Crystal separation method for preparing electronic-grade dimethyl carbonate

Abstract

The invention discloses a purification and separation method for preparing electronic-grade dimethyl carbonate, which comprises the following steps: adding a dimethyl carbonate crude product into a tubular crystallizer, controlling the temperature of a system to be 5 DEG C, and standing for 0.5 hour at a constant temperature; cooling to-2 DEG C to 0 DEG C, standing for 0.33-1 hour at a constant temperature, and adding a dimethyl carbonate seed crystal into the crystallizer; keeping the temperature constant for 0.33-1 hour, then reducing the temperature to-8 DEG C to-4 DEG C, keeping the temperature constant for 1-3 hours, and discharging mother liquor; and raising the temperature of the residual solid in the crystallizer to 1-4 DEG C for sweating, continuously keeping the constant temperature for 0.5-2 hours, and discharging sweat into a sweat storage tank. And rapidly heating the residual solid to 5-20 DEG C for melting, and discharging to obtain a high-purity product. The purity of the electronic-grade dimethyl carbonate product obtained through the melt crystallization technology can reach 99.950-99.996%, and the yield is larger than 78%. The method is simple and convenient to operate, lower in energy consumption, free of environmental pollution and generation of other impurities and wastes, capable of meeting the green and environment-friendly requirements and high in industrial application value.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical engineering separation, and particularly relates to a purification and separation method for preparing electronic-grade dimethyl carbonate. Background Art

[0002] Dimethyl carbonate (DMC) is an organic compound with the chemical formula CH3OCOOCH3. It is a colorless, transparent liquid with a molecular weight of 90.08, a melting point of 4°C, a boiling point of 90.3°C, a viscosity of 0.664mpa·s, and a saturated vapor pressure close to that of ethanol at room temperature. It is volatile. It can be mixed with almost all organic solvents such as alcohols, ketones, and esters in any proportion and is slightly soluble in water. Its structural formula is shown below:

[0003]

[0004] Dimethyl carbonate is considered an environmentally friendly "green chemical" due to its low toxicity. It is also used as a solvent and chemical raw material due to its good chemical reactivity. It is widely used in many fields such as coatings, drug synthesis, polymer and catalyst preparation, and lithium-ion batteries. Due to its excellent compatibility and low dielectric constant, viscosity and melting point, DMC is the most widely used solvent in lithium-ion battery electrolytes. It can be mixed with other carbonate solvents in proportion and is the guarantee for lithium-ion batteries to obtain advantages such as high voltage and high performance. With the large-scale use of lithium-ion batteries, the purity requirements for electrolyte solvents are getting higher and higher. Therefore, electronic-grade DMC has a large development space and demand in terms of production scale and product quality.

[0005] In recent years, under the guidance of the national new energy strategy, the rapid development of my country's electronic products, especially new energy electric assisted bicycles and electric cars, has led to rapid development of the corresponding lithium battery industry output and demand, and my country's DMC production capacity has ushered in rapid expansion. In 2020, the total DMC production capacity reached 1.096 million t / a. However, compared with industrial-grade DMC products (≥99.9%), the production capacity of electronic-grade DMC products (≥99.99%) is seriously insufficient, and its production still has high process thresholds and technical barriers. There are only a handful of companies in my country that are capable of producing electronic-grade DMC.

[0006] At present, the main methods for synthesizing DMC are transesterification, methanol oxidation carbonylation and urea alcoholysis. Among them, transesterification and methanol oxidation carbonylation both obtain binary azeotropes of DMC-methanol (whose mass composition is 70% methanol and 30% DMC), which must be separated and purified to obtain high-purity DMC, but the obtained DMC product is also difficult to reach the electronic grade, which seriously affects the performance of lithium-ion batteries. At present, the purification methods of electronic-grade DMC products (≥99.99%) reported at home and abroad mainly include distillation and cooling crystallization, but the distillation method needs to be coupled with other unit operations, such as adsorption or drying, in order to obtain DMC with a purity of 99.99%. The operation is relatively cumbersome and the requirements for the purity of raw materials are too high. Although the cooling crystallization method can remove trace impurities that cannot be removed by the distillation process, and the energy consumption is low, the existing cooling crystallization process has a long operation cycle, serious mother liquor inclusion, and the obtained product needs further dehydration treatment.

