In production, for some reason, some Ammonium Paratungstate (APT) products are not qualified in terms of academic composition or physical properties.
A small amount of ammonium paratungstate waste may be produced during the production process, which can be recycled. The method of recovering unqualified ammonium paratungstate is mainly by dissolving and filtering, and then recrystallization is carried out. Therefore the factory directly use ammonia as the solvent at high temperature to dissolve APT, but APT in ammonia water is solubility, lead to dilute solution concentration,the dissolution time is long. Thus, the modern industry began to gradually over the use of APT thermal decomposition into ammonium metatungstate for recycle.
The working mechanism of thermal decomposition is to control the appropriate temperature, calcined APT can be converted to ammonium metatungstate (AMT).
According to the high temperature decomposition performance of APT·H2O, when APT·4H2O is heated in air, it is dehydrated first at about 100 °C and then decomposed at 190 °C to 250 °C to dissolve the amorphous ammonium metatungstate formate. Some scholars have proved that the calcination temperature of the unqualified ammonium tungstate is better at 280 °C, and the leaching rate of the product at the concentration of 25% is the highest, up to 99.6%. So that the calcination treatment is carried out at an appropriate temperature in advance so that the unacceptable APT is better soluble in the NH3 solution and can create better recycle conditions for the unqualified ammonium paratungstate.
After calcination treatment of APT by pure water leaching, pH control in 3-4, after leaching filtration, evaporation and crystallization can be obtained ammonium metatungstate.