Analysis of Failed Rotary Cement Kiln

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:

1. Rawmix Preparation: Limestone and clay/shale are ground into fine powder.
2. Pyroprocessing: The rawmix is heated to ~1,450°C in a rotary kiln, forming calcium silicates (clinker).
3. Finish Grinding: Clinker is ground with additives to produce cement.

Rotary Kiln Design and Operation

1. Structure: A steel tube (slightly inclined at 1–4°) lined with refractory bricks.
2. Refractory Function: Protects steel structures from temperatures exceeding 800°C, where steel weakens.
3. 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

1. Initial conveyor issues disrupted the heatup sequence.
2. Resumed heating after a 24-hour pause likely caused uneven thermal expansion.
3. Resulting stresses compromised the refractory lining, exposing the kiln shell to extreme heat (visible as hot spots).

Refractory Zones & Environmental Demand