Refractory Blog

Formulation and Properties of Corundum and High-Alumina Refractory Mortar

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Refractory mortar is an unshaped refractory material composed of powdered materials and binders, used for preparing slurries. The powdered materials are made from fully sintered clinker (such as high-alumina clinker, calcined silica, magnesia, etc.) or other volume-stable refractory raw materials (such as silica, wax stone).

Rongsheng Refractory Mortar Manufacturer

The powder used to make refractory mortar can be fully sintered clinker and other volume-stable refractory raw materials. The particle size of the powder depends on the application requirements, with a generally limited particle size of less than 1mm, and sometimes less than 0.5mm or even finer. A reasonable particle composition has a significant impact on ensuring the mortar’s workability, as do the binders and admixtures. Ordinary refractory mortars use bonded clay as the binder. However, chemical binders are increasingly widely used. Adding various admixtures to refractory mortar can improve its workability. For example, adding water-retaining agents to extend the water loss time and ensure construction quality; adding plasticizers, even in small amounts, can increase the mortar’s plasticity; adding dispersants to improve the mortar’s fluidity, etc.

Refractory mortars can be classified into hydraulic, thermohardening, and air-hardening refractory mortars based on their binder setting and hardening characteristics. Hydraulic refractory mortars use cement as a binder and can be used at room temperature or in places where they may frequently come into contact with water or moisture. Thermosetting refractory mortars are commonly made with thermosetting binders such as phosphoric acid or phosphates. After hardening, these mortars exhibit high strength at various temperatures, low shrinkage, tight joints, and strong erosion resistance. Air-hardening refractory mortars commonly use air-hardening binders such as sodium silicate. These mortars ensure tight joints in masonry.

Depending on the material of the refractory powder used, commonly used refractory mortars can be classified as: clay-based, silica-based, high-alumina-based, magnesia-based, and insulating. Refractory mortar is mainly used as a contact and surface coating for refractory brick masonry. When used as a jointing material, its quality has a significant impact on the lifespan of the masonry. It can adjust dimensional errors and irregular shapes of bricks, making the masonry neat and load-bearing. It also helps the masonry form a strong and tight whole, resisting external damage and preventing the infiltration of molten metal.

Rongsheng Corundum Refractory Mortar
Rongsheng Corundum Refractory Mortar

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    Corundum Refractory Mortar

    The formulation of corundum refractory mortar mainly uses sintered corundum clinker or fused alumina with a diameter of 0.5-0.63 mm and finer. Fused alumina with a diameter of 5-7 μm or industrial alumina with a diameter of less than 5 μm is used as a binder. To ensure the shrinkage of the refractory clay during air and firing, the industrial alumina content in the formulation should not exceed 15%-20%, while the content of ultrafine fused alumina powder should reach 40%-50%. 10% of the refractory mortar is introduced with orthophosphoric acid (density 1.72 g/cm³).

    Using corundum clinker and introducing less than 0.1% fermented alcohol waste liquid can improve the plasticity of the slurry and reduce its water content. This refractory mortar has a normal moisture content of less than 19%, a weight loss of 1.8% when heated to 110°C, and an apparent porosity of 26%. As shown in Table 1, the refractory clay has sufficient shear bond strength after firing at 1000-1500°C.

    Properties of Corundum Refractory Mortar
    Properties of Corundum Refractory Mortar

    Note: Component 1 is a refractory mortar primarily composed of sintered corundum, with particles smaller than 0.5 mm, 10% phosphoric acid, and 0.1% fermented alcohol waste liquid. Component 2 is a refractory mortar primarily composed of industrial alumina, with particles smaller than 30 μm, 5.5% polyphosphoric acid, and 0.1% fermented alcohol waste liquid. Component 3 is a refractory mortar primarily composed of light-burned alumina, bonded with sodium pyrophosphate. Component 4 is a refractory mortar primarily composed of high-alumina clinker, 10% clay, 0.1% fermented alcohol waste liquid, and 0.15% sodium carbonate.

    According to Table 1, the shear bond strength of the refractory mortar is related to the final porosity; as porosity increases, the shear bond strength tends to decrease. To maintain the uniformity of the moisture content of the corundum refractory mortar at 2.5%, some dust needs to be removed, and 2.5% CaCl2 needs to be introduced. This increases the moisture content of the slurry to 24%–27%, without reducing the cohesive strength between the refractory mortar and the corundum refractory material. The shear bond strength after firing at 1000°C is 1.4–2.0 MPa, and reaches 10 MPa after firing at 1500°C.

    Phosphate-bonded corundum refractory mortar can withstand 40 cycles of repeated water exchange at 1300°C without cracking. Corundum refractory mortar containing approximately 96% alumina has a refractoriness of 2000°C. Reducing the alumina content in the refractory mortar to 90%–91% slightly decreases the refractoriness, and the deformation initiation temperature under a 0.2 MPa load drops to 1580°C.

    High Alumina Mortar Material Can Be Used to Fill up Seam
    High Alumina Mortar Material of Rongsheng

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      High-Alumina Refractory Mortar

      The technical properties of high-alumina refractory mortar are shown in Table 2.

      Properties of High-Alumina Refractory Mortar
      Table 2: Properties of High-Alumina Refractory Mortar

      Storage and Usage Principles of High-Alumina Mortar

      High-alumina mortar is the jointing material for high-alumina refractory bricks, mainly used for bonding bricks. Its storage time affects its performance, and its continued usability can be determined.

      Judging the Storage Condition of the Binder

      1. Check the Binder Packaging

      First, check if there is a separately packaged binder in the high-alumina refractory mortar. If there is a separately packaged binder, and the storage time exceeds 6 months, it cannot be used and needs to be replaced.

      If there is no separately packaged binder, take a small amount of refractory mortar and smell it. If there is an odor, it means that the binder was added to the refractory powder during production; if there is no odor, it means that the binder was not added.

      1. Advantages of No Added Binder

      High-alumina mortar without added binder is in its optimal state. It can be remixed with binder and used again, thus ensuring its bonding performance.

      Judgment of Refractory Mortar with Added Binder

      If the high-alumina refractory mortar has had a binder added directly during production and has been stored for more than 6 months, the following test is required to determine its usability:

      1. Test Preparation

      Weigh 1 kg of refractory powder, add 20%-25% drinking water, and stir to form a slurry. Prepare two refractory bricks of the same material.

      1. Test Procedure

      Take a portion of the refractory mortar and evenly spread it on one refractory brick, smooth it out, and place the second refractory brick on top. Rub the two refractory bricks back and forth approximately 15 times.

      1. Result Judgment

      If the two refractory bricks can bond together, the high-alumina refractory mortar is still usable. If the refractory bricks cannot bond, add an appropriate amount of binder in a reasonable proportion and repeat the above test. If the refractory bricks bond successfully, they can be put into use.

      Summary of High-Alumina Refractory Mortar Usage

      The binder in high-alumina refractory mortar primarily promotes the adhesion of the mortar. However, the binder will disappear after high-temperature baking. Therefore, high-alumina mortar that has been stored for a long time needs to be assessed for its continued usability using the methods described above. By checking the storage condition of the binder and conducting simple tests, the feasibility of using high-alumina refractory mortar can be effectively determined, avoiding the impact of material issues on the bonding effect and construction quality of refractory bricks.

