Application Methods of Kyanite, Andalusite and Sillimanite in Monolithic Refractories
In the fields of non-metallic materials and refractories, especially refractory materials, for convenience, three minerals-kyanite, sillimanite, and andalusite-are collectively referred to as the "Three Stones." All of them belong to high-alumina mineral raw materials.
The main applications of the Three Stones in monolithic refractories are as follows:
As refractory aggregate: For example, coarse-grained andalusite is used as a refractory aggregate.
As refractory powder: For instance, concentrates of andalusite and sillimanite are used as powder materials.
As additives: They mainly serve as expansion agents.
Among the three minerals, kyanite is the most widely used. Since the mullitization reaction of kyanite is accompanied by the largest expansion rate, kyanite acts as an excellent expansion agent for monolithic refractories. It can offset the shrinkage of refractory materials at high temperatures and improve their high-temperature performance.
Refractory Castables
In recent years, research on the application of three-stone minerals in refractory castables has mainly focused on four aspects:
1. Application of Kyanite in Refractory Castables
When kyanite is incorporated into castables, kyanite from Yinshan and Shuyang Hanshan is mostly selected as the expansion agent. In terms of expansibility, kyanite from Tongbai has a larger expansion value and is more suitable for use as an expansion agent in unshaped refractory materials.
In ladle castables, adding Nanyang Yinshan kyanite as an expansion agent improves the linear change rate of the castables, thereby eliminating shrinkage cracks that occur during high-temperature service and cooling processes, and extending the service life of the materials. In practical ladle applications, with necessary repairs to the ladle bottom and other parts, the overall service life of a ladle can reach 1200–1300 heats.
High-alumina castables prepared with kyanite as an expansion agent show improved linear change after firing. Without kyanite, the linear change after firing is negative, and shrinkage increases with rising temperature: it is -0.09% at 1300℃ and -1.05% at 1500℃.
After adding kyanite, the shrinkage of castables is reduced or even transformed into positive expansion. For example, when adding 8% or 10% kyanite with different particle sizes (0.175 mm or 0.09 mm), the linear change rate of castables remains positive within 1300–1500℃. This indicates that the material not only offsets high-temperature shrinkage but also exhibits a certain degree of expansion.
In actual production, the particle size of kyanite concentrate should be reasonably selected according to comprehensive performance requirements. This ensures that the material exhibits little or no shrinkage at high temperatures while maintaining high strength. Generally, a particle size range of 0.174–0.074 mm is considered optimal.
2. Effect of Andalusite Addition on Castable Properties
(1) Application of Andalusite in Iron Trough Castables
With the large-scale development of blast furnaces, the scouring and abrasion caused by molten iron and slag have become more severe. In particular, the working conditions of the main iron trough are increasingly harsh, resulting in reduced service life. To address this problem, andalusite is used in the main iron trough castables. By leveraging andalusite's inherent properties, the overall performance of the main trough castables can be significantly improved.
Depending on different service requirements, andalusite with particle sizes of 0–1 mm and 0.074 mm is added to iron trough castables. This addition helps reduce apparent porosity, increase cold crushing strength, and enhance the thermal shock resistance of the castables. The higher the grade of andalusite, the better its overall effect and high-temperature performance.
For a 250 m³ blast furnace with a single tapping hole, the initial iron throughput of andalusite-containing iron trough castables reaches 80,000 to 120,000 tons. With intermediate gunning and lining repairs, the total service throughput can exceed 1.5 million tons, effectively reducing production costs.
Action mechanism: At high temperatures, andalusite decomposes to form a certain amount of mullite and liquid phase. The formed mullite improves thermal shock resistance and refractoriness under load, while the liquid phase promotes sintering, ensures tight bonding between the matrix and aggregate, seals pores, reduces apparent porosity, and further increases the crushing strength of the castables.
