Reduction of Tungsten Oxides by Hydrogen

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

Besides chemical conversion and chemical vapor transport, the reduction process is a purification step, too. Trace impurities, always present in the oxide, may evaporate. On the other hand, foreign phases can be incorporated during the CVT growth of tungsten, finally leading to inclusions in the tungsten powder particles. This is of special interest in the production of non-sag tungsten wire used for incandescent lamps. 

The reduction of WO3 to W as described by the simple equation above is in reality a rather complex process. Most of the scientific backgrovmd and understanding is relatively new, although the process has been in use on a technical scale for more than 80 years. 

In the past an attempt was made to describe the reduction process in terms of temperature, time, diffusion-path lengths, and oxide bulk density, based on theoretical diffusion equations [3.16], but such models are not used in practice due to the complexity of parameter interactions. Industrial production today is still based on extensive empirical knowledge, supported by in-depth understanding of the basic aspects governing the reduction process, aceompanied by a feel for the matter. 

The following subsections will present the major aspects of the reduction process in order to understand the chemical and physical processes that arise during the reduction sequence and determine the quality of the final metal powder. The industrial procedure including technical details will be presented in Section 5.4. 

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