Preparation method of low-sodium high-temperature alpha-phase alumina micro powder

Abstract

The invention provides a preparation method of low-sodium high-temperature alpha-phase alumina micro powder, and belongs to the technical field of alumina powder. The preparation method comprises the steps of acid leaching, coating and post-treatment. The coating step comprises the following steps: adding a silica sol solution into an acid-leached aluminum oxide suspension for the first time, stirring at 74-78 DEG C for 1.0-1.4 hours, then adding the silica sol solution for the second time, stirring for 1.0-1.5 hours while keeping the temperature, then adding the silica sol solution for the third time, and stirring for 0.8-1.2 hours at 62-65 DEG C to obtain silica sol-coated aluminum oxide; the alpha-phase alumina micropowder prepared by the preparation method disclosed by the invention is uniform in particle size, low in sodium oxide content, high in alpha-phase conversion rate and good in powder fluidity.

Description

Technical Field

[0001] The present invention belongs to the technical field of alumina powder, and particularly relates to a method for preparing low-sodium high-temperature α-phase alumina fine powder. Background Art

[0002] High-temperature α-phase alumina powder is one of the ceramic materials widely used in various industries. It has a melting point as high as 2040°C, good formability, stable crystal phase, high hardness, good wear resistance, excellent high-temperature stability, and excellent physical and chemical stability. It is widely used in industries such as ceramics, refractories, rubber, plastics, petrochemicals, etc.

[0003] The sodium oxide content is an important technical index of high-temperature α-alumina fine powder. A higher sodium oxide content will affect the conversion rate of the α-phase. The presence of sodium elements will interfere with the crystal structure of α-alumina. When it enters the lattice interstitial sites, it will cause lattice distortion, thereby affecting the density and other properties of the material; in the electronic field, a high sodium content will reduce its insulation performance, resulting in unstable signal transmission; in the ceramic field, sodium will affect the sintering process of ceramics during high-temperature sintering, resulting in defects such as pores and cracks in the ceramics, which not only reduces the aesthetics of the ceramics but also reduces the strength performance of the ceramic products. Therefore, providing a method for preparing low-sodium high-temperature α-phase alumina fine powder is an urgent research topic in the existing technology.

[0004] The existing methods for preparing low-sodium high-temperature α-phase alumina fine powder mainly include non-mineralization methods and mineralization methods. The non-mineralization methods mainly include the Bayer process, the alcohol-aluminum salt hydrolysis method, etc. Among them, in the Bayer process, bauxite is treated, and alumina reacts with sodium hydroxide to form a sodium aluminate solution, which is decomposed and calcined to obtain α-phase alumina fine powder. This method is difficult to effectively remove sodium impurities in the raw materials, resulting in a higher sodium content in the product and affecting the final performance of the product; the alcohol-aluminum salt hydrolysis method uses the hydrolysis and polycondensation reactions of metal alcoholates to prepare α-phase alumina fine powder, which has a high cost and harsh reaction conditions.

[0005] The mineralization method uses a mineralizing agent to remove sodium impurities and calcines at high temperature to obtain α-phase alumina powder. However, it is difficult to control the addition amount of the mineralizing agent. Adding the mineralizing agent makes the primary crystals of the powder difficult to control, and it is difficult to obtain α-phase alumina at the fine powder level.

[0006] CN115974112A discloses a low-sodium submicron α-alumina powder and its preparation method and application. Specifically, it discloses mixing industrial alumina, organic acid, dispersant and water, adjusting the pH value of the obtained dispersion to 7-7.5 to obtain a slurry; subjecting the slurry to pressure filtration treatment to obtain low-sodium industrial alumina powder; mixing the low-sodium industrial alumina powder with a magnesium-containing compound and rare earth oxide, and calcining to obtain low-sodium submicron α-alumina powder. The low-sodium submicron α-aluminum oxide powder prepared by this method has low sodium content, high purity, the primary crystal size is submicron level, the purity is 99.9 - 99.99 wt%, the α-phase content is 96 - 99%, the primary crystal size is 0.3 - 0.8 μm, and the sodium content is 0.01 - 0.05 wt%; however, the α-aluminum oxide powder has the defect of strong agglomeration force, which limits its subsequent applications. Summary of the Invention

[0007] In order to solve the technical problems existing in the prior art, the present invention provides a method for preparing low-sodium high-temperature α-phase alumina micropowder, which improves the uniformity of the powder, reduces the sodium oxide content, increases the α-phase conversion rate, and enhances the fluidity of the powder.

