Molten CaCl2 Research Articles

Electrochemical Metallurgy in CaCl2-CaO Melts on the Basis of TiO2·RuO2 Inert Anode

Molten CaCl2 has been considered as significant electrolyte for novel electrochemical metallurgy processes due to high solubility of oxygen anions. Some novel processes based on molten-salt electrolysis of oxides in molten chlorides have been proposed, typically including FFC process and OS process. An inert anode capable of sustained oxygen evolution is crucial for a green and efficient electrochemical de-oxidation process. Herein we report the results of a promising inert anode candidate, who shows an excellent performance. Several techniques including anodic polarization, cyclic voltammetry, Tafel polarization and electrochemical impedance have been performed to evaluate the anode behavior of the TiO2·RuO2 anode, revealing that the anode is quite stable during anodic polarization and has a more positive corrosion electrode potential. An in situ analysis of anode gases using the mass spectra proved the capability of the sustained evolution of oxygen at the TiO2·RuO2 anode during electrolysis, meaning that this oxides anode could be a potential candidate as inert anode in calcium chloride–calcium oxide melts for green metallurgical processes, as it showed a nearly negligible solubility of ruthenium dioxide when immersed in the melt and a low rate of corrosion even after long time electrolysis.

Molten salt synthesis of La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) type hydrogen storage alloys

La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys were synthesized directly from sintered mixture of La2O3 + NiO + CoO in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200 °C for 2 h) converted the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic La2NiO4, LaNiO3, La3Ni2O6.5, La3Ni2O6.84 and La4Ni3O9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La2NiO4 was the main La–Ni–O phase to initiate the LaNi5 phase formation. The increase in the Co content of the sample was observed to delay the La2NiO4 formation and thus the reactions initiated by La2NiO4 reduction. LaOCl formed chemically was the last oxygen including phase remained in the alloy before obtaining the final alloy structure. The porous alloy structure was beneficial for higher hydrogen storage capacity and it was observed that La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys had promising discharge capacities changed between 223 mA h g−1 (LaNi5) and 325 mA h g−1 (La(Ni0.7Co0.3)5). This work clearly indicated that the electro-deoxidation was a very effective method in the synthesis of the hydrogen storage materials.

Formation of Si nanowires by the electrochemical reduction of porous Ni/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) were prepared by the electrochemical reduction of solid Ni/SiO2 blocks in molten CaCl2 at 1173K. The SiNWs have diameter distributions ranging from 80 to 350nm, and the nickel–silicon droplets are found on the tips of the nanowires. The growth mechanism of SiNWs was investigated, which confirmed that the nano-sized nickel–silicon droplets formed at the Ni/SiO2/CaCl2 three-phase interline. The droplets lead to the oriented growth of SiNWs. Formation of nano-sized nickel–silicon droplets suggests that this method could be a potential way to produce nano-sized metal silicides.

The Role of Granule Size on the Kinetics of Electrochemical Reduction of SiO2 Granules in Molten CaCl2

As a fundamental study to develop a new process for producing solar-grade silicon, the effect of granule size on the kinetics of the electrochemical reduction of SiO2 granules in molten CaCl2 was investigated. SiO2 granules with different size ranges were electrolyzed in molten CaCl2 at 1123 K (850 °C). The reduction kinetics was evaluated on the basis of the growth rate of the reduced Si layer and the behavior of the current during electrolysis. The results indicated that finer SiO2 granules are more favorable for a high reduction rate because the contact resistance between the bottom Si plate and the reduced Si particles is small and the diffusion of O2− ions in CaCl2 inside the porous Si shell is easy. Electrolysis using SiO2 granules less than 0.1 mm in size maintained a current density of no less than 0.4 A cm−2 within 20 minutes, indicating that the electrochemical reduction of fine SiO2 granules in molten CaCl2 has the potential of becoming a high-yield production process for solar-grade silicon.

