Production practice has indicated that the main factors affecting the service life of refractory castable lining are generally the quality of the castable itself, construction quality of the lining, curing conditions, production operation, and frequent kiln shutdowns caused by mechanical and electrical failures in the calcination system.
1. Quality and Management of Refractory Castable
Refractory castable is composed of refractory aggregate, refractory powder, binders, additives, and ultrafine powder. These components, especially the refractory aggregate, are made from minerals subjected to high-temperature calcination and are mixed in strict particle size and weight ratios. Some castables also contain steel fibers. If the castable is not properly heat-treated or the particle sizes and additives are not appropriately mixed, it can impact the lining's service life.
The components of refractory castable tend to exhibit hydraulic characteristics. If not stored correctly during transportation or storage, exposure to moisture can cause certain hydraulic materials to coagulate, leading to clumping. These clumps differ significantly from properly dried, loose castable in terms of density, mechanical strength, and thermal conductivity. If these clumps enter the casting process, the affected lining area is prone to cracking, ultimately reducing the lining's service life.
Additionally, if refractory castables are not stored separately by type, cross-contamination can occur, which may also affect the lining's service life.
2. Quality of Lining Casting
During construction, if the contractor rushes the work without following the specified timing, the castable may clump after mixing. Also, water content is often manually controlled during construction, leading to imbalances in the water-to-cement ratio. These substandard casting practices result in a lining that fails to meet practical requirements in terms of moisture content, bulk density, and volume stability, and may cause peeling or spalling during kiln heating or operation.
3. Unstable Operation
Operation of a rotary kiln includes both furnace heating and material-running phases. Practice has shown that operational instability is one of the primary reasons for suboptimal service life of the kiln lining.
1. Furnace Heating
The furnace heating phase is meant to dry out moisture in the kiln lining to achieve the strength and thermal performance required for high-temperature operation. The surface temperature of the lining in the area with the most direct fuel heat input and rapid temperature change should be carefully controlled. In actual production, as the temperature of this lining area cannot be measured directly, kiln exhaust gas temperature is often used as a reference. However, as the exhaust gas temperature at the kiln tail is lower than the gas temperature produced by fuel combustion, the lining in direct contact with the combustion gases may experience a more rapid temperature rise than the kiln tail gas, potentially leading to uneven heating.
2. Daily Operation
As a flame kiln, the rotary kiln relies primarily on hot flue gas produced by burning the volatile content of the processed material (e.g., petroleum coke). The feed rate and raw material properties significantly affect kiln temperature and its stability. Limited by control capabilities, feed rates fluctuate, sometimes by as much as ±35t/h from the set feed rate. This fluctuation causes substantial oscillations in flue gas temperature, leading to sudden temperature changes in the lining, resulting in large thermal shocks that compromise lining stability and can lead to collapse.
3. Auxiliary Air Volume
Moreover, if the auxiliary air volume is not controlled properly, excessive wind pressure when the kiln is underloaded can cause the air to blow directly on the lining on the inner wall of the burner, leading to temperature inconsistencies that reduce the lining's service life.
4. Frequent Kiln Shutdowns Due to Electrical and Mechanical Failures
The continuous rotation of the rotary kiln at a set speed is essential for the mechanical movement of materials inside. Any fault in cooling water pipes and valves, combustion air fans and their power supply, the main drive components, support parts, material handling systems, or the waste heat boiler requires stopping the material feed and sometimes reducing temperature before repairs can begin. If the feed stoppage is prolonged, solely relying on auxiliary fuel to maintain temperature may cause temperature drops (particularly noticeable when using gas as an auxiliary fuel). These frequent temperature fluctuations have a significant impact on the lining. Therefore, it is essential to enhance regular inspections and implement a scheduled maintenance system to reduce the frequency and duration of kiln shutdowns.
In summary, the primary factors affecting the lining's service life in production are:
1. Severe or excessive temperature fluctuations in the lining.
2. Prolonged impact and wear from the materials on the lining.
3. High-temperature stress or structural deformation due to repeated heating.