Common production methods of ammonium paratungstate (APT) include evaporation, neutralization, crystallization, freeze-drying and solvent extraction. Among them, the solvent extraction method is a new method that has just been designed in recent years.
1.Evaporation Method
According to the different uses of APT, the evaporation process is also different. For example, the APT used in the production of tungsten wire usually adopts a jacketed steam-heated evaporator, and the steam pressure is generally controlled at (0.5～2.0)×105Pa. There is also a jacket-heated agitation tank, which is equipped with a vacuum system on the top cover. The connected vacuum pipe maintains a vacuum of about 10.67～13 33kPa during evaporation. You can also insert three compressed air pipes from the top cover and a compressed air pipe from the bottom of the reaction tank to pass in the compressed air. While the compressed air is passed through, the vacuum is drawn at the same time, to speed up the discharge of ammonia in the solution.
In the evaporation method, the APT solution can also be treated with hexamethylenetetramine, and then evaporated to obtain APT crystals. The specific steps are as follows: add hexamethylenetetramine to the ammonium tungstate solution, then adjust the pH to 5～2.0 with sulfuric acid, and the temperature cannot exceed 20℃. Then filter, dissolve the filtrate in ammonia water, and then heat to evaporate , When the solution is only 25% of the original solution, stop the evaporation, so that the tungsten content of the product will reach 90%.
The advantages of the evaporation method are simple operation and low cost; the disadvantages are long production cycle, large energy consumption, and low crystallization rate.
2.Crystallization Method
First, evaporate the solution in a constant temperature water bath at 50-70°C for 1 to 2 hours, then neutralize it with hydrochloric acid to a pH of 7.0-7.5, then evaporate to a mother liquor density of 1.08-1.109/mL, and finally discharge and cool.
The method is characterized by high crystal purity, easy control of particle size, and fast crystallization speed.
3.Freeze-drying Method
Ammonium tungstate solution and liquid nitrogen are injected into an enameled dish containing liquid nitrogen in a ratio of 1:2～3 (volume), and a little stirring is used to obtain a paste product, and it will be quickly moved together with the enameled dish to dry In the equipment, the product can be obtained by subliming and removing water under a certain degree of vacuum.
The characteristic of the production method is that uniform fine powder or various uniform composite powders can be obtained, and the impurities and components of the finished product can be better controlled.
4.Solvent Extraction Method
During extraction, the pH of the ammonium tungstate feed solution should be greater than 9, and the concentration of W03 should be 100～2009/L. Mix the feed solution with the extractant EPA and shake to extract the ammonium ion from the feed solution to reduce the pH value of the feed solution. Then control the pH of the raffinate to 6-7. After standing for phase separation, digest the raffinate at 60°C for 5 hours to obtain the product with a crystallization rate as high as 96%.
The production method is characterized by high crystallization conversion rate, high product purity, short production cycle, low energy consumption, but more one-time investment.
