A sheet-like alumina composite material, its preparation method and application

Abstract

This invention provides a sheet-like alumina composite material, its preparation method, and its application. The preparation method includes the following steps: (1) mixing high-alumina fly ash with a fluoride salt solution, subjecting the mixture to a first reaction, followed by solid-liquid separation and drying to obtain a first solid phase; (2) calcining the first solid phase to obtain the sheet-like alumina composite material. The process can also simultaneously recover by-products. The ammonia-containing gas generated in the first reaction is absorbed to form an ammonia solution, and the silicon-containing gas generated during calcination is captured to obtain a silicon-containing liquid. The ammonia solution and the silicon-containing liquid are mixed to carry out a second reaction, and the solid phase is dried after liquid-solid separation to co-produce silica. This invention uses inexpensive and readily available raw materials and a simple process to simultaneously prepare sheet-like alumina composite materials with an alumina content ≥80wt% and nano-sized silica, opening up innovative avenues for the high-value utilization of fly ash.

Description

Technical Field

[0001] This invention relates to the field of fly ash resource utilization technology, and in particular to a sheet-like alumina composite material, its preparation method and application. Background Technology

[0002] Fly ash, the fly ash produced after coal combustion, currently has a comprehensive utilization rate of only about 70%. Large-scale stockpiling of fly ash occupies significant amounts of land and easily pollutes the atmosphere, water resources, and soil resources, necessitating the development of new comprehensive utilization methods. High-alumina fly ash has a significantly higher Al2O3 content than ordinary fly ash, and is also rich in mullite (3Al2O3·2SiO2) and a small amount of corundum (α-Al2O3), demonstrating high potential for comprehensive utilization.

[0003] While traditional alumina possesses high hardness, high melting point, and excellent corrosion resistance, its inherent defects severely limit its high-end applications: its irregular particle morphology leads to insufficient packing density, easily forming numerous pores in applications; sharp particle edges cause significant stress concentration, greatly weakening the impact resistance of composite materials; large particle size differences cause unstable material flowability, easily leading to sedimentation and stratification problems during processing; considerable volume shrinkage occurs during sintering, significantly increasing the risk of product deformation; and when used as a reinforcing material, the overly isotropic nature of the particles makes it difficult to construct efficient stress transfer paths within the matrix. Especially in coatings and composite systems requiring high surface smoothness, directional thermal / electrical conductivity, or toughening reinforcement, it cannot meet the stringent requirements for morphology control and functional orientation. In contrast, the unique two-dimensional lamellar structure of lamellar alumina endows it with advantages such as low electrical conductivity, high strength, high hardness, large specific surface area, and high thermal conductivity, making it highly promising for reinforcing metal / ceramic / polymer-based composite materials.

[0004] Currently, the main technologies for alumina extraction from fly ash include four major routes: pre-desiliconization-alkaline lime sintering, acid leaching, ammonium sulfate sintering, and limestone sintering. Among these, alkaline lime sintering and limestone sintering are the most representative. Limestone sintering is limited by high phase transformation losses and high energy consumption per unit volume, hindering its industrial application. Meanwhile, the pre-desiliconization-alkaline lime sintering method, the only industrially viable route, still suffers from technical bottlenecks such as a lengthy process and high sintering heat consumption. Patent CN201010300300.3 discloses a process for preparing mullite whiskers or flaky alumina from fly ash, which involves adding specific aluminum / silicon additives and molten salt additives and reacting them at high temperatures to obtain the product. However, this method suffers from high energy consumption, environmental risks from hydrofluoric acid dissolution of impurities, and low product purity and insufficient aspect ratio. Patent CN202510655239.0 discloses a method for preparing high-purity flaky alumina from fly ash, but this method suffers from cost issues related to multi-stage acid-alkali treatment and the use of high-value reactants such as cerium dioxide. There is an urgent need to develop innovative processes for the efficient preparation of flake alumina from fly ash at low cost. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a sheet-like alumina composite material, its preparation method, and its applications. The preparation method uses a fluoride salt solution as the reaction medium to treat fly ash, removing silicon-containing components and fully converting aluminum-containing components into sheet-like alumina during calcination. This eliminates the need for costly and complex pretreatment processes, resulting in a sheet-like alumina composite material with a high alumina content. Furthermore, it can optimally co-produce silica and achieve the recycling of the fluoride salt solution, significantly improving the utilization rate of fly ash.

