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Extraction of Iron

Ores of Iron           

Magnetite (Fe3O4)

Red haematite (Fe2O3)

Brown haematic Limonite  (Fe2O3.3H2O)

Spathic iron or sidenite (FeCO3)

Iron pyrites (FeS2), Copper pyrites (CuFeS2)

Ores of Iron

Concentration of the ore

Dressing of the ores: The iron ores are first broken into small pieces 3-5 cm in size.

Roasting or Calcination: During roasting S, As, P are oxidized to the respective oxides.

S + O2 → SO2­↑

4As + 3O2 → 2As2O3­↑

FeCO3 decomposes as,

FeCO3 → FeO + CO2­↑

Fe2O3.3H2O loses water

Fe3O4 is decomposed to ferrous oxide and ferric oxides.

Fe3O4 → FeO + Fe2O3

Ferrous oxide reacts with silica to forms ferrous silicate at high temperature.

FeO + SiO2 → FeSiO3

But the conversion of FeO into Fe2O3 will prevent the formation of FeSiO3. Thus the mass of the ore becomes porous causing the increase in the effective surface area.

Smelting in the Blast furnace: Blast furnace is a shaft furnace made of steel plate of 20-30 in with 4-4.6 diameter.

Extraction of Cast Iron

Iron is usually extracted from the haematite. Concentrated ore after calcinations is reduced with carbon i.e. smelted in the blast furnace.

Reactions taking place in the blast furnace are

Zone of combustion:

C + O2 → CO2
(Coke)

CO2 + C → 2CO; ΔH = +ve (Newmann’s inversion reaction)

Zone of reduction:

The following reduction reactions are called indirect reduction, which is done by CO, which is unstable at higher temperature (See Ellingham diagram).

Fe2O3 + 3CO → 2Fe + 3CO2

FeO + CO → Fe + CO2

Zone of slag formation:

CaCO3 → CaO + CO2

Zone of fusion (lower part of furnace):

Extraction of Cast Iron

Molten iron is heavier than from molten slag. The two liquids are periodically tapped off. The molten iron tapped off from the furnace is solidified into blocks called ‘plags’.

Refer to the following video for blast furnace

Preparation of Wrought Iron

This is done by heating cast iron with haematite (Fe2O3) which oxidizes C to CO, S to SO2, Si to SiO2, P to P4O10 and Mn to MnO

Fe2O3 + 3C → 2Fe + 3CO

Where CO and SO2 escape, manganous oxides (MnO) and Silica (SiO2) combine to form slag.

MnO + SiO2 → MnSiO3

Similarly phosphorous pentoxide combines with haematite to form ferric phosphate slag.

 2Fe2O3 + P4O10 → 4FePO4

Bosh :

The diameter of the furnace gradually increases from the top down wards. Widest part of the furnace is called Bosh. At above 2m tuyers are there through which hot air blast is blown into the furnace.

Hearth:

Below the bosh this region exists. (1) slag notch is at higher height and (2) tap hole for metal passage at lower position from the bottom.
At the top of the furnace the hopper is there which is cup and cone arrangement.
Through this charge is introduced ill the course bed in the furnace is 4/5th the of the furnace. How air at 700oC is forced into the furnace through the tuyers.
The thermal gradient inside exists from 1800oC (hearth) to 400oC–900oC in the upper region. Near the both the temperature varies from 1200o-1300o chemical reactions which take place are:

At 1200oC near the tuyers,

C + O2 → CO2;

CO2 + C → 2CO

Above bosh 600o-900oC, ferric oxide is partially reduced by CO as

Fe2O3 + 3CO ↔ 2Fe + 3CO2

CaCO3 CaO + CO2

2CO CO2 + C

Fe2O3 + 3C → 2Fe + 3CO

CaO + SiO2 → CaSiO3 (slag)

The reaction at 1500oC, MnO2 is reduced to Mn

MnO2 + 2C → Mn + 2CO

Ca3(OH)2 → CaO + P2O5

2P2O5 + 10C → P4 + 10CO

Collection of Cast iron: Metal is cast into ingots or in the ladle for further refining like steel making.

Wrought Iron: Minimum % of carbon is 0.1 – 0.1% and other impurities like S, P, Mn, Si less than 0.3%

Manufacturing Process: Cast iron is taken in pudding furnace and melted by hot blast of air. The chemical reactions, which occur, are

S + O2 → SO2­↑
3S + 2Fe2O3 → 4Fe + 3SO2­↑

3Si + 2Fe2O3 → 4Fe + 3SiO2

Mn + Fe2O3 → 2Fe + 3MnO

MnO + Fe2O3 → MnSiO3 (slag)

3C + Fe2O3 → 2Fe + 3CO

4P + 5O2 → 2P2O5; Fe2O3 + P2O5 → 2FePO4 (slag)

The impurities are removed from ion, the melting point of the metal rises and it becomes as semi solid mass. Metal taken out from the furnace in the form of balls with the help of rubbles. The balls are then beaten under hammer to separate out the slag. The product thus formed is thus called wrought iron.

Some Important Alloy

Name

Composition

Stainless steel

Fe, Cr, Ni

Invar

Fe, Ni

Alnico

Fe, AI, Ni, Co

Brass

Cu, Zn

Bronze

Cu, Zn, Sn

Gun Metal

Cu, Sn

Bell Metal

Cu, Sn

German Silver

Cu, Zn, Ni

Solder, pewter

Pb, Sn

Babbitt metal

Sn, Sb, Cu

Steel

Manufacture from CI (Cast Iron)

Acid Bessemer Process

This process is carried out when percentage of P is very low. The lining of the furnace is made of SiO2. The following reactions take place to lessen the impurities.

