High purity, low sodium, fine grain alpha alumina and method of making same

Abstract

This application proposes a high-purity, low-sodium, fine-grained α-alumina and its preparation method. Water vapor-activated quartz sand and aluminum hydroxide are ball-milled to form core-shell structured seed crystals. A ternary composite mineralizer is added in stages. The mixture is calcined at varying temperatures, followed by acid washing, water washing, and drying to obtain the high-purity, low-sodium, fine-grained α-alumina. This preparation method achieves uniform seed crystal adhesion, precise segmented supply of the mineralizer, and multiple synergistic sodium reduction, solving technical problems such as high conversion temperature, coarse grains, insufficient purity, and unstable product quality. The α-alumina of this application has a sodium oxide content ≤0.022%, a particle size of 0.4-0.6 μm, and an α phase content of over 99.2%, ensuring that the product meets the needs of the high-end electronic ceramic materials field.

Description

Technical Field

[0001] This application belongs to the field of alumina preparation technology, and particularly relates to a high-purity, low-sodium, fine-grained α-alumina and its preparation method. Background Technology

[0002] α-Al2O3 powder has advantages such as high melting point, corrosion resistance, good chemical stability and excellent insulation properties, and is widely used in the manufacture of high-performance ceramic products such as structural ceramics and electronic ceramics. Among them, high-purity α-Al2O3 powder with a purity of over 99% can meet the material purity requirements of different high-end application fields, such as semiconductors and optics, and has broad application prospects and great market potential.

[0003] Currently, α-Al₂O₃ powders with high purity and relatively uniform particle size can be prepared using methods such as sol-gel method, precipitation method, hydrothermal method, and vapor deposition. However, these methods are costly and complex, making them unsuitable for large-scale industrial production. Industrially, calcination is often used to prepare α-Al₂O₃ powders. Appropriate calcination temperature and time are employed to ensure the precursor is fully converted into α-Al₂O₃. However, calcined α-Al₂O₃ powders are prone to particle growth or agglomeration, thus reducing powder uniformity.

[0004] In calcination methods, to lower the phase transition temperature of α-Al₂O₃ and inhibit excessive grain growth, existing technologies typically add mineralizers or introduce seed crystals during the calcination process. Mineralizers such as fluorides (AlF₃, NaF, NH₄F, etc.), chlorides (NaCl, NH₄Cl, etc.), or borides can form gaseous or liquid mesophases during calcination, promoting mass transfer and lowering the nucleation barrier of the α-phase. For example, Chinese patent CN114751435A discloses a method for preparing α-Al₂O₃ in two steps using industrial alumina as raw material and adding a composite mineralizer of fluoride and chloride. However, single or binary mineralizer systems often suffer from a narrow operating temperature range: fluorides are prone to volatilization and dispersion in the low-temperature calcination section, and the mineralization effect is insufficient in the high-temperature calcination section; while chlorides help remove sodium impurities, excessive addition can lead to abnormal particle growth. Most existing technologies add the mineralizer to the raw meal all at once, resulting in severe dispersion in the early stage and excessively low mineralizer concentration in the later stage, making it impossible to maintain a stable mineralization effect throughout the entire calcination cycle.

[0005] Regarding seed induction, existing technologies have used pre-prepared α-Al₂O₃ seeds to provide heterogeneous nucleation sites for the γ→α phase transition, thereby reducing the phase transition temperature and refining the grain size. For example, Chinese patent CN118929712A discloses a method for preparing fine-grained α-Al₂O₃ by adding α-Al₂O₃ seeds to Bayer process aluminum hydroxide and then calcining it. Chinese patent CN112897996B further optimized the seed particle size and addition amount, but in the above schemes, the seed and precursor particles are only simply physically mixed, resulting in poor uniformity of seed distribution in the raw material system. This leads to the need for high-temperature and long-term transformation in local seedless areas, thus failing to maximize the induction effect. In addition, the seed material is limited to α-Al₂O₃ itself. To improve the uniformity of seed distribution, some studies have attempted to combine the seed and precursor through mechanical grinding or spray granulation, but the resulting composite particles are mostly random agglomerations, failing to achieve uniform and dense coating of the seed on the surface of each precursor particle.

[0006] Furthermore, the Bayer process for industrial aluminum hydroxide typically introduces sodium-containing additives during production, resulting in a final product containing 0.2%-0.5% Na₂O impurities. Sodium reacts with α-Al₂O₃ at high temperatures to form β-Al₂O₃ (Na₂O·11Al₂O₃) or sodium aluminate, severely degrading the high-temperature insulation and dielectric properties of the product in the electronic ceramics field. Traditional sodium removal methods include acid washing pretreatment or high-temperature mineralizing agent reaction, but the former only removes surface-adsorbed sodium, while the latter's removal efficiency is unstable due to the type of mineralizing agent and the feeding method. How to efficiently remove sodium impurities while ensuring complete α-phase transformation and a fine-grained structure has long been a challenging problem for those skilled in the art.

