Mullite castable preparation method based on comprehensive utilization of secondary aluminum ash nonferrous smelting slag

Abstract

According to the mullite castable preparation method based on comprehensive utilization of the secondary aluminum ash nonferrous smelting slag, components are regulated and controlled by controlling the molar ratio of Al2O3 to SiO2, so that porous light mullite stably falls into a mullite phase region, and generation of excessive corundum phases is avoided. PMMA microspheres, corn flour, carbonized rice husks and SiO2 coated calcium carbonate composite pore-forming agent systems are adopted, and a layered slurry formula, gradient slip casting and segmented sintering processes are combined, so that the porosity and strength distribution are accurately regulated and controlled, and low-temperature sintering and amorphous phase change control are realized. By optimizing the grading of the porous light mullite aggregate and adding the homologous fine powder, the bonding strength and compressive strength of the castable are improved. And finally, the surface of the castable is coated with a compact mullite coating, so that the porosity is reduced, and the erosion resistance, corrosion resistance and stripping resistance are remarkably improved. The obtained castable has the advantages of light weight, high temperature resistance, high strength, low cost and excellent thermal stability and erosion resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of comprehensive utilization of nonferrous metal smelting waste slag, and particularly relates to a method for preparing a mullite castable based on comprehensive utilization of secondary aluminum ash nonferrous smelting slag. Background Art

[0002] The aluminum casting process is a key link in aluminum processing and consumes a lot of energy. In order to reduce the energy loss in the casting process, people use high-efficiency equipment such as induction electric heating furnaces and regenerative combustion systems to achieve green and energy-saving production. The aluminum casting process requires that the refractory lining has high temperature resistance and thermal insulation performance on the one hand, which can withstand the impact of high-temperature flames, and on the other hand, the lining has a certain ability to resist water vapor and acid-base gas erosion. For refractory lining materials for aluminum casting, an important measure to improve the working efficiency of the kiln and reduce heat loss is to use lightweight and high-temperature resistant refractory materials instead of heavy refractory materials when building the kiln lining. Traditional lightweight insulation materials are mostly based on ceramsite, perlite, and lightweight clay. They can generally only be used as insulation materials in environments below 1200℃ and cannot be used as high-temperature resistant work linings that are in direct contact with flames and atmosphere. Artificially synthesized high-temperature resistant lightweight materials, such as alumina fibers and alumina hollow balls, have a use temperature higher than 1500℃, but are expensive and have low strength. Therefore, there is an urgent need to develop a refractory furnace lining material for aluminum casting that is lightweight, high temperature resistant, low in cost, and high in mechanical strength.

[0003] During the aluminum and recycled aluminum smelting process, floating aluminum slag is formed due to oxidation. After recycling, the remaining aluminum slag usually contains 15-30% metallic aluminum, but the aluminum grade is low, and it is generally discarded without further extraction of aluminum, which is called secondary aluminum ash. Secondary aluminum ash contains heavy metal elements and impurities such as AlN, which pollute the atmosphere and soil and is an industrial hazardous waste. However, secondary aluminum ash contains a large amount of aluminum oxide with economic value and is a renewable resource.

[0004] Using secondary aluminum ash as raw material to produce refractory lining materials for aluminum casting can realize the resource utilization of solid waste and reduce environmental pollution caused by landfill or stacking; it can reduce the dependence of the production of refractory lining materials for aluminum casting on natural raw materials (such as kaolin and bauxite) and reduce resource consumption; and secondary aluminum ash is low in cost, which can significantly reduce production costs; it is in line with the trend of green manufacturing; using secondary aluminum ash as raw material to produce refractory lining materials for aluminum casting can develop new application scenarios for secondary aluminum ash and meet the energy-saving and consumption-reducing needs of aluminum casting production. The technical difficulties of using secondary aluminum ash as raw material to produce refractory lining materials for aluminum casting are the removal of impurities and composition regulation of secondary aluminum ash, precise control of porous structure such as porosity and strength balance, selection of pore-making process, sintering process such as low-temperature sintering requirements, amorphous phase change control, material erosion resistance and corrosion resistance.

[0005] Therefore, it is necessary to develop a lightweight, high-temperature resistant, low-cost, high-mechanical strength, erosion-resistant and corrosion-resistant refractory lining material for aluminum melting and casting, aiming at the technical difficulties of using secondary aluminum ash as raw material to produce refractory lining materials for aluminum melting and casting. Summary of the Invention

[0006] To solve the above technical problems, the present application provides a method for preparing mullite castable based on comprehensive utilization of secondary aluminum ash and non-ferrous smelting slag, which includes the following steps: S1: Pretreat the secondary aluminum ash, and the pretreatment of the secondary aluminum ash includes the following steps: S11: Ball mill the secondary aluminum ash and screen it to obtain fine secondary aluminum ash powder; the particle size of the fine secondary aluminum ash powder is 20 - 45 μm; S12: Add the fine secondary aluminum ash powder obtained in S11 into water, stir it evenly by machine, and assist heating at a temperature of 50 - 100 °C for 4 - 6 h. Stop heating when the pH stabilizes at 9.5 - 10.0, and cool it to 50 - 60 °C to obtain a secondary aluminum ash mixed solution; Preferably, the weight ratio of the fine secondary aluminum ash powder to water is 1:5 - 1:10; Preferably, the mechanical stirring speed is 300 - 500 rpm; S13: Filter the secondary aluminum ash mixed solution obtained in S12 under a certain filtration pressure to obtain a filter cake; control the moisture content of the filter cake ≤ 30%; Preferably, the filtration pressure is 0.4 - 0.6 MPa; Preferably, the moisture content of the filter cake is 20% - 30%; S14: Dry the filter cake obtained in S13, control the moisture content < 1% to obtain pretreated secondary aluminum ash; Preferably, the drying conditions are: temperature 110 °C, time 24 h; The pretreated secondary aluminum ash includes: 1 - 10% of Al, 40 - 70% of Al2O3, 5 - 15% of SiO2, 2% - 5% of CaO, 3 - 8% of alkali metal oxides, 2 - 5% of MgO; the alkali metal oxides are a mixture of K2O and Na2O.

[0007] By reacting fine secondary aluminum ash with water, most soluble salts, alkali metal oxides, calcium and magnesium oxides, metallic aluminum (Al), and other trace impurities can be removed, providing high-purity raw materials for the preparation of high-performance mullite materials. The main purpose of pretreating secondary aluminum ash is to remove aluminum nitride in the aluminum ash, which is a key step in the secondary aluminum ash pretreatment process. The aim is to remove AlN through hydrolysis reaction to avoid the release of harmful gases such as ammonia (NH3) during subsequent high-temperature sintering, which may affect the material properties and production environment. The particle size of the fine secondary aluminum ash directly affects the hydrolysis efficiency of aluminum nitride (AlN), the subsequent filtration rate, and the final composition uniformity. The smaller the particle size, the larger the specific surface area, and the more sufficient the contact between AlN and water, resulting in a more thorough hydrolysis reaction (the reaction rate is proportional to the particle surface area). However, if the particle size is too fine (such as <10μm), it is likely to cause too high viscosity of the suspension, making filtration difficult, and may form colloids, affecting the drying efficiency. Controlling the particle size of the fine secondary aluminum ash to be 20 - 45μm can achieve better hydrolysis efficiency of AlN, filtration rate, and composition uniformity. The reaction between AlN and water will produce NH3, leading to an increase in the pH of the solution (usually >9). When the pH stabilizes at 9 - 10, it indicates that AlN has completely reacted. Therefore, the end point of the reaction can be judged by monitoring the pH value.

[0008] After moderately cooling the secondary aluminum ash mixture to 50 - 60°C and then filtering, due to the moderate liquid viscosity, the filtration rate can be increased, and the precipitation of impurities can be reduced.

[0009] By controlling the water content of the filter cake by increasing the filtration pressure (0.4 - 0.6MPa), the residual amount of soluble impurities (such as CaO, MgO, KO, NaO) in the filter cake can be reduced. During the drying process of the filter cake with high water content, the evaporation of water may cause soluble impurities to precipitate on the particle surface, forming a low-melting-point phase. In the filter cake with low water content, there is less water, and the risk of impurity precipitation is reduced, which is beneficial to improving the refractory performance of the final product. Controlling the water content of the filter cake can also provide a uniform and dense green body structure for the subsequent sintering step. The filter cake with high water content is prone to cracking or porosity after drying, affecting the densification and strength of the sintered green body. The filter cake with low water content has a uniform structure after drying, and the crystal phase develops more completely during sintering, resulting in better material properties.

[0010] Controlling the water content of the pretreated secondary aluminum ash to be <1% can improve the powder fluidity, enhance the sintering performance, reduce the residual impurities, and improve the product purity and refractory performance. The fluctuation of the water content will affect the physical and chemical properties of the powder (such as fluidity, bulk density), making it difficult to control the process parameters. The powder with low water content has stable properties, and the process parameters are easier to regulate, which can improve the product consistency and qualification rate.

[0011] The secondary aluminum ash was pretreated by S1 to remove AlN, soluble impurities, etc. in the secondary aluminum ash. Using the secondary aluminum ash as raw material can reduce production costs and environmental pollution. By strictly controlling the moisture content, it can provide a high-quality raw material basis for the subsequent preparation of mullite materials and ensure the performance and economy of the final product.

