Blue Tungsten Oxide Preparation -- Internal Reduction Method
Blue tungsten oxide is one of the important raw materials for making tungsten powder.
Compared with yellow tungsten oxide, blue tungsten oxide is easy to be doped because of its good activity and large specific surface. It can maintain its original crystalline shape and has been widely used as an upgraded version of yellow tungsten oxide. There are three main methods for the blue tungsten oxide preparation. APT closed calcination method, APT hydrogen mild reduction method, internal reduction. Internal reduction method was proposed by scholars in Fudan University in 1981.
Internal reduction method by means of an internal component of the tungsten compound to make reduction of W. Intrinsic reduced tungstate was synthesized with APT as raw material (Intramo - lecularly Reduching Tunnstate). It has thermal decomposition in an inert atmosphere, blue tungsten oxide with simple phase composition, large specific surface and high purity has been obtained.
The distinguishing feature of internal reduction method is that the yellow tungstic acid is dissolved in ammonia water first, then add proper amount of urea, the intermediate, which is called the " reduced tungstate IRT ", is obtained by adjusting the acidity with oxalic acid. The dried IRT is then decomposed at 500°C to 700°C in the nitrogen stream. Using the ammonia decomposition of molecule internal component to reduce, blue tungsten oxide can be prepared.
The internal reduction method is carried out in a multi-tube furnace. The thermal decomposition temperature of IRT was determined to be 620 ±30°C, and the pyrolysis time was controlled at about 30 minutes.
Two different components of blue tungsten have been prepared by internal reduction method. One is the tetragonal system WO: the particles are very thin, specific surface 22一30m²/g. The other is monoclinic system W20O58, the particles are very crude, specific surface 10m²/g. The blue tungsten oxide prepared by internal reduction method has the advantages of high purity, large specific surface, uniform particle size and simple phase composition. It can be used as a new raw material for tungsten wire, carbide and other products, and its properties and practical use are good.
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