What is the Manufacturing of cement?

Cement manufacturing industry is a huge industry nowadays and production of cement is increasing day by day as the demand of cement is also increasing rapidly.

The cement manufacturing industry is currently experiencing rapid growth due to the high demand for cement. Factors such as globalization, modernization, population growth, and economic development contribute to this increase in demand. As cement plays a critical role as the main ingredient in concrete, it is essential that it possesses all the necessary binding properties and meets the criteria of a high-quality binder.

Manufacturing of cement:

Cement manufacturing is a complex industrial process that involves various stages and adheres to strict safety and environmental regulations. These stages include collecting materials, processing them, grinding them, heating and cooling them, and finally packing the cement.

Stages of Cement Manufacturing:

There are six main stages of the cement manufacturing process.

Stage 1: Raw Material Extraction from quarry

• The primary materials used in cement manufacturing are limestone (Calcium), sand, clay, shale, fly ash, bauxite, etc. The specific combination and proportion of these raw materials depend on the type of cement and its brand.
• These natural rocks are extracted from quarries and then crushed into smaller pieces, typically around 6 inches in size.
• They are further crushed in hammer mills or crushers to reduce the size to around 3 inches.
• The smaller particles are then prepared for pyro processing.

Stage 2: Grinding, Proportioning, and Blending

• The crushed raw ingredients are combined together in proportion depending upon grade, type and use of cement.

• To prepare the raw materials for cement production, they are first crushed and combined with additives. This mixture is then ground to achieve a fine and homogenous consistency. The exact composition of the cement is determined based on the desired properties. Typically, limestone forms around 80% of the mixture, while clay makes up the remaining 20%. At the cement plant, the raw mixture is dried to reduce its moisture content to less than 1%. This dried mixture is then blended using heavy wheel-type rollers and rotating tables. The rollers crush the mixture to a fine powder, which is stored in silos and later fed into the kiln for further processing.

Stage 3: Pre-Heating Raw Material

The pre-heating chamber incorporates a series of cyclones to efficiently utilize the hot gases generated by the kiln.

This innovative approach reduces energy consumption and promotes a more environmentally-friendly cement production process. Within the chamber, the raw materials undergo transformation into oxides, preparing them for subsequent burning within the kiln.

Stage 4: Kiln Phase i

During the kiln phase, clinker production occurs through a series of chemical reactions involving calcium and silicon dioxide compounds. The process can be summarized as follows:

1. Evaporation of free water.

2. Release of combined water in the argillaceous components.

3. Conversion of calcium carbonate (CaCO3) to calcium oxide (CaO) through calcination.

4. Formation of dicalcium silicate through the reaction of CaO with silica.

5. Formation of the liquid phase through the reaction of CaO with aluminum and iron-bearing constituents.

6. Creation of clinker nodules.

7. Evaporation of volatile constituents, such as sodium, potassium, chlorides, and sulfates.

8. Formation of tricalcium silicate as excess CaO reacts with dicalcium silicate.

 

These events occur in four distinct stages based on temperature changes within the kiln:

1. 100°C (212°F): Evaporation of free water.

2. 100°C (212°F) – 430°C (800°F): Dehydration and formation of silicon, aluminum, and iron oxides.

3. 900°C (1650°F) – 982°C (1800°F): Evolution of CO2 and production of CaO through calcination.

4. 1510°C (2750°F): Formation of cement clinker.

 

The kiln is inclined at a 3-degree angle to allow the materials to move through it over a period of 20 to 30 minutes. By the time the raw mix reaches the lower part of the kiln, it has transformed into clinker, which exits the kiln in the form of marble-sized nodules.

Stage 5: Cooling and final grinding

After leaving the kiln, the clinker undergoes rapid cooling from 2000°C to 100°C-200°C by exposing it to air. At this point, various additives are mixed with the clinker and ground together to create the end product, cement. One important additive is gypsum, which is blended with the clinker to control the setting time and enhance the compressive strength of the cement. It also prevents the clinker powder from clumping together and coating the surfaces of the grinding equipment. Grinding aids, such as Triethanolamine, are sometimes added to prevent powder agglomeration. Other additives like ethylene glycol, oleic acid, and dodecyl-benzene sulphonate may also be used.

The heat generated by the clinker is recirculated back into the kiln to maximize energy efficiency. Finally, the clinker passes through a grinding process in rotating drums with steel balls in a cement plant. This process reduces the clinker into an extremely fine powder, where each pound of cement contains billions of grains. This powdered product is the final result of the cement manufacturing process.

Stage 6: Packing and Shipping

After the grinding process, the cement is transferred to silos, which are large storage tanks. From there, it is packaged in bags weighing between 20 to 40 kilograms. While the majority of the cement is transported in bulk quantities using trucks, trains, or ships, a smaller portion is packaged for customers who require smaller quantities.

Chemical Reactions during Cement Manufacturing Process

The reactions that take place (after evaporation of free water) between the reactants in the kilning phase of cement making process are as follows:

1. Clay Decomposition:
   Si₂Al₂O₅(OH)₂ → 2 SiO₂ + Al₂O₃ + 2 H₂O (vapor)
   KAlSi3O8 (orthoclase) + 0.5 SO₂ + 0.25 O₂ → 3 SiO₂ + 0.5 Al₂O₃ + 0.5 K₂SO₄
2. Dolomite Decomposition:
   CaMg(CO₃)₂ → CaCO₃ + MgO + CO₂
   KMg₃AlSi₃O₁₀(OH)₂ + 0.5 SO₂ + 0.25 O₂ → 0.5 K₂SO₄ + 3 MgO + 0.5 Al₂O₃ + 3 SiO₂ + H₂O (vapor)
3. Low-Temperature Calcite Decomposition:
   2 CaCO₃ + SiO₂ → Ca₂SiO₄ + 2 CO₂
   2 MgO + SiO₂ → Mg₂SiO₄
   Ca₅(PO₄)3OH + 0.25 SiO₂ → 1.5 Ca₃(PO₄)₂ + 0.25 Ca₂SiO₄ + 0.5 H₂O (vapour)
4. Alumina and Oxide Reaction:
   12 CaCO₃ + 7 Al₂O₃ → Ca₁₂Al₁₄O₃₃ + 12 CO₂
   4 CaCO₃ + Al₂O₃ + Fe₂O₃ → Ca₄Al₂Fe₂O₁₀ + 4 CO₂
   4 CaCO₃ + Al₂O₃ + Mn₂O₃ → Ca₄Al₂Mn₂O₁₀ + 4 CO₂
5. The reaction of Remaining Calcite:
   CaCO₃ → CaO + CO₂
6. Sintering:
   Ca₂SiO₄ + CaO → Ca₃SiO₅