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Metallurgy
Concepts
(i) What is metallurgy (ii) Minerals and Ores
(iv) Processes involved in extraction of metals
(iii) Occurrence of metals
Introduction
Copper and iron were the first metals put to extensive use by early man. Copper mixed
with tin called bronze was so widely used for many years that this period came to be
known as the ‘Bronze Age’. The increasing use of metals in day to day life aroused the
interest of man in their properties and the sources from which they could be recovered.
This gave birth to a new branch in chemistry called metallurgy.
The science that deals with procedures used in extracting metals from their
ores, purifying and alloying metals and creating useful objects from metals
is called metallurgy.
Metallurgy is also the practice of removing valuable metals from an ore and refining the
extracted raw metals into a purer form.
Mineral and Ore
Natural materials found inside the earth containing metals in their combined states ( as a
single compound or as a mixture of compounds ) mixed with non – metallic impurities of
Earth and rock ( called gangue ) are termed minerals. These metals do not occur in the
native form and need to be extracted for use. All minerals are not suitable for extraction
of metals. Minerals from which the metal can be extracted easily and economically are
called ores. Ores contain metal compounds with a lower percentage of impurities. Thus
all ores are minerals but all minerals are not ores.
Occurrence of metals
Metals occur in nature in the free as well as in the combined states. The most unreactive
metals i.e. which are not affected by air and water, like silver, gold and platinum are
generally found in the free state. In other words, elements which have low chemical
reactivity generally occur native or free or in metallic state and those which are
chemically reactive or affected by air and water generally occur in combined state e.g.
halogens, chalcogens.
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Ores can be divided into four general groups as follows.
(i) Native ores – These ores contain metals in the free state. e.g. Ag, Au, Pt, Hg, Cu etc.
These are usually found associated with rock or alluvilial materials like clay, sand etc.
Sometimes, lumps of pure metals are also found in them. These are termed nuggets.
Iron is found in free state as meteorites which also have 20% to 30 % nickel.
(ii) Sulphurised and arsenical ores - These ores consist of sulphides and arsenides in
simple and complex forms of metals. Some examples of this group are PbS, ZnS,
Ag2S, NiAs, CuFeS2, 3Ag2S. Sb2S3 etc.
(iii) Oxide ores - In these ores, metals are present as their oxides or oxysalts such as
carbonates, nitrates, sulphates, phosphates, silicates etc. The examples include Fe2O3,
Al2O3, BeO.Al2O3, MnO2, CaCO3, FeO.TiO2 , NaNO3, BaSO4 , Zn2SiO4, Ca3(PO4)2
etc.
(iv) Halide ores - Metallic halides are very few in nature. Chlorides are more common.
The examples include common salt, NaCl, , Carnallite, KCl, MgCl2.6H2O ,
Fluospar, CaF2 , Horn silver, AgCl etc.
An overview of the processes
The various processes involved in the extraction of metals from their ores and their
subsequent refining are known as metallurgy. An overview of various processes involved
during metallurgy is given below.
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Enrichment of ore
This is also called as concentration or dressing of the ore. Ore is an impure metal
containing large amount of sand and rocky material. The impurities like sand, rocky
materials, limestone, mica etc. is called gangue or matrix. These impurities must be
removed from the ore before the extraction of metal. Sometimes the ore appears in the
form of big lumps. So it becomes necessary to break those lumps into small pieces.
Crushing and grinding of the ore : These ores occur in nature as huge lumps. They are
broken to small pieces with the help of crushers or grinders. These pieces are then
reduced to fine powder with the help of a ball mill or stamp mill. This process is called
pulverisation.
Depending upon the nature of the ore, one or more of the following steps are taken to
concentrate the ore. These are mostly physical methods of concentration.
Concentration or dressing of the ore : These ores are usually obtained from the ground
and therefore contain large amount of unwanted impurities, e.g., earthing particles, rocky
matter, sand, limestone etc.. It is essential to separate the large bulk of these impurities
from the ore to avoid bulk handling and in subsequent fuel costs. The removal of these
impurities from the ores is known as concentration. The concentration is done by
physical as well as chemical methods
(1) Hydraulic washing ( Gravity separation ) : In this process, the ore particles are
poured over a hydraulic classifier which is a vibrating inclined table with grooves and a
jet of water is allowed to flow over it. The denser ore settles in the grooves while the
lighter gangue particles are washed away. This method is used for concentration of heavy
oxide ores of lead, tin, iron etc. The hydraulic washing method is shown in the following
figure.
Fig.1 - Hydraulic washing
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(2) Froth floatation - This method is especially used for sulphide ores. The method
employs a mixture of water and pine oil which is made to agitate with the ore.
A mixture of water, pine oil, detergent and powdered ore is first taken in a tank. A blast
of compressed air is blown through the pipe of a rotating agitator to produce froth. The
sulphide ore particles are wetted and coated by pine oil and rise up along with the froth
(froth being lighter). The gangue particles wetted by water sink to the bottom of the tank
( water being heavier ). Sulphide being more electronegative attracts the covalent oil
molecules. The gangue being less electronegative is attracted by the water. The froth
containing the sulphide ore is transferred to another container, washed and dried. Thus
sulphide ore is separated from the gangue. The froth floatation process is shown in the
following figure.
Fig.2 – Froth floatation process
(3) Magnetic separation – Magnetic ores like pyrolusite ( MnO2) and chromite
( FeO.Cr2O3 )are enriched by this method by making use of the difference in the
magnetic properties of the ore and gangue particles. The powdered ore is dropped on to
leather or brass conveyer belt, which moves over two rollers one of which is magnetic.
When the ore passes over the magnetic roller, it sticks to the belt due to the force of
attraction and falls nearer to magnetic roller. The gangue falls in a normal way under the
influence of gravity. The magnetic ore and gangue thus form two separate heaps.
Following figure shows the magnetic separation method.
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Fig.3 – Magnetic separation process
(4) Leaching – It means washing. In this process, the ore is washed with some suitable
reagent ( solvent ) so that the main metal passes into its salt solution. This solution is
separated and subjected to further treatment like precipitation. It is then treated further to
recover the metal. This is a chemical method of concentration.
