A method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization

Abstract

The present invention discloses a method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization, which belongs to the field of organic polymer recycling. The BHET recovery and purification method of the present invention includes the steps of reacting waste PET products with ethylene glycol in a hydrogen atmosphere under the catalysis of catalyst I at a specific temperature to obtain mixture I containing partially depolymerized PET and a precipitate formed by dyes; mixing mixture I with catalyst II in ethylene glycol for alcoholysis, filtering while hot to remove insoluble matter, cooling to crystallize, and filtering to obtain purified BHET crystals. The method of the present invention can obtain decolorized, high-purity BHET crystals without using conventional purification methods such as sublimation or purification columns. This method is environmentally friendly, easy to operate, and not only enables ethylene glycol recycling, but also has low recovery costs and is advantageous for industrial production. The present invention can be used to recycle waste PET products.

Description

[Technical Field]

[0001] The present invention relates to a method for recovering and purifying BHET, which is based on stepwise reactions and stepwise crystallization, in the field of recycling organic polymer compounds. [Background technology]

[0002] Polyester fiber, commonly referred to as polyester, is the leading synthetic fiber in the textile industry. Global production of polyester was estimated at approximately 53.63 million tons in 2021, consuming a significant amount of non-renewable petroleum resources and posing a serious threat to the ecosystem due to PET-derived waste. Based on publicly available data, 77.5% of China's PET is used in synthetic fiber / textile products. However, because PET fabrics contain dyes and additives, their economic recovery is poor. At the end of their useful life, they are often landfilled or incinerated. Therefore, how to process this waste PET is currently a focus of research. Effective recovery of waste PET can bring positive social and economic benefits while also reducing environmental and resource burdens.

[0003] In recent years, methods for recovering PET waste have primarily included energy recovery, physical recovery, and chemical recovery. Chemical recovery has attracted considerable attention due to its ability to achieve closed-loop PET recovery. Among these chemical recovery methods, alcoholysis has already been widely adopted. Among these, alcoholysis using ethylene glycol (EG) involves decomposing PET waste at a certain temperature and pressure using EG as the reaction medium under the catalysis of a transesterification catalyst. This process, also known as sugar alcoholysis, has mild reaction conditions and low cost. During depolymerization, the carboxyl carbon of the ester group of PET is attacked by the free electron pair of EG, and the hydroxyethyl group of EG combines with the carboxyl carbon of PET, resulting in cleavage of the long chain of PET into oligomers, which then form BHET. While alcoholysis of waste polyester products using ethylene glycol to recover BHET has proven effective, this recovery method poses a challenge: how to purify the target product, BHET. Waste polyester products generally contain large amounts of impurities such as dyes and other condensation polymers, which react to form various further products during alcoholysis, making it difficult to recover decolorized, high-purity BHET.

[0004] The prior Chinese patent document CN115894223A discloses a chemical recovery method for waste PET products utilizing the phase transition properties of BHET crystals, in which the waste PET product and a catalyst are mixed in ethylene glycol and subjected to an alcoholysis reaction to obtain an alcoholysis solution, which is then cooled and crystallized to obtain crystals containing BHET, which are then sublimated under reduced pressure and condensed to obtain high-purity BHET. However, because this recovery process uses vacuum sublimation, it requires a dedicated vacuum sublimation device and imposes high vacuum and temperature requirements during the operation, making the operation difficult and consuming a lot of energy for recovery. Furthermore, without vacuum sublimation purification, it is usually impossible to effectively separate the BHET crystals from the products produced by the alcoholysis reaction of high molecular weight polymers other than PET contained in the waste PET product, which significantly affects the purity of the target BHET. Summary of the Invention [Problem to be solved by the invention]

[0005] The technical problem to be solved by the present invention is to provide a method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization to overcome the drawback of the conventional alcoholysis method using ethylene glycol, which is that it is difficult to separate BHET obtained by decomposing waste PET products from other oligomers and dyes. [Means for solving the problem]

[0006] In order to achieve the above object, the present invention uses the following technical means.

