Pet depolymerization product purification method

Abstract

The present invention relates to a PET depolymerization product purification method and, specifically, to a PET depolymerization product purification method which can readily obtain, from a PET depolymerization product containing impurities such as an adhesive component and a colored pigment, high-purity terephthalic acid that can be commercialized, and which can collect a used depolymerization catalyst.

Description

Purification method of PET depolymerization product

[0001] The present invention relates to a method for purifying a PET depolymerization product, and more particularly, to a method for purifying a PET depolymerization product, which can easily obtain high-purity terephthalic acid that can be commercialized from a PET depolymerization product containing impurities such as adhesive components and colored pigments, and can recover a used depolymerization catalyst.

[0002] Polyester is a type of plastic used in various applications such as films, containers, paints, machine parts, and fibers. A representative example of this is PET (Polyethylene terephthalate). PET can be manufactured by polymerizing terephthalic acid (TPA) or dimethyl terephthalate (DMT) with ethylene glycol (EG). Since PET is widely used industrially, it generates a lot of waste, so it is a material that is recycled due to regulations in each country. Research is being conducted on depolymerization methods to convert it into monomers.

[0003] There are various known chemical reaction pathways for PET depolymerization methods, including hydrolysis, methanolysis, and glycolysis. Among the above depolymerization methods, the hydrolysis method has the advantage of being able to hydrolyze waste PET containing up to 40 wt% of impurities.

[0004] The above hydrolysis method is a method in which depolymerization is carried out under high temperature conditions in the presence of a high concentration of acid or base catalyst, and TPA is ultimately obtained as a depolymerization product. However, the basic hydrolysis reaction using the base catalyst has the problem that, depending on the use and condition of the PET raw material, an unspecified number of impurities such as pigments, adhesives, minerals, etc. are included in the depolymerization product, making it difficult to purely separate terephthalic acid and ethylene glycol from the impurities, and making it difficult to commercialize the product.

[0005] In addition, the basic hydrolysis method uses an alkaline catalyst, typically sodium hydroxide (NaOH) or potassium hydroxide (KOH), and since the catalyst is consumed for the production of disodium terephthalate or dipotassium terephthalate immediately after the depolymerization reaction, it is introduced in excess to stably maintain the pKa of the basic hydrolysis reaction. However, the catalyst is discarded in the form of an insoluble salt due to the strong acid introduced for the protonation of the terephthalate salt, making it difficult to recover, which increases the unit price of raw materials economically, and can also cause environmental problems due to the increase in waste.

[0006] As a prior art, Korean Patent No. 10-0983349 (announced on September 20, 2010) discloses a technology for obtaining a product by hydrolyzing PET with KOH or NaOH, adding water to the product to separate the aqueous phase and the organic phase, and converting the terephthalate into TPA by adding sulfuric acid to the aqueous phase. Korean Patent No. 10-2269702 (announced on June 28, 2021) discloses a technology for obtaining a solution by dissolving disodium terephthalate (Na2-TPA) in an aqueous solution by adding water to the product by hydrolyzing PET in the presence of sodium carbonate (Na2CO3) and zinc acetate, decolorizing the solution by adding a coagulant, and then obtaining TPA using sulfuric acid.

[0007] Nevertheless, research and development is needed on a method for purifying PET depolymerization products that enables the recovery of high-purity terephthalic acid of 99% or more from the depolymerization products through basic hydrolysis, and further, the recovery of basic catalysts from the depolymerization products.

[0008] The present invention aims to provide a method for purifying a PET depolymerization product, which enables recovery of terephthalic acid with a purity of 99% or higher from a depolymerization product resulting from basic hydrolysis, and further enables recovery of a base catalyst from the depolymerization product.

