Boehmite recrystallization method

Abstract

The invention provides a boehmite recrystallization method, and relates to the technical field of boehmite preparation. The boehmite recrystallization method comprises the following steps: S1, mixing boehmite powder and water to obtain boehmite slurry; s2, the boehmite slurry is ground; s3, adding an ammonium polyacrylate dispersing agent into the ground boehmite slurry, and uniformly mixing to obtain mixed slurry; s4, transferring the mixed slurry into a concentration kettle, and carrying out hydrothermal synthesis recrystallization reaction; and S5, washing the recrystallized mixed slurry to remove impurities, and carrying out spray drying to obtain boehmite powder. According to the preparation method disclosed by the invention, the ammonium polyacrylate is subjected to hydrothermal synthesis reaction, so that the damaged original crystal is re-grown and crystallized, and the initial original crystal state is recovered under the condition that the granularity is not changed, so that the specific surface area is reduced, the water absorption is reduced, the restoration of boehmite which destroys the original crystal is realized, and the boehmite can be continuously applied to lithium battery diaphragm coating and pole piece coating.

Description

Technical Field

[0001] The present application relates to the technical field of boehmite preparation, and particularly relates to a method for recrystallizing boehmite. Background Art

[0002] Boehmite has unique chemical, optical, mechanical and other properties. It is the precursor of γ-Al2O3 and is widely used in fields such as ceramic materials, composite materials, and surface protective layer materials. Especially in the coating of lithium battery separators, it can improve the heat resistance of the separator, enhance the puncture resistance, and improve the rate and cycle performance of the battery. With the rapid development of industries such as lithium batteries, the requirements for indicators such as the purity, particle size, and crystal integrity of boehmite are constantly increasing, which has promoted the development of boehmite recrystallization technology.

[0003] Traditional methods for preparing boehmite include the sol-gel method, hydrothermal synthesis method, etc. Although they can prepare ultrafine oxide powders with a narrow particle size distribution and a small degree of aggregation, there are still some deficiencies. For example, the lithium battery application requires a smaller particle size. During the processing using the above methods, it is necessary to grind and crush the boehmite. The grinding and crushing process is likely to cause damage to the original crystals of boehmite, such as missing corners and broken edges, resulting in a decrease in crystal integrity, an increase in the specific surface area BET, and an increase in water absorption, making it impossible to continue to be used in the coating of lithium battery separators and electrode sheets. Currently, for boehmite with damaged crystals and a too high specific surface area BET, it is generally subjected to high-temperature calcination to convert it into alumina and then applied to other fields. There is currently no report on directly using boehmite with damaged crystals and a too high BET after treatment in the lithium battery field.

[0004] Content of the Application

[0005] The purpose of the present application is to provide a method for recrystallizing boehmite in view of the deficiencies of the prior art. By adding ammonium polyacrylate to the boehmite slurry for hydrothermal synthesis reaction, the damaged original crystals are allowed to regrow and recrystallize, restoring the initial original crystal state, thereby reducing the specific surface area and water absorption, achieving the repair of boehmite with damaged original crystals, and enabling it to continue to be used in the coating of lithium battery separators and electrode sheets.

[0006] To achieve the above purpose, the technical solutions adopted by the present application are as follows:

[0007] A method for recrystallizing boehmite, comprising the following steps:

[0008] S1. Mix boehmite powder and water to obtain a boehmite slurry;

[0009] S2. Grind the boehmite slurry;

[0010] S3. Add an ammonium polyacrylate dispersant to the ground boehmite slurry and mix evenly to obtain a mixed slurry;

[0011] S4. Transfer the mixed slurry to a concentration kettle for hydrothermal synthesis and recrystallization reaction;

[0012] S5. Wash and remove impurities from the recrystallized mixed slurry and spray dry it to obtain boehmite dry powder.

[0013] Furthermore, in step S1, the boehmite powder is the damaged boehmite during the grinding and pulverization process.

[0014] Furthermore, in step S1, the solid content in the boehmite slurry is 50 - 60%.

