Problem 6 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the reactions (a) - (d), the reaction(s) that does/do not occur in the blast furnace during the extraction of iron is/are: (a) \(\mathrm{CaO}+\mathrm{SiO}_{2} \rightarrow \mathrm{CaSiO}_{3}\) (b) \(3 \mathrm{Fe}_{2} \mathrm{O}_{3}+\mathrm{CO} \rightarrow 2 \mathrm{Fe}_{3} \mathrm{O}_{4}+\mathrm{CO}_{2}\) (c) \(\mathrm{FeO}+\mathrm{SiO}_{2} \rightarrow \mathrm{FeSiO}_{3}\) (d) \(\mathrm{FeO} \rightarrow \mathrm{Fe}+\frac{1}{2} \mathrm{O}_{2}\) (a) (a) (b) (a) and (d) (c) (c) and (d) (d) (d)

Step-by-Step Solution

Answer

Reactions (c) and (d) do not occur in the blast furnace.

Step 1: Understand Reactions in a Blast Furnace

In a blast furnace, iron is extracted from its ore, mostly hematite (\(\mathrm{Fe}_2\mathrm{O}_3\)). The primary reactions involve the reduction of iron oxides to iron and the formation of slag to remove impurities. Common reduction reactions are with carbon monoxide (CO) to reduce oxides, and flux is added to form slag with impurities like SiO₂ and CaO.

Step 2: Analyze Reaction (a)

Reaction (a), \(\mathrm{CaO} + \mathrm{SiO}_2 \rightarrow \mathrm{CaSiO}_3\), is a common reaction in the blast furnace, where calcium oxide reacts with silicon dioxide (silica) to form calcium silicate, a component of slag. This reaction does occur in the blast furnace.

Step 3: Analyze Reaction (b)

Reaction (b), \(3\mathrm{Fe}_{2}\mathrm{O}_{3} + \mathrm{CO} \rightarrow 2\mathrm{Fe}_{3}\mathrm{O}_{4} + \mathrm{CO}_{2}\), involves the reduction of hematite with carbon monoxide to form magnetite. This is a step in the gradual reduction process of iron ore, and it does occur in the blast furnace.

Step 4: Analyze Reaction (c)

Reaction (c), \(\mathrm{FeO} + \mathrm{SiO}_{2} \rightarrow \mathrm{FeSiO}_{3}\), involves iron(II) oxide reacting with silicon dioxide to form iron silicate. This specific compound formation doesn't occur in the blast furnace, as silica primarily combines with lime (CaO) to form slag, not with iron oxides.

Step 5: Analyze Reaction (d)

Reaction (d), \(\mathrm{FeO} \rightarrow \mathrm{Fe} + \frac{1}{2}\mathrm{O}_{2}\), is an oxidation of iron instead of reduction. This reaction typically does not happen under blast furnace conditions, where reduction, not oxidation, is favored.

Step 6: Conclusion

From the analysis, reactions (a) and (b) occur in the blast furnace. Reactions (c) and (d) do not fit the typical blast furnace operations focused on reducing iron ore and forming slag.

Key Concepts

Blast Furnace Reactions

Understanding how reactions occur in a blast furnace is crucial in the extraction of iron from its ores. The blast furnace is a towering structure where these reactions take place at different temperature zones. The main goal of these reactions is to separate iron from impurities found in iron ore, such as silica and other gangue materials. Within the blast furnace, several reactions occur simultaneously or consecutively:

• Combustion: Hot air blasted into the furnace reacts with coke (carbon) to generate heat and carbon monoxide.
• Reduction: Carbon monoxide serves as a reducing agent, converting iron oxides into molten iron.
• Formation of Slag: Impurities react with additives to form a secondary product called slag.

These reactions are symbiotic processes that must function together to ensure efficient extraction of iron.

Reduction of Iron Oxides

The fundamental chemical processes in a blast furnace involve the reduction of iron oxides to produce pure iron. Reduction reactions are steps where iron compounds lose their oxygen content, and become elemental iron. The major reactions entail:

• Fe2O3 (hematite) is reduced by CO forming Fe3O4 (magnetite).
• Fe3O4 is further reduced to FeO (wüstite) and then to iron (Fe).

Oxygen is steadily removed from the iron oxide, forming iron and carbon dioxide as by-products. This sequence reduces hematite to metallic iron, moving downwards in the blast furnace where temperature permits these endothermic reactions.

Formation of Slag

The formation of slag plays a critical role in the blast furnace process. Slag is a by-product that helps in purifying the iron from its ore. The materials involved include calcium oxide (CaO) and silicon dioxide (SiO2), among others. The reactions lead to the creation of products like calcium silicate:

• CaO reacts with SiO2 to form CaSiO3, a primary component of slag.
• The slag is less dense than molten iron, allowing it to float on top and be easily removed.

By forming slag, the furnace effectively captures and removes unwanted components from the ore, ensuring high purity of the produced iron.

Iron Ore Reduction Processes

In a blast furnace, the reduction of iron ore involves a series of chemical transformations which gradually reduce iron ore to metallic iron. This process includes several stadia:

• The initial contact of iron ore with carbon monoxide results in magnetite and carbon dioxide (3Fe2O3 + CO → 2Fe3O4 + CO2).
• This is followed by the reduction of magnetite to wüstite, and then further to iron (Fe).

Some steps only happen under specific temperature conditions, ensuring maximum efficacy. Throughout the process, impurities are systematically removed while iron is released in a molten state. Efficient management of these processes is key for optimal iron extraction and economic feasibility.