Molten Halides Research Articles

On the applicability of the Redlich-Kister framework for viscosity estimation of molten halide salt mixtures

For molten halide salt mixtures already being utilized or under consideration for carbon-free energy production systems, it is crucial that their viscosity is well understood so that system thermal hydraulics can be reliably assessed. Because of the difficulty in accurately measuring molten halide viscosity and the sheer size of the matrix of possible higher order salt mixtures that may be of interest to the energy industry, there are several gaps in the quantified understanding of molten halide viscosity across this matrix. As such, both first-principles and semi-empirical modeling techniques may be crucial for rapidly assessing this broad, complex compositional domain. Herein, the Redlich-Kister framework is applied to assess the feasibility of broadly interpolating and estimating the viscosity of several pseudobinary and pseudoternary molten halide salt systems that may be of key interest to the energy industry. The framework is based on the assumption that an ideal component and a nonideal component collectively describe the viscosity as a function of composition and temperature for a given molten halide system. Three different ideal models were considered for the ideal component, including Grunburg-Nissan, Katti-Chaudhri, and Gambill methods. Regarding the pseudobinary interpolations, the Redlich-Kister models with either the Grunburg-Nissan or Katti-Chaudhri models as the ideal component resulted in either highly (average error less than 5%) or reasonably (average error less than 15%) accurate interpolations of pseudobinary halide viscosity; BeF2- or UF4-bearing salts tended to result in reasonably accurate interpolations, whereas other pseudobinary mixtures tended to show high accuracy. Regarding the pseudoternary extrapolations, the Redlich-Kister framework shows reasonable success at estimating the extent to which a pseudoternary system may indicate deviations from ideal Grunburg-Nissan mixing, where discrepancies with comparative experimental data generally stay within 30%. The primary reasons identified for such discrepancies are (1) inaccuracy in the underlying experimental data, (2) different complexation behavior in the higher order systems compared to the pseudobinary subsystems, and (3) extrapolation into temperatures too far out of the domain, which is valid for the underlying experimental data feeding the Redlich-Kister model.

Complex Structure of Molten FLiBe (2 LiF – BeF 2 ) Examined by Experimental Neutron Scattering, X-Ray Scattering, and Deep-Neural-Network Based Molecular Dynamics

The use of molten salts as coolants, fuels, and tritium breeding blankets in the next generation of fission and fusion nuclear reactors benefits from furthering the characterization of the molecular structure of molten halide salts, paving the way to predictive capability of the chemical and thermophysical properties of molten salts. Due to its neutronic, chemical, and thermochemical properties, 2LiF-BeF2 is a candidate molten salt for several fusion- and fission-reactor designs. We performed neutron and x-ray total-scattering measurements to determine the atomic structure of liquid 2LiF-BeF2. We also performed and neural-network molecular-dynamics simulations to predict the structure obtained by neutron- and x-ray-diffraction experiments. The use of machine learning provides improvements to the efficiency in predicting the structure at a longer length scales than is achievable with simulations at significantly lower computational expense while retaining near accuracy. We found that the NNMD simulations accurately predicted the BeF42− oligomer formations seen in the experimental first-structure-factor peak. Our combination of high-resolution measurements with large-scale molecular dynamics provided an avenue to explore and experimentally verify the intermediate-range ordering beyond the first-nearest neighbor that has posed too many experimental and computational challenges in previous works. With a deeper understanding of the salt structure and ion ordering, the evolution of salt chemistry over the lifetime of a reactor can be better predicted, which is crucial to the licensing and operation of advanced fission and fusion reactors that employ molten salts. To this end, this work will serve as a reference for future studies of salt structure and macroscopic properties with and without the addition of solutes. Published by the American Physical Society 2024

Effect of Fission Products on Molten Salt Corrosion for Nuclear Reactor Applications