[0007] Compared with distillation / rectification, the separation and purification of melt crystallization is based on the melting point and solid-liquid phase diagram of the substance, and its operating temperature is generally lower than distillation and rectification. At the same time, the melt crystallization purification process does not involve the separation of the gas phase, the process is safer and more controllable, and no solvent is required, saving the cost and energy consumption required for solvent recovery. The process is safe, low-carbon, and environmentally friendly, which is more in line with the future development trend of the industry. Summary of the invention

[0008] The purpose of the present invention is to obtain an electronic-grade dimethyl carbonate product by a crystallization separation method. The present invention provides a separation and purification method for a crude dimethyl carbonate product containing methanol impurities, wherein the impurities are derived from the production process. The present invention utilizes the difference in melting points between dimethyl carbonate and methanol and utilizes a static melt crystallization method to obtain an electronic-grade dimethyl carbonate product, and the purity of the obtained product is as high as 99.996%. The method does not use solvents, is pollution-free, and has low energy consumption, and is an effective means for safely and low-carbonly producing electronic-grade dimethyl carbonate.

[0009] In order to achieve the above object, the present invention provides a crystallization separation method for preparing electronic grade dimethyl carbonate, comprising the following steps:

[0010] Step 1: Add crude dimethyl carbonate to the crystallizer, control the system temperature to 5°C, and keep it at constant temperature for 0.5h;

[0011] Step 2: After the constant temperature standing is completed, the crude dimethyl carbonate in step 1 is cooled to the crystallization temperature, and the dimethyl carbonate seed crystals are added after the constant temperature standing is continued for 0.33h to 1h at the crystallization temperature; the temperature when the seed crystals are added is -2°C to 0°C; after the seed crystals are added, the crystallizer is continued to maintain the crystallization temperature for 0.33h to 1h; a solid-liquid mixture is obtained; the crystallization temperature is -2°C to 0°C.

[0012] Step 3: Cooling the solid-liquid mixture obtained in step 2 to the melting crystallization endpoint temperature, and maintaining the melting crystallization endpoint temperature for 1 hour to 3 hours; after the constant temperature is completed, solid-liquid separation is performed, and the mother liquor is discharged to the corresponding mother liquor storage tank; the melting crystallization endpoint temperature is -8°C to -4°C;

[0013] Step 4: heating the remaining solid in the crystallizer after the mother liquor is discharged in step 3 to the sweating end temperature for sweating treatment, and after heating to the sweating end temperature, continue to keep the temperature constant for 0.5h to 2h, then separate the solid and liquid, and discharge the sweat into the corresponding sweat storage tank to obtain a solid product; the sweating end temperature is 1°C to 4°C;

[0014] Step 5: heating the solid product obtained in step 4 to the heating end point temperature, melting all the solids in the crystallizer, and discharging from the bottom of the crystallizer to the corresponding product storage tank to obtain the final product; the heating end point temperature is higher than the melting point of dimethyl carbonate, and the heating end point temperature is 5°C to 20°C.

[0015] Preferably, in step 1, the crude dimethyl carbonate contains 96% to 99.5% dimethyl carbonate by mass and 0.5% to 4% impurities. Preferably, the crystallizer is a tubular crystallizer.

[0016] Any of the above is preferably that, in step 2, the crude dimethyl carbonate in step 1 is cooled to the crystallization temperature at a certain cooling rate; preferably, the cooling process is a uniform cooling process; preferably, the cooling rate is 0.5°C / h to 6°C / h; preferably, the cooling rate is 0.5°C / h to 3°C / h; preferably, the cooling rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0°C / h and ranges therebetween.

