A method and system for recovering glycolic acid

Abstract

The application provides a method and system for recycling glycolic acid, and belongs to the technical field of degradable material recycling.The method adopts an organophosphate derivative catalyst to efficiently hydrolyze waste degradable plastics containing polyglycolic acid, so that high-quality glycolic acid crystals are obtained.The method for recycling glycolic acid from polyglycolic acid provided by the application hydrolyzes under the catalysis of a catalyst, and after hydrolysis, an organic solvent is added, so that the glycolic acid obtained by hydrolysis enters the water phase, and the oligomers and colored impurities generated in the hydrolysis process are dissolved in the organic phase.The method has high hydrolysis selectivity and a wide raw material adaptation range, and finally, high-purity and high-quality glycolic acid crystals are obtained, and the obtained glycolic acid can be directly reused.

Description

Technical Field

[0001] This invention belongs to the field of biodegradable material recycling technology, and in particular relates to a method and system for recycling glycolic acid. Background Technology

[0002] Plastics, as an indispensable key material in modern society, have deeply penetrated diverse fields such as daily necessities, industrial manufacturing, packaging materials, and building decoration. However, plastic waste is difficult to degrade naturally, accumulating in the natural environment over a long period and causing continuous damage to soil, water bodies, and ecosystems. Against this backdrop, biodegradable plastics have received widespread attention as a compliant alternative due to their ability to degrade naturally in the environment. Unlike traditional plastics that require decades or even centuries to degrade, biodegradable plastics, through molecular structure design, can completely decompose within months to years under specific conditions. They ultimately transform into non-toxic substances such as water and carbon dioxide, leaving no persistent residue.

[0003] Polyglycolic acid (PGA), as the simplest aliphatic polyester in terms of structure, possesses excellent biodegradability, good biocompatibility, and outstanding heat resistance and mechanical properties. In recent years, with breakthroughs in the process of preparing PGA from coal-based raw materials and the acceleration of industrialization, its application scenarios are rapidly expanding from the single medical field to civilian fields such as tableware and biodegradable packaging.

[0004] Biodegradable plastics degrade and age over time, leading to a decline in performance and eventually rendering them unusable. Simply discarding them and letting them degrade on their own not only takes a long time but also wastes resources.

[0005] Glycolic acid is an important organic synthesis intermediate used in chemical cleaning, cosmetics, and polymer materials. It can also be used as a monomer raw material to synthesize polyglycolic acid, which has wide applications across various industries.

[0006] Traditional hydrolysis methods for recovering polyester have limitations such as long reaction cycles and low raw material conversion rates (Ind. Eng. Chem. Res. 2024, 63, 1864-1874). Without a catalyst, hydrolysis can only process materials with relatively low molecular weights, presenting significant limitations. For example, patent CN115872850B can only process polyglycolic acid with a number average molecular weight of only a few thousand. In recent years, alcohols and ammonia have been used as solvents for depolymerization and polyester recovery. For instance, patent CN116986981A reacts polyglycolic acid with an ammonia source, but this ammonialysis reaction uses large amounts of concentrated ammonia, raising environmental and safety concerns. Patent US5264617 uses ethylene glycol and polylactic acid for alcoholysis, which requires large amounts of ethylene glycol. Patent US10808096 uses methanol as both a solvent and reactant with polyester, achieving high efficiency and high yield, but it involves large solvent consumption and complex recovery processes.

[0007] In terms of catalysts, early research primarily utilized oxides, halides, or organic acid compounds of metals such as magnesium, aluminum, zinc, tin, and antimony. Tin and antimony catalysts suffer from toxicity issues, while magnesium and aluminum catalysts exhibit lower activity. Strong acid and strong base catalysts reduce the selectivity of the hydrolysis reaction, leading to the formation of more impurities.

[0008] Patent CN121107976A uses a magnesium-based catalyst to alcoholyze a series of polyesters, but it requires high temperatures and has strict requirements for catalyst compatibility. Improper compatibility may lead to a sharp drop in efficiency. At the same time, it does not involve the recovery and recycling of the catalyst.

[0009] Patent CN104327260A uses titanium alkali metal coordination compounds as catalysts for the alcoholysis of polyesters, and the catalyst can be used directly for subsequent polymerization without separation from the depolymerization products. However, it has strict requirements for the pretreatment of raw materials and low tolerance for impurities. Patent CN114031756B uses titanium-based acidic catalysts for the alcoholysis of polyesters in polyols, but it requires precise control of temperature fluctuations (≤2℃) and microwave absorber compatibility, which places high demands on equipment, and microwave heating is also relatively energy-intensive.

[0010] In recent years, emerging bifunctional catalysts, such as ionic liquids (ILs) and eutectic solvents (DESs), have shown the potential to catalyze the depolymerization of polyesters through nucleophilic-electrophilic dual-site activation. For example, in patent CN121045642A, diols are used as depolymerization reagents to carry out alcoholysis under the catalysis of ionic liquids. This reaction has high degradation efficiency, and the products can be directly reused. Other researchers have used ionic liquids to catalyze transesterification for the depolymerization and recovery of polyesters (Angew. Chem. Int. Ed. 2025, 64, e202513723). This method has high catalytic activity and is applicable to various esters, such as carboxylic esters and carbonates. Still other researchers have used a eutectic solvent synthesized from the reaction of choline chloride and zinc chloride to catalyze the reaction of polylactic acid with methanol (Polym. Degrad. Stab., 2024, 219, 110625), which exhibits high activity and high selectivity. However, the use of ionic liquids presents challenges such as high synthesis costs and difficulties in recycling and reuse. Eutectic solvents, on the other hand, require stringent synthesis and storage conditions.

[0011] Patent CN 118812342 A proposes the use of Al2O3-doped SO4. 2- A method for hydrolyzing waste plastics containing polyglycolic acid using TiO2 solid superacid catalysts. While this type of solid acid catalyst offers advantages such as easy separation and non-corrosiveness to equipment, it still faces challenges in practical applications, especially when processing complex industrial waste biodegradable plastics (often containing PLA, PBAT, and various fillers): First, the excessive acidity and uneven distribution of acid sites in the solid superacid may lead to insufficient selective attack on polyglycolic acid bonds, while simultaneously exacerbating the non-selective degradation of other coexisting polyesters, introducing various impurity monomers, and increasing the difficulty of subsequent separation and purification. Second, the limited mass transfer efficiency between the solid catalyst and the liquid-phase reaction system may affect the hydrolysis efficiency. Third, for complex waste systems, existing technologies do not provide a systematic and effective separation strategy for efficiently removing oligomers, pigments, and other degradation byproducts generated in the reaction system to ensure the high purity of the final glycolic acid crystals.

[0012] Therefore, developing a novel catalytic system capable of highly selective and efficient hydrolysis of polyglycolic acid in complex waste biodegradable plastics, and matching it with an efficient separation process to obtain high-purity glycolic acid products, has become an important direction for current technological development. Summary of the Invention

[0013] To address the aforementioned technical problems, this invention proposes a method and system for recovering glycolic acid. This method involves applying an organophosphate ester non-metallic catalyst to the catalytic hydrolysis of polyglycolic acid, while simultaneously introducing a matching organic solvent extraction and separation step to achieve the resource recovery and reuse of polyester plastic waste.

[0014] To achieve the above objectives, the present invention provides the following technical solution: This invention proposes a method for recycling glycolic acid, using waste biodegradable plastic as raw material. The method includes the following steps: (1) The waste biodegradable plastic is crushed to obtain granular material, wherein the waste biodegradable plastic is waste biodegradable plastic containing polyglycolic acid; (2) The granular material, the organic phosphate catalyst and water are mixed and hydrolyzed under heating and stirring conditions to obtain a solid-liquid mixture; (3) The solid-liquid mixture obtained after the hydrolysis reaction in step (2) is subjected to solid-liquid separation. The filtrate is mixed with an organic solvent and separated to obtain an aqueous phase rich in glycolic acid. (4) The aqueous phase is post-treated to obtain glycolic acid crystals.

[0015] This invention addresses the complex composition of waste biodegradable plastics (containing polyglycolic acid and other polyesters and fillers), designing a synergistic recovery pathway of specific catalytic hydrolysis and selective extraction separation. Its technical principle is not a single-step improvement, but rather an organic combination of catalyst molecular design, reaction process control, and separation and purification strategies, achieving efficient and highly selective conversion from complex raw materials to high-purity monomers. In step (2), an organophosphate derivative is used as a catalyst, and the catalyst molecule can ionize hydrogen ions (H+). +These hydrogen ions preferentially coordinate with or form hydrogen bonds with the carbonyl (C=O) oxygen atoms on the polyglycolic acid polymer chain, significantly enhancing the positive charge of the carbonyl carbon atom. This process reduces the electron cloud density of the carbon atoms in the ester bond, making them more susceptible to nucleophilic attacks. At the same time, the phosphoryl oxygen atom (P=O) in the catalyst molecule has lone pairs of electrons and strong electronegativity, enabling it to form a stable hydrogen bond network with the water (H2O) molecules in the reaction medium. This interaction not only anchors water molecules near the catalytic active center, but more importantly, it further enhances the nucleophilicity of the oxygen atoms in the water molecules through the polarization of hydrogen bonds, making them more likely to attack the activated carbonyl carbon. After hydrolysis, step (3) introduces an organic solvent extraction step. The system mainly contains the target product glycolic acid (strongly hydrophilic), incompletely hydrolyzed oligomers (with a certain degree of hydrophobicity), as well as the catalyst molecule itself and colored impurities that may come from other plastic components. After adding the organic solvent, the system forms a water-organic two-phase system. Because glycolic acid contains hydroxyl and carboxyl groups, it is highly polar and almost entirely distributed in the aqueous phase. Hydrophobic oligomers, most organic impurities, and the organophosphate catalyst molecules that perform the catalytic function tend to dissolve in the organic phase. A simple liquid-liquid separation process can achieve preliminary purification of the product and removal / recovery of the catalyst. This step directly separates most impurities and catalyst from the glycolic acid aqueous solution, laying a solid foundation for obtaining high-purity crystals and avoiding the impact of catalyst residue on the purity of the final product.

[0016] Furthermore, the waste biodegradable plastics used as raw materials contain polyglycolic acid, and also include at least one of polylactic acid (PLA), polyhydroxyalkanoate (PHA), poly(3-hydroxybutyrate) (PHB), polycaprolactone (PCL), polybutylene succinate (PBS), and polybutylene terephthalate (PBAT), as well as at least one filler selected from starch, bamboo powder, talc, montmorillonite, and calcium carbonate. The sources of the waste biodegradable plastics used as raw materials include, but are not limited to, waste generated during the production of polyglycolic acid, degraded glycolide, waste from commercial use, and waste reaching the end of its service life.

[0017] Further, in step (2), the organophosphate catalyst is selected from at least one of monomethyl phosphate, dimethyl phosphate, trimethyl phosphate, monophenyl phosphate, diphenyl phosphate, triphenyl phosphate, dithiophosphate, and trithiophosphate.

[0018] Furthermore, in step (1), the crushing is carried out at -10℃ to 20℃, and the particle size of the resulting granular material is 0.1mm to 1mm.

[0019] Further, in step (2), the mass ratio of the particulate material to water is 1:(1~2.5), and the amount of the organic phosphate catalyst added is 20~25% of the mass of the particulate material.

[0020] Furthermore, in step (2), the temperature of the hydrolysis reaction is 70℃~160℃, the stirring speed is 10r / min~250r / min, and the reaction time is 0.5h~8h.

[0021] Further, in step (3), the organic solvent is selected from at least one of dichloromethane, chloroform, toluene, n-butanol, and ethyl acetate.

[0022] Further, in step (4), the post-processing includes the sequential steps of concentration, crystallization, solid-liquid separation, washing and drying. The concentration is evaporative concentration, and the water content in the aqueous phase after concentration is ≤50%. The crystallization is cooling crystallization.

[0023] Furthermore, the solution is evaporated and concentrated at a temperature of 50°C to 100°C until crystals precipitate. Since the hydrolysate has a high water content and glycolic acid has high solubility in water, directly crystallizing without concentration would result in a low yield of glycolic acid. Therefore, preferably, the aqueous phase needs to be concentrated before the crystallization operation.

[0024] Furthermore, the cooling crystallization process is as follows: first, the temperature is lowered to 0~10℃ and kept at a constant temperature for 1~2 hours, then the temperature is lowered to -20~-5℃ and kept at a constant temperature for 0.5~2 hours.

[0025] Furthermore, the washing process involves washing 3-7 times with a detergent at a temperature ≤5°C. Since the main impurities in the generated glycolic acid are water and catalyst, the solvent used at this temperature should have low solubility for glycolic acid but still effectively dissolve the impurities, thus improving the yield. Therefore, the detergent is further selected from at least one of alcohols, esters, halogenated hydrocarbons, and aromatic hydrocarbons.

[0026] Furthermore, the drying conditions are: an absolute pressure of 2~20 kPa, a temperature of 50~70℃, and a drying time of 0.5~24 h.

[0027] The present invention also provides a system for implementing the above-described method for recovering glycolic acid, comprising: Crusher, used for crushing waste biodegradable plastics; Hydrolysis reactor, used for catalytic hydrolysis of crushed materials; A primary filter is used to separate the solid-liquid mixture after hydrolysis; An extraction and separation device is used to mix and separate the filtrate from the primary filter with an organic solvent to obtain an aqueous phase rich in glycolic acid. The post-processing unit is used to process the aqueous phase from the extraction and separation device to obtain glycolic acid crystals.

[0028] Furthermore, the post-processing unit includes at least a secondary heater, a condenser, a secondary filter, a washing tank, and a dryer connected in sequence; the extraction and separation device is also provided with an organic solvent recovery branch, which includes a primary heater for recovering organic solvents and returning them to the extraction and separation device or the hydrolysis vessel for reuse.

[0029] Compared with the prior art, the present invention has the following advantages and technical effects: (1) The method for recovering glycolic acid from waste biodegradable plastics involved in this invention uses an organophosphate catalyst to selectively hydrolyze waste biodegradable plastics containing polyglycolic acid, which can be converted into glycolic acid hydrolysate to achieve high-value recycling.

[0030] (2) The method for recovering glycolic acid from waste biodegradable plastics involved in this invention can process polyglycolic acid with a weight average molecular weight of up to 100,000 to 200,000, and the obtained glycolic acid crystals have a purity of more than 99.5 wt%.

[0031] (3) The method for recovering glycolic acid from waste biodegradable plastics involved in this invention has low requirements for raw materials. It can use waste biodegradable materials from various industries, and the raw material sources are wide-ranging, with broad application prospects.

[0032] (4) The system for recovering glycolic acid from waste biodegradable plastics involved in this invention can produce glycolic acid from waste biodegradable plastics, realize the recycling and reuse of waste carbon resources, and has high economic value. Attached Figure Description

[0033] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The diagram shows the equipment flow chart for recovering glycolic acid according to the present invention, wherein 1-crusher, 2-hydrolysis kettle, 3-primary filter, 4-extraction and separation device, 5-primary heater, 6-secondary heater, 7-condenser, 8-secondary filter, 9-washing tank, and 10-dryer. Figure 2 This is a liquid chromatogram of the glycolic acid crystal product obtained from the method of recovering glycolic acid crystals from waste biodegradable plastics in Example 1; Figure 3This is a gel permeation chromatogram of polyglycolic acid in waste biodegradable plastics from Example 1; Figure 4 This is a liquid chromatogram of the glycolic acid crystal product obtained from the method of recovering glycolic acid crystals from waste biodegradable plastics in Example 2; Figure 5 This is a gel permeation chromatogram of polyglycolic acid in waste biodegradable plastics in Example 2. Detailed Implementation

[0034] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0035] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0036] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0037] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0038] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0039] This invention provides a method for recycling glycolic acid, using waste biodegradable plastic as raw material. The method includes the following steps: (1) The waste biodegradable plastic is crushed to obtain granular material. The waste biodegradable plastic used is waste biodegradable plastic containing polyglycolic acid. (2) Mix the granular material, the organic phosphate catalyst and water, and carry out the hydrolysis reaction under heating and stirring conditions to obtain a solid-liquid mixture; (3) The solid-liquid mixture obtained after the hydrolysis reaction in step (2) is subjected to solid-liquid separation. The filtrate is mixed with an organic solvent and separated to obtain an aqueous phase rich in glycolic acid. (4) The aqueous phase is post-treated to obtain glycolic acid crystals.

[0040] In a preferred embodiment of the present invention, the waste biodegradable plastic used as raw material contains polyglycolic acid, and also contains at least one of polylactic acid (PLA), polyhydroxyalkanoate (PHA), poly(3-hydroxybutyrate) (PHB), polycaprolactone (PCL), polybutylene succinate (PBS), and polybutylene terephthalate (PBAT), as well as at least one filler selected from starch, bamboo powder, talc, montmorillonite, and calcium carbonate. The sources of the waste biodegradable plastic used as raw material include, but are not limited to, waste generated during the production of polyglycolic acid, deteriorated glycolide, waste from commercial use, and waste reaching the end of its service life.

[0041] In step (2) of the preferred embodiment of the present invention, the organophosphate catalyst is selected from at least one of monomethyl phosphate, dimethyl phosphate, trimethyl phosphate, monophenyl phosphate, diphenyl phosphate, triphenyl phosphate, dithiophosphate, and trithiophosphate.

[0042] In step (1) of the preferred embodiment of the present invention, the crushing is carried out at -10℃ to 20℃, and the particle size of the resulting granular material is 0.1mm to 1mm.

[0043] In step (2) of the preferred embodiment of the present invention, the mass ratio of particulate material to water is 1:(1~2.5), and the amount of organic phosphate catalyst added is 20%~25% of the mass of particulate material.

[0044] In step (2) of the preferred embodiment of the present invention, the temperature of the hydrolysis reaction is 70℃~160℃, the stirring speed is 10r / min~250r / min, and the reaction time is 0.5h~8h.

[0045] In step (3) of the preferred embodiment of the present invention, the organic solvent is selected from at least one of dichloromethane, chloroform, toluene, n-butanol, and ethyl acetate.

[0046] In step (4) of the preferred embodiment of the present invention, the post-processing includes the sequential steps of concentration, crystallization, solid-liquid separation, washing, and drying. The concentration is evaporative concentration, and the water content in the aqueous phase after concentration is ≤50% by weight. The crystallization is cooling crystallization, wherein: The aqueous phase is evaporated and concentrated at a temperature of 50℃-100℃ until crystals precipitate. Because the hydrolysate has a high water content and glycolic acid has high solubility in water, direct crystallization without concentration would result in a low glycolic acid yield. Therefore, it is preferable to concentrate the aqueous phase before crystallization.

[0047] The cooling crystallization process is as follows: first, lower the temperature to 0~10℃ and keep it at a constant temperature for 1~2 hours, then lower the temperature to -20~-5℃ and keep it at a constant temperature for 0.5~2 hours.

[0048] The washing process involves using a detergent with a temperature ≤5℃ and washing 3-7 times. Since the main impurities in the generated glycolic acid are water and catalyst, the solvent used must have low solubility for glycolic acid at this temperature, yet effectively dissolve the impurities to improve the yield. Therefore, the detergent is further selected from at least one of alcohols, esters, halogenated hydrocarbons, and aromatic hydrocarbons.

[0049] The drying conditions are: absolute pressure 2~20 kPa, temperature 50~70℃, drying time 0.5~24 h.

[0050] The method for recovering glycolic acid according to the present invention is carried out in a system for recovering glycolic acid, and the equipment flow diagram is shown below. Figure 1 The system comprises, in sequence: 1-crusher, 2-hydrolysis kettle, 3-primary filter, 4-extraction and separation device, 5-primary heater, 6-secondary heater, 7-condenser, 8-secondary filter, 9-washing tank, and 10-dryer. Crusher 1 is used for crushing waste biodegradable plastics; Hydrolysis reactor 2 is used for catalytic hydrolysis of crushed materials; Primary filter 3 is used to separate the solid-liquid mixture after hydrolysis; Extraction and separation device 4 is used to mix the filtrate from primary filter 3 with an organic solvent and separate the phases to obtain an aqueous phase rich in glycolic acid; The post-processing unit is used to process the aqueous phase from the extraction and separation device 4 to obtain glycolic acid crystals.

[0051] The post-processing unit includes at least a secondary heater 6, a condenser 7, a secondary filter 8, a washing tank 9, and a dryer 10 connected in sequence; the extraction and separation device 4 is also provided with an organic solvent recovery branch, which includes a primary heater 5 for recovering organic solvents and returning them to the extraction and separation device 4 or the hydrolysis vessel 2 for reuse.

[0052] More specifically, an embodiment of the present invention provides a method for recovering glycolic acid, which is achieved through the following steps and corresponding system devices: (1) Raw material crushing: Waste biodegradable plastic is fed into crusher 1 and crushed at a temperature of -10℃ to 20℃ to obtain granular material with a particle size of 0.1mm-1mm; (2) Catalytic hydrolysis reaction: The granular material obtained in step (1) is transported to hydrolysis reactor 2 and mixed with water and organophosphate catalyst. The mass ratio of granular material to water is 1:(1~2.5). The amount of organophosphate catalyst (selected from at least one of monomethyl phosphate, dimethyl phosphate, trimethyl phosphate, monophenyl phosphate, diphenyl phosphate, triphenyl phosphate, dithiophosphate, and trithiophosphate) added is 20%-25% of the mass of granular material. The hydrolysis reaction is carried out in hydrolysis reactor 2 at a temperature of 70℃-160℃ and a stirring speed of 10-200r / min for 0.5h-8h to obtain a solid-liquid mixture. During this process, the organophosphate catalyst efficiently and selectively catalyzes the hydrolysis of polyglycolic acid to glycolic acid through its unique mechanism. (3) Preliminary solid-liquid separation and extraction phase separation: The solid-liquid mixture after the reaction in the hydrolysis vessel 2 is pumped into the primary filter 3 for solid-liquid separation to remove insoluble residues. The obtained filtrate is collected and introduced into the extraction phase separation device 4. In the extraction phase separation device 4, an organic solvent (selected from at least one of dichloromethane, chloroform, toluene, n-butanol, and ethyl acetate) is added to the filtrate. After thorough mixing, the mixture is allowed to stand for phase separation. Glycolic acid is enriched in the aqueous phase due to its strong hydrophilicity, while catalyst molecules, oligomers produced by hydrolysis, and some colored impurities are distributed to the organic phase, thereby achieving preliminary purification and effective separation of the catalyst. The separated organic phase can be fed into the primary heater 5 for solvent recovery. The recovered organic solvent can be returned to the extraction phase separation device 4 or the hydrolysis vessel 2 for recycling.

[0053] (4) Aqueous phase post-treatment and crystal acquisition: a. Evaporation and concentration: The aqueous phase rich in glycolic acid separated from the extraction and separation device 4 is sent to the secondary heater 6 for evaporation and concentration. The concentration is carried out at a temperature of 50℃-100℃ until the weight content of water in the aqueous phase drops below 50% and crystals begin to precipitate. b. Cooling and crystallization: The concentrated liquid is transferred to condenser 7 for cooling and crystallization. The specific process is as follows: first, the temperature is lowered to 0-10℃ and kept at a constant temperature for 1-2 hours; then, the temperature is further lowered to -20~-5℃ and kept at a constant temperature for 0.5-2 hours to allow glycolic acid crystals to fully separate out.

[0054] c. Crystal separation and washing: The crystal-containing slurry is pumped into a secondary filter 8 for solid-liquid separation, and wet crystals are collected. The wet crystals are transferred to a washing tank 9 and washed 3-7 times with a detergent (selected from at least one of alcohols, esters, halogenated hydrocarbons, and aromatic hydrocarbons) at a temperature not exceeding 5°C to remove small amounts of water and impurities adsorbed on the crystal surface.

[0055] d. Drying: The washed wet crystals are sent to dryer 10 and dried for 0.5-24 hours under absolute pressure of 2-20 kPa and temperature of 50-70℃ to finally obtain high-purity glycolic acid crystal products.

[0056] In the embodiments of the present invention, the methods for recovering glycolic acid are all carried out in the corresponding systems according to the above steps.

[0057] All raw materials used in the embodiments of the present invention were purchased commercially, and the waste biodegradable plastics were biodegradable plastics that had exceeded their shelf life.

[0058] In this invention, the purity of the obtained glycolic acid product was determined by liquid chromatography. Specific test conditions were as follows: the instrument was a Shimadzu LC-2010CHT, and a C18 column was used. The chromatographic conditions were set as follows: column temperature 40℃; mobile phase consisting of a 0.01% (w / w) aqueous solution of phosphoric acid and acetonitrile at a volume ratio of 7:3; flow rate 0.8 mL / min; detection wavelength 210 nm; injection volume 20 μL.