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        Preparation and Properties of Porous Spherical Mullite Castables

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        In high-temperature industries such as iron and steel metallurgy, the poor insulation and rapid heat loss of refractory materials used in industrial kiln linings lead to excessively high kiln surface temperatures, resulting in increased energy consumption during ironmaking and steelmaking processes. This not only hinders the lifespan of refractory materials but also affects production schedules and endangers the safety of production personnel.

        Low Thermal Conductivity Refractory Materials for High-Temperature Kilns

        For example, the high thermal conductivity of refractory materials used in steel ladles leads to accelerated heat loss, which triggers a series of chain reactions during steelmaking:

        1. Rapid heat loss causes excessively high ladle shell temperatures, resulting in severe deformation;
        2. Rapid cooling of molten steel leads to cold forming, nodule formation, and even ladle flow interruption and final pouring.

        Therefore, developing low thermal conductivity refractory materials for high-temperature kilns has become an urgent need for major steel mills. The high porosity and low thermal conductivity of lightweight materials offer new insights into the preparation of insulating refractory materials for high-temperature kiln linings. However, the preparation of lightweight refractories is often achieved by changing process conditions or adding additives, which suffers from poor controllability of pore size distribution. Consequently, functionalized refractory raw materials have rapidly developed, such as lightweight mullite, alumina hollow spheres, and gel powder. These raw materials facilitate the preparation of lightweight thermal insulation refractory materials. However, their excessively light weight and high price also limit their industrial application.

        Meanwhile, with increasing production requirements, lightweight, high-strength refractory materials suitable for higher temperatures have gradually become the focus. Using microporous lightweight mullite, multiphase hollow spheres, high-alumina bauxite, silica, and alumina micropowder as main raw materials, the effects of lightweight aggregate composition, micropowder composition, bonding system, and pore-forming agent on the performance and structure of lightweight high-alumina castables were studied, and a lightweight high-alumina castable with good high-temperature performance was developed. Using mullite microspheres, α-Al₂O₃ micropowder, and silica micropowder as raw materials, and AlF₃·3H₂O and V₂O₅ as additives, a lightweight, high-strength mullite microsphere thermal insulation refractory material was prepared. Lightweight, high-strength, and highly thermally shock resistant corundum-mullite refractory materials were prepared by using alumina hollow spheres as pore-forming agents and AlF3·3H2O as additives to control the pore characteristics of corundum-mullite and the in-situ formation of mullite whiskers.

        Rongsheng Mullite Refractory Castable Manufacturer
        Rongsheng Mullite Refractory Castable Manufacturer

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          Preparation and Properties of Porous Spherical Mullite Castables

          Existing research shows that recent studies on the performance of aluminosilicate materials have largely focused on aggregate substitution or the use of additives. Alumina, a commonly used and relatively inexpensive raw material in aluminosilicate materials, has had its impact on aluminosilicate castables rarely reported.

          • To prepare a high-temperature kiln insulating refractory material with excellent thermal insulation and mechanical properties, porous spherical mullite-based castables were prepared using porous spherical mullite, fine alumina powder, α-Al₂O₃ micropowder, silica powder, and Secar 71 cement as the main raw materials.
          • The effects of fine alumina powder content on the mechanical properties, thermal conductivity, erosion resistance, and thermal shock resistance of the porous spherical mullite-based castables were investigated.

          The results show that changing the content of fine alumina powder can improve the thermal insulation, thermal shock resistance, and erosion resistance of the porous spherical mullite-based castables while maintaining high mechanical properties.

          With increasing bauxite powder content, the mechanical properties of porous spherical mullite-based castables did not change significantly. However, the thermal conductivity decreased slightly, and the erosion resistance showed considerable differences, with thermal shock stability initially increasing and then decreasing. When the bauxite powder content was 28% (mass fraction), the porous spherical mullite-based castable exhibited good mechanical properties, thermal shock stability, and erosion resistance. The thermal conductivity at 1000℃ was 0.905 W·m⁻¹·K⁻¹. The high-temperature flexural strengths of the porous spherical mullite-based castable at 1100℃ and 1400℃ were 22 MPa and 5 MPa, respectively, indicating good thermal insulation performance, high flexural strength, and a certain degree of resistance to ladle slag erosion.

          The thermal conductivity of the porous spherical mullite-based castable is lower than that of high-alumina castables used in tundish and ladle permanent layers, making it a viable alternative to high-alumina castables used in tundish and ladle permanent layers to reduce heat loss.

          Mullite refractory castable price
          Mullite refractory castable price

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            Application Effects of Lightweight Mullite Castables

            Lightweight mullite castables are primarily used as integral linings for regenerators in heating furnaces. This working layer operates continuously at a high temperature of 1350℃, enduring temperature fluctuations within the kiln and the erosion and wear of high-temperature gases. During construction, the casting thickness is 300-400mm, with a water content of 18%-19%. In practical applications, this castable demonstrates advantages such as high workability, high-temperature strength, low thermal conductivity, good insulation, and good thermal shock resistance. Furthermore, its low-cement, micro-powder composite bonding allows for direct contact with the flame.

            Energy-saving and environmentally friendly lightweight mullite castables, with their superior workability and high-temperature performance, have become ideal refractory linings for regenerators in heating furnaces. In addition, their application in thermal kilns such as aluminum melting furnaces, fluidized bed boilers, and gasifiers has also seen widespread development.

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              The Importance of Water Addition in the Fabrication of Precast Refractory Shapes

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              Precast Refractory Shapes are products made according to the shape of the application area to facilitate on-site construction. They are produced by pouring the refractory castable into a mold and vibrating it into various shapes after the refractory castable is produced.

              Rongsheng Precast Refractory Shapes
              Rongsheng Precast Refractory Shapes

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                Water Addition in Precast Refractory Shapes Manufacturing

                The amount of water added during the manufacturing process of Precast Refractory Shapes is crucial. Excessive water leads to a sparse particle size, affecting the strength and performance of the finished product. Insufficient water results in a lack of fluidity, making it impossible to form. The water addition amount for each type of precast refractory castable is determined based on the raw materials and binders used in the mixing process.

                If the binder, such as high-alumina cement, is used in large quantities, resulting in a high calcium content, the water addition amount will also increase. Currently, most castable products utilize low-cement technology to reduce water addition, shorten drainage time, and enhance product strength. A moderate water addition improves product fluidity and facilitates construction. Furthermore, reduced drainage time results in higher product strength, lower porosity, and consequently, a longer service life for the Precast Refractory Shapes.

                Excessive water addition fails to enhance the fluidity of the castable. Moreover, during the manufacturing process, the use of a vibrator can cause water to splash out, quickly rising to the surface while large aggregates sink to the bottom. This also generates a large number of water bubbles, meaning excessive and large pores. This prolongs the demolding cycle of Precast Refractory Shapes, and even if demolding is successful, the product’s strength and usability will suffer from cracking and eventual detachment during use.

