A homogeneous corundum mullite brick and its preparation process

Abstract

This invention discloses a homogeneous corundum mullite brick and its preparation process, belonging to the field of refractory materials technology. The process employs a sol-gel assisted microscopic in-situ homogenization method: a precursor sol film is formed on the submicron-level bauxite surface by constructing a homogeneous lanthanum ion solution and performing colloidal-level contact grinding; a salt bridge network is formed by spray drying and granulation; finally, sintering is carried out under an oxidizing atmosphere, inducing lanthanum ions to preferentially react with impurities to generate high-melting-point lanthanum ferrite or lanthanum titanate-like crystalline phases. This process blocks impurities from participating in the formation of low-melting-point glass phases at the microscopic scale, resulting in clean grain boundaries within the brick and the absence of continuous glass phases. The obtained product exhibits a high-temperature creep rate of less than [value missing] and a load softening temperature of [value missing], achieving a breakthrough in the preparation of high-end refractory materials using low-grade raw materials.

Description

Technical Field

[0001] This invention relates to a homogeneous corundum mullite brick and its preparation process, belonging to the technical field of refractory material preparation. Background Technology

[0002] Corundum-mullite materials, due to their excellent high-temperature mechanical properties, thermal shock resistance, and chemical stability, are widely used in key components of high-temperature industries such as blast furnace hot blast stoves, dry quenching equipment, glass melting furnaces, and ceramic tunnel kilns. Traditional high-performance corundum-mullite bricks rely on high-grade or first-grade high-alumina bauxite clinker (Al2O3>80%) as the main raw material. However, with the long-term exploitation of high-quality bauxite resources globally, high-grade ore resources are becoming increasingly depleted, and the refractory materials industry faces the practical dilemma of having to use large quantities of medium- and low-grade bauxite (Al2O3 content 60%~70%).

[0003] A significant characteristic of low- and medium-grade bauxite is its high impurity content, particularly iron oxide (Fe2O3) and titanium dioxide (TiO2). Under high-temperature operating conditions, these impurities are not inert components but act as strong fluxes. According to phase equilibrium theory, Fe2O3 and TiO2 readily react with SiO2 in the matrix to form low-melting-point liquid phases (such as fir olivine or titanium-rich glass phases). Especially in industrial kilns with reducing atmospheres, Fe³⁺ is easily reduced to Fe²⁺, leading to a significant decrease in the temperature at which the liquid phase appears, sometimes even below 1200°C. This severely damages the high-temperature microstructure of the material, resulting in a lower load softening temperature and a higher high-temperature creep rate, thus significantly shortening the service life of industrial kilns.

[0004] To address the issues of large fluctuations in ore composition and uneven impurity distribution, existing technologies primarily employ a "homogenization" process. Traditional homogenization techniques typically involve crushing, grinding, and mixing the mined ore to produce a composite material with macroscopically uniform chemical composition, which is then calcined at high temperatures to obtain homogeneous clinker. While this method eliminates component segregation on a macroscopic scale and solves the problem of "uneven composition," from a microscopic thermodynamic and kinetic perspective, it still faces the following insurmountable technical bottlenecks:

[0005] First, it cannot fundamentally eliminate the fluxing properties of impurities, i.e., the limitations of physical mixing. Traditional homogenization is a form of "physical dispersion," which merely mechanically disperses harmful impurities such as Fe, Ti, and their compounds throughout the matrix. At the microscopic grain boundaries, these impurities retain their original chemical activity and are adjacent to the SiO2 component in the matrix. During high-temperature sintering or service, the dispersed impurities will still preferentially combine with SiO2 to form a continuous or semi-continuous low-melting-point glass phase network. This physical homogenization actually transforms localized large melt nodules into a grain boundary softening phase that permeates the entire material, making it difficult for the overall high-temperature strength and creep resistance of the material to reach the level of high-grade bauxite products.

[0006] Second, there are limitations imposed by solid-state reaction diffusion kinetics. To mitigate the harmful effects of impurities, some existing improvement technologies attempt to introduce modifiers into the feedstock, such as oxide powders capable of forming high-melting-point phases. However, in traditional dry or semi-dry powder mixing processes, it is difficult to achieve molecular- or nanoscale contact between modifier particles and impurity mineral particles. Limited by the diffusion distance of solid-state reactions, during sintering, modifiers often preferentially react with the main crystalline phase (Al₂O₃ or SiO₂) with a large contact area, failing to effectively "capture" or "fix" fine and dispersed impurity particles before the liquid phase formation temperature. This results in low modification efficiency, allowing impurities to still participate in liquid phase formation.

[0007] In summary, the core technical challenge that the refractory materials industry urgently needs to solve is how to overcome the limitations of traditional physical homogenization when utilizing low-grade bauxite resources, effectively suppress or change the occurrence state of impurities at the microscale, and block their reaction pathway with silica to form a low-melting-point liquid phase, so as to prepare corundum-mullite bricks with performance comparable to high-end synthetic materials using low-cost raw materials. Summary of the Invention

[0008] The technical problem to be solved by this invention is to provide a homogeneous corundum mullite brick and its preparation process. Through a sol-gel assisted microscopic in-situ homogenization process, the technical problem of low load softening temperature and poor high-temperature creep resistance of refractory materials caused by the easy formation of low melting point glass phases by impurities such as iron oxide and titanium dioxide in medium and low grade bauxite at high temperature is solved.

[0009] The technical problem to be solved by this invention is achieved by the following technical solution:

[0010] A homogeneous corundum mullite brick and its preparation process, employing a sol-gel assisted microscopic in-situ homogenization method, includes the following steps:

[0011] a) Construction of ionized dispersion system: Low-grade bauxite raw material is crushed, and water-soluble lanthanum salt is dissolved in a liquid medium containing dispersant and complexing agent to form a homogeneous lanthanum ion solution;

[0012] b) Colloidal contact grinding: The bauxite raw material is added to the homogeneous solution to form a slurry, which is then subjected to wet ultrafine grinding. Lanthanum ions are adsorbed and coated on the surface of submicron bauxite particles using a liquid medium to form a precursor sol film.

