Mullite material, method for its production and use

Abstract

The application relates to a mullite material and a preparation method and application thereof, and the method comprises the following steps: mixing fly ash, a calcium medium and water to perform stirring treatment, and then performing solid-liquid separation to obtain an intermediate product; sequentially performing acid treatment and solid-liquid separation on the obtained intermediate product to obtain slag; sequentially performing alkali treatment and solid-liquid separation on the obtained slag to obtain the mullite material. According to the application, the calcium-containing raw material is selectively reacted with glassy SiO2 and Al2O3 in the fly ash to convert the glassy SiO2 and Al2O3 into a form which is easy to remove, and the glassy SiO2 and Al2O3 are efficiently removed, so that the mullite material with high aluminum-silicon ratio is obtained. The application does not need to add an external aluminum-rich raw material and does not need a high-temperature roasting process, so that the technological process is simplified, and the cost is reduced. The method has the advantages of cheap and easily-obtained raw materials, a simple process and easy implementation, and can realize high-value utilization of high-aluminum fly ash.

Description

Technical Field

[0001] This invention relates to the field of fly ash resource utilization technology, specifically to a mullite material, its preparation method and uses, and more specifically to a method for preparing mullite material using high-alumina fly ash, the resulting mullite material and its uses. Background Technology

[0002] Currently, fly ash, the fly ash produced after coal combustion, has a comprehensive utilization rate of only about 70%. The 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 contains significantly higher Al2O3 content than ordinary fly ash and also contains mullite (3Al2O3·2SiO2), giving it high comprehensive utilization value.

[0003] Mullite possesses many excellent physical properties, such as high fracture toughness, high temperature resistance, oxidation resistance, thermal shock resistance, creep resistance, low thermal conductivity, strong electrical insulation, low dielectric constant, and high chemical stability. It can be used in various composite materials and has been widely researched and applied in chemical, energy, environmental, power, and metallurgical fields. Mullite minerals are extremely rare in nature, and it is mainly synthesized artificially. Artificially synthesized mullite is mainly divided into sintered mullite and fused mullite. It is usually prepared from industrial alumina, bauxite, kaolin, and fused alumina as raw materials, with the addition of certain auxiliary materials. High-alumina fly ash itself is rich in mullite and is a potential raw material for preparing mullite products.

[0004] Currently, the preparation of mullite materials using high-alumina fly ash as raw material mainly involves adding aluminum-rich raw materials such as alumina and bauxite to the high-alumina fly ash, adjusting the ratio of aluminum to silicon in the system to be close to the theoretical ratio of mullite, and then sintering at high temperature after mixing to obtain mullite products. CN102765944A discloses a method for preparing mullite powder. This method uses fly ash as the main raw material, adds an appropriate amount of Al2O3, and loads it into the ball mill jar of a planetary ball mill according to the stoichiometric composition of mullite. Triethanolamine is used as a grinding aid, and the sample is milled for a certain time and then removed. The milled sample is calcined at different temperatures to obtain mullite powder, which is then washed and filtered with hydrofluoric acid to obtain mullite powder. CN101643359A discloses a method for preparing mullite powder using fly ash. The method involves dry grinding fly ash and bauxite raw materials separately in a ball mill to remove iron; placing the iron-removed fly ash and bauxite separately into an alumina crucible and heating them in a resistance furnace, then removing them after natural cooling; mixing the cooled fly ash and bauxite evenly and heating them in a resistance furnace, then removing them after natural cooling to obtain mullite powder with different particle sizes and alumina contents.

[0005] While the above methods can produce mullite products, they require the addition of large amounts of high-alumina raw materials such as bauxite and industrial alumina due to the presence of significant amounts of glassy silica in high-alumina fly ash. Furthermore, a high-temperature calcination process is necessary to react the added high-alumina raw materials with the SiO2 in the fly ash to convert it into mullite, resulting in high costs and the need for large quantities of high-alumina raw materials. Additionally, the methods are less effective at removing impurities such as iron, magnesium, and calcium from fly ash. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of this invention is to provide a mullite material, its preparation method and uses, which realizes the preparation of mullite material without the need for external aluminum-rich raw materials or high-temperature calcination process, thus simplifying the process and reducing costs.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing mullite materials using high-alumina fly ash, the method comprising:

[0009] After mixing and stirring fly ash, calcium medium and water, the intermediate product is obtained through solid-liquid separation.

[0010] The obtained intermediate product was subjected to acid treatment and solid-liquid separation in sequence to obtain slag.

