A method for preparing low-sodium and high-temperature α-phase alumina micropowder

Abstract

The invention provides a method for preparing low-sodium and high-temperature α-phase alumina micropowder, belonging to the technical field of alumina powder. The preparation method comprises acid leaching, coating and post-processing steps. The coating step comprises: adding a silica sol solution to an acid-leached alumina suspension for the first time, stirring at 74-78°C for 1.0-1.4 hours, then adding the silica sol solution for the second time, keeping the temperature and stirring for 1.0-1.5 hours, then adding the silica sol solution for the third time, stirring at 62-65°C for 0.8-1.2 hours, to obtain silica sol-coated alumina. The α-phase alumina micropowder prepared by the preparation method has uniform particle size, low sodium oxide content, high α-phase conversion rate and good powder fluidity.

Description

Technical Field

[0001] The invention belongs to the technical field of alumina powder, and particularly relates to a method for preparing low-sodium and high-temperature α-phase alumina micropowder. 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 of up to 2040°C. It has good formability, stable crystal phase, high hardness, good wear resistance, high thermal stability, and excellent physical and chemical stability. It is widely used in ceramics, refractories, rubber, plastics, petrochemical and other industries.

[0003] Sodium oxide content is an important technical indicator of high-temperature α-alumina micropowders. A high sodium oxide content will affect the conversion rate of the α phase. The presence of sodium will interfere with the crystal structure of α-alumina, entering the lattice gaps and causing lattice distortion, which in turn affects the density and other properties of the material. In the electronics field, high sodium content will reduce its insulation performance and lead to unstable signal transmission. In the ceramic field, sodium will affect the sintering process of ceramics during high-temperature sintering, causing defects such as pores and cracks in the ceramics, which not only reduces the appearance of the ceramics, but also reduces the strength of the ceramic products.

[0004] Therefore, providing a method for preparing low-sodium and high-temperature α-phase alumina micropowder is a topic that needs to be studied urgently in the prior art.

[0005] The existing methods for preparing low-sodium and high-temperature α-phase alumina powder mainly include non-mineralization method and mineralization method;

[0006] The non-mineralization methods mainly include the Bayer process and the alcohol aluminum salt hydrolysis method. Among them, the Bayer process is to process bauxite, react alumina with sodium hydroxide to generate sodium aluminate solution, and then decompose and calcine to obtain α-phase alumina powder. This method is difficult to effectively remove sodium impurities in the raw materials, resulting in a high sodium content in the product, which affects the final performance of the product. The alcohol aluminum salt hydrolysis method is to use the hydrolysis and condensation reaction of metal alcohol salts to prepare α-phase alumina powder, which has a high composition and harsh reaction conditions.

[0007] The mineralization method uses a mineralizer to remove sodium impurities and calcines at high temperature to obtain α-phase alumina powder; however, the amount of mineralizer added is difficult to control. Adding the mineralizer makes the original crystals of the powder difficult to control, making it difficult to obtain micro-powder-level α-phase alumina.

[0008] CN115974112A discloses a low-sodium submicron α-alumina powder and its preparation method and application. Specifically, it discloses that industrial alumina, an organic acid, a dispersant and water are mixed, and the pH value of the resulting dispersion is adjusted to 7-7.5 to obtain a slurry; the slurry is subjected to a filter press to obtain a low-sodium industrial alumina powder; the low-sodium industrial alumina powder is mixed with a magnesium-containing compound and a rare earth oxide, and calcined to obtain a low-sodium submicron α-alumina powder;

[0009] The low-sodium submicron α-alumina powder prepared by this method has a low sodium content and high purity. The original crystal size is submicron, the purity is 99.9-99.99wt%, the α phase content is 96-99%, the original crystal size is 0.3-0.8μm, and the sodium content is 0.01-0.05wt%. However, the α-alumina powder has the defect of strong agglomeration force, which limits its subsequent application. Summary of the Invention

[0010] In order to solve the technical problems existing in the prior art, the present invention provides a method for preparing low-sodium and 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 powder fluidity.

