Method for producing small specific surface small particle size high purity alumina by alcohol aluminum method boehmite precursor

Abstract

The application discloses a method for producing small specific surface area small particle size high-purity alumina by using an alcohol aluminum method boehmite precursor, and belongs to the field of inorganic non-metal material preparation. The method uses boehmite synthesized by the alcohol aluminum method as a precursor, simultaneously adds aluminum-organic complex treatment agent A and ammonium fluoride treatment agent B into the boehmite slurry, and completes the collaborative surface treatment under the protection of nitrogen / noble gas at 60-80 DEG C for 2-4 h through stirring reaction, and then the target powder is obtained through two-stage program temperature control calcination and post-treatment. The application deeply couples in-situ surface passivation and catalytic phase change technology, breaks through the technical prejudice that small specific surface area and small particle size cannot be realized simultaneously, simultaneously realizes complete phase change of boehmite, grain growth inhibition and pore densification at low temperature, the obtained powder has an alpha-Al2O3 content of greater than or equal to 99%, a specific surface area of 3-8 m 2 / g, a D50 of 0.5-1.0 mu m and a purity of greater than or equal to 99.995%, no external impurities are introduced, the process is compatible with the existing production line, and is suitable for the fields of LED substrates, high-end heat-conducting fillers and the like.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic material preparation technology, specifically relating to a method for producing high-purity alumina with small specific surface area and small particle size from boehmite precursors using the aluminum alkoxide method. Background Technology

[0002] High-purity alumina, with its excellent insulation, high thermal conductivity, high hardness, and chemical stability, has become an indispensable key material in high-end electronics, advanced ceramics, and precision manufacturing. Different high-end applications place conflicting and stringent demands on the performance of alumina powder: the LED sapphire substrate field requires the powder to simultaneously possess a median particle size of less than 1.0 μm to ensure the density of the sintered body, and a particle size of less than 10 μm. 2 The low specific surface area of ​​ / g is required to avoid residual pores during sintering. In the field of high-end electronic thermal conductive fillers, powders are required to have both low specific surface area to reduce the viscosity of the resin system and increase the upper limit of filling, as well as small particle size to increase the density of thermal conductive channels. The above contradictory requirements have become the core bottleneck restricting the improvement of the performance of high-purity alumina.

[0003] Aluminum alkoxide hydrolysis is currently the mainstream process for the industrial production of ultra-high purity alumina. It uses high-purity aluminum and high-purity alcohol as raw materials, and prepares boehmite precursors through aluminum alkoxide synthesis, purification, and hydrolysis. Then, it undergoes calcination to achieve a phase transformation to α-Al2O3. However, this process has two inherent contradictions that cannot be overcome: one is the irreversible contradiction between small specific surface area and small particle size. To achieve complete α-phase phase transformation and low specific surface area, high-temperature calcination above 1200℃ is required to promote grain sintering and eliminate porosity. However, high temperature will inevitably lead to abnormal grain growth, making it impossible to obtain small-particle-size powder. To control the grain size, the calcination temperature needs to be lowered, which will lead to incomplete phase transformation, a large amount of residual transition phase, and a persistently high specific surface area, resulting in insufficient product performance stability. Secondly, there is a contradiction between energy consumption and purity. High-temperature calcination not only brings huge energy consumption and equipment costs, but also easily introduces impurities from refractory materials to contaminate the product. Existing technologies such as silane coating, SiO2 heterostructure coating, and template agent control used to suppress grain growth will introduce foreign impurities such as silicon and carbon, which runs counter to the core requirements of ultra-high purity applications.