[0007] Therefore, the existing α-Al2O3 calcination process still has significant shortcomings in terms of mineralizer delivery efficiency, seed distribution uniformity, deep removal of sodium impurities, and development and utilization of heterogeneous seed crystals. Summary of the Invention

[0008] This application provides a high-purity, low-sodium, fine-grained α-alumina and its preparation method to solve the problems existing in related technologies. The technical solution is as follows: In a first aspect, embodiments of this application provide a method for preparing high-purity, low-sodium, fine-grained α-alumina, comprising the following steps: Steam-activated quartz sand and aluminum hydroxide are ball-milled and then mixed with a first mineralizing agent solution to obtain a mixture. The mixture is heated to a first temperature under a first atmosphere, then a second atmosphere is introduced and a second mineralizing agent solution is added; the mixture is then heated to a second temperature and subjected to a first heat preservation process. After the first heat preservation is completed, the temperature is raised to the third temperature, and the second heat preservation is carried out. After the second heat treatment is completed, the product is acid-washed, water-washed, and then dried to obtain the high-purity, low-sodium, fine-grained α-alumina.

[0009] In one embodiment, the average particle size D50 of the steam-activated quartz sand is 0.1-0.5 μm; and the average particle size D50 of the aluminum hydroxide is 5-20 μm.

[0010] In one embodiment, the mass ratio of steam-activated quartz sand to aluminum hydroxide is (0.5-2):100.

[0011] In one embodiment, the ball milling is carried out using a planetary ball mill; the conditions are a rotation speed of 300-500 rpm and a time of 30-60 min.

[0012] In one embodiment, quartz sand is reacted at 850-900°C under a mixture of water vapor and nitrogen. After the reaction is completed, it is cooled under an inert gas atmosphere to obtain water vapor activated quartz sand.

[0013] In one embodiment, the aluminum hydroxide is preheated at 320-350°C for 2-3 hours.

[0014] In one embodiment, the temperature is increased to 850-900°C at a rate of 4-6°C / min, the volume ratio of water vapor to nitrogen is 1:(2-4), and the reaction time is 1-2 hours; the mixture is then cooled under nitrogen protection.

[0015] In one embodiment, the first mineralizing agent solution comprises the following components in parts by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; The second mineralizing agent solution comprises the following components by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; In the first mineralizing agent solution and the second mineralizing agent solution, the mass ratio of the first mineralizing agent to the second mineralizing agent is 1:(0.6-1.5).

[0016] In one embodiment, the sum of the first mineralizing agent and the second mineralizing agent is 1.3-2.1% of the mass of aluminum hydroxide.

[0017] In one embodiment, the first atmosphere is air or nitrogen; the first temperature is 300-400°C; the second atmosphere is a mixture of water vapor and nitrogen, with a volume ratio of water vapor to nitrogen of 1:(2-3); the second temperature is 950-1050°C; the first holding time is 0.5-1h; the third temperature is 1220-1250°C; and the second holding time is 0.5-1.5h.

[0018] In one embodiment, pickling is performed using a dilute hydrochloric acid solution at a temperature of 50-65°C for 30-60 minutes; the concentration of the dilute hydrochloric acid is 0.05-0.1 mol / L, and the mass ratio of the dilute hydrochloric acid solution to the product is (5-10):1.

[0019] In one embodiment, deionized water is used for washing until the conductivity of the washing solution is ≤50μS / cm; drying is carried out at 100-110℃ for 2-10 hours.

[0020] In one embodiment, the temperature is increased to a first temperature at a heating rate of 3-5°C / min; the temperature is increased to a second temperature at a heating rate of 6-8°C / min; and the temperature is increased to a third temperature at a heating rate of 4-6°C / min.

[0021] In one embodiment, after the second heat preservation ends, the furnace temperature is reduced to 700-900°C at a cooling rate of 2-4°C / min, and then cooled to room temperature along with the furnace.

[0022] In one embodiment, the second mineralizer solution is injected into the mixture in a pulsed manner.

[0023] Secondly, embodiments of this application provide a high-purity, low-sodium, fine-grained α-alumina, prepared by the above-described method for preparing high-purity, low-sodium, fine-grained α-alumina.

[0024] In one embodiment, the sodium oxide content is ≤0.022%; the particle size is 0.4-0.6μm; and the α phase accounts for more than 99.2%.

[0025] The advantages or beneficial effects of the above technical solutions include at least the following: This application presents a high-purity, low-sodium, fine-grained α-alumina and its preparation method. Activated silica sand is used as a heterogeneous seed crystal, and the mineralizer is precisely supplied via a time-phased gradient feeding method. The mineralizer decomposes or reacts stepwise at different temperature ranges, forming a stepped mineralization effect of "low-temperature induction - medium-temperature regulation - high-temperature enhancement," significantly reducing the nucleation and growth temperature of α-Al₂O₃ and improving mineralization efficiency. Furthermore, through a multiple synergistic mechanism of pretreatment desodiuming, chemical desodiuming with mineralizer, and deep desodiuming via acid leaching, the sodium content is reduced without the need for expensive low-sodium raw materials, thus lowering costs. The α-alumina prepared by this application has a sodium oxide content ≤0.022%, a particle size of 0.4-0.6 μm, and an α phase content of over 99.2%, ensuring that the product meets the needs of the high-end electronic ceramic materials field.

[0026] The above overview is for illustrative purposes only and is not intended to be limiting in any way. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features of this application will become readily apparent from the accompanying drawings and the following detailed description. Attached Figure Description

[0027] In the accompanying drawings, unless otherwise specified, the same reference numerals throughout the various drawings denote the same or similar parts or elements. These drawings are not necessarily drawn to scale. It should be understood that these drawings depict only some embodiments disclosed in this application and should not be construed as limiting the scope of this application.

