How to Prevent Oxidation of Al2O3-SiC-C Castable?

Technological progress. Rongsheng monolithic refractory manufacturer, environmentally friendly fully automatic monolithic refractory production line, specializes in providing high-quality refractory products for high-temperature industrial furnace linings. After years of hard work, Rongsheng’s refractory products 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, Iran, Iraq, Israel, etc. Rongsheng’s iron trough castable, Al₂O₃-SiC-C castable, has also improved its oxidation resistance compared to before. Contact Rongsheng for detailed information. So, how does Al₂O₃-SiC-C castable prevent oxidation?

Rongsheng Al2O3-SIC-C Castable
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    Preferential Oxidation of Carbon Source

    Antioxidants are added to the castable material of the iron trough. The main principle is that the affinity of the antioxidant with oxygen is greater than the affinity of carbon with oxygen. The added antioxidant reacts with oxygen before carbon to achieve an anti-oxidation effect. Anti-oxidation mechanism of adding borides, Si and Al:

    • (1) The additive itself undergoes oxidation reaction before carbon.
    • (2) Part of the low-melting phase is generated during the high-temperature process, which blocks the porosity inside the material and increases the density of the material, resulting in the material having good anti-oxidation properties.

    The effect of the amount of metal silicon powder added on the performance of the iron trough castable. From the cross-section, as the amount of metal silicon powder added increases, the area of ​​the darker color area in the cross-section increases, indicating that the anti-oxidation effect is better. During the sintering process of the sample, the anti-oxidation mechanism of metal silicon powder as an antioxidant is as follows:

    anti-oxidation mechanism 1

    In the same time, as the amount of metal silicon powder added increases, the amount of carbon reduced in the sample increases. At the same time, the more SiO2 is generated by the reaction, of which SiO2 is in the liquid phase, which can close the pores and increase the density. In this way, the anti-oxidation performance and comprehensive mechanical properties of the castable are improved.

    Figure 1 Cross-section of samples with different amounts of metal silicon powder added after oxidation
    Figure 1 Cross-section of samples with different amounts of metal silicon powder added after oxidation

    Adding metal Al powder to carbon-containing refractory materials can have an antioxidant effect. This is mainly because during the heat treatment process, Al reduces CO to C(s) and generates Al2O3, which inhibits the oxidation of C. The reaction formula is as follows:

    reaction formula 2

    Volume expansion will occur during this reaction process, increasing the density of the material structure, thereby hindering the oxidation of carbon. However, metal Al will react with carbon above 1 000°C to generate Al4C3. And Al4C3 will react with water vapor during the reaction process:

    reaction process 3

    The material will have a significant volume expansion, seriously deteriorating the comprehensive performance of the material.

    During the oxidation process at 1100°C and 1500°C, the antioxidant is BN, and its reaction equation is:

    reaction equation 4

    This shows that BN has an antioxidant effect. In addition, the generated B2O3 promotes the sintering of the sample, blocks the pores of the sample, and makes the structure of the sample denser. But the generated N2 will also cause the volume expansion of the sample.

    In addition, B4C was added to Al2O3-SiC-C castable to study the effect of B4C on its mechanical properties. B4C will undergo oxidation reaction in air atmosphere:

    oxidation reaction 5

    The generated B2O3 liquid phase can promote sintering and improve the anti-oxidation effect of the material. The improvement of anti-oxidation property also further inhibits the oxidation of carbon-containing materials such as SiC, and also has a positive effect on the high-temperature flexural strength of the sample.

    oxidation of carbon-containing materials 6

    It can be seen that the anti-oxidation mechanism of CaB6 is the same as that of B4C and BN. They all react with O2 to generate liquid phase, and then this liquid phase further promotes the sintering of the sample. Improve the density of the material, and then improve the anti-oxidation effect of the material.

    Reaction to form a protective layer

    Unlike additives that preferentially oxidize carbon sources, some low-melting-point additives form a liquid phase with oxygen or components in the material at high temperatures. The generated liquid phase forms a protective layer on the surface of the material and fills the pores inside the material to prevent oxygen from entering the material, thereby achieving a certain anti-oxidation effect.

    Si3N4-Fe was added to Al2O3-SiC-C castables and its effect on material properties was studied. The experiment found that when Si3N4-Fe was added to Al2O3-SiC-C castables, Si3N4 in Si3N4-Fe was first oxidized, and SiO2 was generated on the surface of the sample, which formed a low-melting phase with Al2O3 or impurities in the material, and further formed a glass phase film on the surface of the sample. It prevented O2 from entering the sample, thereby achieving a certain anti-oxidation effect.