[0013] c) In-situ structural reconstruction: The ground slurry is spray-dried and granulated, so that lanthanum salt is precipitated in situ between particles during the drying process, forming a salt bridge network that coats impurity minerals, and thus obtaining composite micro powder.

[0014] d) Oxidation reaction sintering: After the composite micro powder is shaped, it is sintered at high temperature. Before the liquid phase formation temperature, lanthanum ions are induced to preferentially react with impurities in bauxite. and A solid-state reaction occurs, transforming the impurities into lanthanum ferrite or lanthanum titanate-like crystalline phases with melting points above 1700°C, thereby blocking the impurities from participating in the formation of low-melting-point glass phases at the microscale.

[0015] The present invention is further configured such that: in step a), the complexing agent is selected as citric acid or EDTA, and the pH value of the slurry is controlled to maintain the complexation stability of lanthanum ions;

[0016] The amount of lanthanum salt added Control is based on the following non-stoichiometric model:

[0017]

[0018] in, and They are respectively from the raw materials and molar content, The diffusion compensation coefficient ranges from 1.10 to 1.25. It is used to compensate for the loss caused by lanthanum ions dissolving in the mullite lattice or reacting with surface silica at high temperatures, ensuring that impurities are completely captured.

[0019] The present invention is further configured such that, in step b), the particle size distribution of the slurry after wet ultrafine grinding is controlled. and ;

[0020] The specified particle size range is used to break the dissociation of bauxite minerals and expose the iron-containing impurities encapsulated within them, while shortening the solid-phase diffusion distance between lanthanum ions and impurities to the nanoscale, so as to ensure that the chemical anchoring reaction is completed before <1200°C.

[0021] The present invention is further configured such that, in step c), the spray drying granulation process controls the inlet air temperature to be 280-320°C, producing particles with a diameter of 50-150 mm. spherical pseudoparticles;

[0022] The spherical pseudo-particles are composed of lanthanum salt nanocrystals connected to aluminum-silicon particles. During the subsequent sintering process, the pseudo-particles act as independent reaction micro-regions, restricting the migration of the liquid phase on a macroscopic scale.

[0023] The present invention is further configured such that, in step d), the chemically anchored reaction sintering includes a specific thermodynamic control regime:

[0024] Set a constant temperature or slow heating range between 900°C and 1300°C, and utilize... The reaction Gibbs free energy is lower than The characteristics of the reaction make The phase forms prior to the silicate liquid phase;

[0025] The atmosphere control for the oxidation reaction sintering requires that the oxygen volume concentration in the furnace always be greater than 2%, and this oxygen concentration is used to maintain the iron element in the system at a certain level. Valence state, to prevent it from reverting to This leads to the decomposition of the generated lanthanum ferrite perovskite structure and the formation of a low-melting-point fir olivine phase.

[0026] The present invention is further configured such that: the preparation process also includes temperature control during the sintering stage: a solid-state reaction holding section is set in the range of 900°C to 1300°C, so that lanthanum ions complete the reaction before the liquid phase is generated. and The lattice occupancy is then determined; subsequently, the temperature is raised to above 1600°C for densification sintering.

[0027] The present invention is further configured such that the raw materials are selected from... Secondary or tertiary bauxite with a content of 60% to 70%, and industrial alumina powder added to adjust the total content. The content is 72%~75%, and the water-soluble lanthanum salt is selected from lanthanum nitrate, which does not leave anionic impurities after thermal decomposition.

[0028] The present invention is further configured such that: the bricks obtained by the process are crushed and screened, and then recycled as aggregates into the molding process of step d) to form a refractory material system with a fully homogeneous structure.

[0029] A homogeneous corundum mullite brick prepared using the process described in any one of claims 1 to 8 has a microstructure based on a chemical anchoring effect:

[0030] The matrix framework is composed of well-developed interwoven network mullite and granular corundum.

[0031] At the three-way junctions of the matrix grain boundaries, there are dispersed grains with an average size of less than 3 mm. High-brightness second-phase particles, which, according to energy dispersive spectroscopy analysis, are rich in La, Fe, and Ti elements and poor in Si elements;

[0032] The brick body has clean grain boundaries and no continuous network of glassy phase distribution. The invention is further configured as follows:

[0033] The present invention is further configured such that: the main crystalline phase of the second phase particles is lanthanum ferrite or a solid solution of lanthanum titanate or an orthorhombic crystal system; through the pinning effect of the second phase particles on grain boundary slip, the high-temperature creep rate of the brick at 1450°C for 50 hours is less than 0.2%, and the softening temperature under a 0.2MPa load is greater than 1680°C.

[0034] The beneficial effects of this invention are:

[0035] 1. By chemically reconstructing impurity elements in low-grade bauxite raw materials, the high-value utilization of resources is realized. Traditional processes rely on expensive high-grade bauxite, while this invention successfully uses secondary or tertiary bauxite with an alumina content of only 60% to 70% as the main raw material. By introducing precisely calculated lanthanum salt and using a unique sol-gel and wet ultrafine grinding process, harmful impurities in the raw materials are transformed into beneficial high-temperature reinforcing phases, so that the performance of the final product reaches or even surpasses that of traditional products made from premium bauxite, significantly alleviating the industry's dependence on high-quality resources.