[0004] In the prior art, two types of improvement methods are usually adopted to solve the above contradictions: one is to reduce the phase transformation temperature of alumina by adding a phase transformation mineralizer, thereby reducing grain growth caused by high-temperature calcination. A representative solution is Chinese patent CN102502839A, which discloses the use of ammonium fluoride as a phase transformation mineralizer to achieve alumina phase transformation at 1000℃. However, this method can only reduce the phase transformation threshold and cannot fundamentally inhibit grain sintering caused by grain boundary migration during the phase transformation process. It is still difficult to stably obtain high-purity alumina powder with a particle size of less than 1μm. The other method is to inhibit grain growth by surface coating. A representative solution is Chinese patent CN101234839A, which discloses the in-situ generation of an alumina coating by a complex of aluminum sec-butoxide and ethyl acetoacetate to inhibit grain growth. However, this method either introduces heterogeneous impurities that damage the product purity or cannot achieve complete phase transformation of alumina at low temperatures. Neither of these methods can achieve a synergistic balance of small specific surface area, small particle size, high α phase content, and ultra-high purity.

[0005] More importantly, there has long been a prevalent technical bias in this field: those skilled in the art generally believe that fluoride-based phase change mineralizers, while reducing the activation energy of alumina phase change, inevitably accelerate grain boundary mass transfer, leading to grain sintering and growth, a process that cannot be completely suppressed even with surface coating. Furthermore, it is generally believed that in the preparation of high-purity alumina via the aluminum alkoxide method, small specific surface area and small particle size are two performance indicators that cannot be synergistically achieved, exhibiting an irreversible negative correlation. To date, there is no technical solution in this field capable of simultaneously achieving complete boehmite phase change, precise suppression of grain growth, and controllable densification of pore structure at relatively low temperatures without introducing foreign impurities. This has become a critical technical bottleneck hindering the development of high-end, high-purity alumina. Summary of the Invention

[0006] In view of this, the present invention proposes a method for producing high-purity alumina with small specific surface area and small particle size from boehmite precursors using the aluminum alkoxide method. This method aims to overcome the inherent defects in existing high-purity alumina preparation processes using the aluminum alkoxide method, such as the inability to achieve small specific surface area and small particle size synergistically, the difficulty in simultaneously achieving low-temperature phase transformation and grain suppression, and the easy introduction of impurities during modification treatment. This method breaks through long-standing technical biases in the field and achieves low-energy consumption and high-stability preparation of high-performance high-purity alumina.

[0007] The technical solution of this invention is implemented as follows: This invention provides a method for producing high-purity alumina with small specific surface area and small particle size from boehmite precursors using the aluminum alkoxide method, comprising preparing boehmite precursor slurry using high-purity aluminum alkoxide as raw material, and further comprising the following steps:

[0008] (1) Synergistic surface treatment: Treatment agent A and treatment agent B are simultaneously added to the boehmite precursor slurry, and the mixture is stirred and reacted at 60℃-80℃ for 2h-4h under nitrogen or rare gas inert atmosphere protection to obtain the modified precursor; the treatment agent A is an aluminum-organic complex formed by alkoxyaluminum and β-dicarbonyl compound through coordination bond, which can generate an alumina coating in situ on the surface of boehmite during heat treatment, and the amount of treatment agent A added is 1%-3% based on the dry weight of the boehmite precursor; the treatment agent B is ammonium fluoride, and its amount added is 0.01%-0.1% based on the dry weight of the boehmite precursor; (2) Low-temperature calcination: The modified precursor is subjected to two-stage programmed temperature control calcination: first, the temperature is increased to 500℃-600℃ at a rate of 1℃ / min-5℃ / min and held for 1h-3h, and then the temperature is increased to 850℃-980℃ at a rate of 2℃ / min-8℃ / min and held for 2h-6h to obtain the high-purity alumina.

[0009] In some embodiments, the aluminum alkoxy has the general formula Al(OR)3, wherein R is a C1-C6 alkyl group; and the β-dicarbonyl compound has the general formula R'COCH2COR'', wherein R' and R'' are each independently a C1-C6 alkyl or alkoxy group.

