EXTRACTION OF METALS
The extraction of metals is economically important, because metals are useful
materials with a range of desirable properties, for example electrical and thermal
conductivity, strength, malleability, ductility and shiny appearance. The way a
particular metal is extracted depends on a number of factors.
COMPOSITION OF THE EARTH’S CRUST
Element
Oxygen
Silicon
Aluminium
Iron
Calcium
Sodium
Potassium
Magnesium
Hydrogen
Titanium
Mass %
49.50
25.70
7.50
4.70
3.40
2.60
2.40
1.90
0.88
0.58
Element
Chlorine
Phosphorus
Carbon
Manganese
Barium
Sulphur
Chromium
Fluorine
Nitrogen
All others
Mass %
0.19
0.12
0.09
0.08
0.05
0.05
0.03
0.03
0.03
0.15
Of the possible engineering metals, only aluminium and iron can be classed as
abundant, with titanium, manganese and chromium being the next most common
metals. Other metals which are widely used are relatively rare in the Earth’s crust, for
example:
zinc
0.0070%
copper
0.0055%
lead
0.0013%
tin
0.0002%
silver
0.000007%
TOPIC 12.11: EXTRACTION OF METALS
1
USES OF METALS
Metals have been used by Man for thousands of years. Today, more than 60 metallic
elements are extracted and used by us. The use to which a metal is put depends on:

its physical properties, such as tensile strength, compressive strength,
density, hardness, appearance, electrical conductivity, thermal conductivity

its chemical properties, such as resistance to corrosion, reactivity etc.

its cost
All metals are:
 ductile – they can be drawn into wires
 malleable – they can be hammered into shape without breaking
Complete the table below by giving the property of each metal which is being made
use of in the application which has been given in column 2.
METAL
USE
REASON
Silver
Jewellery
.................................................................................
Aluminium
Aircraft construction .................................................................................
Aluminium
Overhead power
lines
.................................................................................
Copper
Electrical cables
.................................................................................
Lead
Roofing
.................................................................................
Gold
Gold leaf
.................................................................................
Copper
Saucepan bases
.................................................................................
Titanium
Artificial hip joints
.................................................................................
Copper
Coins
.................................................................................
Aluminium
Cooking foil
.................................................................................
Aluminium
Window frames
.................................................................................
Zinc
Galvanising steel
.................................................................................
Tin
Coating steel for
use in food cans
.................................................................................
TOPIC 12.11: EXTRACTION OF METALS
2
ORES
Only metals with a low reactivity towards water and oxygen, such as gold and silver,
occur uncombined (NATIVE) in the Earth’s crust. Most metals occur in the form of
compounds. High grade ores, which exist in a few specific locations, are rocks which
metal compounds metals in high enough concentrations for their extraction to be
economic. The economic concentration of the metal compound will vary, depending
on the value of the metal.
Some useful metal ores have become concentrated by the movement of water
through cracks deep in the Earth’s crust. Water which is at a high temperature and
pressure can dissolve minerals which are not normally soluble. As the solution rises
towards the surface, the temperature and pressure fall, and the minerals crystallise
out forming mineral veins.
Most metal ores are oxides or compounds which can easily be converted to oxides,
like sulphides and carbonates.
METAL
PRINCIPAL ORE
FORMULA
TYPE
Aluminium
Chromium
Copper
Iron
Iron
Lead
Mercury
Tin
Uranium
Zinc
Bauxite
Chromite
Malachite
Haematite
Magnetite
Galena
Cinnabar
Cassiterite
Carnotite
Sphalerite
Al2O3
FeO.Cr2O3
CuCO3.Cu(OH)2
Fe2O3
Fe3O4
PbS
HgS
SnO2
K2(UO2)2(VO4)2
ZnS
Oxide
Oxide
Basic carbonate
Oxide
Oxide
Sulphide
Sulphide
Oxide
Oxide
Sulphide
The process of extracting a metal from its ore is reduction. Oxide ores are reduced
directly to the metal, but sulphides are first converted to oxides by roasting them in
air. For example:
2ZnS(s) + 3O2(g)
2ZnO(s) + 2SO2(g)
The sulphur(IV) oxide produced in this process is a pollutant which contributes to
acid rain. Sulphur(IV) oxide dissolves in water in the clouds, to form sulphurous acid,
H2SO3.
H2O(l) + SO2(g)
H2SO3(aq)
Some is oxidised by oxygen in the air to sulphur(VI) oxide.
2SO2(g) + O2(g)
2SO3(g)
Sulphur(VI) oxide dissolves in water to form sulphuric acid
H2O(l) + SO3(g)
H2SO4(aq)
When these acids fall to the ground in rain water, they damage plants and water
courses by lowering the pH and by leaching toxic metals out of rocks.
TOPIC 12.11: EXTRACTION OF METALS
3
EXTRACTION OF A METAL
Metals can be extracted from their ores by reduction in several ways. The method
chosen for a particular metal depends on a number of factors:
1. Energy Requirements of the Process
Metal oxides are stable compounds and their reduction is usually endothermic. The
cost of the energy used in the reduction process is an important factor. A process
which is operated at a lower temperature will usually be cheaper. The energy
required for electrolysis is high, and where possible, cheaper methods such as
carbon reduction will be preferred.
2. Cost of the Reducing Agent
Some reducing agents are easily obtained: carbon is readily available from coal;
hydrogen can be obtained relatively easily by the reaction of natural gas with steam.
These reducing agents will therefore be much cheaper than metals such as sodium
and aluminium, which first have to be extracted from their ores in costly processes.
3. The Required Purity of the Metal
If a metal is required in a high state of purity for a particular use, for example copper
in electrical wiring, it may be necessary to use a more expensive reduction technique
to achieve this result.
Taking all the factors into account, the cheapest method
capable of producing a saleable product will be used.
CARBON REDUCTION
Carbon is a cheap and plentiful reducing agent. The most common form of carbon
used in metal extraction is coke, which is obtained from coal by heating it in the
absence of air. Apart from its high carbon content, coke has the additional benefit of
being structurally strong and porous.
The reduction of a metal oxide with carbon:
MO(s) + C(s)


