Alumina-based corundum-mullite hollow sphere with gradient structure and preparation method thereof

Abstract

The application relates to a bauxite-based corundum mullite hollow sphere with a gradient structure and a preparation method thereof, and belongs to the field of heat-insulating refractory materials. The preparation method comprises the following steps: firstly, high-alumina bauxite is beneficiated and classified and homogenized, then, according to a certain mass percentage, III-grade high-alumina bauxite, silica, mixed additives and an organic binder are uniformly mixed and granulated; taking spherical particles as cores, II-grade high-alumina bauxite and I-grade high-alumina bauxite are sequentially wrapped under the rotation of a granulator to obtain a non-homogeneous spherical particle blank body with a SiO2-rich interior; and finally, the corundum mullite hollow sphere with the gradient structure is obtained through high-temperature calcination. The refractory hollow sphere is designed with the gradient structure through the combined process of high-alumina bauxite classification and homogenization, coating forming and high-temperature calcination in-situ pore forming, the phase performance characteristics of corundum and mullite are fully utilized, and the refractory hollow sphere with the low thermal conductivity, the good corrosion resistance, the high use temperature and the excellent thermal shock resistance is developed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of heat-insulating refractory materials, and particularly relates to alumina-based corundum mullite hollow spheres with a gradient structure and a preparation method thereof. BACKGROUND

[0002] Refractory hollow spheres have the characteristics of low thermal conductivity, high softening temperature under load, good corrosion resistance, excellent high-temperature mechanical properties, etc. They can not only be used as a heat-insulating layer, but also can be directly contacted with a flame as a high-temperature layer, and are one of the most promising high-performance heat-insulating refractory raw materials. At present, the method for industrialized preparation of refractory hollow spheres is relatively single, mainly using a hollow sphere blowing method, that is, high-purity oxides such as Al2O3 and ZrO2 are fused into a molten liquid at a temperature above 2000 ℃, and then the molten liquid is blown by compressed air to obtain the hollow spheres. The hollow spheres obtained by this method have a high service temperature, but the shell structure is uniform and dense, and the thermal shock resistance is poor. At the same time, the production energy consumption is high, and the raw material cost is high, which further limits its large-scale application in high-temperature industries. With the gradual deepening of the demand for energy saving and carbon reduction in high-temperature industries, it is urgent to develop refractory hollow spheres with excellent high-temperature performance, which can meet the requirements of industrialized preparation and have more abundant raw material sources.

[0003] The reserves of high-alumina bauxite resources in China rank in the forefront of the country, but the development and application are still uneven. Among them, high-grade I high-alumina bauxite ore is in short supply, while the reserves of medium and low-grade ores (II and III grade materials) are more abundant, but the development is insufficient and the resource utilization rate is low. In alumina-based refractory raw materials, I high-alumina bauxite is in the corundum phase after high-temperature calcination, has the characteristics of high-temperature resistance and good corrosion resistance; II and III high-alumina bauxite clinker is mainly mullite-high silica glass phase, which has slightly lower refractoriness, poorer wear resistance, but low thermal conductivity, low thermal expansion coefficient and good high-temperature mechanical properties. In recent years, it has been recognized in the industry to develop high-performance, high-value-added heat-insulating refractory raw materials based on the advantages of high-alumina bauxite resources. The publication number CN117964353A discloses a low-grade high-alumina bauxite as the main raw material, and the performance of the porous material is optimized by modifying SiO2 to successfully prepare a low-thermal-conductivity alumina-based mullite synthetic material. The publication number CN10643143A discloses a high-strength, closed-pore mullite material prepared from high-alumina bauxite and silicon dioxide as the main raw materials. The refractory heat-insulating raw material synthesized from medium and low-grade high-alumina bauxite is mainly mullite phase, has the advantages of low thermal conductivity and good mechanical properties, but it is difficult to balance the high-temperature resistance and corrosion resistance performance, and is only suitable for heat-insulating layers and low-temperature working layers. Therefore, it is necessary to explore a new preparation method to design the gradient structure and performance of the heat-insulating refractory raw material by utilizing the characteristics of high-alumina bauxite of different grades, which can greatly improve the service temperature and application range of alumina-based heat-insulating refractory materials, and provide technical support for energy saving and carbon reduction in high-temperature industries. SUMMARY

[0004] The present application aims at overcoming the deficiencies in the prior art, and provides a bauxite-based corundum mullite hollow sphere with a gradient structure and a preparation method thereof.

