Molten LiCl-KCl Eutectic Research Articles

Evaluation of Li Electrodeposition into Al from LiCl–KCl Electrolyte

This study investigated the effects of temperature and the current density on the production of an Al–Li master alloy using electrolysis. The current efficiency and the specific energy consumption were calculated according to Faraday’s law to determine optimal conditions for production of the Al–Li master alloy. Solid aluminum and graphite were used as the cathode and the anode, respectively. The LiCl–KCl eutectic molten salt was used as a lithium-containing electrolyte. Different current densities were applied to obtain the optimum condition. In addition, different temperatures were investigated at the optimum current density during the deposition of lithium. Scanning electron microscopy, optical microscopy, and the X-ray diffraction analysis were used to investigate the microstructure and phase compositions. Atomic absorption spectroscopy was used to measure the existing lithium. The results showed that electrolysis at 620°C and a current density of 0.9 A/cm2 at 4 V for 3 h leads to the formation of an Al–17.43 wt % Li master alloy. The current efficiency and the specific energy consumption for the optimum conditions were found to be 96% and 16.08 kWh/kg, respectively.

Electrochemical Studies on the Redox Behavior of Zr(IV) in the LiCl-KCl Eutectic Molten Salt and Separation of Zr and Hf

Electrochemical behavior of Zr(IV) in LiCl-KCl-ZrCl4 molten salt on molybdenum electrode was systematically studied by a series of electrochemical techniques such as cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was investigated that the Zr(IV) was first reduced to Zr(III) in this system, which is different from most studies in which Zr(IV) is first reduced to Zr(II). The activation energy of Zr(IV) converted to Zr(III) was calculated to be 24.6 kJ mol−1 by measuring the diffusion coefficients of Zr(IV) at 450 °C, 500 °C, and 550 °C. Through research, the nucleation of zirconium belongs to the progressive nucleation on molybdenum electrode in LiCl-KCl-ZrCl4 (0.0359 mol L−1) at 500 °C at −3.0 V vs Cl2/Cl−. Constant current electrolysis (Current density 0.52 A cm−2, duration 500 min) was carried out in LiCl-KCl-ZrCl4 system and the electrolytic product was porous metallic zirconium. More importantly, it is found that the difference in decomposition potential between Zr(II) and Hf(II) is the largest by calculation and test compared with their trivalent and tetravalent states, and they may be separated by electrolysis, which provides a theoretical reference for the electrolytic preparation of nuclear grade zirconium.

Improvement of an Experimental Routine for Electrochemical Composition Measurements of SmCl3 in LiCl-KCl Eutectic Salt Systems

Cyclic voltammetry (CV) was used to study SmCl3 at concentrations of 0.42 to 8.99 wt% in molten eutectic LiCl-KCl (44.2:55.8 wt%) at 773 K. For each sample, CV was repeated at different electrode surface areas to measure the peak current density. By analyzing the measured peak current density and concentration relationship with the Randles-Sevcik equation, the Sm(III) diffusivity for each sample was calculated. These diffusion coefficients ranged from 0.934 × 10−5 to 1.572 × 10−5 cm2‧s−1, showing no noticeable trend with a change in concentration. The samples were then divided into two groups of five. The first group was used to develop a calibration model for concentration prediction, while the second group was used to test and validate the model. The first model was based on the relationship between current density and concentration. This model had a very low limit of detection of 0.14 wt% and very low error as evaluated by the root-mean-square error of calibration of 0.108 wt%. The second model was a multivariate approach utilizing the current density values and laser-induced breakdown spectroscopy (LIBS) intensities as regressors; however, the introduction of LIBS data showed an increase in the model’s prediction error when compared to the first model. The electrode withdrawal method proved to be a preferable option due to a substantial increase in precision.

