Method for preparing glycollic acid solution from polyglycolic acid industrial waste

Abstract

The invention provides a method for preparing a glycolic acid solution by using polyglycolic acid industrial waste, which comprises the following steps: mixing crushed polyglycolic acid waste with water, carrying out hydrothermal hydrolysis to obtain a glycolic acid crude water solution, and then carrying out decoloration and organic impurity adsorption by using microporous activated carbon to obtain the glycolic acid solution. And then carrying out deep purification through cation exchange resin and anion exchange resin in sequence to remove metal ions and inorganic anion impurities so as to obtain the required glycollic acid aqueous solution product. The content of metal ions in the prepared glycollic acid aqueous solution product is less than or equal to 1 ppm, the content of anions is less than or equal to 10 ppm, and the glycollic acid aqueous solution product is not turbid or precipitated after being stored for 30 days at normal temperature, shows good long-term storage stability and can be directly used as a raw material of an industrial cleaning agent; and the technological process is simple, industrial amplification is easy, and good application prospects are achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of recycling of polyester degradable materials, and particularly relates to a method for preparing glycolic acid solution by using industrial waste of polyglycolic acid. BACKGROUND

[0002] Polyglycolic acid (PGA) is a new type of degradable material that has developed rapidly in recent years. It is a kind of polymer with the best mechanical properties and the strongest gas barrier property among aliphatic polyesters. Due to its high crystallinity, biodegradability and excellent biocompatibility, it is widely used in degradable packaging materials, shale oil and gas fracturing proppants, surgical sutures, tissue engineering scaffolds, 3D printing consumables and other fields. With the maturation of the degradable material industry chain, the production capacity of PGA has shown a high growth trend in recent years. At the same time, a large amount of offcuts, failed products, oligomers and intermediate materials containing catalyst residues are generated during the production and processing of PGA. The composition of these materials is complex, the metal content is high, the color is deep, and they are easily degradable, which makes the waste recycling and treatment increasingly difficult.

[0003] In actual industrial systems, the aqueous solution obtained by hydrolysis of PGA waste usually contains glycolic acid, oligomers, organic by-products, and various metal ions and inorganic anion impurities. The types of impurities are complex and interact significantly, which can easily lead to high color and insufficient stability of the solution, making it difficult to be directly used as a functional raw material for chemical or fine applications. Therefore, how to achieve the simultaneous removal of color, organic impurities, metal ions and inorganic anion impurities while ensuring the yield of glycolic acid, and further ensure the stability of the obtained glycolic acid aqueous solution during storage and use, has become a key technical problem that needs to be solved in the recycling process of PGA industrial waste.

[0004] Glycolic acid is an important organic carboxylic acid that is widely used as an acidic functional component in industrial cleaning. It is commonly used for metal surface cleaning, heat exchange equipment descaling, boiler and circulating water system cleaning, and other application scenarios. It removes metal oxides, scale and inorganic deposits through acidic dissolution and complexation, and has good compatibility with various metal substrates. The glycolic acid cleaning agent currently used in industry mainly uses chemical synthesis routes, including chemical oxidation method using formaldehyde as raw material and substitution reaction method using chloroacetic acid as raw material. The above synthesis routes are mature and highly scalable, but in actual production, they are often accompanied by complex reaction by-products, residual metal catalysts and high solution color.

[0005] In cleaning agent applications, glycolic acid aqueous solutions have high requirements for the content of metal ions and inorganic anions. Improper impurity control can easily lead to secondary corrosion of equipment or affect cleaning effectiveness. However, traditional chemical synthesis of glycolic acid is difficult to completely avoid the introduction of metal impurities during the production process. Different batches of products also exhibit fluctuations in impurity composition and color, often requiring additional refining to meet the demands of high-end cleaning applications, resulting in high production costs. In contrast, preparing glycolic acid aqueous solutions from polyglycolic acid waste through hydrolysis theoretically has potential advantages such as renewable raw material sources, a relatively simple reaction path, and low risk of introducing chlorine sources and toxic byproducts. If an industrially feasible refining process can be developed to address the problems of complex impurities, high metal content, and insufficient stability in the hydrolysis system of PGA industrial waste, it is expected to provide a greener, lower-cost, and more stable glycolic acid raw material source for the industrial cleaning agent field.

