EFFECT OF ADDITIVES ON THE FORMATION OF MGAL2O4 FROM MGO AND AL2O3

The mixture of Al2O3 and MgO at a mole ratio of 1:1, was fired between 1173 and 1623 K in air, to investigate the effects of the particle size and type of Al2O3, and the kind of additives on the formation rate of MgAl2O4 and, at the same time, to elucidate the mechanism of the reaction. The results are shown as follows. (1) MgAl2O4 was formed more rapidly from Al2O3 than from Al2O3. (2) The addition of CaB4O7, B2O3 and NaF accelerated the formation of MgAl2O4, but the addition of CaF2, CaCl2 and CaCO3 in an amount of 1 mass% or below had little effect on this reaction. (3) With the progress of the reaction, the Al2O3-MgAl2O4 interface moved from the outside to the inside of Al2O3 particle, whereas the MgAl2O4-MgO interface moved to the outside of the particle. On the basis of this finding, the reaction rate was analyzed using the solid-diffusion controlled equation which includes a correction factor for the change of molecular volume. As a result, the apparent activation energy value was estimated to be 316 kJ/mol for the addition of 1 mass%CaB4O7 and 270 kJ/mol for no addition. These values were approximated by the activation energy value of the interdiffusion of Mg2+ and Al3+ within the defective MgO-Al2O3 spinel.