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Silicon Carbide Refractory Castable for Furnace Lining in Reducing Atmosphere

September 5, 2024September 3, 2024 by rsrefractorycastable

Silicon carbide refractory castable has stable properties and excellent corrosion resistance and wear resistance. It is not corroded by boiling hydrochloric acid, sulfuric acid, or hydrofluoric acid. SiC 75-80% castable. Silicon carbide refractory castable for reducing atmosphere furnace lining. It will not oxidize when used in a reducing atmosphere with a temperature of 1600°C, but oxidation will occur in an oxidizing atmosphere.

The Optimal Operating Temperature of Silicon Carbide Castables in Reducing Atmosphere Furnaces

Silicon carbide castable is used at a temperature of 800-1140℃, rather than 1300-1500℃. Because within the range of 800-1140°C, the oxide film formed by oxidation has a loose structure and cannot play the role of silicon carbide. Above 1140°C, especially between 1300-1500°C, the oxide film formed by oxidation covers the surface of the silicon carbide substrate, preventing oxygen from contacting the silicon carbide. Therefore, the antioxidant capacity is the best within the temperature range of 1300-1500°C. However, when the temperature exceeds 1500°C, the oxidation protective layer will be destroyed. At this time, the silicon carbide castable will be strongly oxidized and the matrix will be decomposed and destroyed.

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Application Advantages of Silicon Carbide Castables

Silicon carbide castable is a hard material with high hardness. It has good creep resistance, thermal shock resistance, and wear resistance, but has high thermal conductivity. However, silicon carbide castables also have different qualities, that is, different proportions of silicon carbide are added. However, no matter what type of castable it is, metal silicon powder must be added to adjust it to resist oxidative decomposition. Including refractory bricks, they must also be adjusted to prevent oxidation.

In fact, silicon carbide castable is a composite castable, which is made of alumina aggregate and powder, mixed with a certain proportion of silicon carbide and binder. So they are all composite.

Generally used in furnace linings with severe erosion. If the temperature is below 1300°C for the furnace lining, do not use silicon carbide refractory castables. Furnace linings in oxidizing atmosphere are also not suitable for this castable. Silicon carbide refractory castables are best used in reducing atmosphere furnace linings at 1300-1500°C.

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Silicon Carbide and Its Application in Refractory Materials

Silicon carbide can maintain high strength and high wear resistance at high temperatures. After adding a certain proportion of silicon carbide, refractory castables have good chemical stability and will not be corroded by acid and alkali solutions. Silicon carbide has a larger wetting angle with molten metal and slag. Compared with oxide refractory castables, it has good corrosion resistance to various furnace lining solids, liquids and gases.

Wear-resistant

The hardness of silicon carbide is second only to diamond, and it has strong wear resistance. It is an ideal material for wear-resistant pipes, impellers, pump chambers, cyclones, and linings of ore hoppers. Its wear resistance is 5-20 times that of cast iron and rubber, and it is also one of the ideal materials for aviation runways. Using a special process to coat silicon carbide powder on the inner wall of the turbine impeller or cylinder block can improve its wear resistance and extend its service life by 1 to 2 times.

Thermal shock resistance

Due to the high thermal conductivity and small thermal expansion coefficient of silicon carbide, silicon carbide refractory materials have good thermal shock resistance. The thermal shock resistance of silicon carbide products is also closely related to the type and nature of the bonding base material. In actual application, since silicate-bonded silicon carbide products can be observed to expand, crack, and deform after being subjected to thermal shock, the service life of the material can be easily predicted.

High thermal conductivity

Since silicon carbide itself has good thermal conductivity, refractory materials with high silicon carbide content have higher thermal conductivity. Most of their thermal conductivity exceeds 14.4W/(m·K). Used in heat exchangers, saggers, water-cooled walls of coal gasification furnaces, indirectly heated kiln furniture products, etc. During the use of silicon carbide products, the thermal conductivity of the particle surface will gradually become smaller. The properties of the bonding base material have a certain impact on the thermal conductivity of silicon carbide products. The thermal conductivity of silicon oxynitride bonded and silicon nitride bonded silicon carbide is higher, and the thermal conductivity of silicate bonded silicon carbide is smaller.

