Classic Chemical Purification Method Producing Ammonium Paratungstate

The classic chemical purification method to prepare ammonium paratungstate (APT) has a long process, which is going through purification of crude sodium tungstate, producing tungstic acid from sodium tungstate and then to get ammonium tungstate, then getting APT by evaporation crystallization, with the process like bellow:

1. Transfer tungsten to liquid phase by the form of sodium tungstate (Na₂WO₄) from tungsten concentrate by sodium carbonate (NaCO₃), and then go on filtration, leaching, pressure filtration process to obtain crude sodium tungstate solution, the reaction equations are as following:
Na₂CO₃+FeWO₄=FeO↓+ Na₂WO₄+CO₂↑
Na₂CO₃+MnWO₄=MnO↓+ Na₂WO₄+CO₂↑;
2. Add calcium chloride (CaCl₂) solution after the crude sodium tungstate solution is purified to deposit calcium tungstate (CaWO₄), which is the artificial scheelite, the reaction is as follows:
Na₂WO₄+ CaCl₂=CaWO₄↓+2NaCl
3. Decompose the artificial scheelite with acid to get tungstic acid, and then add ammonia water to dissolve it and get ammonium tungstate solution, which is the industrial grade ammonium tungstate, the reaction is as follows:
CaWO₄+2HCl=H2WO₄↓+ CaCl₂ H₂WO₄+2NH₄OH=(NH₄)₂WO₄+2H₂O
4. Evaporation and crystallization the ammonium tungstate solution to get APT.