[0007] The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to the present invention comprises the steps of: Step S1: Mixing waste PET products and catalyst I in ethylene glycol, reacting them under a hydrogen atmosphere, cooling and crystallizing them, and then filtering them to obtain mixture I; and step S2 of mixing the mixture I and the catalyst II in ethylene glycol to carry out an alcoholysis reaction, filtering the mixture while hot to remove insoluble matter, cooling the mixture to crystallize the mixture, and filtering the resulting mixture to obtain purified BHET crystals. Mixture I comprises the reaction product of partially depolymerized PET and a dye; The catalyst I is prepared by the in-situ reaction of a metal salt compound with an N- or P-containing organic ligand.

[0008] Preferably, the metal salt compound comprises Fe(OAc)2, FeCl2, Co(OAc)2, CoCl2, Ni(OAc)2, NiCl2, Cu(OAc)2 or CuCl; The organic ligand containing N or P is [ka] , [ka] , [ka] , [ka] or [ka] Includes.

[0009] Preferably, the mass ratio of the metal salt compound to the N- or P-containing organic ligand in the catalyst I is 1:1.5-2.

[0010] Preferably, the reaction in a hydrogen atmosphere involves introducing hydrogen at 10 to 20 atm and reacting at 140 to 150° C. for 40 to 60 minutes.

[0011] Preferably, the waste PET product contains a dye, and the dye is an azo dye or an anthraquinone dye.

[0012] Preferably, in step S1, the mass percentage of PET in the waste PET product is 65 to 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 the ethylene glycol is 1:4 to 1:7, The cooling crystallization is carried out at a temperature of 8 to 12°C for a period of 6 to 18 hours.

[0013] Preferably, in step S2, the catalyst II is potassium carbonate; the mass ratio of the mixture I to the catalyst II is 1:0.005 to 0.03; the mass ratio of the mixture I to the ethylene glycol is 1:4 to 1:7; The alcoholysis reaction is carried out at a temperature of 230 to 260° C. for 1 to 2 hours.

[0014] Preferably, the purified BHET crystals have a purity of 98.78 to 99.42%, a chromaticity L value of 97.5 to 99, an a value of −0.8 to −1.23, and a b value of 0.13 to 0.7.

[0015] The decolorization principle of the present invention is to use the C-H bond of the chromophoric group of the dye to undergo a cross-coupling reaction in the presence of catalyst I to generate a new non-chromophoric group, and then the dye undergoes a polymerization reaction to generate a cross-linked polymer, which is then polymerized from a small molecule to a polymer and precipitates, thereby decolorizing the waste polyester product. The specific reaction principle is as follows:

[0016] The hydrogen used in the present invention reduces the catalyst to a low-valent metal catalytic active center M, For azo dyes, the inductive effect of the N=N double bond of the dye is utilized to carry out an oxidative addition reaction between the low-valent metal catalyst active center M and the C-H bond of the aromatic hydrocarbon to generate a metal-containing heterocyclic compound. Under heating conditions, the metal-containing heterocyclic compound undergoes a series of complex chemical reactions including C-C coupling and reduction of the N=N bond to generate a complex polymer, as shown below.

[0017] [ka]

[0018] The anthraquinone dye undergoes the above-mentioned oxidative addition reaction, utilizing the inductive action of the C=O carbonyl group of the dye, to produce a complex polymer. The reaction is as follows:

[0019] [ka] [Effects of the Invention]

[0020] By using the above technical means, the present invention achieves the following technical advances over the prior art:

[0021] (1) This invention first uses ethylene glycol to treat waste PET products with alcohol, partially depolymerizing the PET in the waste polyester product. During this process, other polymer components (e.g., spandex, nylon, etc.) are completely depolymerized to produce small molecule monomers that dissolve in the ethylene glycol solution. Under these reaction conditions, dye molecules first migrate from between the PET molecules into the ethylene glycol solution. A catalyst I, generated in situ from a metal salt compound and an N- or P-containing organic ligand, catalyzes the reaction of H2 with the dye molecules, converting the dye's color-forming groups to colorless groups. The dye then polymerizes to form a crosslinked polymer, which precipitates from the ethylene glycol. The reaction solution is then cooled and crystallized to yield a mixture I of partially depolymerized PET and a polymer precipitate produced by the dye reaction. Further alcoholysis reaction is carried out under the catalyst of potassium carbonate to produce the target product BHET from the partially depolymerized PET in Mixture I. The colorless precipitate produced by the reaction of the dye molecules in Mixture I is still insoluble and can be removed by filtration while still hot from the alcoholysis solution. After filtration, the filtrate is cooled and crystallized, and then filtered to obtain the decolorized, highly pure target product BHET.

