Molten CaCl2 Research Articles

Direct Electrolytic Reduction of Powdery SiO2 in Molten CaCl2 with Pellet-Type SiO2 Contacting Electrodes

Direct electrolytic reduction of powdery SiO2 has been investigated in molten CaCl2 at 1123 K aiming at a low-cost production of solar-grade Si. Pellet-type SiO2 contacting electrodes composed of a palletized SiO2 powder and a lead material of a Si rod or a Pt wire were newly contrived, and the impurity levels of the produced Si were evaluated. A two-electrode method was also applied, which considerably lowered impurity levels of metal elements in the produced Si. The concentrations of impurities contained in the original SiO2 powders, the prepared SiO2 pellets and the electrolyzed samples were traced with ICP-AES and GD-MS analyses. It was revealed that B and P concentrations were not significantly increased in the electrolysis. Another pellet-type SiO2 contacting electrode, in which three pellets composed of SiO2 and Si powders were attached to a Si rod, was also devised for the improvement of reduction rate.

Direct Deoxidation of ZrOCl<sub>2</sub> Powder to Zirconium With Non-Sintering

Direct electro-deoxidation of zirconium oxychloride powder without sintering in molten CaCl2 and NaCl mixture salt was studied. Molten CaCl2 and NaCl mixture salt as electrolyte, a little graphite crucible filled with zirconium oxychloride powder as novel cathode and a graphite rod as anode, deoxidation experiments were performed at 700 and 3.2V for 8h and 15h, respectively. Results show that using new material of zirconium oxychloride and the method of direct electro-deoxidation without sintering can be applied to the preparation of fine zirconium metal particles, which have a great advantage in cheap cost, short technical process, and energy saving.

Research on Removal of Sulfide Inclusion in Steel Surface by Molten Salt Electrolysis

Steel including sulfide inclusion was used as cathode, while high density graphite rod was used as anode. Sulfide inclusion in steel surface was removed by molten salt electrolysis in molten CaCl2 at 850 under the stationary voltage. The influences of the voltage and electrolytic time on the process of reaction were investigated. The distributions of the sulfide inclusion were investigated by SEM. The desulfurated ratio was investigated by quantitative metallographic analysis, Cyclic voltammetry was performed to characterize reactions of cathode. The results show that the voltage and the time of electrolysis were in direct proportion with desulfurated ratio. The higher voltage and the longer electrolysis time is, the greater the desulfurated ratio is.

Electrochemical decomposition of SiO2 pellets to form silicon in molten salts

▶ Increasing temperature or decreasing SiO 2 particle size increased the reduction rate. ▶ Addition of NaCl to the electrolyte decreased the reduction rate. ▶ Complete reduction of SiO 2 pellets was achieved after 16 h of electrolysis. ▶ The brown color of silicon may be the result of nanometer-scale crystallite size. ▶ Higher purity of Si may be obtained by advancement of cell component materials. Direct electrochemical reduction of porous SiO 2 pellets in molten CaCl 2 salt and CaCl 2 –NaCl salt mixture was investigated by applying 2.8 V potential. The study focused on the effects of temperature, particle size of SiO 2 powder starting material and the behavior of cathode contacting materials during electrochemical reduction process. The starting materials and the electrolysis products were characterized by X-ray diffraction analysis and scanning electron microscopy mainly. The studies showed that smaller particle sizes and higher temperatures had slightly positive effects in increasing the reduction rate within the ranges covered in this study. The results were interpreted from variations of current and accumulative electrical charge that passed through the cell as a function of duration of electrochemical reduction under different conditions. Microstructures and compositions of the reduced pellets were used to infer that electrochemical reduction of SiO 2 in molten salts may become a method to produce silicon that could be used in solar energy utilization. Furthermore, X-ray diffraction analysis results indicated that the silicon produced at the cathode reacts with contacting materials; nickel, and iron in stainless steel to form Ni–Si and Fe–Si compounds due to very reactive nature of silicon especially at high temperatures.

Electrolytic Reduction of Powdery SiO2 in Molten CaCl2 with Pellet-Type SiO2 Contacting Electrodes

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In situ synchrotron diffraction of the electrochemical reduction pathway of TiO 2

Despite over ten years of work into the low-cost electrowinning of Ti direct from the oxide, the reduction sequence of TiO 2 pellets in molten CaCl 2 has been the subject of debate, particularly as the reduction pathway has been inferred from ex situ studies. Here, for the first time white beam synchrotron X-ray diffraction is used to characterize the phases that form in situ during reduction and with ∼100 μm resolution. It is found that TiO 2 becomes sub-stoichiometric very early in reduction, facilitating the ionic conduction of O ions, that CaTiO 3 persists to nearly the end of the process and that, finally, CaO forms just before completion of the process. The method is quite generally applicable to the in situ study of industrial chemical processes. Implications for the industrial scale-up of this method for the low-cost production of Ti are drawn.