[0008] In view of the above technical problems, the present invention adopts the following technical solutions: A method for preparing low-sodium high-temperature α-phase alumina micropowder, comprising acid leaching, coating, and post-treatment steps, and the specific operations are as follows: 1. Acid Leaching Add industrial alumina into the pretreatment solution, raise the temperature to 37 - 42 °C, stir at 200 - 230 rpm for 35 - 43 min, continue to raise the temperature to 60 - 65 °C, add mixed acid, continue to stir for 1.2 - 1.5 h, naturally cool to room temperature, after centrifugation, wash with deionized water until the washing liquid is neutral, and after drying, obtain acid-leached alumina; The particle size of the industrial alumina is 40 - 60 μm, and the sodium oxide content is 0.52 - 0.57 wt%; The mass ratio of the industrial alumina, pretreatment solution, and mixed acid is 400:590 - 610:5.8 - 6.3; The preparation method of the pretreatment solution is as follows: add cellulose nanocrystals CNC into deionized water, stir at 40 - 45 °C for 0.8 - 1.2 h, then add ammonium citrate solution for ultrasonic treatment, the ultrasonic time is 40 - 50 min, the ultrasonic power is 83 - 88 W, the ultrasonic frequency is 23 - 28 kHz, after the ultrasonic treatment, add cerium oxide dispersion liquid, keep warm and stir for 1.4 - 1.8 h, after removing ethanol, obtain the pretreatment solution; The mass ratio of the deionized water, cellulose nanocrystals CNC, ammonium citrate solution, and cerium oxide dispersion liquid is 100:0.07 - 0.10:46 - 53:15.0 - 15.7; The mass concentration of the ammonium citrate solution is 0.42 - 0.48 wt%; The preparation method of the cerium oxide dispersion liquid is as follows: add cerium oxide into absolute ethanol, ultrasonically disperse for 48 - 53 min, the ultrasonic power is 132 - 146 W, the ultrasonic frequency is 32 - 38 kHz, and after the ultrasonic dispersion, obtain the cerium oxide dispersion liquid; The mass ratio of the absolute ethanol to the cerium oxide is 100:1.8 - 2.2; The mixed acid is a mixture of oxalic acid and sulfamic acid, and the mass ratio of the oxalic acid to the sulfamic acid is 4.0 - 4.5:1.0.

[0009] 2. Coating Add the silica sol into deionized water, and then add ammonium citrate. After stirring evenly, a silica sol solution is obtained; add the acid-leached alumina into deionized water, and after stirring evenly, an acid-leached alumina suspension is obtained; Add the silica sol solution to the acid-leached alumina suspension for the first time, control the pH value of the silica sol solution to be 4.8 - 5.2, and the addition rate of the silica sol solution to be 0.4 - 0.6 g / min. After the addition is completed, raise the temperature to 74 - 78 °C at a rate of 1.8 - 2.2 °C / min, keep warm and stir for 1.0 - 1.4 h, then add the silica sol solution for the second time, control the pH value of the silica sol solution to be 7.0, and the addition rate of the silica sol solution to be 0.6 - 1.0 g / min. After the addition is completed, keep warm and stir for 1.0 - 1.5 h. After the stirring ends, add the silica sol solution for the third time, control the pH value of the silica sol solution to be 9.6 - 10.0, and the addition rate of the silica sol solution to be 0.8 - 1.2 g / min. Lower the temperature to 62 - 65 °C at a rate of 0.7 - 1.2 °C / min, keep warm and stir for 0.8 - 1.2 h. After the stirring ends, perform centrifugal separation and then drying to obtain silica sol-coated alumina; The mass concentration of the silica sol is 18 - 22 wt%, and the particle size is 25 - 35 nm; In the silica sol solution, the mass ratio of the silica sol, deionized water, and ammonium citrate is 22 - 27:24 - 26:0.8 - 1.2; In the acid-leached alumina suspension, the mass ratio of the acid-leached alumina to deionized water is 100:240 - 260; The mass ratio of the acid-leached alumina suspension, the first addition amount of the silica sol solution, the second addition amount of the silica sol solution, and the third addition amount of the silica sol solution is 340 - 360:19.3 - 25.5:14.7 - 15.0:12.0 - 12.7.