Conversion of concentrated ore to oxide
It is easier to obtain a metal from its oxide form as compared to its sulphide, carbonate or
any other form. Therefore, prior to reduction usually the metal is converted to its oxide
form. Following methods are used to convert the concentrated ore to its oxide form.
1) Calcination – It is a process in which the ore is heated strongly in absence of air. The
ore is heated at a temperature well below its melting point. The ore gets thermally
decomposed, undergoes phase transformation and eliminates the volatile impurities like
moisture, carbon dioxide etc. Since the ore becomes porous and compact , it easily
undergoes further chemical reactions. This method is generally used for carbonate and
hydrated ores.
2) Roasting – It is a process wherein the ore is heated either alone or with some other
material in excess of air below the fusion point of the ore. Usually, this method is used
for sulphide ores. In roasting, definite chemical changes take place to form oxide or
chloride of the metal. Ores of metals like zinc, lead, copper and nickel, when roasted in
air, are converted to their oxides. Ores of some metals like lead may get partially
oxidized and converted to sulphate. In such case, it is called sulphating roasting or partial
roasting. Ores of metals like silver and gold are mixed with common salt and are heated
in air. They are converted to their chlorides which are easy to reduce. This type of
roasting is called chlorinating roasting. The purpose of roasting is to convert the ore in a
form suitable to reduce. The gaseous product of sulphide roasting, sulphur dioxide, is
often used to produce sulphuric acid.
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Reduction of oxide to a metal
One or more of the following methods can be used to obtain a metal from its oxide.
(i) Heating – Less reactive metals like silver and mercury can be obtained by heating
their oxides alone. These metals are placed at the bottom of the electrochemical series.
2 HgO
→ 2 Hg + O2
;
2 Ag2O
→ 4Ag + O2
(ii) Carbon reduction - Some metals like lead, copper, zinc and iron can be obtained by
reducing their oxides by carbon. When the oxides of these metals are heated with coke,
the oxides are reduced to a metal. Carbon has more affinity for oxygen than the metals
have it for oxygen hence carbon extracts the oxygen leaving behind the free metal.
PbO + C → Pb + CO
ZnO + C → Zn + CO
;
;
CuO + C → Cu + CO
Fe2O3 + 3 C → 2 Fe + 3CO
(iii) Use of carbon monoxide – Oxides of metals like PbO, CuO, FeO can be reduced by
carbon monoxide at high temperature to give the corresponding metals.
CuO + CO → Cu + CO2 :
FeO + CO → Fe + CO2 ; PbO + CO → Pb + CO2
(iv) Use of aluminium – Oxides of metals like ZnO, Cr2O3 and MnO2 can not be
reduced by carbon because these metals have a greater affinity for oxygen than carbon.
An active metal like aluminium ( in the form of powder ) is required to reduce the oxides
of these metals. The reduction of a metal oxide by heating with aluminium is called
aluminothermy. In this process, lot of heat is evolved and hence the metal may melt in
the container. It is tapped from the bottom of the container.
Cr2O3 + 2 Al → Al2O3 + 2 Cr + Heat ; 3MnO2 + 4Al → 2 Al2O3 + 3 Mn + Heat
(v) Electrolysis – The metals like iron, zinc, lead, chromium, manganese lie in the
middle of the electrochemical series. They are somewhat active. So their oxides can be
reduced by carbon or carbon monoxide or reactive metals like aluminium or sodium,
calcium . But the metals like sodium, magnesium, calcium, aluminium which are placed
in the top of the electrochemical series i.e. which are very active, can not be obtained by
the reduction of their oxides by ordinary reducing agents. So they are obtained by passing
an electric current through the purified molten ore. An electric current is passed through
the molten oxide or chloride of the metal. The metal gets deposited at the cathode from
where it is separated.
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Refining of metals
(i) Liquation – It is a technique for separating constituents of an ore, a metal, or an alloy
by partial melting. The technique is used when the melting temperature of the metal is
lower than that of the impurity and the impurities are not miscible with the metal. Metals
like Bi, Sn, Pb, Hg etc. are purified by this technique. The sloping floor of the
reverberatory furnace is used to melt the crude metal, when pure metal flows down and
impurities are left behind.
Fig. 4 - The liquation method
Electro- refining - Electro-refining of metals is a process of obtaining pure metal from
the impure one by the process of electrolysis.
Process – The process of electro-refining of metals involves the following steps :
(i) The electrolyte is usually an aqueous solution of the salt of the metal with some
corresponding acid, if necessary.
(ii) A thick block of impure metal is made as the anode.
(iii) A thin rod or sheet of pure metal is made as the cathode.
(iv) The metal cations being positive, migrate towards the cathode and get discharged.
(v) At anode, the atoms of the metal lose electrons, form cations and enter the solution.
(vi) The less electropositive impurities in the anode, settle down at the bottom and are
removed as anode mud while the more electropositive impurities pass into the solution.
(vii) Anode finally disintegrates while the cathode gains in weight due to the collection of
pure metal. This way pure metal is obtained.
Distillation - Metals like zinc and mercury which boil at low temperature are purified by
this method. The impure metal is taken in iron retort and heated strongly above the
boiling point of the metal. At the boiling point, vapours of the metal are produced which
are led to a condenser. By condensation of the vapours, pure metal is obtained.
Activity 1 – Collect 2 to 3 ores from your teacher. Suggest the steps and draw a flow
sheet type diagram to recover the metal from its ore.
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Test your understanding :
1) What is the difference between a mineral and an ore ?
2) What are the methods used to concentrate an ore ?
3) What is the purpose of the following processes ?
(i) Calcination (ii) Roasting (iii) Magnetic separation (iv) Liquation
4) Which reducing agents are used in metallurgy ?
Aluminium and iron are two important metals which are used as construction materials.
So also they find many applications in domestic life. Hence we will study the metallurgy
of these two metals here.
Metallurgy of aluminium
Concepts
(i) Occurrence of aluminium (ii) Purification of bauxite (iii) Electrolysis of alumina
(iv) Refining of aluminium (v) Physical and chemical properties of aluminium
(vi) Uses of aluminium (vii) Alloys of aluminium
Occurrence
Aluminium is the most abundant ( 8.13 % ) metallic element in the earth’s crust and after
oxygen and silicon, the third most abundant of all elements in the crust. Because of its
strong affinity to oxygen, it is not found in nature in the elemental state but only in
combined forms such as oxide or silicate.
