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Importance of Refractory Aggregate in Refractory Castables

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Refractory aggregate is a key component of refractory castables. Its type, grade, and particle size distribution are key factors influencing various castable properties.

Performance Requirements for Refractory Aggregates

High-Temperature Stability

Refractory aggregates must exhibit excellent chemical stability at high temperatures, resisting chemical reactions with the melt and gases within the furnace and maintaining their structure and properties. They must also possess a low coefficient of thermal expansion and good thermal shock resistance to withstand rapid temperature fluctuations without breaking.

Most refractory aggregate particles are multiphase and polycrystalline. Therefore, their shape is influenced by the crystal structure, crystallization behavior, and impurity content of the various phases in the material, as well as the processing methods used.

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    For example, mullite produced by the electric fusion process, because it precipitates from the melt, is mostly euhedral. Therefore, due to its crystallization behavior, mullite often forms columnar polycrystalline aggregates. During crushing, the grains fracture along the longitudinal direction, where the bonding strength is poor, resulting in the formation of columnar polycrystalline particles.

    Mullite produced by sintering, however, has crystal growth that is constrained by the surrounding environment. This results in morphologies such as needles, columns, plates, and granules, with irregular interlacing growth. Consequently, the crushed particles have irregular shapes, including flakes, needles, columns, and fusiform shapes.

    The morphology of crushed refractory aggregate particles also depends on the density of the material and the crushing method. For example, for ultra-dense and high-density high-alumina clinker, impact or extrusion crushing methods often produce flake or fusiform aggregate particles. Grinding crushing methods often produce irregular granular or nearly spherical shapes. Therefore, to produce aggregate particles that are most suitable for monolithic refractory materials, a more appropriate crushing method should be selected.

    Mechanical Strength

    Under high-temperature operating conditions, refractory aggregates must withstand the impact, abrasion, and pressure of the furnace charge. Therefore, they must possess sufficient mechanical strength, including compressive strength, flexural strength, and wear resistance.

    Particle Size Distribution

    A reasonable particle size distribution is crucial to the performance of refractory materials. Typically, refractory aggregates are composed of particles of varying size grades to form a tightly packed structure, enhancing the material’s density and strength.

    In the production of monolithic refractory materials, refractory aggregates are generally divided into coarse, medium, and fine aggregates. The particle size ranges of coarse, medium, and fine aggregates are related to the critical particle size (i.e., particle size) of the aggregate.

    • For example, if the aggregate particle size is 8 mm, the coarse aggregate particle size range is 8-3 mm, the medium aggregate particle size range is 3-1 mm, and the fine aggregate particle size range is 1-0.088 mm. Particles smaller than 0.088 mm are not considered aggregates but are instead called powder (or matrix).
    • For example, if the aggregate particle size is 3.5mm, the coarse aggregate particle size range is 3.5-1.5mm, the medium aggregate particle size range is 1.5-0.5mm, and the fine aggregate particle size range is 0.5-0.074mm.

    The ideal aggregate particle size distribution should achieve the most compact packing. That is, the gaps left by the coarse particles are filled by the medium particles, and the tiny gaps left by the medium particles are filled by the fine particles, thus forming a skeleton. The remaining gaps are filled by the powder. However, due to the irregular shape of refractory aggregate particles, achieving an ideal aggregate particle size distribution is difficult in actual production. Aggregate gradation is usually determined through experiments, and the typical ratio of coarse, medium, and fine aggregate is (35-45):(30-40):(15-25).

    Refractory Aggregates for Refractory Castables
    Refractory Aggregates for Refractory Castables

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      Refractory Aggregates for Refractory Castables

      1. Castable performance can be improved through refractory aggregate grading.

      The refractory aggregate grade, impurity content, sintering quality, and storage time of the raw materials also affect castable performance. As the Al₂O₃ content increases, the castable’s refractoriness and refractoriness under load also increase. When the post-firing linear change decreases, or when the material is underfired or contains a high impurity content, the refractoriness and refractoriness under load decrease, and the post-firing linear shrinkage increases.

      Before making refractory castables, it is important to select well-sintered, high-purity bauxite and carefully grade the refractory aggregate to improve the various properties of the castable.

      1. The particle size of the refractory aggregate is determined by the thickness of the lining.

      The more irregular the shape of the refractory aggregate particles, the better. Avoid using long, flake-shaped particles. The particle size is determined by the thickness of the lining.

      1. The Importance of Refractory Aggregates in Refractory Castables.

      The grade, impurity content, sintering quality, and raw material quality of the refractory aggregate significantly influence the density of the castable. These are the most basic principles that must be followed when determining the aggregate particle size gradation. The values may change after adding cement and powder, but the basic principles remain the same.

      When high-alumina cement is added to the castable, the material fluidity remains the same, but the water requirement is different. A better aggregate particle size gradation reduces water usage, increases bulk density, and reduces apparent porosity. Furthermore, the compressive strength at room temperature and after firing is also higher. When fine aggregate is present in larger quantities, its particle surface area increases, requiring the addition of more cement and powder to encapsulate the aggregate and prevent the aggregate from becoming densely packed. While a higher cement content also requires more water, encapsulation is crucial. Therefore, a proper mix ratio is crucial to avoid compromising the performance of the refractory castable.

      Refractory aggregate particle grading is generally a three-level gradation, with a high-quality gradation characterized by larger particles at the ends and smaller particles in the middle. When crushing refractory aggregate, the ratio of large, medium, and small particles should be controlled to prevent segregation of oversized particles, thereby creating a denser castable.

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        Applications of Refractory Aggregates

        Refractory aggregates are widely used in high-temperature industries such as steel, nonferrous metallurgy, glass, cement, and ceramics. The following are some specific application examples:

        1. In Steelmaking

        High-alumina refractory aggregates are commonly used in the shaft and waist of blast furnaces to withstand the erosion of high-temperature molten iron and slag, as well as the impact of furnace charge. Magnesia refractory aggregates are often used in electric furnace linings. Their excellent slag resistance and high-temperature resistance effectively extend the service life of electric furnaces.

        1. In the Glass Industry

        Silica refractory aggregates are the primary material for kiln refractory linings. For example, the tin baths in float glass production lines use refractory materials made from high-quality silica refractory aggregates, which can withstand temperatures up to 1600°C and resist the erosion of molten glass.

        1. In Cement Production

        The firing zone of rotary kilns typically utilizes refractory materials made from magnesia-chromium refractory aggregates to withstand the abrasion and chemical attack of high-temperature clinker.

        1. In tunnel kilns in the ceramic industry

        Refractory bricks made from mullite refractory aggregate are used in the kiln roof and walls, ensuring long-term stability and energy efficiency.

        In short, refractory aggregates, as a crucial component of refractory materials, play an irreplaceable role in ensuring the safe and efficient operation of high-temperature industries. Continuous in-depth research and innovation in the performance, preparation processes, and applications of refractory aggregates will provide strong support for the development of high-temperature industries.

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          What Should You Do if the Refractory Plastic Cannot be Cured?

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          Recently, a user of Rongsheng Refractory Material Factory consulted that a batch of refractory plastics purchased before could not solidify after ramming, and wanted to ask what was going on. Rongsheng Factory, based on years of production, sales, and construction experience, analyzed from the following aspects to help customers solve the problem. Rongsheng Refractory Castable Factory, a professional technical team, and an environmentally friendly fully automatic amorphous refractory production line provide material guarantees for the overall refractory lining of high-temperature industrial furnaces. Contact Rongsheng to get free solutions and quotes for refractory linings.

