Method for purifying polyester depolymerization product and purified polyester depolymerization product obtained thereby

Abstract

The present invention provides a method for purifying a polyester depolymerization product in which impurities and poorly soluble dyes and pigments are removed from a material produced by polyester depolymerization so as to minimize impurity content and perform decolorization.

Description

Method for purifying a polyester depolymerization product and purified polyester depolymerization product obtained thereby

[0001] The present invention relates to a method for depolymerizing a polyester containing various additives such as chemical pigments, dyes, adhesives, etc., thereby converting and recovering the monomer, and obtaining a purified polyester depolymerization product having a reduced impurity content by removing various additives contained in the monomer.

[0002] Polyester is the world's most widely purchased and widely used polymer, with annual production exceeding 75 million tons. Its relatively low cost of manufacture and its unique physical, chemical, and thermal properties make it a versatile material for a wide range of applications, including clothing, carpets, and films. Polyethylene terephthalate (PET), a leading polyester, is used in a wide range of applications, including disposable beverage containers, consumer goods, paints, films, and automotive interiors. The commercial success of PET has led to a global surge in the volume of polyester-based plastic waste, leading to a growing interest in recycling waste plastics. Waste polyester has thermoplastic properties and can be converted into low-molecular substances through chemical reactions. It can be utilized in the recycling industry to manufacture regenerated materials through physical and chemical recycling processes. However, most waste polyester discharged after consumption is contaminated with various foreign substances, and this contamination of foreign substances is pointed out as one of the problems that must be resolved in technologies for recycling.

[0003] Physical recycling involves sorting PET waste and melting it at high temperatures to produce recycled PET. While colorless and transparent materials can be physically recycled, colored and composite materials, such as automotive interior materials or textiles, are difficult to recycle due to concerns about the generation of hazardous substances and changes in their physical properties. Consequently, they are often incinerated or landfilled. Consequently, technologies are being developed to recover waste polyester and chemically recycle it as an alternative to conventional treatment methods like incineration or landfilling.

[0004] Chemical recycling involves chemically decomposing PET into its prepolymerized terephthalic acid or its derivatives (dimethyl terephthalate (DMT) or bis(2-hydroxyethyl) terephthalate (BHET)) and ethylene glycol. The resulting monomers are then repolymerized, resulting in the same physical properties as PET made from terephthalic acid.

[0005] This chemical recycling of polyester is called depolymerization technology, and depolymerization technology can be divided into glycolysis, methanolysis, and hydrolysis.

[0006] The above glycolysis is a method of decomposing polyester into bis(2-hydroxyethyl) terephthalate (BHET) by using a glycol such as ethylene glycol (EG) or propylene glycol (PG) and a catalyst (e.g., Zn(OAc)2, an alkaline earth metal-based catalyst, or an organic catalyst) at a high temperature of 150℃ or higher. Glycolysis is the simplest, oldest, and easiest process to attempt, and is a technology that many companies are developing, as it utilizes the transesterification reaction mechanism. BHET manufactured through glycolysis is mainly used in the manufacture of PET, and must be converted into terephthalic acid through hydrolysis to manufacture other types of polyester. However, glycolysis is very slow, and complete depolymerization of PET into BHET cannot be achieved without the use of catalysts such as metal salts, zeolites, or ionic catalysts. Since glycolysis produces a final product containing a significant amount of other oligomers in addition to the BHET monomer, it is difficult to recover the BHET monomer when it is the desired product. In other words, although soluble dye components can be removed through activated carbon and ion exchange resins through glycolysis, colloidal substances or insoluble additives such as carbon black, fine pigments, or oligomers with a micelle structure are not easily removed. Therefore, prior to the glycolysis reaction, it is necessary to sort polyester waste from the beginning to purify waste polyester with a low impurity content, or to perform a separate process to remove colloidal or insoluble substances.

[0007] The above methanolysis is a method of depolymerizing polyester into dimethyl terephthalate (DMT) and ethylene glycol through a transesterification reaction using methanol and a catalyst (e.g., an alkaline (alkaline earth) metal-based catalyst or an organic catalyst). This method has the same transesterification mechanism as glycolysis, but has the advantage of being able to react at low temperatures below 100°C, which has led to extensive commercial development. It has the advantage of being able to produce various polyesters, and the produced monomer, DMT, is also characterized by high versatility. However, methanolysis also requires a pretreatment process to remove heavy metals and moisture in the waste prior to the reaction. In addition, during the refining process, when it undergoes a sublimation process at a high temperature of 200°C or higher, there is a problem that the produced DMT may deteriorate if a large amount of reactive oligomers or organic molecules are present, which may reduce the yield.