Direct Extraction of Ti and Ti Alloy from Ti-Bearing Dust Slag in Molten CaCl2

Abstract Using process of solid oxygen-ion conducting membrane (SOM), titanium metal and its alloy can be prepared directly from Ti-bearing dust slag by immersing it in the molten CaCl2 at 1,100℃, which has been proposed by constant voltage of 3.5 V for 2–6 h. The dust slag was ball-milled and pressed into pellets, then employed as the cathode, while the liquid copper, which was saturated with graphite powder and encased in yttria-stabilized zirconia (YSZ) tube, acted as the anode. The effect of forming pressure and electrolytic time on products was analyzed. The results show that the content of titanium increased with electrolytic time and the characteristic morphology presents as granule. Ti–Fe alloy can be obtained from Ti–Fe residue by 6 h electrolysis. For titanium-rich residue, when the forming pressure of pellets decreased from 6 to 3 MPa, only electrolysis for more than 4 h can completely remove the oxygen, and pure titanium is obtained by 6 h electrolysis. Besides, there is an unprecedented finding that the porous cathode is conducive to the removal of impurity elements.

Electrochemical preparation of silicon nanowires from porous NiO/SiO2 blocks in molten CaCl2

Silicon nanowires (SiNWs) were produced by the electrochemical reduction of porous NiO/SiO2 blocks ([SiO2]/[NiO]=10M ratio) in molten CaCl2 at 1173K for the first time. NiO additives seed the growth of straight SiNWs. The produced SiNWs have diameter distributions ranging from 60 to 330nm and outer amorphous layers with thickness about 2.5–3.1nm. The length of the nanowire can be as long as 30µm. The high length-to-diameter suggests that silicon is incorporated through the tip of the nanowire. Nano-sized nickel silicide particles are observed in the body of a zigzag SiNW. The results indicate that the electrochemical reduction process has the potential to produce nano-sized Si-based composite and metal silicide.

Formation of Si Nanowires By Electroreduction of Porous Ni/SiO2 Blocks in Molten CaCl2

Electroreduction of porous SiO2 in molten CaCl2 is a promising approach to prepare silicon nanowires (SiNWs) in large quantities. However, it is difficult to control the morphologies and surface topologies of the produced SiNWs. In this work, pure nickel metal powders are added into SiO2, and SiNWs with straight morphology are prepared by electroreduction of solid Ni/SiO2 blocks in molten CaCl2 in a wide temperature range. The prepared SiNWs have a diameter distribution of 80 to 350nm, and nickel silicide is found at the tip of the nanowire. Cyclic voltammograms indicate that nickel does not change the mechanism of electrochemical reduction of SiO2. The growth process of SiNWs is investigated by XRD, FESEM, HRTEM, EDS, and SAED. It is confirmed that nano-sized nickel-silicon droplets formed at the Ni/SiO2/CaCl2 three-phase interface lead the growth of SiNWs, and adding nickel metal into SiO2 can promote the growth of straight nanowires during the electroreduction of Ni/SiO2 in molten CaCl2.

Preparation of niobium carbide powder by electrochemical reduction in molten salt

The niobium carbide powder was prepared via electrochemical reduction of the mixture of Nb2O5 and carbon in molten CaCl2–NaCl. The reaction pathway from the sintered precursor to the final product has been investigated. The effect of the working temperature on the reduction of the Nb2O5/C composite precursor was considered. The role of carbon during the electrochemical reduction of the composite pellet was discussed. The samples were analysed by XRD and SEM. The results indicated that the NbC powder was approximately 200 nm after the reduction. Nb2O5 was gradually reduced to Nb, and NbC was subsequently obtained by the reaction of carbon with Nb metal. In addition, Nb2O5 could spontaneously react with CaO in the melt to form a serious of calcium niobates. The participation of carbon was available for the efficiency of electro-reduction of Nb2O5.