Direct Production of Ti–29Nb–13Ta–4.6Zr Biomedical Alloy from Oxide Mixture in Molten CaCl2

A process based on the principle of the Ono–Suzuki process involving electrolysis and the calciothermic coreduction of an oxide mixture consisting of , , , and for producing Ti–29Nb–23Ta–4.6Zr (TNTZ) was proposed. Ca was produced by the electrolysis of CaO in molten by applying a voltage of 3.2 V between a carbon anode and Ti cathode at 1173 K. The residual oxygen concentration in the obtained powder decreased monotonically with an increase in the supplied electric charge. An oxygen concentration of 0.19 mass % was attained by supplying the electrical charge around 3 times larger than that necessary to generate the stoichiometrically required amount of Ca. This sample consisted of a Ti-based body-centered cubic solid solution and a small amount of hexagonal close-packed (hcp) solid solution. The inhomogeneity in the composition was suppressed to a relatively low level so that it could be easily improved by heat-treatment. After sintering at 1300 K for 54 ks, for example, the Ti- and Zr-rich hcp phase disappeared and the composition achieved the targeted value. The results of this study indicate that the OS process can be applied to directly produce TNTZ alloy from an oxide mixture.

Effects of inert anodes in the FFC Cambridge reduction of hematite

The industrial production of iron and steel contributes to a significant share of the global CO2 emissions. Electrochemical methods using an inert oxygen evolving anode can eliminate the CO2 formation. The FFC Cambridge process approach was tried out for the reduction of hematite to produce iron at 915°C, using hematite pellets made for industrial blast furnaces. The electrolyte consisted of pure molten CaCl2 containing 0·5 mol.-%CaO. Different anodes were tested. Graphite anodes were found to halt the reduction of the inner parts of the cathode pellets. Also large amounts of carbon were found in the cathode product in these experiments. Nickel ferrite cermet with intergranular nickel and copper was tried as an inert oxygen evolving anode material. These anodes showed promising behaviour and also made the cathode pellet more active with respect to electro reduction. Iron was found to be formed during the process.

Direct electrochemical preparation of CeNi 5 and La xCe 1− xNi 5 alloys from mixed oxides by SOM process

A novel SOM process was used to prepare CeNi 5 and La x Ce 1− x Ni 5 hydrogen storage alloys directly from their mixed oxides. The electrolytic reduction was carried out in molten CaCl 2 system at 1000 °C. The reduction mechanism was investigated by analyzing the chemical compositions and the phase constitutions of the intermediate products of electrolysis. The results suggested that the reduction of NiO–CeO 2 may take place in two steps: first, NiO was reduced into Ni and CeO 2 reacted with CaCl 2 to form CeOCl, then Ni reacted with CeOCl leading to the formation of CeNi 5. It was found that the reduction rate increased while decreasing the pressure load of the mixed oxide pellets. Furthermore, CeNi 5 could not be produced if the pressure load was lower than 10 MPa. It was also found that the pellets of NiO–CeO 2 could be completely reduced to CeNi 5 alloy by the SOM process, which was greatly excelled than FFC process. The successful preparation of La x Ce 1− x Ni 5 ( x = 0–1) alloy reported here suggests that the SOM process may be promising for the industrial application of producing such alloys.

Electrochemical synthesis and characterization of a NdCo5 permanent magnet

NdCo5 was produced in a simple one-stage process by electro-deoxidising an inexpensive single phase solid oxide solution of Nd2O3 and Co3O4. The reduction was performed in molten CaCl2 at 900 °C and the cell operated at 3.1 V at a cathodic current density of 170 mA cm−2 for 12 hours. The magnetic properties of the product, measured using a vibrating magnetometer, closely matched that of pure and homogenous NdCo5. The reaction mechanism at the cathode is elucidated from partially reduced samples using electron microscopy, X-ray diffraction analysis and conductivity measurements.

