A method for decoloring waste polyester alcoholysis solution

Abstract

The present application relates to waste polyester chemical recycling technology, provide a kind of waste polyester alcoholysis liquid decolorization method.Waste polyester is alcoholized in ethylene glycol and removes part of ethylene glycol under reduced pressure, so that the mass ratio of ethylene glycol and BHET is 1.5-4:1, water and hydroxyl carboxylic acid / salt buffer system are added, pH is adjusted to 4.5-6.5, and decoloring agent of chlorite salt matching with the ethylene glycol content of alcoholysis liquid is added in stages under 100-140 DEG C to decolor, then sodium bisulfite and ascorbic acid combination is used to reduce residual high-valence chlorine anion, and inorganic anion is removed by strong alkaline anion exchange resin, and finally high whiteness, high purity BHET is obtained by crystallization.The method has low oxidant dosage, high decoloring efficiency, can significantly reduce chlorate residue and reduce side reaction.

Description

Technical Field

[0001] This invention relates to the technical field of polyester recycling, and in particular to a method for decolorizing waste polyester alcoholysis solution. Background Technology

[0002] Polyethylene terephthalate (PET) is widely used in packaging, textiles, and engineering plastics, and its waste volume is increasing year by year. In chemical recycling, using ethylene glycol to hydrolyze waste polyester into bis(2-hydroxyethyl) terephthalate (BHET) is an important technical route for achieving high-quality recycling. However, consumer-grade polyester products typically contain various organic dyes, pigments, or auxiliaries. These colorants enter the hydrolysis solution during the hydrolysis process, giving the resulting BHET a significant color, making it difficult to meet the whiteness and purity requirements of downstream re-polyester (re-PET).

[0003] Existing decolorization technologies include activated carbon adsorption, recrystallization decolorization, and peroxide oxidation treatment. Activated carbon adsorption has limited applicability to dye structures, and its slow mass transfer in high-viscosity alcoholysis solutions requires a long adsorption time, and subsequent filtration is difficult, easily leading to BHET loss. Recrystallization decolorization depends on a specific solvent system, but its decolorization effect is greatly affected by solvent selectivity, BHET solubility, and cooling kinetics, making it difficult to balance decolorization efficiency and yield. Decolorization relying solely on oxidants often results in problems such as oxidant disproportionation, the formation of oxidation side reactions, and partial degradation of BHET, making it difficult to achieve stable decolorization while maintaining the structural integrity of the main product.

[0004] Therefore, while maintaining the structural stability and crystal quality of BHET, how to achieve efficient, controllable decolorization treatment with low by-products in the complex multiphase system of alcoholysis solution is a key technical issue in the current waste polyester recycling process. Summary of the Invention

[0005] The purpose of this invention is to address the problems of high chroma, low decolorization efficiency, and the impact of oxidation byproducts on crystal quality caused by residual coloring impurities in waste polyester alcoholysis solutions. This invention provides a decolorization method that achieves high efficiency, controllability, and minimal side reactions while maintaining the structural integrity of the main components. The invention aims to achieve this by synergistically controlling the composition of the alcoholysis solution, solvent environment, acid-base conditions, oxidant addition method, and post-treatment steps, enabling the decolorization process to be completed under mild and stable conditions, thereby obtaining bis(2-hydroxyethyl) terephthalate with high whiteness, low chroma, and stable crystal quality.

[0006] To achieve the above objectives, the present invention provides a method for decolorizing waste polyester alcoholysis solution, comprising the following steps:

[0007] (1) Waste polyester is subjected to alcoholysis in ethylene glycol to obtain an alcoholysis solution containing bis(2-hydroxyethyl) terephthalate and ethylene glycol. The alcoholysis solution is subjected to vacuum evaporation to remove some free ethylene glycol, so that the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate in the alcoholysis solution is 1.5 to 4:1.

[0008] (2) Add water to the alcoholysis solution obtained in step (1) so that the mass fraction of water in the mixed solvent of ethylene glycol and water is 15-35 wt%, and adjust the pH of the alcoholysis solution to 4.5-6.5 by adding a buffer system composed of hydroxycarboxylic acid and its alkali metal salt.