[0017] Any of the above is preferably that in step 2, the crystallization temperature is -2°C, -1°C, 0°C and the range therebetween. Preferably, the seeding temperature is -2°C, -1°C, 0°C and the range therebetween.

[0018] Any of the above items is preferably that in step 2, the constant temperature standing time is 0.33h to 1h, more preferably 0.33h, 0.5h, 1h and the range therebetween.

[0019] Any of the above is preferably that in step 2, the mass of the seed crystals added is 0.25% to 2.8% of the mass of the crude dimethyl carbonate, preferably 0.25%, 0.5%, 1.0%, 1.25%, 1.5%, 1.75%, 2.0%, 2.25%, 2.5%, 2.8% and ranges therebetween.

[0020] Preferably, in any of the above items, in step 2, the maintenance time after adding the seed crystal is 0.33h to 1h, more preferably 0.33h, 0.5h, 1h and the range therebetween.

[0021] Any of the above is preferably that in step 3, the melt crystallization endpoint temperature is -8°C, -7°C, -6°C, -5°C, -4°C and ranges therebetween.

[0022] Any of the above is preferably that, in step 3, the solid-liquid mixture obtained in step 2 is cooled to the melt crystallization endpoint temperature at a certain cooling rate; preferably, the cooling process is a uniform cooling process; preferably, the cooling rate is 0.5°C / h to 6°C / h; preferably, the cooling rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0°C / h and ranges therebetween.

[0023] Any of the above items is preferably that in step 3, the constant temperature time at the melt crystallization endpoint temperature is 1 h to 3 h, and more preferably 1, 1.5, 2, 2.5, 3 h and the range therebetween.

[0024] Preferably, in any one of the above items, in step 3, the content of dimethyl carbonate in the mother liquor is 88.43% to 92.30%.

[0025] Any of the above items is preferably that in step 4, the sweating endpoint temperature is 1°C to 4°C, and more preferably 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0°C and ranges therebetween.

[0026] Preferably, in any of the above items, in step 4, the remaining solid in the crystallizer after the mother liquor is discharged in step 3 is heated to the sweating end temperature at a certain heating rate to carry out sweating treatment; preferably, the heating process is a uniform heating process; preferably, the heating rate is 0.5℃ / h to 6℃ / h; preferably, the heating rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0℃ / h and the range therebetween.

[0027] Preferably, in any of the above items, in step 4, the constant temperature time at the sweating endpoint temperature is 0.5 h to 2 h, and more preferably 0.5, 1, 1.5, 2 h and the range therebetween.

[0028] Any of the above is preferably that in step 5, the heating end point temperature is 5°C to 20°C. More preferably, it is 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20°C and ranges therebetween.

[0029] The present invention utilizes the difference in melting point between impurities and dimethyl carbonate and the advantage of the greater solubility of impurities in dimethyl carbonate, strictly controls the temperature of the crystallization process and the heating and cooling rates, and separates and purifies to obtain electronic-grade dimethyl carbonate products. The purity of the obtained dimethyl carbonate product can be as high as 99.950% to 99.996%, and the yield is greater than 78%. No additional organic solvent is required in the process, and only a tubular crystallizer and a temperature control device are required to complete the efficient separation of dimethyl carbonate and impurities, which is simpler to operate; lower energy consumption, greater yield, higher purity; no environmental pollution and the generation of other impurities and wastes, meet green environmental protection requirements, and have high industrial application value. DETAILED DESCRIPTION

[0030] The present invention is described more clearly and completely through the following examples, but the examples described are only part of the embodiments of the present invention, not all of them. The embodiments are to help understand the present invention and should not be used to limit the scope of protection of the present invention.

