Method for preparing mullite porous ceramics by adding fluorosilicic acid urea

Abstract

This invention discloses a method for preparing mullite porous ceramics by adding urea fluorosilicate. The preparation method includes the following steps: accurately weighing the following basic raw materials by mass fraction: kaolin clinker and alumina micro powder; slowly adding urea solution dropwise to fluorosilicic acid solution while continuously stirring throughout the process, and obtaining urea fluorosilicate solution after the system is homogeneous; adding urea fluorosilicate solution, kyanite, mullite whiskers and water to the basic raw materials, mixing evenly and pouring into a mold; placing the mold in an oven at 60~90℃ for sealing and curing for 2~6 hours; after molding, demolding and drying for 24~48 hours, and then holding at 1450℃~1600℃ for 2~4 hours, a mullite porous ceramic with a through-pore structure, high porosity, low thermal conductivity, good flexural strength and dimensional stability is obtained.

Description

Technical Field

[0001] This invention relates to the field of porous ceramics, and more particularly to a method for preparing mullite porous ceramics by adding urea fluorosilicate. Background Technology

[0002] Porous ceramics are a widely used material in the insulation structures of industrial kilns. Industrial kilns are common equipment in heavy industries such as metallurgy, construction, and chemicals. Their annual energy consumption accounts for more than 25% of my country's total industrial energy consumption. Among them, heat loss, which is highly related to the performance of insulation materials, directly affects energy efficiency.

[0003] Mullite porous ceramics, as a lightweight thermal insulation material with mullite as the main crystalline phase, are highly favored in the selection of kiln insulation materials due to their high melting point, low thermal conductivity, low expansion coefficient, excellent creep resistance, outstanding chemical stability and high strength at high temperatures.

[0004] The methods for preparing high-quality porous refractory materials have diversified with industrial development. Currently, the core contradiction in the preparation process of mullite porous ceramics is that it is difficult to balance porosity and mechanical strength. High porosity formulations tend to lead to a loose material skeleton and insufficient flexural strength, while the means of enhancing strength often sacrifice pore connectivity. At the same time, some processes are prone to problems such as excessive volume shrinkage and cracking and deformation of the green body during sintering, which makes it difficult to meet the dimensional accuracy requirements of precision high-temperature components. In addition, some traditional preparation methods also have defects such as toxic substances remaining in pore-forming agents, low raw material utilization, high production energy consumption or high environmental protection costs, which restrict their industrial application. The starch curing method selected in this patent has significant advantages: the natural starch raw material used has the dual functions of binder and pore-forming agent, eliminating the need for additional binder components, simplifying the formulation system and process steps, and reducing production costs; by adjusting the amount, particle size and type of starch added, the porosity and pore size distribution of ceramics can be precisely controlled to meet the performance requirements of different application scenarios; and after high-temperature combustion of starch, only CO2 and H2O are generated, with no toxic gases or impurities remaining, meeting the requirements of green and environmentally friendly production.

[0005] The paper "A Method for Preparing Highly Oriented Tubular Through-Porous Ceramics" (CN101597177A) discloses a method for preparing porous ceramics. Its advantage is the formation of long-range ordered tubular through-pores, achieving directional control of the pore structure. However, its disadvantage is the use of PVC as a binder, which decomposes during high-temperature processing, releasing toxic substances such as HCl and even dioxins. This not only requires additional exhaust gas treatment equipment but also poses environmental emission risks. The paper "A Method for Preparing Porous SiOC Ceramics and Its Application in Lithium-ion Battery Anode Materials" (CN114057488A) discloses a ceramic forming method using starch as a template, but the volatile organic solvents used still pose environmental and health risks. CN116003158B discloses a method for preparing mullite porous ceramics using lithium slag, which requires extremely high process control, even proposing a heating rate of 0.1℃ / min~1℃ / min to 600℃. This method has low production efficiency and is difficult to adapt to large-scale industrial applications. CN111302773A discloses a low-cost, easily industrialized method for preparing mullite porous ceramics, but its product has an apparent porosity of only 32%~36%, far lower than samples prepared by the starch solidification method. A porosity of 40%~60% can usually be achieved by adjusting the amount of starch added. CN108083789A discloses a method for preparing mullite porous ceramics by manual tamping and molding. This molding method cannot produce complex-shaped products and can only produce simple block products. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing mullite porous ceramics by adding urea fluorosilicate.