Aluminium occurs in igneous rocks chiefly as alumino silicate in feldspar, feldspathoids,
and mica; in the soil derived from them as clay and upon further weathering as bauxite
and iron-rich laterite. Bauxite, a mixture of hydrated aluminium oxides, is the principal
aluminium ore.
Extraction of aluminium
The extraction of aluminium is called elctrometallurgy. It deals with the use of
electricity for smelting or refining of metals. In electrometallurgy, the electrochemical
effect of an electric current brings about the reduction of metallic compounds and thereby
the extraction of metals from their ores ( electro-winning ) or the purification of the
metals ( electro-refining.)
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The principal aluminium ore is bauxite Al2O3 . 2H2O. It is essentially an impure
aluminium oxide. The major impurities include iron oxide, silicon dioxide and titanium
dioxide.
The extraction of aluminium, in principle , should be easy because the ore occurs in
oxide form which can be reduced by a suitable reducing agent to give the metal.
However, in practice, aluminium oxide can not be reduced that easily. Aluminium has
great affinity for oxygen hence it can not be reduced by usual reducing agents
Aluminium is too high in the electrochemical series ( it is a highly reactive element ) so it
can not be reduced by hydrogen or carbon. If at all reduced by carbon, the temperature
required for the reduction is very high. This does not make the process economic. Hence
aluminium is obtained by the electrolysis of pure alumina.
Purification of bauxite
Bauxite contains iron oxide or silica as major impurity. The bauxite containing iron oxide
as major impurity is called red bauxite and the bauxite containing silica as major
impurity is called white bauxite. Iron and silicon both make aluminium metal brittle and
liable for corrosion hence they must be eliminated. If bauxite contains iron oxide, Fe2O3
as the major impurity, it is purified by Baeyer’s process or Hall’s process. . If it
contains silica , SiO2 as the major impurity, it is purified by Serpek’s process.
(i) Serpek’s process – This process is used when bauxite ore contains appreciable
amount of silica (above 7 %) and low amount of Fe2O3 ( less than 1 %) .Powdered
bauxite is mixed with carbon and heated up to 18000C in a current of nitrogen .
Aluminium from bauxite is converted to aluminium nitride while silica is reduced to
silicon.
Al2O3 .n H2O + 3C + N2 → 2 AlN + 3 CO + n H2O
SiO2 + 2C → Si ↑ + 2 CO ↑
Silicon volatilizes at this temperature. Aluminium nitride is hydrolyzed with hot water. It
precipitates aluminium hydroxide.
AlN + 3 H2O → Al(OH)3 ↓ + NH3
The precipitate of Al(OH)3 is washed, dried and ignited at about 15000C to get pure
alumina.
2 Al(OH)3 → Al2O3 + 3 H2O ↑
b) Baeyer’s process – This process is used when bauxite ore contains appreciable
amount of Fe2O3 ( 7 to 10 % ) and low amount of silica ( less than 1 % ). The ore is
first calcined and then finely ground. It is then digested with a hot and strong solution
of caustic soda ( 45 % ) in an autoclave under 80 lb. pressure at 1500C for 2 to 8 hours.
At this stage, aluminium oxide dissolves in NaOH to form sodium meta aluminate
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( NaAlO2) while ferric oxide and titanium dioxide remain undissolved.. They are then
removed by filtration.
Al2O3 + 2 NaOH → 2 NaAlO2 + H2O
Sodium meta aluminate ( soluble )
Silica dissolves in the form of silicate. After filtration, sodium meta aluminate solution
is diluted with water, slowly cooled and then mixed with a little freshly precipitated
aluminium hydroxide which acts as a nucleus for precipitation of aluminium
hydroxide.( Alternatively CO2 can be passed till the solution becomes acidic ) It is
then digested. Sodium meta aluminate, NaAlO2 hydrolyses to give precipitate of
aluminium hydroxide.
NaAlO2 + 2 H2O → NaOH + Al(OH)3 ↓
Aluminium hydroxide precipitate is then washed, dried and ignited to get pure
alumina ( Al2O3 ) .The filtrate containing caustic soda is concentrated and used again.
2 Al(OH)3 → Al2O3 + 3 H2O ↑
c) Hall’s process - This process is used for low grade bauxite ores. In this process,
bauxite ore is fused with sodium carbonate, Na2CO3 to give water soluble sodium meta
aluminate , NaAlO2 leaving behind Fe2O3 and SiO2.
Al2O3 + Na 2CO3 → 2 NaAlO2 + CO2 ↑
The fused mass of sodium meta silicate is extracted with water and filtered. The
impurities Fe2O3 and SiO2
remain on the filter paper. The filtrate containing NaAlO2 is warmed and CO2 is passed
through it, when Al(OH)3 is precipitated.
2 NaAlO2 + CO2 + 3 H2O → 2 Al(OH)3 ↓ + Na2CO3
The precipitate is filtered, washed and ignited to obtain pure alumina.
2 Al(OH)3 → Al2O3 + 3 H2O ↑
Pure alumina
Electrolysis of pure alumina
Aluminium can be obtained by electrolysis of pure alumina but it offers two problems.
(i) Pure alumina is a poor conductor of electricity and melts at about 20000C.
(ii) When fused alumina is electrolyzed at 20000C ,the metal formed vapourises as its
boiling point is 18000C.
Aluminium is usually prepared by Hall- Heroult process. Alumina is fused with
cryolite Na3AlF6. Alumina dissolves in cryolite. Cryolite lowers the temperature of the
mixture. Small amount of CaF2 and AlF3 are also added to lower the temperature of the
mixture. Pure alumina melts at 20000 C while the mixture melts at about 9500C The
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charge consists of cryolite ( 85 % ), CaF2 ( 5 % ), AlF3 ( 5 % ) and Al2O3( 5 % ) .The
electrolysis is carried out at temperature of 9500C and with a voltage of 5.5 volts in a
graphite lined steel tank which acts as a cathode. The anodes are made of graphite. The
Al2O3 is added from feeder at the top. Some coke is thrown on the surface of charge to
control the oxidation of the metal. The electrode reactions are complicated and their exact
nature is not known . The simplified mechanism of electrode reactions is given below.
