Ammonium Paratungstate Preparation by Evaporation Crystallization

Ammonium paratungstate is a kind of chemical substance, mainly white crystal, with two kinds of flake or needle, which is used to produce tungsten trioxide or blue tungsten oxide to make tungsten powder. It is also used to manufacture ammonium metatungstate and other tungsten compounds, and used as additives in petrochemical industry.

It is mainly used for manufacturing tungsten trioxide or blue tungsten oxide metal powder; the downstream products of tungsten powder include tungsten material series, such as tungsten bar, tungsten wire and other electric vacuum materials; alloy series, such as tungsten carbide, hard alloy, alloy blade, alloy drill, alloy mold, etc.; other wear-resistant, pressure resistant, high temperature resistant mechanical equipment parts, etc.

At present, the most important production method of apt in industry is to prepare ammonium paratungstate (APT) by evaporation crystallization, because there is no need to add reagents, no new impurities are introduced, and impurities can be further removed in the crystallization process, which is conducive to ensuring product quality. There are two operation modes: batch intermittent operation and continuous operation. The main equipment used in evaporative crystallization are enamel (or titanium lined) reactor and continuous mold.

Batch intermittent operation is carried out in an enamel (or titanium lined) reactor with a stirring device heated by a jacket. The crystallization rate of ammonium paratungstate can reach 90% ~ 95% by batch intermittent operation. If the purity of the product is high, the crystallization rate can be controlled lower, and the mother liquor density can be controlled at a slightly higher level.

Continuous operation is carried out in continuous mold. The ammonium tungstate solution is heated to a certain temperature outside and then sent to the continuous crystallizer for evaporation to make the solution supersaturated; then, the supersaturated solution is filtered and washed by the continuous filter, mixed with the mother liquor, and stirred with it. In this way, the solution is recycled for secondary crystallization, so as to prepare the solution. APT products with uniform composition and particle size can be obtained by this method. The process is continuous, the quality is stable, the production capacity is large, and ammonia can be recovered. However, the primary crystallization rate is not high, generally between 80% and 90%.

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