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What are the Bulk Density and Indicators of Anti-Scaling Refractory Castables?
Anti-scaling refractory castables are high-aluminum silicon carbide products used in cement kilns. They offer excellent wear and corrosion resistance. Furthermore, they resist scaling and caking, effectively preventing these conditions on the inner walls of high-temperature kilns. castable refractory density. These castables also exhibit exceptional high-temperature strength, high thermal conductivity, low linear expansion coefficient, and excellent thermal shock resistance.
Bulk Density and Physical and Chemical Properties of Anti-Scaling Castables
The bulk density of anti-scaling castables is 2.4-2.5 g/cm³, castable refractory density, and their refractoriness can reach 1790°C. Its chemical composition primarily consists of SiC and SiO2, and its physical and chemical properties vary depending on the SiC and SiO2 content.
When the SiC and SiO2 content is 55%, after continuous use at 110°C for 24 hours, its flexural strength and compressive strength can reach 8 MPa and 70 MPa, respectively. When the temperature is raised to 1110°C and the temperature is continuously used for 3 hours, its flexural strength and compressive strength reach 9 MPa and 80 MPa, respectively.
When the SiC and SiO2 content is 60%, after continuous use at 110°C for 24 hours, its flexural strength and compressive strength reach 11 MPa and 80 MPa, respectively. When heated to 1110°C and used continuously for 3 hours, its flexural strength and compressive strength reached 12 MPa and 100 MPa, respectively.
This indicates that the performance of anti-scaling refractory castables gradually improves with increasing SiC and SiO2 content.
The anti-scaling effect of anti-scaling castables stems from the fact that, at high temperatures, a glaze forms on the refractory surface, preventing the penetration of glass and thus preventing scaling. Precisely due to its anti-scaling properties, it is widely used in areas prone to scaling, such as preheater discharge ramps, flue risers, and calciner cones in large and medium-sized new dry-process cement kilns.
Performance and Applications of Heavy-Weight and Lightweight Alkali-Resistant Refractory Castables
Alkali-Resistant Castables
Alkali-resistant castables are castables that resist corrosion from alkali metal oxides at medium and high temperatures. They can be categorized by operating temperature as medium-temperature and high-temperature.
Medium-temperature alkali-resistant castables typically use aluminum-silicon-based materials as aggregates or fine powders, such as clay clinker, waste porcelain powder, and expanded perlite. Their alkali resistance works by reacting with alkali metal oxides to form a high-viscosity glaze layer on the surface of the refractory castable. This layer prevents further penetration of alkali metal oxides and other materials into the castable, improving corrosion resistance.
High-temperature alkali-resistant castables are divided into high-strength alkali-resistant castables and standard alkali-resistant castables. They are used in large quantities in cement rotary kiln preheaters. They have a SiO2 content of approximately 60% and an Al2O3 content between 45% and 75%. High-strength, alkali-resistant castables use high-voltage ceramics, while standard alkali-resistant castables use low-voltage ceramics. The higher the operating temperature, the higher the Al2O3 content. Depending on the application, the process mix can be adjusted by using calcium aluminate cement, silica powder, and other ingredients.
Alkali-resistant castables can be categorized into lightweight and heavyweight types based on bulk density. castable refractory density.
Aggregates used in lightweight alkali-resistant castables include alkali-resistant ceramsite, clayey porous clinker, waste porcelain material, and high-strength expanded perlite, with ordinary calcium aluminate cement as the binder.
Aggregates used in heavyweight, medium-temperature alkali-resistant castables include high-alumina clinker and clay clinker, with calcium aluminate cement plus microsilica (fume silica) as the binder.
