Anionic Structure in Molten Cryolite–Alumina Systems

Abstract

For aluminum production, the alumina (Al2O3) dissolution in the electrolyte is one of the important step in the industrial process. The electrolyte is a cryolitic bath containing mainly NaF and AlF3 at around 1000 °C. In this liquid, the main anionic species are fluoroaluminates ions such as [AlF6]3–, [AlF5]2–, [AlF4]−, and F–. During the Al2O3 dissolution, different kinds of oxyfluoroaluminates species are formed. However, due to the difficult experimental conditions and the large number of potential ions, no quantitative speciation could be made up to now. Here, we propose a speciation of alumina–cryolite melts combining in situ NMR experiments, classical molecular dynamics simulations, and electronic structure calculations. This allows us to establish the nature and quantities of each species, depending on the Al2O3 concentration, for two different initial cryolitic compositions. In particular, we show that the O atoms are always linked to at least two Al atoms, leading to the formation of polymeric oxyfluorides. From the dynamic point of view, the simulations also show that the lifetime of the O–Al bonds is also much longer than that of the F–Al ones.