Producing Ammonium Metatungstate from Ammonium Tungstate in Silica advantages

ammonium metatungstate product picture

A particular advantage of the ammonium metatungstate (AMT) prepared from ammonium tungstate in silica is that the silica used in digestion and subsequently filtered from the digested solution of ammonium metatungstate (AMT) may be reused repeatedly, that is, its capacity to promote formation of AMT is not diminished with use. While the exact mechanism is not well understood at this time, the role of silica is to prevent the precipitation of insoluble APT and therefore to allow lowering of the pH to the ammonium metatungstate forming region, about 4 to 4.5.
After digestion is completed, the silica is removed, such as by filtering, and the solid AMT may be recovered in the conventional manner, such as by evaporation or spray drying.

The silica used in the invention may be any commercially available silica, provided that its impurities, both in kind and amount, are consistent with the final envisioned application for the ammonium metatungstate. For example, both sodium and phosphorus may be considered objectionable impurities for certain catalyst applications. Residual silica which cannot be removed from the digested AMT solution by filtering is generally below 1 percent by weight, and typically from 0.3 to 0.6 weight percent WO3 basis, and is compatible with many catalyst applications.

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