Na3AlF3 → 3 NaF + AlF3 ;
4 AlF3 →
4Al 3+ + 12 F-
At anode → 2 Al2O3 + 12 F- → 4 AlF3 + 3 O2 + 12 e- ; 4C + 3O2 → 2 CO2 + 2 CO
At cathode → 4 Al3+ + 12 e- → 4 Al
Diagram –
Fig. 5 - Electrolysis of alumina
Refining of aluminium metal
Metal produced by Hall – Herouit’s process is almost 99.9 percent aluminium
and it contains small amounts of iron, silicon from the bath and some alumina and
carbon. So Hoope’s electrolytic refining process is used to refine the metal.
Hoope’s process – In this process, fused salt electrolyte is used. The cell uses three
liquid layers of different densities.
(i) The bottom anode layer consists of impure aluminium.
(ii) The middle layer consists of cryolite , alumina and barium fluoride acting as
electrolyte.
(iii) The top cathode layer is of pure metal. This aluminium layer is connected with
graphite electrode to the mains.
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Fig.. 6 - Hoope’s cell for refining aluminium
The cell is made of iron box. It is lined from inside with carbon. The cell is shown in
Fig. 6 . On passing electric current, aluminium from the middle layer passes into the top
layer and equivalent amount of aluminium passes from the bottom layer to the middle
layer. From time to time , pure aluminium is removed from the top and aluminium of
lower purity is added to the bottom layer. Thus, there is transfer of aluminium from the
base to the top while impurities are left behind. Pure aluminium is tapped from the top.
The refined aluminium has purity of 99.99 %.
Physical and Chemical Properties of Aluminium
(i) Aluminium is a white metal with a slight bluish tinge. In moist air, it becomes dull
owing to the formation of superficial protective layer of its oxide.
(ii) It is a light metal ( sp. Gravity 2.7 ) which melts at 6580 C and boils at 18000C.
(iii) It is malleable and ductile especially between 1000C and 1500C. Near about its
melting point, it becomes brittle and can be ground to powder.
(iv) It is an excellent conductor of heat and electricity.
(v) It is tough and has a moderate tensile strength.
(vi) Finely divided aluminium or thin aluminium foil burns readily in air or oxygen when
heated, forming aluminium oxide ( with little nitride AlN also ) liberating much heat.
4 Al + 3 O2 → 2 Al2O3 + Heat
(vii) Action of acids - Aluminium is above hydrogen in the activity series and it
displaces hydrogen from non-oxidising acids like HCl and dilute H2SO4.
2 Al + 6 HCl → 2 AlCl3 + 3 H2
2Al + 3 H2SO4 → Al2(SO4)3 + 3 H2
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But hot concentrated H2SO4 gives SO2
2 Al + 6 H2SO4 → Al2(SO4)3 + 3 SO2 + 6 H2O
Concentrated HNO3 makes aluminium passive and with dilute HNO3, it produces
ammonium nitrate but no gas is evolved.
8 Al + 30 HNO3 → 8 Al(NO3)3 + 3 NH4NO3 + 9 H2O
(viii) Action of alkalies - Aluminium is an amphoteric metal so it reacts with acids as
well as alkalies. ( The reaction with acids are given above )
.
2 Al + 2 NaOH + 2 H2O → 2 NaAlO2 + 3 H2
(ix) Action of non-metals - Heated aluminium directly combines with halogens, carbon
nitrogen and sulphur.
a) 2Al + 3 Cl2 → 2 AlCl3
c) 2 Al + N2 → 2 AlN
b) 4 Al + 3 C → Al4C3
d) 2 Al + 3 S → Al2S3
(x) Reducing action - At high temperature, aluminium has a strong affinity for oxygen
and hence it reduces oxides of iron, manganese etc.
Fe2O3 + 2 Al → Al2O3 + 2 Fe + Heat
Uses of aluminium
(i) Since it is lighter and has high tensile strength, aluminium is used in making body of
air-ships and motor cars.
(ii) On account of its good electrical conductivity, it is used for making electrical
transmission cables.
(iii) On account of its good thermal conductivity, it is used in making cooking utensils.
(iv) Since it resists corrosion, it is used in aluminium paints.
(v) Aluminium foils are used in wrapping cigaretts, confectionary items etc.
(vi) Aluminium is used as a deoxidizer and for removing blow holes in metallurgy.
(vii) It is used in thermite welding and in the aluminothermic process.
(viii) Salts of aluminium such as alum are used as mordants in dyeing industries.
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Alloys of aluminium
Alloy
Composition
Properties
Uses
-----------------------------------------------------------------------------------------------------------1) Magnalium
98 % Al, 2 % Mg
Hard, tough, light,
For making balance
Can be excellently
beams, light
worked on lathe
instruments, articles
-----------------------------------------------------------------------------------------------------------2) Duralium
95 % Al, 4 % Mn,
Resistance to
For making airships,
0.5 % Mg
corrosion, highly
aeroplanes etc.
ductile, light
-----------------------------------------------------------------------------------------------------------3) Aluminium
10 t0 12 % Al,
Resistance to
For making utensils,
Bronze
88 to 90 % Cu
corrosion, readily
jewellery, decorative
fusible, strong
articles , coins
-----------------------------------------------------------------------------------------------------------4) Nickeloy
95 % Al, 4% Cu,
Extremely light,
For making airships
1 % Ni
great mechanical
strength
-----------------------------------------------------------------------------------------------------------5) Alnico
50% steel, 20%Al,
For making
20% Ni, 10% Cu
permanent magnets
------------------------------------------------------------------------------------------------------------
Activity 2 - Suggest the names of three alloys of aluminium which you use in day to
day life.
Test your understanding
1) Name the ore and give its composition from which aluminium is extracted.
2) Name the processes used to purify aluminium ore.
3) What is the composition of electrolyte when molten alumina is subjected to
electrolysis ?
4) Draw a diagram of Hoop’s cell for refining aluminium metal.
5) Give the names , composition and uses of any two alloys of aluminium.
