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Metallurgy

Concepts

(i) What is metallurgy (ii) Minerals and Ores (iii) Occurrence of metals

(iv) Processes involved in extraction 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 ; CuO + C → Cu + CO

ZnO + C → Zn + 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 1800

0C.

(iii) It is malleable and ductile especially between 1000C and 150

0C. 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 b) 4 Al + 3 C → Al4C3

c) 2 Al + N2 → 2 AlN 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 800

0C - 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 1200

0C – 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

– 1000

0C.

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(iii) Zone of combustion – ( 13000C to 1500

0C – 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 1900

0C )

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 1000

0C, 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 )

23

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.

24

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 ; P4O10 + 6 CaO → 3 Ca3 ( PO4)2 ( Slag )

SiO2 + CaO → CaSiO3 ( 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 (i) The oxidation of impurities is

carried out by hot blast of air. carried out by Fe2O3.

(ii) It is restricted to pig iron of a (ii) It is adapted to pig iron of any

particular composition. composition.

(iii) On account of internal heating, (iii) On account of external heating,

the temperature can not be the temperature can be

controlled very well. controlled very well.

(iv) Since there is no regenerative (iv) Since there is regenerative

system of heat, fuel can not be system of heat, fuel can be

saved. saved .

(v) The process is not continuous (v) The process is continuous and

and the charging is difficult. the charging is easier.

(vi)The steel is not of good quality. (vi) The steel is of uniform and

superior quality.

(vii)It is quick and takes only ten (vii) It is slow and takes twenty

minutes for its completion. hours for its completion.

25

(viii)Approximately 15 % of iron (viii)The loss of iron is not more

is lost in slag. 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 Metal added Alloy Steel Composition Properties Uses

1 Manganese Manganese

steel

12 to 15 %

Mn

Very hard and

resistant to

wear

For rock crushing

machinery, armour

plates, rail road tracks

26

etc.

2 Nickel

steel

Nickel steel 3.2 % Ni Increased

elasticity and

hardness

For making armour

plates, cables,

automobile parts

3 Chromium Chromium

steel

11.5 % Cr Resists

corrosion

For utensils, cycle and

automobile parts

4 Nickel Invar 36 % Ni Coefficient of

expansion

equals that of

glass

For clock pendulums

and measuring tapes

5 Chromium Chromium

steel

1.5 to 2 %

Cr

Extremely hard For cutting tools and

crushing machinery,

armour piercing bullets

6 Vanadium

and

chromium

Chromium

– vanadium

steel

0.15 %V +

1% Cr

Good tensile

strength

For springs, shafts,

axles and frames

7 Tungsten Tungsten

steel

14 to 20%

W

Extremely hard

and strong

For drills and high

speed tools

8 Molybdenum Molybdenu

m steel

0.3 to 3 %

Mo

Retains

hardness even

at high

temperature

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/enrichment-

ores.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-block-

elements/aluminium.php

6) Fig. 6 – Hoope’s cell for refining aluminium

http://www.tutorvista.com/content/chemistry/chemistry-iv/p-block-

elements/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/chemical-

compounds/hearth-process.php

10) Fig. 10 – L.D. Process

http://www.steel.org/AM/images/learning/howmade/images/BOFvesseldwg.jpg