Molten Salt Electrorefining Research Articles

Rational design of pulse-electrolysis protocols promotes molten-salt electrorefining

Molten-salt electrorefining (MSE) is currently the primary method for purifying many elements, offering promise for the sustainable upcycling of spent metals in the future. However, the conventional constant-current electrolysis protocol inevitably leads to deterioration of the microenvironment near the electrode, leading to reaction runaway and process inefficiency. In this work, we comprehensively studied for the first time the effect of constant- and pulse-electrolysis protocols on environmental evolution in electrode–electrolyte interfaces during the MSE of Ti, using a computer model. The results showed that compared to constant electrolysis, pulse-electrolysis protocols could effectively regulate the interfacial microenvironment and improve the electrorefining efficiency. Furthermore, a correlation between interfacial reconstruction and pulse parameters was established, indicating that the duty ratio was the key factor in inhibiting the deterioration of the interfacial microenvironment. To further verify the simulation results, the real-time morphological evolution of the electrodes during electrolysis was monitored by a novel in situ X-ray computed tomography (CT) technology under pulse-electrolysis protocols. The experimental results indicated that the pulse-electrolysis protocols enhanced the interfacial microenvironment and promoted electrolysis efficiency. Collectively, this work demonstrated that MSE could be effectively advanced by rationally designing the pulse-electrolysis protocols, facilitating its large-scale application in industry.

Electrochemical investigations on cathodic reduction processes at graphite electrode in LiCl-KCl eutectic melt

Sequence of redox processes in LiCl-KCl eutectic at graphite electrode was studied in temperature range 698-798 K using cyclic voltammetry and electrochemical impedance spectroscopy. There were two reduction steps observed when potential was scanned from −0.25 to −1.50 V (vs.▪); a single-step two-electron transfer process due to reduction of carbon to ▪ followed by reduction of ▪. Electrolysis at −1.20 V at 773 K led to formation of ▪ on graphite surface. Complex impedance data were modelled using equivalent circuits and it was established that ▪ film formed around graphite electrode restricting both diffusion of ▪ in melt and metallic ▪ into graphite framework. These results are relevant in terms of their consequences in molten salt electrorefining of metal fuels.

Upcycling of Titanium by Molten Salt Electrorefining

Upcycling of Titanium by Molten Salt Electrorefining

Mechano-electrochemical phase field modeling for formation and modulation of dendritic Pattern: Application to uranium recovery from spent nuclear fuel

Dendrite formation is a critical issue in uranium recovery from spent nuclear fuel (SNF) through a molten-salt electrorefining process. To understand and modulate uranium dendritic formation, we developed a computation model that involves all the complexities in the mechano-electrochemical process, such as diffusion–reaction kinetics, interfacial anisotropy and the variations of electric and stress fields. In particular, the lattice mismatch between deposit and substrate is considered which addressed the importance of cathode material. The model explains various morphologies of dendrites, which in a two-dimensional scenario can be demarcated based on the perimeter-to-area ratio, χ/S. Dendrites can be needle-like, tooth-like, or tree-like when χ/S < 2 mm−1, 2 mm−1 ≤ χ/S < 6 mm−1, and χ/S ≥ 6 mm−1, respectively. With these conditions, the parameter maps for modulating dendritic patterns are drawn to elucidate the effects of interfacial anisotropy, nuclei site geometry, diffusivity, electric and stress fields, which can be employed to design a molten-salt electroplating process to minimize failures caused by dendrite formation.

Multi-potential electropurification of a reusable CaCl2-CaF2 eutectic salt for solar-grade Si electrorefining

Molten-salt electrorefining is a promising technique for energy-efficient production of ultrapure solar-grade Si. This work focuses on electropurification to remove impurities from an eutectic CaCl2-CaF2 molten salt for solar-grade Si. Both an as-received salt and a salt used for Si electrorefining have been electropurified by a field-enhanced process and the impurity concentrations before and after purification have been characterized by inductively-coupled plasma mass spectrometry and cyclic voltammetry. More importantly, a new multi-potential electropurification process, enabled by the three-electrode electrolyzer, has been proposed and proven to be more effective in removing all the impurities in the molten salt than a single-potential process. The salt after Si electrorefining has been regenerated to its original purity level before Si electrorefining by the multi-potential process, demonstrating the feasibility of a reusable salt by electropurification.

