A process for producing ammonium metatungstate (AMT) from ammonium tungstate solution is described which involves the addition of about 3.6 percent by weight of silica to an AT solution, digestion for at least about 4 hours at a temperature of at least about 98°C, followed by filtration to remove the silica from the AMT solution. Typically about 0.4 percent by weight of silica remains after filtering. The resulting AMT solution may be further processed to recover solid AMT, such as by evaporation or spray drying.

 

In accordance with the invention, it has been discovered that contacting an ammonium tungstate (AT) solution with at least about 1.5 percent by weight of silica (WO3 basis), and digesting the solution while in contact with the silica at a temperature of at least about 98°C for at least about 4 hours results in substantially complete conversion of AT solution to AMT solution without appreciable formation of APT, which when filtered and crystallized yields solid AMT containing from about 0.3 to about 0.6 percent by weight residual silica.

 

EXAMPLE: To 3600 milliliters of AT solution containing 2.4 pounds WO3 per gram of solution were added 30 grams of silica as a finely divided powder obtainable under the tradename CAB-O-SIL. The resulting slurry was digested for about 6 hours at 98°C after which the volume of the solution had decreased to about 1000 milliliters and the pH of the solution had decreased to about 4. The silica gel which had formed was then filtered from the solution and subsequently recycled. The solution was then spray dried at a temperature within the range of about 350°C to 125°C. Upon analysis it was revealed that the spray dried product contained 0.54 weight percent silica and 0.023 weight percent sodium, WO3 basis. X-ray diffraction analysis confirmed that the solid product was ammonium metatungstate.