Thermogravimetric Analysis of Thermal Insulating Materials Exposed to Sodium Vapor

The bottom thermal insulating layer in aluminum electrolysis cells is normally well protected from chemical attacks by a refractory layer. However, autopsies of spent cathode lining have shown that sodium vapor may reach and react with the thermal insulating layer, thus affecting the thermal balance and structural integrity of the entire electrolysis cell. Here, we report on a new laboratory test, named Na-tg test, where commercial thermal insulating materials are exposed to sodium vapor at controlled temperature and sodium partial pressure. The weight change of three commercial insulation materials was measured as a function of time at various temperatures and partial pressures of sodium. The exposed materials were investigated with respect to changes in the microstructure, chemical and mineralogical composition, due to the reaction with sodium. The findings are discussed with respect to the chemical durability of the three thermal insulating materials.