METAL EXTRACTION
Metals are useful materials, because they have many desirable properties, for
example:
thermal and electrical conductivity, malleability, ductility, strength, hardness,
resistance to corrosion, low density, shiny appearance
Occurrence
The two most common elements found in the earth’s crust are oxygen (45%) and
silicon (27%). These two elements occur mainly as the compound silicon dioxide
which is found in large quantities in most rocks.
The commonest metals in the Earth’s crust are Al(8.1%), Fe(5.0%), Ca(3.6%),
Na(2.8%), K(2.6%), Ti(0.4%). Of these, Ca, Na and K are unsuitable for
engineering purposes. Apart from Al, Fe (which are widely distributed) and Ti,
the other commonly used metals are scarce in the Earth’s crust but occur in
localised high-grade ores.
Ores are rocks which contain a high enough concentration of a desired metal for
its extraction to be economically viable. The fewer the impurities, the higher
grade the ore is. The concentration of metal in the ore which is economically
viable depends on the inherent value of the metal. Extracting a metal is not
cheap and the method used depends on the position of the metal in the
reactivity series.
Most metal ores are sulphides or oxides of the metal, for example:
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galena (PbS)
cinnabar (HgS)
zinc blende (ZnS)
haematite (Fe2O3)
cassiterite (SnO2)
bauxite (Al2O3)
rutile (TiO2)
but some ores are carbonates and hydroxides, such as
 malachite (CuCO3.Cu(OH)2)
 calcite (CaCO3)
Some unreactive metals occur native, i.e. uncombined in the Earth’s crust, for
example copper, silver and gold.
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Extracting a Metal from its Ore
The extraction of a metal from its ore involves the reduction of a metal ion to a
metal atom. Often an ore contains a metal oxide or a substance which can easily
be changed into a metal oxide. Metal oxides are usually used directly in the
extraction process, but sulphide ores are first roasted in air to convert them to
oxides. For example Zinc is obtained from its ore, Zinc blende, which contains
mainly zinc sulphide.
2ZnS(s) + 3O2(g)
2ZnO(s) + 2SO2(g)
This process releases sulfur(IV) oxide into the atmosphere, which is
undesirable because of the formation of acid rain. How a metal is extracted
from its ore depends on how reactive the metal is. The reactivity series of
metals lists metals in order of their reactivity, the most reactive being placed
at the top of the list and the least reactive at the bottom.
A more reactive metal can displace a less reactive metal from its compounds.
The non-metals hydrogen and carbon will also displace less reactive metals from
the oxides of those metals.
K
Na
Li
Ca
Mg
Al
Ti
C
Zn
Fe
Sn
Pb
H
Cu
Ag
Au
Pt
Metals above carbon must be obtained by the electrolysis of a molten
compound (e.g. Al) or by the reaction of a compound with a more
reactive metal (e.g. Ti).
Metals below carbon can be obtained by carbon reduction of the
metal oxide (e.g. Fe and Zn)
Metals below hydrogen can be obtained by hydrogen reduction of the
metal oxide or by electrolysis of an aqueous solution of a salt (e.g. Cu)
Gold, an unreactive element, is found in the Earth’s crust as the metal
itself. Chemical separation is not needed.
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IRON & STEEL
EXTRACTION OF IRON
A metal such as iron, which is less reactive than carbon, can be extracted from
its ore (iron oxide) by reduction using carbon. This is carried out in a blast
furnace
The raw materials are:
 iron ore (haematite)
 coke, which acts as a fuel and a reducing agent
 limestone, which removes impurities
 hot air, which is blown into the base of the blast furnace
Carbon dioxide burns in air to form carbon dioxide:
C + O2
CO2
At the high temperatures of the furnace, carbon dioxide reacts with coke to
form carbon monoxide:
C + CO2
2CO
The carbon monoxide reduces the iron oxide in iron ore by combining with the
oxygen to form carbon dioxide:
Fe2O3 + 3CO
2Fe + 3CO2
This is a redox reaction in which iron oxide is reduced and carbon monoxide is
oxidised. The molten iron which is formed (known as pig-iron) runs to the
bottom of the furnace.
Limestone combines with the acidic impurities in the ore to form a molten slag,
which floats on the surface of the molten iron. This is an acid-base reaction.
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waste gases
EXTRACTION
OF IRON
RAW MATERIALS:
iron ore (haematite)
coke
limestone
stock line
o
300 C
ore reduced
by CO
o
600 C
melting zone
o
900 C
o
1200 C
1500 oC
carbon burns
o
1900 C
slag forms
hot air blown in
through tuyeres
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slag
iron
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BLAST
FURNACE
Cowper stoves
pre-heat the blast
scrubber
removes
impurities
LOCATION OF AN IRON & STEEL PLANT
The
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plant should be located:
near a port for the bulk importation of iron ore
close to a road and rail network for distribution
close to a town for a workforce
STEEL MAKING
The pig-iron produced in the blast furnace has a high carbon content which
makes it brittle. The carbon needs to be removed and most iron is then mixed
with either a little carbon or with other metals to make the alloy steel. The
addition of small quantities of other metals or carbon alters the properties of
steel.
High carbon steel is hard and strong but brittle; it is used to make, for example,
chisels and other cutting tools.
Low carbon steel is soft and easily shaped; it is used to make wire, nails and
rivets. It is also used in the manufacture of car bodies.
Stainless steel is an alloy containing chromium and nickel, which is hard and
resistant to corrosion. It is used to make cutlery, sinks etc.