[0011] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0012] A method for preparing low-sodium, high-temperature α-phase alumina micropowder includes acid leaching, coating, and post-treatment steps. The specific operations are as follows:

[0013] 1. Acid leaching

[0014] Add industrial alumina to the pretreatment solution, raise the temperature to 37-42°C, stir at 200-230 rpm for 35-43 minutes, continue to raise the temperature to 60-65°C, add mixed acid, continue stirring for 1.2-1.5 hours, naturally cool to room temperature, centrifuge, wash with deionized water until the washing liquid is neutral, and dry to obtain acid-leached alumina;

[0015] The particle size of the industrial alumina is 40-60 μm, and the sodium oxide content is 0.52-0.57 wt%;

[0016] The mass ratio of the industrial alumina, pretreatment liquid and mixed acid is 400:590-610:5.8-6.3;

[0017] The preparation method of the pretreatment liquid comprises the following steps: adding cellulose nanocrystals (CNC) to deionized water, stirring at 40-45° C. for 0.8-1.2 hours, then adding ammonium citrate solution for ultrasonic treatment, wherein the ultrasonic treatment time is 40-50 minutes, the ultrasonic power is 83-88W, and the ultrasonic frequency is 23-28kHz. After the ultrasonic treatment, a cerium oxide dispersion is added, the mixture is stirred at this temperature for 1.4-1.8 hours, and the ethanol is removed to obtain the pretreatment liquid.

[0018] 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;

[0019] The mass concentration of the ammonium citrate solution is 0.42-0.48wt%;

[0020] The cerium oxide dispersion is prepared by adding cerium oxide to anhydrous ethanol, performing ultrasonic dispersion for 48-53 minutes, with an ultrasonic power of 132-146 W and an ultrasonic frequency of 32-38 kHz, to obtain a cerium oxide dispersion after the ultrasonic dispersion is completed;

[0021] The mass ratio of anhydrous ethanol to cerium oxide is 100:1.8-2.2;

[0022] The mixed acid is a mixture of oxalic acid and aminosulfonic acid, and the mass ratio of the oxalic acid to the aminosulfonic acid is 4.0-4.5:1.0.

[0023] 2. Coating

[0024] Adding silica sol to deionized water, then adding ammonium citrate, and stirring evenly to obtain a silica sol solution; adding acid-leached alumina to deionized water, and stirring evenly to obtain an acid-leached alumina suspension;

[0025] Adding silica sol solution to the acid-leached alumina suspension for the first time, controlling the pH value of the silica sol solution to 4.8-5.2, and adding the silica sol solution at a rate of 0.4-0.6 g / min. After the addition is completed, the temperature is increased to 74-78 ° C at a rate of 1.8-2.2 ° C / min, and the mixture is kept warm and stirred for 1.0-1.4 hours. Then, adding silica sol solution for the second time, controlling the pH value of the silica sol solution to 7.0, and adding the silica sol solution at a rate of 0.6-1.0 g / min. After the addition is completed, the mixture is kept warm and stirred for 1.0-1.5 hours. After the stirring is completed, adding silica sol solution for the third time, controlling the pH value of the silica sol solution to 9.6-10.0, and adding the silica sol solution at a rate of 0.8-1.2 g / min. The temperature is reduced to 62-65 ° C at a rate of 0.7-1.2 ° C / min, and the mixture is kept warm and stirred for 0.8-1.2 hours. After the stirring is completed, the mixture is dried after centrifugal separation to obtain silica sol-coated alumina;

[0026] The silica sol has a mass concentration of 18-22 wt % and a particle size of 25-35 nm;

[0027] In the silica sol solution, the mass ratio of silica sol, deionized water, and ammonium citrate is 22-27:24-26:0.8-1.2;

[0028] In the acid-leached alumina suspension, the mass ratio of acid-leached alumina to deionized water is 100:240-260;

[0029] 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.

[0030] 3. Post-processing

[0031] After the silica sol coated alumina and the post-treatment agent are uniformly mixed, calcination is carried out under a nitrogen atmosphere, the temperature is increased to 810-830°C at a rate of 1.8-2.2°C / min, and the temperature is kept at this temperature for 30-40 minutes, and then the temperature is increased to 1230-1280°C at a rate of 3.2-3.8°C / min, and the temperature is kept at this temperature for 2.3-2.7 hours to obtain α-phase alumina powder;

[0032] The mass ratio of the silica sol coated alumina to the post-treatment agent is 100:3.3-3.8;

[0033] The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide and yttrium oxide, and the mass ratio of the ammonium molybdate, tetrabutylammonium bromide and yttrium oxide is 0.8-1.2:0.2-0.4:1.1-1.4.

