂ S) gas was investigated. In this study, H ₂ S ( g ) was generated by dissolving iron sulfide (FeS) in a sulfuric acid (H ₂ SO ₄ ) solution at 293 K, and then reacted with Li ₂ SO ₄ at 873–1073 K. When the reduction of Li ₂ SO ₄ with H ₂ S ( g ) was conducted at 1073 K for 6 h, high-purity Li ₂ S was produced. In these experiments, a deposit of excess elemental sulfur (S) was observed. The excess S was produced by the decomposition of H ₂ S ( g ) and the reaction between H ₂ S ( g ) and sulfur dioxide (SO ₂ ) gas generated by the reaction of Li ₂ SO ₄ with H ₂ S ( g ). In addition, the mechanism for the reduction of Li ₂ SO ₄ with H ₂ S ( g ) was elucidated using a chemical potential diagram considering the chemical potentials of the species involved in the reaction. These findings demonstrate the feasibility of producing high-purity Li ₂ S by reducing Li ₂ SO ₄ with H ₂ S ( g ) generated by dissolving FeS in a H ₂ SO ₄ solution. Graphical Abstract

Rare earth elements possess unique chemical, electrical, magnetic, and luminescent properties that can significantly enhance the performance of existing products even when added in small amounts. Moreover, due to the absence of commercially viable substitutes, these elements hold high strategic value in the Fourth Industrial Revolution and advanced technology sectors. Each element exhibits distinct characteristics, enabling diverse applications of rare earth materials in fields such as smartphones, semiconductors, electric vehicle motors, wind turbines, precision-guided weapons, and other advanced equipment. Among these applications, permanent magnets account for the largest market share. South Korea relies on imports for the majority of neodymium permanent magnets, which are essential components used in electric vehicle production, robotics, consumer electronics and other related applications. However, research on recycling these materials remains insufficient. Therefore, this paper provides an overview and analysis of recent global issues surrounding the rare earth supply chain and current research trends in recycling technologies for rare earth permanent magnets.