Square wave voltammetry for real time analysis of minor metal ion concentrations in molten salt reactor fuel

Molten salt mixtures containing LiCl-KCl-UCl3-CeCl3 were used as surrogates for molten salt reactor fuel in a study of electrochemical concentration analysis. Current responses to low concentration CeCl3 (0.5 wt% Ce) in molten eutectic LiCl-KCl with high concentration UCl3 (5 and 10 wt% U) were measured using cyclic voltammetry (CV) and square wave voltammetry (SWV). CV peaks associated with CeCl3 were impossible to quantify because of dominant reducing current contribution from UCl3. SWV results featured peaks from UCl4, UCl3, and CeCl3 with minimal baseline interference. A potential hold at −1.5 V for 100 s was effective at cleaning the working electrode in between each SWV scan, which enabled generating reproducible (with ±1.10% average error) peak current results for CeCl3 over a period of 2 h. The peak current density for CeCl3 increased by 8.3% from doubling the U concentration from 5 to 10 wt%, thus indicating that this method is promising for measuring minor metal ion concentrations in molten salt reactor fuel with high U concentrations.

LiCl-KCl eutectic molten salt as an original and efficient medium to intercalate metals into graphite: Case of europium

We present the successful intercalation of europium into graphite by using an original method based on LiCl-KCl molten salts mixture. After solubilization of europium in the liquid chlorides mixture at 450 °C, reactivity toward graphite is investigated. We evidence the formation of the first stage EuC6 Graphite Intercalation Compound (GIC) with a repeat distance Ic = 487 pm. Using 00l X-ray diffraction and ion beam analysis, we show that europium is intercalated into the bulk with remarkable degree of homogeneity. This is also confirmed by magnetic measurements that clearly put in evidence the antiferromagnetic transition at about TN = 40 K. Below such temperature the characteristic step-like behavior of metamagnetic transitions was found by isothermal magnetization measurements, that is generally visible only in highly pure samples.

Comparison of Exchange Current Density Acquisition Methods for LaCl3 in Molten LiCl-KCl Eutectic Salt

The exchange current density values (i0) of the La/La3+ in LiCl-KCl eutectic salts at different temperatures (723–798 K) and concentrations (0.229–0.597 mol%) were measured by using cyclic voltammetry (CV), linear polarization (LP), and Tafel methods. The Tafel i0 values had the largest relative error on an average of 27%, while the LP values, found at the same scan rates, had a lower relative error, ranging from 10–17%. The LP and Tafel results were found to be highly affected by an unstable surface area. The CV i0 values were less affected by this issue due to higher scan rates, mainly 50–200 mV s−1. The relative error of CV was the least among the three, averaging 6.5–11%. Linear correlations for i0 versus temperature and concentration were developed from these results.

Electrochemical measurements of diffusion coefficients and activity coefficients for MnCl2 in molten eutectic LiCl-KCl

An electrochemical study of manganese chloride in molten salt mixtures of eutectic LiCl-KCl was carried out using a variety of electrochemical methods in a high temperature cell including cyclic voltammetry (CV), chronopotentiometry (CP), chronoamperometry (CA), and open circuit potentiometry. Single step reduction from Mn2+ to Mn(0) was observed on both W and Mo working electrodes. Using a combination of these methods, measurements were made of activity coefficient and diffusion coefficient for MnCl2 in LiCl-KCl as a function of concentration (3.54 × 10−4 to 3.60 × 10−3 mol fraction of MnCl2) at 773K. From OCP measurements, values for activity coefficient varied from 0.014 to 0.0071. Diffusion coefficients varied with concentration and differed based on measurement method (CV, CA, or CP). Based on cyclic Mn(II) ranged from 1.1 to 2.8 × 10−5 cm2/s depending on concentration.

The role of correlations in the determination of the transport properties of LaCl3 in high temperature molten eutectic LiCl–KCl

Abstract It is important to develop an accurate assessment of fundamental data of lanthanides in high temperature molten salts to enable an efficient application of pyroprocessing. This requires a careful consideration of uncertainties in the reported results. In this study, cyclic voltammetry (CV) tests of LaCl3 in KCl–LiCl molten salt were conducted at low concentration levels in the molten salt at 723 K and at several scan rates. The CV signals were subsequently analyzed through the conventional CV analysis and using a BET-based model through a nonlinear least-squares fitting procedure. It was determined that the parameters of the model were strongly correlated and the support plane procedure was implemented to assign joint confidence intervals for the diffusivity of lanthanum. Accounting for the correlations led to a significant increase in the uncertainty of the reported diffusivity which led to better agreement with the literature. Accounting for the correlations may be important for higher concentration levels.