[0036] 2. Through liquid-phase dispersion and nanoscale adsorption and encapsulation of ionic lanthanum salt, the modifier and the iron and titanium impurities on the surface of bauxite particles achieve close contact at the atomic scale. In the subsequent sintering process, lanthanum ions preferentially react with iron oxide and titanium dioxide in the solid phase to generate lanthanum ferrite or lanthanum titanate crystals with melting points exceeding 1700 degrees Celsius in situ. This chemical anchoring process precedes the formation of the low-melting-point silicate glass phase, thereby fixing the impurity elements in the high-melting-point crystal phase and completely blocking their path to form a continuous glass network at the grain boundaries, resulting in a clean microstructure at the grain boundaries.

[0037] 3. The product prepared by this invention has significantly improved its resistance to deformation at high temperatures because the low-melting-point glass phase at the grain boundaries is effectively eliminated and the dispersed high-melting-point second-phase particles exert a strong pinning effect on grain boundary slip. The load softening starting temperature of the product is increased to above 1680 degrees Celsius, and the high-temperature creep rate under long-term load of 1450 degrees Celsius can be reduced to below 0.2%. These key indicators have reached the level of high-end synthetic refractory materials, ensuring its long-term dimensional stability and service life under harsh high-temperature environments. Attached Figure Description

[0038] Figure 1 This is a schematic diagram illustrating the microscopic mechanism evolution of the process of the present invention.

[0039] Figure 2 This is a comparison diagram of the microstructures of the process of this invention and the traditional process.

[0040] Figure 3 This is an elemental surface scan analysis diagram of the second phase particles at the grain boundary in an embodiment of the present invention. Detailed Implementation

[0041] To facilitate a clear understanding of the technical means, creative features, objectives, and effects of this invention, the invention will be further described below in conjunction with specific illustrations.

[0042] In existing refractory material preparation technologies, the utilization of low- and medium-grade bauxite mainly relies on physical homogenization techniques. Although this traditional method eliminates component segregation on a macroscopic scale, at the microscopic scale, impurities such as iron oxide and titanium dioxide retain their original chemical activity and react with silicon dioxide at high temperatures to form a low-melting-point glass phase. This physical dispersion cannot fundamentally eliminate the fluxing properties of impurities, making it difficult to improve the high-temperature performance of the material. Therefore, this embodiment adopts a microscopic in-situ homogenization process based on the chemical anchoring effect.

[0043] This invention provides a homogeneous corundum mullite brick and its preparation process, such as... Figure 1 As shown, this paper illustrates the mechanism evolution of the sol-gel-assisted microscopic in-situ homogenization process of this invention. Stages a) and b) depict the process by which lanthanum ions are uniformly coated on the surface of submicron-sized bauxite particles in the form of chemical adsorption through the construction of an ionized dispersion system and wet ultrafine grinding, forming a precursor sol film. Stage c) shows the state after spray drying granulation, where lanthanum salt is precipitated in-situ in the interparticle gaps and a nanoscale salt bridge network is constructed, forming a spherical pseudo-particle microstructure. Stage d) is a schematic diagram of the microstructure after the final oxidation reaction sintering, showing that lanthanum ions have preferentially captured impurities Fe and Ti to form high-brightness second-phase particles and pinned them at the grain boundaries, thereby blocking the formation of low-melting-point glass phase and achieving grain boundary cleanliness and structural homogenization.

[0044] The technical solutions of each claim are described in detail below with specific steps.

[0045] This embodiment relates to a preparation process for homogeneous corundum mullite bricks, employing a sol-gel assisted microscopic in-situ homogenization method, specifically including the following steps:

[0046] a) Construction of ionized dispersion system: Low-grade bauxite raw material is crushed, and water-soluble lanthanum salt is dissolved in a liquid medium containing dispersant and complexing agent to form a homogeneous solution of lanthanum ions (La3+);

[0047] b) Colloidal contact grinding: bauxite raw material is added to a homogeneous solution to make a slurry, which is then subjected to wet ultrafine grinding. Lanthanum ions are adsorbed and coated on the surface of submicron bauxite particles using a liquid medium to form a precursor sol film.

[0048] c) In-situ structural reconstruction: The ground slurry is spray-dried and granulated, so that lanthanum salt is precipitated in situ between particles during the drying process, forming a salt bridge network that coats impurity minerals, and thus obtaining composite micro powder.

[0049] d) Oxidation reaction sintering: After the composite micro powder is shaped, it is sintered at high temperature. Before the liquid phase formation temperature, lanthanum ions are induced to preferentially react with impurities in bauxite. and A solid-state reaction occurs, transforming the impurities into lanthanum ferrite or lanthanum titanate-like crystalline phases with melting points above 1700°C, thereby blocking the impurities from participating in the formation of low-melting-point glass phases at the microscale.

[0050] In some embodiments of the present invention, step a uses secondary bauxite with an Al2O3 content of 60% to 70% as raw material, which is pre-crushed to below 200 mesh. The water-soluble lanthanum salt is selected as lanthanum nitrate hexahydrate or lanthanum chloride, and the liquid medium is deionized water. In practical applications, the lanthanum salt can also be selected from other water-soluble rare earth salts such as lanthanum acetate. This application does not limit this.

[0051] The core of this process lies in utilizing the sol-gel principle to assist wet ball milling. Through liquid-phase doping, the chemical anchoring agent lanthanum is distributed on the surface of low-grade mineral particles with ionic precision. In step b, ultrafine grinding breaks the dissociation of the minerals, exposing the impurities encapsulated inside. Charged La3+ ions are uniformly coated on the mineral surface through electrostatic adsorption. During the rapid drying process in step c, dissolved lanthanum salt precipitates in the interparticle gaps, forming nanoscale salt bridges, achieving molecular-level contact between the modifier and the impurity minerals. In the early stage of sintering in step d, due to the extremely short diffusion distance, lanthanum ions can quickly capture nearby iron and titanium ions, generating a high-melting-point perovskite phase.