[0028] Figure 1 Transmission electron microscope images of α-Al2O3 were prepared for the embodiments of this application; Figure 2 The energy dispersive spectroscopy (EDS) diagram of the core-shell composite particles prepared in Example 1, which have an Al(OH)3 core-SiO2 shell structure. Detailed Implementation

[0029] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of this application. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0030] Existing α-Al₂O₃ calcination processes still have significant shortcomings in terms of mineralizer delivery efficiency, seed crystal distribution uniformity, deep removal of sodium impurities, and the development and utilization of heterogeneous seed crystals. Therefore, this application provides a high-purity, low-sodium, fine-grained α-alumina and its preparation method.

[0031] A method for preparing high-purity, low-sodium, fine-grained α-alumina includes the following steps: Steam-activated quartz sand and aluminum hydroxide are ball-milled and then mixed with a first mineralizing agent solution to obtain a mixture. The mixture is heated to a first temperature under a first atmosphere, then a second atmosphere is introduced and a second mineralizing agent solution is added; the mixture is then heated to a second temperature and subjected to a first heat preservation process. After the first heat preservation is completed, the temperature is raised to the third temperature, and the second heat preservation is carried out. After the second heat treatment is completed, the product is acid-washed, water-washed, and then dried to obtain the high-purity, low-sodium, fine-grained α-alumina.

[0032] This application uses activated silica sand as a heterogeneous seed crystal. Activation with steam forms abundant Si-OH active groups on the surface of the silica sand, enhancing its inductive ability. More importantly, this invention uses ball milling to uniformly adhere the activated silica sand to the surface of aluminum hydroxide particles, forming a core-shell structure composite particle with aluminum hydroxide as the core and activated silica sand as the shell. This core-shell structure ensures close contact between the seed crystal and the raw material particles, maximizing the inductive effect of the heterogeneous seed crystal.

[0033] This application also designs a time-phased gradient feeding strategy to address the problem of severe early-stage mineralization and insufficient later-stage mineralization caused by single-stage feeding. The composite mineralizer is added during the raw material preparation stage, and the remaining part is added after the furnace is heated; this achieves precise supply of mineralizer during the nucleation stage and significantly improves mineralization efficiency.

[0034] Furthermore, through a multi-synergistic mechanism of pretreatment desodiuming, mineralizing agent chemical desodiuming, and acid washing deep desodiuming, the sodium content can be reduced without the need for expensive low-sodium raw materials, thus lowering costs.

[0035] In one embodiment, the average particle size D50 of steam-activated quartz sand is 0.1-0.5 μm; the average particle size D50 of aluminum hydroxide is 5-20 μm.

[0036] Nanoscale activated silica sand is uniformly attached to the surface of micron-sized aluminum hydroxide particles, forming a core-shell structure composite particle with aluminum hydroxide as the core and activated silica sand as the shell. This core-shell structure ensures close contact between the seed crystal and the raw material particles, maximizing the induction effect of the heterogeneous seed crystal.

[0037] As one implementation method, the mass ratio of steam-activated quartz sand to aluminum hydroxide is (0.5-2):100.

[0038] As one implementation method, the ball milling is carried out using a planetary ball mill; the conditions are a rotation speed of 300-500 rpm and a time of 30-60 min.

[0039] Ball milling or using a planetary ball mill ensures sufficient contact between the activated quartz sand seed crystals and the aluminum hydroxide particles, achieving the ideal state where "each aluminum hydroxide particle is surrounded by a seed crystal." The seed crystals induce the formation of fine crystals, which are evenly distributed. Instead, coarse crystals are spontaneously nucleated in seedless areas, resulting in extremely uneven grain size.

[0040] In one embodiment, quartz sand is reacted at 850-900°C under a mixture of water vapor and nitrogen. After the reaction is completed, it is cooled under an inert gas atmosphere to obtain water vapor activated quartz sand.

[0041] Steam activation creates abundant Si-OH active groups on the surface of quartz sand. On one hand, these Si-OH groups form hydrogen bonds with the hydroxyl groups on the Al(OH)3 surface, resulting in a uniform core-shell structure after ball milling. On the other hand, during calcination, they form Si-O-Al chemical bonds with the Al2O3 precursor, providing a low-energy nucleation interface and significantly reducing the nucleation barrier of the α-phase. Furthermore, seed crystals limit grain growth and prevent sintering neck formation. Unactivated quartz sand, with its inert surface, cannot achieve uniform coating, leading to abnormal grain growth and coalescence in localized seedless areas. Preferably, activation is carried out in a tube furnace, with a mixture of steam and nitrogen introduced into the furnace at a specific flow rate.

[0042] In one embodiment, the aluminum hydroxide is preheated at 320-350°C for 2-3 hours.

[0043] In the raw material pretreatment stage, aluminum hydroxide is heated to volatilize the soluble sodium salts adsorbed on its surface, thus achieving initial sodium reduction. In this embodiment, Bayer process industrial aluminum hydroxide is preferably used.

[0044] As one implementation method, the temperature is increased to 850-900℃ at a rate of 4-6℃ / min, the volume ratio of water vapor to nitrogen is 1:(2-4), and the reaction time is 1-2h; then cooled under nitrogen protection.

[0045] In one embodiment, the first mineralizing agent solution comprises the following components by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; The second mineralizing agent solution comprises the following components by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; In the first mineralizing agent solution and the second mineralizing agent solution, the mass ratio of the first mineralizing agent to the second mineralizing agent is 1:(0.6-1.5).