                The appropriate amount of water added depends on the different raw material matrixes. During mixing, aggregates should be added first, followed by fine powders, then the binder, and finally the appropriate proportion of water. This simplifies the manufacturing process, reduces drainage, increases product strength, and lowers porosity.

                Therefore, the amount of water added during the pre-fired Precast Refractory Shapes manufacturing process is crucial and cannot be arbitrarily increased. The principle is to add less rather than more, which enhances the product’s performance and facilitates faster and more convenient construction.

                Rongsheng Precast Refractory Shapes Manufacturer
                Rongsheng Precast Refractory Shapes Manufacturer

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                  Can Precast Refractory Shapes be Used Without Baking?

                  Yes, Precast Refractory Shapes can be used without baking at a certain temperature. However, this requires the addition of explosion-proof fibers to the pre-made process mix to ensure sufficient drainage of crystal water. Alternatively, a long natural drying time can also allow them to be used without baking.

                  Precast Refractory Shapes are an extension of refractory castables. They are made by mixing raw material granules and powders, binders, and water to form a castable. This mixture is then poured into pre-made molds and undergoes dehydration, baking, and drainage processes before use. Comparatively, precast refractory components that have been baked at 300℃ have a better service life and safety than those that haven’t been baked.

                  However, if the quantity of precast refractory components used is large and the operating conditions are not demanding, adding explosion-proof materials during production and ensuring sufficient drainage of moisture allows them to be used during the industrial furnace lining baking process as the furnace temperature gradually increases. Unbaked pre-fired refractory precast components, while possessing higher strength, are more prone to breakage during transportation, handling, and construction.

                  Currently, the proportion of unbaked Precast Refractory Shapes used in the market is higher than that of baked ones. This is due to lower costs, coupled with improvements in castable technology, resulting in performance comparable to products from earlier technologically advanced periods.

                  However, under harsh operating conditions, such as excessively high furnace lining temperatures and the need for emergency repairs, where the furnace lining temperature rises rapidly, unbaked Precast Refractory Shapes, even those unbaked, offer better performance and a longer service life. Baked precast refractory components, on the other hand, have higher strength, better thermal shock resistance, and are less likely to crack during use.

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                    Basis for Selecting Castable Refractory for Flue Lining

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                    The most important consideration for flue lining is the erosion and corrosion caused by acidic gases. Acidic gases necessitate the use of acid-resistant castables or acid-resistant bricks for the lining. However, flues can be vertical or horizontal; for vertical flues, weight must be considered. If they are too heavy or the chimney is too tall, installation and assembly will be extremely difficult.

                    Flue Lining Acid Castable Refractory
                    Flue Lining Acid Castable Refractory

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                      Flue Lining Using Castable Refractory

                      Flue temperatures are not high, but the gas pressure is high, and the flue gas volume is large. The lining is susceptible to acid erosion and water erosion due to the generation of water vapor. Therefore, careful consideration is needed when choosing castable refractory or refractory bricks for the lining.

                      For example, roasting furnace flues can be vertical or horizontal, and the vertical and horizontal structures are often connected. Therefore, when selecting materials for the lining, both weight and erosion must be considered. If acid bricks are used, the mortar joints will result in poor airtightness. If heavy acid-resistant castable refractory is used, it will be too heavy. If lightweight acid-resistant castable refractory is used, its erosion and scouring resistance is insufficient, which is not suitable for flue lining applications. Furthermore, since the flue lining temperature is not high, insulation is generally not required.

                      Therefore, semi-heavy acid-resistant castable refractory should be considered. Semi-heavy refractory has a bulk density of around 1.5, which provides resistance to erosion and scouring while also offering some insulation. Another reason is that the weight issue is also alleviated. Currently, considering market conditions and chimney characteristics, semi-heavy acid-resistant castable is an ideal material for flue linings. It is particularly suitable for use in large-diameter flues.

                      What is the thickness of the castable lining for a chimney?

                      Fluid ducts typically have small inner diameters. If acid-resistant castable lining is used, the thickness is generally less than 100mm, resulting in a significant weight reduction. However, the small diameter of the lining can cause unnecessary difficulties during construction. For flues with a diameter of less than 2 meters, using a coating material is a better alternative, as semi-heavy coating materials can be 30-50mm thick. This reduces the weight by approximately 50% compared to castable lining. For vertical flues, this further reduces the potential for excessive weight. It also facilitates construction in small-diameter flues. Therefore, semi-heavy acid-resistant coating materials are an ideal choice for small-diameter flues.

                      Construction of Flue Lining Refractory
                      Construction of Flue Lining Refractory

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                        Construction of Flue Lining with Castable Refractory

                        The construction of flue lining with castable refractory can be carried out after installation or after the lining is installed. When constructing the flue lining with castable refractory, a 3mm gap should be left between each cast section for expansion joints. The process involves applying sealant to adjacent sections of the castable refractory before pouring the next section.

                        Construction of Flue Lining with Castable Refractory Before Installation

                        Constructing the lining after installation is generally preferable. Large-diameter flue linings are easier to construct with castable refractory. However, if the large-diameter pipe is heavy, lining before installation will significantly increase the pipe’s weight, increasing installation risk and making component welding difficult. Large-diameter flue linings are best constructed after installation.

                        The construction of flue lining with castable refractory before installation is suitable for small-diameter flue linings due to its lighter weight. Pre-installation construction reduces the difficulty of high-altitude operations, as the refractory lining is poured on the ground, resulting in lower labor intensity and higher construction efficiency. The disadvantage is that the lining construction cannot be carried out continuously, and expansion joints are difficult to handle.

                        Safety is the primary concern during construction.

                        The primary concern in constructing flue linings using refractory lining is safety, followed by the welding of anchors and the refractory pouring method. For large-diameter flues, the anchor nails are welded first, and then the anchors are painted before proceeding with the construction in sections. Sectioning involves constructing two sections within a 180° radius, then rotating the pipe 180° to construct the third and fourth sections. If there is an insulation layer, the insulation refractory lining is constructed first, followed by the working layer of the wear-resistant refractory lining. Additionally, if two types of refractory lining are used, the construction must also be carried out in four sections. However, a 12-hour break must be observed after each section is completed before proceeding to the next section. The lower half is poured first, and then the flue is rotated on the ground to pour the remaining half.

                        Lining Construction on the Ground

                        If lining construction is carried out on the ground, pipe openings must be straightened and anti-deformation measures implemented. This is to prevent pipe opening deformation due to weight differences during construction and rotation. Areas prone to deformation during refractory lining construction should not be constructed initially; lining should be applied to these areas after other parts are completed.

                        For pipe bends, expansion joints, and valves, refractory lining should be applied in sections, with expansion joints reserved in both horizontal and vertical directions. For lower-lying areas, inclined formwork can be used for refractory lining, followed by vibration to ensure compaction, and excess material should be removed tangentially. In areas with limited space or high construction joints, refractory lining should be applied using a flat butt joint, with a 3mm high-temperature resistant ceramic fiber felt sandwiched within the joint before welding.