[0059] The technical solution of the present invention will be further illustrated by the following embodiments.

[0060] Example 1 A method for recycling glycolic acid, using waste biodegradable plastics that have exceeded their shelf life, specifically comprising polyglycolic acid, polybutylene terephthalate (PET), kaolin, starch, and bamboo powder, the method includes the following steps: (1) Raw material crushing: Waste biodegradable plastic is fed into crusher 1 and crushed at 10°C to obtain granular material with a particle size of 0.1mm-1mm; (2) Catalytic hydrolysis reaction: 400g of the granular material obtained in step (1) is transported to the hydrolysis reactor and mixed with 400g of water, 40g of monomethyl phosphate, and 40g of monophenyl phosphate (the amount of organic phosphate catalyst added is 20% of the mass of the granular material). The hydrolysis reaction is carried out in the hydrolysis reactor 2 at a temperature of 90℃ and a stirring speed of 250r / min for 5h to obtain a solid-liquid mixture. (3) Preliminary solid-liquid separation and extraction phase separation: The solid-liquid mixture after the reaction in the hydrolysis kettle 2 is pumped into the first-stage filter 3 for solid-liquid separation to remove insoluble residues. The obtained filtrate is collected and introduced into the extraction and separation device 4. In the extraction and separation device 4, 50g of organic solvent (dichloromethane) is added to the filtrate. After thorough mixing, the mixture is allowed to stand for phase separation. The separated organic phase can be fed into the first-stage heater 5 for solvent recovery. The recovered organic solvent can be returned to the extraction and separation device 4 or the hydrolysis kettle 2 for recycling. (4) Aqueous phase post-treatment and crystal acquisition: a. Evaporation and concentration: The aqueous phase rich in glycolic acid separated from the extraction and separation device 4 is sent to the secondary heater 6 for evaporation and concentration. The evaporation and concentration is carried out at 70°C and 10 kPa absolute pressure for 2 hours until the weight content of water in the aqueous phase drops below 50% and crystals begin to precipitate. b. Cooling and crystallization: The concentrated liquid is transferred to condenser 7 for cooling and crystallization. The specific process is as follows: the temperature is first lowered to 5°C under normal pressure and kept at a constant temperature for 1 hour; then the temperature is further lowered to -10°C and kept at a constant temperature for 2 hours to allow glycolic acid crystals to fully separate out.

[0061] c. Crystal separation and washing: The crystal-containing slurry is pumped into a secondary filter 8 for solid-liquid separation, and wet crystals are collected. The wet crystals are transferred to a washing tank 9 and washed three times with a detergent (ethanol) at -20°C to remove small amounts of water and impurities adsorbed on the crystal surface.

[0062] d. Drying: The washed wet crystals are sent to dryer 10 and dried for 5 hours under an absolute pressure of 10 kPa and a temperature of 50°C to finally obtain high-purity glycolic acid crystal products.

[0063] The liquid chromatogram of the glycolic acid crystal product obtained from the method of recycling glycolic acid crystals from waste biodegradable plastics in Example 1 is shown below. Figure 2 Tests showed that the obtained glycolic acid crystals contained 1928 mg / kg of terephthalic acid, 316 mg / kg of adipic acid, and 132 mg / kg of 1,4-butanediol, with a glycolic acid purity of 99.763%.

[0064] Example 1: Gel transmission chromatogram of polyglycolic acid in waste biodegradable plastics (see Figure 1) Figure 3 The specific GPC test results are shown in Table 1.

[0065] Table 1 GPC Test Results

[0066] Combination Figure 3 As can be seen from Table 1, the weight-average molecular weight of glycolic acid, the product prepared in Example 1, is 150,478 Daltons.

[0067] Example 2 The waste biodegradable plastics used are biodegradable plastics that have exceeded their shelf life, specifically composed of polyglycolic acid, polylactic acid, montmorillonite, and calcium sulfate. A method for recovering glycolic acid from waste biodegradable plastics is the same as in Example 1, except that the following parameter settings are different: 400g of the granular material obtained in step (1) is transported to the hydrolysis reactor and mixed with 600g of water, 30g of dimethyl phosphate and 70g of diphenyl phosphate (the amount of organic phosphate catalyst added is 25% of the mass of the granular material). The hydrolysis reaction was carried out at 110℃ and 200r / min for 4 hours. Add 60g of organic solvent (dichloromethane) to the filtrate; Evaporation and concentration were carried out at 60℃ and 5 kPa absolute pressure for 1.5 h. The specific process of cooling crystallization is as follows: first, the temperature is lowered to 5°C under normal pressure and kept at a constant temperature for 0.5 hours; then, the temperature is further lowered to -10°C and kept at a constant temperature for 1.5 hours. Dry for 3 hours under an absolute pressure of 7 kPa and a temperature of 60°C.

[0068] The liquid chromatogram of the glycolic acid crystal product obtained from the method of recycling glycolic acid crystals from waste biodegradable plastics in Example 2 is shown below. Figure 4 Tests showed that the obtained glycolic acid crystals contained 1275 mg / kg of lactic acid, and the glycolic acid purity was 99.581%.

[0069] Example 2: Gel transmission chromatogram of polyglycolic acid in waste biodegradable plastics (see Figure 2) Figure 5 The specific GPC test results are shown in Table 2.

[0070] Table 2 GPC Test Results

[0071] Combination Figure 5 As can be seen from Table 2, the weight-average molecular weight of glycolic acid, the product prepared in Example 2, is 163,534 Daltons.

[0072] Example 3 The waste biodegradable plastics used are biodegradable plastics that have exceeded their shelf life, specifically composed of polyglycolic acid, polybutylene succinate, talc, and calcium carbonate. A method for recovering glycolic acid from waste biodegradable plastics is the same as in Example 1, except that the following parameter settings are different: 400g of the granular material obtained in step (1) is transported to the hydrolysis reactor and mixed with 1000g of water, 20g of trimethyl phosphate, and 80g of triphenyl phosphate (the amount of organic phosphate catalyst added is 25% of the mass of the granular material). The hydrolysis reaction was carried out at 130℃ and 200r / min for 3 hours. Add 90g of organic solvent (ethyl acetate) to the filtrate; Evaporate and concentrate at 80℃ and 7kPa absolute pressure for 1 hour; The specific process of cooling crystallization is as follows: first, the temperature is lowered to 5°C under normal pressure and kept at a constant temperature for 1.5 hours; then, the temperature is further lowered to -15°C and kept at a constant temperature for 0.5 hours. Dry for 1 hour under an absolute pressure of 15 kPa and a temperature of 90°C.

[0073] Tests showed that the obtained glycolic acid crystals contained 2034 mg / kg succinic acid and 1853 mg / kg butanediol, with a purity of 99.560%.

[0074] Tests show that the weight-average molecular weight of polyglycolic acid is 146,000 Daltons.

[0075] Example 4 The waste biodegradable plastics used are biodegradable plastics that have exceeded their shelf life, specifically composed of polyglycolic acid, polycaprolactone, and wollastonite. A method for recovering glycolic acid from waste biodegradable plastics is the same as in Example 1, except that the following parameter settings are different: 500g of the granular material obtained in step (1) is transported to the hydrolysis reactor and mixed with 1200g of water, 25g of monomethyl phosphate, and 95g of triphenyl phosphate (the amount of organic phosphate catalyst added is 24% of the mass of the granular material). The hydrolysis reaction was carried out at 150℃ and 150r / min for 2 hours. Add 120g of organic solvent (toluene) to the filtrate; Evaporate and concentrate at 80℃ and 7kPa absolute pressure for 1 hour; The specific process of cooling crystallization is as follows: first, the temperature is lowered to 5°C under normal pressure and kept at a constant temperature for 2 hours; then, the temperature is further lowered to -20°C and kept at a constant temperature for 0.5 hours. Dry for 4 hours under an absolute pressure of 10 kPa and a temperature of 70°C.

[0076] Tests showed that the obtained glycolic acid crystals contained 623 mg / kg of 6-hydroxyhexanoic acid and 1520 mg / kg of butanediol, with a purity of 99.510%.

[0077] Tests show that the weight-average molecular weight of polyglycolic acid is 140,450 Daltons.

[0078] Comparative Example 1 Same as Example 1, except that step (3) of preliminary solid-liquid separation and extraction phase separation is omitted, that is, the solid-liquid mixture that has been reacted in the hydrolysis vessel 2 is directly subjected to aqueous post-treatment and crystal acquisition steps.

[0079] Tests showed that the obtained glycolic acid crystals contained 1827 mg / kg terephthalic acid, 298 mg / kg adipic acid, and 146 mg / kg 1,4-butanediol, with a purity of 81.671%. Compared to Example 1, the purity of this comparative example decreased significantly. This is because no organic solvent was used to extract the catalyst from the reaction solution in this comparative example, leaving a significant amount of catalyst in the reaction solution. Consequently, the crystals contained more catalyst, resulting in a decrease in purity.

[0080] Comparative Example 2 Same as Example 1, except that 40g of monomethyl phosphate and 40g of monophenyl phosphate catalyst are replaced with 80g of Al2O3-SO4. 2- / TiO2 solid superacid catalyst, Al2O3-SO4 2- The preparation method of the TiO2 solid superacid catalyst is as follows: Ti compound and Al(NO3)3·9H2O were dissolved in water, and concentrated ammonia was slowly added dropwise under vigorous stirring until the pH of the solution reached 10. The solution was then aged at room temperature for 36 hours to obtain a mixed hydroxide precipitate. The precipitate was eluted with a 4% (w / w) ammonium acetate solution, centrifuged, washed, filtered, and dried at 373 K for 24 hours. It was then soaked in 2 mol / L H2SO4 for 1 hour (liquid-to-solid ratio 15 mL / g). After further filtration and drying at 373 K for 24 hours, the solution was calcined at 873 K in static air for 2 hours to obtain Al2O3-SO4. 2- / TiO2 solid superacid catalyst.