[0009] (3) Under closed conditions, the alcoholysis solution obtained in step (2) is heated to 100-140°C, and an aqueous solution of chlorite decolorizing agent is added to it, so that the amount of chlorite ions added is 0.05-0.5 wt% relative to the mass of bis(2-hydroxyethyl) terephthalate in the alcoholysis solution. The reaction is maintained for 20-120 min to oxidize and decompose the organic dyes and colored impurities in the alcoholysis solution.

[0010] (4) After the reaction in step (3) is completed, the alcoholysis solution is cooled to 60-100°C, and a reducing agent is added to reduce the residual chlorite and chlorate to chloride. Then, the alcoholysis solution is contacted with a strong basic anion exchange resin to remove oxidation byproducts, and the alcoholysis solution is obtained after decolorization and purification.

[0011] (5) The alcoholysis solution obtained in step (4) is subjected to crystallization separation and solid-liquid separation to obtain the finished product bis(2-hydroxyethyl) terephthalate solid.

[0012] This invention controls the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate in the alcoholysis solution, ensuring a high solute concentration after the removal of free ethylene glycol. Maintaining the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate within the range of 1.5 to 4:1 ensures complete dissolution of bis(2-hydroxyethyl) terephthalate when water is subsequently added, preventing uneven decolorization caused by premature crystallization. Simultaneously, it increases the effective concentration of colored impurities such as dyes in the system, thereby achieving higher decolorization efficiency with the same amount of oxidant added.

[0013] Based on the above, by adding water to the alcoholysis solution, the mass fraction of water in the ethylene glycol-water mixed solvent is controlled at 15–35 wt%, creating a two-component solvent environment with good mass transfer performance. This control of water content ensures the dissolution and diffusion of chlorite, and on the other hand, maintains the single-phase system while preventing the large-scale precipitation of bis(2-hydroxyethyl) terephthalate, thus allowing for more complete contact between the oxidant and colored impurities, resulting in higher stability of the decolorization process.

[0014] Furthermore, by employing a buffer system composed of a hydroxycarboxylic acid and its salt, the pH of the alcoholysis solution is adjusted to a controllable range of 4.5–6.5, maintaining a stable acid-base environment during the decolorization reaction. This pH range avoids the problem of excessive oxidation caused by the generation of volatile reactive species from chlorite under lower pH conditions, while also preventing rapid deactivation of chlorite under alkaline conditions. Therefore, stable output of oxidation is achieved, making the decolorization process more controllable, and no significant oxidation byproducts are observed in bis(2-hydroxyethyl) terephthalate after the reaction.

[0015] With proper control of the system composition and pH, the alcoholysis solution was heated to 100–140°C, and a chlorite decolorizing agent was added at 0.05–0.5 wt% of the mass of bis(2-hydroxyethyl) terephthalate, allowing the decolorization reaction to proceed under high temperature and low viscosity conditions. This temperature range significantly improved the mass transfer efficiency between the oxidant and impurities, enabling the decolorization reaction to be completed in a shorter time.

[0016] After decolorization, the system is cooled to 60–100°C, and a reducing agent is added to reduce the remaining high-valence chloride anions. This effectively prevents these substances from continuing to oxidize during subsequent crystallization, avoiding instability in quality such as color reversion. Furthermore, by contacting the alcoholysis solution with a strongly basic anion exchange resin, chlorate, chlorite, and some organic anions are further removed from the system. This ensures that the alcoholysis solution reaches a low level of inorganic residue before entering the crystallization process, which is beneficial for obtaining crystalline products with stable color and high purity.

[0017] By combining and controlling the above steps, the final obtained bis(2-hydroxyethyl) terephthalate solid has high whiteness and low color value, and the loss of the main components is small during the decolorization and purification process, achieving a good balance between decolorization efficiency, product purity and process energy consumption.

[0018] As a further improvement of the present invention, the hydroxycarboxylic acid in step (2) is one or more of lactic acid, citric acid, and tartaric acid, and the alkali metal salt is the corresponding sodium salt and / or potassium salt.

