Question

Pig iron contains around

• A. \( \sim 2.1 \text{ wt\% Carbon} \)
• B. \( \sim 6.7 \text{ wt\% Carbon} \)
• C. \( \sim 0.7 \text{ wt\% Carbon} \)
• D. \( \sim 4.0 \text{ wt\% Carbon} \)

Hint:

Because of its high carbon content, pig iron is extremely brittle and cannot be forged or rolled directly. It must be refined to reduce carbon levels to produce steel.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the typical weight percentage of carbon present in pig iron, which is the immediate liquid iron product tapped from an ironmaking blast furnace.

Step 2: Key Formula or Approach:
Liquid iron in the blast furnace hearth is in direct, prolonged contact with solid carbon (coke) at high temperatures (\( \gt 1400^\circ\text{C} \)).
As a result, the liquid metal becomes saturated with carbon according to the solubility limit of carbon in liquid iron at that temperature.

Step 3: Detailed Explanation:

• High Carbon Saturation: Liquid iron in the blast furnace absorbs carbon until it reaches saturation, which is typically around \( 4.0 \text{ to } 4.5\text{ wt\%} \) carbon at operating temperatures.
This high carbon content lowers the melting point of the alloy to approximately \( 1150^\circ\text{C} \text{ to } 1200^\circ\text{C} \) (near the Fe-C eutectic temperature), allowing the liquid iron to be easily tapped and handled.

• Impurity Levels: In addition to carbon, pig iron contains significant concentrations of other impurities reduced from the ore and flux:
- Silicon (\( \sim 0.5 \text{ to } 1.5\text{ wt\%} \))
- Manganese (\( \sim 0.5 \text{ to } 1.0\text{ wt\%} \))
- Phosphorus (\( \sim 0.1 \text{ to } 1.5\text{ wt\%} \))
- Sulfur (\( \sim 0.03 \text{ to } 0.08\text{ wt\%} \))

• Comparison with Other Options:
- \( 0.7\text{ wt\%} \) (Option C) represents typical medium/high-carbon steels.
- \( 2.1\text{ wt\%} \) (Option A) is the maximum solubility limit of carbon in austenite, marking the theoretical boundary between steel and cast iron.
- \( 6.7\text{ wt\%} \) (Option B) corresponds to stoichiometric cementite (\( \text{Fe}_3\text{C} \)), which is highly brittle and not achieved in standard liquid iron output.


Step 4: Final Answer:
Therefore, pig iron contains around \( 4.0\text{ wt\%} \) carbon, which corresponds to Option (D).