Molten Salt Electrodeposition Behavior and Nucleation Mechanism of Rare Earth Praseodymium AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY24090004

Chinese Library Classification Number:TF803.21

Citation Information:

Given their critical role in permanent magnets for new energy vehicles, wind turbines, communications, and other industries advance, the demand for rare earth metals, such as Pr, and other essential materials is rapidly increasing. Therefore, developing an efficient extraction method for Pr is becoming urgent. The electrolytic reduction methods for rare earths include the ionic liquid method and the high-temperature molten salt method, depending on the electrolyte composition. Ionic liquids offer high electrical conductivity and a broad electrochemical window. However, their complex synthesis, challenging purification, and high cost limit their large-scale industrial application and development. Rare-earth metals are commonly prepared industrially using high-temperature molten salt electrolysis. The chloride system is widely used in laboratory research；however, in industrial production, the chloride system electrolysis of praseodymium faces issues such as low current and electrolysis efficiency, high power consumption, and the generation of chlorine gas. Fluoride molten salts offer better stability and higher current efficiency compared to chloride molten salts. Despite advancements, the electrodeposition kinetics and nucleation mechanisms of rare-earth praseodymium in fluoride molten salt systems remain poorly understood. This study investigated the electrochemical redox behavior and nucleation mechanism of praseodymium (Pr) on an inert tungsten (W) electrode in the LiF-PrF₃ fluoride salt system. The reduced praseodymium was analyzed by using X-ray diffraction (XRD）, scanning electron microscopy (SEM）, energy dispersive spectroscopy (EDS）, and inductively coupled plasma-emission spectroscopy (ICP-AES）. The experiment was conducted in a pit-type furnace using a graphite crucible as the electrolytic cell. A mixture of PrF₃ and LiF in a 9∶1 mass ratio was placed in the crucible, and argon gas was introduced. The mixture was then heated to 1000 ℃ and maintained at this temperature for 1 h with continuous stirring. Following this, the system was connected to an electrochemical workstation to measure cyclic voltammetry and i-t curves. Finally, electrolysis was performed using a direct current supply, with specific electrolysis potential and time settings to prepare praseodymium metal. The electrodeposition behavior of Pr on an inert W electrode in LiF-PrF₃-Pr₆O₁₁ molten salt system was investigated by adding Pr₆O₁₁ to LiF-PrF₃-Pr₆O₁₁ molten salt system, using Pr（Ⅲ) as a representative of the non-variable lanthanide ions：1) The cyclic voltammetry curve displayed two pairs of redox peaks, with one pair for praseodymium emerging around -0.2 V. This indicatedan electrochemical process where solid-phase material formed on a heterogeneous inert electrode, then dissolved into the solution. This pair of signals corresponded to the dissolution and precipitation reactions of Pr（Ⅲ）. The oxidation signal observed around -0.5 V corresponded to Li dissolution, while the reduction signal related to the reduction of Li（Ⅰ）. As the scan rate increased, the reduction peak potentials shifted negatively, while the oxidation peak potentials shifted positively. At scan rates ranging from 0.10 to 0.30 V·s^-1, the oxidation peak potential increased gradually with lgv, indicating that the system didnot exhibit a fully reversible behavior with a zero intercept. The reduction peak potential decreased slowly with lgv within the same scan rate range. This behavior suggested that the praseodymium reduction reaction was quasi-reversible. The average diffusion coefficient of Pr（Ⅲ) in molten LiF-PrF₃-Pr₆O₁₁ with a concentration of 1.8×10⁻⁴ mol·cm⁻³ was 4.667×10⁻⁵~1.414×10⁻⁴ cm²·s⁻¹. 2) The j-t curves of rare earth metal Pr electrodeposited onto W electrodes under varying overpotential pulses aligned with the current transient characteristics of the metal electrodeposition process. Initially, the current decreased rapidly due to bilayer charging, subsequently increased rapidly as nucleation and growth began on the W electrode. As diffusion zones overlap, the current reached a maximum before decaying over time, indicating diffusion-controlled electrodeposition processes. The nucleation rate gradually rose, reflected in larger -j_max values and shorter t_max durations. The dimensionless curves from -0.53 to -0.51 V closely matched the transient nucleation curves, indicating instantaneous nucleation of praseodymium electrodeposition on tungsten electrodes within this range. 3) Overvoltage in the molten salt system supplied the energy needed for the reduction of Pr（Ⅲ) ions to monomers. The deposition rate was influenced by the overvoltage magnitude；a higher overvoltage led to an increased nucleation rate. A 5.08 g sample of praseodymium was obtained after 2 h of constant potential electrolysis at -0.53 V using a direct current power supply. While most peaks correspond to Pr, the presence of Pr₂O₃ peaks suggested oxidation of the praseodymium metal upon exposure to air. The electrolysis efficiency was 87.97%, and the metal sample purity, as determined by ICP, was 98.86%.