(2) Castables for Desulfurization Lances
Molten iron pretreatment desulfurization lances are constantly subjected to severe thermal cycling. Lance damage is mainly caused by thermal stress-induced cracks rather than simple erosion; molten iron penetrates into the cracks and eventually leads to failure.
Improving the thermal shock resistance of the castables can effectively prevent cracking and spalling of the lance body. The addition of andalusite compensates for high-temperature shrinkage, ensures volume stability of the lance, and enhances its service performance.
3. Effect of Sillimanite and Kyanite Addition on Castable Properties
Adding sillimanite alone, or a combination of sillimanite and kyanite concentrate, to castables can improve the linear change of finished products after firing.
In addition, it significantly enhances the refractoriness under load and compressive strength of the products. A higher grade of sillimanite concentrate produces more pronounced performance improvements.
For example, for sample SC-3, which uses sillimanite with 59% Al₂O₃ content and grade-I bauxite as the aggregate, the refractoriness under load (4% deformation) is higher than 1600℃. When using sillimanite with 48% Al₂O₃ content, while still using grade-I bauxite as the aggregate, the refractoriness under load (4% deformation) decreases significantly. For sample SC-12, the value is only 1565℃.
4. Effect of Combined Natural Sillimanite and Andalusite Powder on Castable Properties
The addition of combined mineral powder to castables improves the thermal shock resistance of specimens and reduces the linear change rate after firing. Low-cement, high-alumina castables properly incorporated with natural sillimanite-andalusite composite minerals show improvements in their main technical properties.
The primary reason is the substantial formation of mullite in the matrix. The composite mineral powder forms a liquid phase at a relatively low temperature range of 1000–1300℃. This liquid phase facilitates the formation of in-situ mullite and secondary mullite, thereby enhancing the overall performance of the castables. The appropriate addition amount of the composite mineral powder is 5%.
Traditionally, kyanite has been mainly used as an expansion agent in refractory castables to offset the high-temperature shrinkage of materials. With further research and understanding, the combined application of andalusite, sillimanite, or all three aluminosilicate minerals can effectively optimize and enhance the quality of Al₂O₃-SiO₂ based materials. This applies equally to both shaped refractories and monolithic refractories, fully demonstrating the unique comprehensive properties of the three-stone minerals.
Refractory Plastics
For refractory plastics, a comparison between formulations with and without kyanite shows clear differences in linear change after firing at 1400℃. Samples containing kyanite exhibit larger linear expansion, while those without kyanite show a smaller expansion. In general, the expansion value increases with kyanite addition, and all specimens containing kyanite display expansive linear change after firing at 1400℃.
This behavior is beneficial for the service performance of refractory plastics: it improves the structural stability of the lining, reduces cracking, and slows down spalling and peeling. Compared with the linear change at 1400℃, the linear expansion at 1600℃ increases slightly further.
Refractory Rammables
After adding three-stone minerals, the linear change of high-alumina refractory rammables after firing transforms from linear shrinkage to linear expansion. Among them, kyanite has the best effect; its linear change after firing at 1400℃ increases from -0.40% to +1.60%.
This clearly demonstrates the expansion effect of the three-stone minerals. The addition of three-stone minerals has no significant influence on the compressive strength of high-alumina refractory rammables within the temperature range of 1400–1500℃. This is because kyanite and andalusite decompose rapidly at this temperature range and do not fully undergo mullitization.
Refractory Gunning Mixes
Refractory gunning mixes are unshaped refractory materials applied by mechanical spraying with pneumatic tools. They are composed of refractory aggregate, fine powder, and a binder (or admixture).
According to bulk density, they are classified into three types:
Lightweight gunning mixes (0.5–1.39 g/cm³) are commonly used as thermal insulation linings.
Medium and heavyweight gunning mixes (1.3–1.8 g/cm³) can be used as both insulating linings and working linings for low- and medium-temperature kilns and furnaces.
Heavyweight gunning mixes (above 1.89 g/cm³) are mainly applied as working linings.
