Medium tungsten trioxide can be produced by ammonium paratungstate - sodium paratungstate complex salt method. The apparent density of the obtained tungsten trioxide is 0.8~1.0g/cm3.
Producing Method
Step one: Pour water into the complex salt to make into slurry and heat to boiling. Then, spray the slurry into 30%~31% concentrated hydrochloric acid solution which is simmering. The specific mass ratio:
Complex salt: deionized water: hydrochloric acid = 1:0.36~0.38:1.06. 
It takes 0.5h or so in total reflux reaction under boiling.
Step two: After the reaction, the mother liquor is separated by centrifugation, and the obtained tungstic acid precipitate is carefully washed with hot deionized water to eliminate Cl-. Generally, 8 times is needed. The washing solution should be separated thoroughly every time before the implementation of the next washing process.
Step three: Dry the fully washed tungstic acid precipitate, dehydrate, and then calcine at the temperature range of 750~850°C to obtain tungsten trioxide.
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