          Phosphate Bonded High Alumina Refractory Plastics
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            As we all know, the components of refractory plastics are made of granular and powdered aggregates, plastic clay, and other binders and plasticizers, and then a small amount of water is added. After fully mixing, it is a hard mud-paste-like amorphous refractory material that maintains high plasticity for a long time.

            1. What aggregates and binders are refractory plastic components divided into?

            According to the type of aggregate, it can be divided into clay, high aluminum, mullite, corundum, chromium, silicon carbide, and zirconium-containing refractory plastics. According to the type of binder, it can be divided into refractory plastics combined with aluminum sulfate, phosphoric acid, phosphate, water glass and resin.

            1. The plasticity index of refractory plastics requires that the amount of water added must be limited when adding binders

            Plastic clay is an additional binder for plastics. It only accounts for 10%~25% of the total weight of plastics, but it has a great influence on the bonding strength between plastics and their hardened bodies, the plasticity of plastics, the volume stability and refractoriness of plastics and their hardened bodies.

            The plasticity of refractory materials is related to the characteristics of clay, the amount of clay used and the amount of water. It mainly depends on the amount of water, which increases with the increase of water. However, too much water will bring adverse effects, generally 5%~10% is appropriate.

            1. Necessity of adding appropriate amount of plasticizer to refractory plastic

            In order to control the amount of clay in plastic and reduce the amount of water, it is necessary to add appropriate amount of plasticizer. Its plasticizing effects mainly include: increasing the hygroscopicity of clay particles. Dispersing clay particles and wrapping them with water film. Dispersing humus in clay and making clay particles sol. Increasing the electrostatic repulsion between clay particles in clay-water system. Stabilizing sol. Excluding ions that hinder solification from the system as insoluble salts, etc.

            1. Considering the hardening and strength of plastic, it is necessary to correctly select the binder

            Plastic without any added chemical binder is called ordinary plastic. This kind of plastic has very low strength before sintering, but as the temperature rises, water escapes and the strength increases. After high-temperature sintering, the cold strength increases. But the hot strength at high temperature decreases with the increase of temperature.

            The strength of plastic with sodium silicate increases faster with the increase of temperature after construction, and the mold can be removed faster after construction. However, during the drying process, this binder may migrate to the surface of the structure or product, preventing the smooth removal of moisture and causing stress and deformation of the surface.

            Aluminum phosphate is the most widely used thermosetting binder in plastics. After construction, it can obtain high strength after drying and baking.

            In order to improve the shortcomings of plastics with soft clay as binder, such as slow hardening after construction and low strength at room temperature, it is often chosen to add an appropriate amount of air-hardening and thermosetting binders and their polymers.

            1. High-quality plastics also need to have excellent thermal shock resistance

            The good thermal shock resistance of plastics mainly depends on the following aspects: Plastics made of aluminum silicate refractory raw materials as granular and powdered materials will not produce serious deformation caused by crystal transformation during heating or when used at high temperatures. The mineral composition near the heating surface is fine crystals of mullite and cristobalite, with less glass, and transitions along the heating surface to the low-temperature side. The structure and phase of the plastic are gradual rather than drastic. Due to the uniform porous structure in the plastic component, the expansion coefficient and elastic modulus are generally low.

            1. Correct construction method of refractory plastics

            The construction of plastics requires a tamping machine or manual tamping. The thickness of each addition is 80~100mm. After tamping, the surface is loosened, and then the material is added to continue tamping until it is completed. According to the type of binder, temperature and humidity conditions, the plastic should be naturally dried for a period of time before demolding. Because demolding too early will cause cracks on the stress surface of the anchor brick. When the temperature is around 25℃, the plastic combined with phosphoric acid should be demoulded after 48 hours of natural drying. The plastic combined with aluminum sulfate and clay should be demoulded after 72 hours of natural drying.

            1. The shelf life of refractory plastic should not be too long

            Refractory plastic generally has a shelf life of 15 days. Experienced manufacturers do not use hydraulic binders to ensure that the plasticity of plastic does not decrease significantly during its shelf life. Rongsheng Refractory Material Manufacturers recommends that users use up the products in one construction within 3-5 days after the arrival of the goods, and do not store them for a long time.

            Rongsheng Refractory Material Manufacturer recommends that you purchase high-quality refractory plastic products and read the instructions for the construction and use of refractory plastic in detail. Strictly follow the construction instructions for construction. Choose a professional construction team to carry out the construction of the refractory furnace lining. After the construction is completed, reasonable maintenance of the refractory furnace lining can effectively extend the service life of the furnace lining.

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              Thermodynamic Properties of Al2O3-SiC-C Castables

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              Al2O3-SiC-C castables have excellent thermal shock resistance and slag resistance, and are widely used in blast furnace tapping channels, ladle linings, and slag lines of mixed iron furnaces. Al2O3-SiC-C (hereinafter referred to as ASC) castables mostly use ultra-low cement bonding systems composed of alumina micropowder, silica micropowder, and a small amount of calcium aluminate cement. Compared with ultra-low cement bonding, calcium aluminate cement bonding without silica micropowder and cement-free bonding have the characteristics of high purity and small amount of liquid phase generated at high temperature. Therefore, it has excellent high-temperature mechanical properties and slag resistance.

              Al2O3-SiC-C Castable Refractory
              Al2O3-SiC-C Castable Refractory

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                Bonding Methods of ASC Castables

                For a long time, people have been accustomed to the ultra-low cement bonding method and the application of silica powder in ASC castables. Therefore, there are few applications and research on calcium aluminate cement bonded ASC castables without silica powder. Common cement-free bonding methods for castables mainly include silica powder agglomeration bonding, hydrated alumina bonding and sol bonding. The shortcomings of silica powder agglomeration bonding are low body strength, hydrated alumina bonding body strength, poor burst resistance, etc., which make these two bonding methods less used in ASC castables. In recent years, with the development of silica sol bonded castable technology, a small amount of silica sol bonded ASC castables have appeared on the market. With the development of alumina sol bonding system, alumina sol bonded ASC castables that replace hydrated alumina may have better performance.

                It is well known that the good thermal conductivity of silicon carbide and carbon in ASC castables gives the material better thermal shock resistance, and the non-wettability of carbon and slag gives the material better slag resistance. Carbon plays a key role in the structure and performance of ASC castables. However, carbon is easily oxidized at high temperatures, which deteriorates the material structure and performance. Carbon can be oxidized by oxygen in the atmosphere and by the material’s own oxide components, such as SiO2. When SiO2 and carbon coexist, the generation and escape of SiO2 gas at high temperatures will consume the carbon in the material, increase the porosity of the material, and thus deteriorate the performance of the material. It can be seen that as far as ASC castables are concerned, the SiO2 component may have an adverse effect on its performance. Based on the above analysis, this work studied the changes in phase composition and chemical composition of four common bonding methods of ASC castables: silica sol, silica-free micropowder cement, hydrated alumina and ultra-low cement, and conducted thermodynamic analysis.

