There are various methods for the preparation of high purity Ammonium Metatungstate, and the following are the main methods summarized according to the referenced articles:
Neutralization and nanofiltration coupling process
In this method, the acid is first neutralized to prepare a mixed solution of AMT (ammonium metatungstate).
Subsequently, the AMT mixed solution is treated by nanofiltration to achieve concentration and impurity removal.
This method not only obtains a highly concentrated ammonium metatungstate solution and reduces the cost of the subsequent crystallization process, but also effectively reduces the impurity content and ensures the high purity of ammonium metatungstate.
Solvent extraction method
Use organic extractant (such as di(2-ethylhexyl) phosphate (D₂EHPA) or tributyl phosphate (TBP)) and kerosene solvent to form an organic phase to extract ammonium ion from ammonium tungstate solution.
The pH of the solution was reduced to 2-4 and boiled to produce ammonium metatungstate solution.
Subsequently, the product is obtained by concentration and crystallization, filtration and separation, and drying.
This method has high sensitivity and selectivity, but the extraction solvent may be volatile, flammable and toxic, and special attention is needed when operating.
Ion exchange method
Ammonium paratungstate is used as raw material to prepare ammonium metatungstate by ion exchange method.
Ammonium paratungstate is heated at a specific temperature (e.g. 130-140°C) and time (1-8 hours) to produce ammonium metatungstate solution.
The product is then obtained by concentration and crystallization, filtration and separation, and drying.
Integrated Neutralization-Nano-filtration-Crystallization Process
Industrial grade nitric acid and international zero-grade standard ammonium paratungstate (APT) solution are used as raw materials.
APT is converted to AMT by neutralization reaction under appropriate conditions of acidity, temperature, stirring speed and membrane pressure.
The AMT solution was concentrated and washed using a nanofiltration membrane device to obtain a pure AMT solution.
Finally, high purity ammonium metatungstate crystals are obtained through a cooling crystallization process.
Each of these methods has its own characteristics and can be selected based on factors such as raw material sources, product purity requirements, production costs, and environmental requirements. For example, the neutralization and nanofiltration coupling process can reduce the production cost and environmental pollution while obtaining high purity ammonium metatungstate; while the solvent extraction method has high selectivity and sensitivity, which is suitable for application scenarios with very high purity requirements.
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