Tungsten Trioxide Produced by Ammonium Paratungstate (APT)
Ammonium paratungstate is shorted for APT, a white crystalline substance, which can be divided into two kinds of flake and needle. APT is the most important precursor of tungsten products, which can be used for the preparation of tungsten trioxide, blue tungsten oxide, tungstic acid, ammonium paratungstate and other products. Ammonium paratungstate is separated from the tungsten ore, when heated to 600 ℃, the decomposition product is tungsten trioxide, the chemical reaction equation ammonium paratungstate prepares tungsten trioxide is like follows:
（NH4）10（H2W12O42）•4H2O→12WO3+10NH3+10H2O
Different phase of tungsten oxide will be produced, when ammonium paratungstate calcined under different temperatures in air, when the temperature rose to about 500℃, ammonium paratungstate will be completely converted to tungsten trioxide. By differential thermal analysis and X-ray diffraction method ammonium paratungstate calcining process analysis, and found that when ammonium paratungstate was heated in air, the phase change of course will be:
1. When the temperature is not less than 240 ℃, ammonium paratungstate converted to ammonium metatungstate (AMT);
2. Continue to heat up to 300~350 ℃, amorphous material (ammonium tungsten bronze ATB) will come into being;
3. When temperature rises to 400 ℃, hexagonal WO 3 will come out;
4. When the temperature reaches to 500 ℃, we will obtain the triclinic crystalline form of tungsten trioxide.
In addition, purity of APT determines that of tungsten trioxide, crystalline of tungsten trioxide remains that from APT, and therefore, by controlling the crystal form and purity of raw material ammonium paratungstate can get the ideal polymorph and purity of tungsten trioxide.
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