Preparation method of mullite castable based on comprehensive utilization of secondary aluminum dross colored smelting slag

Abstract

The application provides a preparation method of a mullite castable based on comprehensive utilization of secondary aluminum dross colored smelting slag, the molar ratio of Al2O3 / SiO2 is controlled to regulate the composition, the porous lightweight mullite is stably located in the mullite phase region, and excessive corundum phase is avoided. A PMMA microsphere, corn powder, carbonized rice husk and SiO2 coated calcium carbonate composite pore forming agent system is used, combined with a layered slurry formula, gradient slurry forming and segmented sintering process, the porosity and strength distribution are precisely controlled, low temperature sintering and amorphous phase transformation control are realized. The combination strength and compressive strength of the castable are improved by optimizing the gradation of the porous lightweight mullite aggregate and adding homologous fine powder. Finally, a dense mullite coating is coated on the surface of the castable to reduce the porosity and significantly improve the erosion resistance, corrosion resistance and anti-spalling performance. The obtained castable has the advantages of lightweight, high temperature resistance, high strength, low cost and excellent thermal stability and erosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of comprehensive utilization technology of non-ferrous metal smelting waste residue, specifically relating to a method for preparing mullite castable based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag. Background Technology

[0002] Aluminum smelting and casting is a crucial step in aluminum processing, consuming a significant amount of energy. To reduce energy loss during this process, efficient equipment such as induction furnaces and regenerative combustion systems are used to achieve green and energy-saving production. The aluminum smelting and casting process requires refractory furnace linings to possess both high-temperature resistance and insulation properties to withstand the impact of high-temperature flames, while also resisting the corrosion of moisture and acid / alkali gases. For refractory furnace lining materials used in aluminum smelting and casting, a key measure to improve kiln efficiency and reduce heat loss is to replace heavy refractory materials with lightweight, high-temperature resistant refractory materials during kiln lining construction. Traditional lightweight insulation materials, often based on expanded clay, perlite, and lightweight clay, are generally only suitable for environments below 1200℃ and cannot be used as high-temperature resistant work linings directly exposed to flames and atmospheres. Artificially synthesized high-temperature resistant lightweight materials, such as alumina fibers and hollow alumina spheres, can be used at temperatures above 1500℃, but they are expensive and have relatively low strength. Therefore, there is an urgent need to develop a lightweight, high-temperature resistant, low-cost, and high-mechanical-strength refractory furnace lining material for aluminum casting.

[0003] During the aluminum and recycled aluminum smelting process, floating aluminum dross is formed due to oxidation. After recycling, the remaining aluminum dross typically contains 15-30% metallic aluminum, but the aluminum grade is low, and it is generally discarded without further aluminum extraction; this is called secondary aluminum ash. Secondary aluminum ash contains heavy metals and impurities such as AlN, which pollute the atmosphere and soil, making it a hazardous industrial waste. However, it also contains a large amount of economically valuable alumina, making it a renewable resource.

[0004] Utilizing secondary aluminum ash as a raw material to produce refractory furnace lining materials for aluminum casting can realize the resource utilization of solid waste, reducing environmental pollution caused by landfill or stockpiling; it can reduce the dependence on natural raw materials (such as kaolin and bauxite) in the production of refractory furnace lining materials for aluminum casting, reducing resource consumption; and secondary aluminum ash is inexpensive, which can significantly reduce production costs; it conforms to the trend of green manufacturing; and using secondary aluminum ash as a raw material to produce refractory furnace lining materials for aluminum casting opens up new application scenarios for secondary aluminum ash, meeting the energy-saving and consumption-reducing needs of aluminum casting production. The technical challenges of using secondary aluminum ash as a raw material to produce refractory furnace lining materials for aluminum casting lie in the removal of impurities and the control of composition in secondary aluminum ash, the precise control of porous structure such as the balance between porosity and strength, the selection of pore-forming processes, the sintering process such as the requirement of low-temperature sintering, the control of amorphous phase transformation, and the material's 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 furnace lining material for aluminum casting, addressing the technical challenges of producing refractory furnace lining materials for aluminum casting using secondary aluminum ash as raw material. Summary of the Invention

[0006] To address the aforementioned technical problems, this application provides a method for preparing mullite castables based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag, comprising the following steps: S1: Pre-treatment of secondary aluminum ash, the pre-treatment of secondary aluminum ash includes the following steps: S11: Ball mill secondary aluminum ash, sieve, to obtain secondary aluminum ash fine powder; the particle size of the secondary aluminum ash fine powder is 20-45μm. S12: Add the secondary aluminum ash fine powder obtained in S11 to water, stir mechanically until uniform, and heat with auxiliary heating at 50-100℃ for 4-6 hours. When the pH stabilizes at 9.5-10.0, stop heating and cool to 50-60℃ to obtain a secondary aluminum ash mixture. Preferably, the weight ratio of the secondary aluminum ash fine powder to water is 1:5 to 1:10; Preferably, the mechanical stirring speed is 300-500 rpm; S13: The secondary aluminum ash mixture obtained in S12 is filtered under a certain filtration pressure to obtain a filter cake; the moisture content of the filter cake is controlled to be ≤30%; Preferably, the filtration pressure is 0.4–0.6 MPa; Preferably, the moisture content of the filter cake is 20% to 30%; S14: Dry the filter cake obtained in S13 and control the moisture content to <1% to obtain pretreated secondary aluminum ash; The preferred drying conditions are: temperature 110°C, time 24 hours; The pretreated secondary aluminum ash comprises: 1-10% Al, 40-70% Al2O3, 5-15% SiO2, 2%-5% CaO, 3-8% alkali metal oxides, and 2-5% MgO; the alkali metal oxides are a mixture of K2O and Na2O.

[0007] By reacting secondary aluminum ash fine powder 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 pretreatment of secondary aluminum ash is to remove aluminum nitride (AlN), a key step in the pretreatment process. Its aim is to remove AlN through hydrolysis, preventing the release of harmful gases such as ammonia (NH3) during subsequent high-temperature sintering, which could affect material performance and the production environment. The particle size of the secondary aluminum ash fine powder directly affects the hydrolysis efficiency of AlN, subsequent filtration speed, and the final compositional uniformity. Smaller particle size results in a larger specific surface area, allowing for more thorough contact between AlN and water and a more complete hydrolysis reaction (the reaction rate is directly proportional to the particle surface area). However, excessively fine particles (e.g., <10μm) can lead to excessively high suspension viscosity, making filtration difficult and potentially forming colloids that affect drying efficiency. Controlling the particle size of the secondary aluminum ash fine powder to 20–45μm can achieve better AlN hydrolysis efficiency, filtration speed, and compositional uniformity. The reaction between AlN and water produces NH3, causing the solution pH to rise (usually >9). When the pH stabilizes at 9–10, it indicates that the AlN has completely reacted; therefore, the endpoint of the reaction can be determined by monitoring the pH value.

[0008] After the secondary aluminum ash mixture is appropriately cooled to 50-60℃, it is filtered. Because the liquid viscosity is moderate, the filtration rate can be improved and the precipitation of impurities can be reduced.

[0009] By controlling the moisture content of the filter cake by increasing the filtration pressure (0.4–0.6 MPa), the residual amount of soluble impurities (such as CaO, MgO, KO, and NaO) in the filter cake can be reduced. During the drying process, the evaporation of moisture in a high-moisture filter cake may cause soluble impurities to precipitate on the particle surface, forming a low-melting-point phase. A low-moisture filter cake contains less moisture, reducing the risk of impurity precipitation and improving the refractory properties of the final product. Controlling the moisture content of the filter cake also provides a uniform and dense green body structure for subsequent sintering steps. High-moisture filter cake is prone to cracks or pores after drying, affecting the density and strength of the sintered green body. A low-moisture filter cake has a uniform structure after drying, resulting in more complete crystal phase development during sintering and superior material properties.

[0010] Controlling the moisture content of pretreated secondary aluminum ash to <1% can improve powder flowability, enhance sintering performance, reduce impurity residue, and improve product purity and refractory performance. Fluctuations in moisture content can affect the physicochemical properties of powder (such as flowability and bulk density), making it difficult to control process parameters. Powder with low moisture content has stable properties, and process parameters are easier to adjust, which can improve product consistency and pass rate.

[0011] The secondary aluminum ash pretreatment using S1 removes AlN, soluble impurities, and other contaminants. Using secondary aluminum ash as raw material reduces production costs and environmental pollution. Strict control of moisture content provides a high-quality raw material base for subsequent mullite material preparation, ensuring the performance and economic viability of the final product.