Corrosion of high density graphite anodes during direct electrochemical de-oxidation of solid oxides in molten CaCl2 medium

Graphite anode used in the direct electrochemical de-oxidation of solid oxides in molten calcium chloride medium (FFC Cambridge process) degrades by formation of carbon oxides and erosion. The problem was studied by using graphite as anode in the electrochemical de-oxidation of powder compacted and sintered Nb2O5 pellet electrodes in molten CaCl2 at 1173K and at a constant voltage of 3.1V. The degradation of the graphite anodes, recovered from the electro-de-oxidation cells at predetermined intervals of time (2, 9, 22, 26, 35, 44 and 56h) during the long-duration electrolysis, was determined by mass loss measurements and the morphological changes by SEM imaging. The graphite rods were found corroded differently along the lengths, with the maximum corrosion being at the melt-immersed surface of the electrode close to the melt–gas interface. The prominent carbon consuming reaction C+2O2−→CO2+4e− apart, the impregnation of salt into the defects of the graphite anode and convection currents existing in the melt appeared to enhance the corrosion and hence the mass loss of graphite towards the later stages of electrolysis.

Preparation of TiC/SiC composites from Ti-enriched slag by an electrochemical process in molten salts

TiC/SiC composites were successfully prepared from Ti-enriched slag via an electro-deoxidization process in molten CaCl2–NaCl eutectic melt at 700°C in argon. Electrolysis was conducted at an applied voltage of 3.2V using a graphite anode and a composite cathode composed of graphite powder and Ti-enriched slag. According to the results, a possible reaction pathway was suggested: firstly the complex Ti-compounds were decomposed into TiO2-based simple oxides; then a mass of CaTiO3 and a spot of CaSiO3 formed accompanied by the reduction of titanium dioxide to titanium sub-oxide such as TiO, Ti2O; finally the graphite powder involved in the formation of SiC, TiC1−xOx, which were further converted into SiC and TiC by prolonging electrolysis.

Production of nano-sized chromium carbide powders from Cr2O3/C precursors by direct electrochemical reduction in molten calcium chloride

Chromium carbides were synthesized by direct electrolysis of non-sintering Cr2O3/C precursors in CaCl2 molten at 1123K. A series of electrolysis experiments were conducted with different Cr2O3/C molar ratios, cell voltages and electrolysis time. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques, respectively. The results showed that the single-phase Cr7C3 powders with a mean diameter of <200nm were prepared after electrolysis at 2.8V for 5h in 1123K CaCl2 melt. By prolonging the electrolysis time to 10h, the product was converted into a Cr7C3 and Cr3C2 composite. Carbon was not only used as a raw material, but could also accelerate the rate of electrochemical reduction of Cr2O3. The reaction procedure forming the chromium carbides involved the electro-deoxidation of Cr2O3 and combined with carbon simultaneously.

Synthesis of La2Ni7 hydrogen storage alloy by the electro-deoxidation technique

La2Ni7 alloy was synthesized in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. Sintering at 1200 °C for 3 h was determined as the optimum condition to have the electrode pellet with enough porosity and mechanical strength. The hydroscopic La2O3 disappeared and the non-hydroscopic La2NiO4 formed during the sintering. The X-ray diffraction peaks indicated that the sinter product La2NiO4 and NiO reduced to LaNi5 and Ni, respectively, within 4 h electro-deoxidation process. The reduction kinetics of LaOCl was relatively slow. This sluggish reduction caused formation of the target La2Ni7 phase only after 6 h electro-deoxidation. The final La2Ni7 alloy structure with small amount of retained LaNi5 was obtained after 10 h electro-deoxidation and this structure did not change up to 25 h. It was observed that the synthesized alloy had maximum discharge capacity of 207 mA h g−1 and the alloy kept more than 90% of its discharge capacity within 20 charge/discharge cycles. According to the gathered EIS data La2Ni7 alloy was estimated to have no considerable surface barrier layer after discharging to hinder the atomic hydrogen diffusion on its surface.

