Influence of Rare Earth Oxide Concentration on Electrochemical Co-Deposition of Nd and Pr from NdF3-PrF3-LiF Based Melts

Abstract

The impact of rare earth oxide (REO) concentration on the deposition process and selective recovery of the metal being deposited from a molten fluoride salt system was investigated by applying deposition of Nd and Pr and varying the concentration of REO added to the electrolyte. A ternary phase diagram for the liquidus temperature of the NdF3-PrF3-LiF system was constructed to better predict the optimal electrolyte constitution. Cyclic voltammetry was used to record three redox signals, reflecting the processes involving Nd(III)/Nd and Pr(III)/Pr transformations. A two-step red/ox process for Nd(III) ions and a single-step red/ox process for Pr(III) ions were confirmed by square-wave voltammetry. The cyclic voltammetry results indicated the possibility of neodymium and praseodymium co-deposition. In order to sustain higher co-deposition rates on the cathode and to avoid increased production of PFC greenhouse gases on the anode, a low-overpotential deposition technique was used for Nd and Pr electrodeposition from the electrolyte with varying Nd2O3 and Pr6O11 concentrations. Co-deposited neodymium and praseodymium metals were characterized by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) analysis. After electrodeposition, concentration profiles of neodymium and praseodymium were recorded, starting from the cathode surface towards the electrolyte bulk. The working temperature of 1050 °C of the molten fluoride salt basic electrolyte, in line with the constructed phase diagram, was validated by improved co-deposition and led to a more effective deposition process.

Advances in Understanding Metal Electrolysis Process

“…In particular, the recycling, substitution, and efficient use of REEs is critical for developing a strategy towards a "green RE economy" and supplementing current supply channels. Examples of recent research focusing on several of these aspects are discussed in three papers: [1][2][3]. Articles [1,3] are focused on neodymium and praseodymium deposition, including the complete electroreduction mechanism of both metals, to enable better control over the process, greenhouse gas emission reduction during electrolysis, and better deposition efficiency.…”

“…Examples of recent research focusing on several of these aspects are discussed in three papers: [1][2][3]. Articles [1,3] are focused on neodymium and praseodymium deposition, including the complete electroreduction mechanism of both metals, to enable better control over the process, greenhouse gas emission reduction during electrolysis, and better deposition efficiency. The key objective was to simultaneously deposit neodymium and praseodymium from a fluoride molten bath containing their oxides, as an alloy, by reducing the ecological footprint of the process.…”

“…The environmentally improved process for neodymium and praseodymium co-deposition is performed in these studies under relatively small praseodymium deposition overpotentials on inert Mo and W cathodes resulting in very low levels of carbon oxide, fluorine, and fluorocarbon gas emissions due to the low anodic overpotentials. The application of the Nd and Pr co-deposition in a potentiostatic mode is a highly promising approach for recycling REEs from neodymium iron boron magnets if initially separated from the magnet in the oxide form [1,3]. The conditions for an anode effect during neodymium and didymium oxide electrowinning were discussed in paper [2].…”

Recovery of Rare Earth Elements from Spent NdFeB Magnets: Metal Extraction by Molten Salt Electrolysis (Third Part)

“…Fundamental aspects of the process have also been addressed, it was shown that Nd deposits in two steps, from Nd(III) to Nd(II) at ≈−0.40 V, then Nd(II) + 2e− → Nd(0) at ≈−0.525 V (with tungsten as reference electrode) [17,18]. Further work on the co-deposition leading to Nd-Pr alloy formation revealed that praseodymium can be deposited within one-step 3 electrons exchanged following the neodymium deposition [18][19][20]. Since the main parameters in the Nd-Pr alloy formation via MSE have been discussed [19][20][21], the next step of this development would be implementing this knowledge on a more actual and complex raw material, such as NdFeB-based rare earth oxide.…”

“…Further work on the co-deposition leading to Nd-Pr alloy formation revealed that praseodymium can be deposited within one-step 3 electrons exchanged following the neodymium deposition [18][19][20]. Since the main parameters in the Nd-Pr alloy formation via MSE have been discussed [19][20][21], the next step of this development would be implementing this knowledge on a more actual and complex raw material, such as NdFeB-based rare earth oxide. The attempt to produce NdFeB magnets from recycled rare earth alloy scraps is a feasible concept [2,3,22].…”