Method for preparing high-purity m-tert-butylphenol by layer melt crystallization

Abstract

The present application belongs to the field of chemical purification, and particularly relates to a method for preparing high-purity m-tert-butyl phenol by layer melt crystallization, comprising the following steps: subjecting liquid m-tert-butyl phenol crude material to two-stage layer melt crystallization in a layer melt crystallization device, introducing ultrasonic operation in the cooling process, and finally obtaining high-purity m-tert-butyl phenol with a purity greater than 99.5%. The method is suitable for m-tert-butyl phenol crude material with a mass percentage content of greater than or equal to 75%, and is preferably suitable for m-tert-butyl phenol crude material with a mass percentage content of greater than 95%. No solvent is added, thereby avoiding the pollution of the solvent to the product, reducing the investment and operation cost, and being environmentally friendly and free of three wastes. The introduction of ultrasonic during cooling greatly promotes the nucleation of m-tert-butyl phenol, shortens the operation time, reduces the inclusion of impurities in the crystal layer, and helps to improve the purity of the final product.

Description

Technical Field

[0001] This invention belongs to the field of chemical purification, and in particular to a method for preparing high-purity m-tert-butylphenol by introducing ultrasonic layer-by-layer melt crystallization from m-tert-butylphenol crude material. Background Technology

[0002] m-tert-butylphenol, a derivative of phenol, is an important intermediate in fine chemicals, particularly as a raw material for the synthesis of the acaricide etoxazole. Its chemical name is 3-tert-butylphenol. Currently, m-tert-butylphenol is mainly imported from abroad and is expensive. Therefore, developing a synthesis process for m-tert-butylphenol that uses readily available raw materials, is simple to operate, and has mild reaction conditions has promising market prospects.

[0003] Patent CN110803981A describes a synthesis method using tert-butylbenzene and m-tert-butylphenol as raw materials. The reaction is catalyzed by divalent copper ions and carried out at high temperatures to obtain the product. The product obtained by this method has a purity of 93%–94% and a yield of 89%, which is relatively low. Furthermore, this method requires sophisticated equipment due to the high reaction temperature and produces numerous byproducts.

[0004] Patent CN110563557A discloses an environmentally friendly synthesis process for m-tert-butylphenol. The process involves extracting phenol and isobutylene, byproducts generated during the synthesis of tert-butylphenol, and placing them in a reaction vessel. A positioning aid, pentasilicic acid, is then added to the reaction vessel. Under the action of the pentasilicic acid, phenol and isobutylene are converted into m-tert-butylphenol. Isobutylene is recovered, and crude m-tert-butylphenol is obtained. This crude product is then obtained through distillation to yield the final m-tert-butylphenol. While this method is environmentally friendly and low-cost, it does not address product purity and produces a relatively large number of byproducts, resulting in a low yield.

[0005] Patent CN105198710A discloses a method for synthesizing m-tert-butylphenol, using p-tert-butylchlorobenzene as a raw material. The process involves nitration to obtain 1-chloro-4-tert-butyl-2-nitrobenzene, reduction to generate m-tert-butylaniline, diazotization, and hydrolysis to obtain m-tert-butylphenol. Finally, distillation yields 97% pure m-tert-butylphenol. This method produces a product with low purity, requires the addition of large amounts of concentrated sulfuric acid and nitric acid, has a complex synthesis process, excessively long reaction time, and high costs.

[0006] US Patent 4103096 reports a synthetic route for m-tert-butylphenol, using p-tert-butylphenol as a raw material and 1%–5% trifluoromethanesulfonic acid as a catalyst. The reaction is carried out at 120°C–240°C, with continuous heating until the tert-butylphenol content in the reaction solution reaches its maximum, achieving a yield of up to 35%. However, the purity of the product obtained by this method is only 30%–50%, which is too low. While the reaction steps are simple, the temperature requirements are high, the reaction process is difficult to control, and there are many byproducts.

[0007] Patent CN110903171A uses phenol and tert-butyl chloride as raw materials, and acidic clay and concentrated sulfuric acid as catalysts. Through an alkylation reaction, the main product m-tert-butylphenol and the byproduct p-tert-butylphenol are obtained in one step and separated. Using unreacted phenol and the p-tert-butylphenol byproduct from the previous step as raw materials, and acidic clay and aluminum trichloride as catalysts, a rearrangement reaction is then performed in one step to obtain m-tert-butylphenol. The purity of the product obtained by this method can reach 97%–98%. However, this method has drawbacks such as overly complex synthesis processes, the need for reaction solvents, and solvent recovery.

