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 belongs to the recycling field of organic polymer compounds, relates to a method for recovering and purifying BHET, and specifically is a method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization. [Background Art]

[0002] Polyester fiber mainly refers to polyethylene terephthalate (PET) fiber, which is generally called polyester and ranks first among synthetic fibers in the spinning industry. In 2021, the total global output of polyester was about 53.63 million tons. It consumes a large amount of non-renewable petroleum resources, and PET-derived waste also poses a serious threat to ecosystems. According to published data, 77.5% of PET in China is used for synthetic fibers and spinning products. However, since PET fabrics contain dyes and auxiliaries, the economic effect of recovery is low, and they are often landfilled or incinerated after the end of their service life. Therefore, how to treat these waste PETs is the current research focus. Effective recovery of waste PET can bring good social benefits and economic effects, and can also reduce the load on the environment and resources.

[0003] In recent years, there are mainly three methods for recovering waste PET: energy recovery, physical recovery, and chemical recovery. Among these, chemical recovery methods have attracted considerable attention because they enable closed-loop recovery of PET. Among the chemical recovery methods, the alcohol decomposition method is already being applied on a large scale. In particular, the alcohol decomposition method using ethylene glycol (EG) involves decomposing waste PET under the catalysis of a transesterification catalyst using EG as the reaction medium at a constant temperature and pressure. Also known as sugar alcohol decomposition, it has mild reaction conditions and low costs. 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, causing the long chain of PET to be cleaved and forming an oligomer, which subsequently forms BHET. While the method of recovering BHET by alcohol decomposition of waste polyester products using ethylene glycol is effective to some extent, this recovery method has the problem of how to purify the target product, BHET. Waste polyester products generally contain large amounts of impurities such as dyes and other condensed polymers. These impurities also react during alcohol decomposition to produce various other products, making it difficult to recover decolorized, high-purity BHET.

[0004] Conventional Chinese patent document CN115894223A discloses a chemical recovery method for waste PET products utilizing the phase transition properties of BHET crystals. This method involves mixing waste PET products and a catalyst in ethylene glycol, performing an alcohol decomposition reaction to obtain an alcohol decomposition solution, cooling and crystallizing it to obtain a crystalline product containing BHET, and then sublimating it under reduced pressure and condensing it to obtain high-purity BHET. However, because this recovery process uses reduced-pressure sublimation, it requires a dedicated reduced-pressure sublimation apparatus, demands high vacuum and temperature during the operation, is difficult to perform, consumes a lot of energy, and without reduced-pressure sublimation purification, it is usually impossible to effectively separate the BHET crystals from products generated by the alcohol decomposition reaction of other polymers other than PET contained in the waste PET product, which significantly affects the purity of the target product, BHET. [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The technical problem that this invention aims to solve is to provide a method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization, in order to overcome the drawback of conventional alcohol decomposition methods using ethylene glycol, which makes it difficult to separate BHET obtained by decomposing waste PET products from other oligomers and dyes. [Means for solving the problem]

[0006] To achieve the above objective, the present invention employs the following technical means.

[0007] The method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization according to the present invention is: Step S1 involves mixing waste PET products and catalyst I in ethylene glycol, reacting them under a hydrogen atmosphere, cooling and crystallizing, and then filtering to obtain mixture I. Step S2 involves mixing mixture I and catalyst II in ethylene glycol, carrying out an alcohol decomposition reaction, filtering while hot to remove insoluble matter, cooling and crystallizing, and then filtering to obtain purified BHET crystals. The mixture I comprises a 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.

[0008] Preferably, the metal salt compound includes Fe(OAc)2, FeCl2, Co(OAc)2, CoCl2, Ni(OAc)2, NiCl2, Cu(OAc)2, or CuCl. The aforementioned 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 organic ligand containing N or P in the catalyst I is 1:1.5 to 2.

[0010] Preferably, the reaction is carried out under a hydrogen atmosphere, which involves introducing hydrogen at 10-20 atm and reacting for 40-60 minutes under conditions of 140-150°C.

