Preparation method of high-purity aluminum oxide for sapphire

Abstract

The invention discloses a preparation method of high-purity aluminum oxide for sapphire, which belongs to the technical field of high-purity aluminum oxide preparation, and comprises the following steps: mixing low-carbon alcohol with metal aluminum, and carrying out substitution reaction under the stirring condition to generate aluminum alkoxide; performing reduced pressure distillation and purification on the obtained aluminum alkoxide to obtain high-purity aluminum alkoxide; adding the obtained high-purity aluminum alkoxide into a mixed solution of alcohol and water, carrying out hydrolysis reaction under a stirring condition to obtain slurry, and carrying out solid-liquid separation to obtain a pseudo-boehmite filter cake; adding the obtained pseudo-boehmite filter cake into high-purity water for size mixing, and then heating under a microwave condition for evaporation and steam stripping for multiple times to obtain high-purity pseudo-boehmite; and finally, calcining the high-purity pseudo-boehmite to obtain the high-purity aluminum oxide for sapphire. According to the method, the alcohol entrained in the pseudo-boehmite slurry can be removed and recycled, so that the production cost of the high-purity aluminum oxide is reduced, and the residual carbon content in the high-purity aluminum oxide product is reduced, so that the high-purity aluminum oxide product meets the requirements of producing sapphire single crystals.

Description

Technical Field

[0001] This invention belongs to the technical field of high-purity alumina preparation, specifically relating to a method for preparing high-purity alumina for sapphire. Background Technology

[0002] Sapphire is a single crystal of aluminum oxide (α-Al₂O₃) with excellent properties such as high hardness, high strength, high optical transmittance, and resistance to high temperatures, wind, sand, rain, and salt spray corrosion. It has wide applications in defense, military, microelectronics, and optoelectronics. For example, it can be used as an infrared military device for high-performance weapons such as military aircraft, submarines, ships, and missiles; a window material for satellite space technology and high-power lasers; a gallium nitride-based LED substrate material; and a protective material for smartphones and mobile wearable devices.

[0003] The main raw material for producing sapphire crystals is high-purity alumina of grade 5N or higher. The main production methods for high-purity alumina include the modified Bayer process, the aluminum ammonium sulfate pyrolysis process, the aluminum ammonium carbonate pyrolysis process, the aluminum alkoxide hydrolysis process, and the direct hydrolysis process. The aluminum ammonium carbonate pyrolysis process and the aluminum ammonium sulfate pyrolysis process both employ multiple crystallization techniques for purification; however, it is difficult to remove metal ions such as iron and nickel, as well as halogen elements, resulting in alumina purity that can only reach 4N, which cannot meet the requirements of high-end sapphire. The direct hydrolysis process involves reacting metallic aluminum with water under high temperature and pressure to produce aluminum hydroxide. The product is then dried and calcined at high temperature to obtain high-purity alumina. This process is simple and pollution-free, but purification is impossible during the process, thus requiring high purity raw materials. Aluminum alkoxide hydrolysis is currently the main method for industrial-scale preparation of high-purity alumina. It uses aluminum alkoxide as a raw material, which is purified and hydrolyzed to obtain boehmite and alcohol. The alcohol can be recycled, and the boehmite can be calcined to obtain high-purity alumina. This method features high product purity (up to 5N or higher), flexible production processes, and environmental friendliness. However, the boehmite filter cake obtained after the hydrolysis and alcohol separation of aluminum alkoxide often still contains some alcohol. If this is not removed, it will not only increase the raw material consumption in the boehmite preparation process, thus increasing the production cost, but also result in high-purity alumina with excessively high residual carbon content after calcination. When used in sapphire production, this will cause the sapphire to appear pale yellow or brown, making it unsuitable for optoelectronic devices. Summary of the Invention

[0004] To address the aforementioned shortcomings, this invention discloses a method for preparing high-purity alumina for sapphire, which can remove and recycle alcohols entrained in boehmite, thereby reducing the production cost of high-purity alumina. Moreover, the prepared alumina has a residual carbon content of less than 70 ppm and a purity of greater than 99.999%, meeting the requirements for producing sapphire single crystals.

