Formation of mullite in fused cast AZS refractories

Abstract

Fused cast refractories, based on the system Al2O3-ZrO2-SiO2 (AZS), are used in industrial glass tanks and are in direct contact with the glass melt and/or the furnace atmosphere (combustion gases, evaporation products, and dusts) at temperatures up to 1600 °C. Three samples of AZS materials were studied, (i) the original AZS refractory as delivered from the refractory industry, (ii) AZS refractory thermally treated similar to furnace heat up over days and ending for 10 h at 1580 °C and (iii) AZS refractory thermally cycled 600 times between 1530 and 1580 °C after furnace heat up simulation.The samples were studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) including Energy Dispersive X-ray analysis (EDX) and Electron Backscatter Diffraction (EBSD). In the untreated AZS, monoclinic ZrO2 (m-ZrO2) and Al2O3 were detected, while in AZS thermally treated at 1580 °C, mullite also occurred. Thermal cycling led to further increase in the mullite concentration and to a small concentration of tetragonal ZrO2 (t-ZrO2). The Al2O3 crystals as well as some of the ZrO2 had a blocky morphology. Surprisingly also formed mullite crystals were block and not needle-like. In the mullite as well as in the Al2O3 crystals tiny m-ZrO2 crystals were embedded which were formed during cooling in the production process from t-ZrO2 and were heavily twinned.