White lime is obtained by calcining and decomposing calcium carbonate rock at around 1200℃, producing calcium oxide and carbon dioxide. The highest firing zone temperature in both vertical shaft furnaces and rotary kilns does not exceed 1250℃, while the transition zone and preheating zone are ≤900℃. The temperature gradient is significant, requiring segmented matching of refractory materials for the lining.
I. Phosphate Composite Bricks are the First Choice for High-Temperature Zones
The firing zone directly bears the erosion from flames and materials, and phosphate composite refractory bricks with a bulk density ≥2.8 g·cm⁻³ are commonly used. The high-alumina matrix and appropriate amount of silicon carbide work synergistically to form a dense glassy phase at 1200℃, providing resistance to alkali corrosion and wear, with a service life of over 3 years.
II. Comparison of Transition Zone Schemes
1. Rotary Kiln
New kilns still primarily use high alumina bricks (secondary Al₂O₃ ≥ 65%) or ordinary phosphate bricks, with a brick shape of 345 mm long and 75 mm thick. This facilitates individual brick replacement during maintenance and minimizes kiln body deformation. In recent years, some older kilns have switched to integral castable refractory to achieve roundness due to excessive ellipticity of the kiln shell. While this improves airtightness, it requires large-scale dismantling for later maintenance, and secondary baking can easily lead to additional stress on the kiln shell, causing users to adopt a cautious approach.
2. Vertical Shaft Furnace
The transition zone temperature is lower. Traditionally, 345 mm × 75 mm high alumina refractory bricks are sufficient for two years of operation. Some manufacturers have increased the thickness to 100 mm, increasing the weight of a single brick by 30%, which can extend the lifespan by 6–8 months. Construction still uses the original brick shape, without requiring changes to the furnace shell structure.
III. Emergency Measures – Spray Coating
When production tasks are urgent and shutdown windows are insufficient, refractory spray coatings can be used for online repairs.
Scope of Application: Residual lining thickness ≥ 50 mm, local depression depth ≤ 60 mm.
Construction Key Points:
1. Remove slag and loose layers, retaining a hard base;
2. Spray gun nozzle diameter ≤ 40 mm, aggregate particle size ≤ 5 mm, layered spraying, each layer ≤ 30 mm, total thickness ≤ 80 mm, to prevent delamination under its own weight;
3. After natural curing for 2 hours, the temperature can be increased, and production can be resumed at a rate of 50 ℃·h⁻¹.
Limitations: The sprayed coating has not undergone high-pressure molding and sintering, and its strength and fatigue resistance are only 40–50% of that of fired bricks. It typically only lasts for 3–6 months and is considered a stopgap measure.
IV. Overhaul Decision
During a major overhaul, the entire sprayed coating layer should be removed down to the hard base, and high-alumina bricks or phosphate refractory bricks should be used for re-laying. Fired bricks, sintered at temperatures above 1300℃, exhibit significantly superior erosion and thermal shock resistance compared to sprayed bricks. This allows for restoration of the designed production cycle and reduces hidden production losses caused by frequent emergency repairs.
V. Selection Process
1. Temperature Zoning: High-temperature zones are zoned with phosphate composite bricks; transition zones are zoned with secondary high-alumina bricks or ordinary phosphate bricks.
2. Kiln Deformation Assessment: New kilns should prioritize brick lining; for older kilns, only those with an ellipticity greater than 0.3% of the diameter should be considered for integral castable refractory.
3. Maintenance Level Differentiation: Routine small-area spalling → partial brick replacement; deep wear but tight production schedule → emergency spraying; annual major overhaul → complete replacement and rebuilding.
Following this sequence, safe, long-term, low-maintenance operation of the lime kiln lining can be achieved in alkaline, medium-erosion environments below 1200℃.