[0009] In order to solve the above problem, in the method for purifying a PET depolymerization product of the present invention, (a) a step of depolymerizing a polymer including PET as a raw material through basic hydrolysis in the presence of a metal base catalyst to obtain a PET depolymerization product including a terephthalate metal salt; (b) a step of filtering the obtained PET depolymerization product to obtain a cake, and then adding water to the cake to elute an aqueous solution including a terephthalate metal salt; (c) a step of adding at least one organic solvent among an ether solvent and a ketone solvent having a solubility in water of 20 g / 100 mL or more to the aqueous solution including the terephthalate metal salt of step (b) to separate the phases into an aqueous phase and an organic phase, and then obtaining a solution of an aqueous phase including a terephthalate metal salt; And (d) a step of reacting a strong acid with a solution containing the terephthalate metal salt to precipitate the terephthalate metal salt as a solid terephthalic acid, and obtaining a solution containing a salt in which a base catalyst-derived metal and a conjugate base of the strong acid are combined.

[0010] The metal base catalyst of step (a) above may be one or more of potassium hydroxide, lithium hydroxide, and sodium hydroxide.

[0011] The organic solvent introduced in the above step (c) may be characterized by including at least one of an ether solvent including tetrahydrofuran (THF) and dioxane; a ketone solvent including methyl ethyl ketone (MEK) and acetone;

[0012] The amount of organic solvent introduced in the above step (c) may be characterized by being introduced 1 to 4 times the weight of water introduced in the above step (b).

[0013] As an example of the present invention, after step (c), paper slurry incineration ash can be added to adsorb and remove foreign substances or impurities.

[0014] Another example of the present invention may be characterized by including, after step (d), a step of (e) filtering out the solid terephthalic acid precipitated in step (d), then adding paper slurry incinerator ash to a solution containing a salt in which a base catalyst-derived metal and a conjugate base of a strong acid are combined, recovering a metal base catalyst from the salt in which the base catalyst-derived metal and a conjugate base of a strong acid are combined, and precipitating a salt in which the base catalyst-derived metal and a conjugate base of a strong acid are combined.

[0015] A method for purifying a PET depolymerization product, characterized in that the above-mentioned paper slurry incineration ash is replaced with one or more of calcium oxide, calcium hydroxide, magnesium oxide and magnesium hydroxide.

[0016] According to the present invention, a pure terephthalate metal salt can be separated from a depolymerization product of basic hydrolysis containing impurities such as pigments and adhesives, and by converting it into terephthalic acid, terephthalic acid (TPA) having a high purity of 99% or more required for commercialization can be obtained.

[0017] In addition, the salt formed by combining the conjugate base of a metal and a strong acid derived from the base catalyst, which is the residue after obtaining the terephthalic acid, can be converted back into the form of a metal base catalyst to recover the catalyst.

[0018] Figure 1 is a drawing showing the appearance of TPA and recovered KOH obtained according to comparative examples and examples of the present invention and the HPLC spectrum of TPA.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In general, the nomenclature used herein is well known and commonly used in the art.

[0020] Throughout this specification, whenever a part is said to “include” a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0021] In the existing basic hydrolysis method of the prior art, the method for purifying the depolymerization product only discloses a technique for simply adding water to the depolymerization product to separate the aqueous phase containing the terephthalate salt and the organic phase containing organic component impurities. However, the organic impurities such as adhesives or pigment components may be partially included in the aqueous phase, thereby reducing the purity of the terephthalic acid finally obtained. Accordingly, the applicant discovered a purification method capable of obtaining high-purity terephthalic acid of 99% or more required for commercialization by adding an appropriate ratio of THF to water to effectively separate and remove organic impurities through phase separation, thereby completing the present invention.

[0022] Hereinafter, the purification method of the PET depolymerization product of the present invention will be examined.

