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Extraction
of metals
Francine
Taylor-Campbell, Contributor
YOU
SHOULD BE ABLE TO:
- Relate
the principles underlying the extraction
of a metal to its position in the
electro-chemical series
- Describe
the extraction of aluminium and
iron
POINTS
TO NOTE
- The
reactive metals are found in nature
as compounds called ores. These
compounds contain the metal in combination
with oxygen, sulphur or chlorine
along with other impurities. Metals
that are not very reactive are usually
found in their free state that is,
uncombined, for example gold.
- To
extract a metal Xx+ from its ore,
the process of reduction is used;
that is, the metal ions gain electrons
to become atoms. Xx+ + xe === X
- For
the process of reduction, a reducing
agent such as carbon or hydrogen
is generally used. The reducing
agent works by displacing the metal
from its ore. However, for very
reactive metals, for example sodium
and potassium, the process of electrolysis
is used to supply electrons to the
molten ore to extract the metal.
In
the extraction of aluminium, the ore
(bauxite) is purified and dissolved
in molten cryolite (Na3AlF6) from
which it produces the ions Al3+ and
O2-. The aluminium oxide/cryolite
solution is then electrolysed in a
cell using graphite (carbon) electrodes.
Aluminium ions (+ve) move towards
the cathode where they gain electrons
and form molten aluminium.
Al3+
(l) + 3e ==== Al (l) This molten aluminium
is collected at the bottom of the
cell. The oxide ions (-ve) move towards
the anode where they lose electrons
to form oxygen which is collected.
2O2-(l)
==== O2 (g) + 4e
The
extraction of iron is different from
that of aluminium. This is because
iron is not as reactive, hence a reducing
agent such as carbon monoxide can
be used to extract it from its ore.
In
this process the ore (haematite) is
mixed with limestone (CaCO3) and coke
(carbon) and heated in a blast furnace.
The reaction occurs in different stages:
1.
The carbon burns to form carbon dioxide
C
(s) + O2 (g) === CO2
(g)
2.
The carbon dioxide reacts with more
coke (carbon) to form carbon monoxide
which is the reducing agent in this
method. CO2 (s) + C (s)
=== 2CO (g)
3.
The carbon monoxide reduces the haematite
to iron which is collected at the
bottom of the furnace as molten iron.
Fe2O3 (s) +
3CO (g) ========== 2Fe (s) + 3CO2
(g)
Impurities
in the ore, mainly silica, are removed
by reacting with calcium oxide formed
from the decomposition of limestone.
CaCO3
(s) ========== CaO (s) + CO2
(g)
CaO
(s) + SiO2 (s) =========
CaSiO3 (l) slag or calcium
silicate
This
slag is also collected at the bottom
of the furnace.
This
reaction is facilitated because the
calcium oxide is basic while silica
is acidic.
The
slag produced is used for road building
and also as a fertilizer. The iron
produced is impure containing carbon,
which is removed by reacting it with
oxygen. From this, steel and other
types of iron are produced.
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
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