2Mn + O2 →  2MnO

Si + O2 → SiO2

MnO + SiO2 → MnSiO3

S + O2 → SO2

4Fe + 3O2 →2Fe2O3

Fe2O3 + 3C → 2Fe + 3CO

2C + O2 → 2CO ­

When the blue flame of CO drops out it complete oxidation of carbon.

All the reactions are exothermic. After the complete of the reaction calculated amount of spiegeleisen (an alloy of Fe, Mn, C) is added to the molten mass. After that again air blast is continued for two minutes to engage the thorough mixing of the speigel. The function of speigcleisen is to supply requisite amount of carbon to iron to produce steel of desired quality.

FeO + Mn → MnO + Fe

FeO + C → Fe + CO

Carbon  and mangenese of the spiegel act as deoxidisers to remove any dissolved O2 and reduce any ferrous oxide which may be formed during the reactions. Manganese serves to make the steel harder and to increase its tensile strength. The converter is then tilted and molten steel cast into moulds, or taken in laddle for further processing.

Basic Bessemer Process

The lining of the converter is of basic materials like CaO, MgO etc. The converter is charged with molten cast iron and some lime is added to it and hot blast of air under pressure is continued. Mn,  Si of CI is first oxidised. S from SO2, C, P are then oxidised to form CO and P2O5. The P2O5 thus formed combines with lime to form a basic slag [Ca3(PO4)2.CaO] which is known as Thomas slag.

2Mn + O2 → 2MnO; MnO + SiO2 → MnSiO3 (slag)

Si + O2 → SiO2

S + O2 → SO2

2C + O2 → 2CO­

4P + 5O2 → 2P2O5

After the completion of the reactions the air blast is stopped and the converter is tilted and the basic slag is removed. Spiegeleisen is added to the molten Fe when it is converted and steel.

Open Hearth Process

Open hearth process is the modern process for the manufacture of the high grade steel.

The impurities present in the cast iron  are mainly oxidised by the addition of haematite ores and the rest are oxidised by O2  of air.

The percentage of C, Si is lowered down by the addition of scrap Fe.

Heat generated by burning the mixture of producer gas and air is used to preheat the fuel gas and air.

This regenerative system causes an economic fuel consumption and maintains high temperature (1800°C) in the furnace.

Reactions taking place in the hearth:

Mn, Si, P, S and carbon are oxiidsed by haematite (Fe2O3) to form their respective oxides. The reactions occuring are as follows.

2Fe2O3 + 3Si → 4Fe + 3SiO2

Fe2O3 + 3Mn →2Fe + 3MnO

2Fe2O3 + 3S → 4Fe + 3SO2­

Fe2O3 + 3C → 2Fe + 3CO­

5Fe2O3 + 6P → 10 Fe + 3P2O5

MnO + SiO2 → MnSiO3

CaO + SiO2 → CaSiO3

3CaO + P2O5 → Ca3(PO4)2

3MgO + P2O5 → Mg3(PO4)2 (Slag mixture)

C, P are also oxidised by air to forms carbon monoxide and phosphorus pentoxide.

2C + O2 → 2CO

4P + 5O2 → 2P2O5

After the removal of impurities slag is separated, requisite amount of spiegeleisen is added to the molten mass to obtain desired quality of steel by deoxidation and recarburisation. While molten ferrosilicon or Al are also sometiems added for removal of any dissolved gas in it.

Duplex process:

This is actually a combination of acid besemer process and open hearth process. Si, Mn, S, C (partially) are oxidised and removed or slag or volatite oxides. This process is applied in TISCO.

Varieties of Steel

Carbon steel can be divided into the following groups.

Low carbon (mild steel)    0.15 - 0.3%.

Medium carbon steel  0.3 - 0.6%.

High carbon steel  0.6 - 0.8%.

Tool steel   0.8 - 1.4%.

Alloy Steels

Nickel Steel

(2 - 5%) Ni

Very hard, tough, resistant to corrosion

For making shafts, gears, etc.

Chrome steel

(13 - 14%) Cr, 0.7% Ni, 0.3% C

Resistant to corrosion

For making house hold materials.

Manganese steel

9 - 14%Mn

Very hard, resistant to ware

For making helmets

Inver

30 - 36% Ni 0.3% C

Coefficient of expansion is very low

Measuring instruments

Alnico

20% Ni, 12% Al, 5% Co

High magnetic property

For making strong magnets.

Durairon

16% Si

Resistant to acids

Vessels for keeping HCl, H2SO­4 etc.

Question 1:  Fe3O4 is formula of

(A)  magnetite

(B)  red haematite

(C)  brown haematite

(D)  iron pyrites

Question 2: Which one of the following ores is sulphite one?

(A)  magnetite

(B)  red haematite

(C)  brown haematite

(D)  iron pyrites

Question 3: Bell Metal is alloy of

(A)  Cu and Sn

(B)  Cu and Cr

(C)  Sn and Cr

(D)  Cu and Fe

Q.1

Q.2

Q.3

a

d

a

Related Resources

To read more, Buy study materials of General Principles & Isolation of Elements comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.

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