Aluminium ore is called bauxite. Bauxite contains aluminium oxide, water, iron oxide and other impurities. The purified dry ore, called alumina, is aluminium oxide (Al2O3). The alumina must be molten for electrolysis to work, since the ions are not free to move in the solid state. Unfortunately, alumina has a high melting point (2040 °C) and it is not practical to do electrolysis at such a high temperature. In the middle of the nineteenth century it was found that alumina dissolved in cryolite. Cryolite is sodium aluminium fluoride (Na3AlF6). A solution of alumina in cryolite melts at about 900 °C and electrolysis is done at about 950 °C. Extraction of Aluminium - Electrolysis Cell
The steel container is coated with carbon (graphite) and this is used as the negative electrode (cathode). Aluminium oxide (Al2O3) is an ionic compound. When it is melted the Al3+ and O2- ions are free to move and conduct electricity. Electrolysis of the alumina/cryolite solution gives aluminium at the cathode and oxygen at the anode.
4Al3+ + 12e- 4Al (aluminium metal at the (-) cathode) reduction. 6O2- - 12e- 3O2 (oxygen gas at the (+) anode) oxidation. Aluminium is more dense than the alumina/cryolite solution and so it falls to the bottom of the cell where it can be tapped off as pure liquid metal. The overall reaction is aluminium oxide aluminium + oxygen. 2Al2O3(l) 4Al(l) + 3O2(g) Oxygen is given off at the positive carbon anode. Carbon dioxide is also given off at the carbon anode because hot oxygen reacts with the carbon anode to form carbon dioxide gas. Carbon + oxygen carbon dioxide
C(s) + O2(g) CO2(g)
The carbon anodes slowly disappear because each molecule of carbon dioxide which is given off takes a little piece of carbon away with it. The carbon anodes need to be replaced when they become too small. Extraction of Iron