[0005] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0006] A preparation method of a bauxite-based corundum mullite hollow sphere with a gradient structure comprises the following steps:

[0007] Firstly, high-alumina bauxite is beneficiated and graded to obtain I, II and III grade high-alumina bauxite fine powder;

[0008] Secondly, the III grade high-alumina bauxite, the silica and the mixed additive are mixed according to the mass percentage of 45-65%, 33-53% and 1-4% respectively, and are ground into fine powder with a particle size of ≤600 mesh, and then the mixed raw materials are mixed uniformly with the organic binder with a percentage of 2-4wt% to obtain mixed raw materials;

[0009] Thirdly, the mixed raw materials are granulated in a disc granulator to obtain spherical particles with a particle size of 0.2-0.9mm;

[0010] Fourthly, the spherical particles are used as cores, and the II grade high-alumina bauxite is wrapped around the cores under the rotation of the granulator, and the I grade high-alumina bauxite is wrapped around the outermost layer to obtain a non-homogeneous spherical particle blank with a SiO2-rich inner part;

[0011] Fifthly, the non-homogeneous spherical particle blank is dried sufficiently and is placed in a high-temperature rotary kiln for calcination at 1550-1650℃ for 1-3h to obtain a corundum mullite hollow sphere with a gradient structure.

[0012] Further, the particle size of the high-alumina bauxite fine powder in the first step is ≤200 mesh; the Al2O3 content of the III grade high-alumina bauxite is 42-52wt%, and the SiO2 content is 32-45wt%; the Al2O3 content of the II grade high-alumina bauxite is 53-68wt%, and the SiO2 content is 15-31wt%; and the Al2O3 content of the I grade high-alumina bauxite is ≥69wt%, and the SiO2 content is ≤15wt%.

[0013] Further, the SiO2 content of the silica in the second step is ≥99.56wt%, and the particle size of the silica raw material is ≤180 mesh.

[0014] Further, the second step mixing additive is one of lanthanum cerium carbonate, cerium carbonate and lanthanum carbonate mixed with SiC, and the particle size of the mixed additive is ≤200 mesh.

[0015] Further, the mass ratio of lanthanum cerium carbonate, cerium carbonate or lanthanum carbonate to SiC in the mixed additive is 1-10:1.

[0016] Further, the organic binder in the second step is one of polyvinyl alcohol, dextrin and corn starch.

[0017] Further, in the fourth step, the mass of the second grade bauxite is 25-35% of the spherical particles, and the mass of the first grade bauxite is 10-15% of the spherical particles.

[0018] Further, the bauxite-based corundum mullite hollow sphere prepared by the preparation method is formed in situ during the heating and calcination process, and the phase of the hollow sphere shell has a gradient distribution structure, the outer shell is mainly corundum phase, and the mullite phase gradually increases from the surface of the shell inward.

[0019] Further, the corundum phase on the surface of the shell of the bauxite-based corundum mullite hollow sphere accounts for 70-90%.

[0020] Further, the volume density of the bauxite-based corundum mullite hollow sphere is 1.0-1.6 g / cm 3 , and the average shell thickness is 0.1-0.2 mm.

[0021] Further, the thermal conductivity of the bauxite-based corundum mullite hollow sphere is 0.3-1.0 W / (m·K), and the use temperature is 1550-1650℃.

[0022] Compared with the prior art, the bauxite-based corundum mullite hollow sphere has the following beneficial effects:

[0023] 1. The bauxite-based corundum mullite hollow sphere is prepared by using the in-situ diffusion reaction of the prepared non-homogeneous spherical particle blank during the calcination process, and the hierarchical homogenization-coating forming-high temperature calcination in-situ pore forming process is adopted to prepare the refractory hollow sphere, so that the pore forming efficiency is high, the energy consumption is low, and the scale production requirements can be met.

[0024] 2. The bauxite-based corundum mullite hollow sphere is designed based on the characteristics of different grades of bauxite, the outer layer of the hollow sphere shell is mainly corundum phase, which ensures the corrosion resistance and high temperature resistance of the refractory raw material, and the inner layer of the hollow sphere shell is cross-linked columnar mullite, which improves the mechanical properties and high temperature volume stability of the refractory hollow sphere.