[0052] This implementation method overcomes the limitations of traditional solid-phase reaction diffusion kinetics, ensuring that the chemical anchoring reaction occurs before the liquid-phase generation reaction. By converting impurities in situ into high-melting-point crystalline phases, the fluxing effect of impurities is fundamentally eliminated, enabling refractory materials prepared from low-grade raw materials to achieve high-temperature mechanical properties comparable to high-end synthetic materials.

[0053] During wet grinding, fluctuations in the pH of the slurry can easily lead to premature hydrolysis and precipitation of lanthanum ions, resulting in uneven distribution. In addition, if the amount of lanthanum salt added is not properly controlled, too little will lead to incomplete capture of impurities, while too much may react with the matrix silica to form low-melting-point lanthanum silicate, which will reduce performance.

[0054] In some embodiments of the present invention, the complexing agent in step a is citric acid or EDTA, and the pH value of the slurry is controlled to maintain the complexed stable state of lanthanum ions.

[0055] The amount of lanthanum salt added, nLa, is controlled according to the following non-stoichiometric model:

[0056]

[0057] That is, n La Equals the coefficient α multiplied by the raw material The molar content plus twice the amount The sum of the molar contents.

[0058] Among them, C Fe and C Ti They are respectively from the raw materials and The molar content is α, which is the diffusion compensation coefficient, ranging from 1.10 to 1.25. The coefficient α is used to compensate for the loss caused by lanthanum ions dissolving in the mullite lattice or reacting with surface silica at high temperatures, ensuring that impurities are completely captured.

[0059] In specific implementation, ammonium polyacrylate (PAA-NH4) is selected as the dispersant, and citric acid is selected as the complexing agent. In the ingredient calculations, if the raw materials contain... The content is 3.0 wt%. The content is 2.0 wt%. The theoretical lanthanum atom requirement is calculated based on the above model, and the final sample weight of lanthanum nitrate is determined by combining the coefficient α. Usually, the experimentally optimized amount of lanthanum oxide added accounts for 4% to 6% of the total weight.

[0060] Citric acid, as a complexing agent, can form a stable chelate with La3+, preventing it from precipitating due to excessive local alkalinity during grinding. Regarding the stoichiometric model, the coefficient α is set to take into account the competitive reactions in the multi-component system. Although La has a very strong binding force with Fe, some La will inevitably dissolve in the mullite lattice or react with the most active surface SiO2. Setting an excess coefficient of 1.10 to 1.25 ensures that even if there are side reactions that consume La, there will still be enough La to completely anchor harmful impurities Fe and Ti.

[0061] By controlling the complexing agent in a steady state and using precise non-stoichiometric proportions, the uniformity of the modifier in the slurry and its effectiveness during the reaction are ensured, avoiding defects caused by local lanthanum enrichment, while maximizing the efficiency of impurity removal.

[0062] Impurities in bauxite clinker often exist within mineral particles in the form of fine inclusions. If the grinding fineness is insufficient, the impurities cannot be exposed, making it difficult for chemical anchoring agents to contact them. At the same time, the rate of solid-phase reaction is limited by the diffusion distance, and excessively coarse particles will cause the reaction to lag behind the appearance of the liquid phase.

[0063] In some embodiments of the present invention, step b controls the particle size distribution of the slurry after wet ultrafine grinding to be less than or equal to 0.8 micrometers and less than or equal to 2.0 micrometers;

[0064] The particle size range is used to break the dissociation of bauxite minerals and expose the iron-containing impurities encapsulated inside, while shortening the solid-phase diffusion distance between lanthanum ions and impurities to the nanoscale, so as to ensure that the chemical anchoring reaction is completed before the liquid phase appears at a temperature below 1200°C.

[0065] In practice, a high-flow circulating sand mill is used, and the grinding media is high-purity zirconia beads with a diameter of 0.5 to 1.0 mm. The mixed slurry is pumped into the sand mill for circulating grinding until the laser particle size analyzer test results meet the requirement that D90 is less than 2.0 micrometers. If D90 is greater than 5 micrometers, it is considered a defective product and needs to be re-ground.

[0066] Submicron-level grinding can fully break down the dissociation of mineral particles, exposing hematite and anatase impurities encased within aluminosilicates. More importantly, this particle size shortens the diffusion distance between reactants to the nanometer level. According to solid-state reaction kinetics, shortening the diffusion distance can significantly reduce the activation energy of the reaction, allowing the La-Fe reaction to proceed rapidly at a lower temperature (approximately 900°C), thus completing solidification before the formation of the 1200°C ferroolitic liquid phase.

[0067] Strict particle size control is a prerequisite for achieving low-temperature chemical anchoring, ensuring that all impurities can be captured by the modifier, avoiding any impurities that slip through the net due to poor local diffusion, thereby guaranteeing the uniformity of the microstructure of the final product.

[0068] If the slurry after wet grinding is dried directly, component migration and agglomeration are likely to occur, resulting in uneven distribution of active ingredients. In addition, traditional dry pressing requires good particle flowability, and it is extremely difficult to directly form ultrafine powders.

[0069] In some embodiments of the present invention, step c, spray drying granulation, controls the inlet air temperature to be 280 to 320°C to prepare spherical pseudo-particles with a particle size of 50 to 150 micrometers.

[0070] The spherical pseudo-particles are composed of lanthanum salt nanocrystals connected to aluminum-silicon particles. During the subsequent sintering process, the pseudo-particles act as independent reaction micro-regions, restricting the migration of the liquid phase on a macroscopic scale.

[0071] The specific equipment uses a high-speed centrifugal spray drying tower, controlling the outlet air temperature at 100 to 110°C. The slurry is atomized into tiny droplets by an atomizer, and then dried instantly in hot air, agglomerated into spherical powder with good flowability.