[0010] 3. Post-treatment Mix the silica sol-coated alumina and the post-treatment agent evenly, and then perform calcination in a nitrogen atmosphere. Raise the temperature to 810 - 830 °C at a rate of 1.8 - 2.2 °C / min, keep warm for 30 - 40 min, and then raise the temperature to 1230 - 1280 °C at a rate of 3.2 - 3.8 °C / min, keep warm for 2.3 - 2.7 h to obtain α-phase alumina micropowder; The mass ratio of the silica sol-coated alumina to the post-treatment agent is 100:3.3 - 3.8; The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 0.8 - 1.2:0.2 - 0.4:1.1 - 1.4.

[0011] Compared with the prior art, the present invention has achieved the following beneficial effects: 1. The present invention prepares α-phase alumina micropowder by adopting specific acid leaching, coating, and post-treatment steps; among them, in the acid leaching step, first, it is treated with a pretreatment solution, which is a mixture of cellulose nanocrystals CNC, ammonium citrate, and cerium oxide. Ammonium citrate can chelate sodium on the surface. Combining cerium oxide and cellulose nanocrystals CNC can increase steric hindrance, enhance the dispersion of industrial alumina, avoid agglomeration phenomena, provide activation sites for subsequent steps, and increase the conversion rate of the α-phase. Then, a combination of two acids is used, which can accelerate the replacement of sodium ions and reduce the sodium content; the second step is to coat the acid-leached alumina with silica sol, and the silica sol is treated with ammonium citrate, which can chelate sodium on the surface of alumina. During the coating process, a gradient coating method is adopted. First, a relatively dense silica sol is formed on the surface of alumina to enhance the fluidity performance of alumina particles. The amount of silica sol in the middle layer is moderate, which can adsorb residual sodium and block the entry of impurities. The outermost layer has the least amount of silica sol, which promotes the escape of sodium at high temperatures. It can also inhibit the abnormal growth of alumina grains during high-temperature calcination, which is beneficial to the uniform conversion of the α-phase; finally, the post-treatment agent is used to calcine alumina at high temperatures. Ammonium molybdate in the post-treatment agent can reduce the phase transition temperature of alumina and promote the formation of α-phase alumina. Tetrabutylammonium bromide can undergo an ion exchange reaction with sodium ions to remove sodium. Combining with the yttrium oxide component can improve the lattice structure of alumina and inhibit grain growth. Finally, alumina micropowder with uniform particle size, good fluidity, low sodium content, and high α-phase conversion rate is obtained; 2. The α-phase alumina micropowder obtained by the method of the present invention has an α-phase conversion rate of 99.1 - 99.7%, an original crystal grain size of 0.3 - 0.5 μm, a repose angle of 28 - 32°, and according to the detection and analysis of an inductively coupled plasma atomic emission spectrometer ICPE - 9820, the sodium oxide content is 0.02 - 0.06 wt%, and the Al2O3 content is 99.90 - 99.95 wt%. Specific Embodiments

[0012] In order to more clearly understand the technical features, objectives, and effects of the present invention, the specific embodiments of the present invention are now described.