Metallurgy of Iron
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Concepts
(i) Occurrence of iron (ii) Commercial forms of iron (iii) Manufacture of cast iron
(iv) Blast furnace (v) Physical and chemical properties of cast iron
(vi) Manufacture of steel (vi) Alloys of iron
Occurrence
Next to aluminium, iron is the most abundant metal in earth’s crust. Iron is the fourth
most abundant ( about 5 % ) metallic element in the earth’s crust . Because of its strong
affinity to oxygen, it is not found in nature in the elemental state but only in combined
forms such as oxide. Iron is easily attacked by humid atmosphere. It is generally found
associated with other metals like copper, cobalt and nickel. Principally iron occurs as
oxides, much less as sulphide and sometimes as the carbonate.
The chief sources of iron are –
(i) Red Haematite – Fe2O3, Brown Haematite or Limonite – 2 Fe2O3 . H2O,
Magnetite – Fe3O4
(ii) Siderite – or Spathic Iron ore - FeCO3
(iii) Iron pyrites – FeS2
(iv) Chalcopyrites – CuFeS2
Commercial forms of iron
There are three commercial forms of iron .
(i) Cast or Pig iron – It is the most impure form of iron. It contains about 1.5 % to 4.5
% carbon. Other impurities like Si, P, Mn and S are present upto about 1.5 %.
(ii) Wrought or Malleable iron – It is the purest form of iron. It contains about 0.2 %
carbon.
(iii) Steel – It is an alloy of iron with carbon and other elements like manganese, silicon
and phosphorus. It is midway between cast and wrought iron as far as impurities are
concerned. It contains 0.1 to 1.5 % carbon.
The three varieties differ from each other mainly in their carbon content.
The first step in the extraction of iron is the production of pig or cast iron which is
subsequently used in making wrought iron or steel.
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Extraction of iron
The extraction of iron is pyrometallurgy. It is the extractive metallurgy which consists
of the thermal treatment given to minerals or ores to recover the metal. The process
involves chemical reactions at elevated temperature.
The process of extraction of iron is fundamentally very simple as it consists essentially of
the reduction of iron oxide by carbon. But as molten iron dissolves carbon and other
impurities, iron obtained is impure and is known as pig iron or cast iron. The ore ( red
haematite or hydrated oxide or carbonate ) is calcined in shallow kilns to remove
moisture, carbon dioxide etc. The ore thereby becomes porous and is then more easily
reduced in the blast furnace.
Iron is normally extracted from its oxide ore called haematite and rarely extracted from
carbonate ore called siderite. Iron pyrite is an important source of sulphur and therefore
it is not used in the extraction of iron. The extraction of iron involves following steps.
Manufacture of cast ion
It is done in following steps.
(i) Washing and concentration or dressing of the ore
Haematite ore is washed with water. It is subjected to magnetic separation. The
ore being magnetic in nature, falls apart as a separate heap. This way the ore becomes
rich in oxide of iron. It is then broken into small pieces of 1” to 2” size, screened and
shifted. This helps to remove gangue. Due to washing, silicious impurities are removed.
The ore is thus concentrated.
(ii) Preliminary roasting and calcinations
The concentrated ore is roasted and calcined with a little coal in shallow kiln
( furnace ) in excess air. Following changes take place during roasting and calcinations.
(i) Moisture escapes as steam and organic matter present burns off to give CO2 and
sulphur and arsenic are oxidized to form their volatile oxides SO2 and As2O3 respectively.
(ii) Ferrous oxide is converted to ferric oxide which avoids formation of ferrous silicate
in the slag during smelting and (iii) The mass becomes porous and thus makes it more
suitable for reduction to metallic iron. Following reactions take place.
Fe2O3. 3 H2O → Fe2O3 + 3 H2O ↑ ; FeCO3 → FeO + CO2 ↑ ;
4 FeO + O2 → 2 Fe2O3
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(iii) Reduction or smelting in a Blast Furnace
The roasted and calcined ore ( 8 parts ) is mixed with coke ( 4 parts ) which acts
as a reducing agent and limestone ( 1 part ) which acts as a flux. The mixture is
introduced in a tall Blast Furnace. The blast furnace has two functions (i) to reduce the
ore to metallic iron and (ii) to remove the impurities in the form of slag.
Description of Blast Furnace
a) It consists of an outer shell which is made of steel plates riveted ( fastened ) together.
From inside it sis lined with fire bricks. ( Refer Figure )
b) It is about 15’ to 120’ tall. ( The height varies from place to place. ) It is 15’ to 24’ in
diameter at the wider end near the bottom. ( boshes, lower cone ) It is kept in a vertical
position with the help of iron columns.
c) In its lower part, the furnace gradually narrows down below the boshes. The lower part
of the furnace is called hearth (floor ) or crucible where molten iron and slag is
collected.
d) The mouth of the furnace i.e. the top is closed by a double cup and cone arrangement
through which the mixture ( called charge ) of the calcined ore, limestone and coke is
fed from time to time. The hot gases escape through the flue. ( Refer diagram )
e) The cone is made of iron and is kept tight against the top of the furnace being
counterpoised by weights.
Diagram –
Fig. 7 -
Blast Furnace
18
f) A blast of hot and dry air, obtained by hot gases, escaping from the blast furnace itself
and freed from dust in a scrubber , is blown into the furnace , just above the hearth
by a number of water-cooled pipes, called tuyeres or twyers ( pipe rings).
g) Near the bottom of the furnace, there are two outlets on opposite sides, one for the
removal of slag ( called slag hole ) and the other for taking out molten metal ( called
metal hole or tap hole ).
h) The temperature of the furnace is not constant in all the parts. It is maximum at the
hearth ( about 17000C ) and decreases slowly towards the throat of the furnace ( about
3000C )
The Process
The charge ( roasted ore + coke + limestone in the proportion 8: 4:1 ) is
introduced in the furnace from the top. Simultaneously, the furnace is lit and a hot blast
of air is passed through the tuyeres. The different heat zones of the blast furnace are
shown in the figure.
Reactions in the blast furnace
Following chemical reactions take place in different zones of the blast furnace.
