Which of the following is used in forming slag in the blast furnace for the extraction of iron?

Analyze the question: The question asks which substance is used to form slag in the blast furnace. Slag is the waste product that removes impurities from the iron ore.

Concept: The iron ore (haematite, Fe₂O₃) contains impurities, mainly silica (SiO₂, sand). These must be removed. This is done by adding limestone (CaCO₃) to the furnace charge, along with coke and the ore.

Step-by-step solution:

1. Limestone decomposes in the heat of the furnace: CaCO₃ → CaO + CO₂

2. The calcium oxide (a basic oxide) reacts with silica (an acidic oxide) to form calcium silicate, which is the slag: CaO + SiO₂ → CaSiO₃ (slag)

3. The slag is molten and less dense than molten iron, so it floats on top of the iron and is tapped off separately.

Which option is correct?

• A. Ca(CO₃)₂ is not a valid formula, since calcium carbonate is CaCO₃.
• B. Ca(OH)₂ (slaked lime) is not added to the blast furnace.
• C. CaCO₃ is limestone, the substance that is charged into the furnace.
• D. CaO is the actual reactant with silica, but it is formed inside the furnace from limestone. It is not the material added.

The question asks what is used in forming slag, and the raw material added to the furnace is limestone, CaCO₃.

Tip: Remember the blast furnace charge as "ore, coke, limestone". Also remember the logic: limestone → CaO (basic) + SiO₂ (acidic) → slag. If JAMB asks what reacts directly with silica, then CaO is the answer. If it asks what is added to the furnace, then CaCO₃ is the answer.