Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

(a) slag

Step 1: Understanding the Process

In the blast furnace, iron ore is reduced to produce iron. Limestone, or calcium carbonate, is added as a fluxing agent to this process.

Step 2: Role of Limestone

The addition of limestone to the blast furnace results in thermal decomposition, where limestone breaks down into calcium oxide (CaO) and carbon dioxide (CO₂) at high temperatures.

Step 3: Formation of Slag

The calcium oxide functions as a flux to react with silica (SiO₂) in the ore. This reaction forms calcium silicate, a compound that melts and collects impurities to form slag.

Step 4: Identifying the Product

The compound calcium silicate, which contains calcium ions, is part of the molten slag which is separated from iron.

Key Concepts

Blast Furnace

A blast furnace is a towering structure used in the metal extraction process, especially for extracting iron from its ore. Inside the blast furnace, several reactions take place at varying temperature zones to reduce the iron oxide present in the ore to metallic iron. The process begins by charging the furnace with a mixture of iron ore, coke (a form of carbon), and limestone. The primary purpose of a blast furnace is to provide the extreme heat necessary to facilitate these reactions.

Here’s how it works:

• The combustion of coke produces carbon monoxide, which reacts with iron ore to form molten iron.
• A hot air blast is introduced to support continuous combustion and maintain high temperatures.
• The heat generated promotes the breakdown of calcium carbonate (limestone) into calcium oxide and carbon dioxide.

The presence of limestone is crucial as it adds efficiency to the separation of impurities, ensuring the end product, iron, is of higher quality.

Limestone

Limestone, primarily composed of calcium carbonate (\(CaCO_3\)), is a critical ingredient in the metallurgy of iron. Introduced as a fluxing agent in the blast furnace, limestone has a distinct role in facilitating the separation of impurities from the desired metal.

When added to the furnace, limestone undergoes thermal decomposition due to high furnace temperatures, producing calcium oxide (\(CaO\)) and releasing carbon dioxide (\(CO_2\)). This decomposition is an essential step as it transforms limestone into a more reactive form.

• Calcium oxide, commonly known as quicklime, engages actively with non-metallic impurities present in iron ore.
• The reaction between calcium oxide and silica (common impurity) results in the formation of calcium silicate.
• This newly formed compound is a key component of the molten slag.

Thus, limestone not only contributes to the breakdown of impurities but also assists in their removal by converting them into a liquid slag, which is much easier to separate from the metal.

Slag

Slag is the by-product of the iron smelting process in the blast furnace, with a vital role in ensuring impurities are effectively removed from the metallic iron. Formed during reactions inside the furnace, it consists mainly of calcium silicate and other compounds produced from impurities.

As the furnace operates:

• Calcium silicate forms when calcium oxide combines with silicon dioxide, present in the ore.
• Slag is less dense than molten iron, allowing it to float on top of the liquid metal.
• This floating layer helps in simplifying the separation process as slag can be easily extracted.

The creation and removal of slag are essential, ensuring the purity of iron.

Additionally, beyond removing impurities, slag has practical applications too. It can be processed and used in construction, as a component in cement production, or as an aggregate material, showcasing its versatility beyond the blast furnace.