Phase Relations in the MgO-Rich Region of the System MgO-Fe₂O₃

Abstract

Fe₂O₃-added magnesia-spinel brick, in replacing magnesia-chrome brick, has been recently used as a basic refractory for the burning zone in cement rotary kiln. According to the system MgO-Fe₂O₃ phase diagram, the phase change and the separation with temperature fluctuation in the kiln may damage the microstructure of the magnesia-spinel brick, which leads to the quality deterioration. The MgO-rich region of this system has been investigated by using a high-temperature X-ray diffraction (XRD) apparatus in air. The change from MgOss to MgO accompanied by the exsolution of Fe³⁺ occurs possibly around 1010°C based on the phase diagram. However, MgOss was found to still exist at room temperature in this study. Also, it was proved that the formation temperature of the solid-solution including 5-20mass%Fe₂O₃ shifted to a higher temperature compared to the conventional result. The unknown edge of the boundary line between MgOss and MgOss+MgFe₂O₄ on the MgO-Fe₂O₃ composition line was estimated in the system MgO-FeO-Fe₂O₃ phase diagram by considering the oxygen partial pressure of the atmosphere as 0.1MPa. The lattice constant of MgOss was altered by the change of the electronic spin state of Fe³⁺ from low-spin state to high-spin state around 1100°C on a temperature-elevated process, together with the solution quantity of Fe³⁺ and heating temperature. Furthermore, the crystal strain of MgOss and MgFe₂O₄ increased remarkably upon this spin alternation. However, the reverse-change of the spin state could not be recognized on a cooling process, then MgOss was considered to be metastable due to this slow reverse-change. The existence of MgOss single phase in the system MgO-Fe₂O₃ became clear under expanded conditions, which certainly contributes to the realization of the long life time of magnesia-spinel bricks.