                        What Type of Refractory Castable is Best for a Kiln Operating at 1500℃?

                        When the operating temperature of a kiln is 1500℃, corundum castable should be selected based on the temperature. However, many users don’t understand this and think that because it’s expensive, they should choose general high-alumina castable.

                        However, if the operating temperature is directly at 1500℃, general high-alumina castable is not suitable. Brown corundum castable is required to meet the requirements. Some users have suggested using steel fiber castable. However, this is also not an option because steel fibers begin to melt above 1300℃. Therefore, steel fiber castable cannot be used.

                        However, some users suggest using stainless steel, but this is also not suitable. Because the lining only uses rust as an anchor, it does not provide high-temperature resistance or corrosion resistance. Corundum castable is characterized by the highest bulk density, the lowest apparent porosity, and good high-temperature resistance, corrosion resistance, and thermal shock resistance. Moreover, the characteristics of corundum castable are fully realized at 1500℃. If steel fiber castable is used, it cannot withstand temperatures up to 1500°C. General high-alumina castables, however, can be used at temperatures of 1300°C-1400°C.

                        However, for different kilns with varying gas fields and degrees of corrosion, corundum wear-resistant castables or high-strength wear-resistant castables can be selected. This provides both high-temperature resistance and corrosion resistance. For example, in waste incinerators, the temperature is not high, but the degree of corrosion is strong, sometimes involving both acid and alkali corrosion. In such cases, corundum castable is unsuitable because the temperature is too low for the corundum to be fully utilized. Using clean, wear-resistant castables can resist acid or alkali corrosion. Moreover, they are cheaper than corundum castables, making them cost-effective and suitable for the special conditions of incinerators.

                        In short, regardless of the material of the refractory castable chosen, it is essential to select the appropriate refractory castable based on the specific kiln, temperature, and degree of corrosion.

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                          Application of Dry Ramming Refractory Material in Electric Furnace Bottom

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                          Currently, the working layer of high-power and ultra-high-power electric furnaces is commonly constructed using magnesia dry ramming refractory materials. The working layer, made of refractory materials, comes into direct contact with molten steel and slag, bearing the high-temperature heat load and slag erosion. The erosion of molten steel, the mechanical impact of scrap steel, and the high-temperature oxidation and reduction processes within the furnace cause slag to penetrate the furnace floor, resulting in a thinning of the furnace floor. During discontinuous operation, the dicalcium silicate in the slag absorbs atmospheric moisture and disintegrates, reducing the material’s durability and service life.

                          Magnesia Refractory Ramming Mass Material from Rongsheng
                          Refractory Ramming Mass Material from Rongsheng

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                            Common dry ramming refractory materials use high-iron, high-calcium synthetic magnesia and fused magnesia as aggregates, with the synthetic and fused magnesia powders used as fine powders, with a critical particle size of 5-6 mm. C2F (dicalcium ferrite) in the synthetic magnesia is used as a sintering aid, without the addition of any binders, and are produced using a multi-stage batching process. Through vigorous ramming, the resulting density is guaranteed, allowing it to sinter into a solid, integral structure at the appropriate temperature. Its service life is several times longer than that of previous knotting and bricklaying methods. The dry ramming mass used in medium frequency furnaces has higher quality requirements. Some scholars have studied the use of fused corundum, white corundum, plate-shaped corundum, and magnesia as granular materials, magnesia fine powder and corundum fine powder as powder, adding high-temperature sintering agents and boric acid as admixtures. The various raw materials are mixed evenly and the particle size distribution is adjusted to produce dry ramming mass.

                            Dry Ramming Material for Electric Furnace Bottom Construction

                            Before applying dry ramming refractory material for electric furnace bottoms, be sure to clean away any foreign matter such as permanent layer residue, dust, wire, and plastic sheeting. Calculate the knot size; the actual knot thickness is the desired knot thickness multiplied by 1.09. Prepare a sufficient amount of ramming material based on the required furnace slope and bottom dimensions. Upon receipt, inspect the ramming material for debris and moisture. Remove any debris and do not use damp material. Prepare ramming tools such as a rammer and jackhammer.

                            For specific construction steps, refer to the following plan:

                            After shoveling the material flat and laying it flat, tamp it down with your feet to remove any air. After tamping, insert a steel chisel into the material and shake it repeatedly, then tamp it down further with your feet. The ideal thickness for each layer of ramming refractory material is 150-200mm. Then, use a knotter to tamp the material three times in a spiral motion from the perimeter to the center.

                            The quality of the knot is usually checked by placing a 5mm diameter round steel bar on the ramming layer and pressing it down with a 10kg pressure to a depth of no more than 30mm. During on-site construction, you can forcefully insert the chisel, but the depth should not exceed 30mm.

                            The method for knotting the furnace slope is the same as for the furnace bottom: first tamp it down with your feet, then tamp it with a knotter. The maximum angle between the slope and the furnace bottom should not exceed 40°. This is to prevent rolling or collapse caused by excessive slopes.

                            In areas where molten steel is agitated and eroded, such as the taphole base bricks and the furnace door, ramming should be more vigorous and appropriate thickness may be added to maximize the service life of the refractory material in these damaged areas.

                            After ramming, a 5-10mm thick steel plate is placed on the ramming refractory material to prevent the scrap from damaging the furnace bottom or piercing the ramming material layer, which could cause steel leakage. If steelmaking cannot be carried out in time, a 100-200mm thick layer of lime is placed on the iron plate to prevent the ramming material from hydrating.

                            When knotting the ramming material, strict adherence to construction requirements is crucial to ensure its density. Failure to do so will result in significant shrinkage during use, leading to numerous cracks and spalling, shortening its lifespan. The first furnace smelting is crucial. During oxygen decarburization, the oxygen lance must not be inserted too deeply, as this can cause the ramming material to flip upwards and create large pits at the furnace bottom. During the first furnace smelting, a layer of lime can be applied to the furnace bottom. This not only prevents scrap steel from directly impacting the furnace bottom, but also prevents the ramming material from hydrating and prematurely forming slag.

                            Generally, dry ramming refractory material has a lifespan of over 300 furnaces. This can be extended to 500-600 furnaces through hot repair, with some commercially available products even exceeding 600 furnaces.

                            Repair of ramming material at the bottom of an electric furnace

                            After a certain period of use, the ramming refractory material at the bottom of an electric furnace will be damaged to varying degrees due to various reasons. Therefore, the lining should be repaired according to the damage of the lining.

                            • (1) Hot repair is carried out at regular intervals during smelting, but the bottom dynamics must be closely monitored after each batch of steel is discharged. If a pit with a depth greater than 150mm is found, it must be repaired.
                            • (2) Before repairing, use oxygen (immediately after the steel and slag are discharged) to blow the surface to be repaired to completely remove the residual steel and slag in the area.
                            • (3) Hoist the ramming material to the top of the area to be repaired and drop it down. Move the crane to distribute it reasonably.
                            • (4) Hoist iron blocks or other heavy objects and compact them.
                            • (5) It should be emphasized that if a large area of ​​the furnace bottom or furnace slope is hot-repaired, in order to ensure the service life after the repair, shorten the number of repairs, and reduce the consumption of dry ramming refractory material per ton of steel, the first batch of steel after hot repair can be smelted in accordance with the “Operation Requirements for Smelting New Furnaces”.