[0010] The alkoxyaluminum defined by this general formula can form a stable six-membered ring chelate with the β-dicarbonyl compound through coordination bonds. This chelate can achieve uniform adsorption at the molecular level by bonding with the hydroxyl groups on the surface of boehmite, avoiding the problems of easy hydrolysis and aggregation and uneven coating thickness of conventional inorganic aluminum salt precursors. At the same time, this chelate structure can precisely control the pyrolysis rate of the aluminum source, ensuring that a continuous, dense, ultrathin alumina coating is formed in situ on the surface of boehmite particles during subsequent heat treatment, rather than discrete alumina particles, thus ensuring the stability of the passivation and isolation effect.

[0011] In some embodiments, the aluminum alkoxy is aluminum sec-butoxide, the β-dicarbonyl compound is ethyl acetoacetate, and the complexation molar ratio of the two is 1:1 to 1:2.

[0012] The hydrolytic activity of aluminum sec-butoxide is highly matched with the reactivity of the hydroxyl groups on the boehmite surface, enabling a mild and controllable adsorption reaction in an aqueous slurry system without agglomeration due to excessively rapid hydrolysis. Ethyl acetoacetate, as a bidentate ligand, can form a stable complex with aluminum sec-butoxide with moderate steric hindrance, effectively preventing premature hydrolysis of aluminum alkoxy in the aqueous system. The complexation molar ratio of 1:1 to 1:2 ensures that the coordination sites of aluminum alkoxy are fully complexed, achieving precise control of hydrolytic activity, while also avoiding excessive β-dicarbonyl compounds that would generate carbon residue during subsequent heat treatment, perfectly balancing the uniformity of the coating layer and the ultra-high purity requirements of the final product.

[0013] In some embodiments, the treatment agent B is replaced by a composition of at least one of ammonium fluoride and ammonium bifluoride, yttrium oxide, and lanthanum oxide, and the total amount of phase change catalyst added is 0.01%-0.1% based on the dry mass of the boehmite precursor.

[0014] Ammonium bifluoride has a similar catalytic mechanism to ammonium fluoride. It can slowly release fluoride ions during heat treatment, which replace oxygen ions in the alumina lattice to form anionic vacancies, significantly reducing the lattice diffusion barrier of aluminum ions and thus lowering the phase transition activation energy. Rare earth oxides such as yttrium oxide and lanthanum oxide have ionic radii much larger than aluminum ions and cannot enter the alumina lattice. They can selectively segregate at grain boundaries to form a low-melting-point rare earth aluminate eutectic phase, providing a grain boundary channel for rapid diffusion of aluminum ions. When combined with ammonium fluoride, they can further reduce the phase transition temperature. At the same time, the rare earth ions segregated at the grain boundaries can pin the grain boundary migration, forming a dual grain inhibition effect with the in-situ generated alumina coating, further enhancing the synergistic control capability of small particle size and low specific surface area.

[0015] In some implementations, in the two-stage programmed temperature control calcination in step (2), the temperature is finally raised to 900℃-950℃ and held for 3h-5h.

[0016] This temperature range closely matches the critical phase transformation temperature of the modified system and the stable temperature window of the in-situ alumina coating. It can provide sufficient thermodynamic driving force for the reconstructive phase transformation of boehmite to stable α-Al2O3, ensuring the complete phase transformation, while avoiding excessive temperature that could cause the isolation effect of the in-situ alumina coating to fail and abnormal grain growth. The matched holding time can guide the orderly collapse and elimination of the pore structure inside the particles, rather than the formation of new microporous structures, while ensuring the complete phase transformation. This allows for precise control of the specific surface area of ​​the product without increasing the grain size.

[0017] In some embodiments, the reaction temperature of the synergistic surface treatment in step (1) is 70°C, the reaction time is 3 hours, and the protective atmosphere is nitrogen.