[0012] S2: Preparation of porous lightweight mullite includes the following steps: S21: Raw material pretreatment, including the following steps: S211: Matrix material preparation, using pretreated secondary aluminum ash obtained in S1 as the aluminum source and natural silica or quartz sand as the silicon source, adjusting the amount of aluminum and silicon sources added, and controlling the molar ratio of Al2O3 in the aluminum source to SiO2 in the silicon source to be 1.5 to 2.0 to obtain the matrix material. Preferably, the amount of secondary aluminum ash added during the aluminum source pretreatment is 80-100 parts; the amount of natural silica or quartz sand added 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; the prepared porous lightweight mullite is placed in the mullite phase region, avoiding the formation of too much corundum phase, which would affect the heat resistance or mechanical strength of the material, and enabling the prepared porous lightweight mullite to have a higher service temperature. S212: Preparation of nano-CaCO3 coated with SiO2; Preferably, the SiO2-coated nano-CaCO3 is prepared by a sol-gel method, in which nano-CaCO3 is dispersed in a tetraethyl orthosilicate (TEOS) solution, subjected to ammonia-catalyzed hydrolysis at pH 8.5–10.5, stirred at 50–70°C for 4–8 hours, and calcined at 500–700°C for 1–3 hours to form a dense SiO2 coating layer, thereby obtaining 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 conflict with other pore-forming agents. Furthermore, SiO2 can react with CaO to form more stable compounds, such as calcium silicate, thus reducing the impact of CaO on material properties.

[0013] S213: Preparation of carbonized rice husks, rice husks are calcined at 500-700℃ for 1-3 hours (N2 protection), and ball-milled to D50 30-70μm.

[0014] The preparation of carbonized rice husks involves removing organic matter from the husks through high-temperature pyrolysis while retaining their inorganic components (such as silicon dioxide). This avoids the problem of uncarbonized rice husks being used as pore-forming agents, where the rapid decomposition of organic matter during high-temperature sintering releases large amounts of gases (such as CO2 and H2O), leading to uneven pore structure or even collapse, and residual carbon impurities. The residual SiO2 framework in the carbonized rice husks can react with Al2O3 in the matrix material during subsequent sintering to form mullite (3Al2O3·2SiO2), enhancing the material's high-temperature stability and mechanical strength. During the carbonization process, the carbon framework oxidizes to form open pores, while the SiO2 framework provides closed pores, forming a multi-level porous structure to optimize material properties.

[0015] During the preparation of carbonized rice husks, nitrogen protection can prevent the oxidation of the carbon skeleton after high-temperature pyrolysis of organic matter in the rice husks; maintain the SiO2 skeleton structure; and avoid the generation of CO and NO during air calcination. x The release of polluting gases.

[0016] Carbonized rice husks are ball-milled to a D50 of 30–70 μm to form uniform mesopores, balancing porosity and strength. This particle size range is suitable for gradient structure design, with smaller particles filling the matrix gaps in the surface layer to enhance compactness, and larger particles forming interconnected pores in the inner layer to improve thermal insulation performance. This particle size range is easy to disperse and matches the size of PMMA microspheres and corn flour, avoiding slurry stratification.

[0017] S22: Preparation of layered slurry, wherein the layered slurry includes a surface slurry, a transition layer slurry, and an inner layer slurry; the preparation of the surface slurry includes: ball milling 82-88 parts of matrix material, 0.5-1 parts of dispersant, 12-20 parts of surface pore-forming agent, 2-4 parts of binder, and an appropriate amount of water for 1-3 hours, adjusting the viscosity to 3000-3500 mPa·s; the preparation of the transition layer slurry includes: mixing 75-85 parts of matrix material, 1.0-1.5 parts of dispersant, 25-35 parts of transition layer pore-forming agent, and 1.5-2.5 parts of layered slurry... The mixture consists of a binder, 0.7–1.3 parts of a slow-release agent, and an appropriate amount of water. It is ball-milled for 1–3 hours to adjust the viscosity to 3000–3500 mPa·s. The inner layer slurry comprises 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 an appropriate amount of water. It is ball-milled for 1–3 hours to adjust the viscosity to 3000–3500 mPa·s. The surface slurry, transition layer slurry, and inner layer slurry are filtered through a sieve to obtain a slurry with a particle size of 25–38 μm. The layered design of matrix material content in layered slurries is a core control method for gradient porosity structures. The selection of dosage is primarily based on the synergistic ratio of matrix material and pore-forming agent to control porosity and strength gradients, reduce interlayer stress, and ensure process compatibility for slurry molding and sintering stability. The surface layer requires high strength and density; a high matrix content ensures a dense structure after sintering and can withstand mechanical loads. The transition layer provides a gradient connection, requiring a balance between strength and porosity to alleviate the thermal stress / shrinkage difference between the surface and inner layers. The inner layer requires ultra-lightweight and high porosity; a low matrix content maximizes porosity and achieves ultra-low thermal conductivity. There is a synergistic relationship between the matrix dosage and the pore-forming agent. A low pore-forming dosage in the surface layer creates a small number of closed pores, allowing the matrix material to directly contact and sinter, forming a high-strength network. This ensures that the surface layer, as a load-bearing layer, is free of structural defects and resists external impacts or high-temperature erosion. In the transition layer, a high pore-forming dosage increases the proportion of micropores, and the interlayer binder strengthens the grain boundaries. The matrix material still dominates, but the porosity is increased, achieving a smooth transition from strength to thermal insulation performance. In the inner layer, a high pore-forming dosage creates multi-level pores, with the matrix acting only as a "skeleton" to maintain the basic structure. This sacrifices some strength for ultra-low thermal conductivity, making it suitable for non-load-bearing thermal insulation areas. The selection of the matrix material requires consideration of process adaptability, such as slurry rheology control. Higher matrix dosage results in higher slurry viscosity, which can be adjusted by using dispersants and water. Inner layers with less matrix and more pore-forming agent require more dispersant to prevent particle sedimentation. For sintering shrinkage matching, a gradient design of the matrix dosage can reduce interlayer shrinkage differences and avoid cracking.

[0018] The dispersant is an anionic dispersant; preferably, the dispersant is ammonium polyacrylate. The mechanism of dispersants in layered slurries is to stabilize the powders (matrix, pore-forming agents, nano-ZrO2, etc.) in the slurry through electrostatic repulsion (such as ammonium polyacrylate) or steric hindrance, preventing particle agglomeration; reducing slurry viscosity; and adjusting rheology to ensure defect-free filling of the mold during injection molding. The selection of dispersant dosage for each layer depends on the raw material composition and ratio of each layer. The surface layer has a high matrix content, making particle dispersion easier; excessive dispersant may lead to excessively low slurry viscosity, therefore 0.5–1 part dispersant is used. The transition layer contains interlayer binders, increasing the proportion of pore-forming agents, requiring more dispersant to prevent nanoparticle agglomeration. The inner layer has a high total amount of pore-forming agents, requiring enhanced dispersion to prevent sedimentation while maintaining fluidity under high solids content.

[0019] Preferably, the sustained-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℃. Boric acid slows down the decomposition rate by forming a protective film, preventing overlap with the gas release from PMMA microspheres (which decompose at 300–400℃), thus avoiding uneven pore size. It also reduces concentrated gas production, preventing excessively high local pressure that could cause cracks or interconnected pores. The dosage of the slow-release agent should be controlled between 0.7 and 1.3 parts to ensure uniform pore distribution and sufficient porosity. Too little agent results in insufficient slow-release effect and uneven pore distribution; too much agent may excessively inhibit decomposition, leading to insufficient porosity.

