High Purity Ammonium Metatungstate Crystal Preparation by Nanofiltration

With the rapid development of petroleum processing industry, the research and development of tungsten catalysts have been paid more and more attention. The active component of the catalyst, tungsten trioxide, is used as catalyst carrier, such as spherical silicon and aluminum oxide, which are made by leaching and pyrolysis of ammonium metatungstate. This is the direct driving force to promote the development of AMT in recent years. There are many processes for AMT production. Nanofiltration technology is a membrane separation technology separated from reverse osmosis technology, which is the continuation and development branch of ultra-low pressure reverse osmosis technology.

Now, nanofiltration technology has been separated from reverse osmosis technology, which has become an independent separation technology between ultrafiltration and reverse osmosis technology. It has been widely used in many fields, such as desalination, ultra pure water manufacturing, food industry, environmental protection, and has become an important branch of membrane separation technology. The integrated process of neutralization, nanofiltration and crystallization consists of three consecutive steps: 1) acid neutralization to prepare AMT mixed solution; 2) nanofiltration of AMT mixed solution; 3) evaporation and crystallization of AMT solution after nanofiltration.

When dilute nitric acid is added into ammonium paratungstate solution, ammonium paratungstate is gradually converted into ammonium metatungstate under appropriate reaction conditions such as acidity and temperature. Ammonium metatungstate solution was concentrated and washed through nanofiltration membrane device to obtain pure ammonium metatungstate solution. At a certain time interval, samples are taken to analyze the concentration of tungsten and nitrate ions in the feed solution and the permeate solution, and the pressure of the permeate solution is directly read by the pressure gauge. The content of tungsten was determined by thiocyanate colorimetry, and the content of nitrate ion was determined by ferrous sulfate potassium dichromate titration.

In the dissolution process, the solubility of ammonium paratungstate increases with the increase of temperature, which is also conducive to the transformation of ammonium paratungstate to ammonium metatungstate. Therefore, in this process, the reaction temperature should be increased as much as possible. However, the reaction temperature should not exceed 100 ℃, otherwise it will cause water evaporation and increase energy consumption, so the temperature can be controlled at 80 ℃ ~ 95 ℃ in the process of dissolution and transformation.

Neutralization nanofiltration coupling process is a promising way to produce high purity ammonium metatungstate crystal. It can not only obtain high concentration of ammonium metatungstate solution, reduce the cost for the later crystallization process, but also ensure the preparation of high purity ammonium metatungstate with less impurities.

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