Method for recovering and purifying BHET on basis of stepwise reaction and stepwise crystallization

Abstract

A method for recovering and purifying BHET on the basis of stepwise reaction and stepwise crystallization, relating to the field of the recycling of organic polymer compounds. The method for recovering and purifying BHET comprises: at a specific temperature and under the catalysis of a catalyst I, reacting a waste PET product in an ethylene glycol and hydrogen atmosphere to prepare a mixture I containing a precipitate which is generated by partially depolymerized PET and a dye; and mixing the mixture I and a catalyst II in ethylene glycol for an alcoholysis reaction, performing filtration while hot to remove insoluble substances, and performing cooling crystallization and filtration to obtain a purified BHET crystal. The method can obtain a decolored BHET crystal having a relatively high purity, without using conventional purification means such as sublimation or passing through purification columns. The method is not only green, environmentally friendly, simple and convenient to operate, and capable of achieving recycling of ethylene glycol, but also has low recovery cost and facilitates industrial production. The method can be used for recycling waste PET products.

Description

Method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization Technical Field

[0001] The invention belongs to the field of recycling of organic polymer compounds and relates to a method for recovering and purifying BHET, in particular to a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization. Background Art

[0002] Polyester fiber mainly refers to polyethylene terephthalate (PET) fiber, commonly known as polyester, which ranks first among synthetic fibers in the textile industry. In 2021, global polyester production was approximately 53.63 million tons, consuming a large amount of non-renewable petroleum resources. At the same time, PET-derived waste also poses a serious threat to the ecosystem. According to published data, 77.5% of PET in my country is used for synthetic fiber / textile products. However, because PET textiles contain dyes and auxiliaries, the economic benefits of recycling are low, and most of them are landfilled or incinerated after the end of their service life. Therefore, how to deal with these waste PET is the focus of current research. Effectively recycling waste PET can produce good social and economic benefits, while also reducing pressure on the environment and resources.

[0003] In recent years, the main methods for recycling waste PET include energy recovery, physical recovery, and chemical recovery. Chemical recovery has attracted considerable attention due to its ability to achieve closed-loop PET recycling. Alcoholysis, a chemical recovery method, is now widely used. Ethylene glycol (EG) alcoholysis involves the degradation of waste PET under a transesterification catalyst at a specific temperature and pressure using EG as the reaction medium. This process, also known as sugar alcoholysis, offers relatively mild reaction conditions and low cost. During depolymerization, the carboxyl carbon of the ester group on the PET is attacked by free electron pairs from EG. The hydroxyethyl group of EG bonds with the carboxyl carbon of the PET, breaking the long PET chain into oligomers and subsequently forming BHET. While ethylene glycol alcoholysis of waste polyester products has achieved some success in recovering BHET, this recovery method remains challenging due to the purification of the target BHET product. Waste polyester products typically contain a large number of impurities, such as dyes and other condensed polymers. These impurities react during the alcoholysis process to form a variety of products, making the recovery of high-purity, decolorized BHET difficult.

[0004] Existing Chinese patent document CN115894223A discloses a chemical recovery method for waste PET products that utilizes the phase change properties of BHET crystals. This method involves mixing the waste PET products with a catalyst in ethylene glycol, performing an alcoholysis reaction to obtain an alcoholysis solution, cooling and crystallizing the resulting BHET-containing crystals, and then subjecting them to vacuum sublimation and condensation to obtain high-purity BHET. However, this recovery process requires specialized vacuum sublimation equipment and high vacuum and temperature requirements during operation, making it difficult to operate and requiring high energy consumption. Furthermore, without vacuum sublimation purification, the BHET crystals cannot be effectively separated from the alcoholysis products of other non-PET polymers commonly contained in the waste PET products, significantly affecting the purity of the target BHET product.

[0005] Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, so as to overcome the deficiency that the BHET obtained by the existing ethylene glycol alcoholysis method for degrading waste PET products is difficult to separate from other oligomers and dyes.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] The present invention provides a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, which comprises the following steps performed in sequence:

[0009] S1. Mixing waste PET products and catalyst I in ethylene glycol, reacting under a hydrogen atmosphere, cooling and crystallizing, and filtering to obtain mixture I;

[0010] S2. Mixing mixture I and catalyst II in ethylene glycol, performing alcoholysis reaction, filtering while hot to remove insoluble matter, cooling and crystallizing, and filtering to obtain purified BHET crystals;

[0011] The mixture I includes a reaction product of partially depolymerized PET and a dye;

[0012] The catalyst I is generated by in-situ reaction of a metal salt compound with an organic ligand containing N or P.

