Reaction of Molten Aluminum with MgO and Formation of MgAl<sub>2</sub>O<sub>4</sub> Spinel at 1000°C

The reaction of MgO with molten Al was investigated at 1000°C. When the MgO pellet was heated with Al in an alumina tube, MgAl2O4 spinel formed on both the MgO pellet and the inner surface of the alumina tube. Mg ions were reduced at the interface of the molten Al and MgO pellet, and spinel was formed, although the reaction demonstrated a positive change in the standard Gibbs free energy. The reduced Mg was dissolved in molten Al and reacted at the inner surface of the alumina tube to form the spinel. When Al was melted in the MgO tube, spinel formed at the inner surface of the tube. These reactions can be understood by considering an equilibrium reaction, 4MgO + 2Al ⇄ MgAl2O4 + 3Mg, with activities of Mg and Al in the metal liquid. The formed spinel was black, which was due to the metal aluminum particles inside the formed spinel layer.