A method for purifying dioxanone by solution crystallization-melt crystallization coupling

Abstract

The present application relates to a kind of solution crystallization-melting crystallization coupling method for purifying p-dioxanone, which first obtains crude product with higher purity by solution crystallization, and then further purifies by melting crystallization to obtain high-purity p-dioxanone product. Using this coupling process to purify p-dioxanone, the product has high purity, can save a lot of energy consumption, reduce production cost, meet the requirements of industrial large-scale production, and the p-dioxanone product refined has a purity of more than 99.9%, which can meet the requirements of downstream applications.

Description

Technical Field

[0001] This invention relates to the field of p-dioxanone purification technology, specifically to a method for purification of p-dioxanone using a solution crystallization-melt crystallization coupling process. Background Technology

[0002] p-Dioxane (PDO), with its six-membered ring structure, is the monomer for synthesizing poly(p-dioxane). PDO, along with polylactic acid, polycaprolactone, and polyglycolic acid, are all aliphatic polyesters. Their unique biodegradability, biocompatibility, and bioabsorbability have led to their widespread application in the field of medical biodegradable materials. PDO is an ideal material for surgical sutures and can also be used to manufacture bone plates and tissue repair materials, such as surgical instruments like screws, hooks, plates, and forceps, demonstrating broad application prospects.

[0003] The currently widely used method for producing poly(p-dioxanone) is a one-step synthesis using diethylene glycol as a raw material via cyclization and dehydrogenation. However, the purity of the synthesized PDO is relatively low due to factors such as reaction efficiency, byproducts, and the purity of the raw materials. The large number of impurities containing active hydrogen in the PDO monomer generates numerous active centers during polymerization, causing continuous chain transfer and eventual termination of the polymer's active chains. Therefore, improving the molecular weight of poly(p-dioxanone) is crucial to increasing the purity of the PDO monomer.

[0004] Currently, there are four main methods for purifying PDO monomers: The first is to repeatedly dissolve the crude PDO monomer at high temperatures and recrystallize it at low temperatures using appropriate organic solvents. European patents EP0617029A1 and EP0617029B1 disclose the specific process steps of this purification method. This method, which achieves the required purity through repeated recrystallization, results in excessively high preparation costs for PDO monomers. The second method involves repeated vacuum distillation of the PDO monomer, collecting distillate fractions within a certain boiling range under a specific vacuum. This method is not very effective for purifying PDO monomers, as some impurities with boiling points close to PDO, such as ethylene glycol, are difficult to remove by distillation. The third method involves adding strong active hydrogen removal reagents such as calcium hydride (CaH2), sodium hydride (NaH), lithium hydride (LiH), aluminum hydride (AlH3), and lithium aluminum hydride (AlH4Li) to the PDO and combining this with distillation or fractional distillation to effectively remove water and active hydrogen groups such as hydroxyl and carboxyl groups from the crude PDO monomer. Patents CN110028658A and CN105622912A report purification methods for PDO of this type. However, the strong active hydrogen removal reagents used in this method are unstable, costly, and promote PDO polymerization, while also generating solid waste. The fourth method involves adding reagents and catalysts to PDO to react with impurities, followed by distillation to remove the impurities. For example, KR100196097B1 reports a method of adding MDI [diphenylmethane diisocyanate or methylene bis-(4-)phenyl-isocyanate] to PDO, and adding trivalent amine DMDEE (dimorpholinodiethyl ether) as a catalyst. After the reaction, vacuum distillation is performed to remove hydroxyl-containing compounds from the PDO. This method is cumbersome, costly, and introduces other impurities, requiring further treatment to remove the added substances.

[0005] In summary, existing PDO purification processes suffer from drawbacks such as high preparation costs, poor purification efficiency, and the generation of solid waste during the process. Therefore, developing a more efficient, feasible, and economical PDO purification method is of great significance in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for the purification of p-dioxanone using a solution crystallization-melt crystallization coupling process that features low energy consumption, low production cost, convenient operation, and high product purity, and can meet the requirements of large-scale industrial production.