Electrochemistry of Hf(IV) in NaCl–KCl–NaF–K2HfF6 molten salts

The cathodic reduction mechanism of Hf(IV) ions in a fused NaCl–KCl–NaF–K2HfF6 salt system was studied in various NaF concentrations at 1073 K to obtain a purified dendritic Hf metal. The results of cyclic voltammetry and square wave voltammetry indicated that the reduction process comprised two steps of Hf(IV) → Hf(II) and Hf(II) → Hf at low NaF concentrations (0 < molar ratio of [F−Hf4+] ≤ 17.39) and one step of Hf(IV) → Hf at high NaF concentrations (17.39 < molar ratio of [F−/Hf4+] < 23.27). The structure and morphology of the deposits obtained in potentiostatic electrolysis in the one-step reduction process were analyzed and verified by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry. In the one-step reduction process, the disproportionation reaction between the Hf metal and Hf complex ions was inhibited, and a large dendrite Hf metal was achieved in molten salt electrorefining.

Computational model‐based design of molten salt electrorefining process for high‐purity zirconium metal recovery from spent nuclear fuel

Computational model‐based design of molten salt electrorefining process for high‐purity zirconium metal recovery from spent nuclear fuel

Material balance evaluation of pyroprocessing for minor actinide transmutation nitride fuel

ABSTRACT Fuel cycle technology for the transmutation of long-lived minor actinides (MAs) using an accelerator-driven system has been developed under the double-strata fuel cycle concept. A mononitride solid solution of MAs and plutonium diluted with zirconium nitride is a prime fuel candidate for transmutation of MAs. Pyroprocessing is suitable for recycling the residual MAs in spent nitride fuel with high radiation doses and decay heat. The spent nitride fuel is anodically dissolved, and the actinides are recovered simultaneously into a liquid cadmium cathode via molten salt electrorefining, which is the core step of the process. The process should be designed to achieve the target recovery yield of MAs and the acceptable impurity level of rare earth fission products in the recovered material. In this study, we evaluated the material balance during the pyroprocessing of spent nitride fuel to gain important insight on the design process. We examined the effects of changing the stage numbers in the countercurrent extraction and the zeolite treatment on material flow and quantity of waste. The results show that multistage extraction is necessary to meet the targets and the stage number of the zeolite treatment is recommended to be more than three from the aspect of the amounts of glass-bonded sodalite wastes.

Minimising oxygen contamination through a liquid copper-aided group IV metal production process

This paper demonstrates for the first time the fabrication of Zr-Cu alloy ingots from a Hf- free ZrO2 precursor in a molten CaCl2 medium to recover nuclear-grade Zr. The reduction of ZrO2 in the presence of CaO was accelerated by the formation of Ca metal in the intermediate stage of the process. Tests conducted with various amounts of ZrO2 indicate that the ZrO2 was reduced to the metallic form at low potentials applied at the cathode, and the main part of the zirconium was converted to a CuZr alloy with a different composition. The maximum oxygen content values in the CuZr alloy and Zr samples upon using liquid Cu were less than 300 and 891 ppm, respectively. However, Al contamination was observed in the CuZr during the electroreduction process. In order to solve the Al contamination problem, the fabrication process of CuZr was performed using the metallothermic reduction process, and the produced CuZr was used for electrorefining. The CuZr alloy was further purified by a molten salt electrorefining process to recover pure nuclear-grade Zr in a LiF-Ba2ZrF8-based molten salt, the latter of which was fabricated from a waste pickling acid of a Zr clad tube. After the electrorefining process, the recovered Zr metal was fabricated into nuclear-grade Zr buttons through arc melting following a salt distillation process. The results suggest that the removal of oxygen from the reduction product is a key reason for the use of a liquid CaCu reduction agent.

