Tungsten Trioxide

Tungsten Trioxide

In a technical scale, WO3 is almost exclusively produced by calcination of APT under oxidizing conditions (in air). Usual equipment consists of rotary furnaces operating at 500-700℃. Sufficient air supply must be provided to suppress any reducing reaction by the partly cracked ammonia. The ammonia evolved can be recovered by absorption in cold water and concentrated by subsequent distillation.

The WO3 particles are pseudomorphous to APT. This means that the shape and size of the particles are the same as the APT crystals, but they are built of very small WO3 grains (Fig. 5.18) forming a large oxide sponge with a high degree of microporosity (specific surface area). Their grain size and agglomerate structure depend on the calcinations condition (heating rate, temperature, and time). Higher temperature and low heating rates result in coarser grains. Above 700℃, coarse, faceted WO3 single crystals form due to enhanced chemical vapor transport of the oxide.

Low-temperature calcined WO3 (approximately 500-550℃) is highly reactive an dissolves easily in water, which is not the case for higher-temperature calcined WO3.

For special purposed, especially in the case where a high specific surface area is necessary and APT pseudomorphology is undesirable, WO3 can be produced also by calcinations of tungstic acid.

As precursor for the W and WC powder production, WO3 lost its importance mainly to tungsten blue oxide. WO3 is also used as a yellow pigment.

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