[0020] The layered slurry, in which no slow-release agent is added to the surface layer, is based on the fact that the low amount of corn flour and high matrix resistance of the surface layer can suppress the concentrated release of gas; the absence of a slow-release agent is to avoid excessive suppression of porosity by the slow-release agent. Preferably, the binder is PVA; the binder provides viscosity and adhesion to each layer of the 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 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 thermal stress; the interlayer binder inhibits excessive grain growth at high temperature 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 to meet the fluidity requirements of grouting and molding, while taking into account particle suspension and degassing. Controlling the particle size of each layer of slurry can eliminate agglomerates and ensure uniform pore size. Preferably, the surface pore-forming agent comprises 2-4 parts SiO2-coated CaCO3, 4-6 parts PMMA microspheres, 4-6 parts corn flour, and 2-4 parts carbonized rice husk; the transition layer pore-forming agent comprises 4-6 parts SiO2-coated CaCO3, 6-8 parts PMMA microspheres, 7-9 parts corn flour, and 8-12 parts carbonized rice husk; and the inner layer pore-forming agent comprises 7-9 parts SiO2-coated CaCO3, 9-11 parts PMMA microspheres, 9-11 parts corn flour, and 13-17 parts carbonized rice husk. In the gradient pore structure design of porous lightweight mullite, the differences in the proportions of pore-forming agents in the surface, transition, and inner layers are based on the functional layering adaptation of porosity gradient control, strength / insulation / transition connection, decomposition temperature matching, and sintering shrinkage coordination from a process compatibility perspective. For example, the surface layer requires high strength, high-temperature stability, and process compatibility; therefore, a low total pore-forming dosage (12–20 parts) ensures matrix density and compressive strength. PMMA microspheres and corn flour form a small number of closed pores, reducing stress concentration. SiO2-CaCO3 and carbonized rice husk (SiO2 skeleton) enhance temperature resistance, and the low-temperature decomposition of PMMA microspheres / corn flour does not interfere with the surface densification sintering. The transition layer has requirements for pore gradient transition, interface strengthening, and decomposition synergy. The coating layer, with its increased proportion of carbonized rice husk, provides more micropores and a SiO2 framework, connecting to the highly porous inner layer. The PMMA microsphere / corn flour ratio is slightly higher than the surface layer, forming medium-sized pores. An interlayer binder is added to pin grain boundaries and prevent interlayer delamination. High-temperature decomposition of SiO2-CaCO3 and carbonized rice husk form multi-level pores. Ultra-low thermal conductivity is required: the inner layer demands ultra-low thermal conductivity, structural stability, and lightweight design; therefore, a high pore-forming dosage is used to achieve high porosity and low thermal conductivity. The SiO2 framework and micropores of the carbonized rice husk in the inner layer significantly improve thermal insulation performance. The PMMA microspheres / corn flour in the inner layer form large interconnected pores, while the carbonized rice husk / SiO2-CaCO3 provides microporous support. The low matrix proportion allows the density to be reduced to 0.8 g / cm³. 3 Below, it still maintains a certain compressive strength.

[0021] The ball milling media is corundum with a particle size of 5-10 mm; In the preparation of porous lightweight mullite, balancing porosity (which determines the material's lightweight and thermal insulation properties) with mechanical strength (which determines its load-bearing capacity) is a key challenge. Materials with high porosity are lighter and have better thermal insulation properties, but their strength is reduced, while materials with high strength typically have lower porosity. In the preparation of porous lightweight mullite, pore-forming agents have a significant impact on both porosity and mechanical strength.

[0022] This technical solution aims to obtain porous lightweight mullite with good porosity and mechanical strength. It employs a composite pore-forming agent system consisting of PMMA microspheres, corn flour, carbonized rice husks, and SiO2-coated calcium carbonate. Through temperature gradient decomposition and functional complementarity, it achieves precise control and synergistic optimization of multi-level pores. The pore-forming characteristic of PMMA microspheres lies in their low-temperature (typically 300–400℃) thermal decomposition during sintering, generating gases (such as CO2 and H2O) to form uniform closed or open pore structures with main pores (20–50 μm). The PMMA microspheres have a wide particle size range (3–70 μm), and the pore size distribution can be customized (40%–70%). Corn flour assists in controlling the pore size distribution of PMMA microspheres (closed pores, 10–30 μm), with a decomposition temperature of 250–400℃. Carbonized rice husk powder decomposes at 400–800℃, leaving a residual SiO2 framework (micropores 1–5 μm) in the mid-temperature range, reinforcing the matrix and improving strength and specific surface area. Corn flour and rice husks are low-cost and environmentally friendly biomass pore-forming agents, and their different pore structures can enhance the diversity and performance of pore structures. Nano-CaCO3 decomposes at high temperatures (800-900℃) into CaO and CO2, with the gas forming secondary pores. Introducing nano-calcium carbonate as a pore-forming agent, nano-calcium carbonate begins to decompose upon heating at 800℃ to form nanopores. Its decomposition temperature partially overlaps with that of carbonized rice husks, which may lead to uneven pore structure or collapse. For example, if PMMA microspheres and corn flour decompose at lower temperatures, the gas generated during the high-temperature decomposition of calcium carbonate may create pressure within the already formed pore structure, leading to structural damage. This application uses SiO2 to coat calcium carbonate, delaying the formation of micropores (1-5 μm) at 1000–1100 °C, avoiding premature decomposition and conflict with other pore-forming agents, and reducing the influence of residual CaO. This helps to more precisely control pore formation at high temperatures. During the heating process, calcium carbonate reacts with SiO2 and Al2O3 respectively to generate high-temperature resistant, high-strength calcium silicate and calcium aluminate. This reaction causes volume shrinkage of the material, forming nanoscale pores, increasing the specific surface area of ​​the material, and giving it superior surface activity. Simultaneously, the generated calcium silicate and calcium aluminate have hydration activity, resulting in superior bonding strength between the aggregate and matrix in the high-temperature resistant lightweight castable prepared from this lightweight porous mullite aggregate.

[0023] S23: Gradient injection molding: First, inject the surface slurry to a thickness of 2mm into a mold with a release agent sprayed on the inner wall, and then remove bubbles under vacuum; then inject the transition layer slurry to a thickness of 5mm and vibrate; then inject the inner layer slurry to a thickness of 10mm and allow it to cure statically; dry; and obtain a porous lightweight mullite preform. The thickness design of each layer in gradient grouting molding is based on the core functions and performance requirements of each layer. The surface layer is the main stress surface of the material (such as mechanical wear), which needs to be achieved through high matrix content and low thickness. A thin layer (2mm) can ensure complete densification after sintering and avoid internal pore residue due to excessive thickness. The thin layer can reduce the depth of heat penetration and, together with 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 thickness of 5mm can fully disperse thermal stress and prevent interlayer delamination. Furthermore, the interlayer binder is evenly distributed within a thickness of 5mm to achieve the best grain boundary pinning effect. A thickness of 5mm allows for a smooth transition of porosity and avoids abrupt changes in performance (too thin a layer can easily lead to interface stress concentration). The inner layer considers the optimization of thermal insulation performance. The thermal insulation effect is proportional to the thickness of the thermal insulation layer. A 10mm inner layer can reduce the thermal conductivity to meet the thermal insulation requirements. The high porosity of the inner layer requires sufficient thickness to maintain structural stability. In addition, the inner layer accounts for a high proportion of the overall thickness and has a low density, achieving overall weight reduction.

[0024] Preferably, the vacuum degassing pressure 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 of the surface layer completely removes air bubbles, preventing the high-strength surface layer from cracking due to pore defects; the transition layer uses vibration to evenly disperse the interlayer binder, eliminating local agglomeration and ensuring interlayer bonding strength; the inner layer is left to cure statically, naturally expelling residual gas, stabilizing the high porosity structure, and preventing external forces from damaging the fragile porous skeleton.

[0025] The molding process is also a significant factor affecting the balance between porosity and mechanical strength. This application constructs a layered molding process with a gradient porosity structure consisting of a surface layer, a transition layer, and an inner layer. By adjusting the composition of the pore-forming agent and the proportion of raw materials in each layer, a gradient structure is achieved between a dense surface layer (low porosity and high strength) and a porous interior layer (high porosity and low thermal conductivity). The design of a transition layer between the surface and inner layers allows for gradient adjustment of shrinkage stress, solving the problem of interlayer bonding strength issues caused by interface cracking due to the difference in shrinkage rates between the surface and inner layers.