[0006] However, existing publicly available technologies lack mature process solutions that systematically address the characteristics of PGA industrial waste hydrolysate systems, such as deep decolorization, demetallization, removal of inorganic anions, and long-term storage stability control of glycolic acid aqueous solutions. Therefore, it is not yet possible to directly convert PGA industrial waste into high-purity glycolic acid aqueous solutions that meet the requirements for industrial cleaning agents. Summary of the Invention

[0007] This invention addresses the problem that polyglycolic acid (PGA) industrial waste, during polymerization, extrusion, degradation, and processing, suffers from catalyst residues (such as Sn, Zn, Fe, Ca, etc.), pigment formation, and the coexistence of oligomers and inorganic anionic degradation byproducts. This results in hydrolysates with high metal ion content, deep color, complex impurity systems, and poor stability, making them unsuitable for direct use in high-value-added products. The invention proposes a recovery method that can efficiently convert PGA industrial waste into a high-purity glycolic acid aqueous solution that can be directly used in industrial cleaning agents.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] A method for preparing an aqueous solution of glycolic acid using polyglycolic acid industrial waste includes the following steps:

[0010] (1) After mixing the crushed polyglycolic acid waste with water at a mass ratio of 1: (3~10), the mixture is subjected to hydrothermal hydrolysis under closed conditions. After solid-liquid separation, a crude aqueous solution of glycolic acid is obtained.

[0011] (2) Add activated carbon to the crude aqueous solution of glycolic acid for decolorization treatment;

[0012] (3) The decolorized glycolic acid aqueous solution is passed through cation exchange resin and anion exchange resin in sequence for ion exchange treatment to finally obtain the desired glycolic acid aqueous solution product.

[0013] The present invention is further configured such that, in step (1), the hydrothermal hydrolysis reaction temperature is 90~150 ℃, the reaction time is 2~12 h, and the pressure is 0.1~1.0 MPa.

[0014] The present invention is further configured such that, in step (1), the hydrothermal hydrolysis reaction temperature is 100~130 ℃.

[0015] The present invention is further configured such that, in step (2), the activated carbon is a powdered microporous activated carbon.

[0016] The present invention is further configured such that, in step (2), the pore volume of the powdered microporous activated carbon accounts for 50-70% of the total pore volume, the specific surface area is 800-1600 m² / g, the average pore size is ≤2 nm, and the particle size is ≤50 μm.

[0017] The present invention is further configured such that the average pore size of the powdered microporous activated carbon is 0.5~2.0 nm, preferably 0.8~1.8 nm; and the particle size of the powdered microporous activated carbon is 5~50 μm, preferably 10~40 μm.

[0018] The present invention is further configured such that, in step (2), the temperature of the decolorization treatment is 60~90℃ and the treatment time is 20~60 min.

[0019] The present invention is further configured such that, in step (3), the cation exchange resin is a macroporous strong acid cation exchange resin; and the anion exchange resin is a macroporous weak base anion exchange resin.

[0020] The present invention is further configured such that, in step (1), a solid heterogeneous acid catalyst is added during the hydrolysis reaction to improve the hydrolysis reaction rate; the solid heterogeneous acid catalyst includes sulfonic acid type ion exchange resin and supported metal oxide catalyst.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The technical solution of the present invention achieves efficient conversion of high-impurity PGA waste into high-purity glycolic acid aqueous solution by hydrothermal hydrolysis, decolorization and ion exchange, and by designing the conditions of each step, and the glycolic acid loss rate is small in the treatment steps.

[0023] (2) The technical solution of the present invention can directly obtain an aqueous solution of glycolic acid that can be used as an industrial cleaning agent. The resulting aqueous solution of glycolic acid has low metal content, low anion content, low color and storage stability, and can be directly used for cleaning, complexing and descaling applications.

[0024] (3) The technical solution of the present invention is simple to operate and easy to scale up industrially, realizing the large-scale treatment of PGA waste. It has significant practical advantages, and the process equipment is highly versatile and the operating conditions are mild, which has good prospects for industrial application. Detailed Implementation

[0025] The present invention will now be described in detail with reference to specific embodiments. It should be understood that the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of the present invention.