[0007] This invention is achieved through the following technical solution: A method for purifying p-dioxanone using a solution crystallization-melt crystallization coupling process includes the following steps: (1) PDO raw material is mixed with low-boiling-point aliphatic ester and low-boiling-point liquid alkane, and cooled to -15~-8℃ at a cooling rate of 0.5~2℃ / min. PDO crystals precipitate out, and then the temperature is maintained for a period of time for isothermal crystallization. (2) Filtration and separation yielded filtrate and PDO crystals A; (3) Melt the PDO crystal A obtained in step (2) into a liquid and transfer it to a melt crystallizer. Cool it to the crystallization temperature of 20-27℃ at a cooling rate of 0.5-5℃ / min. Turn on the ultrasound to initiate PDO crystallization. Turn off the ultrasound and maintain it for a certain time to allow PDO to precipitate in crystal form, and obtain PDO crystal B and crystallization mother liquor. (4) Transfer out the crystallization mother liquor, raise the PDO crystal B obtained in step (3) to the sweating temperature of 29~35℃ at a heating rate of 0.5~5℃ / min, and sweat. The PDO crystal B partially melts to obtain further purified PDO crystal C and sweating liquid. (5) Transfer the sweat, raise the temperature, and melt all the PDO crystals C obtained in step (4) to obtain the final purified product.

[0008] As an improvement of the present invention, it is preferred that the purity of the PDO raw material in step (1) is 90~99%. If the purity of the raw material is too low, the purity of the solution crystallization product will also be low, and after melt crystallization, the purity cannot reach a polymerization purity of more than 99.9%.

[0009] As an improvement of the present invention, in step (1), the low-boiling-point aliphatic ester is preferably selected from one or more of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, and methyl propionate.

[0010] As an improvement to the present invention, in step (1), the low-boiling-point liquid alkane is preferably selected from one or more of n-pentane, n-hexane, and n-heptane.

[0011] As an improvement to the present invention, in step (1), the preferred mass ratio of PDO raw material to solvent is (1~2):1; wherein the mass ratio of low-boiling-point aliphatic ester to low-boiling-point liquid alkane in the solvent is (1~2.5):1. Within this range, PDO can be kept close to saturation at the mixing temperature, thereby improving product yield and purity.

[0012] As an improvement to the present invention, it is preferred that in step (1), the mixing temperature is 20~35℃ and the isothermal crystallization time is 2~15h. The mixing temperature is close to room temperature, which is convenient to operate and does not require additional energy consumption. If the crystallization time is too short, insufficient crystal precipitation will lead to a decrease in yield. If the time is too long, excessive crystallization will occur, and impurities will be trapped in the crystals, resulting in a poor purification effect.

[0013] As an improvement to the present invention, in step (3), the crystal melting temperature is preferably 35~100℃, more preferably 50~80℃. If the temperature is too low, the dissolution is slow; if the temperature is too high, the raw material will produce oligomers, and the purity will decrease.

[0014] As an improvement to the present invention, in step (3), the crystallization time is preferably controlled between 1 and 50 h, more preferably between 2 and 22 h. If the crystallization time is too short, the crystals will not precipitate sufficiently, which will lead to a decrease in yield; if the time is too long, the crystallization will be excessive, and impurities will be trapped in the crystals, resulting in a poor purification effect.

[0015] As an improvement to the present invention, it is preferable that the sweating time in step (4) is controlled between 2 and 18 hours. If the time is too short, the sweating will be insufficient and the purity will be low; if the time is too long, too much crystal will dissolve and the yield will be low.

[0016] As an improvement to the present invention, it is preferable that in step (5), the crystal melting temperature is controlled at 50~80°C. If the temperature is too low, the dissolution is slow; if the temperature is too high, the raw material will produce oligomers, and the purity will decrease.

[0017] The beneficial effects of the present invention are as follows: 1. The present invention uses a combination of solution crystallization and melt crystallization processes to purify p-dioxanone. Solution crystallization is used to purify PDO from a low purity (e.g., 90-99%) to above 99%, and then melt crystallization is used to increase the purity of PDO from above 99% to above 99.9%. This overcomes the shortcomings of existing single solution crystallization methods, such as low purity, the need for further processing after adding reaction reagents to remove impurities, high cost, and complex steps.

[0018] 2. The crystallization conditions for solution crystallization and melt crystallization are relatively mild, and the energy consumption is greatly reduced compared to multiple distillation or recrystallization, thus solving the problems of high energy consumption and high production costs of existing multiple distillation or recrystallization methods.