Novel Synthesis of High Pure Titanium Trichloride in Molten CaCl2

Novel Synthesis of High Pure Titanium Trichloride in Molten CaCl2

Synthesis of La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) Hydrogen Storage Alloys by the Electro-Deoxidation Technique

La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) alloys were synthesized directly from sintered mixture of La2O3 + NiO + CoO in the molten CaCl2 electrolyte by the electro-deoxidation method at 850°C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200°C for 3 h) converted the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic LaNiO3, La3Ni2O6.5 and La4Ni3O9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La2NiO4 was the main La-Ni-O phase formed as a result of partial reduction or dissociation reaction to initiate the LaNi5 phase formation. The target La2Ni7 phase formed much later than LaNi5 phase probably due to the sluggish reduction of LaOCl. It was observed that La2(Ni1-xCox)7 (x = 0.05, 0.1, 0.2) alloys had promising discharge capacities changed between 227 mA h g−1 (La2(Ni0.95Co0.05)7) and 332 mA h g−1 (La2(Ni0.8Co0.2)7) depending on the alloy Co content. This work clearly indicated that the electro-deoxidation was very effective method in the synthesis of the hydrogen storage materials.

Direct electrochemical synthesis of zirconium carbide from zirconia/C precursors in molten calcium chloride

ZrC was successfully synthesized by direct electrolysis of non-sintering ZrO2/C mixture in molten CaCl2 electrolyte. The process of the electrochemical solid state reduction was investigated by characterizing the composition and microstructure of the products under different experimental conditions, in conjunction with analyzing the voltammetric features of initial powders in molten CaCl2. The results showed that the pure ZrC powder could be obtained by direct electrolysis of non-sintering ZrO2/C mixture in 1123K CaCl2 melt at 3.1V for 7h when carbon content in mixture was over stoichiometric ratio of ZrC. Besides the role of carbon source, carbon also promoted the electrochemical reduction of ZrO2. The reaction process involved the electrochemical reduction of ZrO2 in the presence of carbon to form ZrC with CaZrO3 as intermediate.

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl2 salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl2 or CaCl2–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO2−x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.

Electrochemical deoxidation of ZrSiO4 in molten calcium chloride

The electrochemical deoxidation of ZrSiO4 pellets in molten CaCl2 at 850°C in an argon atmosphere was studied. The sintered ZrSiO4 pellet was cathodically polarized against a graphite counter electrode under a constant applied potential. XRD and EDX analyses revealed that ZrSiO4 became ZrSi. Reduction from ZrSiO4 to ZrSi started at the location on the surface around the cathodic current-collector Mo wire and extended to the entire surface. As the electro-deoxidation reaction continued, the oxygen content in the product decreased.

Niobium powder synthesized by calciothermic reduction of niobium hydroxide for use in capacitors

Metallic niobium powder was produced for applications in electric capacitors via calciothermic reduction of niobium hydroxide in molten CaCl2. Sub-micrometer spherical metallic particles with coral-like morphologies reflected the particle size of the starting oxide powder. A fine powder was obtained from the mixtures of niobium hydroxide and CaO or Ca(OH)2, respectively.Sintered pellets of the metallic powder showed a higher capacitance (CV) than those of the simply reduced powder without pre-treatment, because the shrinkage during sintering was smaller. The CV was as large as that of commercially sintered pellets for tantalum capacitors. Therefore, this niobium powder would act as a higher-voltage capacitor by applying chemical anodic treatment at higher voltages, and lower oxygen content in the reduced power could realize a lower leak current.

Influences of graphite anode area on electrolysis of solid metal oxides in molten salts

Laboratory studies of electrochemical reduction of refractory metal oxides, e.g. TiO2 and Ta2O5, in molten CaCl2 often involve a graphite anode and a cell voltage of 3.0 V or higher, which deviates significantly from thermodynamic predictions. The causes considered in the past have included mechanistic, kinetic and dynamic complications of cathode reactions, but little was considered on anodic processes. This paper shows that oxidation of the O2− ion on the graphite anode is also a significant contributor to the high cell voltages applied. Cyclic voltammetry in molten CaCl2 containing added CaO (up to 2.51 mol%) suggested that O2− oxidation on graphite proceeds dominantly in two steps as previously observed on glassy carbon. With increasing CaO concentration, the second step became rate-limiting over a wide range of potentials before the processes reached at diffusion controlled high current density. This understanding led to the proposal and experimental confirmation of a “low anode current density strategy” in potentiostatic reduction of thin cylindrical pellets of TiO2 and Ta2O5 in molten CaCl2 at 850 °C. It was observed that a 10-fold increase of the graphite anode area could decrease the cell voltage by about 1.0 V, which should save energy consumption by up to one third.

