Chemical Stability of Thermal Insulating Materials in Sodium Vapour Environment

The most typical thermal insulating materials used in the cathode lining in aluminium electrolysis cells are Moler, calcium silicate, or vermiculite. The thermal insulation is important for the overall thermal and dimensional stability of the cell. The chemical stability of the thermal insulating materials is important, especially in cases where the refractory layer above the thermal insulation layer becomes fully penetrated by sodium vapour. The chemical degradation of thermal insulating materials by sodium vapours has been investigated in a laboratory test resembling the environments in the cathode lining. The exposed materials were investigated with respect to changes in the microstructure and chemical and mineralogical composition by a combination of optical and electronic microscopy and powder X-ray diffraction. These investigations revealed different reaction patterns for the three materials and the formation of new mineralogical phases were identified. Finally, these findings were compared with chemical reactions with sodium based on computational thermodynamics.