[0031] Example 1

[0032] (1) 180 g of crude dimethyl carbonate, wherein the mass fraction of dimethyl carbonate is 96% and the content of impurities is 4%, is poured into a tubular crystallizer, the temperature in the tubular crystallizer is controlled at 5° C., and the mixture is allowed to stand at the constant temperature for 0.5 h;

[0033] (2) After the constant temperature standing is completed, the crude dimethyl carbonate product at 5° C. in step (1) is cooled to -2° C. at a rate of 6° C. / h, and the temperature is maintained at the same temperature for 0.33 h, after which 5.04 g of dimethyl carbonate seed crystals are added; after the seed crystals are added, the temperature is maintained at the same temperature for another 0.33 h;

[0034] (3) After the constant temperature is completed, the solid-liquid mixture obtained in step (2) is cooled to -8°C at a cooling rate of 4°C / h, and then kept at the constant temperature for 1 hour; after the constant temperature is completed, the stopcock at the bottom of the crystallizer is opened to discharge the mother liquor into the mother liquor storage tank, and the mother liquor is detected by gas chromatography, and the content of dimethyl carbonate is 89.61%;

[0035] (4) heating the remaining solid in the crystallizer after the mother liquor is discharged in step (3) to 1° C. at a heating rate of 2° C. / h for sweating treatment, and after reaching the sweating operation temperature, maintaining the temperature for 0.5 h; then performing solid-liquid separation and discharging the sweat into a sweat storage tank;

[0036] (5) After the sweat is completely drained, the temperature is raised to 5°C within 0.5 h, the product is melted and discharged into the product storage tank.

[0037] The final obtained dimethyl carbonate product was detected by gas chromatography, and the purity was 99.994%, and the product yield was 82.16%.

[0038] Example 2

[0039] (1) 180 g of crude dimethyl carbonate, wherein the mass fraction of dimethyl carbonate is 99.5% and the content of impurities is 0.5%, is poured into a tubular crystallizer, the temperature in the tubular crystallizer is controlled at 5° C., and the mixture is allowed to stand at the constant temperature for 0.5 h;

[0040] (2) After the constant temperature standing is completed, the crude dimethyl carbonate product at 5° C. in step (1) is cooled to -1° C. at a rate of 1° C. / h, and the temperature is kept constant for 0.5 h, after which 0.45 g of dimethyl carbonate seed crystals are added; after the seed crystals are added, the temperature is kept constant for another 0.5 h;

[0041] (3) After the constant temperature is completed, the solid-liquid mixture obtained in step (2) is cooled to -6°C at a cooling rate of 2°C / h, and then kept at the constant temperature for 2h; after the constant temperature is completed, the stopcock at the bottom of the crystallizer is opened to discharge the mother liquor into the mother liquor storage tank, and the mother liquor is detected by gas chromatography, and the content of dimethyl carbonate is 92.30%;

[0042] (4) heating the remaining solid in the crystallizer after the mother liquor is discharged in step (3) to 2° C. at a heating rate of 6° C. / h for sweating treatment, and after reaching the sweating operation temperature, maintaining the temperature for 1 hour; then performing solid-liquid separation, and discharging the sweat into a sweat storage tank;

[0043] (5) After the sweat is completely drained, the temperature is raised to 18°C ​​within 0.5 h, the product is melted and discharged into the product storage tank.

[0044] The final obtained dimethyl carbonate product was detected by gas chromatography, and the purity was 99.996%, and the product yield was 79.47%.