M(l) + CO(g)
is endothermic (H is +ve)
since a gas is produced in the process, the entropy change (S) is +ve
The free energy change (G) is given by the equation:
G = H -TS
For a reaction to be feasible, G must be negative. G becomes negative when the
value of TS is greater than H. Therefore, if the temperature is high enough,
carbon reduction of any metal oxide is feasible.
Carbon reduction is used to extract, among others, iron and zinc. The temperature
required to extract aluminium by this method is too high to be practical and
economic.
TOPIC 12.11: EXTRACTION OF METALS
4
IRON & STEEL
EXTRACTION OF IRON
The most important example of carbon reduction is the manufacture of iron. This is a
continuous process which is carried out in a blast furnace.
The raw materials are:
 iron ore (haematite, Fe2O3)
 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
Haematite, coke and limestone are heated together strongly to produce a uniform
mixture known as sinter, which is fed into the top of the blast furnace. Sinter is
porous and structurally strong.
Hot air is blown into the base of the furnace through jets called tuyeres.
Carbon burns in air to form carbon dioxide:
C(s) + O2(g)
CO2(g)
This reaction is highly exothermic and produces the heat needed for the reduction of
iron(III) oxide.
At the high temperature produced, carbon dioxide reacts with coke to form carbon
monoxide:
C(s) + CO2(g)
2CO(g)
The carbon monoxide reduces most of the iron(III) oxide to iron at around 1200 oC:
Fe2O3(s) + 3CO(g)
2Fe(l) + 3CO2(g)
In the hotter part of the furnace, coke also reduces iron(III) oxide:
Fe2O3(s) + 3C(g)
2Fe(l) + 3CO(g)
The molten iron which is formed runs to the bottom of the furnace and is tapped off.
The product, known as cast iron, is impure. It contains up to 4% carbon, together
with sulphur, silicon, phosphorus and manganese in quantities which vary with the
ore used and with the operating conditions.
Limestone decomposes at the high temperatures in the furnace:
CaCO3(s)
CaO(s) + CO2(g)
Calcium oxide is a basic substance which combines with the acidic impurities in the
ore, mainly silicon dioxide, to form a molten slag, which runs to the bottom of the
furnace and floats on the surface of the molten iron.
CaO(s) + SiO2(s)
CaSiO3(l)
Slag is used:
to make breeze blocks for the construction industry
as road chippings.
TOPIC 12.11: EXTRACTION OF METALS
5
waste gases
EXTRACTION
OF IRON
RAW MATERIALS:
iron ore (haematite)
coke
limestone
stock line
o
300 C
o
600 C
slag forms
o
900 C
o
1200 C
1500 oC
ore reduced by CO
melting zone
ore reduced by C
o
1900 C
carbon burns
hot air blown in
through tuyeres
TOPIC 12.11: EXTRACTION OF METALS
slag
iron
6
BLAST
FURNACE
Cowper stoves
pre-heat the blast
scrubber
removes
impurities
THE BASIC OXYGEN STEEL-MAKING PROCESS
Molten iron from the blast furnace is mixed with recycled scrap steel, then powdered
magnesium is added. The magnesium reacts with sulphur impurities in the iron, forming
magnesium sulphide, which floats as a slag on the surface of the iron.
Mg + S
MgS
Pure oxygen is then blown into the molten iron through a water-cooled lance. Calcium
oxide is also added. The non-metal impurities are converted into oxides. Carbon is
oxidised to carbon monoxide, which is given off as a gas. The oxides of silicon and
phosphorus, which are solid and acidic, react with calcium oxide to form a slag. Sulphur
cannot be removed by treatment with oxygen, because iron would be oxidised in
preference to the sulphur.
Calculated quantities of carbon and/or other elements are then added to make a range
of steels with particular properties.
oxygen
water cooled
lance
desulphurised, molten
cast iron containing
scrap steel
TOPIC 12.11: EXTRACTION OF METALS 7
