Problem 13 - A Complete Resource Book in Chemistry for JEE Main

Question

In the manufacture of iron from haematite, lime stone is added to act as an (a) slage (b) reducing agent (c) oxidizing agent (d) flux

Step-by-Step Solution

Answer

(d) flux

Step 1: Understanding the Role of Limestone

In the extraction of iron from haematite (Fe2O3) in a blast furnace, limestone is added along with coke and the iron ore. Each of these components has a specific purpose in the chemical reactions that take place.

Step 2: Defining 'Flux' Role

A flux is a substance that is added to help remove impurities from ore. Limestone (CaCO3) serves as a flux in the iron-making process. It reacts with the silica (SiO2) impurities present in haematite to form calcium silicate (CaSiO3), which is a liquid slag.

Step 3: Recognizing Slag Formation

The calcium silicate (CaSiO3) formed by the reaction between limestone and silica is referred to as slag. However, the initial role of limestone is to act as a flux, not a slag directly. The slag is the resultant by-product due to its interaction as a flux.

Step 4: Eliminating Other Options

- Reducing agents remove oxygen from ore, typically carbon or carbon monoxide in this process. - Oxidizing agents add oxygen. Limestone does not act as a reducing or oxidizing agent. Thus, limestone's primary role is in facilitating slag formation as a flux.

Step 5: Conclusion and Choice

Evaluating these points, it is clear that limestone is added primarily to function as an aid to form slag by acting as a flux in the blast furnace.

Key Concepts

Iron Extraction

In the heart of industrial processes, iron extraction from haematite (\( \text{Fe}_2\text{O}_3 \)) stands as a pivotal operation, primarily conducted in a blast furnace. This large steel tower is generally over 30 meters tall, and its goal is to transform iron ore into usable metallic iron. To achieve this, specific reactants perform key roles:

• Iron ore, typically haematite, which provides the iron.
• Coke, a form of carbon that serves two purposes: as a source of heat when burned and as a reducing agent that strips oxygen from the iron ore.
• Limestone, which helps to remove impurities.

This mix is fed into the top of the furnace while air is blasted in from the bottom, creating intense heat. At these high temperatures, coke reduces haematite to pure iron, which collects at the bottom as liquid iron, ready to be tapped and further refined.

Limestone Role

Limestone, or calcium carbonate (\( \text{CaCO}_3 \)), plays a crucial supporting role in the iron extraction process. Understanding its purpose is key to mastering how the process operates efficiently in the blast furnace. As the furnace heats up, limestone decomposes to form lime (\( \text{CaO} \)) and carbon dioxide (\( \text{CO}_2 \)). The lime then reacts with silica impurities present in the iron ore. If left untreated, these impurities would lead to structural weaknesses in the produced iron.Thus, limestone's transformation and its subsequent chemical reactions facilitate the removal of these impurities. This allows for the extraction of higher-quality iron.

Flux in Metallurgy

In metallurgy, a flux is essential for simplifying the purification of metals by aiding in the removal of impurities. The concept of a flux is straightforward but hugely impactful. During the extraction of iron, limestone performs this vital job by acting as a flux. As temperatures in the blast furnace rise, the limestone decomposes and the resulting lime reacts with silica. This interaction produces calcium silicate (\( \text{CaSiO}_3 \)), which is a compound with a lower melting point. Because it melts away from the rest of the material, it can easily be removed, leaving behind the purified metal. The process is seamless, ensuring that slag formation does not hinder efficiency and helps maintain the quality of the iron produced.

Slag Formation

Slag formation is the noticeable by-product of the purification process in iron extraction. Although slag itself isn't the main objective, its creation is crucial, stemming from the reaction between silica (\( \text{SiO}_2 \)) impurities and lime (\( \text{CaO} \)) derived from limestone.This reaction yields calcium silicate, also known as slag. It is a molten mass that floats above molten iron because it is less dense. A major advantage of slag is its ability to be easily separated from the purified iron, aiding in the maintenance of high yield and purity of the extracted metal.Moreover, the formation of slag has benefits outside iron-making, as it can be repurposed in industries like construction, forming bricks and other materials due to its durability.