[0026] Preferably, the drying is a gradient heating drying process: heating to 60-80°C at a rate of 1-2°C / min and holding for 6-12 hours; then heating to 90-120°C at a rate of 0.5-1°C / min and holding for 8-16 hours; then heating to 180-200°C at a rate of 0.3-0.5°C / min and holding for 10-24 hours; controlling the cooling rate to <1°C / min and cooling in the furnace; thus obtaining a porous lightweight mullite preform. Low-temperature dehumidification at 60–80℃ slowly removes free water, preventing surface hardening and blockage of internal moisture channels; evaporation of bound water at 90–120℃ stabilizes the embryo skeleton structure and prevents shrinkage and cracking; and thorough removal of residual moisture at 180–200℃ prepares for subsequent sintering and prevents cracking during high-temperature sintering. S24: Staged sintering. The porous lightweight mullite preform obtained in S22 is heated to 300-400℃ at a heating rate of 3-5℃ / min and held for 30-60min under N2 atmosphere protection. Then, it is heated to 800-1000℃ at a heating rate of 2-3℃ / min and held for 60-120min under air atmosphere. Then, it is heated to 1100-1400℃ at a heating rate of 4-6℃ / min and held for 120-180min under air atmosphere. The cooling rate is controlled to be <5℃ / min, and the material is cooled in the furnace to 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 staged sintering process to achieve synergistic optimization of pore structure and matrix properties through a staged process of "low temperature slow release - medium temperature transition - high temperature strengthening". This minimizes defects and reduces energy consumption, and improves the repeatability and reliability of industrial production. Precise control of the decomposition behavior of pore-forming agents allows PMMA microspheres and corn flour to decompose at low temperatures (300–400℃), forming large-sized closed pores (20–50 μm) and avoiding gas interference at high temperatures. Carbonized rice husks undergo organic matter decomposition at medium temperatures (800–1000℃), with the residual SiO2 framework providing micropores (1–5 μm). SiO2-coated CaCO3 undergoes delayed decomposition at high temperatures (1000–1100℃), generating uniform micropores and preventing structural collapse due to concentrated gas production. The matrix is ​​more uniform, and the porosity gradient is precisely controllable. The gradient heating segmented sintering process optimizes matrix sintering and crystal phase formation, avoiding premature densification. The slow heating in the low-temperature section (3-5℃ / min) prevents premature matrix sintering and ensures the full decomposition of the pore-forming agent. The long-term holding time (120-180 minutes) in the high-temperature section (1100-1400℃) promotes the complete formation of mullite (3Al2O3·2SiO2), reduces impurities (such as free Al2O3), improves compressive strength, and enhances temperature resistance. The gradient heating and segmented sintering process reduces defects and stress concentration. The gradient heating matches the differences in thermal expansion coefficients at different stages of the material, preventing crack formation. The nano-ZrO2 in the transition layer diffuses uniformly during heating, pinning grain boundaries and inhibiting interlayer delamination, thus improving material yield and interlayer bonding strength. This process also achieves energy savings and process stability. Slow heating in the low-temperature stage reduces thermal shock, while rapid heating in the high-temperature stage shortens sintering time, resulting in lower overall energy consumption. A fixed heating curve (e.g., 400℃ for 30–60 minutes → 800℃ for 60–120 minutes) ensures batch stability. This phased sintering process meets the requirements for low-temperature sintering and provides better control over amorphous phase transformation.

[0027] This technical solution, through layered slurry formulation design, gradient grouting molding, and segmented sintering process, can precisely control the porosity and strength distribution of porous materials to meet the differentiated requirements for material performance.

[0028] S25: The porous lightweight mullite blocks obtained in S24 are crushed by jaw crusher and sieved through a screen to obtain porous lightweight mullite of different particle sizes; The porous lightweight mullite of different particle sizes includes porous lightweight mullite aggregate and porous lightweight mullite fine powder; the particle size of the porous lightweight mullite aggregate is 0.15mm to 5mm; the particle size of the porous lightweight mullite fine powder is 44 to 74μm; the porous lightweight mullite aggregate, according to the proportion of the porous lightweight mullite aggregate, includes 25 to 40 parts of porous lightweight mullite aggregate 1 with a particle size of 1mm to 5mm, 35 to 45 parts of porous lightweight mullite aggregate 2 with a particle size of 0.5mm to 1mm, and 20 to 35 parts of porous lightweight mullite aggregate 3 with a particle size of 0.15mm to 0.5mm; In some embodiments, the preparation of porous lightweight muryl in step S2 further includes the following steps: The porous lightweight mullite blocks obtained in S24 are subjected to post-treatment, which includes acid washing to remove impurities and surface impregnation. The acid washing to remove impurities involves immersing the porous lightweight mullite blocks obtained in S23 in an acid solution. The surface impregnation involves impregnating the surface layer with silica sol to fill the surface openings, followed by secondary sintering at 1100-1200℃ to form a dense protective layer. The post-treated porous lightweight mullite blocks are then obtained. The acid solution is a 1 mol / L HCl solution; the porous lightweight mullite blocks are immersed in the acid solution for 30 minutes. The silica sol contains 20 wt% SiO2. The secondary sintering time is 50–60 min; By immersing the block material in an acid solution, residual CaO is dissolved, thus improving acid resistance. The surface pores of the block material are filled by impregnation with silica sol, which enhances the surface density. During secondary sintering, SiO2 reacts with the matrix to form a glassy phase, which blocks the penetration of high-temperature oxidizing / corrosive media.

[0029] S3: Preparation of high-temperature resistant lightweight mullite castable includes 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 the water-retaining agent, 5-10 parts of the expanding agent, and 10-25 parts of the binder in a mixer and stir until fully mixed to obtain 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, stir evenly, and obtain high-temperature resistant lightweight mullite wet material. S33: Pour the high-temperature resistant lightweight mullite wet material obtained in S32 into the mold, force vibration molding, curing, and demolding to obtain high-temperature resistant lightweight mullite castable; The appropriate amount of water mentioned in step S32 is 15-30% of the dry weight of the high-temperature resistant lightweight mullite material; Preferably, the water-retaining agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; and the expanding agent is kyanite or andalusite. This technical solution optimizes the gradation of porous lightweight mullite aggregate by adding aggregates of 1mm-5mm, 0.5mm-1mm, and 0.15mm-0.5mm in size, along with fine powder of 44-74μm, to improve bulk density and strength. The 1-5mm aggregate provides the main load-bearing framework; the 0.5-1mm aggregate reduces voids between large particles; and the 0.15-0.5mm aggregate further densifies the aggregate. The fine powder encapsulates the particles and fills the gaps between the aggregates, enhancing bonding strength and compressive strength. All components (aggregates 1 / 2 / 3 and fine powder) retain their original porous structure, resulting in a lower overall density and further reduced thermal conductivity of the castable. The aggregates and fine powder are sourced from the same place and have the same coefficient of thermal expansion, avoiding high-temperature cracking caused by component differences.

[0030] This technical solution uses porous lightweight mullite aggregate, which significantly reduces material density and improves thermal insulation, making it suitable for high-temperature kiln linings. The mullite aggregate is combined with pure calcium aluminate cement, resulting in excellent high-temperature resistance. Kyanite / andalusite acts as an expanding agent, decomposing at high temperatures to generate mullite and expanding in volume, effectively offsetting sintering shrinkage. Bentonite / Guangxi white clay acts as a water-retaining agent, improving the plasticity and moisture retention capacity of the castable. The wide range of component proportions (e.g., 40-70 parts aggregate, 10-25 parts binder) facilitates performance adjustment.

[0031] In some embodiments, the method for preparing mullite castable further includes the following steps: S4: To prepare a dense mullite coating, 40-60 parts of calcined alumina micro powder, 20-40 parts of nano-silica micro 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 parts of boron anhydride, 0.1-0.5 parts of nano-ZnO, and 0.1-0.5 parts of polyvinyl alcohol fiber are placed in an appropriate amount of water and mixed thoroughly to obtain the dense mullite coating. S5: Apply the dense mullite coating described in S4 to 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 it by heating it to 1000-1200℃ at a heating rate of 2-5℃ / min, and hold it at that temperature for 1-3 hours. Preferably, the thickness of the dense mullite coating is 0.1 mm to 0.5 mm; A 0.1mm coating can basically completely cover surface defects, but if the coating is too thin, localized through-holes will appear that are not covered. When the coating thickness is too thick, the thermal stress increases, which can lead to premature peeling of the coating.

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

[0033] The castable surface has numerous micropores, making it unable to resist the corrosion of high-temperature flue gas and acid / alkali atmospheres when used as the working layer of an aluminum refractory lining. This technical solution solves this problem by preparing a dense mullite coating on the castable surface. In this solution, calcined alumina micropowder is the aluminum source, and nano-silicon micropowder is the silicon source. The two form mullite during high-temperature sintering. Fluorite powder is introduced into the mullite coating material, and the fluorite powder is uniformly dispersed between the alumina and silica micropowder, forming a strong bond during the mullite formation process. Utilizing the non-wetting property of CaF2, the main component of fluorite, on aluminum, a dense mullite coating is prepared on the surface of furnace lining materials made of high-temperature resistant lightweight mullite castable. This coating effectively resists the corrosion of acid and alkaline gases and aluminum slag. This technical solution uses calcium zirconate, which reduces the amount of fluorite powder used, thereby reducing the volatilization of F- from the fluorite powder and improving corrosion resistance. This technical solution introduces boron anhydride, a low-temperature sintering promoter, and nano-ZnO composite sintering aid into the mullite coating material to promote densification, thereby lowering the mullite formation reaction temperature of alumina and silica. This allows the coating to sinter at 1000–1200℃, forming a dense mullite coating 0.1 mm–0.5 mm thick. This coating significantly improves the material's resistance to thermal stress and spalling. This technical solution also incorporates polyvinyl alcohol fiber, which provides crack resistance and reinforcement, improving the long-term stability and thermal shock resistance of the coating.

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

[0035] This application controls the composition by adjusting the molar ratio of Al2O3 in the pretreatment secondary aluminum source to SiO2 in the silicon source, so that the prepared porous lightweight mullite falls into the mullite phase region, avoiding the formation of too much corundum phase, which would affect the thermal shock resistance or mechanical strength of the material, and enabling the prepared porous lightweight mullite to have a higher service temperature.

