Vaporization of Fluorides from Aluminium Cells, Part I: Theoretical Study on Replacement of Aluminium Fluoride and Soda

Commercial alumina contains Na2O and CaO, which must be "neutralised" by regular additions of AlF3 to the bath in aluminium electrolysis cells. As a result, most smelters are net bath producers. The present paper suggests that part of the AlF3 can be replaced by the condensate formed by partial vacuum vaporization of tapped-off bath, thereby reducing the amounts of deposited bath. The condensate may have a composition close to the main evaporation product (NaAlF4), while the remaining bath will be rich in NaF and can be used for replacing soda. Data for vapour pressure and condensate composition are presented. A bath mass balance is derived in terms of Na2O and CaO in the alumina, comprising replacement of AlF3 with condensate from produced bath. Different process options are discussed, including repeated cycles of vaporization and condensation.