[0072] During the instantaneous evaporation of the droplets, the lanthanum salt originally dissolved in the water rapidly precipitates at the contact points and surface of the bauxite microparticles, forming a nanoscale salt film or salt bridge. This self-assembled structure tightly binds the aluminum-silicon particles together, forming countless tiny reaction units. Each pseudo-particle contains reactants in stoichiometric equilibrium, which confines the chemical reaction within the pseudo-particles during sintering, blocking the long-distance migration and aggregation of the liquid phase on a macroscopic scale.

[0073] This step not only solves the molding process problem of ultrafine powder, but more importantly, it constructs a microscopic prestructure that is conducive to in-situ reaction, providing a physical basis for subsequent uniform sintering and effectively preventing macroscopic segregation.

[0074] In multi-component systems, lanthanum competes with silicon to form silicates and with iron to form ferrates. If the temperature is increased too rapidly, it may bypass the solid-phase reaction temperature range and directly enter the liquid-phase region. Furthermore, lanthanum ferrate... It has a perovskite structure, and its stability is highly dependent on the valence state of iron. If the oxygen partial pressure in the atmosphere is insufficient, Restore to This will lead to When the LA is decomposed, it will be converted into low-melting-point silicates, leading to anchoring failure.

[0075] In some embodiments of the present invention, step d, chemically anchored reaction sintering, includes a specific thermodynamic control regime:

[0076] A constant temperature or slow heating range is set between 900°C and 1300°C. Taking advantage of the fact that the Gibbs free energy of the reaction between La and Fe is lower than that of the reaction between La and Si, [the following is possible:] The phase forms prior to the silicate liquid phase;

[0077] Atmosphere control for oxidation reaction sintering requires that the oxygen volume concentration in the furnace always be greater than 2%. This oxygen concentration is used to maintain the iron content in the system. Valence state, to prevent it from reverting to This leads to the formation of lanthanum ferrite. Decomposition of perovskite structure and formation of low-melting-point olivine phase.

[0078] In actual production, tunnel kilns are used, equipped with forced ventilation systems to ensure an oxidizing atmosphere. In the temperature range of 900°C to 1300°C, the heating rate is controlled at a low level or a heat preservation platform is set up to allow sufficient time for the solid-phase reaction.

[0079] According to thermodynamic calculations, The Gibbs free energy of formation is more negative than that of silicates, meaning that the bonding force between La and Fe is stronger. By slowly increasing the temperature in the solid-state reaction region, the more thermodynamically stable components are preferentially formed using kinetic control. Sufficient oxygen partial pressure ensures that iron maintains its +3 valence, which is essential for maintaining... The necessary condition for the stability of the perovskite lattice is that once the environment is reduced, Upon decomposition, the free FeO will immediately combine with SiO2 to form low-melting-point glass, rendering all previous efforts futile.

[0080] By precisely controlling the temperature and atmosphere, the direction of the competing reaction was successfully manipulated, ensuring that impurities were "locked" in the high-melting-point compound rather than entering the liquid phase, thereby significantly increasing the high-temperature softening temperature of the material.

[0081] The sintering process needs to balance the completion of the chemical reaction and the densification of the material. If the temperature is directly and rapidly increased to a high temperature, the pores will close prematurely, hindering the release of reacting gases, and a liquid phase will appear before the chemical reaction is complete. If the temperature is too low, the material cannot be densified and will lack strength.

[0082] In some embodiments of the present invention, the preparation process further includes temperature control during the sintering stage: a solid-state reaction holding section is set in the range of 900°C to 1300°C, so that lanthanum ions complete the reaction before the liquid phase is generated. and The lattice occupancy is then determined; subsequently, the temperature is raised to above 1600°C for densification sintering.

[0083] The specific process curve design is as follows: room temperature to 900°C is the nitrate decomposition period, 900°C to 1300°C is the chemical anchoring reaction period, which requires slow heating, 1300°C to 1500°C is the murolization period; finally, densification is carried out at 1600°C to 1650°C for 4 to 6 hours to make the green body density reach more than 2.65 g / cm3.

[0084] The segmented sintering strategy separates the "reaction" and "densification" processes along the timeline. The low-temperature segment focuses on allowing La atoms to diffuse into the Fe and Ti lattices, completing the "capture" process and generating... In the La2TiO5 microcrystal section, a small amount of high-temperature liquid phase (mainly formed by impurities in the matrix and a small amount of La-Si phase) is used to promote particle rearrangement and mass transfer, eliminate pores, promote grain development, and finally obtain a dense ceramic body.

[0085] This system ensures that the material has both excellent high-temperature chemical stability (impurities are fixed) and high density and high strength physical properties, achieving synergistic optimization of structure and performance.

[0086] The alumina content in bauxite raw materials fluctuates greatly, making it difficult to guarantee the proportion of mullite and corundum in the final mineral phase when used directly. Furthermore, the residues from the decomposition of lanthanum nitrate need to be considered for their environmental and material impacts.

[0087] In some embodiments of the present invention, the raw material is selected as secondary or tertiary bauxite with an Al2O3 content of 60% to 70%, and industrial alumina micro powder is added to adjust the total Al2O3 content to 72% to 75%. The water-soluble lanthanum salt is selected as lanthanum nitrate, which does not leave anionic impurities after thermal decomposition.

[0088] In practice, industrial α-Al₂O₃ micro powder with a purity greater than 99% is used as an aluminum supplement. Lanthanum nitrate La(NO₃)₃·6H₂O decomposes upon heating to produce La₂O₃, NO₂, and O₂. The gas is discharged without introducing harmful alkali metal ions such as sodium and potassium.

[0089] The total alumina content was adjusted to 72%-75%, slightly higher than the theoretical composition of mullite (71.8%). This was to ensure that the main crystalline phase of the matrix was mullite while containing an appropriate amount of corundum phase, thereby improving the material's corrosion resistance and high-temperature strength. Lanthanum nitrate was chosen because its anionic groups completely volatilize at high temperatures, unlike sulfates or chlorides, which leave harmful residues or corrode kiln equipment. Through fine-tuning of the composition, the mineral phase composition of the matrix was optimized. By selecting high-purity precursors, the introduction of secondary impurities was avoided, ensuring the high-temperature cleanliness of the material.

