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

Abstract

The possibility of neodymium electrowinning from neodymium oxide using a reducing agent (RA) produced in-situ by solvent reduction was investigated in molten LiF-CaF 2-Li 2 O between 900 C and 1040 C. Nd 2 O 3 galvanostatic electrolyses were performed and reaction products were analyzed by SEM, XRD and electron-probe microscopy. Metallic neodymium was not directly obtained, due to several limitations for Nd 2 O 3 reduction: low electrical conductivity, unfavourable Pilling-Bedworth coefficient and formation of a dense insulating CaO layer on the sample surface. Consequently, the reduction of a pellet made of a neodymium oxide-metallic oxide mixture was carried out as an alternative pathway. Nd 2 O 3-CuO reduction led to metallic neodymium production in the form of liquid Cu-Nd alloys. A reaction mechanism was proposed based on these experimental results:

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

“…However, Nd 2 O 3 has very limited solubility in fluoride melts (LiF-CaF 2 -NdF 3 , LiF-KF-NdF 3 , etc.). Its concentration in the melts is 1-5 mass% at 1373 K and much smaller than the concentrations of LiF, CaF 2 , or NdF 3 [31]. Nevertheless, when added to the fluoride melt, Nd 2 O 3 dissolves in the presence of LiF and NdF 3 and forms oxyfluorides, NdOF or [NdOF 5 ] 4− [28,29,31,32]:…”

“…It should be noted that there are several other oxidation side reactions that may take place in the electrolyte used in this work [31]:…”

“…When the solidified electrolyte seen in Figure 8a was removed from the electrode and the Mo electrode surface in Figure 9a and analyzed again by XRD, metal neodymium was recognized on the electrode surface in addition to Mo, NdF 2 , and NdF 3 (Figure 9b) Similar results were obtained in XRD analysis of the W working electrode exposed to the similar potentiostatic regime in the same electrolyte ( Figure 10). The deposition was done in the same time period and at the potential of −0.78 V. References to chronopotentiometric electrodeposition found in literature describe getting neodymium metal in the form of drops found in the electrolyte after cooling off the system [6,20,21,24,26,31]. There is only one example of Nd metal being identified on the inert cathode (Mo and W) surface by a deposition similar to the potentiostatic deposition from a fluoride molten salt electrolyte of the same composition in the literature available [18].…”

“…There is only one example of Nd metal being identified on the inert cathode (Mo and W) surface by a deposition similar to the potentiostatic deposition from a fluoride molten salt electrolyte of the same composition in the literature available [18]. There are, however, papers showing XRD results of neodymium being deposited from fluoride melts on the working electrode, but only in the form of alloys [6,29,31]. References to chronopotentiometric electrodeposition found in literature describe getting neodymium metal in the form of drops found in the electrolyte after cooling off the system [6,20,21,24,26,31].…”

“…There are, however, papers showing XRD results of neodymium being deposited from fluoride melts on the working electrode, but only in the form of alloys [6,29,31]. References to chronopotentiometric electrodeposition found in literature describe getting neodymium metal in the form of drops found in the electrolyte after cooling off the system [6,20,21,24,26,31]. References to chronopotentiometric electrodeposition found in literature describe getting neodymium metal in the form of drops found in the electrolyte after cooling off the system [6,20,21,24,26,31].…”