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                              Bauxite Calcination Process

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                              We are a professional manufacturer of calcined bauxite aggregate, offering a comprehensive range of products including lumps, aggregates, and fine powders. We process and sell a wide range of aggregates and fine powders (containing 60-90% aluminum) and can also customize them to meet customer needs. Particle sizes include: 0-1mm; 1-3mm; 3-5mm; 5-8mm; 8-15mm; and 80-325 mesh. Our aggregate products are high-quality and competitively priced. Contact us for free samples and a quote.

                              Calcined Bauxite Aggregate Material Can be Applied to Make Monolithic Refractory
                              Calcined Bauxite Aggregate Material of Rongsheng

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                                Bauxite Calcination Process

                                The bauxite calcination process follows: raw materials entering the factory → crushing → screening → calcination → crushing → finished product delivery.

                                Equipment for Calcining Bauxite

                                In the process of producing calcined bauxite aggregate clinker, the calcination of bauxite has a great influence on the quality of the clinker. Therefore, it is very important to choose a kiln for calcining bauxite. The main equipment for calcining bauxite includes downdraft kiln, rotary kiln, tunnel kiln, etc.

                                (1) Downdraft kiln is an intermittent kiln. Its name is derived from the flow of flame. The flames generated by combustion all rise from the fire nozzle of the combustion chamber to the top of the kiln. Since the kiln top is sealed, the flames cannot continue to rise. In the absence of an air outlet at the top of the kiln, they are pulled downward by the suction force of the chimney. After passing through the gaps in the sagger columns, they enter the branch flue and the main flue from the fire-absorbing hole at the bottom of the kiln and are finally discharged from the chimney. Because hot gases are light in weight and always float on the surface, people are accustomed to calling the flames flowing from bottom to top “shun”. The flames flowing from top to bottom are called “reverse”. This is the origin of the name “downdraft kiln”.

                                The advantages of the downdraft kiln are that the kiln volume can be large or small, which is flexible in production and can produce high-quality products. The construction investment cost is low and the metal material consumption is low. The disadvantages are that it is intermittent operation and the exhaust gas temperature is very high when it leaves the kiln, so the fuel consumption per unit product is high. The production scale is small and the working conditions are poor. The mechanization level is low, the operation and control are difficult, and the production efficiency is low. The environmental performance is poor. Generally, the thermal efficiency of the downdraft kiln is only 6.48%, while the coal consumption per ton of finished product is as high as 1.456 tons.

                                Rotary Kiln for Bauxite Calcining Production
                                Rotary Kiln for Bauxite Calcining Production

                                (2) The rotary kiln refers to a rotary calcining kiln, which originated in cement production. Rotary kiln equipment is now widely used in many production industries such as building materials, metallurgy, chemical industry, and environmental protection to perform mechanical, physical or chemical treatment on solid materials. Most rotary kilns for calcining bauxite have changed from heavy oil as fuel to coal powder as fuel. The coal consumption for calcining 1 ton of high-bauxite is 200-250kg. The characteristics of rotary kilns are: simple structure, few vulnerable parts, easy to control the production process and high operating rate. It is easy to mass produce and easy to realize automatic control, and the product quality is stable.

                                (3) Tunnel kilns are modern continuous firing thermal equipment, widely used in the roasting production of ceramic products, and also used in metallurgical industries such as abrasives. Tunnel kilns are mainly used for the firing of products. The refractory industry can be divided into three types according to the different operating temperatures. Low-temperature tunnel kilns are mainly used for roasting skateboard bricks, etc., with a firing temperature of about 1000℃. Medium-temperature tunnel kilns are mainly used for firing ordinary alkaline bricks, clay bricks, high-alumina bricks, silica bricks, etc., with a firing temperature of 1300℃-1650℃. High-temperature tunnel kilns are mainly used for firing mid-range magnesia bricks, high-purity magnesia bricks, magnesia-alumina and corundum products, with a firing temperature greater than 1700℃, generally between 1800℃-1900℃. The labor intensity is high, the production efficiency is low, the heat consumption is high, and it is difficult to realize automatic control.

                                At present, for calcined bauxite aggregate, large-scale bauxite calcining production systems at home and abroad all use rotary kiln equipment and technology.

                                Advantages and Disadvantages of Ceramic Aggregates and High-Alumina Aggregates

                                As two important refractory and construction materials, ceramic aggregates and high-alumina aggregates each possess unique advantages and disadvantages. Below is a detailed comparison of their advantages and disadvantages:

                                Ceramic Aggregate

                                Advantages

                                • Reduced Density: Compared to traditional concrete aggregates, ceramic fine aggregate has a lower density, effectively reducing the overall density of concrete, thereby reducing the load on the building itself.
                                • Increased Strength and Durability: Ceramic fine aggregate has higher strength and hardness, effectively improving the compressive strength and durability of concrete, making it stronger and more durable.
                                • Reduced Environmental Pollution: Traditional concrete production produces large amounts of carbon dioxide and other pollutants, while ceramic fine aggregate concrete can effectively reduce environmental pollution and improve production efficiency.

                                Disadvantages

                                • Higher Cost: Because the production cost of ceramic fine aggregate is higher than that of traditional aggregates, the cost of ceramic fine aggregate concrete is also higher, increasing the overall cost of concrete structure construction.
                                • Difficult Construction: The construction of ceramic fine aggregate concrete is relatively difficult, requiring high technical requirements and construction experience, as well as special construction techniques and equipment.
                                • Poor Seismic Resistance: Due to its lower density, ceramic fine aggregate concrete has inferior seismic performance compared to traditional concrete.

                                High-Alumina Aggregate

                                Advantages

                                • High Strength and Hardness: High-alumina aggregates offer high strength and hardness, capable of withstanding high temperatures and mechanical stress, ensuring a stable and safe production process.
                                • High-Temperature Resistance: High-alumina aggregates, calcined bauxite aggregates are primarily made from bauxite calcined at high temperatures. They contain a high proportion of aluminum oxide, resulting in excellent high-temperature resistance.
                                • Chemical Stability: High-alumina aggregates exhibit excellent chemical stability at high temperatures and are resistant to reactions with acids and alkalis, maintaining excellent stability in aggressive media.
                                • Wide Applications: High-alumina aggregates are widely used in ceramics, metallurgy, and the chemical industry. They are an indispensable raw material for applications such as refractory bricks, castables, and gunning mixes.
                                • Thermal Insulation: High-bauxite aggregates are rich in alumina and have low thermal conductivity. They reduce indoor heat loss in winter and block external heat conduction in summer, providing greater building comfort and contributing to energy conservation and environmental protection.
                                • Durability: Due to its high content of alumina, high-alumina aggregates are impervious to deliquescence, moisture, and water erosion, and are also resistant to acid and alkali corrosion. This extends the lifespan of buildings and reduces maintenance costs.

