Sustainable Semi-Continuous Rare Earth Alloy Production in Chloride Based Molten Salts

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Rare earth elements have numerous uses ranging from green energy to defense applications. Nd metal production uses a neodymium/lithium fluoride molten salt with a consumable carbon anode where dissolved neodymium oxide and carbon are converted to neodymium metal and CO2. In addition to producing CO2, the process releases PFCs, making it environmentally unsustainable. We have shown that neodymium chloride can be electrolyzed to produce Nd metal at the cathode and chlorine gas at the anode. The chlorine can be recycled in the conversion of the neodymium oxide (from ore) to neodymium chloride. In our process, the anode is non-consumable which allows for continuous processing without producing deleterious greenhouse gases. Chloride rare earth electrowinning historically suffered from low Coulombic efficiency obtained in lab-scale efforts (10-50%). The electrolytic reduction of neodymium chloride is a two-step reaction where a stable intermediate (Nd2+) is formed,1-2 The intermediate species can diffuse away from the cathode resulting in efficiency losses. Further, anodic overpotentials for the chlorine evolution reaction are significant due to the somewhat sluggish kinetics and poor wetting characteristics of the salt on traditional graphite anodes. Solid metal electrowinning in molten salts tends to produce rough deposits which trap significant salt impurities necessitating downstream purification. However, producing liquid metal allows for easy density separation from the salt, leading to a purer as-deposited product as well as faster metal deposition kinetics (i0 ~ 1 A/cm2) which reduces the effects of Nd2+ out diffusion, and chlorine evolution kinetics (i0 ~ 0.2 A/cm2) because of the higher temperature. In this talk, we report our work on improvements to Coulombic efficiency (~85%), and a stable anode that catalyzes chlorine evolution. The combination of improved Coulombic efficiency and reduced anode overpotentials results in low specific energy consumption (~3 kWh/kg) and thus lower operating costs. We will also report on efforts to produce liquid metal in the form of an NdFe alloy at ~800 °C which can be collected semi-continuously at rates of >1 A/cm2. References : R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022). D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017). Holcombe et al., ACS Sustainable Chem. Eng. 2024, 12, 10, 4186–4193, (2024). Holcombe et al., Holcombe et al 2024 Electrochem. Soc. Interface 33 49, (2024). Rohan Akolkar. System and Process for Sustainable Electrowinning of Metals. US Patent 20230279572. Sept. 2023 Portions of this work were performed by LLNL under Contract DE-AC52-07NA27344.