[0034] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0035] 1. The present invention adopts specific acid leaching, coating and post-treatment steps to prepare α-phase alumina powder; wherein, in the acid leaching step, it is first treated with a pretreatment liquid, which is a mixture of cellulose nanocrystals CNC, ammonium citrate and cerium oxide. Ammonium citrate can chelate the sodium on the surface. Combined with cerium oxide and cellulose nanocrystals CNC, it can improve steric hindrance, enhance the dispersibility of industrial alumina, avoid agglomeration, provide activation sites for subsequent steps, and improve the conversion rate of α phase. Then, two acids are compounded, 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 the sodium on the surface of the alumina. A gradient coating method is adopted in the coating process, firstly oxidizing A relatively dense silica sol is formed on the surface of the aluminum, which enhances the fluidity of the 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 silica sol in the outermost layer is the least, which promotes the escape of sodium at high temperature. It can also inhibit the abnormal growth of alumina grains during high-temperature calcination, which is conducive to the uniform conversion of α phase. Finally, a post-treatment agent is used to calcine the alumina at high temperature. The 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 ion exchange with sodium ions to remove sodium, combine with yttrium oxide components, improve the lattice structure of alumina, inhibit grain growth, and ultimately obtain alumina micropowder with uniform particle size, good fluidity, low sodium content and high α phase conversion rate.

[0036] 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 grain size of 0.3-0.5 μm, an angle of repose of 28-32°, and according to the detection and analysis of an inductively coupled plasma atomic emission spectrometer ICPE-9820, a sodium oxide content of 0.02-0.06wt%, and an Al2O3 content of 99.90-99.95wt%. DETAILED DESCRIPTION

[0037] In order to more clearly understand the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described.

[0038] Example 1 A method for preparing low-sodium high-temperature α-phase alumina powder

[0039] 1. Acid leaching

[0040] 400 g of industrial alumina was added to 590 g of pretreatment solution, the temperature was raised to 37°C, and the mixture was stirred at 200 rpm for 35 min. The temperature was further raised to 60°C, 5.8 g of mixed acid was added, and the mixture was stirred for 1.2 h. The mixture was naturally cooled to room temperature. After centrifugation, the mixture was washed with deionized water until the washing solution was neutral, and dried to obtain acid-leached alumina.

[0041] The particle size of the industrial alumina is 40 μm and the sodium oxide content is 0.52 wt%;

[0042] The pretreatment liquid is prepared by adding 0.07 g of cellulose nanocrystals (CNC) to 100 g of deionized water, stirring at 40° C. for 1.2 h, then adding 46 g of ammonium citrate solution for ultrasonic treatment, wherein the ultrasonic treatment time is 40 min, the ultrasonic power is 88 W, and the ultrasonic frequency is 23 kHz. After the ultrasonic treatment, 15.0 g of cerium oxide dispersion is added, the mixture is stirred at this temperature for 1.4 h, and the ethanol is removed to obtain the pretreatment liquid.

[0043] The mass concentration of the ammonium citrate solution is 0.42wt%;

[0044] The cerium oxide dispersion is prepared by adding 1.8 g of cerium oxide to 100 g of anhydrous ethanol, and performing ultrasonic dispersion for 48 minutes at an ultrasonic power of 132 W and an ultrasonic frequency of 32 kHz to obtain a cerium oxide dispersion.

[0045] The mixed acid is a mixture of oxalic acid and aminosulfonic acid, and the mass ratio of the oxalic acid to the aminosulfonic acid is 4.0:1.0.

[0046] 2. Coating

[0047] 22 g of silica sol was added to 24 g of deionized water, and then 0.8 g of ammonium citrate was added and stirred to obtain a silica sol solution; 100 g of acid-leached alumina was added to 240 g of deionized water and stirred to obtain an acid-leached alumina suspension;

[0048] 19.3 g of silica sol solution was added to 340 g of the acid-leached alumina suspension, the pH value of the silica sol solution was controlled to be 4.8, the addition rate of the silica sol solution was 0.4 g / min, and after the addition was completed, the temperature was raised to 74 ° C. at a rate of 1.8 ° C. / min, and the mixture was stirred for 1.0 h. Then, 14.7 g of silica sol solution was added, the pH value of the silica sol solution was controlled to be 7.0, the addition rate of the silica sol solution was 0.6 g / min, and after the addition was completed, the mixture was stirred for 1.0 h. After the stirring was completed, 12.0 g of silica sol solution was added, the pH value of the silica sol solution was controlled to be 9.6, the addition rate of the silica sol solution was 0.8 g / min, the temperature was lowered to 62 ° C. at a rate of 0.7 ° C. / min, and the mixture was stirred for 1.2 h. After the stirring was completed, it was centrifuged and dried to obtain silica sol-coated alumina;

[0049] The silica sol has a mass concentration of 18 wt % and a particle size of 35 nm.