[0045] Example 3

[0046] (1) 180 g of crude dimethyl carbonate, wherein the mass fraction of dimethyl carbonate is 96% and the content of impurities is 4%, is poured into a tubular crystallizer, the temperature in the tubular crystallizer is controlled at 5° C., and the mixture is allowed to stand at the constant temperature for 0.5 h;

[0047] (2) After the constant temperature standing is completed, the crude dimethyl carbonate product at 5° C. in step (1) is cooled to 0° C. at a cooling rate of 0.5° C. / h, and the temperature is maintained for 1 hour, after which 0.5 g of dimethyl carbonate seed crystals are added; after the seed crystals are added, the temperature is maintained for another 1 hour;

[0048] (3) After the constant temperature is completed, the solid-liquid mixture obtained in step (2) is cooled to -4°C at a cooling rate of 0.5°C / h, and then kept at the constant temperature for 3 hours; after the constant temperature is completed, the stopcock at the bottom of the crystallizer is opened to discharge the mother liquor into the mother liquor storage tank, and the mother liquor is detected by gas chromatography, and the content of dimethyl carbonate is 88.43%;

[0049] (4) heating the remaining solid in the crystallizer after the mother liquor is discharged in step (3) to 4° C. at a heating rate of 3° C. / h for sweating treatment, and after reaching the sweating operation temperature, maintaining the temperature for 2 hours; then performing solid-liquid separation, and discharging the sweat into a sweat storage tank;

[0050] (5) After the sweat is completely drained, the temperature is raised to 20°C within 0.5 h, the product is melted and discharged into the product storage tank.

[0051] The final obtained dimethyl carbonate product was detected by gas chromatography, and the purity was 99.959%, ​​and the product yield was 78.26%.

[0052] Example 4

[0053] (1) 180 g of crude dimethyl carbonate, wherein the mass fraction of dimethyl carbonate is 98.4% and the content of impurities is 1.6%, is poured into a tubular crystallizer, the temperature in the tubular crystallizer is controlled at 5° C., and the mixture is allowed to stand at the constant temperature for 0.5 h;

[0054] (2) After the constant temperature standing is completed, the crude dimethyl carbonate product at 5° C. in step (1) is cooled to -2° C. at a rate of 3° C. / h, and the temperature is maintained at this temperature for 0.5 h, after which 1.8 g of dimethyl carbonate seed crystals are added; after the seed crystals are added, the temperature is maintained at this temperature for another 1 h;

[0055] (3) After the constant temperature is completed, the solid-liquid mixture obtained in step (2) is cooled to -8°C at a cooling rate of 6°C / h, and then kept at the constant temperature for 3 hours; after the constant temperature is completed, the stopcock at the bottom of the crystallizer is opened to discharge the mother liquor into the mother liquor storage tank, and the mother liquor is detected by gas chromatography, and the content of dimethyl carbonate is 92.07%;

[0056] (4) heating the remaining solid in the crystallizer after the mother liquor is discharged in step (3) to 3° C. at a heating rate of 0.5° C. / h for sweating treatment, and after reaching the sweating operation temperature, maintaining the temperature for 0.5 h; then performing solid-liquid separation and discharging the sweat into a sweat storage tank;

[0057] (5) After the sweat is completely drained, the temperature is raised to 20°C within 0.5 h, the product is melted and discharged into the product storage tank.

[0058] The final obtained dimethyl carbonate product was detected by gas chromatography, and the purity was 99.976%, and the product yield was 80.21%.

[0059] Example 5

[0060] (1) 180 g of crude dimethyl carbonate, wherein the mass fraction of dimethyl carbonate is 96.7% and the content of impurities is 3.3%, is poured into a tubular crystallizer, the temperature in the tubular crystallizer is controlled at 5° C., and the mixture is allowed to stand at the constant temperature for 0.5 h;

[0061] (2) After the constant temperature standing is completed, the crude dimethyl carbonate product at 5° C. in step (1) is cooled to 0° C. at a rate of 2° C. / h, and the temperature is maintained at the same temperature for 1 hour, after which 2.7 g of dimethyl carbonate seed crystals are added; after the seed crystals are added, the temperature is maintained at the same temperature for another 0.33 hour;