                                Disadvantages

                                • High Cost. Due to the complex production process and high raw material costs, high-alumina aggregates command a relatively high market price.
                                • High production equipment and processes are required. The production of high-alumina aggregates requires advanced equipment and processes to ensure product quality and performance.

                                In summary, ceramic aggregates and high-alumina aggregates each have their own unique advantages and applications. Choosing which aggregate to use requires careful consideration based on the specific project requirements and budget.

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                                  Monolithic Refractories in Various Shapes, Such as Powder, Granular, Mortar, and Block

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                                  Monolithic refractories, also known as bulk refractories, are made by mixing refractory aggregates and powders of a specific grade with binders and admixtures. These refractory linings materials are used directly without undergoing forming and firing processes. Rongsheng Castable Refractory Factory, a manufacturer of unshaped refractory materials, operates an advanced, environmentally friendly, fully automatic monolithic refractory production line with an annual output of 80,000 tons. They primarily produce monolithic refractory products, including refractory castables, refractory plastics, refractory ramming materials, refractory clay, high-temperature mortar, refractory cement, and prefabricated refractory blocks. These products provide reliable support for the integral lining of high-temperature industrial furnaces.

                                  Refractory Castables

                                  Refractory castables are a new type of refractory material that exhibits excellent fluidity after mixing with water without calcining. They are an important type of amorphous refractory material. They are a mixture of refractory aggregate, refractory powder, and binder (or admixtures) in a specific proportion. They can be shipped in bulk form or prefabricated.

                                  Castable Refractory in Bulk
                                  Castable Refractory in Bulk

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                                    The composition of castable refractory is as follows:

                                    1. Refractory aggregate is the main component of refractory castables and functions similarly to the crushed stone and sand in conventional building concrete. Refractory aggregate can be obtained from calcined clinker of various refractory materials (clay, high-alumina, siliceous, magnesian, etc.) or from various waste bricks that have been crushed to a certain degree. Aggregate particle size significantly impacts product quality. Coarse aggregate (5-20 mm) generally accounts for 35%-45% of the mix, while fine aggregate (0.15-5 mm) accounts for 30%-35%.
                                    2. Binders: These act as a bonding and hardening agent, imparting a certain strength to the product. Common binders include ordinary Portland cement, alumina cement (high-alumina cement), magnesia cement, water glass, and phosphoric acid. To ensure refractoriness and minimize volume shrinkage during use, the binder dosage should be kept as low as possible, generally 10% to 25%. Furthermore, the binder and aggregate should not form excessive low-melting products.
                                    3. Admixtures: To improve the physical and chemical properties and workability of refractory castables, appropriate amounts of these additives are often added. These include plasticizers, dispersants, accelerators, retarders, dilution agents, and gelling agents. For refractory castables exposed to high mechanical forces or intense thermal shock, adding an appropriate amount of stainless steel fiber can significantly increase the material’s toughness. Adding inorganic fiber to insulating refractory castables not only enhances toughness but also helps improve thermal insulation.

                                    Refractory Mortar

                                    Refractory mortar, composed of refractory aggregate, binders, and admixtures, is used as a joint material for shaped products. It is delivered in dry or wet form. During construction, a mixing liquid (water or other liquid) is added to the mortar to a specified consistency. Masonry or pouring is then performed using a trowel or specialized machinery (such as a pressure grouting machine).

                                    Refractory Mortar of two Categories
                                    Refractory Mortar of Two Categories

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                                      Refractory mortar is divided into two categories: heavy and light. Based on the binder, it can be categorized as phosphate slurry, water glass slurry, and organic binder slurry. Based on the material, it can be categorized as clay, high-alumina, magnesia, silica, or carbonaceous slurry. Its components are as follows:

                                      01 Refractory Powder

                                      Refractory powder is typically fine particles of refractory materials such as high-alumina, silica, and magnesia, and determines the basic properties of the refractory mortar.

                                      02 Binder

                                      Binders, such as Portland cement, phosphate, and water glass, are used to allow the refractory mortar to harden and develop a certain strength after application.

                                      03 Admixtures

                                      Admixtures are added according to specific application requirements, such as setting accelerators, setting retarders, and plasticizers.

                                      Performance Characteristics

                                      • Excellent Adhesion: Securely bonds refractory bricks and other refractory materials, forming a tight masonry structure and preventing the penetration of high-temperature gases and slag.
                                      • High Refractoriness: Remains stable at high temperatures without softening or melting, maintaining masonry stability.
                                      • Suitable Plasticity: Facilitates construction and can fill brick joints and irregularities.
                                      • Excellent Volume Stability: Minimal volume change at high temperatures, preventing expansion or contraction that could damage the masonry structure.

                                      Refractory Precast Blocks

                                      Precast blocks, which cannot be mass-produced for certain reasons, are typically prefabricated refractory block products composed of refractory aggregate, refractory powder, binders, and admixtures.

                                      Precast Shapes with Refractory Anchor Bricks for Heating Furnace Top
                                      Precast Shapes with Refractory Anchor Bricks for Furnace

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                                        01 Refractory Aggregates

                                        Refractory aggregates, such as high-alumina bauxite, corundum, and mullite, form the primary skeleton of precast blocks. They determine their refractoriness and high-temperature strength.

                                        Refractory powder: Refractory powder has a finer particle size and fills the gaps between aggregates, increasing the density and strength of the precast blocks.

                                        02 Binders

                                        Common binders include cement, phosphate, and water glass. They are used to bind the aggregate and powder together, ensuring sufficient strength after curing under certain conditions.

                                        03 Admixtures

                                        Additional additives, such as accelerators, retarders, and explosion-proofing agents, are added as needed to improve the construction and performance of precast blocks.

                                        Performance Characteristics

                                        • Excellent refractory properties: Maintains structural stability in high-temperature environments without softening or melting. High refractoriness and refractoriness under load.
                                        • Good mechanical strength: Through a rational formulation and manufacturing process, the precast blocks possess high compressive and flexural strength, capable of withstanding mechanical loads at high temperatures.
                                        • High-dimensional accuracy: Prefabricated in the factory, dimensional deviations can be strictly controlled, facilitating installation and ensuring the quality and tightness of the masonry.
                                        • Easy and convenient construction: The precast blocks can be transported directly to the construction site for installation, reducing on-site construction time and workload. They are particularly suitable for the construction of complex structures such as large industrial furnaces.
                                        • Good thermal stability: They can withstand rapid temperature fluctuations without cracking or flaking, ensuring safety during furnace startup and shutdown.

                                        Refractory Cement

                                        Refractory cement, also known as aluminate cement, is a specialty cement with high refractory properties. It is used as a binder in refractory materials for the production of refractory bricks, refractory castables, and refractory spray coatings. It enables refractory materials to maintain excellent strength and integrity at high temperatures. It is also used in the linings of various industrial furnaces, such as those used in steelmaking, cement, and glassmaking.