[0046] In one embodiment, the first atmosphere is air or nitrogen; the first temperature is 300-400°C; the second atmosphere is a mixture of water vapor and nitrogen, with a volume ratio of water vapor to nitrogen of 1:(2-3); the second temperature is 950-1050°C; the first holding time is 0.5-1h; the third temperature is 1220-1250°C; and the second holding time is 0.5-1.5h.

[0047] Traditional mineralizers are generally single fluorides or fluorides + chlorides. The mineralizer in this application is a ternary composite mineralizer system composed of ammonium fluoride (NH4F), ammonium chloride (NH4Cl), and trace amounts of aluminum fluoride (AlF3). The composite mineralizer in this application corresponds to the processing temperature. The three components of the mineralizer decompose or react stepwise at different temperature ranges: NH4F releases HF at low temperatures (~250℃), NH4Cl releases HCl at medium temperatures (~340℃), and AlF3 sublimates at high temperatures (>800℃) and reacts with the Al2O3 surface to form the AlOF mesophase. These three components form a stepped mineralization effect of "low-temperature induction - medium-temperature regulation - high-temperature enhancement," significantly reducing the nucleation and growth temperature of α-Al2O3.

[0048] The mixture of this application is heated to a first temperature under a first atmosphere, the first temperature being 300-400℃. When the temperature is raised to (~250℃), NH4F releases HF, and when the temperature is raised to the first temperature, NH4Cl releases HCl. During the calcination process at a second temperature, AlF3 sublimates and reacts with the surface of Al2O3 to form an AlOF mesophase.

[0049] In addition, the ammonium chloride, ammonium fluoride, and aluminum fluoride in the composite mineralizer can react with sodium impurities during calcination to generate volatile sodium salts, which are then discharged, thus achieving chemical desodiumification.

[0050] As one embodiment, the reaction of this application is carried out in a tubular furnace, with different atmospheres introduced into the tubular furnace or muffle furnace at different flow rates.

[0051] In one embodiment, the sum of the first mineralizing agent and the second mineralizing agent is 1.3-2.1% of the mass of aluminum hydroxide.

[0052] In one implementation method, the second mineralizer solution is injected into the mixture in a pulsed manner.

[0053] The time-division gradient feeding strategy of this application involves adding a portion of the composite mineralizer during the raw material preparation stage, and the remainder after the furnace is heated. Simultaneously, a small amount of steam is introduced at the moment of feeding to promote the rapid conversion of SiF4 to SiO2 seed crystals. The SiO2 seed crystals participate in the HF cycle, and the Al-OF intermediate phase works synergistically, achieving precise supply of the mineralizer during the nucleation period, significantly improving mineralization efficiency. Finally, after the steam is consumed, the silicon content in SiO2 is removed by SiF4. A second mineralizer solution is pulsed into the mixture while simultaneously being stirred at high speed, ensuring uniform mixing of the second mineralizer and the mixture, resulting in a uniform coating of the mineralizer on the particle surface.

[0054] In one implementation method, a dilute hydrochloric acid solution is used for pickling. The pickling temperature is 50-65℃ and the pickling time is 30-60 min. The concentration of the dilute hydrochloric acid is 0.05-0.1 mol / L, and the mass ratio of the dilute hydrochloric acid solution to the product is (5-10):1.

[0055] Pickling is a method of sodium removal. This application not only uses acid for sodium removal in the subsequent process, but also achieves a multi-synergistic mechanism of pretreatment sodium removal, chemical sodium removal with mineralizing agent, and deep sodium removal by preheating aluminum hydroxide at 320-350℃ for 2-3 hours and chemical sodium removal with mineralizing agent.

[0056] As one implementation method, deionized water is used for washing until the conductivity of the washing solution is ≤50μS / cm; drying is carried out at 100-110℃ for 2-10 hours.

[0057] In one implementation, the temperature is increased to a first temperature at a rate of 3-5°C / min; the temperature is increased to a second temperature at a rate of 6-8°C / min; and the temperature is increased to a third temperature at a rate of 4-6°C / min.

[0058] The heating in this application is carried out at a certain heating rate; the heating of the first temperature is carried out at a certain heating rate, and NH4F in the mineralizer can release HF at low temperature (~250℃).

[0059] In one implementation method, after the second heat preservation ends, the furnace temperature is reduced to 700-900℃ at a cooling rate of 2-4℃ / min, and then cooled to room temperature along with the furnace.

[0060] As one implementation method, a jet mill is used to perform mild deagglomeration under an air pressure of 0.4-0.6 MPa, and the product α-alumina powder is obtained by passing it through a 325-mesh sieve.

[0061] This application provides a high-purity, low-sodium, fine-grained α-alumina, which is prepared by the above-described method for preparing high-purity, low-sodium, fine-grained α-alumina.

[0062] In one embodiment, the sodium oxide content is ≤0.022%; the particle size is 0.4-0.6μm; and the α phase accounts for more than 99.2%.

[0063] The following specific examples will provide further details.