[0008] The hydrolysis process depolymerizes polyester into terephthalic acid and ethylene glycol through hydrolysis using water or a solvent and a catalyst. It is broadly divided into acid hydrolysis, neutral hydrolysis, and alkaline hydrolysis. Acid hydrolysis uses water and sulfuric acid, heating the reaction to 100°C to directly produce terephthalic acid. This reaction has the advantage of producing terephthalic acid directly and eliminating the need for organic solvents. However, it carries the risk of using over 85% sulfuric acid, and the resulting terephthalic acid must be re-alkaliized to purify it. Neutral hydrolysis utilizes high-temperature steam to produce terephthalic acid at temperatures ranging from 200 to 300°C or using microwaves. However, this process requires specialized equipment and consumes significant energy. Similarly to acid hydrolysis, re-alkaliization is required for purification. Alkaline hydrolysis involves forming an intermediate, a terephthalate salt, using an alkaline catalyst and water or a solvent. This is followed by a purification process and acid treatment to form the final terephthalic acid. While relatively simple reaction conditions and facilities, no separate management of moisture or heavy metals, and mild conditions are advantageous, hydrolysis uses large amounts of water and solvent, resulting in lower economic and environmental impacts. Compared to glycolysis or methanolysis, hydrolysis struggles to remove insoluble substances such as carbon black, fine pigments, or adhesives. This is because the hydrolysis purification process takes place in an aqueous phase, and the free-form form of terephthalic acid has very low water solubility, making purification only possible under limited alkaline conditions. Hydrolysis also faces the challenge of requiring multiple steps, utilizing membranes, expensive adsorbents, defoaming agents, or expensive solvents to recover the terephthalic acid monomer.

[0009] In the chemical recycling method using depolymerization of polyester, there is a need for a purification method that increases the efficiency of removing insoluble impurities such as chemical pigments, dyes, and adhesives, and is economical and does not cause environmental pollution.

[0010] [Prior Art Literature]

[0011] [Patent Document]

[0012] Domestic Publication Patent No. 10-2023-0027276

[0013] The problem to be solved by the present invention relates to a method for removing impurities such as chemical pigments, dyes, and adhesives contained in a polyester depolymerization product and a purified polyester depolymerization product obtained thereby.

[0014] In order to solve the above problem, the present invention provides a method for purifying a polyester depolymerization product, comprising the steps of: pulverizing polyester; treating the pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product; filtering the polyester depolymerization product to obtain a sludge cake; and treating the filtered sludge cake with a solvent to prepare a sludge cake dispersion solution; adding an adsorbent to the sludge cake dispersion solution and stirring it to obtain a sludge cake mixed solution; filtering the sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer; and filtering the solution containing the polyester depolymerization monomer, washing it with purified water, and drying it to obtain a polyester depolymerization monomer.

[0015] The method for removing impurities from a polyester depolymerization product according to the present invention can effectively remove chemical pigments, fine pigments having a size of 100 microns or less, adhesives, colloidal impurities, or insoluble impurities. Furthermore, the method for removing impurities from a polyester depolymerization product according to the present invention can recover monomers such as terephthalic acid, DMT, and BHET with a purity of 99% or higher.

[0016] Figure 1 shows the appearance of polyester fiber containing carbon black.

[0017] Figure 2 shows the appearance of a composite polyester film containing urethane acrylate.

[0018] Figure 3 shows the appearance of polyester fiber containing fine pigments.

[0019] Figure 4 shows white terephthalic acid obtained from a depolymerization product according to Example 1 of the present invention.

[0020] Figure 5 shows black terephthalic acid obtained from a depolymerization product according to Example 2 of the present invention.

[0021] Figure 6 shows gray terephthalic acid obtained from a depolymerization product according to Example 3 of the present invention.

[0022] Figure 7 shows white terephthalic acid obtained from a depolymerization product according to Example 4 of the present invention.

[0023] Figure 8 shows white terephthalic acid obtained from a depolymerization product according to Example 5 of the present invention.

[0024] Figure 9 shows white terephthalic acid obtained from a depolymerization product according to Example 6 of the present invention.

[0025] Figure 10 shows black terephthalic acid obtained from a depolymerization product according to Example 7 of the present invention.

[0026] Figure 11 shows white terephthalic acid obtained from a depolymerization product according to Example 8 of the present invention.

[0027] Figure 12 shows dark yellow terephthalic acid obtained from the depolymerization product according to Example 9 of the present invention.