[0008] The above-mentioned process has technical problems such as low product purity, a large number of by-products, high temperature requirements, high cost, and difficulty in recovering the reaction solvent. There is an urgent need to develop a new, environmentally friendly, simple to operate, and high-purity process. Summary of the Invention

[0009] To solve the above-mentioned technical problems, the present invention provides a method for preparing high-purity m-tert-butylphenol by layer-by-layer melt crystallization. The method uses two-stage layer-by-layer melt crystallization to purify the crude m-tert-butylphenol. Ultrasonic waves are introduced during the crystallization process, which greatly shortens the process time.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A method for preparing high-purity m-tert-butylphenol by layer-by-layer melt crystallization includes the following steps:

[0012] Step 1) Liquid phase feeding: Add liquid m-tert-butylphenol crude material with a mass percentage of ≥75% to the primary layer melt crystallizer to the designed liquid level;

[0013] Step 2) Cooling and crystallization: Cool the liquid m-tert-butylphenol crude material with a mass percentage of ≥75%. During the cooling process, turn on the ultrasonic probe for 1 to 5 minutes every 5 to 15 minutes. Control the cooling rate to 1 to 10 K / h, the final cooling temperature to 0 to 20℃, and hold the temperature for 0.5 to 1 hour. The first-stage mother liquor is discharged and processed separately.

[0014] Step 3) Primary sweating: After the non-crystallized primary mother liquor is discharged, the crystals are subjected to primary sweating. The heating rate is 1-20K / h, the temperature is raised to 40-46℃, and the temperature is held for 0.5-1h to discharge the primary sweating liquid.

[0015] Step 4) Material processing: After the primary sweating liquid is discharged, the material is heated to melt it completely and discharge it to obtain a primary product of m-tert-butylphenol with a purity of ≥95%.

[0016] Step 5) Liquid phase feed: Add the primary product of m-tert-butylphenol with a purity ≥95% from step 4) to the secondary melt crystallizer to the designed liquid level;

[0017] Step 6) Cooling and crystallization: Cool the liquid m-tert-butylphenol primary product with a purity ≥ 95% by turning on the ultrasonic probe for 1 to 5 minutes every 5 to 15 minutes during the cooling process; control the cooling rate to 1 to 10 K / h, and the final cooling temperature to 10 to 30℃; hold the temperature for 0.5 to 1 hour, and then discharge the secondary mother liquor.

[0018] Step 7) Secondary sweating: After the uncrystallized secondary mother liquor is discharged, the crystals are subjected to secondary sweating. The heating rate is 1-20 K / h, the temperature is raised to 43-48℃, and the temperature is held for 0.5-1h. The secondary sweating liquid is then discharged.

[0019] Step 8) Material processing: After the secondary sweating liquid is discharged, the material is heated to melt it completely and discharge it, finally obtaining high-purity m-tert-butylphenol with a purity greater than 99.5%.

[0020] Furthermore, in steps 2) and 6), the ultrasound probe is turned on for 1 to 2 minutes every 5 to 10 minutes.

[0021] Furthermore, in step 2), the cooling rate is 5–10 K / h, and the final cooling temperature is 5–20 °C.

[0022] Further, in step 3), the heating rate is 8–15 K / h, and the final temperature is 40–44 °C.

[0023] Furthermore, in step 6), the cooling rate is 5–10 K / h, and the final cooling temperature is 10–20 °C.

[0024] Furthermore, in step 7), the heating rate is 1–10 K / h, and the final heating temperature is 43–46 °C.

[0025] Furthermore, the primary sweating fluid is used as raw material in step 1).

[0026] Furthermore, the secondary mother liquor that did not crystallize after cooling in step 6) is returned to step 1) as raw material for reuse.

[0027] Furthermore, the crystals in the two-stage layered melt crystallizer are heated to the final temperature and the uncrystallized secondary sweating liquid is returned to step 5) as raw material.

[0028] Furthermore, a layer-by-layer melt crystallization process incorporating ultrasound was used to purify crude m-tert-butylphenol to a high-purity product with a purity >99.5% and an impurity content <0.5%.

[0029] Furthermore, according to process requirements, a two-step crystallization method is adopted, in which the primary product enters the secondary crystallizer as raw material.

[0030] Advantages and beneficial effects of the present invention:

[0031] 1) This invention uses a two-stage layered melt crystallization method for purification, which is suitable for crude m-tert-butylphenol with a mass percentage of ≥75%, and preferably for crude m-tert-butylphenol with a mass percentage of ≥95%. It purifies the product to a high purity of ≥99.5%, and has the advantages of no solvent, safe process, no waste, and high product purity. It is a green separation and purification process.

