Reactivity of Alumina toward Phosphoric Acid

Reaction between alumina and 33.3 wt% orthophosphoric acid was investigated by monitoring the heat liberated under isothermal conditions at temperatures from 25° to 90°C. In a separate set of experiments, the H 3 PO 4 concentration was varied from 0 to 50 wt%, at 25°C. Reactivities of five aluminas (three anhydrous and two hydrated) differing in particle size, surface area, and crystallinity were studied. Relationships between the properties of the aluminas and their reactivities toward phosphoric acid were established. The aluminas with the highest surface areas and the lowest degrees of crystallinity react more rapidly and produce overall more heat. Increasing the temperature and phosphoric acid concentration were also shown to increase heat evolution. However, increasing the H 3 PO 4 concentration beyond 33.3 wt% (molar Al/P ratio = 1.0) for the anhydrous aluminas, and beyond 40 wt% for boehmite, does not result in a significant increase in the amount of heat evolved. Gibbsite continues to release greater amounts of heat when reacting with increasing concentrations of H 3 PO 4 (up to 50 wt%). The anhydrous aluminas generally react faster than do the hydrates. Within the range of H 3 PO 4 concentrations from 0 to 33.3 wt% the hydrates and the most reactive anhydrous alumina exhibit approximately the same degree of reactivity on a per mole of Al 2 O 3 basis.