[0064] Example 1 Bayer process industrial aluminum hydroxide was placed in a heating vessel, heated to 320°C, and held for 3 hours to obtain pre-activated aluminum hydroxide powder (D50 of 12 μm); quartz sand was placed in a tube furnace, heated to 850°C at 5°C / min, and a mixture of steam (2 L / min) and nitrogen (6 L / min) was introduced and held for 1.5 hours, then cooled under nitrogen protection to obtain pre-activated quartz sand powder (D50 of 0.3 μm). 100 parts of activated aluminum hydroxide powder and 1.0 part of activated quartz sand powder were added to a planetary ball mill and ball-milled at 400 rpm for 45 min to form core-shell structured composite particles; the elemental distribution diagram of transmission electron microscopy is shown below. Figure 2 As shown; Take 0.6 parts of ammonium chloride, 0.8 parts of ammonium fluoride, and 0.3 parts of aluminum fluoride, dissolve them in 15 parts of deionized water, and stir until completely dissolved to obtain a composite mineralizer solution; Take 50% of the total amount of composite mineralizer and atomize it into the mixed powder while stirring at high speed to make the mineralizer evenly coat the surface of the particles. The mixture was loaded into an alumina boat and placed in a temperature-controlled tube furnace. Nitrogen gas (flow rate 3 L / min) was introduced to maintain positive pressure inside the furnace. The temperature was increased from room temperature to 350°C at a rate of 4°C / min. After reaching 350°C, the remaining composite mineralizer was injected in a pulsed manner, while a mixture of water vapor (1.5 L / min) and nitrogen gas (4 L / min) was introduced for 8 minutes. The heating rate was then adjusted to 7°C / min, and the temperature was rapidly increased to 1000°C and held for 0.8 hours. The temperature was increased from 1000℃ to 1230℃ at a rate of 5℃ / min and held at that temperature for 1 hour; then the temperature was decreased to 800℃ at a rate of 3℃ / min and cooled to room temperature in the furnace to obtain crude α-alumina. The crude product was added to 8 times its mass of 0.08 mol / L dilute hydrochloric acid solution, stirred at 60°C for 45 min, filtered, and the filter cake was washed with deionized water until the conductivity of the washing solution was ≤50 μS / cm. It was then dried at 110°C for 6 h, and slightly depolymerized using an air jet mill under 0.5 MPa air pressure. The product was then passed through a 325-mesh sieve to obtain high-purity, low-sodium, fine-crystalline α-alumina. High-resolution transmission electron microscopy was used for analysis. Figure 1 As shown.

[0065] Example 2 Bayer process industrial aluminum hydroxide was placed in a heating kettle, heated to 350°C, and held for 2 hours to obtain pre-activated aluminum hydroxide powder (D50 is 8 μm). Quartz sand was placed in a tube furnace and heated to 900°C at a rate of 5°C / min. A mixture of steam (2L / min) and nitrogen (6L / min) was introduced and kept at this temperature for 1.5 hours. The mixture was then cooled under nitrogen protection to obtain pre-activated quartz sand powder (D50 of 0.15μm). Take 100 parts of activated aluminum hydroxide powder and 0.8 parts of activated quartz sand powder, add them to a planetary ball mill, and ball mill them at 350 rpm for 50 min to form core-shell structured composite particles; Take 0.5 parts ammonium chloride, 0.6 parts ammonium fluoride, and 0.2 parts aluminum fluoride, dissolve them in 12 parts deionized water, stir to dissolve, and obtain a composite mineralizing agent solution; Take 55% of the total amount of composite mineralizer and atomize it into the mixed powder while stirring at high speed to make the mineralizer evenly coat the surface of the particles. The mixture was loaded into an alumina boat and placed in a temperature-controlled tube furnace. Air was introduced (flow rate 2.5 L / min) to maintain positive pressure inside the furnace. The temperature was increased from room temperature to 350°C at a rate of 3.5°C / min. After reaching 350°C, the remaining mineralizing agent was pulsed in, and a mixture of water vapor (1.5 L / min) and nitrogen (4 L / min) was introduced for 8 min. The heating rate was adjusted to 6.5°C / min, and the temperature was rapidly increased to 1000°C and held for 0.6 h. The temperature was increased from 1000℃ to 1220℃ at a rate of 4.5℃ / min and held at that temperature for 1.2 hours. The temperature was then decreased to 800℃ at a rate of 2.5℃ / min and cooled to room temperature in the furnace to obtain crude α-alumina. The crude product was added to 6 times its mass of 0.06 mol / L dilute hydrochloric acid, stirred at 60℃ for 35 min, filtered, and washed with water until the conductivity was ≤50 μS / cm; dried at 110℃ for 6 h, and slightly depolymerized under 0.45 MPa in an air jet mill, and passed through a 325 mesh sieve to obtain high-purity, low-sodium, fine-grained α-alumina.