                Rongsheng Al2O3-SIC-C Castable
                Rongsheng Al2O3-SIC-C Castable

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                  Thermodynamic Properties of Al2O3-SiC-C Castables

                  (1) After calcination at 1350℃, the mass of all samples with different bonding methods increased significantly. Compared with the samples calcined at 1350℃, the mass change rate of the samples calcined at 1500℃ decreased significantly except for the pure calcium aluminate cement bonding method without silica powder.

                  (2) Thermodynamic analysis showed that under the carbon coexistence calcination conditions in the Si-O-C-N system, SiO2 is a stable phase when the temperature is below 1431℃, and SiC is a stable phase when the temperature is above 1431℃. Si and SiC in Al2O3-SiC-C castables are oxidized by CO at low temperatures, which increases the C and SiO2 contents in the castables.

                  (3) Under the carbon coexistence calcination conditions, the stability order of SiO2-containing compounds is: cristobalite < mullite < anorthite. Under the condition of 1500℃ carbonization, both cristobalite and mullite can be reduced by C, while anorthite can only be reduced by C when there is enough Al2O3 around it. When the temperature does not exceed 1550℃, Si can prevent C from being oxidized by SiO2-containing compounds, and SiC can only prevent C from being oxidized when the temperature is lower than 1431℃.

                  (4) When the SiO2 content in the material is high, more SiO gas will escape from the reaction, which will lead to a reduction in sample mass and unstable structure. Considering that silica can promote the formation of SiC, the amount of silica powder added to ASC castables should be minimized.

                  Low Cement Silicon Carbide Castable in RS Factory
                  Low Cement Silicon Carbide Castable in RS Factory

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                    Low Cement SiC Refractory Castable

                    Low cement SiC refractory castable is characterized by low linear expansion coefficient, high thermal conductivity, high strength and good wear resistance. It has been applied in thermal equipment such as power generation boilers, non-ferrous metallurgical furnaces and incinerators, and the use effect is good. Low cement SiC refractory castable uses silicon carbide with SiC greater than 97% as refractory aggregate and powder, adds SiO2 ultrafine powder and metal silicon antioxidant, uses CA-70 cement as binder and adds polyphosphate water reducer. The main properties of this material are SiC of 85%, 110℃ drying bulk density, pressure resistance and flexural strength are 2.5g/cm3, 45MPa and 9MPa respectively. After burning at 1000℃, the linear change, pressure resistance and flexural strength are -0.2%, 107MPa and 24MPa respectively. After firing at 1450℃, the linear change, compressive strength and flexural strength are +0.3%, 130MPa and 54MPa respectively, and the thermal conductivity at 400℃ is 12.2W/(m·K).

                    Rongsheng Refractory Castable Manufacturer

                    Rongsheng Refractory Castable Manufacturer, advanced environmentally friendly fully automatic amorphous refractory production line, specializes in providing refractory products for high-temperature industrial furnace linings. Rongsheng’s refractory products have been sold to more than 120 countries and regions around the world, including 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. If you need to buy high-quality silicon carbide refractory castables, low-cement silicon carbide refractory castables, Al2O3-SiC-C castables, please contact Rongsheng manufacturers. Get free samples and quotes.

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

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                      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
                      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 Calcium Hexaaluminate Castable in Permanent Lining of Ladle

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                          As competition in the steel industry intensifies, steel companies are pursuing extreme energy efficiency to reduce production costs while developing new products to increase profits. Molten iron or steel will drop in temperature during transportation, and the materials and thermal insulation of ladles, torpedo tanks, ladles, and tundishes are closely related to the temperature drop.

                          At present, most manufacturers use the method of sticking insulation boards along the inner surface of the steel shell for the insulation of ladles and tundishes. However, insulation boards or nano insulation boards have problems of powdering and shrinkage during use, and thermal conductivity increases accordingly, and the insulation effect becomes worse. Some manufacturers use lightweight insulation bricks as insulation layers, but because of their low density and many pores, there are safety hazards when the working lining is damaged and the permanent layer cracks. Moreover, the thickness of lightweight insulation bricks will not be too thick due to the limitation of inclusion. Although there are studies on semi-lightweight castables containing porous spherical mullite or corundum, laboratory analysis results show that the thermal conductivity of semi-lightweight castables at around 1000°C is higher than that at room temperature. The refractory materials used in the water-cooled beams of the rolling mill heating furnace of Nippon Steel Corporation have developed from refractory bricks in the 1970s to plastics, ceramic fibers and thermal insulation castables, and then gradually developed into calcium hexaaluminate castables and prefabricated blocks, which are now called the fourth generation of thermal insulation materials. The main reason is that calcium hexaaluminate (CA₆) thermal insulation refractory materials have micron-level pores, and their thermal conductivity is kept at a low level from room temperature to 1500℃, and they have excellent high-temperature volume stability, thermal shock resistance, slag resistance, etc.

                          According to the characteristics of a 200t ladle in a certain factory, thermal insulation panels are used to assist calcium hexaaluminate castables as permanent layer materials. The effects of the permanent layer materials and structural changes of the ladle on reducing the temperature drop of molten steel are analyzed, and the economic and use effects of its application are analyzed and discussed.

                          Main Components of the Ladles
                          Main Components of the Ladles

                          Application of Calcium Hexaaluminate Castable in Permanent Layer of Ladle

                          After the capacity of 200t ladle of a steel plant was expanded, its permanent layer used high-aluminum castable, which was thinned from 120mm to 90mm. The working layer of the ladle wall uses corundum prefabricated bricks, and the thickness of the ladle wall is thinned from 200mm to 170mm, and the thickness of the slag line magnesium carbon brick is thinned from 220mm to 190mm. After the expansion, the heat dissipation of the ladle increases, the steel shell is deformed, and the temperature drop of the molten steel increases. In order to improve the insulation effect of the ladle, the thickness of the permanent layer remains unchanged, and the rest does not change. Only the material and structure of the permanent layer are changed to reduce its thermal conductivity. The permanent layer is insulated by an insulation board close to the steel shell, and calcium hexaaluminate castable is used inside. The thermal conductivity of the insulation board is ≤0.2W·m⁻¹·K⁻¹, there is no obvious crack after burning, the strength is high, and the maximum use temperature is 1200℃. According to the matching of the thickness of the insulation board of 10mm and the thickness of the castable of 80mm, the thermal simulation calculation is carried out.

                          The simulation results show that the temperature drop of the molten steel of a single ladle using 80mm thick calcium hexaaluminate castable + 10mm thick insulation board as the permanent layer is 45℃. The temperature drop of the molten steel of a single ladle using 80mm thick high-aluminum castable + 10mm thick insulation board as the permanent layer is 49℃. The temperature drop of the molten steel of a single ladle using only 90mm thick high-aluminum castable and no insulation board as the permanent layer is 86℃.

                          Therefore, using calcium hexaaluminate castable + insulation board as the permanent layer reduces the temperature drop of the molten steel by 4℃ compared with using high-aluminum castable + insulation board, and reduces the temperature drop of the molten steel by 41℃ compared with using only high-aluminum castable.