    ZrB2 was added to Al2O3-SiC-C castables. It was found that the thickness of the oxide layer gradually decreased with the increase of the amount of ZrB2 added. In the presence of carbon, carbon is first oxidized to CO, and CO continues to react with ZrB2 as follows:

    CO react with ZrB2 7

    On the one hand, it hinders the oxidation of carbon. On the other hand, the generated B2O3 liquid phase and the liquid phase generated by the reaction of B2O3 and CaO form a protective layer on the surface of the material. It hinders O2 from entering the interior of the sample and further prevents the oxidation of carbon.

    The effects of andalusite particle size, composition and addition amount on the performance of aluminum silicon carbide refractory materials were studied. The experiment found that when andalusite was added to Al2O3-SiC-C castables, the silicon-rich glass phase generated blocked the pores inside the material due to the transformation of andalusite into mullite. In addition, the generated glass phase formed secondary mullite with the original matrix and aggregate of the material, which enhanced the dense structure of the material. Both of these help to prevent O2 from entering the material. The effect of andalusite particle size and purity on the anti-oxidation effect of Al2O3-SiC-C castables. The anti-oxidation effect of andalusite with a particle size of 1 to 3 mm is better than that of 5 to 8 mm and 3 to 5 mm. This is because the smaller the particle size of andalusite, the lower the purity, and the faster the mullite transformation rate. Based on the fact that andalusite has a certain anti-oxidation ability. Replace part of the antioxidant in the castable with andalusite. The experiment found that replacing part of the antioxidant silicon in the castable with andalusite has a better anti-oxidation effect than the antioxidant silicon powder. At present, there have been many studies on the addition of andalusite to Al2O3-SiC-C castables. The anti-oxidation effect of andalusite on castables mainly comes from the high-temperature phase transformation of andalusite to produce mullite and silicon-rich glass phase. As an aluminosilicate mineral, kyanite also has high corrosion resistance and good mechanical properties when converted into mullite after calcination. By adding kyanite powder to Al2O3-SiC-C castables, it was found that mullite phase was detected in the sample with kyanite powder added, and the formation of liquid phase reduced the apparent porosity of the sample and increased the density.

    Add calcium aluminotitanate to the iron ditch castable. Replace the brown corundum in the iron ditch castable with calcium aluminotitanate, and the content of Al2O3, CaO and TiO2 in the sample increases. During the high-temperature reaction process, CaO, SiO2 and Al2O3 in the matrix react to generate low-melting phases such as CaSi2Al2O8 and mullite, which increases the density of the sample, hinders O2 from entering the material, and improves the anti-oxidation effect. However, when the substitution rate of calcium aluminate titanate is too high, the increase in the amount of low-melting phase and mullite generated causes the volume expansion of the sample, leading to the proliferation and expansion of cracks, which in turn reduces the anti-oxidation effect and mechanical properties of the sample.

    Optimizing the Form of Carbon Source

    Common carbon sources for iron trough castables include high-temperature modified asphalt, spherical asphalt, flake graphite, etc. Researchers are trying to find a way to modify graphite, that is, to optimize the form of carbon source to improve the anti-oxidation effect. At present, one way to optimize the form of carbon source is to add a coating on the surface of graphite so that oxygen cannot directly react with carbon. The first thing to consider when choosing a coating is whether the thermal expansion coefficient between the coating and the carbon-containing refractory material matches. The most commonly used coatings are silicon nitride and silicon carbide, which have similar thermal expansion coefficients to the carbon matrix and good chemical compatibility. According to the actual use environment of the coating, the following factors should also be considered when formulating the coating:

    • (1) Diffusion rate of oxygen.
    • (2) Fluidity of the coating raw material at high temperature.
    • (3) Chemical stability.

    The second form is to change the carbon source by adding modified graphite, such as composite carbon source form, granulated graphite, carbon black, etc. Carbon oxidation is reduced by increasing the fixed carbon content of the carbon source and reducing the carbon volatility. The effect of the amount of granulated graphite added on the properties of Al2O3-SiC-C castables was studied. Granulated graphite with a carbon mass fraction of 35% was used as the carbon source, and its effect on the anti-oxidation properties of Al2O3-SiC-C castables was investigated by changing its addition amount.

