During the process of tungsten metallurgy, ammonium paratungstate is the necessary intermediate raw material. When crystallize ammonium paratungstate from solution, orthorhombic ammonium paratungstate can be obtained when temperature is below 323K. When temperature is higher than 323K, monoclinic ammonium paratungstate is obtained. In industrial production, monoclinic APT is the most common seen product.
Orthorhombic ammonium paratungstate thermal decomposition mechanism: Dehydrate under 335K and 376K, decompose to uncertain ammonium tungsten bronze under 475K, under 563K forms into crystalline ammonium tungsten bronze, under 640K ammonium tungsten bronze starts decomposing. Until temperature reaches 755K, it will generate WO3. Chemical equation:
（NH4）10H2W12O40•10H2O = 12WO3 + 10NH3 + 12H2O