[0028] Figure 13 shows white terephthalic acid obtained from a depolymerization product according to Example 10 of the present invention.

[0029] Figure 14 shows white terephthalic acid obtained from a depolymerization product according to Example 11 of the present invention.

[0030] Figure 15 shows white terephthalic acid obtained from a depolymerization product according to Example 12 of the present invention.

[0031] Figure 16 shows white terephthalic acid obtained from a depolymerization product according to Example 13 of the present invention.

[0032] Figure 17 shows a pale yellow terephthalic acid obtained from a depolymerization product according to Example 14 of the present invention.

[0033] Figure 18 shows white terephthalic acid obtained from a depolymerization product according to Example 15 of the present invention.

[0034] Figure 19 shows red terephthalic acid obtained from a depolymerization product according to Example 16 of the present invention.

[0035] Figure 20 shows a red terephthalic acid obtained from a depolymerization product according to Example 17 of the present invention.

[0036] Figure 21 shows white terephthalic acid obtained from a depolymerization product according to Example 18 of the present invention.

[0037] Figure 22 shows white terephthalic acid obtained from a depolymerization product according to Example 19 of the present invention.

[0038] Figure 23 shows white terephthalic acid obtained from a depolymerization product according to Example 20 of the present invention.

[0039] Figure 24 shows red terephthalic acid obtained from a depolymerization product according to Example 21 of the present invention.

[0040] Hereinafter, various embodiments of the present invention will be described with reference to the accompanying drawings. It should be understood that the present invention is not limited to specific embodiments, but rather encompasses various modifications, equivalents, and / or alternatives of the embodiments of the present invention. In the description of the drawings, similar reference numerals may be used for similar components.

[0041] In this document, the expressions “has”, “may have”, “includes”, or “may include” indicate the presence of a feature (e.g., a number, function, operation, or component such as a part), but do not exclude the presence of additional features.

[0042] In this document, the expressions "A or B," "at least one of A and / or B," or "one or more of A and / or B" can include all possible combinations of the listed items. For example, "A or B," "at least one of A and B," or "at least one of A or B" can all refer to cases where (1) at least one A is included, (2) at least one B is included, or (3) at least one A and at least one B are included.

[0043] The expression "configured to" as used herein can be used interchangeably with, for example, "suitable for", "having the capacity to", "designed to", "adapted to", "made to", or "capable of". The term "configured to" does not necessarily mean "specifically designed to".

[0044] The terms used in this document are used only to describe specific embodiments and may not be intended to limit the scope of other embodiments. The singular expression may include the plural expression unless the context clearly indicates otherwise. Terms used herein, including technical or scientific terms, may have the same meaning as commonly understood by those of ordinary skill in the art described in this document. Terms defined in general dictionaries among the terms used in this document may be interpreted as having the same or similar meaning in the context of the relevant technology, and shall not be interpreted in an idealized or overly formal sense unless explicitly defined in this document. In some cases, even if a term is defined in this document, it cannot be interpreted to exclude the embodiments of this document.

[0045] The embodiments disclosed in this document are presented for the purpose of explaining and understanding the disclosed technical content and are not intended to limit the scope of the present invention. Therefore, the scope of this document should be interpreted to include all modifications or various other embodiments based on the technical concept of the present invention.

[0046] Hereinafter, preferred embodiments of the present invention will be described in detail. Prior to this, it should be noted that the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention, they should be interpreted in a way that aligns with the technical spirit of the present invention.

[0047] Accordingly, the configurations of the embodiments described in this specification are only some of the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention, so it should be understood that there may be various equivalents and modified examples that can replace them at the time of filing this application.

[0048] Throughout the 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 stated.

[0049] Hereinafter, the present invention will be described in detail.

[0050]

[0051] A method for purifying a polyester depolymerization product according to one embodiment of the present invention may include the steps of: pulverizing polyester; treating the pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product; filtering the polyester depolymerization product to obtain a sludge cake; treating the filtered sludge cake with a solvent to prepare a sludge cake dispersion solution; adding an adsorbent to the sludge cake dispersion solution and stirring it to obtain a sludge cake mixed solution; filtering the sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer; and filtering the solution containing the polyester depolymerization monomer, washing it with purified water, and drying it to obtain a polyester depolymerization monomer.

[0052] The step of crushing the polyester above is to produce an opaque or colored polyester obtained from a polyester molded product in the form of flakes or a finely divided solid using a crusher to form a polyester feedstock.

