A Study of the Equilibrium Adsorption of Hydrogen Fluoride on Smelter Grade Aluminas

Abstract

Equilibrium adsorption isotherm data at 80°C were fitted to the linear form of the Langmuir equation for the reaction of five typical smelter grade aluminas with low concentrations of HF (20–1100 ppm vol.) in dry air. This shows that multilayers of fluoride are adsorbed on the active surface, since a monolayer site area of 3.3 to 5.4 Ų is calculated from this procedure, which is not consistent with 11.3 Ų found from conventional molecular theory. Heating tests and extractions with water and sodium hydroxide show that at least 95% of the fluoride adsorbed from dry or humid gas is strongly held on the alumina surface. The amount of fluoride adsorbed is at least doubled by the presence of about 5% volume of water vapour in the gas. The effect of water vapour is non-linear, rising rapidly at 1–3% and levelling off at 5–8%. Neither the equilibrium nor the reaction rate is significantly affected by temperature change from 20° to 80°C, suggesting that the adsorption rate is probably controlled by mass transfer of HF to the reacting surface, rather than by chemical reaction. The fluoride concentration adsorbed at equilibrium provides a laboratory method for measuring and comparing alumina reactivities to HF.