[0062] (3) After the constant temperature is completed, the solid-liquid mixture obtained in step (2) is cooled to -7°C at a cooling rate of 1°C / h, and then kept at the constant temperature for 2h; after the constant temperature is completed, the stopcock at the bottom of the crystallizer is opened to discharge the mother liquor into the mother liquor storage tank, and the mother liquor is detected by gas chromatography, and the content of dimethyl carbonate is 89.34%;

[0063] (4) heating the remaining solid in the crystallizer after the mother liquor is discharged in step (3) to 3.5° C. at a heating rate of 2° C. / h for sweating treatment, and after reaching the sweating operation temperature, maintaining the temperature for 1 hour; then performing solid-liquid separation and discharging the sweat into a sweat storage tank;

[0064] (5) After the sweat is completely drained, the temperature is raised to 15°C within 0.5 h, the product is melted and discharged into the product storage tank.

[0065] The final obtained dimethyl carbonate product was detected by gas chromatography, and the purity was 99.950%, and the product yield was 78.99%.

[0066] Example 6

[0067] Example 6 is similar to Example 1-5, except that in step 2, the crystallization temperature is -2°C, -1°C, 0°C and the range therebetween. The seeding temperature is -2°C, -1°C, 0°C and the range therebetween.

[0068] Example 7

[0069] Example 7 is similar to Examples 1-5, except that, in step 2, the crude dimethyl carbonate in step 1 is cooled to the crystallization temperature at a certain cooling rate; the cooling rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0°C / h and ranges therebetween.

[0070] In step 2, the constant temperature standing time is 0.33h, 0.5, 1h and the range therebetween.

[0071] In step 2, the maintenance time after adding the seed crystal is 0.33h, 0.5, 1h and the range therebetween.

[0072] Example 8

[0073] Example 8 is similar to Examples 1-5, except that in step 3, the melt crystallization endpoint temperature is -8°C, -7°C, -6°C, -5°C, -4°C and ranges therebetween.

[0074] Example 9

[0075] Example 9 is similar to Examples 1-5, except that, in step 3, the solid-liquid mixture obtained in step 2 is cooled to the melt crystallization endpoint temperature at a certain cooling rate; the cooling rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0°C / h and ranges therebetween.

[0076] In step 3, the constant temperature time at the terminal temperature of the melt crystallization is 1, 1.5, 2, 2.5, 3 hours and the range therebetween.

[0077] Example 10

[0078] Example 10 is similar to Examples 1-5, except that in step 4, the sweating endpoint temperature is 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0°C and ranges therebetween.

[0079] Embodiment 11

[0080] Example 11 is similar to Examples 1-5, except that, in step 4, the remaining solid in the crystallizer after discharging the mother liquor in step 3 is heated to the sweating endpoint temperature at a certain heating rate for sweating treatment; the heating rate is 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0℃ / h and the range therebetween.

[0081] In step 4, the constant temperature time at the sweating endpoint temperature is 0.5, 1, 1.5, 2°C and the range therebetween.

[0082] Example 12

[0083] Example 12 is similar to Examples 1-5, except that in step 5, the heating end temperature is 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20°C and ranges therebetween.

[0084] In Examples 6 to 12, the obtained dimethyl carbonate product has a purity of up to 99.950% to 99.996%, and a yield of more than 78%.

[0085] Comparative Example 1

[0086] The difference from Example 1 is that the temperature of adding seed crystals is -3°C. The purity of the dimethyl carbonate product finally obtained is 99.87% and the product yield is 83.02% as determined by gas chromatography. The other conditions are the same.

[0087] Comparative Example 2

[0088] The difference from Example 1 is that the end point temperature of the melt crystallization is -10° C. The purity of the dimethyl carbonate product finally obtained is 99.38% as determined by gas chromatography, and the product yield is 85.02%.

[0089] The product indexes in the comparative example are much lower than those in the embodiment. This is because the temperature of adding seed crystals affects the assembly rate of dimethyl carbonate molecules when forming crystals. Too fast an assembly rate leads to the inclusion of more impurities. In addition, a lower melting crystallization endpoint temperature leads to more impurity enrichment and entry into the crystal lattice, resulting in reduced product purity.