[0032] 2) Introducing ultrasound into the crystallization process can significantly affect the performance of the crystallized product. Ultrasound typically causes radiation and microbeams, resulting in localized high-temperature and high-pressure zones. This leads to mass transfer and cavitation during the crystallization process. These phenomena accelerate the collision rate between solute molecules and speed up the nucleation rate, thereby greatly shortening the process time and reducing operating costs. Attached Figure Description

[0033] Figure 1 This is a process flow diagram of the method of the present invention. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the best implementation examples.

[0035] Example 1

[0036] In a 500ml single-stage melt crystallizer, 508.19g of crude m-tert-butylphenol with a purity of 85% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 10K / h, with the ultrasonic probe turned on for 1 minute every 10 minutes during the initial cooling process. The final temperature was 10℃, and the temperature was held for 0.5 hours. The first-stage mother liquor was discharged and processed separately. The material was then heated to 42℃ at a rate of 15K / h and held for 0.5 hours for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 334g of solid-phase m-tert-butylphenol product with a purity of 95% was obtained.

[0037] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. Cooling was carried out at a rate of 8K / h, with the ultrasonic probe activated for 1 minute every 10 minutes during the initial cooling process. The final temperature was 10°C, and the temperature was held for 0.5 hours. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 44°C at a rate of 10K / h and held for 0.5 hours for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After melting, the product was discharged, yielding 215.5g of high-purity m-tert-butylphenol with a purity of 99.5%. The overall yield was 49.64%.

[0038] Comparative Example 1

[0039] In a 500ml single-stage melt crystallizer, 508.19g of crude m-tert-butylphenol with a purity of 85% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 10K / h until the final temperature was 10℃. The temperature was held for 0.5h, and the first-stage mother liquor was discharged for further processing. The material was then heated to 40℃ at a rate of 15K / h and held for 0.5h for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 311g of solid-phase m-tert-butylphenol product with a purity of 90% was obtained.

[0040] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. The liquid was cooled at a rate of 8K / h to a final temperature of 10°C and held at that temperature for 0.5h. The secondary mother liquor was then discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 44°C at a rate of 10K / h and held at that temperature for 0.5h for a second-stage sweating process. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization process as raw material. After melting, the product was discharged, yielding 190g of high-purity m-tert-butylphenol with a purity of 98%. The overall yield was 43.11%.

[0041] Example 2

[0042] In a 500ml single-stage melt crystallizer, 505.22g of crude m-tert-butylphenol with a purity of 80% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 8K / h, with the ultrasonic probe turned on for 1 minute every 10 minutes during the initial cooling process. The final temperature was 5℃, and the temperature was held for 0.5h. The first-stage mother liquor was discharged and processed separately. The material was then heated to 42℃ at a rate of 15K / h and held for 0.5h for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 364g of solid-phase m-tert-butylphenol product with a purity of 93% was obtained.

[0043] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. Cooling was carried out at a rate of 6K / h, with the ultrasonic probe activated for 1 minute every 10 minutes during the initial cooling process, reaching a final temperature of 10°C, and held at that temperature for 0.5 hours. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 44°C at a rate of 10K / h and held at that temperature for 0.5 hours for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After melting, the material was discharged, yielding 233g of high-purity m-tert-butylphenol with a purity of 99.5%. The overall yield was 57.36%.

[0044] Comparative Example 2

[0045] In a 500ml single-stage melt crystallizer, 505.22g of crude m-tert-butylphenol with a purity of 80% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 8K / h, with a final temperature of 5℃, and held at that temperature for 0.5h. The first-stage mother liquor was discharged and processed separately. The material was then heated to 42℃ at a rate of 15K / h and held at that temperature for 0.5h for a first-stage sweating process. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization process. Finally, 334g of solid-phase m-tert-butylphenol product with a purity of 88% was obtained.

[0046] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then transferred to a 300ml second-stage layer-by-layer melt crystallizer. The liquid was cooled at a rate of 6K / h to a final temperature of 10°C and held at that temperature for 0.5h. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 44°C at a rate of 10K / h and held at that temperature for 0.5h for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After melting, the product was discharged, yielding 200.5g of high-purity m-tert-butylphenol with a purity of 95%. The overall yield was 47.13%.

