A Sustainable Process for Rare Earth Electrowinning in Chloride Based Molten Salts

Abstract

Neodymium production has become increasingly important due to use of neodymium–iron–boron (NdFeB) magnets in green energy, consumer technology and defense applications. The state-of-the-art practice features a neodymium fluoride and lithium fluoride molten salt with a consumable carbon anode converting dissolved neodymium oxide and carbon to neodymium metal and CO2.1-2 The anode effect results in PFCs and CO emissions, which make the current neodymium electrowinning process unsustainable.3-4 Some attempts have been made to electrolyze the chloride salt of neodymium to produce chlorine at the anode. Chlorine is a value-added product which neither consumes the anode nor produces other deleterious GHGs. Additionally, the chloride of neodymium can be non-carbothermically produced through a spontaneous reaction with hydrochloric acid, producing only water as a side product. However, a common problem in chloride rare earth electrowinning is the low Coulombic efficiency (10-50%) obtained in lab-scale and pilot-scale efforts.5 The neodymium chloride electrolysis process notoriously encounters a two-step reduction producing an intermediate divalent (Nd2+) oxidation state which is stable in the chloride media.6-7 The intermediate neodymium species diffuses away from the cathode leading to Coulombic efficiency loss. Additionally, the kinetics of the chlorine evolution reaction (CER) in chloride molten salts are known to be relatively sluggish, causing significant anodic overpotentials which elevate the specific energy consumption during electrowinning. Furthermore, electrowinning from molten salts often produces spongy or dendritic deposits of metal requiring energy-intensive purification. In this talk, we will address all aforementioned technical hurdles and report on our efforts achieving high Coulombic efficiency (~85%), and compact deposits with superior as-electrowon neodymium metal purity (>99 wt.%). Furthermore, we will report the development of a new anode which catalyzes chlorine evolution, lowering the anodic overpotential considerably during electrolysis at high current densities of practical interest for electrowinning. Specific energy consumption is calculated for some example cases and found to be between 2.1 and 3.5 kWh/kg representing a significant improvement over the conventional oxyfluoride process. Figure 1