Different types of refractory gunning mixes can be used for the inner steel shell above the middle belly of blast furnaces, the combustion chamber, regenerator, and mixing chamber of hot blast stoves, as well as the inner wall of various hot blast pipelines. Their functions include thermal insulation, improving the air tightness of the furnace body, and protecting the furnace steel shell.
Refractory gunning mixes are also used in the hot blast stoves of China's largest blast furnaces with a volume over 5500 m³. The addition of three-stone minerals can not only optimize the linear change rate of refractory gunning mixes, but also form new mullite phases, further improving the comprehensive performance indicators of the materials. The added three-stone minerals can be used either individually or in compound blends.
Good application results can be achieved by incorporating three-stone minerals into unshaped refractories such as refractory gunning mixes and refractory castables. The single or compound types of three-stone minerals can be selected according to the performance requirements of shaped or unshaped refractory materials.
Refractory Mortar
With the development of monolithic refractories, refractory mortar has made extensive progress in research and development, production, inspection, and other areas.
As the application scope of refractory mortar continues to expand, conventional refractory mortars can no longer fully meet the service and construction requirements of industrial kilns in terms of working performance.
In new types of refractory mortars, the addition of kyanite plays a significant role. When kyanite concentrate is added to refractory mortar, the mullitization and expansion reaction of kyanite can compensate for the high-temperature shrinkage of the mortar.
High-Strength Dam Wall Precast Blocks
High-strength slag dam wall precast blocks for blast furnace iron troughs are incorporated with expansion agents. This allows the material to expand uniformly at various temperatures, especially in the range of 1000–1500℃, which offsets or reduces shrinkage at different temperature stages.
Materials containing expansion agents can fill microcracks caused by matrix shrinkage and internal stress, improving the volume stability of the refractory.
In summary, all types of monolithic refractories, including castables, plastics, rammables, and refractory mortars, incorporate kyanite, andalusite, and sillimanite to varying degrees, with kyanite being the most widely used.
The primary mechanism is that the three-stone minerals decompose at high temperatures and undergo mullitization accompanied by volume expansion. This compensates for high-temperature shrinkage, converts linear change into positive expansion, alleviates structural spalling, and enhances volume stability.
Additionally, mullite formed by the decomposition of three-stone minerals positively affects the refractoriness under load and the mechanical strength of the refractories.
Quartz (SiO₂) is also used as an expansion agent for monolithic refractories. The polymorphic transformation of quartz generates expansion, which compensates for high-temperature shrinkage. In particular, the expansion from the phase transition of α-quartz (high-temperature quartz) to α-cristobalite is mainly utilized due to its large volume variation.
Compared with quartz, kyanite provides better performance. It features a relatively larger expansion rate, and the mullite crystal phase formed by its decomposition improves the high-temperature performance of refractories. Therefore, kyanite is commonly used as an expansion agent in monolithic refractories, either alone or combined with andalusite or sillimanite as functional additives.
To achieve favorable performance improvements through the decomposition and mullitization of kyanite, andalusite, and sillimanite, it is essential to select the appropriate grade, dosage, and particle size according to the service temperature. Improper selection can deteriorate the internal structure, induce expansion cracks, and reduce bulk density and strength.
The methods described above mainly utilize the expansion effect from the mullitization of three-stone minerals and the phase transformation of quartz to compensate for high-temperature shrinkage and enhance volume stability. However, the benefits of expansion reactions extend beyond these effects.
For example, low-creep bricks and modified high-alumina bricks for hot blast stoves rely on internal expansion reactions to improve refractoriness under load, creep resistance, and thermal shock resistance. Therefore, it is crucial to rationally use three-stone minerals to optimize the comprehensive performance of refractory materials.
It is also important to note that expansion reactions are widespread in refractory materials. For instance, the matrix design of low-creep hot blast stove bricks involves significant expansion effects. In conclusion, besides acknowledging the ubiquitous expansion reactions in refractories, it is even more critical to rationally control these reactions to enhance material performance.