Molten halide salts are used in various technologically important applications like molten salt reactors and concentrated solar power. A major challenge of molten salts is their corrosivity towards structural materials. However, most of the complex interfacial phenomena related to molten salt corrosion are not well understood, primarily due to the complex impact of salt chemistry on structural material interface. Structural material corrosion is influenced by salt redox potential, fission product buildup, presence of impurities and moisture. Given the huge dependence of operations on salt chemistry, understanding the interactions between soluble fission products and structural alloys is of great importance. EuCl3 is one of the primary fission products and shown EuCl3 to cause accelerated corrosion of structural alloys in molten chloride salts. Given the more positive formal oxidation potential of Eu(III)/Eu(II) compared to nickel, iron, and chromium that are major components of the structural alloys, Eu(III) species act as a strong oxidant to cause the dissolution of these metals. This talk will highlight utilization of novel in situ optical spectroscopy technique to study time-dependent evolution of corrosion damage of metals in presence of EuCl3 as well as the EELS technique to identify changes in oxidation states of dissolved alloying elements.

The high temperature oxidation and thermal shock behavior of a dense WSi2–TaSi2 coating on Ta substrate prepared by a novel two-step process

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na2WO4-WO3 system) and halide activated pack cementation was adopted to prepare a WSi2–TaSi2 coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm2, 40 mA/cm2 and 50 mA/cm2. It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm2, the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi2–TaSi2 coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO2 and Ta2O5, which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO2, the four-layer SiO2–W5Si3–WSi2–Ta5Si3 coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature.

Impact of Catholyte Lewis Acidity at the Molten Salt–NaSICON Interface in Low-Temperature Molten Sodium Batteries

Low-temperature molten sodium batteries comprising molten sodium anodes, a NaSICON solid-state separator, and molten halide salt catholytes offer promise as low-cost, earth-abundant energy storage technologies. The emergence of a specific, high-voltage, sodium iodide (NaI)-based catholyte chemistry has prompted the evaluation of chemical and electrochemical properties of the molten salts, particularly at critical interfaces with high-performance NaSICON separators. Herein, batteries operated at 110 °C with NaI-AlCl3-based catholytes of differing Lewis acidities were evaluated. Batteries with >50 mol % AlCl3 (acidic catholytes) experienced a linear decline in energy efficiency during cycling, whereas batteries with >50 mol % NaI (basic catholytes) maintained >95% energy efficiency for 50 cycles (>80 days) at 2.5 mA cm–2. A three-electrode cell was developed, enabling identification of the NaSICON–catholyte interface as the source of increased battery impedance. Complementary physical and chemical characterization of the NaSICON exposed to acidic and basic catholytes showed no changes in crystallinity, bulk morphology, or bulk chemical composition, but surface sensitive X-ray photoelectron spectroscopy (XPS), however, revealed subtle changes in local NaSICON surface chemistry. In addition, Raman spectroscopy indicated that stably performing basic catholytes lack the dimer species Al2Cl6I– present in acidic catholytes. Select thermodynamic and formal charge assessments suggest that preferential interactions between these acidic dimeric species and the NaSICON surface may be responsible for the observed increases in electrochemical impedance and degraded battery performance. These results indicate that maintaining a Lewis basic catholyte avoids such potentially deleterious interactions, enabling efficient and stable battery cycling.

Simultaneously tuning interlayer spacing and termination of MXenes by Lewis-basic halides

The surface and interface chemistry are of significance on controlling the properties of two-dimensional transition metal carbides and nitrides (MXenes). Numerous efforts have been devoted to the regulation of Ti3C2Tx MXene, however, tuning interlayer spacing and surface halogen termination of other MXenes (besides Ti3C2Tx) is rarely reported while demanded. Here we propose a Lewis-basic halides treatment, which is capable of simultaneously engineering the interlayer spacing and surface termination of various MXenes. Benefited from the abundant desolvated halogen anions and cations in molten state Lewis-basic halides, the -F termination was substituted by nucleophilic reaction and the interlayer spacing was enlarged. Ti3C2Tx MXene treated by this method showed a high specific capacity of 229 mAh g−1 for Li+ storage, which is almost 2 times higher than pristine one. Considering the universality, our method provides an approach to regulating the properties of MXenes, which may expand their potential applications in energy storage, optoelectronics and beyond.