[0012] S2: Prepare porous lightweight mullite, including the following steps: S21: Raw material pretreatment, including the following steps: S211: Preparation of matrix material. Using the pretreated secondary aluminum ash obtained by S1 as the aluminum source and natural silica or quartz sand as the silicon source, adjusting the addition amounts of the aluminum source and the silicon source, and controlling the molar ratio of Al2O3 in the aluminum source to SiO2 in the silicon source to be 1.5 - 2.0 to obtain the matrix material; Preferably, the addition amount of the aluminum source pretreated secondary aluminum ash is 80 - 100 parts; the addition amount of the silicon source natural silica or quartz sand is 10 - 30 parts; By controlling the molar ratio of Al2O3 in the aluminum source to SiO2 in the silicon source, the composition is regulated; to make the prepared porous lightweight mullite fall within the mullite phase region, avoiding the formation of excessive corundum phase, which affects the heat resistance or mechanical strength performance of the material, and enabling the prepared porous lightweight mullite to have a higher use temperature; S212: Preparation of SiO2-coated nano-CaCO3; Preferably, the preparation of SiO2-coated nano-CaCO3 is carried out by the sol-gel method. The nano-CaCO3 is dispersed in tetraethyl orthosilicate (TEOS) solution, and ammonia water is used for catalytic hydrolysis at pH 8.5 - 10.5, and stirred at 50 - 70 °C for 4 - 8 hours; calcined at 500 - 700 °C for 1 - 3 hours to form a dense SiO2 coating layer to obtain SiO2-coated nano-CaCO3; Preferably, the thickness of the SiO2 coating layer is 10 - 20 nm; Coating nano-CaCO3 with SiO2 can delay the decomposition temperature of CaCO3, avoiding premature decomposition and conflicts with other pore-forming agents. For SiO2-coated nano-CaCO3, SiO2 can react with CaO to form more stable compounds, such as calcium silicate, reducing the influence of CaO on the material performance.

[0013] S213: Preparation of carbonized rice husk. The rice husk is calcined at 500 - 700 °C for 1 - 3 hours (under N2 protection) and ball-milled to D50 of 30 - 70 μm.

[0014] Preparation of carbonized rice husk: The organic matter in the rice husk is removed by high-temperature pyrolysis, while its inorganic components (such as silicon dioxide) are retained. When uncarbonized rice husk is used as a pore-forming agent, the rapid decomposition of organic matter during the high-temperature sintering process will release a large amount of gases (such as CO2, H2O), resulting in uneven pore structure or even collapse, and residual carbon impurities. The carbonized rice husk retains the SiO2 skeleton, which can react with Al2O3 in the matrix material to form mullite (3Al2O3·2SiO2) during the subsequent sintering process, enhancing the high-temperature stability and mechanical strength of the material. During the carbonization of rice husk, open pores are formed after the oxidation of the carbon skeleton, and the SiO2 skeleton provides closed pores, forming a hierarchical pore structure to optimize the material properties.

[0015] During the preparation of carbonized rice husk, N2 protection can prevent the oxidation of the carbon skeleton after the high-temperature pyrolysis of the organic matter in the rice husk; maintain the SiO2 skeleton structure; and avoid the release of polluting gases such as CO and NO produced by air calcination. x

[0016] The carbonized rice husk is ball-milled to D50 of 30 - 70 μm to form uniform mesopores, taking into account both porosity and strength; this particle size range is suitable for gradient structure design. Smaller particle sizes in the surface layer fill the matrix gaps to enhance densification; larger particle sizes in the inner layer form connected pores to improve heat insulation performance; this particle size range is easy to disperse and matches the sizes of PMMA microspheres and corn starch, avoiding slurry stratification.

[0017] S22: Preparation of layered slurry. The layered slurry includes surface layer slurry, transition layer slurry, and inner layer slurry. The preparation of the surface layer slurry includes: ball-milling 82 - 88 parts of matrix material, 0.5 - 1 part of dispersant, 12 - 20 parts of surface layer pore-forming agent, 2 - 4 parts of binder, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. The preparation of the transition layer slurry includes: ball-milling 75 - 85 parts of matrix material, 1.0 - 1.5 parts of dispersant, 25 - 35 parts of transition layer pore-forming agent, 1.5 - 2.5 parts of interlayer binder, 0.7 - 1.3 parts of slow-release agent, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. The inner layer slurry includes 65 - 75 parts of matrix material, 1.0 - 1.5 parts of dispersant, 34 - 46 parts of inner layer pore-forming agent, 0.7 - 1.3 parts of slow-release agent, 2 - 4 parts of binder, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. The surface layer slurry, transition layer slurry, and inner layer slurry are respectively filtered through a sieve to obtain a slurry with a particle size of 25 - 38 μm. The hierarchical design of the amount of matrix material in the hierarchical slurry is the core control method for the gradient pore structure. The amount selection is mainly carried out from the perspective of the process compatibility of coordinating the proportion of matrix material - pore former to control the porosity and strength gradient, reduce the interlayer stress, and ensure the forming and sintering stability of the slurry. The surface layer requires high strength and density. A high proportion of the matrix can ensure a dense structure after sintering and bear mechanical loads; the transition layer has a gradient connection, and it is necessary to balance strength and porosity to relieve the thermal stress / shrinkage difference between the surface layer and the inner layer; the inner layer requires ultra-light and high porosity, and a low proportion of the matrix can achieve the maximum porosity and realize an ultra-low thermal conductivity. There is a synergistic relationship between the amount of matrix and the pore former. A low amount of pore former in the surface layer forms a small number of closed pores, and the matrix material directly contacts and sinters to form a high-strength network, ensuring that there are no structural defects when the surface layer serves as a load-bearing layer and resisting external impacts or high-temperature scouring; in the transition layer, a medium-high amount of pore former increases the proportion of micropores, and the interlayer binder strengthens the grain boundaries. The matrix material still dominates, but the porosity increases, realizing a smooth transition of strength - heat insulation performance; a high amount of pore former in the inner layer forms a multi-level pore structure, and the matrix only serves as a "skeleton" to maintain the basic structure, sacrificing some strength in exchange for ultra-low thermal conductivity, which is suitable for non-load-bearing heat insulation areas. The selection of the matrix material needs to consider process adaptability. For example, for the control of slurry rheology, the higher the amount of matrix, the greater the viscosity of the slurry. The viscosity is adjusted by dispersants and the amount of water; in the inner layer, there is less matrix and more pore former, so more dispersants are needed to prevent particle sedimentation; for example, for sintering shrinkage matching, the gradient design of the amount of matrix can reduce the interlayer shrinkage difference (, avoiding cracking.

[0018] The dispersant is an anionic dispersant; preferably, the dispersant is ammonium polyacrylate; The action mechanism of the dispersant in the hierarchical slurry is to stabilize the powder (matrix, pore former, nano-ZrO2, etc.) in the slurry through electrostatic repulsion (such as ammonium polyacrylate) or steric hindrance, preventing particle agglomeration; reducing the viscosity of the slurry and adjusting the rheology to ensure that there are no defects when filling the mold during slip casting. For the selection of the amount of dispersant for each layer, it is selected according to the raw material composition and ratio of each layer. The proportion of the matrix in the surface layer is high, and the difficulty of particle dispersion is relatively low. An excessive amount of dispersant may cause the viscosity of the slurry to be too low. Therefore, 0.5 - 1 part of the dispersant is selected; the transition layer contains an interlayer binder, and the proportion of the pore former increases, so more dispersants are needed to prevent nanoparticle agglomeration; the total amount of the pore former in the inner layer is high, and it is necessary to strengthen the dispersion to avoid sedimentation and take into account the fluidity under a high solid content.

[0019] Preferably, the slow-release agent is boric acid; The slow-release agent is mainly used to delay the decomposition of corn flour. Corn flour decomposes at 250-400°C. Boric acid forms a protective film to slow down its decomposition rate, avoiding the overlap of gas release with that of PMMA microspheres (which decompose at 300-400°C) and preventing uneven pores. Moreover, it can reduce the concentrated gas production and prevent cracks or pore connectivity caused by excessive local air pressure. The dosage of the slow-release agent is controlled at 0.7-1.3 parts, which is beneficial to the uniform distribution of pores and sufficient porosity. If the dosage of the slow-release agent is too low, the slow-release effect is insufficient and the pore distribution is uneven. If the dosage of the slow-release agent is too high, it may overly inhibit the decomposition, resulting in insufficient porosity.

[0020] For the layered slurry described above, no slow-release agent is added to the surface slurry because the low corn flour dosage and high matrix resistance on the surface can inhibit the concentrated gas release. Not adding the slow-release agent is to avoid the excessive inhibition of the porosity by the slow-release agent. Preferably, the binder is PVA; the binder provides viscosity and adhesion for each layer of slurry. Preferably, the interlayer binder is nano-ZrO2 or SiC; the interlayer binder is distributed at the grain boundaries of the transition layer, which can inhibit crack propagation, improve the interlayer bonding strength, and have a good pinning effect; the thermal expansion coefficient of the interlayer binder is between that of the surface layer (dense) and the inner layer (porous), which can buffer the thermal stress; the interlayer binder inhibits the excessive growth of grains at high temperatures and can maintain the stability of the pore structure of the transition layer. The viscosity of each layer is adjusted to 3000-3500 mPa·s, which can meet the requirements of the fluidity of slip casting and take into account particle suspension and degassing. Controlling the particle size of each layer of slurry can eliminate agglomerates and ensure pore uniformity. Preferably, the surface layer pore-forming agent includes 2-4 parts of SiO2-coated CaCO3, 4-6 parts of PMMA microspheres, 4-6 parts of corn flour, and 2-4 parts of carbonized rice husk; the transition layer pore-forming agent includes 4-6 parts of SiO2-coated CaCO3, 6-8 parts of PMMA microspheres, 7-9 parts of corn flour, and 8-12 parts of carbonized rice husk; the inner layer pore-forming agent includes 7-9 parts of SiO2-coated CaCO3, 9-11 parts of PMMA microspheres, 9-11 parts of corn flour, and 13-17 parts of carbonized rice husk. In the gradient pore structure design of porous lightweight mullite, the ratio differences of pore-forming agents in the surface layer, transition layer, and inner layer are based on the process compatibility perspectives of porosity gradient control, functional hierarchical adaptation of strength / thermal insulation / transition connection, decomposition temperature matching, and sintering shrinkage coordination. For example, the surface layer has requirements for high strength, high-temperature stability, and process adaptability. Therefore, a low total pore-forming agent dosage (12–20 parts) is used to ensure the densification and compressive strength of the matrix. PMMA microspheres and corn flour form a small number of closed pores to reduce stress concentration; SiO2-CaCO3 and carbonized rice husk (SiO2 skeleton) enhance the temperature resistance, and the low-temperature decomposition of PMMA microspheres / corn flour does not interfere with the densification sintering of the surface layer. The transition layer has requirements for pore gradient transition, interface strengthening, and decomposition coordination. The transition layer increases the proportion of carbonized rice husk to provide more micropores and SiO2 skeletons to connect the high-porosity structure of the inner layer; the proportion of PMMA microspheres / corn flour is slightly higher than that of the surface layer to form medium-sized pores; an interlayer binder is added to pin the grain boundaries and prevent interlayer peeling; the high-temperature decomposition of SiO2-CaCO3 and carbonized rice husk form multi-level pores; for the ultra-low thermal conductivity requirement: the inner layer has requirements for ultra-low thermal conductivity, structural stability, and light weight. Therefore, a high pore-forming agent dosage is used in the inner layer to achieve a high porosity and a low thermal conductivity; the SiO2 skeleton and micropores of the carbonized rice husk in the inner layer significantly improve the thermal insulation performance; the PMMA microspheres / corn flour in the inner layer form large-sized connected pores, and the carbonized rice husk / SiO2-CaCO3 provides micropore support; the low matrix proportion enables the density to be reduced to 0.8 g / cm 3 Below, a certain compressive strength is still maintained.