Transient modeling of spent nuclear fuel electrorefining with liquid metal electrode

During the molten salt electrorefining of spent nuclear fuel, multiple phases such as oxide, solid metal, liquid metal, and molten salt often co-exist. Computational modeling can be a useful tool for understanding the reaction mechanism across the multiple phases. The new model has been developed and applied to a lab-scale electrorefining with liquid metal anode and solid cathode LiCl-KCl molten salt. The benchmark study predicts anodic dissolution and cathodic deposition of U and Pu with minor disagreements. In particular, the on-set of Pu deposition on the surface of the solid cathode is well estimated, which is important for the quality of U ingot and the safeguards of process. The underestimation of U deposition (∼6%) and the overestimation of Pu dissolution (∼7%) at the end of simulation are explained by unconsidered reaction species such as Np and Am from the liquid Cd anode, which overestimates the dissolution of Pu from the anode compared to the measured data. The sensitivity study also reveals the transition behaviors of electrochemical reactions for U and Pu on the solid cathode are changed by diffusion boundary layer thickness, transfer coefficients, and the difference of electrochemical potentials more sensitively than those of the liquid metal anode. For this specific experiment case, the thinner diffusion boundary layer improves the prediction of cathodic reactions particularly at the end of electrorefining.

Modeling the Molten Salt Electrorefining Process for Spent Metal Fuel Using COMSOL

Molten salt electrorefining process is one of the key steps of the pyrochemical reprocessing flow sheet for the spent metallic fuel from fast reactors. The electrorefining process is simulated using COMSOL Multiphysics. This involves solving multiple equations corresponding to electrochemical reactions at the electrode surfaces, mass transfer of metal ions in the electrolyte, potential distribution in the electrolyte, and overall material balance of metal ions in a coupled manner. The model is validated using the data of laboratory scale electrorefining experiments from literature. The results show a good agreement with the present experimental data, the variation being less than 10% for the U and Pu concentration changes in liquid Cd anode and molten salt, and U deposit on solid cathode.

Morphology Change Of Si Deposit In Molten Salt Electrorefining

Abstract The effects of processing parameters on the morphology change in a Si deposit recovered by means of molten salt electrorefining are evaluated using electrochemical techniques such as cyclic voltammetry and chronopotentiometry at 800°C. It was found that concentration of K2SiF6 and current density were important parameters in determining deposit size. Higher concentrations of K2SiF6 were effective in coarsening the silicon deposit and decreasing the cell potential. Silicon nanofiber was recovered at 5 wt% of K2SiF6 whereas dense particles were prepared at 30 and 50 wt% of K2SiF6. The morphology of the Si deposit was determined by the concentration of Si in the electrolyte which is related to the formation of crystal and growth of Si. The formation mechanism of the Si deposit was interpreted by using high resolution TEM as well as electrochemical properties.

Novel Approach to Extracting Transuranic Elements in Molten Salt Electrorefining

A novel approach to extracting transuranic elements (TRUs) from molten salt into liquid Cd using U metal as a reductant was investigated for the molten salt electrorefining process. We considered two methods of adding U metal: direct extraction (DE) and electrochemical extraction (EE). In the DE method, U metal added to Cd is dissolved and exchanged for TRU ions in the salt. The EE method is based on the principle of a concentration cell. When U metal and Cd separately placed in the salt are electrically connected, the U metal is anodically dissolved in the salt, and U and TRU ions are reduced at the Cd. The advantages of these methods over the conventional electrolytic method are as follows: The container for Cd can be made of steel, dendritic U metal does not form on the surface of the Cd or the crucible, and the operation is simple and stable. It was experimentally demonstrated that Pu and Am could be extracted from LiCl-KCl melt into liquid Cd by both the DE and EE methods when U metal collected at the solid cathode was used as a reductant. Crucibles made of steel could be used as containers for Cd, and a total of ∼3 wt% of U, Pu, and Am in the Cd was collected in 10 h. In the EE tests, the separation factors among U, Pu, and Am were always equal to the values at equilibrium. The rate-determining step for the extraction was not the mass transfer in the Cd or salt phase but the electron transfer at the Cd-salt interface. Then, a concept high-performance electrorefiner equipped with two anode–solid cathode modules and an EE or DE module was preliminarily designed.

Effects of operating conditions on molten-salt electrorefining for zirconium recovery from irradiated Zircaloy-4 cladding of pressurized water reactor

To reduce the final waste volume from used nuclear fuel assembly, it is significant to decontaminate irradiated cladding. Electrorefining in high temperature molten salt could be one of volume decontamination processes for the cladding. This study examines the effect of operating conditions on decontamination factor in electrorefining of irradiated Zircaloy-4 cladding of pressurized water reactor. One-dimensional time-dependent electrochemical reaction code, REFIN, was utilized for simulating irradiated cladding electrorefining. Composition of irradiated Zircaloy was estimated based on ORIGEN-2 and other literatures. Co and U were considered in electrorefining simulation with major elements of Zircaloy-4 to represent activation products and actinides penetrating into the cladding respectively. Total 240 cases of electrorefining are simulated including 8 diffusion boundary layer thicknesses, 10 concentrations of contaminated molten salt and 3 termination conditions. Decontamination factors for each case were evaluated and it is revealed that the radioactivity of Co-60 in recovered zirconium on cathode could decrease below the clearance level when initial concentration of chlorides except ZrCl4 is lower than 1×10−11 weight fraction if electrorefining is finished before anode potential reaches −1.8V (vs. Cl2/Cl−).

