Extraction of Iron: Blast Furnace and Slag | Class 10

Quick answer: Iron is extracted from haematite ($\mathrm{Fe_2O_3}$) in a blast furnace. The charge is haematite, coke (carbon) and limestone. Burning coke produces heat and carbon monoxide, which reduces iron oxide to iron: $\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$. Limestone breaks down to calcium oxide, which removes sand (silica) as molten slag: $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$. The slag floats on the molten iron and is removed separately.

Iron is used everywhere, from bridges and buildings to tools and trains, but it is not found as a pure metal. It occurs in ores such as haematite ($\mathrm{Fe_2O_3}$), mixed with sandy impurities (silica, $\mathrm{SiO_2}$).

In industry, iron is extracted from its oxide in a tall furnace called a blast furnace.

Syllabus note: The NCERT Class 10 chapter Metals and Non-metals explains that metals in the middle of the reactivity series, such as iron, are obtained by reducing their oxides with carbon. The blast furnace, its zones and the types of iron go beyond the Class 10 textbook. They are useful background and help with higher-level questions.

What You Will Learn

• The main ore of iron and the materials fed into the blast furnace
• What coke and limestone do in the furnace
• The main reactions: combustion, reduction and slag formation
• Why slag floats on top of the molten iron
• The difference between pig iron, cast iron, wrought iron and steel

1. Materials Fed into the Blast Furnace (the Charge)

The charge is added at the top of the furnace, and a blast of hot air is blown in near the bottom.

2. Reactions in the Blast Furnace

The temperature is highest at the bottom of the furnace and lower towards the top.

Burning of Coke (Near the Bottom)

Hot air reacts with coke, producing carbon dioxide and a lot of heat:

$\mathrm{C + O_2 \longrightarrow CO_2} \quad (\text{heat released})$

As the carbon dioxide rises through more hot coke, it is converted into carbon monoxide:

$\mathrm{CO_2 + C \longrightarrow 2CO}$

Reduction of Iron Oxide (Upper and Middle Parts)

Carbon monoxide removes oxygen from the iron oxide, reducing it to iron:

$\mathrm{Fe_2O_3 + 3CO \longrightarrow 2Fe + 3CO_2}$

Key point: In the blast furnace, iron oxide is reduced mainly by carbon monoxide. Carbon monoxide is the main reducing agent.

Slag Formation (Removing Silica)

Limestone breaks down on heating to form calcium oxide:

$\mathrm{CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2}$

Calcium oxide (a basic oxide) reacts with silica (an acidic impurity) to form calcium silicate, called slag:

$\mathrm{CaO + SiO_2 \longrightarrow CaSiO_3}$

Collecting the Iron

Near the bottom, the iron melts and collects as molten iron. The molten slag is lighter, so it floats on top of the iron. It also helps protect the molten iron from being oxidised again by the hot air. The iron and the slag are drained out separately.

3. Types of Iron

4. Summary Table

5. Exam Tips and Common Mistakes

Exam Tip: When asked about the role of limestone, write: limestone breaks down to calcium oxide, which reacts with silica impurities to form calcium silicate slag, $\mathrm{CaO + SiO_2 \rightarrow CaSiO_3}$.

Exam Tip: Name carbon monoxide as the main reducing agent and write the equation $\mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2}$.

Common Mistake: Saying that slag sinks. Slag is less dense than molten iron, so it floats on top.

Common Mistake: Calling the product from the blast furnace pure iron. It is pig iron, which contains about 4% carbon.

Frequently Asked Questions

What is the main ore of iron?

Haematite, which is mainly iron(III) oxide, Fe₂O₃.

What is the reducing agent in the blast furnace?

Carbon monoxide, formed from coke, is the main reducing agent. It reduces iron oxide to iron.

What is the role of limestone in the extraction of iron?

Limestone breaks down to calcium oxide, which reacts with silica impurities to form calcium silicate slag that can be removed.

Why does slag float on molten iron?

Slag is less dense than molten iron, so it floats on top and can be drained off separately.

What is the difference between pig iron and wrought iron?

Pig iron contains about 4% carbon and is hard and brittle. Wrought iron contains very little carbon and is soft, malleable and easily welded.