Chemical Degradation of Cathode Linings in Hall-H,roult Cells-An Autopsy Study of Three Spent Pot Linings

Cathode autopsies are used frequently in the aluminum industry to investigate pot failure and
the degradation of the cathode lining. The materials observed in spent pot lining (SPL) has so
far been assumed to reflect the sequence of layers from the cathode to the nonreacted refractory
lining as present during the operation of the cell. Here, we demonstrate that the thermal gradient
in the lining is reversed during cooling and that the physical appearance of the SPL is
caused both by processes taking place during operation and cooling of the shutdown cell. X-ray
diffraction and microscopy of the SPL from three shutdown cells revealed that sodium metal is
the main component responsible for the chemical degradation of the refractory lining. Two
distinct reaction fronts were identified in the three SPL showing that sodium is penetrating
deeper down into the lining than the molten fluorides from the electrolyte. The mechanisms for
the transport of sodium and bath components in the refractory lining are proposed based on the
experimental observations. The sodium penetration is inhibited by the formation of a viscous
barrier as suggested previously, but the current findings suggest that the barrier retards diffusion
of O₂– and F– anions rather than Na+ as proposed previously.