[0022] (2) The reasons for selecting ethylene glycol as the solvent in step S1 of the present invention are as follows: a) At room temperature, ethylene glycol has a very high solubility for impurities but a low solubility for PET or partially depolymerized PET. Therefore, the reaction solution can be cooled and crystallized to separate PET from the impurities, thereby obtaining a relatively high-purity partially depolymerized PET, which can then be used as a raw material for the alcoholysis reaction to obtain BHET; b) By controlling the appropriate temperature and time, ethylene glycol can effectively remove impurities (e.g., sucrose) from the waste PET product. c) Under the conditions of step S1, the azo dyes or anthraquinone dyes that make up a large proportion of textile dyes are dissolved in ethylene glycol, and then catalyst I catalyzes the reaction between H2 and the dye molecules, causing the color-forming groups of the azo dyes or anthraquinone dyes that make up a large proportion of organic pigments to react and form colorless groups, which then undergo a polymerization reaction to form cross-linked polymers that precipitate from the ethylene glycol, thereby achieving decolorization of waste polyester products.

[0023] (3) This invention proposes a method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization by rationally selecting reaction solvents and catalysts, rationally controlling reaction conditions, and optimizing the proportionality of raw materials. This method involves alcoholysis followed by cooling and crystallization, followed by filtration. This method achieves high decolorization efficiency and high purity (98.78-99.42%) of BHET crystals without the need for conventional purification methods such as sublimation or column purification. This method is environmentally friendly, easy to operate, and has relatively low recovery costs. It also allows for the recycling of ethylene glycol. Therefore, the BHET recovery and purification method of this invention is advantageous for industrial production.

[0024] The present invention can be used for recycling waste PET products, and the obtained BHET can be further applied for industrial production.

[0025] The present invention will now be described in more detail with reference to the drawings and specific examples. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 2 is a chromaticity measurement diagram of the BHET crystal in Example 1 of the present invention. [Figure 2] 1 is a GPC curve of the waste PET fabric in Example 1 of the present invention. [Figure 3] 1 is a GPC curve of mixture I1 of Example 1 of the present invention. [Figure 4] 1 is a photograph of filtrate I of Example 1 of the present invention. [Figure 5] 1 is an HPLC spectrum of the BHET crystal in Example 1 of the present invention. [Figure 6] FIG. 10 is a chromaticity measurement diagram of a BHET crystal in Example 2 of the present invention. [Figure 7] 1 is an HPLC spectrum of BHET crystals in Example 2 of the present invention. [Figure 8] FIG. 10 is a chromaticity measurement diagram of a BHET crystal in Example 3 of the present invention. [Figure 9] 1 is an HPLC spectrum of BHET crystals in Example 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in more detail below with reference to specific examples. It should be understood that the examples described are preferred examples of the present invention and are only for the purpose of explaining the present invention, and are not intended to limit the present invention.

[0028] All materials, reagents and the like used in the examples of the present invention are commercially available unless otherwise specified.

[0029] Example 1: A method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization

[0030] (1) The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to this embodiment includes the following steps S1 to S2 in order:

[0031] In step S1, 200 g of washed, dried, and sliced ​​waste PET fabric (PET content: 99 wt%, brown azo dye content: 1 wt%) was placed in a reaction vessel containing 1,200 g of ethylene glycol, and 1 g of catalyst I1 was added. After uniform stirring, the mixture was reacted at 140°C for 60 minutes under a hydrogen atmosphere of 10 atm. After the reaction was completed, the reaction solution was gradually cooled to room temperature, left to crystallize in an environment of 10°C for 12 hours, and then filtered to obtain 200.6 g of mixture I1 and filtrate I.

[0032] Here, catalyst I1 is a compound consisting of Fe(OAc)2 and [ka] were produced by in-situ reaction of the two in a mass ratio of 1:2.

