Question

Hot metal from a blast furnace is treated with mill scale prior to oxygen steelmaking for _____.

• A. dephosphorization
• B. decarburization
• C. desulphurization
• D. desiliconization

Hint:

In steelmaking, the order of impurity removal usually begins with silicon, since it has the highest affinity for oxygen. Mill scale (FeO) is ideal for early-stage desiliconization.

Answer 1

The Correct Option is D

Step 1: Recall purpose of mill scale.
Mill scale is a waste oxide layer (\(\text{FeO, Fe}_2\text{O}_3, \text{Fe}_3\text{O}_4\)) obtained during rolling of steel. It is rich in iron oxides and acts as a strong oxidizing agent when mixed with hot metal. Step 2: Effect on hot metal impurities.
- Silicon oxidizes very easily compared to other impurities (phosphorus, sulphur, carbon). - When mill scale (iron oxides) is added to molten iron, the oxides preferentially oxidize silicon in the hot metal to form silica (\(\text{SiO}_2\)), which then goes to slag. Step 3: Eliminate other options.
- Dephosphorization: requires a basic slag and controlled oxygen blowing, not just mill scale.
- Decarburization: done during oxygen blowing in BOF steelmaking, not by mill scale treatment.
- Desulphurization: needs basic fluxes (CaO, Mg), not mill scale.
- Desiliconization: happens first and most readily via Fe-oxides in mill scale. Step 4: Conclude.
Therefore, mill scale is used for desiliconization before oxygen steelmaking. \[ \boxed{\text{Desiliconization}} \]