Long-term heat treatment of collector bars for aluminium electrolysis: impact on microstructure and electrical properties

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In order to identify possible optimizations regarding the electrical energy efficiency of an aluminium electrolysis cell, the impact of service temperature on microstructure and electrical properties of the cell cathode was investigated. The investigations include experiments regarding the chemical composition, especially the content of carbon, the electrical conductivity and the microstructure at selected positions. Thermodynamic calculations were used to estimate local service temperatures and explain phase transformations and formations. It was found that due to the increased service temperature diffusion processes of carbon took place to a particular extent between cast iron and collector bar. As a result, the carbon content in the collector bar changed from 0.06 to 1.05–1.4 wt%, while in the cast iron a reduction from 3.47 to < 1.50 wt% took place. These processes led to isothermal phase transformations and formations, that changed the matrix of the collector bar from austenitic with low content of ferrite to an austenitic matrix accompanied by precipitation of secondary, predominantly allotriomorphic cementite at service temperature. It was then shown that this has a negative effect on collector bar and decreases the electrical conductivity by up to 26 %. It was also discovered that graphite spheroidization within the grey cast iron has a positive effect on its electrical conductivity, which has increased by 52 %. The results provide the basis to gain an understanding of the carbon diffusion related processes within the cathode of an electrolysis cell and reveal further potential to increase the energy efficiency of primary aluminium production.