Application of Kyanite, Andalusite, and Sillimanite in Monolithic Refractories

In the field of non-metallic and refractory materials, especially refractories, kyanite, sillimanite, and andalusite are collectively referred to as the "three minerals," all belonging to high-alumina mineral raw materials.

The main applications of these three minerals in monolithic refractories include: (1) as refractory aggregates, such as using coarse andalusite as a refractory aggregate; (2) as refractory powders, such as using andalusite and sillimanite concentrates as powders; (3) as additives, primarily used as expansion agents. Among the three minerals, kyanite is the most widely used. This is because the expansion value associated with the mullitization reaction of kyanite is the highest, making it an excellent expansion agent for monolithic refractories, helping to offset the shrinkage of refractories at high temperatures and improving their high-temperature performance.

I. Application of the Three Minerals in Refractory Castables

Recent studies on the application of the three minerals in refractory castables have focused on four main areas:

1. Application of Kyanite in Refractory Castables Adding kyanite to castables primarily involves using kyanite sourced from Hushan and Shuyang, with the kyanite from Tongbai showing the highest expansion value, making it particularly suitable as an expansion agent in monolithic refractories. When kyanite from Nanyang is added to ladle castables, it improves the linear change rate of the castables, eliminating shrinkage cracks that can occur during high-temperature use and cooling, thus extending the lifespan of the material. In practice, with necessary repairs to the ladle's bottom, a ladle can be used for 1,200 to 1,300 cycles. High-alumina castables formulated with kyanite as an expansion agent show improved post-firing linear change. Without kyanite, the post-firing linear changes are all negative, increasing with temperature. At 1,300°C, it is -0.09%, and at 1,500°C, it is -1.05%. However, when kyanite is added, the shrinkage decreases or becomes positive expansion; for different grades (0.175 mm or 0.09 mm) of kyanite added at 8% or 10%, the castables show positive values at temperatures between 1,300°C and 1,500°C, effectively offsetting high-temperature shrinkage while also demonstrating expansion. In production, selecting the appropriate kyanite concentrate particle size ensures minimal shrinkage (or very little) at high temperatures, while maintaining high strength. Generally, a particle size of 0.174–0.074 mm is considered moderate.

2.Impact of Adding Andalusite on Castable Properties (1) Application of Andalusite in Iron Trough Materials

(1)With the large-scale operation of blast furnaces, the scouring force of molten iron and slag increases, especially in the main trough where working conditions are becoming increasingly harsh, resulting in reduced lifespans. Andalusite is utilized in the production of main trough materials to enhance their performance. Depending on different usage requirements, different particle sizes (0–1 mm, 0.074 mm) of andalusite are added to the iron trough castables, which helps reduce porosity and improve compressive strength at room temperature, enhancing thermal shock stability. The better the quality of the andalusite, the more superior its performance at high temperatures. Castables with added andalusite can achieve a single iron output of 80,000 to 120,000 tons from a 250 m³ blast furnace, with repairs and patching in between, leading to a lifespan exceeding 1.5 million tons and reduced production costs. The mechanism primarily utilizes the decomposition of andalusite at high temperatures to produce a certain amount of mullite and liquid phase, which improves the thermal shock stability and softening temperature under load of the castables. The generated liquid phase not only promotes sintering to tightly bond the matrix and aggregate but also fills voids, reducing the apparent porosity and enhancing the compressive strength of the castables.

(2) Desulfurization Gun Pre-treatment with Castables Desulfurization guns experience extreme temperature changes, where damage typically arises from thermal stress cracks rather than erosion. Enhancing the thermal shock resistance of the castables helps prevent cracks and spalling in the gun body. Adding andalusite compensates for shrinkage, ensuring volume stability and improving performance. In trials at a steel plant in Ningbo, the guns reached over 200 cycles with minimal repairs.

1. Impact of Adding Sillimanite and Kyanite on Castable Properties Adding sillimanite or a combination of sillimanite and kyanite concentrates to the castables improves post-firing linear changes. This also significantly affects the softening temperature under load and compressive strength. High-grade sillimanite concentrates yield more noticeable effects. For example, using sillimanite with an Al2O3 content of 59% and first-grade bauxite as aggregate, the softening temperature (4%) exceeds 1,600°C; whereas using sillimanite with 48% Al2O3 reduces the softening temperature, as seen in the SC-12 sample at 1,565°C.

2. Impact of Adding Natural Sillimanite and Andalusite Composite Mineral Powder on Castable Properties Incorporating composite mineral powders improves the thermal shock resistance of the castables and reduces post-firing linear changes. Appropriately adding natural sillimanite and andalusite composite mineral powder to low-cement high-alumina castables leads to improvements in key technical indicators. This is mainly due to the significant generation of mullite within the matrix. Composite mineral powders form a liquid phase at lower temperatures (1,000–1,300°C), facilitating the formation of in-situ mullite and secondary mullite, thereby positively impacting the performance of the castables. The optimal addition amount for composite mineral powder is about 5%. Historically, kyanite has primarily been used as an expansion agent to offset shrinkage in refractory castables. However, with increased understanding, the combination of andalusite, sillimanite, or the three minerals effectively improves the quality of Al2O3-SiO2 based materials, applicable to both shaped and unshaped refractories.