[0021] The ball milling medium for the ball milling is corundum with a particle size of 5–10 mm; In the preparation of porous lightweight mullite, the balance between porosity (determining the lightweight and thermal insulation properties of the material) and mechanical strength (determining the load-bearing capacity) is the core challenge. Materials with a high porosity will be lighter and have good thermal insulation performance, but their strength will decrease, while materials with high strength usually have a lower porosity. In the preparation of porous lightweight mullite, the pore-forming agent has a great influence on the porosity and mechanical strength.

[0022] This technical solution aims to obtain porous lightweight mullite with good porosity and mechanical strength. A composite pore-forming agent system of PMMA microspheres, corn flour, carbonized rice husk, and SiO2-coated calcium carbonate is used. Through temperature gradient decomposition and functional complementarity, precise control of multi-level pores and synergistic optimization of properties are achieved. The pore-forming characteristics of PMMA microspheres are that during sintering, PMMA microspheres thermally decompose at a low temperature (usually 300 - 400°C) to generate gases (such as CO2, H2O), forming main pores (20 - 50μm) with a uniform closed-pore or open-pore structure. The particle size range of PMMA microspheres is wide (3 - 70μm), and the pore size distribution can be customized (40% - 70%). Corn flour assists PMMA microspheres in regulating the pore size distribution (closed pores, 10 - 30μm), and the decomposition temperature is 250 - 400°C. Carbonized rice husk powder decomposes at 400 - 800°C, leaving a SiO2 skeleton in the middle temperature range (micropores 1 - 5μm), enhancing the matrix and improving strength and specific surface area. Corn flour and rice husk, as biomass pore-forming agents, are low-cost and environmentally friendly. Different pore structures can enhance the diversity and performance of the pore structure. Nano-CaCO3 decomposes at high temperature (800 - 900°C) into CaO and CO2, and the gas forms secondary pores. Introducing nano-calcium carbonate as a pore-forming agent, nano-calcium carbonate begins to thermally decompose at 800°C to form nano-pores. Its decomposition temperature partially overlaps with that of carbonized rice husk, which may lead to non-uniform pore structure or collapse. For example, if PMMA microspheres and corn flour decompose at a lower temperature, the gas generated during the high-temperature decomposition of calcium carbonate may cause pressure within the already formed pore structure, resulting in structural damage. In this application, SiO2-coated calcium carbonate is used to delay the decomposition of calcium carbonate to form micropores (1 - 5μm) at 1000 - 1100°C, avoiding premature decomposition and conflicts with other pore-forming agents, and reducing the influence of residual CaO, which helps to more precisely control the formation of pores at the high-temperature stage. During the heating process, calcium carbonate reacts with SiO2 and Al2O3 respectively to form high-temperature-resistant and high-strength calcium silicate and calcium aluminate. This reaction causes volume shrinkage of the material, forming nano-scale small pores, increasing the specific surface area of the material, and making the material have more excellent surface activity. At the same time, the generated calcium silicate and calcium aluminate have hydration activity, so that in the high-temperature-resistant lightweight castable prepared from this lightweight porous mullite aggregate, the aggregate and the matrix have more excellent bonding strength.

[0023] S23: Gradient grouting molding. First, inject the surface slurry into a mold with a demoulding agent sprayed on the inner wall to a thickness of 2 mm, and remove air bubbles under vacuum; then inject the transition layer slurry to a thickness of 5 mm and vibrate; then inject the inner layer slurry to 10 mm, and let it stand and cure; dry; to obtain a porous lightweight mullite embryo; The thickness design of each layer in gradient grouting forming is based on the core functions and performance requirements of each layer. The surface layer is the main stress-bearing surface of the material (such as mechanical wear) and needs to be achieved through a high matrix content and a low thickness. The thin layer (2 mm) can ensure complete densification after sintering and avoid residual internal pores caused by excessive thickness. The thin layer can reduce the depth of heat penetration and, combined with a low porosity, effectively reflect high-temperature radiation. The transition layer needs to coordinate the difference in thermal expansion coefficients between the surface layer and the inner layer. A 5-mm thickness can fully disperse thermal stress and prevent interlayer delamination. And the interlayer binder is evenly distributed within a 5-mm thickness to achieve the best grain boundary pinning effect. A 5-mm thickness allows a smooth transition of porosity and avoids sudden changes in performance (too thin is likely to cause stress concentration at the interface). The inner layer considers optimizing the heat insulation performance. The heat insulation effect is proportional to the thickness of the heat insulation layer. A 10-mm inner layer can reduce the thermal conductivity to meet the heat insulation requirements. The high porosity of the inner layer requires a sufficient thickness to maintain structural stability. And the inner layer accounts for a high proportion of the overall thickness, and the density of the inner layer is small, achieving overall weight reduction.

[0024] Preferably, the pressure for vacuum degassing is -0.1 MPa and the time is 5 min; the vibration frequency is 50 Hz and the time is 3 min; the static curing is carried out at room temperature for 24 to 72 hours. Vacuum degassing is carried out on the surface layer to completely remove air bubbles and avoid cracking of the high-strength surface layer due to pore defects. The transition layer evenly disperses the interlayer binder through vibration to eliminate local agglomeration and ensure the interlayer bonding strength. The inner layer is statically cured to naturally discharge residual gases and stabilize the high-porosity structure, avoiding external force damage to the fragile porous framework.

[0025] The forming process is also one of the important factors affecting the balance between porosity and mechanical strength. In this application, a gradient pore structure of the surface layer, transition layer, and inner layer is constructed and formed layer by layer. By adjusting the composition of the pore-forming agent and the raw material ratio in each layer, a gradient structure of a dense surface layer (low porosity and high strength) and a porous interior (high porosity and low thermal conductivity) is achieved. Designing a transition layer between the surface layer and the inner layer can gradiently adjust the shrinkage stress and solve the problem of the interlayer bonding strength caused by the difference in shrinkage rates between the surface layer and the inner layer resulting in interface cracking.

[0026] Preferably, the drying is gradient heating drying. Heat up to 60 - 80 °C at a heating rate of 1 - 2 °C / min and hold for 6 - 12 hours; then heat up to 90 - 120 °C at a heating rate of 0.5 - 1 °C / min and hold for 8 - 16 hours; then heat up to 180 - 200 °C at a heating rate of 0.3 - 0.5 °C / min and hold for 10 - 24 hours; control the cooling rate < 1 °C / min and cool with the furnace; obtain a porous lightweight mullite embryo. Through low-temperature dehumidification at 60-80°C, free water is slowly removed to prevent surface hardening from clogging the internal water vapor channels; through evaporation of bound water at 90-120°C, the skeleton structure of the embryo is stabilized to avoid shrinkage and cracking; through thorough removal of residual moisture at 180-200°C, preparation for subsequent sintering is made to avoid bursting during high-temperature sintering; S24: Sinter in stages. Heat the porous lightweight mullite embryo obtained in S22 to 300-400°C at a heating rate of 3-5°C / min, hold for 30-60 min under N2 atmosphere protection; then heat to 800-1000°C at a heating rate of 2-3°C / min, hold for 60-120 min under air atmosphere; then heat to 1100-1400°C at a heating rate of 4-6°C / min, hold for 120-180 min under air atmosphere; control the cooling rate <5°C / min and cool with the furnace; obtain porous lightweight mullite blocks. The sintering process is an important factor affecting the balance between porosity and mechanical strength. This application adopts a gradient heating and staged sintering process through a staged process of "low-temperature slow release - medium-temperature transition - high-temperature strengthening" to achieve the synergistic optimization of pore structure and matrix properties; defect minimization and energy consumption reduction; repeatability and reliability of industrial production. Precisely regulate the decomposition behavior of the pore-forming agent. PMMA microspheres and corn starch decompose in the low-temperature stage (300-400°C) to form large-sized closed pores (20-50 μm) to avoid gas interference in the high-temperature stage; the organic matter in carbonized rice husk decomposes in the medium-temperature stage (800-1000°C), and the residual SiO2 skeleton provides micropores (1-5 μm); SiO2-coated CaCO3 decomposes at a delayed rate in the high-temperature stage (1000-1100°C) to generate uniform micropores, avoiding structural collapse caused by concentrated gas production, with a more uniform pore distribution and a precisely controllable porosity gradient; adopting the gradient heating and staged sintering process can optimize matrix sintering and crystal phase formation, avoid premature densification, and slowly heat up in the low-temperature stage (3-5°C / min) to prevent premature sintering of the matrix and ensure sufficient decomposition of the pore-forming agent; in the high-temperature stage (1100-1400°C), long-term holding (120-180 minutes) promotes the complete formation of mullite (3Al2O3·2SiO2), reduces impurity phases (such as free Al2O3), improves compressive strength, and enhances heat resistance. Adopting the gradient heating and staged sintering process can reduce defects and stress concentration. The gradient heating matches the differences in the thermal expansion coefficients of the material at each stage to avoid crack generation. The transition layer of nano-ZrO2 diffuses uniformly during heating, pins the grain boundaries, inhibits interlayer delamination, improves the material yield rate, and enhances the interlayer bonding strength; adopting the gradient heating and staged sintering process can achieve energy conservation and process stability. Slowly heating up in the low-temperature stage reduces thermal shock, and quickly heating up in the high-temperature stage shortens the sintering time, resulting in reduced comprehensive energy consumption; by fixing the heating curve (such as holding at 400°C for 30-60 minutes → holding at 800°C for 60-120 minutes), batch stability is ensured. The staged sintering process of this technical solution meets the requirements of low-temperature sintering and better controls the non-crystalline phase transformation.

