A controllable preparation method of flaky aluminum oxide

Abstract

The invention discloses a controllable preparation method of flaky aluminum oxide. The invention aims to provide a method for preparing flaky α-Al2O3 micron seed crystals at low temperature, and realizes controllable preparation of seed crystal particle size by a hydraulic classification method. The method comprises the following steps: uniformly mixing aluminum salt and urea, grinding the mixture, and drying the mixture to obtain a urea-aluminum complex; uniformly mixing the mixture with a soluble molten salt, and calcining the mixture at 550°C to 1100°C for 2-10 hours to obtain flaky α-Al2O3 seed crystals; screening the flaky α-Al2O3 seed crystals with different particle size ranges by hydraulic classification; uniformly mixing the seed crystals, aluminum salt, soluble molten salt, and a crystal growth regulator solution to obtain a suspension A; adding a soluble phosphate to an alkaline aqueous solution and mixing the mixture to obtain a solution B; adding the solution B to the suspension A, mixing the mixture, and drying the mixture to obtain a gel mixture solid; and washing the solid obtained by high-temperature calcination with water to remove the soluble molten salt, and drying the solid to obtain a flaky α-Al2O3 powder.

Description

Technical Field

[0001] The invention discloses a method for preparing aluminum oxide; specifically, it is a controllable preparation method for flaky aluminum oxide, belonging to the technical field of inorganic materials. Background Art

[0002] α-Al2O3, also known as corundum, possesses properties such as high hardness, strength, wear resistance, heat resistance, and corrosion resistance, making it widely used. Compared to conventional α-Al2O3, flake α-Al2O3 has a unique two-dimensional structure, with radial dimensions of micrometers and longitudinal dimensions of nanometers, resulting in a high aspect ratio. Therefore, it is widely used in pearlescent pigments, grinding and polishing, thermally conductive composite materials, and other fields.

[0003] Currently, the main methods for preparing flaky α-Al2O3 powders include high-temperature solid-phase methods, hydrothermal methods, sol-gel methods, and molten salt methods. The molten salt method involves dissolving the reaction product in a molten salt liquid at high temperature to form a saturated solution. This solution is then slowly cooled or the flux evaporated to form a supersaturated solution from which crystals precipitate. The molten salt acts as both a flux and a reaction medium. The molten salt method offers advantages such as a short reaction cycle, controllable crystal morphology, uniform and non-segregated powder components, high phase purity, and a simple process, making it the primary method for preparing flaky alumina.

[0004] US5702519 uses a water-soluble aluminum salt as the aluminum source, sulfate as the molten salt, and small amounts of phosphate and titanyl sulfate as crystal growth regulators to produce alumina powder with a hexagonal platelet structure, ranging in size from 3 to 22 μm and a thickness of approximately 200 to 300 nm. The methods disclosed in patent applications WO2006101306A1, WO2008026829A1, CN1150165A, and CN104925843A mostly use water-soluble aluminum salts or composite aluminum salts as the aluminum source, sodium sulfate and potassium sulfate as the molten salt, and use additives such as titanium salts, phosphates, zinc salts, and tin salts to control the shape and size of the alumina. CN104986786B prepares a precursor by adding nano-alumina seeds and / or flaky α-Al2O3 seeds and unstable alumina to water-soluble aluminum salts and soluble salts. The flaky alumina prepared by this method has a relatively small particle size and diameter-to-thickness ratio of about 3.8 to 9.5 μm. CN114590827A obtains a white precipitate by adding sulfuric acid dropwise to a mixture of aluminum hydroxide and alkali solution, calcining it at 1100°C to obtain flaky alumina seeds, and then adding the obtained seeds to the precursor to obtain a flaky alumina powder with an average particle size of about 38 μm and a thickness of about 0.5 μm by a molten salt method. Therefore, the controllable preparation of large-sized, high-diameter-to-thickness flaky α-Al2O3 powder by introducing seeds of suitable size into the molten salt method has become the focus of people's attention. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for preparing flaky α-Al2O3 micron seeds at low temperature based on the molten salt method, and to achieve controllable preparation of seed particle size through hydraulic classification.

