Kiln Process
Thermochemical Reactions in Cement Manufacturing
Kiln Process Thermochemical Reactions
Temperature ranges and chemical reactions in the kiln process
| Process | Reactions | Temperature °C | Temperature °F |
|---|---|---|---|
| Drying/Pre-heat | Free water evaporates | 20 - 100 | 68 - 212 |
| Crystallization water driven out | 100 - 300 | 212 - 572 | |
| Chemical water driven out | 400 - 900 | 752 - 1652 | |
| Calcining | CO₂ Driven out CaCO₃ → CaO + CO₂ |
600 - 900 | 1112 - 1652 |
| Sintering/Clinkering | Formation of Liquid Phase, Formation of alite and belite | 1450 (exothermic) | 2642 (2440 - 2642) |
| Cooling | Crystallization of aluminates and ferrites | 1300 - 1240 | 2264 - 2372 |
Important Note:
Carbon Steel Temperature limit: 800°F (427°C). Refractory lining is required above this temperature.
Generalized Diagram of Long Dry Process Kiln
Dehydration Zone
Free water evaporates, formation of initial compounds
Gas Temp:
450°C / 840°F
450°C / 840°F
Material Temp:
50°C / 120°F
50°C / 120°F
Calcination Zone
Limestone decomposition, CO₂ release
Gas Temp:
800°C / 1470°F
800°C / 1470°F
Material Temp:
600°C / 1110°F
600°C / 1110°F
Clinkering Zone
Formation of C₂S and C₃S, liquid phase formation
Gas Temp:
1200°C / 2190°F
1200°C / 2190°F
Material Temp:
1000°C / 1830°F
1000°C / 1830°F
Cooling Zone
Rapid cooling of clinker, crystallization
Gas Temp:
1750°C / 3180°F
1750°C / 3180°F
Material Temp:
1350°C / 2460°F
1350°C / 2460°F
Preheater System
Preheater Tower
Preheaters perform 40-80% calcination
- Multiple stages for heat exchange
- Counter-current gas flow
- Efficient heat recovery system
- Reduces fuel consumption
Precalciner
Precalciners perform 70-96% calcination
- Higher calcination efficiency
- Separate combustion chamber
- Reduces kiln thermal load
- Increases production capacity
Factors Affecting Calcination
The difference between preheater and precalciner performance depends on:
- Number of preheater stages
- Type of calciner design
- Available kiln length
- Fuel type and quality
Rotary Kiln Specifications
Rotary Kiln
Key Components
Physical Structure
- Thick steel cylinder lined with refractory materials
- Diameter: 3m to 8m
- Length: 30m to 200m
- Mounted on roller bearings
- Rotates about its own axis at specified speed
Fuel Types
- Powdered coal
- Fuel oil
- Natural gas
- Alternative fuels (waste-derived)
Operating Temperature:
Up to 1500°C in the hottest zone
Physical and Chemical Changes in the Rotary Kiln
150°C
Limestone loses water in its combination
450°C
Clay decomposes
2SiO₂·Al₂O₃·2H₂O → 2SiO₂ + Al₂O₃ + 2H₂O ↑
650°C
Magnesium carbonate decomposes
MgCO₃ → MgO + CO₂ ↑
950°C
Calcium carbonate decomposes
CaCO₃ → CaO + CO₂ ↑
1250°C
20-30% of raw materials become slurry, clinker balls start to form
Final Step
Grinding clinker with 2-3% gypsum to produce cement
Clinker Cooling Process
Cooling Requirements
Moderate rate of cooling results in higher strength clinker.
Stage 1: Rapid Cooling
From 1200°C to 500°C in approximately 15 minutes
Stage 2: Slow Cooling
From 500°C to normal atmospheric temperature in approximately 10 minutes
Rate of Cooling Influences:
- The degree of crystallization
- The size of the crystal
- The amount of amorphous materials present in clinker
Clinker Cooler
Controlled cooling for optimal clinker quality
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