[0013] Example 1 A method for preparing low-sodium high-temperature α-phase alumina micropowder 1. Acid leaching Add 400 g of industrial alumina to 590 g of the pretreatment solution, raise the temperature to 37 °C, stir at 200 rpm for 35 min, continue to raise the temperature to 60 °C, add 5.8 g of the mixed acid, continue stirring for 1.2 h, naturally cool to room temperature, after centrifugation, wash with deionized water until the washing liquid is neutral, and after drying, obtain acid-leached alumina; The particle size of the industrial alumina is 40 μm, and the sodium oxide content is 0.52 wt%; The preparation method of the pretreatment solution is as follows: add 0.07 g of cellulose nanocrystal CNC to 100 g of deionized water, stir at 40 °C for 1.2 h, then add 46 g of ammonium citrate solution for ultrasonic treatment, the ultrasonic time is 40 min, the ultrasonic power is 88 W, the ultrasonic frequency is 23 kHz. After the ultrasonic treatment, add 15.0 g of cerium oxide dispersion, keep stirring for 1.4 h, remove ethanol, and obtain the pretreatment solution; The mass concentration of the ammonium citrate solution is 0.42 wt%; The preparation method of the cerium oxide dispersion is as follows: add 1.8 g of cerium oxide to 100 g of absolute ethanol, ultrasonically disperse for 48 min, the ultrasonic power is 132 W, the ultrasonic frequency is 32 kHz, and obtain the cerium oxide dispersion after the ultrasonic dispersion ends; The mixed acid is a mixture of oxalic acid and sulfamic acid, and the mass ratio of oxalic acid to sulfamic acid is 4.0:1.0.

[0014] 2. Coating Add 22 g of silica sol to 24 g of deionized water, then add 0.8 g of ammonium citrate, and stir evenly to obtain a silica sol solution; add 100 g of acid-leached alumina to 240 g of deionized water, stir evenly to obtain an acid-leached alumina suspension; Add 19.3 g of the silica sol solution to 340 g of the acid-leached alumina suspension, control the pH value of the silica sol solution to 4.8, the addition rate of the silica sol solution is 0.4 g / min. After the addition is completed, raise the temperature to 74 °C at a rate of 1.8 °C / min, keep stirring for 1.0 h, then add 14.7 g of the silica sol solution, control the pH value of the silica sol solution to 7.0, the addition rate of the silica sol solution is 0.6 g / min. After the addition is completed, keep stirring for 1.0 h. After the stirring ends, add 12.0 g of the silica sol solution, control the pH value of the silica sol solution to 9.6, the addition rate of the silica sol solution is 0.8 g / min, lower the temperature to 62 °C at a rate of 0.7 °C / min, keep stirring for 1.2 h. After the stirring ends, after centrifugal separation and drying, obtain silica sol-coated alumina; The mass concentration of the silica sol is 18 wt%, and the particle size is 35 nm.

[0015] 3. Post-treatment After mixing 100 g of silica sol-coated alumina and 3.3 g of a post-treatment agent evenly, calcination is carried out under a nitrogen atmosphere. The temperature is raised to 810 °C at a rate of 1.8 °C / min, held for 40 min, and then the temperature is raised to 1230 °C at a rate of 3.2 °C / min and held for 2.3 h to obtain α-phase alumina micropowder; The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 0.8:0.2:1.1.

[0016] The α-phase alumina micropowder obtained by the method of Example 1 has an α-phase conversion rate of 99.1%, an original crystal grain size of 0.5 μm, and a repose angle of 32°. According to the detection and analysis by an inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content is 0.06 wt%, and the Al2O3 content is 99.90 wt%.

