Electrochemical Recovery of Rare-Earth Elements Using Liquid Metals in Molten Salts

Abstract

This work presents electrodeposition of rare-earth elements (Nd and Gd) in molten salts using liquid metal cathodes (Bi and Sn) at 773–973 K with an aim enhance the recovery of rare-earth elements. Recovery of rare-earth elements from molten salts is technically challenging due to high reactivity of rare-earth metals, multivalency of rare-earth cations in electrolytes that can results in a disproportionation reaction (e.g., Nd + 2NdCl3 → 3NdCl2), and dissolution of rare-earth metals in molten salts (e.g., 9 mol% La metal in LaCl3 at 1099 K). Improved rare-earth recovery is discussed based on round-trip coulombic efficiencies using liquid metal electrodes in molten salts and the electromotive force (emf) measurements of rare-earth alloys (e.g., Nd-Bi, Nd-Sn, and Gd-Bi). The rare-earth alloys are also characterized using X-ray diffraction, electron microscopy, and differential scanning calorimetry to determine their phase behavior during electrodeposition processes. The use of two-phase rare-earth alloys, e.g., Nd-Sn alloy (10-90 mol%), as a stable reference electrode is discussed within the context of reliable electrochemical characterization of rare-earth alloys.