[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 65-99%. The weight percentage of 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 7. The aforementioned cooling crystallization is performed at a temperature of 8-12°C for a duration of 6-18 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 7. The aforementioned alcohol decomposition reaction takes place at a temperature of 230-260°C for 1-2 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 that in the presence of catalyst I, a cross-coupling reaction is carried out by using the C-H bond of the chromophore group of the dye to generate a new non-chromophore group; meanwhile, the dye undergoes a polymerization reaction to form a crosslinked polymer, which polymerizes from a small molecular substance into a high molecular substance and precipitates, thereby realizing decolorization of waste polyester products. The specific reaction principle is as follows.

[0016] The hydrogen used in the present invention reduces the catalyst to a low-valent metal catalyst active center M, For azo dyes, by utilizing the induction effect of the N=N double bond of the dye, an oxidative addition reaction between the low-valent metal catalyst active center M and the C-H bond of the aromatic hydrocarbon is carried out to generate a metal-containing heterocyclic compound; under heating conditions, a series of complex chemical reactions including C-C coupling and reduction of the N=N bond are further carried out on the metal-containing heterocyclic compound to generate a complex polymer. The reaction is as follows.

[0017]

Chemical Formula

[0018] For anthraquinone dyes, by utilizing the induction effect of the C=O carbonyl group of the dye, the above oxidative addition reaction is carried out to generate a complex polymer. The reaction is as follows.

[0019]

Chemical Formula

Effects of the Invention

[0020] Due to the adoption of the above technical solutions, the present invention achieves the following technical progress compared with the prior art.

[0021] (1) In the present invention, first, an alcohol treatment reaction is carried out on waste PET products using ethylene glycol to partially depolymerize the PET in the waste polyester product. In this process, other polymer components other than PET (e.g., spandex, nylon, etc.) are completely depolymerized to produce small molecule monomers which are dissolved in the ethylene glycol solution. Under these reaction conditions, dye molecules first migrate from between PET molecules to the ethylene glycol solution. Catalyst I, which is produced in situ from a metal salt compound and an organic ligand containing N or P, catalyzes the reaction between H2 and the dye molecules, causing the chromogenic groups of the dye to react and produce colorless groups. The dye then undergoes a polymerization reaction to produce a crosslinked polymer, which precipitates from the ethylene glycol. The reaction solution is cooled and crystallized to obtain a mixture I of partially depolymerized PET and a polymer precipitate produced by the reaction of the dye. The alcohol decomposition reaction is further carried out under potassium carbonate catalysis to produce the target product BHET from the partially depolymerized PET in mixture I. The colorless precipitate produced by the reaction of dye molecules in mixture I is still insoluble and can be removed by filtering while hot with the alcohol decomposition solution. After filtration, the filtrate is cooled, crystallized, and filtered to obtain the decolorized, high-purity target product BHET.

[0022] (2) The basis for selecting ethylene glycol as the solvent in step S1 of the present invention is as follows: a) At room temperature, ethylene glycol has very high solubility for impurities but low solubility for PET or partially depolymerized PET, so by cooling and crystallizing the reaction solution, PET and impurities can be separated, thereby obtaining PET with relatively high purity and partially depolymerized, which can then be used as a raw material for the alcohol decomposition reaction to obtain BHET; b) By controlling the temperature and time appropriately, ethylene glycol can be used to remove impurities in waste PET products (e.g., s) The catalyst effectively participates in the depolymerization reaction of materials such as Pandex and nylon, and the depolymerization product dissolves in ethylene glycol. Under these conditions, PET is only partially depolymerized. In step S1, after an azo dye or anthraquinone dye, which accounts for a large proportion of the spinning dye, dissolves in ethylene glycol, catalyst I catalyzes the reaction between H2 and the dye molecules, causing the chromogenic groups of the azo dye or anthraquinone dye, which accounts for a large proportion of the organic pigment, to react and produce colorless groups. These groups then undergo polymerization to form a crosslinked polymer, which precipitates from the ethylene glycol, thereby achieving decolorization of the waste polyester product.

