One embodiment of the invention resides in a process for ammonium metatungstate production consisting essentially of fluid bed roasting ammonium paratungstate at a temperature over the range of about 275℃ to 305℃. for a time at least sufficient to provide an ammonium metatungstate "precursor" consisting essentially of an ammonium tungstate of an average ammonia to tungsten mole ratio of about 0.5, forming an aqueous slurry of the precursor and digesting the slurry at a pH of about 3 to 4 for a time at least sufficient to effect substantially complete dissolution of said contained precursor, filtering the digested slurry to produce a clear filtrate, and then further processing the filtrate by evaporation to provide crystals of substantially pure ammonium metatungstate.

 

The time at temperature may be relatively short and range from over 10 minutes to about 2 hours, for example, from about 15 to 100 minutes, e.g. about 20 to 60 minutes. In a preferred embodiment, the temperature may range from about 280℃ to 295℃.

 

Previous methods do not define the relationship between roasting time and temperature and, in fact, these methods usually indicate that tungstic acid and unconverted ammonium paratungstate generally result from roasting. Also, the digestion temperatures are specified as ranging from 70℃ to below the boiling point at slurry concentrations of 9 to 10% by weight.

 

On the other hand, the time-temperature relationship is such for fluid bed roasting that consistently high conversion is achieved. Moreover, it has been discovered how over/under roasting can be corrected during digestion at the relatively high slurry or pulp densities of about 10% to 35% by weight at digestion temperatures in the range of about 90℃ to the boiling point.

 

EXAMPLE 

APT was calcined in a fluid-bed reactor at 283° C. for a residence time of about 60 minutes. The roasted product was then digested by adding 10 grams of calcine to 100 ml of water (i.e., 100 grams) for 20 minutes at the boiling point of the solution. The solution had a pH of 3.6. The recovery efficiency of soluble tungsten came to 99.7%. The solution was filtered and the clear filtrate spray-dried at 110° C. to obtain crystalline AMT containing 0.013% of insolubles by weight.