[0019] As a further improvement of the present invention, the pH of the alcoholysis solution in step (2) is 5.0 to 6.0.

[0020] As a further improvement of the present invention, the chlorite decolorizing agent is added in a segmented incremental manner, that is, during the decolorization reaction, it is added in three or more times at a single addition amount of 0.1 to 0.3 wt% of the alcoholysis solution mass, and the time interval between each addition is 3 to 15 minutes, so that the chlorite ions in the alcoholysis solution are maintained at a stable concentration at 100 to 140°C.

[0021] This invention employs a segmented, incremental addition of chlorite, ensuring the decolorization process remains within a suitable oxidative intensity range. Segmented addition avoids the instantaneous strong oxidizing environment caused by excessively high initial chlorite concentrations, reducing the potential risk of over-oxidation of BHET molecules. Compared to processes involving one-time addition or blind addition at fixed time intervals, this invention's control strategy achieves comparable or even better decolorization results while significantly reducing the total amount of chlorite used and preventing color recovery and byproduct formation due to excessive oxidation.

[0022] As a further improvement of the present invention, the reaction temperature in step (3) is 110-130°C and the reaction time is 30-90 min.

[0023] As a further improvement of the present invention, the alcoholysis catalyst in step (1) is selected from one or more of zinc acetate, manganese acetate and cobalt acetate, and a metal chelating agent is added to the alcoholysis solution before step (2) or step (3) so that the total content of iron, copper, manganese and zinc in the alcoholysis solution is not higher than 10 ppm.

[0024] As a further improvement of the present invention, the metal chelating agent is selected from one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and iminodiacetonitrile pentamethylphosphonic acid and their sodium salts.

[0025] In chlorite decolorization systems, residual metal catalysts such as zinc acetate, manganese acetate, and cobalt acetate from the alcoholysis step promote the disproportionation of chlorite ions to chlorate ions. This not only consumes effective oxidants but also generates additional inorganic anions, affecting the purity of subsequent crystallization. This invention addresses this issue by adding specific metal chelating agents to the alcoholysis solution before adding chlorite, thereby controlling the total concentration of metal ions such as iron, copper, manganese, and zinc at a low level. This significantly suppresses the disproportionation tendency of chlorite ions, improves the utilization efficiency and reaction selectivity of chlorite, and avoids the negative impacts of upstream alcoholysis catalysts.

[0026] As a further improvement of the present invention, the reducing agent in step (4) is a combination of sodium bisulfite and ascorbic acid, the molar ratio of sulfite to chlorite is 1.2 to 2.0:1, the strong basic anion exchange resin is a type I or type II quaternary ammonium resin, the filling amount is 5 to 30 wt% of the alcoholysis liquid, and the residence time of the alcoholysis liquid in the resin bed is 10 to 60 min.

[0027] As a further improvement of the present invention, after the reduction treatment and anion exchange treatment in step (4), the alcoholysis solution is subjected to ion chromatography detection, and the peak area of ​​chlorate ions does not exceed 2.0 × 10⁻⁶. -6 μS·min.

[0028] As a further improvement of the present invention, the chlorite decolorizing agent in step (3) is a mixed solvent solution composed of water, ethylene glycol and a co-solvent, wherein the mass fraction of ethylene glycol in the mixed solvent is 20-55 wt%, the mass fraction of the co-solvent in the mixed solvent is 0.05-0.5 wt%, and the co-solvent is selected from one or more of methanol, ethanol or propylene glycol.

[0029] This invention further refines the decolorizing agent into a ternary system of ethylene glycol, water, and chlorite, ensuring that the mass fraction of ethylene glycol in the decolorizing agent is within the same or similar range as the ethylene glycol content in the alcoholysis solutions obtained in steps (1) and (2). Thus, when the decolorizing agent is added to the alcoholysis solution, the polarity and solvation environment of the two systems are highly matched. Chlorite ions preferentially allocate to the ethylene glycol-rich phase of the alcoholysis solution, fully contacting the hydrophobic dye molecules enriched in this phase, significantly improving the selective oxidation ability of the dye molecules. Simultaneously, by avoiding a distinct phase interface between the highly polar aqueous phase and the high-ethylene glycol phase, excessive aggregation and autocatalytic disproportionation of chlorite ions in the aqueous phase are suppressed, reducing the formation of byproducts such as chlorate ions, thereby reducing oxidant loss while achieving a high decolorization rate.

