Super grade yellow tungsten oxide, also known as super grade tungsten trioxide, is a grade of yellow tungsten oxide with highest purity under Chinese standard. Its grade is WO3-0. The complex salt method can be used to produce yellow tungsten oxide with different particle size. 
Super grade WO3 can generally be prepared by calcining pure tungstate or ammonium paratungstate (APT). So, the particle size of tungstic acid or APT will directly affect the particle size of WO3. It should be properly controlled according to needs in actual production as the impurities in the system is often easy to absorb on the active site of crystal growth, hinder the grain growth, and make the grain thinning.
The freezing crystallization method can be applied for the preparation of fine ammonium paratungstate. APT, with the smallest and uniform particle size, is available by using this method.
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