[0011] The resulting slag was subjected to alkali treatment and solid-liquid separation in sequence to obtain mullite material.

[0012] In this invention, the high-alumina fly ash used contains mullite, as well as a large amount of glassy SiO2 and Al2O3, which coat the surface of the mullite and have a relatively dense structure. The presence of glassy SiO2 and Al2O3 affects the performance of mullite and needs to be removed. If acid solution is used for leaching directly, SiO2 cannot be effectively leached, and due to the dense coating of glassy SiO2, only a portion of Al2O3 can be leached, resulting in poor removal efficiency. If alkaline solution is used for leaching directly, SiO2 and Al2O3 easily react to form dense sodium aluminosilicate, which forms a secondary coating on the fly ash particles, inhibiting the leaching process and causing alkali loss. Therefore, the method provided by this invention first treats fly ash with calcium-containing raw materials. By controlling specific reaction conditions, calcium-containing compounds can selectively react with glassy SiO2 and Al2O3 in the fly ash to generate loose and porous calcium-aluminum-silicon compounds. These compounds do not form a dense coating on the fly ash, which is beneficial for the complete conversion of glassy SiO2 and Al2O3 in the fly ash. The calcium-aluminum-silicon compounds readily react with acid solutions, and subsequent acid leaching can efficiently remove aluminum and calcium from them.

[0013] As a preferred technical solution of the present invention, the aluminum content in the fly ash, calculated as Al2O3, is ≥35% by mass. For example, it can be 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, or 45%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0014] Preferably, the fly ash undergoes decarbonization and deironization treatment.

[0015] In this invention, the decarbonization treatment can be achieved through screening and / or air classification. The sieve size used for screening is 60-120 mesh, for example, 60, 70, 80, 90, 100, 110, or 120 mesh, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. Fly ash contains a small amount of large-particle unburned carbon. Pre-decarbonization is preferred, as it removes most of the unburned carbon, avoiding impact on mullite products, and allows for efficient recovery of the unburned carbon, enabling its utilization as low-quality coal. Simultaneously, it removes some large-particle impurities mixed in with the fly ash, further improving product quality.

[0016] In this invention, the iron removal process can be dry magnetic separation and / or wet magnetic separation. This is because fly ash generally contains a certain amount of iron-containing phases, which, if they enter mullite products, will affect the product's color, melting point, light transmittance, and other properties. Therefore, it is preferable to remove the iron-containing phases beforehand. The magnetic induction intensity used in the magnetic separation is 3000-20000 Gs, for example, 3000 Gs, 3500 Gs, 4000 Gs, 4500 Gs, 5000 Gs, 5500 Gs, 60... 00Gs, 6500Gs, 7000Gs, 7500Gs, 8000Gs, 8500Gs, 9000Gs, 9500Gs, 10000Gs, 11000Gs, 12000Gs, 13000Gs, 14000Gs, 15000Gs, 16000Gs, 17000Gs, 18000Gs, 19000Gs, or 20000Gs, etc., but not limited to the listed values; other unlisted values ​​within this range also apply.

[0017] Preferably, the calcium medium includes one or a combination of at least two of calcium oxide, calcium hydroxide, or calcium chloride; for example, it may be a combination of calcium oxide and calcium hydroxide, a combination of calcium oxide and calcium chloride, a combination of calcium hydroxide and calcium chloride, or a combination of calcium oxide, calcium hydroxide, and calcium chloride.

[0018] Preferably, the amount of calcium medium added, calculated as calcium oxide, has a molar ratio of (0.7-1.3):1 to the glassy SiO2 in the fly ash. For example, it can be 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.05:1, 1.1:1, 1.15:1, 1.2:1, 1.25:1, or 1.3:1, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. If the molar ratio of CaO to SiO2 is too small, it cannot provide sufficient calcium source, resulting in incomplete silicon-aluminum conversion. If the molar ratio of CaO to SiO2 is too large, the resulting calcium-aluminum-silicon compound structure becomes dense, which is not conducive to the reaction, and there is also an excess of CaO, leading to waste.

[0019] As a preferred embodiment of the present invention, the solid-liquid ratio of the mixture is 1:(3-15) g / mL, for example, it can be 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, or 1:15, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the liquid-solid ratio is too small, the reaction mass transfer effect is poor, the conversion is incomplete, and the resulting calcium aluminum silicon compound has a dense structure; if the liquid-solid ratio is too large, the energy consumption for heating and stirring increases, and the cost rises.