다상유동 전산모사를 통한 공학 규모의 cathode processor의 성능평가

용융염 전해정련공정은 사용후핵연료로부터 전기화학적인 방법을 통해 음극에서 우라늄을 회수 하는 공정이다. 이 때 우라늄은 약 30wt%의 염을 포함하고 있어 순수한 우라늄을 얻기 위해서는 염을 제거하는 Cathode Process (CP)가 필수적이다. CP는 대량의 우라늄을 처리해야 하므로 파이로공정의 난관중의 하나로 인식되고 있으며, 우라늄의 순도가 최종적으로 결정되는 단계이므로 매우 중요한 공정이다. 현재, 이에 대한 연구는 주로 실험적 방법에 근거 하고 있어 염 제거 공정 중 온도, 압력, 염 가스의 거동을 관찰하기 어렵다. 따라서 본 연구에서는, 공정의 운전 조건에 대해 적합한 수학적 모델을 이용하여 전산모사 해석을 진행하였다. 본 연구는 증류부에서 염 가스의 증류 량, 확산계수에 의해 계산된 장치 내 염 가스의 이동 그리고 응축부에서의 응결속도를 중점적으로 연구하였다. 장치내의 각각의 염 가스 거동을 정의하기 위해 Hertz-Langmuir 관계식, Chapman-Enskog Theory, ANSYS-CFX의 상용 코드를 사용하였다. 그리고 HSC Chemistry에서 염의 물성 값을 이용하여 모델을 구성하였다. 본 연구의 전산모사 해석을 통해 얻은 연구 결과를 이용하여 염 가스의 거동과 장치의 최적 운전조건을 예측하였다. 따라서 본 해석 결과는 CP의 물리적 현상을 깊게 이해하는데 쓰일 뿐 아니라, 공학규모의 CP 장치를 상용규모로 확장하는데 이용 할 수 있다. Molten salt electrorefining process achieves uranium deposits at cathode using an electrochemical processing of spent nuclear fuel. In order to recover pure uranium from cathode deposit containing about 30wt% salt, the adhered salt should be removed by cathode process (CP). The CP has been regarded as one of the bottle-neck of the pyroprocess as the large amount of uranium is treated in this step and the operation parameters are crucial to determine the final purity of the product. Currently, related research activities are mainly based on experiments consequently it is hard to observe processing variables such as temperature, pressure and salt gas behavior during the operation of the cathode process. Hence, in this study operation procedure of cathode process is numerically described by using appropriate mathematical model. The key parameters of this research are the amount of evaporation at the distillation part, diffusion coefficient of gas phase salt in cathode processor and phase change rate at condensation part. Each of these conditions were composed by Hertz-Langmuir equation, Chapman-Enskog theory, and interphase mass flow application in ANSYS-CFX. And physical properties of salt were taken from the data base in HSC Chemistry. In this study, calculation results on the salt gas behavior and optimal operating condition are discussed. The numerical analysis results could be used to closely understand the physical phenomenon during CP and for further scale up to commercial level.

High-temperature distillation and consolidation of U–Zr cathode product from molten salt electrorefining of simulated metallic fuel

High-temperature distillation experiments were performed using U–Zr cathode products of various compositions to obtain knowledge on suitable operation conditions and equipment design such as the container material. The LiCl–KCl–UCl3 electrolyte adhering to the U–Zr cathode products was almost completely vaporized at 1273–1573K, under pressure of 10–300Pa. Massive ingots were obtained from the remaining cathode products by heating them at 1573–1673K. Three different phases were identified in a distillation product of a higher Zr content. A U-rich bulk (3.9wt% Zr) and a deposit of a relatively low Zr content (17.2wt% Zr) were considered to be formed during the cooling process of the distillation product. Another Zr-rich deposit (64.7wt% Zr), which might cause the inhomogeneity of product ingots, was expected to result from Zr-rich spots that originally existed in the cathode product. The Cl content in the cathode product was decreased by distillation to less than 1/200 of that after electrorefining, while it was markedly larger at a higher Zr concentration. To limit the amount of Zr-rich deposit and the Cl content, the amount of Zr in the distillation product should be controlled to a sufficiently low level by optimization of the operating procedures and conditions in the electrorefining and distillation steps. The zirconia coating material developed in this study showed superior performance in inhibiting reaction between the melted U–Zr alloy melt and the graphite crucible and also in the easy release of the U–Zr ingot from the crucible.

