Orthorhombic ammonium paratungstate is the necessary intermediate raw material with the chemical formula of (NH4)10H2W12O42•10H2O during the process of tungsten metallurgy. The decomposition reaction of orthorhombic APT may be determined in a HT-1000 nicrocalorimeter by three step calorimetry. The chemical equation is as follows:
(NH4)10H2W12O42•10H2O(s) = 12WO3(s) + 10NH3(g) + 12H2O(g)
Thermal Decomposition Mechanism of Orthorhombic Ammonium Paratungstate
1. Dehydrate at 335K and 376K.
2. Decompose into amorphous ammonium tungsten bronze at around 475K.
3. Form into crystalline ammonium tungsten bronze at about 563K.
4. Ammonium tungsten bronze start to decomposing at about 640K until temperature reaches 755K to generate WO3.
Orthorhombic ammonium paratungstate is obtained when ammonium paratungstate is crystallized from solution at a relatively low temperature (lower than 323K). At a relatively high temperature (higher than 323K), ammonium paratungstate is monoclinic with chemical formula of (NH4)10H2W12O42•4H2O. Monoclinic ammonium paratungstate is the most common seen product in industrial production, but it is unavoidable to obtain orthorhombic APT.
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