[0023] In the method for purifying a PET depolymerization product of the present invention, (a) a step of depolymerizing a polymer including PET as a raw material through basic hydrolysis in the presence of a metal base catalyst to obtain a PET depolymerization product including a terephthalate metal salt; (b) a step of filtering the obtained PET depolymerization product to obtain a cake, and then adding water to the cake to elute an aqueous solution including a terephthalate metal salt; (c) a step of adding at least one organic solvent among an ether solvent and a ketone solvent having a solubility in water of 20 g / 100 mL or more to the aqueous solution including a terephthalate metal salt of step (b) to separate the aqueous phase and the organic phase, and then obtaining a solution of an aqueous phase including a terephthalate metal salt; And (d) a step of reacting a strong acid with a solution containing the terephthalate metal salt to precipitate the terephthalate metal salt as a solid terephthalic acid, and obtaining a solution containing a salt in which a base catalyst-derived metal and a conjugate base of the strong acid are combined.

[0024] Step (a) above is a step for performing hydrolysis to depolymerize a polymer including PET. Conditions such as temperature and pressure can be applied without limitation to conditions applied to conventional hydrolysis methods. However, from the perspective of catalyst recovery, a basic hydrolysis method in the presence of a metal base catalyst is preferably applied.

[0025] In an example of the present invention, when basic hydrolysis is performed in the presence of the metal base catalyst, the metal base catalyst may be used without limitation as long as it is a catalyst used for basic hydrolysis, and preferably, an alkali metal hydroxide, more preferably, at least one of potassium hydroxide, lithium hydroxide, and sodium hydroxide may be used.

[0026] The above terephthalate metal salt is a metal derived from a metal base catalyst combined with terephthalate, and the type of metal depends on the type of metal of the base catalyst, and may be, for example, one or more of dipotassium terephthalate (K2-TPA), disodium terephthalate (Na2-TPA), and dilithium terephthalate (Li2-TPA).

[0027] The above step (b) is a step of eluting an aqueous solution containing a terephthalate metal salt from the PET depolymerization product of step (a).

[0028] In order to remove foreign substances contained in the above PET depolymerization product, the depolymerization product is filtered to remove a filtrate containing an organic solvent and impurities dissolved therein, and a cake containing a terephthalate metal salt is obtained. Since the cake still contains organic impurity components such as pigments and adhesives, water is added to elute an aqueous solution containing a terephthalate metal salt.

[0029] The amount of water added to the cake can be 4 to 10 times the weight of the polymer, including the raw PET. If the amount of water added is less than 4 times the weight of the raw PET, the PET will become supersaturated. If it exceeds 10 times, it is not economical and has a negative impact on the environment.

[0030] The above step (c) is a step of separating the aqueous solution containing the terephthalate metal salt of step (b) into an aqueous phase and an organic phase by introducing an organic solvent, separating and removing the organic phase containing organic impurities such as the pigment and adhesive, and selectively obtaining only the aqueous solution containing the terephthalate metal salt.

[0031] The organic solvent may be one that is easily miscible with water and can be separated into an aqueous phase and an organic phase due to a salting out effect when a terephthalate metal salt is present in a solution of water and an organic solvent, and one that does not undergo an additional reaction may be preferably used. The organic solvent that is easily miscible with water that can be used in the present invention includes an organic solvent that has a solubility in water of 20 g / 100 mL or more, and when an organic solvent that has a solubility of less than 20 g / 100 mL is used, there is a problem that the purity of the TPA finally obtained is reduced because phase separation is not easy.

[0032] As the organic solvent, an ether solvent and a ketone solvent can be used. When the ketone solvent is not easily miscible with water, that is, when the solubility in water is less than 20 g / 100 mL, the purification efficiency due to phase separation decreases, so it is preferable to use a solvent having a solubility of 20 g / 100 mL or more. As an example of the organic solvent, one or more of an ether solvent including tetrahydrofuran (THF) and dioxane; a ketone solvent including methyl ethyl ketone (MEK) and acetone; can be used.

[0033] The amount of organic solvent input in the above step (c) may be 1 to 4 times the weight of water input in the above step (b). If the amount of organic solvent input is less than 1 time the weight of water, there is a problem that the purity of the TPA finally obtained is reduced because the phase separation of the aqueous phase and the organic phase is not easy, and if it exceeds 4 times, some TPA is separated into the organic phase, reducing the purity of the TPA, and the amount of organic solvent discarded is excessive, which reduces the economic feasibility.