[0018] The reaction temperature of 70℃ can balance the adsorption and bonding efficiency of the complex on the boehmite surface with the structural stability of the complex. This ensures that the treatment agent is fully adsorbed on the particle surface without causing premature decomposition and failure of the complex. The reaction time of 3 hours can achieve uniform co-distribution of treatment agent A and treatment agent B on the surface of boehmite particles. This ensures that the in-situ formation of the passivation layer and the phase change catalysis process are completely synchronized during the subsequent heat treatment, thus isolating and inhibiting the formation of new grains from the initial stage of phase change. The nitrogen protective atmosphere can effectively prevent the oxidative decomposition of β-dicarbonyl compounds during heating, while isolating the dissolved oxygen in the slurry from interfering with the complexation reaction, ensuring the integrity and uniformity of the coating layer.

[0019] In some embodiments, the boehmite precursor slurry is prepared using a gradient hydrothermal crystallization process of "low-temperature nucleation - medium-temperature crystallization - isothermal ripening".

[0020] This gradient hydrothermal crystallization process can achieve uniform nucleation and controllable growth of boehmite particles through staged temperature control, producing near-spherical boehmite particles with complete crystals, concentrated particle size distribution, and uniform surface hydroxyl distribution. This provides a uniform reaction substrate for subsequent surface treatment, effectively avoiding problems such as uneven coating and asynchronous phase transformation caused by uneven precursor particle size and excessive crystallinity differences, and further ensuring the uniformity of particle size distribution and performance stability of the final product.

[0021] In some embodiments, the modified precursor is washed until the conductivity of the filtrate is <10 μS / cm before calcination, and then spray-dried.

[0022] Washing to this conductivity threshold can effectively remove soluble impurities such as free fluoride ions and unreacted small organic molecules from the modified slurry, preventing impurities from forming low-melting-point phases during calcination and causing abnormal grain growth, while ensuring the ultra-high purity of the final product. Spray drying can achieve rapid and uniform drying of modified precursor particles, avoiding particle agglomeration and coating layer structure damage caused by capillary forces during conventional static drying, and ensuring the stable performance of passivation and isolation during subsequent calcination.

[0023] The present invention has the following advantages over the prior art: This invention deeply couples in-situ surface passivation technology with catalytic phase change technology through a synchronous and synergistic surface treatment process and a specially matched two-stage low-temperature calcination process. It fundamentally breaks the inherent contradiction in the field that small specific surface area and small particle size cannot be synergistically achieved. It overcomes the common technical prejudice that fluoride phase change catalysts inevitably lead to grain growth while promoting low-temperature phase change. Compared with the conventional schemes of single modification and step-by-step modification in existing technologies, this invention achieves complete phase change of boehmite to α-Al2O3 at low temperature, precise suppression of grain growth, controllable densification of pore structure, and simultaneous achievement of ultra-high product purity. It does not require the introduction of heterogeneous impurities or major modifications to existing aluminum alkoxide process industrial production lines. While significantly reducing production energy consumption and carbon emissions, it can stably prepare high-purity alumina powder that meets the stringent performance requirements of high-end application scenarios, and has outstanding industrial application value and performance advantages. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] I. Experimental Materials and Equipment 1. Experimental materials: 99.995% pure aluminum isopropoxide, distillation grade aluminum sec-butoxide, ethyl acetoacetate, acetylacetone, ammonium fluoride, ammonium bifluoride, yttrium oxide, lanthanum oxide, high-purity aluminum ingots, distillation grade isopropanol, and citric acid, all of which are commercially available high-purity reagents; high-purity deionized water (conductivity <1μS / cm).

[0026] 2. Experimental equipment: stainless steel reactor, hydrothermal reactor (PTFE liner), high-speed centrifuge, spray dryer, box-type programmable temperature controlled furnace, air jet mill, turbine air classifier, X-ray diffractometer (XRD), surface area and porosity analyzer (BET), laser particle size analyzer, inductively coupled plasma optical emission spectrometer (ICP-OES), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM).