[0006] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a sheet-like alumina composite material, the method comprising the following steps: (1) Mix high-alumina fly ash with fluoride salt solution, and after the mixture undergoes a first reaction, it is separated into solid and liquid phases and dried to obtain the first solid phase; (2) The first solid phase is calcined to obtain a sheet-like alumina composite material.

[0007] This invention employs a fluoride salt medium system to pretreat fly ash under low-temperature reaction conditions. During this process, amorphous aluminum in the fly ash reacts with fluoride salts to form low-solubility fluoroaluminates, which remain in the solid phase and are directionally converted into lamellar alumina in the subsequent calcination reaction. The silicon component in the fly ash enters the liquid phase in the form of fluorosilicates, which, after ammoniation precipitation, can be used to prepare high-purity silica.

[0008] Preferably, the iron removal pretreatment method in step (1) includes dry magnetic separation or wet magnetic separation. Fly ash generally contains a certain amount of iron-containing phases. If these phases enter alumina products, they will seriously affect the product's color, melting point, light transmittance, and other properties. Therefore, it is necessary to remove the iron-containing phases in advance.

[0009] Preferably, the magnetic induction intensity used in the dry or wet magnetic separation is 3000~20000 Gs, for example, it can be 3000 Gs, 3500 Gs, 4000 Gs, 4500 Gs, 5000 Gs, 5500 Gs, 6000 Gs, 6500 Gs, 7000 Gs, 7500 Gs, 8000 Gs, 8500 Gs, 9000 Gs, 9500 Gs, 10000 Gs, 11000 Gs, 12000 Gs, 13000 Gs, 14000 Gs, 15000 Gs, 16000 Gs, 17000 Gs, 18000 Gs, 19000 Gs or 20000 Gs, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable. Too low a magnetic induction intensity cannot effectively remove iron, resulting in a low iron removal rate; while too high a magnetic induction intensity can achieve a higher iron removal rate, it will also entrain too much non-magnetic material, causing loss of fly ash.

[0010] Preferably, the iron removal pretreatment includes: firstly, using a magnetic induction intensity of 3000~12000Gs for initial selection 1~3 times, and then using a magnetic induction intensity of 12000~20000Gs for fine selection 1~3 times.

[0011] Preferably, the reaction temperature in step (1) is 50~150℃, for example, it can be 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃ or 150℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0012] Preferably, the reaction time is 0.5 to 10 hours, for example, it can be 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours, 9.5 hours, or 10 hours, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0013] Preferably, the fluoride solution in step (1) is an ammonium fluoride solution or / and an ammonium hydrogen fluoride solution.

[0014] Preferably, the concentration of the fluoride salt solution is 10-80 wt%, for example, it can be 10 wt%, 15 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, or 80 wt%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0015] Preferably, the volume-to-mass ratio of the fluoride salt solution to the first portion of fly ash is 3~15 ml / g, for example, it can be 3 ml / g, 4 ml / g, 5 ml / g, 6 ml / g, 7 ml / g, 8 ml / g, 9 ml / g, 10 ml / g, 11 ml / g, 12 ml / g, 13 ml / g, 14 ml / g or 15 ml / g, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0016] Preferably, the Al2O3 content in the fly ash in step (1) is ≥35wt%, for example, it can be 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, 40wt%, 42wt% or 45wt%, etc., and its main phase is whisker-like mullite and a small amount of platy corundum. The surface of the mullite is coated with a large amount of amorphous SiO2 and a small amount of amorphous Al2O3.

[0017] Preferably, the fly ash is fly ash that has undergone decarbonization pretreatment.

[0018] Preferably, the decarbonization pretreatment method includes screening and / or air separation.

[0019] Preferably, the sieve aperture size used for screening is 60-120 mesh, for example, 60 mesh, 70 mesh, 80 mesh, 90 mesh, 100 mesh, 110 mesh, or 120 mesh, but not limited to the listed values; other unlisted values ​​within this range are also applicable. Fly ash contains a small amount of large particles of unburned carbon, which can adversely affect the application of alumina products. Pre-decarbonization can remove most of the unburned carbon, avoiding its impact on alumina products, and also achieve efficient recovery of the unburned carbon, allowing it to be used as low-quality coal. Simultaneously, it can remove some large particulate impurities mixed in with the fly ash, improving product quality.

[0020] Preferably, before the drying in step (1), the first solid phase is further washed.