[0013] Preferably, the metal salt compound comprises Fe(OAc)2, FeCl2, Co(OAc)2, CoCl2, Ni(OAc)2, NiCl2, Cu(OAc)2 or CuCl;

[0014] The organic ligand containing N or P includes

[0015] Preferably, the molar ratio of the metal salt compound to the organic ligand containing N or P in the catalyst I is 1:1.5-2.

[0016] Preferably, the reaction is carried out under a hydrogen atmosphere, comprising introducing 10 to 20 atm of hydrogen and reacting at 140 to 150° C. for 40 to 60 minutes.

[0017] Preferably, the waste PET products contain dyes, and the dyes are azo dyes and anthraquinone dyes.

[0018] Preferably, in step S1, the mass proportion of PET in the waste PET products is 65-99%;

[0019] The weight percentage of the catalyst I in the waste PET product is 0.1 to 1 wt%;

[0020] The mass ratio of the waste PET product to ethylene glycol is 1:4-7;

[0021] The temperature of the cooling crystallization is 8 to 12° C., and the time is 6 to 18 hours.

[0022] Preferably, in step S2, the catalyst II is potassium carbonate;

[0023] The mass ratio of the mixture I to the catalyst II is 1:0.005-0.03;

[0024] The mass ratio of the mixture I to ethylene glycol is 1:4-7;

[0025] The temperature of the alcoholysis reaction is 230-260° C., and the time is 1-2 hours.

[0026] Preferably, the purified BHET crystals have a purity of 98.78-99.42%, a chromaticity L value of 97.5-99, an a value of -0.8--1.23, and a b value of 0.13-0.7.

[0027] The decolorization principle of the present invention is: in the presence of catalyst I, the dye's chromogenic group undergoes a cross-coupling reaction through the CH bond to generate a new non-colorogenic group. Simultaneously, the dye undergoes polymerization to form a cross-linked polymer, and small molecules are polymerized into macromolecules that are then precipitated, achieving decolorization of waste polyester products. The specific reaction principle is as follows:

[0028] The hydrogen used in the present invention reduces the catalyst to a low-valent metal catalytic active center M;

[0029] For azo dyes: The N=N double bond of the dye is used to guide the oxidation-addition reaction between the low-valent metal catalytic active center M and the aromatic hydrocarbon C-H bond to form a metal-containing heterocyclic compound. Under heating conditions, the metal-containing heterocyclic compound further undergoes a series of complex chemical reactions including C-C coupling and N=N bond reduction to form a complex polymer. The reaction is as follows:

[0030] For anthraquinone dyes: Using the guiding effect of the C=O carbonyl group of the dye, an oxidation-addition reaction similar to the above occurs to generate a complex polymer. The reaction is as follows:

[0031] Due to the adoption of the above technical solution, the present invention has the following technical advancements compared with the prior art:

[0032] ① The present invention first uses ethylene glycol to perform an alcohol treatment reaction on waste PET products, partially depolymerizing the PET in the waste polyester products. During this process, other non-PET polymer components (such as spandex and nylon) undergo complete depolymerization to form small monomers that dissolve in the ethylene glycol solution. Simultaneously, under these reaction conditions, dye molecules migrate from the PET molecules into the ethylene glycol solution. Catalyst I, generated in situ from a metal salt compound and an organic ligand containing nitrogen or phosphorus, catalyzes the reaction between hydrogen and the dye molecules, causing the dye's chromophore groups to react to form colorless groups. Simultaneously, the dye polymerizes to form a cross-linked polymer that precipitates from the ethylene glycol. The reaction solution is cooled and crystallized to produce a mixture I of partially depolymerized PET and a macromolecular precipitate generated by the dye reaction. Further alcoholysis is catalyzed by potassium carbonate, where the partially depolymerized PET in mixture I produces the target product, BHET. The colorless precipitate generated by the dye reaction in mixture I remains insoluble and can be removed by hot filtration of the alcoholysis solution. After filtration, the filtrate is cooled, crystallized, and filtered to obtain the decolorized, high-purity target product, BHET.

