
Rotary Kilns Operate 24/7 and are Stopped only for a Few Days for Maintenance
Overview
A rotary cement kiln failure occurred during startup operations following conveyor system malfunctions. Initial heatup attempts were suspended due to feeding issues but resumed 24 hours later. This restart led to observable hot spots on the kiln shell, signaling refractory lining degradation.
Cement Kiln Fundamentals
Cement production involves three stages:
- Rawmix Preparation: Limestone and clay/shale are ground into fine powder.
- Pyroprocessing: The rawmix is heated to ~1,450°C in a rotary kiln, forming calcium silicates (clinker).
- Finish Grinding: Clinker is ground with additives to produce cement.
Rotary Kiln Design and Operation
- Structure: A steel tube (slightly inclined at 1–4°) lined with refractory bricks.
- Refractory Function: Protects steel structures from temperatures exceeding 800°C, where steel weakens.
- Process Flow:
- Rawmix enters the upper end; rotation moves material downhill.
- Fuel (gas, oil, or pulverized solids) is injected through a burner pipe at the lower end, creating a high-temperature flame.
- Clinker exits into a cooler, where air is preheated (400–800°C) before entering the kiln to optimize combustion.
Startup Challenges & Thermal Management
Kilns operate continuously (24/7) to avoid thermal stress from repeated heating/cooling cycles. Startup requires:
- Gradual temperature increases to minimize thermal gradients.
- Controlled rotation and feed introduction.
- Strict monitoring to prevent refractory damage from rapid expansion.
Root Cause of Failure
- Initial conveyor issues disrupted the heatup sequence.
- Resumed heating after a 24-hour pause likely caused uneven thermal expansion.
- Resulting stresses compromised the refractory lining, exposing the kiln shell to extreme heat (visible as hot spots).
Refractory Zones & Environmental Demand
Zone | Conditions | Refractory Type |
---|---|---|
Upper Transition | Moderate Thermal Cycling | High Thermal Shock Resistance |
Burning Zone | Peak Temperatures (~1,450 C) | Dense, High Alumina Bricks |
Lower Transition | Abrasion + Chemical Exposure | Abrasion Resistant Materials |
Key Takeaways
Refractory Integrity: Critical to kiln longevity; localized failures risk shell deformation.
- Startup Protocols: Interruptions demand reevaluation of heating curves to prevent stress buildup.
- Monitoring: Infrared imaging during startups can detect early hot spot formation.
- This incident underscores the need for rigorous procedural adherence during kiln restarts, particularly after unplanned shutdowns.
More can be Learned about Rotary Kilns at ScienceDirect.
O’Donnell Consulting Performs Failure Analysis Services on Equipment including Vessels, Heat Exchangers, Cyclones, Conveyors and Kilns.
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