[0015] Furthermore, in step S1, in the ground boehmite slurry, the particle size D50 of boehmite is 0.8 - 0.9 μm.

[0016] Furthermore, in step S2, in the ground boehmite slurry, the BET specific surface area of boehmite is 6.5 - 8.5 m 2 / g, and the crystal integrity is 50% - 70%.

[0017] Furthermore, in step S3, the addition amount of the ammonium polyacrylate dispersant is 0.1 - 0.15% of the dry weight of boehmite; the molecular weight of ammonium polyacrylate is 5000 - 6000.

[0018] In this application, ammonium polyacrylate dispersant is added to the boehmite slurry. Compared with other dispersants, the addition of ammonium polyacrylate can effectively prevent the agglomeration of the original crystals of boehmite during the high-temperature and high-pressure recrystallization process, enhance the dispersibility, thereby improving the efficiency of the hydrothermal synthesis and recrystallization reaction, and helping to obtain boehmite powder with controllable particle size and specific surface area and higher cleanliness.

[0019] Furthermore, in step S4, the temperature of the hydrothermal synthesis and recrystallization reaction is 200 - 205 °C, the pressure is 1.5 - 1.8 MPa, and the reaction time is 4 - 5 h.

[0020] During the hydrothermal synthesis and recrystallization reaction, if the temperature is too high and the pressure is too high, agglomerates will be generated during the recrystallization of boehmite, and the crystal nuclei will obtain greater energy to make the original crystals larger, which is not conducive to the subsequent processing of boehmite; if the temperature is too low and the pressure is too low, the driving force during the recrystallization of boehmite is insufficient, and the damaged original crystals cannot be regrown and crystallized. Therefore, this application controls the recrystallization reaction temperature and pressure to make the damaged original crystals regrow and crystallize, and improves the crystal integrity and reduces the specific surface area without changing the particle size.

[0021] Furthermore, in step S5, the washing liquid during the washing and impurity removal process is deionized water.

[0022] On the other hand, a boehmite dry powder is prepared by the boehmite recrystallization method described above. The particle size D50 of the boehmite powder is 0.8 - 0.9 μm, the specific surface area < 6 m 2 / g, the crystal integrity > 98%, and the purity reaches above 3N level.

[0023] On the other hand, the boehmite dry powder prepared by the above method is applied in the coating of lithium battery separators and electrodes.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The present application provides a boehmite recrystallization method. By adding an organic solvent to the boehmite slurry for hydrothermal synthesis reaction, the damaged primary crystals during the grinding process are regrown and crystallized to restore the initial primary crystal state, improve the crystal integrity, thereby reducing the specific surface area and water absorption, realizing the repair of the damaged boehmite primary crystals, and enabling it to be applied in the coating of lithium battery separators and electrodes.

[0026] 2. During the boehmite recrystallization process of the present application, by adding a certain amount of ammonium polyacrylate dispersant, the agglomeration of primary crystals during the high-temperature and high-pressure recrystallization process of boehmite can be effectively prevented, and the dispersibility can be enhanced. This auxiliary agent is easily decomposed at high temperatures, has little residue, has little impact on the subsequent processing of separator alumina, and does not introduce sodium elements. Other auxiliary agents such as sodium polyacrylate are prone to residue and introduce sodium impurity elements.

[0027] 3. During the hydrothermal synthesis recrystallization process of the present application, by controlling the temperature and pressure of the hydrothermal synthesis recrystallization reaction, the damaged primary crystals are regrown and crystallized, and the crystal integrity is improved while the particle size remains unchanged, reducing the specific surface area. Description of the Drawings

[0028] Figure 1 is the SEM image of the boehmite before recrystallization in Example 2 of the present application;

[0029] Figure 2 is the XRD pattern of the boehmite before recrystallization in Example 2 of the present application;

[0030] Figure 3 is the SEM image (2000 times) of the boehmite after recrystallization in Example 2 of the present application;

[0031] Figure 4 is the SEM image (10000 times) of the boehmite after recrystallization in Example 2 of the present application;

