The surface chemistry of secondary alumina from the dry scrubbing process

Abstract

The primary role of dry scrubbing systems at modern aluminium smelters is to prevent discharge of particulate material and, by adsorption on the primary alumina, prevent emission of gaseous hydrogen fluoride to the environment. Dry scrubbers generally return all of the collected material to the electrolytic cell and thus they form an important element in the smelter's mass balance. Return of fluoride is beneficial as it reduces the smelter's requirement to add aluminium fluoride to maintain constant bath ratio. On the other hand return of hydrogen, in the form of adsorbed water, HF, or chemically bound hydroxyl groups, is not beneficial as it provides a pathway for re-generation of HF in the electrolyte. The products of the dry scrubbing reaction are not well described in the literature. In particular, it has not been possible to determine the form and relative distribution of hydrogen in the dry scrubber product. This paper presents the results of spectroscopic investigations of hydrofluorinated alumina that show water and fluoride to be bound in the form of a hydrated aluminium-hydroxyfluoride phase of variable stoichiometry. Hydrogen therefore occurs as water of hydration that is easily removed at relatively low temperatures, and as hydroxyl groups that are stable to higher temperatures. Hydrogen fluoride release may occur as a result of self hydrolysis of the hydroxyfluoride, or by reaction with the molten electrolyte.