[0026] All ppm values ​​mentioned below are by mass fractions and are applicable to aqueous solutions (density approximately 1 g / mL). The activated carbon, cation exchange resin, and anion exchange resin used in this invention were all commercially available.

[0027] Example 1

[0028] A method for preparing glycolic acid solution using polyglycolic acid industrial waste includes the following steps:

[0029] (1) Take 500 g of polyglycolic acid (PGA) scrap produced in the polymerization extrusion section, crush it to 1-3 mm, mix it with water at a mass ratio of 1:5, and add it to a hydrothermal reactor. React at 120 ℃ for 6 h. The system is under autogenous pressure, and the reaction pressure is usually in the range of 0.4-0.6 MPa. After the reaction, perform solid-liquid separation to obtain a crude glycolic acid aqueous solution. The content of glycolic acid in the crude glycolic acid aqueous solution is about 17 wt%, the content of metal ions is about 198 ppm, and the content of inorganic anions is about 55 ppm.

[0030] (2) Add 0.5wt% of powdered microporous activated carbon to the crude aqueous solution of glycolic acid, stir and decolorize at 75 °C for 30 min, and then filter to obtain the decolorized aqueous solution of glycolic acid.

[0031] (3) The decolorized glycolic acid aqueous solution is sequentially passed through a macroporous strong acid cation exchange resin and a macroporous weak base anion exchange resin to obtain the final glycolic acid aqueous solution product.

[0032] In this embodiment, the pore volume of the powdered microporous activated carbon accounts for 50-70% of the total pore volume, the specific surface area is 800-1600 m² / g, the average pore size is 0.8-1.8 nm, and the particle size is 10-40 μm; the macroporous strong acid cation exchange resin is selected from macroporous strong acid cation exchange resins with styrene-divinylbenzene as the backbone and sulfonic acid groups introduced; the macroporous weak basic anion exchange resin is selected from macroporous anion exchange resins with styrene-divinylbenzene as the backbone and secondary or tertiary amine weak basic groups introduced.

[0033] Testing revealed that the final glycolic acid aqueous solution contained 16.4 wt% glycolic acid, with a glycolic acid loss rate of 0.6%, a metal ion content of 0.12 ppm, and an inorganic anion content of 3.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for metal ion content, inorganic anion content, and storage stability in industrial cleaning agents.

[0034] Example 2

[0035] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that, in step (1), the mass ratio of water to PGA is adjusted to 1:3, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 26.2 wt% glycolic acid, approximately 350 ppm metal ions, and approximately 98 ppm inorganic anions.

[0036] Testing revealed that the final glycolic acid aqueous solution contained 25.6 wt% glycolic acid, with a glycolic acid loss rate of 0.6%, a metal ion content of 0.23 ppm, and an inorganic anion content of 6.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0037] Example 3

[0038] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that, in step (1), the mass ratio of water to PGA is adjusted to 1:6, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 15.2 wt% glycolic acid, approximately 185 ppm metal ions, and approximately 50 ppm inorganic anions.

[0039] Testing revealed that the final glycolic acid aqueous solution contained 14.5 wt% glycolic acid, with a glycolic acid loss rate of 0.7%, a metal ion content of 0.18 ppm, and an inorganic anion content of 5.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0040] Example 4

[0041] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that, in step (1), the mass ratio of water to PGA is adjusted to 1:10, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 10.8 wt% glycolic acid, approximately 96 ppm metal ions, and approximately 26 ppm inorganic anions.

[0042] Testing revealed that the final glycolic acid aqueous solution contained 10.1 wt% glycolic acid, with a glycolic acid loss rate of 0.7%, a metal ion content of 0.15 ppm, and an inorganic anion content of 3.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0043] The processes provided in Examples 1-4 can stably produce aqueous glycolic acid solutions with low impurities, low loss rates, and good storage stability, indicating that the processes have good adaptability to changes in the water ratio.

