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50.8 EXTRACTION OF IRON

Iron is the second most abundant metal occurring in the earth’s crust. It is an element of the first transition series.

OCCURRENCE

The important ores of iron are:

1.      Haematite, Fe2O3 (red oxide of iron)

2.      Magnetite, Fe3O4 (magnetic oxide of iron)

3.      Limonite, Fe2O3.3H20 (hydrated oxide of iron)

4.      Iron pyrites, FeS2

5.      Siderite or Spathic ore, FeCO3.

The cast iron is generally extracted from haematite (Fe2O3). The various steps involved in the process are as  follows:

1.      Concentration. The are is crushed with the help of jaw crushers into small pieces of about 5 cm size. The crushed ore is washed with a stream of water whereby lighter sand particles are washed away and the heavier are particles settle down.

2.      Calcination. The concentrated are is heated strongly in the presence of air. This is called calcination. During calcination following changes take place:

  • Moisture is driven out.
  • Sulphur, arsenic and phosphorus impurities are expelled as their volatile oxides.
  • Carbonate are changes into oxide are.

FeCO3             →          FeO + CO2

  • Ferrous oxide changes into ferric oxide.

4FeO + O2                   2Fe2O3

 

3. Smelting. After calcination the are is subjected to reduction with carbon in a blast furnace (Fig. 50.4). The process is called smelting. Blast furnace is made up of steel, lined inside with fire resistant bricks. It has a cup and cone arrangement for the introduction of charge at the top, and at
the base it has:

(i)  a ‘tapping hole for removing molten iron from time to time.

(ii)   an arrangement for the introduction of hot air.

(iii)  an outlet for slag.

The calcined ore, coke and lime stone are mixed in the ratio 8: 4 : 1 and are fed into the furnace. through cup and cone arrangement. Blast of hot air at about 1000 K is blown in through narrow pipes (tuyeres) at the base of the furnace. Coke here serves as a fuel as well as a reducing agent while

lime is a flux. Reactions which take place in the furnace are as follows:

50.8 EXTRACTION OF IRON

 

(i)                 Coke bums at the base to produce CO2 which rises up.

C + O2             →             CO2 , H = – 406 kJ mol-1

 

The reaction is exothermic and temperature here is upto about 2173 K. This region is called combustion zone.

 

(ii)               As CO2 rises up it is reduced to CO with the coke.

CO2 + C         →       2CO, H = + 173 kJmol-1

 

Due to the endothermic nature of this reaction, the temperature in this region falls to 1475-1575 K. In this region Fe2O3, if present, gets reduced to iron by hot coke and the spongy iron produced in the upper region gets melted. This is known as fusion zone.

 

(iii)             Near the top of the furnace, where temperature is about 873 K, the oxides of iron (Fe2O3 and Fe3O4) are reduced to iron and Fe by carbon (II) oxide

 

Fe2O3 + CO              →                    2FeO + CO2

3Fe2O3 + CO                        →                    2Fe3O4 + CO2

Fe3O4 + 4CO                        →                    3Fe + 4CO2

 

(iv)              In the middle portion of the furnace where temperature is about 1073 to 1273 K, FeO is reduced to iron while coke is oxidized to CO.

 

CO2 + C                   →                    2CO

FeO + CO                →                    Fe + CO2

FeO + C                   →                    Fe + CO

 

Lime stone decomposes to produce CaO which combines with silica (impurity) to form slag.

 

CaCO3                            →                             CaO + CO2

CaO + SiO2                  →                             CaSiO3

 

Molten slag is lighter than molten iron and forms the upper layer. This region is called slag formation zone.

 

Iron formed moves down and melts in the fusion zone. Molten iron dissolves in it some carbon, silicon, and phosphorus and forms the lower layer at the base of the furnace. Iron thus formed is called pig iron.

 

Pig iron contains about 4% of carbon and many other impurities such as phosphorus, silicon, sulphur and manganese.

Iron works are located close to coal fields so that plentiful of coal is available at cheap rate. Coal is used as reducing agent and as fuel.

Pig iron is converted into cast iron by heating molten pig iron with scrap iron and coke in specially designed furnaces. A blast of hot air is blown through the mixture. Cast iron contains about 3% carbon. It slightly expands on solidifying and hence reproduces the shape of the mould. Cast iron is
very hard and brittle. Its melting point is about 1473 K

CAST IRON FROM IRON PYRITES

Iron pyrites (FeS2) is not directly used as an ore for the extraction of iron because it contains quite high percentage of sulphur. Due to high content of sulphur it is primarily us for the manufacture of H2SO4, For this purpose it is burnt in air to get SO2:

 

4FeS2 + 11O2                                               2Fe2O3 + 8SO2

SO2 is converted into SO3 which is absorbed in water get H2SO4. The residual FeO3 formed during burning of FeS2 is reduced to iron in blast furnace as already discussed.

COMMERCIAL FORMS OF IRON

There are three commercial forms of iron which differ from each other mainly in their carbon content.