A High Throughput Experimentation & Material Informatics Platform for the Discovery of Molten Salt Reactor Candidate Structural Materials

ASME Sec(III) Div(5) code-certified structural alloys for Molten (halide) Salt Reactors (MSRs) such as 800H, SS316, and IN617 have significant Cr content. This makes them readily-degradable in molten halide salts due to the thermodynamic favorability of soluble chromium halide formation. It is therefore imperative to discover alloys with corrosion resistance that exceeds those of currently-certified alloys, which also possess the necessary high hardness and irradiation-resistance at reactor operating temperatures as a prerequisite for the licensing and deployment of MSRs.The largely-unexplored quasi-infinite quarternary FeCrMnNi High Entropy Alloy space shows promise to yield desired alloys. We present a high-throughput (HTP) process demonstrating a turnaround time from fabrication to corrosion-testing and analysis of 1 week for 25 samples. 70 alloy samples of 1cm2 were each corrosion-tested on 0.3g salt droplets in isolated corrosion environments. The HTP platform includes the development of an in-situ high-temperature electrochemical sensor system capable of automating the analysis of dissolved corrosion product analytes in the salt.The results of the 70 corrosion tests were used to train a Machine Learning model to predict corrosion performance metrics (eg. Elemental corrosion concentration into salt) based on an input vector parametrizing the physical properties (“descriptors”) of the alloys in the sample set. The model was tested on 20 additional samples and demonstrated conservative predictive capability as well as facilitated the reduction of input feature space from 62 dimensions to 4 dimensions with negligible loss in predictive accuracy.

Electrodeposited NiMo Alloy Coatings for Improved Molten Salt Compatibility

The use of molten halide salts as a primary coolant within next generation molten salt nuclear reactors (MSRs) has the potential to improve efficiency and nuclear reactor safety by operating at low pressures and high temperatures without boiling. However, these coolants will require the development of new corrosion resistant material systems that will have to meet or supersede existing standard codes for these systems. Therefore, new MSRs will require validation and testing of new material systems that can produce robust component structures and enable them to withstand these corrosive environments. Furthermore, any next generation process to apply protective coatings must allow for a conformal, thick, dense, and bonded material to be formed onto an ASME code approved material (e.g. type 316H stainless steel) that will comprise the structure of heat exchanger and containment vessel components.Within this context, Faraday Technology Inc. in collaboration with Oak Ridge National Lab and Quintus Technologies is working to develop and demonstrate the effectiveness of a low cost and high value corrosion-resistant alloy coating for next generation molten salt reactors. The manufacturing process involves the application of a functionally graded NiMo coating by electrodeposition onto 316H alloy substrates and their subsequent high temperature corrosion evaluation in molten fluoride salt at 700°C. After application the components were hot isostatically pressed (HIPped; at Quintus) to create a diffusion bond between the coating and alloy substrate. Initially, 316H alloy coupons were coated for evaluation (Figure 1 Left) in Mo capsules with static FLiNaK salt known to be corrosive due to metallic impurities. The next stage of testing involved evaluating coatings on the inner diameter of 316H tubes (25mm outer diameter; Figure 1 Middle). Coated tubes (75mm long) had coated endcaps welded to either end and were filled with the same FLiNaK salt and tested for 500 h at 700°C and 1000 h at 750°C. In addition, the microstructure and salt performance of butt welds were evaluated for joints between coated 316H tubes and joints between coated and uncoated 316H tubes. Results from the static capsule testing will be presented including the effect of coating composition and heat treating conditions on the depth of attack in the molten salt as feedback for the coating development process. The final phase of testing will include 200 mm long coated 316H tube sections in the hot and cold legs of a flowing FLiNaK thermal convection loop experiment which is currently being fabricated (Figure 1 R). Acknowledgements: The financial support of DOE Contract No. DE-SC0019602 is acknowledged. Figure 1: Corrosion characterization test plan. (Left) static coupon studies, (Middle) static pipe studies, and (Right) continuous flow loop pipe studies. Figure 1

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy-Functionalized Pyridinium Cations.

