Adding Kyanite to these refractory castables significantly improves their performance!

　　Kyanite is a high-alumina mineral raw material that decomposes into mullite and SiO2 at high temperatures ₂ This is accompanied by significant volume expansion, a characteristic not found in other high-alumina raw materials. Therefore, kyanite is often added to products as a raw material, utilizing its expansion effect to offset the shrinkage of certain matrix materials or the firing shrinkage of products. This results in products with relatively stable high-temperature volume, good thermal shock resistance, and good creep resistance, thereby improving the high-temperature performance and extending the service life of the products. Kyanite also features acid and alkali resistance, corrosion resistance, high impact resistance, and good electrical insulation properties. Kyanite is widely used in metallurgy, ceramics, refractory materials, glass, machinery, power, and chemical industries. This article summarizes the literature on the application progress of kyanite in monolithic refractory materials and systematically introduces the progress in the utilization of kyanite materials.

　　Castable is a monolithic refractory material with good fluidity after being mixed with water. It can be directly cast into a lining or made into prefabricated blocks using casting or compaction methods. Castables are widely used as lining materials for metallurgical, cement, petroleum, chemical, building materials, and machinery industry kilns and thermal equipment.

　　1. Alumina-based refractory castable

　　With technological advancements, castables are developing towards lower cement content. Low-cement castables, compared to traditional castables, exhibit superior high-temperature performance and slag resistance. Among them, alumina-based low-cement castables have made some progress. To address the structural spalling caused by volume shrinkage at high temperatures in alumina-based low-cement castables, researchers such as Zhang Ling, Zhu Xinwei, and Shan Yuxiang have studied the effects of kyanite on the performance of alumina-based low-cement castables. The results show that kyanite raw materials do not expand significantly below 1300℃ and expand rapidly in the temperature range of 1325℃～1390℃. The kyanite expansion temperature range is narrow, only 65℃. As the temperature continues to rise, the expansion rate decreases due to the formation of liquid phase and sintering. Only when a sufficient amount of kyanite is added can it compensate for the shrinkage caused by sintering and improve the high-temperature volume stability of the material. Adding an appropriate amount of kyanite within the temperature range where kyanite has a large expansion value is beneficial for improving the load softening temperature of the material. The amount of kyanite added should vary depending on the operating temperature to ensure improved material strength. The addition of kyanite, on the one hand, can decompose to produce mullite, improving the strength of the material; on the other hand, the expansion generated by decomposition limits the increase in strength. Therefore, only by introducing an appropriate amount of kyanite can the room-temperature mechanical properties of the material be improved.

　　2. Refractory castable for sintering ignition furnace

　　The sintering ignition furnace is thermal equipment for preparing sintered ore and pellet ore for blast furnaces. The lining materials used often experience severe chemical corrosion at the bottom of the furnace wall and high temperatures around the burner at the top of the furnace during use, resulting in a service life of less than 2 years.

　　To improve the service life of the ignition furnace, Xia Changyong et al. used alumina, mullite, SiC, kyanite, and sillimanite as raw materials, SiO2 micropowder, active α-Al2O3 micropowder, and calcium aluminate cement as binders, and added sodium tripolyphosphate as a water reducer to develop a refractory castable for sintering ignition furnaces. The results show that the addition of SiO2 micropowder not only increases the strength after drying but also increases the room-temperature flexural strength after firing at 1350℃, but the linear shrinkage rate of the material after high-temperature firing increases with the increase in the amount of SiO2 micropowder added, affecting the high-temperature volume stability of the material. The introduction of kyanite ensures that the material achieves an ideal expansion effect.

　　3. Alumina-based cement-free castable

　　Heating furnaces and homogenizing furnaces are thermal equipment used for heating and homogenizing steel billets or ingots during steel rolling or forging. The refractory materials used in the burner area require good high-temperature refractory performance, thermal shock resistance, and excellent resistance to high-temperature airflow erosion and dust erosion. However, the currently used alumina-cement bonded high-alumina refractory castables generally have poor thermal shock resistance and spalling resistance, resulting in a short service life of 3-6 months. At the same time, due to the use of alumina cement as a binder, the CaO component in the cement forms low-melting substances at high temperatures, reducing the refractory performance of the castable, causing severe deformation of the material, changing the temperature field inside the furnace, and thus affecting the heating quality of the steel billets.

　　4. Castable for iron pretreatment desulfurization gun

　　Iron pretreatment is an important process for improving the quality of steel products, expanding the variety of steel, and reducing costs. The desulfurization gun is an important component of the pretreatment equipment. During use, the desulfurization gun castable is subjected to high-temperature erosion and wear due to the stirring of slag and molten iron. It also needs to withstand mechanical damage caused by gun body vibration during spraying and sudden temperature changes during intermittent operation. Therefore, the desulfurization gun castable often suffers from phenomena such as perforation, cracking, and melting damage, which damages the inner core of the steel pipe, reduces the service life of the desulfurization gun, hinders production, and increases production costs.