[0032] Figure 5 is the XRD pattern of the boehmite after recrystallization in Example 2 of the present application;

[0033] Figure 6It is the SEM image of boehmite after recrystallization in Example 6 of this application;

[0034] Figure 7 It is the SEM image of boehmite after the recrystallization reaction in Comparative Example 2 of this application. Detailed implementation manners

[0035] The following non-limiting examples can enable those of ordinary skill in the art to understand this application more comprehensively, but do not limit this application in any way. The following content is merely an exemplary illustration of the scope claimed in this application. Those skilled in the art can make various changes and modifications to the application based on the disclosed content, and it should also fall within the scope claimed in this application.

[0036] In this application, the "dry basis weight of boehmite" refers to the dry weight of boehmite in the boehmite slurry as the benchmark. For example, "the addition amount of the ammonium polyacrylate dispersant is 0.1 - 0.15% of the dry basis weight of boehmite" means that based on the dry weight of boehmite in the boehmite slurry, the dosage of the ammonium polyacrylate dispersant is 0.1 - 0.15% of the weight of boehmite.

[0037] The following further illustrates this application in the form of specific examples. All chemical reagents used in the examples of this application are obtained through conventional commercial channels unless otherwise specified.

[0038] Example 1

[0039] A method for recrystallizing boehmite, comprising the following steps:

[0040] S1. Mix boehmite powder and water to obtain a boehmite slurry with a solid content of 50%;

[0041] S2. Grind the boehmite slurry with a grinder until the particle size D50 = 0.8 - 0.9 μm;

[0042] S3. Add 0.1% of ammonium polyacrylate (molecular weight 5000) based on the dry basis weight of boehmite to the ground boehmite slurry, and mix evenly to obtain a mixed slurry;

[0043] S4. Transfer the mixed slurry to a concentration kettle for hydrothermal synthesis recrystallization reaction, with a reaction temperature of 200 °C, a pressure of 1.5 MPa, and a time of 4 h;

[0044] S5. Wash and remove impurities from the recrystallized mixed slurry, and spray dry to obtain boehmite dry powder.

[0045] Example 2

[0046] A method for recrystallizing boehmite, comprising the following steps:

[0047] S1. Mix the boehmite powder and water to obtain a boehmite slurry with a solids content of 55%.

[0048] S2. Grind the boehmite slurry with a grinder until the particle size D50 = 0.8 - 0.9 μm. The SEM image and XRD pattern of this boehmite are shown in Figure 1 , 2 respectively.

[0049] S3. Add ammonium polyacrylate (molecular weight 6000) accounting for 0.15% of the dry weight of the boehmite to the ground boehmite slurry, and mix evenly to obtain a mixed slurry.

[0050] S4. Transfer the mixed slurry to an autoclave for hydrothermal synthesis and recrystallization reaction at a reaction temperature of 205 °C, a pressure of 1.8 MPa, and a time of 4 h.

[0051] S5. Wash and remove impurities from the recrystallized mixed slurry, and then spray-dry it to obtain boehmite dry powder.

[0052] The SEM image and XRD pattern of the boehmite dry powder obtained after recrystallization are shown in Figures 3 - 5 respectively. It can be seen from the figures that for the boehmite before recrystallization, due to mechanical treatments such as grinding and ball milling, the shear force caused damage to the crystal edges, forming surface defects and fine fragments, and the crystal integrity was only 63%; while the recrystallized boehmite restored its typical crystal structure as a rhombohedral hexahedron (pseudo-cubic structure), and the crystal integrity was increased to 99%.

[0053] Example 3

[0054] The difference from Example 1 is that in this example, the dosage of ammonium polyacrylate is 0.08% of the dry weight of the boehmite.

[0055] Example 4

[0056] The difference from Example 1 is that in this example, the dosage of ammonium polyacrylate is 0.2% of the dry weight of the boehmite.

[0057] Example 5

[0058] The difference from Example 1 is that in this example, the reaction temperature is 180 °C and the reaction pressure is 1 MPa.