[0027] II. Performance Testing Methods 1. α-Al2O3 phase content: The relative content of the α phase was determined by X-ray diffraction (XRD) and calculated using phase quantitative analysis software. 2. Specific surface area: The liquid nitrogen adsorption method (BET) was used for testing. Before the test, the sample was degassed at 150℃ for 4 hours. 3. Particle size and distribution: A laser particle size analyzer was used for testing. The samples were ultrasonically dispersed in anhydrous ethanol before testing. The median particle size D50 and the particle size distribution Span value (Span=(D90-D10) / D50) were recorded. 4. Purity and impurity content: Inductively coupled plasma optical emission spectrometry (ICP-OES) was used for testing. The samples were pretreated by alkali fusion method before testing, and the purity of Al2O3 and the content of Na and Fe elements were recorded. 5. Microstructure and interface layer characterization: The grain morphology and agglomeration state of the powder were observed using scanning electron microscopy (SEM); the thickness and elemental composition of the amorphous alumina interface layer on the particle surface were detected using high-resolution transmission electron microscopy (HRTEM).

[0028] III. Examples Example 1 1. Preparation of boehmite precursor: 5 kg of 99.995% pure aluminum ingot and 30 L of distillation grade isopropanol were added to a 50 L stainless steel reactor, along with 0.1 wt% mercuric chloride catalyst. The mixture was refluxed at 90 °C for 8 h. The reaction solution was then subjected to three-stage vacuum distillation to obtain high-purity aluminum isopropoxide. The aluminum isopropoxide was slowly added to a reactor containing 20 L of 90 °C high-purity deionized water at a molar ratio of 1:4. The mixture was vigorously stirred and hydrolyzed for 2 h to obtain boehmite sol. The sol was transferred to a hydrothermal reactor, and 0.5 wt% (based on aluminum alkoxide) citric acid was added. The mixture was crystallized at 200 °C for 4 h, then cooled to 130 °C and matured for 10 h. After natural cooling, a boehmite precursor slurry with a solid content of 10% was obtained. 2. Synergistic surface treatment: Preparation of treatment agent A: Aluminum sec-butoxide and ethyl acetoacetate are mixed in a molar ratio of 1:1.5 and stirred at 60°C for 2 hours to obtain a clear solution; 10 kg (on a dry basis) of the above boehmite slurry is taken, and 2% (on a dry basis) of treatment agent A and 0.05 wt% of ammonium fluoride (dissolved in a small amount of high-purity water) are added simultaneously under stirring. The mixture is stirred continuously at 70°C for 3 hours under nitrogen protection to obtain the modified precursor slurry; 3. Washing and drying: The modified precursor slurry was washed three times with high-purity deionized water using a high-speed centrifuge until the conductivity of the filtrate was <10μS / cm; the washed slurry was spray-dried at 120℃ to obtain a modified boehmite precursor powder with good flowability. 4. Two-stage calcination: The modified boehmite precursor powder is placed in a high-purity alumina crucible and then placed in a box-type programmable temperature control furnace. The temperature is increased to 550℃ at a heating rate of 2℃ / min and held for 2 hours. Then, the temperature is increased to 920℃ at a heating rate of 3℃ / min and held for 4 hours. The furnace is then allowed to cool naturally to room temperature to obtain the calcined powder. 5. Post-processing: The calcined powder is placed in an air jet mill and pulverized and deagglomerated at a working pressure of 0.7 MPa. Then, it is classified by a turbine air classifier and the target powder is collected.

[0029] 6. Interface layer characterization: HRTEM analysis showed that the powder particles prepared in this embodiment were coated with an amorphous alumina interface layer with a thickness of about 2.3 nm, containing only Al and O elements, which is consistent with the chemical composition of the matrix.

[0030] Example 2 Except for the addition of treatment agent A, which is 1% of the dry weight of boehmite, the preparation steps and process parameters in this embodiment are completely the same as in Example 1.

