Chemistry 2008 JAMB Past Questions - Question 31: The substance that is used in the steel industry for the removal of carbon, sulphur and phosphorus impurities from pig iron is

Correct Answer

A

Explanation

The substance used in the steel industry for the removal of carbon, sulfur, and phosphorus impurities from pig iron is **calcium oxide (CaO)**, commonly known as **lime**.

Here's how it works:

- **Carbon:** During the steelmaking process, impurities like carbon are removed by **oxidation**. CaO reacts with carbon to form carbon monoxide (CO):

  CaO + C -> CaC₂O₄ + CO

  The CO gas bubbles out of the molten metal, taking a significant amount of carbon with it.

- **Sulfur:** CaO reacts with sulfur to form calcium sulfide (CaS), which then floats to the surface as slag and is removed:

  CaO + S -> CaS

- **Phosphorus:** Similar to sulfur, CaO reacts with phosphorus to form calcium phosphate (Ca₃(PO₄)₂) that becomes part of the slag and is removed:

  3CaO + P₄ -> Ca₃(PO₄)₂

**Other materials** can also be used along with CaO in specific situations, such as:

- **Fluorspar (CaF₂)**: Helps improve the fluidity of slag and promote further removal of sulfur and phosphorus.
- **Dolomite (CaMg(CO₃)₂)**: Similar to CaO, acts as a flux and provides magnesium oxide (MgO) which also aids in desulfurization.
- **Iron ore**: Can be added to oxidize excess silicon present in pig iron.

However, **CaO remains the primary and most widely used substance for removing these impurities due to its effectiveness, affordability, and readily available sources.**