[0047] Example 3

[0048] In a 500ml single-stage melt crystallizer, 510g of crude m-tert-butylphenol with a purity of 90% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 8K / h, with the ultrasonic probe turned on for 1 minute every 10 minutes during the initial cooling process. The final temperature was 10℃, and the temperature was held for 0.5h. The first-stage mother liquor was discharged and processed separately. The material was then heated to 43℃ at a rate of 10K / h and held for 0.5h for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 378g of solid-phase m-tert-butylphenol product with a purity of 96% was obtained.

[0049] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. Cooling was carried out at a rate of 6K / h, with the ultrasonic probe activated for 1 minute every 10 minutes during the initial cooling process, reaching a final temperature of 12°C, and held at that temperature for 0.5 hours. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 44°C at a rate of 8K / h and held at that temperature for 0.5 hours for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After heating and melting, the material was discharged, yielding 239.6g of high-purity m-tert-butylphenol with a purity of 99.7%. The overall yield was 52.04%.

[0050] Comparative Example 3

[0051] In a 500ml single-stage melt crystallizer, 510g of crude m-tert-butylphenol with a purity of 90% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 8K / h, with a final temperature of 10℃. The temperature was held for 0.5h, and the first-stage mother liquor was discharged for further processing. The material was then heated to 42℃ at a rate of 10K / h and held for 0.5h for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 352.5g of solid-phase m-tert-butylphenol product with a purity of 95% was obtained.

[0052] The solid product obtained from the primary layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml secondary layer melt crystallizer. The liquid was cooled at a rate of 6K / h to a final temperature of 12°C and held at that temperature for 0.5h. The secondary mother liquor was then discharged and reused as raw material for the primary layer melt crystallization. The material was then heated to 44°C at a rate of 8K / h and held at that temperature for 0.5h for secondary sweating. The secondary sweating liquid was discharged and returned to the secondary layer melt crystallizer for reuse. After melting, the product was discharged, yielding 221.75g ​​of high-purity m-tert-butylphenol with a purity of 99%. The overall yield was 47.83%.

[0053] Example 4

[0054] In a 500ml single-stage melt crystallizer, 500.3g of crude m-tert-butylphenol with a purity of 75% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 10K / h, with the ultrasonic probe turned on for 1 minute every 10 minutes during the initial cooling process. The final temperature was 5℃, and the temperature was held for 0.5 hours. The first-stage mother liquor was discharged and processed separately. The material was then heated to 40℃ at a rate of 8K / h and held for 0.5 hours for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 310.21g of solid-phase m-tert-butylphenol product with a purity of 85% was obtained.

[0055] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. Cooling was carried out at a rate of 6K / h, with the ultrasonic probe activated for 1 minute every 10 minutes during the initial cooling process, reaching a final temperature of 10°C, and held at that temperature for 0.5 hours. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 43°C at a rate of 8K / h and held at that temperature for 0.5 hours for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After melting, the material was discharged, yielding 210g of high-purity m-tert-butylphenol with a purity of 99.3%. The overall yield was 55.57%.

[0056] Comparative Example 4

[0057] In a 500ml single-stage melt crystallizer, 500.3g of crude m-tert-butylphenol with a purity of 75% was added. The temperature was raised to 55℃ until the material was completely melted into a liquid state. The temperature was lowered at a rate of 10K / h, with a final temperature of 5℃. The temperature was held for 0.5h, and the first-stage mother liquor was discharged for further processing. The material was then heated to 40℃ at a rate of 8K / h and held for 0.5h for first-stage sweating. The first-stage sweating liquid was discharged and reused as raw material for the first-stage melt crystallization. Finally, 295g of solid-phase m-tert-butylphenol product with a purity of 80% was obtained.

[0058] The solid product obtained from the first-stage layer-by-layer melt crystallization was heated to 55°C until it was completely melted into a liquid state, and then placed into a 300ml second-stage layer-by-layer melt crystallizer. The liquid was cooled at a rate of 6K / h to a final temperature of 10°C and held at that temperature for 0.5h. The secondary mother liquor was discharged and reused as raw material for the first-stage layer-by-layer melt crystallization. The material was then heated to 43°C at a rate of 8K / h and held at that temperature for 0.5h for secondary sweating. The secondary sweating liquid was discharged and returned to the second-stage layer-by-layer melt crystallization as raw material. After heating and melting, the product was discharged, yielding 171.25g of high-purity m-tert-butylphenol with a purity of 90%. The overall yield was 41.08%.

