Molten CaCl2 Research Articles

A study of the reaction pathways during electrochemical reduction of dense Nb2O5 pellets in molten CaCl2 medium

Electrochemical behaviour of powder compacted and sintered Nb2O5 pellet electrodes (~4mm thick, open porosity ~10%) in molten CaCl2 medium at 1173K was studied using cyclic voltammetry and polarisation measurements. Similar dense Nb2O5 pellets were subsequently electrolysed by using them as cathodes against graphite counter electrode in the molten CaCl2 electrolyte medium with an aim to gain insight into the electro-reduction mechanism. The chemical changes that are taking place in the cathode and also in the electrolyte melt at different stages of electrolysis were analysed. The electro-reduction experiments were carried out at an applied voltage of 3.1V and terminated after 2, 9, 22, 26 and 35h of electrolysis and the visibly distinct layers present in the partially reduced samples were analysed. The analysis showed presence of CaxNbyOz compounds, sub-oxides of Nb like NbO2, NbO, etc. and metallic Nb in the samples. A pellet, which was electrolysed for 44h was found to be reduced completely to Nb metal. The CaO, Ca and CaCO3 produced and released to the melt during the cathodic polarisation was found to be associated with the electrochemical changes that are taking place on the electrode and this was ascertained by determination of the ‘total solubilised basicity (TSB)’ and ‘total insolubilised basicity (TIB)’ of the melt as a function of time during electrolysis. Based on the results, a three-stage mechanism comprising (i) cathodic insertion of Ca2+ into the oxide pellet, (ii) reduction of the oxide electrode via formation and decomposition of ternary intermediates and (iii) chemical reduction of the cathode with electro-generated calcium metal has been proposed for electro-reduction of dense Nb2O5 pellets.

ChemInform Abstract: Electrodeposition of Crystalline and Photoactive Silicon Directly from Silicon Dioxide Nanoparticles in Molten CaCl2.

AbstractPure, polycrystalline, and photoactive silicon is directly obtained from SiO2 nanoparticles by electrodeposition in molten CaCl2 on a silver electrode (quartz tube, 850 °C).

Electrochemical Near-Net-Shape Production Via the FFC Cambridge Process --- Dedication to the Special Session for the 2012 Max Bredig Award

The energy- and labour-intensive extraction processes, and the complex and expensive downstream processes for alloying and manufacturing are the main causes for the high cost of engineering components. These are particularly true for reactive metals based alloys, such as titanium alloys. This paper demonstrates the feasibility of using the FFC Cambridge Process for near-net-shape production of Ti-6Al-4V based engineering components in different shapes, including hollow sphere and miniature hollow golf driver head, from their corresponding metal oxide precursors. A series of investigations were carried out in order to attain a fundamental understanding of the relevant electrochemical reduction mechanism in molten CaCl2. Additive oxide was found to assist the electro-reduction of TiO2 by preventing in situ formation of perovskites during electrolysis in molten CaCl2.

Fundamental Study on Reduction Rate for Electrolytic Reduction of SiO2 Powder in Molten CaCl2

Direct electrolytic reduction of powdery SiO2 has been investigated in molten CaCl2 at 1123 K aiming at a low cost and mass production of solar-grade Si. SiO2 powder was set on a Si plate which was installed at the bottom of the crucible and connected to a graphite rod used as a current lead. SiO2 powder was reduced to Si by cathodic polarization of the Si plate. The reduction rate was evaluated by the magnitude of the reduction current, in which three parameters were varied: electrode potential, SiO2 layer thickness, and SiO2 particle size. As a result, the rate-determining step of the reduction is suggested to be the diffusion of O2- ions inside the reduced porous Si particles filled with the electrolyte. During the reduction process, however, other factors such as contact resistance between Si particles and porosity of SiO2 particles possibly influence the reduction rate.

Electrochemical Fabrication of Niobium Silicon Alloys from Oxide Powder Mixtures

NbSi alloys were prepared by direct electrochemically reducing four mixed Nb2O5 and SiO2 powders (Nb-10Si, Nb-20Si, Nb-30Si, and Nb-37.5Si) in molten CaCl2 electrolyte at 900°C. The samples were characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). No oxidized phases were remained by XRD tests. Under SEM, Nb phase was scattered in Nb5Si3 phase for the samples of Nb-10Si, Nb-20Si, and Nb-30Si. For the sample of Nb-37.5Si, only Nb5Si3 was obtained after electrolysis.