[0020] Preferably, the stirring temperature is 150-250℃, for example, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, or 250℃, but not limited to the listed values; other unlisted values ​​within this range are also applicable. If the reaction temperature is too low, the reaction rate is too slow, and the aluminum-silicon conversion is incomplete; if the reaction temperature is too high, energy consumption increases, and the resulting calcium-aluminum-silicon compound has a complete crystal structure and enhanced stability, which is detrimental to the subsequent acid leaching process.

[0021] Preferably, the stirring time is 1-10 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, 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 not proceed completely, and the aluminum-silicon conversion will be incomplete. Increasing the reaction time also increases energy consumption, and the resulting calcium-aluminum-silicon compound will have a complete crystal structure and enhanced stability, which is detrimental to the subsequent acid leaching process.

[0022] The calcium aluminum silicon compound generated in the above process is insoluble and remains mixed with mullite. To separate it from the mullite and obtain a material mainly composed of mullite, this invention uses an acid solution to leach the reaction product. The calcium aluminum silicon compound readily reacts with acid and decomposes, with calcium and aluminum transforming into water-soluble calcium and aluminum salts respectively and dissolving, while silicon transforms into a highly reactive silica gel remaining in the solid phase. By controlling specific acid leaching conditions, a highly polymerized, macroporous silica gel can be generated, facilitating the leaching and liquid-solid separation of calcium and aluminum.

[0023] As a preferred embodiment of the present invention, the acid solution used for the acid treatment includes hydrochloric acid and / or nitric acid.

[0024] Preferably, the mass concentration of the acid solution used for acid treatment is 10-25%, for example, it can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the acid concentration is too low, the calcium, aluminum, and silicon compounds will not decompose completely, and silicon will easily form unstable silica sol, which will easily clog the filter cloth, seriously affecting the liquid-solid separation efficiency. If the acid concentration is too high, the generated silica gel particles will be too small and too dense, which is not conducive to the leaching and washing of calcium and aluminum. In addition, if the acid concentration is too high, the hydrochloric acid will be too volatile and corrosive, which is not conducive to operation and will easily cause the loss of hydrochloric acid.

[0025] Preferably, the solid-liquid ratio for acid treatment is 1:(3-8) g / mL, for example, it can be 1:3, 1:4, 1:5, 1:6, 1:7, or 1:8, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the liquid-solid ratio is too low, the mass transfer effect of the reaction is poor, affecting the leaching effect; if the liquid-solid ratio is too high, there is a large excess of acid, and the energy consumption for heating and stirring increases, leading to higher costs.

[0026] As a preferred embodiment of the present invention, the acid treatment temperature is 40-70℃, for example, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, or 70℃, but not limited to the listed values; other unlisted values ​​within this range are also applicable. If the leaching temperature is too low, the reaction rate is too slow, the reaction is incomplete, and silica sol that is difficult to filter and separate is easily formed; if the leaching temperature is too high, energy consumption increases, acid corrosivity is enhanced, and equipment requirements are increased.

[0027] Preferably, the acid treatment time is 0.5-5 hours, for example, 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the leaching time is too short, the leaching reaction will not be complete, affecting the leaching effect; if the reaction time is too long, energy consumption will increase.

[0028] The liquid phase obtained after acid treatment mainly consists of soluble calcium and aluminum salts. The calcium salts can be converted into calcium sulfate crystals by adding sulfuric acid to produce gypsum byproducts, while simultaneously regenerating hydrochloric acid / nitric acid. Since the aluminum concentration is low, it can be recycled back to the acid leaching process in step (2). Once the aluminum concentration is increased to a certain level, it can be used as a raw material for preparing aluminum chloride / aluminum nitrate.

[0029] In this invention, the slag is washed before alkali treatment. The mass ratio of water to slag during the washing process is (3-5):1, for example, it can be 3:1, 3.5:1, 4:1, 4.5:1 or 5:1, etc. The washing temperature is 20-80℃, for example, it can be 20℃, 30℃, 40℃, 50℃, 60℃, 70℃ or 80℃, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] As a preferred embodiment of the present invention, the alkaline solution used in the alkaline treatment includes sodium hydroxide solution and / or potassium hydroxide solution. After the above acid leaching, the glassy SiO2 in the fly ash is transformed into highly active silica gel, which can be leached with alkaline solution under relatively mild conditions to obtain sodium silicate and / or potassium silicate solution, which can be used as raw material for the wet preparation of water glass.

