Abstract:
Interaction between electrolyte and carbon cathodes during the electrolytic production of
aluminium decreases cell life. This paper describes the interaction between carbon cathode
materials and electrolyte, based on industrial and laboratory data. It also reports on the
degree of expansion of semi-graphitic and graphitised materials when exposed to a sodium
rich environment. Phase relations in the slow cooled bath electrolyte, spent industrial cathodes
and laboratory scale cathode samples were similar: all contained Na3AlF6, NaF, CaF2
and NaAl11O17. Al4C3, AlN and NaCN were only detected in the spent industrial cathodes.
The inability to locate Al4C3 in the laboratory scale samples could be due to very low concentrations
of Al4C3 which could not be detected by XRD, or to the limited direct contact
between the produced aluminium and carbon material. X-ray diffraction analysis confirmed
that sodium intercalation into graphite did not take place. Wear of the examined
carbon cathodes proceeded due to penetration of electrolyte and sodium into the cathode,
followed by reactions with carbon and N2 whereby AlN and NaCN formed. Once electrolysis
started the carbon cathodes expanded rapidly, but slowed down after approximately an
hour. Sodium expansion decreased with degree of graphitisation of the carbon cathode
material.