[0006] To this end, the technical solution provided by the present invention is as follows:

[0007] A controllable preparation method of flaky aluminum oxide comprises the following steps in sequence:

[0008] Step 1) Preparation of flaky α-Al2O3 seed crystals

[0009] (1) Mix aluminum salt and urea in a mass ratio of 1:1-1:4 and grind them evenly, then dry them to obtain urea aluminum complex.

[0010] (2) the urea-aluminum complex prepared in step (1) and the soluble molten salt are mixed uniformly in a molar ratio of 1:4 and then calcined at a temperature of 550° C. to 1100° C. for a holding time of 2 to 10 hours to obtain flaky α-Al2O3 powder;

[0011] (3) screening the prepared flaky α-Al2O3 powder into particle sizes within different ranges using a hydraulic classification method to obtain flaky α-Al2O3 seeds within different particle size ranges;

[0012] The hydraulic classification method refers to a method of using water as a medium, utilizing the different settling velocities of particles in the classification equipment, and the water moving upward against the settling direction of the particles, so that the particles move either upward or downward according to the difference in the interference settling velocities, thereby achieving a method of separating particles of different particle sizes.

[0013] The classification equipment is a wet classification equipment for ultrafine powders, preferably but not limited to a hydrocyclone and a horizontal spiral centrifugal classifier.

[0014] Step 2) Synthesis of flake α-Al2O3 powder

[0015] (1) Mixing flaky α-Al2O3 seed crystals, aluminum salt, soluble molten salt and crystal growth regulator solution to obtain suspension A.

[0016] The amount of the flake α-Al2O3 seed crystals added is 0.1-2wt% of the suspension A raw material;

[0017] The molar ratio of the soluble molten salt to the aluminum salt is 4:1.

[0018] The molar ratio of the crystal growth regulator to the aluminum salt is 1:50-1:200.

[0019] (2) adding a soluble phosphate to the alkaline aqueous solution and mixing to obtain solution B;

[0020] The mass ratio of the solid content of the alkaline aqueous solution to the phosphate is 60:1.

[0021] (3) adding solution B to suspension A under continuous stirring, controlling the pH at the end of the hydrolysis reaction to be 6.5-7.5, to obtain a gel mixture containing a hydrolyzate, which is then dried to obtain a gel mixture solid;

[0022] The mass ratio of the solution B to the suspension A is 1:3-1:5.

[0023] (4) calcining the gel mixture solid at 900-1300°C for 2-6 hours at a heating rate of 1-10°C / min; washing the calcined solid with water to remove soluble molten salt and then drying to obtain flaky α-Al2O3 powder.

[0024] Furthermore, in the controllable preparation method of the above-mentioned flaky aluminum oxide, the aluminum salt in step 1) and step 2) is an aluminum oxyacid salt and / or an aluminum halide.

[0025] Furthermore, in the controllable preparation method of the above-mentioned flaky alumina, the soluble molten salt in step 1) and step 2) is a mixture of one or more of sodium chloride, sodium sulfate, sodium fluoride, potassium chloride, potassium sulfate, potassium fluoride, lithium chloride, lithium sulfate and lithium fluoride.

[0026] Furthermore, in the controllable preparation method of the above-mentioned flaky aluminum oxide, the crystal growth regulator solution in step 2) is one or more of titanyl sulfate, titanium tetrachloride, tin chloride, and zinc sulfate.

[0027] Furthermore, in the controllable preparation method of the above-mentioned flaky aluminum oxide, the alkaline aqueous solution in step 2) is an aqueous solution of one or a mixture of two or more of sodium carbonate, potassium carbonate, lithium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide, potassium hydroxide and lithium hydroxide.

[0028] Furthermore, in the controllable preparation method of the above-mentioned flaky alumina, the soluble phosphate described in step 2) includes one or more of phosphates and metaphosphates, such as sodium phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, and sodium trimetaphosphate.

