Extraction of Aluminium - Hall (Electrolytic) Cell.

                 


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.

Al3+  +  3e-    arrow   Al (aluminium metal at the (-)cathode)
2O2-  -  4e-    arrow   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 oxide   arrow    aluminium + oxygen
2Al23+O32-(l)         arrow        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  arrow  carbon dioxide
C(s)  +    O2(g)   arrow          CO2(g)

The carbon anode slowly reacts to make carbon dioxide and needs to be replaced regularly.