                          After the insulation board is pasted on the inner wall of the ladle shell, the bottom permanent layer is poured first, then the inner formwork is installed, and then the straight section of the hexaaluminate calcium castable is poured. The inner wall of the permanent layer after demoulding is smooth, crack-free, and has good integrity. After using a working layer overhaul cycle (170 furnaces), the inner wall of the permanent layer after overhaul was checked. Except for a small number of discontinuous cracks, there was no peeling damage and it can continue to be used. After the permanent layer uses low thermal conductivity hexaaluminate calcium castable, the shell temperature of the ladle for the 1st to 3rd heats is 199℃, and the slag line temperature is 251℃. The shell temperature of the ladle for the 6th heat is 224℃, and the slag line temperature is 262℃. Compared with the original high-aluminum castable permanent lining, the average temperature of the stabilized ladle shell is reduced by 30 to 50℃. The data of tracking the ladle with permanent layer using calcium hexaaluminate castable and high alumina castable shows that the temperature of the ladle with low thermal conductivity calcium hexaaluminate castable in the permanent layer drops by 0.46℃·min⁻¹ during vacuum treatment. The temperature of the ladle with high alumina castable drops by 0.97℃·min⁻¹ during vacuum treatment, and the insulation effect is significantly improved.

                          Steel Ladle Refractory Lining
                          Steel Ladle Refractory Lining

                          Economic Analysis of the Application of Calcium Hexaaluminate Castables in Permanent Layers

                          Due to the fierce competition among steel companies, they need to reduce production costs and develop low-carbon and zero-carbon products, so the ladle is under pressure to add scrap steel and expand capacity. However, adding more scrap steel will cause the temperature of the molten steel to drop, and the scrap steel needs to be supplemented or preheated. From the perspective of the enterprise, reducing the temperature drop of the molten steel can add more scrap steel, reduce supplementary heat, and reduce costs. Through the LF process, the electrode heats the molten steel. It is estimated that the power consumption per ton of steel is 0.4kWh for 1℃ increase in temperature, and the cost of 1℃ increase in temperature per ton of steel is about 0.3 yuan. In comparison, reducing the temperature drop of the molten steel by 1℃ will reduce the cost of 1 ton of steel by 0.3 yuan. Taking a 200t ladle as an example, according to the existing permanent layer overhaul furnace service, the cost of a ton of steel using insulation board + calcium hexaaluminate castable is about 0.49 yuan, and the cost of a ton of steel using high-aluminum castable is about 0.25 yuan, so the cost of a ton of steel is increased by 0.24 yuan. For steel mills, it is a favorable choice to use new permanent layer materials (insulation board + calcium hexaaluminate castable) to reduce the temperature drop of molten steel. For example, according to calculations, the temperature drop of molten steel can be reduced by 4°C, which reduces the cost of ton steel by 1.2 yuan. After deducting the cost of ton steel of new materials of 0.49 yuan, it is still 0.46 yuan less than the cost of ton steel of 0.25 yuan using high-aluminum castables, which is very beneficial to steel mills. However, for refractory suppliers, if there is no price compensation, they are not actively using new permanent layer materials.

                          The volume density of the calcium hexaaluminate castable in this test is 2.82g·cm⁻³. If moderately lightweight aggregates are used to reduce the volume density to 2.0~2.5g·cm⁻³, the cost will be reduced by about 11%~29%. If the density is moderately reduced, the thermal conductivity of the castable will also decrease, and the safety as a permanent layer will also increase. As for the insulation layer, the price will be lower if aggregates with lower bulk density (such as 0.8-1.5g·cm⁻³) are used as spray coatings. Under this condition, nano insulation boards or insulation boards can be eliminated. Through the use of calcium hexaaluminate materials and changes in structure, the cost per ton of steel with insulation layer refractory materials is expected to be reduced to 0.32-0.39 yuan, which is close to the cost of 0.25 yuan per ton of steel using high-aluminum castables as permanent layers. Coupled with the benefit of reducing the temperature drop of molten steel, steel mills and refractory suppliers can achieve a win-win situation. Therefore, calcium hexaaluminate materials have good development potential and application prospects.

                          Advantages of Calcium Hexaaluminate Castables in Permanent Layers

                          (1) The thermal conductivity of calcium hexaaluminate castables for permanent layers is about 0.631W·m⁻¹·K⁻¹ at 500, 800, and 1000℃, and the influence of temperature changes is small. The thermal conductivity of high-aluminum castables increases with increasing temperature, and the thermal conductivity at 1000℃ is 1.028W·m⁻¹·K⁻¹. Calcium hexaaluminate castables have better thermal insulation effect than high-aluminum castables at high temperatures.

                          (2) After the permanent layer of a 200t ladle is made of low-thermal-conductivity calcium hexaaluminate castable + thermal insulation board, the temperature of the ladle wall and slag line is reduced by an average of 30 to 50℃ compared with the original permanent lining of high-aluminum castables.

                          (3) It is expected that the volume density of calcium hexaaluminate castables can be reduced to 2.0-2.5 g·cm⁻³ by using moderately lightweight aggregates, and the cost will be reduced by 11%-29%. The reduced density can also reduce thermal conductivity. The moderate lightweighting of aggregates can reduce the amount of materials used, and combined with structural changes, it is expected that the cost of calcium hexaaluminate insulation materials can be reduced by 20%-35%. At the same time, the temperature drop of molten steel is reduced, the benefits are increased, and the synergistic effect can significantly reduce the production cost of steel mills.

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                            Construction Requirements of Refractory Castables for Tubular Heating Furnace Linings

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                            Tubular heating furnace lining construction regulations. Construction Requirements for Refractory Castables and Ceramic Fibers for Tubular Heating Furnace Linings. In the lining design of tubular heating furnaces for petrochemicals, the main application is refractory castables and ceramic fiber products. Its structure can be divided into castable lining structure, ceramic fiber structure, and composite lining structure. During the construction and installation process, it must be carried out in accordance with the following regulations:

                            1.  Under ambient temperature of 27℃ and windless conditions, the design temperature of the outer surface of the furnace body and hot smoke duct should not exceed 80℃.
                            2.  The design of the lining structure should allow all components to expand appropriately. When using multi-layer or composite linings, the joints should not continuously penetrate the lining.
                            3.  Unless otherwise specified, the allowable operating temperature of any layer of refractory material should be at least 165℃ higher than its calculated hot surface temperature, and the minimum allowable operating temperature of the refractory material of the radiation and shielding section should be 980℃.
                            4.  The minimum operating temperature of the burner brick should be 1650℃.
                            5.  The manhole door should be protected by refractory materials with the same thermal insulation performance as the surrounding refractory layer to avoid direct radiation.
                            6.  In addition to the cast structure lining, an anti-corrosion layer should be applied to the inner side of the furnace wall steel plate.

                            Monolithic Refractory Castables Lining Structure

                            1.  Hydraulic castable lining is applicable to all parts of the heating furnace. The type of castable material should be selected according to its use temperature and should comply with the relevant provisions of SH/T3115.
                            2.  For double-layer castable lining, the minimum thickness of the hot surface layer should be 75mm. The anchor should support each layer of lining.
                            3.  When the thickness of the castable lining is greater than 50mm, the height of the anchor should penetrate 70% of the thickness of the lining. The distance between its top and the hot surface should not be less than 12mm.
                            4.  The anchors should be arranged in a square shape, and the maximum spacing should be 3 times the total thickness of the lining, but should not exceed 300mm on the furnace wall and 225mm on the furnace roof. To avoid the formation of continuous shear surfaces, the fork direction of the anchors should be staggered.
                            5.  When the total thickness of the lining does not exceed 150mm, the minimum diameter of the anchor nail should be 5mm; when it exceeds 150mm, the minimum diameter of the anchor nail should be 6mm.
                            6.  The lining thickness of elbow box, tail flue, smoke duct and chimney should not be less than 50mm.
                            7.  Expansion joints should be left around burner brickwork and pre-burned molded products.
                            8.  Linings with a density greater than or equal to 970kg/m3 are allowed to be reinforced with metal fibers, and the amount of metal fibers added should not exceed 3% of the dry mix.
                            9.  When the total amount of heavy metals including sodium in the fuel exceeds 250mg/kg, the exposed hot surface layer should use low iron (iron content not more than 1%) or heavy castables. The density of heavy castables is at least that the content of AI2O3 in its aggregate should not be less than 40%, and the content of SiO2 should not be greater than 35%.

