Ammonium Paratungstate Preparing Tungsten Trioxide

By drying and calcining sodium tungstate and ammonium paratungstate (APT) can prepare tungsten trioxide. Dried at temperature of about 100 ℃, with the main vaporisation of moisture the material is physically adsorbed and removed. Only at higher temperatures the material is calcined to remove molecule compounds and ammonia water of sodium tungstate and ammonium paratungstate.

When tungstic acid is usually calcined at 180 ℃ ,it is dehydrated dehydration at 500 ℃ completely, producing tungsten trioxide:
H₂WO₄=WO₃+H₂O

When ammonium paratungstate is heated and calcined in air at different temperatures , which will produce different phase, it transfers to tungsten trioxide when the temperature rose to about 500 ℃. Analyzing APT by differential thermal analysis and X-ray diffraction method calcination , which shows APT is heated in the air , its course as follows: at the temperature is not less than 240 ℃, APT is converted to ammonium metatungstate (AMT), continuing to have sublimation to 300 ~ 350 ℃, the conversion of the amorphous material (ammonium tungsten bronze ATB), the temperature rose to 400 ℃, it became into the hexagonal WO₃, the temperature reached 500 ℃.

APT purity determines the purity of tungsten trioxide, APT crystalline form has hereditary to crystalline tungsten trioxide, the temperature of calcination and the rate of thermal decomposition has significant impact on the physical and chemical properties of the product tungsten trioxide. calcining the same batch of APT at different temperatures, the product WO₃ surface area and particle size vary greatly, the calcination temperature is higher, resulting WO₃ is more coarse.

Physicochemical properties of tungsten trioxide, not only depend on the calcination temperature, but also depend on the thermal decomposition rate and APT the duration time. It was found that rapid warming, rapid decomposition, the output of tungsten trioxide has wide and deep cracks, large specific surface area.

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