[0021] Preferably, the liquid-to-solid ratio of the washing process is 1~10 ml / g, for example, it can be 1 ml / g, 2 ml / g, 3 ml / g, 4 ml / g, 5 ml / g, 6 ml / g, 7 ml / g, 8 ml / g, 9 ml / g, or 10 ml / g, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The washing method is starch washing or online rinsing, with 1-3 washing stages, preferably 3-stage countercurrent washing.

[0022] Preferably, the first washing is performed 1 to 5 times, for example, 1 time, 2 times, 3 times, 4 times or 5 times.

[0023] Preferably, the drying temperature in step (1) is 80~140°C, for example, it can be 80ºC, 90ºC, 100ºC, 110ºC, 120ºC or 140ºC, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. If the drying temperature is too low, the drying efficiency is too low, the drying time is significantly prolonged, and the equipment investment is increased; if the drying temperature is too high, the fluoride salt in the first solid phase is easily decomposed, resulting in fluorine-containing compounds being carried in the drying water vapor, increasing the difficulty of processing.

[0024] Preferably, the calcination temperature is 600~1200℃, for example, it can be 600ºC, 700ºC, 750ºC, 800ºC, 850ºC, 900ºC, 950ºC, 1000ºC, 1050ºC, 1100ºC, 1150ºC, or 1200ºC, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The first solid phase contains a large amount of fluoroaluminate. If not treated, it will affect the preparation of subsequent alumina products and cause the loss of fluorine and aluminum, increasing costs. This invention calcines the first solid phase containing fluoroaluminate at a specific temperature, causing the fluoroaluminate to react with the remaining silica in the fly ash. The fluoroaluminate is converted into lamellar corundum, and the silica is converted into silicon-containing gas, which can be absorbed and utilized by an absorbent liquid. Through the roasting process, all Al2O3 in fly ash can be converted into lamellar corundum, realizing the high-value utilization of aluminum. It also reduces the fluorine content in the product to below 2%, without affecting its utilization. The roasting temperature has a significant impact on the conversion efficiency of fluoroaluminates. If the roasting temperature is too low, the conversion reaction is difficult to occur, failing to achieve the conversion objective; if the roasting temperature is too high, energy consumption will increase, process costs will rise, and equipment investment will also increase.

[0025] Preferably, the calcination time is 10-300 min, for example, it can be 10 min, 30 min, 60 min, 90 min, 120 min, 150 min, 180 min, 210 min, 240 min, 270 min, or 300 min, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the calcination time is too short, the conversion reaction will be incomplete, and the fluoroaluminate conversion will be incomplete; if the calcination time is too long, energy consumption will increase, and costs will rise.

[0026] In a second aspect, the present invention provides a method for the co-production of a lamellar alumina composite material and silica, the co-production method comprising the method for preparing the lamellar alumina composite material as described in the first aspect. The co-production method further comprises: The ammonia gas generated in the first reaction of step (1) is absorbed by the first absorbent to obtain an ammonia solution; the silicon-containing gas generated in the calcination of step (2) is absorbed by the second absorbent to obtain a silicon-containing absorbent. The silicon-containing liquid and the ammonia solution are mixed, the pH is adjusted, and a second reaction is carried out. Then, a second solid phase is obtained by a second solid-liquid separation. The silicon-containing liquid includes the silicon-containing absorbent and the silicon-containing leachate obtained by the first solid-liquid separation in step (1). The second solid phase is subjected to a second washing and a third solid-liquid separation in sequence. The resulting third solid phase is dried to obtain silica.

[0027] Preferably, the concentration of the ammonia solution is 5-25 wt%, for example, it can be 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5% or 25%, etc.

[0028] Preferably, the concentration of SiO2 in the silicon-containing liquid is 5~100 g / L, for example, it can be 5 g / L, 10 g / L, 15 g / L, 20 g / L, 25 g / L, 30 g / L, 35 g / L, 40 g / L, 45 g / L, 50 g / L, 55 g / L, 60 g / L, 65 g / L, 70 g / L, 75 g / L, 80 g / L, 85 g / L, 90 g / L, 95 g / L or 100 g / L, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] Preferably, the pH is adjusted to 7-14, for example, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, or 14, but not limited to the listed values; other unlisted values ​​within this range are also applicable. A pH that is too low will prevent the formation of silica, while a pH that is too high will waste ammonia and affect the circulation efficiency of the fluoride solution.