[0050] 3. Post-processing

[0051] 100 g of silica sol-coated alumina and 3.3 g of a post-treatment agent were mixed uniformly, and then calcined under a nitrogen atmosphere. The temperature was increased to 810°C at a rate of 1.8°C / min and kept at this temperature for 40 min. Then, the temperature was increased to 1230°C at a rate of 3.2°C / min and kept at this temperature for 2.3 h to obtain α-phase alumina powder.

[0052] The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of the ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 0.8:0.2:1.1.

[0053] The α-phase alumina powder obtained by the method of Example 1 had an α-phase conversion rate of 99.1%, an original grain size of 0.5 μm, an angle of repose of 32°, and according to analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content was 0.06 wt %, and the Al 2 O 3 content was 99.90 wt %.

[0054] Example 2 A method for preparing low-sodium high-temperature α-phase alumina powder

[0055] 1. Acid leaching

[0056] 400 g of industrial alumina was added to 600 g of pretreatment solution, the temperature was raised to 40°C, and the mixture was stirred at 220 rpm for 40 min. The temperature was further raised to 63°C, 6.0 g of mixed acid was added, and the mixture was stirred for 1.3 h. The mixture was naturally cooled to room temperature. After centrifugation, the mixture was washed with deionized water until the washing solution was neutral, and dried to obtain acid-leached alumina.

[0057] The particle size of the industrial alumina is 50 μm and the sodium oxide content is 0.54 wt%;

[0058] The pretreatment liquid is prepared by adding 0.08 g of cellulose nanocrystals CNC to 100 g of deionized water, stirring at 42° C. for 1.0 h, then adding 50 g of ammonium citrate solution for ultrasonic treatment, wherein the ultrasonic treatment time is 45 min, the ultrasonic power is 85 W, and the ultrasonic frequency is 25 kHz. After the ultrasonic treatment, 15.3 g of cerium oxide dispersion is added, the mixture is stirred at this temperature for 1.5 h, and the ethanol is removed to obtain the pretreatment liquid.

[0059] The mass concentration of the ammonium citrate solution is 0.45wt%;

[0060] The cerium oxide dispersion is prepared by adding 2.0 g of cerium oxide to 100 g of anhydrous ethanol, and performing ultrasonic dispersion for 50 minutes at an ultrasonic power of 140 W and an ultrasonic frequency of 36 kHz to obtain a cerium oxide dispersion.

[0061] The mixed acid is a mixture of oxalic acid and aminosulfonic acid, and the mass ratio of the oxalic acid to the aminosulfonic acid is 4.2:1.0.

[0062] 2. Coating

[0063] 25 g of silica sol was added to 25 g of deionized water, and then 1.0 g of ammonium citrate was added and stirred to obtain a silica sol solution; 100 g of acid-leached alumina was added to 250 g of deionized water and stirred to obtain an acid-leached alumina suspension;

[0064] To 350 g of the acid-leached alumina suspension, 22.5 g of a silica sol solution was added, the pH value of the silica sol solution was controlled to be 5.0, and the silica sol solution was added at a rate of 0.5 g / min. After the addition was completed, the temperature was raised to 76 ° C. at a rate of 2.0 ° C. / min, and the mixture was stirred at this temperature for 1.2 h. Then, 15.0 g of a silica sol solution was added, and the pH value of the silica sol solution was controlled to be 7.0. The silica sol solution was added at a rate of 0.8 g / min. After the addition was completed, the mixture was stirred at this temperature for 1.2 h. After the stirring was completed, 12.5 g of a silica sol solution was added, and the pH value of the silica sol solution was controlled to be 9.8. The silica sol solution was added at a rate of 1.0 g / min. The temperature was lowered to 63 ° C. at a rate of 1.0 ° C. / min, and the mixture was stirred at this temperature for 1.0 h. After the stirring was completed, the mixture was centrifuged and dried to obtain silica sol-coated alumina.

