Role of limestone in the blast furnace — function of the lime produced In the blast furnace burden, limestone (CaCO3) decomposes to lime (CaO) and carbon dioxide. What is the principal function of the lime thus formed during smelting?

Introduction / Context:
Fluxes are added to blast furnace burdens to transform gangue and ash into a fluid, separable slag. Understanding each ingredient’s role streamlines burden design and ensures efficient separation of impurities from molten iron.

Given Data / Assumptions:

• Limestone is the common flux; it decomposes to CaO + CO2 in the stack.
• Gangue includes silica and alumina from ore and coke ash.
• Goal is to form a low-melting, fluid slag for easy tapping and desulfurization.

Concept / Approach:
The lime (CaO) from limestone is a basic oxide that reacts with acidic oxides such as SiO2 and Al2O3 to form calcium and alumino-silicate slags. This captures impurities, adjusts slag basicity, and assists in removing sulfur from hot metal via CaS formation. While limestone affects thermal balance via its endothermic decomposition, its critical role is chemical: creating a fluid slag that separates cleanly from iron.

Step-by-Step Solution:

Verification / Alternative check:
Slag basicity indices (CaO/SiO2) are controlled in practice to achieve desulfurization and fluidity targets, confirming lime’s principal function as a flux.

Why Other Options Are Wrong:

• “Controls the grade” is indirect; chemistry separation is the main purpose.
• Lime is not an iron-bearing mineral.
• Limestone does not “supply heat”; its calcination consumes heat.
• Catalysis of gas reactions is not its main role.

Common Pitfalls:
Believing limestone is a fuel; overlooking its endothermic decomposition and primary fluxing function.

Final Answer: