Production line and method for preparing glycollic acid through glycolic acid ester hydrolysis

Abstract

The invention discloses a production line and method for preparing glycollic acid through glycolate hydrolysis, and the production line comprises a mixing kettle used for mixing and heating glycolate and water; a rectifying tower; a heat end inlet of the heat exchanger is connected with the methanol steam outlet; a circulating outlet of the concentration kettle is connected with a cold end inlet of the heat exchanger, a circulating inlet is connected with a cold end outlet, and the concentration kettle is used for heating and concentrating the glycollic acid aqueous solution to obtain a high-concentration glycollic acid aqueous solution; the cooling kettle is used for carrying out gradient cooling crystallization on the high-concentration glycollic acid aqueous solution; a centrifuge; a dryer; and the impurity treatment tank is used for carrying out impurity treatment on the glycollic acid mother liquor. By optimizing the rectification temperature and pressure, the methanol water vapor obtained by rectification can meet the heating concentration requirement of the glycollic acid solution, and the energy consumption is remarkably reduced; glycollic acid mother liquor is recycled, so that the energy consumption of the device is reduced and the yield is increased; impurities in the glycollic acid mother liquor are treated and reused, so that the product purity is improved.

Description

Technical Field

[0001] The present application belongs to the technical field of glycolic acid preparation, and specifically relates to a production line and method for preparing glycolic acid by hydrolyzing glycolic acid lipids. Background Art

[0002] Glycolic acid is widely used in organic synthesis, cleaning, electroplating, textile, leather, sterilization and other industries. At present, the widely used industrial synthesis processes of glycolic acid mainly include: nitrous acid oxidation of glycine, chloroacetic acid hydrolysis, and formaldehyde carbonylation. Among them, chloroacetic acid hydrolysis is the best method for industrial synthesis of glycolic acid, but this synthesis method requires acetic acid and sulfur powder, which is very corrosive to equipment and environmental pollution, resulting in increased production costs.

[0003] In order to solve the above problems, there are currently literature reports on a method for preparing glycolic acid by hydrolyzing glycolic acid methyl ester, but this method has the problems of high energy consumption, low product quality and yield, mainly manifested in: 1. Methanol needs to be evaporated for the reaction to proceed forward, resulting in high energy consumption; 2. The vacuum concentration of glycolic acid needs to be controlled at a relatively low temperature. In order to evaporate and separate water, the vacuum degree needs to be controlled at a relatively low level; 3. During the crystallization process of glycolic acid, it is difficult to accurately control the cooling amplitude. If the cooling amplitude is too large, a large amount of solids will appear, causing pipeline blockage. If the cooling amplitude is too small, the glycolic acid output will be insufficient and the production capacity will be too low; 4. A large amount of impurities are contained in the production process of glycolic acid, which affects the quality of glycolic acid. Summary of the invention

[0004] The purpose of the present application is to provide a production line and method for preparing glycolic acid by hydrolyzing glycolic acid ester, so as to solve the problems of high energy consumption, low product quality and low yield in the method for preparing glycolic acid by hydrolyzing glycolic acid methyl ester in the prior art.

[0005] In order to achieve the above-mentioned object, the first aspect of the present application provides a production line for preparing glycolic acid by hydrolyzing glycolic acid lipids, comprising:

[0006] A mixing kettle, comprising a raw material inlet for injecting water and methyl acetate and a mixed solution outlet, wherein the mixing kettle is used to mix and heat glycolic acid lipid and water in the mixing kettle to hydrolyze the glycolic acid lipid to obtain a glycolic acid methanol mixed solution;

[0007] A distillation tower, comprising a raw material inlet, a methanol water vapor outlet at the top of the tower and a glycolic acid aqueous solution outlet at the bottom of the tower, wherein the raw material inlet is connected to the mixed solution outlet;

[0008] A heat exchanger, wherein a hot end inlet is connected to the methanol water vapor outlet, and a hot end outlet is connected to the top of the distillation tower;

[0009] A concentrator, comprising an inlet connected to the outlet of the glycolic acid aqueous solution, a circulation outlet, a circulation inlet and a water vapor outlet, wherein the circulation outlet is connected to the cold end inlet of the heat exchanger, and the circulation inlet is connected to the cold end outlet of the heat exchanger, and the concentrator is used to heat and concentrate the glycolic acid aqueous solution to obtain a high-concentration glycolic acid aqueous solution;

[0010] A cooling kettle, comprising an inlet connected to the circulation outlet and a crystallization mixed solution outlet, the cooling kettle being used to perform gradient temperature reduction crystallization on the high-concentration glycolic acid aqueous solution;

[0011] A centrifuge connected to the crystallization mixed solution outlet, the centrifuge comprising a precipitation outlet and a supernatant outlet, and the centrifuge is used to centrifuge the crystallization mixed solution to obtain crude glycolic acid crystals and glycolic acid mother liquor;

[0012] A dryer connected to the precipitation outlet, the dryer is used to dry the crude glycolic acid crystals to obtain fine glycolic acid crystals;

[0013] An impurity treatment tank, the inlet of which is connected to the supernatant outlet, and the outlet of which is connected to the raw material inlet of the mixing kettle. The impurity treatment tank is used to perform impurity treatment on the glycolic acid mother liquor.

[0014] In one or more embodiments, it further includes:

[0015] a condenser, an inlet of which is connected to the water vapor outlet;

[0016] The condensate storage tank has an inlet connected to the outlet of the condenser, and an outlet connected to the raw material inlet of the mixing kettle.

[0017] In one or more embodiments, the operating pressure of the distillation tower is 90-150 KPa, and the operating temperature of the tower bottom is 80-120°C; the operating pressure of the concentration tower is 10-50 KPa, and the operating temperature is 20-60°C.

[0018] In order to achieve the above object, the second aspect of the present application provides a method for preparing glycolic acid by hydrolyzing glycolic acid lipids, using the production line described in any of the above embodiments, the method comprising:

[0019] Mixing glycolic acid lipid and water in a mixing kettle and heating them to hydrolyze the glycolic acid lipid to obtain a glycolic acid-methanol mixed solution;

[0020] performing a rectification operation on the glycolic acid-methanol mixed solution to obtain methanol water vapor and glycolic acid aqueous solution;

[0021] Using the methanol vapor as a heat source to heat and concentrate the glycolic acid aqueous solution to obtain a high-concentration glycolic acid aqueous solution;

[0022] The high-concentration glycolic acid aqueous solution is subjected to gradient cooling crystallization and step-by-step centrifugation to obtain crude glycolic acid crystals, and the glycolic acid mother liquor obtained by centrifugation is treated for impurities and then reintroduced into the mixing kettle;

[0023] The crude glycolic acid crystals are dried to obtain fine glycolic acid crystals.

[0024] In one or more embodiments, the impurity treatment includes removing glycolide from the acetic acid alcohol mother liquor, and the glycolide removal method includes:

[0025] adding alkaline solid to the glycolic acid mother liquor to decompose glycolide into glycolate;

[0026] Wherein, the alkaline solid includes one or more combinations of sodium hydroxide, calcium hydroxide, magnesium hydroxide, calcium carbonate and sodium carbonate.

[0027] In one or more embodiments, the impurity treatment includes removing oxalic acid from the acetic acid alcohol mother liquor, and the method for removing oxalic acid includes:

[0028] adding a precipitant to the glycolic acid mother liquor to form oxalate precipitate from the oxalic acid, and then filtering and removing the precipitate;

[0029] Wherein, the precipitant is one or a combination of calcium carbonate and calcium chloride, and the dosage of the precipitant is 0.8 to 2.0 eq.

[0030] In one or more embodiments, the molar ratio of glycolic acid to water is 1:(2-8).

[0031] In one or more embodiments, the gradient cooling crystallization specifically includes two cooling crystallizations, the temperature of the first cooling crystallization is 0 to 20°C, and the temperature of the second cooling crystallization is -20 to 0°C.

[0032] In one or more embodiments, it further includes:

[0033] The water vapor generated during the heating and concentration of the glycolic acid aqueous solution is condensed and recovered, and then reintroduced into the mixing kettle.

[0034] In one or more embodiments, the concentration of the high-concentration glycolic acid aqueous solution is 60-95 wt %.

[0035] Different from the prior art, the beneficial effects of this application are:

[0036] The present application optimizes the distillation temperature and pressure so that the methanol vapor obtained by distillation can meet the heating and concentration requirements of the glycolic acid solution, and significantly reduces energy consumption through thermal coupling technology;

[0037] The present application utilizes the glycolic acid mother liquor obtained by centrifugation, thereby improving the hydrolysis efficiency, improving the selectivity of the reaction, avoiding the loss of glycolic acid polymerization during the reaction, reducing the energy consumption of the device, increasing the product production capacity and single-pass yield, having good fluidity and being stirrable, and obtaining more high-quality glycolic acid products;

[0038] The present application effectively improves the purity of the glycolic acid product by treating the impurities in the glycolic acid mother liquor and applying the operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0040] Figure 1 This is a schematic structural diagram of an embodiment of a production line for preparing glycolic acid by hydrolyzing glycolic acid lipids in the present application;

[0041] Figure 2 It is a schematic flow chart of an embodiment of a method for preparing glycolic acid by hydrolyzing glycolic acid lipids of the present application.

[0042] Description of reference numerals:

[0043] Mixing kettle 10; raw material inlet 101; mixed solution outlet 102;

[0044] Distillation tower 20; raw material inlet 201; methanol water vapor outlet 202; glycolic acid aqueous solution outlet 203;

[0045] Heat exchanger 30;

[0046] Concentrator 40; circulation outlet 401; circulation inlet 402; water vapor outlet 403;

[0047] Cooling kettle 50; crystallization mixed liquid outlet 501;

[0048] Centrifuge 60; sedimentation outlet 601; supernatant outlet 602;

[0049] Dryer 70;

[0050] Impurity treatment tank 80;

[0051] Condenser 90;

[0052] Condensate storage tank 100. DETAILED DESCRIPTION

[0053] In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work should fall within the scope of protection of the present application.

[0054] In order to solve the problems of high energy consumption, low product quality and yield in the current method of preparing glycolic acid by hydrolysis of methyl glycolate, the applicant has developed a new production line for preparing glycolic acid by hydrolysis of glycolate. The production line improves the energy utilization rate in the production process through thermal coupling technology, and improves the quality and yield of the product through mother liquor reuse and impurity treatment processes.

[0055] Specifically, see Figure 1 , Figure 1 It is a structural schematic diagram of an embodiment of a production line for preparing glycolic acid by hydrolyzing glycolic acid lipids in the present application.

[0056] like Figure 1 As shown, the production line includes a mixing kettle 10, a distillation tower 20, a heat exchanger 30, a concentration kettle 40, a cooling kettle 50, a centrifuge 60, a drying machine 70 and an impurity treatment tank 80.

[0057] Specifically, the mixing kettle 10 includes a raw material inlet 101 for injecting water and methyl acetate and a mixed solution outlet 102. The mixing kettle 10 is used to mix and heat glycolic acid lipid and water in the mixing kettle 10 to hydrolyze the glycolic acid lipid to obtain a glycolic acid methanol mixed solution.

[0058] In one embodiment, the molar ratio of glycolic acid to water can be 1:(2-8), more preferably, the molar ratio of glycolic acid to water can be 1:(3-6.5), illustratively, the molar ratio of glycolic acid to water can be 1:5, 1:5.5, 1:3, 1:3.5 or 1:4, all of which can achieve the effects of this embodiment.

[0059] The distillation tower 20 includes a raw material inlet 201, a methanol water vapor outlet 202 at the top of the tower, and a glycolic acid aqueous solution outlet 203 at the bottom of the tower. The raw material inlet 201 is connected to the mixed solution outlet 102, so that the glycolic acid-methanol mixed solution can be distilled in the distillation tower 20 to generate methanol water vapor and glycolic acid aqueous solution.

[0060] The hot end inlet of the heat exchanger 30 is connected to the methanol water vapor outlet 202, and the hot end outlet is connected to the top of the distillation tower 20.

[0061] The concentrator 40 includes an inlet connected to the glycolic acid aqueous solution outlet 203, a circulation outlet 401, a circulation inlet 402 and a water vapor outlet 403. The circulation outlet 401 is connected to the cold end inlet of the heat exchanger 30, and the circulation inlet 402 is connected to the cold end outlet of the heat exchanger 30. The concentrator 40 is used to heat and concentrate the glycolic acid aqueous solution to obtain a high-concentration glycolic acid aqueous solution.

[0062] In one embodiment, the concentration of the high-concentration glycolic acid aqueous solution may be 60 to 95 wt %. More preferably, the concentration of the high-concentration glycolic acid aqueous solution may be 70 to 85 wt %.

[0063] Based on the above design, methanol vapor evaporated by distillation is used as the heat source for concentrating the ethanolic acid aqueous solution. The methanol vapor after heat exchange returns to the distillation tower 20, and the ethanolic acid aqueous solution in the concentration kettle 40 circulates through the heat exchanger 30 and is heated by the methanol vapor, thereby realizing thermal coupling.

[0064] In one embodiment, the operating pressure of the distillation tower 20 is 90-150 KPa, and the operating temperature of the bottom of the tower is 80-120°C.

[0065] More preferably, the operating pressure of the distillation tower 20 may be normal pressure to 130 KPa, and the operating temperature of the bottom of the tower may be 90 to 110°C.

[0066] Under the above operating pressure and operating temperature, the methanol vapor in the distillation tower 20 can meet the heat requirement for heating and concentrating the glycolic acid solution. Therefore, there is no need to add a heat source in the concentration kettle 40, thereby further reducing energy consumption.

[0067] In one embodiment, the operating pressure of the concentration tower may be 10-50 KPa, and the operating temperature may be 20-60° C., so as to facilitate the concentration of the aqueous acetate solution and the evaporation of water vapor.

[0068] More preferably, the operating pressure of the concentration tower may be 20-40 KPa, and the operating temperature may be 30-50°C.

[0069] The cooling kettle 50 includes an inlet connected to the circulation outlet 401 and a crystallization mixed solution outlet 501. The cooling kettle 50 is used to perform gradient temperature reduction crystallization on the high-concentration glycolic acid aqueous solution.

[0070] It is understandable that after the circulation outlet 401 of the concentration kettle 40 passes through the heat exchanger 30, part of it can be refluxed to the circulation inlet 402, and part of it can enter the cooling kettle 50 for crystallization. In this embodiment, the cooling kettle 50 adopts a gradient cooling crystallization method to improve the production capacity of glycolic acid, while avoiding the problem of excessive cooling amplitude causing a large amount of solids to cause pipeline blockage, and excessively small cooling amplitude causing insufficient glycolic acid production and low production capacity.

[0071] In one embodiment, the gradient cooling crystallization specifically includes two cooling crystallizations, the temperature of the first cooling crystallization is 0 to 20°C, and the temperature of the second cooling crystallization is -20 to 0°C.

[0072] More preferably, the temperature of the first cooling crystallization is 5 to 15°C, and the temperature of the second cooling crystallization is -15 to -5°C.

[0073] The centrifuge 60 is connected to the crystallization mixed solution outlet 501, and the centrifuge 60 includes a precipitation outlet 601 and a supernatant outlet 602. The centrifuge 60 is used to centrifuge the crystallization mixed solution to obtain crude glycolic acid crystals and glycolic acid mother liquor. During the step-by-step cooling and crystallization process in the cooling kettle 50, the bottom crystallization mixed solution obtained by each cooling and crystallization enters the centrifuge 60 and is centrifuged to obtain crude glycolic acid crystals and glycolic acid mother liquor.

[0074] The dryer 70 is connected to the precipitation outlet 601 and is used to dry the crude glycolic acid crystals to obtain 99% glycolic acid crystal fine products.

[0075] The inlet of the impurity treatment tank 80 is connected to the supernatant outlet 602, and the outlet is connected to the raw material inlet 201 of the mixing kettle 10. The impurity treatment tank 80 is used to treat the impurities of the ethanol acid mother liquor and re-introduce the ethanol acid mother liquor that has been treated with impurities into the mixing kettle 10.

[0076] Based on the above scheme, on the one hand, the ethanol acid yield is increased by applying the ethanol acid mother liquor, that is, the hydrolysis efficiency is improved, the selectivity of the reaction is improved, and the polymerization loss of ethanol acid during the reaction is avoided, the energy consumption of the device is reduced, the product production capacity and single-pass yield are increased, the fluidity is good and it can be stirred, and more high-quality ethanol acid products are obtained; on the other hand, the purity of the ethanol acid product is also effectively improved by the impurity treatment operation of the ethanol acid mother liquor.

[0077] In one embodiment, the impurity treatment may include removing glycolide from the acetate alcohol mother liquor, and removing oxalic acid from the acetate alcohol mother liquor.

[0078] Specifically, the method for removing glycolide includes: adding alkaline solid to glycolic acid mother liquor to decompose glycolide into glycolate.

[0079] Wherein, the alkaline solid may include one or more combinations of sodium hydroxide, calcium hydroxide, magnesium hydroxide, calcium carbonate and sodium carbonate. More preferably, the alkaline solid may include one or more combinations of sodium hydroxide, calcium hydroxide and sodium carbonate.

[0080] The method for removing oxalic acid comprises: adding a precipitant to a glycolic acid mother liquor to form oxalate precipitate from the oxalic acid, and then filtering and removing the oxalic acid;

[0081] The precipitant is one or a combination of calcium carbonate and calcium chloride, and the amount of the precipitant is 0.8 to 2.0 eq.

[0082] In order to further improve the utilization rate of raw materials, the production line in this embodiment also includes a condenser 90 and a condensate storage tank 100. The inlet of the condenser 90 is connected to the water vapor outlet 403 of the concentration kettle 40, and the outlet is connected to the condensate storage tank 100. The outlet of the condensate storage tank 100 is connected to the raw material inlet 101 of the mixing kettle 10, thereby collecting the water vapor generated during the concentration process of the ethanol acid solution as the reaction raw material for the next batch.

[0083] Based on the production lines of the above-mentioned embodiments, the energy consumption in the preparation process of glycolic acid is effectively reduced through thermal coupling technology, while the quality and yield of the glycolic acid product are improved.

[0084] The present application also provides a method for preparing glycolic acid by hydrolyzing glycolic acid esters, the method using any of the above-mentioned production lines, specifically, please refer to Figure 2 , Figure 2 It is a schematic flow chart of an embodiment of a method for preparing glycolic acid by hydrolyzing glycolic acid lipids of the present application.

[0085] like Figure 2 As shown, the method includes:

[0086] S100, mixing glycolic acid lipid and water in a mixing kettle and heating them to hydrolyze the glycolic acid lipid to obtain a glycolic acid-methanol mixed solution;

[0087] S200, performing a rectification operation on the glycolic acid-methanol mixed solution to obtain methanol water vapor and glycolic acid aqueous solution;

[0088] S300, heating the concentrated glycolic acid aqueous solution using methanol vapor as a heat source to obtain a high-concentration glycolic acid aqueous solution;

[0089] S400, condensing and recovering the water vapor generated during the heating and concentration of the glycolic acid aqueous solution, and reintroducing the water vapor into the mixing kettle;

[0090] S500, performing gradient cooling crystallization and step-by-step centrifugation on the high-concentration glycolic acid aqueous solution to obtain crude glycolic acid crystals, and re-introducing the glycolic acid mother liquor obtained by centrifugation into the mixing kettle after impurity treatment;

[0091] S600, drying crude glycolic acid crystals to obtain fine glycolic acid crystals.

[0092] It should be noted that the above detailed description of the production line is also applicable to the method of the present application. By adopting the above scheme, on the one hand, the heat of methanol water vapor is fully utilized, and the energy consumption is significantly reduced through thermal coupling. On the other hand, the hydrolysis efficiency is improved by applying the ethanol acid mother liquor, the selectivity of the reaction is improved, and the polymerization loss of ethanol acid during the reaction is avoided, the energy consumption of the device is reduced, the single-pass yield of the product production capacity is increased, the fluidity is good and it can be stirred, and more high-quality ethanol acid products are obtained. By treating the impurities of the ethanol acid mother liquor, the purity of the ethanol acid product is effectively improved.

[0093] It is obvious to those skilled in the art that the present application is not limited to the details of the exemplary embodiments described above, and that the present application can be implemented in other specific forms without departing from the spirit or essential features of the present application. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present application. Any reference numeral in a claim should not be regarded as limiting the claim to which it relates.

[0094] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A production line for preparing glycolic acid by hydrolysis of glycolic acid lipids, characterized in that: include: A mixing kettle, comprising a raw material inlet for injecting water and methyl acetate and a mixed solution outlet, wherein the mixing kettle is used to mix and heat glycolic acid lipid and water in the mixing kettle to hydrolyze the glycolic acid lipid to obtain a glycolic acid methanol mixed solution; A distillation tower, comprising a raw material inlet, a methanol water vapor outlet at the top of the tower and a glycolic acid aqueous solution outlet at the bottom of the tower, wherein the raw material inlet is connected to the mixed solution outlet; A heat exchanger, wherein a hot end inlet is connected to the methanol water vapor outlet, and a hot end outlet is connected to the top of the distillation tower; A concentrator, comprising an inlet connected to the outlet of the glycolic acid aqueous solution, a circulation outlet, a circulation inlet and a water vapor outlet, wherein the circulation outlet is connected to the cold end inlet of the heat exchanger, and the circulation inlet is connected to the cold end outlet of the heat exchanger, and the concentrator is used to heat and concentrate the glycolic acid aqueous solution to obtain a high-concentration glycolic acid aqueous solution; A cooling kettle, comprising an inlet connected to the circulation outlet and a crystallization mixed solution outlet, the cooling kettle being used to perform gradient temperature reduction crystallization on the high-concentration glycolic acid aqueous solution; A centrifuge connected to the crystallization mixed solution outlet, the centrifuge comprising a precipitation outlet and a supernatant outlet, and the centrifuge is used to centrifuge the crystallization mixed solution to obtain crude glycolic acid crystals and glycolic acid mother liquor; A dryer connected to the precipitation outlet, the dryer is used to dry the crude glycolic acid crystals to obtain fine glycolic acid crystals; An impurity treatment tank, the inlet of which is connected to the supernatant outlet, and the outlet of which is connected to the raw material inlet of the mixing kettle. The impurity treatment tank is used to perform impurity treatment on the glycolic acid mother liquor.

2. The production line according to claim 1, characterized in that: Also includes: a condenser, an inlet of which is connected to the water vapor outlet; The condensate storage tank has an inlet connected to the outlet of the condenser, and an outlet connected to the raw material inlet of the mixing kettle.

3. The production line according to claim 1, characterized in that: The operating pressure of the distillation tower is 90-150 KPa, and the operating temperature of the tower bottom is 80-120°C. The operating pressure of the concentration tower is 10-50 KPa, and the operating temperature is 20-60°C.

4. A method for preparing glycolic acid by hydrolyzing glycolic acid lipids, characterized in that: Using the production line according to any one of claims 1 to 3, the method comprises: Mixing glycolic acid lipid and water in a mixing kettle and heating them to hydrolyze the glycolic acid lipid to obtain a glycolic acid-methanol mixed solution; performing a rectification operation on the glycolic acid-methanol mixed solution to obtain methanol water vapor and glycolic acid aqueous solution; Using the methanol vapor as a heat source to heat and concentrate the glycolic acid aqueous solution to obtain a high-concentration glycolic acid aqueous solution; The high-concentration glycolic acid aqueous solution is subjected to gradient cooling crystallization and step-by-step centrifugation to obtain crude glycolic acid crystals, and the glycolic acid mother liquor obtained by centrifugation is treated for impurities and then reintroduced into the mixing kettle; The crude glycolic acid crystals are dried to obtain fine glycolic acid crystals.

5. The method according to claim 4, characterized in that The impurity treatment includes removing glycolide from the acetic acid alcohol mother liquor, and the glycolide removal method includes: adding alkaline solid to the glycolic acid mother liquor to decompose glycolide into glycolate; Wherein, the alkaline solid includes one or more combinations of sodium hydroxide, calcium hydroxide, magnesium hydroxide, calcium carbonate and sodium carbonate.

6. The method according to claim 4, characterized in that The impurity treatment includes removing oxalic acid from the acetic acid alcohol mother liquor, and the method for removing oxalic acid includes: adding a precipitant to the glycolic acid mother liquor to form oxalate precipitate from the oxalic acid, and then filtering and removing the precipitate; Wherein, the precipitant is one or a combination of calcium carbonate and calcium chloride, and the dosage of the precipitant is 0.8 to 2.0 eq.

7. The method according to claim 4, characterized in that The molar ratio of the glycolic acid lipid to water is 1:(2-8).

8. The method according to claim 4, characterized in that The gradient cooling crystallization specifically includes two cooling crystallizations, the temperature of the first cooling crystallization is 0 to 20°C, and the temperature of the second cooling crystallization is -20 to 0°C.

9. The method according to claim 4, characterized in that Also includes: The water vapor generated during the heating and concentration of the glycolic acid aqueous solution is condensed and recovered, and then reintroduced into the mixing kettle.

10. The method according to claim 4, characterized in that The concentration of the high-concentration glycolic acid aqueous solution is 60-95 wt %.

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