The neutralization method in ammonium metatungstate production is an important production process, which can be divided into the following steps:
Raw material preparation
The neutralization method mainly uses secondary ammonium metatungstate and nitric acid as raw materials. Add secondary ammonium metatungstate to deionized water and ammonium citrate to form a slurry containing 12% to 30% WO₃.
Acidification process
Add 3% to 5% dilute nitric acid to the ammonium paratungstate slurry according to the ratio of ammonium paratungstate to nitric acid (such as 1:20). The purpose of this step is to adjust the pH value of the solution by acidification, usually in the range of 2 to 4.
Heating and stirring
Heat the mixed solution to 80 to 90°C and stir continuously. The stirring speed can be set to, for example, 80 r/min. This step can promote the reaction and the uniformity of the solution.
Control pH value
During the heating and stirring process, the pH value of the solution needs to be controlled at a constant temperature and volume to ensure that it is within the predetermined range.
Reaction and aging
The stirring reaction lasts for about 4 hours, and then the solution is aged for several hours to ensure that the reaction is fully carried out.
Concentration and crystallization
The aged solution is concentrated until the WO₃ content reaches about 50%. Subsequently, crystals of ammonium metatungstate are obtained by cooling and crystallization.
Filtration and drying
The insoluble matter in the solution is filtered out, and then the clear liquid is further concentrated, cooled and crystallized, and dried at 80°C to finally obtain the ammonium metatungstate product.
The neutralization method is characterized by a relatively simple process and a high solution conversion rate, but it consumes a large amount of acid, is costly, and has environmental risks. Despite this, it is still one of the commonly used processes for preparing ammonium tungstate in the early days.
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