[0019] 3. Using a mixture of low-boiling-point aliphatic esters and low-boiling-point liquid alkanes as the PDO solvent allows the PDO raw material to reach saturation at room temperature, resulting in high yield and low energy consumption after cooling crystallization. After obtaining PDO crystals A through solution crystallization filtration, the residual solvent has a large difference in melting point from the product and will not crystallize during the melt crystallization stage. Therefore, no further separation is required, and the next step of melt crystallization can be carried out to obtain high-purity PDO crystals, reducing the number of operation steps.

[0020] 4. During melt crystallization, PDO crystallization is initiated by ultrasound, without introducing foreign impurities, thus further improving crystal purity.

[0021] In summary, the present invention employs a coupled refining process, resulting in a product with high purity, which can save a significant amount of energy consumption, reduce production costs, and meet the requirements of large-scale industrial production. The purity of the refined p-dioxanone can reach over 99.9%, which can meet the requirements of downstream applications. Detailed Implementation

[0022] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention. Example 1

[0023] 200 g of PDO raw material with a purity of 92.3% was mixed with 80 g of methyl formate and 100 g of n-heptane at 20 °C. The mixture was then cooled to -8 °C at a cooling rate of 0.5 °C / min, causing PDO crystals to precipitate and begin crystallization. This temperature was maintained for 2 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 27 °C at a cooling rate of 0.5 °C / min, and sonication was initiated for 30 s at a power of 500 W. The sonication was then turned off and maintained for 3 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 30 °C at a heating rate of 0.5 °C / min and maintained for 2 h. The PDO crystals partially melted, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 50°C to melt all the crystals, yielding 123 g of the final purified product, with a yield of 61.5%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.93%. Example 2

[0024] 200 g of PDO raw material with a purity of 90.2% was mixed with 120 g of methyl propionate and 50 g of n-pentane at 20 °C. The mixture was then cooled to -10 °C at a cooling rate of 1 °C / min, causing PDO crystals to precipitate and begin crystallization. This temperature was maintained for 2 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 60 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 23 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 30 s at a power of 500 W. The ultrasonication was then turned off and maintained for 2 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 29 °C at a heating rate of 1 °C / min and maintained for 3.5 h. The PDO crystals partially melted, yielding further purified PDO crystals and the sweating liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 107 g of the final purified product, with a yield of 50.4%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.91%. Example 3

[0025] 1000 g of PDO raw material with a purity of 94.5% was mixed with 350 g of methyl formate and 200 g of n-pentane at 25 °C. The mixture was then cooled to -15 °C at a cooling rate of 1 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 5 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 50 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 25 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 1 min at a power of 500 W. The ultrasonication was then turned off and maintained for 3 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 30 °C at a heating rate of 2 °C / min and maintained for 4 h. This caused partial melting of the PDO crystals, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 527 g of the final purified product, with a yield of 52.7%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.95%. Example 4

[0026] 1000 g of PDO raw material with a purity of 98.9% was mixed with 500 g of methyl acetate and 300 g of n-hexane at 25 °C. The mixture was then cooled to -13 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 7 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 25 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 1 min at a power of 500 W. The ultrasonication was then turned off and maintained for 5 h, allowing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 31 °C at a heating rate of 2 °C / min and maintained for 2 h. This partially melted the PDO crystals, yielding further purified PDO crystals and the resulting liquid. The sweat was transferred and the temperature was raised to 60°C to melt all the crystals, yielding 652 g of the final purified product, with a yield of 65.2%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.995%. Example 5

[0027] 1000 g of PDO raw material with a purity of 95.2% was mixed with 600 g of propyl formate and 400 g of n-pentane at 30 °C. The mixture was then cooled to -12 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 8 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 20 °C at a cooling rate of 5 °C / min, and ultrasonication was initiated for 1 min at a power of 500 W. The ultrasonication was then turned off and maintained for 2 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 35 °C at a heating rate of 5 °C / min and maintained for 1 h. This partially melted the PDO crystals, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 543 g of the final purified product, with a yield of 54.3%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.92%. Example 6

