ELECTROCHEMICAL RECOVERY OF RARE EARTH ELEMENTS IN MOLTEN SALT SYSTEMS: DESIGN AND DEVELOPMENT OF ELECTRODE MATERIALS
Restricted (Penn State Only)
- Author:
- Castro Baldivieso, Stephanie
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 20, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Feifei Shi, Outside Unit & Field Member
Zi-Kui Liu, Major Field Member
Hojong Kim, Chair & Dissertation Advisor
Susan Sinnott, Major Field Member - Keywords:
- electrochemical recovery
electromotive force measurements
Gd-Bi alloys
oxide reduction
recovery efficiency of rare earth elements
Gd-Bi kinetic properties - Abstract:
- Fission products are generated from the production of nuclear energy via the nuclear fission of uranium atoms. Used nuclear fuel (UNF) constitutes of fission products, where U and Pu isotopes represent the majority, and a small portion of this fuel contains actinides, rare-earth elements, and alkaline earth elements. The UNF is not recycled: it is stored in cooling pools due to its heat emanation and radioactivity from the actinides, and later it is kept in dry casting underground storage. Pyroprocessing of UNF allows the recycling of U and Pu from the waste, and it consists of 4 different stages: (1) oxide reduction unit, (2) electro refiner, (3) cathode processor and (4) fuel fabrication furnace. In the first stage, the oxide reduction unit, the UNF oxide gets reduced to metal at the cathode in molten LiCl-Li<sub>2</sub>O. This process is feasible for most of the fission products in the molten salt. However, rare-earths are very stable in their oxide form (e.g., Nd<sub>2</sub>O<sub>3</sub>) in chloride salts and it is very challenging to reduce them to metal. Electrochemical recovery and reduction pathways in which rare earth oxides (e.g., La<sub>2</sub>O<sub>3</sub>) can be readily reduced are investigated in this dissertation by mixing these oxides with transition metal oxides (e.g., NiO). At first, the feasibility of reducing transition metal oxides (e.g., NiO) was evaluated to characterize the standard reduction potentials and how efficient the transition metal oxide can be reduced in molten LiCl-Li<sub>2</sub>O electrolyte at 650 °C. Secondly, NiO was mixed with La<sub>2</sub>O<sub>3</sub> respectively, using their atomic ratios (1:5) according to its phase diagram (La-Ni) and sintered into pellet materials. Furthermore, the reduction pathways of this alloy by electrolysis were investigated in LiCl, and LiCl-Li<sub>2</sub>O. In the next stage of the pyroprocess, the metallic UNF is used in the electrorefiner as the anode, and it is dissolved into a LiCl-KCl molten salt electrolyte, which allows the recovery of U and Pu at the cathode using an inert metal rod. However, the process becomes inefficient over time due to the accumulation of impurities (e.g., remaining fission products) in the molten salt electrolyte. The accumulation of these impurities reduces the recovery efficiency of U over time. The recovery of the rare earth fission products from the molten LiCl-KCl-RECl<sub>3</sub> electrolyte used in the electrorefiner unit is investigated in this dissertation by evaluating the recovery efficiency of rare earth elements (specifically Gd) using liquid metal electrodes (e.g., Bi). At first, thermodynamic properties of Gd-Bi were determined using electromotive force (emf) measurements, in which enthalpies, entropies, activity and activity coefficients of Gd-Bi alloys were evaluated. The inconsistencies of the Gd-Bi phase diagrams were also evaluated using x-ray diffraction (XRD), differential scanning calorimetry (DSC) and coulometric titrations to examine the solubility of Gd in liquid Bi. The low activity of Gd-Bi alloys indicate strong interactions between the rare earths and liquid metals, and the kinetic properties (e.g., diffusivity, coulombic efficiencies, resistances) probed by electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and deposition-removal cycles suggest a reversible process of Gd(III) ions in the molten LiCl-KCl-GdCl<sub>3</sub> salt using an inert cathode; and Gd(III) ions maintain a constant ohmic and charge transfer resistances when deposited into Bi at different range of temperatures. This work will provide fundamental knowledge on how rare earth oxides can be reduced in a feasible manner to metals by alloy formation with transition metals, and how feasible is to recover Gd using liquid Bi electrode by studying its thermodynamic and kinetic properties in the molten salt electrolyte. Rare earth oxide reduction also provides cheaper alternatives to fabricate alloy systems used in Ni-metal hydride batteries (LaNi<sub>5</sub> alloys).