Silicon mullite brick for rotary kiln and production process of silicon mullite brick

Abstract

The invention discloses a silicon mullite brick for a rotary kiln and a production process of the silicon mullite brick, and belongs to the technical field of refractory materials. According to the product, core-shell structure SiC-coated Al2O3 particles are adopted as a base material, mullite, corundum and a rare earth additive in a special proportion are matched, and a beta-Si3N4 fiber reinforced network is generated in situ, so that the thermal shock resistance and erosion resistance of the material are remarkably improved. In the aspect of a preparation process, a sol-gel coating technology, ultrasonic dispersion treatment and isostatic pressing vibration molding are innovatively combined, and a composite process of staged atmosphere control sintering and microwave-assisted heating is adopted, so that precise regulation and control of a microstructure of the material are realized. The prepared silicon mullite brick has excellent comprehensive performance, can meet the use requirements of rotary kilns in the industries of cement, metallurgy and the like under the extreme working conditions of high temperature, strong erosion and the like, solves the technical problems that a traditional product is easy to oxidize, poor in thermal stability and the like, and has the characteristics of energy conservation, environmental protection and high production efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of refractory materials, in particular to a silica-mudstone brick for a rotary kiln and a production process thereof. Background Art

[0002] As a key refractory material for rotary kiln linings, silica-mullite bricks must withstand long-term service in extreme environments subject to high temperatures (>1400°C), frequent thermal shock, and strong alkali / sulfur corrosion. Traditional silica-mullite bricks are typically composed of silicon carbide (SiC) and mullite, formed with a phosphate binder and sintered at high temperatures. However, existing technology still faces the following bottlenecks: Silicon carbide oxidation and interface defects: In traditional formulations, exposed SiC particles are prone to forming a SiO2 layer in a high-temperature oxidizing environment, leading to volume expansion and structural spalling. Furthermore, the interface bonding between SiC and the substrate is insufficient, exacerbating the risk of thermal shock cracking. Inadequate erosion and thermal shock resistance: Existing additives (such as single rare earth or metal oxides) have limited control over liquid phase formation and phase stability, making it difficult to form an anti-erosion barrier. Furthermore, they lack microstructural reinforcement mechanisms (such as fiber toughening), resulting in thermal shock cycles typically below 15. Low sintering process efficiency: Conventional sintering relies on a single atmosphere (such as air or nitrogen) and resistance heating, which makes it difficult to balance the requirements of SiC oxidation resistance and grain densification. It also consumes a lot of energy and is prone to structural inhomogeneity (such as porosity > 15%). Poor structural adaptability: The homogeneous brick structure is difficult to match the thermal stress distribution in different sections of the rotary kiln, resulting in premature local damage.

[0003] To address the above problems, it is urgent to develop a new generation of silica-molybdenum bricks with high thermal shock resistance, corrosion resistance and structural adaptability through material component innovation and process innovation to meet the harsh working conditions of cement, metallurgy and hazardous waste treatment rotary kilns. Summary of the Invention

[0004] The main purpose of the present invention is to provide a silica-molybdenum brick for a rotary kiln and a production process thereof, which can effectively solve the problems mentioned in the background technology.

[0005] To achieve the above object, the technical solution adopted by the present invention is: A silica-molybdenum brick for a rotary kiln comprises the following raw materials in percentage by mass: 30-40% silicon carbide (SiC) particles, the surface of which is coated with a nano-aluminum oxide (Al2O3) layer to form a SiC@Al2O3 core-shell structure; High purity mullite (Al2O3-SiO2) 25-35%; Corundum powder (Al2O3≥99%) 10-15%; The total addition amount of composite rare earth oxide Y2O3+La2O3 is 1.5-2.5%; Silicon powder 1-2% and boron nitride (BN) 0.5-1%; Modified silica sol and aluminum phosphate composite binder 4-6%; The silicon carbide particles are arranged in a 3:4:3 ratio of coarse (1-3mm): medium (0.1-1mm): fine (<0.1mm) particles. The core-shell SiC@Al2O3 structure inhibits high-temperature oxidation of silicon carbide, while the β-Si3N4 fiber network enhances thermal shock resistance. The gradient particle size distribution optimizes density.

