The Electrical Conductivity of Molten Oxide-Fluoride Cryolite Mixtures

A new way to reduce the energy consumption during the operation of powerful aluminum reduction cells is suggested via reducing the resistance of the electrolyte, i.e., increasing its electrical conductivity. The electrical conductivity of molten cryolite mixtures NaF-AlF₃-CaF₂-Al₂O₃ with cryolite ratio (CR) of 2.1-3.0 and content of CaF₂ and Al₂O₃, up to 8 wt%, was measured at the temperatures from liquidus to 1300 K. Based on the experimental results, a multifunctional equation for the electrical conductivity of oxide-fluoride cryolite melts was evaluated. The experimental and calculated values of the electrical conductivity agree within 1.5%. The activation energy of the electrical conductivity of the NaF-AlF₃-CaF₂-Al₂O₃ melts was estimated. The activation energy of electrical conductivity for molten NaF-AlF₃ mixtures with CR 3.0 and 2.1, determined by the most mobile cations Na⁺, increased from 15.8 kJ/mol up to 18.5 kJ/mol. It was found that CR had a greater impact on the activation energy than the changes in the Al₂O₃ or CaF₂ concentrations. Based on the ratio of the activation energies of the electrical conductivity and the viscous flow, the correlation between the electrical conductivity and viscosity of molten cryolite mixtures NaF-AlF₃-CaF₂-Al₂O₃ was illustrated.