[0017] Example 2 A preparation method of low-sodium high-temperature α-phase alumina micropowder 1. Acid leaching Add 400 g of industrial alumina to 600 g of a pretreatment solution, raise the temperature to 40 °C, stir at 220 rpm for 40 min, continue to raise the temperature to 63 °C, add 6.0 g of a mixed acid, continue to stir for 1.3 h, naturally cool to room temperature, after centrifugation, wash with deionized water until the washing liquid is neutral, and after drying, obtain acid-leached alumina; The particle size of the industrial alumina is 50 μm, and the sodium oxide content is 0.54 wt%; The preparation method of the pretreatment solution is as follows: add 0.08 g of cellulose nanocrystals CNC to 100 g of deionized water, stir at 42 °C for 1.0 h, then add 50 g of ammonium citrate solution for ultrasonic treatment. The ultrasonic time is 45 min, the ultrasonic power is 85 W, and the ultrasonic frequency is 25 kHz. After the ultrasonic treatment, add 15.3 g of cerium oxide dispersion liquid, keep stirring for 1.5 h, remove ethanol, and obtain the pretreatment solution; The mass concentration of the ammonium citrate solution is 0.45 wt%; The preparation method of the cerium oxide dispersion liquid is as follows: add 2.0 g of cerium oxide to 100 g of absolute ethanol, ultrasonically disperse for 50 min, the ultrasonic power is 140 W, and the ultrasonic frequency is 36 kHz. After the ultrasonic dispersion, obtain the cerium oxide dispersion liquid; The mixed acid is a mixture of oxalic acid and sulfamic acid, and the mass ratio of oxalic acid and sulfamic acid is 4.2:1.0.

[0018] 2. Coating Add 25 g of silica sol to 25 g of deionized water, then add 1.0 g of ammonium citrate, and stir evenly to obtain a silica sol solution; add 100 g of acid-leached alumina to 250 g of deionized water, and stir evenly to obtain an acid-leached alumina suspension; Add 22.5 g of the silica sol solution to 350 g of the acid-leached alumina suspension, control the pH value of the silica sol solution to be 5.0, and the addition rate of the silica sol solution to be 0.5 g / min. After the addition is completed, raise the temperature to 76 °C at a rate of 2.0 °C / min, keep stirring for 1.2 h, then add 15.0 g of the silica sol solution, control the pH value of the silica sol solution to be 7.0, and the addition rate of the silica sol solution to be 0.8 g / min. After the addition is completed, keep stirring for 1.2 h. After the stirring is completed, add 12.5 g of the silica sol solution, control the pH value of the silica sol solution to be 9.8, and the addition rate of the silica sol solution to be 1.0 g / min. Lower the temperature to 63 °C at a rate of 1.0 °C / min, keep stirring for 1.0 h. After the stirring is completed, perform centrifugal separation and then drying to obtain silica sol-coated alumina; The mass concentration of the silica sol is 20 wt%, and the particle size is 30 nm.

[0019] 3. Post-treatment Mix 100 g of the silica sol-coated alumina and 3.5 g of the post-treatment agent evenly, and then calcine in a nitrogen atmosphere. Raise the temperature to 820 °C at a rate of 2.0 °C / min, keep warm for 35 min, then raise the temperature to 1250 °C at a rate of 3.5 °C / min, and keep warm for 2.5 h to obtain α-phase alumina micropowder; The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 1.0:0.3:1.3.

[0020] For the α-phase alumina micropowder obtained by the method of Example 2, the α-phase conversion rate is 99.7%, the primary crystal grain size is 0.3 μm, the angle of repose is 28°. According to the detection and analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content is 0.02 wt%, and the Al2O3 content is 99.95 wt%.

[0021] Example 3 A preparation method of low-sodium high-temperature α-phase alumina micropowder 1. Acid leaching Add 400 g of industrial alumina to 610 g of the pretreatment solution, raise the temperature to 42 °C, stir at 230 rpm for 43 min, continue to raise the temperature to 65 °C, add 6.3 g of the mixed acid, continue to stir for 1.5 h, naturally cool to room temperature, after centrifugation, wash with deionized water until the washing liquid is neutral, and after drying, obtain acid-leached alumina; The particle size of the industrial alumina is 60 μm, and the sodium oxide content is 0.57 wt%. The preparation method of the pretreatment solution is as follows: add 0.10 g of cellulose nanocrystals CNC to 100 g of deionized water, stir at 45 °C for 0.8 h, then add 53 g of ammonium citrate solution and perform ultrasonic treatment. The ultrasonic time is 50 min, the ultrasonic power is 83 W, and the ultrasonic frequency is 28 kHz. After the ultrasonic treatment, add 15.7 g of cerium oxide dispersion liquid, keep warm and stir for 1.8 h, and remove ethanol to obtain the pretreatment solution. The mass concentration of the ammonium citrate solution is 0.48 wt%. The preparation method of the cerium oxide dispersion liquid is as follows: add 2.2 g of cerium oxide to 100 g of absolute ethanol, perform ultrasonic dispersion for 53 min, the ultrasonic power is 146 W, and the ultrasonic frequency is 38 kHz. After the ultrasonic dispersion, the cerium oxide dispersion liquid is obtained. The mixed acid is a mixture of oxalic acid and sulfamic acid, and the mass ratio of oxalic acid to sulfamic acid is 4.5:1.0.

