Process for Producing Ammonium Metatungstate from Ammonium Tungstate by Digestion in Silica - Patent US3956474 A
A process for producing ammonium metatungstate (AMT) from ammonium tungstate (AT) 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.

Production of Ammonium Metatungstate - Patent EP0039135
Ammonium metatungstate is produced by contacting the anolyte side of a cation exchange membrane with an aqueous solution of ammonium tungstate and applying an electrical potential across the membrane such as to effect the transport of ammonium ions from said solution through the membrane to lowerthe pH of the solution to a level at which ammonium metatungstate is formed.

Method of Producing Crystalline Ammonium Metatungstate - Patent 4626424
This invention relates to a process for producing a relatively pure solid ammonium metatungstate by precipitating the solid from a mixture of ammonium metatungstate solution and methyl alcohol.

Process for Preparing Ammonium Metatungstate - Patent 20120141359
A process for preparing ammonium metatungstate using ammonium paratungstate as raw material comprising: (A) leaching wet ammonium paratungstate with nitric acid to obtain a dilute solution of ammonium metatungstate; (B) concentrating the dilute solution of ammonium metatungstate to be a concentrated solution of ammonium metatungstate; and (C) spray-drying the concentrated solution of ammonium metatungstate to obtain powder of ammonium metatungstate.

Method of Producing Crystalline Ammonium Metatungstate - Patent US 3175881 A
This invention relates to an improved method for producing ammonium metatungstate. More particularly, the invention is concerned with a method for preparing ammonium metatungstate in crystalline form employing ammonium paratungstate as the starting material.

Process for Producing Ammonium Metatungstate from Ammonium Tungstate by Ion Exchange - Patent US 3857928 A
A process for producing crystalline ammonium metatungstate (AMT) from ammonium tungstate (AT) solution by introducing AT solution into an ion exchange column containing a weak acid (carboxylic group) cation exchange resin, and collecting the effluent from the column until a pH of about 3.5 is reached. The effluent solution is then digested at about 98 DEG C for about 5 hours, followed by crystallization of AMT by conventional processes, such as, evaporation or spray drying.

Method for Making Ammonium Metatungstate - Patent 8435483
A method for producing ammonium metatungstate from ammonium paratungstate includes preparing an ammonium paratungstate mixture including solid ammonium paratungstate and water. The mixture is contacted with a cation exchange material to lower a pH of the mixture to a pH range wherein metatungstate ion is stable and an insoluble tungstic acid is not formed. The pH of the mixture may be maintained in the pH range until the ammonium paratungstate mixture is converted into an ammonium metatungstate solution.

Method of Producing Crystalline Ammonium Metatungstate - Patent 4612182
A method is disclosed for producing crystalline ammonium metatungstate from ammonium paratungstate. The method involves heating theammonium paratungstate at from about 100° C. to about 250° C. for from about 1 hour to about 8 hours to remove some ammonia and water, digesting a solution of the heated ammonium paratungstate in water at from about 80° C. to about 100° C. for from about 2 hours to about 6 hours at relatively constant volume while maintaining the pH of the slurry at from about 4.2 to about 3.0 by addition of ammonia as necessary to form a solution of ammonium metatungstate in water. The ammonium metatungstate solution is then evaporated to concentrate it. Any insoluble material is separated from the solution. Ammonium metatungstate is then crystallized from the resulting concentrated ammoniummetatungstate solution.

Process for Producing Ammonium Metatungstate - Patent 4504461
A process is provided for producing ammonium metatungstate comprising fluid bed roasting ammonium paratungstate at a temperature over the range of about 275° C. to 305° C. for a time at least sufficient to provide a roasted product in the form of a precursor of ammonium metatungstate, forming an aqueous slurry of the precursor and digesting the precursor at a pH of about 3 to 4 for a time at least sufficient to effect substantially complete dissolution of said precursor, filtering the digested slurry to produce a clear filtrate, and further processing the clear filtrate by evaporation to provide crystals of substantially pure ammonium metatungstate.

Method for Producing Ammonium Metatungstate from Ammonium Paratungstate - Patent EP0200170A3
A method is disclosed for producing ammonium metatungstate from ammonium paratungstate. The method involves first roasting the ammonium paratungstate at from about 275 DEG C. to about 300 DEG C. for from about 10 hours to about 20 hours to remove some ammonia and some water. The roasted ammonium paratungstate is then added to dilute solution of an ammonium metatungstate which is at a pH of from about 3.6 to about 4.2, the rate of addition being such that a slurry is produced having a pH of from about 3.6 to about 4.2. The pH of the resulting slurry is then adjusted to stabilize it at from about 3.6 to about 4.2 followed by evaporation to form a relatively concentrated solution of ammonium metatungstate which is then separated from any insolubles. Ammonium metatungstate is then crystallized from the relatively concentrated ammonium metatungstate solution.