[0007] The innovation of this invention lies in the introduction of urea fluorosilicate as an additive in the slurry. The urea fluorosilicate solution itself is acidic. When added to the slurry during the mixing stage, it adjusts the pH value of the slurry. The acidic conditions affect the dispersibility of the ceramic slurry through interaction with the charges on the surface of the ceramic powder, while simultaneously improving its sintering reactivity. During the curing stage, it advances starch gelatinization, resulting in more uniform curing. During the high-temperature sintering stage, the HF produced by its decomposition acts as a mineralizer, promoting the formation of columnar and acicular mullite, improving mechanical strength, and forming a significant bimodal pore structure with the pores formed by starch removal.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] (1) Accurately weigh the following basic raw materials by mass fraction: 50~60wt% kaolin clinker, 40~50wt% alumina powder; (2) The urea solution is slowly added dropwise to the fluorosilicic acid solution, and the mixture is stirred continuously throughout the process. After the system is homogeneous, the fluorosilicic acid urea solution is obtained. (3) Add urea fluorosilicate solution, kyanite, mullite whiskers and water to the basic raw materials. The mass of urea fluorosilicate solution is 1~15wt% of the basic raw materials, the mass of potato starch is 5~15wt% of the basic raw materials, the mass of kyanite is 4~10wt% of the basic raw materials, the mass of mullite whiskers is 1~4wt% of the basic raw materials, and the mass of water is 30~40wt% of the basic raw materials. Mix evenly and pour into the mold. (4) Place the mold in an oven at 60~90℃ and seal it for 2~6 hours; after molding, demolding and drying for 24~48 hours, keep it at 1450℃~1600℃ for 2~4 hours to obtain the finished product.

[0010] Furthermore, the particle size of the kaolin clinker in the basic raw materials is all less than 125µm.

[0011] Furthermore, the particle size of the alumina micro powder in the basic raw materials is all less than 105µm.

[0012] Furthermore, the kaolin clinker in the basic raw materials has an Al2O3 content ≥48wt% and a SiO2 content ≥48wt%.

[0013] Furthermore, the α-Al2O3 content of the alumina micro powder in the basic raw material is ≥99wt%.

[0014] The concentration of the fluorosilicic acid solution is 30-35%, and the concentration of the urea solution is 35-40%; the molar ratio of urea to fluorosilicic acid is 4:1-1.5.

[0015] The amount of the fluorosilicone urea solution added is 1 to 15 wt% of the total mass of the base raw materials, preferably 3 to 8 wt%.

[0016] Furthermore, the potato starch particles are all less than 180µm in size.

[0017] Furthermore, the kyanite grain size is all less than 420µm.

[0018] Furthermore, the aspect ratio of the mullite whiskers in the additive is 2.5 to 13.2.

[0019] Furthermore, the molding method described in step five is in-situ starch curing molding, with a curing temperature of 60~90℃.

[0020] The beneficial effects of this invention are: Introducing fluorosilane as a multifunctional synergistic agent into ceramic slurry: 1. fluorosilane slowly releases H in aqueous solution. +1. The urea fluorosilicate effectively controls the pH value of the slurry, and in the curing stage, it works with potato starch to complete in-situ gelatinization and cross-linking, which not only maintains the uniformity of the green body but also significantly reduces the risk of drying cracking. 2. Urea fluorosilicate begins to decompose under heat during the curing stage and continues to release gas during the subsequent sintering heating process, forming micron / submicron-sized pores. At the same time, potato starch burns away during sintering, leaving large pores. The combination of these two types of pores constructs a through-hole porous structure with a bimodal pore size distribution. 3. The HF produced by the decomposition of urea fluorosilicate reacts with ceramic powder to generate fluorides such as aluminum fluoride (AlF3), which act as a highly efficient mineralizer to promote the formation of needle-like mullite and produce a synergistic reinforcing effect with the added mullite whiskers, significantly improving mechanical strength.

[0021] In addition, the high-temperature phase transformation expansion of kyanite is used to offset sintering shrinkage, further improving dimensional stability.

[0022] This invention successfully balances the strength and thermal insulation properties of porous ceramics while taking into account both high porosity and excellent thermal insulation performance, resulting in products with excellent mechanical properties and dimensional stability. Attached Figure Description

[0023] Figure 1 This is a morphological feature diagram of the product obtained in Example 1. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below: Example 1 Accurately weigh the following basic raw materials by mass fraction: 56.68 wt% kaolin clinker and 43.32 wt% alumina micro powder; according to the molar ratio of fluorosilicic acid to urea of ​​1:4, slowly add 788.3 g of urea solution to 500 ml of fluorosilicic acid solution, stirring continuously throughout the process, and after the system is homogeneous, fluorosilicic acid urea solution is obtained. A fluorosilane urea solution, kyanite, mullite whiskers, and water are added to the basic raw materials. The mass of the fluorosilane urea solution is 3 wt% of the basic raw materials, the mass of potato starch is 25 wt% of the basic raw materials, the mass of kyanite is 10 wt% of the basic raw materials, the mass of mullite whiskers is 3 wt% of the basic raw materials, and the mass of water is 30 wt% of the basic raw materials. After mixing evenly, the mixture is poured into a mold. The mold is placed in an 80℃ oven and sealed and cured for 2 hours. After molding, demolding, and drying for 24 hours, the product is kept at 1500℃ for 3 hours to obtain the finished product.

[0025] The bulk density of the finished product is 1.25~1.28 g / cm³. 3 The flexural strength is 7.81~8.22 MPa, the apparent porosity is 57.32~58.25%, the vertical dimensional shrinkage is 0.92~0.95%, and the thermal conductivity is 0.33~0.36 W / (m·K).