[0025] 3. The hollow sphere layer crystal morphology formed by the bauxite-based corundum mullite hollow sphere can deflect cracks inside the hollow sphere, hinder the expansion of large cracks, and improve the thermal shock resistance of the refractory hollow sphere. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 Microstructure of the section of the bauxite-based corundum-mullite hollow sphere with gradient structure prepared in Example 1 of the present application.

[0027] Figure 2 Microstructure and energy spectrum element distribution of the section of the bauxite-based corundum-mullite hollow sphere with gradient structure prepared in Example 2 of the present application after resin curing. DETAILED DESCRIPTION

[0028] The technical solutions and effects of the present application will be further described below in combination with the drawings and specific examples, but the protection scope of the present application is not limited thereto. Example 1

[0029] The present example provides a preparation method of a bauxite-based corundum-mullite hollow sphere with gradient structure, comprising the following steps:

[0030] The high-alumina bauxite is beneficiated, graded and homogenized to obtain I, II and III grade high-alumina bauxite fine powder; the III grade high-alumina bauxite, the silica and the mixed additive are proportioned according to the mass percentage of 60wt% of III grade high-alumina bauxite, 38wt% of silica and 2wt% of mixed additive, and are co-ground into fine powder with particle size of ≤600 mesh, and the above-mentioned raw materials are mixed uniformly with 4wt% of corn starch to obtain mixed raw materials;

[0031] The above-mentioned mixed raw materials are granulated in a disc granulator to obtain spherical particles with particle size of 0.2-0.6mm;

[0032] The above-mentioned spherical particles are used as cores, and are wrapped with II grade high-alumina bauxite based on 35wt% of the spherical particles under the rotation of the granulator, and the outermost layer is wrapped with I grade high-alumina bauxite with 10wt% to obtain a non-homogeneous spherical particle blank with SiO2-rich inside;

[0033] The above-mentioned non-homogeneous spherical particle blank is dried sufficiently, is placed in a high-temperature rotary kiln, is calcined at 1600℃ for 3h, and finally a corundum-mullite hollow sphere with gradient structure is obtained.

[0034] The high-alumina bauxite in the present example is graded, homogenized and finely ground, and the particle size of the raw materials is ≤200 mesh, wherein the Al2O3 content of the III grade high-alumina bauxite is 45wt%, the SiO2 content is 39wt%; the Al2O3 content of the II grade high-alumina bauxite is 55wt%, the SiO2 content is 30wt%; the Al2O3 content of the I grade high-alumina bauxite is 78wt%, and the SiO2 content is 8wt%. The SiO2 content of the silica is 99.65wt%, and the particle size of the raw materials is ≤180 mesh. The mixed additive is lanthanum cerium carbonate and SiC, and the mass ratio of lanthanum cerium carbonate to SiC is 3:1, and the particle size of the raw materials is ≤200 mesh.

[0035] The technical specifications of the bauxite-based corundum mullite hollow spheres prepared in this embodiment are as follows: the bulk density of the hollow spheres is 1.25 g / cm³. 3 The average shell thickness is 0.15 mm. The corundum phase content on the surface of the hollow sphere is 75.5%, while the interior of the shell is composed of mullite-high silica glass phase. The thermal conductivity of the hollow sphere is 0.55 W / (m·K), and the operating temperature is ≥1600℃. (See attached image) Figure 1 The microstructure shown is obtained according to the steps in Example 1. (See attached image.) Figure 1 As can be seen, the crystal morphology and phase composition of the hollow sphere shell are distributed in a gradient: the inner layer of the spherical shell is composed of interlocking columnar mullite, the middle layer of the shell is a corundum-mullite composite structure, and the surface of the hollow sphere shell is mainly composed of granular and platy corundum. Example 2

[0036] This embodiment provides a method for preparing bauxite-based corundum mullite hollow spheres with a gradient structure, including the following steps:

[0037] High-alumina bauxite is beneficiated, graded and homogenized to obtain grade I, II and III high-alumina bauxite fine powder; grade III high-alumina bauxite, grade III high-alumina bauxite, grade II, grade III high-alumina bauxite, grade III high-alumina bauxite, grade III silica, grade III mixed additives are mixed and ground into fine powder of ≤600 mesh, and dextrin of grade III high-alumina bauxite, grade III, grade III, grade IV, grade III, is added and mixed evenly to obtain mixed raw material.