[0006] Preferably, the nano-alumina layer is coated using a sol-gel method, with a coating thickness of 50-200 nm and an addition amount of nano-alumina of 2-3% of the total mass of the silicon carbide. The sol-gel method ensures uniform coating of the nano-alumina on the silicon carbide, avoids interfacial defects, and enhances the bonding strength between particles at high temperatures.

[0007] Preferably, the mass ratio of Y2O3 to La2O3 in the composite rare earth oxide is 1:1 to 2:1. The specific ratio of rare earth oxides synergistically reduces the sintering temperature and promotes the formation of mullite phase, thereby improving high-temperature phase stability.

[0008] Preferably, the silica brick has an in-situ generated β-Si3N4 fiber network inside, with a fiber diameter of 0.1-1 μm and a length of 5-20 μm. The β-Si3N4 fibers significantly improve the resistance to alkali corrosion and thermal shock by bridging cracks and absorbing stress.

[0009] Preferably, a production process of silica-molybdenum bricks for rotary kiln comprises the following steps: (1) Raw material pretreatment: The silicon carbide particles are coated with nano-aluminum oxide by a sol-gel method and dried for later use; Pre-sinter the mullite and corundum powder at 600-800℃ for 2 hours; (2) Mixing and molding: The pretreated raw materials were mixed with composite rare earth oxides, silicon powder, boron nitride and composite binder, and subjected to ultrasonic dispersion treatment for 30 minutes; The green body was prepared by isostatic pressing (pressure 150-200 MPa) combined with high-frequency vibration (50-100 Hz); (3) Staged atmosphere sintering: Low temperature section (200-800℃): debinding in air atmosphere; Medium temperature section (800-1350℃): nitrogen atmosphere (flow rate 5-10L / min); High temperature section (1350-1480℃): weak reducing atmosphere (CO / H2 volume ratio 9:1), keep warm for 2-4 hours; (4) Microwave assisted sintering: microwave heating is used in the high temperature range (>1200℃), with a frequency of 2.45GHz and a power density of 5-10W / cm 3 .

[0010] Phased atmosphere control combined with microwave sintering reduces energy consumption while avoiding SiC oxidation, achieving precise control of the material's microstructure.

[0011] Preferably, the ultrasonic frequency in step (2) is 20-40 kHz, and the power density is 0.5-1.5 W / cm 3 Ultrasonic treatment effectively disperses nanomaterials and rare earth oxides, reduces agglomeration, and improves green body uniformity and sintering activity.

[0012] Preferably, during the high-temperature sintering process, the CO / H2 ratio and holding time are controlled to allow the silicon powder to react with boron nitride to form β-Si3N4 fibers. By controlling the reaction gas ratio, the silicon powder and BN are fully reacted to form β-Si3N4 fibers, thereby avoiding the formation of by-products.

[0013] Preferably, after the green body is formed, 3D printing technology is used to create a gradient pore structure with a pore size range of 50-200 μm and a porosity gradient of 5-8% on the surface and 10-15% in the interior. The gradient pore structure design alleviates thermal stress concentration and adapts to the temperature and corrosive environment requirements of different areas of the rotary kiln.

[0014] Compared with the prior art, the present invention has the following beneficial effects: The present invention suppresses the high-temperature oxidation of silicon carbide through the innovative design of core-shell structured SiC@Al2O3 particles, combines the synergistic reinforcement of rare earth oxides with the in-situ generation of β-Si3N4 fiber network, and significantly improves the thermal shock resistance and corrosion resistance of silica mullite bricks; the production process adopts staged atmosphere-controlled sintering and microwave-assisted heating to achieve the coordinated optimization of densification and grain regulation, and at the same time introduces ultrasonic dispersion and 3D printing gradient pore structure, taking into account material uniformity and thermal stress adaptability. The final silica mullite bricks have low porosity, high strength and excellent thermal stability, which can meet the long-term service requirements under the extreme conditions of high temperature and high corrosion of rotary kilns, and have the advantages of energy saving, environmental protection and customization. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the production process of the present invention. DETAILED DESCRIPTION

[0016] In order to make the technical means, creative features, objectives and effects achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0017] like Figure 1 The production process flow of silica-molybdenum bricks for rotary kiln is shown in the figure, which is described below in conjunction with a detailed embodiment.