[0036] In the preparation of porous lightweight mullite, this application employs a layered slurry formulation design, gradient slurry casting, and segmented sintering process, including PMMA microspheres, corn flour, carbonized rice husks, and a SiO2-coated calcium carbonate composite pore-forming agent system, to precisely control the porosity and strength distribution of the porous lightweight mullite. The segmented sintering process meets the requirements for low-temperature sintering and effectively controls the amorphous phase transformation.

[0037] In the process of making high-temperature resistant lightweight mullite castable, this application optimizes the gradation of porous lightweight mullite aggregate and adds porous lightweight mullite fine powder, which significantly enhances the bonding strength and compressive strength of the castable. The aggregate and fine powder are from the same source and have the same coefficient of thermal expansion, making the castable more resistant to high temperatures.

[0038] This application involves preparing a dense mullite coating and applying it to the surface of a high-temperature resistant lightweight mullite castable. This significantly reduces the surface porosity, improves the erosion and corrosion resistance of the high-temperature resistant lightweight mullite castable, and significantly enhances its thermal properties and anti-stripping properties.

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

[0040] The present disclosure will be further described in detail below with reference to the accompanying drawings and preferred embodiments. However, those skilled in the art will appreciate that these drawings are drawn for the purpose of explaining the preferred embodiments only and should therefore not be construed as limiting the scope of the present disclosure. Furthermore, unless specifically indicated, the drawings are only schematic representations of the composition or structure of the described objects and may contain exaggerated depictions, and the drawings are not necessarily drawn to scale.

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

[0042] The following is in conjunction with the appendix Figure 1 This disclosure will be explained in detail.

[0043] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this disclosure.

[0044] This application provides a method for preparing mullite castables based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag, such as... Figure 1As shown, it includes the following steps: S1: Pre-treatment of secondary aluminum ash, the pre-treatment of secondary aluminum ash includes the following steps: S11: Ball mill secondary aluminum ash, sieve, to obtain secondary aluminum ash fine powder; the particle size of the secondary aluminum ash fine powder is 20-45μm. S12: Add the secondary aluminum ash fine powder obtained in S11 to water, wherein the weight ratio of the secondary aluminum ash fine powder to water is 1:5 to 1:10. Stir evenly with mechanical stirring at a speed of 300 to 500 rpm. Add auxiliary heating at 50 to 100°C and react for 4 to 6 hours. Stop heating when the pH stabilizes at 9.5 to 10.0. Cool to 50 to 60°C to obtain a secondary aluminum ash mixture. S13: The secondary aluminum ash mixture obtained in S12 is filtered under a filtration pressure of 0.4 to 0.6 MPa to obtain a filter cake; the moisture content of the filter cake is 20% to 30%. S14: The filter cake obtained in S13 is dried at 110℃ for 24 hours, and the moisture content is controlled to be <1% to obtain pretreated secondary aluminum ash; The preferred drying conditions are: temperature 110°C, time 24 hours; The pretreated secondary aluminum ash comprises: 1-10% Al and 40-70% Al2O3. 3、 5-15% SiO2, 2-5% CaO, 3-8% alkali metal oxides, and 2-5% MgO; wherein the alkali metal oxides are a mixture of K2O and Na2O.

[0045] S2: Preparation of porous lightweight mullite includes the following steps: S21: Raw material pretreatment, including the following steps: S211: Preparation of matrix material, using the pretreated secondary aluminum ash obtained in S1 as the aluminum source, and natural silica or quartz sand as the silicon source, wherein the amount of aluminum source added is 80-100 parts; and the amount of natural silica or quartz sand added is 10-30 parts; to obtain the matrix material. S212: Preparation of SiO2-coated nano-CaCO3, wherein the SiO2-coated nano-CaCO3 is prepared by sol-gel method, in which nano-CaCO3 is dispersed in tetraethyl orthosilicate (TEOS) solution, and subjected to ammonia-catalyzed hydrolysis at pH 8.5-10.5, stirred at 50-70℃ for 4-8 hours; calcined at 500-700℃ for 1-3 hours to form a dense SiO2 coating layer, thereby obtaining SiO2-coated nano-CaCO3; the thickness of the SiO2 coating layer is 10-20 nm. S213: Preparation of carbonized rice husks, rice husks are calcined at 500-700℃ for 1-3 hours (N2 protection), and ball-milled to D50 30-70μm.

[0046] S22: Preparation of layered slurry, wherein the layered slurry includes a surface slurry, a transition layer slurry, and an inner layer slurry; the preparation of the surface slurry includes: ball milling 82-88 parts of matrix material, 0.5-1 parts of dispersant, 12-20 parts of surface pore-forming agent, 2-4 parts of binder, and an appropriate amount of water for 1-3 hours, adjusting the viscosity to 3000-3500 mPa·s; the preparation of the transition layer slurry includes: mixing 75-85 parts of matrix material, 1.0-1.5 parts of dispersant, 25-35 parts of transition layer pore-forming agent, and 1.5-2.5 parts of layered slurry... The mixture consists of a binder, 0.7–1.3 parts of a slow-release agent, and an appropriate amount of water. It is ball-milled for 1–3 hours to adjust the viscosity to 3000–3500 mPa·s. The inner layer slurry comprises 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 an appropriate amount of water. It is ball-milled for 1–3 hours to adjust the viscosity to 3000–3500 mPa·s. The surface slurry, transition layer slurry, and inner layer slurry are filtered through a sieve to obtain a slurry with a particle size of 25–38 μm. The surface pore-forming agent comprises 2-4 parts SiO2-coated CaCO3, 4-6 parts PMMA microspheres, 4-6 parts corn flour, and 2-4 parts carbonized rice husk; the transition layer pore-forming agent comprises 4-6 parts SiO2-coated CaCO3, 6-8 parts PMMA microspheres, 7-9 parts corn flour, and 8-12 parts carbonized rice husk; the inner layer pore-forming agent comprises 7-9 parts SiO2-coated CaCO3, 9-11 parts PMMA microspheres, 9-11 parts corn flour, and 13-17 parts carbonized rice husk. The ball milling media is corundum with a particle size of 5-10 mm; S23: Gradient injection molding: First, inject the surface slurry to a thickness of 2mm into a mold with a release agent sprayed on the inner wall, and then remove bubbles under vacuum; then inject the transition layer slurry to a thickness of 5mm and vibrate; then inject the inner layer slurry to a thickness of 10mm and allow it to cure statically; dry; and obtain a porous lightweight mullite preform. The vacuum degassing pressure 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. The drying process is a gradient heating drying, wherein the temperature is increased to 60-80°C at a rate of 1-2°C / min and held for 6-12 hours; then the temperature is increased to 90-120°C at a rate of 0.5-1°C / min and held for 8-16 hours; then the temperature is increased to 180-200°C at a rate of 0.3-0.5°C / min and held for 10-24 hours; the cooling rate is controlled to be <1°C / min, and the material is cooled in the furnace; a porous lightweight mullite preform is obtained. S24: Staged sintering. The porous lightweight mullite preform obtained in S22 is heated to 300–400℃ at a heating rate of 3–5℃ / min and held for 30–60 min under N2 atmosphere protection. Then, the temperature is increased to 800℃–1000℃ at a heating rate of 2–3℃ / min and held for 60–120 min under air atmosphere protection. Finally, the temperature is increased to 1100–1400℃ at a heating rate of 4–6℃ / min and held for 120–180 min under air atmosphere protection. Atmosphere; control the cooling rate to <5℃ / min, cool with the furnace; obtain porous lightweight mullite blocks; perform post-treatment on the obtained porous lightweight mullite blocks, the post-treatment including acid washing to remove impurities and surface impregnation; the acid washing to remove impurities involves immersing the porous lightweight mullite blocks obtained in S23 into an acid solution; the surface impregnation involves impregnating the surface layer with silica sol to fill the surface openings, and secondary sintering at 1100~1200℃ to form a dense protective layer; obtain post-treated porous lightweight mullite blocks; The acid solution is a 1 mol / L HCl solution; the porous lightweight mullite blocks are immersed in the acid solution for 30 minutes. The silica sol contains 20 wt% SiO2. The secondary sintering time is 50–60 min; S25: Porous lightweight mullite blocks are crushed by jaw crusher and sieved through a screen to obtain porous lightweight mullite of different particle sizes; The porous lightweight mullite of different particle sizes includes porous lightweight mullite aggregate and porous lightweight mullite fine powder; the particle size of the porous lightweight mullite aggregate is 0.15mm to 5mm; the particle size of the porous lightweight mullite fine powder is 44 to 74μm; the porous lightweight mullite aggregate, according to the proportion of the porous lightweight mullite aggregate, includes 25 to 40 parts of porous lightweight mullite aggregate 1 with a particle size of 1mm to 5mm, 35 to 45 parts of porous lightweight mullite aggregate 2 with a particle size of 0.5mm to 1mm, and 20 to 35 parts of porous lightweight mullite aggregate 3 with a particle size of 0.15mm to 0.5mm; S3: Preparation of high-temperature resistant lightweight mullite castable includes 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 the water-retaining agent, 5-10 parts of the expanding agent, and 10-25 parts of the binder in a mixer and stir until fully mixed to obtain 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, stir evenly, and obtain high-temperature resistant lightweight mullite wet material. S33: Pour the high-temperature resistant lightweight mullite wet material obtained in S32 into the mold, force vibration molding, curing, and demolding to obtain high-temperature resistant lightweight mullite castable; The appropriate amount of water mentioned in step S32 is 15-30% of the dry weight of the high-temperature resistant lightweight mullite material; The water-retaining agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; the expanding agent is kyanite or andalusite; S4: To prepare a dense mullite coating, 40-60 parts of calcined alumina micro powder, 20-40 parts of nano-silica micro 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 parts of boron anhydride, 0.1-0.5 parts of nano-ZnO, and 0.1-0.5 parts of polyvinyl alcohol fiber are placed in an appropriate amount of water and mixed thoroughly to obtain the dense mullite coating. S5: Apply the dense mullite coating described in S4 to 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 it at a heating rate of 2-5℃ / min to 1000-1200℃, and hold it at that temperature for 1-3 hours to obtain the mullite castable. The thickness of the dense mullite coating is 0.1 mm to 0.5 mm; The binder is aluminum sol powder; the particle size of the calcined alumina micro powder is D50 2μm; the particle size of the nano-silicon micro powder is D50 0.5μm; the particle size of the aluminum sol powder binder is 10-30μm; the particle size of the boron anhydride is 74μm; the particle size of the nano-ZnO is 10-50nm; and the length of the polyvinyl alcohol fiber is 1-5mm. The medium temperature conditions described in step S5 are 100–200°C; the drying time is 24–48 h. Specific Implementation