(i) Zone of reduction – ( 3000C to 8000C - i.e. dull red heat )
This is the uppermost zone of the blast furnace. It is called the zone of reduction. Here
the iron oxide from the charge is reduced by carbon monoxide to spongy iron.
Fe2O3 + 3 CO → 2 Fe + 3 CO2 ↑
The reduction of Fe2O3 actually takes place in following three stages.
a) Conversion of ferric oxide to ferroso - ferric oxide
3 Fe2O3 + . CO → CO2 + 2 Fe3O4
b) Conversion of ferroso – ferric oxide to ferrous oxide.
Fe3O4 + CO → CO2 + 3 FeO
c) Conversion of ferrous oxide to metallic iron.
FeO + CO → CO2 + Fe
When the spongy iron falls in the middle region ( zone of heat absorption ), limestone,
CaCO3 decomposes to give CaO ( lime ) and CO2. Lime thus obtained acts as a flux.
It combines with silica to form a fusible ( meltable ) slag.
CaCO3 → CaO + CO2 ; CaO + SiO2 → CASiO3 ( slag )
(ii) Zone of heat absorption – ( 8000C to 12000C – i.e. bright red heat )
This is the middle part or zone of the blast furnace. In this zone, the ascending CO2 is
reduced to carbon monoxide when it reacts with carbon ( coke ).
CO2 + C → 2 CO - 39 kcal
As the reaction is endothermic, the temperature in this region falls and comes in the
range 8000C – 10000C.
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(iii) Zone of combustion – ( 13000C to 15000C – i.e. white heat )
This is the zone near the tuyeres . Here the carbon burns to form CO2 producing
tremendous amount of heat.
C + O2 → CO2 ↑ + 97 kcal
The heat evolved raises the temperature to 15000C. As the hot gases go up and meet
the descending charge, the temperature falls slowly to about 13000C.
(iv) Zone of fusion – (15000C to 19000C )
In this zone, the spongy iron melts and dissolves some carbon, phosphorus and silica.
The molten iron collects at the bottom of the furnace while the fusible slag floats on
it and protects the iron from oxidation. The layers of molten iron and slag are
withdrawn through separate tapping holes from time to time.
The process is economical as it is continuous one. The waste gases containing
about 25% CO, 15% CO2, 56% N2 and 4% H2 are let out through the outer pipe. These
are burnt with air to produce heat which is used for preheating the air blast passed
through the tuyeres. The blast furnace can work day and night for years together. Iron so
obtained is known as Pig Iron. It is remelted in a vertical furnace ( known as cupola )and
can be cast or poured into moulds. It is then called cast iron. Thus cast iron is obtained
after remelting pig iron.
Varieties of cast iron
When pig iron in the blast furnace is suddenly cooled, crystalline cast iron is
obtained. It is known as white cast iron. In this form, carbon is present in the combined
state as iron carbide. It is very hard and white in colour. On the other hand, if molten iron
is slowly cooled in sand moulds a graphite coloured iron is formed. It is known as grey
cast iron. In this type, a part of carbon separates out as graphite and gives grey colour to
the metal. It is softer and more coarse grained than the white form.
Products of Blast Furnace
The products of blast furnace are (i) Pig iron (ii) Slag (iii) Flue gases .
(i) Pig iron – Average composition of pig iron is : a) Iron – 92 to 95 % , b) Carbon – 2.5
to 4.5 % c) Silicon – 0.7 to 3% d) Phosphorus –0.5 to 1 % e) Manganese - 0.2 to1 %
f) Sulphur – 0.1 to 0.3 %
(ii) Slag - It is mostly calcium silicate containing some amount of aluminium silicate. It
Contains 55 % SiO2 , 30 % CaO and 15 % CO2 and 15 % Al2O3. It is useful for road
making and cement manufacture.
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(iii)Flue gases - The gases leaving the blast furnace through the flue are known as flue
gases. The average composition of the gaseous mixture is a) CO – 25 % b) CO2 –
10 % c) N2 – 58 to 60 % d) H2 – 1 to 2 % Due to the appreciable proportion of
carbon monoxide, the gaseous mixture has a fuel value and hence it is used for
heating the air blast in Cowper’s stoves.
Major part of cast iron is used to manufacture steel. It is also used for casting metal
objects such as pipes, railings, weights and heavier parts of machinery.
Physical and Chemical Properties of Cast Iron
(i) Since cast iron is impure, it has melting point ( about 12000C ) lower than that of pure
iron ( about 15300 C).
(ii) On solidifying, it expands
(iii) It is harder due to the presence of carbon and silicon.
(iv) Due to sulphur, it is brittle when red hot ( red short ) and due to phosphorus, it is
brittle when cold ( cold short ).
(v) It can not be welded and can not be permanently magnetized.
(vi) Action of air - Pure iron is not affected by dry air. In presence of moist air and
carbon dioxide, it begins to rust readily forming reddish brown hydrated ferric oxide.
When heated in air, it is oxidized and gets covered with thick bluish black scales of
ferroso – ferric oxide Fe3O4.
(vii) Action of steam – When steam is passed over iron heated to 8000C to 10000C, iron
is oxidized to Fe3O4.
3 Fe + 4 H2O → Fe3O4 + 4 H2 ↑
(viii) Action of acids - Non-oxidising acids react with iron and form ferrous salts
evolving hydrogen. Hot and concentrated sulphuric acid oxidizes iron forming a mixture
of ferrous and ferric sulphates with the evolution of SO2.
Fe + 2 H2SO4 → FeSO4 + SO2 + 2 H2O
2 FeSO4 + 2 H2SO4 → Fe2(SO4 )3 + SO2 + 2 H2O
Cold and dilute HNO3 forms a mixture of ferrous nitrate and NH4NO3.
4 Fe + 10 HNO3 → 4 Fe(NO3 )3 + NH4NO3 + 3 H2O
Highly concentrated and pure nitric acid of specific gravity 1.45 has apparently no action
on iron. Iron is made passive in such acid. It is suggested that passivity of iron is due to
the formation of a thin protective invisible oxide film on the surface of iron. Iron also
becomes passive by other oxidizing agents like dichromates, nitrates, etc. Passive iron is
not affected by dilute acids. Passivity is lost when such a passive metal is rubbed or
treated with reducing agents.