[0034] As an example of the present invention, by adding paper sludge incineration ash to the aqueous solution containing the terephthalate metal salt obtained in step (c), trace amounts of impurities contained in the raw polymer can be adsorbed and removed.

[0035] The above paper sludge ash (PSA) is obtained by incinerating sludge generated in the paper manufacturing process from the perspective of resource recycling.

[0036] The above paper sludge incinerator ash may include at least one of an adsorbent metal oxide and a metal hydroxide, and for example, may include at least one of a metal oxide of calcium oxide, magnesium oxide, aluminum oxide, titanium oxide, and silica; a metal hydroxide of calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and titanium hydroxide.

[0037] The above step (d) is a step of reacting a strong acid with a solution containing the terephthalate metal salt to precipitate the terephthalate metal salt as a solid terephthalic acid, and obtaining a solution containing a salt in which a base catalyst-derived metal and a conjugate base of a strong acid are combined.

[0038] In order to convert the above terephthalate metal salt into terephthalic acid, a strong acid is reacted, and a strong acid including hydrochloric acid and sulfuric acid used in a conventional hydrolysis method can be used, and is not limited to the type of sulfuric acid described, but sulfuric acid can be preferably used.

[0039] From the above reaction, high purity terephthalic acid of 99% or more that can be commercialized can be obtained.

[0040] In addition, a salt in which terephthalic acid and a base catalyst-derived metal and a conjugate base of a strong acid are combined is generated from the reaction of the terephthalate metal salt and a strong acid. As an example of a salt in which a base catalyst-derived metal and a conjugate base of a strong acid are combined, in the case where the metal base catalyst used for depolymerization in the step (a) is potassium hydroxide (KOH) and the strong acid used in the step (d) is sulfuric acid (H2SO4), the base catalyst-derived metal (K + ) and the conjugate base of strong acid (SO4 2- ) is a salt combined with potassium sulfate (K2SO4).

[0041] The above potassium sulfate may be treated as waste after purification, but since it contains the metal of the metal base catalyst, if it can be recovered in the form of a catalyst, resource recycling is possible, and cost reduction is possible in the production of terephthalic acid through PET hydrolysis, and environmental pollution due to disposal can be prevented.

[0042] In this regard, as another example of the present invention, after the step (d), (e) the solid terephthalic acid precipitated in the step (d) is filtered out, and then paper slurry incinerator ash is added to a solution containing a salt in which a metal derived from a base catalyst and a conjugate base of a strong acid are combined, thereby recovering a metal base catalyst from the salt in which a metal derived from a base catalyst and a conjugate base of a strong acid are combined, and precipitating a salt in which a metal derived from the paper sludge incinerator ash and a conjugate base of a strong acid are combined.

[0043] The above paper sludge incineration ash is added to recover the salt, which is a combination of a base catalyst-derived metal and a conjugate base of a strong acid, contained in a solution from which solid terephthalic acid precipitated in step (d) has been filtered out, in the form of a metal base catalyst.

[0044] The above-mentioned paper sludge incinerator ash may contain one or more of metal oxides and metal hydroxides, and for example, may contain one or more of metal oxides of calcium oxide, magnesium oxide, aluminum oxide, titanium oxide, and silica; metal hydroxides of calcium hydroxide, magnesium hydroxide, aluminum hydroxide, and titanium hydroxide; and preferably, may contain one or more of calcium oxide, calcium hydroxide, magnesium oxide, and magnesium hydroxide.

[0045] Additionally, the above-mentioned paper slurry incinerator ash can be used by replacing it with one or more of calcium oxide, calcium hydroxide, magnesium oxide, and magnesium hydroxide.

