Question

Lime (CaO) is usually added to the blast furnace in the metallurgy of iron where it reacts with impurities present in iron ore. What of the following best describes its function? (a) \(\mathrm{CaO}\) acts as an oxidizing agent. (b) CaO acts as a reducing agent. (c) \(\mathrm{CaO}\) acts as an acid. (d) \(\mathrm{CaO}\) acts as a base.

Short answer

Lime (CaO) acts as a base.

Step-by-step solution

Understanding Lime Function

Lime, also known as calcium oxide (CaO), is used in metallurgy, particularly in blast furnaces during iron production. Its primary role is to interact chemically with impurities in ore, particularly to remove them in the form of slag.

Chemical Nature of Lime (CaO)

Calcium oxide (CaO) is characterized by its basic chemical properties. It is classified as a basic oxide, meaning it tends to react with acidic substances. The key feature of CaO in iron metallurgy is its basicity.

Reaction in Blast Furnace

In the blast furnace, lime reacts with impurities that are acidic in nature. These impurities often include oxides such as silica (SiO extsubscript{2}). CaO reacts with SiO extsubscript{2} to form calcium silicate, a component of slag, thereby removing the impurities from the iron.

Conclusion

Given that CaO reacts with acidic impurities to form slag, it acts as a base during the iron extraction process. Thus the correct description of CaO's role in the blast furnace context is as a base.

Key concepts

Blast Furnace

The blast furnace is an essential part in the process of extracting iron from iron ore. It's a towering structure that operates continuously, subjecting raw materials to high temperatures to produce molten iron. The key goal of the blast furnace is to reduce iron oxide present in iron ore into liquid iron.
The process involves several steps where raw materials such as iron ore, coke, and limestone (CaCO extsubscript{3}) are loaded from the top. As the materials descend, they are subjected to a blast of hot air, intensifying the temperature inside the furnace. This environment enables chemical reactions that facilitate the extraction of iron.
The blast furnace functions:

• Reduces iron oxides to produce liquid iron
• Separates impurities from raw iron ore
• Operates at extremely high temperatures to melt materials

Iron Ore

Iron ore is the primary raw material used in the production of iron. It is naturally rich in iron oxides. The most common forms include hematite (Fe extsubscript{2}O extsubscript{3}) and magnetite (Fe extsubscript{3}O extsubscript{4}). Extracting valuable iron involves multiple processes, of which the blast furnace is a pivotal part.
These ores contain not just iron but also varying impurities, which include silica, phosphorus, and sulfur. To produce pure iron, these impurities need to be removed efficiently.
Essential roles of iron ore in metallurgy:

• Provides the fundamental material (iron) for further processing
• Contains both iron and impurities that should be handled
• Serves as a primary input in the blast furnace

Calcium Oxide (CaO)

Calcium oxide, commonly known as lime, plays a crucial role in the metallurgical processes in blast furnaces. It is characterized as a basic oxide due to its tendency to react with acidic oxides found in iron ore impurities.
Its primary role in the blast furnace is as an impurity remover. By adding CaO, the acidic impurities in the iron ore, such as silica, are neutralized. This reaction forms compounds like calcium silicate that do not mix with molten iron, thus can be easily separated. This is an essential step in purifying the iron.
Functions of Calcium Oxide in a blast furnace:

• Acts as a basic agent to neutralize acidic impurities
• Facilitates the formation of slag
• Enhances the purity of extracted iron by removing waste

Slag Formation

Slag formation is a critical outcome of the reactions occurring in the blast furnace during iron extraction. When calcium oxide reacts with impurities present in the iron ore, they transform into slag, a byproduct that can be easily separated due to its different properties from iron.
This process involves lime binding with impurities such as silica to form calcium silicate. Because this compound is less dense than molten iron, it floats on top, allowing for easy extraction.
Importance of Slag Formation:

• Removes impurities from molten iron
• Improves the quality of the produced iron
• Enables the recycling of waste materials into useful byproducts

Slag not only aids in purification but can be processed further for use in various industries, showcasing its versatility beyond the blast furnace.