[0031] Example 3 Except for the addition of treatment agent A, which is 3% of the dry weight of boehmite, the preparation steps and process parameters in this embodiment are completely the same as in Example 1.

[0032] Example 4 Except for the complexation molar ratio of aluminum sec-butoxide to ethyl acetoacetate being 1:1, the preparation steps and process parameters in this embodiment are completely consistent with those in Example 1.

[0033] Example 5 Except for the complexation molar ratio of aluminum sec-butoxide to ethyl acetoacetate being 1:2, the other preparation steps and process parameters in this embodiment are completely consistent with those in Example 1.

[0034] Example 6 Except for the amount of ammonium fluoride added being 0.01 wt% of boehmite dry basis and the final calcination temperature being 980°C, the other preparation steps and process parameters in this embodiment are completely the same as in Example 1.

[0035] Example 7 Except for the addition of ammonium fluoride at a dry basis of 0.1 wt% and the final calcination temperature of 850°C, the preparation steps and process parameters in this embodiment are completely the same as those in Example 1.

[0036] Example 8 Except for the synergistic surface treatment temperature of 60℃ and the stirring reaction time of 2h, the preparation steps and process parameters in this embodiment are completely the same as those in Example 1.

[0037] Example 9 Except for the synergistic surface treatment temperature of 80℃ and the stirring reaction time of 4h, the preparation steps and process parameters in this embodiment are completely the same as those in Example 1.

[0038] Example 10 Except for the fact that treatment agent A is a complex of aluminum isopropoxide and acetylacetone in a molar ratio of 1:1.5, the other preparation steps and process parameters in this embodiment are completely the same as those in Example 1.

[0039] Example 11 Except for the fact that the treatment agent B is a mixture of ammonium fluoride and yttrium oxide in a mass ratio of 1:1 and the total amount added is 0.05 wt% of boehmite dry basis, the other preparation steps and process parameters are completely consistent with those in Example 1.

[0040] Example 12 Except for the fact that the treatment agent B is a mixture of ammonium fluoride and ammonium bifluoride in a mass ratio of 2:1 and the total addition amount is 0.05 wt% of boehmite dry basis, the other preparation steps and process parameters are completely consistent with those in Example 1.

[0041] Example 13 Except for the fact that the treatment agent B is a mixture of ammonium fluoride and lanthanum oxide in a mass ratio of 1:1 and the total addition amount is 0.05 wt% of boehmite dry basis, the other preparation steps and process parameters are completely consistent with those in Example 1.

[0042] Example 14 Except for the first stage of calcination, where the heating rate is 1℃ / min and the holding time is 550℃ for 1h, and the second stage heating rate is 2℃ / min, the other preparation steps and process parameters are completely the same as in Example 1.

[0043] Example 15 Except for the first stage of calcination, where the heating rate is 5℃ / min and the holding time is 550℃ for 3 hours, and the second stage, where the heating rate is 8℃ / min and the holding time is 920℃ for 6 hours, the other preparation steps and process parameters are completely consistent with those in Example 1.

[0044] IV. Comparative Examples Comparative Example 1 Except for the absence of treatment agent A and the addition of only 0.05 wt% ammonium fluoride, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0045] Comparative Example 2 Except for the absence of ammonium fluoride and the addition of only 2% (by dry weight) of treatment agent A, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0046] Comparative Example 3 Except for the addition of 2% (by dry weight) of treatment agent A, stirring at 70°C under nitrogen protection for 3 hours, and then adding 0.05 wt% ammonium fluoride and stirring for another 1 hour, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0047] Comparative Example 4 Except for replacing treatment agent A with an equal mass of silane coupling agent KH-550, the preparation steps and process parameters in this comparative example are completely consistent with those in Example 1.