The thermodynamic and transport properties of GdCl3 in molten eutectic LiCl-KCl derived from the analysis of cyclic voltammetry signals

Pyroprocessing technology is a promising tool for recycling nuclear fuel and producing high purity gadolinium for industrial applications. An efficient implementation of pyroprocessing entails a careful characterization of the electrochemical and transport properties of lanthanides in high temperature molten salts. In this work, the cyclic voltammetry signals of Gd in molten LiCl-KCl salt were recorded for a combination of three temperatures (723 K, 773 K, and 823 K) and three concentration levels (3 wt. %, 6 wt. %, and 9 wt. %) including concentration levels higher than previously reported and relevant for a realistic application of pyroprocessing for molten salt recycle, and the concentration effects were investigated. Four scan rates (200 mV/s to 500 mV/s) were used for each condition, and the signals were examined using conventional Cyclic Voltammetry (CV) analysis equations and by utilizing a two-plate Brunauer, Emmett, and Teller (BET) model accounting for mass diffusion, kinetics, adsorption, and the evolution of electrode morphology via a nonlinear least squares procedure for fitting the model to the experimental signals. It was determined that the redox process is quasi-reversible for the scan rates being used. Furthermore, the applicability of the conventional equations for CV analysis was shown to be problematic for the conditions used, and this is thought to be due to the fact that these equations were derived under the assumption of reversible conditions. The model-derived values for diffusivity are consistent with the literature and are shown to decrease with increasing concentration. This may be due to increased interactions at higher concentration levels. It was also shown that the formal redox potential increased with a concentration and was slightly more positive on the covered electrode.

Preliminary Study of Optimization of Normal Pulse Voltammetry for Actinide Measurement in Molten Salt Electrorefiners Using LiCl-KCl-UCl3-MgCl2 as a Surrogate Salt

Normal pulse voltammetry has been applied and optimized to develop correlations between diffusional current and concentration for UCl3 and MgCl2 in molten eutectic LiCl-KCl. This method was selected to mitigate working electrode area growth in concentrated molten salt mixtures in which a fairly wide potential range is needed to analyze all actinides. MgCl2 has been selected as a surrogate for PuCl3 based on similar standard reduction potential. Diffusional current plateaus were successfully attained by adjusting the step size to 0.03 V. Relaxation time of 15 seconds was found to be sufficient to recover after each pulse. A matrix of mixtures were prepared and tested with UCl3 concentration varying between 1 to 9 wt% and MgCl2 concentration varying between 0.3 and 1.5 wt%. The measured diffusional currents correlated well with concentration and matched a model that includes both diffusion and migration from the bulk molten salt to the working electrode surface. Concentrations of MgCl2 do not appear to affect the UCl3 diffusional current and vice versa. This is, thus, a promising method for simultaneous analysis of UCl3 and PuCl3 in molten salt electrolytes used for spent nuclear fuel electrorefining.

Exchange Current Density and Diffusion Layer Thickness in Molten LiCl-KCl Eutectic: A Modeling Perspective for Pyroprocessing of Metal Fuels

