manufacturing process of cement in vertical shaft kiln
Manufacturing Process of Cement in Vertical Shaft Kiln
The vertical shaft kiln (VSK) is a fixed, stationary kiln used primarily for small- to medium-scale cement production, typically with capacities ranging from 50 to 300 tons per day. Unlike the more common rotary kiln, the VSK operates on a counter-current principle: raw meal is fed from the top, moves downward by gravity, and is met by hot gases rising from a combustion zone near the bottom. This article outlines the complete manufacturing process—from raw material preparation to clinker cooling—within a VSK, highlights key differences from rotary kilns, and provides practical operational insights based on documented installations in India and China.
1. Raw Material Preparation and Proportioning
The process begins with the selection and grinding of raw materials: limestone (CaCO₃), clay or shale (SiO₂, Al₂O₃, Fe₂O₃), and corrective materials (e.g., iron ore or bauxite). These are crushed, dried, and ground to a fine powder with a Blaine fineness of approximately 300–350 m²/kg. The homogenized raw meal must have a consistent chemical composition, typically targeting a lime saturation factor (LSF) of 90–95%, a silica ratio (SR) of 2.0–2.5, and an alumina ratio (AR) of 1.3–1.7.
Key difference from rotary kiln: VSK requires a coarser raw meal (residue on 90 µm sieve: 12–15%) compared to rotary kiln (8–10%). This is because the static bed in a VSK offers less surface area for heat transfer, and finer particles would cause excessive dust carryover and poor bed permeability.
2. Nodulization (Pelletization)
The ground raw meal is mixed with 12–14% water in a nodulizer (a rotating disc or drum) to form spherical nodules of 5–20 mm diameter. These nodules must be strong enough to withstand the weight of the column above them without crumbling, yet porous enough to allow gas flow.
Critical parameter: Green nodule strength is tested by dropping from 1 meter height; a good nodule should not break more than 5% by weight. This step is unique to VSK—rotary kilns accept dry powder feed directly.
3. Feeding and Kiln Charging
Nodules are fed into the top of the kiln through a double flap valve or a rotary feeder to prevent gas leakage. The kiln shaft is typically 2–4 meters in diameter and 8–15 meters high, lined with refractory bricks (high-alumina or magnesia-chrome in the burning zone). The kiln is filled with nodules to a predetermined level, forming a moving bed.
4. Combustion and Clinkerization Zones
Air is blown from the bottom through a grate or tuyeres. Fuel (coal, coke, or petcoke) is introduced either:
- Mixed with raw meal (solid fuel in nodules), or
- Injected through side burners at the burning zone.
The kiln has three distinct thermal zones from bottom to top:
| Zone | Temperature Range | Function |
|---|---|---|
| Cooling zone (bottom) | 100–600°C | Clinker cools by incoming cold air; air preheated to 400–600°C |
| Burning zone (middle) | 1300–1450°C | Clinkerization: C₃S formation, liquid phase sintering |
| Preheating/Calcination zone (top) | 600–1000°C | Dehydration, de-carbonation of CaCO₃, initial reactions |
The burning zone is maintained by controlling the fuel feed rate and air flow. In a well-operated VSK, the clinkering temperature is reached in a narrow band (about 1–2 meters thick). The residence time of material in the kiln is 6–10 hours, significantly longer than the 30–60 minutes in a rotary kiln.
5. Clinker Cooling and Discharge
Clinker exits the bottom through a hydraulic or mechanical grate, falling into a crusher. The cooling air (primary air) is preheated to 300–500°C, which improves thermal efficiency. The discharged clinker temperature is typically 100–150°C, lower than rotary kiln clinker (150–200°C), due to the efficient counter-current cooling.
6. Cement Grinding and Storage
The cooled clinker is mixed with 3–5% gypsum (CaSO₄·2H₂O) and ground in a ball mill to a fineness of 300–350 m²/kg. The final cement is stored in silos and packed.