                                        High-Temperature Refractory Cement
                                        High-Temperature Refractory Cement

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                                          01 Composition

                                          It is primarily composed of calcium aluminate minerals, such as monocalcium aluminate (CaO·Al₂O₃) and monocalcium dialuminate (CaO·2Al₂O₃). It may also contain small amounts of dicalcium silicate (2CaO·SiO₂).

                                          Performance Characteristics

                                          • High refractoriness: It maintains structural stability in high-temperature environments, resisting softening and melting. Its refractoriness generally exceeds 1580°C.
                                          • Early Strength and Rapid Hardening: It exhibits a high rate of early strength development, achieving high strength in a short period of time, facilitating rapid construction.
                                          • Excellent Corrosion Resistance: Strong resistance to corrosive media such as slag and molten salt at high temperatures.
                                          • Excellent Thermal Stability: Minimal volume change with temperature fluctuations, making it less susceptible to cracking and flaking.

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                                            Can Lightweight Castables Used as Insulation Layers for Industrial Furnaces Reduce Heat Consumption?

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                                            Lightweight castables have lower thermal conductivity than heavy castables and significantly reduce the weight of the furnace lining. The diffusion rate of combustion temperature in industrial furnaces can be reduced by about 30% compared to heavy castables. Therefore, using lightweight insulating castables as insulation layers can effectively reduce the heat loss of industrial furnace linings.

                                            Lightweight Castable Refractory for Furnaces
                                            Lightweight Castable Refractory for Furnaces

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                                              Lightweight Castables as Insulation for Industrial Furnaces

                                              Lightweight castables are the first layer of material that adheres to the furnace shell in the kiln lining. High-end lightweight castables, such as hollow alumina sphere castables and lightweight mullite refractory castables, can also be used directly as a working layer on the lining of some lightweight kilns. This reduces furnace weight, lowers heat consumption, and saves energy costs.

                                              The thickness of lightweight castables used as insulation should be 80mm-150mm. Thicknesses below 50mm will result in poor installation and ineffective insulation. Using lightweight castables for lining pipes and chimneys is also a poor choice. Castables with a specific gravity of approximately 1.5 can serve as both an insulation layer and a working layer. This reduces weight while meeting the requirements for chimney and pipe linings.

                                              Lightweight castables are used at different temperatures: low temperatures (600-900°C), medium temperatures (900-1200°C), and high temperatures (1200°C). High-temperature insulation layers require high-quality lightweight materials, such as lightweight mullite and hollow alumina sphere castables. Commonly used materials include vermiculite, perlite, and ceramsite. These materials have an operating temperature of approximately 1000°C and cannot be used at high temperatures. Lightweight mullite and corundum-mullite castables can be used at temperatures between 1350°C and 1500°C, allowing for direct use in the working layer.

                                              Lightweight castables lack the refractoriness, compressive strength, and flexural strength of heavier castables. However, their low thermal conductivity provides excellent insulation. Choosing the right lightweight material for different temperatures and kiln linings can effectively insulate and reduce energy consumption.

                                              Lightweight Insulation Castable
                                              Lightweight Insulation Castable

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                                                Significant Features of Low Thermal Conductivity Castables

                                                Low thermal conductivity castables are unique refractory materials that maintain low bulk density and low thermal conductivity even at high temperatures, while also offering excellent thermal insulation. Unlike conventional lightweight castables, whose bulk density increases with increasing temperature, low thermal conductivity castables maintain a low bulk density even at high temperatures. Their light weight effectively reduces the weight of equipment, which is crucial for the installation and operation of large-scale industrial equipment. Furthermore, their excellent thermal insulation properties ensure more uniform furnace temperature distribution, thereby saving energy and reducing production costs.

                                                In terms of technical performance, low thermal conductivity castables offer the dual advantages of low bulk density and high refractory strength, a true innovation in themselves. They offer strong overall performance, excellent spalling resistance, a long service life, and can be repaired and processed into various shapes as needed. They can also be used to construct integral furnace linings, greatly facilitating the maintenance and construction of industrial furnaces.

                                                Wide Application Areas and Energy-Saving Advantages

                                                Low thermal conductivity castables have a wide range of applications across numerous industries. They are widely used in industrial furnaces and thermal equipment in industries such as metallurgy, machinery, power generation, chemicals, and petroleum. Compared to conventional lightweight castables, using low thermal conductivity castables can achieve energy savings of over 20%, a significant saving for energy-intensive industries. Especially in applications such as petroleum and petrochemical furnaces, where material performance requirements are more stringent, low thermal conductivity castables require a lower bulk density and adhere to stricter quality standards. Therefore, compared to lightweight castables used in other industries, low thermal conductivity castables for petroleum and petrochemical furnaces are of higher quality and relatively higher price. However, the energy-saving benefits and equipment protection they provide are well worth the effort.

                                                Easy Construction

                                                Low thermal conductivity castables are relatively easy to apply. Manufacturers carefully select the aggregate particle size. An appropriate particle size helps enhance the compressive strength of lightweight castables. Excessively large particle sizes can negatively impact the construction process. By optimizing the particle size, low thermal conductivity castables can be constructed more smoothly, reducing difficulties and problems during construction, improving construction efficiency, and ensuring construction quality.

                                                Considerations for Lightweight Castable Pipe Installation

                                                Lightweight castables provide insulation. For pipe use, their bulk density generally must be no less than 1.2 kg/m³. Specific gravity of 1.5 is more common. When used in pipes, installation is limited by the pipe diameter, making it difficult for workers to work within the pipe.

                                                The Pipe Insulation
                                                The Pipe Insulation

                                                1. Pay attention to the insulation layer.

                                                Applying the insulation layer to the insulation is also challenging. The pipe insulation in the image above is applied irregularly, and the anchors are not suitable for use. Anchors must be painted. If not, plastic caps must be placed on them. Otherwise, rust will occur, shortening their lifespan.

                                                If this occurs, there is a remedy: apply a layer of plastic film to the surface of the insulation. This prevents moisture from penetrating the insulation during baking after the lightweight castable is applied. If moisture seeps into the insulation, even after drying, the insulation’s effectiveness will be reduced.

                                                2. Pay attention to the proportions added during construction.

                                                When applying lightweight materials, the water ratio must be carefully considered. If too little water is added, some of the base materials, due to their light weight, will be difficult to apply and will not set easily. Adding too much water will affect the strength of the material in the final application.

                                                3. Mix the aggregate and powder before construction.

                                                During on-site construction, first mix the aggregate and powder, then add the binder. Then, strictly follow the manufacturer’s water ratios and mix the lightweight castable. After mixing, verify that the castable has good fluidity before vibrating. Generally, the water content for lightweight castables is 15-17%. Some manufacturers, due to the light weight of the lightweight castable, may sprinkle some water during production to facilitate production control.

