Ammonium paratungstate is produced by separating tungsten from its ore. Once the ammonium paratungstate is prepared, it is heated to its decomposition temperature, 600 °C. Left over is WO3, tungsten(VI) oxide. From there, the oxide is heated in an atmosphere of hydrogen, reducing the tungsten to elemental powder, leaving behind water vapor. From there, the tungsten powder can be fused into any number of things, from wire to bars to other shapes.

 

Adding Na3PO4 in alkali decomposition process is common for the impurities in calcium (Ca) in  tungsten concentrates, which makes it as insoluble Ca3 (PO4) 2 go into the leach residue, thereby avoiding its combination with WO4- and formation of CaWO4 precipitate remaining in the slag, to improve the leaching rate of tungsten. 

 

In wolframite concentrate leaching process, impurities Sn, Si, P, As, also have a certain amount of leaching. In alkaline solution, they exist as SnO3 2-, SiO3 2-, HPO4 2-, HAsO4 2- and other forms, these ions can form insoluble compounds with Ca2 + and settle down. Thus, under certain conditions (theoretical amount of base in an amount of 170 to 190, a temperature of 160 ~ 170 ℃), without or with a small mount of Na3PO4 (even add some calcium compounds such as lime), calcium and the reaction with these anions can be taken great advantage, to reduce the tin content of sodium tungstate solution. Some minerals take other measures to reduce the amount of sodium phosphate, tin leaching rate can be reduced by 25% to 40%.

 

The solubility product of CaWO4 is small, so some people think it controversial to inhibit impurities with calcium compound as additives.