                            Ceramic Fiber Structure

                            1) Ceramic fibers with layered or modular structures can be used in all parts of the heating furnace except chimneys and flues.

                            2) The minimum thickness of the ceramic fiber blanket for the hot surface layer should be 20mm, and the density should not be less than 128kg/m3. The thickness of the ceramic fiber board used for the hot surface layer should not be less than 38mm, and the density should not be less than 240kg/m3. The minimum density of the ceramic fiber blanket used for the back layer should be

                            3) The allowable operating temperature of any layer of ceramic fiber should be 280℃ higher than the hot surface temperature of the acid meter.

                            4) The maximum distance from the anchor of the hot surface layer of the ceramic fiber blanket to all edges should be 75mm.

                            5) The furnace roof anchors are arranged in a rectangular shape. The spacing between them should not exceed the following values:

                            • The ceramic fiber blanket is 300mm wide and the spacing is 150mm×225mm
                            • The ceramic fiber blanket is 600mm wide and the spacing is 225mm×225mm
                            • The ceramic fiber blanket is 900mm wide and the spacing is 225mm×250mm
                            • The ceramic fiber blanket is 1200mm wide and the spacing is 225mm×270mm

                            6) The furnace wall anchors are arranged in a rectangular shape. The spacing between them should not exceed the following values:

                            • The ceramic fiber blanket is 300mm wide and the spacing is 150mm×300mm
                            • The ceramic fiber blanket is 600mm wide and the spacing is 225mm×300mm
                            • The ceramic fiber blanket is 1200mm wide and the spacing is 270mm×300mm

                            7) Metal anchors that are not covered by furnace tubes should be completely covered by ceramic fiber modules or protected by ceramic fiber blankets.

                            8) When the flue gas velocity exceeds 12m/s, the ceramic fiber blanket cannot be used for the hot surface layer:

                            • When the velocity is greater than 12m/s and less than 24m/s, the hot surface layer should use wet blanket, ceramic fiber board or ceramic fiber module.
                            • When the velocity exceeds 24m/S, the hot surface layer should use castable or outer protective layer.

                            9) When the ceramic fiber blanket is constructed, the maximum dimension direction should be consistent with the flue gas flow direction, and the connection of the blanket on the hot surface layer should be overlapped, and the overlap direction is along the flue gas flow direction. When the hot surface layer uses ceramic fiber board, it should be butt-jointed and the joints should be tight.

                            10) The ceramic fiber blanket used for the back layer should use a butt joint with a compression amount of at least 25mm at the joint, and all joints of adjacent layers should be staggered.

                            11) The ceramic fiber module should be constructed according to the vertical seam vertical masonry method, and the staggered mosaic method is only applicable to the furnace top.

                            12) When the ceramic fiber module is constructed, each side should be compressed to avoid shrinkage cracks.

                            13) The ceramic fiber module on the furnace roof should be designed so that its anchoring range should be at least greater than 80% of the module width.

                            14) The anchors should be fixed on the wall panels before the ceramic fiber module is constructed.

                            15) The anchor assembly should be installed less than 50mm from the module cold surface.

                            16) The metal parts in the module should be at least austenitic + stainless steel or nickel alloy.

                            17) When the ceramic fiber structure is used for fuel with a sulfur content greater than 10mg/kg, the inner surface of the shell should be coated with a layer of anti-corrosion paint, and the allowable operating temperature of the anti-corrosion paint should not be less than 180℃.

                            18) When the sulfur content in the fuel exceeds 500mg/kg, an austenitic stainless steel foil gas barrier layer should be set. The position of the gas barrier layer should be such that the temperature of the gas barrier layer should be 55℃ higher than the calculated dew point under any operating conditions. The edges of the gas barrier layer should overlap by at least 175mm, and the edges and openings should be sealed.

                            19) Ceramic fiber structures should not be used when the heavy metal content in the fuel exceeds 100 mg/kg.

                            20) Ceramic fiber structures should not be used in convection sections equipped with soot blowers, steam spray guns or water washing facilities.

                            21) Anchors should be installed before the wall panels are coated with anti-corrosion paint. The paint should cover the anchors, and the temperature of the uncovered part should be above the acid dew temperature.

                            Composite Lining Structure

                            1.  When using a cast hot surface layer, its minimum thickness is 75mm.
                            2.  The anchoring system should have a fixing and supporting function for each layer.
                            3.  For each lining, the type and installation of anchors shall comply with the requirements of Articles 2 and 3.
                            4.  The allowable operating temperature of any layer of material shall comply with the requirements of Articles 2 and 3.
                            5.  The insulation block shall be made of calcium silicate or slag wool with an operating temperature of at least 900°C. The insulation block can only be used as a backing material. However, it is not allowed to be used when the sulfur content in the liquid fuel exceeds 1% (mass fraction) or the hydrogen sulfide content in the gas fuel exceeds 100mg/kg.
                            6.  If the sulfur content in the fuel exceeds 10mg/kg, and insulation blocks or ceramic fibers are used as backing insulation, the wall panels should be coated with protective coatings, and the allowable operating temperature of the protective coatings should not be less than 180°C.
                            7.  If insulation blocks or ceramic fibers are used as the backing layer of the castable, they should be isolated to prevent water from seeping out of the castable.
                            8.  The density of the ceramic fiber module used as the backing material shall not be less than 190kg/m3, and the density of the ceramic fiber blanket shall not be less than 128kg/m3.

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                              Insulation Compression Non-Sintered High-Strength Low-Temperature Castable for Galvanizing Furnace

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                              Since 2003, when China Jiangnan Shipbuilding (Group) Co., Ltd. introduced China’s first high-speed pulse flame galvanizing furnace (Weistek®) to Dalian Xinyongshang Technology Co., Ltd., all structural galvanizing furnaces launched by Dalian Xinyongshang Technology Co., Ltd. to the world have been using heat-insulating and pressure-resistant non-sintered low-temperature castables as the bottom insulation material for the furnace bottom insulation structure for more than ten years.

                              In the hot-dip galvanizing industry in mainland China, whether it is a flat flame galvanizing furnace or a high-speed pulse galvanizing furnace, except for the furnace type launched by Dalian Xinyongshang Technology Co., Ltd. for the hot-dip galvanizing industry, as of the time of publication, all use heavy and lightweight refractory brick masonry structures.

                              Lightweight High-Strength Castable for Steel Ladle
                              Lightweight High-Strength Castable for Steel Ladle

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                                Advantages of lightweight heat-insulating, pressure-resistant, non-sintered low-temperature castable insulation layer compared to traditional insulation layer

                                Comparison between “lightweight heat-insulating, pressure-resistant, non-sintered low-temperature castable” and “refractory brick masonry insulation layer”, the application advantages are obvious:

                                (1) “Refractory brick masonry insulation layer” is built with ordinary refractory bricks. When the galvanizing furnace needs to heat up, the refractory bricks absorb heat, resulting in a slow heating rate. However, when the furnace needs to cool down, the refractory bricks release heat to the outside, resulting in a very slow cooling rate, which affects work efficiency and wastes energy.