The paradigm of supramolecular chemistry relies on the delicate balance of noncovalent forces. Here we present a systematic approach for controlling the structural versatility of halide salts by the nature of hydrogen bonding interactions. We synthesized halide salts with hydroxy‐functionalized pyridinium cations [HOC n Py]+ (n=2, 3, 4) and chloride, bromide and iodide anions, which are typically used as precursor material for synthesizing ionic liquids by anion metathesis reaction. The X‐ray structures of these omnium halides show two types of hydrogen bonding: ‘intra‐ionic’ H‐bonds, wherein the anion interacts with the hydroxy group and the positively charged ring at the same cation, and ‘inter‐ionic’ H‐bonds, wherein the anion also interacts with the hydroxy group and the ring system but of different cations. We show that hydrogen bonding is controllable by the length of the hydroxyalkyl chain and the interaction strength of the anion. Some molten halide salts exhibit a third type of hydrogen bonding. IR spectra reveal elusive H‐bonds between the OH groups of cations, showing interaction between ions of like charge. They are formed despite the repulsive interaction between the like‐charged ions and compete with the favored cation‐anion H‐bonds. All types of H‐bonding are analyzed by quantum chemical methods and the natural bond orbital approach, emphasizing the importance of charge transfer in these interactions. For simple omnium salts, we evidenced three distinct types of hydrogen bonds: Three in one!

Hastelloy G 35 сorrosion tests for intergranular corrosion resistance

The article is devoted to the analysis of corrosion behavior and assessment of the resistance of Hastelloy G 35. Nickel-based alloys (steels) containing chromium, molybdenum, tungsten, possibly iron are widely used as materials for the manufacture of equipment in high-temperature technological processes using solutions of highly hygroscopic halides (chlorides) in molten halides (chlorides) of alkali metals. During the operation of the equipment, the instability of the surface of apparatus and pipelines to corrosion is determined. In this test, 4 samples of 80×20×3 mm were used, of which 2 samples were subjected to provoking heating, and 2 samples were examined in their normal state. Our tests were carried out in a boiling aqueous solution of iron sulfate and sulfuric acid for 48 ± 0.25 h. Further, the bent samples were examined using an MBS-9 microscope with a magnification of х8. The results of corrosion tests are also presented. As a result of the study, the rate of penetration of corrosion of the alloy was calculated. Hastelloy microstructure is presented. The average depth of destruction was determined from the six maximum values detected in six fields of view. According to the research results, the corresponding conclusions were made.

Interaction between nucleant particles and a solid-liquid interface in Al-4.5Cu alloy

Al3Ti and TiB2 particles were made in-situ from adding halide salts in molten Al-4.5 wt % Cu alloys. The molten halide salts also dissolved the oxide in the molten metal to ensure that the newly formed Al3Ti and TiB2 particles had intimate interfaces with the molten metal. Excess amounts of titanium were added into the melt to ensure that the wetting angles between the particles and the molten alloy were small and the particles were able to significantly reduce the grain sizes of the aluminum alloy. Such particle/matrix systems should have a negative total interfacial energy change when a particle is engulfed into the solid by an advancing solid-liquid interface, resulting in particles being engulfed by an advancing solid-liquid interface according to widely accepted theories on particle pushing. Experimental results indicate that these particles are actually pushed by the growing dendrites into the interdendritic regions, regardless of the growth rates being positive, close to nil, or even negative. It seems that the particle pushing mechanisms based on interfacial energy interactions are not applicable for these particle/matrix systems under experimental conditions of this study. Analytical analysis suggests that these particles are pushed by the growing dendrites due to fluid flow before they could get into close contact with the solid-liquid interface within a several interatomic layers where the van der Waals forces become significant. The shrinkage driven flow in the liquid of the mushy zone is sufficient to dislodge a particle from a depression and rolls the particle on the solid-liquid interface.

Electrolytic Formation of Solid Crystalline UO2–ThO2 and UO2–ThO2–ZrO2 Solutions from Salt Melts

For the first time, crystalline solid solutions of UO2–ThO2 and UO2–ThO2–ZrO2 have been electrolytically prepared. The mechanism of their formation in molten halides is presented. The influence of the electrolysis current density on the compositions of the cathode deposits has been established and explained. We also established the effect of zirconium ions on the ThO2 content in the UO2–ThO2–ZrO2 cathode deposits.

