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Aluminium
is obtained by the electrolysis of alumina, using the
process discovered simultaneously in 1886 in France
by Paul Héroult and in the USA by Charles Hall.
The process of making
metallic aluminium is carried out in two successive
stages: a chemical process to extract alumina from the
bauxite, and an electrolytic reduction process to convert
alumina into aluminium. (Known as the Hall-Heroult process,
named after its inventors).
An aluminium smelter
comprises three main sectors: Carbon, Potlines and Casthouse.
It is in the potlines that aluminium is actually produced,
in each of the 336 reduction cells that will be installed.
The liquid metal is transferred
to the Casthouse, where it is solidified. The Sohar
Aluminium Smelter will initially produce 22kg ingots
and 650kg sow ingots, destined for overseas markets. |
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The
Potline
A potline is a long building
or two buildings which contain a series of ‘pots’,
or large electrolytic cells, in which aluminium is made.
Each pot is a large rectangular cell, lined with carbon
blocks and insulating bricks. The pots are connected
electrically in a series, so that direct electric current
flows through one pot, then on to the next and so on,
to the end of the line.
Inside the pot, alumina is dissolved in a ‘bath’
of molten cryolite (sodium aluminium fluoride) and other
materials. As the electric current is passed through
the bath it generates the heat to keep the bath molten
and causes the alumina to separate into two constituent
elements, aluminium and oxygen.
Just under two tonnes
of alumina and about half a tonne of carbon products
are needed to make one tonne of aluminium. The Sohar
Aluminium smelter will have the capacity to produce
350,000 tonnes of finished product every year. |
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The
Casthouse
Molten metal is carried
from the potline to the Casthouse in transport ladles
on specially designed vehicles.
Overhead cranes are used
to lift the ten tonne ladles and pour the molten aluminium
into holding furnaces where it is mixed with specific
amounts of alloy materials to produce special qualities
and strength according to customer requirements.
The aluminium is then
cast into slabs, billets or ingots. Some of the different
aluminium processes are:
- Casting:
Pouring molten metal into moulds.
- Forging:
Hot metal is hammered or squeezed into the shape
of a die, used extensively in highly stressed structural
parts.
- Drawing
or Pressing: Forcing a flat metal
blank into a die under press from a metal ram, mainly
used to manufacture 'holloware'.
- Spinning:
A circular metal blank is rotated at high speed
and pushed into the shape required by the pressure
from a tool.
- Extruding:
Pushing the aluminium through a die to form longer
tube, rod, or wire products.
- Rolling:
Squeezing the aluminium between two rollers to form
a flat plate sheet or foil product at very fine
gauges (as thin as .0006mm).
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Rodded
Anodes (Carbon)
Carbon anodes, made from
petroleum coke and pitch, are manufactured on site.
They are used to conduct electricity into the smelting
cells/pots in the potroom.
Anodes are consumed in
the smelting process and the remaining portions (known
as butts) are recycled.
Petroleum coke is crushed,
mixed with liquid pitch and vibrated into a rectangular
block weighing more than 1400kg.
These anode blocks are
baked in a natural gas fired baking furnace for several
weeks to improve electrical conductivity characteristics.
Anodes are attached to
rods and suspended into the electrolytic cells in the
potroom where they are slowly consumed in the aluminium
process. They are replaced on a rotating schedule every
few weeks.
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