Steels are iron-carbon alloys containing 0.04 –1.0% carbon. The addition of small
quantities of other metals or carbon alters the properties of steel.
High carbon steel (~1% carbon) is hard and strong but brittle; it is used to make, for
example, chisels.
Mild steel (~0.1% carbon) is the commonest steel and is used, for example, to make car
bodies.
Low carbon steel (~0.04% carbon) is soft and easily shaped; it is used to make wire,
nails and rivets.
Stainless steel is an alloy containing chromium and nickel, which is hard and resistant to
corrosion. It is used to make cutlery, sinks etc.
PROBLEMS WITH THE CARBON REDUCTION PROCESS
1. Pollution
A by-product of this process is carbon dioxide, which causes problems because it is a
greenhouse gas and contributes to global warming.
2. Carbide Formation
Instead of the oxide being reduced to the metal in this process, some metals form metal
carbides instead. The method is therefore impractical for such metals. An important
example is titanium; when titanium(IV) oxide is heated with carbon:
TiO2 + 3C
TiC + 2CO
Other metals which give carbides are vanadium, tungsten and molybdenum.
Carbides are, however, important materials, some of which can be used as catalysts.
Tungsten carbide is very hard and is used as an abrasive and to tip drills.
TOPIC 12.11: EXTRACTION OF METALS 8
ALUMINIUM
Aluminium is the commonest metal in the Earth’s crust. It is a reactive metal which can
only be reduced by carbon at impractically high temperatures. Instead, it is obtained by
electrolysis of the molten metal oxide.
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.
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.
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).
For electrolysis to be carried out, the aluminium oxide must be molten, but because its
melting point is so high (>2000oC), it is dissolved in molten cryolite (Na3AlF6). This
lowers the melting point of the melt to 970oC.
pipe for sucking out
molten aluminium
(+) graphite anodes
crust of solid
electrolyte
molten electrolyte
molten aluminium
TOPIC 12.11: EXTRACTION OF METALS 9
(-) graphite cathode
The process is run continuously. As aluminium oxide is used up, more is added at
regular intervals from a hopper above the cell, after first breaking the crust of electrolyte.
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+ + 3e-
Al
The negative oxide ions (O2-) 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 monoxide and
carbon dioxide. This means that the anode burns away and has to be replaced
regularly.
2C + O2
2CO
C + O2
CO2
The process consumes large amounts of electricity, both to melt the cryolite and to
decompose the aluminium oxide, which makes aluminium an expensive metal. The
process is economical only where relatively cheap electricity is available.
Tiny amounts of fluorine are produced in the process, but great care is taken to ensure
that this does not pollute the area around the plant. Waste cryolite could cause fluoride
pollution.
TOPIC 12.11: EXTRACTION OF METALS 10
TITANIUM
Titanium is a relatively abundant metal in the Earth’s crust, but the difficulties in
extracting it limit its uses, despite many desirable properties. It has high strength, low
density and a high resistance to corrosion. It is non-toxic.
Carbon reduction is not a feasible method of extracting titanium. Traces of carbon,
oxygen and nitrogen in titanium render it brittle and unusable.
The only practical way of extracting titanium is by the reduction of the metal halide with
a reactive metal. This is an expensive method.
Extraction
Titanium is a reactive metal which is extracted from its ores by reaction with a more
reactive metal.
 The raw material is the ore, RUTILE, which contains titanium dioxide, TiO2.