[0053] The above polyester is a polymer having an ester (RO-C(=O)-R') chemical functional group in the main chain, and is also called polyester. The polyester may be at least one selected from the group consisting of polyesters formed by polymerizing dicarboxylic acid and dialcohol, for example, polyethylene terephthalate (PET), polypropylene terephthalate (PPT), polyglycolide or polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), and Vectran. Preferably, the polyester may be polyethylene terephthalate (PET).

[0054] The dicarboxylic acid, which is a product obtainable by depolymerizing the above polyester, may be any one selected from the group consisting of terephthalic acid, naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, cyclohexanedicarboxylic acid, trimellitic acid, and pyromellitic acid. Preferably, the dicarboxylic acid may be terephthalic acid.

[0055] The dialcohol, which is a product obtainable by depolymerizing the above polyester, may be any one selected from the group consisting of ethylene glycol, trimethylene glycol, 1,2-propanediol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, decanmethylene glycol, dodecamethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, polypropylene glycol, di(tetramethylene) glycol, tri(tetramethylene) glycol, polytetramethylene glycol, pentaerythritol, and 2,2-bis(4-β-hydroxyethoxyphenyl)propane. Preferably, the dialcohol may be ethylene glycol.

[0056] The polyester may be an opaque or colored polyester molded article containing a coloring agent such as a chemical pigment, dye, or an additive such as an adhesive. The opaque or colored polyester molded article may be polyester waste generated after use, residue generated during the production of the molded article, defective products, etc. For example, it may be, but is not limited to, used PET bottles, cups, strings, packaging packs, sheets, fibers, fabrics, clothes, films, sheets, etc.

[0057] When the polyester is polyester waste, the polyester content in the waste may be 50 wt% to 100 wt% based on the total weight of the polyester waste. Preferably, it may be 75 wt% to 100 wt%. When the polyester content of the polyester waste is less than the above numerical range, the amount of by-products and non-recyclable separated and purified waste obtained through the depolymerization process may be too large compared to the raw materials, making it practically uneconomical.

[0058] The chemical pigment that the above polyester may contain may be an organic pigment or an inorganic pigment.

[0059] The organic pigment comprises at least one selected from the group consisting of monoazo, disazo, lacid azo, β-naphthol, naphthol AS, benzimidazolone, disazo condensates, azo metal complex pigments and polycyclic pigments such as phthalocyanine, quinacridone, perylene, perinone, thioindigo, antanthrone, anthraquinone, flavanthrone, indanthrone, isobiolanthrone, pyranthrone, dioxazine, quinophthalone, isoindolinone, isoindoline and diketopyrrolopyrrole pigments, and carbon black.

[0060] The above inorganic pigment may include at least one selected from the group consisting of titanium dioxide, zinc sulfide, iron oxide, chromium oxide, ultramarine, nickel or chromium antimony titanium oxide, cobalt oxide, mixed oxides of cobalt and aluminum, bismuth vanadate, and extenders.

[0061] The dye that the above polyester may contain may be an acid dye, a direct dye, a sulfur dye, a metal complex dye or a reactive dye.

[0062] The adhesive that the polyester may contain may be urethane acrylate. Urethane acrylate is used as a curable material in paper, wood, plastics, etc., and can be used to improve the adhesion and bonding between monomers and to prevent formability and breakage. Urethane acrylate can provide formability while also providing breakage and thermal shock stability when manufacturing PET film, and can improve adhesion and bonding, thereby producing a PET film without cracks. Since the depolymerization process of waste polyester is purified under limited conditions such as an aqueous system, and the depolymerization product terephthalic acid has low solubility in water in its free structure, expensive adsorbents and solvents such as antifoaming agents must be used separately to separate and purify the adhesive components contained therein, such as urethane acrylate.

[0063] In the step of crushing the polyester, the opaque or colored polyester obtained from the polyester molded product may be mechanically cut, crushed and processed into pieces of 1 mm to 10 mm in size using a crusher in the form of processed flakes or in the form of an undifferentiated solid. Preferably, pieces of 2.5 mm to 10 mm in size may be used. If the processed polyester in the form of flakes or in the form of an undifferentiated solid is crushed to a size less than the above numerical range, impurities such as dyes, pigments or colorants in the depolymerization product of the polyester cannot be substantially separated, and if pieces having a size exceeding the above numerical range are provided for the depolymerization reaction, the reaction speed may be slow, which may reduce economic feasibility.

[0064] In the step of crushing the polyester, the polyester may contain at least one insoluble impurity selected from the group consisting of fine pigments having a size of 100 microns or less, carbon black, and urethane acrylate.