                                Ordinary refractory brick masonry insulation layer has many gaps in the masonry, and employees of hot-dip galvanizing companies cannot meet the precision requirements of thermal kiln masonry. The large number of gaps increases the penetration of heat to the foundation.

                                Since the zinc pot is placed on the insulation layer, if it is a refractory brick masonry, the zinc pot will expand during the heating process. Due to the strong friction between the pot bottom and the insulation layer masonry, the existing gaps in the refractory brick masonry will be pulled open, causing the gaps to expand further. During the cooling process of the zinc pot, the friction between the pot bottom and the refractory bricks is much smaller than the pressure friction between the refractory bricks. Therefore, the existing cracked masonry gaps cannot be restored. Therefore, the heating and stopping processes that often occur in hot-dip galvanizing production intensify the penetrating transfer of heat to the insulation layer at the bottom of the furnace. This causes a large amount of heat loss and reduces the thermal efficiency of the galvanizing furnace system.

                                (2) “Lightweight heat-insulating and pressure-resistant non-sintered low-temperature castable” is a refractory material made of refractory aggregates, powders, binders, and admixtures with a certain particle size distribution, also known as bulk refractory materials. It is used for the lining of thermal equipment and is directly baked without going through the firing process. Compared with refractory bricks, it has the characteristics of simple process (because the firing process is omitted), energy saving, low cost, and easy mechanized construction. It is used in the zinc pot base to meet the pressure requirements and has better thermal insulation performance than refractory bricks, reducing the heat dissipation of the galvanizing furnace foundation and significantly improving thermal efficiency.

                                Rongsheng Ultra High-Strength Nanopore Insulating Castable for Sale
                                Ultra High-Strength Nanopore Insulating Castable

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                                  Correct application of castables

                                  For the correct application of castables, its characteristics must be mastered. The smaller the bulk density of the castable, the better its thermal insulation performance, but the decrease in bulk density will definitely reduce the compressive strength in the non-sintered state. For the galvanizing furnace system, when using castables, the compressive strength in the non-sintered state must be met first, and the thermal insulation protection of the furnace bottom heat must be met secondly. Therefore, the thermal insulation performance of the furnace bottom can only be improved by reducing the bulk density on the premise that the compressive strength meets the requirements.

                                  The castable is constructed by one-time integral casting, and the micropores are enriched in the insulation layer of the base to form dense air pockets, so it has excellent thermal insulation performance. At the same time, the entire insulation layer has no gaps, which can ensure the integrated shrinkage of the entire insulation layer and reduce the heat transfer path to the earth. The heat is isolated and collected in the furnace combustion chamber to the maximum extent, thereby greatly improving the thermal efficiency.

                                  The thermal insulation structure design of the zinc pot base adopts lightweight, heat-insulating, and compressive non-sintered low-temperature castables, which puts forward more requirements for the application of castables. The smaller the bulk density, the better the thermal insulation performance, but the decrease in bulk density will definitely greatly reduce the compressive strength in the non-sintered state. This will increase the thermal stability of the zinc pot after it is in place. In order to solve this contradiction, Tangshan Kaiping Xinde Hot-dip Galvanizing Technology Co., Ltd. has made gradual improvements in response to the development trend of thermal efficiency, energy saving and consumption reduction of the galvanizing furnace system. Relying on large state-owned enterprise groups and professional refractory research institutes, lightweight heat-insulating and compressive non-sintered low-temperature castables for zinc pot bases have been developed. This castable belongs to a new generation of high-grade amorphous heat-insulating refractory materials without beads.

                                  Lightweight heat-insulating and compressive non-sintered low-temperature castables replace floating beads with “lightweight aggregates“. After the raw materials are burned and crushed, the third-grade aggregates are obtained. When manufacturing “lightweight aggregate + high-aluminum system” castables, multi-level (guaranteed to be above level three) particle grading and pre-mixed finished products are used for construction. Solved the technical problems of water drift and low strength in the construction of lightweight materials.

                                  “Lightweight aggregate + high aluminum series” castables belong to alkaline refractory materials, which are used for heavy-load insulation structures of large tunnel kilns, roller kilns, shuttle kilns, anti-seepage insulation of aluminum electrolytic cells, and load insulation of galvanizing furnace bottoms. It solves the defects of water enrichment and cavitation segregation of floating beads. During the construction process, vibrating equipment can be used to accelerate the construction progress. Under natural air drying conditions, the process from initial setting to reaching the strength of the zinc-carrying pot is shortened to less than one week. The construction and galvanizing furnace renovation period is shortened, ensuring the timeliness of the production line to obtain economic benefits.

                                  Conditions of use

                                  1100℃, lightweight. Construction methods: pouring, coating and spraying. Low-temperature castable for hot surfaces.

                                  Product features

                                  (1) Aggregates are pre-sintered with special refractory materials to produce third-grade particles for secondary grading and pre-mixed finished product construction. This solves the technical problems of light material construction water drift and low strength.

                                  (2) The material belongs to alkaline refractory materials, which are used for heavy-load insulation structures of large tunnel kilns, roller kilns, shuttle kilns, anti-seepage insulation of aluminum electrolytic cells, and load insulation of galvanized furnace bottoms.

                                  (3) A new generation of beadless, high-grade, heat-insulating, and pressure-resistant non-sintered low-temperature castable developed by a professional refractory materials research institute.

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                                    Erosion of Na2O and Reducing Atmosphere on 70 Refractory Castables in Aluminum Melting Furnace

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                                    Attack of aluminosilicate refractories by molten aluminum. For example, in aluminum melting furnaces and holding furnaces, this usually results in the formation of aluminum oxide deposits on the refractory. The degree of attack is increased in the presence of alkalis and in reducing atmospheres. This is related to the conversion of alumina bricks into sodium aluminate in a reducing atmosphere. The formation of sodium aluminate promotes the formation of aluminum nitride. In melting furnaces and holding furnaces, the alkali may come from the metal charge.

                                    70 Refractory Castables in Aluminum Melting Furnace
                                    70 Refractory Castables in Aluminum Melting Furnace

                                    Erosion on 70 Refractory Castables in Aluminum Melting Furnace

                                    In aluminum melting furnaces and holding furnaces, the contact of aluminosilicate refractories with liquid aluminum usually results in the formation of bonded interfacial deposits containing mainly aluminum oxide. Most of this aluminum oxide is the result of the reaction of aluminum with oxides in the refractory, especially with silicon dioxide. The reaction formula is as follows:

                                    4AL+3SiO2→2AL2O3+3Si  (1)

                                    The kinetics of the reaction decreases rapidly after the deposit appears. It is concluded that this deposit becomes a barrier that prevents aluminum from penetrating into the refractory. The kinetics of this deposit are increased in a reducing atmosphere and in the presence of alkali. There are two sources of alkaline sodium oxides, in the aluminum ingots produced by the electrolytic cell or in the refractory. In the latter case, the active presence of Na2O cannot be determined. On the other hand, the relevant aspects of the reducing atmosphere have not yet been explained.

