An alternative for producing WC powders on an industrial scale is the direct carburization process, which was patented in Japan. It has been exclusively used there for several years to produce submicron and ultrafine WC powders of high quality. Other than the conventional process, this is done through the quality of the oxide and carbon source as well as the process parameters.
The necessary equipment consists of mixer, pelletizer, drier, and two rotary furnaces.
Raw material such as WO3 derived from tungstic acid or low-temperature calcined WO3 from APT (via AMT) can be used. The procedure is as follows.
Eighty-four parts per weight WO3 and 16.5 parts carbon are mixed for 1 hour in a Hensell mixer. Twenty-three parts water are added and the mixture is extruded (1.2 mm diameter) and cut every 3rd mm and dried (<0.1% H2O). The pellets are fed into the first rotary furnace and flow automatically from there into the second by gravity.
Typical furnace conditions are: stage 1—nitrogen atmosphere, 1350℃(reduction); stage 2—hydrogen atmosphere, 1650℃ (carburization).
The exclusion of hydrogen in the reduction step avoids any tungsten crystal growth via the volatile WO2（OH）2. The reaction path follows the sequence: WO3→(WO2.9)→WO2.72→WO2→W→W2C→WC. The intermediately formed, needlelike WO2.72 decomposes to ultrafine WO2 nuclei, which are further transformed to WC of approximately the same size. To meet the carbon balance (around 6.13 wt% C), precise control of temperature and atmosphere is necessary throughout the two-stage procedure.
The finest powders available (Cr3C2 doped or undoped) are characterized by a close particle size distribution and a specific surface area between 3.0 and 3.5cm2•g-1. The real particle size ranges around 0.15um.
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