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(ix) Other reactions - Iron is not affected by alkalies. When heated with (a) sulphur it
forms ferrous sulphide and (b) dry chlorine it forms ferric chloride.
Fe + S → FeS
;
2 Fe + 3 Cl2 → 2 FeCl3
Wrought Iron
It is the purest form of iron. The carbon content in it is very low (less than 0.2
%).It has very small amount of impurities. It melts at a higher temperature ( 15000C)
than cast iron. It is obtained by purifying cast iron by the process known as puddling
( i.e. stirring ) It is grey in colour. It is soft, ductile and malleable. It is used to prepare
chains, wires, bolts, couplings for railway carriages etc.
Uses of iron
1) Major part of cast iron is used to manufacture steel.
2) Cast iron is used for casting metal objects such as pipes, railings, weights, heavier
parts of machines etc.
3) Iron is used as material of construction.
Manufacture of steel
Steel is manufactured from pig or cast iron.. Iron containing 0.1 to 1.5 % carbon is called
steel. It is manufactured from cast iron by burning out carbon, silicon, phosphorus and
sulphur. Then a calculated quantity of carbon is added to it to get a required quality of
steel. Following methods are used for the manufacture of steel from pig or cast iron.
(i) Bessemer process (ii) Open Hearth process (iii) L. D .process
1) Bessemer process
Bessemer process was invented in 1855 by an English steel maker Henry
Bessemer. The process is carried out in a special kind of egg-shaped or pear-shaped
furnace. The furnace is called Bessemer converter.
Bessemer converter is 20 feet high and 10 feet in diameter. This is made of steel
plates. It is lined with silica ( SiO2 ) or a mixture of lime (CaO ) and magnesia ( MgO)
depending upon the nature of impurities present in cast iron. If the impurities are basic,
like MnO, then a lining of silica bricks is used and the process is known as Acid
Bessemer process. On the other hand, if the impurities are acidic like P2O5 or SO2, then
a lining of CaO and MgO is used and the process is known as Basic Bessemer process.
Bessemer converter is provided with a number of fine holes at the bottom through which
a hot blast of air can be forced in fine jets. It is supported on two horizontal arms
( trunnions ) so that it can be tilted in a vertical plane. The converter can hold a charge of
20 tons at a time. The Bessemer converter is shown in the following figure.
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Figure 8 - Bessemer Converter
a) Acid Bessemer Process - First the converter is turned into a horizontal position. Then
molten pig or cast iron containing little or no phosphorus is introduced into it. The
converter is then brought almost to a vertical position and a hot blast of air is introduced.
The air is used to oxidize the impurities. Silicon and manganese are partly oxidized and
their oxides pass into slag. Sulphur burns off as SO2. A portion of iron is also oxidized.
The iron oxide ( Fe2O3 ) formed oxidizes Mn and Si, if any. The iron oxide now reacts
with the carbon present evolving CO which burns at the mouth of the converter with a
blue flame. When the flame dies out, the molten metal in the converter becomes wrought
iron containing little Fe2O3. To get steel, the converter is again tilted, the blast of hot air
is stopped and a calculated quantity of molten spiegeleisen ( an alloy of Fe, C and Mn )
is added. The hot blast of air is turned on again for few minutes for thorough mixing. The
molten steel is then poured out and cast into moulds. The reactions taking place in the
Bessemer converter are summarized below.
Si + O2 → SiO2 ;
2 Mn + O2 → 2 MnO ;
MnO + SiO2 → MnSiO3
2 C + O2 → 2 CO ;
4 Fe + 3 O2 → 2 Fe2O3 ;
Fe2O3 + 3 C → 2 Fe + 3 CO
b) Basic Bessemer process ( Thomas Gilchrist process ) - This process is only used to
treat pig iron containing phosphorus. A charge of limestone and coke is first introduced
into the converter. The hot blast of air is turned on. The molten cast iron containing
phosphorus is then added and the blast continued.
Silicon and manganese are first oxidized and pass into slag. Then phosphorus and carbon
are oxidized simultaneously. Carbon monoxide burns at the mouth of the converter.
Phosphorus pentoxide reacts with lime and forms a slag containing calcium phosphate. It
is known as Thomas Slag and is used as a fertilizer. The molten iron is poured out into a
ladle ( deep spoon with long handle ) separated from the slag and then mixed with the
charge of requisite amount of spiegeleisen for recarburisation and deoxidation to form
steel. The reactions taking place in the converter are summarized below.
P4 + 5 O2 → P4O10
;
6 CaO + P4O10 → 2 Ca3(PO4)2 ( Thomas Slag )
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Merits of Bessemer process
(i) Time required for the production of steel is less
(ii) Production cost is low.
Demerits of Bessemer process
(i) Steel produced is of poor quality
(ii) Loss of iron in slag is more.
2) Open hearth process ( Siemens - Martine process )
Most of the high grade steel is made by this process. This is a modern method of
manufacture of steel. The furnace used for this purpose is shown in following figure. The
process is carried out in a large shallow hearth. When the hearth ( floor ), is lined with
silica, the process is called Acid Open Hearth process. When the hearth is lined with
lime and magnesia, the process is called Basic Open Hearth process. The furnace is
heated by means of producer gas ( a mixture of CO + N2 with little amount of H2 , CO2
and CH4 ) and air. There are two generators – one for the hot air and one for the producer
gas. The spent hot gases are made to leave the hearth through two other generators.
Meanwhile, the first two generators get cooled and the producer gas and air are switched
over to the second set of generators and the spent hot gases are led through the first two
generators. The process, thus, works alternatively
Fig. 9 – Open hearth furnace
a) Acid Open Hearth Process - When pig iron contains no phosphorus, then this
method is used. Pig or cast iron, rusted steel scrap and iron ore are introduced on
to the hearth of the furnace. It melts due to hot producer gas. Iron oxide ( Fe2O3 )
acts as an oxidizing agent.
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Fe2O3 + 3 C → 2 Fe + 3 CO ↑ ; 2 Fe2O3 + 3 S → 4 Fe + 3 SO2↑
The oxides of Mn (i.e. MnO ) and Si ( i.e. SiO2 ) react with each other to form slag
(MnSiO3 ). The amount of carbon is adjusted to get desired quality of steel.
b) Basic Open Hearth Process - When pig iron contains phosphorus, then this method
is used. Carbon, sulphur and silicon get oxidized to their respective oxides.