[0033] After the waste PET fabric is treated and reacted in this step, the azo dye in the fabric dissolves in ethylene glycol and reacts with H2 under the catalysis of catalyst I1, causing the color-forming groups in the azo dye to react with colorless groups, which then polymerize to form crosslinked polymers and precipitate from the ethylene glycol. During this step, the PET is partially depolymerized, and a small amount of the partially depolymerized PET dissolves in ethylene glycol. However, most of the partially depolymerized PET does not dissolve in ethylene glycol, and catalyst I1 does not dissolve in ethylene glycol. Since the mixture I1 obtained in this step is filtered, a small amount of ethylene glycol adheres to its surface. Therefore, it is reasonable that the mass of the resulting mixture I1 is slightly greater than the mass of the waste PET fabric.

[0034] In step S2, 200.6 g of mixture I1 and 4.0 g of potassium carbonate were placed in a reaction vessel containing 1203.6 g of ethylene glycol and subjected to an alcoholysis reaction at 250°C for 1.5 hours. After the reaction was completed, the mixture was filtered while still hot to remove insoluble matter, yielding filtrate II, which consisted of the crosslinked polymer formed by the azo dye, catalyst I1, and potassium carbonate. The filtrate II was gradually cooled to room temperature and then left to crystallize at 10°C for 12 hours to precipitate white crystals. This was filtered to yield crude BHET 1 and filtrate III, which was then left to crystallize at 10°C for 2 hours to precipitate white crystals, which was then filtered to yield crude BHET 2.

[0035] Crude BHET 1 and crude BHET 2 were mixed, washed with water, filtered, and then dried at a temperature of 60 to 80°C for 1 to 2 hours. In this example, the drying conditions were 60°C and 2 hours, and purified BHET crystals were obtained.

[0036] As a result of calculation, the yield of BHET recovered by applying the BHET recovery and purification method based on stepwise reactions and stepwise crystallization according to this example is 86.5%.

[0037] (2) In this example, the color of the waste PET fabric, mixture I1, and purified BHET crystals was measured, and the molecular weight distribution of the raw materials and intermediates was also characterized. Specifically, the results are as follows:

[0038] (1) Measurement of chromaticity

[0039] In this example, a LAB colorimeter was used to measure the color of the waste PET fabric, which was the raw material in step S1, the mixture I1 obtained in step S1, and the purified BHET crystals, which was the target product obtained in step S2. The specific results are as follows:

[0040] The waste PET fabric was brown with a color index of L*=40.1, a*=7.09, b*=10.04. Mixture I1, which was mainly partially depolymerized PET, was nearly white and lumpy with a color index of L*=77.64, a*=-1.47, b*=-0.29. The purified BHET crystals were white and granular with a color index measurement diagram shown in Figure 1, with a specific color index of L*=97.5, a*=-0.98, b*=0.26.

[0041] As can be seen from the above color results, the method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to the present invention can effectively remove the original color of waste polyester.

[0042] (2) Characterization of molecular weight distribution

[0043] In this example, gel chromatography was used to characterize the molecular weight distributions of the raw materials, intermediates, and products. The GPC curves of the waste PET fabric are shown in Figure 2, and the GPC curves of mixture I1 are shown in Figure 3.

[0044] The statistical table of molecular weight distribution of PET in the waste PET fabric and the partially depolymerized PET in mixture I1 is shown in Table 1 below.

[0045] Table 1. Statistical table of molecular weight distribution of raw materials and different intermediates [Table 1]

[0046] Note that GPC measures the molecular weight and distribution of a polymer (e.g., PET), and other impurities are filtered out before measurement, so the GPC curve for mixture I1 represents the molecular weight and distribution of partially depolymerized PET.

[0047] As can be seen from Figures 2-3 and Table 1, PET was partially depolymerized during the reaction in step S1, but there was sufficient molecular weight to precipitate all of the PET without loss after the reaction solution was cooled.

[0048] (3) Color of filtrate I

[0049] Filtrate I is a colorless, transparent liquid, specifically as shown in FIG.

[0050] (3) The purity of the purified BHET crystals obtained in step S2 of (1) of this Example was measured using HPLC. The HPLC spectrum of the purified BHET crystals is shown in Figure 5.

[0051] As a result of detection and calculation, the purified BHET crystals obtained have a purity of 99.3%.

