Ammonium metatungstate (AMT) is an important intermediate compound during the tungsten wet smelting process, white or yellowish crystals, which has the highest solubility in all tungsten compounds (303g/100mL under 20°C in water). There are many preparation methods of ammonium metatungstate, such as thermal decomposition, tungsten acid, electrodialysis, solvent extraction, ion exchange method, while the thermal decomposition method based on the raw material of APT is the main process for manufacturing AMT in industrial currently.

 

Being one class of preparation method, acid neutralization method has been focused and widely used in the industry because of its simple process and equipment. Some studies have followed on the nitric acid neutralization method, taken ammonium paratungstate (APT) as raw material to convert and generate AMT in an environment of nitric acid, which is heating a certain amount of deionized water, and slowly adding APT and dilute nitric acid solution with the ratio of 1:20 with stirring, then controlling the transformation temperature within limits to convert APT to AMT in the liquid phase and generate AMT solution, and finally get ammonium metatungstate crystal after the steps of ageing, preconcentration, concentration, crystallization, solid-liquid separation and drying. Wherein, the reaction equations of APT converts to AMT by nitric acid neutralization method is like:

(NH₄)₁₀W₁₂O₄₁.xH₂0+4HNO₃= (NH₄)₆H₂W₁₂O₄₀.yH₂0+4NH₄NO₃+H₂O

 

The nitric acid in this method can be replaced by organic acids, such as formic acid, acetic acid, citric acid, lactic acid, with the advantages of moderate acidity and easy to operate; but, the expansive price of organic acids makes them hardly to be used in the mass production of AMT in industrial. However, although the nitric acid being used in the nitric acid neutralization method has the benefits of easy to get and low cost; but, because nitric acid has a strong acidity which will make the generation of tungstic acid colloid becoming easily, and introduces impurity of nitrate into AMT, thereby to decrease the converting rate and property of AMT.