II. Application of the Three Minerals in Refractory Plastics

In refractory plastics, comparing samples with and without kyanite shows that the former exhibit greater linear change after firing at 1,400°C, indicating increased expansion, which benefits the stability of the structural framework and reduces cracking and spalling. The linear changes at 1,600°C show slight expansion compared to those at 1,400°C.

III. Application of the Three Minerals in Refractory Rammed Materials

After adding the three minerals, the linear change in high-alumina ramming materials shifts from shrinkage to expansion post-firing. Among these, kyanite shows the best effect, with linear changes at 1,400°C increasing from -0.40% to +1.60%, demonstrating the expansion agent role of the three minerals. However, the addition of the three minerals has no significant effect on the compressive strength of high-alumina ramming materials at 1,400–1,500°C, as kyanite and andalusite decompose quickly and are not fully mullitized at this temperature stage.

IV. Application of the Three Minerals in Refractory Spraying Materials

Refractory spraying materials are amorphous refractories applied using pneumatic tools. Composed of refractory aggregates, powders, and binders (or additives), they are classified into lightweight, medium, and heavy spraying materials based on bulk density. Lightweight spraying materials (0.5–1.39 g/cm³) are commonly used as insulating linings, while medium and heavy materials (1.3–1.8 g/cm³ and greater than 1.89 g/cm³, respectively) can serve as working linings in low to medium temperature kilns. Various types of refractory spraying materials are used in regions such as the upper belly of blast furnaces, combustion chambers of hot blast stoves, regenerator chambers, mixing chambers, and various hot air duct inner walls, serving to insulate, retain heat, enhance gas-tightness, and protect the iron skin of the furnace. China’s largest blast furnace (with a volume greater than 5,500 m³) also employs refractory spraying materials. The introduction of the three minerals not only improves the linear change rates of refractory spraying materials but also enhances material performance by introducing new phases of mullite. The added minerals can be single types or composite. In both refractory spraying materials and castables, the addition of the three minerals yields positive results. Depending on the specific requirements for shaped or unshaped refractories, the appropriate type or composite type of the three minerals should be chosen.

V. Application of the Three Minerals in Refractory Slurries

With the development of unshaped refractories, refractory slurries have seen broad advancements in R&D, production, and testing. As the application scope of refractory slurries expands, conventional slurries can no longer fully meet the requirements of kiln construction. The incorporation of kyanite plays a significant role in new refractory slurries. By adding kyanite concentrates to the slurries, the mullitization expansion reaction compensates for shrinkage at high temperatures.

VI. Application of the Three Minerals in High-Strength Slag Block Prefabricated Parts

In high-strength slag block precast components for blast furnace tapping channels, the addition of expansion agents allows the material to expand uniformly across various temperatures, especially within the range of 1000 to 1500°C. This expansion compensates for or reduces the shrinkage that occurs at different temperature stages. The use of expansion agents can fill micro-cracks caused by matrix shrinkage and internal stress, thus improving the volume stability of the material.

In summary, various types of amorphous refractory materials, such as castables, plastics, ramming materials, and refractory slurries, utilize three minerals—andalusite, kyanite, and sillimanite—to varying degrees, with kyanite being the most widely used. The primary mechanism involves the decomposition of these minerals at high temperatures, resulting in volume expansion due to the mullitization reaction, which compensates for the shrinkage of amorphous refractory materials at elevated temperatures. This process leads to positive linear expansion, mitigates structural spalling, and enhances the volume stability of the materials.

Additionally, the decomposition of these three minerals contributes positively to the load softening temperature and strength of the materials. Other expansion agents for amorphous refractory materials include quartz (SiO2), which also compensates for high-temperature shrinkage through phase transformations, primarily the expansion associated with the transformation of α-quartz (high-temperature quartz) to β-quartz, as this transition exhibits the most significant volume change.

However, kyanite outperforms quartz in this application. Kyanite provides a relatively greater expansion value and, upon decomposition, yields mullite crystals that benefit the high-temperature performance of the material. Consequently, kyanite is commonly used as an expansion agent in amorphous refractory materials, either alone or in combination with andalusite or sillimanite.

The effective use of kyanite, andalusite, and sillimanite through their mullitization reactions can significantly enhance material properties. Careful selection of grade, dosage, and particle size based on operating temperatures is essential; otherwise, it may lead to internal deterioration, causing expansion cracks and reduced density and strength in the materials.

These methods leverage the decomposition and accompanying expansion reactions of the three minerals, as well as the expansion associated with quartz phase transformations, to compensate for high-temperature shrinkage in amorphous refractory materials and improve volume stability. However, the benefits of expansion reactions extend beyond this; for instance, low-creep bricks and modified high-alumina brick series used in hot blast stoves employ internal expansion reactions to enhance load softening temperature, creep resistance, and thermal shock resistance. Therefore, it is crucial to effectively utilize these three minerals to improve the performance of refractory materials.