[0027] Through the design of hierarchical slurry formula, gradient grouting forming and segmented sintering process, this technical solution can accurately control the porosity and strength distribution of porous materials to meet the differentiated requirements for material properties.

[0028] S25: Jaw-crush and screen the porous lightweight mullite block materials obtained in S24 to obtain porous lightweight mullite with different particle sizes; The porous lightweight mullite with different particle sizes includes porous lightweight mullite aggregate and porous lightweight mullite fine powder; the particle size of the porous lightweight mullite aggregate is 0.15 mm to 5 mm; the particle size of the porous lightweight mullite fine powder is 44 to 74 μm; the porous lightweight mullite aggregate, calculated according to the share of the porous lightweight mullite aggregate, includes 25 to 40 parts of porous lightweight mullite aggregate 1 with a particle size of 1 mm to 5 mm, 35 to 45 parts of porous lightweight mullite aggregate 2 with a particle size of 0.5 mm to 1 mm, and 20 to 35 parts of porous lightweight mullite aggregate 3 with a particle size of 0.15 mm to 0.5 mm; In some embodiments, when preparing porous lightweight mullite in S2, the following steps are further included: Perform post-treatment on the porous lightweight mullite block materials obtained in S24. The post-treatment includes acid washing for impurity removal and surface impregnation; the acid washing for impurity removal is to immerse the porous lightweight mullite block materials obtained in S23 in an acid solution; the surface impregnation is to impregnate the surface layer with silica sol to fill the surface openings, and then perform secondary sintering at 1100 - 1200 °C to form a dense protective layer; obtain the post-treated porous lightweight mullite block materials; The acid solution is 1 mol / L HCl solution; the immersion time of the porous lightweight mullite block materials in the acid solution is 30 min; The SiO2 content in the silica sol is 20 wt%; The time for the secondary sintering is 50 - 60 min; By immersing the block materials in the acid solution, residual CaO is dissolved to improve acid resistance; by impregnating the surface pores of the block materials with silica sol, the surface density is enhanced. SiO2 reacts with the matrix during secondary sintering to form a glass phase, blocking the penetration of high-temperature oxidation / corrosion media.

[0029] S3: Prepare a high-temperature resistant lightweight mullite castable, including the following steps: S31: Place 40 - 70 parts of the porous lightweight mullite aggregate, 5 - 25 parts of the porous lightweight mullite fine powder, 2 - 8 parts of a water retention agent, 5 - 10 parts of an expansion agent, and 10 - 25 parts of a binder in a mixer and stir to mix evenly to obtain a high-temperature resistant lightweight mullite dry material; S32: Add an appropriate amount of water to the high-temperature resistant lightweight mullite dry material obtained in S31 and stir evenly to obtain a high-temperature resistant lightweight mullite wet material; S33: Pour the high-temperature resistant lightweight mullite wet material obtained in S32 into a mold, perform forced vibration molding, cure, and demold to obtain a high-temperature resistant lightweight mullite castable; The appropriate amount of water described in step S32 is 15-30% of the weight of the high-temperature resistant lightweight mullite dry material; Preferably, the water retention agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; the expansion agent is kyanite or andalusite; This technical solution optimizes the grading of the porous lightweight mullite aggregate, and adds aggregates of 1 mm - 5 mm, 0.5 mm - 1 mm, and 0.15 mm - 0.5 mm, and fine powder of 44 - 74 μm in stages to improve the bulk density and strength; the 1 - 5 mm aggregate provides the main load-bearing framework; the 0.5 - 1 mm aggregate reduces the voids between large particles; the 0.15 - 0.5 mm aggregate further densifies; the fine powder wraps the particles and fills the gaps between the aggregates, enhancing the bonding strength and compressive strength; all components (aggregate 1 / 2 / 3, fine powder) retain the original porous structure, the overall density of the castable is lower, and the thermal conductivity is further reduced; the aggregate and the fine powder are from the same source and have the same coefficient of thermal expansion, avoiding high-temperature cracking caused by component differences.

[0030] This technical solution uses a porous lightweight mullite aggregate, significantly reducing the material density and improving the heat insulation effect, and is applicable to the lining of high-temperature kilns; the mullite aggregate is combined with pure calcium aluminate cement, with excellent high-temperature resistance; kyanite / andalusite is used as the expansion agent, which decomposes into mullite and expands in volume at high temperatures, effectively offsetting the sintering shrinkage; bentonite / Guangxi white clay is used as the water retention agent to improve the plasticity and water retention capacity of the castable; the component ratio range is wide (such as 40 - 70 parts of aggregate, 10 - 25 parts of binder), facilitating performance adjustment.

[0031] In some embodiments, the method for preparing the mullite castable further includes the following steps: S4: Prepare a dense mullite coating by placing 40 - 60 parts of calcined alumina fine powder, 20 - 40 parts of nano silicon fine powder, 1 - 5 parts of fluorite powder, 2 - 6 parts of calcium zirconate, 2 - 10 parts of binder, 0.1 - 2 parts of carboxymethyl cellulose, 0.2 - 1 part of boric anhydride, 0.1 - 0.5 part of nano ZnO, and 0.1 - 0.5 part of polyvinyl alcohol fiber in an appropriate amount of water and mixing evenly to obtain the dense mullite coating; S5: Brush the dense mullite coating obtained in S4 onto the surface of the high-temperature resistant lightweight mullite castable obtained in S3, dry it under medium-temperature conditions, and then cool it to room temperature; heat it to 1000 - 1200 °C at a heating rate of 2 - 5 °C / min, perform sintering, and keep the temperature for 1 - 3 hours; Preferably, the coating thickness of the dense mullite coating is 0.1 mm - 0.5 mm; The 0.1 mm coating can basically completely cover the surface defects. For too thin a coating, there will be uncoated through - pores locally; when the coating thickness is too thick, the thermal stress increases, which will cause early peeling of the coating.

[0032] Preferably, the binder is aluminum sol powder; the particle size of the calcined alumina micropowder is D50 2 μm; the particle size of the nano - silicon micropowder is D50 0.5 μm; the particle size of the aluminum sol powder binder is 10 - 30 μm; the particle size of the boric anhydride is 74 μm; the particle size of the nano - ZnO is 10 - 50 nm; the length of the polyvinyl alcohol fiber is 1 - 5 mm; The medium - temperature condition in step S5 is 100 - 200 °C; the drying time is 24 h - 48 h.

[0033] There are many micropores on the surface of the castable. When it is used as the working layer of the aluminum melting refractory lining, it cannot resist the erosion of high - temperature flue gas and acid - base atmosphere. This technical solution solves this problem by preparing a dense mullite coating on the surface of the castable. In this technical solution, the calcined alumina micropowder is the aluminum source and the nano - silicon micropowder is the silicon source, and they form mullitization during the high - temperature sintering process; fluorite powder is introduced into the mullite coating material, and the fluorite powder is evenly dispersed between the alumina and silica micropowders and forms a strong bonding strength during the mullitization process. Utilizing the non - wetting property of the main component CaF2 of fluorite to aluminum, a dense mullite coating that can effectively resist the erosion of acid - base gases and aluminum slag is prepared on the surface of the furnace lining material made of high - temperature resistant lightweight mullite castable; this technical solution selects to add calcium zirconate, which can reduce the usage amount of fluorite powder, thereby reducing the volatilization of F - in the fluorite powder and improving the corrosion resistance; this technical solution introduces a low - temperature sintering promoter boric anhydride into the mullite coating material and adds a nano - ZnO composite sintering aid to promote densification, thereby reducing the temperature of the mullitization reaction between alumina and silica, so that the coating undergoes a sintering reaction at 1000 - 1200 °C to form a dense mullite coating with a thickness of 0.1 mm - 0.5 mm. This coating can significantly improve the material's resistance to thermal stress and spalling resistance. This technical solution; this technical solution introduces polyvinyl alcohol fiber, which plays an anti - cracking and strengthening effect, improving the long - term stability and thermal shock resistance of the coating.

[0034] This application pre - treats secondary aluminum ash to remove AlN, most soluble salts, alkali metal oxides, calcium and magnesium oxides, metallic aluminum (Al) and other trace impurities, providing high - purity raw materials for the preparation of high - performance mullite materials and ensuring the performance and economy of the final product.

[0035] This application controls the molar ratio of Al2O3 in the pre - treated secondary aluminum source to SiO2 in the silicon source for composition regulation, so that the prepared porous lightweight mullite falls within the mullite phase region, avoiding the formation of too much corundum phase, which affects the thermal shock resistance or mechanical strength performance of the material, and enabling the prepared porous lightweight mullite to have a higher service temperature.