4D in Situ Temperature-Dependent Study on Morphological and Chemical Evolution of Metals in Molten Salt Environments By Multimodal Microscopy

To further the development of sustainable energy, it is necessary to understand material behavior under extreme environments. The need to transport high-temperature heat in next-generation nuclear or large-scale solar thermal power plants drives increased interest in molten halide salts as a heat transfer fluid. Specifically, molten KCl-MgCl2 and related mixtures are attractive, as they have some of the highest volumetric heat capacities of viable systems at 700 oC and lowest cost. However, the prevalent challenge for molten KCl-MgCl2 is corrosion with the encasing structural Ni-based alloys. Literature has shown that this corrosion generally proceeds through the selective dissolution of Cr along with intergranular corrosion. Our prior work showed that in addition to intergranular corrosion, a percolation dealloying phenomenon can contribute to the morphological changes of metal corrosion in molten salt; this dealloying process leads to a porous structure formation instead of corrosion along the grain boundaries. However, the underlying conditions and kinetics that determine the dominating phenomenon remain unclear.In our present study, binary Ni-20Cr (wt. %) alloy was studied in molten KCl-MgCl2 (50 – 50 mol. %) at elevated temperatures (500, 600, 700 and 800 oC) by multimodal X-ray and electron microscopy techniques. In situ synchrotron X-ray nano-tomography was used to observe the real-time morphological evolution. 3D visualizations present apparently different pathways for the material’s morphological evolution at different temperatures. With quantification of morphological characteristics, including relative volume change, porosity, feature size distribution and corrosion distance propagation as a functions of temperature and time, possible rate-determining factors are discussed, including long-range diffusion-control (such as dissolved Cr ions diffusing outward or salt ions diffusing inward) vs. interface-control (such as surface diffusion or reaction at the interfaces). Furthermore, by X-ray Absorption Near-Edge Structure (XANES) spectroscopic imaging, the oxidation state of Ni in the remaining sample was analyzed. In addition, high-resolution Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy was applied to observe the corrosion products at the surface of the alloy and salt penetration at the nanoscale. Overall, this study reveals the temperature-dependent corrosion behavior of Ni-20Cr alloy in molten KCl-MgCl2 salts, which could be applied to improve the materials used in large-scale energy facilities such as nuclear and solar power plants, highlighting the importance of considering temperature-dependent kinetics routes. On the broader impact, the knowledge could be applied to a wider community, such as when utilizing the molten salt dealloying phenomena to fabricate nano-/mesoporous metals for functional applications. This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy Office of Science.

Electrochemical reduction and dissolution of liquid aluminium in thin layers of molten halides

New methods of liquid metals refining and separation operated with low energy consumption and environmental impact are highly desirable nowadays. This work presents an approach of Al refining and extraction from scrap in a thin layer of the multiple-pore molten salt electrochemical system, which appears promising. The new single-capillary cell design with a quasi-reference electrode on the TiB2 substrate was used to study the kinetics of aluminium reduction and dissolution in a narrow 1 mm-diameter channel filled with KF-AlF3 (1.1<[KF]/[AlF3]<1.5 mole/mole) or equimolar NaCl-KCl with N wt.% of AlF3 (3<N<20) at 700–850 °C. It was found that the chloride-based melts are more preferable than the fluoride mixtures due to the much bigger electrode potential window between Al and Alkali metal (1.3 vs. 0.4 V), higher apparent limiting currents (0.9 vs. 0.3 A cm−2) and lower overvoltage (4–16 vs. 66–247 mV at 0.1 A cm−2) resulted from the diffusion kinetics dominance. However, anodic dissolution of aluminium in NaCl-KCl-AlF3 may be chemical reaction-controlled with the exchange current density of 105 mA cm−2, and the reaction order of 0.34 observed in the experiment. The temperature drastically affected the whole kinetics picture with the activation energy for alkali metal reduction found to be 94.762 kJ mole−1. The preferable temperature for liquid metal refining was found to be 800 °C. The cathodic process in the KF-AlF3 melt has mixed kinetics. It possesses surprisingly low apparent diffusion limiting current (0.3 A cm−2 at 800 °C and [KF]/[AlF3]=1.1 mole/mole) and chemical reaction control at higher [KF]/[AlF3] (with exchange current density of 50 mA cm−2 and the reaction order of 0.08 at [KF]/[AlF3]=1.5 mole/mole). It may happen due to the co-deposition of Al and K with the evolution of solid cryolite. The composition of the melt plays a crucial role in refining performance. The optimal [KF]/[AlF3] for Al refining is 1.2 mole/mole. It allows operating at a current density not higher than 0.4 A cm−2 with a thickness of a thin-layer system less than 60 μm. The preferable AlF3 concentration in the NaCl-KCl-AlF3 composition is 10%. It allows operating at 1.4 A cm−2 with a thickness of 22 μm or lower.

