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Case study on how to select durable alkaline bricks for different zones of a cement kiln.
2026-03-16
Cement rotary kilns not only produce cement but also play a crucial role in processing waste and by-products from other industries. In Japanese cement kiln production, the unit amount of waste and by-products used has been increasing year by year, and this trend is expected to continue. Furthermore, in recent years, the price of coal used as fuel has remained high, leading to an increase in the use of fuels such as petroleum coke and waste plastics in cement kilns. Consequently, unstable coating and changes in the gas composition within the furnace sometimes occur, resulting in increasingly harsh operating conditions for refractory materials. To address these changing conditions, the furnace materials have been repeatedly improved. This paper introduces alkaline bricks used in cement rotary kilns.
1. The inner lining of a cement rotary kiln
Figure 1 shows the lining of a typical cement rotary kiln. In the lower-temperature calcining zone, high-alumina bricks or clay bricks are generally used. In the burning zone, where the thermal load is highest, magnesia-chrome bricks with good corrosion resistance and coating adhesion were previously widely used. However, due to increased environmental awareness, magnesia-chrome bricks have largely disappeared from cement kilns, and magnesia-spinel bricks are now mainly used in the cooling zone, burning zone, and transition zone.
2. The quality of alkaline bricks used in rotary kilns
Table 1 shows the performance and characteristics of alkaline bricks used in cement rotary kilns. ECOLOK (ELK) is a chromium-free brick designed for the burning zone, while SPILOK (SP) is a spinel brick primarily used in the cooling and transition zones. All bricks have been adjusted in terms of thermal and physical properties to meet the operating conditions of various cement rotary kiln types and zones. The performance and application history of the bricks used in each zone are described below.
① Bricks for the Cooling Zone
The operating temperature of the cooling zone is approximately 1300℃, which is quite high. Because the surface of the bricks is difficult to protect with a coating, thermal wear of the cement clinker becomes a significant cause of damage. In addition, the furnace shell near the cooling zone is continuously heated for extended periods, gradually deforming into a flared shape. This causes the bricks to loosen, and sometimes leads to brick slippage and damage during operation.
Bricks used in the cooling zone require good wear resistance and thermal shock resistance. To improve coating adhesion, SP-8DC containing a suitable amount of impurities and SP-8D made from high-purity raw materials were used. Figure 2 shows the condition of SP-8DC used in the cooling zone. The left image shows damage due to thermal wear, and the right image shows damage due to crushing. Although the causes of damage differ, the amount of damage is less than that of other bricks adjacent to the SP-8DC.
② Chrome-Free Bricks for the Burning Zone
The burning zone is the area with the highest thermal load in a cement rotary kiln, with temperatures reaching up to 1450°C. For the magnesia-spinel bricks used in the burning zone, the Al2O3 content is optimized considering corrosion resistance and thermal shock resistance. Trace amounts of special components are added to improve coating adhesion and further enhance corrosion resistance. To compare the corrosion resistance of the burning zone bricks shown in Table 1, a rotary drum corrosion test was conducted (1700°C, 5 hours, corrosive agent: silicate cement). Figure 3 shows the damage amount for each material. ELK-11CW, with the least damage, is suitable for the hottest burning point, while ELK-12CXR, with the second least damage, is suitable for the support points where mechanical stress occurs. Furthermore, ELK-12CX-1 offers a superior balance of corrosion resistance and coating adhesion, making it versatile for a wide range of areas in the burning zone.
Figure 4 shows the durability of ELK-12CX-1 in a harsh location within the cement kiln's burning zone (left image) and in a less demanding location (right image). ELK-12CX-1 not only exhibits less damage than adjacent bricks, but its coating also adheres appropriately, contributing to stable operation of the burning zone.
③ Bricks for the Transition Zone
Due to the increased consumption of waste and by-products in recent years, the transition zone, where the furnace atmosphere changes most significantly, has seen a decrease in brick lifespan in most kilns. Alkaline sulfates [K3Na(SO4)2, K2SO4] and alkali metal chlorides (KCl, NaCl), as volatile components in the furnace gases, infiltrate and deposit into the bricks, causing embrittlement and cracking. This is one of the reasons for the damage to bricks in the transition zone. The impurities contained in the bricks react with the volatile components in the furnace gases, accelerating the deterioration of these bricks. Therefore, to suppress these reactions, bricks used in the transition zone are characterized by having very few impurities. Based on the general-purpose SP-8L material with good sulfur resistance in the transition zone, SP-8LDF with superior stress relaxation properties for support points and SP-8LS with high hot strength and good wear resistance have been developed.
Figure 5 shows the durability of SP-8L used in the transition zone; compared to other adjacent bricks, it has a greater remaining thickness. Observation of the used SP-8L shows slight surface embrittlement and good durability.
3. Conclusion
Although the number of cement kilns has decreased compared to its peak, the equipment utilization rate remains high, supported by strong cement demand. When the equipment utilization rate is high, an emergency shutdown due to equipment failure can significantly impact kiln operation. Failures caused by insufficient refractory brick durability, in particular, require several days of repair, so such failures should be avoided as much as possible. Improving the lifespan of refractory bricks remains an important research topic. The refractory bricks described in this article, due to their excellent durability, not only reduce the risk of kiln shutdown failures but also offer long-term cost advantages. Both their quality and durability have received consistent praise from users. In the future, we will strive to ensure stable operation and improve cost competitiveness for all our customers.