[0036] In the process of preparing porous lightweight mullite in this application, through the design of a hierarchical slurry formula including a PMMA microsphere, corn starch, carbonized rice husk, and SiO2-coated calcium carbonate composite pore-forming agent system, gradient grouting molding, and segmented sintering process, the porosity and strength distribution of the porous lightweight mullite are precisely controlled; the segmented sintering process meets the requirements of low-temperature sintering and effectively controls the amorphous phase transformation.

[0037] In the process of preparing the high-temperature-resistant lightweight mullite castable in this application, the grading of the porous lightweight mullite aggregate is optimized, and porous lightweight mullite fine powder is added, significantly enhancing the bonding strength and compressive strength of the castable; the aggregate and the fine powder have the same source and the same coefficient of thermal expansion, and the castable has stronger high-temperature resistance.

[0038] In this application, by preparing a dense mullite coating and applying the coating to the surface of the high-temperature-resistant lightweight mullite castable, the porosity of the surface is significantly reduced, the erosion resistance and corrosion resistance of the high-temperature-resistant lightweight mullite castable are improved, and the thermal performance and spalling resistance of the high-temperature-resistant lightweight mullite castable are significantly improved.

[0039] The high-temperature-resistant lightweight mullite castable prepared by the technical solution of this application is lightweight, high-temperature-resistant, low-cost, has high mechanical strength, and good erosion resistance and corrosion resistance. Brief Description of the Drawings

[0040] The following will further describe the present disclosure in detail with reference to the drawings and preferred embodiments. However, those skilled in the art will understand that these drawings are only drawn for the purpose of explaining the preferred embodiments and should not be construed as limiting the scope of the present disclosure. In addition, unless otherwise specified, the drawings only schematically show the composition or structure of the described object and may include exaggerated displays, and the drawings are not necessarily drawn to scale.

[0041] Figure 1 It is a schematic diagram of a method for preparing a mullite castable based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag. Detailed Description of the Embodiments

[0042] The following will be described in detail with reference to the attached Figure 1 , and make a detailed description of the present disclosure.

[0043] In order to make the purpose, technical solutions, and advantages of the present disclosure clearer, the following will further describe the present disclosure in detail with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present disclosure and are not used to limit the present disclosure.

[0044] This application provides a method for preparing a mullite castable based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag, as Figure 1As shown, it includes the following steps: S1: Pretreat secondary aluminum ash, and the pretreatment of secondary aluminum ash includes the following steps: S11: Ball-mill secondary aluminum ash, and then screen it to obtain fine secondary aluminum ash powder; the particle size of the fine secondary aluminum ash powder is 20 - 45 μm; S12: Add the fine secondary aluminum ash powder obtained in S11 into water, and the weight ratio of the fine secondary aluminum ash powder to water is 1:5 - 1:10. Stir mechanically and evenly, with the mechanical stirring speed being 300 - 500 rpm, and assist heating at a temperature of 50 - 100 °C. React for 4 - 6 h, and stop heating when the pH stabilizes at 9.5 - 10.0. Cool to 50 - 60 °C to obtain a secondary aluminum ash mixed solution; S13: Filter the secondary aluminum ash mixed solution obtained in S12 under a filtration pressure of 0.4 - 0.6 MPa to obtain a filter cake; the moisture content of the filter cake is 20% - 30%; S14: Dry the filter cake obtained in S13 at 110 °C for 24 h, and control the moisture content to be < 1% to obtain pretreated secondary aluminum ash; Preferably, the drying conditions are: temperature 110 °C, time 24 h; The pretreated secondary aluminum ash includes: 1 - 10% Al, 40 - 70% Al2O 3、 5 - 15% SiO2, 2% - 5% CaO, 3 - 8% alkali metal oxides, 2 - 5% MgO; the alkali metal oxides are a mixture of K2O and Na2O.

[0045] S2: Prepare porous lightweight mullite, including the following steps: S21: Pretreat the raw materials, including the following steps: S211: Prepare the matrix material. Use the pretreated secondary aluminum ash obtained in S1 as the aluminum source and natural silica or quartz sand as the silicon source. The addition amount of the aluminum source is 80 - 100 parts; the addition amount of the silicon source, natural silica or quartz sand, is 10 - 30 parts; to obtain the matrix material; S212: Prepare SiO2-coated nano-CaCO3. The preparation of SiO2-coated nano-CaCO3 is carried out by the sol-gel method. Disperse nano-CaCO3 in tetraethyl orthosilicate (TEOS) solution, and carry out ammonia-catalyzed hydrolysis at pH 8.5 - 10.5, and stir at 50 - 70 °C for 4 - 8 hours; calcine at 500 - 700 °C for 1 - 3 hours to form a dense SiO2 coating layer to obtain SiO2-coated nano-CaCO3; the thickness of the SiO2 coating layer is 10 - 20 nm; S213: Prepare carbonized rice husk. Calcinate rice husk at 500 - 700 °C for 1 - 3 hours (under N2 protection), and ball-mill it to D50 of 30 - 70 μm.

[0046] S22: Preparation of layered slurry. The layered slurry includes surface slurry, transition layer slurry, and inner layer slurry. The preparation of the surface slurry includes: ball milling 82 - 88 parts of matrix material, 0.5 - 1 part of dispersant, 12 - 20 parts of surface pore-forming agent, 2 - 4 parts of binder, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. The preparation of the transition layer slurry includes: ball milling 75 - 85 parts of matrix material, 1.0 - 1.5 parts of dispersant, 25 - 35 parts of transition layer pore-forming agent, 1.5 - 2.5 parts of interlayer binder, 0.7 - 1.3 parts of sustained-release agent, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. The inner layer slurry includes 65 - 75 parts of matrix material, 1.0 - 1.5 parts of dispersant, 34 - 46 parts of inner layer pore-forming agent, 0.7 - 1.3 parts of sustained-release agent, 2 - 4 parts of binder, and appropriate amount of water for 1 - 3 hours, and adjusting the viscosity to 3000 - 3500 mPa·s. Respectively screen-filter the surface slurry, transition layer slurry, and inner layer slurry to obtain slurries with a particle size of 25 - 38 μm. The surface pore-forming agent includes 2 - 4 parts of SiO₂-coated CaCO₃, 4 - 6 parts of PMMA microspheres, 4 - 6 parts of corn flour, and 2 - 4 parts of carbonized rice husk. The transition layer pore-forming agent includes 4 - 6 parts of SiO₂-coated CaCO₃, 6 - 8 parts of PMMA microspheres, 7 - 9 parts of corn flour, and 8 - 12 parts of carbonized rice husk. The inner layer pore-forming agent includes 7 - 9 parts of SiO₂-coated CaCO₃, 9 - 11 parts of PMMA microspheres, 9 - 11 parts of corn flour, and 13 - 17 parts of carbonized rice husk. The ball milling medium for the ball milling is corundum with a particle size of 5 - 10 mm. S23: Gradient grouting molding. First, inject the surface slurry into a mold with a demoulding agent sprayed on the inner wall to a thickness of 2 mm, and remove air bubbles under vacuum. Then inject the transition layer slurry to a thickness of 5 mm and vibrate. Then inject the inner layer slurry to 10 mm and let it stand for curing. Dry to obtain a porous lightweight mullite embryo. The pressure for vacuum degassing is -0.1 MPa and the time is 5 min. The vibration frequency is 50 Hz and the time is 3 min. The static curing is to stand at room temperature for 24 - 72 hours. The drying is gradient heating drying. Heat up at a heating rate of 1 - 2 °C / min to 60 - 80 °C and keep warm for 6 - 12 hours. Then heat up at a heating rate of 0.5 - 1 °C / min to 90 - 120 °C and keep warm for 8 - 16 hours. Then heat up at a heating rate of 0.3 - 0.5 °C / min to 180 - 200 °C and keep warm for 10 - 24 hours. Control the cooling rate <1 °C / min and cool with the furnace to obtain a porous lightweight mullite embryo. S24: Phase sintering. Heat the porous light-weight mullite green body obtained in S22 from room temperature to 300 - 400 °C at a heating rate of 3 - 5 °C / min, hold for 30 - 60 min under N₂ atmosphere protection; then heat to 800 - 1000 °C at a heating rate of 2 - 3 °C / min, hold for 60 - 120 min under air atmosphere; then heat to 1100 - 1400 °C at a heating rate of 4 - 6 °C / min, hold for 120 - 180 min under air atmosphere; control the cooling rate < 5 °C / min and cool with the furnace to obtain porous light-weight mullite blocks. Post-treat the obtained porous light-weight mullite blocks, and the post-treatment includes pickling for impurity removal and surface impregnation. The pickling for impurity removal is to immerse the porous light-weight mullite blocks obtained in S23 into the acid solution. The surface impregnation is to impregnate the surface layer with silica sol to fill the surface pores, and then sinter at 1100 - 1200 °C for the second time to form a dense protective layer, thus obtaining the post-treated porous light-weight mullite blocks. The acid solution is 1 mol / L HCl solution. The immersion time of the porous light-weight mullite blocks in the acid solution is 30 min. The SiO₂ content in the silica sol is 20 wt%. The time for the second sintering is 50 - 60 min. S25: Jaw-crush and screen the porous light-weight mullite blocks to obtain porous light-weight mullite with different particle sizes. The porous light-weight mullite with different particle sizes includes porous light-weight mullite aggregate and porous light-weight mullite fine powder. The particle size of the porous light-weight mullite aggregate is 0.15 mm - 5 mm. The particle size of the porous light-weight mullite fine powder is 44 - 74 μm. The porous light-weight mullite aggregate, calculated according to the share of the porous light-weight mullite aggregate, includes 25 - 40 parts of porous light-weight mullite aggregate 1 with a particle size of 1 mm - 5 mm, 35 - 45 parts of porous light-weight mullite aggregate 2 with a particle size of 0.5 mm - 1 mm, and 20 - 35 parts of porous light-weight mullite aggregate 3 with a particle size of 0.15 mm - 0.5 mm. S3: Prepare a high-temperature resistant light-weight mullite castable, including the following steps: S31: Place 40 - 70 parts of the porous light-weight mullite aggregate, 5 - 25 parts of the porous light-weight mullite fine powder, 2 - 8 parts of the water-retaining agent, 5 - 10 parts of the expansion agent, and 10 - 25 parts of the binder into a mixer and stir to mix evenly to obtain a high-temperature resistant light-weight mullite dry material. S32: Add an appropriate amount of water to the high-temperature resistant light-weight mullite dry material obtained in S31 and stir evenly to obtain a high-temperature resistant light-weight mullite wet material. S33: Pour the high-temperature resistant light-weight mullite wet material obtained in S32 into a mold, vibrate it forcibly to form, cure, and demold to obtain a high-temperature resistant light-weight mullite castable. The appropriate amount of water described in step S32 is 15-30% of the weight of the high-temperature resistant lightweight mullite dry material; The water retention agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; the expansion agent is kyanite or andalusite; S4: Prepare a dense mullite coating. Put 40-60 parts of calcined alumina micropowder, 20-40 parts of nano silicon micropowder, 1-5 parts of fluorite powder, 2-6 parts of calcium zirconate, 2-10 parts of binder, 0.1-2 parts of carboxymethyl cellulose, 0.2-1 part of boric anhydride, 0.1-0.5 part of nano ZnO, and 0.1-0.5 part of polyvinyl alcohol fiber into an appropriate amount of water, and mix them evenly to obtain the dense mullite coating; S5: Brush the dense mullite coating described in S4 onto the surface of the high-temperature resistant lightweight mullite castable. After drying under medium temperature conditions, cool it to room temperature; heat it to 1000-1200°C at a heating rate of 2-5°C / min, carry out sintering, and keep it warm for 1-3 hours to obtain the mullite castable; The coating thickness of the dense mullite coating is 0.1 mm - 0.5 mm; The binder is aluminum sol powder; the particle size of the calcined alumina micropowder is D50 2 μm; the particle size of the nano silicon micropowder is D50 0.5 μm; the particle size of the aluminum sol powder binder is 10-30 μm; the particle size of the boric anhydride is 74 μm; the particle size of the nano ZnO is 10-50 nm; the length of the polyvinyl alcohol fiber is 1-5 mm; The medium temperature condition described in step S5 is 100-200°C; the drying time is 24 h - 48 h. Specific embodiments