[0005] This invention is achieved using the following technical solution: A method for preparing high-purity alumina for sapphire, comprising the following steps: (1) Mix low-carbon alcohol, metallic aluminum and catalyst, and a substitution reaction occurs under stirring conditions to generate aluminum alkoxide; (2) The aluminum alkoxide obtained in step (1) is purified by vacuum distillation to remove impurities and obtain high-purity aluminum alkoxide. (3) The high-purity aluminum alkoxide obtained in step (2) is added to a mixture of alcohol and water, and hydrolyzed under stirring conditions to obtain a slurry. The slurry is then subjected to solid-liquid separation to obtain a pseudo-boehmite filter cake. (4) Add the pseudoboehmite filter cake obtained in step (3) to high-purity water for slurry preparation, and then place it under microwave conditions with a microwave power of 600W to 4000W for evaporation stripping 3 to 5 times to obtain high-purity pseudoboehmite; after each evaporation stripping, add high-purity water to the dried material again for slurry preparation, and then perform evaporation stripping again. The time for each evaporation stripping is 2 to 10 minutes. (5) Calcine the high-purity boehmite obtained in step (4) to obtain high-purity alumina for sapphire.

[0006] Preferably, the aluminum in step (1) is one or more of aluminum shavings, aluminum wire or aluminum powder, and the purity of the aluminum is greater than 99.5%; the catalyst is anhydrous aluminum chloride or aluminum alkoxide.

[0007] Preferably, the low-carbon alcohol mentioned in step (1) is a C3 to C5 isomeric alcohol, and the water content of the low-carbon alcohol is less than 0.2%.

[0008] Preferably, in the substitution reaction described in step (1), the molar ratio of metallic aluminum to lower alcohol is 1:(4-8); the amount of catalyst used is 0.1% to 1% of the molar amount of aluminum; the temperature of the substitution reaction is 80℃ to 120℃; and the time of the substitution reaction is 2 to 8 hours.

[0009] Preferably, in the vacuum distillation purification in step (2), the temperature of vacuum distillation is 130℃~180℃, and the vacuum degree of vacuum distillation is 150Pa~1000Pa.

[0010] Preferably, in the hydrolysis reaction described in step (3), the mass ratio of high-purity aluminum alkoxide to water is 1:(2-10), and the volume ratio of water to alcohol is 1:(1-8).

[0011] Preferably, in the hydrolysis reaction described in step (3), the water is high-purity water or deionized water, the alcohol is a low-carbon alcohol, the hydrolysis reaction temperature is 50℃~80℃, the hydrolysis reaction time is 2~6 hours, and the hydrolysis reaction is carried out under the condition of a stirring speed of 120~200rpm.

[0012] Preferably, the solid-liquid separation method described in step (3) includes any one of centrifugal separation, vacuum filtration or pressure filtration.

[0013] Preferably, the slurry preparation step (4) controls the solid content to be 20% to 40%.

[0014] Preferably, the calcination temperature in step (5) is 800℃~1300℃, and the holding time is 2~6 hours.

[0015] Compared with existing technologies, this technical solution has the following advantages: Microwaves are a type of non-ionized radiation energy that induces molecular motion through ion migration and dipole rotation. When microwaves act on polar molecules, they undergo instantaneous polarization under the influence of the microwave electromagnetic field, and undergo polarity-reversal motion at a rate of 245 million times per second, resulting in bond vibration, tearing, and interparticle friction and collision, rapidly generating a large amount of heat energy. This invention applies the selective, rapid, and overall heating characteristics of microwaves to remove alcohols entrained in boehmite filter cakes. Specifically, metallic aluminum undergoes a substitution reaction with lower carbon alcohols to generate aluminum alkoxides, which are then purified by vacuum distillation. These alkoxides are then added to a mixture of water and alcohol for hydrolysis to obtain a boehmite-alcohol-water slurry. After solid-liquid separation, the boehmite filter cake is mixed with high-purity water and then subjected to microwave evaporation stripping to remove the alcohols entrained in the boehmite. The resulting slurry is then calcined to obtain high-purity alumina for sapphire.

