Geology Reference
In-Depth Information
minerals. A tonne of primary aluminium needs approximately 2 tonnes of alumina
which in turns requires 4 to 6 tonnes of bauxite. A representative aluminium smelter
will produce some 125,000 tonnes of aluminium annually and consists of around 300
pots.
The Bayer process involves the digestion of bauxite in caustic soda at 280
o
C and
its subsequent precipitation at 55-70
o
C. The obtained hydroxide is then calcined to
obtain alumina at temperatures of around 1000
o
C. The mix of materials that does
not dissolve in the digestion process is known as red mud. This is washed with
condensate to reduce any free alkali to the lowest level.
Alumina is a very stable material whose direct reduction requires very high
temperatures. To avoid the need for that, alumina is dissolved in molten cryolite
(Na
3
AlF
6
) to 960
o
C leaving Al to be extracted by electrolysis using Hall-Heroult
cells. These are made of carbon and are connected in series. The direct current
passes from the anode through the bath (fed with alumina to maintain a content
of 2 to 6%) to the cathode and from there to the next cell at low voltage but very
high current (typically some 150,000 A). Current density at the anode face is 0.6 -
1.3 A=cm
2
. The cathode is insulated by refractory bricks inside a rectangular steel
shell container known as a pot, where the aluminium ion is reduced to an aluminium
metal that flows to the bottom and is siphoned off. At the carbon anode, oxygen is
formed and rapidly reacts to produce carbon dioxide and thus the anode needs to be
replaced regularly. Aluminium fluoride (AlF
3
) is added to the cells to decrease the
bath temperature and neutralise any sodium oxide impurities. This positive effect
is however offset by an increase in fluoride emissions. Process gases are collected by
dry alumina scrubbers to remove the perfluorocarbons and hydrogen fluoride. Tars
but not sulphur dioxide nor carbon dioxide are additionally retained. Any alumina
leaving the scrubbers is amassed in fabric filters and recycled to the cells.
The way in which alumina is fed into the system together with the specific
anode used are the two main factors which serve to characterise the cell type. Two
major types are currently in use: firstly and most commonly are those employing
prebaked carbon anodes followed by the self-baking Soderberg anodes, which are
composed of a mixture of calcined petroleum coke and coal tar pitch. In the former,
anodes are manufactured in a separate production plant, whilst the anodes of the
latter are made in situ to take advantage of the heat arising from the molten bath
whilst saving on capital and labour. Soderberg anodes are however less e
cient and
experience greater di
culties in collecting baking fumes (IPPC, 2009).
Fig. 8.3 shows schematically the processes associated with the aluminium pro-
duction.
8.3.2 Energy and environmental issues
Generating primary aluminium is extremely energy intensive. Around half of the
electricity used in manufacturing non-ferrous metals is assigned to producing it,
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