[0028] 1000 g of PDO raw material with a purity of 97.2% was mixed with 400 g of methyl acetate and 400 g of n-hexane at 30 °C. The mixture was then cooled to -14 °C at a cooling rate of 1 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 6 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 70 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 21 °C at a cooling rate of 4 °C / min, and ultrasonication was initiated for 1 min at a power of 500 W. The ultrasonication was then turned off and maintained for 3 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 32 °C at a heating rate of 4 °C / min and maintained for 3 h. The PDO crystals partially melted, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 602 g of the final purified product, with a yield of 60.2%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.992%. Example 7

[0029] 2500 g of PDO raw material with a purity of 93.8% was mixed with 1000 g of methyl acetate and 900 g of n-hexane at 35°C. The mixture was then cooled to -14°C at a cooling rate of 2°C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 10 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 80°C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 25°C at a cooling rate of 3°C / min, and sonicated for 1 min at a power of 500 W to initiate PDO crystallization. The sonication was then turned off and maintained for 10 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 32°C at a heating rate of 3°C / min and maintained for 7 h. The PDO crystals partially melted, yielding further purified PDO crystals and the sweating liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 1433 g of the final purified product, with a yield of 57.3%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.94%. Example 8

[0030] 2500 g of PDO raw material with a purity of 98.2% was mixed with 1000 g of methyl acetate and 1000 g of n-pentane at 30 °C. The mixture was then cooled to -15 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 9 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 26 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 1 min at a power of 500 W. The ultrasonication was then turned off and maintained for 12 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 33 °C at a heating rate of 2 °C / min and maintained for 6 h. The PDO crystals partially melted, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 1310 g of the final purified product, with a yield of 52.4%. Gas chromatography analysis by Agilent Technologies showed the final product purity to be 99.993%. Example 9

[0031] 5000 g of PDO raw material with a purity of 95.7% was mixed with 3000 g of propyl formate and 2000 g of n-hexane at 35 °C. The mixture was then cooled to -15 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 15 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 22 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 90 s at a power of 500 W. The ultrasonication was then turned off and maintained for 20 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 33 °C at a heating rate of 2 °C / min and maintained for 15 h. The PDO crystals partially melted, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 2570 g of the final purified product, with a yield of 51.4%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.96%. Example 10

[0032] 5000 g of PDO raw material with a purity of 98.7% was mixed with 2000 g of methyl acetate and 2000 g of n-hexane at 35 °C. The mixture was then cooled to -15 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and crystallize. This temperature was maintained for 15 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 24 °C at a cooling rate of 2 °C / min, and ultrasonication was initiated for 90 s at a power of 500 W. The ultrasonication was then turned off and maintained for 22 h, causing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was removed, and the obtained PDO crystals were heated to 32 °C at a heating rate of 2 °C / min and maintained for 18 h. The PDO crystals partially melted, yielding further purified PDO crystals and the precipitated liquid. The sweat was transferred and the temperature was raised to 80°C to melt all the crystals, yielding 2660 g of the final purified product, with a yield of 53.2%. Gas chromatography analysis by Agilent confirmed a final product purity of 99.996%. Example 11

[0033] 1000 g of PDO (96.0% purity) raw material was mixed with 250 g of methyl formate, 100 g of propyl formate, 200 g of n-hexane, and 100 g of n-pentane at 35 °C. The mixture was then cooled to -15 °C at a rate of 1 °C / min, causing PDO crystals to precipitate and begin crystallization. This temperature was maintained for 6 h. After crystallization, the mixture was filtered to obtain the filtrate and PDO crystals. The obtained PDO crystals were heated to 70 °C to melt them into a liquid and transferred to a melt crystallizer. The mixture was then cooled to the crystallization temperature of 22 °C at a rate of 3 °C / min, and sonicated for 2 min at a power of 500 W to initiate PDO crystallization. The sonication was then turned off and maintained for 4 h, allowing PDO to precipitate in crystalline form, yielding PDO crystals and the mother liquor. The mother liquor was transferred out, and the obtained PDO crystals were subjected to a sweating process by heating at a rate of 2°C / min to 32°C and holding for 3 hours. The PDO crystals partially melted, yielding further purified PDO crystals and the sweating liquid. The sweating liquid was then transferred out, and the temperature was increased to 70°C to completely melt the obtained crystals, yielding 588g of the final purified product, with a yield of 58.8%. Gas chromatography analysis by Agilent Technologies showed that the purity of the final product was 99.97%. Example 12

