Reduction of Doped Tungsten Oxides

Powder properties can be significantly influenced by the addition of a third element (see also Section 3.3). This can be used industrially for the production of special W grades, as mentioned below. 

TUngsten blue oxide is treated with aqueous solutions of the respective element compounds. 

Lithium Doping. Lithium compounds are preferred to sodium for very coarse tungsten powder grades (50-100 pm) Fig. 5.29. Reduction conditions are the same as above. Due to the high lithium boiling point, it is not volatilized during reduction like 

FIGURE 5.29. Coarse W powder obtained by Lithium doping (average particle size 50 pm). By courtesy of Wolfram Bergbau- und Hiittenges. m.b.H., Austria. 

Chromium Doping. Additions of chromium compounds can be used to produce very fine metal powder grades ( 0.5 pm). Chromium oxide is formed during reduction and acts as a grain refiner. Such powders are subsequently carburized to ultrafine WC. Special precautions have to be taken because of the high pyrophoricity of the metal powder. 

---

Forms partially during the hydrogen reduction of doped tungsten oxide (NS-W production). 

---