[0044] Comparative Example 1

[0045] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that, in step (1), the mass ratio of water to PGA is adjusted to 1:2, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution is deep yellow, with high viscosity, a glycolic acid mass fraction of approximately 35.2 wt%, a metal ion content of approximately 380 ppm, and an inorganic anion content of approximately 113 ppm.

[0046] The final glycolic acid aqueous solution product was found to contain 31.6 wt% glycolic acid, with a glycolic acid loss rate of 3.6%, a metal ion content of 2.4 ppm, and an inorganic anion content of 36 ppm. The solution was dark yellow and became turbid after being stored at room temperature for 10 days.

[0047] The comparative example shows that when the mass ratio of water to PGA is less than 1:3, the oligomers and dark-colored byproducts in the hydrolysis system increase significantly, leading to increased decolorization load, decreased subsequent purification stability, and difficulty in obtaining an aqueous glycolic acid solution that meets the requirements of industrial cleaning agents.

[0048] Comparative Example 2

[0049] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that, in step (1), the mass ratio of water to PGA is adjusted to 1:12, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 8.9 wt% glycolic acid, approximately 79 ppm metal ions, and approximately 26 ppm inorganic anions.

[0050] The final glycolic acid aqueous solution product was found to contain 11.3 wt% glycolic acid, with a glycolic acid loss rate of 2.4%, a metal ion content of 0.12 ppm, and an inorganic anion content of 8.0 ppm. The solution was colorless and clear, and no turbidity or precipitation was observed after 30 days of storage at room temperature.

[0051] The comparative example shows that when the mass ratio of water to PGA is higher than 1:10, although it is beneficial to suppress the formation of dark-colored byproducts, the relative loss of glycolic acid during decolorization and ion exchange is significantly increased, the mass fraction of glycolic acid in the obtained product is low, and the energy consumption and processing cost per unit product are significantly increased, which is not conducive to industrial economic operation.

[0052] Example 5

[0053] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis temperature in step (1) is adjusted to 90 °C, while all other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 15.2 wt% glycolic acid, approximately 186 ppm metal ions, and approximately 50 ppm inorganic anions.

[0054] Testing revealed that the final glycolic acid aqueous solution contained 14.2 wt% glycolic acid, with a glycolic acid loss rate of 1.0%, a metal ion content of 0.20 ppm, and an inorganic anion content of 3.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0055] Example 6

[0056] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis temperature in step (1) is adjusted to 150 °C, while all other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 17.6 wt% glycolic acid, approximately 214 ppm metal ions, and approximately 62 ppm inorganic anions.

[0057] Testing revealed that the final glycolic acid aqueous solution contained 16.1 wt% glycolic acid, with a glycolic acid loss rate of 1.5%, a metal ion content of 0.68 ppm, and an inorganic anion content of 9.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0058] Comparative Example 3

[0059] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis temperature in step (1) is adjusted to 80°C, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 12.3 wt% glycolic acid, approximately 146 ppm metal ions, and approximately 41 ppm inorganic anions, and is deep yellow in color.

[0060] Testing revealed that the final glycolic acid aqueous solution contained 11.2 wt% glycolic acid, with a glycolic acid loss rate of 1.1%, a metal ion content of 0.31 ppm, and an inorganic anion content of 7.0 ppm. The solution was yellowish and contained precipitate. This comparative example indicates that hydrolysis is insufficient and oligomer residues are high at temperatures below 90 ℃, making it difficult to stably obtain a long-term clear glycolic acid aqueous solution.

[0061] Comparative Example 4

[0062] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis temperature in step (1) is adjusted to 160 °C, while all other process conditions remain the same. In step (1), the resulting crude glycolic acid aqueous solution contains approximately 16.9 wt% glycolic acid, approximately 200 ppm metal ions, and approximately 57 ppm inorganic anions, and is deep yellow in color.

[0063] The final glycolic acid aqueous solution contained 16.2 wt% glycolic acid, with a glycolic acid loss rate of 0.7%, a metal ion content of 0.72 ppm, and an inorganic anion content of 8.0 ppm. The solution was light yellow and precipitated after 5 days of storage at room temperature.

[0064] The comparative example shows that when the temperature is above 150 °C, excessive degradation leads to an increase in color and byproducts, thereby reducing the synergistic purification effect of decolorization and ion exchange and exacerbating the loss of glycolic acid.

