Tungsten trioxide, also known as tungsten(VI) oxide or tungstic anhydride, WO3, is a chemical compound containing the transition metal tungsten and oxygen. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.
2 WO3 + 3 C → 2W + 3 CO2 (high temperature)
WO3 + 3 H2 → W + 3 H2O (550 - 850 °C)
Tungsten trioxide appears usually in form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite (of same composition as meymacite, however sometimes written as H2WO4). These minerals are very rare secondary tungsten minerals.