[0026] Example 2 The method is the same as in Example 1, except that the mass fraction of the fluorosilicic acid urea solution is 8 wt%.

[0027] The bulk density of the finished product is 1.19~1.21 g / cm³. 3 The flexural strength is 8.71~9.32 MPa, the apparent porosity is 60.23~61.07%, the vertical dimensional shrinkage is 0.85~0.90%, and the thermal conductivity is 0.30~0.32 W / (m·K).

[0028] SEM images reveal numerous micron-sized macropores formed by the removal of potato starch granules within the ceramic matrix. These interconnected pores provide the material with high porosity and excellent permeability. Simultaneously, the addition of urea fluorosilicate promotes the growth of mullite columnar and needle-like grains, resulting in a highly compact pore wall framework with tight intergranular bonding and no obvious loosening or collapse. Furthermore, submicron-sized micropores, created by the decomposition and release of gases during the curing and sintering process of urea fluorosilicate, exist within the pore walls, ultimately forming a hierarchical porous structure. This structure retains the high porosity advantage of starch-based pore formation while enhancing the mechanical strength of the pore wall framework through the liquid-phase sintering and whisker-promoting effects of urea fluorosilicate, achieving a synergistic optimization of high porosity and structural stability in porous ceramics.

[0029] Example 3 The method is the same as in Example 1, except that the mass fraction of the fluorosilicic acid urea solution is 15 wt%.

[0030] The bulk density of the finished product is 1.13~1.16 g / cm³. 3 The flexural strength is 7.44~7.69 MPa, the apparent porosity is 61.23~62.09%, the vertical dimensional shrinkage is 0.79~0.83%, and the thermal conductivity is 0.28~0.31 W / (m·K).

[0031] Example 4 The method is the same as in Example 1, except that the mass fraction of the fluorosilicic acid urea solution is 0 wt%.

[0032] The bulk density of the finished product is 1.10~1.12 g / cm³. 3 The flexural strength is 5.03~5.63 MPa, the apparent porosity is 62.55~63.31%, the vertical dimensional shrinkage is 0.73~0.78%, and the thermal conductivity is 0.35~0.38 W / (m·K).

[0033] The described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing mullite porous ceramics by adding urea fluorosilicate, characterized in that, Includes the following steps: Step 1: Prepare basic raw materials, which include aluminum source and silicon source; Step 2: Slowly add urea solution dropwise to fluorosilicic acid solution, stirring continuously throughout the process. Once the system is homogeneous, a fluorosilicic acid urea solution is obtained. Step 3: Mix the basic raw materials with urea fluorosilicate solution, pore-forming agent, volume expansion agent, reinforcing phase and water to obtain ceramic slurry; Step 4: Inject the ceramic slurry into the mold and solidify it in situ to obtain a ceramic green body; Step 5: After demolding and drying the ceramic green body, it is sintered at high temperature to obtain the mullite porous ceramic.

2. The preparation method according to claim 1, characterized in that, The basic raw materials include 50-60 wt% kaolin clinker and 40-50 wt% alumina micro powder. The kaolin clinker has a particle size of less than 125 µm, with an Al2O3 content of ≥48 wt% and a SiO2 content of ≥48 wt%. The alumina micro powder has a particle size of less than 105 µm, with an α-Al2O3 content of ≥99 wt%.

3. The preparation method according to claim 1, characterized in that, The concentration of the fluorosilicic acid solution is 30-35%, and the concentration of the urea solution is 35-40%; the molar ratio of urea to fluorosilicic acid is 4:1-1.

5.

4. The preparation method according to claim 1, characterized in that, The amount of the fluorosilicone urea solution added is 1 to 15 wt% of the total mass of the base raw materials, preferably 3 to 8 wt%.

5. The preparation method according to claim 1, characterized in that, The pore-forming agent is potato starch, and its addition amount is 5-15 wt% of the total mass of the base raw materials; The volume expander is kyanite, and its addition amount is 4-10 wt% of the total mass of the base raw materials; The reinforcing phase is mullite whiskers, and its addition amount is 1-4 wt% of the total mass of the base raw materials; The amount of water added is 30-40 wt% of the total mass of the base raw materials.

6. The preparation method according to claim 1, characterized in that, The potato starch has a particle size of less than 180µm; the kyanite has a particle size of less than 420µm; and the mullite whiskers have an aspect ratio of 2.5 to 13.

2.

7. The preparation method according to claim 1, characterized in that, The in-situ curing molding is starch in-situ curing molding, with a curing temperature of 60~90℃ and a curing time of 2~6 hours.

8. The preparation method according to claim 1, characterized in that, The high-temperature sintering temperature is 1450℃~1600℃, and the holding time is 2~4 hours.

9. The preparation method according to claim 1, characterized in that, The drying time for the ceramic green body is 24 to 48 hours.

10. A mullite porous ceramic, characterized in that, It is prepared by any one of claims 1 to 9.

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