    Al2O3-SiC-C castables have been widely used in castables for iron troughs, and currently have excellent use effects. However, how to further extend the service life of the material and improve the performance of the material depends to a large extent on its anti-oxidation effect. Al and Si powders will undergo hydration reactions. Although the liquid phase generated by low-melting phase additives will block the pores inside the sample and increase the density. However, excessive liquid phase causes the volume expansion and crack propagation of the sample. At the same time, the liquid phase generated by the additives during the heat treatment process will form a thin film on the surface of the material to prevent oxygen from entering the material and reduce the oxygen partial pressure inside the material. Secondly, adding a coating on the surface of the carbon source, although this achieves a certain anti-oxidation effect, there are also problems such as deterioration of material performance and cumbersome production. In order to further improve the anti-oxidation technology of Al2O3-SiC-C iron trough castables and extend the service life of iron trough castables, studying low-cost anti-oxidation technology that can simultaneously improve the anti-oxidation effect and material performance is one of its important development directions.

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      Application of Ferrosilicon Nitride in Al2O3-SiC-C Blast Furnace Iron Trench Castables

      The blast furnace iron trench is a channel for guiding high-temperature molten iron and molten slag. Its refractory lining is subject to high temperature, mechanical and chemical erosion by molten iron and slag liquid. The first is the scouring effect on the surface of the refractory material when iron and slag flow. Secondly, due to the frequent alternation of hot and cold during tapping or the long-term action of high temperature, micro-cracks occur in the refractory material. Such microcracks promote the penetration and structural damage of molten iron and slag. Therefore, the iron trench lining material is required to have strong resistance to the erosion of molten iron and slag, good thermal expansion stability, small change in burnt volume, and strong oxidation resistance. It is easy to construct and does not adhere to slag iron, making it easy to repair and dismantle. It has a uniform, highly dense structure and high strength. It does not produce harmful gases and is conducive to environmental protection.

      Rongsheng Low Cement Silicon Carbide Castable (Al2O3-48%,SiC-30%)
      Rongsheng Silicon Carbide Castable Al2O3-SiC-C Castable

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        Service Life of Blast Furnace Tap Trough Refractory Materials

        After the blast furnace became larger, the usage conditions of the tap trough also changed, and the erosion and wear of the trough wall by molten iron increased. Increasing the generation gap, reducing costs and reducing the work pressure of workers behind the furnace are the main goals of the work behind the furnace. For the main iron trench and different parts, the erosion conditions are different, and the selection of materials should be different to achieve a comprehensive and balanced economic life. For iron ditches, efforts should be made to improve the anti-penetration properties of the material. As far as the construction technology and maintenance process are concerned, some operating methods are also closely related to the service life of the material.

        Damage Mechanism of Blast Furnace Iron Trench Refractory Materials

        The damage to refractory materials used in blast furnace iron trenches is mainly due to cracks, chemical erosion, penetration, and erosion caused by thermal shock of high-temperature molten iron and slag.

        The first is the effect of high-temperature molten iron. Since the blast furnace has a certain pressure, the molten iron with a temperature of up to 1500°C spurting out from the blast furnace directly impacts the working surface of the iron trench. This results in erosion and damage of trench lining materials, especially within the impact zone. The vortex formed here not only intensifies the thermal impact of molten iron and slag on the trench lining, but also intensifies the erosion and erosion of the iron trench lining. The most severe damage is to the trench lining material. Therefore, the damage here determines the service life of the tap hole.

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          Secondly, in the metallurgical industry, the damage caused by molten slag to refractory materials is the most serious. Since the slag contains relatively complex chemical components, it is easy to react with components in refractory materials to form low melting point substances. Causes damage to the structure of the refractory material and reduces the corrosion resistance, erosion resistance and other properties of the refractory material. When the blast furnace taps iron, in addition to high-temperature molten iron, there is also high-temperature slag entering the tap trough. Slag not only aggravates the erosion of the iron trench lining, but also tends to adhere to the trench wall. It is the main factor that damages the trench lining material. In addition, the damage to the iron trench lining material caused by molten slag also varies depending on the structure of the tap trench. For molten iron tap trenches, the main damage caused by molten slag is erosion and erosion. As for the non-molten iron tap trough, in addition to the above functions, more and more slag adheres to the trench wall and is deposited, which hinders the normal use of the tap trough. Therefore, slagging operations must be carried out in front of the furnace. When slag removal is carried out, the refractory material of the trench wall will be removed together with the bonded slag, which is the greatest damage to the non-molten iron tapping trench lining.

          Third, although the blast furnace is a high-temperature equipment that operates continuously, tapping is an intermittent operation, causing the trench lining refractory material to withstand rapid changes in temperature. At this point, the molten iron tap trough also has advantages over the non-molten iron tap trough. Therefore, rapid changes in temperature reduce the thermal shock stability of iron trench lining refractory materials.

