DISSOLUTION OF AMMONIUM PARATUNGSTATE TETRAHYDRATE IN AQUEOUS AMMONIA BEFORE AND AFTER LOW-TEMPERATURE CALCINATION

Ammonium paratungstate tetrahydrate, APT.4H2O dissolves rather slowly in aqueous ammonia at room temperature. Dissolution at higher temperatures increases the rate of dissolution, but the amount of dissolved tungsten decreases due to the lower solubility of ammonia at higher temperatures which results in a lower pH of the solution. The rate of dissolution was found to be independent of the crystal size. It is therefore concluded that the rate-controlling step occurs in the solution, and is probably H2W124210- --> 2HW6O215-. Calcination of APT.4H2O in air, nitrogen and hydrogen/nitrogen mixtures, into an X-ray amorphous product, is shown to be one way to improve the dissolution rate and to increase the dissolved amount of APT.4H2O. The highest solubilities can be obtained by calcining APT.4H2O in air or nitrogen at temperatures ranging from 250 to 310-degrees-C for approximately 2 h. When the temperature is too low or the residence time is too short some crystalline APT.4H2O will still exist; WO3 is formed when the temperature is too high or the residence time too long. It is supposed that the large H2W12O4210- structure is broken during the calcination process.