Reduction Of Tungsten Trioxide Research Articles

(Digital Presentation) Electrochemical Reduction of Oxygen-Containing Tungsten Compounds on a Liquid Gallium Cathode in an Eutectic Melt of Calcium and Sodium Chlorides

This paper presents the results of studies of electrochemical reduction of oxygen-containing tungsten compounds (WO3, CaWO4) in a molten eutectic mixture of calcium and sodium chlorides. In contrast to the known methods of direct electrochemical reduction of refractory metal oxides, it is proposed to reduce the initial oxygen-containing tungsten compounds in powder form on a liquid gallium cathode without resorting to pre-pressing and sintering operations.In the temperature range 600-1000 °C, at which the electrochemical reduction of these compounds takes place, gallium does not evaporate, does not interact with the components of the melt, or with the initial compounds of tungsten or pure tungsten. The liquid gallium cathode not only provides reliable contact with highly dispersed initial tungsten powder compounds and their uniform polarization but also protects the reduced metal from contamination by products of electrolysis and electrolyte. In addition, gallium, due to its lower specific weight than tungsten oxide, protects WO3 from unwanted interactions with calcium chloride, which causes the formation of volatile tungsten oxochlorides and the loss of the initial tungsten trioxide. Due to the large difference in crystallization temperatures of the molten electrolyte (over 500 °C) and the gallium cathode (~ 30 °C), as well as the difference in specific weights of the electrochemical cell components, fine tungsten powder isolated during electrolysis is easily separated from melt and gallium.During the reduction of tungsten trioxide at the beginning of electrolysis on the surface of the gallium cathode, sodium and calcium cations, which are part of the electrolyte mixture, are discharged. In the range of reduction temperatures (700-800 °C) the discharge potentials of sodium and calcium cations are almost the same. As the gallium cathode is saturated with isolated metals (sodium and calcium), tungsten trioxide is reducedWO3 + 3nM → W + 3MnO, where М = Na, Ca,and n = 2 when sodium is involved in the reduction and n = 1 when the reducing agent is calcium. The formed metal oxides (MnO) react with tungsten trioxide, forming a mixture of tungstates of the appropriate metalsWO3 + MnO → MnWO4 Both oxides and tungstates of calcium and sodium have a lower specific weight than gallium, so as they accumulate, they float to the interface: cathode/melt, where at the appropriate values of the electrode potential, also participate in electrode processesMnWO4 + 6е− → W + MnO + 3О2− MnO + 2е− → nM + О2−.Reduced tungsten settles to the bottom of the gallium cathode container, and calcium and sodium contribute to the reduction of tungstates.The results of voltammetric studies of oxidized and unoxidized monocrystalline tungsten, tungsten cathode, on the surface of which is applied a film of WO3, gallium cathode with tungsten starting compounds (WO3, CaWO4) in a molten eutectic mixture of sodium and calcium chlorides indicate in favor of the fact that this process takes place in stages in several stages. This is also supported by the results of electrolysis under potentiostatic conditions at different potentials of the cathode relative to the unpolarized tungsten electrode. The mixture of tungsten and calcium tungstate is the reduction product at potentials lower than the reduction potentials of sodium and calcium tungstates. For example, with a reduction potential of −1.3 V, the reduction products of both tungsten trioxide and calcium tungstate contain no more than 2.0% of tungsten, the rest is calcium tungstate. Reduction at a potential of −2.0 V provides a product with a tungsten content of not less than 20.0%. Finally, the reduction at a potential equal to or greater than −2.4 V at the cathode produces tungsten without the content of extraneous phases. The degree of extraction of tungsten from tungsten trioxide is not less than 65.0%, from calcium tungstate is about 72.8%.The morphology of reduced tungsten was studied by scanning electron microscopy. The resulting tungsten powder has the form of dendritic crystals with a size of several tens of micrometers.