Corrosion Behavior of Pre-Carburized Hastelloy N, Haynes 244, Haynes 230, and Incoloy 800H in Molten FLiNaK

Austenitic alloys such as austenitic stainless steels and Ni-based alloys have been specified as container materials for molten salt reactors (MSRs). In MSR environments, these alloys are susceptible to carburization because (1) graphite components provide a source of carbon, (2) oxide films are not protective in molten halide salts, and (3) MSR operating temperatures fall within the temperature range of carbide formation. Carburization may occur simultaneously with corrosion by the selective dissolution of Cr. To study the corrosion behavior of a carburized microstructure, Hastelloy N, Haynes 244, Haynes 230, and Incoloy 800H samples were pre-carburized at 900°C in a hydrocarbon atmosphere prior to exposure to molten FLiNaK at 700°C. Pre-carburized samples featured a carbide-rich microstructure, particularly near the sample surface. The face-centered-cubic matrix of pre-carburized samples was found to be depleted in Cr, Mo, and to a lesser extent W, which are strong carbide-forming elements. Chromium dissolution in pre-carburized samples was suppressed compared to untreated samples due to the reduction in Cr activity. However, selective attack of the connected carbide structure along grain boundaries was observed in pre-carburized Haynes 230 and Incoloy 800H, which are alloys with high Cr content.

Uranium Electrorefining in 3LiCl-2KCl Based Melts

Molten halides can be effectively employed for electrorefining of refractory and radioactive metals with relatively negative reduction potentials and high melting temperatures. In the present study electrorefining of uranium was studied for developing pyrochemical reprocessing technology of metallized spent nuclear fuels. 3LiCl-2KCl molten salt was chosen as the working electrolyte. Uranium containing electrolytes were prepared by dissolving UCl3 in the solvent salt melt in an argon filled glove box to obtain various desired concentrations of uranium in the electrolyte. The working temperature in the refining experiments varied between 450 and 750 °C.The experiments on uranium electrorefining were carried out in a stainless steel semi-industrial water-cooled electrolyser. Crude uranium metal was loaded in a specially designed molybdenum anodic basket. Glassy-carbon crucible was used to hold the melt. Molybdenum rod was employed as a cathode.The main goal of the electrolytic refining process is obtaining high-purity uranium metal. Specific energy consumption is also very important. The results of the electrolytic refining depend on various parameters. In the present study current density, temperature, uranium concentration in the electrolyte, and specific quantity of electricity passed through the cell were selected as variables, and the current efficiently was chosen as a response factor.Dense and compact uranium deposits were obtained under low current densities, while application of relatively high current densities led to the formation of uranium dendrites. Intermediate values of current density allowed producing more coherent deposits that could be easily scraped off the cathode. Increasing temperature led to a sharp change of the cathodic deposits morphology. The reason for this is the formation of tetragonal beta-uranium instead of the low-temperature orthorhombic alpha modification.The values of cathodic current efficiency in the most of experiments confirmed the three-electron scheme of uranium reduction. Cathodic current density under certain conditions exceeded 95 %. The amount of salt retained in the cathodic deposit varied from 7 to 25 %, values typical for dendrite electrolytic metals.

Extraction and optimization of neodymium from molten fluoride electrolysis

Most rare earth elements (REEs) are critical materials of extreme prominence. Because of their criticality, their convalescence into metal from rare earth oxide (REO) and their recycling from end-of-life materials are essential for the proficient use and safeguard of natural resources. With pyrometallurgical approaches using potential versus partial oxidation pressure (E-pO2-) diagrams, the conditions for making neodymium are established by which REOs are dissolved in a molten halide bath to form pure Nd metal in the electrolysis process. This paper also expands on how the diagrams are used and verified through design of experiment studies, which result in statistical rejoin models based on molten salt electrolysis for extracting neodymium from its oxide in a fluoride bath.