[0065] The silica sol has a mass concentration of 20 wt % and a particle size of 30 nm.

[0066] 3. Post-processing

[0067] 100 g of silica sol-coated alumina and 3.5 g of a post-treatment agent were mixed uniformly, and then calcined under a nitrogen atmosphere. The temperature was increased to 820°C at a rate of 2.0°C / min, and kept at this temperature for 35 min. Then, the temperature was increased to 1250°C at a rate of 3.5°C / min, and kept at this temperature for 2.5 h to obtain α-phase alumina powder.

[0068] The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of the ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 1.0:0.3:1.3.

[0069] The α-phase alumina powder obtained by the method of Example 2 had an α-phase conversion rate of 99.7%, an original grain size of 0.3 μm, an angle of repose of 28°, and according to analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content was 0.02 wt %, and the Al 2 O 3 content was 99.95 wt %.

[0070] Example 3 A method for preparing low-sodium high-temperature α-phase alumina powder

[0071] 1. Acid leaching

[0072] 400 g of industrial alumina was added to 610 g of pretreatment solution, the temperature was raised to 42 ° C, and the mixture was stirred at 230 rpm for 43 minutes. The temperature was further raised to 65 ° C, 6.3 g of mixed acid was added, and the mixture was stirred for 1.5 hours. The mixture was naturally cooled to room temperature. After centrifugation, the mixture was washed with deionized water until the washing liquid was neutral, and dried to obtain acid-leached alumina.

[0073] The particle size of the industrial alumina is 60 μm and the sodium oxide content is 0.57 wt%;

[0074] The pretreatment liquid is prepared by adding 0.10 g of cellulose nanocrystals CNC to 100 g of deionized water, stirring at 45° C. for 0.8 h, then adding 53 g of ammonium citrate solution for ultrasonic treatment, wherein the ultrasonic treatment time is 50 min, the ultrasonic power is 83 W, and the ultrasonic frequency is 28 kHz. After the ultrasonic treatment, 15.7 g of cerium oxide dispersion is added, the mixture is stirred at this temperature for 1.8 h, and the ethanol is removed to obtain the pretreatment liquid.

[0075] The mass concentration of the ammonium citrate solution is 0.48wt%;

[0076] The cerium oxide dispersion is prepared by adding 2.2 g of cerium oxide to 100 g of anhydrous ethanol, and performing ultrasonic dispersion for 53 minutes at an ultrasonic power of 146 W and an ultrasonic frequency of 38 kHz to obtain a cerium oxide dispersion.

[0077] The mixed acid is a mixture of oxalic acid and aminosulfonic acid, and the mass ratio of the oxalic acid to the aminosulfonic acid is 4.5:1.0.

[0078] 2. Coating

[0079] 27 g of silica sol was added to 26 g of deionized water, and then 1.2 g of ammonium citrate was added and stirred to obtain a silica sol solution; 100 g of acid-leached alumina was added to 260 g of deionized water and stirred to obtain an acid-leached alumina suspension;

[0080] To 360 g of the acid-leached alumina suspension, 25.5 g of a silica sol solution was added, the pH value of the silica sol solution was controlled to be 5.2, and the silica sol solution was added at a rate of 0.6 g / min. After the addition was completed, the temperature was raised to 78 ° C. at a rate of 2.2 ° C. / min, and the mixture was stirred for 1.4 h. Then, 14.8 g of a silica sol solution was added, and the pH value of the silica sol solution was controlled to be 7.0. The silica sol solution was added at a rate of 1.0 g / min. After the addition was completed, the mixture was stirred for 1.5 h. After the stirring was completed, 12.7 g of a silica sol solution was added, and the pH value of the silica sol solution was controlled to be 10.0. The silica sol solution was added at a rate of 1.2 g / min. The temperature was lowered to 65 ° C. at a rate of 1.2 ° C. / min, and the mixture was stirred for 0.8 h. After the stirring was completed, the mixture was centrifuged and dried to obtain silica sol-coated alumina.

[0081] The silica sol has a mass concentration of 22 wt % and a particle size of 25 nm.

