Ammonium paratungstate powder (APT) may be produced by secondary reduction process. In other words, APT is reduced to blue tungsten oxide (BTO) in first stage, and BTO is reduced to tungsten powder (W powder) in the second stage.
There are two methods for reducing APT to BTO - hydrogen reduction and self-reduction. And there are also two types of blue tungsten oxide - doped blue tungsten oxide and undoped blue tungsten oxide. The W powder using undoped blue tungsten oxide as raw material may be obtained by hydrogen reduction and is mainly applied for producing tungsten carbide.
The differences between hydrogen reduction of the doped blue tungsten oxide and hydrogen reduction of the undoped blue tungsten oxide are as follows:
1. (1) Doped blue tungsten oxide is directly reduced to WO2.72 or WO2 depending on the temperature.
(2) Undoped blue tungsten oxide first converted to WO2.90.
2. (1) Doped blue tungsten oxide is reduced into the intermediate compounds WO2.72 in the range of 1023~1173 K.
(2) The undoped blue tungsten oxide is reduced into WO2.72 in the range of 873~1023K.
3. (1) Doped blue tungsten oxide firstly forms β-W at a temperature higher than 1023K and then changes to α-W.
(2) β-W occurs relatively early if using undoped blue tungsten oxide as raw material.
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