Thermal Insulation Materials in Aluminum Electrolysis Cells
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Aluminum is produced by an energy intensive process known as the Hall-Héroult process, which has been developed extensively since it was first invented in 1886. Recent trends in the aluminum industry has been to strive for increased production output by increasing the amperage of the electrolysis cells. This has led to an increased carbon cathode wear and resulting in a reduction in cell lifetime. In addition, with increasing energy costs, the focus on lower energy consumption has resulted in higher demand on the thermal insulation in Hall-Héroult cells. Previous research efforts on the bottom lining of the electrolysis cells have focused mainly on the refractory layer, which protects the thermal insulation layer, and its resistance to attack by the molten electrolyte. However, it is established that sodium is at the forefront of this attack, but the impact of sodium vapor on the thermal insulation materials has up until now not been investigated and constitute the motivation of this thesis work. The effect of sodium vapor on the mineralogy, structural stability, and microstructure of commercial thermal insulation materials used in the electrolysis cells were investigated in this work by two laboratory tests. The materials were commercial products based on diatomaceous earth (Moler), calcium silicate, and vermiculite. The final part of this work aimed to develop an improved thermal insulation material that would have better degradation resistance towards sodium vapor as well as the cryolite melt. At the same time, the new material should have considerably lower thermal conductivity than traditional refractory bricks and better mechanical properties than common thermal insulation materials. Such a material could be used in different ways in the bottom lining, one of which could be to replace, partially or fully, the traditional refractory bricks to allow an increase in carbon cathode thickness for enhanced cell lifetime.