Figure 3-88. Transformations occurring in the Hall-Héroult process.
At the cathode, aluminum metal is produced due to the reduction of the Al3+
cations. At the anode, the oxide anions, O2-
, are discharged to form oxygen. The O2
formed oxidizes the carbon, the material that the anode is made of, to CO and CO2
. As the electrochemical process occurs, the carbon anode is consumed and has to be periodically replaced.
Aluminum is produced on a huge scale and is the second most widely used metal (after iron). Being non-toxic, non-carcinogenic, very light (density 2.7 g/cm3
), resistant to corrosion, greatly conductive to electricity, readily available, and inexpensive aluminum is a very attractive material for a broad variety of applications.
Pure aluminum is rarely used as a material because of its softness. However, mechanical properties of aluminum can be vastly improved by alloying it with just a fraction to a few percent of Cu, Mg, Zn, Mn, and Si. Aluminum alloys are used to make cans and foil, parts and bodies of cars, trucks, aircraft skins, spacecraft, and railway cars, electric motors and generators, windows and doors, cooking utensils, and electric wire. The list of applications of aluminum compounds is as impressive. Anhydrous aluminum chloride (AlCl3
) is broadly employed as a catalyst and promoter in petrochemical and chemical industries as well as in the manufacturing of pharmaceuticals and agrochemicals. Alumina (Al2
) is also used as a catalyst and support for a broad variety of catalysts. Being just slightly less hard than diamond, alumina is used to make abrasives. Powdered alumina is an excellent adsorbent and desiccant. Synthetic ruby and sapphire are made of Al2
. Aluminum compounds are also used in the production of deodorants, in water treatment, paper manufacturing, and leather tanning. 3.7.3. Chemical Properties and Amphoterism of Aluminum. Is Aluminum a Metal or Nonmetal?
An aluminum atom has three valence electrons in the 3rd
electron shell: 1s2
2p6 3s2 3p1
. These three electrons are easily discarded by an aluminum atom in order to attain the stable electron configuration of neon. Aluminum is very easily oxidized by O2
and even by water. Would you question this statement, given the fact that aluminum wire, foil, and other aluminum items are virtually indefinitely stable in moist air and even in water, exhibiting no sign of oxidation?
As a matter of fact, aluminum is immediately oxidized to alumina on exposure to air (Figure 3-89). However, the very thin film of the Al2
produced on the metal surface is strikingly good at blocking access of the oxygen to the aluminum atoms underneath. In this way, the bulk of the aluminum metal is remarkably well protected from oxidation. Watch this video
to see the melting of an aluminum rod with a torch in the air. Note the thin Al2
film on the surface of the molten metal. Also note that due to this film of alumina the aluminum rod does not catch a fire in spite of the very high temperature of the flame and plenty of oxygen around.