[0030] Compared to the conventional method of preparing chlorite decolorizing agents using pure aqueous solutions, the above-mentioned decolorizing agent formulation, matched with the ethylene glycol content of the alcoholysis solution, achieves higher color improvement and lower chlorate generation at the same chlorite dosage. Comparative examples show that under pure water sodium chlorite solution conditions, the detection peak area of ​​chlorate in the decolorized alcoholysis solution is significantly higher than that of the formulation of this invention, and the crystals exhibit yellowing and a decrease in whiteness during subsequent crystallization. In contrast, the chlorate content is significantly reduced under the scheme of this invention, and the whiteness of the BHET crystals remains stable.

[0031] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0032] This invention controls the composition, pH, temperature, oxidant addition method, and post-treatment steps of the alcoholysis solution to ensure a stable, homogeneous, and selectively oxidizing reaction environment throughout the decolorization process. Firstly, by limiting the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate, the alcoholysis solution, after removing some free ethylene glycol, forms a homogeneous system with high solute content and low viscosity. Even after adding water, it maintains a single-phase distribution, avoiding uneven decolorization caused by localized dye enrichment. Simultaneously, it increases the relative concentration of impurities, thus achieving good decolorization results with a lower oxidant dosage.

[0033] Based on this, by controlling the water content of the ethylene glycol-water system and maintaining a stable pH in the weakly acidic region, the disproportionation rate of chlorite under high-temperature conditions can be significantly reduced, allowing its oxidizing capacity to be released in a constant manner. This improves decolorization efficiency while preventing degradation of the main components. Furthermore, the decolorizing agent is formulated as a mixture of ethylene glycol, water, and a co-solvent that matches the polarity of the alcoholysis solution. This allows the oxidant to rapidly distribute to the ethylene glycol-rich phase after addition, ensuring sufficient contact with dye molecules, improving the selective oxidation capacity of impurities, and significantly reducing the formation of byproducts such as chlorate, thus improving the stability of decolorization.

[0034] Furthermore, by adding chlorite in stages, the concentration of the oxidant can be kept under control during the reaction, avoiding ineffective consumption and side reactions caused by instantaneous strong oxidation, thus achieving a more efficient oxidation utilization rate. After the reaction is complete, high-valence chloride anions are removed by a reducing agent, and residual inorganic anions and oxidation byproducts are further removed by a strongly basic anion exchange resin. This ensures that the alcoholysis solution reaches a low-impurity state before entering the crystallization step, effectively preventing quality problems such as decreased whiteness and yellowing of crystals during crystallization.

[0035] By controlling the above process steps, this invention can obtain bis(2-hydroxyethyl) terephthalate solid with high whiteness, low chroma and complete structure with a lower oxidant dosage and shorter processing time. Its purity and thermal stability are significantly improved, and the decolorization efficiency, product quality and process energy consumption are comprehensively optimized. Attached Figure Description

[0036] Figure 1 This is an ion chromatogram of the alcoholysis solution after decolorization in Example 1.

[0037] Figure 2 This is an ion chromatogram of the alcoholysis solution after decolorization in Example 2.

[0038] Figure 3 This is an ion chromatogram of the alcoholysis solution after decolorization in Comparative Example 1.

[0039] Figure 4 This is the ion chromatogram of the alcoholysis solution after decolorization in Comparative Example 3.

[0040] Figure 5 The image shows the high-performance liquid chromatogram of the BHET product obtained in Example 1.