[0066] Example 3 Bayer process industrial aluminum hydroxide was placed in a heating kettle, heated to 335°C, and held for 2.5 hours to obtain pre-activated aluminum hydroxide powder (D50 of 18 μm). Quartz sand was heated to 875°C at 5°C / min, and a mixture of water vapor (2L / min) and nitrogen (6L / min) was introduced and kept at this temperature for 1.5h. After cooling, activated quartz sand powder (D50 of 0.45μm) was obtained. Take 100 parts of activated aluminum hydroxide powder and 1.8 parts of activated quartz sand powder, add them to a planetary ball mill, and ball mill them at 480 rpm for 35 minutes to form core-shell structured composite particles. Take 0.75 parts of ammonium chloride, 0.9 parts of ammonium fluoride, and 0.45 parts of aluminum fluoride, dissolve them in 18 parts of deionized water, stir to dissolve, and obtain a composite mineralizing agent solution; Take 45% of the total amount of composite mineralizer and atomize it into the mixed powder while stirring at high speed to make the mineralizer evenly coat the surface of the particles. The material was loaded into a crucible and placed in a muffle furnace. Nitrogen gas (flow rate 4.5 L / min) was introduced to maintain positive pressure. The temperature was raised from room temperature to 350°C at a rate of 4.5°C / min. After reaching 350°C, the remaining mineralizing agent was pulsed in, and a mixture of water vapor (1.5 L / min) and nitrogen gas (4 L / min) was introduced for 8 min. The heating rate was adjusted to 7.5°C / min, and the temperature was rapidly raised to 1000°C and held for 1.2 h. The temperature was increased from 1000℃ to 1245℃ at a rate of 5.5℃ / min and held at that temperature for 0.7h; then the temperature was decreased to 800℃ at a rate of 3.5℃ / min and cooled to room temperature in the furnace to obtain crude α-alumina. The crude product was added to 9 times its mass of 0.09 mol / L dilute hydrochloric acid, stirred at 60℃ for 55 min, filtered, and washed with water until the conductivity was ≤50 μS / cm; dried at 110℃ for 6 h, and slightly depolymerized under 0.55 MPa in an air jet mill, and passed through a 325 mesh sieve to obtain high-purity, low-sodium, fine-grained α-alumina.

[0067] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that no quartz sand seeds are added, specifically: Bayer process industrial aluminum hydroxide was placed in a heating kettle, heated to 320°C, and held for 3 hours to obtain pre-activated aluminum hydroxide powder (D50 of 12 μm); 0.6 parts ammonium chloride, 0.8 parts ammonium fluoride, and 0.3 parts aluminum fluoride were dissolved in 15 parts deionized water and stirred until completely dissolved to obtain a composite mineralizer solution; 50% of the total amount of composite mineralizer was atomized and sprayed into the mixed powder while stirring at high speed to ensure that the mineralizer was uniformly coated on the particle surface; The mixture was loaded into an alumina boat and placed in a temperature-controlled tube furnace. Nitrogen gas (flow rate 3 L / min) was introduced to maintain positive pressure inside the furnace. The temperature was increased from room temperature to 350°C at a rate of 4°C / min. After reaching 350°C, the remaining composite mineralizer was injected in a pulsed manner, while a mixture of water vapor (1.5 L / min) and nitrogen gas (4 L / min) was introduced for 8 minutes. The heating rate was then adjusted to 7°C / min, and the temperature was rapidly increased to 1000°C and held for 0.8 hours. The temperature was increased from 1000℃ to 1230℃ at a rate of 5℃ / min and held at that temperature for 1 hour; then the temperature was decreased to 800℃ at a rate of 3℃ / min and cooled to room temperature in the furnace to obtain crude α-alumina. The crude product was added to 8 times its mass of 0.08 mol / L dilute hydrochloric acid solution, stirred at 60℃ for 45 min, filtered, and the filter cake was washed with deionized water until the conductivity of the washing solution was ≤50 μS / cm. It was dried at 110℃ for 6 h, and then slightly depolymerized using an air jet mill under 0.5 MPa air pressure. The product was then passed through a 325 mesh sieve to obtain high-purity, low-sodium, fine-grained α-alumina.

[0068] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that ammonium chloride and aluminum fluoride are not added; ammonium fluoride is used as the single mineralizing agent. Specifically: Take 1.7 parts of ammonium fluoride, dissolve it in 15 parts of deionized water, and stir until completely dissolved to obtain a composite mineralizer solution.

[0069] Everything else is the same as in Example 1.

[0070] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the composite mineralizer is added all at once, specifically: Bayer process industrial aluminum hydroxide was placed in a heating vessel, heated to 320°C, and held for 3 hours to obtain pre-activated aluminum hydroxide powder (D50 of 12 μm); quartz sand was placed in a tube furnace, heated to 850°C at 5°C / min, and a mixture of steam (2 L / min) and nitrogen (6 L / min) was introduced and held for 1.5 hours, then cooled under nitrogen protection to obtain pre-activated quartz sand powder (D50 of 0.3 μm). Take 100 parts of activated aluminum hydroxide powder and 1.0 part of activated quartz sand powder, add them to a planetary ball mill, and ball mill them at 400 rpm for 45 min to form core-shell structured composite particles; Take 0.6 parts of ammonium chloride, 0.8 parts of ammonium fluoride, and 0.3 parts of aluminum fluoride, dissolve them in 15 parts of deionized water, and stir until completely dissolved to obtain a composite mineralizer solution; The composite mineralizer is atomized and sprayed into the mixed powder while being stirred at high speed to ensure that the mineralizer is evenly coated on the particle surface. The mixture is then loaded into a corundum boat and placed in a programmable temperature-controlled tube furnace. Nitrogen gas (flow rate 3L / min) is introduced to maintain positive pressure inside the furnace. The temperature is raised from room temperature to 350°C at a rate of 4°C / min. After reaching 350°C, the heating rate is adjusted to 7°C / min, and the temperature is rapidly raised to 1000°C and held for 0.8 hours. The temperature was increased from 1000℃ to 1230℃ at a rate of 5℃ / min and held at that temperature for 1 hour; then the temperature was decreased to 800℃ at a rate of 3℃ / min and cooled to room temperature in the furnace to obtain crude α-alumina. The crude product was added to 8 times its mass of 0.08 mol / L dilute hydrochloric acid solution, stirred at 60℃ for 45 min, filtered, and the filter cake was washed with deionized water until the conductivity of the washing solution was ≤50 μS / cm. It was dried at 110℃ for 6 h, and then slightly depolymerized using an air jet mill under 0.5 MPa air pressure. The product was then passed through a 325 mesh sieve to obtain high-purity, low-sodium, fine-grained α-alumina.