Lightweight alkali-resistant refractory castables use aggregates such as alkali-resistant ceramsite, clayey porous clinker, porcelain waste, and high-strength expanded perlite. Calcium aluminate cement or water glass is used as the binder. Alkali-resistant castables can be made from high-silica refractory raw materials. These materials react with alkali metal oxides at high temperatures to form a highly viscous liquid phase, forming a dense, axialized protective layer that prevents further penetration and erosion by molten alkali metals. Their chemical composition is generally: Al₂O₃ 30%-55%, SiO₂ 25%-45%. Traditional alkali-resistant castables use 25%-30% cement and 20%-25% water. Due to the breakdown of the cement bond structure at moderate temperatures (800-1000°C), the strength drops significantly, reaching only about 50% of the oven-dried strength. Therefore, the introduction of ultrafine powder and appropriate dispersants can reduce the cement content to 10%-20% and the water content to 15%-20%. The physical properties of a typical low-cement lightweight alkali-resistant refractory castable are as follows: at 110°C, after 16 hours of drying, the bulk density is 1.5-1.6 g/cm³, the flexural strength is 3-6 MPa, and the compressive strength is 30-40 MPa. After sintering at 1100°C for 3 hours, the bulk density is 1.4-1.5 g/cm³, the flexural strength is 6.0-6.5 MPa, and the compressive strength is 40-45 MPa. The linear change rate after sintering is -0.3%-0.5%. At 350°C, the thermal conductivity is 0.4-0.5 W/(m·K).
Heavy-Duty Alkali-Resistant Refractory Castables
Heavy-Duty Alkali-Resistant Refractory Castables use aggregates such as alumina clinker and clay clinker. The binder and alkali-resistant powder are the same as those used for lightweight alkali-resistant castables. Its main chemical composition is: Al₂O₃ 35%-60%, SiO₂ 35%-60%. Traditional heavy-duty alkali-resistant refractory castables use 20% cement and 10%-15% water. Low-duty alkali-resistant refractory castables use 5%-15% cement and 6.5%-7.5% water. The binder for low-duty alkali-resistant castables is calcium aluminate cement plus fine silica powder (fume silica). The addition of fine silica powder not only improves the castable’s bonding strength at medium temperatures but also helps form an anti-permeability barrier on the castable’s lining during use. Its characteristics are that the strength after firing at medium temperatures (1000-1200°C) is equivalent to the strength after drying (110°C), and it has good alkali corrosion resistance. The physical indicators of a typical low-cement heavy-duty alkali-resistant refractory castable are as follows: at 110°C, after 16 hours of drying, the bulk density is 2.20-2.59g/cm3, the flexural strength is 4-8MPa, and the compressive strength is 40-60MPa; at 1100°C, after 3 hours of firing, the bulk density is 2.20-2.40g/cm3, the flexural strength is 6-10MPa, and the compressive strength is 35-55MPa. The linear change rate after firing is -0.3%-0.4%, and the thermal shock stability (1100°C → water cooling) is greater than 20 times; at 350°C, the thermal conductivity is 1.2-1.3W/(m·K).
What is the Kiln Part Where Alkali-Resistant Castables are Used?
The alkali resistance of alkali-resistant castables is achieved by reacting with alkali metal oxides at high temperatures to form a highly viscous liquid phase, forming a glaze-like protective layer that prevents penetration and corrosion by molten alkali metals.
Alkali-resistant castables are primarily used for refractory and alkali-resistant linings in hot spots of kilns subject to alkali corrosion, such as cement kiln preheater systems, piping systems, and the tail end of dry rotary kilns. They can also be used as linings for various kilns not subject to alkali corrosion, and high-performance grades can be used for areas requiring high strength and wear resistance. Heavy-weight alkali-resistant castables are primarily used in the tail end, head end, preheater, discharge port, and tuyere of alumina and cement rotary kilns. Lightweight alkali-resistant refractory castables are primarily used for the preheater roof, drum, and thermal insulation lining of these rotary kilns. Alkali-resistant castables are also used in industrial kilns subject to alkali corrosion in industries such as steel, nonferrous metals, glass, machinery, and petrochemicals.
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