Anti-oxidation

Silicon carbide has good oxidation resistance. Oxidation is weak below 1300°C. Significant oxidation does not occur until the temperature is above 1300°C. The oxidation generates a SiO2 glass protective film, which can inhibit the oxidation of silicon carbide.

The oxidation resistance of silicon carbide refractory products also varies significantly with the type of binding base material. The lower oxidation resistance of silicon nitride-bonded silicon carbide products can be explained by their microstructural characteristics. Because the base material of silicon nitride combined with silicon carbide products is in the form of interwoven fibers and has high air permeability, it plays a small protective role on silicon carbide particles. In silicate bonded and silicon oxynitride bonded silicon carbide products, the surface of the silicon carbide particles is wrapped by a continuous base material. Therefore, it has strong antioxidant properties. The antioxidant properties of silicate-bonded silicon carbide and silicon oxynitride-bonded silicon carbide show similar properties in the above tests, but the differences between them can be clearly shown in long-term use.

Resistance to slag

SiC is a compound with strong covalent bonds and maintains high bonding strength at high temperatures. Therefore, SiC has good chemical stability and will not be corroded by most acid and alkali solutions. Silicon carbide has a larger wetting angle with molten metal and slag. Compared with oxide refractory materials, it has good corrosion resistance to various solids, liquids and gases. Such as Al2O3-SiC-C castables and products used in iron-making systems, silicon molybdenum bricks and silicon carbide-containing castables used in cement kilns, various acid-base reaction vessels, etc.

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Application of Carbonaceous Castables Containing Silicon Carbide Raw Materials in Blast Furnace Bottom

Blast furnaces are the basis for stable and smooth ironmaking production, and the most important ones are carbon bricks and water cooling systems. Carbon bricks and molten iron are not easily wetted and have strong corrosion resistance. Water cooling is the key to ensuring the longevity of carbon bricks. During the blast furnace bottom masonry process, after the water-cooling pipes are laid, the existing technology mostly uses carbon ramming material to level and fill the carbon brick joints. In order to ensure the water cooling effect, the carbon ramming material is required to have high thermal conductivity, a certain strength, and good construction performance. Since the performance of ramming materials is closely related to the construction process and quality, and there are many human-influenced factors, the thermal conductivity often fails to meet the expected requirements. In recent years, the results of analyzing carbon castables show that the thermal conductivity of castables mainly made of carbon raw materials is relatively low, and some are less than 10W·(m·K)⁻¹. The thermal conductivity of carbonaceous castables containing silicon carbide raw materials is greater than 15W·(m·K)⁻¹. Its construction is more convenient than ramming material, and its operation is simple. It only needs vibration forming and quick smoothing, which can save a lot of man-hours. Performance of carbonaceous castables containing silicon carbide raw materials in blast furnace bottoms.

(1) Carbonaceous castables using silicon carbide as the main raw material have higher strength. The compressive strength after 24 hours of insulation at 110°C is greater than 17MPa, and the compressive strength after 3 hours of insulation at 1450°C is greater than 70MPa. The thermal conductivity at 700°C measured using the laser method is 7.2W·(m·K)⁻¹. The compressive strength of imported carbon castables using carbon as the main raw material after being kept at 110°C for 24 hours is 18.3MPa. The compressive strength after being kept at 1450°C for 3 hours was significantly reduced. The thermal conductivity at 700℃ measured by laser method is 5.08W·(m·K)⁻¹. Its thermal conductivity is relatively low compared to the technical requirements of blast furnace bottom carbonaceous castables.
(2) After the sol-bonded carbon castable with silicon carbide as the main raw material is soaked in water, the compressive strength and volume density increase, while the shape remains unchanged. After soaking in 2% (w) hydrochloric acid solution, the compressive strength after 30 days decreased. After immersion in 2% (w) sulfuric acid solution, the compressive strength decreased after 30 days, and the decrease was smaller than that in hydrochloric acid solution. After soaking in 2% (w) sodium hydroxide solution, it was completely loosened into powdered materials in 30 days. Carbon castables made of silicon carbide as the main raw material are not resistant to immersion in alkali solutions. In an environment where alkali metal vapor may penetrate into the blast furnace bottom, such carbonaceous castables should be used with caution if there is cooling water leakage.

