Electrochemical Formation of Nd-Fe Alloys in Molten LiF-CaF₂-NdF₃

Abstract

In the present study, as a fundamental study for recycling Nd from Nd-Fe-B permanent magnet scrap, the electrochemical Nd-alloying behavior of Fe was investigated in a molten LiF-CaF₂-NdF₃ (0.3 or 0.5 mol%) system at 1123 K. Herein, the equilibrium potential of Nd³⁺/Nd was determined as 0.18 V (vs Li⁺/Li) by open-circuit potentiometry using a Mo electrode. Cyclic voltammetry and open-circuit potentiometry were conducted using an Fe electrode, and the results suggested the formation of multiple phases within the Nd-Fe alloys. Alloy samples were prepared via one- or two-step potentiostatic electrolysis of Fe electrodes at various potentials. The formation of liquid Nd-Fe alloy was confirmed through one-step potentiostatic electrolysis at 0.10 V. The solid Nd₂Fe₁₇ alloy was formed via a two-step potentiostatic electrolysis, wherein the potential was changed to 0.25 V after the initial electrolysis at 0.10 V, indicating that the Nd₂Fe₁₇ phase is thermodynamically stable at 0.25 V. The equilibrium potentials of the coexistence states of liq. Nd-Fe + Nd₂Fe₁₇ and Nd₂Fe₁₇ + Fe were determined as 0.19 and 0.31 V, respectively, which are in good agreement with the previously reported thermodynamic data.