[0016] This invention removes and recycles alcohols entrained in boehmite using microwave heating stripping. Compared to methods that remove alcohols from alumina precursor slurries by washing or steam stripping, this significantly reduces water consumption and energy consumption for alcohol removal, effectively lowering the cost of producing high-purity alumina via the aluminum alkoxide method. Furthermore, it significantly reduces the residual carbon content in alumina, avoiding the problem of sapphires produced from high-purity alumina prepared by the aluminum alkoxide method exhibiting discoloration and thus being unsuitable for optoelectronic devices. Attached Figure Description

[0017] Figure 1 This is a flowchart of the preparation method of high-purity alumina for sapphire described in Example 1.

[0018] Figure 2 This is an electron microscope image of the high-purity alumina prepared by the method described in Example 1.

[0019] Figure 3 The image shows the XRD pattern of the high-purity alumina prepared by the method described in Example 1. Detailed Implementation

[0020] The following examples further illustrate the present invention, but are not intended to limit the invention. Specific experimental conditions and methods not specified in the following examples are generally conventional methods well-known to those skilled in the art. The method for determining the carbon content of alumina is as follows: An alumina sample is placed in a high-frequency induction furnace, oxygen is introduced, and with the assistance of a flux, the carbon in the sample is oxidized to carbon dioxide. Carbon dioxide has a selective absorption spectrum in the infrared band; after being absorbed by the corresponding absorption cell, it is converted into a signal by a detector, processed by a computer, and the result is output.

[0021] Example 1: A method for preparing high-purity alumina for sapphire, comprising the following steps: (1) Mix 250 mL of isopropanol with a water content of less than 0.2 wt% with 20 g of aluminum wire with a purity of 99.9% in a three-necked flask, purge with inert gas for 10 min to remove air, then place on a reflux flask and heat to 85 °C under stirring. Then add 0.5 g of aluminum isopropoxide and maintain reflux for 6 h until the aluminum wire is completely dissolved. Cut the aluminum wire into 1 cm long segments. (2) Transfer the three-necked flask to a vacuum distillation apparatus and continue heating. First, distill off the excess isopropanol at atmospheric pressure at a temperature of 85°C. Then, transfer the material to an Erlenmeyer flask and evacuate it to 250Pa. Then, heat it until the temperature reaches 150°C. A colorless viscous liquid distills out from the end of the distillation tube. Then, redistill and purify it repeatedly for 3 times. After standing for 8 days, solidify it to obtain white blocky high-purity aluminum isopropoxide. (3) Add 40g of high-purity aluminum isopropoxide obtained in step (2) into a three-necked flask containing 160mL of isopropanol and 80mL of high-purity water, and carry out hydrolysis reaction for 4h at a temperature of 65℃ and under stirring to obtain a slurry. Centrifuge the slurry and discard the clear liquid to obtain wet powder (boehmite). (4) Add 100 mL of high-purity water to the wet powder obtained in step (3) to make a slurry, and then heat it under microwave conditions with a microwave power of 2000W to evaporate and strip it three times to obtain high-purity boehmite; after each evaporation and stripping, the dried material obtained is added to high-purity water to make a slurry again, and then evaporated and stripped again. The evaporation and stripping time is 3 minutes each time. (5) The high-purity boehmite obtained in step (4) was placed in a muffle furnace and calcined at 1250℃ for 3 hours to obtain alumina powder. After testing, its purity was ≥99.999% and its carbon content was 0.025%.

