The manufacture of tungsten metal powder is a crucial step in tungsten metal and alloy production, since the powder properties significantly affect the properties in subsequent operations, such as pressing, sintering, and metalworking. Between 70 to 80% of tungsten worldwide is produced via powder metallurgy and thus passes through this important stage. In the past, advances in powder technology have greatly contributed to the development of tungsten and its alloys, as well as today’s high standard of product quality. Powder grades are tailormade for the subsequent applications, and the powder industry is facing a competitive market where the stringent fulfillment of exacting demands is an important part of business success.
The powder is characterized by chemical (purity), physical (grain size, size distribution, shape, agglomeration, etc.), and technological properties (fluidity, compaction density, etc.), which are influenced by the production process and which can be controlled—to a certain extent—by the process parameters.
Today, production of tungsten metal powder is accomplished almost exclusively by the hydrogen reduction of high-purity tungsten oxides. Reduction of the oxides by carbon, common in the early years of metal production, is presently used only for the production of tungsten carbide (direct carburization). The hydrogen reduction of tungsten halides (Axel Johnson process) has not become established on a large scale.
The common starting materials are tungsten trioxide (WO3) and tungsten blue oxide(WO3-X), the latter being the most widely used material. Tungstic acid (H2WO4) is used only for selected metal grades.
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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