[0038] The above-mentioned mixed raw materials are granulated in a disc granulator to obtain spherical particles with a particle size of 0.3~0.8mm;

[0039] Using the above spherical particles as the core, the pellets are wrapped with 25 wt% of Grade II high-alumina bauxite based on the spherical particles under the rotation of the pelletizer, and the outermost layer is wrapped with 15 wt% of Grade I high-alumina bauxite, to obtain a heterogeneous spherical particle green body rich in SiO2 inside.

[0040] The above-mentioned heterogeneous spherical particle blanks were thoroughly dried and placed in a high-temperature rotary kiln. After being calcined at 1650℃ for 2 hours, hollow corundum mullite spheres with a gradient structure were finally obtained.

[0041] The high-alumina bauxite described in this embodiment, after grading, homogenization, and fine grinding, has a raw material particle size ≤200 mesh. Specifically, Grade III high-alumina bauxite contains 43wt% Al2O3 and 41wt% SiO2; Grade II high-alumina bauxite contains 60wt% Al2O3 and 26wt% SiO2; and Grade I high-alumina bauxite contains 85wt% Al2O3 and 5.5wt% SiO2. The silica contains 99.70wt% SiO2 and has a raw material particle size ≤180 mesh. The mixed additives are cerium carbonate and SiC, with a mass ratio of cerium carbonate:SiC = 3.5:0.5, and a raw material particle size ≤200 mesh.

[0042] The technical index of the bauxite-based corundum mullite hollow sphere prepared in this embodiment is that the volume density of the hollow sphere is 1.35 g / cm 3 , the average shell thickness is 0.20 mm, the content of corundum phase in the shell surface of the hollow sphere is 80.5%, the inside of the shell is mullite-high silica glass phase, the thermal conductivity of the hollow sphere is 0.63 W / (m·K), and the use temperature is ≥1600℃. The microstructure and energy spectrum shown in the attached Figure 2 are prepared according to the steps of this embodiment 2. It can be seen from the attached Figure 2 that the elements in the shell of the hollow sphere are distributed in a gradient, and the SiO2 element gradually increases from the shell surface to the inside. Embodiment 3

[0043] The embodiment provides a preparation method of a bauxite-based corundum mullite hollow sphere with a gradient structure, comprising the following steps:

[0044] The high-alumina bauxite is beneficiated, graded and homogenized to obtain I, II and III grade high-alumina bauxite fine powder; the raw materials are mixed according to the mass percentage of 45wt% of III grade high-alumina bauxite, 53wt% of silica and 2wt% of mixed additives, and are ground into fine powder with a particle size of ≤600 mesh; and 3wt% of polyvinyl alcohol is added to the above-mentioned raw material percentage to obtain mixed raw materials.

[0045] The above-mentioned mixed raw materials are granulated in a disc granulator to obtain spherical particles with a particle size of 0.4-0.9 mm;

[0046] The above-mentioned spherical particles are used as cores, and are wrapped with 25wt% of II grade high-alumina bauxite based on the spherical particles under the rotation of the granulator, and the outermost layer is wrapped with 15wt% of I grade high-alumina bauxite to obtain a non-homogeneous spherical particle blank with SiO2-rich inside;

[0047] The above-mentioned non-homogeneous spherical particle blank is sufficiently dried and placed in a high-temperature rotary kiln, and is calcined at 1550℃ for 3h to finally obtain a corundum mullite hollow sphere with a gradient structure.

[0048] The high-alumina bauxite is graded, homogenized and finely ground, and the particle size of the raw materials is ≤200 mesh, wherein the Al2O3 content of the III grade high-alumina bauxite is 42wt%, and the SiO2 content is 43wt%; the Al2O3 content of the II grade high-alumina bauxite is 55wt%, and the SiO2 content is 30wt%; and the Al2O3 content of the I grade high-alumina bauxite is 75wt%, and the SiO2 content is 10wt%. The SiO2 content of the silica is 99.70wt%, and the particle size of the raw materials is ≤180 mesh. The mixed additives are lanthanum carbonate and SiC, and the mass ratio of lanthanum carbonate to SiC is 3.5:0.5, and the particle size of the raw materials is ≤200 mesh.