[0022] 2. Coating Add 27 g of silica sol to 26 g of deionized water, then add 1.2 g of ammonium citrate, and stir evenly to obtain a silica sol solution; add 100 g of acid-leached alumina to 260 g of deionized water, and stir evenly to obtain an acid-leached alumina suspension. Add 25.5 g of silica sol solution to 360 g of acid-leached alumina suspension, control the pH value of the silica sol solution to 5.2, and the addition rate of the silica sol solution is 0.6 g / min. After the addition is completed, raise the temperature to 78 °C at a rate of 2.2 °C / min, keep warm and stir for 1.4 h, then add 14.8 g of silica sol solution, control the pH value of the silica sol solution to 7.0, and the addition rate of the silica sol solution is 1.0 g / min. After the addition is completed, keep warm and stir for 1.5 h. After the stirring is completed, add 12.7 g of silica sol solution, control the pH value of the silica sol solution to 10.0, and the addition rate of the silica sol solution is 1.2 g / min. Lower the temperature to 65 °C at a rate of 1.2 °C / min, keep warm and stir for 0.8 h. After the stirring is completed, perform centrifugal separation and drying to obtain silica sol-coated alumina. The mass concentration of the silica sol is 22 wt%, and the particle size is 25 nm.

[0023] 3. Post-treatment Mix 100 g of silica sol-coated alumina and 3.8 g of post-treatment agent evenly, and perform calcination in a nitrogen atmosphere. Raise the temperature to 830 °C at a rate of 2.2 °C / min, keep warm for 30 min, then raise the temperature to 1280 °C at a rate of 3.8 °C / min, and keep warm for 2.7 h to obtain α-phase alumina micropowder. The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 1.2:0.4:1.4.

[0024] The α-phase alumina micropowder obtained by the method of Example 3 has an α-phase conversion rate of 99.5%, an original crystal grain size of 0.4 μm, a repose angle of 30°, and according to the detection and analysis of an inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content is 0.03 wt%, and the Al2O3 content is 99.92 wt%.

[0025] The present invention prepares α-phase alumina micropowder by specific acid leaching, coating, and post-treatment steps; among them, in the acid leaching step, first, it is treated with a pretreatment solution, which is a mixture of cellulose nanocrystals CNC, ammonium citrate, and cerium oxide. Ammonium citrate can chelate sodium on the surface. Combining cerium oxide and cellulose nanocrystals CNC can increase steric hindrance, enhance the dispersibility of industrial alumina, avoid agglomeration, provide activation sites for subsequent steps, and increase the conversion rate of the α-phase. Then, a mixture of two acids is used for compounding, which can accelerate the replacement of sodium ions and reduce the sodium content; the second step is to coat the acid-leached alumina with silica sol, and the silica sol is treated with ammonium citrate, which can chelate sodium on the surface of alumina. During the coating process, a gradient coating method is adopted. First, a relatively dense silica sol is formed on the surface of alumina to enhance the fluidity performance of alumina particles. The amount of silica sol in the middle layer is moderate, which can adsorb residual sodium and block the entry of impurities. The outermost layer has the least amount of silica sol, which promotes the escape of sodium at high temperatures. It can also inhibit the abnormal growth of alumina grains during high-temperature calcination, which is beneficial to the uniform conversion of the α-phase; finally, the post-treatment agent is used to calcine alumina at high temperatures. Ammonium molybdate in the post-treatment agent can lower the phase transition temperature of alumina and promote the formation of α-phase alumina. Tetrabutylammonium bromide can undergo an ion exchange reaction with sodium ions to remove sodium. Combining with the yttrium oxide component can improve the lattice structure of alumina and inhibit grain growth. Finally, alumina micropowder with uniform particle size, good fluidity, low sodium content, and high α-phase conversion rate is obtained.