[0046] For example, in the step (d) above, the metal (K) derived from the base catalyst (KOH) + ) and the conjugate base of strong acid (SO4 2-) is combined with a salt to produce potassium sulfate (K2SO4), when paper sludge incineration ash containing calcium oxide is introduced, potassium hydroxide (KOH) is produced together with calcium sulfate (CaSO4). The potassium hydroxide can be recycled and may be reintroduced as a metal base catalyst in the purification method of the present invention.

[0047] The paper sludge ash (PSA) introduced in the above step (e) is obtained by incinerating sludge generated in the paper manufacturing process from the perspective of resource recycling, like the incinerator ash introduced in step (c).

[0048] For example, the amount of paper sludge incinerator ash input in the above step (e) can be input by referring to the stoichiometric ratio of the metal contained in the paper sludge incinerator ash and the conjugate base of the strong acid in the salt generated in the above step (d) after component analysis, and in the example described above, the single metal species contained in the paper sludge incinerator ash is divalent calcium ion (Ca 2+ ), and the conjugate base of the strong acid contained in the salt is sulfate ion (SO4 2- ) paper sludge incinerator ash can be added considering the theoretical reaction molar ratio of 1:1.

[0049] Hereinafter, preferred embodiments of the method for purifying PET depolymerization products of the present invention will be described. It should be noted that the following examples are provided to illustrate one or more preferred embodiments of the present invention, but the present invention is not limited to these embodiments. Numerous modifications to the following examples may be made within the scope of the present invention.

[0050] In the comparative examples and examples below, the formula for deriving the conversion rate, which is the rate at which waste plastic becomes monomer due to depolymerization, is as shown in Equation 1 below.

[0051] [Formula 1]

[0052] (m0: initial high molecular weight, m: unreacted high molecular weight)

[0053] <Example 1: Depolymerization of PET film (75% PET) containing adhesive followed by monomer purification and catalyst recovery>

[0054] PET raw material is prepared by finely crushing PET film containing adhesive. 60 g of ethanol, 60 g of MIBK, and 11.7 g of KOH are added to a 250 mL round-bottomed flask, followed by 10 g of finely crushed PET film, and stirred at 60°C for 2 hours. After the reaction is complete, the reaction product is filtered to obtain a cake, and 80 g of water is added to the cake to obtain an aqueous solution from which dipotassium terephthalate has been eluted. The process of adding 80 g of THF per time to the aqueous solution, separating the phases, and recovering only the aqueous phase is repeated twice. Sulfuric acid is added to the recovered aqueous phase to adjust the pH to 3 or lower, and a white solid is obtained, which is then filtered. After washing five times with 80 g of water and drying, white terephthalic acid (4.91 g) was obtained (conversion: 99%, purity: 99.88% (HPLC), see Figure 1).

[0055] The above white terephthalic acid was filtered out, and paper sludge incinerator ash was added to the filtered filtrate to adjust the pH to 11 or higher, and then concentrated under reduced pressure to recover 4.2 g of KOH (yield: 95%).

[0056] <Example 2: Monomer purification and catalyst recovery following depolymerization of beer PET containing colored pigments>

[0057] Prepare PET raw material by finely crushing beer PET containing a large amount of pigment. Add 60 g of ethanol, 60 g of MIBK, and 11.7 g of KOH to a 250 mL round-bottomed flask, add 10 g of finely crushed PET, and stir at 60°C for 2 hours. After the reaction is complete, filter the reaction product to obtain a cake, and add 80 g of water to the cake to obtain an aqueous solution from which dipotassium terephthalate has been eluted. Repeat the process of adding 80 g of THF per time to the aqueous solution, separating the phases, and recovering only the aqueous phase twice. Adjust the pH of the recovered aqueous phase to 3 or lower by adding sulfuric acid, and then filtering the white solid. After washing five times with 80 g of water and drying, terephthalic acid (6.05 g) was obtained (conversion: 99%, purity: 99.99% (HPLC), see Figure 1).