Electrochemical Methods for Analysis of Hydroxide and Oxide Impurities in Li, Mg/Na, and Ca Based Molten Chloride Salts

With applications that include thermal energy storage in Gen3 concentrated solar power (CSP), molten salt batteries in grid scale energy storage, fuel and coolant in molten salt reactors (MSR), and oxide reduction/electrorefining of spent nuclear fuel; molten salts are potentially the most important all around medium for advanced energy systems. In particular, molten halide salts are known to be extremely hygroscopic, typically resisting complete thermal dehydration. Incomplete removal of water leads to hydrolysis; promoting formation of volatile HCl and insoluble oxide/hydroxide impurities in the molten salt. Depending on the application, this can accelerate corrosion, lower product yield through formation of insoluble oxides and oxychlorides, and/or reduce cell efficiency in electrolytic processes. Consequently, the generation, speciation, and electrochemical response of oxide and hydroxide impurities in various molten chloride salts have been widely studied. In this presentation, previously published work with LiCl-Li2O, MgCl2-KCl-NaCl, CaCl2, and CaCl2-CaO-CeCl3 molten chloride salts will be reviewed and discussed to reveal underlying commonality and lessons learned. In LiCl-Li2O and MgCl2-KCl-NaCl, LiOH and NaOH, respectively, were found in the salt after melting water-contaminated starting material. Electrode response signals were ascribed to both LiOH and NaOH contamination, and a nondestructive, in-situ method for determining their concentrations using cyclic voltammetry (CV) was developed. In CaCl2, Ca(OH)2 was found to be unstable, reverting to CaO at operating temperatures. Electrode signals were ascribed to the CaO, and an in situ measurement technique using CV to measure the CaO concentration was developed. The effect of CaO contamination on an electrorefining process was further studied in CaCl2-CaO-CeCl3, where CaO presence was shown to promote the formation of both insoluble Ce2O3 and CeClO. Electrochemical methods in all cases were further substantiated through support analyses including titration, thermogravimetric analysis, quadrupole mass spectrometry, and inductively coupled plasma mass spectrometry. These quantitative electroanalytical methods are shown to be both effective and versatile, showing promise as the development of real-time, in-line sensors for many molten salt processes continues to be of great consequence. Moreover, their versatility in different molten chloride salt systems is encouraging, as they can become an integral step in the evaluation of new use-cases for other molten halide salts and/or their suitability for new applications.

Molten iodide salt electrolyte for low-temperature low-cost sodium-based liquid metal battery

Using low-melting-point electrolytes could overcome various key challenges of low-cost sodium-based liquid metal batteries (Na-LMBs), e.g. high rates of self-discharge and degradation of structural materials, by lowering their operating temperatures. Molten halide salts are considered promising electrolyte candidates for Na-LMBs due to their high stability and electrical conductivity. In this work, thermodynamic simulation via FactSageTM and thermal analysis via e.g. Differential Scanning Calorimeter (DSC) were carried out to explore the NaI-LiI-KI system, since it could be a promising electrolyte for Na-LMBs due to its low melting point and Na solubility. The results show that the eutectic NaI-LiI-KI performs as a pseudo-binary salt with a melting point of ~290 °C. In this pseudo-binary salt, the solubility of NaI in the eutectic LiI-KI is ~7 mol%. Using the eutectic NaI-LiI-KI electrolyte, Na-LMBs could be operated at < 350 °C. Moreover, the Na solubility and Na+ conductivity of the eutectic NaI-LiI-KI electrolyte, which are vital to the battery performance, were estimated by calculation based on the literature data. Additionally, its applicability and economy were also discussed based on cost pre-analysis of salt materials, salt treatment and structural materials regarding salt corrosivity.

Synthesis of a Nanocrystalline α-Al2O3 Powder in Molten Halides in the Temperature Range 700–800°С

The chemical interaction between molten aluminum and glucose in molten alkaline halides at temperatures of 700–800°С is investigated. This interaction forms α-Al2O3 powder consisting of 20–100-nm nanocrystals in the salt melt. α-Al2O3 nanocrystalline formation is confirmed by scanning electron microscopy and Raman spectroscopy.