Top Causes Of Refractory Shedding in Lime Kilns & Proven Solutions

Part 1: Cause Analysis of Refractory Shedding in Lime Kilns

1.1 Refractories in the Preheating Zone

The original refractory configuration in the preheating zone consists of lightweight high-alumina bricks for the insulation layer and high-quality fire clay bricks for the working layer. Post-shutdown inspections inside the kiln revealed three main failure modes of fire clay bricks: collapse, bulging, and the formation of bottom cavities, all of which ultimately lead to shedding. The root causes are as follows:

1)The lightweight high-alumina bricks have low compressive strength and bulk density, which results in premature damage before the fire clay bricks.

2)The installation of 6 (or 3) lifters per brick ring in the preheating zone (see Figure 1) requires reprocessing of 12 (or 6) bricks per ring. Without lifters, only 2-3 fire clay bricks would need reprocessing. The increased number of joints and reprocessed bricks (with reduced dimensions) make the structure more susceptible to shedding due to uneven thermal expansion.

3)Shedding occurs due to inconsistent thermal expansion along the thickness of the fire clay bricks. Temperature gradients across the kiln lining thickness create layered structures with varying thermal expansion rates. Over time, this induces internal stress in specific layers, leading to shedding.

1.2 High-Alumina Bricks in the Calcining Zone

1.2.1 Chemical Erosion

The mineral composition of spallation-resistant high-alumina bricks primarily includes mullite (3Al₂O₃·2SiO₂), corundum (Al₂O₃), and a glassy phase. Solid-state reactions occur between the calcareous components and the brick lining, altering the original microstructure of the refractory bricks. When the composition of the feedstock entering the kiln is improper or the kiln temperature is excessively high, excessive liquid phase penetration and deposition occur, leading to loose and brittle structures in the high-alumina bricks. This reduces the service life of the brick lining. On the other hand, unstable composition or feeding rates of raw materials can lead to overheating of the kiln. The SiO₂ component in the materials reacts with the Al₂O₃ component in the kiln bricks to form secondary mullite, causing recrystallization. This volume expansion of the brick lining results in cracking.

1.2.2 Mechanical Damage

Observation of the surface of cracked refractory bricks reveals cracks, edge/chip damage, and even partial breakage of many bricks. This indicates that the refractory bricks were subjected to torsional forces during the transportation of the kiln shell. These significant torsional forces exert complex combined stresses (compression, bending, tension, and shear) on the brick lining, which expands internal cracks and causes structural damage to the refractory bricks.

1.2.3 Thermal Stress Damage

During kiln shutdown or equipment failure, cold air rapidly enters the kiln, causing a sudden drop in the temperature of the brick lining. The surface layer of the bricks contracts, and excessive thermal stress between the surface layer and the hot inner layer of the kiln body leads to detachment. As a result, large areas of the brick surface, which have been embrittled by chemical erosion, fracture and spall at the interface.

Part 2: Selection of New Refractories and Improvement Measures

2.1 New Refractories for the Preheating Zone

To address the causes of fire clay brick shedding in the preheating zone, the following two improvement measures have been adopted:

Eliminate the lightweight high-alumina bricks in this area and redesign the fire clay bricks for this position. The old and new brick types are shown in Figures 2 and 3, respectively.

Remove the lifters in this area, as shown in Figure 4.

Through the two improvements mentioned above, the service life of the fire clay bricks in this area has been extended from 4 months to over 1 year. Furthermore, no brick shedding has occurred, even during multiple fault-handling processes involving significant temperature fluctuations.

2.2 New Refractories for the Calcining Zone

Based on the aforementioned cause analysis, it is essential to select a suitable material to replace the high-alumina bricks. From the perspective of on-site application, magnesia-alumina spinel (chemical formula: MgO·Al₂O₃, with MgO content of 28.2% and Al₂O₃ content of 71.8%) is an excellent alternative. It features a high melting point (2135℃), a low thermal expansion coefficient, superior thermal shock resistance, and strong alkali corrosion resistance. However, its thermal conductivity is higher than that of high-alumina bricks. Therefore, an insulation felt is used in the lower part during masonry. This insulation felt has a certain level of compressive strength, excellent thermal insulation properties, and a service life comparable to that of magnesia-alumina bricks. The masonry method is illustrated in Figure 5. A comparison of the physical and chemical properties of several materials is presented in Table 1.

The insulation felt demonstrates excellent performance: compared to the same area without insulation felt, the temperature is reduced by 20℃. For the kiln shell surface where the insulation felt is applied, the maximum temperature is lowered by 65℃, with an average reduction of 30℃, allowing the kiln shell surface temperature of the lime kiln to drop below 350℃. A comparison of the kiln shell surface temperatures before and after the application of the insulation felt is shown in Figure 6.

2.3 Application Effects

By optimizing the configuration and masonry of refractory materials in the lime kiln, the operation rate of the rotary kiln has been improved, ensuring a steady supply of steelmaking lime and sintered lime. The service life of the refractory materials in the rotary kiln has been extended to over 1 year, and the surface temperature of the lime kiln shell has been maintained below 350℃. Additionally, this optimization has reduced both gas consumption and the heat consumption per ton of lime (by approximately 2%). With an annual output of 250,000 tons of lime per kiln, the application of new thermal insulation materials has lowered the cost by 9.14 yuan per ton, resulting in an annual cost reduction of about 2.3 million yuan per kiln.

Part 3: Conclusion

Based on the service conditions of refractory materials in different parts of the lime kiln, and through research on the re-selection of refractory materials, redesign of brick types, application of new thermal insulation materials, and improvement measures, strategies to extend the service life of the rotary kiln have been proposed. As a result, the service life of refractory materials in the lime kiln has been increased from 4 months to over 1 year. Meanwhile, the surface temperature of the calcining zone on the rotary kiln shell has been reduced to below 350℃, heat consumption per ton of lime has decreased by approximately 2%, and production costs have been reduced by 9.14 yuan per ton. Additionally, the safety of the refractory materials used in the rotary kiln has been improved and guaranteed.