[0033] ② The basis for selecting ethylene glycol as the solvent in step S1 of the present invention is: a. At room temperature, ethylene glycol has a high solubility for impurities and a low solubility for PET or partially depolymerized PET. In this way, by cooling and crystallizing the reaction solution, PET can be separated from impurities, thereby obtaining partially depolymerized PET with relatively high purity as a raw material for the alcoholysis reaction to further obtain BHET; b. By controlling the appropriate temperature and time, ethylene glycol effectively participates in the depolymerization reaction of impurities (such as spandex, nylon, etc.) in waste PET products, and its depolymerization products are soluble in ethylene glycol. Under these conditions, PET only partially depolymerizes; c. Under the conditions of step S1, after azo or anthraquinone dyes, which account for a large proportion of textile dyes, are dissolved in ethylene glycol, catalyst I can catalyze the reaction between H2 and the dye molecules, causing the chromophores of azo or anthraquinone dyes, which account for a large proportion of organic pigments, to react to form colorless groups, and also undergo polymerization to form cross-linked polymers that are precipitated from ethylene glycol, thereby achieving decolorization of waste polyester products;

[0034] ③ The present invention proposes a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization by rationally selecting the reaction solvent and catalyst, rationally controlling the reaction conditions, and optimizing the ratio between the raw materials. This method, after the alcoholysis reaction, is cooled, crystallized, and filtered, without conventional purification methods such as sublimation or purification columns, to obtain BHET crystals with good decolorization effect and high purity (98.78-99.42%). This method is not only environmentally friendly, simple to operate, and has relatively low recovery costs, but also ethylene glycol can be recycled in this method. Therefore, the method for recovering and purifying BHET provided by the present invention is conducive to industrial production.

[0035] The invention can be used for recycling waste PET products, and the prepared BHET can be further applied to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

[0037] FIG1 is a chromaticity detection diagram of the BHET crystal in Example 1 of the present invention;

[0038] FIG2 is a GPC curve of waste PET textiles in Example 1 of the present invention;

[0039] FIG3 is a GPC curve of the mixture I1 of Example 1 of the present invention;

[0040] FIG4 is a picture of filtrate 1 of Example 1 of the present invention;

[0041] FIG5 is an HPLC spectrum of BHET crystals in Example 1 of the present invention;

[0042] FIG6 is a chromaticity detection diagram of the BHET crystal in Example 2 of the present invention;

[0043] FIG7 is an HPLC spectrum of BHET crystals in Example 2 of the present invention;

[0044] FIG8 is a chromaticity detection diagram of the BHET crystal in Example 3 of the present invention;

[0045] FIG9 is an HPLC spectrum of BHET crystals in Example 3 of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be further described in detail below through specific embodiments. It should be understood that the embodiments described are preferred examples of the present invention and are only used to explain the present invention and are not intended to limit the present invention.

[0047] Unless otherwise specified, the materials, reagents, etc. used in the examples of the present invention can be obtained from commercial sources.

[0048] Example 1 A method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization

[0049] (1) This embodiment provides a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, which comprises the following steps performed in sequence:

[0050] S1. 200 g of washed, dried, and sliced ​​waste PET textiles (99 wt% PET content, 1 wt% brown azo dye content) were placed in a reaction vessel containing 1200 g of ethylene glycol. 1 g of catalyst I1 was added and stirred uniformly. The mixture was reacted at 140°C under a hydrogen atmosphere of 10 atm for 60 min. After the reaction, the reaction solution was slowly cooled to room temperature and then placed in an environment of 10°C for 12 h for crystallization. The mixture was then filtered to obtain 200.6 g of mixture I1 and filtrate I.

[0051] Among them, catalyst Ⅰ1 is composed of Fe(OAc)2 and It is generated by in-situ reaction with a molar ratio of 1:2.

[0052] After the waste PET textiles are treated and reacted in this step, the azo dyes therein are dissolved in ethylene glycol and then react with H2 under the catalytic action of catalyst I1, causing the chromophore groups of the azo dyes to react to form colorless groups. Simultaneously, a polymerization reaction occurs to form a cross-linked polymer that forms a precipitate from the ethylene glycol. This step is accompanied by partial depolymerization of the PET. In addition, a small amount of partially depolymerized PET will also dissolve in ethylene glycol, while most of the partially depolymerized PET is insoluble in ethylene glycol. Catalyst I1 is also insoluble in ethylene glycol. The mixture I1 obtained in this step is obtained by filtration, and its surface is not only stained with a small amount of ethylene glycol. Therefore, it is reasonable that the mass of the mixture I1 obtained here is slightly greater than the mass of the waste PET textiles.