[0029] Compared with the prior art, the technical solution provided by the present invention has the following technical advantages:

[0030] 1. The technical solution provided by the present invention prepares urea-aluminum complex by solid phase method, and obtains flaky α-Al2O3 seed crystals by calcining at 550℃~1100℃ by molten salt method, which has the advantages of simple process and low energy consumption.

[0031] 2. The technical solution provided by the present invention uses a hydraulic classification method to classify the particle size of flaky α-Al2O3 seeds, thereby achieving controllable adjustment of the seed particle size, which is conducive to the preparation of flaky α-Al2O3 powder with uniform particle size and high aspect ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a scanning electron microscope image of the flaky aluminum oxide prepared in Example 3;

[0033] Figure 2 This is a cross-sectional electron microscope image of the flaky aluminum oxide prepared in Example 3;

[0034] Figure 3 The XRD diffraction pattern of the flaky aluminum oxide prepared in Example 3;

[0035] Figure 4 This is the particle size distribution diagram of the flaky aluminum oxide prepared in Example 3. DETAILED DESCRIPTION

[0036] The following further describes the claims of the present invention in detail in conjunction with specific embodiments, but does not constitute any limitation to the present invention. Any limited modifications made by anyone within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0037] Example 1

[0038] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0039] In 300 g of water, 0.13 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate 12-hydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0040] The gel mixture solid was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to obtain a flaky α-Al2O3 powder, designated as A1. Characterization revealed an average particle size of approximately 16.64 μm, an average thickness of approximately 0.22 μm, and an aspect ratio of approximately 76.

[0041] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0042] Example 2

[0043] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0044] In 300 g of water, 0.65 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate 12-hydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0045] The gel mixture solid was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to obtain flaky α-Al2O3 powder, designated as A2. Characterization revealed an average particle size of approximately 17.24 μm, an average thickness of approximately 0.21 μm, and an aspect ratio of approximately 82.

[0046] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0047] Example 3

[0048] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0049] In 300 g of water, 1.3 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate dodecahydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0050] The gel mixture was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to yield flaky α-Al2O3 powder, designated A3. Characterization revealed an average particle size of approximately 17.26 μm, an average thickness of approximately 0.21 μm, and an aspect ratio of approximately 82.

[0051] The morphology and thickness of the powders were analyzed by field emission scanning electron microscopy (SEM, ZEISS GeminiSEM450). Figure 1 and Figure 2 The powder phase was analyzed by X-ray powder diffractometer (XRD, PANalytical X'Pert Pro), and the measured data were as follows: Figure 3 As shown in the figure, the powder’s D is tested by using the Truth Optical LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1, and the measured particle size distribution is shown in Table 1. Figure 4 shown.

[0052] Example 4

[0053] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0054] In 300 g of water, 2.6 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate 12-hydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0055] The gel mixture was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to yield flaky α-Al2O3 powder, designated A4. Characterization revealed an average particle size of approximately 16.86 μm, an average thickness of approximately 0.21 μm, and an aspect ratio of approximately 80.

[0056] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0057] Example 5

[0058] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0059] In 300 g of water, 6.5 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate 12-hydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0060] The gel mixture solid was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to obtain a flaky α-Al2O3 powder, designated A5. Characterization revealed an average particle size of approximately 16.58 μm, an average thickness of approximately 0.21 μm, and an aspect ratio of approximately 79.

[0061] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0062] Example 6

[0063] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly and placed in an alumina crucible. The mixture was kept at 900°C for 5 hours, then cooled to room temperature and dissolved in 60°C hot water to remove the added sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was then hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 5.8μm.

[0064] In 300 g of water, 13 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate dodecahydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0065] The gel mixture was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to yield a flaky α-Al2O3 powder, designated A6. Characterization revealed an average particle size of approximately 14.82 μm, an average thickness of approximately 0.22 μm, and an aspect ratio of approximately 67.

[0066] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0067] Example 7

[0068] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly in an alumina crucible and kept at 900°C for 5 hours. After cooling to room temperature, the sulfate was dissolved in 60°C hot water to remove the sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of approximately 3.5μm.