Rationalisation and optimisation of solid state electro-reduction of SiO2 to Si in molten CaCl2 in accordance with dynamic three-phase interlines based voltammetry

Abstract The cyclic voltammograms of a silica sheathed tungsten disc (W-SiO2) electrode in molten CaCl2 at 900 °C exhibited an unusually increasing reduction current with decreasing the potential scan rate. When the cathodic limit was less negative than −1.00 V (vs. a quartz sealed Ag/AgCl reference electrode), the reduction current was also smaller in the forward (negative) potential scan than that in the reversed (positive) scan. However, at a given reduction charge, the reduction current increased with the scan rate, following approximately a logarithm law. These unique features have been elaborated according to the dynamic model of the conductor (silicon)/insulator (silica)/electrolyte (molten salt) three-phase interlines (3PIs). Combining the voltammetric observations with the composition analysis of the products from potentiostatic electrolysis of porous silica pellets, the optimal potential window was identified to be from −0.65 V to −0.95 V. In this potential range, silica was converted to pure silicon with the oxygen content being less than 0.5 wt.%. At potentials more negative than −0.95 V, the reduction of Ca2+ ions in the reduction-generated porous silicon layer led to the formation of various calcium silicides. These findings can help the development of an electrolytic process for clean, efficient and inexpensive production of high purity silicon.

Phase diagram of the system Ca-Ti-O at 1200 K

Phase relations in the system Ca-Ti-O have been established by equilibration of several samples at 1200 K for prolonged periods and identification of phases in quenched samples by optical and scanning electron microscopy, XRD and EDS. Samples representing 20 compositions in the ternary system were analyzed. There was negligible solid solubility of Ca in the phases along the binary Ti-O, and of Ti in CaO. Four ternary oxides were identified: CaTiO3, Ca4Ti3O10 and Ca3Ti2O7 containing tetravalent titanium, and CaTi2O4 containing trivalent titanium. Tie-lines link calcium titanite (CaTi2O4) with the three calcium titanates (CaTiO3, Ca4Ti3O10 and Ca3Ti2O7), CaO, oxygen excess TiO1+δ and stoichiometric TiO. Tie-lines connect CaTiO3 with TiO2−x , Magneli phases Ti n O2n−1 (28 ≥ n ≥ 4), Ti3O5, Ti2O3 and TiO1+δ . CaO was found to coexist with TiO, and Ti-O solid solutions α and β. The phase diagram is useful for understanding the mechanisms and kinetics of direct calciothermic reduction of TiO2 to metal and electrochemical reduction of TiO2 using graphite anode and molten CaCl2 electrolyte.

Cyclic Voltammetry of ZrO2 Powder in the Metallic Cavity Electrode in Molten CaCl2

The electrochemical reduction of the insulative powder in molten was investigated using the metallic cavity electrode (MCE) in molten at . Cyclic voltammograms (CVs) revealed two consecutive reduction peaks corresponding to (i) to and (ii) to Zr. The intermediate, , was metastable and underwent disproportionation to and Zr, which was responsible for the detection of Zr metal in the potentiostatic reduction at less negative potentials. In the anodic scan, four main oxidation processes were observed. The relevant reactions were rationalized as the reoxidation of (iii) to , (iv) Zr to , (v) Zr to , and (vi) Zr to . The metastable intermediate also contributed to the unique current variations in the anodic potential scans under different conditions. Potentiostatic electrolysis of the powder in the MCE at the feature potentials of the CVs and analyses of the electrolysis products by scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed the electroreduction mechanism and revealed the localized conversion of the dense aggregates of the submicrometer particles of to cauliflower-like aggregates of the nanoparticulates of Zr in the early stage of the electroreduction process.

Phase-Tunable Fabrication of Consolidated (α+β)-TiZr Alloys for Biomedical Applications through Molten Salt Electrolysis of Solid Oxides

Electrochemical reduction of a solid TiO2−ZrO2 mixture (molar ratio = 1:1) to the TiZr alloys in a consolidated porous structure was studied by constant cell voltage electrolysis in molten CaCl2 at 900 °C. For the first time, and surprisingly, it was found that tuning the α- and β-phases in the TiZr alloys could be easily realized by controlling the electrolysis time or, specifically, the oxygen content in the alloys. Oxygen acted as a phase-transition inhibitor from the high-temperature β-TiZr to the low-temperature α-TiZr, leading to metastable β-TiZr or (α+β)-TiZr as the product at room temperature. However, if the oxygen content decreased to <1100 ppm, only α-TiZr was collected, even when the electrolytic alloy was directly quenched in water. The consolidated porous (α+β)-TiZr alloys possessed elastic moduli in the range of 20−40 GPa, matching closely to that of natural bone (10−30 GPa). They also exhibited excellent corrosion resistance in the Ringer solution. These novel findings promise a simple, one-step, low-energy, and environmentally friendly electrochemical process for the production of β-phase containing TiZr alloys as medical implant materials without any added expensive and/or bio-incompatible elements.