[0071] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the activated aluminum hydroxide powder and activated quartz sand powder were simply mixed for 30 minutes at low speed (20 rpm) using a V-type mixer, without high-energy ball milling; specifically: Take 100 parts of activated aluminum hydroxide powder and 1.0 part of activated quartz sand powder, add them to a V-type mixer and mix them simply at a low speed (20 rpm) for 30 minutes; the rest is the same as in Example 1.

[0072] Comparative Example 5 The quartz sand is not activated. Unactivated ordinary quartz sand (D50=0.3μm) is used instead of preactivated quartz sand powder. The remaining steps are the same as in Example 1.

[0073] The alumina from Examples 1-3 and Comparative Examples 1-5 were tested, and the contents of Fe2O3, Na2O, and SiO2 are shown in Table 1; the original grain size, specific surface area, and α phase content are also shown in Table 1.

[0074] Table 1

[0075] Table 1 shows a performance comparison of α-Al₂O₃ powders in Examples 1-3 and Comparative Examples 1-5. The sodium oxide content in Examples 1-3 is ≤0.022%, far lower than that of conventional Bayer process products, meeting the requirements for electronic ceramic grade (≤0.05%). Furthermore, the iron content is low, meeting the requirements for high-purity alumina. The SiO₂ is derived from activated silica sand seeds, with controllable and stable content. The original grain size is between 0.4-0.6 μm, which is fine and uniform, significantly smaller than that of traditional processes (1.0-2.0 μm). The specific surface area is moderately high, indicating fine grains, no severe sintering agglomeration, and near-complete α-phase transformation, far exceeding that of traditional processes (typically 92-96%). In contrast, the performance of each comparative example shows significant deterioration due to the absence or improper application of key process steps.

[0076] Comparative Example 1 lacks activated quartz sand seeds, resulting in insufficient seed induction, obstructed sodium volatilization channels, and low sodium removal efficiency. Without seeds to provide nucleation sites, higher temperatures are required for transformation, while the highest temperature in this application is only 1250℃, leading to a large amount of residual transition phase. Spontaneous nucleation is minimal, resulting in long grain growth time, coarse grains, dense sintering, and poor dispersibility.

[0077] Comparative Example 5 (without activated quartz sand) was significantly coarser than Example 1 (0.45 μm), demonstrating that the unactivated sand had weak nucleation induction ability and was prone to grain growth. Specific surface area (2.8 m²) 2 / g): significantly lower than Example 1 (8.2m 2 / g), the powder was severely sintered and poorly dispersed; the α phase (84%) was much lower than that of Example 1 (99.5%), and a large amount of transition phase remained, indicating a low seed induction effect. The alumina of Comparative Example 5 was slightly better than that of Comparative Example 1.

[0078] Comparative Example 2 uses a single NH4F mineralizer, with an α-phase conversion rate of only 78%. NH4F decomposes at ~250℃ to produce HF, which is effective for low-temperature mineralization. However, above 350℃, a large amount of HF escapes, resulting in insufficient mineralization in the high-temperature zone and the retention of the transition phase.

[0079] In Comparative Example 3, the alumina Na₂O content was as high as 0.11%. The mineralizer was added only once. During the calcination process after addition, significant volatilization occurred in the early stage of heating, and sodium removal was not aided by the mineralizer in the later stage. The α-phase conversion rate was 85%, which, although higher than that of Comparative Example 2, still contained 15% transition phase. The primary crystals were coarse (approximately 1.20 μm); the excessive mineralizer in the early stage led to localized excessive liquid phase sintering, resulting in abnormally large grains. The specific surface area was only 3.0 m². 2 / g: Severe sintering and agglomeration.

[0080] Comparative Example 4 (simple mixture of quartz sand and aluminum hydroxide, without forming a core-shell structure): α-phase conversion rate 82%: Insufficient contact between seed crystals and aluminum hydroxide particles, failing to achieve the ideal state of "seed crystals surrounding every aluminum hydroxide particle." In some areas, seed crystals induced the formation of fine crystals, while in seedless areas, coarse crystals spontaneously nucleated, resulting in extremely uneven grain size. Na₂O content 0.08%: Although higher than the example, the desodium removal effect was slightly better than in Comparative Example 1 without seed crystals because some seed crystals were still in contact.

[0081] The crystal forms of alumina in Example 1 and Comparative Examples 1, 4 and 5 were further examined to determine the role of quartz sand as a heterogeneous seed crystal. The crystal forms of alumina are shown in Table 2; the particle size distribution is also shown in Table 2.

[0082] Table 2

[0083] In Example 1, the total residual transition phase was only 0.5%, almost completely converted into the α phase; in Comparative Example 5, the residual transition phase reached 11.5%, of which the θ-Al2O3 content was as high as 8.2%, indicating that the conversion was hindered in the 1000-1200℃ range.