[0030] Preferably, the reaction time for the second reaction is 5 to 300 minutes, for example, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 75 minutes, 90 minutes, 105 minutes, 120 minutes, 150 minutes, 180 minutes, 240 minutes, or 300 minutes, but not limited to the listed values. Other unlisted values ​​within this range are also applicable. If the reaction time is too short, the reaction will be incomplete, affecting the yield and quality of silica; if the reaction time is too long, severe agglomeration will occur between silica nanoparticles, resulting in a significant increase in the average particle size and a significant decrease in the specific surface area of ​​the silica product, seriously affecting product performance.

[0031] Preferably, the temperature of the second reaction is 20~70℃, for example, it can be 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃ or 70℃, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable. Excessively high reaction temperatures will lead to a significant decrease in the yield of silica, and at the same time, a significant increase in silica particle size and a significant decrease in specific surface area.

[0032] Preferably, the slurry from the second reaction can be allowed to stand and age before the second solid-liquid separation.

[0033] Preferably, the settling and aging time is 1 to 48 hours, for example, it can be 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, 28 hours, 30 hours, 32 hours, 34 hours, 36 hours, 38 hours, 40 hours, 42 hours, 44 hours, 46 hours, or 4 hours and 8 hours, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] Preferably, the second washing is at least two stages of countercurrent washing, and more preferably, the number of washing stages is ≥3 stages, such as stages 3, 4, 5, 6 or 7.

[0035] Preferably, the liquid phase obtained from the second solid-liquid separation is returned to step (1) as a fluoride salt solution after being evaporated and concentrated.

[0036] Preferably, the washing liquid from the second washing is returned for use as the first absorbent.

[0037] Preferably, the second absorbent comprises water and / or the washing liquid after the first washing. Preferably, the equipment for the second and third solid-liquid separations comprises any one or a combination of at least two of a plate and frame filter press, a belt filter, a pressure filter, or a centrifuge, such as a combination of a centrifuge and a plate and frame filter press, a belt filter and a pressure filter, or a belt filter and a centrifuge.

[0038] Preferably, the drying method includes spray drying, flash drying, microwave drying, or freeze drying.

[0039] Thirdly, this application provides a sheet-like alumina composite material as described in the first aspect.

[0040] Preferably, the alumina content of the sheet-like alumina composite material is >80%, for example, it can be 80%, 83%, 86%, 90%, 95% or 99%, etc.

[0041] Preferably, the F content of the sheet-like alumina composite material is <2%, for example, it can be 1.9%, 1.6%, 1.2%, 0.9% or 0.5%, etc.

[0042] Fourthly, the present invention provides an application of the sheet-like alumina composite material according to the first aspect in ceramic materials, refractory materials, catalyst carriers, microwave absorbing materials, abrasive materials, polishing materials or fillers.

[0043] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The preparation method of the sheet-like alumina composite material provided by the present invention uses fluoride salt solution to selectively leach and separate SiO2 from fly ash under low temperature conditions. The solid phase after leaching is calcined to obtain the sheet-like alumina composite material, so that Al2O3 in fly ash is almost completely converted into corundum; at the same time, it can co-produce silica, and almost completely convert SiO2 in fly ash into nano-sized silica products with a specific surface area ≥200m². 2 The average particle size of the silica is within 90nm, and the purity of the silica can reach over 95%, enabling high-value utilization of aluminum and silicon in fly ash. Simultaneously, the fluoride salt medium used can be recycled, generating no fluoride-containing waste liquid or residue; the product contains >80% alumina and <2% sulfur dioxide, achieving high-value utilization of fly ash solid waste.

[0044] (2) The preparation method of the sheet-like alumina composite material of the present invention uses fly ash, coal gangue and other solid wastes as the main raw materials. The raw materials are cheap and readily available, the process is simple, the operating conditions are mild, and it is easy to implement. Detailed Implementation

[0045] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0046] It should be understood that in the description of this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0047] Example 1

