To choose high alumina bricks suitable for kilns, you need to consider many factors comprehensively:
1. Temperature conditions: The temperature condition of the kiln is one of the key factors in choosing high alumina refractory bricks. If the kiln is in a continuous high-temperature environment, such as the temperature is above 1500℃ for a long time, such as the top of the steelmaking open-hearth furnace, the cover of the electric furnace, etc., you should give priority to special-grade (Al₂O₃≥85%) or first-grade (75%-85%) high-alumina bricks. They have extremely high refractoriness, and the special-grade refractoriness exceeds 1790℃. They can withstand such extreme high temperatures. At the same time, their load softening temperature is also high, and they can maintain good structural stability at high temperatures and are not easy to deform and collapse. If the temperature of the kiln fluctuates greatly, such as the soaking furnace, the temperature changes frequently between 1350-1500℃, then the second-grade (60%-75%) or third-grade (48%-60%) high-alumina bricks may be more suitable, but you should pay attention to their thermal shock resistance. You can choose high-alumina bricks that have been treated with special processes, such as adding ZrO₂ and other substances to improve thermal shock resistance. ZrO₂ will undergo phase change when the temperature changes, absorb thermal stress, and reduce cracks and spalling caused by thermal shock in the brick body.
2. Chemical environment: The chemical atmosphere in the kiln cannot be ignored for its erosion effect on high alumina bricks. In an alkaline slag environment, such as the transition zone of a cement kiln, there is CaO-Fe₂O₃ slag. High-alumina bricks contain a high content of Al₂O₃, which can form a solid solution with CaO and inhibit the formation of low-melting-point phases, thereby resisting the erosion of alkaline slag. At this time, conventional high-alumina bricks can be used. However, in an acidic gas environment, such as the presence of H₂S gas in a synthetic ammonia converter, the erosion resistance of ordinary high-alumina bricks is limited. High-alumina bricks containing SiC whiskers and other components can be selected. SiC whiskers can enhance the acid corrosion resistance of the brick body. Or when using ordinary high-alumina bricks, surface protection measures, such as acid-resistant coating, can be used to improve its resistance to acidic gas erosion.
3. Mechanical stress: Consider the scouring and mechanical stress of materials in the kiln. If the material flow rate is fast, the scouring and wear of the brick body are serious. For example, the material flow rate of the cement kiln preheater discharge pipe may exceed 5m/s. In this case, high-alumina bricks with high compressive strength should be selected. Generally, the compressive strength of special and first-grade high-alumina bricks can reach more than 50MPa, which can withstand the scouring of materials and reduce wear. For kilns that are frequently started and stopped, such as the feeding port of a glass kiln, the temperature changes dramatically and the brick body is prone to spalling. At this time, high-alumina bricks with strong anti-stripping performance should be selected. This type of high-alumina brick usually has a low thermal expansion coefficient. For some specially treated high-alumina bricks, the thermal expansion coefficient can be as low as 5.8×10⁻⁶/℃, which can effectively reduce the thermal stress caused by temperature changes and reduce the risk of spalling.
4. Cost factor: Cost is also an important aspect to consider when choosing high alumina bricks. The prices of high-alumina bricks of different grades and performances vary greatly. Special-grade and first-grade high-alumina bricks are relatively expensive due to their excellent performance; second-grade and third-grade high-alumina bricks are relatively cheap. In addition to the initial purchase cost, the later maintenance cost should also be considered. Although the purchase cost of special-grade and first-grade high-alumina bricks is high, they have a long service life and can reduce the number of kiln overhauls and downtime. In the long run, the comprehensive cost may be lower. Although the initial purchase cost of second-grade and third-grade high alumina firebricks is low, if they are used under unsuitable working conditions, they may lead to frequent replacement, increased maintenance costs and downtime losses. Therefore, it is necessary to comprehensively calculate the full life cycle cost, that is, the purchase cost plus the maintenance cost and downtime loss, to determine the most economical and suitable high alumina fire brick.
5. Other factors: The operation cycle of the kiln and the difficulty of maintenance and other factors should also be considered. If the kiln needs to run continuously for a long time, the durability of the high alumina fire bricks is higher, and high aluminum bricks with more reliable quality and more stable performance should be selected. In terms of maintenance, if the maintenance is difficult and it is inconvenient to replace high aluminium bricks, products with long service life should also be selected.