[0065] Example 7

[0066] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis reaction time in step (1) is adjusted to 2 hours, while all other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 14.2 wt% glycolic acid, approximately 164 ppm metal ions, and approximately 46 ppm inorganic anions.

[0067] Testing revealed that the final glycolic acid aqueous solution contained 13.4 wt% glycolic acid, with a glycolic acid loss rate of 0.7%, a metal ion content of 0.22 ppm, and an inorganic anion content of 6.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0068] Example 8

[0069] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps are the same as those in Example 1, except that in step (1), the hydrolytic reaction time is adjusted to 4 hours, while the other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 16.3 wt% glycolic acid, approximately 172 ppm metal ions, and approximately 50 ppm inorganic anions.

[0070] Testing revealed that the final glycolic acid aqueous solution contained 15.6 wt% glycolic acid, with a glycolic acid loss rate of 0.7%, a metal ion content of 0.16 ppm, and an inorganic anion content of 4.0 ppm. The solution was colorless and transparent. No turbidity or precipitation was observed after 30 days of storage at room temperature. The obtained glycolic acid aqueous solution meets the requirements for use as a raw material in industrial cleaning agents.

[0071] Comparative Example 5

[0072] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis reaction time in step (1) is adjusted to 1 hour, while all other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 10.8 wt% glycolic acid, approximately 150 ppm metal ions, and approximately 30 ppm inorganic anions. The obtained crude glycolic acid aqueous solution is light yellow in color.

[0073] The final glycolic acid aqueous solution product was found to contain 8.0 wt% glycolic acid, with a glycolic acid loss rate of 2.0%, a metal ion content of 0.13 ppm, and an inorganic anion content of 3.0 ppm. The solution was light yellow in color. Precipitation appeared in the glycolic acid aqueous solution product after 3 days of storage at room temperature.

[0074] Comparative Example 6

[0075] A method for preparing glycolic acid solution using polyglycolic acid industrial waste is disclosed. The preparation steps differ from those in Example 1 only in that the hydrothermal hydrolysis reaction time in step (1) is adjusted to 13 h, while all other process conditions remain the same. In step (1), the obtained crude glycolic acid aqueous solution contains approximately 17.2 wt% glycolic acid, approximately 202 ppm metal ions, and approximately 58 ppm inorganic anions. The obtained crude glycolic acid aqueous solution is deep yellow.

[0076] The final glycolic acid aqueous solution contained 15.1 wt% glycolic acid, with a glycolic acid loss rate of 2.1%, a metal ion content of 0.16 ppm, and an inorganic anion content of 5.0 ppm. The solution was light yellow in color. Precipitation appeared in the glycolic acid aqueous solution after 7 days of storage at room temperature.

[0077] The comparative example shows that when the reaction time exceeds 6 h, the deep degradation byproducts and color increase, leading to an increase in decolorization load and a greater relative loss of glycolic acid, which is detrimental to the economic efficiency and stability of the process.

[0078] Comparative Example 7

[0079] Compared with Example 1, the difference is that the microporous powdered activated carbon used in step (2) is replaced with mesoporous activated carbon, and the rest of the operation is the same; wherein, the specific surface area of ​​the mesoporous activated carbon is 900-1200 m² / g, the mesoporous pore volume accounts for 40-60% of the total pore volume, the average pore size is 3-10 nm, and the particle size is 10-75 μm.

[0080] When mesoporous activated carbon is used, the glycolic acid aqueous solution still appears light yellow after decolorization; in the final glycolic acid aqueous solution product, the glycolic acid mass fraction is 16.5 wt%, the loss rate is 0.7%, and slight turbidity appears after 15 days of storage at room temperature; the metal ion content decreases from 180 ppm to 0.32 ppm, and the anion content decreases from 49 ppm to 6 ppm.

[0081] Comparative Example 8

[0082] Compared with Example 1, the difference is that the microporous powdered activated carbon used in step (2) is replaced with macroporous activated carbon, and the rest of the operation is the same; wherein, its specific surface area is 600-1000 m² / g, the macroporous pore volume accounts for 30-60% of the total pore volume, the average pore size is 50-200 nm, and the particle size is 10-75 μm.