[0034] 5000 g of PDO raw material with a purity of 98.4% was mixed with 2000 g of methyl formate, 500 g of methyl acetate, 1000 g of n-hexane, and 500 g of n-heptane at 35 °C. The mixture was then cooled to -14 °C at a cooling rate of 2 °C / min, causing PDO crystals to precipitate and begin crystallization. This temperature was maintained for 14 h. After crystallization, the mixture was filtered to separate the filtrate and PDO crystals. The obtained PDO crystals were heated to 80 °C to melt them into a liquid and transferred to a melt crystallizer. Then, the mixture was cooled to the crystallization temperature of 25 °C at a cooling rate of 4 °C / min, and ultrasonication was initiated for 2 min at a power of 500 W. The ultrasonication was then turned off and maintained for 20 h, allowing PDO to precipitate in crystalline form, yielding PDO crystals and a mother liquor. The mother liquor was transferred out, and the obtained PDO crystals were subjected to a sweating process by heating at a rate of 3°C / min to 33°C and holding for 16 h. Partial melting of the PDO crystals resulted in further purified PDO crystals and the sweating liquid. The sweating liquid was then transferred out, and the temperature was increased to 80°C to completely melt the obtained crystals, yielding 2815 g of the final purified product, with a yield of 56.3%. Gas chromatography analysis by Agilent Technologies confirmed a final product purity of 99.99%.

Claims

1. A method for purifying p-dioxanone using a solution crystallization-melt crystallization coupling process, characterized in that, Includes the following steps: (1) PDO raw material is mixed with low-boiling-point aliphatic ester and low-boiling-point liquid alkane, and cooled to -15~-8℃ at a cooling rate of 0.5~2℃ / min. PDO crystals precipitate out, and then the temperature is maintained for a period of time for isothermal crystallization. (2) Filtration and separation yielded filtrate and PDO crystals A; (3) Melt the PDO crystal A obtained in step (2) into a liquid and transfer it to a melt crystallizer. Cool it to the crystallization temperature of 20-27℃ at a cooling rate of 0.5-5℃ / min. Turn on the ultrasound to initiate PDO crystallization. Turn off the ultrasound and maintain it for a certain time to allow PDO to precipitate in crystal form, and obtain PDO crystal B and crystallization mother liquor. (4) Transfer out the crystallization mother liquor, raise the PDO crystal B obtained in step (3) to the sweating temperature of 29~35℃ at a heating rate of 0.5~5℃ / min, and sweat. The PDO crystal B partially melts to obtain further purified PDO crystal C and sweating liquid. (5) Transfer the sweat, raise the temperature, and melt all the PDO crystals C obtained in step (4) to obtain the final purified product.

2. The method for purifying dioxane according to claim 1, characterized in that, In step (1), the purity of the PDO raw material is 90-99%.

3. The method for purifying dioxane according to claim 1, characterized in that: In step (1), the low-boiling-point aliphatic ester is selected from one or more of methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, and methyl propionate.

4. The method for purifying dioxane according to claim 1, characterized in that, In step (1), the low-boiling-point liquid alkane is selected from one or more of n-pentane, n-hexane, and n-heptane.

5. The method for purifying dioxane according to claim 1, characterized in that, In step (1), the mass ratio of PDO raw material to solvent is (1~2):1; wherein, the mass ratio of low-boiling-point aliphatic ester to low-boiling-point liquid alkane in the solvent is (1~2.5):

1.

6. The method for purifying dioxane according to claim 1, characterized in that, In step (1), the mixing temperature is 20~35℃ and the isothermal crystallization time is 2~15 h.

7. The method for purifying dioxane according to claim 1, characterized in that, In step (3), the melting temperature of PDO crystal A is 35~100℃.

8. The method for purifying dioxane according to claim 1, characterized in that, In step (3), the PDO crystallization time is controlled between 1 and 50 h.

9. The method for purifying dioxane according to claim 1, characterized in that, In step (4), the sweating time is controlled between 2 and 18 hours.

10. The method for purifying dioxane according to claim 1, characterized in that, In step (5), the melting temperature of PDO crystal C is controlled at 50~80℃.