Properties of Low-Temperature Melting Electrolytes for the Aluminum Electrolysis Process: A Review

The traditional electrolyte for aluminum electrolysis is composed of molten cryolite (Na3AlF6) and alumina (Al2O3). One of the objectives of the industry is to lower the operating temperature of the electrolytic process, which is currently around 960 °C. The benefits commonly evoked of a temperature decrease in the aluminum plotlines are multiple: reduction of the energetic consumption and thus of the global environmental footprint, increase of the cell life due to limited corrosion, reduction of production costs, etc. In this work, an overview of some properties of molten fluoride systems (Na−cryolite-based electrolytes (NaF−AlF3) and K−cryolite-based electrolytes (KF−AlF3)) is presented. To a lesser extent, the case of all-chloride and mixed chloro-fluoride salts is also discussed. Many physicochemical properties of salt mixtures are of major importance to evaluate the potential of a given electrolyte. When available, the following properties are reported and compared: the liquidus temperature, the electrical conductivity, the density, the vapor pressure, the solubility of alumina, and the solubility of liquid aluminum.