Structure of High-Temperature NaF−AlF₃−Al₂O₃ Melts: A Multinuclear NMR Study

Abstract

Fluoroaluminate molten salts are used in the Hall-Heroult industrial process for the production of aluminum by electrolysis. To better understand the mechanism of the dissolution of alumina (Al2O3) in cryolitic melts, we have studied the structure of these liquids up to 1025 °C by in-situ NMR of 27Al, 23Na, 19F, and 17O, using a laser-heated experimental setup. From these experiments we can propose a quantification of the different aluminum-bearing species AlF4-, AlF52-, and AlF63- in the liquid and give a direct experimental evidence of the existence of at least two different oxy-fluoride species, Al2OF62- and Al2O2F42-, when dissolving alumina in the fluoride liquid. The obtained results are compared with other spectroscopic data obtained by in-situ Raman spectroscopy.