[0031] Preferably, the concentration of the alkali solution used for the alkali treatment is 80-150 g / L, for example, it can be 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, 105 g / L, 110 g / L, 115 g / L, 120 g / L, 125 g / L, 130 g / L, 135 g / L, 140 g / L, 145 g / L, or 150 g / L, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. If the alkali concentration is too low, the silicon leaching reaction will not proceed completely, resulting in a low leaching rate; if the alkali concentration is too high, there will be excessive alkali, leading to poor leaching selectivity, and some mullite will also be decomposed by the alkali.

[0032] Preferably, the solid-liquid ratio in the alkali treatment is 1:(2-5) g / mL, for example, it can be 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5 or 1:5, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] As a preferred embodiment of the present invention, the temperature of the alkali treatment is 30-70℃, for example, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, or 70℃, but not limited to the listed values; other unlisted values ​​within this range are also applicable. The silica sol generated after acid leaching has extremely high activity, therefore it can be leached with alkali solution under relatively mild reaction conditions. If the temperature is too low, the leaching rate is too slow and the leaching rate is too low; if the temperature is too high, energy consumption increases.

[0034] Preferably, the alkali treatment time is 0.5-3 hours, for example, 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours, but not limited to the listed values; other unlisted values ​​within this range are also applicable. If the leaching time is too short, the reaction will be incomplete, resulting in a low silicon leaching rate; if the reaction time is too long, energy consumption will increase.

[0035] As a preferred technical solution of the present invention, the method includes:

[0036] Fly ash, calcium medium, and water are mixed and stirred, followed by solid-liquid separation to obtain an intermediate product. The fly ash contains ≥35% aluminum (Al2O3 by mass), and undergoes decarbonization and deironization treatment. The calcium medium includes one or a combination of at least two of calcium oxide, calcium hydroxide, or calcium chloride. The amount of calcium medium added is calculated as the molar ratio of calcium oxide to glassy SiO2 in the fly ash (0.7-1.3):1. The solid-liquid ratio of the mixture is 1:(3-15) g / mL. The stirring temperature is 150-250℃, and the stirring time is 1-10h.

[0037] The obtained intermediate product is subjected to acid treatment and solid-liquid separation in sequence to obtain slag; the acid solution used for acid treatment includes hydrochloric acid and / or nitric acid, the mass concentration of the acid solution used for acid treatment is 10-25%, the solid-liquid ratio of acid treatment is 1:(3-8) g / mL, the temperature of acid treatment is 40-70℃, and the time of acid treatment is 0.5-5h.

[0038] The obtained slag is subjected to alkali treatment and solid-liquid separation in sequence to obtain mullite material; the alkali solution used for alkali treatment includes sodium hydroxide solution and / or potassium hydroxide solution, the concentration of the alkali solution used for alkali treatment is 80-150 g / L, the solid-liquid ratio in alkali treatment is 1:(2-5) g / mL, the temperature of alkali treatment is 30-70℃, and the time of alkali treatment is 0.5-3h.

[0039] In a second aspect, the present invention provides a mullite material, which is prepared by the method described in the first aspect.

[0040] Thirdly, the present invention provides uses of the mullite material as described in the second aspect, the uses including the preparation of ceramic materials, refractory materials, catalyst supports, microwave absorbing materials, abrasive materials, polishing materials or fillers using the mullite material.

[0041] Compared with existing technical solutions, the present invention has the following beneficial effects:

[0042] (1) Based on the characteristics of mullite encapsulated by dense glassy SiO2 and Al2O3 in high-alumina fly ash, this invention first selectively reacts with glassy SiO2 and Al2O3 by adding calcium-containing substances to transform them into loose and porous calcium-aluminum-silicon compounds. Then, through acid leaching and alkali leaching treatment, glassy SiO2 and Al2O3 can be effectively removed to obtain mullite material. The product has an Al2O3 content >68%, an aluminum-silicon ratio ≥3.2, and a Na2O content ≤0.3%, which can realize the high-value utilization of fly ash solid waste.

[0043] (2) The present invention uses fly ash solid waste as the main raw material. The raw material is cheap and readily available, the process is simple, the operating conditions are mild, and it is easy to implement. Detailed Implementation

[0044] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows:

[0045] Example 1

[0046] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0047] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pretreated for decarbonization by sieving with an 80-mesh sieve. The undersize material was then decarbonized fly ash. The decarbonized fly ash was further pretreated for iron removal by magnetic separation using a dry magnetic separator with a magnetic induction intensity of 3500 Gs, yielding iron-removed fly ash.