Valence states, impurities and electrocrystallization behaviors during molten salt electrorefining for preparation of high-purity titanium powder from sponge titanium

High-purity titanium powder was prepared by molten salt electrorefining from sponge titanium in NaCl–KCl–TiClx salts. The titanium valence, purity and electrocrystallization during electrolysis process were studied. The XPS analysis showed that the titanium valences are mainly +4, +3 and +2 at the earlier, medium and later stages of electrolysis, respectively. During the electrolysis process, the contents of impurities Si, Cr, Mn, Al vary little, and the contents of impurities Fe, Cu, Ni decrease markedly, while the contents of impurities O, N, H increase obviously. The residual impurities are usually distributed in small tunnel of dendritic crystals. Enhancing the electrolysis temperature and prolonging the electrolysis time can increase the titanium particle size. The TEM analysis showed that the electrodeposited titanium is not a single crystal, but contains many nanostructured grains and subgrains, with grain size of 100–500 nm. The electrolysis mechanisms were also discussed.

Development of Treatment Process for Anode Residue from Molten Salt Electrorefining of Spent Metallic Fast Reactor Fuel

A new treatment process was proposed for the anode residue from a molten salt electrorefining step in the pyrometallurgical reprocessing of spent metallic fast reactor fuel. This treatment process consists of two steps: (a) oxidation of the remaining actinides in the anode residue by the addition of CdCl2 and (b) removal of the accompanying chloride by high-temperature distillation. The oxidation of the remaining uranium by CdCl2 was studied using anode residue from previous electrorefining experiments using U-Zr alloys. The reaction between uranium and CdCl2 was completed in [approximately]2 days with a satisfactory chlorine balance among the species in the molten chlorides solvent. A high uranium oxidation rate was attained by appropriately controlling the rate of CdCl2 addition. The high-temperature distillation tests were carried out at 1473 K with pressure of [approximately]300 Pa to remove the solvent accompanying the anode residue. The chloride content in the anode residue was lowered to 1% to 2.5% by the distillation operation. Although the anode residue was heated to 1673 to 1773 K at a pressure of [approximately]50 kPa after the distillation, it was not melted completely. The remaining ratio of uranium after the electrorefining and the above treatment process was evaluated to be 0.04% to 0.20%. Material flow calculations were performed for a pyrometallurgical reprocessing facility equipped with the anode residue treatment process. It showed that (a) the chlorine and uranium supply/demand balance is maintained unless the remaining ratio of uranium after electrorefining exceeds a certain value and (b) the addition of the anode residue treatment process does not have an adverse effect on either the performance of the overall process or the facility design.

Formation of high purity Si nanofiber from metallurgical grade Si by molten salt electrorefining

We studied the purification of metallurgical grade Si from solutions of K2SiF6 in fluoride melts using a molten salt electrorefining process at 700 °C. Electrorefining close to the deposition potential gave dense, coherent, and well-adherent deposits. It was shown that the deposition rate and microstructure of Si strongly depend on the process temperature. Deposited polycrystalline silicon has a well defined rod shape and crooked structure that varies with current density. The anodic dissolution rate is affected by the initial concentrations of K2SiF6 and the applied current density. The results of an inductively coupled plasma (ICP) analysis indicated that recovered silicon fiber deposits with purities greater than 99.98% can be obtained using the developed technique. The morphology of the electrodeposited silicon on silver substrates is discussed in the context of a cathodic reaction on the electrode surface, and a comprehensive explanation of the purification mechanism with salt removal is provided.

Development of Treatment Process for Anode Residue from Molten Salt Electrorefining of Spent Metallic Fast Reactor Fuel

Development of Treatment Process for Anode Residue from Molten Salt Electrorefining of Spent Metallic Fast Reactor Fuel