[0048] This embodiment provides a method for preparing sheet-like alumina composite material, the method comprising the following steps: (1) High-alumina fly ash (alumina content of 52.5 wt% and silica content of 33.2 wt%) was pretreated by sieving and decarbonization with a sieve size of 90 mesh. The undersize material was decarbonized fly ash. The decarbonized fly ash was then subjected to deironization pretreatment by passing it through a dry magnetic separator with a magnetic induction intensity of 4000 Gs to obtain deironized fly ash with an Al2O3 content of 54.6%. The deironized fly ash was then added to a 40% ammonium fluoride solution for reaction (desiliconization reaction). The liquid-solid ratio of the ammonium fluoride solution to the fly ash was 5 mL / g. The reaction was stirred at 90°C for 3 h. After the reaction was completed, a plate and frame filter press was used for liquid-solid separation to obtain the first solid phase and a silica-containing leachate. (2) The first solid phase was washed three times online with water in a countercurrent manner, with a washing liquid-to-solid ratio of 3 mL / g. After washing, the first solid phase was dried at 90°C and then calcined at 1050°C for 240 min to obtain a sheet-like alumina composite material and silicon-containing gas. The material performance indicators are shown in Table 1.

[0049] A method for co-producing silica: The ammonia-containing gas generated in the first reaction process is drawn out by an induced draft fan and sent to a spray tower for absorption to obtain an ammonia solution with a concentration of 15%; the silicon-containing gas generated in the calcination process is introduced into a heat exchanger to cool to 40°C and then sent to a spray tower for absorption by water spray to obtain a silicon-containing absorbent liquid, which is then combined with the desilication reaction leachate to obtain a silicon-containing liquid with a SiO2 concentration of 34 g / L. The ammonia solution and the silicon-containing solution are rapidly mixed evenly to carry out a second reaction. The temperature of the second reaction is controlled at 30°C, and the pH value of the reaction system is adjusted to 11. The reaction is stirred for 180 minutes at this pH value. After the second reaction is completed, the reaction slurry is subjected to a second solid-liquid separation using a plate and frame filter press to obtain a second solid phase and filtrate. The filtrate is concentrated and returned to the first reaction process. The second solid phase is subjected to a three-stage countercurrent slurry washing with water. After a fourth solid-liquid separation, a fourth solid phase (washed silica) and a silica washing liquid are obtained. The silica washing liquid is used as an absorbent for ammonia-containing gas. The fourth solid phase is dried using a spray dryer to obtain nano-sized silica product. The product performance indicators are shown in Table 1.

[0050] There is no specific order between the preparation method of the sheet-like alumina composite material and the co-production of silica described in this embodiment; the two can be carried out simultaneously.

[0051] Example 2

[0052] This embodiment provides a method for preparing sheet-like alumina composite material, the method comprising the following steps: (1) High-alumina fly ash (alumina content of 50.6 wt% and silica content of 37.2 wt%) was pretreated by decarbonization through sieving with a sieve size of 70 mesh. The undersize material was decarbonized fly ash. The decarbonized fly ash was then pretreated by deironization through two magnetic separations using a dry magnetic separator with a magnetic induction intensity of 3000 Gs to obtain deironized fly ash with an Al2O3 content of 52.1%. The deironized fly ash was then added to a 30% ammonium fluoride solution for reaction (desiliconization reaction). The liquid-solid ratio of the ammonium fluoride solution to the fly ash was 8 mL / g. The reaction was stirred at 80°C for 2.5 h. After the reaction was completed, a plate and frame filter press was used for liquid-solid separation. The solid phase was dried at 110°C to obtain the first solid phase and a silica-containing leachate. (2) The first solid phase was calcined at 1000°C for 150 min to obtain a sheet-like alumina composite material and silicon-containing gas. The material performance indicators are shown in Table 1.

[0053] A method for co-producing silica: The ammonia-containing gas generated in the first reaction process is drawn out by an induced draft fan and sent to a spray tower for absorption to obtain an ammonia solution with a concentration of 15%; the silicon-containing gas generated in the calcination process is introduced into a heat exchanger to cool to 30°C and then sent to a spray tower for absorption with water to obtain a silicon-containing absorbent liquid, which is then combined with the desilication reaction leachate to obtain a silicon-containing liquid with a SiO2 concentration of 33.6 g / L. The ammonia solution and the silicon-containing solution are rapidly mixed evenly to carry out a second reaction. The temperature of the second reaction is controlled at 40°C, and the pH value of the reaction system is adjusted to 10. The reaction is stirred for 180 minutes at this pH value. After the second reaction is completed, the reaction slurry is subjected to a second solid-liquid separation using a plate and frame filter press to obtain a second solid phase and filtrate. The filtrate is concentrated and returned to the first reaction process. The second solid phase is subjected to five stages of online countercurrent washing with water. After a fourth solid-liquid separation, a fourth solid phase (washed silica) and a silica washing liquid are obtained. The silica washing liquid is used as an absorbent for ammonia-containing gas. The fourth solid phase is dried using a flash dryer to obtain nano-sized silica product. The product performance indicators are shown in Table 1.