Molten Salt Assisted Electrochemical Separation of Spent Fuel Surrogates by Partial Direct Reduction and Selective Anodic Dissolution

The remediation of spent nuclear fuels is increasingly seen as a requisite for sustainable nuclear power generation. As of yet, very few nations have navigated the social, technical and economic difficulties of installing operational geological repositories [1], despite the legacy of waste accumulation becoming ever more present. Current aqueous reprocessing technology (e.g. PUREX) offers a viable solution to the partitioning of neutron poisoning species (typically highly radiotoxic fission products (FPs) and minor actinides (MAs)), allowing the reuse of approximately 97% of spent fuel species [2]. However significant concern over the proliferation risk of a non-discriminatory separation process (one where plutonium can be selectively produced) coupled with the perceived economic costs of such technology has hindered wider acceptance of reprocessing. Pyroprocessing, involving a precise and controlled means of solid/liquid phase species separation in molten salts, can provide an appealing alternative.A current UK based research consortium (REFINE) aims to deliver the materials science for a sustainable pyroprocessing mechanism in harmony with the next generation of nuclear reactors (Gen III+, Gen IV). Work at the University of Nottingham is focused on exploiting the FFC Cambridge Process [3], renowned for the direct electro-reduction of metal oxides such as TiO2 or Cr2O3 [4, 5], as a means for a controlled selective direct reduction and anodic dissolution of spent fuel species.In this novel approach to partitioning using potentiostatic electrolysis in a three electrode cell, the mixed oxide preforms (spent fuel surrogates) are pressed with 2 tonnes uniaxial pressure and sintered for 2 hours at 1173K to form a palletised electrode which is then selectively reduced in a molten CaCl2 electrolyte (at 1083K). By reversing electrode polarisation, any formed metallic phases are selectively separated via anodic dissolution and electroplated to form a pure (or alloyed) metallic deposit on the counter electrode for recovery. Feasibility was first assessed by a 2NiO-CeO2 mixed oxide pellet (Ni2Ce molar ratio) because of the large difference in redox potentials of NiO and CeO2, the relevant working electrode reactions given as:2NiO-CeO2 + 4e- = 2Ni-CeO2 + 2O2- (1)2Ni-CeO2 = 2Ni2+ + CeO2 + 4e- (2)CO/CO2 is produced from a consumable graphite counter electrode. This process was shown to be successful in the partitioning of the produced nodular Ni from 2NiO-CeO2 using a reduction potential of +1.53 V (vs. Ca/Ca2+) and then an oxidation potential of +2.19V vs. Ca/Ca2+; metallic phases and potentials emulate those shown elsewhere in the literature [6, 7]. Surface atomic presence of Ni was lowered to 8.26% from 29.85%, as examined by EDX and morphology by SEM (figure 1). Intermediate species formation (i.e. CeOCl [CeIV to CeIII]) observed in some early experiments, and which is parasitic to the selective partial reduction mechanism, could be avoided by precise potential control.More representative electrochemical surrogate materials (ZrO2 for UO2, CeO2 for PuO2 and Gd2O3 for various MAs) were then selected for use in a series of concept experiments. In this paradigm, the benefits of solid phase pyroreprocessing can be assessed on the ability to separate U and Pu from MAs, seen as an integral requirement for non-proliferation and hence more global acceptance of the technology. Cyclic voltammetry was used to establish the hierarchy of reduction between oxide components and what, if any, species interactions exist during electrochemical treatment. Results herein will determine the applicability of a multifaceted, direct and proliferation resistant process for current and future nuclear fuel recycling.