The DIFAC (DIFfusion of Actinides in EleCtrorefiner) computer code for pyroprocessing, developed earlier by the authors, is modified in the present work to model electrorefining at the liquid cadmium electrode. The modeling of electrorefining of metal fuels requires accurate knowledge of two important kinetic parameters: exchange current density io and diffusion layer thickness δ. These are estimated in the present work by polarization methods and employing Tafel and Allen-Hickling analysis for Gd3+/Gd, U3+/U, and Zr2+/Zr couples in LiCl-KCl eutectic at 773 K for an inert cathode and compared with literature data, wherever possible. The equilibrium potentials for these couples at an inert electrode are found to be –1.94, –1.52, and –1.22 V, respectively, at 773 K. Electrochemical studies are also carried out in LiCl-KCl eutectic to estimate io and δ for the anodic dissolution of Na-bonded U-Zr and Gd-U-Zr alloy and are compared with the anodic dissolution of U-Pu-Zr alloy. The equilibrium potential of Na-bonded U-Zr alloy in LiCl-KCl-UCl3 was found to be –1.46 V, and those for Gd-U-Zr alloy in blank LiCl-KCl and LiCl-KCl-UCl3 were –1.56 and –1.34 V, respectively, at 773 K. The exchange current densities of Na-bonded U-Zr and Gd-U-Zr alloy were found to be in the range of 40.1 to 46.5 mA · cm−2 and 16.8 to 27.3 mA · cm−2 at 773 K, respectively.A preliminary design of the liquid cadmium electrode suitable for laboratory-scale experiments on uranium- and plutonium-based systems is also reported in the present work. The io and δ of gadolinium, uranium, and zirconium are subsequently estimated at the liquid cadmium electrode at 773 K. The equilibrium potentials for Gd3+/Cd6Gd, U3+/[U]Cd, and Zr2+/Cd3Zr couples in LiCl-KCl eutectic at 773 K for the liquid cadmium electrode are found to be –1.35, –1.13, and −1.12 V, respectively. Finally, a few algorithms are proposed for modeling electrorefining data at the liquid cadmium electrode for multicomponent systems.

Electrochemical Behavior of Samarium in Molten LiCl-KCl

The mass transport behavior of samarium was investigated in molten eutectic LiCl-KCl in order to develop real-time concentration monitoring capabilities for the pyrochemical reprocessing of used nuclear fuel. Pyrochemical reprocessing relies on electrolysis in molten LiCl-KCl to recycle fissile material from used nuclear fuel. Without the capabilities to monitor concentrations of various actinides and fission products, pyrochemical reprocessing cannot be commercialized due to strict material accountability requirements set by the International Atomic Energy Agency (IAEA) [1]. Therefore, physical electrochemical properties of these species in molten salts is extremely important. Cyclic voltammetry was utilized to investigate electrochemical behavior of samarium in LiCl-KCl. By establishing valid relationships between these parameters and the peak current, the diffusion coefficient of samarium in our system can be reported with confidence [2]. Experiments were conducted in a high-purity argon atmosphere glove box using a three-electrode cell contained in an alumina crucible inside a resistive furnace. A high-resolution translation stage equipped with ceramic electrode mounts was used to accurately control electrode positions inside the melt. Tungsten was used as the counter and working electrodes, and a silver wire was used as a quasi-reference (Ag|AgCl2). The variation in peak current that resulted from changing the concentration, scan rate and electrode surface area will be reported, along with calculated diffusion coefficient values for samarium in molten LiCl-KCl at 500°C. The effects of using molybdenum as a working electrode will also be reported and compared to results obtained using tungsten. A comparison between reported diffusion coefficient values obtained by separate research organizations and results obtained in these studies will also be discussed. Acknowledgements: This work was performed under the auspices of the Department of Energy (DOE) under contracts DE-NE0008262 and DE-NE0008236 as well as the US Nuclear Regulatory Commission (USNRC) under contracts NRCHQ-11-G-38-0039. Dr. Kenny Osborne serves as the program manager for the DOE award and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC awards.

Effect of H2O on Corrosion of Ni in Molten Eutectic LiCl-KCl

In the absence of water and oxygen, it is known that molten chloride salts are relatively non-corrosive to a wide range of metals. Corrosion rates are expected to increase when water is present, however. To understand the threshold at which very small concentrations of water can cause corrosion of metals such as nickel and iron, a series of electrochemical measurements were made in molten eutectic LiCl-KCl and LiCl-KCl-UCl3. The presence of UCl3 is relevant for the use of this electrolyte in nuclear fuel electrorefining systems. Such systems are normally operated in inert atmosphere glove boxes or hot cells, where water concentration ranges from 1 to 50 ppm. In this study, argon gas containing up to 900 ppm H2O was bubbled into molten LiCl-KCl or LiCl-KCl-UCl3. Either a nickel or iron rod was inserted into the salt along with a Ag/AgCl reference electrode encased in a close-ended ceramic tube. The open circuit potential (OCP) was measured between the metal rod and the reference electrode as the gaseous Ar+H2O was bubbled into the salt. The temperature was held constant at 773 K. This is expected to result in formation of HCl, which will corrode the metal and cause a metal chloride to form and partition into the salt phase. Based on the Nernst Equation, this should cause a positive rise in the OCP. Based on changing OCP as a function of time, the rates of corrosion were compared for different concentrations of H2O in the gas stream. Cyclic voltammetry scans and samples of the salt were taken to verify the concentration of nickel and iron chloride in the salt at the end of each experiment. Noticeable corrosion was achieved in a time frame of 4 hours with water concentration as low as 120 ppm. This result demonstrates the importance of achieving a very high level of dryness in the salt prior to heating and melting it. Water concentration in the contacting atmosphere should also be well below 100 ppm to ensure long-term stability of the steels and similar metal alloys in molten LiCl-KCl or LiCl-KCl-UCl3. Implications and strategies for long term operation of electrorefiners using LiCl-KCl will be discussed.