[0065] In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the polyester depolymerization product may be produced by any one depolymerization reaction selected from the group consisting of glycolysis, methanolysis, and hydrolysis. The step is to obtain a dicarboxylic acid and a dialcohol, which are polyester depolymerization products, from the glycolysis, methanolysis, and hydrolysis reactions. Preferably, the step of obtaining the above-mentioned polyester depolymerization product may utilize a hydrolysis reaction, but is not limited thereto.

[0066] In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the step may be performed by treating a solvent comprising a basic hydrolysis solvent composed of an alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst. The solvent is used for the basic hydrolysis of polyester and can sufficiently dissolve the alkaline catalyst and can be reused. The alkylated aromatic compound has a structure that is highly stable against the basic hydrolysis reaction of polyester and can produce high-purity terephthalic acid. The polarity adjusting compound can increase the solubility of the catalyst and easily affect the destruction of the ester functional group. The alkaline catalyst can form a terephthalate metal salt from the polyester and be dissolved in purified water introduced after the filtration step to create an alkaline environment. In this alkaline environment, monoesters, which are by-products of the hydrolysis reaction, can be completely decomposed.

[0067] In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the solvent is any one selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol ethylene glycol, propylene glycol, butylene glycol, methylene chloride, chloroform, methyl isobutyl ketone, methyl ethyl ketone, acetone, tetrahydrofuran, dioxane, methyl-t-butyl ether, diethyl ether, ethyl acetate, butyl acetate, methoxybenzene, ethoxybenzene, dimethoxybenzene, toluene, xylene, trimethylbenzene, ethylbenzene, diethylbenzene, triethylbenzene, propylbenzene, dipropylbenzene, and butylbenzene. The above may include. Preferably, the solvent may be a combination of ethanol and toluene, but is not limited thereto. Toluene has a stable structure even under alkaline conditions, so it does not generate contaminants even through a chain reaction and enables solvent reuse. The ethanol serves to increase the solubility of the catalyst and can adjust the polarity of the solvent to facilitate swelling of the polyester.

[0068] In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the reaction catalyst may include any one selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium oxide, sodium oxide, and lithium oxide. The reaction catalyst prevents a part of the dicarboxylic acid (e.g., terephthalic acid), which is a polyester depolymerization product, from acting as an acid catalyst, and forms a form including a dicarboxylate-metal salt (e.g., terephthalate-metal salt) so that it can be easily dissolved in water. The metal salt varies depending on the type of the reaction catalyst, and the dicarboxylate-metal salt may include any one selected from the group consisting of dipotassium terephthalate (K2-TP), disodium terephthalate (Na2-TP), and dilithium terephthalate (Li2-TP).

[0069] In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the weight ratio of the solvent and the reaction catalyst may be 1:0.01 to 0.5. Preferably, the weight ratio may be 1:0.05 to 0.3. If the weight of the reaction catalyst is less than the above-mentioned numerical range, the metal content derived from the reaction catalyst may be low, thereby lowering the yield of the polyester depolymerization product. In addition, if the weight exceeds the above-mentioned numerical range, the solvent content increases to dissolve the catalyst, thereby requiring a separate process to remove the solvent, etc. later, which reduces economic feasibility and may lower the conversion rate into the polyester depolymerization product.

[0070] In the step of treating the pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the weight ratio of the polyester, the solvent, and the reaction catalyst may be 1:1 to 50:0.1 to 5. Preferably, the weight ratio may be 1:5 to 25:0.5 to 2.5. If the weight of the solvent is less than the above numerical range, the viscosity of the solvent-treated polyester increases, so that depolymerization cannot occur smoothly. If the weight exceeds the above numerical range, too much solvent is added, resulting in poor economic efficiency. If the weight of the reaction catalyst is less than the above numerical range, the content of metals derived from the reaction catalyst may be low, so that the yield of the polyester depolymerization product may be low. If the weight exceeds the above numerical range, the content of the solvent may increase in order to dissolve the catalyst, so that a separate process for removing the solvent, etc. is required later, resulting in poor economic efficiency and a low conversion rate to the polyester depolymerization product.

[0071] The step of filtering the polyester depolymerization product to obtain a sludge cake and treating the filtered sludge cake with a solvent to prepare a sludge cake dispersion solution is to obtain a monomer produced by the polyester depolymerization reaction through phase separation. The monomer produced by the polyester depolymerization reaction may include impurities including insoluble substances or colloidal substances such as carbon black, fine pigments, and adhesives. The monomer produced by the polyester depolymerization reaction may be any one selected from the group consisting of HET, DMT, and terephthalic acid.

