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Aluminum in abundance...
Aluminum is one of world's most useful and used metals. Its qualities of
lightness, strength, corrosion resistance, formability, conductivity and its many
attractive surface finishes have made it the second most widely used metal in the
world; only iron is used in greater quantity.
Aluminum can be cast, extruded, rolled, forged and stamped. It is readily
recyclable, requiring only five per cent of the energy initially used to produce it.
When alloyed with elements such as magnesium, copper and silicon, its
characteristics of strength and lightness are enhanced. It can also be combined
with ceramic particles to form composites stiffer than steel or titanium and as
durable as cast iron. These composite products can then be cast or extruded to
near-finished shapes to reduce costly machining.
Typical Kitimat Works potroom
Aluminum is used in products ranging from automotive applications to outer
space. Transportation, construction, packaging and energy are all industries
which have been changed significantly by using the properties of aluminum to
their advantage.
Aluminum is the earth's most abundant metallic element, making up
approximately eight per cent of the planet's crust. However, in nature, aluminum
is always found in combination with other elements, never in its free state. While
aluminum never occurs in its pure form in nature, it is commonly found in the
form of oxides. The most commercially viable source of aluminum is bauxite,
which is predominantly found in tropical and sub-tropical regions of the world.
Bauxite can be found as a fine powder, granules or rocks and can be pink,
cream, red, brown, yellow or gray in colour, depending on the composition of the
bauxite. While the ore is generally easy to mine, the process to extract the
aluminum from the bauxite is quite complex. The process of making metallic
aluminum is carried out in two successive stages: a chemical process to extract
anhydrous aluminum oxide from the bauxite, and an electrolytic process to
reduce the alumina to aluminum.
Commercial-grade bauxite typically contains about 75 per cent anhydrous
aluminum oxide. To extract the aluminum oxide, the bauxite is crushed and
ground into a powder and mixed with a solution of caustic soda. This paste is
mixed with further amounts of caustic soda solution in autoclaves or digesters.
There, under pressure and at a high temperature, the caustic soda dissolves the
hydrated alumina and leaves a solution called sodium aluminate. Any impurities
remain as dissolved residue. The residue or 'red mud' - comprised mainly of the
oxides of iron, silicon and titanium - is removed by sedimentation and filtration.
The inert red mud is washed to recover the chemicals and is disposed of by a
'wet stacking' technique to reduce the risk of effluent.
The sodium aluminate solution is then pumped into precipitator tanks where very
fine and pure aluminum trihydrate is added as 'seed'. Under agitation and with
gradual cooling, the alumina trihydrate contained in the solution precipitates (that
is, it is deposited in solid form from solution) on the 'seed'. The trihydrate solids
are then separated from the caustic soda solution by settling and vacuum
filtering. The caustic soda solution is recovered and returned to the start of the
process to be reused.
The trihydrate is then passed through high-temperature (900 - 1100 C.) kilns
(calciners) that drive off the chemically-combined water. The aluminum oxide that
results is a fine, white powder and is known as calcined alumina. Four to five
tonnes of bauxite are required to produce approximately two tonnes of calcined
alumina, which in turn yields one tonne of aluminum.
Bauxite is not shipped to Kitimat. Instead, Alcan imports annually some 500,000
tonnes of calcined alumina by deep-sea vessel from Australia to Kitimat.
Although aluminum is the most abundant metallic element in the earth's crust,
comprising some eight per cent, it was only first extracted in 1807. Still rare in the
mid-1800s, aluminum was more highly prized than silver or gold. It was in 1886
that an economic way was found to free aluminum from its tenacious molecular
bond with oxygen. Working independently, two young scientists, Paul Louis
Toussaint-Heroult in France and Charles Martin Hall in the United States, almost
simultaneously came up with the same discovery.
Both believed that an electrolytic process would be the key. However, alumina by
itself does not conduct electricity. The two scientists overcame the problem by
dissolving alumina in molten cryolite, a brittle translucent rock, which is a good
conductor of electricity.
This graphic illustrates the aluminum manufacturing process
Aluminum produced at Kitimat Works is made via the Hall-Heroult process in
large cells commonly referred to as pots.
The pot is comprised of two sections. The negative section is called the cathode,
while the positive section is called the anode. Inside the pot is a molten
electrolyte composed primarily of cryolite and aluminum fluoride. It is into this
molten electrolyte that the alumina ore is dissolved. However, to break the bond
between the aluminum and oxygen molecules, electricity must first pass through
the electrolyte, flowing from the positive anode to the negative cathode. As the
reduction process takes place, the heavier aluminum molecules sink to the
bottom of the pot while the lighter oxygen molecules rise to the surface of the
electrolyte to be drawn off by gas collection systems known as