Rongsheng Refractory Castable Manufacturer is a powerful refractory material manufacturer. Rongsheng Manufacturer has an environmentally friendly, fully automatic unshaped refractory material production line. The refractory lining materials products of Rongsheng manufacturer have been sold to more than 120 countries around the world, such as South Africa, Chile, Egypt, Colombia, Uzbekistan, Italy, Indonesia, Ukraine, Hungary, Spain, Kenya, Syria, Zambia, Oman, Venezuela, India, Peru, the United States, Ethiopia, etc. We can customize refractory lining materials according to the actual working conditions of high-temperature furnace linings. To purchase high-quality silicon carbide refractory materials, please choose Rongsheng Refractory Castable Manufacturer.

Long Service Life Silicon Nitride Combined Silicon Carbide Thermal Shock Resistant Castables

July 16, 2024 by rsrefractorycastable

With the development of the cement industry, in order to adapt to the requirements of large-scale, long-lasting, energy-saving and environmentally friendly cement industry, the requirements for refractory materials for the lining of large and medium-sized cement kilns are becoming increasingly stringent. Existing refractory materials can no longer meet production needs in actual use. Especially at the kiln mouth, grate cooler, coal injection pipe, smoke chamber, top of preheater and other parts. Due to high-temperature thermal stress, rapid cooling and rapid hot air flow impact, high-temperature cement clinker wear and high-temperature gas alkali corrosion, it is easy to cause cracking, peeling and wear of the lining, making it difficult to meet the requirement of the kiln age being more than one year. Moreover, the corundum-silicon nitride-silicon carbide composite castable used in the tuyere belt of the ironmaking blast furnace can be poured and constructed quickly. However, its alkali resistance and thermal shock stability cannot meet the requirements for refractory materials for large and medium-sized cement kiln linings.

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Therefore, it has become a primary task to prepare a refractory material with strong thermal shock stability, high compressive strength, small thermal expansion coefficient, low thermal conductivity, good alkali resistance and long service life. Long-life silicon nitride combined with silicon carbide thermal shock-resistant castables solves the above technical problems for the blast furnace and cement industry.

Silicon Nitride Combined with Silicon Carbide Thermal Shock Resistant Castable

This castable uses silicon nitride and silicon carbide as the main raw materials and is also supplemented with corundum, activated alumina and other materials. Using composite explosion-proof materials and additives, silicon nitride combined with silicon carbide thermal shock-resistant castables are made. Its raw material composition is as follows:

Silicon nitride 150-300 mesh, 5-20%.
Black silicon carbide 100-200 mesh, 10-30%.
Green silicon carbide 150-300 mesh, 5-20%.
Tabular corundum, 20-30%.
White corundum, 15-30%.
Activated alumina 300-600 mesh, 3-12%.
Metal aluminum powder 80-150 mesh, 0. 5-3%.
Heat-resistant steel fiber 5%.
Metal silicon powder 100-200 mesh, 2-8%
Aluminate cement 3-6%.

The silicon nitride combined with silicon carbide castable has excellent high-temperature stability, high thermal shock stability (water cooling, 1100°C ≥ 30 times), and high alkali corrosion resistance (level 2 or above). Excellent normal and high-temperature strength and high-temperature material wear resistance. Suitable as long-life (more than one year) large and medium-sized cement kiln lining materials. It is especially suitable for key parts with harsh working conditions such as the kiln mouth, coal injection pipe, cement kiln smoke chamber, grate cooler, and top of the preheater of large and medium-sized cement kilns.