[0026] Comparative Example 1 1. Acid leaching The pretreatment solution is equally replaced with an ammonium citrate solution, and the mass concentration of the ammonium citrate solution is 0.45 wt%; the mixed acid is equally replaced with oxalic acid, and the rest of the operations are exactly the same as in Example 2; 2. Coating It is exactly the same as in Example 2.

[0027] 3. Post-treatment The post-treatment agent is equally replaced with a mixture of boric acid and magnesium chloride, and the mass ratio of boric acid to magnesium chloride is 1:1; the rest of the operations are exactly the same as in Example 2.

[0028] The α-phase alumina fine powder obtained by the method of Comparative Example 1 has an α-phase conversion rate of 94.6%, an original crystal grain size of 1.1 μm, a repose angle of 38°. According to the detection and analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content is 0.15 wt%, and the Al2O3 content is 98.02 wt%.

[0029] According to the results of Comparative Example 1, in the acid leaching step, only ammonium citrate was used as the pretreatment solution, and the mixed acid was only oxalic acid component, which would lead to uneven acid washing, and the lack of sulfamic acid component resulted in more sodium residue in the deep layer, affecting the removal of sodium, and changing the mineralizer component, resulting in abnormal grain growth, hindering the transformation of α-phase, the surface of the transformed α-phase alumina was rough, with poor fluidity, and finally the α-phase conversion rate of the product was low, the repose angle was high, the sodium content was high, and the original crystal grain size was large.

[0030] Comparative Example 2 1. Acid leaching It is exactly the same as that in Example 2; 2. Coating 25 g of silica sol was added to 25 g of deionized water, and after stirring evenly, a silica sol solution was obtained; 100 g of acid-leached alumina was added to 250 g of deionized water, and after stirring evenly, an acid-leached alumina suspension was obtained; 50 g of silica sol solution was added to 350 g of acid-leached alumina suspension, the pH value of the silica sol solution was controlled to be 7.0, the addition rate of the silica sol solution was 0.8 g / min. After the addition was completed, the temperature was raised to 63 °C at a rate of 2.0 °C / min, and stirred for 3.4 h. After the stirring was completed, it was centrifuged and dried to obtain silica sol-coated alumina; 3. Post-treatment It is exactly the same as that in Example 2.

[0031] The α-phase alumina fine powder obtained by the method of Comparative Example 2 has an α-phase conversion rate of 95.5%, an original crystal grain size of 0.8 μm, a repose angle of 36°. According to the detection and analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content is 0.10 wt%, and the Al2O3 content is 98.37 wt%.

[0032] According to the results of Comparative Example 2, in the coating step, only a single silica sol component was used, and uniform coating was carried out under the condition of pH 7.0, and the total addition amount of silica sol remained unchanged, which would lead to some sodium being sealed and unable to be removed, and affect the transformation to α-phase, affecting the local sintering performance, increasing the roughness of the product, with larger particles and poor fluidity.

[0033] Unless otherwise specified, the percentages used in the present invention are all mass percentages.

[0034] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing low-sodium high-temperature α-phase alumina micropowder, characterized in that, It includes an acid leaching step, a coating step, and a post-treatment step; In the acid leaching step, industrial alumina is added to the pretreatment solution, stirred evenly, heated to 60 - 65 °C, and mixed acid is added and stirred to obtain acid-leached alumina; The preparation method of the pretreatment solution is as follows: cellulose nanocrystals CNC are added to deionized water, stirred evenly at 40 - 45 °C, ammonium citrate solution is added for ultrasonic treatment, and then cerium oxide dispersion is added and stirred for 1.4 - 1.8 h. After removing ethanol, the pretreatment solution is obtained; The mixed acid is a mixture of oxalic acid and sulfamic acid; In the coating step, silica sol solution is first added to the acid-leached alumina suspension, stirred at 74 - 78 °C for 1.0 - 1.4 h, then silica sol solution is added for the second time, stirred while keeping warm for 1.0 - 1.5 h, and then silica sol solution is added for the third time, stirred at 62 - 65 °C for 0.8 - 1.2 h to obtain silica sol-coated alumina; In the post-treatment step, the silica sol-coated alumina and the post-treatment agent are mixed evenly and then calcined to obtain α-phase alumina micropowder; The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide.

2. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that, In the acid leaching step, the particle size of the industrial alumina is 40 - 60 μm, and the sodium oxide content is 0.52 - 0.57 wt%; The mass ratio of the industrial alumina, the pretreatment solution, and the mixed acid is 400:590 - 610:5.8 - 6.

3.

3. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that, In the preparation method of the pretreatment solution, the mass ratio of the deionized water, cellulose nanocrystals CNC, ammonium citrate solution, and cerium oxide dispersion is 100:0.07 - 0.10:46 - 53:15.0 - 15.7; The preparation method of the cerium oxide dispersion is as follows: cerium oxide is added to absolute ethanol, ultrasonically dispersed for 48 - 53 min, the ultrasonic power is 132 - 146 W, the ultrasonic frequency is 32 - 38 kHz, and after the ultrasonic dispersion is completed, the cerium oxide dispersion is obtained; The mass ratio of the absolute ethanol to the cerium oxide is 100:1.8 - 2.2; The mass concentration of the ammonium citrate solution is 0.42 - 0.48 wt%.

4. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that, In the mixed acid, the mass ratio of the oxalic acid to the sulfamic acid is 4.0 - 4.5:1.

0.

5. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that, The preparation method of the silica sol solution is as follows: silica sol is added to deionized water, and then ammonium citrate is added, and after stirring evenly, the silica sol solution is obtained; The mass concentration of the silica sol is 18 - 22 wt%, and the particle size is 25 - 35 nm; The mass ratio of the silica sol, the deionized water, and the ammonium citrate is 22 - 27:24 - 26:0.8 - 1.2; The preparation method of the acid-leached alumina suspension is to add acid-leached alumina to deionized water, and after stirring evenly, obtain the acid-leached alumina suspension; The mass ratio of the acid-leached alumina to the deionized water is 100:240 - 260.

6. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that In the coating step, when the silica sol solution is added for the first time, the pH value of the silica sol solution is 4.8 - 5.2, the addition rate of the silica sol solution is 0.4 - 0.6 g / min, and the heating rate is 1.8 - 2.2 °C / min; When the silica sol solution is added for the second time, the pH value of the silica sol solution is 7.0, and the addition rate of the silica sol solution is 0.6 - 1.0 g / min; When the silica sol solution is added for the third time, the pH value of the silica sol solution is 9.6 - 10.0, the addition rate of the silica sol solution is 0.8 - 1.2 g / min, and the heating rate is 0.7 - 1.2 °C / min.

7. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that In the coating step, the mass ratio of the acid-leached alumina suspension, the first addition amount of the silica sol solution, the second addition amount of the silica sol solution, and the third addition amount of the silica sol solution is 340 - 360:19.3 - 25.5:14.7 - 15.0:12.0 - 12.

7.

8. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that In the post-treatment step, the calcination is carried out in a nitrogen atmosphere, the temperature is raised to 810 - 830 °C at a rate of 1.8 - 2.2 °C / min, held for 30 - 40 min, and then the temperature is raised to 1230 - 1280 °C at a rate of 3.2 - 3.8 °C / min and held for 2.3 - 2.7 h to obtain the α-phase alumina micropowder; The mass ratio of the silica sol-coated alumina to the post-treatment agent is 100:3.3 - 3.

8.

9. The preparation method of a low-sodium high-temperature α-phase alumina micropowder according to claim 1, characterized in that In the post-treatment agent, the mass ratio of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 0.8 - 1.2:0.2 - 0.4:1.1 - 1.4.