[0048] Comparative Example 5 Except for the calcination process, which involves directly heating to 920°C at 3°C / min and holding for 6 hours, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0049] Comparative Example 6 Except for the addition of ammonium fluoride, which is 0.2 wt% of boehmite dry weight, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0050] Comparative Example 7 Except for the addition of treatment agent A, which was 0.5 wt% of boehmite dry basis, the preparation steps and process parameters of this comparative example were completely consistent with those of Example 1.

[0051] Comparative Example 8 Except for the synergistic surface treatment temperature of 50°C, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0052] Comparative Example 9 1. Preparation of boehmite precursor: completely consistent with Example 1; 2. Surface treatment: Take 10 kg (dry basis) of boehmite slurry, add 2% (dry basis) of silane coupling agent KH-550, stir at 70℃ for 3 h under nitrogen protection, then add 0.05 wt% ammonium fluoride and continue stirring for 1 h to obtain modified precursor slurry; 3. Washing and drying: exactly the same as in Example 1; 4. Calcination: The precursor powder is heated to 1100℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then cooled in the furnace. 5. Post-processing: Completely consistent with Example 1.

[0053] Comparative Example 10 Except for the absence of treatment agent A and ammonium fluoride, the preparation steps and process parameters of this comparative example are completely consistent with those of Example 1.

[0054] V. Performance Test Results Following the performance testing methods described above, all powders prepared in the examples and comparative examples were comprehensively tested, and the test results are shown in the table below:

[0055] VI. Microstructure and Interface Layer Characterization Results

[0056] 1. The powders prepared in all examples were nearly spherical grains with uniform size, no obvious hard agglomerates, and good dispersibility. Comparative Examples 1, 3, 4, 6, 7, and 9 all showed problems of coarse grains, obvious sintering necks, and severe agglomeration. Comparative Examples 2 and 10 showed problems of a large amount of soft agglomerates and incomplete phase transformation.

[0057] 2. The powders prepared in Examples 1, 2, 5, 11, and 13 all had an amorphous alumina interface layer of 1-5 nm thickness as detected by HRTEM. The chemical composition was consistent with the matrix, containing only Al and O elements. No continuous and uniform amorphous alumina interface layer was detected in any of the comparative examples.

[0058] VII. Results Analysis 1. The high-purity alumina powders prepared in all embodiments meet the core performance indicators of α-Al2O3 content ≥99%, specific surface area 3-8m² / g, D50 0.5-1.0μm, and Al2O3 purity ≥99.995%, which completely cover all parameter ranges defined in the claims. This proves that the technical solution of the present invention can stably achieve the expected effect, and the summary of the claims is completely reasonable and fully supported by the specification.

[0059] 2. Comparative Examples 1-2 and 10 demonstrate that using treatment agent A or treatment agent B alone, or neither, cannot simultaneously achieve low-temperature complete phase transformation and grain growth inhibition. This highlights the core necessity of the synergistic surface treatment of this invention and directly breaks through the technical bias in this field.

[0060] 3. Comparative Example 3 demonstrates that the conventional step-by-step combination method of the prior art cannot achieve the synergistic effect of the present invention. The synchronous surface treatment of the present invention is not a simple superposition of the prior art, but the key to achieving unexpected technical effects. The prior art has no combination inspiration.

[0061] 4. Comparative Examples 4 and 9 demonstrate that conventional silane coating is fundamentally different from the aluminum-organic complex passivation system of the present invention. Silane coating is prone to introducing impurities and has poor effects. The homogeneous in-situ coating of the present invention has irreplaceable advantages and is not a direct replacement by conventional means.

[0062] 5. Comparative Example 5 demonstrates that the two-stage programmed temperature-controlled calcination of the present invention is a necessary process strongly bound to the synergistic modification system. It is an unconventional calcination method, and direct calcination will lead to the failure of the passivation layer and a sharp drop in performance.

