Question

(a) In the smelting process carried out in blast furnaces for the extraction of iron, the lower region of the furnace has the highest temperature.
(b) Formation of $CO$ from $C{O_2}$ and coke reduces the temperature of the middle region of the blast furnace.
(c) Limestone is taken from a blast furnace.
(d) Silica present in the ore as gangue is converted to fusible slag by the process of reduction
which of the following regarding the process of smelting is correct?
A.a, b and c
B.b and c
C.only a
D.only d

Answer 1

We need to understand the complete procedure of smelting used in metallurgy. Smelting is the process of extraction of metals from its oxide by fusing it with carbon. It is also known as smelting – reduction with carbon. Carbon is basically oxidised to $C{O_2}$​ at temperature below $973K$ and to $CO$ at temperature above $973K$ . For example, copper, zinc, lead, iron, etc are obtained from their oxides by reduction with coke and the reducing agent Carbon combines with the oxygen of the metal oxide and metals are either obtained in the molten state of vapour state.

Complete step by step answer:
We need to know that the iron is chiefly extracted from its oxide ore by heating it with coke and limestone in a blast furnace. The furnace contains (i) tuyeres through which hot air is passed, (ii) tapping holes through which molten iron is drawn and (iii) a slag hole from which slag is drawn. At the top, the furnace carries a hopper which rests over a cup and cone-like arrangement to prevent the escape of gases during the charging of the furnace. The charge contains calcined ore, coke and limestone in the ratio 4:2:1 and introduced from the top.
We have to remember that the blast furnace is divided into three different regions:
Lower part of the furnace is known as the zone of fusion. The reaction occurring in this region is the oxidation step as: $C + {O_2} \to C{O_2} + heat$
Middle part of the furnace is known as the zone of heat absorption. The reactions occurring in this region are $C{O_2} + C \to 2CO + heat$ and $CaC{O_3} \to CaO + C{O_2} + heat$ .In this step, formation of $CO$ from $C{O_2}$ reduces the temperature in this region as higher temperature is required for this step hence the heat is absorbed.
Top part of the furnace known as zone of reduction. The reactions occurring in this region are $CaO + Si{O_2} \to CaSi{O_3}$ and $F{e_2}{O_3} + 3CO \to 2Fe + 3C{O_2}$
We must need to know that the coke burns at the bottom of the furnace to give $C{O_2}$ . This gas rises up, combines with C to give $CO$ which reduces the temperature in this region which is the middle part of the furnace. Limestone is used as a flux. Flux is a chemical substance which combines with gangue (unwanted parts of the ore) at higher temperatures to form easily fusible material called slag which is not soluble in the molten metal. The slag formed is CaSiO3. The gangue present in the ore is silica which is converted to slag by the process of oxidation in the top part of the furnace. The slag is removed through the slag hole and the molten iron is drawn through the tapping hole.
Hence the options (a), (b) and (c) are correct but (d) is incorrect since the Silica present in the ore as gangue is converted to fusible slag by the process of oxidation and not reduction.
Therefore, the correct option is option (A).

Note:
We must be noted that only the metals whose reduction potentials are between $- 1.5V$ and $+ 0.5V$ are generally extracted by smelting. Also for carbon to be used as a reducing agent for the extraction of iron from its oxide, temperature should be above $710^\circ C$ as the Gibbs free energy for the reaction will be negative and the reaction will occur spontaneously.