[0083] When macroporous activated carbon is used, the glycolic acid aqueous solution still appears dark yellow after decolorization; the final glycolic acid aqueous solution product has a glycolic acid mass fraction of 17.0 wt% and a loss rate of 0.8%, and becomes slightly turbid after being stored at room temperature for 5 days; the metal ion content decreases from 192 ppm to 0.2 ppm, and the anion content decreases from 59 ppm to 5 ppm.

[0084] Comparative Example 9

[0085] Compared with Example 1, the difference is that the cation exchange resin used in step (3) is replaced with a gel-type strong acid cation exchange resin, while the rest of the operations are the same.

[0086] The final glycolic acid aqueous solution product was colorless and transparent, but gradually became slightly turbid within 10-15 days of storage at room temperature; the glycolic acid mass fraction decreased from 17.6wt% to 16.9wt%, with a loss rate of 0.7%; the metal ion content decreased from 192ppm to 18ppm; and the anion content decreased from 49ppm to 32ppm.

[0087] Comparative Example 10

[0088] Compared with Example 1, the difference is that the cation exchange resin used in step (3) is replaced with a gel-type weak acid cation exchange resin, while the rest of the operations are the same.

[0089] The final glycolic acid aqueous solution product was colorless and transparent, but gradually became slightly turbid within 5 days of storage at room temperature; the glycolic acid mass fraction decreased from 17.9 wt% to 16.2 wt%, with a loss rate of 1.7%; the metal ion content decreased from 192 ppm to 32 ppm; and the anion content decreased from 63 ppm to 18 ppm.

[0090] Comparative Example 11

[0091] Compared with Example 1, the difference is that the cation exchange resin used in step (3) is replaced with a macroporous weak acid cation exchange resin, while the rest of the operations are the same.

[0092] The final glycolic acid aqueous solution was colorless and transparent, but gradually became slightly turbid within 10 days of storage at room temperature. The glycolic acid mass fraction decreased from 16.8 wt% to 16.2 wt%, a loss rate of 0.6%. The metal ion content decreased from 195 ppm to 19 ppm, and the anion content decreased from 56 ppm to 23 ppm. It is speculated that this decrease is due to the reduced cation removal efficiency leading to an increase in complexed impurities in the solution, thus affecting the effective loading and selectivity of the subsequent anion exchange process.

[0093] Comparative Example 12

[0094] Compared with Example 1, the difference is that the anion exchange resin used in step (3) is replaced with a gel-type strong basic anion exchange resin, while the rest of the operations are the same.

[0095] The final glycolic acid aqueous solution was initially colorless, but gradually became slightly turbid within 10 days of storage at room temperature; the glycolic acid mass fraction decreased from 16.8 wt% to 15.9 wt%, with a loss rate of approximately 0.9%; the metal ion content decreased from 189 ppm to 0.89 ppm, and the anion content decreased from 63 ppm to 35 ppm.

[0096] Comparative Example 13

[0097] Compared with Example 1, the difference is that the anion exchange resin used in step (3) is replaced with a gel-type weakly basic anion exchange resin, while the rest of the operations are the same.

[0098] The final glycolic acid aqueous solution product was colorless and transparent, but became turbid within 7 days of storage at room temperature; the glycolic acid mass fraction decreased from 17.1 wt% to 15.8 wt%, with a loss rate of approximately 1.3%; the metal ion content decreased from 193 ppm to 0.54 ppm, and the anion content decreased from 70 ppm to 32 ppm.

[0099] Comparative Example 14

[0100] Compared with Example 1, the difference is that the anion exchange resin used in step (3) is replaced with a macroporous strong base anion exchange resin, while the rest of the operations are the same.

[0101] The final glycolic acid aqueous solution was initially colorless, but gradually became slightly turbid within 10 days of storage at room temperature; the glycolic acid mass fraction decreased from 17.4 wt% to 15.9 wt%, with a loss rate of approximately 1.5%; the metal ion content decreased from 199 ppm to 0.69 ppm, and the anion content decreased from 53 ppm to 21 ppm.