[0090] To manufacture large-sized refractory bricks, particles of different sizes need to be graded and packed together. If only fine powder is used for sintering, the shrinkage rate is too high, making it difficult to control the size and prone to cracking. Traditional aggregates are mostly ordinary bauxite, which has poor performance and will become the weak point of the refractory bricks.

[0091] In some embodiments of the present invention, the bricks produced by the process are crushed and screened, and then recycled as aggregates into the molding process of step d to form a refractory material system with a fully homogeneous structure.

[0092] The specific method involves crushing the homogeneous clinker or defective bricks prepared by the above process, sieving them into particles of different sizes such as 5-3 mm, 3-1 mm, and 1-0 mm, using these particles as aggregates (accounting for 65-70% of the total amount), mixing them with the homogeneous fine powder prepared by this process (accounting for 30-35% of the total amount), adding a binder, pressing them into shape, and then sintering them again to prepare refractory bricks of standard size.

[0093] This strategy of "growing one's own bones from one's own flesh" ensures that the aggregates and matrix in refractory bricks have completely identical chemical compositions and coefficients of thermal expansion. At high temperatures, a perfect ceramic bond can be formed between the aggregates and matrix, without interfacial stress or chemical erosion, thus achieving a truly "completely homogeneous" structure.

[0094] It greatly improves the overall uniformity and thermal shock stability of refractory bricks, eliminates the shortcomings of mismatch between aggregate and matrix properties in traditional refractory materials, and significantly extends service life.

[0095] This embodiment prepares a homogeneous corundum mullite brick with a microstructure based on the chemical anchoring effect:

[0096] The matrix framework is composed of well-developed interwoven network mullite and granular corundum.

[0097] At the three-way point of the matrix grain boundary, there are high-brightness second-phase particles with an average particle size of less than 3 micrometers. Energy dispersive spectroscopy analysis shows that the second-phase particles are rich in La, Fe and Ti elements and poor in Si elements.

[0098] The internal grain boundaries of the brick are clean, with no continuous network of glass phase distribution.

[0099] like Figure 2 As shown, observations were made using scanning electron microscopy (SEM) in backscattered electron (BSE) mode. Figure 2 As shown in (A), the microstructure of this invention is characterized by the presence of highly bright second-phase particles (rich in La, Fe, and Ti) dispersed at grain boundaries within its well-developed mullite and corundum matrix framework, and clean internal grain boundaries without a continuous network of glassy phase distribution. In contrast, Figure 2 (B) shows that in traditional processes, because impurities are not chemically anchored, they react with the matrix silica at high temperatures, resulting in a large amount of dark gray low-melting-point glass phase network filling the grain boundaries. This is the main reason for the decline in the high-temperature performance of the material.

[0100] Further analysis using EDS energy dispersive spectroscopy Figure 2 The second-phase particle composition observed in the results are as follows: Figure 3 As shown in the figure, the chemical anchoring effect is clearly demonstrated: the bright particle region in the SEM morphology shows a high enrichment of lanthanum (La), iron (Fe), and titanium (Ti) in the elemental distribution map. Crucially, the silicon (Si) elemental distribution map in the same region shows a significant depletion zone (void), which strongly proves that lanthanum ions successfully induced impurities Fe and Ti to deviate from the reaction pathway with SiO2, preferentially forming silicon-free, high-melting-point compounds, thus avoiding the formation of a low-melting-point silicate liquid phase.

[0101] This unique microstructure is direct evidence of the chemical anchoring effect; the high-brightness particles are generated in situ. It forms a solid solution with La2TiO5, which is fixed at the grain boundaries like nails and does not form low-melting-point co-alloys with silicon dioxide. Therefore, the grain boundaries appear very clean, without the thick glass phase layer commonly found in traditional materials.

[0102] This microstructure directly determines the improvement in macroscopic performance. Clean grain boundaries and a rigid framework ensure high-temperature strength, while dispersed second-phase particles provide an additional enhancement mechanism.

[0103] The specific performance data of materials are key indicators for verifying the effectiveness of the technology, especially the high-temperature creep rate and the load softening temperature, which are core parameters for measuring the ability of refractory materials to resist deformation under high temperature and high pressure.

[0104] In some embodiments of the present invention, the main crystalline phase of the second phase particles is orthorhombic lanthanum ferrite. Or a solid solution of it with lanthanum titanate La2TiO5, through the pinning effect of the second phase particles on grain boundary slip, the high-temperature creep rate of the brick at 1450°C for 50 hours is less than 0.2%, and the softening temperature under 0.2MPa load is greater than 1680°C.

[0105] X-ray diffraction (XRD) analysis revealed that the primary crystalline phases of the material are mullite and corundum, while the secondary crystalline phases are perovskite. Performance tests showed that, under a pressure of 0.2 MPa, the sample did not soften or deform when heated to 1680°C (T0.6>1680°C), which is much higher than the 1450-1500°C of ordinary high-alumina bricks.

[0106] Due to the disappearance of the low-melting-point iron-silicon glass phase, the softening point of the material increases to near the eutectic temperature of the matrix mullite-corundum (>1800°C). Simultaneously, hard particles dispersed at the grain boundaries... The grains exert a strong "pinning effect" on grain boundary sliding and dislocation movement, which greatly hinders plastic deformation at high temperatures and thus significantly reduces the high-temperature creep rate.