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

[0048] Comparative Example 2: Compared with Example 2, Comparative Example 2 had no interlayer binder, but the other components and processes were the same.

[0049] Comparative Example 3: Compared with Example 2, Comparative Example 3 had no sustained-release agent, but the other components and processes were the same.

[0050] Comparative Example 4: Compared with Example 2, Comparative Example 3 had no dispersant, but the other components and processes were the same.

[0051] Porous lightweight mullite was prepared according to the steps of S2: preparing porous lightweight mullite in Examples 1-3 and Comparative Examples 1-4 respectively. The technical indicators of the obtained porous lightweight mullite were tested, and the test results are shown in Table 1.

[0052] Table 1. Technical specifications of porous lightweight mullite obtained in Examples 1-3 and Comparative Examples 1-4 As shown in Table 1, in Examples 1-3, the porosity increased and the density decreased with increasing pore-forming agent content; the gradient pore-forming agent design formed a uniform gradient pore size; based on staged sintering and matrix ratio optimization, high-purity mullite (>85%) was obtained; the transition layer in Examples 1-3 significantly improved the pore gradient, and the interlayer porosity difference remained stable at 10-15%; Example 2 achieved a bulk density of (0.75-0.95 g / cm³). 3 It exhibits the best performance in terms of crystal phase purity (pure mullite) and pore gradient (continuous distribution). In Examples 1-3, the surface pore-forming agent corn flour inhibits the expansion of PMMA microspheres, while the inner pore-forming agent has a high content of PMMA microspheres, which synergistically form pores of 50-80 μm with carbonized rice husk; in Example 2, the pore size is limited to 40-70 μm by a slow-release agent.

[0053] Comparative Example 1: The absence of a transition layer resulted in a porosity difference of up to 20% between the surface and inner layers, and the interface was prone to fracture, with direct contact between the surface and inner layers and the disappearance of the density gradient. Comparative Example 2: The absence of an interlayer binder resulted in poor interlayer bonding, the appearance of amorphous phases (interlayer fracture zones) in some areas, and increased density inhomogeneity. Comparative Example 3: The absence of a slow-release agent led to an increase in closed pores and poor pore connectivity. Comparative Example 4: The absence of a dispersant led to the aggregation of pore-forming agents, large porosity fluctuations (±15%), poor uniformity, and densification in some areas.

[0054] In summary, the transition layer and interlayer binder are key to maintaining gradient pore size and mechanical strength (comparative example 1 / 2 shows a significant decrease in performance); the slow-release agent optimizes pore connectivity (comparative example 3 shows an increase in closed-pore ratio); and the dispersant ensures uniform distribution of the pore-forming agent (comparative example 4).

[0055] Examples 4-6 Examples 4-6 are studies on layered pore-forming agents. The specific composition of the layered pore-forming agents is shown in the table below: Comparative Example 5: Compared with Example 5, Comparative Example 5 uses CaCO3 instead of SiO2 to coat CaCO3, while the other components and processes are the same.

[0056] Comparative Example 6: Compared with Example 5, Comparative Example 6 had no PMMA microsphere pore-forming agent in each layer; the other components and processes were the same.

[0057] Comparative Example 7: Compared with Example 5, each layer of Comparative Example 7 did not have SiO2-coated CaCO3 pore-forming agent, but the other components and processes were the same.

[0058] Comparative Example 8: Compared with Example 5, the transition layer pore-forming agent and the surface layer pore-forming agent were interchanged, while the other components and processes remained the same.

[0059] Comparative Example 9: Gradient injection molding. First, a surface layer slurry to a thickness of 5 mm was injected into a mold with a release agent sprayed on the inner wall, and then vacuum degassing was performed. Next, the transition layer slurry to a thickness of 10 mm was injected and vibrated. Then, the inner layer slurry to a thickness of 15 mm was injected and allowed to stand for curing. After drying, a porous lightweight mullite preform was obtained. Comparative Example 10: Gradient injection molding. First, a surface layer slurry to a thickness of 5 mm was injected into a mold with a release agent sprayed on the inner wall, and then vacuum degassing was performed. Next, the transition layer slurry to a thickness of 2 mm was injected and vibrated. Then, the inner layer slurry was injected to a thickness of 5 mm and allowed to stand for curing. After drying, a porous lightweight mullite preform was obtained. Comparative Example 11S23 replaced the staged sintering with a gradient heating process, with a heating rate of 3-5℃ / min to 1100-1400℃ and calcination for 24-72 hours; Porous lightweight mullite was prepared according to the steps of S2: preparing porous lightweight mullite in Examples 4-6 and Comparative Examples 5-11 respectively. The technical indicators of the obtained porous lightweight mullite were tested, and the test results are shown in Table 2.

[0060] Table 2. Technical specifications of porous lightweight mullite obtained in Examples 4-6 and Comparative Examples 5-11 Examples 4-6 show a reasonable gradient of pore-forming agents in the surface, transition, and inner layers, with PMMA microspheres dominating the main pores and exhibiting excellent uniformity. SiO2 coating of CaCO3 inhibits CaO residue. Example 6 shows an increase in the content of carbonized rice husks and an increase in the micropores of the carbonized rice husks. Example 5 has the lowest bulk density, the highest porosity, and a uniform pore size distribution, and its crystal phase is pure, representing the best balance between formulation and process.

[0061] Comparative Example 5: Uncoated CaCO3 decomposes prematurely (below 800℃), leading to increased closed pores, decreased effective porosity, CaO residue blockage, and reduced high-temperature stability due to anorthite impurities. Comparative Example 6: The absence of PMMA microspheres results in the loss of primary pores in the PMMA microspheres, relying on corn flour and carbonized rice husks, with micropores dominating and losing lightweight properties. Comparative Example 7: The lack of SiO2 coating on CaCO3 leads to CaCO3 decomposition, reducing secondary pores and resulting in poor primary pore connectivity in the PMMA microspheres. Comparative Example 8: The pore-forming agent gradient is reversed (surface layer > transition layer), causing sintering stress concentration and localized cracking. Example 9: Increased grout thickness (5mm for the surface layer) led to uneven drying shrinkage, resulting in microcracks and uneven pore size. Comparative Example 10: Increased surface layer thickness led to uneven drying shrinkage and an increase in localized closed pores. The transition layer was too thin (2mm) and could not effectively buffer interlayer stress, resulting in direct contact between the surface and inner layers, localized densification, stress concentration at the sintering interface, and localized amorphization. The effect of the pore-forming agent in the transition layer was limited, the pore gradient was broken, and the effective porosity decreased. Comparative Example 11: Gradient heating sintering resulted in insufficient decomposition of the pore-forming agent, leading to an increase in residual carbon and closed pores. Insufficient sintering resulted in incomplete carbon oxidation, inhibiting mullitization.