[0072] In the step of filtering the polyester depolymerization product to obtain a sludge cake and treating the filtered sludge cake with a solvent to prepare a sludge cake dispersion solution, the solvent is water, methanol, ethanol, propanol, isopropanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol ethylene glycol, propylene glycol, butylene glycol, methylene chloride, chloroform, methyl isobutyl ketone, methyl ethyl ketone, acetone, tetrahydrofuran, dioxane, methyl-t-butyl ether, diethyl ether, ethyl acetate, butyl acetate, methoxybenzene, ethoxybenzene, dimethoxybenzene, toluene, xylene, trimethylbenzene, ethylbenzene, diethylbenzene, triethylbenzene, propylbenzene, It may include at least one selected from the group consisting of dipropylbenzene and butylbenzene.

[0073] The step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution is to remove insoluble or colloidal impurities through adsorption, substitution reaction, or ion exchange reaction. The adsorbent can be added by controlling stable pH and temperature conditions depending on the type of monomer. When an adsorbent is used, impurities can be removed without using an additional organic solvent.

[0074] The above adsorbent may include at least one selected from the group consisting of incinerator ash, activated carbon, zeolite, silicate, calcium carbonate, calcium oxide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, and alumina. The incinerator ash may be at least one selected from the group consisting of sewage sludge incinerator ash or paper mill sludge incinerator ash.

[0075] In the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution, in the case of fine pigments contained in the monomers generated by the polyester depolymerization reaction, removal is possible through adsorption or chelation in the porous structure of the adsorbent. In the case of carbon black contained in the monomer, substitution-reactive functional groups such as -COOH and -OH of the carbon black react with the metal oxide of the adsorbent to convert them into a bulk structure, thereby allowing removal. In the case of adhesives, surfactants, emulsifiers, or oligomers contained in the monomer, such as urethane acrylate, precipitation or adsorption occurs through ion exchange with the cations contained in the adsorbent, thereby allowing removal. It is desirable that these reactions occur under stable temperature and pH conditions for each monomer, and it is desirable to optimize the conditions according to the characteristics of the impurities. In addition, if the stirring time is too long, the monomers bind to the adsorbent, which may reduce the yield.

[0076] In the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution, the weight ratio of the sludge cake dispersion solution and the adsorbent may be 1:0.01 to 5. Preferably, the weight ratio may be 1:0.01 to 2. Most preferably, the weight ratio may be 1:0.05 to 1. If the content of the adsorbent is less than the above numerical range, chelation of carbon black and the adsorbent does not occur sufficiently, making it difficult to remove impurities such as carbon black, and ion exchange of urethane acrylate and the adsorbent does not occur sufficiently, making it difficult to remove such impurities. If the content of the adsorbent exceeds the above numerical range, the yield may decrease due to adsorption of the adsorbent and the monomer generated by the polyester depolymerization reaction.

[0077] The step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution may further include a step of changing the pH by adding an acid or a base and stirring while applying temperature. The acid may include at least one selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, and citric acid. The base may include at least one selected from the group consisting of calcium carbonate, calcium oxide, calcium hydroxide, sodium hydroxide, sodium carbonate, sodium bicarbonate, magnesium carbonate, magnesium oxide, and magnesium hydroxide. The pH may be adjusted within a range of 1 to 14.

[0078] If the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution further includes a step of changing the pH by adding an acid or a base and stirring at a temperature, the temperature may be controlled to 4 to 100°C. The stirring time may be 10 minutes to 12 hours. If the stirring time is less than the above range, the adsorbent and impurities may not be sufficiently adsorbed, and if it exceeds the above range, the bond between the monomer and the adsorbent becomes strong, making it difficult to remove impurities.

[0079] The step of filtering the above sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer is to obtain a monomer from which impurities have been removed by an adsorbent. The step may be performed at a temperature in the range of 1 to 100°C to obtain a solution containing a polyester depolymerization monomer. In addition, the step may be performed at a pH of 1 to 7 to obtain a polyester depolymerization monomer. In the step of filtering the above sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer, the polyester depolymerization monomer may include any one selected from the group consisting of BHET, BHET oligomers, dimethyl terephthalate, monomethyl terephthalate, terephthalic acid, and derivatives of terephthalic acid.