[0047] Examples 1-3 Examples 1-3 are studies on the preparation of layered slurries. The compositions of the specific layered slurries are as follows in the table: Comparative example 1 Compared with Example 2, Comparative example 1 has no transition layer, and the other components and processes are the same.

[0048] Comparative example 2 Compared with Example 2, Comparative example 2 has no interlayer binder, and the other components and processes are the same.

[0049] Comparative example 3 Compared with Example 2, Comparative example 3 has no slow-release agent, and the other components and processes are the same.

[0050] Comparative example 4 Compared with Example 2, Comparative example 4 has no dispersant, and the other components and processes are the same.

[0051] Examples 1-3 and Comparative Examples 1-4 were respectively used to prepare porous lightweight mullite according to the steps of S2: preparing porous lightweight mullite, and the technical indexes of the obtained porous lightweight mullite were detected. The detection results are shown in Table 1.

[0052] Table 1 Technical indexes of porous lightweight mullite obtained from Examples 1-3 and Comparative Examples 1-4 As can be seen from Table 1, in Examples 1-3, with the increase of the content of the pore-forming agent, the porosity increases and the density decreases; the gradient pore-forming agent design forms a uniform gradient pore size; based on the staged sintering and the optimization of the matrix ratio, high-purity mullite (>85%) is obtained; in Examples 1-3, the transition layer significantly improves the pore gradient, and the difference in interlayer porosity is stable at 10-15%; Example 2 shows the best performance in terms of bulk density (0.75-0.95 g / cm 3 ), crystal phase purity (pure mullite) and pore gradient (continuous distribution). In Examples 1-3, the corn starch as the surface pore-forming agent inhibits the pore expansion of PMMA microspheres, and the content of PMMA microspheres in the inner layer pore-forming agent is high, and they cooperate with carbonized rice husk to form pores of 50-80 μm; in Example 2, it is limited to 40-70 μm through the slow-release agent.

[0053] In Comparative Example 1, the absence of a transition layer results in a 20% difference in porosity between the surface layer and the inner layer, and the interface is prone to fracture. The surface layer and the inner layer are in direct contact, and the density gradient disappears; in Comparative Example 2, the absence of an interlayer binder results in poor interlayer bonding force, and an amorphous phase appears locally (interlayer fracture zone), and the density inhomogeneity increases; in Comparative Example 3, due to the absence of a slow-release agent, the closed pores increase and the pore connectivity is poor; in Comparative Example 4, the absence of a dispersant results in the agglomeration of the pore-forming agent, large fluctuations in porosity (±15%), poor uniformity, and densification in some areas.

[0054] In summary, the transition layer and the interlayer binder are the keys to maintaining the gradient pore size and mechanical strength (the performance of Comparative Examples 1 / 2 decreases significantly); the slow-release agent optimizes the pore connectivity (the closed pore rate increases in Comparative Example 3); the dispersant ensures the uniform distribution of the pore-forming agent (Comparative Example 4).

[0055] Examples 4-6 Examples 4-6 are studies on the hierarchical pore-forming agent. The specific composition of the hierarchical pore-forming agent is as follows in the table: Comparative Example 5 Compared with Example 5, CaCO3 is used instead of SiO2 to coat CaCO3, and the other components are the same and the process is the same.

[0056] Comparative Example 6 Compared with Example 5, there is no PMMA microsphere pore-forming agent in each layer; the other components are the same and the process is the same.

[0057] Comparative Example 7: Compared with Example 5, there is no SiO2-coated CaCO3 pore former in each layer, and the remaining components and processes are the same.

[0058] Comparative Example 8: Compared with Example 5, the transition layer pore former and the surface layer pore former are interchanged, and the remaining components and processes are the same.

[0059] Comparative Example 9: Gradient grouting molding. First, inject the surface layer slurry into a mold with a demoulding agent sprayed on the inner wall to a thickness of 5 mm, and remove air bubbles under vacuum; then inject the transition layer slurry to a thickness of 10 mm and vibrate; then inject the inner layer slurry to 15 mm, let it stand and cure; dry to obtain a porous lightweight mullite embryo. Comparative Example 10: Gradient grouting molding. First, inject the surface layer slurry into a mold with a demoulding agent sprayed on the inner wall to a thickness of 5 mm, and remove air bubbles under vacuum; then inject the transition layer slurry to a thickness of 2 mm and vibrate; then inject the inner layer slurry to 5 mm, let it stand and cure; dry to obtain a porous lightweight mullite embryo. Comparative Example 11: S23 uses a gradient heating process to replace staged sintering, and the heating rate is 3-5 °C / min to be calcined at 1100-1400 °C for 24-72 hours. Examples 4-6 and Comparative Examples 5-11 are respectively prepared into porous lightweight mullite according to S2: the steps of preparing porous lightweight mullite, and the technical indexes of the obtained porous lightweight mullite are detected. The detection results are shown in Table 2.

[0060] Table 2 Technical indexes of porous lightweight mullite obtained from Examples 4-6 and Comparative Examples 5-11 In Examples 4-6, the gradient of the pore formers in the surface layer, transition layer, and inner layer is reasonable. The PMMA microspheres dominate the main pores, and the uniformity is good. The SiO2 coating on CaCO3 inhibits the residual CaO. In Example 6, the content of carbonized rice husk increases, and the micropores of carbonized rice husk increase; Example 5 has the lowest bulk density, the highest porosity, a uniform pore size distribution, and a pure crystal phase, which is the best balance of the formula and process.

[0061] In Comparative Example 5, the uncoated CaCO3 decomposed prematurely (below 800 °C), resulting in an increase in closed pores, a decrease in effective porosity, blockage by residual CaO, and a reduction in the high-temperature stability of the anorthite impurity phase; in Comparative Example 6, the absence of PMMA microspheres led to the absence of main pores of PMMA microspheres, relying on corn flour and carbonized rice husks, with micro-pores dominant and the loss of light-weight characteristics; in Comparative Example 7, the absence of SiO2 coating on CaCO3 led to a reduction in secondary pores formed by the decomposition of CaCO3 and poor connectivity of the main pores of PMMA microspheres; in Comparative Example 8, the gradient of the pore former was reversed (surface layer > transition layer), and the reversed pore gradient led to stress concentration during sintering and local cracking; in Comparative Example 9, the thickness of the grouting increased (5 mm for the surface layer), and uneven drying shrinkage resulted in micro-cracks and uneven pore diameters; in Comparative Example 10, the increase in the thickness of the surface layer led to uneven drying shrinkage and an increase in local closed pores; the transition layer was too thin (2 mm) to effectively buffer the interlayer stress, resulting in direct contact between the surface layer and the inner layer, local densification, stress concentration at the sintering interface, and local amorphization; the pore-forming effect of the pore former in the transition layer was limited, the pore gradient was fractured, and the effective porosity decreased; in Comparative Example 11, gradient heating sintering led to insufficient decomposition of the pore former, an increase in residual carbon and closed pores; incomplete sintering, incomplete oxidation of carbon, and inhibition of mullitization.