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Analysis of the Causes of Peeling, Cracking, and Peeling of Castables for Cement Kiln Burner Linings

The burner is an important process equipment of the cement kiln firing system. It has a great relationship with clinker output and quality, the life of refractory materials in the kiln, clinker coal consumption, environmental protection emissions, etc. The main reason that affects the service life of the burner is the damage of the burner lining castable. How to extend the service life of the burner lining castable is particularly important. The four-channel pulverized coal burner of a cement manufacturer uses Al₂O₃-SiC series low-cement refractory castable as a protective lining with a thickness of 100mm. Serious damage occurred to the burner head and bottom 2 months after it was put into use. The main modes of damage are peeling, cracking and peeling.

Burner Lining Castable Damage

The damage to the burner lining castable is shown in Figure 1. Take a piece of lining castable from the burner that is completed from outside to inside as a sample, and observe the overall condition of the sample. Divide the residual lining sample into four layers from outside to inside according to different colors. The division of residual lining layers is shown in Figure 2.

Figure 1 Damage to Burner Lining Castables

Figure 2 Analysis of 4 Levels of Lining Castables

From the appearance of the damaged castable, it can be clearly seen that the first layer is light yellow and has obvious erosion, indicating that a liquid phase has been produced on the surface of the castable. The second layer appears white and has the loosest structure. The castable has completely deteriorated and is basically a crystallized accumulation of alkali salts. Layer 3 appears light black and has a dense structure, and the aggregate particles in the castable components can be seen. Layer 4 appears gray and has a dense structure. No crystalline material is found. The matrix part of the castable and the aggregate particles can be clearly seen.

After combining experiments and analyzing its components, a conclusion is drawn.

(1) The outer surface of the burner lining castable produces low melting point anorthite and potassium feldspar, which produces a liquid phase at high temperatures, reducing the surface strength of the castable and causing corrosion damage to the castable.
(2) The middle brushing of high-temperature secondary air damages the SiO₂ film formed by the oxidation of SiC on the surface of the castable, causing alkali salts to enter the castable to form continuous erosion.
(3) The chemical corrosion damage of the burner mainly comes from K salt, which mainly has two aspects: on the one hand, the K salt that penetrates into the castable reacts with the matrix to generate new compounds, destroying the matrix composition of the castable. On the other hand, due to the density difference of the potassium salt itself, volume expansion occurs after deposition and cooling in the castable matrix. The combined effect of two reasons leads to damage to the castable lining.

Cost-Effective Low-Cement Castable with Excellent Thermal Shock Stability

July 10, 2024July 9, 2024 by rsrefractorycastable

Low cement castable is one of the most widely used castables in thermal kiln linings, with Cost-Effective performance and easy construction. Low-cement castables have the advantages of less water addition, high density, good volume stability, and high strength at high temperatures. However, low-cement castables have poor air permeability and poor thermal shock resistance at high temperatures. When the castable is baked after being demolded, the low-cement castable is prone to bursting, peeling, peeling, etc. In severe cases, it may even damage the entire kiln lining. Therefore, how to improve and enhance the explosion-proof performance of low-cement castables is of great significance to the large-scale application and development of low-cement castables.

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How to Increase the Explosion-Proof and Thermal Shock Stability of Low-Cement Castables?

Rongsheng castable manufacturer, in order to improve the explosion-proof properties of low-cement castables, uses the method of adding explosion-proof fibers to low-cement castables to increase the permeability of the castables. The addition of explosion-proof fiber significantly improves the explosion-proof performance of low-cement castables. After the explosion-proof fiber is added to the castable, the explosion-proof fiber will shrink and melt when the castable is heated and baked after demoulding. Elongated pores are formed in the castable, increasing the opening vents inside the castable and improving the air permeability of the castable. This accelerates the discharge of water vapor, thereby improving the burst resistance of the castable and improving the explosion-proof performance of the castable.