[0081] Tests showed that the obtained glycolic acid crystals contained 101 g / kg terephthalic acid, 42 g / kg adipic acid, and 67 g / kg 1,4-butanediol, with a purity of 77.231%. Compared to Example 1, the purity of this comparative example decreased significantly, possibly due to the Al2O3-SO4... 2- The TiO2 solid superacid catalyst is too acidic. Although it accelerates the hydrolysis rate of polyglycolic acid, the excessive acidity also causes other polyesters to degrade rapidly, resulting in an increase in the content of degradation products of the remaining polyesters. Therefore, the purity of the final glycolic acid decreases.

[0082] Comparative Example 3 Same as Example 1, except that in step (4), the mixture is evaporated and concentrated at 90°C and 10 kPa absolute pressure for 2 hours; The specific process of cooling crystallization is as follows: first, the temperature is lowered to 5°C under normal pressure and kept at a constant temperature for 1 hour; then, the temperature is further lowered to -10°C and kept at a constant temperature for 2 hours. Dry for 2.5 hours under an absolute pressure of 10 kPa and a temperature of 90°C.

[0083] Tests showed that the obtained glycolic acid crystals contained 3124 mg / kg of terephthalic acid, 458 mg / kg of adipic acid, and 211 mg / kg of 1,4-butanediol, with a purity of 87.212%. Compared with Example 1, the purity of this comparative example was significantly reduced. This is because the excessively high temperature in the final drying step caused the glycolic acid monomers to polymerize, forming oligomers such as dimers and trimers. Therefore, although the degradation of the remaining polyesters was relatively low, the purity of the crystals still decreased.

[0084] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for recovering glycolic acid, characterized in that, The raw material is waste biodegradable plastic, and the method includes the following steps: (1) The waste biodegradable plastic is crushed to obtain granular material, wherein the waste biodegradable plastic is waste biodegradable plastic containing polyglycolic acid; (2) The granular material, the organic phosphate catalyst and water are mixed and hydrolyzed under heating and stirring conditions to obtain a solid-liquid mixture; (3) Separate the solid-liquid mixture obtained after the hydrolysis reaction in step (2), mix the obtained filtrate with an organic solvent, and separate the liquid to obtain an aqueous phase rich in glycolic acid; (4) The aqueous phase is post-treated to obtain glycolic acid crystals.

2. The method for recovering glycolic acid according to claim 1, characterized in that, In step (2), the organophosphate catalyst is selected from at least one of monomethyl phosphate, dimethyl phosphate, trimethyl phosphate, monophenyl phosphate, diphenyl phosphate, triphenyl phosphate, dithiophosphate, and trithiophosphate.

3. The method for recovering glycolic acid according to claim 1, characterized in that, In step (1), the crushing is carried out at -10℃ to 20℃, and the particle size of the resulting granular material is 0.1mm to 1mm.

4. The method for recovering glycolic acid according to claim 1, characterized in that, In step (2), the mass ratio of the granular material to water is 1:(1~2.5), and the amount of the organic phosphate catalyst added is 20%~25% of the mass of the granular material.

5. The method for recovering glycolic acid according to claim 1, characterized in that, In step (2), the temperature of the hydrolysis reaction is 70℃~160℃, the stirring speed is 10r / min~250r / min, and the reaction time is 0.5h~8h.

6. The method for recovering glycolic acid according to claim 1, characterized in that, In step (3), the organic solvent is selected from at least one of dichloromethane, chloroform, toluene, n-butanol and ethyl acetate.

7. The method for recovering glycolic acid according to claim 1, characterized in that, In step (4), the post-processing includes the sequential steps of concentration, crystallization, solid-liquid separation, washing and drying.

8. The method for recovering glycolic acid according to claim 7, characterized in that, The concentration is evaporative concentration, and the water content in the aqueous phase after concentration is ≤50% by weight; the crystallization is cooling crystallization; the drying is carried out at an absolute pressure of 2~20kPa and a temperature of 50~70℃ for 0.5~24h.

9. A system for implementing the method for recovering glycolic acid as described in any one of claims 1 to 8, characterized in that, Including those connected sequentially: Crusher (1), used for crushing waste biodegradable plastics; Hydrolysis reactor (2) is used to carry out catalytic hydrolysis reaction on crushed materials; A primary filter (3) is used to separate the solid-liquid mixture after hydrolysis; Extraction and separation device (4) is used to mix the filtrate from the primary filter (3) with an organic solvent and separate the phases to obtain an aqueous phase rich in glycolic acid; The post-processing unit is used to process the aqueous phase from the extraction and separation device (4) to obtain glycolic acid crystals.

10. The system according to claim 9, characterized in that, The post-processing unit includes at least a secondary heater (6), a condenser (7), a secondary filter (8), a washing tank (9), and a dryer (10) connected in sequence; the extraction and separation device (4) is also provided with an organic solvent recovery branch, which includes a primary heater (5) for recovering organic solvents and returning them to the extraction and separation device (4) or the hydrolysis vessel (2) for reuse.

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