                                    The main experiment this time was to determine the reaction of the aluminum silicate refractory to aluminum corrosion caused by alkali and reducing atmosphere, and to determine the reaction of high-aluminum castables (70% Al2O3) with aluminum fluoride (ALF3) as an infiltrant to such corrosion. The possible effects caused by the presence of alkali in the refractory, this product is a representative of amorphous refractory with non-wetting additives. It is used in industry as aluminum insulation furnace and aluminum smelting furnace lining.

                                    Rongsheng High Alumina Castables Refractory
                                    Rongsheng High Alumina Castables Refractory

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                                      According to the operating temperature of the aluminum insulation furnace and aluminum smelting furnace, the test temperature is determined, and the temperature at the contact with the metal reaches 850℃, and the temperature at the flame radiation site reaches 1200℃~1500℃. The corrosion experiments on industrial high-alumina refractory castables with aluminum fluoride (ALF3) as a non-wetting agent have shown that the corrosion caused by aluminum and the role of basic oxides contained in the refractory material have been speculated.

                                      It appears that, especially in the presence of a reducing atmosphere, beta alumina is the active phase in the refractory material in contact with liquid aluminum. From a thermodynamic point of view, the action of beta alumina on liquid aluminum leads to the formation of metallic sodium. The reaction equation is as follows:

                                      6NaAL11O17+2AL→6Na+34AL2O3 (2)

                                      When the oxygen partial pressure is higher than 10-19atm, the metallic sodium produced in the aluminum solution (aNa~0.1) will be oxidized. The reaction is as follows:

                                      2Na+1/2O2→Na2O (3)

                                      When the oxygen partial pressure is lower than 10-19atm, the presence of Na2O must be related to the action of refractory oxides (especially in silica) on metallic sodium. The reaction equation is as follows:

                                      4Na+SiO2→2Na2O+Si (4)

                                      On the other hand, in the presence of ALF3, metallic sodium may also be produced. The reaction formula is as follows:

                                      6NaAL11O17+2ALF3→6NaF+34AL2O3 (5)

                                      3NaF+AL→3Na+ALF3 (6)

                                      Through analysis, when the liquid electrolyte with a temperature higher than 888℃ exists, reaction equations (5) and (6) can produce metallic sodium, which is conducive to the mutual exchange between the reactants. Under such conditions, a higher Na2O generation kinetics can be expected. Therefore, if the latter contains ALF3 as a wetting agent, aluminum will corrode the refractory material faster. The interface deposit mainly contains corundum (a-AL2O3) and also contains aluminum (AL) and aluminum nitride (ALN). It is believed that the aluminum nitride present in the deposit may participate in this corrosion process. This participation of aluminum nitride is consistent with the analysis of the reaction products obtained between aluminum and sodium carbonate in air and nitrogen. It was identified that at 900℃, the main reaction product obtained was sodium aluminate (NaALO2), which exists in the form of hydrate (NaALO2·3H2O). The specific gravity of sodium aluminate is 2.69g/cm³, while that of aluminum oxide is 3.96g/cm³. Therefore, the protective aluminum deposit should be accompanied by an increase in volume when it is converted into sodium aluminate. The increase in volume is conducive to the formation of cracks, which is conducive to the penetration of aluminum and thus the refractory is susceptible to corrosion.

                                      Application of Cr2O3-Al2O3 Refractory Castables in Melting Furnaces

                                      In principle, although melting furnaces can be lined with Cr2O3-Al2O3 bricks, they can also be lined with Cr2O3-Al2O3 refractory castables. However, for furnaces with complex structures, Cr2O3-Al2O3 refractory castables are often used for lining when considering cost and construction (convenience). Especially for thermal decomposition gasification melting furnaces, since the furnace walls are mostly boiler water pipes, it is very difficult to use refractory bricks for lining, but it is very convenient to use Cr2O3-Al2O3 refractory castables for lining.

                                      When the melting furnace is lined with corundum refractory castables mixed with Cr2O3-Al2O3 powder and Al2O3 powder, it is believed that the Cr2O3-Al2O3 refractory castable with a high Cr2O3 content in the matrix material will also have high durability.

                                      In order to confirm this, the corrosion resistance of refractory castables was compared and studied. The corrosion test was conducted in a rotary corrosion test furnace (1750℃). The results confirmed that the corrosion resistance of these refractory castables increased with the increase of Cr2O3 content in the matrix. The following conclusions can be drawn from the results of the slag resistance test:

                                      • (1) The fact that the concentration of Cr2O3 near the working surface of the Cr2O3-Al2O3 refractory castable decreased confirmed that Cr2O3 had transferred to the slag. The slag contained Cr2O3 and increased its viscosity, which prevented the slag from further penetrating and corroding the Al2O3-Gr2O3 refractory castable.
                                      • (2) At high temperatures, Cr2O3 reacted with Al2O3 to form a solid solution, which improved the high-temperature performance of the material while also improving the corrosion resistance of the material.
                                      • (3) Since the slag containing Cr2O3 has a high viscosity, it also improves the slag resistance of the Al2O3-Cr2O3 refractory castable.

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                                        Low Cement Castable Refractory Cement For Industrial Furnaces High Dense

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                                        Low cement castables have the characteristics of high density, low porosity, high strength, low wear, thermal shock resistance and erosion resistance. Low cement castables not only have the characteristics of traditional refractory materials, but also adopt the most advanced ultrafine powder application technology to improve the medium temperature strength and high temperature volume stability of the castables. The organizational structure of the product forms a firm anchor-shaped multi-directional crystal structure at high temperature. The overall structural connection stability of the lining is ensured, and the service life is increased by 3 times compared with traditional castables.

                                        Low Cement Castable Directly from Factory
                                        Low Cement Castable Directly from Factory

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                                          Application of Different Types of Low-Cement Castables

                                          Low-cement and ultra-low-cement refractory castables have been widely used in industrial kilns in the fields of metallurgical industry, petrochemical equipment, machinery equipment manufacturing and decorative building materials at this stage, and have shown good use effects and economic benefits. Different types of low-cement castables play specific roles in various application fields as follows:

                                          1. Clay and high-alumina low-cement castables can be used as lining materials for various heat treatment furnaces such as hot blast furnaces, heating furnaces, soaking furnaces, and rotary kilns.
                                          2. Mullite low-cement castables are used for various high-temperature burner linings, x water-cooling pipe cladding and other applications.
                                          3. Corundum and chromium-containing corundum low-cement castables are suitable for the refining equipment parts outside the molten steel furnace, the lining of the power frequency induction furnace, the high-temperature wear-resistant lining of the petrochemical catalytic cracking reactor, and other occasions.
                                          4. Al2O3-SiC-C Low cement castables can be used for the lining of blast furnace tapping troughs, cupola tapping troughs, integral powder spraying guns for molten iron normalization treatment and other parts.
                                          5. Low cement castables of refractory and Cr-containing refractory can be used as linings of the triangular area of ​​the top of the iron-making electric arc furnace and the linings of the components of the external refining equipment of the molten steel furnace. For example, the overall spray gun lining for the spouting metallurgical industry, the RH and DH vacuum pump degassing device and the prepreg tube lining, the DC induction furnace lining, and the high temperature wear-resistant lining of the catalytic cracking tubular reactor of the petrochemical equipment.
                                          6. Low cement castables of MgO-Al2O3 or Al2O3-MgO-Al2O3 are used as lining materials for equipment such as ladles and tapping troughs.

