Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the follwoing reaction occurs in the combustion zone of the blast furnace during the extraction of iron? (a) \(\mathrm{CO}_{2}+\mathrm{C} \longrightarrow 2 \mathrm{CO}\) (b) \(\mathrm{CaO}+\mathrm{SiO}_{2} \longrightarrow \mathrm{CaSiO}_{3}\) (c) \(\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2}\) (d) \(\mathrm{FeO}+\mathrm{CO} \longrightarrow \mathrm{Fe}+\mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

(c) \( \mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} \) occurs in the combustion zone of the blast furnace.

Step 1: Understanding the Blast Furnace Zones

In a blast furnace, several zones exist where different reactions occur during the extraction of iron. The main zones include the combustion zone, the reduction zone, and the slag formation zone.

Step 2: Identifying the Combustion Zone

The combustion zone, also known as the tuyere zone, is where the hot air is blown into the furnace. In this zone, the primary reaction is the combustion of coke (carbon) with oxygen to form carbon dioxide and release heat. This reaction is crucial as it provides the necessary temperature for the process.

Step 3: Examining Possible Reactions

The reactions given are: (a) \( ext{CO}_{2} + ext{C} ightarrow 2 ext{CO}\) (b) \( ext{CaO} + ext{SiO}_{2} ightarrow ext{CaSiO}_{3}\) (c) \( ext{C} + ext{O}_{2} ightarrow ext{CO}_{2}\), and (d) \( ext{FeO} + ext{CO} ightarrow ext{Fe} + ext{CO}_{2}\). In the combustion zone, the most relevant reaction involves carbon and oxygen.

Step 4: Analyzing the Reaction with Oxygen

The combustion zone primarily involves the reaction \( ext{C} + ext{O}_{2} ightarrow ext{CO}_{2}\). Here, carbon (coke) reacts with incoming oxygen to form carbon dioxide, thereby providing the energy required for further reactions in the blast furnace.

Key Concepts

Reduction Zone Reactions

In the blast furnace, the reduction zone is where essential chemical transformations occur to extract iron from its ores. This zone lies above the combustion zone and is vital for the reduction of iron oxides into metallic iron. The temperature in the reduction zone is highly controlled due to the heat generated below in the combustion zone. As the gases ascend and the solid materials move downward, specific reactions occur.

• The predominant reaction in the reduction zone is between iron(III) oxide or iron(II) oxide and carbon monoxide: \[ \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \]
• This reaction reduces the iron ore into metallic iron and carbon dioxide gas.
• The reduction phases begin with Fe₂O₃ and proceed through FeO before producing pure iron.

As carbon monoxide is produced from the combustion zone, it plays a crucial part in transforming iron oxides throughout the reduction zone. This step is a cornerstone in obtaining iron, ready for further use in industry.

Coke Combustion

Coke combustion is a critical part of the iron extraction process within the blast furnace. It occurs in the combustion zone and is primarily responsible for generating the heat required to drive other chemical reactions.
Coke, a form of carbon derived from coal, interacts with oxygen to create carbon dioxide:

• \( \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \)

This exothermic reaction releases a substantial amount of heat, achieving the high temperatures crucial for other reactions in the upper zones.

• As carbon dioxide moves upward, it reacts further in the reduction zone to form carbon monoxide: \( \text{CO}_2 + \text{C} \rightarrow 2\text{CO} \)
• This newly formed carbon monoxide is pivotal for the reduction process, acting as a reducing agent for iron oxides.

Coke not only aids in combustion but also contributes to the structure and permeability of the blast furnace charge, aiding in the overall efficiency of the iron extraction process.

Iron Extraction Process

The iron extraction process in a blast furnace is an amalgamation of several complex reactions and is carefully orchestrated to maximize efficiency. The extraction is based on turning iron ore, primarily hematite (Fe₂O₃), into usable iron.
The steps in a broad brush view are:

• Raw materials like iron ore, coke, and limestone are loaded into the furnace.
• In the combustion zone, coke burns to form carbon dioxide, generating necessary heat.
• Carbon dioxide further reacts with coke, forming carbon monoxide, which then flows upward.
• In the reduction zone, carbon monoxide reduces iron ore to metallic iron. Here, chemical reactions transform iron oxides into metallic iron.

The molten iron collects at the bottom and can be tapped off. Waste products, like slag, which consist of impurities from the ore and added flux, are also formed and removed.
The systematic progression through different zones ensures a continuous and efficient extraction, delivering high-quality iron ready for industrial application.