[0063] 6. Comparative Examples 6-8 demonstrate that the parameters such as the amount of treatment agent added and the synergistic treatment temperature specified in this invention are critical ranges for achieving synergistic effects. Exceeding these ranges will lead to a sharp drop in core performance, which is not a conventional parameter optimization in this field.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing high-purity alumina with small specific surface area and small particle size from boehmite precursor using the aluminum alkoxide process, comprising preparing a boehmite precursor slurry using high-purity aluminum alkoxide as raw material, characterized in that, It also includes the following steps: (1) Synergistic surface treatment: Treatment agent A and treatment agent B are simultaneously added to the boehmite precursor slurry, and the mixture is stirred and reacted at 60℃-80℃ for 2h-4h under nitrogen or rare gas inert atmosphere protection to obtain the modified precursor; the treatment agent A is an aluminum-organic complex formed by alkoxyaluminum and β-dicarbonyl compound through coordination bond, which can generate an alumina coating in situ on the surface of boehmite during heat treatment, and the amount of treatment agent A added is 1%-3% based on the dry weight of the boehmite precursor; the treatment agent B is ammonium fluoride, and its amount added is 0.01%-0.1% based on the dry weight of the boehmite precursor; (2) Low-temperature calcination: The modified precursor is subjected to two-stage programmed temperature control calcination: first, the temperature is increased to 500℃-600℃ at a rate of 1℃ / min-5℃ / min and held for 1h-3h, and then the temperature is increased to 850℃-980℃ at a rate of 2℃ / min-8℃ / min and held for 2h-6h to obtain the high-purity alumina.

2. The method according to claim 1, characterized in that, The general formula of the aluminum alkoxy is Al(OR)3, wherein R is a C1-C6 alkyl group; the general formula of the β-dicarbonyl compound is R'COCH2COR'', wherein R' and R'' are each independently a C1-C6 alkyl or alkoxy group.

3. The method according to claim 2, characterized in that, The alkoxyaluminum is sec-butoxide aluminum, and the β-dicarbonyl compound is ethyl acetoacetate, with a complexation molar ratio of 1:1 to 1:

2.

4. The method according to claim 1, characterized in that, The treatment agent B is replaced by a composition of at least one of ammonium fluoride and ammonium bifluoride, yttrium oxide, and lanthanum oxide, and the total amount of phase change catalyst added is 0.01%-0.1% based on the dry mass of the boehmite precursor.

5. The method according to claim 1, characterized in that, In the two-stage programmed temperature control calcination described in step (2), the temperature is finally raised to 900℃-950℃ and held for 3h-5h.

6. The method according to claim 1, characterized in that, The reaction temperature for the synergistic surface treatment in step (1) is 70°C, the reaction time is 3 hours, and the protective atmosphere is nitrogen.

7. A high-purity alumina powder, characterized in that, The product is prepared by the method described in claim 1, and simultaneously satisfies the following conditions: α-Al₂O₃ phase content ≥ 99%, and specific surface area ≥ 3 m². 2 / g-8 m 2 / g, with a median particle size D50 of 0.5μm-1.0μm.

8. A high-purity alumina powder, characterized in that, Simultaneously satisfying the following conditions: α-Al2O3 phase content ≥99%, specific surface area 3m² 2 / g-8 m 2 / g, with a median particle size D50 of 0.5μm-1.0μm; the surface of the powder particles is coated with an amorphous alumina interface layer with a thickness of 1 nm-5 nm. The chemical composition of the interface layer is consistent with that of the matrix α-Al2O3, containing only Al and O elements. The thickness of the interface layer is detected by high-resolution transmission electron microscopy (HRTEM).

9. The high-purity alumina powder according to claim 7 or 8, characterized in that, The powder has an alumina purity of ≥99.995%, a sodium content of ≤30 ppm, and an iron content of ≤20 ppm.

10. The high-purity alumina powder according to claim 7 or 8, characterized in that, The particle size distribution Span value of the powder satisfies: (D90 - D10) / D50 ≤ 1.5.

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