Neodymium Metal Production Via Chloride Based Molten-Salt Electrolysis

Abstract

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO2).1–3 Similar molten salt techniques exist for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium has seen significant increases in demand from increased production of wind turbines, electric vehicles, and hard disk drives that use neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The current procedure for neodymium processing uses a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd2O3) and carbon to neodymium metal and CO2.4,6 As an unfortunate byproduct of the fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously. The formation of PFCs combined with the emission of other greenhouse gases (CO,CO2) makes the current process dangerous and undesirable from an environmental standpoint. Due to this, few places currently produce neodymium metal and virtually none is produced in the United States, creating supply chain risks with dire consequences.5 An alternative molten salt process has been proposed where rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into a chloride salt based melt and neodymium is electroplated via the below set of reactions.7,8 Cathode: 2NdCl3 + 6e- → 2Nd(solid) + 6Cl- Anode: 6Cl- → 3Cl2 + 6e- Overall: 2NdCl3 → 2Nd(solid) + 3Cl2 This process has several distinct advantages. The chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine is then recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride.This talk focuses on recent advances to improve purity and lower energy consumption (kWh/kg). We evaluate anode and cathode behavior during this neodymium chloride molten salt process. Over-potential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. A new stable analytic cathode is demonstrated which is lowers chlorine over-potential by more than 200 mV at current density of 250 mA/cm2.The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Highly porous Nd deposits are known to form in chloride molten salt but are undesirable due to the energy required to separate out the high volume fraction of salt remaining in the solid sponge. Effects of temperature and electrolyte composition on deposit morphology are investigated and improvements toward densifying the sponge to minimize post electrolysis purification are reported. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Portions of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. T. R. Beck, Electrochem. Soc. Interface, 23, 36–37 (2014).G. G. Botte, Electrochem. Soc. Interface, 23, 49–55 (2014).W. E. Haupin, J. Chem. Educ., 60, 279–282 (1983).M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte.B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol., 48, 9506–9513 (2014).V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576, 10, 576 (2020).R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022).D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017).