[0080] After the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution, a step of adding paper sludge incineration ash may be further included. The paper sludge incineration ash may be effective in removing impurities not removed by the adsorbent. The paper sludge incineration ash is an industrial byproduct of incinerating sludge generated in a paper making process, contains a large amount of calcium, and exhibits high alkalinity of pH 12 or higher when reacted with water. In addition, the particles are fine and have a large surface area, making it suitable for mineral carbonation, and since calcium exists in the form of calcium oxide, it can improve the efficiency of the carbonation reaction. The paper sludge incineration ash has a particle diameter of 100 to 150㎛ and can capture and remove fine pigments included in polyester depolymerization monomers, and can aggregate and remove impurities such as carbon black and urethane acrylate.

[0081] The weight ratio of the above sludge cake mixture solution and the paper sludge incineration ash may be 1:0.01 to 5. Preferably, the weight ratio may be 1:0.01 to 2. Most preferably, the weight ratio may be 1:0.05 to 1. If the weight of the paper sludge incineration ash is less than the above numerical range, colloidal or insoluble impurities cannot be sufficiently coagulated, and if it exceeds the above numerical range, an additional neutralization step is required, which reduces economic feasibility.

[0082] The step of filtering the solution containing the polyester depolymerization monomer, washing it with purified water, and drying it to obtain a polyester depolymerization monomer is to remove impurities such as carbon black, fine pigments, and urethane acrylate from the polyester to obtain a polyester depolymerization monomer. When obtaining a solid polyester depolymerization monomer in the step of filtering the solution containing the polyester depolymerization monomer, washing it with purified water, and drying it to obtain a polyester depolymerization monomer, the precipitation method may vary depending on whether the monomer is obtained by a process of glycolysis, methanolysis, or hydrolysis. When the solution containing the polyester depolymerization monomer is obtained by a glycolysis process, BHET is generated as a monomer, and the BHET can be precipitated as a solid by lowering the temperature or changing the solvent composition. If the solution containing the polyester depolymerization monomer is obtained from a methanolysis process, DMT is generated as a monomer, and the DMT can be precipitated as a solid by removing the solvent or changing the solvent composition. If the solution containing the polyester depolymerization monomer is obtained from hydrolysis, the terephthalate salt can be precipitated by adding an acid and lowering the pH to convert it to terephthalic acid.

[0083] A polyester depolymerization monomer produced by any one of the above methods for purifying a polyester depolymerization product may have a purity of 99.00% or higher. A monomer purified by any one of the above purification methods may have impurities removed to form a polyester depolymerization product.

[0084]

[0085] Hereinafter, the present invention will be described in more detail through examples. These examples are intended solely to illustrate the present invention more specifically, and it will be apparent to those skilled in the art that the scope of the present invention is not limited by these examples, in accordance with the gist of the present invention.

[0086] Examples and Comparative Examples

[0087] Example 1

[0088] Black polyester fibers containing carbon black were finely ground (S1).

[0089] A solvent consisting of 40 g of toluene and 40 g of ethanol and 8.3 g of NaOH was added to a 250 mL flask, and then 10 g of the pulverized polyester fiber obtained in step S1 was added and stirred at 60°C for 3 hours (S2).

[0090] After the above stirring, the reaction product was filtered to obtain 20 g of sludge cake, and 80 g of water was added to the sludge cake to obtain 100 g of a solution from which disodium terephthalate was eluted (S3).

[0091] 10 g of paper sludge incineration ash was added once to 100 g of the solution that had gone through the above S3 step, and the resulting solution was purified and the mixed solution was recovered (S4).

[0092] Sulfuric acid was added to the mixed solution recovered in step S4 to adjust the pH to 3 or lower to obtain a white solid, which was then filtered, washed three times with 30 g of water, and dried to obtain white terephthalic acid (8.32 g) (S5).

[0093] Example 2

[0094] Terephthalic acid was obtained in the same manner as in Example 1, except that 1 g of paper sludge incineration ash was added in the above step S4.

[0095] Example 3

[0096] Terephthalic acid was obtained in the same manner as in Example 1, except that 5 g of paper sludge incineration ash was added in the above S4 step.

[0097] Example 4

[0098] Terephthalic acid was obtained in the same manner as in Example 1, except that 10 g of paper sludge incineration ash was added 5 times in the above step S4.

[0099] Example 5

[0100] The same procedure as Example 1 was performed except that 100 g of paper sludge incineration ash was added in the above S4 step.

[0101] Example 6

[0102] The same procedure as Example 1 was performed except that 200 g of paper sludge incineration ash was added in step S4.

[0103] Example 7

[0104] The same procedure as Example 1 was followed, except that the above S4 step was not performed.

