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)

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

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:

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 4/5^th the of the furnace. How air at 700^oC is forced into the furnace through the tuyers.
The thermal gradient inside exists from 1800^oC (hearth) to 400^oC–900^oC in the upper region. Near the both the temperature varies from 1200^o-1300^o chemical reactions which take place are:

At 1200^oC near the tuyers,

C + O₂ → CO₂;

CO₂ + C → 2CO

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

Fe₂O₃ + 3CO ↔ 2Fe + 3CO₂↑

CaCO₃ CaO + CO₂

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

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

Related Resources

• Click here for past year papers of IIT JEE

• Click here to have a lookj at syllabus of IIT JEE

• You can also refer to refining of metals

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