Metallurgy — flux selection in iron making If iron ore contains clay (siliceous) as the main impurity, which flux should be added during calcination to promote slag formation?

Introduction / Context:
In extractive metallurgy, choosing the correct flux is essential for removing gangue from iron ore. When the impurity is clay (largely silica), the furnace must be provided with a basic flux so that a fluid, separable slag forms, improving metal quality and furnace efficiency.

Given Data / Assumptions:

• Ore contains clay (siliceous gangue, mainly SiO2 and aluminosilicates).
• Objective: convert acidic gangue into a basic slag that can be tapped off.
• Standard ironmaking practice using limestone as a basic flux.

Concept / Approach:
Silica (acidic) requires a basic flux. Limestone (CaCO3) decomposes to CaO and CO2 on heating. The calcium oxide then reacts with silica to form calcium silicate slag. This slag is fluid at furnace temperature and separates from molten iron due to density differences.

Step-by-Step Solution:
Thermal decomposition: CaCO3 → CaO + CO2.Acid–base reaction: CaO + SiO2 → CaSiO3 (slag).Slag floats above iron and is removed, taking siliceous impurities with it.Therefore, add limestone when the gangue is clay/silica rich.

Verification / Alternative check:
Basic slag formation is standard in blast furnace practice; basic fluxes neutralize acidic gangue. Process flow sheets and slag analyses confirm predominance of calcium silicates when limestone is used.

Why Other Options Are Wrong:
Clay is the impurity itself; adding more does not help.

Argillaceous iron ore adds further silica/alumina, worsening slagging.

“All of the above” cannot be correct since basic–acid pairing is required.

Dolomite provides CaO and MgO and can be used, but limestone is the conventional first choice for siliceous gangue in the context of the question.

Common Pitfalls:
Confusing acidic and basic slags; attempting to remove silica with acidic flux; ignoring the decomposition of limestone and slag fluidity requirements.

Final Answer:
limestone (CaCO3)