Effects of MgCl2 on Solid State Reaction in MgO-Al2O3 System

Abstract

The solid state reaction in the MgCl2.6H2O-α -Al2O3 system was carried out and the promoting action of MgCl2 on the formation of MgAl2O, was discussed. Firstly, the mixed powders of α -Al2O3 and MgCl2.6H2O were heated up to about 600° C in air. In this process, MgCl2.6H2O was decomposed to MgO and MgCl2. Then, the MgO- MgCl2-α -Al2O, mixture obtained above was treated isothermally in the temperature range of 650-925° C. The extent of conversion to MgAl2O4 was measured by means of chemical analysis and X-ray diffraction analysis. Kinetic data were fitted to the diffusion-controlled Jander's equation (Fig.7). The curve of log k vs.1/T was divided into two regions. E was 70 kcal/ mol in the lower temperature region and 20 kcal/mol in the higher temperature region. In both regions, active MgO was obtained during the thermal decomposition of MgCl2.6 H2O. Furthermore, in the high-temperature region, the formation rate of MgAl2O4 was considered to be promoted greatly by molten MgCl2. In order to examine actions of molten MgCl2, the model experiments by the use of MgO and α -Al2Os pellets were made in vacuum (Fig.1). Table 1 shows the weight change of two pellets after reaction. Fig.9 and Fig.10 show the X-ray diffraction pattern of the reaction surface of two pellets. These results suggest that MgO dissolved in molten MgCl2 spread uniformly over the surface of α -Al2O3 particles, then MgO is transported to the inner part of α -Al2O3 particles and rapidly reacts with α -Al2O, crystallites to form the spinel-layer through which the counter diffusion of Mg2+ and Al3+ occur (Fig.11). This reaction model was supported by the observations of the optical microscope (Fig.12) and the scanning electron microscope (Fig.13).