[0048] Iron-removed fly ash, calcium oxide, and water were mixed evenly, wherein the molar ratio of CaO to glassy SiO2 in the fly ash was 1.1:1, and the volume ratio of water to the liquid-to-solid ratio of fly ash and calcium oxide (mL / g) was 10:1. The mixed slurry was stirred at 200℃ for 4 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0049] The intermediate product was leached with a 16 wt% hydrochloric acid solution at 60°C and a liquid-to-solid ratio of 4:1 (mL / g) with stirring for 2 hours. The mixture was then filtered to separate the residue and the second liquid phase. The residue was washed with water at 30°C, with a wash water-to-residue liquid-to-solid ratio of 4 (mL / g). The washed solid phase was then leached with a 120 g / L NaOH solution at 50°C and a liquid-to-solid ratio of 3:1 (mL / g) with stirring for 1.5 hours. The mixture was then filtered to separate the solid phase. The solid phase was washed with water at 40°C, with a wash water-to-solid ratio of 3:1 (mL / g). The washed solid phase was dried to obtain the mullite material.

[0050] The test indicators of the obtained mullite material are detailed in Table 1.

[0051] Example 2

[0052] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0053] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pretreated for decarbonization by sieving with a 60-mesh sieve. The undersize material was then decarbonized fly ash. The decarbonized fly ash was then subjected to iron removal pretreatment by dry magnetic separation with a magnetic induction intensity of 5000 Gs to obtain iron-removed fly ash.

[0054] Iron-removed fly ash, calcium oxide, and water were mixed evenly, with the molar ratio of CaO to glassy SiO2 in the fly ash being 0.7:1, and the volume ratio of water to the total mass of fly ash and calcium oxide (mL / g) being 15:1. The mixed slurry was stirred at 250℃ for 1 hour. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0055] The intermediate product was leached with a 20 wt% hydrochloric acid solution at 50°C and a liquid-to-solid ratio of 4:1 (mL / g) with stirring for 1 hour. The mixture was then filtered to separate the residue and the second liquid phase. The residue was washed with water at 40°C, with a wash water-to-residue liquid-to-solid ratio of 4:1 (mL / g). The washed solid phase was then leached with a 100 g / L NaOH solution at 60°C and a liquid-to-solid ratio of 4:1 (mL / g) with stirring for 1 hour. The mixture was then filtered to separate the residue. The solid phase was washed with water at 60°C, with a wash water-to-solid ratio of 4:1 (mL / g). The washed solid phase was dried to obtain the mullite material.

[0056] The test indicators of the obtained mullite material are detailed in Table 1.

[0057] Example 3

[0058] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0059] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pretreated for decarbonization by sieving with a 120-mesh sieve. The undersize material was then decarbonized fly ash. The decarbonized fly ash was then subjected to iron removal pretreatment by dry magnetic separation with a magnetic induction intensity of 7000 Gs to obtain iron-removed fly ash.

[0060] Iron-removed fly ash, calcium oxide, and water were mixed evenly, with the molar ratio of CaO to glassy SiO2 in the fly ash being 1.3:1, and the volume ratio of water to the total mass of fly ash and calcium oxide (mL / g) being 5:1. The mixed slurry was stirred and reacted at 150℃ for 10 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0061] The intermediate product was leached with a 12 wt% hydrochloric acid solution at 65°C and a liquid-to-solid ratio of 6:1 (mL / g) with stirring for 4 hours. The mixture was then filtered to separate the residue and the second liquid phase. The residue was washed with water at 20°C at a solid-to-liquid ratio of 6:1 (mL / g). The washed solid phase was then leached with a 150 g / L NaOH solution at 30°C and a liquid-to-solid ratio of 2:1 (mL / g) with stirring for 3 hours. The mixture was then filtered to separate the solid phase. The solid phase was washed with water at 20°C at a mass ratio of 2 (wash water to solid phase). The washed solid phase was dried to obtain the mullite material.

[0062] The test indicators of the obtained mullite material are detailed in Table 1.

[0063] Example 4

[0064] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0065] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pre-treated for decarbonization by cyclone classification, with a classification particle size of 70 mesh, to obtain decarbonized fly ash. The decarbonized fly ash was then pre-treated for iron removal by wet magnetic separation using a wet magnetic separator with a magnetic induction intensity of 12000 Gs, to obtain iron-removed fly ash.