[0107] These performance indicators demonstrate that this invention utilizes low-cost, low-grade bauxite to prepare high-end refractory materials with performance comparable to or even surpassing that of expensive synthetic raw materials. In the initial stage of industrial production, the composite micro powder obtained in step c) can be pressed into raw blocks, sintered and densified at 1600°C, and then crushed and sieved to obtain 'pre-synthesized homogeneous aggregate' with completely consistent matrix composition. Subsequently, this aggregate is mixed with the composite micro powder in a 7:3 ratio and then enters the normal brick production cycle.

[0108] To further illustrate the technical effects of the present invention, the following comparative experiment was conducted:

[0109] Example 1 was carried out strictly according to the preferred process provided by the present invention, using secondary bauxite raw material, the chemical composition of which was tested and found to be: 65.0%, 2.5% (M) Fe ≈160), 2.0% (M) Ti ≈80).

[0110] In order to make the total The content was adjusted to 74%, and the matrix formula was determined to be: 80kg of secondary bauxite + 20kg of industrial alumina powder.

[0111] According to the formula n of this invention La =α×(C Fe +2×C Ti The diffusion compensation coefficient α is set to 1.20.

[0112] in matrix Number of moles: (80kg×2.5%) / 159.69g / mol≈12.53mol.

[0113] in matrix Number of moles: (80kg×2.0%) / 79.87g / mol≈20.03mol.

[0114] Theoretically required number of La moles: 1.20 × (12.53 + 2 × 20.03) ≈ 63.11 mol.

[0115] If lanthanum nitrate hexahydrate (La(NO3)3⋅6H2O, molar mass 433 g / mol) is selected, then the required mass of lanthanum nitrate is: 63.11 mol × 433 g / mol ≈ 27.3 kg.

[0116] The above 27.3 kg of lanthanum nitrate was dissolved in an aqueous solution containing 0.5 kg of ammonium polyacrylate and 0.3 kg of citric acid (pH adjusted to 6.5). 80 kg of bauxite raw material was added and wet ultrafine grinding was carried out to control the particle size of the slurry to D90 of 1.5 micrometers. After spray drying and granulation, it was sintered in an oxidizing atmosphere and subjected to a slow heating and holding process from 900°C to 1300°C.

[0117] Comparative Example 1 aims to verify the necessity of introducing lanthanum and its chemical anchoring effect. No lanthanum salts were added in this group, and instead of a sol-gel wet process, a conventional dry ball milling mixing process was used, with the particle size D90 controlled at 45 micrometers. The remaining sintering regime was consistent with Example 1.

[0118] Comparative Example 2 aims to verify the superiority of "sol-gel assisted micro-homogenization" over traditional "physical mixing". Although an equal amount of lanthanum was added to this group, micron-sized lanthanum oxide powder was used instead of ionic lanthanum nitrate, and a dry direct mixing method was adopted, which could not form a precursor sol film encapsulating impurities. The other conditions were the same as in Example 1.

[0119] Comparative Example 3 aims to verify the effect of sintering atmosphere on the stability of chemically anchored products. The raw material ratio and process flow of this group are completely consistent with Example 1, but the poor ventilation conditions are simulated during the sintering process, so that the furnace is in a weak reducing atmosphere, resulting in insufficient oxygen content.

[0120] Comparative Example 4 aims to verify the effect of particle size distribution on solid-phase reaction kinetics. Although this group used wet grinding and lanthanum nitrate addition, the grinding time was relatively short, and the slurry particle size D90 was controlled to be 10 micrometers, which is much larger than the upper limit of 2.0 micrometers required by this invention, so as to test the reaction effect after increasing the diffusion distance.

[0121] Table 1

[0122]

[0123] As shown in Table 1, the corundum mullite brick prepared using the integrated process of this invention in Example 1 exhibits the best performance. Its load softening temperature exceeds 1680°C, and its high-temperature creep rate is only 0.15%. This is mainly attributed to the combination of wet ultrafine grinding and the sol-gel process, which allows lanthanum ions to be encapsulated on the surface of impurity minerals with nanoscale precision. In the solid-phase reaction range of 900°C to 1300°C, lanthanum ions preferentially react with iron and titanium impurities to generate high-melting-point lanthanum ferrite and lanthanum titanate. These high-melting-point phases act as grain boundary pins at the microscopic level, while simultaneously blocking the formation path of low-melting-point glass phases, thereby endowing the material with excellent high-temperature mechanical properties.

[0124] Comparative Example 1 clearly demonstrates the harm caused by impurities. In the absence of lanthanum intervention, iron oxide and titanium dioxide in bauxite act as fluxes, reacting with silica in the matrix to generate a large amount of liquid phase, causing the material to soften at around 1420°C and exhibiting a high-temperature creep rate as high as 2.8%. This proves that simply relying on physical mixing cannot solve the problem of impurities in low-grade raw materials.

[0125] The results of Comparative Example 2 reveal the essential difference between physical homogenization and chemical homogenization. Although lanthanum oxide was introduced as a modifier, the dry physical mixing method prevented molecular-level contact between the modifier particles and the impurity minerals. Limited by the solid-phase diffusion distance, lanthanum reacted more readily with the matrix silica, which had a larger contact area, to form low-melting-point lanthanum silicate, rather than capturing dispersed iron impurities. This failure of the competing reaction resulted in improved material properties compared to Comparative Example 1, but still far from reaching the level of Example 1.

[0126] Comparative Example 3 highlights the thermodynamic necessity of atmosphere control. Under a reducing atmosphere, iron is reduced from +3 to +2, causing the lanthanum ferrite in the perovskite structure to decompose. The released ferrous oxide rapidly enters the liquid phase, which not only renders the chemical anchoring effect ineffective but also increases the amount of liquid phase due to the introduction of additional components. This results in material performance that is even worse than that of ordinary bricks without lanthanum. This verifies the critical importance of emphasizing that the oxygen concentration in the furnace must always be greater than 2% in this invention.

