Abstract
The bottom lining in aluminium cells can be degraded by several mechanisms. In the present work, reaction with sodium vapour that diffuses through the cathode was studied. The reaction paths depend on the oxygen level. At reducing conditions, metallic silicon is formed, whereas the presence of oxygen gives sodium oxide bound in the reaction product. Based on available thermodynamic data for the compounds present in the system SiO2-Al2O3-Na2O, each alkemade triangle in the phase diagram could be supplied with numbers for the equilibrium pressure of sodium (or sodium and oxygen), as well as lines showing the change in composition during attack. The calculations show that chamotte bricks normally end up mainly as nepheline. This is in accordance with practical observations. The reducing nature of sodium was verified through laboratory experiments. Autopsies of old linings have revealed the presence of silicon metal particles due to the reduction of SiO2-containing compounds.