[0022] Example 2: A method for preparing high-purity alumina for sapphire, comprising the following steps: (1) Mix 500 mL of isopropanol with a water content of less than 0.2 wt% with 20 g of aluminum wire with a purity of 99.9% in a three-necked flask, purge with inert gas for 10 min to remove air, then place on a reflux flask and heat to 85 °C under stirring. Then add 0.5 g of aluminum chloride and maintain reflux for 6 h until the aluminum wire is completely dissolved. Cut the aluminum wire into 1 cm long segments. (2) Transfer the three-necked flask to a vacuum distillation apparatus and continue heating. First, distill off the excess isopropanol at atmospheric pressure at a temperature of 85°C. Then, transfer the material to an Erlenmeyer flask and evacuate it to 200Pa. Then, heat it until the temperature reaches 140°C. A colorless viscous liquid distills out from the end of the distillation tube. Then, redistill and purify it repeatedly for 3 times. After standing for 10 days, solidify it to obtain white blocky high-purity aluminum isopropoxide. (3) Add 40g of high-purity aluminum isopropoxide obtained in step (2) into a three-necked flask containing 160mL of isopropanol and 80mL of high-purity water, and carry out hydrolysis reaction for 4h at a temperature of 65℃ and under stirring to obtain a slurry. Centrifuge the slurry and discard the clear liquid to obtain wet powder (boehmite). (4) Add 100 mL of high-purity water to the wet powder obtained in step (3) to make a slurry, and then heat it under microwave conditions with a microwave power of 4000W to evaporate and strip it 5 times to obtain high-purity boehmite; after each evaporation and stripping, the dried material obtained is added to high-purity water to make a slurry again, and then evaporated and stripped again. The time for each evaporation and stripping is 2 minutes. (5) The high-purity boehmite obtained in step (4) is placed in a muffle furnace and calcined at 1250℃ for 3 hours to obtain high-purity alumina for sapphire. After testing, its purity is ≥99.999% and its carbon content is 70ppm.

[0023] Example 3: A method for preparing high-purity alumina for sapphire, comprising the following steps: (1) Mix 400 mL of isobutanol with a water content of less than 0.2 wt% with 20 g of aluminum wire with a purity of 99.9% in a three-necked flask, purge with inert gas for 10 min to remove air, then place on a reflux flask and heat to 90 °C under stirring. Then add 0.5 g of aluminum chloride and maintain reflux for 6 h until the aluminum wire is completely dissolved; cut the aluminum wire into 1 cm long segments. (2) Transfer the three-necked flask to a vacuum distillation apparatus and continue heating. First, distill off the excess isopropanol at atmospheric pressure at a temperature of 90°C. Then, transfer the material to an Erlenmeyer flask and evacuate it to 300Pa. Then, heat it until the temperature reaches 160°C. A colorless viscous liquid distills out from the end of the distillation tube. Then, redistill and purify it repeatedly for 3 times. After standing for 5 days, solidify it to obtain white blocky high-purity aluminum isobutoxide. (3) Add 40g of high-purity aluminum isobutoxide obtained in step (2) into a three-necked flask containing 80mL of isobutanol and 80mL of high-purity water, and carry out hydrolysis reaction for 4h at a temperature of 65℃ and under stirring to obtain a slurry. Centrifuge the slurry and discard the clear liquid to obtain wet powder (boehmite). (4) Add 100 mL of high-purity water to the wet powder obtained in step (3) to make a slurry, and then place it under microwave conditions with microwave power of 600W to heat and evaporate and strip 4 times to obtain high-purity boehmite; after each evaporation and stripping, the dried material obtained is added to high-purity water to make a slurry again, and then evaporated and stripped again. The time for each evaporation and stripping is 10 minutes. (5) The high-purity boehmite obtained in step (4) is placed in a muffle furnace and calcined at 1250℃ for 3 hours to obtain high-purity alumina for sapphire. After testing, its purity is ≥99.999% and its carbon content is 103ppm.

