Molten salt electrolysis of neodymium: electrolyte selection and deposition mechanism

Abstract

Molten salt electrolysis of neodymium from Nd 2 O 3 can be a viable alternative to calciothermic reduction. Two important issues merit attention: selection of an appropriate electrolyte and the mechanism of the neodymium deposition reaction. Cyclic voltammetry experiments and melting point considerations point to the eutectic LiF-CaF 2 as an appropriate electrolyte. The presence of CaF 2 polarises the Li deposition reaction to more cathodic potentials, thereby providing a larger potential window for the deposition of neodymium. Experiments with the LiF-CaF 2 -NdF 3 -Nd 2 O 3 and LiF-CaF 2 -LaF 3 -Nd 2 O 3 melts suggest that both [NdF 6 ] 32 and [NdOF 5 ] 42 , depending on their molar ratios, can be reduced to neodymium.

Electrochemical Extraction of Rare Earth Metals in Molten Fluorides: Conversion of Rare Earth Oxides into Rare Earth Fluorides Using Fluoride Additives

“…With the addition of Nd 2 O 3 , complexes that are likely to be formed in the melt would be [NdOF 4 ] 3-and [NdOF 5 ] 4-. These authors have suggested reaction (14) for the solubility of neodymium oxide in the LiF-NdF 3 eutectic melt. Taxil et al [15] showed that the oxyfluoride is an insoluble compound.…”

“…Due to the fast cooling using liquid nitrogen in all four systems, a fine eutectic structure with small alternations LiF and NdF 3 lamellar is formed (Figs. 5,8,11,14). In LiF-Nd 2 O 3 -ZnF 2 and LiF-Nd 2 O 3 -FeF 3 systems, XRD analysis shows that most of the neodymium oxide is converted into neodymium fluoride.…”

“…It has been confirmed that neodymium oxyfluoride participates in the anodic reaction to generate oxygen [13]. However, the role of neodymium oxyfluoride in the anodic and cathodic reactions is not very clear and needs to be clarified further in detail [12][13][14][15][16].…”

Neodymium electrowinning into copper-neodymium alloys by mixed oxide reduction in molten fluoride media

“…Several authors suggested different reduction mechanisms of Nd 2 O 3 in NdF 3 -based molten salt [16][17][18][19], but no agreement has been found yet. Stefanidaki et al [15,16] detailed neodymium production in LiF-Nd 2 O 3 and LiF-NdF 3 -Nd 2 O 3 systems.…”

“…Thudum et al [17] proposed a different reduction pathway for Nd 2 O 3 in LiF-CaF 2 -NdF 3 , suggesting both [NdFO 5 ] 4À and [NdF 6 ] 3À complexes were reduced to metallic neodymium, depending on the OF/F molar ratio: for low ratios, [NdF 6 ] 3À was reduced, whereas above a critical value, [NdFO 5 ] 4À was reduced.…”