[0069] In 300 g of water, 1.3 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate dodecahydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0070] The gel mixture was calcined at 1200°C for 2 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to yield a flaky α-Al2O3 powder, designated A7. Characterization revealed an average particle size of approximately 15.32 μm, an average thickness of approximately 0.19 μm, and an aspect ratio of approximately 80.

[0071] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0072] Example 8

[0073] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly in an alumina crucible and kept at 900°C for 5 hours. After cooling to room temperature, the sulfate was dissolved in 60°C hot water to remove the sulfate, resulting in flaky alumina seed crystals. The flaky alumina powder was hydraulically classified to obtain alumina seed crystals with a thickness of 0.09-0.15μm and an average particle size of 1.8μm.

[0074] In 300 g of water, 1.3 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate dodecahydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 6.5-7.5 to obtain a gel mixture containing the hydrolyzate, which was then dried to obtain a solid gel mixture.

[0075] The gel mixture was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to yield a flaky α-Al2O3 powder, designated A8. Characterization revealed an average particle size of approximately 12.32 μm, a thickness of approximately 0.18 μm, and an aspect ratio of approximately 68.

[0076] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 5o and D 9o The measured data are shown in Table 1.

[0077] Comparative Example 1

[0078] To 300 g of deionized water, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were added and mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.74 g of sodium phosphate dodecahydrate were added to 100 g of water and stirred to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0079] The gel mixture solid was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove soluble molten salts and then dried to obtain a flaky alumina powder, designated as B1. Characterization revealed an average particle size of approximately 10.72 μm, an average thickness of approximately 0.36 μm, and an aspect ratio of approximately 30.

[0080] The D of alumina flakes was evaluated by using a Truth Optics LT2200E laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0081] Comparative Example 2

[0082] 33.6g of aluminum chloride hexahydrate and 18.0g of urea were weighed and ground in a mortar for 1 hour, then dried at 80°C to obtain a white powder of urea-aluminum complex. 58.3g of the urea-aluminum complex, 28.4g of sodium sulfate, and 34.8g of potassium sulfate were mixed uniformly in an alumina crucible and heated at 900°C for 5 hours. After cooling to room temperature, the sulfate was dissolved in 60°C hot water to obtain flaky alumina seeds.

[0083] In 300 g of water, 1.3 g of alumina seed crystals, 66.6 g of aluminum sulfate 18-hydrate, 28.4 g of sodium sulfate, 34.8 g of potassium sulfate, and 0.14 g of titanyl sulfate were mixed to obtain suspension A. 35.3 g of sodium carbonate and 0.76 g of sodium phosphate dodecahydrate were dissolved in 100 g of water and mixed to obtain solution B. Solution B was added to suspension A with continuous stirring. The pH at the end of the hydrolysis reaction was controlled to be 7.0, resulting in a gel mixture containing the hydrolyzate. The gel mixture was then dried to obtain a solid gel mixture.

[0084] The gel mixture was calcined at 1200°C for 5 hours at a heating rate of 5°C / min. The calcined solid was washed with hot water to remove the molten salt and dried to yield a flaky α-Al2O3 powder, designated as B2. Characterization revealed an average particle size of approximately 15.21 μm, an average thickness of approximately 0.26 μm, and an aspect ratio of approximately 60.

[0085] The D of alumina flakes was evaluated by using the Truth Optics LT3600 laser particle size analyzer. 10 、D 50 and D 90 The measured data are shown in Table 1.