[0018] 1. Raw material composition (mass percentage) raw material content Specific specifications and ratios Silicon carbide (SiC) particles 35% Coarse particles (1-3mm): medium particles (0.1-1mm): fine particles (<0.1mm) = 3:4:3, with a surface coating of 50-200nm nano-aluminum oxide layer (2.5% of the total mass of SiC) High purity mullite 30% Al2O3-SiO2 phase, purity ≥95% Corundum powder 12% Al2O3≥99%, average particle size 5μm Composite rare earth oxides 2% Y2O3:La2O3=1:1 (mass ratio) Silicon powder 1.5% Purity ≥98%, particle size <50μm Boron nitride (BN) 0.8% Purity ≥95%, particle size <10μm Modified silica sol-aluminum phosphate composite binder 5% Silica sol solid content 30% (modulus 2.5), aluminum phosphate solution concentration 15% (Al / P molar ratio 1:1.2) Key raw material pretreatment instructions Preparation of SiC@Al2O3 core-shell particles (sol-gel method) Prepare a 0.1 mol / L aluminum isopropoxide solution (the solvent is anhydrous ethanol, containing 5% deionized water), and add a nano-alumina precursor at 2.5% of the mass of SiC.

[0019] SiC particles less than 3 mm (coarse: medium: fine = 3:4:3 pre-mixed) were added to the solution and magnetically stirred at 60 °C for 2 h to form a uniform sol.

[0020] The solvent was evaporated in a water bath at 80°C until it became gel-like, dried at 100°C for 12 hours, and calcined at 500°C for 2 hours to obtain uniformly coated SiC@Al2O3 particles (coating layer thickness detection: transmission electron microscopy (TEM) observation, average thickness 120 nm).

[0021] Pre-sintering of mullite and corundum powder: Mix the two and place them in a box-type resistance furnace, heat them up to 700℃ at 5℃ / min, and keep them warm for 2 hours to remove adsorbed water and impurities and improve sintering activity (after pre-sintering, the surface roughness of the particles increases by 30%, and the specific surface area increases from 2.5m 2 / g increased to 4.2m 2 / g).

[0022] 2. Detailed explanation of production process steps (1) Mixing and molding (key parameter control) Ultrasonic dispersion mixing SiC@Al2O3 particles, pre-calcined mullite, corundum powder, rare earth oxide, silicon powder and BN were added in sequence according to the formula and dry mixed for 15 minutes.

[0023] Add composite binder (silica sol: aluminum phosphate = 2:1 volume ratio), wet mix for 30 minutes, and turn on ultrasonic dispersion (frequency 30kHz, power density 1W / cm 3 , amplitude 50 μm).

[0024] Principle: Ultrasonic cavitation effect breaks up nano-agglomerates, allowing rare earth oxides (average particle size 50nm) and binders to evenly wrap the aggregate, improving the uniformity of the green body (the viscosity of the slurry after mixing is stabilized at 500-600mPa・s).

[0025] Isostatic pressing-high frequency vibration molding The mixture was poured into a mold (φ100mm×200mm), a pre-pressure of 50MPa was applied to expel the air, and then an isostatic pressure of 180MPa was applied for 3 minutes.

[0026] During the pressing process, synchronous high-frequency vibration (frequency 80Hz, amplitude 0.5mm) is used to rearrange and densify the particles (the initial density of the green body reaches 75% of the theoretical density, which is 10% higher than that of traditional pressing).

[0027] After forming, the size accuracy of the blank is controlled: diameter error ±0.1mm, height error ±0.2mm.