[0053] S2. 200.6 g of mixture Ⅰ 1 and 4.0 g of potassium carbonate were placed in a reaction vessel containing 1203.6 g of ethylene glycol and subjected to alcoholysis at 250 ° C for 1.5 h.

[0054] After the reaction is completed, the insoluble matter is removed by hot filtration to obtain filtrate II, in which the insoluble matter is the cross-linked polymer generated by the azo dye, catalyst I1 and potassium carbonate;

[0055] After filtrate II was slowly cooled to room temperature, it was placed in a 10°C environment for 12 hours for crystallization, white crystals precipitated, and filtered to obtain BHET crude product 1 and filtrate III; filtrate III was placed in a 10°C environment for 2 hours, white crystals precipitated, and filtered to obtain BHET crude product 2.

[0056] BHET crude product 1 and BHET crude product 2 were mixed, washed with water, filtered, and then dried at a temperature of 60-80°C for 1-2 hours. In this example, the drying conditions selected were 60°C for 2 hours to obtain purified BHET crystals.

[0057] According to calculation, the yield of BHET recovered by the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided in this embodiment is 86.5%.

[0058] (II) This example not only measured the chromaticity of the above-mentioned waste PET textiles, mixture I1, and purified BHET crystals, but also characterized the molecular weight distribution of the raw materials and intermediates, as follows:

[0059] (1) Determination of chromaticity

[0060] In this example, the colorimeter was used to measure the color of the raw material in step S1, the waste PET textile, the mixture I1 obtained in step S1, and the target product obtained in step S2, the purified BHET crystals. The specific results are as follows:

[0061] The color of waste PET textiles is brown, and its chromaticity is: L*=40.1, a*=7.09, b*=10.04;

[0062] Mixture I1 mainly contains partially depolymerized PET, which is nearly white and blocky, with a chromaticity of L*=77.64, a*=-1.47, and b*=-0.29.

[0063] The purified BHET crystals are white and granular. The colorimetric detection chart is shown in FIG1 . The specific colorimetric values ​​are: L*=97.5, a*=-0.98, and b*=0.26.

[0064] From the above chromaticity results, it can be seen that the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided by the present invention can effectively remove the original color in waste polyester.

[0065] (2) Characterization of molecular weight distribution

[0066] In this example, gel permeation chromatography was used to characterize the molecular weight distribution of raw materials, intermediates, and products. The GPC curve of waste PET textiles is shown in FIG2 , and the GPC curve of mixture I1 is shown in FIG3 .

[0067] The statistical table of molecular weight distribution of PET in waste PET textiles and partially depolymerized PET in mixture Ⅰ1 is shown in Table 1 below.

[0068] Table 1 Molecular weight distribution statistics of raw materials and different intermediates

[0069] It should be noted that because GPC tests the molecular weight and distribution of macromolecules (such as PET), other impurities have been filtered out before testing. Therefore, the GPC curve of mixture I1 represents the molecular weight and distribution of partially depolymerized PET.

[0070] As shown in Figures 2-3 and Table 1, PET partially depolymerized during the reaction in step S1. However, the molecular weight was still large enough to allow all PET to precipitate after the reaction solution was cooled without loss.

[0071] (3) Color of filtrate I

[0072] Filtrate I is a colorless, transparent liquid, as shown in FIG4 .

[0073] (3) The purity of the purified BHET crystals obtained in step S2 of part (1) of this example was determined by HPLC. The HPLC spectrum of the purified BHET crystals is shown in FIG5 .

[0074] After testing and calculation, the purity of the purified BHET crystals was 99.3%.

[0075] Example 2 A method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization

[0076] (1) This embodiment provides a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, which comprises the following steps performed in sequence:

[0077] S1. 200 g of washed, dried, and sliced ​​waste PET products (PET content: 89 wt%, spandex content: 10 wt%, brown anthraquinone dye content: 1 wt%) were placed in a reaction vessel containing 1200 g of ethylene glycol, 2 g of catalyst I2 was added, and the mixture was stirred uniformly. The mixture was reacted at 150°C under a hydrogen atmosphere of 14 atm for 50 min. After the reaction, the reaction solution was slowly cooled to room temperature and placed in an environment of 10°C for 12 h for crystallization. The mixture was filtered to obtain 181.8 g of mixture I2 and filtrate IV.