[0024] Comparative Example 1: A method for preparing high-purity alumina for sapphire, comprising the following steps: (1) Mix 250 mL of isopropanol with a water content of less than 0.2 wt% with 20 g of aluminum wire with a purity of 99.9% in a three-necked flask, purge with inert gas for 10 min to remove air, then place on a reflux flask and heat to 85 °C under stirring. Then add 0.5 g of aluminum isopropoxide and maintain reflux for 6 h until the aluminum wire is completely dissolved. Cut the aluminum wire into 1 cm long segments. (2) Transfer the three-necked flask to the vacuum distillation apparatus and continue heating. First, distill off the excess isopropanol at atmospheric pressure at a temperature of 85°C. Then, transfer the material to the triangular flask and evacuate it to 300Pa. Then, heat it until the temperature reaches 180°C. The colorless viscous liquid distilled from the end of the distillation tube is high-purity aluminum isopropoxide. (3) Add 40g of high-purity aluminum isopropoxide obtained in step (2) into a three-necked flask containing 80mL of isopropanol and 80mL of high-purity water, and carry out hydrolysis reaction for 4h at a temperature of 65℃ and under stirring to obtain a slurry. Centrifuge the slurry and discard the clear liquid to obtain wet powder (boehmite). (4) The wet powder obtained in step (3) is washed three times with high-purity water, and then placed in a drying oven at 80°C for 24 hours to dry. Then it is placed in a muffle furnace and calcined at 1200°C for 2 hours to obtain alumina powder. After testing, its purity is ≥99.999% and its carbon content is 0.05%.

[0025] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing high-purity alumina for sapphire, characterized in that: Includes the following steps: (1) Mix low-carbon alcohol, metallic aluminum and catalyst, and a substitution reaction occurs under stirring conditions to generate aluminum alkoxide; (2) The aluminum alkoxide obtained in step (1) is purified by vacuum distillation to remove impurities and obtain high-purity aluminum alkoxide. (3) The high-purity aluminum alkoxide obtained in step (2) is added to a mixture of alcohol and water, and hydrolyzed under stirring conditions to obtain a slurry. The slurry is then subjected to solid-liquid separation to obtain a pseudo-boehmite filter cake. (4) Add the pseudoboehmite filter cake obtained in step (3) to high-purity water for slurry preparation, and then place it under microwave conditions with a microwave power of 600W to 4000W for evaporation stripping 3 to 5 times to obtain high-purity pseudoboehmite; after each evaporation stripping, add high-purity water to the dried material again for slurry preparation, and then perform evaporation stripping again. The time for each evaporation stripping is 2 to 10 minutes. (5) Calcine the high-purity boehmite obtained in step (4) to obtain high-purity alumina for sapphire.

2. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: The aluminum metal mentioned in step (1) is one or more of aluminum shavings, aluminum wire or aluminum powder, and the purity of the aluminum metal is greater than 99.5%; the catalyst is anhydrous aluminum chloride or aluminum alkoxide.

3. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: The low-carbon alcohol mentioned in step (1) is a C3 to C5 isomeric alcohol, and the water content of the low-carbon alcohol is less than 0.2%.

4. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: In step (1), the molar ratio of metallic aluminum to low-carbon alcohol in the substitution reaction is 1:(4-8), the amount of catalyst is 0.1% to 1% of the number of aluminum moles, the temperature of the substitution reaction is 80℃ to 120℃, and the time of the substitution reaction is 2 to 8 hours.

5. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: In step (2), the vacuum distillation temperature is 130℃~180℃ and the vacuum degree is 150Pa~1000Pa.

6. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: In the hydrolysis reaction described in step (3), the mass ratio of high-purity aluminum alkoxide to water is 1:(2-10), and the volume ratio of water to alcohol is 1:(1-8).

7. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: In the hydrolysis reaction described in step (3), the water is high-purity water or deionized water, the alcohol is low-carbon alcohol, the hydrolysis reaction temperature is 50℃~80℃, the hydrolysis reaction time is 2~6 hours, and the hydrolysis reaction is carried out under the condition of stirring speed of 120~200 rpm.

8. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: The solid-liquid separation method described in step (3) includes any one of centrifugal separation, vacuum filtration and pressure filtration.

9. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: In step (4), the slurry is prepared with a solid content of 20% to 40%.

10. The method for preparing high-purity alumina for sapphire according to claim 1, characterized in that: The calcination temperature in step (5) is 800℃~1300℃, and the holding time is 2~6 hours.

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