[0054] There is no specific order between the preparation method of the sheet-like alumina composite material and the co-production of silica described in this embodiment; the two can be carried out simultaneously.

[0055] Example 3

[0056] This embodiment provides a method for preparing sheet-like alumina composite material, the method comprising the following steps: (1) High-alumina fly ash (alumina content of 54.8 wt% and silica content of 33.5 wt%) was pretreated by decarbonization through sieving with a sieve size of 120 mesh. The undersize material was decarbonized fly ash. The decarbonized fly ash was then pretreated by deironization through a dry magnetic separator with a magnetic induction intensity of 12000 Gs to obtain deironized fly ash with an Al2O3 content of 57.1%. The deironized fly ash was then added to a 40% ammonium fluoride solution for reaction (desiliconization reaction). The liquid-solid ratio of the ammonium fluoride solution to the fly ash was 4 mL / g. The reaction was stirred at 150°C for 0.5 h. After the reaction was completed, a plate and frame filter press was used for liquid-solid separation. The solid phase was dried at 60°C to obtain the first solid phase and a silica-containing leachate. (3) The second solid phase was calcined at 900°C for 150 min to obtain a sheet-like alumina composite material and silicon-containing gas. The material performance indicators are shown in Table 1.

[0057] A method for co-producing silica: The ammonia-containing gas generated in the first reaction process is drawn out by an induced draft fan and sent to a spray tower for absorption to obtain an ammonia solution with a concentration of 15%; the silicon-containing gas generated in the calcination process is introduced into a heat exchanger to cool to 30°C and then sent to a gravity absorber for absorption by a first solid-phase washing liquid to obtain a silicon-containing absorbent liquid, which is then combined with the desilication reaction leachate to obtain a silicon-containing liquid with a SiO2 concentration of 39.6 g / L; The ammonia solution and the silicon-containing solution were rapidly and uniformly mixed to initiate a second reaction. The temperature of the second reaction was controlled at 25°C, and the pH of the reaction system was adjusted to 10.5. The reaction was stirred for 90 minutes at this pH value. After the second reaction was completed, the reaction slurry was allowed to stand and age for 12 hours. After aging, the reaction slurry was subjected to a second solid-liquid separation using a plate and frame filter press to obtain a second solid phase and filtrate. The filtrate was concentrated and returned to the first reaction process. The second solid phase is subjected to a 5-stage countercurrent online washing with water to obtain a fourth solid phase (washed silica) and silica washing liquid. The silica washing liquid is used as an absorbent for ammonia-containing gas. The fourth solid phase is then dried using a spray dryer to obtain nano-sized silica product. The product performance indicators are shown in Table 1.

[0058] There is no specific order between the preparation method of the sheet-like alumina composite material and the co-production of silica described in this embodiment; the two can be carried out simultaneously.

[0059] Example 4

[0060] Compared with Example 1, the difference is that the 40% ammonium fluoride solution was replaced with a 2% ammonium fluoride solution; all other aspects are the same as in Example 1. Product performance indicators are shown in Table 1. As can be seen from Table 1, due to the low concentration of ammonium fluoride and insufficient amount of ammonium fluoride, the desilication effect is significantly reduced, resulting in a lower alumina content in the product and affecting product quality.

[0061] Example 5

[0062] Compared with Example 1, the difference lies in that the first solid phase was calcined at 400°C after drying; otherwise, it was the same as Example 1. Product performance indicators are shown in Table 1. As can be seen from Table 1, due to the excessively low calcination temperature, the fluoroaluminates in the first solid phase could not be fully decomposed, resulting in a fluorine content in the alumina composite material still reaching as high as 15%, which affects both product quality and causes media loss.

[0063] Comparative Example 1 Compared with Example 1, the difference is that the 40% ammonium fluoride solution was replaced with a 40% sodium fluoride solution; all other aspects are the same as in Example 1. Product performance indicators are shown in Table 1. As can be seen from Table 1, the fly ash basically did not react, and the Al2O3 content in the product was very close to that of the raw material.

[0064] Test methods: This invention uses the alkali fusion digestion-ICP-OES method to test the Al2O3 content in the flake alumina composite material; and the alkali fusion-ion selective electrode method to test the fluorine content in the flake alumina composite material. A laser particle size analyzer is used to test the average particle size of the silica product, and the GB / T 10722 method is used to test the specific surface area of ​​the silica product.

