Question

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

Short answer

Limestone is not directly used in the Solvay process (d).

Step-by-step solution

Identify the Substance's Role

Limestone is primarily composed of calcium carbonate (CaCO₃). In order to solve this problem, we first need to understand the role of limestone in each of the given manufacturing processes. We will examine each option to determine where limestone is used.

Evaluate Each Option

- **Portland cement (a):** Limestone is a primary ingredient, providing calcium oxide after heating. - **Phosphorus from phosphorite (b):** Limestone is used as a flux in removing impurities in the production of phosphorus. - **Iron from haematite (c):** Limestone is used in the blast furnace as a flux to remove impurities from iron ore. - **Solvay process of sodium carbonate (d):** Limestone provides CO₂, which is necessary for this process; however, it is not directly used in the manufacturing of the end product, sodium carbonate, without being converted.

Determine the Outcome

The Solvay process is the one where limestone is not directly used in its original form for manufacturing the end product (sodium carbonate). In the Solvay process, limestone is initially converted to lime (calcium oxide) and carbon dioxide, and it is these products that are used in the process.

Key concepts

Portland Cement

Portland cement is a key binder in the construction industry, known for its durability and strength. The process begins with the mining of raw materials such as limestone, clay, and sand. These materials are heated in a kiln at high temperatures up to 1450°C to form clinker.
The main chemical reaction occurring here is the transformation of calcium carbonate from limestone into calcium oxide (lime), releasing carbon dioxide. The lime then combines with other elements like silica, alumina, and iron oxide from clay and sand to form clinker minerals.
After cooling, the clinker is mixed with gypsum and ground into a fine powder, forming Portland cement. This cement, when mixed with water, forms a paste that hardens over time due to hydration reactions, offering structural stability for buildings and infrastructure.

Solvay Process

The Solvay process is a widely used method for producing sodium carbonate (soda ash), a key chemical in glassmaking, detergents, and paper production. This process occurs in a series of chemical reactions that involve salt (sodium chloride), ammonia, and limestone.
Limestone provides the CO₂ needed for the process. Initially, limestone is heated to produce lime (calcium oxide) and carbon dioxide.
The CO₂ is bubbled through a concentrated solution of brine saturated with ammonia, causing the precipitation of sodium bicarbonate. This compound is then heated to form sodium carbonate.

• In the Solvay process, ammonia is recycled, making it economical and environmentally friendly.
• The overall chemical equation is NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl.

It is important to note that while limestone facilitates carbon dioxide production, it is not directly used to make the final product.

Iron Extraction

Iron extraction is a critical industrial process, necessary for producing steel and other iron products that are foundational to modern infrastructure. The extraction occurs in a blast furnace where a combination of iron ore (mainly haematite), coke, and limestone are fed into the top.
As the materials descend in the furnace, a series of reactions reduce iron ore to iron and remove impurities. Limestone acts as a flux, reacting with the impurities (silica, alumina) in the ore to form slag, which is less dense and floats on the molten iron.
The iron tapped off at the bottom of the furnace is then further refined to produce steel. This process is energy-intensive but crucial for extracting the most commonly used metal globally. Key reactions involve the reduction of iron oxides, typically represented by Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Phosphorus Production

Phosphorus production is primarily conducted using phosphate rocks like phosphorite, which are rich in phosphorus minerals. These rocks are often processed in high-temperature furnaces with silica and a carbon source to extract phosphorus.
Limestone is typically included as a flux in this process to help remove impurities from the melted rock. During the thermal processing, the phosphorus component is chemically reduced and vaporized, then condensed into a solid form, often known as "white" or "yellow" phosphorus.

• This form of phosphorus is then used as a base for various chemical compounds like phosphoric acid, which is essential for fertilizers and other chemical manufacturing.
• Due to its reactivity, phosphorus must be handled carefully, stored underwater or within inert gases to prevent inadvertent combustion.

The production of phosphorus is vital for agriculture and various industrial applications.