[0059] Example 6

[0060] The difference from Example 1 is that in this example, the reaction temperature is 210 °C and the reaction pressure is 2 MPa. The SEM image of the boehmite after the recrystallization reaction is shown in Figure 6 respectively. It can be seen that the original crystal agglomeration is serious.

[0061] Example 7

[0062] The difference from Example 1 is that in this example, the molecular weight of ammonium polyacrylate is 3000.

[0063] Example 8

[0064] The difference from Example 1 is that in this example, the molecular weight of ammonium polyacrylate is 8000.

[0065] Comparative Example 1

[0066] The difference from Example 1 is that in this comparative example, the dispersant ammonium polyacrylate is replaced with an equal amount of sodium polyacrylate.

[0067] Comparative Example 2

[0068] The difference from Example 1 is that in this comparative example, the dispersant ammonium polyacrylate is not added. The SEM image of boehmite after the recrystallization reaction is as Figure 7 shown, and it can be seen that the original crystal agglomeration is serious.

[0069] Test Example

[0070] Test the particle size, BET, water absorption rate, and crystal integrity of boehmite before and after recrystallization in the above examples and comparative examples. Among them, the particle size and BET of boehmite before recrystallization are tested with the slurry after grinding in step S2 as the test sample. Among them, the particle size is tested with a Malvern laser particle size analyzer; BET is tested with a specific surface area tester; the water absorption rate is calculated by the ratio of the increased weight of saturated water absorption of boehmite to the dry weight; the crystal integrity is tested with a scanning electron microscope and an XRD diffractometer. The results are shown in Tables 1 and 2 below.

[0071] Table 1. Parameters of Boehmite before Recrystallization

[0072] Group Particle Size (μm) <![CDATA[BET(m 2 / g)]]> Crystal Integrity Water Absorption Rate (%) Example 1 0.835 7.128 63.0％ 6.3 Example 2 0.833 8.251 52.0％ 8.5 Example 3 0.835 7.128 63.0％ 6.3 Example 4 0.835 7.128 63.0％ 6.3 Example 5 0.835 7.128 63.0％ 6.3 Example 6 0.835 7.128 63.0％ 6.3 Example 7 0.835 7.128 63.0％ 6.3 Example 8 0.835 7.128 63.0％ 6.3 Comparative Example 1 0.835 7.128 63.0％ 6.3 Comparative Example 2 0.835 7.128 63.0％ 6.3

[0073] Table 2. Parameters of Boehmite after Recrystallization

[0074]

[0075]

[0076] The results show that during the recrystallization process of boehmite in Comparative Example 2, without adding a dispersant, the primary crystals of boehmite are prone to agglomeration during the recrystallization process, and its particle size increases from 0.8 - 0.9 μm to 1.1 - 1.3 μm. Continuing the processing will result in a large specific surface area and cannot be directly used for the coating of lithium battery separators and electrode sheets. Compared with Comparative Example 2, in Comparative Example 1, sodium polyacrylate is added as a dispersant. Although it can reduce the BET and water absorption rate and improve the crystal integrity, the introduction of sodium impurity elements leads to an increase in the pH of the recrystallized boehmite product and a purity of less than 3N grade, affecting the application of boehmite powder in the coating of lithium battery separators and electrode sheets. Compared with the comparative examples, in Examples 1 and 2, a certain amount of ammonium polyacrylate is added as a dispersant. The pH of the recrystallized boehmite remains unchanged and the particle size is almost unchanged, and the BET decreases significantly (the BET decreases by more than 2.6 m 2 / g in Example 2 before and after recrystallization), and the water absorption rate decreases. Compared with Examples 1 and 2, in Example 3, the amount of dispersant used is too small, and the primary crystals are prone to agglomeration during the recrystallization process, resulting in an increase in particle size; in Example 4, the amount of dispersant used is too large, resulting in a large amount of dispersant residue after subsequent processing and drying, reducing its purity. And compared with Example 2, there is no obvious change in the BET and water absorption rate of Example 4, indicating that after the amount of dispersant increases to a certain extent, continuing to increase the amount of dispersant will not improve the recrystallization effect. Therefore, the optimal amount of dispersant is 0.1% - 0.15% of the dry basis weight of boehmite; in Example 5, the temperature and pressure of the recrystallization reaction are too low, resulting in the inability to achieve recrystallization, and there is no obvious change in the BET and water absorption rate. In Example 6, the temperature and pressure of the recrystallization reaction are too high, and agglomeration occurs during the recrystallization process, resulting in an increase in particle size, and neither can be directly used in the coating of lithium battery separators and electrode sheets, and subsequent grinding treatment is required, which will damage the primary crystals again. In Example 7, the molecular weight of ammonium polyacrylate is 3000, and its stability is relatively poor, and the dispersion effect is also poor. It is prone to decomposition and failure during the recrystallization reaction process, resulting in agglomeration during the recrystallization process, resulting in an increase in particle size, and neither can be directly used in the coating of lithium battery separators and electrode sheets, and subsequent grinding treatment is required, which will damage the primary crystals again. In Example 8, the molecular weight of ammonium polyacrylate is 8000, and its stability is good, but there is a flocculation effect, resulting in flocculation and agglomeration during the recrystallization process, resulting in an increase in particle size, and neither can be directly used in the coating of lithium battery separators and electrode sheets.