[0058] The above white terephthalic acid was filtered out, and paper sludge incineration ash was added to the filtered filtrate to adjust the pH to 11 or higher, and then concentrated under reduced pressure to recover 5.5 g of KOH (yield: 95%).

[0059] <Comparative Example 1: Monomer purification following depolymerization of adhesive PET film>

[0060] PET raw material is prepared by crushing PET film containing adhesive. 60 g of ethanol, 60 g of MIBK, and 11.7 g of KOH are added to a 250 mL round-bottomed flask, 10 g of the crushed PET film is added, and the mixture is stirred at 60°C for 2 hours. After the reaction is complete, the reaction product is filtered to obtain a cake, and 80 g of water is added to the cake to obtain an aqueous solution from which dipotassium terephthalate is eluted. Sulfuric acid is added to the aqueous solution to adjust the pH to 3 or lower, and a white solid is obtained, which is then filtered. The solid is washed five times with 80 g of water, and then dried to obtain terephthalic acid (5.11 g) (conversion: 99%, purity: 97.98% (HPLC), see Figure 1).

[0061] <Comparative Example 2: Monomer purification following depolymerization of adhesive PET film>

[0062] PET raw material is prepared by crushing PET film containing adhesive. 60 g of ethanol, 60 g of MIBK, and 11.7 g of KOH are added to a 250 mL round-bottomed flask, 10 g of the crushed PET film is added, and the mixture is stirred at 60°C for 2 hours. After the reaction is complete, the reaction product is filtered to obtain a cake, and 80 g of water is added to the cake to obtain an aqueous solution from which dipotassium terephthalate has been eluted. The process of adding 80 g of MIBK per time to the aqueous solution, separating the phases, and recovering only the aqueous phase is repeated twice. Sulfuric acid is added to the recovered aqueous phase to adjust the pH to 3 or lower to obtain a white solid, which is then filtered. After washing five times with 80 g of water and drying, terephthalic acid (4.97 g) was obtained (conversion: 99%, purity: 98.66% (HPLC), see Figure 1).

[0063] <Comparative Example 3: Monomer Purification Following Depolymerization of Beer PET Containing Color Pigments>

[0064] Prepare PET raw material by finely crushing beer PET containing a large amount of pigment. Add 60 g of ethanol, 60 g of MIBK, and 11.7 g of KOH to a 250 mL round-bottomed flask, add 10 g of finely crushed PET, and stir at 60°C for 2 hours. After the reaction is complete, filter the reaction product to obtain a cake, and add 80 g of water to the cake to obtain an aqueous solution from which dipotassium terephthalate has been eluted. Repeat the process of adding 80 g of MIBK per time to the aqueous solution, separating the phases, and recovering only the aqueous phase twice. Add sulfuric acid to the recovered aqueous phase to adjust the pH to 3 or lower to obtain a white solid, which is then filtered. After washing five times with 80 g of water and drying, terephthalic acid (6.14 g) was obtained (conversion: 98%, purity: 91.55% (HPLC), see Figure 1).

[0065] <Comparative Example 4: Monomer purification and catalyst recovery following depolymerization of PET film (75% PET) containing adhesive>

[0066] The same method as Example 1 was performed, except that 30 g of THF was added per reaction to separate the phases of an aqueous solution containing dipotassium terephthalate. As a result, white terephthalic acid (4.93 g) was obtained (conversion: 99%, purity: 97.11% (HPLC)), and 4.15 g of KOH, a base catalyst (yield: 93%), was recovered.

[0067] <Comparative Example 5: Monomer purification and catalyst recovery following depolymerization of PET film (75% PET) containing adhesive>

[0068] The same method as Example 1 was performed, except that 170 g of THF was added per reaction to separate the phases of an aqueous solution containing dipotassium terephthalate. As a result, white terephthalic acid (4.90 g) was obtained (conversion: 99%, purity: 98.55% (HPLC)), and 4.17 g of KOH, a base catalyst (yield: 94%), was recovered.