The particles in a pure metal are all the same and are arranged in regular
pattern (lattice). When a force is applied, the layers can slide over each other
and the metal is soft and malleable.
In an alloy, other particals of different size disrupt the lattice and the layers
can not slide easily anymore. The alloy is therefore harder.
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Other alloys
Alloys are mixtures of metals. The properties of the alloy are different to
that of the metal and are tailored to specific applications. Examples are given in
the table below.
ALLOY
COMPOSITION
PARTICULAR PROPERTIES
Stainless steel
Iron
Chromium
Nickel
74%
18%
8%
Resists corrosion
High speed steel
Iron
Tungsten
Chromium
Vanadium
75.7%
18.0%
6.0%
0.3%
Bronze
Copper
Tin
90%
10%
Brass
Copper
Zinc
60%
40%
Solder
Lead
Tin
50%
50%
Low melting point
Coinage bronze
Copper
Tin
Zinc
95.0%
3.5%
1.5%
Harder than copper
Cupronickel
Copper
Nickel
75%
25%
Silvery appearance.
Used for coinage.
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Very strong even at high temperature
Much harder than copper
Easy to work. Resistant to corrosion
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ALUMINIUM
Aluminium is the commonest metal in the Earth’s crust. It is a reactive metal.
Since it is above carbon in the reactivity series, it cannot be extracted by
carbon reduction and must, instead, be obtained by electrolysis.
ELECTROLYSIS
When ionic substances, which are made of ions, are melted or dissolved in water,
the ions are free to move about. Therefore, the ions are able to carry charge
from one place to another: this means that these substances can conduct
electricity.
When an electric current is passed through an ionic substance which has been
melted or dissolved in water, the substance is broken down (decomposed) into
simpler substances through chemical reactions at the electrodes. This is the
process called electrolysis.
During electrolysis, positively-charged ions are attracted to the negative
electrode, which is called the cathode. Here they gain electrons (reduction) and
are converted to neutral atoms. For example:
Cu2+ + 2eCu
The metallic copper formed would be deposited on the electrode.
Negatively-charged ions are attracted to the positive electrode, which is called
the anode. Here they lose electrons (oxidation) and are converted to neutral
atoms. For example:
2Cl- - 2eCl2
The chlorine gas would be given off at the electrode.
THE EXTRACTION OF ALUMINIUM
The raw material for the production of aluminium is the ore, bauxite. This is an
impure form of aluminium oxide, the main impurity being iron(III) oxide. The ore
is purified to give aluminium oxide (sometimes called alumina).
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For electrolysis to be carried out, the aluminium oxide must be molten, but
because its melting point is so high, it is dissolved in molten cryolite. Cryolite is
another compound of aluminium. The role of cryolite is:
 to lower the melting point to 970oC
 to act as a solvent
pipe for sucking out
molten aluminium
(+) graphite anodes
crust of solid
electrolyte
molten electrolyte
molten aluminium
(-) graphite cathode
The molten aluminium oxide/cryolite mixture is electrolysed using graphite
(carbon) electrodes. The positive aluminium ions (Al3+) are attracted to the
cathode, where they gain electrons (reduction) to form aluminium atoms.
Al3+ + 3eAl
2The negative oxide ions (O ) are attracted to the anode, where they lose
electrons (oxidation) to form oxygen molecules.
2O2- - 4e-
O2
At the high temperatures of the electrolysis, the oxygen produced at the anode
reacts with the graphite (carbon) that the electrode is made of to form carbon
dioxide. This means that the anode burns away and has to be replaced regularly.
C + O2
CO2
The process of electrolysis consumes large amounts of electricity, which makes
aluminium an expensive metal.
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PROPERTIES OF ALUMINIUM
Aluminium is a useful structural metal. It can be made harder, stronger and
stiffer by mixing in small amounts of other metals to make alloys.
Aluminium has a low density, which makes it useful in the construction of
aircraft. It can be made harder, stronger and stiffer by mixing with small
amounts of other metals to make alloys.
Aluminium is a good conductor of electricity and is used to make overhead
power lines.
Aluminium is a good conductor of heat and is used to make car engines,
saucepans and kitchen foil.
Aluminium is a useful structural metal, because it is ductile and can be shaped
easily to make, for example, window frames and greenhouses.
Aluminium is a reactive metal which is high in the reactivity series. However, it
is resistant to corrosion and does not oxidise (corrode) as quickly as its
reactivity would suggest. This is because aluminium reacts to form a very thin
layer of aluminium oxide on its surface, which then protects the aluminium below
from further corrosion by acting as a barrier to water and oxygen.
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COPPER
Uses of copper
Copper is a very useful metal. Of the more common metals, it is one
of the best thermal and electrical conductors and is also unreactive.
Copper is used in electrical wiring and plumbing around the home with
the average home containing nearly a quarter of a tonne of copper.
Copper is also used to make pots and pans which conduct heat very
well.
Copper is also mixed with zinc to make the alloy brass from
which many musical instruments are made.
Recycling of metals
The reserves of many important metals are limited and recycling is necessary to
prevent the depletion of ore deposits. Recycling uses much less energy than the
primary extraction process. Since electrolysis of aluminium uses so much energy,
melting and re-shaping aluminium can save up to 95% of the energy used in
extraction.
Much less carbon dioxide is produced during recycling compared with
extraction from an ore and, if metals are recycled, the old material does not go
to landfill. We currently scrap 1.5 million cars a year in the UK.
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Summary questions
Topic 14 Uses of metals
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