[0077] In summary, through the method provided by the present application, it is possible to ensure that the particle size remains almost unchanged, the damaged primary crystals can regrow and crystallize, restore the initial primary crystal state, reduce the specific surface area, reduce the water absorption, and achieve the repair of the damaged boehmite primary crystals, enabling it to be used in the coating of lithium battery separators and electrode sheets.

[0078] The above description of the embodiments is provided to enable those of ordinary skill in the art to understand and use the application. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, this application is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of this application according to the disclosure of this application should be within the protection scope of this application.

Claims

1. A boehmite recrystallization method, characterized in that: The following steps are involved: S1, mixing boehmite powder and water to obtain boehmite slurry; S2, grinding the boehmite slurry; S3, adding ammonium polyacrylate dispersant to the ground boehmite slurry, and mixing to obtain a mixed slurry; S4, transferring the mixed slurry to a concentration kettle for hydrothermal synthesis and recrystallization reaction; S5, washing and removing impurities from the mixed slurry after recrystallization, and spray drying to obtain boehmite powder.

2. The boehmite recrystallization method according to claim 1, characterized in that: In step S1, the solid content of the boehmite slurry is 50-60%.

3. The boehmite recrystallization method according to claim 1, characterized in that: In step S2, in the ground boehmite slurry, the boehmite particle size D50 is 0.8-0.9 μm, and the specific surface area BET is 6.5-8.5 m 2 / g, crystallinity completeness 50-70%.

4. The boehmite recrystallization method according to claim 1, characterized in that: In step S3, the amount of ammonium polyacrylate added is 0.1-0.15% of the dry weight of boehmite.

5. The boehmite recrystallization method according to claim 1, characterized in that: In step S3, the molecular weight of the ammonium polyacrylate is 5000-6000.

6. The boehmite recrystallization method according to claim 1, characterized in that: In step S4, the temperature of the hydrothermal synthesis recrystallization reaction is 200-205° C. and the pressure is 1.5-1.8 MPa.

7. A boehmite powder, characterized in that: The boehmite is prepared by the boehmite recrystallization method according to any one of claims 1 to 6.

8. The boehmite powder according to claim 7, characterized in that: The particle size D50 of the boehmite powder is 0.8-0.9 μm, and the specific surface area is less than 6 m 2 / g, crystal integrity>98%.

9. The boehmite powder according to claim 8, characterized in that: The specific surface area of ​​the boehmite powder is 5 to 6 m 2 / g.

10. Use of the boehmite powder prepared by the boehmite recrystallization method according to any one of claims 1 to 6 or the boehmite powder according to any one of claims 7 to 9 in lithium battery separator coating and pole piece.

Images