[0052] Example 2: BHET recovery and purification method based on stepwise reaction and stepwise crystallization

[0053] (1) The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to this embodiment includes the following steps S1 to S2 in order:

[0054] In step S1, 200 g of washed, dried, and sliced ​​waste PET product (PET content: 89 wt%, spandex content: 10 wt%, brown anthraquinone dye content: 1 wt%) was placed in a reaction vessel containing 1200 g of ethylene glycol, and 2 g of catalyst I2 was added. After uniform stirring, the mixture was reacted at 150°C for 50 minutes under a hydrogen atmosphere of 14 atm. After the reaction was completed, the reaction solution was gradually cooled to room temperature and then left to crystallize at 10°C for 12 hours. After filtration, 181.8 g of mixture I2 and filtrate IV were obtained.

[0055] Here, the catalyst I2 is CuCl and [ka] were produced by in-situ reaction at a mass ratio of 1:1.5.

[0056] In step 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 an alcoholysis reaction at 250°C for 1.5 hours. After the reaction was completed, the mixture was filtered while still hot to remove insoluble matter, yielding filtrate V. Filtrate V was gradually cooled to room temperature and then allowed to crystallize at 10°C for 12 hours to precipitate white crystals, which were then filtered to yield crude BHET product 3 and filtrate VI. Filtrate VI was allowed to stand at 10°C for 2 hours to precipitate white crystals, which were then filtered to yield crude BHET product 4.

[0057] Crude BHET 3 and crude BHET 4 were mixed, washed with water, filtered, and then dried at 60°C for 2 hours to obtain purified BHET crystals.

[0058] As a result of calculation, the yield of BHET recovered by applying the BHET recovery and purification method based on stepwise reactions and stepwise crystallization according to this example is 83.9%.

[0059] (2) In this example, the chromaticity of the above waste PET product, mixture I2 and BHET crystals was measured, specifically as follows:

[0060] In this example, a LAB colorimeter was used to measure the color of the waste PET product, which is the raw material in step S1, the mixture I2 obtained in step S1, and the purified BHET crystals, which is the target product obtained in step S2. The specific results are as follows:

[0061] The waste PET product was brown in color with a color index of L*=16.50, a*=2.21, b*=9.25. Mixture I2 was nearly white and lumpy with a color index of L*=74.39, a*=2.02, b*=8.31. The BHET crystals were white and particulate with a color index of L*=99, a*=-1.23, b*=0.38. The colorimetric diagram of the BHET crystals is shown in Figure 6.

[0062] As can be seen from the above color results, the method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to the present invention can effectively remove the original color in waste PET products.

[0063] (2) The purity of the purified BHET crystals obtained in step S2 of (1) of this Example was measured using HPLC. The HPLC spectrum of the purified BHET crystals is shown in Figure 7. The results of the detection and calculation showed that the purity of the purified BHET crystals obtained was 98.8%.

[0064] Example 3: BHET recovery and purification method based on stepwise reaction and stepwise crystallization

[0065] (1) The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to this embodiment includes the following steps S1 to S2 in order:

[0066] In step 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, and 0.6 g of catalyst I3 was added. After uniform stirring, the mixture was reacted at 145°C for 40 minutes under a hydrogen atmosphere of 20 atm. After the reaction was completed, the reaction solution was gradually cooled to room temperature, left to stand at 10°C for 12 hours to crystallize, and then filtered to obtain 132.4 g of mixture I3.

[0067] Here, the catalyst I3 is FeCl2 and [ka] were produced by in-situ reaction in a mass ratio of 1:1.7.

[0068] In step 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 an alcoholysis reaction at 230°C for 2.0 hours. After the reaction was completed, the mixture was filtered while still hot to remove insoluble matter (mainly cotton cellulose, catalyst I3, precipitates produced by the dye, and potassium carbonate) to obtain filtrate VII. Filtrate VII was gradually cooled to room temperature and then left to crystallize at 8°C for 10 hours to precipitate white crystals, which were then filtered to obtain crude BHET 5 and filtrate VIII. Filtrate VIII was then left to crystallize at 10°C for 2 hours to precipitate white crystals, which were then filtered to obtain crude BHET 6.

[0069] Crude BHET 5 and crude BHET 6 were mixed, washed with water, filtered, and then dried at 70°C for 1.5 hours to obtain purified BHET crystals.

[0070] As a result of calculation, the yield of BHET recovered by the method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to this example is 86.0%.

