Electrolytic Reduction of Solid Al₂O₃ to Liquid Al in Molten CaCl₂

Abstract

The electrochemical reduction of Al₂O₃ has been investigated in molten CaCl₂ at 1123 K. To predict the electrochemical reduction behavior depending on the activity of O²⁻ ions, the potential–pO²⁻ diagram for the Al–Ca–O–Cl system is constructed from thermochemical data. In a Mo box-type electrode, an Al₂O₃ tube is successfully reduced to liquid Al with a maximum purity of 98 at%. However, in the electrolysis of Al₂O₃ powder in an Fe box-type electrode, Al₂Ca is produced through the formation of Ca₁₂Al₁₄O₃₃ as an intermediate product. The different electrochemical reduction behaviors of the tube and the powder are explained by the different diffusion path lengths for O²⁻ ions from three-phase zone (Al₂O₃/CaCl₂/cathode metal) to bulk CaCl₂.