                                          In summary, different types of low cement castables play an important role in various industries and have been widely used and promoted.

                                          Rongsheng Low Cement Castable Materials
                                          Rongsheng Low Cement Castable Materials

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                                            Advantages and Characteristics of Rongsheng High-Strength Low-Cement Castables

                                            Rongsheng Refractory Castables Manufacturer has rich experience in the production and sales of refractory materials. Rongsheng’s environmentally friendly fully automatic amorphous refractory production line provides a reliable guarantee for the timely delivery of Rongsheng’s high-quality refractory castable products. After years of practical experience, Rongsheng has discovered the application advantages and characteristics of high-strength low-cement castables, which can effectively extend the service life of high-temperature industrial furnace linings. If you need to buy high-strength low-cement refractory castables, please contact Rongsheng for free samples and quotes. Happy to share and make progress together. Next is the advantages and characteristics of Rongsheng’s high-strength low-cement castables.

                                            1. Improve the refractoriness and high temperature strength of the refractory lining.

                                            Due to the low CaO content in low cement castables, the formation of low eutectic phases in the material can be reduced. Thereby improving the refractoriness, high temperature strength and resistance to slag erosion, especially the cement-free castable has better performance.

                                            1. The refractory lining has low porosity and high bulk density.

                                            The amount of mixing water used in the construction of low cement castables is only 1/2 to 1/3 (about 4% to 6%) of ordinary low cement castables. Therefore, the porosity is low and the bulk density is high.

                                            1. After the construction of the refractory lining is completed, less cement hydrate is generated.

                                            After the low cement castable is formed and cured, less cement hydrate is generated, and even during heating and baking, there is no phenomenon of a large number of hydration bonds being destroyed and the medium temperature strength being reduced. The low cement castable will gradually sinter as the heat treatment temperature increases, and the strength will gradually increase.

                                            1. It can improve the structural stability of the refractory lining.

                                            Low cement castable combines the characteristics of traditional refractory materials, adopts ultrafine powder application technology, and significantly improves the medium-temperature strength and high-temperature volume stability of the castable, so that the product’s organizational structure forms a firm anchor-shaped multi-directional crystal structure at high temperatures. It ensures the overall structural connection stability of the lining, and its service life can be increased by 3 times compared to traditional castables.

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                                              Why doesn’t the Ladle Containing Molten Steel Melt?

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                                              In steelmaking, the container for molten steel is generally called a ladle, which is mainly used to store and transport molten steel. When tapping steel, 1650℃ molten steel is directly poured into the ladle. At this time, the steel is melted, so why is the container for molten steel (ladle) safe and sound?

                                              First, we need to understand the main components of the ladle. From the outside to the inside, the ladle is composed of a metal shell, an insulation layer, a permanent layer, and a working layer.

                                              Main Components of the Ladles
                                              Main Components of the Ladles

                                              The insulation layer is close to the steel plate of the ladle shell. Its function is to keep warm, prevent the ladle shell from deforming, and reduce the heat conduction of the ladle wall lining.

                                              The function of the permanent layer is to prevent the molten steel from burning through the metal shell when the refractory material of the working layer in the innermost part of the ladle, which is in direct contact with the molten steel, is partially eroded or thinned.

                                              The working layer is in direct contact with the high-temperature molten steel, and at the same time it is subjected to the mechanical scouring of the molten steel and slag, so the refractory material of the working layer is the most important thing in the ladle lining. The height of a ladle also depends on the selection of working layer materials and masonry technology.

                                              After the temperature of the molten steel passes through the above layers, the temperature that is finally transmitted to the ladle shell wall is only about 300℃. The temperature of the working layer is 1600℃, the temperature of the permanent layer is 800℃, and the temperature of the insulation layer is 500℃. This is why the ladle shell will not be melted.

                                              Simply put, the container (ladle) that holds molten steel will not melt, all thanks to the refractory lining inside. Therefore, the selection of durable, high temperature and wear-resistant refractory materials is the key factor to ensure the service life of the ladle. The refractory materials of the ladle lining working layer are generally magnesia-alumina-carbon bricks and carbon composite refractory materials. The thickness of magnesia-alumina-carbon bricks is about 110~120mm. It needs to withstand high temperature, molten steel and mechanical scouring.

                                              The Use Temperature and Requirements of the Castables for the Ladles of Foundry

                                              The selection of castables for the ladle of the foundry is crucial. It not only affects the service life of the ladle, but also affects the efficiency and safety of production. The selection of suitable castables needs to be determined according to the specific working conditions to ensure its refractory performance and construction convenience.

                                              Rongsheng Steel Ladle Refractory Castable
                                              Rongsheng Steel Ladle Refractory Castable

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                                                • Refractoriness of castables for ladle. The refractoriness of the AI2O3-SiC series of materials is between 1580-1770℃, which ensures the stability of the ladle in a high temperature environment.
                                                • Compressive strength. The compressive strength of the castable reaches 80MPA, which means that it can withstand the erosion of molten iron and extend its service life.
                                                • Use temperature. The use temperature of the castable for the ladle is generally not less than 1650℃ to meet the needs of high temperature operations.
                                                • Construction and maintenance. The construction of the castable should be convenient and easy to maintain to adapt to the daily maintenance work of small and medium-sized foundries.
                                                • Thermal shock stability. The castable needs to have good thermal shock resistance to cope with the thermal stress caused by temperature changes.
                                                • Performance requirements. The amount of water added to the castable should not exceed 7.3%, the number of thermal shocks should reach more than 35 times, the line change after firing should be between 0.3% and 0.8%, and the line change after re-firing should be between 0 and 0.2%.

                                                Through these requirements, we can select the castable suitable for the foundry ladle to ensure that it is safe and efficient during use.

                                                Steel Ladle Castables Rongsheng Manufacturer

                                                Ladle castables are a special material used in the ladle, which can withstand high temperature and high pressure, and needs to have good fire resistance and chemical stability.

                                                Performance Characteristics of Rongsheng Steel Ladle Castables

                                                1. Good fire resistance. Ladle castables need to withstand high temperature and high pressure, so they need to have good fire resistance. Its refractoriness can reach above 1650℃, and it can work stably at high temperature.
                                                2. Good chemical stability. Ladle castables need to resist various chemicals in molten steel, so they need to have good chemical stability. It can work stably in molten steel and will not react with substances in molten steel.
                                                3. Good thermal insulation. Ladle castables need to keep the temperature of molten steel stable, so they need to have good thermal insulation. It can transfer heat in molten steel and keep the temperature of molten steel stable.
                                                4. Good compactness. Ladle castables need to fill the internal space of the ladle, so they need to have good compactness. It needs to fill the inner space of the ladle tightly to achieve the best insulation effect.
                                                5. Good unloading. Ladle castables need to be easily unloaded, so they need to have good unloading properties. It needs to be easily washed and peeled off by molten steel to facilitate the discharge of molten steel.

                                                Rongsheng Refractory Castables Factory‘s ladle castables have good fire resistance, good chemical stability, good thermal insulation, good compactness and good unloading properties. These properties can make the ladle castable work stably in the ladle and provide guarantee for the efficient smelting of molten steel. To purchase ladle and iron ladle castables, please contact Rongsheng for free samples and quotations.

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