[0086] Table 1

[0087]

[0088]

[0089] It can be seen from the above table that in Examples 1-6 of the present application, as the amount of micron seed crystals added under the same conditions increases, the particle size of the prepared flaky alumina powder first increases and then decreases, which shows that the amount of micron seed crystals added needs to be within an appropriate range to prepare large-sized crystals. This is mainly because when the amount of seed crystals added increases to a certain extent, the aluminum salts and molten salts involved in the reaction are not sufficient to provide conditions for the continued growth of the crystals. In Examples 3, 7, and 8 of the present application, the micron seeds were graded by hydraulic classification to obtain seeds of three different particle sizes, and the particle size of the prepared flaky alumina powder was proportional to the particle size of the micron seeds, which shows that controllable grading of the micron seeds helps to obtain large-sized flaky alumina powder. Comparative Example 1 did not add micron seeds to participate in the preparation, and the flaky alumina powder obtained had a small particle size and a large thickness. Comparative Example 2 did not perform controllable grading of the micron seeds, and the flaky alumina powder prepared had an average particle size and an uneven thickness.

[0090] This demonstrates that by introducing a urea-aluminum complex as an aluminum source, micronized seed crystals can be prepared at low temperatures (550-850°C) and their particle size can be sized using hydraulic classification, achieving controllable seed grading. Comparative analysis reveals that the particle size of the seed crystals is proportional to the particle size of the resulting flaky alumina powder, and that adding a suitable amount of urea-aluminum complex can yield large-sized flaky alumina powder.

Claims

1. A controllable preparation method of flaky aluminum oxide, characterized in that: The method includes the following steps in sequence: Step 1) Preparation of flake α-Al2O3 seed crystals (1) Mix aluminum salt and urea in a mass ratio of 1:1-1:4 and grind them evenly. After drying, obtain urea aluminum complex. (2) the urea-aluminum complex prepared in step (1) and the soluble molten salt are uniformly mixed in a molar ratio of 1:4 and then calcined at a calcination temperature of 550°C to 900°C for a holding time of 2-10 hours to obtain flaky α-Al2O3 seed crystals; (3) The prepared flaky α-Al2O3 powder is screened into particle sizes within different ranges by hydraulic classification to obtain flaky α-Al2O3 seeds within different particle size ranges; Step 2) Synthesis of flake α-Al2O3 powder (1) mixing flaky α-Al2O3 seed crystals, aluminum salt, soluble molten salt and crystal growth regulator solution to obtain suspension A; The amount of the flake α-Al2O3 seed crystals added is 0.1-2% of the mass of the suspension A raw material; The molar ratio of the soluble molten salt to the water-soluble aluminum salt is 4:1; The molar ratio of the crystal growth regulator to the water-soluble aluminum salt is 1:50-1:200; (2) Add phosphate to the alkaline aqueous solution and mix well to obtain solution B; The mass ratio of the solid content of the alkaline aqueous solution to the phosphate is 60:1; (3) adding solution B to suspension A under continuous stirring, controlling the pH at the end of the hydrolysis reaction to be 6.5-7.5, to obtain a gel mixture containing a hydrolyzate, which is then dried to obtain a gel mixture solid; The mass ratio of solution B to suspension A is 1:3-1:5; The gel mixture solid is calcined at 900-1300°C for 2-6 hours at a heating rate of 1-10°C / min; the calcined solid is washed with water to remove soluble molten salt and then dried to obtain flaky α-Al2O3 powder; The soluble molten salts in step 1) and step 2) are sodium sulfate and potassium sulfate.

2. The controllable preparation method of flaky aluminum oxide according to claim 1, characterized in that: The aluminum salts described in step 1) and step 2) are aluminum oxyacid salts and / or aluminum halides.

3. The controllable preparation method of flaky aluminum oxide according to claim 1, characterized in that: The crystal growth regulator solution in step 2) is one or more of titanyl sulfate, titanium tetrachloride, tin chloride, and zinc sulfate.

4. The controllable preparation method of flaky aluminum oxide according to claim 1, characterized in that: The alkaline aqueous solution in step 2) is an aqueous solution of one or a mixture of two or more of sodium carbonate, potassium carbonate, lithium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydroxide, potassium hydroxide and lithium hydroxide.

5. The controllable preparation method of flaky aluminum oxide according to claim 1, characterized in that: The phosphate described in step 2) is one or more of sodium phosphate, sodium dihydrogen phosphate, sodium hexametaphosphate, and sodium trimetaphosphate.

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