[0069] Figure 1 is a drawing showing the appearance of TPA and recovered KOH obtained according to comparative examples and examples of the present invention and the HPLC spectrum of TPA.

[0070] Referring to FIG. 1, according to Examples 1 and 2, ultra-high purity terephthalic acid of 99% or more was obtained, and it was confirmed that colored pigment impurities, etc. were completely removed from the pure white terephthalic acid in terms of appearance and HPLC spectrum.

[0071] In addition, unlike the comparative examples, only Examples 1 and 2, which performed a catalyst recovery process using incinerator ash, were able to recover the base catalyst KOH with a high yield of 95%.

[0072] Meanwhile, Comparative Examples 4 and 5 are examples in which the amount of organic solvent introduced for phase separation is 0.75 times and 4.25 times, respectively, compared to the weight of water introduced to elute dipotassium terephthalate (K2-TPA), and the purity of the finally obtained TPA was confirmed to be 97% and 98%, respectively, which falls below the purity standard of 99% or more required for commercialization.

[0073] While the present invention has been described above with reference to the embodiments described herein and illustrated in the accompanying drawings, these are merely exemplary, and those skilled in the art will understand that various modifications and equivalent embodiments are possible. Accordingly, the technical protection scope of the present invention should be defined by the following claims.

Claims

1. In the purification method of PET depolymerization product, (a) a step of depolymerizing a polymer including PET as a raw material through basic hydrolysis in the presence of a metal base catalyst to obtain a PET depolymerization product including a terephthalate metal salt; (b) a step of filtering the obtained PET depolymerization product to obtain a cake, and then adding water to the cake to elute an aqueous solution containing a terephthalate metal salt; (c) a step of adding at least one organic solvent among ether solvents and ketone solvents having a solubility in water of 20 g / 100 mL or more to the aqueous solution containing the terephthalate metal salt of step (b) to separate the solution into an aqueous phase and an organic phase, and then obtaining a solution of the aqueous phase containing the terephthalate metal salt; and (d) A method for purifying a PET depolymerization product, characterized by comprising the step of reacting a strong acid with a solution containing the terephthalate metal salt to precipitate the terephthalate metal salt as a solid terephthalic acid, and obtaining a solution containing a salt in which a base catalyst-derived metal and a conjugate base of the strong acid are combined.

2. In paragraph 1, A method for purifying a PET depolymerization product, characterized in that the metal base catalyst is at least one of potassium hydroxide, lithium hydroxide, and sodium hydroxide.

3. In paragraph 1, A method for purifying a PET depolymerization product, characterized in that the organic solvent introduced in the step (c) includes at least one of an ether solvent including tetrahydrofuran (THF) and dioxane; a ketone solvent including methyl ethyl ketone (MEK) and acetone; 4. In paragraph 1, A method for purifying a PET depolymerization product, characterized in that the amount of organic solvent introduced in step (c) is 1 to 4 times the weight of water introduced in step (b).

5. In paragraph 1, After step (c) above, A method for purifying a PET depolymerization product, characterized by adding paper slurry incineration ash to adsorb and remove foreign substances or impurities.

6. In paragraph 1, After step (d) above, (e) A method for purifying a PET depolymerization product, characterized by comprising the step of: after the solid terephthalic acid precipitated in step (d) is filtered off, adding paper slurry incinerator ash to a solution containing a salt in which a base catalyst-derived metal and a conjugate base of a strong acid are combined, recovering a metal base catalyst from the salt in which the base catalyst-derived metal and a conjugate base of a strong acid are combined, and precipitating a salt in which the base catalyst-derived metal and a conjugate base of a strong acid are combined; 7. In paragraph 6, A method for purifying a PET depolymerization product, characterized in that the above-mentioned paper slurry incinerator ash is replaced with one or more of calcium oxide, calcium hydroxide, magnesium oxide and magnesium hydroxide.

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