Preparation of Ammonium Paratungstate (APT)

Ammonium paratungstate (APT) is a known intermediate to produce metallic tungsten, tungsten-containing catalysts, or tungsten-based hard materials (such as tungsten carbide) or sputtering targets. The latter application requires particularly high purity APT. There are two major source of tungsten which include tungsten ores and recycle of tungsten scraps.

Tungsten is extracted from the ore and converted to the highly pure intermediate product ammonium paratungstate (APT) by hydrometallurgical methods, i.e. processing in aqueous solutions. The ore containing scheelite (CaWO4) or wolframite ((Mn, Fe) WO4) is crushed and enriched by flotation technology to obtain a concentrate containing 10-75% WO3. These raw materials are first calcined in an oxidizing atmosphere at 500-600°C to remove surfactants added to facilitate the flotation process. Wolframite concentrate is leached in sodium hydroxide solution, while scheelite stabilized in NaoOH (aq) is treated with sodium carbonate solution. The obtained sodium tungstate solution is further purified through many precipitation reactions to remove elements such as Si, P, F and Mo. The next step is to exchange ammonium with sodium in a solvent extraction or ion exchange step. The resulting ammonium tungstate solution is then heated to evaporate water and ammonia, thereby increasing the tungstate concentration, and lowering the pH of the solution. The paratungstate ion is formed with ammonium and crystallized as APT.

Another source of raw material is tungsten scrap, which can be converted into sodium tungstate by oxidation and into APT in the same way. An environmentally friendly method can also save costs, because the waste is rich in tungsten and low in impurities. The purity of tungsten intermediates (such as APT) is very important because it also limits the purity of the final tungsten material. Due to the improvement of the hydrometallurgical process, the level of polluting elements in tungsten materials has been partially reduced. An important step is to introduce modern solvent extraction or ion exchange technology to convert sodium tungstate into ammonium tungstate solution. APT is also used as a starting material for the manufacture of other tungsten compounds, such as tungstic acid, ammonium metatungstate (AMT), tungsten trioxide (WO3) and slightly reduced tungsten oxide blue (TBO), or tungsten compound.

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