The steel container is coated with carbon (graphite) and this is the negative electrode (cathode).
Electrolysis of the alumina/cryolite solution gives aluminium at the cathode and oxygen at the anode.
Al (aluminium metal at the (-)cathode)
2O2- - 4e- O2 (oxygen gas at the (+)anode)
Aluminium is more dense than the alumina/cryolite solution, and so falls to the bottom of the cell where it can be tapped off as pure liquid metal.
The overall reaction is
aluminium + oxygen
2Al23+O32-(l) 4Al(l) + 3O2(g)
Oxygen is discharged at the positive carbon (graphite) anode.Oxygen reacts with the carbon anode to form carbon dioxide gas.
carbon + oxygen
C(s) + O2(g) CO2(g)
The carbon anode slowly reacts to make carbon dioxide and needs to be replaced regularly.