What are the applications of kyanite, andalusite, and sillimanite in amorphous refractory materials?

　　In the field of non-metallic and refractory materials, especially refractory materials, for convenience, the three minerals Kyanite, Sillimanite, and Andalusite are referred to as "three stones", all belonging to high-alumina mineral raw materials.

　　The main applications of the three stones in monolithic refractory materials are:

　　(1) As refractory aggregate, such as using coarse-grained Andalusite as refractory aggregate;

　　(2) As refractory powder, such as using Andalusite and Sillimanite concentrates as powder;

　　(3) As an additive, mainly used as an expanding agent. Kyanite is used very frequently among the three stones minerals. Because the expansion value accompanying the mullitization reaction of Kyanite is large, Kyanite is a good expanding agent for monolithic refractory materials, used to offset the shrinkage of refractory materials at high temperatures and improve their high-temperature performance.

　　I. Refractory Castables

　　In recent years, research on the application of the three stones in refractory castables has focused on four aspects:

　　1. Application of Kyanite in Refractory Castables

　　When adding Kyanite to castables, Kyanite from Yinshan and Shuyang Hanshan is mainly selected as the expanding agent for refractory castables. In terms of expansibility, Kyanite from Tongbai has a larger expansion value and is more suitable as an expanding agent for monolithic refractory materials. In iron ladle castables, adding Nanyang Yinshan Kyanite as an expanding agent improves the linear change rate of the castable, thereby eliminating shrinkage cracks generated during high-temperature and cooling processes, and improving the service life of the material. In iron ladle use, the bottom of the ladle is repaired as necessary, and a ladle can be used for 1200-1300 cycles in total. The high-alumina castable prepared with Kyanite as an expanding agent improves the post-firing linear change of the castable. When no Kyanite is added to the castable, the post-firing linear change is negative, and the shrinkage increases with increasing temperature. At 1300℃, it is -0.09%, and at 1500℃, it is -1.05%. However, after adding Kyanite, the shrinkage of the castable is reduced or positive expansion occurs. For example, when adding 8% or 10% of different particle sizes (0.175 mm or 0.09 mm) Kyanite, the castable is positive at 1300-1500℃, that is, after offsetting the shrinkage of the material at high temperatures, it still shows expansion.

　　In production, the particle size of Kyanite concentrate is reasonably selected based on the comprehensive material index requirements, ensuring that the material does not shrink (or shrinks very little) at high temperatures while ensuring high strength. Generally, a particle size of 0.174-0.074 mm is considered appropriate.

　　2. Influence of Adding Andalusite on the Properties of Castables

　　(1) Application of Andalusite in Iron Trough Materials

　　With the large-scale development of blast furnaces, the scouring force of molten iron and slag and wear and tear have increased, especially the working conditions of the main trough are becoming increasingly harsh, and the service life is reduced. To change this phenomenon, Li Yuepu and others used Andalusite in the production of iron trough main trough materials, using the characteristics of Andalusite to improve the performance of the iron trough main trough castable. In production, adding Andalusite of different particle sizes (0-1 mm, 0.074 mm) to the iron trough castable according to different usage requirements is beneficial to reducing the apparent porosity of the castable, improving the compressive strength at room temperature, and improving the thermal shock resistance of the castable. The better the quality of Andalusite, the better the effect and high-temperature performance.

　　Using iron trough castables with added Andalusite, the one-time iron tapping amount at a single tapping hole of a 250 m3 blast furnace reaches 80,000 to 120,000 tons, and the service life can reach more than 1.5 million tons after intermediate spraying and pouring repairs, reducing production costs. The mechanism mainly utilizes the fact that when Andalusite decomposes at high temperatures, it can generate a certain amount of mullite and liquid phase. The generated mullite helps to improve the thermal shock resistance and refractoriness under load of the castable, while the liquid phase not only promotes sintering to make the matrix and aggregate closely combined, but also closes pores, reduces the apparent porosity of the product, and improves the compressive strength of the castable.

　　(2) Desulfurization Gun Castables

　　Iron pretreatment desulfurization spray guns are always subjected to drastic cold and hot temperature changes. At this time, the general damage to the spray gun is not erosion, but cracks caused by thermal stress, which penetrate molten iron and cause damage. By improving the thermal shock resistance of the castable, cracking and peeling of the gun body can be prevented. By adding Andalusite, shrinkage is compensated, ensuring the volume stability of the gun body and improving its performance. When Li Yuepu and others tested it at a steel plant in Ningbo, it reached more than 200 times at once, with minor repairs in between.

　　3. Influence of Adding Sillimanite and Kyanite on the Properties of Castables

　　In castables, adding Sillimanite, or a composite of Sillimanite and Kyanite concentrates, improves the post-firing linear change of the product. In addition, the refractoriness under load and compressive strength of the product are also significantly improved. The higher the grade of Sillimanite concentrate, the more obvious the effect. For example, for SC-3 samples with Sillimanite containing 59% Al2O3 and first-grade bauxite as aggregate, the refractoriness under load (4%) is greater than 1600℃; when Sillimanite contains 48% Al2O3 and the aggregate is still first-grade bauxite, the refractoriness under load (4%) is significantly reduced, such as SC-12 samples, which is 1565℃.

　　4. Influence of Adding Natural Sillimanite and Andalusite Composite Mineral Powder on the Properties of Castables

　　Adding composite mineral powder to the castable improves the thermal shock resistance of the sample and reduces the post-firing linear change rate. Adding an appropriate amount of natural Sillimanite-Andalusite composite mineral to low-cement high-alumina castables improves the main technical indicators. The main reason is the generation of a large amount of mullite in the matrix. The composite mineral powder forms a liquid phase at a lower temperature (1000-1300℃), and the liquid phase is beneficial to the formation of in-situ mullite and secondary mullite, thereby having a beneficial effect on the performance of the castable. The suitable addition amount of composite mineral powder is 5%.

　　In the past, the main role of Kyanite in refractory castables was as an expanding agent to offset the shrinkage of the material at high temperatures. With the improvement of understanding, the application of Andalusite, Sillimanite, or a composite of the three stones can effectively improve the quality of Al2O3-SiO2 materials. This is true for shaped refractory materials, and it is also true for monolithic refractory materials, showing the unique properties of the three stones.