 

  1. Cast Iron or Pig Iron. It contains about 2 to 5% of carbon. along with impurities such as sulphur, silicon, phosphorus. manganese, etc. It is the least pure form of iron. It is brittle
    and is very difficult to weld. It is resistant to corrosion and is used for sewage pipes.
  2. Steel. It contains 0.1 to 1.5% carbon and other impurities.
  3. Wrought Iron. It is the purest form of iron and contains carbon and other impurities less than 0.2%. It is malleable and can be easily welded. This type of iron is structurally weak and hence cannot be magnetised permanently. It is used for making wires, chains, electromagnets, etc.

 

4. CONVERSION OF CAST IRON INTO WROUGHT IRON

Wrought iron which is comparatively pure form of is obtained by refining cast iron. The cast iron is heated haematite (Fe2O3) so that the impurities are oxidized.

Fe2O3 + 3C                   →                    2Fe + 3CO

CO escapes while MnO present as an impurity, combines with SiO2 to form slag.

MnO + SiO2                   →                             MnSiO3

(slag)

Wrought iron thus formed contains about 0.2 – 0.5% carbon and traces of P and Si in the form of slag.

 Wrought iron ismalleable and ductile. Its melting point ut 1400°C.

STEEL

The amount of carbon present in steel is nearly intermediate between that contained in cast and wrought iron.

The various steels contain carbon from 0.1 to 1.5%. The increase in carbon content in the steel decreases its ductility and increases its tensile strength. The following methods commonly used for the production of steel:

  1.       Bessemer Process
  2.       The Basic Oxygen Process
  3.       Open Hearth Process
  4.       The Electric Arc Process
  5.      The High Frequency Induction Process.

All these methods are based on the oxidation of impurities pig iron,

Two of these methods are discussed below:

1. Bessemer Process. The Bessemer process involves blowing a strong blast of air through the molten pig iron.

The Bessemer process is accomplished in large pear-shaped iron vessels called converters which are lined on the inside with silica bricks and can hold upto 50 tons of pig iron at a time. The converter is mounted on horizontal pivots around which it can be tilted. The converter is filled with molten pig iron at a temperature of about 1473 K and air is blown into the chamber. Oxygen reacts with impurities and raises the temperature to 2173 K. The impurities are oxidised by air.

 

Si + O2                     →                    SiO2

2Mn + O2                     →                    2MnO

C + O2                     →                    CO2

SiO2 is slagged off with lime and MnO with silica in the form of their respective silicates.

 

CaO + SiO2                            →                             CaSiO3

SiO2 + MnO                     →                    MnSiO3

 

The slag floats on the surface whereas purified iron being than impure iron sinks to the bottom.

The Bessemer process produces iron containing less than 0.3 per cent carbon. If it is desired to obtain steel, the air blast is either shut off before all the carbon has burnt out, or a definite amount of pig iron rich in carbon is added to the iron produced in the converter. Air is again blown through it for a

short period in order to mix the ingredients.

2. The Basic Oxygen Process. The Basic Oxygen process is a development of the Bessemer Process. In this process pure oxygen is used instead of air. Initially, the furnace is charged with hot molten pig iron from the blast furnace lime and scrap steel. A jet of oxygen is blown onto the surface of the metal at a great speed through a water cooled pipe.

The oxygen oxidizes impurities rapidly. Since the reactions taking place in (he furnace are highly exothermic, the heat evolved keeps the contents of the furnace in the molten state. When steel of desired composition is obtained, the oxygen is turned off and molten steel is poured by tilting
the furnace.

ALLOY STEELS

 

Alloy Steel

Special Components

Chief Properties

Uses

1. Nickle

Ni 3.5% Hard, flexible,

rust resistant

For cables armour plates
2. Stainless steel

Cr 18%,

Ni 8%

Does not rust

or corrode

For household utensils, shaving, blades, watch cases

3. Chrome-vanadium steel

Cr 1%

V 0.15%

Tenacious and

load bearing

For axles, springs and cog-wheels

4. Manganese steel

Mn 12 to 15% Extremely

hard and high melting

For rock crusher, burglar proof safes

5. Tungsten steel

W 14 to 20%

Cr 3 to 8%

Very hard and

strong

For cutting tools, springs

6. Invar

Ni36% Extremely low

expansion on heating

For clock pendulums

7. Alnico

Al 12%,

Ni 20%,

Co 5%

Highly magnetic For permanent magnets

 

Steel and alloy steels are preferred to the pure iron because they are stronger and resistant to corrosion. Iron undergoes corrosion (rusting).

 

SOLVED EXAMPLES

 

Example 50.4 Giving examples, differentiate between ‘roasting’ and ‘calcination’.

Solution. Roasting is the process of heating the ore in the excess supply of air below its melting point. This process is applied for sulphide ores. As a result sulphide ore changes into oxide. Calcination is the process of heating the ore in limited supply of air below its melting point. This process is applied for oxide and carbonate ores.

 

For example, zinc blende (sulphide ore of zinc) is roasted whereas haematite (oxide ore of iron) is subjected to calcination.

Heat

2ZnS + 3O2      →        2ZnO + SO2                                         … Roasting

 

Heat

Fe2O3.xH2O     →        Fe2O3 + xH2O                                       …Calcination

 

Example 50.5 How is ‘cast iron’ different from ‘pig iron?

Solution. Cast iron contains about 3% carbon whereas pig iron contains about 4% carbon. Pig iron is harder than cast iron.