Although the reducing agent-carbon-is very cheap, and carbon reduction was the early basis of tungsten powder production, so far none of the numerous carbothermic procedures has been established in the production of pure tungsten.
The reasons for that are:
·The carbon balance is difficult to control; furthermore, the metal forms carbides. Although the theoretical carbon consumption to reduce W03 to W is 15.5 wt%, only 12-14 wt% are normally sufficient at 1200℃ t0 1400℃because the reaction does not only proceed via WO3 + 3C→ W +3CO but also to a certain extent via 2W03+3C W+3.C02. Therefore, in most cases the reduction is either incomplete or the metal powder is contaminated by carbides.
·Carbon is always a source of increased tungsten contamination because it contains impurities like Ca, Si, Fe, S, and P.
·Another disadvantage of the carbothermic reduction is the limited possibility of steering the grain size by varying the reduction conditions as compared to the hydrogen reduction process.
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