[0066] Iron-removed fly ash, calcium hydroxide, and water were mixed evenly, with the molar ratio of converted CaO to glassy SiO2 in the fly ash being 0.9:1, and the volume ratio of water to the total mass of fly ash and calcium hydroxide (mL / g) being 7:1. The mixed slurry was stirred at 180℃ for 5 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0067] The intermediate product was leached with a 10 wt% hydrochloric acid solution at 70°C and a liquid-to-solid ratio of 8:1 (mL / g) with stirring for 5 hours. The mixture was then filtered to separate the residue and the second liquid phase. The residue was washed with water at 80°C, with a wash water-to-residue liquid-to-solid ratio of 5:1 (mL / g). The washed solid phase was then leached with an 80 g / L KOH solution at 70°C and a liquid-to-solid ratio of 5:1 (mL / g) with stirring for 2.5 hours. The mixture was then filtered to separate the residue. The solid phase was washed with water at 70°C, with a wash water-to-solid ratio of 5:1 (mL / g). The washed solid phase was dried to obtain the mullite material.

[0068] The test indicators of the obtained mullite material are detailed in Table 1.

[0069] Example 5

[0070] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0071] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pretreated for decarbonization by sieving with a 90-mesh sieve. The undersize material was then decarbonized fly ash. The decarbonized fly ash was then subjected to iron removal pretreatment by dry magnetic separation with a magnetic induction intensity of 20000 Gs to obtain iron-removed fly ash.

[0072] Iron-removed fly ash, calcium chloride, and water were mixed evenly, with the molar ratio of converted CaO to glassy SiO2 in the fly ash being 1:1, and the volume ratio of water to the total mass of fly ash and calcium chloride (mL / g) being 3:1. The mixed slurry was stirred at 220℃ for 2 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0073] The intermediate product was leached with a 25 wt% hydrochloric acid solution at 40°C and a liquid-to-solid ratio of 3:1 (mL / g) with stirring for 0.5 h. The mixture was then filtered to separate the residue and the second liquid phase. The residue was washed with water at 60°C at a liquid-to-solid ratio of 5:1 (mL / g). The washed solid phase was leached with a 110 g / L KOH solution at 55°C and a liquid-to-solid ratio of 3.5:1 (mL / g) with stirring for 0.5 h. The mixture was then filtered to separate the residue. The solid phase was washed with water at 50°C at a liquid-to-solid ratio of 3.5:1 (mL / g). The washed solid phase was dried to obtain the mullite material.

[0074] The test indicators of the obtained mullite material are detailed in Table 1.

[0075] Example 6

[0076] This embodiment provides a method for preparing mullite-corundum composite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0077] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pre-treated for decarbonization by cyclone classification, with a classification particle size of 110 mesh, to obtain decarbonized fly ash. The decarbonized fly ash was then pre-treated for iron removal by wet magnetic separation using a wet magnetic separator with a magnetic induction intensity of 3000 Gs, to obtain iron-removed fly ash.

[0078] Iron-removed fly ash, calcium oxide, calcium hydroxide, and water were mixed evenly (the mass ratio of calcium oxide to calcium hydroxide was 1:1). The molar ratio of converted CaO to glassy SiO2 in the fly ash was 0.8:1, and the volume ratio of water to the total mass of fly ash and calcium-containing raw materials (mL / g) was 9:1. The mixed slurry was stirred at 170℃ for 6 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0079] The intermediate product was leached with a 22 wt% hydrochloric acid solution at 45°C and a liquid-to-solid ratio of 4 mL / g for 3 hours with stirring. The mixture was then filtered to obtain a residue and a second liquid phase. The residue was washed with water at 30°C at a liquid-to-solid ratio of 3:1 (wash water to residue). The washed solid phase was then leached with a 90 g / L NaOH solution at 65°C and a liquid-to-solid ratio of 4.5:1 (wash water to solid phase) for 2 hours with stirring. The mixture was then filtered to separate the solid phase. The solid phase was washed with water at 65°C at a liquid-to-solid ratio of 2.5:1 (wash water to solid phase). The washed solid phase was dried to obtain the mullite material.

[0080] The test indicators of the obtained mullite material are detailed in Table 1.

[0081] Example 7

[0082] This embodiment provides a method for preparing mullite materials using high-alumina fly ash, the preparation method comprising the following steps:

[0083] Fly ash (Al2O3 content 49.78%, SiO2 content 38.75%) was pretreated for decarbonization by sieving with a 100-mesh sieve. The undersize material was then decarbonized fly ash. The decarbonized fly ash was then pretreated for iron removal by magnetic separation using a dry magnetic separator with a magnetic induction intensity of 6000 Gs, yielding iron-removed fly ash.

