At present, evaporation crystallization method to prepare ammonium paratungstate (APT) is the most important production method for APT, because it is no need to add extra agent, and will not introduce new impurity, besides, the crystallization process can further remove impurity to ensure the quality of products. The operating methods are batch intermittent and continuous operation two forms. Evaporation and crystallization will use the main equipments of enamel (or lining titanium) reactor and continuous mold.
The batch intermittent operation is carried out in a jacketed, heated enamel (or lined titanium) reactor equipped with a stirrer. The ammonium paratungstate crystallization rate of batch intermittent preparation can generally reach 90% to 95%, if the product purity requirements are higher, the crystallization rate can be controlled lower, and the mother liquor density controlled at a slightly higher level.
The continuous operation is carried out in a continuous crystallizer. The ammonium tungstate solution is externally heated to a certain temperature and then sent to a continuous crystallizer to evaporate the solution to supersatize the solution. Then, the supersaturated solution is filtered and washed through a continuous filter, mixed with the mother liquor and then stirred, cycled, then carried out secondary crystallization, thereby to prepare ammonium paratungstate. This method can obtain APT products with uniform composition and particle size, while the other advantages of continuous process, stable product quality, high production capacity and ammonia recyclable, but the primary crystallization rate is not high (about 80% to 90%).
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