[0041] Figure 6 The image shows the high-performance liquid chromatogram of the BHET product obtained in Comparative Example 3. Detailed Implementation

[0042] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0043] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0045] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0046] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0047] Example 1

[0048] This embodiment provides a method for decolorizing waste polyester alcoholysis solution, as detailed below:

[0049] (1) The waste polyester is subjected to alcoholysis in ethylene glycol, and the ethylene glycol content is adjusted.

[0050] Weigh 100 g of green waste polyester bottle flakes, add 300 g of ethylene glycol (analytical grade, Sinopharm Group) and 0.15 g of zinc acetate (analytical grade, ≥99%, Sinopharm Group), and place them in a stirred reactor equipped with a mechanical stirrer and reflux condenser. Heat to 200 ℃ and stir for 3 h to obtain an alcoholysis solution containing bis(2-hydroxyethyl) terephthalate and free ethylene glycol.

[0051] After alcoholysis, the temperature was lowered to 150 °C, and the mixture was evaporated under reduced pressure at 200 Pa for 40 min to remove some free ethylene glycol, resulting in a mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate in the alcoholysis solution of 2.5:1. After evaporation, the alcoholysis solution was a pale yellow transparent liquid without crystallization.

[0052] (2) Add water and use a buffer system to adjust the pH of the alcoholysis solution.

[0053] Add 80 g of deionized water to the above alcoholysis solution to make the mass fraction of water in the ethylene glycol-water solvent 22 wt%.

[0054] Preparation of lactate-sodium lactate buffer: Lactic acid (≥85%, Sinopharm Group) and sodium lactate (analytical grade, Aladdin) were dissolved in water at a molar ratio of 1:1 to obtain a 10 wt% buffer.

[0055] The above buffer solution was added dropwise at 100 °C to bring the pH of the alcoholysis solution to 5.5.

[0056] Then, 0.10 g of disodium ethylenediaminetetraacetate (EDTA-2Na, analytical grade) was added, and the mixture was stirred at 100 °C for 10 min to complex the iron, manganese, copper, zinc, and other metal ions in the alcoholysis solution. ICP-OES testing confirmed that the total metal content was ≤10 ppm.

[0057] (3) Add chlorite decolorizing agent and carry out decolorization reaction under high temperature conditions.

[0058] The decolorizing agent is prepared as follows:

[0059] Add 60 g of deionized water to a three-necked beaker and stir with a magnetic stirrer; add 0.15 g of sodium chlorite (NaClO2, ≥80%, Aladdin) and dissolve completely; add 40 g of ethylene glycol; add 0.10 g of propylene glycol (analytical grade, Sinopharm Group) and stir evenly to form a homogeneous decolorizing agent mixture.

[0060] The alcoholysis solution obtained in step (2) was heated to 120 °C. The decolorizing agent was added in three portions under sealed conditions:

[0061] Dosage per addition: 6 g (the amount of decolorizing agent added each time is approximately 0.15 wt% of the mass of the alcoholysis solution);

[0062] Feeding interval: 7 min;

[0063] The total decolorization reaction time is approximately 45 minutes.

[0064] The total amount of chlorite added is approximately 0.2 wt% of the mass of BHET.

[0065] (4) Perform reduction treatment and purify using a strong basic anion exchange resin.

[0066] Cool the alcoholysis solution to 80°C and add:

[0067] Sodium bisulfite (NaHSO3, analytical grade) 0.25 g

[0068] Ascorbic acid (vitamin C, ≥99%, Aladdin) 0.05 g

[0069] The molar ratio of the two substances is 1.5:1. Stir for 10 min to reduce the residual chlorate and chlorite to chloride.

[0070] The reduced alcoholysis solution was passed through a strongly basic type I quaternary ammonium anion exchange resin (exchange capacity of approximately 1.3 meq / mL) with a loading of 15 wt% of the alcoholysis solution mass. The flow rate was adjusted so that the residence time of the alcoholysis solution in the resin bed was 30 min, thus completing the purification.

[0071] (5) The purified alcoholysis solution was crystallized to obtain bis(2-hydroxyethyl) terephthalate.