SHS Synthesis and SPS Densification of Nanometric Tungsten

Recent studies show that low grain sizes are favorable to improve ductility and machinability in tungsten, as well as the resistance to ablation and spallation if this material is to be used in thermonuclear fusion environment. However, current production routes are not suitable for the fabrication of large bulk nanostructured tungsten samples. The authors propose here a new methodology based on powder metallurgy, including the powder synthesis by the reduction of tungsten trioxide by magnesium using the Self‐propagating High‐temperature Synthesis process in the presence of a reaction moderator, and the densification procedure. Results show that full tungsten densification may be obtained by SPS at a temperature lower than 1800 °C and that the resulting morphology, keeping a partial nanostructure inherited from the synthesized powders, seems indeed favorable to the use of these materials in fusion environments.

Creation of surface catalytic active sites on tungsten oxide(s) and a study of the catalytic behavior in isopropanol and 1-hexene reactions

Catalytically active sites formed after the partial reduction of tungsten trioxide using hydrogen gas at different reduction temperatures were studied by measuring the catalytic activity of the system in reactions with isopropanol and 1-hexene. These reactions were activated by either metal, acid, or acid-metal (bifunctional) functionalities. The identification of the chemical composition of the specific catalytic active sites was achieved by in situ characterization of the system using the XPS-UPS surface technique carried out at the same time as the experimental catalytic measurements. Tungsten trioxide species possess acidic character before applying the reduction treatment. However, when WO3 is reduced to WO2 a metallic character is obtained, imparting the delocalization of π electrons over the tungsten atoms located along the C-axis of the deformed rutile structure of WO2. Changes in the spin–orbit coupling of W(4f) and the structure of the density of states (DOS) at the Fermi level confirmed the development of metallic properties. The bonding of active H, obtained from the dissociation of molecular hydrogen during the reduction procedure, to the oxygen surface sites results in a build-up of Bronsted W-OH acid functionalities. Therefore, a metal–acid bifunctional catalyst is obtained on the surface of the sample in the form of WO2−x(OH)y/TiO2. Titanium dioxide is utilized as a support to mechanically strengthen the catalyst and enhance the surface area.

Reduction of tungsten trioxide with ethanol

This study aims to investigate pyrometallurgical reduction behavior of tungsten trioxide by ethanol at the temperature range 900–1500K. Thermodynamic analysis predicted single tungsten phase formation from tungsten trioxide and ethanol at the temperature range studied. The experiments were carried out in a flowing ethanol (7.1vol.%)-Ar atmosphere in a tube furnace for various reaction periods of time. The extent of the reduction was followed by mass measurements. XRD and SEM were used to carry out phase and morphological studies on the products obtained at various stages of the reduction. It was found that at the temperature range 900–1200K, oxide reduction was incomplete owing to sluggish reaction kinetics between the reactants. At higher temperatures of 1300K, 1400K, and 1500K, tungsten trioxide reduction to metallic tungsten was complete within ~45min, ~30min and ~20min, respectively. Well-facetted tungsten particles with mean sizes of 1.6μm, 2.1μm and 3.3μm were observed at these conditions. At 1500K, there was a significant tungsten loss by the flowing gas stream via the formation of gaseous WO2(OH)2. Possible reaction pathways leading to metallic tungsten were discussed.

Production of tungsten via leaching of scheelite with sulfuric acid

A new process for leaching scheelite ore consists of leaching low- and high-grade concentrates with H2SO4 in the presence of H3PO4 and NaCl at atmospheric pressure. The resulting tungstic acid solution is then evaporated to produce ammonium paratungstate (APT): (NH4)10H2W12O42·4H2O. To obtain the yellow oxide (WO3), the APT is heated at 600° to 700°C for one hour to evaporate ammonia and water. The reduction of tungsten trioxide with H2 can be controlled by the temperature, time and water vapor partial pressure to produce α-W.

