Solubility of Ammonium Paratungstate in Aqueous Ammonia after Low Temperature Calcination

Ammonium paratungstate (APT), is a prevalent starting material in the production of tungsten oxides, catalysts, tungsten alloys, electrochromic materials and so on. It is produced by crystallization from purified ammonium tungstate solutions in batch and continuous processes. Commonly, repeated crystallization is used as a method of improving the quality of the product.

APT dissolves extremely slowly in aqueous ammonia at room temperature; no equilibrium is reached even after 60 days reaction time. Increasing the solution temperature speeds up the reaction, but the total amount of dissolved APT decreases due to the lower solubility of ammonia at higher temperatures, which results in a solution with a lower pH. As the rate of dissolution of APT particles in aqueous ammonia is independent of the size of the particles.

To increase the dissolution rate, low temperature calcination has been applied to the APT to investigate its solubility in aqueous ammonia. The experimental procedures are as follows:

2.5 g of APT were calcined at temperatures varying from 220 to 340 ° C in air and nitrogen (100l/h). In addition, to evaluate the effect of a reducing environment, calcination experiments were performed under hydrogen gas (101 Hz/h) and in an atmosphere of hydrogen and nitrogen with a ratio 1:4 (total flow 50 l/h) for a residence time of 2 h. A horizontal tube furnace with an internal diameter of 3 cm was used. The height of the powder layer was approximately 3 mm.

The calcined product was dissolved in a 100 ml vessel for 30 min. A 25% aqueous ammonia solution was used as lixiviant. The WO3 content of the solution was determined using a gravimetrical method. The oxygen index of the powder was determined using the method of Kiss and Tisza, in order to determine whether reduction of tungsten W 6+ occurred during calcination.

it is believed that the rate-controlling step of APT dissolution in aqueous ammonia takes place in the solution. Calcination of APT at temperatures ranging from 220 to 300 °C will produce a product which dissolves relatively quickly in aqueous ammonia. Over 98% of the product can be dissolved within 30 min in aqueous ammonia when a weight loss of approximately 8.5-9.5 wt% is reached.

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