[0072] The purified alcoholysis solution was cooled to 25°C and allowed to stand for 1 hour, during which BHET precipitated. The solid was collected by centrifugation (3000 rpm, 10 min), washed twice with cold ethylene glycol, and dried under vacuum at 60°C for 12 hours to obtain a white BHET solid product.

[0073] Example 2

[0074] This embodiment provides a decolorization method for waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated here. The only difference between this embodiment and Example 1 is the change in the solvent composition of the decolorizing agent and the ethylene glycol content, as detailed below:

[0075] In this embodiment, the reduced pressure evaporation time was adjusted to 30 min, so that the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate in the alcoholysis solution was 2:1 (ethylene glycol mass fraction was about 65%).

[0076] The preparation method of the decolorizing agent is as follows:

[0077] Add 55g of deionized water to a beaker; add 0.15g of sodium chlorite (≥80%) and dissolve completely; add 45g of ethylene glycol and 0.1wt% propylene glycol as a co-solvent; stir well to obtain a decolorizing agent solution.

[0078] Comparative Example 1

[0079] This comparative example provides a decolorization method for waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated here. The only difference between this comparative example and Example 1 is that the decolorizing agent solvent does not contain ethylene glycol, as detailed below:

[0080] The preparation method of the decolorizing agent is as follows:

[0081] Add 100 g of deionized water to a beaker; add 0.15 g of sodium chlorite (≥80%) and dissolve completely; do not add ethylene glycol or any co-solvent to the decolorizing agent to obtain a sodium chlorite decolorizing agent solution with pure water as the solvent.

[0082] Comparative Example 2

[0083] This comparative example provides a method for decolorizing waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated here. The only difference between this comparative example and Example 1 is the ethylene glycol content in the decolorizing agent, as detailed below:

[0084] The preparation method of the decolorizing agent is as follows:

[0085] Add 90 g of deionized water to a beaker; add 0.15 g of sodium chlorite (≥80%) and dissolve completely; add 10 g of ethylene glycol; add 0.1 wt% propylene glycol as a co-solvent; stir well to obtain a decolorizing agent solution.

[0086] Example 3

[0087] This embodiment provides a decolorization method for waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated. The only difference between this embodiment and Example 1 is the type and amount of chelating agent, as detailed below:

[0088] The chelating agent used was 0.12 g of diethylenetriaminepentaacetic acid pentasodium (DTPA-5Na, ≥98%, Aladdin).

[0089] Comparative Example 3

[0090] This comparative example provides a method for decolorizing waste polyester alcoholysis solution. The similarities with Example 1 will not be repeated. The only difference between this comparative example and Example 1 is that no metal chelating agent was added to the alcoholysis solution in step (2). All other operation steps are the same.

[0091] Comparative Example 4

[0092] The only difference between this comparative example and Example 1 is that the decolorizing agent is added all at once in step (3), and the reaction is maintained for 42 minutes after the one-time addition. The total amount of decolorizing agent added at once is the same as in Example 1, so that the total amount of chlorite added to BHET remains unchanged.

[0093] Comparative Example 5

[0094] This comparative example provides a method for decolorizing waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated here. The only difference between this comparative example and Example 1 is that the amount of water added to the alcoholysis solution in step (2) is reduced, as detailed below:

[0095] In this comparative example, only 20 g of deionized water was added to the alcoholysis solution obtained in step (1), and then lactate / sodium lactate buffer was added to adjust the pH to 5.5.

[0096] Comparative Example 6

[0097] This comparative example provides a decolorization method for waste polyester alcoholysis solution. The similarities to Example 1 will not be repeated here. The only difference between this comparative example and Example 1 is that the amount of water added in step (2) is significantly increased, as detailed below:

[0098] In this comparative example, 180 g of deionized water was added to the alcoholysis solution, and lactate / sodium lactate buffer was added to adjust the pH to 5.5.

[0099] Performance testing

[0100] To evaluate the effectiveness of the decolorization method for waste polyester alcoholysis solution of the present invention, the bis(2-hydroxyethyl) terephthalate products obtained in Examples 1-3 and Comparative Examples 1-6 were selected for performance testing. The test items included crystal color parameters, chlorate residue, total metal ions, purity of main components and yield.

