High-purity ammonium metatungstate may be prepared by a coupling process of neutralization-nanofiltration-crystallization. AMT solution is obtained by neutralization-nanofiltration, which can not only reduce cost as high concentration of AMT solution is obtained for the subsequent crystallization process, but also ensure to prepare high purity AMT as the impurity content is low.
In the above figure, 1. High pressure pump; 2. Primary liquid tank; 3. Nanofiltration membrane device; 4. Valve; 5. Valve.
The neutralization-nanofiltration-crystallization process consists of the following three steps:
1. Acid neutralization to prepare AMT mixed solution.
2. Nanofiltration of the AMT mixed solution.
3. Evaporation and crystallization of AMT solution after nanofiltration.
Nanofiltration technology is a membrane separation technology, which is separated from reverse osmosis technology and is a continuation and branch of ultra-low pressure reverse osmosis technology. Nowadays, nanofiltration technology has been separated from reverse osmosis technology and has become an independent separation technology between ultrafiltration and reverse osmosis technologies. It has been widely used in seawater desalination, ultrapure water production, food industry, environmental protection and many others fields. 
In the above figure, 1. Electric magnetic stirrer; 2. Beaker; 3. Magnetic stirrer; 4. Dropper; 5. Iron support.
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http://www.ammonium-metatungstate.com/english/index.html
