Aluminum is found in varying amounts in nature as aluminosilicates
(contains aluminum, silicon, and oxygen) in various types of clay. As the
minerals are weathered they gradually breakdown into various forms of hydrated
aluminum oxide, Al2O3.xH2O, known as bauxite.
The bauxite is purified by the Bayer Process. First the ore
is mixed with a hot concentrated solution of sodium hydroxide. The NaOH
will dissolve the oxides of aluminum and silicon but not other impurities
such as iron oxides, which remains insoluble. The insoluble materials are
removed by filtration.
The solution which now contains the oxides of aluminum and silicon
are next treated by bubbling carbon dioxide gas through the solution. Carbon
dioxide forms a weak acid solution of carbonic acid which neutralizes the
sodium hydroxide from the first treatment. This neutralization selectively
precipitates the aluminum oxide, but leaves the silicates in solution.
Again filtration is used for the separation. After this stage the purified
aluminum oxide is heated to evaporate the water.
Aluminum in the metal form is very difficult to obtain by using some
of the traditional chemical methods involving carbon or carbon monoxide
as reducing agents to reduce the aluminum ions to aluminum metal. One of
the earliest and costly methods in 1850 was to reduce aluminum chloride
with sodium metal to obtain aluminum metal and sodium chloride. (Sodium
metal is not easy to obtain either). As a result some of the earliest aluminum
metal was made into jewelry.
Hall-Heroult Process: In 1886, Charles Hall, an American (23
yrs. old), and Paul Heroult, a Frenchmen (23 yrs old), simultaneously and
independently developed the process still in use today to make aluminum
metal. The purified aluminum oxide is mixed with cryolite, a mixture of
sodium fluoride and aluminum fluoride, and heated to about 980 degrees
Celsius to melt the solids. The mixture melts at a much lower temperature
than aluminum oxide would by itself.
The hot molten mixture is electrolyzed at a low voltage of 4-5 volts,
but a high current of 50,000-150,000 amps. Aluminum ions are reduced to
aluminum metal at the cathode (the sides and bottom of the electrolysis
cell). At the anode, oxygen is produced from the oxide ions. The anode
material is carbon in the form of graphite, which also is oxidized and
must be replaced quite frequently.
The electricity used to produce aluminum is relatively high. One pound
of aluminum requires 6-8 kilowatts of electrical energy. This amount of
aluminum can be used to make 23 pop cans or one 300 watt light bulb burning
for one hour is required to make one pop can.