|Wolframite and scheelite concentrates with 50-60% WO3 are the main source from which ferro-tungsten, the alloy of iron and tungsten, which is used in steel production, is fused. Then tungstic anhydride is extracted. There are several methods by which WO3 may be obtained. Scheelite concentrates are decomposed in autoclave by soda solution at 180-200°C (obtaining the sodium tungstate solution).|
Wolframite concentrates are decomposed by sintering with soda 800-900°C followed by Na2WO4 water leaching or treating by caustic soda, after which contaminated Na2WO4 solution is obtained. After their precipitating H2WO4 is extracted. For more crude, easier filtered and higher purified substance CaWO4 is precipitated from Na2WO4 solution with following hydrochloric acid decomposition.
Dried H2WO4 contains 0.2-0.3% of contaminations. Hard alloys are produced From WO3 obtained by H2WO4 roasting at 700-800°C. In W metal processing H2WO4 is put to an additional purification by ammonia method: solving in ammonia and crystallizing of ammonium paratungstate 5(NH4)2O-12WO3-nH2O, the roasting of which yields pure WO3.
W powder is obtained via WO3 reduction by hydrogen (and, in hard alloys production, also by carbon) in tube-type furnaces at 700-850°C.