[0101] 1. Crystal colorimetry was performed using a portable colorimeter under D65 light source and 10° viewing angle conditions, measuring the L*, a*, and b* values. Before testing, the sample was vacuum-dried at 60℃ for 12 hours, ground, and passed through a 100-mesh sieve before being placed in a standard sample cup. The instrument was set up according to the standard illuminator D65 and standard observation conditions of 10° as specified in GB / T3978, and calibrated using a standard white plate. A higher L* value indicates a sample closer to white, while a lower b* value indicates a weaker yellow hue.

[0102] 2. The residual chlorate ion was determined by ion chromatography. A sample of the decolorized and purified alcoholysis solution was taken, diluted, and filtered through a 0.22 μm microporous membrane. The injection volume was 25 μL. An anion exchange column was used, with a carbonate / bicarbonate mixed solution as the eluent. Detection was performed using a conductivity detector, and the peak area of ​​chlorate ion was obtained by integration, expressed in μS·min.

[0103] For the final BHET product, the mother liquor sample can be collected and measured directly after crystallization and centrifugation.

[0104] 3. The total metal ion concentration was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). A certain amount of the decolorized and purified alcoholysis solution was taken, digested with nitric acid, and then diluted to a final volume. The concentrations of iron, copper, manganese, and zinc were determined, and the total metal concentration was obtained by summing the contents of the four metals, expressed in ppm.

[0105] 4. The purity of the main component was determined by high performance liquid chromatography (HPLC). A C18 reversed-phase column was used, with a methanol / water mixture of 70 / 30 as the mobile phase, a flow rate of 1.0 mL / min, a column temperature of 30 ℃, and a detection wavelength of 240 nm. The ratio of the BHET main peak area to the total peak area was used as the HPLC purity.

[0106] 5. The BHET yield is calculated as the ratio of the mass of BHET solid after crystallization, centrifugation, and drying to the theoretically obtainable mass of BHET, expressed as a mass fraction.

[0107] The test results of the above tests are as follows Figures 1-6 And as shown in Table 1 below.

[0108] Table 1

[0109]

[0110] Based on the test results of Examples 1-3 and Comparative Examples 1-6, it can be seen that the present invention exhibits significant advantages in decolorization effect, chlorate control, main component purity, crystal whiteness, and product yield. Under the conditions of the present invention, the L* value of the obtained BHET crystals is higher than 84, the b* value is controlled at around 1.1, and the chlorate peak area is maintained at 1.0 × 10⁻⁶. -6 ~1.4×10 -6 With low levels of μS·min, stable metal ion concentrations below 10 ppm, HPLC purity exceeding 99.6%, and BHET yields of approximately 93%–94%, these results demonstrate that this invention, through synergistic control of ethylene glycol content, moisture, acid-base environment, and chlorite addition method, can achieve uniform and stable decolorization at lower oxidant dosages, yielding BHET products with low color, high purity, and good crystallinity.

[0111] In contrast, the decolorizing agent in Comparative Examples 1 and 2 showed a polarity mismatch with the alcoholysis solution, resulting in uneven dye dispersion, decreased oxidation efficiency, significantly increased crystal b* value, and a noticeably larger chlorate peak area. Comparative Example 3, which did not use a metal chelating agent, saw the metal ion concentration in the alcoholysis solution increase to 38 ppm, exacerbating chlorite disproportionation and achieving a chlorate peak area of ​​6.2 × 10⁻⁶. -6 Above μS·min, both the purity and whiteness of BHET decreased significantly. Comparative Example 4 used a one-time addition of the decolorizing agent; the excessively high initial concentration caused instantaneous strong oxidation, leading to an increase in the b* value and a chlorate peak area of ​​approximately 5.1 × 10⁻⁶. -6 μS·min, the decolorization stability was significantly weakened. When the moisture content in Comparative Examples 5 and 6 deviated from the control range of 15-35 wt%, the system viscosity increased, leading to uneven decolorization, or the system became too dilute, causing premature crystallization of BHET, resulting in a significant increase in color and a decrease in yield.