[0084] Iron-removed fly ash, calcium oxide, calcium hydroxide, calcium chloride, and water were mixed evenly (the mass ratio of calcium oxide, calcium hydroxide, and calcium chloride was 1:1:1). The molar ratio of converted CaO to glassy SiO2 in the fly ash was 1.2:1, and the volume ratio of water to the total mass of fly ash and calcium-containing raw materials (mL / g) was 12:1. The mixed slurry was stirred at 190℃ for 5 hours. After the reaction was complete, the mixture was filtered to obtain an intermediate product and a first liquid phase.

[0085] The intermediate product was leached with an 18 wt% hydrochloric acid solution at 55°C and a liquid-to-solid ratio of 5:1 (mL / g) with stirring for 1.5 h. The mixture was then filtered to obtain a residue and a second liquid phase. The residue was washed with water at 50°C at a solid-to-liquid ratio of 5:1 (mL / g). The washed solid phase was then leached with a 130 g / L NaOH solution at 40°C and a liquid-to-solid ratio of 2.5:1 (mL / g) with stirring for 3 h. The mixture was then filtered to separate the solid phase. The solid phase was washed with water at 30°C at a liquid-to-solid ratio of 2.5:1 (mL / g). The washed solid phase was dried to obtain the mullite material.

[0086] The test indicators of the obtained mullite material are detailed in Table 1.

[0087] Comparative Example 1

[0088] The only difference from Example 1 is the absence of calcium-containing raw materials; all other aspects are the same as in Example 2. The test indicators of the obtained mullite material are detailed in Table 1. As shown in Table 1, due to the absence of calcium-containing raw materials, the dense glassy SiO2 and Al2O3 in the fly ash were not converted into loose and porous calcium-aluminum-silicon compounds, resulting in poor subsequent acid and alkali leaching effects. The alumina content and aluminum-silicon ratio in the product were significantly reduced, while the sodium oxide content was significantly increased, leading to a substantial decline in product quality.

[0089] Comparative Example 2

[0090] The only difference from Example 1 is that water is used instead of hydrochloric acid solution; all other aspects are the same as in Example 1. The test indicators of the obtained mullite material are detailed in Table 1. As shown in Table 1, water cannot effectively decompose calcium aluminum silicon compounds, causing them to enter the alkaline leaching process in step (3). Since calcium aluminum silicon compounds do not undergo decomposition in alkaline solutions, the removal of aluminum and silicon is not achieved. Furthermore, the added calcium oxide remains in the solid phase as a calcium aluminum silicon compound, resulting in a significant decrease in the alumina content and aluminum-silicon ratio of the final product, making it impossible to obtain mullite material. In addition, due to the porous nature of calcium aluminum silicon compounds, the adsorbed sodium ions increase, leading to a higher sodium oxide content in the product.

[0091] Comparative Example 3

[0092] The only difference from Example 1 is that water was used instead of NaOH solution; all other aspects are the same as in Example 1. The test indicators of the obtained mullite material are detailed in Table 1. As shown in Table 1, because water could not effectively leach the silica gel, it remained in the product, failing to achieve the desired desilication effect. Simultaneously, because the acid leaching process removed the glassy Al2O3, the final product had very low alumina content and aluminum-silicon ratio.

[0093] Comparative Example 4

[0094] The only difference from Example 1 is that calcium oxide is replaced with an equal amount of sodium oxide. The test parameters of the resulting mullite material are detailed in Table 1.

[0095] Comparative Example 5

[0096] The only difference from Example 1 is that the hydrochloric acid solution was replaced with an equal volume of hydrofluoric acid solution of equal concentration. The test parameters of the obtained mullite material are detailed in Table 1. Hydrofluoric acid reacts with calcium-containing compounds to generate a large amount of insoluble calcium fluoride, resulting in very low alumina content and aluminum-silicon ratio in the final product.

[0097] Comparative Example 6

[0098] The only difference from Example 1 is the change in the order of acid and alkali treatments; that is, alkali treatment is performed first, followed by acid treatment. The test indicators of the resulting mullite material are detailed in Table 1. Performing alkali treatment followed by acid treatment essentially fails to achieve desilication; instead, it removes some aluminum, leading to a decrease in both alumina content and the aluminum-silicon ratio.

[0099] Table 1

[0100]

[0101]

[0102] Note: The aluminum-silicon ratio in mullite products is the mass ratio of Al2O3 to SiO2.

[0103] In summary, this invention selectively reacts with glassy SiO2 and Al2O3 in fly ash by adding calcium-containing raw materials, transforming them into easily removable forms. These are then efficiently removed through acid and alkali leaching, yielding mullite material with an Al2O3 content >68%, an aluminum-silicon ratio ≥3.2, and a Na2O content <0.3%. This material can be applied in ceramic materials, refractory materials, catalyst supports, microwave absorbing materials, abrasive materials, polishing materials, and fillers. Simultaneously, the alkaline silicate solution obtained during the desilication process can be used as a raw material for preparing water glass. This invention utilizes inexpensive and readily available raw materials, employs a simple process, is easy to implement, and has broad application prospects.

