In the production, sometimes for some reason lead to part of the APT products in the classics or physical performance on the failure, may also occur in the production process a small amount of ammonium paratungstate waste recycle, which are recyclable.
Recycle of unqualified ammonium paratungstate method is mainly after the dissolution of the filter, and recrystallization, for this reason, the APT is dissolved at a higher temperature directly with ammonia as a solvent, however, APT solubility in aqueous ammonia is limited, resulting in dilute solution concentration, it takes for a long time to dissolution. Therefore, the modern industry began to use of APT thermal decomposition into ammonium metatungstate for recycle.
Thermal decomposition of the working mechanism is to control the appropriate temperature, calcined APT can be converted to ammonium metatungstate (AMT).
According to the high temperature decomposition performance of APT·4H2O, when heated APT·4H2O in the air, First dehydrated at about 100 °C, and then decomposed at 190 °C to 250 °C to dissolve the amorphous ammonium formate. Some scholars have experimented with unqualified calcined tungstate calcination temperature at 280 °C is better, the product has the highest leaching rate in aqueous ammonia at 25% concentration, it can up to 99.6%. Therefore the calcination treatment is carried out at an appropriate temperature in advance so that the unqualified APT can better soluble in the NH3 solution and can create better recycle conditions for the unqualified ammonium paratungstate.
After the calcination treatment of APT by pure water leaching, leaching pH control in 3-4, after leaching filtration, evaporation crystallization can be obtained ammonium metatungstate.