Production of Fe–Ti alloy from mixed ilmenite and titanium dioxide by direct electrochemical reduction in molten calcium chloride

It is important to develop technologies for the production of high-purity metals and alloys for industrial applications. With the aim of addressing the urgent problem of depletion of resources and the commercial challenges associated with the development of new technologies, detailed research on the electrolysis of mixed FeTiO3/TiO2 in a molten CaCl2 electrolyte was conducted to prepare a high-purity, and high-opacity ferrotitanium (Fe–Ti) intermetallic alloy. Mixed solid oxides of FeTiO3 and TiO2 with FeTiO3:TiO2 molar ratios of 2.78:1.00 and 0.58:1.00 were used for the preparation of Fe–Ti intermetallic alloys (required molar ratios: Ti:Fe=1.36:1.00 and 2.72:1.00). A mixed solid oxide of FeTiO3 and TiO2 was reduced electrochemically in a molten CaCl2 electrolyte at 950°C and a constant voltage of 3.0V. The metallic samples formed by the electrochemical reduction were analyzed using X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy, electron-probe microanalysis, inductively coupled plasma atomic-emission spectroscopy, ion chromatography, and oxygen and carbon analyzers. The mixed oxide pellets were strongly and successfully reduced to a high-purity Fe–Ti intermetallic alloy of β-Ti (FeTi4) and FeTi phases of high-opacity and with small amounts of detected impurities.

Direct electrochemical extraction of Ti5Si3 from Ti/Si-containing metal oxide compounds in molten CaCl2

Direct electrochemical extraction of Ti5Si3 from pressed cathode pellets comprising of powdered Ti/Si-containing metal oxide compounds was investigated by using molten salt electro-deoxidation technology. Three groups of mixtures including TiO2 mixed with SiO2, Ti-bearing blast furnace slag (TBFS) mixed with TiO2, and TBFS mixed with high-titanium slag (HTS) were prepared at the same stoichiometric ratio (Ti: Si = 5: 3) corresponding to the target composition of Ti5Si3, and used as the starting materials in this experiment, respectively. The pressed porous cylindrical pellet of the Ti/Si-containing compounds served as a cathode, and two different anode systems, i.e., the inert solid oxide oxygen-ion-conducting membrane (SOM) based anode system and graphite-based anode system were used contrastively. The electrochemical experiment was carried out at 900–1050°C and 3.0–4.0 V in molten CaCl2 electrolyte. The results show that the oxide components were electro-deoxidized effectively and Ti5Si3 could be directly extracted from these complex Ti/Si-containing metal oxide compounds.

Formation of niobium powder by electrolysis in molten salt

Fine Nb2O5 powder was filled into a basket-type cathode and immersed in molten CaCl2–0.5mol% CaO at 1173K. Nb2O5 immediately reacted with CaO in the molten CaCl2 and formed calcium niobites that exhibited rod- or plate-like morphology. After the electrolysis of CaO with a graphite consumable anode, spherical particles of metallic niobium powder containing a residual oxygen concentration of 0.673mass% O were obtained.

An Investigation of Electro-deoxidation Process for Producing Titanium from Dense Titanium Dioxide Cathode

The electrolytic method for directly extracting metals from the cathode of their its oxide has been called as “electro-deoxidation”. TiO2 cathode in molten CaCl2 has been developed more than 10 years. Two different mechanisms (“FFC Cambridge” and “OS”) were proposed to describe this electrochemical process. In order to further explore the mechanism, a dense cathode was designed in the present study, and an electrical conductor (Mo bar) was connected with the cathode, but without direct connection with molten salt to get rid of the possible electrochemical reaction between electrical conductor and melt. The experiments were carried out in a vertical electric resistance furnace at 900 °C in argon atm osphere w ith molten calciu chloride as electrolyte. The cathode was cylindrical in shape (about 35mm in height, and 8mm in diameter). The cathode after the electrolytic experiment was examined by using SEM technique. The SEM observation of the cathode showed a layer-structure with dense core and porous product in outer layer(s). The electrical resistance of the core was found much lower than that of the original cathode. The result of electro-deoxidation experiments by present authors indicated that the “FFC-Cambridge” mechanism may give a better explanation for the electro-deoxidation process of dense cathode at the initial time.

