Tungsten trioxide can be prepared in several different ways. CaWO4, or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to WO3 and water at high temperatures.
CaWO4 + 2HCl → CaCl2 + H2WO4
H2WO4 + heat → H2O + WO3
Another common way to synthesize WO3 is by calcination of ammonium paratungstate (APT) under oxidizing conditions:
(NH4)10[H2W12O42]•4H2O → 12 WO3 + 10NH3 + 10H2O