Electrodeposition of Neodymium from NdCl₃-Containing Eutectic LiCl–KCl Melts Investigated Using Voltammetry and Diffusion-Reaction Modeling

Abstract

Electrodeposition of neodymium (Nd) metal from NdCl 3 -containing molten LiCl-KCl eutectic melts was investigated using voltammetry and diffusion-reaction modeling. Voltammetry studies confirmed that Nd electrodeposition is a two-step reduction process involving first a reversible one-electron transfer reduction of Nd 3+ to Nd 2+ , followed by quasi-reversible reduction of Nd 2+ to Nd metal. In the electrode potential range where Nd 3+ is reduced to Nd 2+ , the peak current density measured in a voltammetry scan showed good agreement with the classical Randles-Sevcik model for reversible soluble-soluble redox transitions. However, in the potential range where Nd 2+ is reduced to Nd metal, the experimentally measured peak currents in the voltammogram were substantially lower than those predicted by applying the Berzins-Delahay model for reversible soluble-insoluble redox transitions. In the present work, this discrepancy was addressed using transient diffusion-reaction modeling, which accounted for the multivalent (Nd 2+ and Nd 3+ ) species transport and their multi-step reduction to Nd metal. The diffusion-reaction model accurately predicts the voltammetric response during Nd electrodeposition in a broad range of operating conditions (species concentrations and voltammetry scan rates), while providing access to the kinetic parameters governing Nd electrodeposition from halide melts. The approach presented herein may also be applied to other electrodeposition systems which undergo multivalent redox transitions at the electrode-electrolyte interface. Rare earth elements enable important functions in numerous applications. They are used in permanent magnets in electric motors and electronic memory storage, in catalysts for automobile catalytic converters and for petroleum refining, and in phosphors for energyefficient lighting and color displays. 1 Due to their rare occurrence in nature and the complexity associated with extracting and refining them, rare earth metals are very expensive and face potential supply risks. For future sustainable use of rare earth metals, the recovery and recycling of these materials from product waste is of great practical importance. 2 For energy-efficient recovery of rare earth elements, one promising technique is the electrolytic refining of these metals from waste streams such as electronic waste. 3 Electrochemical processing, i.e., electrowinning or electrorefining, using high temperature molten salt electrolytes is particularly attractive, because molten salts offer high ionic conductivity, 4 good electrochemical stability, 5,6 and low interfacial charge transfer resistance. 7,8 These are desirable attributes for industrial scale process implementation. Furthermore, high-temperature molten salt electrochemical systems offer industrially relevant rates of processing (high current densities) and ease of scalability to large-volume manufacturing at low cost. 9,10 Consequently, the characterization of the electrochemical behavior of rare earth metals in molten salts is important, bec...

Investigation on the Electrochemical Behaviour and Deposition Mechanism of Neodymium in NdF3–LiF–Nd2O3 Melt on Mo Electrode

“…Rare earth metals are increasing in importance, particularly in the production of permanent magnets, energy, high technology, and electronic devices [1][2][3][4]. Due to their rare occurrence in nature and the difficulties encountered in extracting and refining them, rare earth metals are expensive.…”

“…Due to their rare occurrence in nature and the difficulties encountered in extracting and refining them, rare earth metals are expensive. Potential supply risks appear to be an additional important issue [3]. To meet long-term sustainable supply, efforts to improve recycling and recovery of product waste from magnets or electronic scraps and spent nuclear fuels are of great importance [4][5][6].…”

“…Recycling of this waste can result in recovery of rare earth metals, including neodymium, a metal which constitutes roughly one third of rare earths [2]. Meanwhile, it was confirmed that electrolytic production and refining are promising techniques for achieving energy-efficient recovery of rare earth elements [3]. As a result, high-temperature rare earths molten salt electrolysis has become a subject of intensive research and development [3,7,8].…”

“…Meanwhile, it was confirmed that electrolytic production and refining are promising techniques for achieving energy-efficient recovery of rare earth elements [3]. As a result, high-temperature rare earths molten salt electrolysis has become a subject of intensive research and development [3,7,8]. Characterization of electrochemical behaviour of rare earth metals in molten salts should provide a further guide for the next steps in development and optimization of the recycling industrial processes.…”

“…Commonly investigated salt combinations include chlorides, fluorides, and oxides of alkali and alkaline earth metals and neodymium [8,10,11]. The electrolytes made of molten chlorides are mainly composed of NdCl 3 + LiCl + KCl [1][2][3]7,[11][12][13][14][15][16]. Fluoride electrolytes are considered more suitable because of their higher conductivity, lower hygroscopy, and higher current efficiency of the neodymium deposition compared to the chloride melts [17,18].…”