Question

Name two important ores of iron. Write the method of extraction of iron and the chemical reaction involved in it.

Answer 1

Two important ores of iron are follows:

(i) Haematite (Fe₂O₃)

(ii) Magnetite (Fe₃O₄)

Extraction of iron from its oxides-

Concentrated ores of iron are mixed with lime stone and coke and fed into a Blast furnace from top. Here coke reduces oxides to the metal as follows:

FeO(s) + C(s) → Fe(sort) + CO(g)    ...(i)

In two steps:

(i) FeO(s) → Fe(s) + 1/2 O₂(g) [ΔG (FeO, Fe)]

(ii) C(s) + 1/2 O₂(g) → CO(g) [ΔG (C, CO)]    ...(ii)

From (i) and (ii), The net Gibbs energy change becomes

ΔG (C, CO) + ΔG (FeO, Fe) = Δr G ...(iii)

∴ The resultant reaction will take place when the r.h.s of equation (iii) become negative.

The reaction occurring in the blast furnance at different temperature are as follows:

At 500 - 800 K (lower temp. range)

3Fe₂O₃ + CO → 2Fe₃O₄ +CO₂

Fe₃O₄ + 4CO → 3Fe - 4CO₂

Fe₂O₃ + CO → 2FeO + CO₂

At 900 - I500 K (higher temp. in the blast furnace)

C + CO₂ → 2CO

FeO + CO₂ → FeCO₂

Lime stone is also decomposed to CaO which remove sillicate impurity of the ore as slag. The slag is in molten state and separates out from iron.

CaCO₃(s) → CaO(s)  + CO₂(g)

CaO(s) + SiO₂(s) → CaSiO₃(fusible slag)

Iron obtained from blast furance contains 4% carbon and many impurities in smaller amount (e.g. SP, Si, Mn) is called big iron. Cast iron is different from pig iron. It has slightly lower carbon content (3%) and it is extremely hard and brittle.

Further Reductions-

Wrought iron or mallecable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reyerberatory furnace line with haematite. This haematite oxidises carbon to carbon monoxide:

Fe₂O₃ + 3C → Fe + 3CO

Limestone is added as a flux and sulphur, sillicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers.