Wear of Silicon Nitride Bonded SiC Bricks in Aluminium Electrolysis Cells

摘要

Samples from a sidewall of an aluminium electrolysis cell were submitted for study of the wear mechanism of the Si_3N_4 bonded SiC-brick. Samples were studied using a combination of chemical analysis, microscopy, SEM-EDS and powder-XRD. Unused bricks from two suppliers were used as reference materials. The unused bricks are very similar. Differences are found in the grain size of the SiC and in the binder (β-Si_3N_4 content varies between bricks in also internally in individual samples). All bricks have a coarse matrix and higher porosity in the core. The wear mechanism of the bricks seems a combination of attack by Na, HF, oxidation and moisture. Below bath level oxidation and Na play a major role but bricks are least altered. At bath level attack is severe but observations were complicated through a repair, which caused secondary reactions in addition to oxidation. Above bath level the bricks disintegrate but reaction products are almost absent. Here we hypothesize that HF-vapor transformed the oxidation products into volatile fluorides and other gases. This study shows that the corrosion tests currently used for Si_3N_4 bonded SiC-bricks, do not simulate the wear mechanism described in this paper. In order to evaluate the performance of these bricks it is important to develop a new corrosion test, where water, Na, HF and oxidation play a role.