[0062] In Examples 1 to 3 and Examples 4 to 6, the pore distribution and pore diameter showed a gradually decreasing trend, which was related to the gradient design of the pore former ratio, the gradient change of the matrix material ratio, the synergistic mechanism of the pore former, and the interlayer differences in process parameters. The content of PMMA microspheres increased layer by layer. However, since spherical closed pores might be formed after its high-temperature decomposition, during the sintering shrinkage process, the higher content of PMMA microspheres in the inner layer, due to the decrease in the matrix material ratio and the action of sintering stress, led to more significant pore compression, and finally the pore diameter decreased; more CO2 was generated by the decomposition of CaCO3 in the inner layer, but the SiO2 coating layer delayed the decomposition, forming more uniform nano-scale secondary pores (1 - 5 μm), filling the gaps between the main pores of PMMA microspheres, and shifting the overall pore size distribution towards smaller sizes; the matrix material in the inner layer was the least, with a higher shrinkage rate during sintering and a stronger compression effect on pores, resulting in the pore diameter of the inner layer being smaller than that of the transition layer and the surface layer; the gas (CO2 / H2O) generated by the combustion of corn flour formed irregular open pores. However, after its content gradient was synergistic with PMMA microspheres, the gas release pressure in the inner layer was higher, which instead promoted the enhancement of pore connectivity and partially offset the pore expansion; during the grouting molding process, the inner layer was thicker, and the moisture migration path was long during drying, with shrinkage stress concentration, resulting in pore compression; the pore former in the inner layer decomposed more violently in an N2 atmosphere (300 - 400 °C) during segmented sintering, but subsequently, in the air atmosphere, the SiO2 coating layer and the sintering shrinkage of the matrix compressed the pores together, forming a denser pore wall structure. For example, after the gradient reversal in Example 5 and Comparative Example 8, the pore diameter of the inner layer decreased instead, indicating that the matrix material ratio and sintering compression were the main reasons. Therefore, the low content of the pore former and the high ratio of the matrix material in the surface layer retained more of the original pore structure; theoretically, the high content of the pore former in the inner layer should increase the pore diameter, but the insufficient matrix material and sintering shrinkage led to pore compression.

[0063] Example 7 Prepare a high-temperature resistant lightweight mullite castable according to step S3; Comparative Example 12 The particle size of the porous lightweight mullite aggregate is 0.15 mm to 5 mm, without gradation optimization, and prepare a high-temperature resistant lightweight mullite castable according to step S3; Test the high-temperature resistant lightweight mullite castables obtained in Example 7 and Comparative Example 12, and the test results are shown in Table 3.

[0064] Table 3 Technical indicators of the high-temperature resistant lightweight mullite castables obtained in Example 7 and Comparative Example 12 Technical indicators Example 7 Comparative Example 12 Maximum service temperature (°C) 1550～1650 1350～1500 Bulk density (g / cm3) 1.2～1.5 1.5～1.8 Thermal conductivity (W / mK) 0.15～0.35 0.35～0.55 High temperature flexural strength (MPa) 3.0～4.0 1.0～2.0 Linear change rate after re-firing (%) -0.1～+0.5％ -1.0～+0.5％ In Example 7, the gradation of the particle size of the porous lightweight mullite aggregate is optimized, and the maximum service temperature is increased by 50 - 150 °C compared with Comparative Example 12 without gradation optimization. In Example 7, close packing is formed through the aggregate gradation, reducing the penetration channels of the glass phase at high temperatures; the expansive agent (kyanite) is transformed into mullite and SiO2 above 1200 °C, which can compensate for the sintering shrinkage and inhibit the crack propagation; in Comparative Example 12, the use of ungraded aggregates results in relatively large local pores, and the high-temperature molten slag is easily infiltrated, reducing the fire resistance limit; In Example 7, the fine powder can fill the gaps between the aggregates, reducing the density; through the optimization of the aggregate gradation, the continuous path of heat flow can be blocked, and the thermal conductivity decreases; in Comparative Example 12, the aggregate gradation is not optimized, and the particle sizes are randomly distributed, and the heat is quickly conducted through the pore walls; In Example 7, the aggregate gradation is optimized, and the multi-level aggregates and the fine powder form a "skeleton - filling" structure, and the hydration products of calcium aluminate cement can generate an intertwined nanocrystalline network at the interface, and the high-temperature flexural strength is significantly enhanced compared with Example 12; in Comparative Example 12, due to the uneven particle size distribution, there are stress concentration points at the interface, and cracking occurs preferentially at high temperatures; The near-zero linear change on reheating (-0.1 to +0.5%) in Example 7 indicates that it is suitable for the lining of high-temperature kilns and is not prone to cracking during long-term use; the shrinkage (-1.0%) in Comparative Example 12 will cause the expansion of the masonry joints, accelerate the infiltration of molten slag, and reduce the service life; in Example 7, the kyanite expansive agent is transformed into mullite and SiO2 at high temperatures to compensate for the sintering shrinkage; the graded aggregates form uniform pores to inhibit local deformation; in Comparative Example 12, the use of ungraded aggregates leads to the concentration of shrinkage stress, resulting in sintering shrinkage.

[0065] Examples 8 - 10 Examples 8 - 10 are studies on dense mullite coatings. The raw material compositions of the specific dense mullite coatings are as follows in the table: Comparative Example 13 S4 was used to prepare a dense mullite coating. 40 - 60 parts of calcined alumina fine powder, 20 - 40 parts of nano silicon fine powder, 2 - 8 parts of fluorite powder, 2 - 10 parts of binder, 0.1 - 2 parts of carboxymethyl cellulose, 0.2 - 1 part of boric anhydride, and 0.1 - 0.5 part of polyvinyl alcohol fiber were placed in an appropriate amount of water and mixed thoroughly to obtain the dense mullite coating; compared with Example 8, Comparative Example 13 had no calcium zirconate and no nano ZnO.

[0066] Comparative Example 14 S4 was used to prepare a dense mullite coating. 40 - 60 parts of calcined alumina fine powder, 20 - 40 parts of nano silicon fine powder, 1 - 5 parts of fluorite powder, 2 - 6 parts of calcium zirconate, 2 - 10 parts of binder, 0.1 - 2 parts of carboxymethyl cellulose, 0.2 - 1 part of boric anhydride, and 0.1 - 0.5 part of nano ZnO were placed in an appropriate amount of water and mixed thoroughly to obtain the dense mullite coating; compared with Example 8, Comparative Example 14 had no polyvinyl alcohol fiber.

[0067] The dense mullite coatings obtained in Examples 8 - 9 and Comparative Examples 13 - 14 were brushed onto the surface of the high - temperature resistant lightweight mullite castable described in S3 according to S5 and tested. The test results are shown in Table 4.

[0068] Table 4 Technical indicators of the high - temperature resistant lightweight mullite castables obtained in Examples 8 - 9 and Comparative Examples 13 - 14 In Examples 8 - 10, nano ZnO was used to refine grains, and calcium zirconate generated CaO - ZrO2 to fill pores, resulting in a gradual decrease in porosity; in Comparative Example 13, there was no calcium zirconate and nano ZnO, the grains were coarsened and there was no ZrO2 filling, and the porosity increased; in Comparative Example 14, there was no polyvinyl alcohol fiber, the number of dry microcracks increased, and the porosity increased.

[0069] In Examples 8 - 10, an organic - inorganic composite network was formed by the binder and carboxymethyl cellulose, the polyvinyl alcohol fiber enhanced the interfacial toughness, and the presence of nano ZnO could also enhance the interfacial bonding; in Comparative Example 13, the lack of calcium zirconate led to the absence of the chemical anchoring effect of CaO - ZrO2 at the interface; in Comparative Example 14, there was no polyvinyl alcohol fiber, the number of dry microcracks increased, thus weakening the mechanical interlocking between the coating and the matrix, without fiber - enhanced interfacial toughness, and the brittleness increased relatively; In Examples 8 - 10, calcium zirconate generated CaO - ZrO2, which synergistically reduced the overall thermal expansion coefficient with mullite; the presence of nano silicon fine powder made the mullitization of the coating more complete; ZnO and boric anhydride formed a viscous glass phase, filled pores and promoted particle rearrangement, improving the sintering shrinkage uniformity; in Comparative Example 13, without calcium zirconate, it only relied on mullite and the glass phase, so the overall thermal expansion coefficient was relatively high. In Comparative Example 14, without polyvinyl alcohol fiber, the number of dry microcracks increased, resulting in an uneven microstructure and a relatively high expansion coefficient.

[0070] In Examples 8 to 10, calcium zirconate generates CaO-ZrO2, enhancing toughness; polyvinyl alcohol fibers can buffer thermal stress; nano-ZnO refines grains and grain boundaries, so the thermal shock resistance is enhanced; in Comparative Example 13, there is no calcium zirconate and nano-ZnO, and in Comparative Example 14, there is no polyvinyl alcohol fiber to buffer thermal stress, so the thermal shock resistance ability is weakened.

[0071] In Examples 8 to 10, calcium zirconate generates CaO-ZrO2, and ZrO2 can resist the wetting of molten aluminum and the dissolution of molten salt; fluorite powder is evenly dispersed between alumina and silica micropowders and forms a strong bonding strength during the mullitization process. Using the non-wetting property of the main component CaF2 of fluorite to aluminum, a dense mullite coating that can effectively resist the erosion of acid-base gases and aluminum slag is prepared on the surface of the refractory material prepared from high-temperature lightweight mullite castable; therefore, the dense mullite coating prepared in Examples 8 to 10 has good resistance to molten aluminum erosion and molten salt corrosion; in Comparative Example 14, due to the absence of polyvinyl alcohol fibers, the number of drying microcracks increases and the porosity increases, and the prepared dense mullite coating is more susceptible to erosion and corrosion.

