Electrochemical and Thermodynamic Properties of Neodymium Alloys in Molten Salts for Their Recovery
Open Access
- Author:
- Im, Sanghyeok
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 08, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Zi-Kui Liu, Major Field Member
Sarma Pisupati, Outside Unit & Field Member
Hojong Kim, Chair & Dissertation Advisor
Feifei Shi, Outside Field Member - Keywords:
- Molten Salt Electrochemistry
Neodymium
Nd alloy
Thermodynamic Property
Emf Measurement
Rare-Earth Recovery - Abstract:
- Recovery of rare-earth elements is an essential process technology to achieve energy sustainability for nuclear power by closing the fuel cycle and for renewable energy by securing rare-earth supply. In detail, rare-earth fission products that accumulate into molten salts (e.g., LiCl-KCl) during reprocessing for used nuclear fuel must be recovered to reuse the salts and minimize nuclear waste. Rare-earth permanent magnets are an essential component for wind turbines and electric vehicle motors but face supply concerns with the rapid deployment of clean energy technologies. In the design of electrochemical processes for rare-earth recovery, thermodynamic and electrochemical properties of rare-earth elements are important to achieve high efficacy but are not widely available in the literature due to the high reactivity of rare-earth metals in molten salt electrolytes. Thus, the primary objective of this thesis is to comprehensively examine the essential fundamentals of rare-earth alloys in molten salt electrolytes with reliable property measurements, focusing on the binary Nd-Sn, Nd-Bi, and Nd-Fe systems. To determine the thermodynamic properties of Nd in molten salts, electromotive force (emf) measurement is introduced. Based on the established emf relation between Nd-based alloys and pure Nd via a solid fluoride electrolyte or a transient technique, less reactive two-phase alloys were employed as a stable reference electrode for reliable emf measurements, instead of pure Nd having uncertainty in molten salts. From an electrochemical cell including the Nd-Sn (xNd = 0.10) reference electrode with a two-phase (liquid + NdSn3), the measured emf values of Nd-Bi alloys (xNd = 0.15–0.40) and Nd-Sn alloy (xNd = 0.10) were stable and reproducible without an indication of cell degradation during the measurements. Furthermore, the potential difference between two identical Nd-Sn reference electrodes was measured before/after the electrochemical evaluation for 45 days, and the stability of the cell for long-term operation was verified by showing only 3 mV difference. The thermodynamic properties for the selected liquid Bi and Sn electrodes to overcome side reactions using their strong chemical interaction with Nd were investigated through coulometric titration emf measurement. Both liquid metals exhibited similar emf values, which translate to extremely low activity of Nd (aNd) at 973 K as 1.1 × 10-13 in Bi and 5.8×10–13 in Sn, confirming their strong chemical interactions with Nd. In addition, using the emf trajectory at each temperature, the solubility of Nd at 973 K was estimated at 1.46 mol% for liquid Sn, compared to 5.65 mol% for liquid Bi. For the Bi electrode with high recovery capacity, the thermodynamic description (Phase diagram) of the Nd-Bi system was updated via CALPHAD modeling with the addition of new data, including activity and solubility by emf measurements, phase transition temperatures by DSC, and the recently observed Nd3Bi7 compound. Highly efficient recovery of Nd into liquid metals of Bi and Sn was achieved in molten LiCl-KCl-NdCl3 electrolyte at 773-973 K by leveraging the strong interactions of Nd. Both liquid metals demonstrated high round-trip coulombic efficiencies (>99.3%) during deposition-removal cycles of 10‒50 mA cm‒2 and high recovery capacity up to approximately 20 mol% Nd beyond the solubility limit. In addition, high recovery yield (84–90%) was confirmed based on chemical analysis of electrolysis products in Bi after constant current electrolysis (–50 mA cm-2) at 873‒973 K. Overpotentials during the Nd deposition process were attributed to charge-transfer and mass-transport resistances by the current-potential curve and electrochemical impedance spectroscopy. The charge-transfer kinetics of Nd deposition into liquid metals was facile with high exchange current densities at ~220 mA cm‒2. Due to the importance of permanent NdFeB magnet alloys for renewable energy technologies, the thermodynamic properties of binary Nd-Fe alloys with varying mole fractions of Nd (xNd = 0.05‒0.78) were investigated via electromotive force measurements in molten chloride at 773–1073 K. The Nd-Fe alloys were heat-treated at 923 K for 7 days to stabilize equilibrium phases and then the measured emf values were consistent between alloy compositions, as shown by equivalent thermal trajectories in distinct two-phase regions of [Fe + Nd2Fe17], [Nd2Fe17 + Nd5Fe17], [Nd5Fe17 + Nd], [L + Nd2Fe17], and [L + Nd5Fe17]. The measured emf results were further validated using open-circuit potentiometry in which Nd was deposited onto a Fe electrode at 973 K, resulting in a difference of less than 18 mV for the identical two-phase alloy between the two methods. It is believed that the data are crucial in developing electrolytic processes for rare-earth alloys (e.g., Nd-Fe) and in materials simulation for reliable prediction of rare-earth alloys and compounds.