[0102] Comparative Example 15

[0103] Compared with Example 1, the difference is that the activated carbon decolorization treatment in step (2) was not performed, while the rest of the operations are the same.

[0104] The final glycolic acid aqueous solution product was dark yellow and contained precipitate; the glycolic acid mass fraction decreased from 17.6 wt% to 17.2 wt%, with a loss rate of 0.4%; the metal ion content decreased from 195 ppm to 7 ppm; and the anion content decreased from 53 ppm to 18 ppm.

[0105] Comparative Example 16

[0106] Compared with Example 1, the difference is that the resin treatment in step (3) was not performed, while the rest of the operations are the same.

[0107] The final glycolic acid aqueous solution product was colorless and transparent, but precipitation occurred after 7 days of storage at room temperature; the glycolic acid mass fraction decreased from 17.1 wt% to 16.9 wt%, with a loss rate of 0.2%; the metal ion content decreased from 195 ppm to 152 ppm; and the anion content decreased from 53 ppm to 42 ppm.

[0108] Comparative Example 17

[0109] Compared with Example 1, the difference is that in step (3), only the cation exchange resin is used, and the rest of the operations are the same.

[0110] The final glycolic acid aqueous solution product was colorless and transparent, and precipitation occurred after 3 days; the glycolic acid mass fraction decreased from 17.9wt% to 17.2wt%, with a loss rate of 0.7%; the metal ion content decreased from 189ppm to 0.91ppm; and the anion content decreased from 53ppm to 46ppm.

[0111] Comparative Example 18

[0112] Compared with Example 1, the difference is that in step (3), only the anion exchange resin is used, and the rest of the operations are the same.

[0113] The final glycolic acid aqueous solution product was colorless and transparent, and precipitation occurred after 5 days; the glycolic acid mass fraction decreased from 17.3wt% to 16.2wt%, with a loss rate of 1.1%; the metal ion content decreased from 188ppm to 151ppm; and the anion content decreased from 53ppm to 7ppm.

[0114] Comparative Example 19

[0115] Compared with Example 1, the difference is that the resin treatment order in step (3) is reversed. First, it is treated with macroporous weak basic anion exchange resin, and then it is treated with macroporous strong acid cation exchange resin. All other operations are the same.

[0116] The final glycolic acid aqueous solution product was colorless and transparent, and precipitation occurred after 10 days; the glycolic acid mass fraction decreased from 17.6 wt% to 15.2 wt%, with a loss rate of 2.4%; the metal ion content decreased from 195 ppm to 9 ppm; and the anion content decreased from 53 ppm to 40 ppm.

[0117] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of this patent.

Claims

1. A method for preparing an aqueous solution of glycolic acid using polyglycolic acid industrial waste, characterized in that, Includes the following steps: (1) After mixing the crushed polyglycolic acid waste with water at a mass ratio of 1: (3~10), the mixture is subjected to hydrothermal hydrolysis under closed conditions. After solid-liquid separation, a crude aqueous solution of glycolic acid is obtained. (2) Add activated carbon to the crude aqueous solution of glycolic acid for decolorization treatment; (3) The decolorized glycolic acid aqueous solution is passed through cation exchange resin and anion exchange resin in sequence for ion exchange treatment to finally obtain the desired glycolic acid aqueous solution product.

2. The method according to claim 1, characterized in that, In step (1), the hydrolysis reaction temperature is 90~150 ℃, the reaction time is 2~12 h, and the pressure is 0.1~1.0MPa.

3. The method according to claim 1, characterized in that, In step (2), the activated carbon is powdered microporous activated carbon.

4. The method according to claim 1, characterized in that, In step (2), the decolorization treatment temperature is 60~90℃ and the treatment time is 20~60 min.

5. The method according to claim 1, characterized in that, In step (3), the cation exchange resin is a macroporous strong acid cation exchange resin, and the anion exchange resin is a macroporous weak base anion exchange resin.

6. The method according to claim 1, characterized in that, In step (1), a solid heterogeneous acid catalyst is added during the hydrolysis reaction to increase the hydrolysis rate; the solid heterogeneous acid catalyst includes sulfonic acid ion exchange resin and supported metal oxide catalyst.