Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

In the metallurgy of iron, when limestone is added to the blast furnace, the calcium ions end up in (a) gangue (b) calcium carbonate (c) slag (d) metallic calcium

Step-by-Step Solution

Answer

The calcium ions end up in slag (c).

Step 1: Understanding the Role of Limestone

In iron metallurgy, limestone is added to the blast furnace to remove impurities in the form of slag. Limestone, primarily composed of calcium carbonate (CaCO₃), decomposes when heated.

Step 2: Decomposition of Limestone

When limestone (CaCO₃) is heated in the blast furnace, it decomposes into calcium oxide (CaO) and carbon dioxide (CO₂).\[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]

Step 3: Formation of Slag

The calcium oxide (CaO) from the decomposed limestone reacts with silica (SiO₂) and other impurities present in the ore. This reaction results in the formation of calcium silicate, which is a major component of the slag formed:\[ \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \]

Step 4: Composition of Slag

Slag, which forms as a by-product in the blast furnace, primarily contains calcium silicate ( CaSiO₃). This indicates that the calcium ions initially present in the limestone end up in slag.

Key Concepts

Limestone Decomposition

In the process of iron metallurgy, limestone plays a pivotal role inside the blast furnace. This natural rock, mainly composed of calcium carbonate (\(\text{CaCO}_3\)), is added not to become part of the iron, but to undergo a chemical change. The intense heat inside the blast furnace initiates the decomposition of limestone.

• When heated, limestone breaks down into calcium oxide, known as lime (\(\text{CaO}\)), and carbon dioxide (\(\text{CO}_2\)).
• The reaction can be simply noted as: \[\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\]

This decomposition is crucial because it transforms the limestone into a reactive form that can interact with other elements in the furnace. Understanding this transformation is key as it sets the stage for the next steps in the blast furnace process.

Formation of Slag

As the blast furnace operates, impurities present in the iron ore need to be removed to yield pure iron. The calcium oxide (\(\text{CaO}\)) formed from the decomposition of limestone serves as an important agent in this purification process.

• Calcium oxide reacts with silica (\(\text{SiO}_2\)) and other impurities present in the iron ore.
• This chemical reaction forms a compound known as calcium silicate (\(\text{CaSiO}_3\)), which is a cornerstone of slag.\[\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3\]

The slag is a molten mixture of calcium silicate and other byproducts, which floats above the denser, liquid iron. It can easily be removed, taking away the impurities with it. This step ensures that the final iron product is purer and has improved quality.

Blast Furnace Process

The blast furnace is an engineering marvel where the magic of metallurgy happens, converting raw materials like iron ore into usable metallic iron. At its heart, the blast furnace combines high temperatures and multiple reactions to achieve this transformation.

• Inside, layers of iron ore, coke (a carbon source), and limestone are introduced.
• Air is blasted into the furnace to sustain high temperatures and promote combustion.
• The blast furnace operates continuously, interacting all components in intricate chemical reactions.

Several reactions take place, notably the reduction of iron ore to iron, complemented by limestone's role in forming slag, which removes impurities. By separating the valuable metallic iron from unwanted substances, the blast furnace secures its place as a key component in the metallurgy of iron.