[0082] 3. Post-processing

[0083] 100 g of silica sol-coated alumina and 3.8 g of a post-treatment agent were mixed uniformly, and then calcined under a nitrogen atmosphere. The temperature was increased to 830°C at a rate of 2.2°C / min and kept at this temperature for 30 min. Then, the temperature was increased to 1280°C at a rate of 3.8°C / min and kept at this temperature for 2.7 h to obtain α-phase alumina powder.

[0084] The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide, and the mass ratio of the ammonium molybdate, tetrabutylammonium bromide, and yttrium oxide is 1.2:0.4:1.4.

[0085] The α-phase alumina powder obtained by the method of Example 3 had an α-phase conversion rate of 99.5%, an original grain size of 0.4 μm, an angle of repose of 30°, and according to analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content was 0.03 wt %, and the Al 2 O 3 content was 99.92 wt %.

[0086] The present invention adopts specific acid leaching, coating and post-treatment steps to prepare α-phase alumina powder; wherein, in the acid leaching step, it is first treated with a pretreatment liquid, which is a mixture of cellulose nanocrystals CNC, ammonium citrate and cerium oxide. Ammonium citrate can chelate the sodium on the surface, and combined with cerium oxide and cellulose nanocrystals CNC, it can improve the steric hindrance, enhance the dispersibility of industrial alumina, avoid agglomeration, provide activation sites for subsequent steps, and improve the conversion rate of the α phase. Then, two acids are compounded, 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 the sodium on the surface of the alumina. A gradient coating method is adopted in the coating process, first the alumina A relatively dense silica sol is formed on the surface, which enhances the fluidity of the 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 silica sol in the outermost layer is the least, which promotes the escape of sodium at high temperature. It can also inhibit the abnormal growth of alumina grains during high-temperature calcination, which is beneficial to the uniform conversion of α phase. Finally, a post-treatment agent is used to calcine alumina at high temperature. The 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 ion exchange with sodium ions to remove sodium, combine with yttrium oxide components, improve the lattice structure of alumina, inhibit grain growth, and finally obtain alumina micropowder with uniform particle size, good fluidity, low sodium content and high α phase conversion rate.

[0087] Comparative Example 1

[0088] 1. Acid leaching

[0089] The pretreatment liquid was replaced with an ammonium citrate solution in an equal amount, and the mass concentration of the ammonium citrate solution was 0.45 wt %; the mixed acid was replaced with oxalic acid in an equal amount, and the remaining operations were exactly the same as in Example 2;

[0090] 2. Coating

[0091] Exactly the same as Example 2.

[0092] 3. Post-processing

[0093] The post-treatment agent was replaced by an equal amount of a mixture of boric acid and magnesium chloride, with the mass ratio of boric acid to magnesium chloride being 1:1; the remaining operations were exactly the same as in Example 2.

[0094] The α-phase alumina powder obtained by the method of Comparative Example 1 had an α-phase conversion rate of 94.6%, an original grain size of 1.1 μm, an angle of repose of 38°, and according to the detection and analysis by inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content was 0.15 wt% and the Al2O3 content was 98.02 wt%.

[0095] According to the results of Comparative Example 1, only ammonium citrate is used as the pretreatment liquid in the acid leaching step, and the mixed acid is only oxalic acid, which will lead to uneven pickling. In addition, the lack of aminosulfonic acid component results in more sodium residue in the deep layer, which affects the removal of sodium. It also changes the mineralizer composition, causing abnormal grain growth and hindering the conversion of the α phase. The surface of the α-phase alumina generated by the conversion is rough and the fluidity is poor, which ultimately results in a low α-phase conversion rate, a high angle of repose, a high sodium content, and a large original grain size of the product.

[0096] Comparative Example 2

[0097] 1. Acid leaching

[0098] Exactly the same as Example 2;

[0099] 2. Coating

[0100] 25 g of silica sol was added to 25 g of deionized water and stirred to obtain a silica sol solution; 100 g of acid-leached alumina was added to 250 g of deionized water and stirred to obtain an acid-leached alumina suspension;

[0101] 50 g of silica sol solution was added to 350 g of the acid-leached alumina suspension, the pH value of the silica sol solution was controlled to 7.0, and the silica sol solution was added at a rate of 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 the mixture was stirred at this temperature for 3.4 hours. After the stirring was completed, the mixture was centrifuged and dried to obtain silica sol-coated alumina;

[0102] 3. Post-processing

[0103] Exactly the same as Example 2.