[0062] In Examples 1-3 and 4-6, the pore distribution and pore size gradually decreased, which is related to the gradient design of the pore-forming agent ratio, the gradient change of the matrix material ratio, the synergistic effect mechanism of the pore-forming agent, and the interlayer differences in process parameters. The PMMA microsphere content increased layer by layer, but because it may form spherical closed pores after high-temperature decomposition, during sintering shrinkage, the higher PMMA microsphere content in the inner layer actually resulted in more significant pore compression due to the reduced matrix material ratio and sintering stress, ultimately leading to a smaller pore size. The inner layer had more CaCO3 decomposing to generate CO2, but the SiO2 coating layer delayed decomposition, forming more uniform nanoscale secondary pores (1-5 μm) that filled the gaps between the main pores of the PMMA microspheres, causing the overall pore size distribution to shift towards smaller sizes. The inner layer had the least matrix material, resulting in a higher shrinkage rate during sintering and a stronger compression effect on the pores, leading to a smaller pore size in the inner layer. The inner layer pore size is actually smaller than that of the transition layer and the surface layer. The gas generated by the combustion of corn flour (CO2 / H2O) forms irregular openings, but after its content gradient synergizes with PMMA microspheres, the gas release pressure in the inner layer is higher, which promotes enhanced pore connectivity and partially offsets pore expansion. During the slurry casting process, the inner layer is thicker, and the moisture migration path is longer during drying, resulting in concentrated shrinkage stress and pore compression. The pore-forming agent in the segmented sintering inner layer decomposes more violently in an N2 atmosphere (300-400℃), but the subsequent sintering shrinkage of the SiO2 coating layer and the matrix in the air atmosphere compresses the pores together, forming a denser pore wall structure. As in Example 5 and Comparative Example 8, after the gradient is reversed, the inner layer pore size actually decreases, indicating that the matrix material ratio and sintering compression are the main reasons. Therefore, the low pore-forming agent content and high matrix material ratio in the surface layer actually retain more of the original pore structure. Theoretically, the high pore-forming agent content in the inner layer should increase the pore size, but insufficient matrix material and sintering shrinkage lead to pore compression.

[0063] Example 7: High-temperature resistant lightweight mullite castable was prepared according to step S3; Comparative Example 12: The porous lightweight mullite aggregate has a particle size of 0.15 mm to 5 mm. Without gradation optimization, a high-temperature resistant lightweight mullite castable is prepared according to step S3. The high-temperature resistant lightweight mullite castables obtained in Example 7 and Comparative Example 12 were tested, and the test results are shown in Table 3.

[0064] Table 3. Technical specifications of the high-temperature resistant lightweight mullite castable obtained in Example 7 and Comparative Example 12. Maximum operating 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 Reheating line change rate (%) -0.1～+0.5％ -1.0～+0.5％ Example 7 optimized the particle size distribution of porous lightweight mullite aggregate, resulting in a maximum service temperature that was 50–150°C higher than that of Comparative Example 12, which did not undergo gradation optimization. Example 7 achieved a close packing of aggregates through gradation, reducing glass phase penetration channels at high temperatures. The expanding agent (kyanite) transforms into mullite and SiO2 above 1200°C, which can compensate for sintering shrinkage and inhibit crack propagation. Comparative Example 12, which used ungraded aggregates, resulted in larger local pores, allowing high-temperature slag to easily penetrate and reducing the refractoriness limit. Example 7: Fine powder can fill the gaps between aggregates, reducing density; by optimizing aggregate gradation, the continuous heat flow path can be blocked, reducing thermal conductivity; Comparative Example 12: No aggregate gradation optimization was performed, resulting in random particle size distribution, and heat is rapidly conducted through the pore walls. Example 7 uses aggregate gradation optimization, with multi-grade aggregates and finer particles forming a "skeleton-filler" structure. Furthermore, the hydration products of pure calcium aluminate cement can generate an interwoven nanocrystalline network at the interface, resulting in significantly enhanced high-temperature flexural strength compared to Example 12. In contrast, Comparative Example 12 has uneven particle size distribution and stress concentration points at the interface, leading to preferential cracking at high temperatures. The near-zero reheat line change (-0.1% to +0.5%) in Example 7 indicates that it is suitable for high-temperature kiln linings and is not prone to cracking during long-term use; the shrinkage (-1.0%) in Comparative Example 12 will lead to the expansion of masonry joints, accelerate slag penetration, and reduce service life; Example 7 compensates for sintering shrinkage by converting kyanite expansion agent into mullite and SiO2 at high temperature; the graded aggregate forms uniform pores and suppresses local deformation; Comparative Example 12 uses ungraded aggregate, which leads to shrinkage stress concentration and sintering shrinkage.

[0065] Examples 8-10 are studies on dense mullite coatings. The specific raw material composition of the dense mullite coatings is shown in the table below: Comparative Example 13S4 prepared a dense mullite coating by placing 40-60 parts of calcined alumina micro powder, 20-40 parts of nano-silica micro powder, 2-8 parts of fluorite powder, 2-10 parts of binder, 0.1-2 parts of carboxymethyl cellulose, 0.2-1 parts of boron anhydride, and 0.1-0.5 parts of polyvinyl alcohol fiber in an appropriate amount of water and mixing them thoroughly to obtain the dense mullite coating. Compared with Example 8, Comparative Example 13 did not contain calcium zirconate or nano-ZnO.

[0066] Comparative Example 14S4 prepared a dense mullite coating by placing 40-60 parts of calcined alumina micro powder, 20-40 parts of nano-silica micro 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 parts of boron anhydride, and 0.1-0.5 parts of nano-ZnO in an appropriate amount of water and mixing them thoroughly to obtain the dense mullite coating. Compared with Example 8, Comparative Example 14 did not contain polyvinyl alcohol fibers.

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

[0068] Table 4. Technical specifications of the high-temperature resistant lightweight mullite castables obtained in Examples 8-9 and Comparative Examples 13-14 In Examples 8-10, the porosity decreased progressively by refining the grains with nano-ZnO and generating CaO-ZrO2 from calcium zirconate to fill the pores. In Comparative Example 13, which did not contain calcium zirconate or nano-ZnO, the grains were coarsened and there was no ZrO2 filling, resulting in increased porosity. In Comparative Example 14, which did not contain polyvinyl alcohol fibers, the number of drying microcracks increased, resulting in increased porosity.

[0069] Examples 8-10 use binders and carboxymethyl cellulose to form an organic-inorganic composite network. Polyvinyl alcohol fibers enhance interfacial toughness, and the presence of nano-ZnO also enhances interfacial bonding. Comparative Example 13 lacks calcium zirconate, resulting in a lack of chemical anchoring effect of CaO-ZrO2 at the interface. Comparative Example 14 lacks polyvinyl alcohol fibers, leading to an increase in drying microcracks, which weakens the mechanical interlocking of the coating and substrate. Without fiber-enhanced interfacial toughness, brittleness is relatively increased. Examples 8-10 utilize calcium zirconate to generate CaO-ZrO2, which synergistically reduces the overall coefficient of thermal expansion with mullite. The presence of nano-silica powder ensures more complete mullite formation in the coating. ZnO and boron anhydride form a viscous glassy phase, filling pores and promoting particle rearrangement, thus improving the uniformity of sintering shrinkage. Comparative Example 13, lacking calcium zirconate and relying solely on mullite and the glassy phase, exhibits a higher overall coefficient of thermal expansion. Comparative Example 14, lacking polyvinyl alcohol fibers, shows increased drying microcracks, leading to an uneven microstructure and a higher coefficient of thermal expansion.

[0070] In Examples 8-10, calcium zirconate generates CaO-ZrO2, which enhances toughness; polyvinyl alcohol fibers can buffer thermal stress; nano-ZnO refines the grains and grain boundaries, thus enhancing thermal shock resistance; comparative example 13 has no calcium zirconate or nano-ZnO, and comparative example 14 has no polyvinyl alcohol fibers to buffer thermal stress, thus weakening thermal shock resistance.

[0071] In Examples 8-10, calcium zirconate generates CaO-ZrO2, which resists wetting by molten aluminum and dissolution by molten salt. Fluorite powder is uniformly dispersed between alumina and silica micropowder, forming a strong bond during the mullite formation process. Utilizing the non-wetting properties of CaF2, the main component of fluorite, on aluminum, a dense mullite coating is prepared on the surface of the furnace lining material made of high-temperature resistant lightweight mullite castable, which can effectively resist the corrosion of acid and alkaline gases and aluminum slag. Therefore, the dense mullite coating prepared in Examples 8-10 has good resistance to molten aluminum corrosion and molten salt corrosion. In Comparative Example 14, due to the absence of polyvinyl alcohol fiber, the number of drying microcracks increases, the porosity increases, and the prepared dense mullite coating is more susceptible to erosion and corrosion.