[0090] The method for purifying and separating dimethyl carbonate disclosed and proposed in the present invention is only a preferred embodiment of the present invention. For those skilled in the art, by referring to the content of this article, it is obvious that without departing from the method of the present invention, the raw materials, process parameters and other links can be appropriately changed to achieve some improvements and supplements. Those skilled in the art who are familiar with this profession can obviously change or appropriately change and combine the methods and products described herein without departing from the content, spirit and scope of the present invention to realize the technology of the present invention. It is particularly important to point out that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the spirit, scope and content of the present invention.

[0091] The above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A crystallization separation method for preparing electronic grade dimethyl carbonate, comprising the following steps: Step 1: Add crude dimethyl carbonate to the crystallizer, control the system temperature to 5°C, and keep it at constant temperature for 0.5h; Step 2: After the constant temperature standing is completed, the crude dimethyl carbonate in step 1 is cooled to the crystallization temperature, and the dimethyl carbonate seed crystals are added after the constant temperature standing is continued for 0.33h to 1h at the crystallization temperature; the temperature when the seed crystals are added is -2°C to 0°C; after the seed crystals are added, the crystallizer is continued to maintain the crystallization temperature for 0.33h to 1h; a solid-liquid mixture is obtained; the crystallization temperature is -2°C to 0°C. Step 3: Cooling the solid-liquid mixture obtained in step 2 to the melting crystallization endpoint temperature, and maintaining the melting crystallization endpoint temperature for 1 hour to 3 hours; after the constant temperature is completed, solid-liquid separation is performed, and the mother liquor is discharged to the corresponding mother liquor storage tank; the melting crystallization endpoint temperature is -8°C to -4°C; Step 4: heating the remaining solid in the crystallizer after the mother liquor is discharged in step 3 to the sweating end temperature for sweating treatment, and after heating to the sweating end temperature, continue to keep the temperature constant for 0.5h to 2h, then separate the solid and liquid, and discharge the sweat into the corresponding sweat storage tank to obtain a solid product; the sweating end temperature is 1°C to 4°C; Step 5: heating the solid product obtained in step 4 to the heating end point temperature, melting all the solids in the crystallizer, and discharging from the bottom of the crystallizer to the corresponding product storage tank to obtain the final product; the heating end point temperature is higher than the melting point of dimethyl carbonate, and the heating end point temperature is 5°C to 20°C.

2. The crystallization separation method according to claim 1, characterized in that In step 1, the crude dimethyl carbonate contains 96% to 99.5% dimethyl carbonate by mass and 0.5% to 4% impurities.

3. The crystallization separation method according to claim 1, characterized in that In step 2, the crude dimethyl carbonate in step 1 is cooled to the crystallization temperature at a certain cooling rate, wherein the cooling process is a uniform cooling process, and the cooling rate is 0.5° C. / h to 6° C. / h.

4. The crystallization separation method according to claim 3, characterized in that: In step 2, the mass of the seed crystals added is 0.25% to 2.8% of the mass of the crude dimethyl carbonate.

5. The crystallization separation method according to claim 1, characterized in that: In step 3, the solid-liquid mixture obtained in step 2 is cooled to the melting crystallization endpoint temperature at a certain cooling rate, and the cooling process is a uniform cooling process, and the cooling rate is 0.5°C / h to 6°C / h.

6. The crystallization separation method according to claim 5, characterized in that In step 3, the content of dimethyl carbonate in the mother liquor is 88.43% to 92.30%.

7. The crystallization separation method according to claim 1, characterized in that: In step 4, the remaining solid in the crystallizer after the mother liquor is discharged in step 3 is heated to the sweating end temperature at a certain heating rate for sweating treatment, and the heating process is a uniform heating process, and the heating rate is 0.5°C / h to 6°C / h.

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