[0028] (2) Staged atmosphere sintering (with temperature-atmosphere control curve) Sintering stage Temperature range Heating rate Atmospheric conditions Main Function Low temperature section 200-800℃ 3℃ / min Air (flow rate 20L / min) Eliminate organic matter from the binder (weight loss rate is about 5%) to form preliminary particle connections Medium temperature section 800-1350℃ 5℃ / min Nitrogen (purity ≥99.9%, flow rate 7L / min) Inhibit SiC oxidation and promote initial sintering of the interface between mullite and corundum powder High temperature section 1350-1480℃ 2℃ / min Weak reducing atmosphere (CO / H2=9:1, total flow rate 10L / min) Triggering the reaction between silicon powder and BN to form β-Si3N4 fibers, while microwave-assisted heating accelerates densification Key temperature control points High-temperature microwave-assisted sintering: When the temperature reaches 1200°C, a microwave generator (frequency 2.45 GHz, power density 8 W / cm³) is activated, synergizing with resistance heating to increase the temperature. The microwaves selectively heat the SiC particles, raising the internal temperature 30-50°C higher than the surface, promoting the growth of necks between the particles. After sintering, grain size uniformity improves by 40%, with an average grain size of 5-8 μm.

[0029] In-situ formation of β-Si₃N₄ fibers: Silicon powder (Si) reacts with boron nitride (BN) at 1400°C for 3 hours, forming a reaction: 3Si + 2BN → Si₃N₄ + 2B (ΔG < 0, thermodynamically feasible). By controlling the CO / H₂ ratio to inhibit Si oxidation (CO partial pressure 0.9 atm, H₂ partial pressure 0.1 atm), a fiber network with a diameter of 0.5 μm and a length of 10-15 μm is generated. Scanning electron microscopy (SEM) observations show an 85% interweaving degree, forming a crack-bridging structure.

[0030] (III) 3D printing gradient pore structure (optional step, bricks for high erosion areas) Green body pretreatment: After forming, the surface of the green body is sprayed with nano-silica binder to enhance the bonding strength between 3D printing layers.

[0031] Gradient pore design: Using photocuring 3D printing technology (the material is sinterable ceramic slurry, the main components of which are mullite micropowder + silica sol), a honeycomb structure with a pore size of 50-100μm and a porosity of 6% is printed on the surface layer of the green body (thickness 5mm); interconnected pores with a pore size of 150-200μm and a porosity of 12% are printed in the internal area (the porosity gradient is controlled by layered slicing software, and the printing thickness of each layer is 0.3mm).

[0032] Mechanism of action: low porosity on the surface resists slag penetration (permeability reduced by 40%), high porosity inside relieves thermal stress (gradient distribution of thermal expansion coefficient, surface 10×10 -6 / ℃, internal 8×10 -6 / ℃).

[0033] (4) Cooling and post-processing After sintering, cool to room temperature at a rate of 10°C / min, take out the bricks, inspect their dimensions (shrinkage rate is controlled at 1.5-2.0%), and polish the surface to a roughness of Ra ≤ 1.6μm, meeting the assembly accuracy requirements of the rotary kiln lining.

[0034] 3. Performance test and results (compared with traditional silica-molybdenum bricks) Test items This embodiment Traditional silica bricks Test Method Bulk density 2.85g / cm3 2.60g / cm3 Archimedes drainage method Apparent porosity 12% 18% Vacuum extraction method Flexural strength at room temperature 55MPa 40MPa Three-point bending method (span 100mm) Thermal shock resistance (1100℃ water cooling) >30 times 12 times Cycle until mass loss>5% Alkali corrosion resistance (1450℃, K2CO3 slag) Erosion depth 1.2mm 3.5mm Static crucible method (keeping warm for 4 hours) β-Si3N4 fiber content 8vol% - X-ray diffraction (XRD) quantitative analysis Performance improvement principle Core-shell structure anti-oxidation: The oxidation weight gain rate of SiC@Al2O3 at 1400℃ is only 0.8% (5% for bare SiC). The Al2O3 layer isolates O2 and inhibits the formation of SiO2 expansion layer.

[0035] Fiber toughening mechanism: β-Si3N4 fiber generates an additional stress of 20MPa when bridging cracks, which increases the fracture energy by 60% (120J / m 2 vs75J / m 2 ).