          In addition to the above damage, the penetration of molten iron into the iron trench lining material is also one of the factors that destroys the material structure. It causes the material to erode and peel off, thereby reducing erosion resistance.

          Rongsheng Silicon Nitride Combined Silicon Carbide Castable
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            Ferrosilicon Nitride is Used in Al2O3-SiC-C Blast Furnace Iron Trench Castables

            In the past, the iron trenches of small and medium-sized blast furnaces were mainly artificial ramming materials mixed with coke, clay clinker and clay, mixed with tar or syrup, and were manufactured by each ironworks. This kind of iron trench lining has a short service life and the iron flow capacity is less than 10,000 tons. In long-term production practice, people have gradually realized that adding silicon carbide and graphite can significantly increase the service life of the tap hole. In terms of refractory aggregates, corundum materials, high-aluminum vanadium clay, magnesia-aluminum spinel materials, etc. have been tested. In terms of binders, various iron trench materials including resin bond, cement bond, and clay bond were tested. After continuous attempts, various types of iron trench materials suitable for large, medium and small blast furnaces have been gradually formed. For high-temperature and high-pressure operations, the tapping volume is large, the flow rate is fast, the temperature is high (1500°C), the tapping times are frequent, the time is long, the amount of slag is large, and the tap trough lining material is used in harsh conditions. To this end, a new generation of iron trench materials based on Al2O3-SiC-C has been developed, which basically meets the production requirements, and the iron throughput ranges from tens of thousands to 100,000 tons. Blast furnace trench castables.

            Al2O3-SiC-C castable has good slag erosion resistance and erosion resistance. It has been widely used in the main trench, skimmer and branch trench of blast furnace tap trench. However, due to the development of smelting technology, the continuous improvement of blast furnace utilization coefficient and the requirement of longevity of blast furnace, it is urgent to further improve the service life of Al2O3-SiC-C iron trench castables. However, the current Al2O3-SiC-C iron trench castables are prone to falling off due to periodic chemical erosion of slag and molten iron, thermal shock and erosion of slag and iron. At the same time, the oxidation of silicon carbide and carbonaceous materials in the castables at high temperatures will also cause structural damage to the materials, which will lead to the damage of the castables.

            Ferrosilicon Nitride Al2O3-SiC-C Iron Trench Castable

            In recent years, research on the use of ferrous silicon nitride in Al2O3-SiC-C iron trench castables has been increasing. Si3N4 in ferrosilicon nitride has the advantage of not being completely wetted with slag and iron, which can improve the corrosion resistance of iron trench castables. The oxidation product of Si3N4 will form a SiO2 protective film on the surface of the sample, hindering further oxidation of the material and enhancing its anti-oxidation performance. The metallic phase Fe has the effect of assisting sintering and can improve the mechanical properties of the castable. It has better antioxidant properties than pure Si3N4. At the same time, the N2 generated by the oxidation of Si3N4 in ferrosilicon nitride at high temperatures and the CO generated by the oxidation of carbon materials will block the internal pores of the material, thus effectively preventing further oxidation.

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              Iron Ditch Anti-Scouring Silicon Carbide Refractory Castable

              The blast furnace tap trough system consists of a main tap trough, branch troughs, and slag troughs. It is an important system to realize the separation of molten iron and slag in the blast furnace. With the enlargement of iron-making blast furnaces and the development of high-temperature and high-pressure technology, higher requirements have been placed on the service life and iron flux of the blast furnace tap trough. The trough material is the main factor affecting the performance of the blast furnace tap trough. Its high-temperature resistance, thermal shock resistance, erosion resistance, erosion resistance, and other properties directly affect the iron flux and service life of the blast furnace tap channel. This in turn affects the production cost and labor intensity of the entire production process.

              Aluminum Silicon Carbide Castables
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                Refractory Material Al2O3-SiC-C Castable for Tap Trough

                At present, the refractory materials used in the blast furnace tap trough are mainly Al2O3-Si C-C castables. Although it has the advantages of good high-temperature mechanical strength, good resistance to slag erosion and permeability, good thermal shock resistance, and small changes in reburning. However, in the face of technological advancements such as increased molten iron temperature, increased iron tapping volume, prolonged iron tapping time, and faster-molten iron flow rate caused by modern large-scale blast furnace smelting. The original iron trench materials cannot meet production needs. Therefore, improving Al2O3-SiC-C castables or developing new refractory materials for blast furnace tap troughs are the research directions of many scholars.

