Key reactions in the blast furnace (temperature range $900-1500\,\mathrm{K}$)

In extraction of iron, the important roles are:

• Coke acts as fuel and reducing agent.
• Limestone acts as flux.
• Iron oxides are reduced mainly by $\mathrm{CO}$.

Check Option A: Limestone is used to remove silicate impurity.

Limestone decomposes as: $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$

The lime formed reacts with silica impurity: $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$

$\mathrm{CaSiO_3}$ is slag and removes silicate impurity.

Hence, A is correct.

Check Option B: Pig iron obtained from blast furnace contains about $4\%$ carbon.

Pig iron produced in blast furnace is impure iron and typically contains about $4\%$ carbon along with other impurities like S, P, Si, Mn.

Hence, B is correct.

Check Option C: Coke $(\mathrm{C})$ converts $\mathrm{CO_2}$ to $\mathrm{CO}$.

This occurs by: $\mathrm{CO_2 + C \rightarrow 2CO}$

This is an important reaction in the furnace because $\mathrm{CO}$ is the main reducing agent for iron oxides.

Hence, C is correct.

Check Option D: Exhaust gases consist of $\mathrm{NO_2}$ and $\mathrm{CO}$.

Blast furnace gases mainly contain:

• $\mathrm{N_2}$
• $\mathrm{CO}$
• $\mathrm{CO_2}$

$\mathrm{NO_2}$ is not a usual major constituent of blast furnace exhaust gas.

Hence, D is incorrect.

Final conclusion

Correct options are: $\boxed{A,\ B,\ C}$

Comparison with stored answer

Stored correct answer: $A, B, C$

My derived answer matches the stored answer.