Freezing crystallization method to prepare ammonium paratungstate (APT) is carried out in liquid nitrogen cooling vacuum dryer with the process of:
1. Spray the ammonium tungstate solution on the wall of dryer with liquid nitrogen cooling down the temperature, then the water and APT will crystallize at the same time;
2. Remove the water and free ammonia under the vacuum environment by sublimation, and the left substance is the dried APT.
Compared with the feed liquid, the ammonium paratungstate get by freezing crystallization method has nothing change in the chemical purity; however, the process will generate superfine ammonium paratungstate powder with the average particle size (FSSS) during 0.6~1.3μm. In addition, the obtained APT has advantages of uniform distribution of particle size, impurities or additives, and thus the freezing crystallization method is often used for preparing W-Re, W-Cu composite powder with uniform composition.
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