Neutralization method to prepare ammonium paratungstate (APT) is carried out in a enamel reactor with jacket heating or lining rubber reaction tank. The process is:
1. Inject ammonium tungstate (NH4)2WO4 solution into the reactor and raise the temperature to the required value (The temperature is controlled at different value according to the particle size of APT, generally, the 70 ~ 75°C will generate medium particles, upper than 90°C will get coarse particles, while temperature bellow 50°C will get fine particles);
2. Slowly add 10% ~ 20% concentrate of hydrochloric acid under vigorous stirring, and finish the acid adding until the pH value is neutralized to 7 ~ 7.5;
3. Stirring 0.5 hour under heat insulation to make the pH value stable, and then standing for 8 ~ 10h until 90% ~ 95% of tungsten in the ammonium tungstate separate out in the form of ammonium paratungstate;
4. Then go on a filtration, and wash the crystal with 1% concentration of ammonium nitrate solution.
Compared with the evaporation crystallization method, the neutralization method has the advantages of small energy consumption, high crystallization speed, easy to obtain fine particles APT; however, since the neutralization method need to add hydrochloric acid which will is bound to introduce impurities, and the crystallization of large mother liquor volume will increase burden of mother liquor recovery system.