[0036] Advantages of gradient pore structure: The dense surface layer hinders the penetration of Na⁺ and K⁺, and the internal porous structure absorbs thermal stress through elastic deformation of pores, reducing the crack growth rate by 50% during thermal shock.

[0037] This embodiment utilizes four core technologies: SiC@Al2O3 core-shell particle preparation, in-situ β-Si3N4 fiber formation, microwave-assisted staged sintering, and 3D-printed gradient pore structures. These technologies address the issues of traditional silica-molybdenum bricks, such as oxidation failure, insufficient thermal shock resistance, and poor structural adaptability. The resulting bricks boast a service life at 1480°C that is more than double that of existing products. They are suitable for demanding operating conditions such as cement kiln firing zones (maximum temperature 1450°C, alkali vapor concentration 8-12 vol%) and reduction stages in metallurgical rotary kilns. They offer both energy-saving and environmentally friendly features (reducing sintering energy consumption by 25%) and customized design.

[0038] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A silica brick for a rotary kiln, characterized in that: The raw material composition includes the following mass percentages: 30-40% silicon carbide (SiC) particles, the surface of which is coated with a nano-aluminum oxide (Al2O3) layer to form a SiC@Al2O3 core-shell structure; High purity mullite (Al2O3-SiO2) 25-35%; Corundum powder (Al2O3≥99%) 10-15%; The total addition amount of composite rare earth oxide Y2O3+La2O3 is 1.5-2.5%; Silicon powder 1-2% and boron nitride (BN) 0.5-1%; Modified silica sol and aluminum phosphate composite binder 4-6%; The particle size distribution of the silicon carbide particles is coarse particles: medium particles: fine particles = 3:4:

3.

2. The silica-molybdenum brick for rotary kiln according to claim 1, wherein: The coating method of the nano-aluminum oxide layer is a sol-gel method, the coating layer thickness is 50-200nm, and the addition amount of the nano-aluminum oxide is 2-3% of the total mass of the silicon carbide.

3. The silica-molybdenum brick for rotary kiln according to claim 1, characterized in that: The mass ratio of Y2O3 to La2O3 in the composite rare earth oxide is 1:1 to 2:

1.

4. The silica-molybdenum brick for rotary kiln according to claim 1, characterized in that: The silica brick has an in-situ generated β-Si3N4 fiber network inside, with a fiber diameter of 0.1-1 μm and a length of 5-20 μm.

5. A process for producing silica-mud bricks for rotary kiln according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Raw material pretreatment: The silicon carbide particles are coated with nano-aluminum oxide by a sol-gel method and dried for later use; Pre-sinter the mullite and corundum powder at 600-800℃ for 2 hours; (2) Mixing and molding: The pretreated raw materials were mixed with composite rare earth oxides, silicon powder, boron nitride and composite binder, and subjected to ultrasonic dispersion treatment for 30 minutes; The green body is prepared by isostatic pressing combined with high-frequency vibration; (3) Staged atmosphere sintering: Low temperature section (200-800℃): debinding in air atmosphere; Medium temperature section (800-1350℃): nitrogen atmosphere; High temperature section (1350-1480℃): weak reducing atmosphere, with CO / H2 volume ratio of 9:1, keeping warm for 2-4 hours; (4) Microwave assisted sintering: Microwave heating is used in the high temperature section, with a frequency of 2.45 GHz and a power density of 5-10 W / cm 3 .

6. The production process of silica-molybdenum bricks for rotary kiln according to claim 5, characterized in that: The ultrasonic frequency in step (2) is 20-40 kHz, and the power density is 0.5-1.5 W / cm 3 .

7. The production process of silica-molybdenum bricks for rotary kiln according to claim 4, characterized in that: During the high-temperature sintering process, the CO / H2 ratio and the holding time are controlled to allow silicon powder to react with boron nitride to generate β-Si3N4 fibers.

8. The process for producing silica-mud bricks for rotary kiln according to claim 5, wherein: After the green body is formed, a gradient pore structure is constructed using 3D printing technology, with a pore size range of 50-200 μm and a porosity gradient of 5-8% on the surface and 10-15% in the interior.

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