[0023] (3) The present invention proposes a method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization by rationally selecting the reaction solvent and catalyst, rationally controlling the reaction conditions, and optimizing the proportional relationship between the raw materials. This method can obtain BHET crystals with a high decolorization effect and high purity (98.78-99.42%) without using conventional purification methods such as cooling and crystallization after alcohol decomposition, filtration, and purification by sublimation or purification column. This method is not only environmentally friendly, easy to operate, and has relatively low recovery costs, but also enables the recycling of ethylene glycol in this method. Therefore, the method for recovering and purifying BHET according to the present invention is advantageous for industrial production.

[0024] This invention can be used for recycling waste PET products, and the resulting BHET can be further applied to industrial production.

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

[0026] [Figure 1] This is a color measurement diagram of a BHET crystal in Example 1 of the present invention. [Figure 2] This is the GPC curve of waste PET fabric in Example 1 of the present invention. [Figure 3] This is the GPC curve for mixture I1 of Example 1 of the present invention. [Figure 4] This is a photograph of filtrate I from Example 1 of the present invention. [Figure 5] This is the HPLC spectrum of the BHET crystal in Example 1 of the present invention. [Figure 6] This is a color measurement diagram of a BHET crystal in Example 2 of the present invention. [Figure 7] This is the HPLC spectrum of the BHET crystal in Example 2 of the present invention. [Figure 8] This is a color measurement diagram of a BHET crystal in Example 3 of the present invention. [Figure 9] This is the HPLC spectrum of the BHET crystal in Example 3 of the present invention. [Modes for carrying out the invention]

[0027] The present invention will be described in more detail below with reference to specific examples. The examples described are preferred embodiments of the present invention and are intended solely to illustrate the invention; they do not limit it.

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

[0029] Example 1: Method for recovering and purifying BHET based on stepwise reaction 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 waste PET fabric (containing 99 wt% PET and 1 wt% brown azo dye) after washing, drying, and slicing was placed in a reaction vessel containing 1200 g of ethylene glycol, 1 g of catalyst I1 was added, and the mixture was stirred uniformly. The reaction was then carried out at 140°C for 60 min under a hydrogen atmosphere of 10 atm. After the reaction was complete, the reaction solution was gradually cooled to room temperature, and then allowed to crystallize for 12 hours in an environment of 10°C. The mixture was filtered to obtain 200.6 g of mixture I1 and filtrate I.

[0032] Here, catalyst I1 is composed of Fe(OAc)2 and [ka] It is produced by an in-situ reaction in a mass ratio of 1:2.

[0033] After the waste PET fabric is treated and reacted in this step, the azo dyes within it dissolve in ethylene glycol and then react with H2 under the catalytic action of catalyst I1. The chromogenic groups of the azo dyes react to form colorless groups, which then undergo polymerization to produce a crosslinked polymer, which precipitates from the ethylene glycol. In this step, the PET is partially depolymerized, and a small amount of the partially depolymerized PET dissolves in ethylene glycol, but most of the partially depolymerized PET does not dissolve in ethylene glycol, and catalyst I1 also does not dissolve in ethylene glycol. Since the mixture I1 obtained in this step is obtained by filtration, a small amount of ethylene glycol adheres to its surface, so it is reasonable that the mass of the obtained mixture I1 is slightly larger 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 the alcohol decomposition reaction was carried out at 250°C for 1.5 hours. After the reaction was complete, the mixture was filtered while still hot to remove insoluble matter and obtain filtrate II, which consisted of a crosslinked polymer produced by the azo dye, catalyst I1, and potassium carbonate. After gradually cooling filtrate II to room temperature, it was left at 10°C for 12 hours to crystallize, precipitating white crystals, which were then filtered to obtain crude BHET product 1 and filtrate III. Fillet III was left at 10°C for 2 hours to precipitate white crystals, which were then filtered to obtain crude BHET product 2.

[0035] Crude BHET 1 and crude BHET 2 were mixed, washed with water, filtered, and then dried. The drying temperature was set to 60-80°C and the drying time to 1-2 hours. In this example, 60°C and 2 hours were selected as the drying conditions to obtain purified BHET crystals.

[0036] Calculations show that the BHET recovered using the stepwise reaction and stepwise crystallization recovery and purification method for BHET according to this embodiment yields 86.5%.

