Electrolytic synthesis of TbFe2 from Tb4O7 and Fe2O3 powders in molten CaCl2

Fine magnetic powder of a terbium–iron intermetallic compound, TbFe2, was prepared by electrochemical reduction of thermally compounded powders of Tb4O7 and Fe2O3 (atomic Tb/Fe ratio, 1:2) in molten CaCl2. The electro-reduction potential of mixed Tb4O7 and Fe2O3 powders was more positive than that of Ca2+ in molten CaCl2 as revealed by cyclic voltammetry using a novel metallic cavity electrode. Porous pellets of mixed Tb4O7 and Fe2O3 powders were prepared by sintering in air, and were reduced to various Tb–Fe intermetallic compounds by constant voltage electrolysis at typically 2.6–3.2 V and 900 °C for 10–12 h. The electrolytic product could be cleaned effectively in dimethylsulfoxide under magnetic stirring, leading to a pure magnetic powder with a low oxygen content (<1300 ppm) and the desired TbFe2 stoichiometry as confirmed by XRD and ICP-AAS. It was also found that raising the sintering temperature (1200 °C, 2 h) encouraged compound formation between Tb4O7 and Fe2O3 powders and led to better stoichiometry control in the electrolytic product. Energy consumption of the electrolysis could be lower than 14 kWh/kg TbFe2, and the Tb recovery yield reached beyond 97%.