Estimation of key physical properties for LaCl3 in molten eutectic LiCl–KCl by fitting cyclic voltammetry data to a BET-based electrode reaction kinetics model

Understanding the electrochemical properties of rare earth elements is important for developing efficient techniques for separating rare earth elements from actinides recovered during the electrodeposition process. In this study the cyclic voltammetry for lanthanum in molten LiClKCl eutectic was recorded at 773 K for different scan rates and different bulk concentrations. A model accounting for mass transport, kinetics and adsorption was applied to analyze the experimental data via performing a nonlinear least squares fit. The results of the simulation are compared against the results of a conventional analysis of the cyclic voltammograms and against the existing literature. At the scan rates used, the reduction/oxidation process is quasi-reversible. The values of diffusivities derived from simulation were larger than the ones derived commonly using equations for diffusion-limited processes. However, those equations were derived based on an assumption of reversibility. This simulation-based approach may provide a more accurate option for analyzing systems that do not exhibit reversibility.

First principles study of the thermodynamic and kinetic properties of U in an electrorefining system using molybdenum cathode and LiCl-KCl eutectic molten salt

Using first principles density functional theory (DFT) calculations we obtain thermodynamic and kinetic properties of U in an electrorefining process for spent nuclear fuels using a LiCl-KCl eutectic molten salt and Mo as a cathode. The thermodynamic stability of electrodeposited U from the molten salt onto the Mo(110) surface electrode is evaluated by activity coefficients as function of surface coverages of U and Cl. Additionally, ab-initio molecular dynamic simulations combined with the Stokes-Einstein-Sutherland relation enables us to calculate the viscosity of the LiCl-KCl eutectic molten salt. Our results well agree with previously reported experimental data endorsing the credibility. Based on our atomic-level mechanical understanding we propose that an accurate computational model system incorporating the electrochemical conditions of the electrorefining process essential for the purpose of establishing thermodynamic and kinetic database of U, otherwise critical deviations are inevitable. More interestingly, the effect of coadsorption of Cl with U on the Mo(110) surface plays a key role in stabilizing electrodeposited U on the cathode. Our approach can be useful for validating published experimental database and for identifying key factors guiding a rational design of highly efficient electrorefining system for spent nuclear fuels, and thus reducing high-level radioactive nuclear wastes.

Electroanalytical Measurements of Binary-Analyte Mixtures in Molten LiCl-KCl Eutectic: Gadolinium(III)- and Lanthanum(III)-Chloride

Electroanalytical measurements, such as cyclic voltammetry and chronopotentiometry, have been applied by other researchers to many eutectic LiCl-KCl mixtures containing a single analyte, typically a rare earth or actinide ion. These measurements have demonstrated the applicability of electroanalytical measurements of concentration in molten LiCl-KCl eutectic, which is prevalently used as an electrolyte in the electrorefining of used nuclear fuel (UNF). However, UNF electrorefiners contain multiple actinide and rare earth ions, necessitating the ability to make electroanalytical measurements in multi-analyte mixtures. The presence of multiple analytes creates interferences and overlapping signals. Chronoamperometry, cyclic voltammetry (CV), chronopotentiometry and open-circuit potential measurements were performed in eutectic LiCl-KCl mixtures containing a range of GdCl3 and LaCl3 concentrations to explore the issues associated with applying electroanalytical techniques in multi-analyte molten salt mixtures and to develop possible solutions. CV was found to be most promising for separating and analyzing each ion's signal. CV measurements were semi-differentiated and correlated to concentration using peak heights and principal component regression (PCR) to determine the concentration from electrochemical signals. The lowest average relative error was obtained using PCR for Gd3+ (6.21%) and using peak height correlations for La3+ (10.4%).