Phosphorus is oxidized to form P2O5 which combines with CaO ( lime ) to form slag.
The reactions are as follows.
10 Fe2O3 + 12 P → 20 Fe + 3 P4O10 ;
SiO2 + CaO → CaSiO3 ( Slag )
P4O10 + 6 CaO → 3 Ca3 ( PO4)2 ( Slag )
;
SiO2 + MnO → MnSiO3 ( Slag )
A small quantity of charge is taken out after certain interval of time and analysed for
carbon content. When a chemical test indicates that metal contains the minimum required
amount of carbon, then calculated quantity of spiegeleisen is added. A little aluminium
or ferrosilicon is added to the molten steel which is drawn out from the furnace. This
removes any dissolved O2 and N2 in the molten steel.
Comparison of Bessemer and Open Hearth process
Open hearth process has many advantages over Bessemer process. A comparison of both
these processes is given in the following table.
Bessemer Process
Open Hearth Process
(i) The oxidation of impurities is
carried out by hot blast of air.
(ii) It is restricted to pig iron of a
particular composition.
(iii) On account of internal heating,
the temperature can not be
controlled very well.
(iv) Since there is no regenerative
system of heat, fuel can not be
saved.
(i) The oxidation of impurities is
carried out by Fe2O3.
(ii) It is adapted to pig iron of any
composition.
(iii) On account of external heating,
the temperature can be
controlled very well.
(iv) Since there is regenerative
system of heat, fuel can be
saved .
(v) The process is not continuous
and the charging is difficult.
(vi)The steel is not of good quality.
(v) The process is continuous and
the charging is easier.
(vi) The steel is of uniform and
superior quality.
(vii) It is slow and takes twenty
hours for its completion.
(vii)It is quick and takes only ten
minutes for its completion.
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(viii)Approximately 15 % of iron
is lost in slag.
(viii)The loss of iron is not more
than 4 %
3) L.D. ( Linz and Donawitze ) Process
In this process, highly pure oxygen is blown from the top of the converter. A jet of
99.5 % pure oxygen is blown at supersonic speed on to the molten bath of pig or cast iron
when all the impurities including phosphorus and sulphur are burnt away at high
temperature, leaving low carbon content and high quality steel. The density of the metal
is higher than impure metal. Therefore, it sinks to the bottom. The process requires about
30 to 40 minutes. The process is much useful for getting low carbon steel. The converter
for l. D. process is shown in the following figure.
Fig. - 10 - L.D. Process
Advantages of L.D. process
(i) Due to liberation of large amount of heat, scrap iron can be used as starting material.
(ii) Carbon and phosphorus are removed at the same time.
(iii) Superior quality of steel is obtained.
(iv) Process is very cheap and quick.
(v) Nitrogen content in the finished steel is very low.
Alloy Steels
N
1
Metal added Alloy Steel
Manganese Manganese
steel
Composition Properties
12 to 15 %
Very hard and
Mn
resistant to
wear
Uses
For rock crushing
machinery, armour
plates, rail road tracks
26
2
Nickel
steel
Nickel steel
3.2 % Ni
3
Chromium
11.5 % Cr
4
Nickel
Chromium
steel
Invar
5
Chromium
Chromium
steel
1.5 to 2 %
Cr
6
Vanadium
and
chromium
Tungsten
7
8
Chromium
– vanadium
steel
Tungsten
steel
Molybdenum Molybdenu
m steel
36 % Ni
Increased
elasticity and
hardness
Resists
corrosion
Coefficient of
expansion
equals that of
glass
Extremely hard
0.15 %V +
1% Cr
Good tensile
strength
14 to 20%
W
0.3 to 3 %
Mo
Extremely hard
and strong
Retains
hardness even
at high
temperature
etc.
For making armour
plates, cables,
automobile parts
For utensils, cycle and
automobile parts
For clock pendulums
and measuring tapes
For cutting tools and
crushing machinery,
armour piercing bullets
For springs, shafts,
axles and frames
For drills and high
speed tools
For cutting tools and
axles
Activity 3 – (i) Find out the places in India where (a) you get iron ore. (b) manufacture
of cast iron or pig iron is done. (ii) Mention names of three alloys of iron which you use
in day to day life.
Test your understanding
1) How does iron occur in nature ? Give any three source of iron.
2) What are the commercial forms of iron ? In which respect do they differ ?
3) Draw and describe the blast furnace and the reactions taking place in various zones in
it during the manufacture of cast iron.
4) Name the processes used to manufacture steel.
5) Give the names, composition and uses of any two alloy steels .
References / Figures / Diagrams :
1) Fig. 1 - Hydraulic washing
http://www.tutorvista.com/topic/iron-sulphide-separate
2) Fig. 2 – – Froth floatation process
http://cbseportal.com/exam/Important-Topics/Chemistry-Froth-Floatation-Process
27
3) Fig. 3 – Magnetic separation process
http://www.tutorvista.com/content/science/science-ii/metals-non-metals/enrichmentores.php
4) Fig. 4 – The liquation method
http://www.tutorvista.com/content/chemistry/chemistry-iii/metals/metals-refining.php
5) Fig. 5 – Electrolysis of alumina
http://www.tutorvista.com/content/chemistry/chemistry-iv/p-blockelements/aluminium.php
6) Fig. 6 – Hoope’s cell for refining aluminium
http://www.tutorvista.com/content/chemistry/chemistry-iv/p-blockelements/aluminium.php
7) Fig. 7 – Blast Furnace
http://www.tutorvista.com/content/chemistry/chemistry-ii/metals/iron.php
8) Fig. 8 – Bessemer Converter
http://qwickstep.com/search/the-bessemer-converter.html
9) Fig. 9 – Open hearth furnace
http://www.tutorvista.com/content/chemistry/chemistry-ii/chemicalcompounds/hearth-process.php
10) Fig. 10 – L.D. Process
http://www.steel.org/AM/images/learning/howmade/images/BOFvesseldwg.jpg