Yttria-stabilized zirconia as membrane material for electrolytic deoxidation of CaO–CaCl2 melts

This article is devoted to the study of the stability of an yttria-stabilized zirconia membrane used in the electrolysis of molten CaCl2–CaO mixtures at 850 °C. Intensiostatic and potentiostatic electrolysis were carried for periods ranging from 10 to 20 h. Post-mortem composition profiles across the zirconia membrane were determined using Raman spectroscopy and microprobe analysis. The membrane degradation was analyzed in terms of synergetic parameters, i.e., chemical, electrochemical, and thermomechanical effects.

The regular features of chlorine formation in the systems NaCl-Me x O y -O2 and Ca(Zn)Cl2-Me x O y -O2

The reaction of sodium, calcium, and zinc chlorides with atmospheric oxygen in the presence of transition metal oxides and antimony oxide at temperatures exceeding the melting temperatures of chlorides was studied. The content of chorine, product of oxidation of chloride ions in molten NaCl and CaCl2, was determined as a function of the polarization force of cations of transition metal oxides.

Direct Reduction of Liquid V2O5 in Molten CaCl2

An alternative process of the reduction of V2O5 was studied. It consists of thermochemical and electrochemical reactions in molten CaCl2 + CaO. V2O5 is thermochemically reduced by Ca. The by-product CaO is electrolyzed. Liquid V2O5 was reduced to vanadium metal through intermediate CaV2O4, and oxygen concentration was 1860ppm at minimum. By deoxidation of the sample with a smaller specific surface area, the lowest oxygen concentration, 460ppm, could be achieved. A major component of ejected gas from the anode was CO2 at the initial stage and CO at the later stage. O2 gas was not detected because its concentration was lower than detection limit.

Electrolytic Reduction of Solid SiO2 in Molten CaCl2 for the Production of Solar-grade Silicon

The direct electrolytic reduction of solid SiO2 has been investigated in molten CaCl2 at 1123 K aiming at the production of solar grade Si. The target impurity levels for Si in the present study were calculated from the acceptable impurity levels for SOG-Si and the segregation coefficients for the impurity elements. New types of SiO2 contacting electrodes, in which SiO2 directly contacts with only Si, were devised to reduce metal contaminations in the produced Si. By using the new SiO2 contacting electrodes and a quartz vessel, most of the metal impurities were significantly reduced to be below the target levels. When NH4OH/H2O2/H2O, HCl/H2O2/H2O and HF/H2O2/H2O were used as a cleaning solution, remaining CaCl2 and some metal impurities in the produced Si were removed effectively. A basket type electrode was also tested for the reduction of granular SiO2.

Optimization of the FFC Cambridge Process for NiTi Production

The FFC Cambridge process is ideal for NiTi production, as it avoids the Ni segregation associated with pyrometallurgical routes to manufacture NiTi, and is able to produce a homogenous low-oxygen product. The process involves the direct electrodeoxidation of NiTiO3 (first stable oxide to form upon mixing and sintering of NiO and TiO2) to NiTi in a molten CaCl2 salt bath. This work builds on previous literature that has elucidated the reduction pathway by investigating the effect of reduction temperature and current collector material on the microstructure of the final reduction product. It was found that the use of a Ni current collector caused Ni-enrichment at the surface of the product, stabilizing the high temperature B2 cubic form to room temperature. Ni-rich phases Ni4Ti3 and Ni3Ti were observed after 24h reductions. The former is favoured at higher reduction temperatures, and the latter at lower temperatures. Mechanisms for formation are proposed.

Calciothermic Reduction of TiCl4 Gas during the Electrolysis of CaCl2 Melt

The calciothermic reduction of TiCl4 gas was studied firstly by using Ca-CaCl2 melt, to whose surface the gas was blown. The fine Ti particles settled down in the crucible, when the concentration of dissolved Ca was > 2 mol% at 1173 K. When < 2mol%Ca, a part of TiCl4 gas was dissolved probably as TiCl3 or TiCl2 into the CaCl2 melt, and it precipitated as the lower Ti oxides. When TiCl4 gas was blown to the molten CaCl2, which was electrochemically decomposed into Cl2 gas at the carbon anode and Ca at the Ti cathode, TiCl4 gas was reduced to the dendritic Ti on the cathode surface. Because the electrolyzed Ca reacted preferentially with the oxide crucibles, most of the oxygen ions combined with Ti2+ and Ti3+ and they formed TiO and Ti2O3.