Effects of Process Conditions on the Fluidised Cathode Electrochemical Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic

The electrochemical reduction of metal oxides is important in the production of refractory metals. Conventional electroreduction routes have limitations related to oxide build-up near the surface of solid precursors, as well as operative electrolyte access to the inner parts of the precursors. The fluidised cathode process offers a promising substitute. The process is demonstrated on a WO3 and LiCl-KCl model system. Cyclicvoltammetry and chronoamperometry were used to investigate the process. Process conditions such as fluidisation rate and concentration of particles in the melt, and their effects, are investigated.

Effects of Process Conditions on the Fluidised Cathode Electrochemical Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic

The direct electrochemical reduction of titanium oxide to titanium metal has been evinced by the FFC Cambridge process. [1] Numerous studies have been carried out to assess the performance and applicability of the process to the reduction of titanium, other refractory metals and metal alloys. However, there are some limitations to the FFC Cambridge process. For example, at least in the case of titanium, the current efficiency is quite low (10-40% to achieve 0.3% oxygen content in the final titanium product). [2]There are two main reasons that hinder the reduction process: oxide product build-up in the electrolyte close to the surface and within the pores of the precursor, resulting in anomalous electro-migration of the oxygen vacancies, creating a barrier to the reduction process; and limited diffusion of the electrolyte within the precursor porous matrix. [3]Thus, innovative new designs of the process are needed. The fluidised cathode process [4] offers a solution to these problems. Figure 1 is a schematic of the electrolytic cell used. The process is potentially applicable to the range of metals and molten salts systems and has been exemplified for the reduction of WO3 in LiCl-KCl eutectic. However, there is still much to be investigated to establish a full understanding and optimise the process. This paper discusses experimental findings of the effects the fluidisation rate and the void fraction have on the fluidised cathode applied to the Li-K-W-O-Cl system.Figure 2. is a chronoamperogram of the process at different rates of fluidisation (argon flow rate). Coulometric analyses have been carried out to assert the extent of its effect. Electrochemical impedance spectroscopy (EIS) studies have also been conducted to establish the effects of the fluidisation rate and the void fraction (concentration of WO3 in the melt) on the diffusion properties between the working and counter electrodes.Applications to more technologically relevant systems such as the electrochemical reduction of uranium oxide to uranium, as being investigated in the EPSRC REFINE consortium programme, are also discussed.

A Fluidised Cathode Process for the Electrochemical Reduction of Tungsten Oxide in A Molten LiCl-KCl Eutectic

The electrochemical reduction of metal oxides is important in the production of refractory metals. The conventional electroreduction routes have low efficiencies and limitations associated with oxide scaling on the surfaces of solid precursors, as well as issues related to effective electrolyte access. The ‘fluidised cathode’ process offers a promising substitute route to the direct electrochemical reduction. The process is demonstrated on a tungsten and LiCl-KCl model system. Cyclicvoltammetry was used to characterise the reduction electrochemistry and chronoamperometry showed the evolution of the electrodeposition process with time. The product samples were analysed using SEM and EDS. Preliminary particle coulometry measurements were also performed.

Modeling the Anodic Behavior of U, Zr, and U-Zr Alloy in Molten LiCl-KCl Eutectic

The DIFfusion of Actinides in EleCtrorefiner (DIFAC) computer code has been developed and is used to calculate the variation of the anode potential with time during constant current anodic dissolution of U, Zr, and U-Zr alloy in molten LiCl-KCl eutectic. A few algorithms are proposed within the framework of the DIFAC code for modeling the activation and concentration overpotentials during anodic dissolution. These algorithms are based on an iterative search procedure and would later be applied to modeling the electrorefining of a multicomponent metallic fuel system.