[0072] The present application has been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present application. The descriptions of the embodiments above are only for the purpose of helping to understand the present disclosure and its core ideas. It should be noted that those skilled in the art can make several improvements and modifications to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A method for preparing mullite castable based on the comprehensive utilization of secondary aluminum ash non-ferrous smelting slag, characterized in that, Includes the following steps: S1: Pre-treated secondary aluminum ash; S2: Preparation of porous lightweight mullite includes the following steps: S21: Raw material pretreatment, which includes matrix material preparation, SiO2-coated nano-CaCO3 preparation, and carbonized rice husk preparation; the matrix material uses the pretreated secondary aluminum ash obtained in S1 as the aluminum source; S22: Preparation of layered slurry, including preparation of surface slurry, preparation of transition layer slurry, and preparation of inner layer slurry; the layered slurry includes raw materials pretreated in S21; S23: Gradient grouting molding, in which a certain thickness of surface grout, transition grout, and inner grout prepared in S22 are injected into the mold in sequence, and the mold is formed to obtain a porous lightweight mullite preform; S24: Staged sintering, the porous lightweight mullite preform obtained in S23 is sintered in stages to obtain porous lightweight mullite blocks. S25: The porous lightweight mullite blocks obtained in S24 are crushed and sieved to obtain porous lightweight mullite of different particle sizes; the porous lightweight mullite of different particle sizes includes porous lightweight mullite aggregate and porous lightweight mullite fine powder. S3: Prepare high-temperature resistant lightweight mullite castable, using the porous lightweight mullite aggregate and porous lightweight mullite fine powder obtained in S25 as raw materials; The preparation of the surface slurry includes: milling 82-88 parts of matrix material, 0.5-1 parts of dispersant, 12-20 parts of surface pore-forming agent, 2-4 parts of binder, and an appropriate amount of water to adjust to a certain viscosity; the preparation of the transition layer slurry includes: 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 an appropriate amount of water to adjust 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-forming agent, 0.7-1.3 parts of slow-release agent, 2-4 parts of binder, and an appropriate amount of water to adjust to a certain viscosity; filtering the surface slurry, transition layer slurry, and inner layer slurry through a sieve to obtain a slurry with a specific particle size; The gradient injection molding process involves first injecting a surface slurry to a thickness of 2 mm into a mold with a release agent sprayed on the inner wall, followed by vacuum degassing; then injecting the transition layer slurry to a thickness of 5 mm and vibrating; finally injecting the inner layer slurry to a thickness of 10 mm and allowing it to stand and cure; and then drying to obtain a porous lightweight mullite preform. The staged sintering process involves heating the porous lightweight mullite preform obtained in S23 to 300-400℃ at a heating rate of 3-5℃ / min, holding it at that temperature for 30-60min under N2 atmosphere protection; then heating it to 800-1000℃ at a heating rate of 2-3℃ / min, holding it at that temperature for 60-120min under air atmosphere; then heating it to 1100-1400℃ at a heating rate of 4-6℃ / min, holding it at that temperature for 120-180min under air atmosphere; and finally controlling the cooling rate to <5℃ / min for furnace cooling. The surface pore-forming agent comprises 2-4 parts SiO2-coated CaCO3, 4-6 parts PMMA microspheres, 4-6 parts corn flour, and 2-4 parts carbonized rice husk; the transition layer pore-forming agent comprises 4-6 parts SiO2-coated CaCO3, 6-8 parts PMMA microspheres, 7-9 parts corn flour, and 8-12 parts carbonized rice husk; the inner layer pore-forming agent comprises 7-9 parts SiO2-coated CaCO3, 9-11 parts PMMA microspheres, 9-11 parts corn flour, and 13-17 parts carbonized rice husk; the viscosity is 3000-3500 mPa·s; the specific particle size is 25-38 μm; The surface slurry features a gradient design of the pore-forming agent ratio and a gradient change in the matrix material proportion.

2. The method for preparing mullite castable as described in claim 1, characterized in that, The pretreatment of secondary aluminum ash includes the following steps: S11: Ball mill secondary aluminum ash, sieve to obtain secondary aluminum ash fine powder; S12: Add the secondary aluminum ash fine powder obtained in S11 to water, stir mechanically until uniform, heat with auxiliary heating, react completely, cool, and obtain secondary aluminum ash mixture. S13: Filter the secondary aluminum ash mixture 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 method for preparing mullite castable as described in 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 auxiliary heating temperature is 50 to 100℃; and the cooling temperature is 50 to 60℃.

4. The method for preparing mullite castable as described in claim 3, characterized in that, The filter cake has a moisture content of ≤30%; the pretreated secondary aluminum ash has a moisture content of <1%.

5. The method for preparing mullite castable as described in claim 1, characterized in that, The matrix material is prepared using pretreated secondary aluminum ash obtained in S1 as the aluminum source and natural silica or quartz sand as the silicon source. The amount of aluminum and silicon sources added is adjusted to control the molar ratio of Al2O3 in the aluminum source to SiO2 in the silicon source to be 1.5~2.

0. The SiO2-coated nano-CaCO3 is prepared with a SiO2 coating layer thickness of 10~20 nm. The carbonized rice husk is prepared with a particle size of D50 of 30~70 μm.

6. The method for preparing mullite castable as described in claim 1, characterized in that, The drying process is a gradient heating drying, with the temperature increased to 60-80℃ at a rate of 1-2℃ / min and held for 6-12 hours; then increased to 90-120℃ at a rate of 0.5-1℃ / min and held for 8-16 hours; then increased to 180-200℃ at a rate of 0.3-0.5℃ / min and held for 10-24 hours; the cooling rate is controlled to be <1℃ / min, and the material is cooled in the furnace.

7. The method for preparing mullite castable as described in claim 1, characterized in that, The porous lightweight mullite aggregate has a particle size of 0.15 mm to 5 mm; the porous lightweight mullite fine powder has a particle size of 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.

8. The method for preparing mullite castable as described in claim 1, characterized in that, The preparation of high-temperature resistant lightweight mullite castable includes 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 the water-retaining agent, 5-10 parts of the expanding agent, and 10-25 parts of the binder in a mixer and stir until fully mixed to obtain 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, stir evenly, and obtain high-temperature resistant lightweight mullite wet material. S33: Pour the high-temperature resistant lightweight mullite wet material obtained in S32 into the mold, force vibration molding, curing, and demolding to obtain high-temperature resistant lightweight mullite castable.

9. The method for preparing mullite castable as described in claim 8, characterized in that, The appropriate amount of water mentioned in step S32 is 15-30% of the dry weight of the high-temperature resistant lightweight mullite; the water-retaining agent is bentonite or Guangxi white clay; the binder is pure calcium aluminate cement; and the expansion agent is kyanite or andalusite.

10. The method for preparing mullite castable as described in claim 1, characterized in that, The process also includes the following steps: post-processing the porous lightweight mullite blocks obtained in S24; the post-processing includes acid washing to remove impurities, surface impregnation, and secondary sintering; the acid washing to remove impurities involves immersing the porous lightweight mullite blocks obtained in S24 in an acid solution; The surface impregnation involves impregnating the surface layer with silica sol to fill the surface openings; secondary sintering at 1100~1200℃ to form a dense protective layer; and obtaining post-treated porous lightweight mullite blocks.

11. The method for preparing mullite castable as described in claim 1, characterized in that, It also includes the following steps: S4: To prepare a dense mullite coating, 40-60 parts of calcined alumina micro powder, 20-40 parts of nano-silica micro 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 parts of boron anhydride, 0.1-0.5 parts of nano-ZnO, and 0.1-0.5 parts of polyvinyl alcohol fiber are placed in an appropriate amount of water and mixed thoroughly to obtain the dense mullite coating. S5: Apply the dense mullite coating described in S4 to the surface of the high-temperature resistant lightweight mullite castable described in S3, dry it under medium temperature conditions, cool it to room temperature, and then sinter it.

12. The method for preparing mullite castable as described in claim 11, characterized in that, The thickness of the dense mullite coating is 0.1 mm to 0.5 mm; the sintering is carried out at a heating rate of 2 to 5 °C / min to 1000 to 1200 °C, and held for 1 to 3 hours; the binder is aluminum sol powder; the medium temperature condition is 100 to 200 °C; and the drying time is 24 to 48 hours.

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