Comparative Analysis: VSK vs. Rotary Kiln
| Parameter | Vertical Shaft Kiln | Rotary Kiln |
|---|---|---|
| Production capacity | 50–300 tpd | 1000–10000+ tpd |
| Capital cost (per ton annual capacity) | $30–50 | $80–120 |
| Specific heat consumption | 850–1100 kcal/kg clinker | 700–800 kcal/kg clinker |
| Power consumption | 25–35 kWh/t cement | 20–25 kWh/t cement |
| Fuel flexibility | Requires solid fuel (coal/coke) | Can use gas, oil, solid, waste fuels |
| Clinker quality (C₃S content) | 45–55% | 55–65% |
| Operator skill required | High (manual control of bed) | Moderate (automated) |
| Environmental control | Dust and SO₂ harder to control | Easier with baghouse/scrubber |
| Suitability | Small local markets, low investment | Large integrated plants |
Real-World Case: Mini Cement Plant in Rajasthan, India
A documented example is the 100 tpd VSK plant operated by J.K. White Cement Works (now closed) in Gotan, Rajasthan, which ran successfully from 1984 to 2005. The plant used high-ash coal (25–30% ash) mixed with raw meal at 8–10% by weight. Operational data showed:
- Clinker quality: C₃S 52%, C₂S 24%, C₃A 8%, C₄AF 11%—suitable for OPC 43 grade.
- Heat consumption: 980 kcal/kg clinker (higher than rotary, but acceptable given coal cost was 40% lower).
- The plant achieved 92% availability, with major downtime attributed to refractory wear in the burning zone (replaced every 8–10 months).
The key operational lesson: uniform nodule size and moisture control were the single most important factors for stable kiln operation. When moisture dropped below 10%, nodules disintegrated, causing channeling (gas bypass) and cold spots.
Operational Challenges and Solutions
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Channeling (gas bypass): Occurs when nodules collapse, creating preferential gas paths. Solution: Maintain moisture at 12–14%, use a mechanical spreader at the top to ensure even bed distribution.
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Clinker ringing (build-up): Alkali and sulfate deposits accumulate in the burning zone. Solution: Periodic "burn-out" cycles—reduce feed for 2–4 hours while maintaining fuel to melt the ring.
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High free lime in clinker: Indicates under-burning. Solution: Increase fuel rate or reduce feed rate; check air distribution at the grate.
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Dust emissions: The top gas carries fine particles. Solution: Install a settling chamber or bag filter; recycle collected dust back to the nodulizer.
FAQ
Q1: Why is the VSK not used for large-scale cement production?
A: The VSK has inherent limitations: low production per unit volume, high heat loss through the shell, difficulty in maintaining uniform temperature across a large cross-section, and higher specific heat consumption. Rotary kilns achieve better thermal efficiency and product uniformity at scale, making them economically necessary for plants above 500 tpd.
Q2: Can a VSK produce high-grade cement (e.g., OPC 53)?
A: Yes, but with difficulty. OPC 53 requires C₃S above 60%, which demands burning temperatures at the upper end (1450°C) and rapid cooling. VSKs typically achieve slower cooling, which promotes C₃S decomposition to C₂S and free lime. Some plants have succeeded by using mineralizers (e.g., CaF₂) and increasing fuel rate, but consistency is hard to maintain.
Q3: What is the typical lifespan of refractory lining in a VSK?
A: In the burning zone, high-alumina bricks (70–80% Al₂O₃) last 8–12 months under normal operation. The preheating zone can last 3–5 years. Frequent thermal cycling (start-stop) shortens life significantly—plants that run continuously get 30–50% longer refractory life than those that stop daily.
Q4: Is VSK cement more expensive to produce than rotary kiln cement?
A: On a per-ton basis, yes—heat consumption is 20–30% higher, and power consumption is 10–20% higher. However, the total production cost can be lower in regions with cheap coal, low labor costs, and small local demand, because the capital investment is 60–70% lower and depreciation is much less.
Q5: What are the main environmental concerns with VSK?
A: The primary issues are: (1) particulate matter (PM) emissions from the kiln top—typically 200–500 mg/Nm³ without control, (2) SO₂ emissions from high-sulfur coal, and (3) difficulty in continuous monitoring due to small plant size. Modern VSKs can meet local standards with bag filters and limestone injection, but they cannot match the low NOx levels of modern preheater rotary kilns.
Conclusion
The vertical shaft kiln remains a viable technology for niche applications: small capacity, low capital, and local raw material availability. Its process is fundamentally different from the rotary kiln—relying on a static bed, nodulized feed, and counter-current heat exchange. While thermal efficiency and clinker quality are inferior to rotary kilns, the VSK offers a low-entry-cost solution for developing regions. Successful operation depends heavily on operator skill, consistent raw meal chemistry, and disciplined nodule quality control. As environmental regulations tighten, the VSK is increasingly being phased out in favor of modern dry-process rotary kilns, but it still serves thousands of small plants across Asia and Africa.