Question

Name different zones in the blast furnace. Write the reactions taking place in them.

Write the reducing agents used to convert Fe₂O₃ to ‘Fe’ in the reduction zone of the blast furnace.

Answer 1

Reduction (Smelting): This step is carried out in a blast furnace. The charge containing ore and limestone is introduced into the furnace through a cup and cone arrangement. In this arrangement, the cone enables uniform distribution of charge and the cup prevents the loss of gases. A blast of preheated air is introduced into the furnace below the bosh. The charge and hot air come in contact with each other and various reactions take place.

Chemical reactions taking place in different zones of the blast furnace:

a. Zone of combustion: This is 5-10 m from the bottom. The hot air blown through the tuyers reacts with coke from the charge to form CO.

\[\ce{C + 1/2O2 -> CO}\], ΔH = -220 kJ

The reaction is highly exothermic; thus, the temperature of this zone is around 2000 K. Some of the CO formed dissociates to form finely divided carbon.

\[\ce{2CO -> 2C + O2}\]

The hot gas rich in CO rises upwards in the blast furnace. The charge coming down gets heated and reacts with CO. Thus, CO acts as a fuel and also as a reducing agent.

b. Zone of reduction (22-25 m near the top): Here, the temperature is around 900 K. Fe₂O₃ is reduced to spongy iron by CO.

\[\ce{Fe2O3 + 3CO ->[900K] 2Fe + 3CO2}\]

Some amount of Fe₂O₃ is reduced to iron by carbon.

\[\ce{Fe2O3 + 3C -> 2Fe + 3CO}\]

c. Zone of slag formation (20 m height from the bottom): The gangue present in the ore is converted to slag. This slag can be used for making a road foundation. The temperature of this zone is 1200 K. The gangue contains silica, alumina, and phosphates. Removal of this gangue is effected by adding limestone in the charge, which acts as a flux. Limestone decomposes to give CaO (quick lime).

\[\ce{CaCO3 ->[\Delta]CaO + CO2}\]

CaO combines with gangue to form molten slag of calcium silicate and calcium aluminate.

\[\ce{CaO + SiO2 -> CaSiO3}\]

\[\ce{12CaO + 2Al2O3 -> 4Ca3AlO3 (slag) + 3O2}\]

d. Zone of fusion (15 m height from the bottom): MnO₂ and Ca₃(PO₄)₂ present in the iron ore are reduced to Mn and P. Some of the silica is also reduced to Si. The spongy iron coming down in the furnace melt absorbs impurities like C, Si, Mn, P, and S. This molten iron collects at the bottom in the furnace. The slag is lighter floats on the surface of molten iron. Molten slag and iron are collected through separate outlets. Molten iron is poured into moulds. These solid blocks are called pigs. This iron contains about 4% of carbon. When pig iron is remelted, run into moulds, and cooled, it becomes cast iron. The waste gases containing N₂, CO, and CO₂ escape through the outlet at the top. These hot gases are used for preheating the blast of air.

Answer 2

In the reduction zone of the blast furnace, iron ore (Fe₂O₃) is reduced to iron (Fe) by the following reducing agents:

1. Carbon monoxide (CO):
   \[\ce{Fe2O3 + 2CO -> 2Fe + 3CO2}\]
   Carbon monoxide acts as a primary reducing agent.
2. Coke (C) (at high temperature):
   \[\ce{Fe2O3 + 3C -> 2Fe + 3CO}\]
   At higher temperatures, carbon (coke) also acts as a reducing agent by directly reducing Fe₂O₃.

Therefore, the main reducing agents in the blast furnace reduction zone are carbon monoxide (CO) and carbon (Coke, C).