[0065] Table 1 Example 1 85.8% 1.12% 73.2 243.1 Example 2 86.3% 0.89% 79.5 253.9 Example 3 82.6% 1.20% 82.1 267.2 Example 4 55.3% 0.52% / / Example 5 58.5% 15% / / Comparative Example 1 56.1% 0.28% / / In summary, this invention effectively removes silica from fly ash through leaching with fluoride solution, liquid-solid separation, and calcination, generating high-quality flake corundum. This produces a flake alumina composite material with an Al2O3 content ≥80% and a fluorine content <2%, which can be applied in ceramic materials, refractory materials, catalyst carriers, microwave absorbing materials, abrasive materials, polishing materials, fillers, and other fields. Furthermore, it can prepare amorphous silica from fly ash into nano-sized silica products and achieve the recycling of fluoride solution, demonstrating broad application prospects.

[0066] This invention illustrates the detailed process equipment and flow through the above embodiments. However, this invention is not limited to the detailed process equipment and flow described above, meaning that this invention does not necessarily depend on the detailed process equipment and flow to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials for the product of this invention, additions of auxiliary components, and selection of specific methods, all fall within the protection and disclosure scope of this invention.

Claims

1. A method for preparing a sheet-like alumina composite material, characterized in that, The preparation method includes the following steps: (1) Mix high-alumina fly ash with fluoride salt solution, and after the mixture undergoes a first reaction, it is separated into solid and liquid phases and dried to obtain the first solid phase; (2) The first solid phase is calcined to obtain a sheet-like alumina composite material.

2. The method according to claim 1, characterized in that, The Al2O3 content in the high-alumina fly ash described in step (1) is ≥35wt%; Preferably, the fly ash is high-alumina fly ash that has undergone decarbonization pretreatment and iron removal pretreatment; Preferably, the decarbonization pretreatment method includes screening and / or air separation; Preferably, the sieve aperture size used for sieving is 60~120 mesh; Preferably, the iron removal pretreatment method includes dry magnetic separation or wet magnetic separation; Preferably, the magnetic induction intensity used in the dry or wet magnetic separation is 3000~20000 Gs.

3. The preparation method according to claim 1 or 2, characterized in that, The fluoride solution mentioned in step (1) is an ammonium fluoride solution and / or an ammonium hydrogen fluoride solution with a concentration of 5~50 wt%; Preferably, the volume-to-mass ratio of the fluoride salt solution to the high-alumina fly ash is 3~15 ml / g.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The reaction temperature is 50~150℃, and the reaction time is 0.5~10h; Preferably, the reaction is a stirred reaction, and the stirring speed is 50~400 r / min; Preferably, the drying conditions are drying at 80~150℃ for 2~12 hours.

5. The preparation method according to any one of claims 1 to 4, characterized in that, Step (1) also includes washing the first solid phase, with a washing liquid-to-solid ratio of 1-10 ml / g and washing 1-5 times.

6. The preparation method according to any one of claims 1 to 5, characterized in that, The roasting temperature in step (3) is 800~1500℃ and the roasting time is 0.5~5h.

7. A method for the co-production of flake alumina composite material and silica, characterized in that, The co-production method includes the method for preparing sheet-like alumina composite material according to any one of claims 1 to 6; the co-production method further includes: The ammonia gas generated in the first reaction of step (1) is absorbed by the first absorbent to obtain an ammonia solution; the silicon-containing gas generated in the calcination of step (2) is absorbed by the second absorbent to obtain a silicon-containing absorbent. The silicon-containing liquid and the ammonia solution are mixed, the pH is adjusted, and a second reaction is carried out. Then, a second solid phase is obtained by a second solid-liquid separation. The silicon-containing liquid includes the silicon-containing absorbent and the silicon-containing leachate obtained by the first solid-liquid separation in step (1). The second solid phase is subjected to a second washing and a third solid-liquid separation in sequence. The resulting third solid phase is dried to obtain silica.

8. A sheet-like alumina composite material, characterized in that, The material is prepared by any one of the preparation methods described in claims 1 to 7, wherein the aluminum oxide content in the material is ≥80wt%.

9. The application of the sheet-like alumina composite material according to claim 8 in refractory materials, ceramic matrices, grinding fillers, coatings and catalyst supports.