The overall strength of low cement castables is very high at high temperatures. However, in some thermal kilns that work intermittently, frequent shutdowns and restarts will affect the service life of the low-cement castable working lining. In the case of rapid cooling and heating, it is very easy for the castable lining to peel off and crack, affecting the service life of the low cement castable.

Rongsheng Unshaped Refractory Castable Manufacturer, adding a certain amount of silicon carbide to low-cement castables will significantly improve the thermal shock stability of low-cement castables when facing rapid cooling and rapid heating conditions. However, the amount of silicon carbide incorporated needs to be strictly controlled. Because silicon carbide has a high thermal conductivity, when too much silicon carbide is added, the overall thermal expansion coefficient of low cement castables will increase, which is detrimental to the thermal shock stability of low cement castables. Castable manufacturers strictly control the amount of silicon carbide added to optimize the thermal stress of low-cement castables caused by excessively high thermal conductivity of silicon carbide. Only then will the low-cement castables have the best thermal shock stability when subjected to thermal shock. .

Whether low-cement castables need to improve their explosion protection or thermal shock stability, a comprehensive analysis needs to be made based on the working environment, operating temperature, erosion conditions, etc. of the thermal kiln. Then, make a reasonable formula based on the actual use needs of the enterprise’s kiln. The refractory material industry is an industry with very strong practical applications. There is no perfect refractory material. You can only find the most suitable refractory material based on the actual situation.

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Silicon Carbide Composite Refractory Castable

What happens when silicon carbide is added to low-cement refractory castables? That turns into silicon carbide low cement castable!

The biggest feature of silicon carbide castable is wear resistance, followed by strong corrosion resistance. Its technological feature is that based on the process of refractory castables, a certain proportion of silicon carbide is added to form a composite refractory castable.

Because silicon carbide has strong wear resistance, adding it to refractory castables can improve the wear resistance of the castables. Moreover, the wear resistance coefficient of the castable is reduced, and the service life can be increased by 1 to 2 times. Silicon carbide has high thermal conductivity and small thermal expansion coefficient. Adding an appropriate proportion can increase the thermal shock resistance of the castable, and the thermal shock resistance of the castable is also relatively good. Especially for silicon carbide combined with silicon nitride products, the thermal shock resistance will be increased by 2 times compared to the thermal shock of the castable before adding it. It also prevents slagging or skin formation on the surface of the castable.

However, refractory castables with high silicon carbide content have high thermal conductivity, and the disadvantage of silicon carbide castables is that they are easily oxidized. Manufacturers add metallic silicon powder to the castable to inhibit the oxidation of silicon carbide. Castables combining silicon nitride with silicon carbide have lower oxidation resistance. Because the base material of silicon nitride combined with silicon carbide castable is in the form of interwoven fibers, it has high air permeability and plays a small protective role on silicon carbide particles. In silicate-bonded and silicon oxynitride-bonded silicon carbide products, the surface of the silicon carbide particles is wrapped by a continuous base material. Therefore, it has strong antioxidant properties.

Silicon carbide castables can maintain high strength and wear resistance at high temperatures, and have good chemical stability and will not be eroded by acid and alkali solutions. In addition, the wetting angle of silicon carbide with molten metal and slag is large. It has good corrosion resistance to various furnace lining solids, liquids and gases.

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Rongsheng Unshaped Refractory Castable Materials Manufacturer is a powerful manufacturer of refractory materials production and sales. Our environmentally friendly, fully automatic unshaped refractory material production line ensures the smooth delivery of unshaped refractory materials. Moreover, our professional technical team can customize the refractory castable formula according to the actual working conditions of the kiln lining to ensure the long service life of the refractory lining. Contact us to get free samples and quotes.