                Aluminum silicon carbide carbonaceous castables, which are unshaped refractory materials, are often used in blast furnace tap troughs, so they are also called trough castables. The blast furnace tap trough castable is made of fused corundum (or sintered corundum, special high-aluminum clinker) and silicon carbide. It is a high-performance refractory castable containing 90%-95% fixed carbon (or flake graphite), binder, and various admixtures.

                Aluminum silicon carbide carbonaceous castable. The aggregate part can be made of one or two types of fused or sintered corundum, brown corundum, spinel, and bauxite clinker. Carbonaceous raw materials can be asphalt, coke, graphite, or electrode powder. The binding agent can be bonded clay, oxide micro powder or ultrafine powder, and pure calcium aluminate cement with good dispersion properties.

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                  The blast furnace tap trough is a channel through which high-temperature molten iron or molten slag flows. Its refractory lining should have the following characteristics. These features are complementary to each other and should be compatible with each other.

                  • ① Strong resistance to erosion and erosion from molten iron and slag.
                  • ② Good thermal shock resistance and small change in volume after reheating.
                  • ③ Strong antioxidant capacity.
                  • ④ Easy to construct, can be baked quickly without bursting, and is easy to repair and dismantle.
                  • ⑤ When used, it does not produce harmful gases, does not stick to slag iron, and has a long service life.

                  Iron trench castable is a high-tech refractory castable with exquisite preparation technology and excellent performance, which can significantly increase the service life of the trench lining. The key issue is that the amount of construction water must be strictly controlled, operated carefully, and maintained carefully to achieve the expected purpose.

                  At the same time, the specific selection of tap trough castables also depends on the size of the blast furnace. Generally speaking, high-aluminum silicon carbide carbonaceous castables can be used for the main ditch, slag ditch, and slag line of small blast furnaces. Corundum silicon carbide carbonaceous castables can be selected for medium and large blast furnaces.

                  Wear-Resistant Corundum Silicon Carbide Castable
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                    Erosion-resistant Molten Iron-Clad Aluminum Silicon Carbide Castable

                    Spheroidized ladle castable is an unshaped refractory material with high aluminum and silicon carbide materials as the main raw materials and refractory cement as the binder. It is widely used in molten running ladles, launders, and working linings of kilns. It has high strength and refractory resistance and has good impact resistance, corrosion resistance, wear resistance, and other properties. It has a long service life, convenient construction, and easy maintenance. It is suitable for the overall lining of ladles and is the refractory material of choice for spheroidized ladles.

                    Molten iron and slag react with the castable, resulting in chemical slag sticking. According to ASTM C01-91 Al2O3-SiO2 series castables, those with CaO content >2.5 are traditional cement-bound castables. The CaO content between 1.0% and 2.5% is low cement castable. The CaO content <1.0% is cement-castable. The lower the CaO content, the higher the temperature at which the liquid phase appears, and therefore the higher the fire resistance. The CaO content of the existing castable is 3.78%, which is a traditional high cement content castable. Aluminum silicon carbide castables are erosion-resistant, heat-exchange-resistant, refractory castables, and ladle castables.

                    The liquid phase of this material appears at a low temperature, about 1300°C. The temperatures of molten iron and slag are generally >1300°C, and the interface is prone to reaction at this time. For example, the components in the refractory material, especially the liquid phase in the matrix, easily react with CaO, SiO2, Fe2O3, etc. in the slag to form Al2O3-SiO2-CaO-Fe2O3 series low-melting materials. The molten iron, molten slag, and refractory materials are melted together to form sticky slag. The sticky residue that occurs in this form is strong and difficult to clean.

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                      Development of Aluminum Silicon Carbide Refractory Castables for Tap Trench

                      Al2O3-SiC-C castable is the most widely used iron trench refractory material in my country. Improving the quality of Al2O3-Si C-C castables is an important factor in reducing the cost of molten iron and achieving efficient production. RS Unshaped Refractory Castable Manufacturer puts forward several suggestions based on the current development situation.

                      (1) Different companies have different compositions of molten iron and slag. The reasons why molten slag and molten iron corrode the iron trench materials should be analyzed based on the actual situation to achieve “prescribing the right medicine”.

                      (2) Using slag or other industrial waste residues as raw materials to prepare Al2O3-Si C-C castables has extremely high added value. At the same time, it can also solve problems such as waste residue accumulation and environmental pollution.

                      (3) Improve the production of Al2O3-Si C-C castables in the direction of green and environmental protection, and remove or replace toxic and harmful components in the raw materials. It not only achieves green production but also protects the health of workers.

                      (4) Many new Al2O3-Si C-C castables have been applied in enterprise blast furnace production, achieving mutual benefit and win-win results between scientific research and enterprises.

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