Kinetics of and an adsorption model for the dissolution of synthetic WO 3 in aqueous ammonia

Abstract

Ammonium paratungstate tetrahydrate (APT) dissolves slowly in aqueous ammonia. The complete calcination of APT into crystalline WO 3, as a way to improve the dissolution characteristics of APT in aqueous ammonia solutions was studied. Calcination experiments were carried out in air at 550°C. The dissolution rate of this calcine was investigated as a function of various solution temperatures, ammonia concentrations, particle sizes and agitator speeds. The dissolution rate was found to be controlled by chemical reaction and has an activation energy of 73±3 kJ/mol. A model based on the adsorption of ammonia ions at the particle surface could adequate describe the asymptotic increase of the dissolution rate at higher ammonia concentrations. Complete calcinations at 650°C and 800°C resulted in a decreased dissolution rate. This could be attributed to a decreasing specific surface area of the calcine with increasing temperature. Calcination at 450°C resulted in a mixture of hexagonal WO 3 and a phase which is iso-structural with hexagonal ammonium tungsten bronze. This mixture dissolves more slowly in aqueous ammonia than does pure WO 3.