[0104] The α-phase alumina powder obtained by the method of Comparative Example 2 had an α-phase conversion rate of 95.5%, an original grain size of 0.8 μm, an angle of repose of 36°, and according to the detection and analysis of an inductively coupled plasma atomic emission spectrometer ICPE-9820, the sodium oxide content was 0.10 wt% and the Al2O3 content was 98.37 wt%.

[0105] According to the results of Comparative Example 2, only a single silica sol component is used in the coating step, and uniform coating is performed under the condition of pH 7.0. The total amount of silica sol added remains unchanged, which will cause part of the sodium to be sealed and unable to be removed, and affect the conversion to the α phase, affecting the local sintering performance, increasing the roughness of the product, making the particles larger, and having poor fluidity.

[0106] Unless otherwise specified, all percentages used in the present invention are by mass.

[0107] Finally, it should be noted that the above descriptions are merely 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 aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing low-sodium high-temperature α-phase alumina powder, characterized in that: including acid leaching, coating and post-processing steps; The acid leaching step comprises adding industrial alumina to the pretreatment solution, stirring evenly, heating to 60-65° C., adding mixed acid and stirring to obtain acid-leached alumina; The preparation method of the pretreatment liquid comprises the following steps: adding cellulose nanocrystals CNC to deionized water, stirring uniformly at 40-45° C., adding ammonium citrate solution for ultrasonic treatment, then adding cerium oxide dispersion and stirring for 1.4-1.8 hours, and removing ethanol to obtain the pretreatment liquid; The mixed acid is a mixture of oxalic acid and sulfamic acid; The coating step comprises adding a silica sol solution to the acid-leached alumina suspension for the first time, stirring at 74-78° C. for 1.0-1.4 hours, then adding the silica sol solution for the second time, keeping warm and stirring for 1.0-1.5 hours, then adding the silica sol solution for the third time, stirring at 62-65° C. for 0.8-1.2 hours, to obtain silica sol-coated alumina; The post-treatment step comprises: uniformly mixing the silica sol-coated alumina and the post-treatment agent and then calcining the mixture to obtain α-phase alumina powder; The post-treatment agent is a mixture of ammonium molybdate, tetrabutylammonium bromide and yttrium oxide.

2. The method for preparing low-sodium and high-temperature α-phase alumina powder 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 liquid and the mixed acid is 400:590-610:5.8-6.

3.

3. The method for preparing low-sodium and high-temperature α-phase alumina powder according to claim 1, characterized in that: In the preparation method of the pretreatment liquid, 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 cerium oxide dispersion is prepared by adding cerium oxide to anhydrous ethanol, performing ultrasonic dispersion for 48-53 minutes, with an ultrasonic power of 132-146 W and an ultrasonic frequency of 32-38 kHz, to obtain a cerium oxide dispersion after the ultrasonic dispersion is completed; The mass ratio of anhydrous ethanol to cerium oxide is 100:1.8-2.2; The mass concentration of the ammonium citrate solution is 0.42-0.48 wt %.

4. The method for preparing low-sodium and high-temperature α-phase alumina powder according to claim 1, characterized in that: In the mixed acid, the mass ratio of the oxalic acid to the aminosulfonic acid is 4.0-4.5:1.

0.

5. The method for preparing low-sodium and high-temperature α-phase alumina powder according to claim 1, characterized in that: The silica sol solution is prepared by adding silica sol to deionized water, then adding ammonium citrate, and stirring to obtain the silica sol solution; The silica sol has a mass concentration of 18-22 wt % and a particle size of 25-35 nm; The mass ratio of the silica sol, deionized water, and ammonium citrate is 22-27:24-26:0.8-1.2; The method for preparing the acid-leached alumina suspension comprises adding the acid-leached alumina into deionized water and stirring the mixture to obtain the acid-leached alumina suspension; The mass ratio of the acid-leached alumina to deionized water is 100:240-260.

6. The method for preparing low-sodium and high-temperature α-phase alumina powder according to claim 1, characterized in that: In the coating step, 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; The second addition of silica sol solution has a pH value of 7.0 and an addition rate of 0.6-1.0 g / min; 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 method for preparing low-sodium and high-temperature α-phase alumina powder 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.

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

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9. The method for preparing low-sodium and high-temperature α-phase alumina powder 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.