Study of the reduction of tungsten trioxide doped with MCl (M=Li, Na, K)

Thermodynamic calculations predict the formation of hydrochloric acid gas and alkali tungstates during hydrogen reduction of WO3 doped with alkali chlorides MCl (M=Li, Na, K). The formation of HCl was proved experimentally by simultaneously coupled TG-MS measurements from RT to 1200°C. The formation of HCl is the result of the reaction between MCl, WO3 and water. Ubiquitous traces of moisture in the gas are sufficient for reaction according to WO3+(2+2n)MCl +(1+n)H2O→M2+2nWO4+n+(2+2n)HCl (n=0, 1, 2). Laboratory reduction tests showed that the formed tungstates differ. NaCl and KCl form monotungstates (n=0), while LiCl produces more lithium-rich compounds (n=1, 2). Temperature and humidity, among other process factors, control subsequent reduction of the tungstates to metals.

Diffusion during electron-beam-induced reduction of tungsten trioxide

Abstract We report experimental results on the kinetics of the electron-beam-induced reduction of WO3 by in situ high-resolution electron microscopy. Electron-beam irradiation of WO3 results in complete reduction to W. The process is surface-initiated through desorption of oxygen from the crystal surface, and the subsequent growth of tungsten was found to be a diffusion-controlled process. The damage rate follows a para-linear curve, that is, an initially parabolic damage rate degenerating into a linear rate. The epitaxial relationship between WO3 and W was found to be (001)wo3//(110)w. The WO3 lattice has a critical nucleus size of about 2·6 nm.

Kinetics of tungsten teioxide reduction by dey and humidified hydrogen

An investigation into the kinetics of reduction of tungsten trioxide by dry and humid hydrogen has shown that water concentrations of up to 3 vol.% in the hydrogen have no effect on the kinetic characteristics of the process. At a temperature of 500°C slightly higher reaction rates are obtained at water contents of 1.5–3 vol.%. In the presence of palladium additions initial reaction rates grow. It has been established that, when the water content of the hydrogen does not exceed 3 vol.%, the reaction in the low-temperature reduction phase passes through a stage of/gb- W phase formation. Under these conditions the degree of dispersion of the end metallic product is practically independent of the degree of humidity of the hydrogen in the temperature range 450–600°C.

Effect of starting specific surface on the kinetics of tungsten trioxide reduction with hydrogen

1. The lower the process temperature, the more pronounced is the effect of the starting specific surface of tungsten trioxide on its reduction rate. 2. At approximately equal rates of low-temperature reduction of tungsten trioxide the specific surface of the metallic end product is not strongly affected by the starting specific surface of the trioxide and its reduction temperature.

Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis

Tungsten carbide catalyzes the formation of water from hydrogen and oxygen at room temperature, the reduction of tungsten trioxide by hydrogen in the presence of water, and the isomerization of 2,2-dimethylpropane to 2-methylbutane. This catalytic behavior, which is typical of platinum, is not exhibited at all by tungsten. The surface electronic properties of the latter are therefore modified by carbon in such a way that they resemble those of platinum.

三酸化タングステンのH2還元に関する速度論的研究

三酸化タングステンのH<SUB>2</SUB>還元に関する速度論的研究

(285)高純タングステンの製造に関する研究(第1報)酸化ダングステンの水素還元(I)

(285)高純タングステンの製造に関する研究(第1報)酸化ダングステンの水素還元(I)

Volatility of the Oxides of Tungsten and Molybdenum in the Presence of Water Vapour

ACCORDING to L. Wohler and O. Balz1,2, the oxides WO3, W2O5 (W4O11) and WO2 volatilize considerably above 800–900° C., and the vapour pressure of tungsten trioxide is, according to O. Ruff and Grieger3,2, 0·206 mm. at 1,023° C. It is generally believed that this phenomenon explains the formation of much larger metal grains during the reduction of tungsten trioxide in hydrogen at 800–900° C. from much finer oxide grains.