[0127] Comparative Example 4 verified the limiting effect of particle size control on kinetics. Despite suitable chemical composition and atmospheric conditions, the diffusion distance between lanthanum ions and impurity centers exceeded the nanoscale due to the D90 reaching 10 micrometers. Before the critical temperature for the appearance of the liquid phase, the solid-phase reaction could not be completely completed, resulting in some impurities "slipping through" and participating in the formation of the glass phase. This result supports the technical requirement of this invention that the particle size should be controlled at D90 less than or equal to 2.0 micrometers to ensure the advantage of reaction kinetics.

[0128] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention, all of which fall within the scope of protection claimed by the present invention. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A preparation process for homogeneous corundum mullite bricks, characterized in that, The sol-gel assisted microscopic in-situ homogenization method includes the following steps: a) Construction of ionized dispersion system: Low-grade bauxite raw material is crushed, and water-soluble lanthanum salt is dissolved in a liquid medium containing dispersant and complexing agent to form a homogeneous lanthanum ion solution; b) Colloidal contact grinding: The bauxite raw material is added to the homogeneous solution to form a slurry, which is then subjected to wet ultrafine grinding. Lanthanum ions are adsorbed and coated on the surface of submicron bauxite particles using a liquid medium to form a precursor sol film. c) In-situ structural reconstruction: The ground slurry is spray-dried and granulated, so that lanthanum salt is precipitated in situ between particles during the drying process, forming a salt bridge network that coats impurity minerals, and thus obtaining composite micro powder. d) Oxidation reaction sintering: After the composite micro powder is shaped, it is sintered at high temperature. Before the liquid phase formation temperature, lanthanum ions are induced to preferentially react with impurities in bauxite. and A solid-state reaction occurs, transforming the impurities into lanthanum ferrite or lanthanum titanate-like crystalline phases with melting points above 1700°C, thereby blocking the impurities from participating in the formation of low-melting-point glass phases at the microscale.

2. The preparation process according to claim 1, characterized in that, In step a), the complexing agent is selected from citric acid or EDTA, and the pH value of the slurry is controlled to maintain the complexed stable state of lanthanum ions; The amount of lanthanum salt added Control is based on the following non-stoichiometric model: in, and They are respectively from the raw materials and molar content, The diffusion compensation coefficient ranges from 1.10 to 1.

25. It is used to compensate for the loss caused by lanthanum ions dissolving in the mullite lattice or reacting with surface silica at high temperatures, ensuring that impurities are completely captured.

3. The preparation process according to claim 1, characterized in that, In step b), the particle size distribution of the slurry after wet ultrafine grinding is controlled. and ; The specified particle size range is used to break the dissociation of bauxite minerals and expose the iron-containing impurities encapsulated within them, while shortening the solid-phase diffusion distance between lanthanum ions and impurities to the nanoscale, so as to ensure that the chemical anchoring reaction is completed before <1200°C.

4. The preparation process according to claim 1, characterized in that, In step c), the spray drying granulation process controls the inlet air temperature to be 280-320°C, producing particles with a diameter of 50-150 mm. spherical pseudoparticles; The spherical pseudo-particles are composed of lanthanum salt nanocrystals connected to aluminum-silicon particles. During the subsequent sintering process, the pseudo-particles act as independent reaction micro-regions, restricting the migration of the liquid phase on a macroscopic scale.

5. The preparation process according to claim 1, characterized in that, In step d), the chemically anchored reaction sintering involves a specific thermodynamic control regime: Set a constant temperature or slow heating range between 900°C and 1300°C, and utilize... The reaction Gibbs free energy is lower than The characteristics of the reaction make The phase forms prior to the silicate liquid phase; The atmosphere control for the oxidation reaction sintering requires that the oxygen volume concentration in the furnace always be greater than 2%, and this oxygen concentration is used to maintain the iron element in the system at a certain level. Valence state, to prevent it from reverting to This leads to the decomposition of the generated lanthanum ferrite perovskite structure and the formation of a low-melting-point fir olivine phase.

6. The preparation process according to claim 1, characterized in that, The preparation process also includes temperature control during the sintering stage: a solid-state reaction holding section is set in the range of 900°C to 1300°C, so that lanthanum ions complete the reaction before the liquid phase is generated. and The lattice occupancy is then determined; subsequently, the temperature is raised to above 1600°C for densification sintering.

7. The preparation process according to claim 1, characterized in that, The raw materials are selected Secondary or tertiary bauxite with a content of 60% to 70%, and industrial alumina powder added to adjust the total content. The content is 72%~75%, and the water-soluble lanthanum salt is selected from lanthanum nitrate, which does not leave anionic impurities after thermal decomposition.

8. The preparation process according to claim 1, characterized in that, The bricks produced by the process are crushed and screened, and then recycled as aggregates in the molding process of step d) to form a refractory material system with a fully homogeneous structure.

9. A homogeneous corundum mullite brick prepared using the process described in any one of claims 1 to 8, characterized in that, It has a microstructure based on the chemical anchoring effect: The matrix framework is composed of well-developed interwoven network mullite and granular corundum. At the three-way junctions of the matrix grain boundaries, there are dispersed grains with an average size of less than 3 mm. High-brightness second-phase particles, which, according to energy dispersive spectroscopy analysis, are rich in La, Fe, and Ti elements and poor in Si elements; The internal grain boundaries of the brick are clean, with no continuous network of glass phase distribution.

10. The homogeneous corundum mullite brick according to claim 9, characterized in that, The main crystalline phase of the second phase particles is lanthanum ferrite or a solid solution of lanthanum titanate or an orthorhombic crystal system. Through the pinning effect of the second phase particles on grain boundary slip, the high-temperature creep rate of the brick at 1450°C for 50 hours is less than 0.2%, and the softening temperature under 0.2MPa load is greater than 1680°C.

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