[0105] Obtained TPA(g) Yield(%) Conversion(%) Purity%(Acid value) Color of TPA Example 18.3296.24100.0099.67 White Example 28.2695.55100.0099.63 Black Example 38.3896.94100.0099.67 Gray Example 47.8590.80100.0099.68 White Example 57.5286.99100.0099.59 White Example 67.1282.36100.0099.51 White Example 78.4197.28100.0099.15 Black

[0106] Examples 8 to 14

[0107] The same procedure as in Examples 1 to 7 was followed, except that a yellow composite polyester film containing urethane acrylate was used instead of a black polyester fiber containing carbon black in the above step S1.

[0108] Obtained TPA(g) Yield(%) Conversion(%) Purity%(Acid value) Color of TPA Example 86.2596.40100.0099.74 White Example 96.2396.09100.0099.66 Dark yellow Example 106.1294.39100.0099.67 White Example 115.9892.23100.0099.59 White Example 125.5385.29100.0099.56 White Example 135.2580.97100.0099.56 White Example 146.86105.80100.0099.25 Light yellow

[0109] <PET함량 75%기준>

[0110] Examples 9 to 21

[0111] The same procedure as in Examples 1 to 7 was followed, except that red polyester fibers containing fine pigments were used instead of black polyester fibers containing carbon black in the above S1 step.

[0112] Obtained TPA(g) Yield(%) Conversion(%) Purity%(Acid value) Color of TPA Example 156.7992.4010099.78 White Example 166.6590.5010099.69 Red Example 176.5489.0010099.87 Light Red Example 186.3586.4210099.67 White Example 196.1583.6910099.51 White Example 205.9881.3810099.56 White Example 216.9794.8510099.02 Red

[0113] <PET함량 85%기준>

Claims

1. Step of crushing polyester; A step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product; A step of filtering the above polyester depolymerization product to obtain a sludge cake and treating the filtered sludge cake with a solvent to prepare a sludge cake dispersion solution; A step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution; A step of filtering the above sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer; A step of filtering a solution containing the above polyester depolymerization monomer, washing it with purified water, and drying it to obtain a polyester depolymerization monomer. A method for purifying a polyester depolymerization product comprising:

2. In claim 1, A method for purifying a polyester depolymerization product, wherein the polyester depolymerization product is produced from any one depolymerization reaction selected from the group consisting of glycolysis, methanolysis, and hydrolysis, in a step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product.

3. In claim 1, In the step of treating the above-mentioned pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product, the solvent is any one selected from the group consisting of water, methanol, ethanol, propanol, isopropanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol ethylene glycol, propylene glycol, butylene glycol, methylene chloride, chloroform, methyl isobutyl ketone, methyl ethyl ketone, acetone, tetrahydrofuran, dioxane, methyl-t-butyl ether, diethyl ether, ethyl acetate, butyl acetate, methoxybenzene, ethoxybenzene, dimethoxybenzene, toluene, xylene, trimethylbenzene, ethylbenzene, diethylbenzene, triethylbenzene, propylbenzene, dipropylbenzene, and butylbenzene. A method for purifying a polyester depolymerization product comprising the above.

4. In claim 1, A method for purifying a polyester depolymerization product, wherein in the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution, the adsorbent includes at least one selected from the group consisting of incinerator ash, activated carbon, zeolite, silicate, calcium carbonate, calcium oxide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, and alumina.

5. In claim 1, A method for purifying a polyester depolymerization product, wherein the weight ratio of the solvent and the reaction catalyst in the step of treating the pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product is 1:0.01 to 0.

5.

6. In claim 1, A method for purifying a polyester depolymerization product, wherein the weight ratio of the polyester, solvent and reaction catalyst in the step of treating the pulverized polyester with a solvent and a reaction catalyst to obtain a polyester depolymerization product is 1:1 to 50:0.1 to 5.

7. In claim 1, A method for purifying a polyester depolymerization product, further comprising a step of adding paper sludge incineration ash after the step of adding an adsorbent to the above sludge cake dispersion solution and stirring to obtain a sludge cake mixed solution.

8. In claim 7, A method for purifying a polyester depolymerization product, wherein the weight ratio of the above sludge cake mixed solution and the paper sludge incineration ash is 1:0.01 to 2.

9. In claim 1, A method for purifying a polyester depolymerization product, wherein in the step of filtering the above sludge cake mixed solution to obtain a solution containing a polyester depolymerization monomer, the polyester depolymerization monomer contains any one selected from the group consisting of BHET, BHET oligomer, dimethyl terephthalate, monomethyl terephthalate, terephthalic acid, and derivatives of terephthalic acid.

10. A polyester depolymerization product purified by any one of the purification methods of claims 1 to 9, wherein the polyester depolymerization product has a purity of 99.00% or higher.

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