[0112] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for decolorizing waste polyester alcoholysis solution, characterized in that, Includes the following steps: (1) Waste polyester bottle flakes are subjected to alcoholysis in ethylene glycol to obtain an alcoholysis solution containing bis(2-hydroxyethyl) terephthalate and ethylene glycol. The alcoholysis solution is subjected to vacuum evaporation to remove some free ethylene glycol, so that the mass ratio of ethylene glycol to bis(2-hydroxyethyl) terephthalate in the alcoholysis solution is 1.5 to 4:

1. (2) Add water to the alcoholysis solution obtained in step (1) so that the mass fraction of water in the mixed solvent of ethylene glycol and water is 15-35 wt%, and adjust the pH of the alcoholysis solution to 4.5-6.5 by adding a buffer system composed of hydroxycarboxylic acid and its alkali metal salt. (3) Under closed conditions, the alcoholysis solution obtained in step (2) is heated to 100-140°C, and an aqueous solution of chlorite decolorizing agent is added to it, such that the amount of chlorite ions added is 0.05-0.5 wt% relative to the mass of bis(2-hydroxyethyl) terephthalate in the alcoholysis solution. The reaction is maintained for 20-120 min to oxidize and decompose the organic dyes and colored impurities in the alcoholysis solution. The chlorite decolorizing agent is a mixed solvent solution composed of water, ethylene glycol and a co-solvent, wherein the mass fraction of ethylene glycol in the mixed solvent is 20-55 wt%, the mass fraction of the co-solvent in the mixed solvent is 0.05-0.5 wt%, and the co-solvent is selected from one or more of methanol, ethanol or propylene glycol. The chlorite decolorizing agent is added in three or more times during the decolorization reaction at a single addition amount of 0.1-0.3 wt% of the mass of the alcoholysis solution, with a time interval of 3-15 min between each addition. (4) After the reaction in step (3) is completed, the alcoholysis solution is cooled to 60-100°C, and a reducing agent is added to reduce the residual chlorite and chlorate to chloride. Then, the alcoholysis solution is contacted with a strong basic anion exchange resin to remove oxidation byproducts, and the alcoholysis solution is obtained after decolorization and purification. (5) Perform crystallization and solid-liquid separation on the alcoholysis solution obtained in step (4) to obtain the finished product bis(2-hydroxyethyl) terephthalate solid.

2. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, The hydroxycarboxylic acid mentioned in step (2) is one or more of lactic acid, citric acid, and tartaric acid, and the alkali metal salt is the corresponding sodium salt and / or potassium salt.

3. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, The pH of the alcoholysis solution in step (2) is 5.0 to 6.

0.

4. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, In step (3), the reaction temperature is 110-130℃ and the reaction time is 30-90min.

5. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, In step (1), the alcoholysis catalyst is selected from one or more of zinc acetate, manganese acetate and cobalt acetate, and a metal chelating agent is added to the alcoholysis solution before step (2) or step (3) so that the total content of iron, copper, manganese and zinc in the alcoholysis solution is not higher than 10 ppm.

6. The decolorization method for waste polyester alcoholysis solution according to claim 5, characterized in that, The metal chelating agent is selected from one or more of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and their sodium salts.

7. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, The reducing agent in step (4) is a combination of sodium bisulfite and ascorbic acid, with a molar ratio of sulfite to chlorite of 1.2 to 2.0:

1. The strong basic anion exchange resin is a type I or type II quaternary ammonium resin, with a loading amount of 5 to 30 wt% of the alcoholysis solution mass, and the residence time of the alcoholysis solution in the resin bed is 10 to 60 min.

8. The decolorization method for waste polyester alcoholysis solution according to claim 1, characterized in that, After the reduction and anion exchange treatment in step (4), the alcoholysis solution was subjected to ion chromatography, and the peak area of ​​chlorate ions did not exceed 2.0 × 10⁻⁶. -6 μS·min.

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