[0072] The above has introduced this application in detail. Specific examples are used in this application to elaborate on the principle and implementation mode of this application. The description of the above examples is only used to help understand this disclosure and the core idea. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of this application, several improvements and modifications can still be made to this application, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A preparation method of mullite castable based on comprehensive utilization of secondary aluminum ash and non-ferrous smelting slag, characterized in that, It includes the following steps: S1: Pretreat secondary aluminum ash; S2: Prepare porous lightweight mullite, including the following steps: S21: Pretreat the raw materials, where the raw material pretreatment includes preparing the matrix material, preparing SiO2-coated nano-CaCO3, and preparing carbonized rice husk; the matrix material uses the pretreated secondary aluminum ash obtained in S1 as the aluminum source; S22: Prepare the layered slurry, including preparing the surface slurry, the transition layer slurry, and the inner layer slurry; the layered slurry includes the raw materials pretreated in S21; S23: Gradient grouting molding, sequentially injecting the surface slurry, the transition layer slurry, and the inner layer slurry prepared in S22 with a certain thickness into the mold, molding to obtain a porous lightweight mullite embryo; S24: Stage sintering, subject the porous lightweight mullite embryo obtained in S23 to stage sintering to obtain porous lightweight mullite blocks; S25: Crush and sieve the porous lightweight mullite blocks obtained in S24 through a screen to obtain porous lightweight mullite with different particle sizes; the porous lightweight mullite with different particle sizes includes porous lightweight mullite aggregate and porous lightweight mullite fine powder; S3: Prepare a high-temperature resistant lightweight mullite castable, which is prepared from the porous lightweight mullite aggregate and porous lightweight mullite fine powder obtained in S25 as raw materials.

2. The preparation method of the mullite castable according to claim 1, characterized in that, The pretreatment of the secondary aluminum ash includes the following steps: S11: Ball mill the secondary aluminum ash, sieve it to obtain secondary aluminum ash fine powder; S12: Add the secondary aluminum ash fine powder obtained in S11 into water, mechanically stir evenly, assist in heating, react completely, and cool to obtain a secondary aluminum ash mixed solution; S13: Filter the secondary aluminum ash mixed solution obtained in S12 under a certain filtration pressure to obtain a filter cake; S14: Dry the filter cake obtained in S13 to obtain pretreated secondary aluminum ash.

3. The preparation method of the mullite castable according to claim 2, characterized in that, The weight ratio of the secondary aluminum ash fine powder to water is 1:5 to 1:10; the mechanical stirring speed is 300 to 500 rpm; the particle size of the secondary aluminum ash fine powder is 20 to 45 μm; the temperature of the auxiliary heating is 50 to 100 °C; the cooling temperature is 50 to 60 °C.

4. The preparation method of the mullite castable according to claim 3, characterized in that, The moisture content of the filter cake ≤ 30%; the moisture content of the pretreated secondary aluminum ash < 1%.

5. The preparation method of the mullite castable according to claim 1, characterized in that, The preparation of the matrix material uses the pretreated secondary aluminum ash obtained in S1 as the aluminum source and natural silica or quartz sand as the silicon source, adjusts the addition amounts of the aluminum source and the silicon source, and controls the molar ratio of Al2O3 in the aluminum source to SiO2 in the silicon source to be 1.5 to 2.0; for the preparation of SiO2-coated nano-CaCO3, the thickness of the SiO2 coating layer is 10 to 20 nm; for the preparation of carbonized rice husk, the particle size is D50 30 to 70 μm.

6. The preparation method of the mullite castable according to claim 1, characterized in that, The preparation of the surface layer slurry includes: ball milling 82 - 88 parts of matrix material, 0.5 - 1 part of dispersant, 12 - 20 parts of surface layer pore former, 2 - 4 parts of binder, and appropriate amount of water, and adjusting to a certain viscosity; the preparation of the transition layer slurry includes: ball milling 75 - 85 parts of matrix material, 1.0 - 1.5 parts of dispersant, 25 - 35 parts of transition layer pore former, 1.5 - 2.5 parts of interlayer binder, 0.7 - 1.3 parts of sustained release agent, and appropriate amount of water, and adjusting to a certain viscosity; the inner layer slurry includes 65 - 75 parts of matrix material, 1.0 - 1.5 parts of dispersant, 34 - 46 parts of inner layer pore former, 0.7 - 1.3 parts of sustained release agent, 2 - 4 parts of binder, and appropriate amount of water for ball milling; adjusting to a certain viscosity; respectively filtering the surface layer slurry, the transition layer slurry, and the inner layer slurry through a sieve to obtain slurries with specific particle sizes.

7. The preparation method of the mullite castable according to claim 6, characterized in that, The surface layer pore former includes 2 - 4 parts of SiO₂-coated CaCO₃, 4 - 6 parts of PMMA microspheres, 4 - 6 parts of corn flour, and 2 - 4 parts of carbonized rice husk; the transition layer pore former includes 4 - 6 parts of SiO₂-coated CaCO₃, 6 - 8 parts of PMMA microspheres, 7 - 9 parts of corn flour, and 8 - 12 parts of carbonized rice husk; the inner layer pore former includes 7 - 9 parts of SiO₂-coated CaCO₃, 9 - 11 parts of PMMA microspheres, 9 - 11 parts of corn flour, and 13 - 17 parts of carbonized rice husk; the viscosity is 3000 - 3500 mPa·s; the specific particle size is 25 - 38 μm.

8. The preparation method of the mullite castable according to claim 1, characterized in that, For the gradient grouting forming, first inject the surface layer slurry into a mold with a demoulding agent sprayed on the inner wall to a thickness of 2 mm, and remove air bubbles under vacuum; then inject the transition layer slurry to a thickness of 5 mm and vibrate; then inject the inner layer slurry to 10 mm, and let it stand for curing; dry; to obtain a porous lightweight mullite embryo.

9. The preparation method of the mullite castable according to claim 8, characterized in that, The drying is gradient temperature rising drying, heating at a heating rate of 1 - 2 °C / min to 60 - 80 °C and holding for 6 - 12 hours; then heating at a heating rate of 0.5 - 1 °C / min to 90 - 120 °C and holding for 8 - 16 hours; then heating at a heating rate of 0.3 - 0.5 °C / min to 180 - 200 °C and holding for 10 - 24 hours; controlling the cooling rate < 1 °C / min and cooling with the furnace.

10. The preparation method of the mullite castable according to claim 1, characterized in that, For the staged sintering, heat the porous lightweight mullite embryo obtained in S23 at a heating rate of 3 - 5 °C / min to 300 - 400 °C, hold for 30 - 60 min under N₂ atmosphere protection; then heat at a heating rate of 2 - 3 °C / min to 800 - 1000 °C, hold for 60 - 120 min in air atmosphere; then heat at a heating rate of 4 - 6 °C / min to 1100 - 1400 °C, hold for 120 - 180 min in air atmosphere; control the cooling rate < 5 °C / min and cool with the furnace.

11. The preparation method of the mullite castable according to claim 1, characterized in that, The particle size of the porous lightweight mullite aggregate is 0.15 mm to 5 mm; the particle size of the porous lightweight mullite fine powder is 44 to 74 μm; the porous lightweight mullite aggregate includes 25 to 40 parts of porous lightweight mullite aggregate 1 with a particle size of 1 mm to 5 mm, 35 to 45 parts of porous lightweight mullite aggregate 2 with a particle size of 0.5 mm to 1 mm, and 20 to 35 parts of porous lightweight mullite aggregate 3 with a particle size of 0.15 mm to 0.5 mm.

12. The preparation method of the mullite castable according to claim 1, characterized in that, The preparation of the high-temperature resistant lightweight mullite castable includes the following steps: S31: Place 40 to 70 parts of the porous lightweight mullite aggregate, 5 to 25 parts of the porous lightweight mullite fine powder, 2 to 8 parts of the water retention agent, 5 to 10 parts of the expansion agent, and 10 to 25 parts of the binder in a mixer and stir to mix evenly to obtain the high-temperature resistant lightweight mullite dry material; S32: Add an appropriate amount of water to the high-temperature resistant lightweight mullite dry material obtained in S31 and stir evenly to obtain the high-temperature resistant lightweight mullite wet material; S33: Pour the high-temperature resistant lightweight mullite wet material obtained in S32 into a mold, perform forced vibration molding, cure, and demold to obtain the high-temperature resistant lightweight mullite castable.

13. The preparation method of the mullite castable according to claim 12, characterized in that, The appropriate amount of water in step S32 is 15 to 30% of the weight of the high-temperature resistant lightweight mullite dry material; the water retention agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; the expansion agent is kyanite or andalusite.

14. The preparation method of the mullite castable according to claim 1, characterized in that, It also includes the following steps: post-treat the porous lightweight mullite block material obtained in S24; the post-treatment includes acid washing for impurity removal, surface impregnation, and secondary sintering; the acid washing for impurity removal is to immerse the porous lightweight mullite block material obtained in S24 in an acid solution; The surface impregnation is to impregnate the surface layer with silica sol to fill the surface openings; perform secondary sintering at 1100 to 1200 °C to form a dense protective layer; obtain the post-treated porous lightweight mullite block material.

15. The preparation method of the mullite castable according to claim 1, characterized in that, It also includes the following steps: S4: Prepare a dense mullite coating. Place 40 to 60 parts of calcined alumina micro powder, 20 to 40 parts of nano silicon micro powder, 1 to 5 parts of fluorite powder, 2 to 6 parts of calcium zirconate, 2 to 10 parts of binder, 0.1 to 2 parts of carboxymethyl cellulose, 0.2 to 1 part of boric anhydride, 0.1 to 0.5 part of nano ZnO, and 0.1 to 0.5 part of polyvinyl alcohol fiber in an appropriate amount of water and mix evenly to prepare the dense mullite coating; S5: Brush the dense mullite coating described in S4 onto the surface of the high-temperature resistant lightweight mullite castable described in S3, dry it under medium-temperature conditions, and then cool it to room temperature; sinter.

16. The preparation method of the mullite castable according to claim 15, characterized in that, The coating thickness of the dense mullite coating is 0.1 mm to 0.5 mm; the sintering is carried out by heating at a heating rate of 2 to 5 °C / min to 1000 to 1200 °C, holding for 1 to 3 hours; the binder is aluminum sol powder; the medium-temperature condition is 100 to 200 °C; the drying time is 24 h to 48 h.

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