The oxide is converted into titanium(IV) chloride, TiCl4, by heating it with coke
and chlorine at about 900oC.
TiO2 + 2C + 2Cl2
TiCl4 + 2CO

Titanium(IV) chloride is a colourless liquid. It fumes in moist air as it is
hydrolysed. It is purified from other chlorides, such as those of Fe, Si and Cr, by
fractional distillation under an atmosphere or argon.

Titanium(IV) chloride is heated with sodium under an inert atmosphere or
argon, forming sodium chloride and titanium. The reaction begins at about
550oC, but because of the exotherm, the temperature rises to about 1000 oC
during the reaction.(Magnesium could be used instead of sodium).
TiCl4 + 4Na
Ti + 4NaCl (H is -ve)

During the reaction sodium, the more reactive metal, is oxidised (loses electrons
to form Na+). Titanium(IV) chloride is reduced. Argon prevents the contamination
of the titanium with oxygen or nitrogen.

Sodium chloride is dissolved out in water to leave titanium as a grey, granular
powder.
The process is a batch process and is expensive because:
 chlorine and sodium have to be produced first by electrolysis
 high temperatures are involved in both steps
 an argon atmosphere is needed
 TiCl4 needs to be handled carefully because it reacts violently with cold water
TOPIC 12.11: EXTRACTION OF METALS 11
TUNGSTEN
Tungsten is the metal which has the highest melting point (3422oC). Like aluminium and
titanium, it is protected from corrosion by a thin oxide layer on it's surface. However, this
layer only protects is up to about 400oC after which it rapidly oxidises in the presence of
air.
Extraction of tungsten
Tungsten is extracted from its ores Wolframite (Fe/Mn)WO4, Huebnerite MnWO4,
Ferberite FeWO4 and Scheelite CaWO4 by reduction with hydrogen.
The ores are processed to produce WO3 which is then heated with hydrogen gas at
850oC
WO3 (s) + 3H2 (g))
→
W (s)) + 3H2O (l))
This process is potentially dangerous to the very flammable nature of hydrogen.
Tungsten could be extracted from its ore by carbon reduction but this also leads to the
formation of tungsten carbide (WC) which is less useful and makes the metal brittle.
Tungsten carbide is however extremely hard and is used to make drills, grinders, and
cutting tools.
TOPIC 12.11: EXTRACTION OF METALS 12
ECONOMIC FACTORS & RECYCLING
The reserves of many important metals (e.g. Cu) are limited and recycling is necessary
to prevent the depletion of ore deposits. Since the collection and transportation of scrap
metal to the recycling plant have an energy requirement, it is not economic to recycle all
metal, but in general, the greater the value of the metal, the more economic it is for
even small pieces to be collected and recycled.
Recycling of Iron
Iron is extensively recycled, and up to 30% scrap steel is used in each batch in the
basic oxygen process. Despite the abundance of iron, it is sensible to recycle it
because:
 scrap iron and steel contain a higher percentage of iron than even high grade
ores
 recycling uses less energy than the primary extraction process
 carbon dioxide, a greenhouse gas, is produced in the primary extraction process.
Recycling, which involves only melting and re-shaping, does not produce carbon
dioxide
 iron is magnetic and can easily be separated from other scrap metal
 a serious pollution problem is avoided. Without recycling, discarded iron and
steel objects, which would include about 1.5 million cars per year in the U.K.,
would pollute the environment
Scrap iron can also be used to extract copper from low grade copper ores. When these
ores are processed the liquid waste still contains very small amounts of copper
dissolved in it. Scrap iron can be used to reduce the dissolved copper ions to copper
metal.
Cu2+ (aq) + Fe (s) → Fe2+ (aq) + Cu (s)
The cheaper iron dissolves in this process and the more expensive and increasingly
scarce copper is obtained. This process is very efficient since little energy is needed
compared to the high temperature carbon reduction process.
Recycling of Aluminium
Aluminium too is an abundant metal, but the large energy consumption makes up a high
percentage of its extraction cost. It is therefore sensible to recycle, even if aluminium is
harder to separate from other scrap metal than iron is, because:
 recycling uses much less energy than the primary extraction process. Since
electrolysis uses so much energy, melting and re-shaping aluminium can save up
to 95% of the energy used in extraction.
 carbon dioxide, a greenhouse gas, is produced in the primary extraction process,
by the burning away of anodes. Recycling, which involves only melting and reshaping, does not produce carbon dioxide.
 scrap aluminium contains a higher percentage of aluminium than bauxite
 as with iron, recycling avoids pollution by dumped objects.
TOPIC 12.11: EXTRACTION OF METALS 13