[0078] Among them, catalyst Ⅰ2 is composed of CuCl and It is generated by in-situ reaction at a molar ratio of 1:1.5.

[0079] S2. 181.8 g of mixture Ⅰ2 and 3.6 g of potassium carbonate were placed in a reaction vessel containing 909 g of ethylene glycol and subjected to alcoholysis at 250 ° C for 1.5 h.

[0080] After the reaction is completed, the insoluble matter is removed by hot filtration to obtain filtrate V. After the filtrate V is slowly cooled to room temperature, it is placed in a 10°C environment for 12 hours for crystallization to precipitate white crystals, which are filtered to obtain BHET crude product 3 and filtrate VI; the filtrate VI is placed in a 10°C environment for 2 hours to precipitate white crystals, which are filtered to obtain BHET crude product 4.

[0081] The crude BHET products 3 and 4 were mixed, washed with water, filtered, and then dried at 60° C. for 2 h to obtain purified BHET crystals.

[0082] According to calculation, the yield of BHET recovered by the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided in this embodiment is 83.9%.

[0083] (II) This example not only measured the chromaticity of the above-mentioned waste PET products, mixture I2, and BHET crystals, but also measured the chromaticity of the above-mentioned waste PET products, mixture I2, and BHET crystals, as follows:

[0084] In this example, the colorimetry of the raw material in step S1, the waste PET product, the mixture I2 obtained in step S1, and the target product, the purified BHET crystals obtained in step S2, was measured using a LAB colorimeter. The specific results are as follows:

[0085] The color of the waste PET product is brown, and its chromaticity is: L*=16.50, a*=2.21, b*=9.25;

[0086] Mixture I2 is nearly white and blocky, with chromaticity of L*=74.39, a*=2.02, b*=8.31;

[0087] The BHET crystal is white and granular, and its chromaticity is: L*=99, a*=-1.23, b*=0.38; wherein, the chromaticity detection chart of the BHET crystal is shown in FIG6 .

[0088] From the above colorimetric results, it can be seen that the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided by the present invention can effectively remove the original color in waste PET products.

[0089] (2) The purity of the purified BHET crystals obtained in step S2 of part (1) of this example was determined by HPLC. The HPLC spectrum of the purified BHET crystals is shown in Figure 7. After testing and calculation, the purity of the purified BHET crystals was 98.8%.

[0090] Example 3 A method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization

[0091] (1) This embodiment provides a method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, which comprises the following steps performed in sequence:

[0092] S1. 200 g of washed, dried, and sliced ​​waste PET fabric (PET content: 65 wt%, cotton content: 34 wt%, red azo dye content: 1 wt%) was placed in a reaction vessel containing 1000 g of ethylene glycol. 0.6 g of catalyst I3 was added and stirred. The mixture was reacted at 145°C under a hydrogen atmosphere of 20 atm for 40 min. After the reaction, the reaction solution was slowly cooled to room temperature and then crystallized at 10°C for 12 h. The mixture was filtered to obtain 132.4 g of mixture I3.

[0093] Among them, catalyst I3 is composed of FeCl2 and It is generated by in-situ reaction at a molar ratio of 1:1.7.

[0094] S2. 132.4 g of mixture Ⅰ3 and 4.0 g of potassium carbonate were placed in a reaction vessel containing 662 g of ethylene glycol and subjected to alcoholysis at 230 ° C for 2.0 h.

[0095] After the reaction is completed, the insoluble matter (mainly cotton cellulose, catalyst I3, precipitate generated by the dye and potassium carbonate) is removed by hot filtration to obtain filtrate VII. After the filtrate VII is slowly cooled to room temperature, it is placed in an 8°C environment for 10 hours for crystallization to precipitate white crystals, which are filtered to obtain BHET crude product 5 and filtrate VIII. The filtrate VIII is placed in a 10°C environment for 2 hours to precipitate white crystals, which are filtered to obtain BHET crude product 6.

[0096] The crude BHET product 5 and the crude BHET product 6 were mixed, washed with water, filtered, and then dried at 70° C. for 1.5 h to obtain purified BHET crystals.

[0097] According to calculation, the yield of BHET recovered by the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided in this embodiment is 86.0%.

