TUNGSTEN METAL POWDER PRODUCTION--GENERAL--2

In principle, APT can also be directly reduced without any prior calcinations step. The disadvantage of direct reduction is the formation of ammonia which has to be scrubbed, but a certain amount of ammonia cracks and dilutes the hydrogen by nitrogen. Consequently, from time to time, part of the contaminated, circulating hydrogen must be vented, thus increasing costs.

Reduction is carried out in pusher furnaces in which the powder passes through the furnace in boats or in rotary furnaces (see below). Walking beam furnaces or furnaces with internal band conveyors are less often used. Fluidized-bed reactors are still not in commercial use, except for the production of nanophase W or WC/Co powder precursors. Furnaces are provided with several temperature zones controlled between 600 and 1100℃. A large excess of hydrogen is used, which is recycled to the furnace after purification. The flow of hydrogen is usually in a countercurrent direction, more rarely concurrent. The hydrogen acts not only as a reducing agent but also carries away the water formed.

The reduction of tungsten oxides by hydrogen to tungsten metal is, in some respect, a unique process. It offers the possibility to produce tungsten powder of any desired average grain size between 0.1 and 10 µm (and, in the case of doped oxides, even up to 100µm), starting from the same oxide precursor. Individual tungsten particles form during reduction as a result of chemical vapor transport of tungsten (vaporization/deposition process), which is responsible for the final powder characteristics.

By changing the reduction parameters, powder characteristics like average grain size, grain size distribution, etc. can be regulated. Temperature and humidity (i.e., the water vapor partial pressure prevalent during reduction) are the two main parameters in steering the average grain size of the W powder, the latter being related to a number of oxide and process-related variables as indicated in Fig. 5.19 and discussed briefly below. The reason for the strong influence of the humidity on powder grain size originates in the strong dependence of humidity on the nucleation rate of the metal phase and the high mobility of tungsten due to the presence of a volatile tungsten compound ([WO₂(OH)₂]). The lower the humidity, the higher the nucleation rate (under isothermal conditions) and the smaller the grain size.

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