Electrosynthesis of Ti2COn from TiO2/C Composite in Molten CaCl2: Effect of Electrolysis Voltage and Duration

Ti2COn powders with controllable oxygen contents have been successfully synthesized by electro-reduction of solid TiO2-graphite composite precursors in molten CaCl2. The effect of electrolysis time and applied cell voltage on the carbon and oxygen content of the product was systematically investigated. The reduction pathway of the composite electrode was studied based on the analysis of intermediate products using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It is suggested that the reduction starts with converting TiO2 and graphite to CaTiO3 and Ti2COn. The resulting CaTiO3 is then reduced to CaTi2O4 which is subsequently reduced to Ti2COn. The present graphite significantly accelerates the electro-reduction of TiO2. The oxygen content decreases with increasing the electrolysis cell voltage and prolonging the electrolysis time. It is found that the cell voltage and electrolysis duration should be strictly controlled to avoid oxygen-depletion in the final product. The obtained product shows good electronic conductivity at room temperature. This process exhibits a relatively high current efficiency for production of Ti2COn from TiO2-graphite composite.

Up-scalable and controllable electrolytic production of photo-responsive nanostructured silicon

The electrochemical reduction of solid silica has been investigated in molten CaCl2 at 900 °C for the one-step, up-scalable, controllable and affordable production of nanostructured silicon with promising photo-responsive properties. Cyclic voltammetry of the metallic cavity electrode loaded with fine silica powder was performed to elaborate the electrochemical reduction mechanism. Potentiostatic electrolysis of porous and dense silica pellets was carried out at different potentials, focusing on the influences of the electrolysis potential and the microstructure of the precursory silica on the product purity and microstructure. The findings suggest a potential range between −0.60 and −0.95 V (vs. Ag/AgCl) for the production of nanostructured silicon with high purity (>99 wt%). According to the elucidated mechanism on the electro-growth of the silicon nanostructures, optimal process parameters for the controllable preparation of high-purity silicon nanoparticles and nanowires were identified. Scaling-up the optimal electrolysis was successful at the gram-scale for the preparation of high-purity silicon nanowires which exhibited promising photo-responsive properties.

Electrolytic Reduction of SiO2 Granules in Molten CaCl2

Electrolytic Reduction of SiO2 Granules in Molten CaCl2

Facile synthesis of freestanding Si nanowire arrays by one-step template-free electro-deoxidation of SiO2 in a molten salt

This communication presents a novel kind of silicon nanomaterial: freestanding Si nanowire arrays (Si NWAs), which are synthesized facilely by one-step template-free electro-deoxidation of SiO2 in molten CaCl2. The self-assembling growth process of this material is also investigated preliminarily.

Electrolysis of TiO2 or TiCl2 Using Bi Liquid Cathode in Molten CaCl2

A new continuous smelting process of Ti using Bi-Ti liquid alloys is examined. In this process, titanium ore (TiO2) are reduced on the Bi liquid cathode to form Bi-Ti liquid alloys and, subsequently, electrorefining or vacuum distillation is performed to get pure Ti metal. Bi was used as a solvent to dissolve Ti since it has a large solubility of Ti and a relatively low vapor pressure suitable for the proposed process. Electrochemical reduction of TiO2 in molten CaCl2 produced the Bi-Ti alloys containing 0.6–2.2 at% of Ti and 17.9--34.9 at% of Ca. Calcium contamination in the alloys is attributed to its small activity coefficient in Bi. However, in case of electrolysis of TiCl2, Ti concentration in the alloy reached 29.0 at% and Ca contamination was suppressed. Thermodynamic consideration using E-pO2− diagram for the Ti-Ca-O-Cl system indicates that it is important to keep high pO2−, in other words, low O2− concentration in the electrolyte to obtain the Bi-Ti alloys to keep low level of Ca contamination.

Purification of metallurgical grade silicon by electrorefining in molten salts

Electrochemical studies on silicon deposition were performed in molten salt electrolytes. Purification of metallurgical grade silicon by electrorefining was carried out in molten Si–chloride salts at temperatures from 973 K to 1223 K. It was found that the use of a liquid alloy anode of silicon and copper was beneficial in molten CaCl2 with NaCl, CaO and dissolved Si. ICP-AES analysis results showed efficient removal of metal impurities, such as titanium, aluminum and iron, which are present in significant quantities in the feedstock. The contents of boron and phosphorus in the silicon after electrorefining were reduced from 36×10−6 and 25×10−6 to 4.6×10−6 and 2.8 ×10−6, respectively. The energy consumption of electrorefining was estimated to be about 9.3 kW·h/kg.

