Civil Engineering Reference
In-Depth Information
rather than refine it themselves. This is, of course, also because of the enormous
amount of energy which is required to produce aluminium. As far as future
development is concerned, it is safe to assume that there will be an expansion of
energy resources and expertise in these countries, and that today's large alu-
minium producing plants in USA, Canada and Northern Europe are just an inter-
mezzo.
Aluminium is produced from bauxite in two stages after extraction of the
bauxite ore. Aluminium oxide is first extracted from the ore by heating it to
between 1100°C and 1300°C with an increased air flow. This is called calcination.
The oxide is then broken down in an electrolytic bath at around 950°C with sodi-
um and fluorides. The pure aluminium is deposited on the negative pole, the
cathode. On the positive pole, the anode, oxygen is released which combines
with carbon monoxide, (CO) and carbon dioxide (CO 2 ). The anode consists of a
paste mixture of powdered coal and tar - for every kilo of aluminium, half a kilo
of paste is required. A huge amount of water is used.
The processes in the aluminium industry release huge amounts of carbon diox-
ide and acidic sulphur dioxide, along with polyaromatic hydrocarbons (PAHs),
flourine and dust. These pollutants are washed off with water and then rinsed
out into the sea or water courses without treatment. Some sulphur dioxide,
hydrocarbons and fluorine escape the washing down with water and come out
as air pollutants instead. Emissions into both air and water can have very nega-
tive consequences for the local environment and its human population. PAH
substances, fluorine and aluminium ions remain in the sludge and slag from the
production processes. This causes problems in the ground water when deposits
have to be stored on site.
The amount of energy needed for the process from ore to aluminium is very
high. Aluminium produced from bauxite ore is used to produce sheeting.
Recycled, it can be used a great deal in cast products (known as downcycling).
Aluminium waste is recycled by smelting in a chloride salt bath at 650°C, which
at best only requires 7 per cent of the energy needed for production from ore. The
waste aluminium has to be pure, not mixed with other materials. Recycling of
aluminium requires a great deal of transport because of its centralized produc-
tion. Most aluminium goods are relatively thin and easily damaged during
demolition or removal, so local re-use is seldom practical. Aluminium is suscep-
tible to corrosion, but less so than steel.
Copper
Copper was most likely the first metal used by mankind. The oldest copper arti-
cles that have been found were made about 7000 years ago in Mesopotamia. An
early development was the invention of bronze, produced by adding tin to cre-
ate a harder metal.
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