Question

Limestone is not used in which of the following manufacturing processes? (a) Portland cement (b) Phosphorus from phosphorite (c) Iron from haematite (d) Solvay process of sodium carbonate

Short answer

Limestone is not used in the extraction of phosphorus from phosphorite (b).

Step-by-step solution

Understand the Role of Limestone

Limestone is a sedimentary rock primarily composed of calcium carbonate (CaCO3). It is used in various industrial processes due to its chemical properties. Understanding where limestone is typically used and its functionality in each process is crucial for solving the exercise.

Analyze Each Option

Evaluate the role of limestone in the given options: (a) Portland cement - Limestone is a primary raw material used to provide calcium. (b) Phosphorus from phosphorite - Limestone is not used in extracting phosphorus through this process. (c) Iron from haematite - Limestone acts as a flux, removing impurities in the iron-making process. (d) Solvay process of sodium carbonate - Limestone is used to produce carbon dioxide, which plays a critical role in the Solvay process.

Identify the Exception

Compare the role of limestone in each process. Notably, in option (b), phosphorus extraction from phosphorite does not utilize limestone, while all the other processes mentioned use limestone in various capacities.

Conclusion from Analysis

Based on the analysis, it is clear that limestone is not a part of the phosphorus extraction from phosphorite. Therefore, option (b) is the correct answer.

Key concepts

Limestone Uses

Limestone is a versatile mineral, predominantly comprising calcium carbonate ( CaCO_3 ). It plays an essential role in various industries due to its chemical and physical properties.
Here are some of the primary uses of limestone:

• Construction Material: Limestone is widely used as a building material. Its durability and aesthetic appeal make it a favorite for architectural applications such as in the construction of buildings and monuments.


• Industrial Applications: It serves as a key raw material in the production of Portland cement, providing essential calcium for the mixture.


• Iron Production: In steelmaking, limestone acts as a flux, helping remove impurities from molten iron.


• Sodium Carbonate Manufacture: In the Solvay process, limestone is used to produce carbon dioxide, a critical component for creating sodium carbonate.

Limestone has also other, more specialized uses such as in agriculture for soil conditioning or in the paper industry. Its diverse applicability makes it indispensable in modern industrial operations.

Solvay Process

The Solvay process is an important industrial method for producing sodium carbonate ( Na_2CO_3 ), also known as soda ash. This compound is widely used in glass manufacturing and as a cleaning agent. The process is named after Belgian chemist Ernest Solvay, who developed it in the mid-19th century.

The core reactions in the Solvay process are:

• Carbonation: Limestone (calcium carbonate) is heated to produce carbon dioxide ( CO_2 ), which is then bubbled into a solution of ammonia and saltwater. This reacts to form ammonium bicarbonate.


• Precipitation: Ammonium bicarbonate reacts with salt to form sodium bicarbonate, which precipitates out due to its lower solubility.


• Calcination: The sodium bicarbonate is then heated to produce sodium carbonate, releasing water and carbon dioxide in the process.

Ammonia is recovered and recycled back into the process, making the Solvay process economically efficient. The use of limestone is critical in producing the carbon dioxide necessary for the initial carbonation step.

Portland Cement

Portland cement is a fundamental material used in construction, known for its strength and resistance. It is the key ingredient in concrete, which is used globally in infrastructure projects such as roads, bridges, and buildings.

The manufacture of Portland cement involves several steps:

• Raw Material Preparation: Limestone and other materials are crushed and blended to create a specific chemical composition necessary for cement quality.


• Heating: The mixture is heated in a rotary kiln at high temperatures, leading to the formation of clinkers. This chemical reaction involves decomposing limestone to release carbon dioxide and form calcium oxide ( CaO ).


• Grinding: Clinkers are cooled and ground with gypsum to produce the fine powder known as Portland cement.

Each step is meticulously controlled to ensure the quality of the final product. The use of limestone, with its high calcium content, is crucial for achieving the necessary chemical composition and physical properties of Portland cement.

Phosphorus Extraction

Phosphorus is a non-metal element essential for life, playing a vital role in biological processes and agricultural applications. It is usually extracted from phosphate rocks, such as phosphorite, rather than relying on limestone.

The extraction process generally involves:

• Mining: Phosphate rock is extracted and then processed to remove impurities.


• Chemical Processing: The rock is treated with sulfuric acid to produce phosphoric acid, which reacts with other compounds to generate phosphorus fertilizers.

Limestone is not used in the extraction of phosphorus from phosphorite. Instead, the process relies on other chemical reactions and treatments to purify and extract phosphorus compounds. The distinction makes phosphorus extraction unique compared to other industrial processes involving limestone.

Iron Production

Iron production is a critical industrial operation used to create iron, an essential metal for various applications, including construction and manufacturing. The process starts with the extraction of iron from its ore, typically haematite ( Fe_2O_3 ).

One of the key steps in iron production occurs in a blast furnace, where:

• Iron Ore Processing: Haematite is combined with coke (a carbon source) and limestone.


• Smelting: The mixture is heated in a blast furnace. As the temperature rises, the carbon from the coke reduces the iron oxide to form molten iron.


• Role of Limestone: Limestone acts as a flux, removing impurities such as silica from the ore by forming a slag. This helps in the efficient and clean extraction of molten iron.

Limestone’s function in this process is crucial, as it not only helps enhance the purity of the iron but also contributes to the efficiency of the entire operation. Without limestone, the removal of impurities would be challenging and would compromise the quality of the iron produced.