[0037] (ii) In this embodiment, in addition to measuring the chromaticity of the waste PET fabric, mixture I1, and purified BHET crystals, the molecular weight distribution of the raw materials and intermediates was characterized, specifically as follows.

[0038] (1) Measurement of chromaticity

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

[0040] The waste PET fabric is brown, with a chromaticity of L*=40.1, a*=7.09, and b*=10.04. Mixture I1 is mainly partially depolymerized PET, is nearly white, and lumpy, with a chromaticity of L*=77.64, a*=-1.47, and b*=-0.29. The purified BHET crystals are white and particulate, with a chromaticity measurement diagram as shown in Figure 1, and specific chromaticity values ​​of L*=97.5, a*=-0.98, and b*=0.26.

[0041] As can be seen from the above chromaticity results, the BHET recovery and purification method 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, the molecular weight distribution of the raw materials, intermediates, and products was characterized using gel chromatography. The GPC curve for the waste PET fabric is shown in Figure 2, and the GPC curve for mixture I1 is shown in Figure 3.

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

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

[0046] Furthermore, since GPC measures the molecular weight and distribution of polymers (e.g., PET), and other impurities are filtered out before measurement, the GPC curve of 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 partially depolymerized during the reaction in step S1. However, after cooling the reaction mixture, there was still enough molecular weight to precipitate all of the PET without loss.

[0048] (3) Color of filtrate I

[0049] Filtrate I is a colorless, transparent liquid, specifically as shown in Figure 4.

[0050] (iii) The purity of the purified BHET crystals obtained in step S2 of (i) of this example was measured by combining HPLC, and the HPLC spectrum of the purified BHET crystals is shown in Figure 5.

[0051] Based on the detection and calculation results, the purified BHET crystals obtained have a purity of 99.3%.

[0052] Example 2: Method for recovering and purifying BHET 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 waste PET product (containing 89 wt% PET, 10 wt% spandex, and 1 wt% brown anthraquinone dye) after washing, drying, and slicing was 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 reaction was then carried out at 150°C for 50 min under a hydrogen atmosphere of 14 atm. After the reaction was complete, the reaction solution was gradually cooled to room temperature, and then allowed to crystallize for 12 hours in an environment of 10°C. The mixture was filtered to obtain 181.8 g of mixture I2 and filtrate IV.

[0055] Here, catalyst I2 is composed of CuCl and [ka] It is produced by an in-situ reaction in 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 the alcohol decomposition reaction was carried out at 250°C for 1.5 hours. After the reaction was complete, the mixture was filtered while still hot to remove insoluble matter and obtain filtrate V. Filtrate V was gradually cooled to room temperature and then left at 10°C for 12 hours to crystallize, allowing white crystals to precipitate. This was then filtered to obtain crude BHET product 3 and filtrate VI. Filtrate VI was left at 10°C for 2 hours to precipitate white crystals, and then filtered to obtain 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] Calculations show that the BHET recovered using the BHET recovery and purification method based on the stepwise reaction and stepwise crystallization described in this embodiment yields 83.9%.

[0059] (ii) In this embodiment, the chromaticity of the above-mentioned waste PET product, mixture I2, and BHET crystal was measured, specifically as follows:

[0060] In this example, the chromaticity 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, was measured using a LAB colorimeter. The specific results are as follows.

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

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

[0063] (ii) The purity of the purified BHET crystals obtained in step S2 of (i) of this example was measured using HPLC, and the HPLC spectrum of the purified BHET crystals is shown in Figure 7. Based on the detection and calculation results, the purity of the obtained purified BHET crystals is 98.8%.

[0064] Example 3: Method for recovering and purifying BHET 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 waste PET fabric (containing 65 wt% PET, 34 wt% cotton, and 1 wt% red azo dye) after washing, drying, and slicing was placed in a reaction vessel containing 1000 g of ethylene glycol. 0.6 g of catalyst I3 was added, and the mixture was stirred uniformly. The reaction was then carried out at 145°C for 40 min under a hydrogen atmosphere of 20 atm. After the reaction was complete, the reaction solution was gradually cooled to room temperature, then allowed to crystallize for 12 hours at 10°C. The mixture was filtered to obtain 132.4 g of mixture I3.