[0049] The technical index of the bauxite-based corundum mullite hollow sphere prepared in the embodiment is as follows: the volume density of the hollow sphere is 1.10 g / cm 3 , the average shell thickness is 0.15 mm, the corundum phase content of the hollow sphere shell is 70.0%, and the inside of the sphere shell is mullite-high silica glass phase. The thermal conductivity of the hollow sphere is 0.40 W / (m·K), and the use temperature is ≥ 1550 ℃.

[0050] While the embodiments of the application have been illustrated and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the spirit and scope of the application, which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing alumina-based corundum mullite hollow spheres with a gradient structure, characterized in that, The preparation method comprises the following steps: Firstly, high bauxite is beneficiated and classified and homogenized to obtain I, II and III grade high bauxite fine powder; Secondly, the III grade high bauxite, the silica and the mixed additive are mixed according to the mass percentage of 45-65%, 33-53% and 1-4% respectively, and are co-ground into fine powder with a particle size of ≤600, and then the fine powder is mixed with the organic binder with a percentage of 2-4wt% to obtain mixed raw materials; Thirdly, the mixed raw materials are granulated in a disc granulator to obtain spherical particles with a particle size of 0.2-0.9mm; Fourthly, the spherical particles are used as cores, and the II grade high bauxite is wrapped under the rotation of the granulator, and the I grade high bauxite is wrapped as the outermost layer to obtain a non-homogeneous spherical particle blank with a SiO2-rich inner part; Fifthly, the non-homogeneous spherical particle blank is dried and is calcined in a high-temperature rotary kiln at 1550-1650℃ for 1-3h to obtain corundum mullite hollow spheres with a gradient structure; The high bauxite fine powder in the first step has a particle size of ≤200; The III grade high bauxite contains 42-52wt% of Al2O3 and 32-45wt% of SiO2; The II grade high bauxite contains 53-68wt% of Al2O3 and 15-31wt% of SiO2; The I grade high bauxite contains ≥69wt% of Al2O3 and ≤15wt% of SiO2; The mixed additive in the second step is a mixture of one of lanthanum cerium carbonate, cerium carbonate and lanthanum carbonate and SiC, and has a particle size of ≤200; In the fourth step, the II grade high bauxite has a mass of 25-35% of the spherical particles, and the I grade high bauxite has a mass of 10-15% of the spherical particles; The bauxite-based corundum mullite hollow spheres prepared by the preparation method are formed in situ during the heating and calcining process, the phase of the hollow sphere shell has a gradient distribution structure, the inner part of the shell is mullite-high silica glass phase, the inner layer of the shell is cross-linked columnar mullite, the middle layer of the shell is corundum mullite composite structure, the outer shell is mainly corundum phase, and the mullite phase gradually increases from the surface of the shell to the inside; The corundum phase accounts for 70-90% of the surface of the shell of the bauxite-based corundum mullite hollow spheres; The volume density of the bauxite-based corundum mullite hollow sphere is 1.0-1.6 g / cm 3 The average shell thickness is 0.1-0.2 mm. The thermal conductivity of the bauxite-based corundum mullite hollow spheres is 0.3-1.0W / (m·K), and the use temperature is 1550-1650℃.

2. The method of claim 1, wherein the alumina-based corundum mullite hollow sphere having a gradient structure is prepared by the steps of: preparing a first slurry by mixing alumina, mullite, and a binder; preparing a second slurry by mixing alumina, mullite, and a binder; and mixing the first slurry and the second slurry. The mass ratio of lanthanum cerium carbonate, cerium carbonate or lanthanum carbonate to SiC in the mixed additive is 1-10:

1.

3. The method of claim 1, wherein the alumina-based corundum mullite hollow sphere having a gradient structure is prepared by the steps of: preparing a first slurry by mixing alumina, mullite, and a binder; preparing a second slurry by mixing alumina, mullite, and a binder; and mixing the first slurry and the second slurry. The organic binder in the second step is one of polyvinyl alcohol, dextrin and corn starch.

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