Preparation of niobium nanoparticles by sodiothermic reduction of Nb2O5 in molten salts

Niobium nanoparticles with high purity were prepared by a sodiothermic reduction process using Nb2O5 as the raw material, LiCl, NaCl, KCl and CaCl2 as the diluents and sodium as the reducing reagent. The effects of the different molten salt systems, CaCl2 content, reaction time, excessive sodium and reaction temperature on the characteristics of the obtained niobium powder were discussed. The as-prepared niobium nanoparticles under the optimum experimental conditions were obtained by sodiothermic reduction with 20% excessive sodium in the NaCl-52mol%CaCl2-2mol%Nb2O5 molten salts at 650 °C for 6 h, and the molar ratio of the oxygen element in Nb2O5 and the CaCl2 of the molten salts is less than 19.23% at 750 °C. Moreover, the particle size of niobium nanoparticles is ranged about 40∼240 nm with increasing of reaction temperature from 650 to 800 °C.

Production of Nb–Ti–Ni alloy in molten CaCl2

A Nb–Ti–Ni hydrogen permeable alloy was synthesized as a powder using the OS process, which involves the simultaneous reduction of a mixture of Nb, Ti, and Ni oxides by Ca that is generated from molten CaCl2 via electrolysis. The mechanism confirmed that a BCC-(Nb, Ti) phase easily formed at an early stage of the reaction followed by the generation of a B2-TiNi phase from the lower oxides such as NbO and Ti2O. The oxygen content of the synthesized alloy decreased significantly to 0.27mass% with increased electric charge. The three elements were uniformly distributed at the macroscopic level.

Electrodeposition of Crystalline and Photoactive Silicon Directly from Silicon Dioxide Nanoparticles in Molten CaCl2

Silicon for solar cells: Relatively pure, polycrystalline, and photoactive silicon was directly obtained from silicon dioxide nanoparticles (NP) by electrodeposition in molten CaCl2 salt on a silver electrode (see picture). This process is based on the formation of liquid droplets of a silver–silicon eutectic alloy and the continuous reduction of SiO2 to silicon. The deposited silicon shows a p-type behavior in photoelectrochemical measurements.

Calciothermic co-reduction of TiO2-Cr2O3 oxides in molten CaCl2: Effect of particle size of starting materials

In this research, the effect of particle size of starting materials (TiO2 and Cr2O3) was investigated on calciothermic co-reduction of oxides and in-situ dissolution of the CaO by-product in molten CaCl2. The particle size of starting oxides, molar ratio of molten salt and holding time of reaction at elevated temperature affected the progress of reduction reactions and the remaining oxygen. In the best optimized condition, the remaining oxygen in the obtained TiCr2 was 1020 and 802 ppm using 388 and 120 nm particle sizes of starting oxides, respectively. The formation of TiCr2 in calciothermic co-reduction process was considered to be mutual diffusion phenomena. Also, the kinetics of hydrogen adsorption for the co-reduced sample was better than that of the arc-melted sample. Particle sizes of the obtained products decreased with-the decrease of the particle sizes of the starting oxides.

Electrolytic reduction of mixed (Fe, Ti) oxide using molten calcium chloride electrolyte

The production of high-purity metals or alloys using effective technologies is critical for future industrialization. With this aim in mind, a fundamental study of electrolysis in molten CaCl2 electrolytes was conducted to develop a new production process for ferrotitanium (Fe–Ti) intermetallic alloys. Mixed solid oxides of TiO2 and Fe2O3 were used in a molar ratio of 5.44:1.00. In this composition of Ti and Fe, FeTi and β-Ti containing iron can co-exist in equilibrium. A mixed solid (Fe, Ti) oxide was reduced electrochemically in a molten CaCl2 electrolyte at 950°C. The metallic samples formed by electrolytic reduction of the mixed solid (Fe, Ti) oxide were analyzed using X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy, electron-probe microanalysis, X-ray fluorescence spectrometry, inductively coupled plasma atomic emission spectroscopy, ion chromatography, and oxygen and carbon analyzers. The oxide pellets were successfully reduced to a highly purified dense intermetallic solid of β-Ti (FeTi4) and FeTi containing low levels of impurities, for example, less than 0.01mass% of carbon.