Mechanistic Studies of HF Adsorption on Alumina

Abstract

Aluminium smelters emit upwards of 6 kg of gaseous hydrogen fluoride per tonne of aluminium metal produced. Since the 1960's, many aluminium smelters have used the dry scrubbing process to capture the HF on the surface of smelter grade alumina. In this way, dispersion of the emitted HF is prevented and the fluoride is returned to the aluminium electrolysis cells. The surface adsorption reactions on which the dry scrubbing process relies have been studied by various researchers. It is common knowledge that there occurs a relatively strong bond between HF and alumina and that the quantity of water in the gas effects the maximum fluoride adsorption capacity of the alumina. It has also been considered that FIF adsorption is a combination of strong chemisorption and reversible physisorption at the temperatures at which the dry scrubbing process operates. Some researchers have postulated the formation of Al-F bonds at the alumina surface, while others have postulated the existence of hydrogen bonds between surface fluoride or hydroxyl ions and molecular H2O and HF. However, these models do not agree with the experimental data which was gathered during the course of researching this thesis, nor do they agree with much of the experimental data presented earlier. The major finding of this research was that hydrogen fluoride adsorption is irreversible under the temperature and gas composition conditions of the dry scrubbing process. The maximum fluoride adsorption capacity of smelter grade alumina depends on the relative humidity during adsorption, as well as the specific surface area of the alumina. Both factors indicate that HF adsorption is a surface process. The most likely product of reaction is a thin layer of crystalline aluminium hydroxyfluoride, AIFx(OH)3-x 6H2O, formed in an aqueous reaction at the alumina surface. The reaction mechanism involves the steps of: H2O adsorption to form an aqueous layer on the alumina surface; HF adsorption to acidify the surface water layer; dissolution of the alumina surface to form AlO2- and AlO2-; precipitation of AlFx(OH)3-x.nH2O. The overall adsorption rate appears to be controlled by the rate of the surface chemical reactions rather than by transport phenomena such as fluid phase mass transfer and intraparticle diffusion. At relative humidity greater than 35% the adsorption capacity of smelter grade alumina increases dramatically and small crystallites of AIFx(OH)3-x.6H2O and AIF3.3H2O are formed. Under these conditions the reaction mechanism is similar, except that water-filled pores provide an environment which is conducive to the formation of larger crystallites of product. At temperatures below 450°C the aluminium hydroxyfluoride hydrate phase dehydrates, and at temperatures above 450°C hydrolysis of aluminium hydroxyfluoride results in the release of HF. Samples which are produced under dry scrubbing conditions and aged under ambient conditions, are indefinitely stable. However, when aged at high humidity the product layer is transformed into distinct crystallites of aluminium hydroxyfluoride by a dissolution/re-precipitation mechanism involving water filled pores. Samples which are hydrofluorinated at greater than 35% humidity show continued growth of A1Fx(OH)3-x 6H2O and AlF3.3H2O under all storage conditions due to residual water condensed within the alumina pores.