Abstract
The most common thermal insulating materials used in the cathode lining in aluminum electrolysis cells are Moler (diatomaceous earth), calcium silicate, or vermiculite based materials. The thermal insulation layer is critical for the overall thermal stability of the cell and is vulnerable to volatile species, such as sodium vapor, that may penetrate through the carbon cathode and refractory layer. Here, we present an investigation of the chemical degradation of typical thermal insulating materials by exposure to sodium vapor in a laboratory test. Changes in microstructure and chemical and mineralogical composition of the exposed materials were characterized by electronic microscopy and powder X-ray diffraction. The materials possess different reaction patterns, ranging from deformation by creep to formation of a glassy layer reducing further sodium penetration. The results from the laboratory test were compared with chemical reactions with sodium predicted by computational thermodynamics and discussed with respect to relevant ternary phase diagrams.
Highlights
The production of aluminum by molten salt electrolysis is energy demanding with a considerable amount of input energy lost in the form of heat release to the surroundings
Rx-layer the brick changed colorthefrom the state, pristine state, butmacroscopic no further crucible
As the experimental temperature was lower for Moler than for calcium silicate and vermiculite, it is not possible to do a direct comparison
Summary
The production of aluminum by molten salt electrolysis is energy demanding with a considerable amount of input energy lost in the form of heat release to the surroundings. The continued move towards low energy cells (lean cells) will require better insulated cells, as less heat will be generated to maintain operational temperature. This will increase the heat gradient towards the bottom insulation layer. One of the most important factors for the increase in the potline amperages was the shift from anthracitic carbon to graphitized carbon in the cathode blocks While this shift ensured lower energy consumption through the increased electrical conductivity of the cathode, the wear resistance of the cathode blocks decreased, negatively affecting the cell lifetime [4]. This will put higher demand on the thermal insulating layer and may increase the exposure to volatile species such as Na and NaAlF4
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