Adsorption of hydrogen fluoride on alumina

In the smelting of aluminium, HF fumes are produced that are subsequently trapped by absorption onto alumina. The factors that affect the adsorption capacity of alumina have been studied previously and are weU established, but the mechanism by which HF adsorbs onto the alumina surface is not well understood. In this study, x-ray photoelectron spectroscopy (XPS) was used to investigate the nature of the surface adsorp tion of HF on alumina. XPS is particularly weU suited to the study of this type of gas adsorption process. Laboratory-prepared samples were studied with particular interest in evidence for AI-F bonding, or in fluoride species formed by reaction with -OH or -0. Also of interest was the role of sodium, since it is segregated to the surface of the alumina during calcination. AI-F bonding was observed on only one sample type. An AI-F interaction was identified when the alumina had been predried and dry HF was adsorbed. When moisture was present no AIF, formation was observed. This suggests that under conventional conditions (i.e. moisture present) the adsorption of HF involves a weak interaction, probably hydrogen bonding, with intermediate layers of water. After heating the samples containing weakly bound HF to 500 “C, no Al-F interaction was observed. Much of the HF was desorbed at 700 OC. An Na-F interaction was observed in all fluoride-adsorbed samples; however, this can only account for a small proportion of the total fluoride adsorbed.