[0084] During calcination, the Si-OH groups on the surface of activated quartz sand form Si-O-Al chemical bonds with the Al2O3 precursor, providing a low-energy nucleation interface and significantly reducing the nucleation barrier of the α phase. Unactivated quartz sand, lacking active groups, exists only as a physical foreign body and cannot provide effective lattice-matched heterogeneous nucleation sites, resulting in a delayed γ→θ→α transformation and a large amount of residual θ phase. Example 1 has a span of only 1.05, indicating an extremely narrow particle size distribution and excellent dispersibility. Comparative Example 5 has a span of 1.60, a wide distribution, and significant agglomeration. The Si-OH groups on the surface of activated quartz sand form hydrogen bonds with the hydroxyl groups on the Al(OH)3 surface, resulting in a uniform core-shell structure after ball milling. During calcination, seed crystals limit grain growth and prevent sintering neck formation. Unactivated quartz sand, with its inert surface, cannot uniformly coat the grains, leading to abnormal particle growth and fusion in localized seedless areas.

[0085] In summary, this invention systematically solves the long-standing common technical problems in α-Al2O3 production, such as high conversion temperature, high sodium residue, and coarse grains, through four synergistic mechanisms: pretreatment for sodium removal, core-shell structure seed crystals, ternary composite mineralizers, and timed feeding. It achieves the goal of directly preparing high-end electronic ceramic α-Al2O3 powder from industrial aluminum hydroxide at low cost.

[0086] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of those different embodiments or examples.

[0087] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly specified.

[0088] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in this application, and these should all be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for preparing high-purity, low-sodium, fine-grained α-alumina, characterized in that, Includes the following steps: Steam-activated quartz sand and aluminum hydroxide are ball-milled and then mixed with a first mineralizing agent solution to obtain a mixture. The mixture is heated to a first temperature under a first atmosphere, then a second atmosphere is introduced and a second mineralizing agent solution is added; the mixture is then heated to a second temperature and subjected to a first heat preservation process. After the first heat preservation is completed, the temperature is raised to the third temperature, and the second heat preservation is carried out. After the second heat treatment is completed, the product is acid-washed, water-washed, and then dried to obtain the high-purity, low-sodium, fine-grained α-alumina.

2. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, The average particle size D50 of steam-activated quartz sand is 0.1-0.5 μm; the average particle size D50 of aluminum hydroxide is 5-20 μm. The mass ratio of water vapor-activated quartz sand to aluminum hydroxide is (0.5-2):100; The ball milling was carried out using a planetary ball mill; the conditions were a rotation speed of 300-500 rpm and a time of 30-60 min.

3. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, Quartz sand is reacted at 850-900℃ under a mixture of water vapor and nitrogen. After the reaction is completed, it is cooled under an inert gas atmosphere to obtain water vapor activated quartz sand. The aluminum hydroxide is preheated at 320-350℃ for 2-3 hours.

4. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 3, characterized in that, The temperature is increased to 850-900℃ at a rate of 4-6℃ / min, the volume ratio of water vapor to nitrogen is 1:(2-4), and the reaction time is 1-2h; then cooled under nitrogen protection.

5. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, The first mineralizing agent solution comprises the following components by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; The second mineralizing agent solution comprises the following components by weight: 0.4-0.8 parts ammonium chloride, 0.5-1.0 parts ammonium fluoride, 0.2-0.5 parts aluminum fluoride, and 10-20 parts water; In the first mineralizing agent solution and the second mineralizing agent solution, the mass ratio of the first mineralizing agent to the second mineralizing agent is 1:(0.6-1.5). The sum of the first and second mineralizing agents is 1.3-2.1% of the mass of aluminum hydroxide.

6. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, The first atmosphere is air or nitrogen; the first temperature is 300-400℃; the second atmosphere is a mixture of water vapor and nitrogen, with a volume ratio of water vapor to nitrogen of 1:(2-3); the second temperature is 950-1050℃; the first holding time is 0.5-1h. The third temperature is 1220-1250℃; the second heat preservation time is 0.5-1.5h.

7. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, Pickling is performed using a dilute hydrochloric acid solution at a temperature of 50-65℃ for 30-60 minutes. The concentration of the dilute hydrochloric acid is 0.05-0.1 mol / L, and the mass ratio of the dilute hydrochloric acid solution to the product is (5-10):

1. Use deionized water for washing until the conductivity of the washing solution is ≤50μS / cm; dry at 100-110℃ for 2-10 hours.

8. The method for preparing high-purity, low-sodium, fine-grained α-alumina according to claim 1, characterized in that, The temperature is increased to the first temperature at a rate of 3-5℃ / min; the temperature is increased to the second temperature at a rate of 6-8℃ / min; and the temperature is increased to the third temperature at a rate of 4-6℃ / min. After the second heat preservation is completed, the furnace temperature is reduced to 700-900℃ at a cooling rate of 2-4℃ / min, and then cooled to room temperature along with the furnace. The second mineralizer solution is injected into the mixture in a pulsed manner.

9. A high-purity, low-sodium, fine-grained α-alumina, characterized in that, It is prepared by the method for preparing high-purity, low-sodium, fine-grained α-alumina according to any one of claims 1-8.

10. The high-purity, low-sodium, fine-grained α-alumina according to claim 9, characterized in that, Sodium oxide content ≤0.022%; particle size 0.4-0.6μm; α phase accounts for more than 99.2%.

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