[0059] The purity of m-tert-butylphenol prepared in Examples 1 to 4 was compared with that prepared by existing processes, and the results are shown in the table below:

[0060]

[0061]

[0062] As can be seen from the table above, Examples 1 to 4, which use the preparation process described in this application, show a significant improvement in the purity of m-tert-butylphenol compared to the prior art. Among them, the process steps in Example 3 are the best embodiment in this application.

[0063] Furthermore, a comparison between two-stage layered melt crystallization and two-stage layered melt crystallization processes incorporating ultrasound reveals that two-stage layered melt crystallization without ultrasound cannot achieve the purity of melt crystallization with ultrasound. To achieve the same purity, a subsequent stage of layered melt crystallization is required, significantly increasing investment and operating costs. In contrast, introducing ultrasound at the initial stage of the two-stage layered melt crystallization process accelerates the collision rate between solute molecules, speeds up nucleation, reduces impurity inclusions in the crystal, and promotes crystal growth, thereby improving crystal purity. Simultaneously, it greatly shortens process time and reduces operating costs.

[0064] The above description is only a preferred example of the present invention. For those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. The content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing high-purity m-tert-butylphenol by layer-by-layer melt crystallization, characterized in that: Includes the following steps: Step 1) Liquid phase feeding: Add liquid m-tert-butylphenol crude material with a mass percentage of ≥75% to the primary layer melt crystallizer to the designed liquid level; Step 2) Cooling and crystallization: Cool the liquid m-tert-butylphenol crude material with a mass percentage of ≥75%. During the cooling process, turn on the ultrasonic probe for 1 to 2 minutes every 5 to 10 minutes. Control the cooling rate to 1 to 10 K / h and the final cooling temperature to 0 to 20℃. Hold the temperature for 0.5 to 1 hour. The uncrystallized primary mother liquor is treated separately. Step 3) Primary sweating: After the non-crystallized primary mother liquor is discharged, the crystals are subjected to primary sweating. The heating rate is 1-20K / h, the temperature is raised to 40-46℃, and the temperature is held for 0.5-1h to discharge the primary sweating liquid. Step 4) Material processing: After the primary sweating liquid is discharged, the material is heated to melt it completely and discharge it to obtain a primary product of m-tert-butylphenol with a purity of ≥95%. Step 5) Liquid phase feed: Add the primary product of liquid m-tert-butylphenol with a purity ≥95% from step 4) to the secondary layer melt crystallizer to the designed liquid level; Step 6) Cooling and crystallization: Cool the primary product of liquid m-tert-butylphenol with a purity ≥95%. During the cooling process, turn on the ultrasonic probe for 1 to 2 minutes every 5 to 10 minutes; control the cooling rate at 1 to 10 K / h, and the final cooling temperature at 10 to 30℃; hold the temperature for 0.5 to 1 hour, and then drain the secondary mother liquor. Step 7) Secondary sweating: After the uncrystallized secondary mother liquor is discharged, the crystals are subjected to secondary sweating. The heating rate is 1-20 K / h, the temperature is raised to 43-48℃, and the temperature is held for 0.5-1h. The secondary sweating liquid is then discharged. Step 8) Material processing: After the secondary sweating liquid is discharged, the material is heated to melt it completely and discharge it, finally obtaining high-purity m-tert-butylphenol with a purity greater than 99.5%. The introduction of ultrasound in the layer-by-layer melting crystallization process accelerates the collision rate between solute molecules, speeds up the nucleation rate, reduces the inclusion of impurities in the crystal, is conducive to crystal growth, and improves crystal purity.

2. The method according to claim 1, characterized in that: Step 2) The cooling rate is 5-10 K / h, and the final cooling temperature is 5-20℃.

3. The method according to claim 1, characterized in that: Step 3) The heating rate is 8-15 K / h, and the final heating temperature is 40-44℃.

4. The method according to claim 1, characterized in that: Step 6) The cooling rate is 5-10 K / h, and the final cooling temperature is 10-20℃.

5. The method according to claim 1, characterized in that: Step 7) The heating rate is 1-10 K / h, and the final temperature is 43-46℃.

6. The method according to claim 1, characterized in that: The first-stage sweating fluid is used as raw material in step 1).

7. The method according to claim 1, characterized in that: The secondary mother liquor that did not crystallize after cooling in step 6) is returned to step 1) as raw material for reuse.

8. The method according to claim 1, characterized in that: The crystals in the two-stage layered melt crystallizer are heated to the final temperature and then allowed to sweat. The uncrystallized secondary sweat liquid is returned to step 5) and used as raw material.

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