Reduction of tungsten oxides

Tungsten forms several oxides, and the reactions pertinent to the reduction of tungsten oxide to tungsten metal are expressed by the following equations ... 

Oxidation of tungsten with water and reduction of tungsten oxides with hydrogen are quite similar, with the high partial pressure of water or hydrogen driving the reaction in a particular direction. The oxidation reaction has been found to follow this sequence of reactions ... 

The final aspect of tungsten oxide reduction chemistry that needs to be considered is the kinetics of the reactions. Under most circumstances, the reduction of tungsten oxides is a transport limited process limited by the rate of transport of the water vapor product out of the material. Under such conditions, no shortcuts in the reduction path may be taken, with the WO3 oxide being reduced according to the following path ... 

EIGURE 8.3. High temperature reduction of tungsten oxide. H2O partial pressure is indicative of the rate of reduction. The four reduction steps are distinctly separated, indicating that the material passes through each equilibrium composition as it is reduced. Erom Lessner and Schubert. 

FIGURE 8.4. Low temperature reduction of tungsten oxide. Note that there are only two peaks, indicating that there must be some steps skipped in the WO3 W reduction. These skipped steps occur because of local variations in oxygen concentration. From Lessner and Schubert. 

As another example, we look at the carbothermic reduction of tungsten oxide, which follows a reaction mechanism... 

Today s only technically important method for tungsten powder production is the hydrogen reduction of tungsten oxides according to the overall equation... 

The importance of the powder layer in the hydrogen reduction of tungsten oxides, although long known, is still today sometimes not altogether understood and therefore its influence is here, once again, briefly summarized. 

REDUCTION OF TUNGSTEN OXIDES BY CARBON OR CARBON-CONTAINING COMPOUNDS. [3.2,3.43,3.44]... 

The reduction of tungsten oxides by carbon or carbon-containing compoimds can be easily performed. Statements about the starting temperature for the reaction between WO3 and solid carbon (carbon blacks, graphite) vary in the current literature between 655 °C and 783 °C. Differences in WO3 and C properties (particle size of the powders, preparation history, crystallinity, etc.) as well as in atmospheres may be responsible for that. The temperature range coincides with the begirming of self-conductivity and sublimation of WO3. Carbon monoxide starts to react witii WO3 at 535 °C (reduction pressure 1 bar, PcoJPco equilibrium ratio 8.52) [3.45]. 

Only recently was a new caibothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process) [3.49]. Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor (WCi. c) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then imdergoes a second heat treatment to convert the precursor into substantially pure WC. 

Carbothermal reduction of tungsten oxides with carbon monoxide [3.47], or gas mixtures of CO/CO2, CO/H2, CH4/H2 [3.50], C2H4/H2, and C2H4/H2 [3.51], as well as by reaction between metal oxide vapor and solid carbon [3.52] have recently attracted attention for producing high surface area tungsten carbides (up to lOOm /g), for use as catalyst (see Section 10.4), and for nanophase WC/Co composite powders (see also Section 9.2.1.4) [3.53]. 

The particle size distribution of the WC is very similar to that of the corresponding W powder. The latter is a consequence of the powder layer height during hydrogen reduction of tungsten oxides and can be somewhat fiirther influenced by the reduction conditions (see Section 5.4.3). 

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