[0104] The present invention is described in detail through the above embodiments, but the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0105] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0106] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0107] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing mullite materials using high-alumina fly ash, characterized in that, The method includes: After mixing and stirring fly ash, calcium medium and water, the intermediate product is obtained through solid-liquid separation. The obtained intermediate product was subjected to acid treatment and solid-liquid separation in sequence to obtain slag. The obtained slag was subjected to alkali treatment and solid-liquid separation in sequence to obtain mullite material; The calcium medium includes one or a combination of at least two of calcium oxide, calcium hydroxide, or calcium chloride. The stirring process is carried out at a temperature of 150-250℃.

2. The method as described in claim 1, characterized in that, The aluminum content in the fly ash, calculated as Al2O3, is ≥35% by mass.

3. The method as described in claim 1, characterized in that, The fly ash undergoes decarbonization and deironization treatment.

4. The method as described in claim 1, characterized in that, The amount of calcium medium added, calculated as calcium oxide, is in a molar ratio (0.7-1.3):1 with the glassy SiO2 in fly ash.

5. The method as described in claim 1, characterized in that, The solid-liquid ratio of the mixture is 1:(3-15) g / mL.

6. The method as described in claim 1, characterized in that, The stirring process takes 1-10 hours.

7. The method as described in claim 1, characterized in that, The acid solution used in the acid treatment includes hydrochloric acid and / or nitric acid.

8. The method as described in claim 1, characterized in that, The mass concentration of the acid solution used in the acid treatment is 10-25%.

9. The method as described in claim 1, characterized in that, The solid-liquid ratio for acid treatment is 1:(3-8) g / mL.

10. The method as described in claim 1, characterized in that, The acid treatment temperature is 40-70℃.

11. The method as described in claim 1, characterized in that, The acid treatment time is 0.5-5 hours.

12. The method as described in claim 1, characterized in that, The alkaline solution used in the alkaline treatment includes sodium hydroxide solution and / or potassium hydroxide solution.

13. The method as described in claim 1, characterized in that, The concentration of the alkali solution used in the alkali treatment is 80-150 g / L.

14. The method as described in claim 1, characterized in that, The solid-liquid ratio in the alkali treatment is 1:(2-5) g / mL.

15. The method as described in claim 1, characterized in that, The temperature for the alkali treatment is 30-70℃.

16. The method as described in claim 1, characterized in that, The alkali treatment time is 0.5-3 hours.

17. The method as described in claim 1, characterized in that, The method includes: Fly ash, calcium medium, and water are mixed and stirred, followed by solid-liquid separation to obtain an intermediate product. The fly ash contains ≥35% aluminum (Al2O3 by mass), and undergoes decarbonization and deironization treatment. The calcium medium includes one or a combination of at least two of calcium oxide, calcium hydroxide, or calcium chloride. The amount of calcium medium added is calculated as the molar ratio of calcium oxide to glassy SiO2 in the fly ash (0.7-1.3):

1. The solid-liquid ratio of the mixture is 1:(3-15) g / mL. The stirring temperature is 150-250℃, and the stirring time is 1-10h. The obtained intermediate product is subjected to acid treatment and solid-liquid separation in sequence to obtain slag; the acid solution used for acid treatment includes hydrochloric acid and / or nitric acid, the mass concentration of the acid solution used for acid treatment is 10-25%, the solid-liquid ratio of acid treatment is 1:(3-8) g / mL, the temperature of acid treatment is 40-70℃, and the time of acid treatment is 0.5-5h. The obtained slag is subjected to alkali treatment and solid-liquid separation in sequence to obtain mullite material; the alkali solution used for alkali treatment includes sodium hydroxide solution and / or potassium hydroxide solution, the concentration of the alkali solution used for alkali treatment is 80-150 g / L, the solid-liquid ratio in alkali treatment is 1:(2-5) g / mL, the temperature of alkali treatment is 30-70℃, and the time of alkali treatment is 0.5-3h.

18. A mullite material, characterized in that, The mullite material is prepared by the method described in any one of claims 1-17.

19. A use of the mullite material as described in claim 18, characterized in that, The applications include using the mullite material to prepare ceramic materials, refractory materials, catalyst carriers, microwave absorbing materials, abrasive materials, polishing materials, or fillers.

Images