[0071] (2) In this example, the color of the waste PET fabric, the first partially depolymerized PET, and the BHET crystals were measured. The specific results are as follows:

[0072] In this example, a LAB colorimeter was used to measure the color of the waste PET fabric, which is the raw material in step S1, the mixture I3 obtained in step S1, and the BHET crystals, which are the target product obtained in step S2. The specific results are as follows:

[0073] The waste PET fabric was red in color with a chromaticity of L*=25.05, a*=27.87, b*=0.90. Mixture I3 was nearly white and lumpy with a chromaticity of L*=80.41, a*=-0.27, b*=4.57. The BHET crystals were white and granular with a chromaticity of L*=98.82, a*=-0.97, b*=0.13. The colorimetric diagram of the BHET crystals is shown in Figure 8.

[0074] As can be seen from the above color results, the method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to the present invention can effectively remove the original color in waste PET products.

[0075] (3) The purity of the purified BHET crystals obtained in step S2 of (1) of this Example was measured using HPLC. The HPLC spectrum of the purified BHET crystals is shown in Figure 9. The results of detection and calculation indicate that the purity of the purified BHET crystals obtained is 99.4%.

[0076] Examples 4 to 8: BHET recovery and purification method based on stepwise reaction and stepwise crystallization

[0077] Examples 4 to 8 are methods for recovering and purifying BHET based on stepwise reaction and stepwise crystallization, respectively, and the steps are basically the same as those of the method according to Example 1, differing only in that the control parameters of different steps are different, which are specifically shown in Table 2.

[0078] Table 2: List of control parameters for different steps in Examples 4 to 8 [Table 2] [Table 3]

[0079] The purity, color, and yield of BHET crystals obtained by applying different BHET recovery and purification methods based on stepwise reactions and stepwise crystallization in Examples 4 to 8 were statistically analyzed, and the statistical table is shown in Table 3 below.

[0080] Table 3: Statistical table of yields and purity of the obtained BHET crystals in Examples 4 to 8 [Table 4]

[0081] Obviously, the above-described embodiments of the present invention are merely examples for clarifying the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art may make various changes or modifications in other forms based on the above description. It is not possible to cover all embodiments here. Any obvious changes or modifications made by the technical means of the present invention still fall within the protection scope of the present invention.

Claims

1. Step S1: mixing a waste PET product and a catalyst I in ethylene glycol, reacting them under a hydrogen atmosphere, cooling them to crystallize them, and then filtering them to obtain a mixture I; and step S2 of mixing the mixture I and the catalyst II in ethylene glycol to carry out an alcoholysis reaction, filtering the mixture while hot to remove insoluble matters, cooling the mixture to crystallize the mixture, and filtering the resulting mixture to obtain purified BHET crystals. Mixture I comprises the reaction product of partially depolymerized PET and a dye; The catalyst I is produced by an in-situ reaction between a metal salt compound and an organic ligand containing N or P, and the method for recovering and purifying BHET is based on stepwise reactions and stepwise crystallization.

2. The metal salt compound is Fe(OAc) ２ , FeCl ２ , Co(OAc) ２ , CoCl ２ , Ni(OAc) ２ , NiCl ２ , Cu(OAc) ２ or CuCl, The organic ligand containing N or P is 【Chemistry 1】 、 【Chemistry 2】 、 【Transformation 3】 、 【Chemistry 4】 or 【Transformation 5】 2. The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to claim 1, comprising:

3. 2. The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to claim 1, wherein the mass ratio of the metal salt compound to the N- or P-containing organic ligand in the catalyst I is 1:1.5-2.

4. The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to any one of claims 1 to 3, characterized in that the reaction under a hydrogen atmosphere comprises introducing hydrogen at 10 to 20 atm and reacting for 40 to 60 minutes under conditions of 140 to 150°C.

5. 5. The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to claim 4, wherein the waste PET products contain dyes, and the dyes are azo dyes and anthraquinone dyes.

6. In step S1, the mass percentage of PET in the waste PET product is 65 to 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 the ethylene glycol is 1:4 to 1:7; The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to claim 4, wherein the cooling crystallization is carried out at a temperature of 8 to 12°C for a time of 6 to 18 hours.

7. 5. The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to claim 4, wherein in step S2, the catalyst II is potassium carbonate.

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

9. The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to claim 8, characterized in that the purified BHET crystals have a purity of 98.78 to 99.42%, a color L value of 97.5 to 99, an a value of -0.8 to -1.23, and a b value of 0.13 to 0.7.

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