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                                                  High Temperature Performance of Silicon Carbide Refractories

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                                                  SiC is widely used in key areas of the steel and nonferrous metallurgy industries due to its stable high-temperature chemical properties, excellent high-temperature strength, high wear resistance, and good thermal shock resistance. Examples include blast furnace tuyere, inner wall, ceramic cups, various furnace wall linings, and kiln furniture. Compared to metals and intermetallic compounds, silicon carbide refractories have higher high-temperature strength and creep resistance. Compared to oxide ceramics, they have higher thermal conductivity and thermal shock resistance. The high-temperature properties of silicon carbide refractories are as follows:

                                                  Silicon Carbide Refractory Castable
                                                  Silicon Carbide Refractory Castable

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                                                    High-Temperature Oxidation Resistance of SiC Refractories

                                                    Although SiC offers excellent performance, it is susceptible to oxidation during long-term use. Extensive research has been conducted on the oxidation process of SiC. Results indicate that high-temperature oxidation of SiC can be divided into two types: passive oxidation and active oxidation. When the O₂ partial pressure is below 10⁻⁴Pa, SiC undergoes active oxidation, with the product being SiO₂ gas and the net weight decreasing. When the O₂ partial pressure is above 10⁻⁴Pa, SiC undergoes passive oxidation, with the product being SiO₂ and the net weight increasing. The formation of a SiO₂ protective film prevents further oxidation. However, at oxidation temperatures above 1473K, SiO₂ converts to cristobalite at high temperatures. This volume expansion destroys the structure of the oxide film, causing cracks, which in turn leads to internal oxidation, severely shortening the service life of the SiC material. Therefore, improving the oxidation resistance of SiC materials is an essential consideration in the design and preparation of SiC refractories.

                                                    The high-temperature oxidation behavior of a self-bonded SiC material with a porosity of 11.5% in air at 1573K was studied. Research results show that the amorphous SiO2 formed during the initial oxidation process can passivate pores and crack tips within the material, resulting in an increase in the room-temperature strength of the material with increasing oxidation time. The refractory material achieved its highest strength, reaching 293 MPa, at an oxidation time of 22.5 h. As oxidation time continues, the amorphous SiO2 crystallizes to form cristobalite, destroying the structure of the oxide film and generating new surface cracks during cooling, resulting in a decrease in the room-temperature strength of the material.

                                                    The effects of adding varying amounts of calcium oxide, aluminum oxide, and zirconium oxide to SiC on its oxidation resistance at different temperatures were investigated. Experimental results show that a 2 wt% aluminum oxide addition yields the best oxidation resistance. Mullite coatings were generated on recrystallized SiC materials of varying particle sizes using a sol-gel method. The effects of coating thickness and particle size on the high-temperature oxidation behavior of the recrystallized SiC at 1773 K were investigated. Results show that the formation of mullite coatings significantly improves the high-temperature oxidation resistance of the recrystallized SiC. Furthermore, increasing coating thickness increases the oxidation resistance of the recrystallized SiC material. Al₂O₃, SiO₂, and Mullite coatings were applied to SiC whiskers using a sol-gel method. Anti-oxidation experiments showed that all three coatings improved the oxidation resistance of SiC.

                                                    In summary, while silicon carbide refractories possess relatively good oxidation resistance, oxidation to a certain degree can be fatal to the material’s structure. Therefore, studying the mechanism and control of the oxidation process in silicon carbide refractories, as well as its impact on the material’s structure and properties, is of great significance.

                                                    Silicon Carbide Lining Refractory for Furnaces
                                                    Silicon Carbide Lining Refractory for Furnaces

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                                                      Thermal Shock Resistance of SiC Refractories

                                                      As an important high-temperature industrial structural material, silicon carbide refractories have high requirements for thermal shock resistance. The thermal shock resistance of SiC materials is not only related to their microstructure, grain size, and the shape and distribution of internal defects, but also to physical properties such as strength, elastic modulus, thermal conductivity, thermal expansion coefficient, Poisson’s ratio, and porosity. Improving and enhancing the thermal shock resistance of SiC materials is crucial for their safe and stable use.

                                                      The Effect of Different Bonding Methods on the Thermal Shock Resistance of SiC Kiln Furniture. Si2N2O-bonded SiC kiln furniture exhibits superior thermal shock resistance to mullite- and Si3N4-bonded SiC kiln furniture. When the Si2N2O content is ≤20%, the thermal shock resistance of Si2N2O-bonded SiC samples improves with increasing Si2N2O content. When the Si2N2O content exceeds 20%, the thermal shock resistance of the samples decreases.

                                                      Si3N4-SiC and Sialon-SiC materials were prepared by reaction sintering. The results show that the in-situ formation of Si3N4 or Sialon bonding phases can increase the toughness of SiC materials, influence crack propagation, and modulate stress distribution at high temperatures, improving the material’s plastic deformation capacity at high temperatures and, consequently, enhancing the thermal shock resistance of SiC materials.

                                                      The thermal shock resistance of reaction-sintered SiC materials was also studied. The results show that materials with low residual Si content and small SiC particles exhibit superior thermal shock resistance to materials with high residual Si content and large SiC particles.

                                                      Aluminum Silicon Carbide Castables
                                                      Aluminum Silicon Carbide Castables

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                                                        SiC Refractories’ Resistance to Cryolite Attack

                                                        SiC is used as the bottom material for aluminum electrolytic cells because it is non-wettable by molten aluminum and exhibits high thermal conductivity, chemical stability, and excellent oxidation resistance. Aluminum smelting typically uses cryolite (Na₃AlF₆) as a flux, and is carried out by electrolytic reduction of aluminum oxide in the electrolytic cell at a temperature typically between 1173 and 1273 K. Therefore, studying the resistance of SiC materials to cryolite attack is of great practical significance.

                                                        The attack of cryolite on various non-oxides, such as BN, SiC, Si₃N₄, Al₁N, and TiN, was studied. The results showed that these non-oxides exhibited good resistance to cryolite attack. Cryolite melt enters the pores of Si₃N₄-bonded SiC refractory materials and reacts with the binder phase. Erosion of the binder phase causes SiC particles to fall into the cryolite melt, resulting in corrosion of the material.

                                                        The crucible method was used to study the cryolite melt corrosion resistance of Si3N4-bonded SiC refractory materials. The results showed that after 20 hours of corrosion at 1273K, only a small amount of corrosion was observed on the inner wall of the crucible of the Si3N4-bonded SiC refractory prepared in an air atmosphere. This indicates that the material has good cryolite corrosion resistance.

                                                        The static crucible method was also used to study the cryolite corrosion resistance of alumina-based Sialon-bonded corundum-SiC composite refractory materials at 1273K. The results showed that under these conditions, cryolite erosion of the composite material was minimal, with an erosion layer thickness of approximately 1mm, and the erosion product being NaAlSiO4. The penetration layer depth was approximately 6mm, and the penetration rate decreased with increasing Sialon content.

                                                        In summary, silicon carbide refractory materials have excellent cryolite corrosion resistance. The corrosion mechanism suggests that corrosion primarily occurs between cryolite and oxides in the binder phase. Reducing or eliminating the oxide content in the binder phase could further improve the cryolite corrosion resistance of silicon carbide refractory materials.

                                                        To purchase high-quality silicon carbide refractory materials, please contact Rongsheng Refractory Materials Factory for free samples and quotes.

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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.

                                                          Rongsheng Refractory Aggregates
                                                          Rongsheng Refractory Aggregates

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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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