Chemistry Reference
In-Depth Information
crust of solid electrolyte
The molten aluminium collects at the bottom of the
cell and it is siphoned out at regular intervals. No
problems arise with other metals being deposited,
since the cryolite is largely 'unaffected' by the flow
of electricity. Problems do arise, however, with the
graphite anodes. At the working temperature of the
cell, the oxygen liberated reacts with the graphite
anodes, producing carbon dioxide.
carbon
C(
s
)
graphite anodes
aluminium
out
4m
wide
1m
deep
siphon
+
+
oxygen
O
2
(
g
)
carbon dioxide
CO
2
(
g
)
→
→
8m long
molten aluminium
collecting on
floor of cell
tank lined
with graphite
cathode
The anodes burn away and have to be replaced on a
regular basis.
The electrolysis of aluminium oxide is a continuous
process in which vast amounts of electricity are used.
Approximately 15 kWh of electricity are used to
produce 1 kg of aluminium. In order to make the
process an economic one, a cheap form of electricity
is required. Hydroelectric power (HEP) is usually
used for this process. The plant shown in Figure 5.7
uses an HEP scheme to provide some of the electrical
energy required for this process. Further details about
HEP are given in Chapter 6, p. 94.
molten electrolyte
Figure 5.6
The Hall-Héroult cell is used in industry to extract aluminium
by electrolysis.
saving in the energy requirements of the process.
In recent years it has become necessary to
manufacture the cryolite.
•
The molten mixture is then electrolysed in a cell
similar to that shown in Figure 5.6.
The anodes of this process are blocks of graphite
which are lowered into the molten mixture from
above. The cathode is the graphite lining of the steel
vessel containing the cell.
Aluminium oxide is an ionic compound. When
it is melted the ions become mobile, as the strong
electrostatic forces of attraction between them
are broken by the input of heat energy. During
electrolysis the negatively charged oxide ions are
attracted to the anode (the positive electrode), where
they lose electrons (oxidation).
oxide ions
2O
2−
(
l
)
electrons
4e
−
The positive aluminium ions are attracted to the
cathode (the negative electrode). They gain electrons
to form molten aluminium metal (reduction).
oxygen molecules
O
2
(
g
)
+
+
→
→
aluminium ions
Al
3+
(
l
)
+
+
electrons
3e
−
aluminium metal
4Al(l)
l
)
→
→
A handy way of remembering it is
OIL RIG
(
O
xidation
I
s
L
oss,
R
eduction
I
s
G
ain of electrons).
The overall reaction which takes place in the cell is:
electrolysis
aluminium
4Al(l)
+
oxygen
3O
2
(g)
aluminium oxide
2Al
2
O
3
(l)
Figure 5.7
An aluminium smelting plant.