[0098] (II) This example not only measured the chromaticity of the above-mentioned waste PET fabric, the first partially depolymerized PET, and the BHET crystals, but also determined the chromaticity of the above-mentioned waste PET fabric, the first partially depolymerized PET, and the BHET crystals, as follows:

[0099] In this example, the colorimeter was used to measure the color of the raw material in step S1, the waste PET fabric, the mixture I3 obtained in step S1, and the target product, the BHET crystal, obtained in step S2. The specific results are as follows:

[0100] The color of the waste PET fabric is red, and its chromaticity is: L*=25.05, a*=27.87, b*=0.90;

[0101] Mixture I3 is nearly white and blocky, and its chromaticity is: L*=80.41, a*=-0.27, b*=4.57;

[0102] The BHET crystal is white and granular, and its chromaticity is: L*=98.82, a*=-0.97, b*=0.13; wherein, the chromaticity detection chart of the BHET crystal is shown in FIG8 .

[0103] From the above colorimetric results, it can be seen that the method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided by the present invention can effectively remove the original color in waste PET products.

[0104] (3) The purity of the purified BHET crystals obtained in step S2 of part (1) of this example was determined by HPLC. The HPLC spectrum of the purified BHET crystals is shown in Figure 9. After testing and calculation, the purity of the purified BHET crystals was 99.4%.

[0105] Examples 4 to 8 Methods for recovering and purifying BHET based on step-by-step reaction and gradual crystallization

[0106] Examples 4 to 8 are methods for recovering and purifying BHET based on step-by-step reaction and gradual crystallization. Their steps are basically the same as the method provided in Example 1, and the only difference is the different control parameters between different steps. See Table 2 for details.

[0107] Table 2 List of control parameters for different steps in Examples 4 to 8

[0108] The purity and chromaticity of BHET crystals obtained by applying different methods for recovering and purifying BHET based on step-by-step reaction and gradual crystallization provided in Examples 4 to 8, as well as the yields of different methods, are statistically analyzed. The statistical table is shown in Table 3 below.

[0109] Table 3 Statistics of the yields of Examples 4 to 8 and the purity of the obtained BHET crystals

[0110] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications based on the above description are possible. It is not possible to enumerate all embodiments here. Any obvious variations or modifications arising from the technical solution of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization, characterized in that: The method comprises the following steps performed in sequence: S1. Mixing waste PET products and catalyst I in ethylene glycol, reacting under a hydrogen atmosphere, cooling and crystallizing, and filtering to obtain mixture I; S2. Mixing mixture I and catalyst II in ethylene glycol, performing alcoholysis reaction, filtering while hot to remove insoluble matter, cooling and crystallizing, and filtering to obtain purified BHET crystals; The mixture I includes a reaction product of partially depolymerized PET and a dye; The catalyst I is generated by in-situ reaction of a metal salt compound with an organic ligand containing N or P.

2. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 1, wherein: The metal salt compound includes Fe(OAc)2, FeCl2, Co(OAc)2, CoCl2, Ni(OAc)2, NiCl2, Cu(OAc)2 or CuCl; The organic ligand containing N or P includes 3. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 1, wherein: The molar ratio of the metal salt compound to the organic ligand containing N or P in the catalyst I is 1:1.5-2.

4. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to any one of claims 1 to 3, characterized in that: The reaction is carried out under a hydrogen atmosphere, comprising introducing 10 to 20 atm of hydrogen and reacting at 140 to 150° C. for 40 to 60 minutes.

5. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 4, wherein: The waste PET products contain dyes, which are azo dyes and anthraquinone dyes.

6. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 4, wherein: In step S1, the mass proportion of PET in the waste PET products is 65-99%; The weight percentage of the catalyst I in the waste PET product is 0.1 to 1 wt%; The mass ratio of the waste PET product to ethylene glycol is 1:4-7; The temperature of the cooling crystallization is 8 to 12° C., and the time is 6 to 18 hours.

7. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 4, characterized in that: In step S2, the catalyst II is potassium carbonate.

8. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to any one of claims 5 to 7, characterized in that: In step S2, The mass ratio of the mixture I to the catalyst II is 1:0.005-0.03; The mass ratio of the mixture I to ethylene glycol is 1:4-7; The temperature of the alcoholysis reaction is 230-260° C., and the time is 1-2 hours.

9. The method for recovering and purifying BHET based on step-by-step reaction and gradual crystallization according to claim 8, characterized in that: The purified BHET crystal has a purity of 98.78-99.42%, a chromaticity L value of 97.5-99, an a value of -0.8--1.23, and a b value of 0.13-0.7.

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