Electrochemical Separation of Lanthanum Oxide in Molten FLiNaK Salt

The electrochemical behavior of La₂O₃ was investigated in LiF-NaF-KF (FLiNaK, 46.5-11.5-42.0 mol %) eutectic at 700°C. In the electrochemical tests, two kinds of working electrodes, i.e., tungsten and graphite, were utilized. The present study showed that La³⁺ ions can be deposited in the form of La metal on a tungsten cathode or LaC₂ on a graphite cathode, and O²⁻ can be removed in the form of CO/CO₂ using a graphite anode. Therefore, a graphite or tungsten cathode (for La³⁺ removal), and a graphite anode (for O²⁻ removal) are good options to remove both La³⁺ and O²⁻ from the molten salts. In addition to the electrochemical tests, inductively coupled plasma mass spectroscopy analysis was used to measure the concentration of the lanthanum element and X-ray powder diffraction techniques were applied to determine the chemical forms of lanthanum in the salt. It turned out that the solubility of La³⁺ in the molten FLiNaK was 6.81 × 10⁻⁴ wt% at 700°C and LaOF was formed by the chemical reactions between La₂O₃ and alkali fluorides during the heating process.