[0067] Here, catalyst I3 is FeCl2 and [ka] It is produced by an 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 the alcohol decomposition reaction was carried out at 230°C for 2.0 hours. After the reaction was complete, 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, then left at 8°C for 10 hours to crystallize, allowing white crystals to precipitate. This was then filtered to obtain crude BHET product 5 and filtrate VIII. Filtrate VIII was left at 10°C for 2 hours to precipitate white crystals, and then filtered to obtain crude BHET product 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] Calculations show that the BHET recovered using the BHET recovery and purification method based on the stepwise reaction and stepwise crystallization described in this embodiment has a yield of 86.0%.

[0071] (ii) In this embodiment, the chromaticity of the above-mentioned waste PET fabric, the first partially depolymerized PET, and the BHET crystals was measured, specifically as follows.

[0072] In this example, the chromaticity 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, was measured using a LAB colorimeter. The specific results are as follows.

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

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

[0075] (iii) The purity of the purified BHET crystals obtained in step S2 of (i) of this example was measured using HPLC, and the HPLC spectrum of the purified BHET crystals is shown in Figure 9. Based on the detection and calculation results, the purity of the obtained purified BHET crystals is 99.4%.

[0076] Examples 4-8: Method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization.

[0077] Examples 4 to 8 are methods for recovering and purifying BHET based on stepwise reactions and stepwise crystallization, respectively. The steps are basically the same as those in Example 1, differing only in the control parameters of the different steps, which are 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 the different stepwise reaction and stepwise crystallization recovery and purification methods described in Examples 4 to 8 were statistically analyzed and are shown in Table 3 below.

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

[0081] Clearly, the above embodiments of the present invention are merely examples to clearly illustrate the present invention and are not intended to limit the embodiments of the present invention. Those skilled in the art can make various other variations or modifications based on the above description. Not all embodiments can be covered here. Any obvious variations or modifications by the technical means of the present invention still fall within the scope of protection of the present invention.

Claims

1. Step S1 involves mixing waste PET products and catalyst I in ethylene glycol, reacting them under a hydrogen atmosphere, cooling and crystallizing them, and then filtering to obtain mixture I. The process comprises, in order, step S2: mixing mixture I and catalyst II in ethylene glycol, carrying out an alcohol decomposition reaction, filtering while hot to remove insoluble matter, cooling and crystallizing, and then filtering to obtain purified BHET crystals. The mixture I comprises a reaction product of partially depolymerized PET and a dye. The catalyst II is potassium carbonate, The catalyst I is produced by an in-situ reaction between a metal salt compound and an organic ligand containing N or P. The aforementioned metal salt compound includes Fe(OAc)₂, FeCl₂, Co(OAc)₂, CoCl₂, Ni(OAc)₂, NiCl₂, Cu(OAc)₂, or CuCl. The aforementioned organic ligand containing N or P is 【Chemistry 1】 、 【Chemistry 2】 、 【Transformation 3】 、 【Chemistry 4】 or 【Transformation 5】 including, A method for recovering and purifying BHET based on stepwise reactions and stepwise crystallization, characterized by the above.

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

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

4. The method for recovering and purifying BHET based on stepwise reaction and stepwise crystallization according to claim 3, characterized in that the waste PET product contains a dye, and the dye is an azo dye and an anthraquinone dye.

5. In step S1, the mass percentage of PET in the waste PET product is 65 to 99%. The weight percentage of 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 7. The method for recovering and purifying BHET based on a stepwise reaction and stepwise crystallization according to claim 3, characterized in that the cooling crystallization is performed at a temperature of 8 to 12°C for a duration of 6 to 18 hours.

6. 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 7. A method for recovering and purifying BHET based on a stepwise reaction and stepwise crystallization according to any one of claims 3 to 5, characterized in that the alcohol decomposition reaction is carried out at a temperature of 230 to 260°C and for a duration of 1 to 2 hours.

7. The method for recovering and purifying BHET based on a stepwise reaction and stepwise crystallization according to claim 6, 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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