Method for recovering terephthalic acid

Abstract

To provide a recovery method of terephthalic acid capable of recovering terephthalic acid having a necessary and sufficient particle diameter by a simplified procedure.SOLUTION: A recovery method of terephthalic acid includes: a depolymerization reaction step S1 of depolymerizing a fractured body of a polyethylene terephthalate resin in an alkali solution; and a neutralization and crystallization step S4 of adding an acid of an acid dissociation constant of 2.9 or more as a neutralizing acid or phosphoric acid at an atmospheric temperature as a neutralizing acid to a processing liquid obtained in the depolymerization reaction step S1 and thus neutralizing and crystallizing the processing liquid to deposit terephthalic acid.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for recovering terephthalic acid from recyclables such as containers made of polyethylene terephthalate (hereinafter abbreviated as PET).

Background Art

[0002] As a method for recovering terephthalic acid from recyclables such as PET bottles, a method is known in which a pulverized product of the recyclables is subjected to a depolymerization reaction in the presence of an alkali to obtain a terephthalate, and a strong acid such as sulfuric acid or hydrochloric acid is added to an aqueous solution of the obtained terephthalate, and a neutralization crystallization reaction is allowed to proceed under heating at 50 to 95°C to precipitate terephthalic acid (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventionally, when using a strong acid and precipitating terephthalic acid under heating, crystals with a relatively small particle size tend to precipitate. Therefore, after the neutralization crystallization step, it is necessary to apply a treatment for heating and melting the terephthalic acid crystals to grow the crystals, for example, an autoclave treatment. Therefore, there are restrictions in reducing the number of steps, and thus in reducing the equipment scale and cost.

[0005] Therefore, an object of the present invention is to provide a method for recovering terephthalic acid that can recover terephthalic acid with a necessary and sufficient particle size by a simpler procedure than conventional methods.

Means for Solving the Problems

[0006] A method for recovering terephthalic acid according to one aspect of the present invention includes a depolymerization step of depolymerizing crushed polyethylene terephthalate resin in an alkaline solution, and a neutralization crystallization step of adding an acid with an acid dissociation constant of 2.9 or higher as a neutralizing acid to the treatment solution obtained in the depolymerization step, and then neutralizing and crystallizing the treatment solution to precipitate terephthalic acid.

[0007] Another embodiment of the present invention relates to a method for recovering terephthalic acid, which includes a depolymerization step of depolymerizing crushed polyethylene terephthalate resin in an alkaline solution, and a neutralization crystallization step of neutralizing the treated solution obtained in the depolymerization step by adding phosphoric acid as a neutralizing acid to the treated solution while keeping it at room temperature, thereby precipitating terephthalic acid by neutralizing and crystallizing the treated solution. [Brief explanation of the drawing]

[0008] [Figure 1] A diagram showing a series of steps for a recovery method according to one embodiment of the present invention. [Figure 2] This figure shows an example of the results of an investigation into the relationship between the molar concentration of acetic acid and the pH of a solution when acetic acid is continuously added to a solution containing terephthalate. [Figure 3] This figure shows an example of the results of an investigation into the relationship between the molar concentration of sulfuric acid and the pH of a solution when sulfuric acid is continuously added to a solution containing terephthalate. [Figure 4] A schematic diagram illustrating the reaction that occurs in the processing solution. [Figure 5] This figure shows the results of an experiment in which terephthalic acid was recovered while appropriately changing the conditions in the neutralization crystallization process. [Modes for carrying out the invention]

[0009] The following describes a method for recovering terephthalic acid according to one embodiment of the present invention. In this embodiment, crushed waste materials such as PET bottles recovered from the market are depolymerized in the presence of alkali to obtain terephthalate, and the obtained terephthalate is neutralized and crystallized with acid to obtain terephthalic acid crystals. This recovery method is positioned as a type of so-called chemical recycling method. In this embodiment, by optimizing the conditions for neutralization and crystallization, terephthalic acid with a necessary and sufficient particle size is precipitated by neutralization and crystallization. This eliminates the step of growing crystals by applying a heat melting treatment. Figure 1 shows the steps of the recovery method according to one embodiment. The recovery method in Figure 1 includes a depolymerization reaction step S1, first and second filtration and solid-liquid separation steps S2 and S3, a neutralization and crystallization step S4, a solid-liquid separation step S5, a washing step S6, a drying step S7, and first and second distillation steps S8 and S9.

[0010] Depolymerization reaction step S1 is a step in which crushed pre-treated PET resin is subjected to a depolymerization reaction in an alkaline solution. Pretreatment of the PET resin may be carried out by, for example, disposing of the contents of collected PET bottles, removing labels and caps, washing, drying, etc., and then crushing the bottles into a shape and size suitable for the depolymerization reaction step. The crushed PET resin obtained in the pretreatment is used as the raw material for the depolymerization reaction. As an example of the alkaline solution used in the depolymerization reaction, an aqueous solution of sodium hydroxide is used. Enzymes may be added as appropriate to promote the depolymerization reaction. In any case, in depolymerization reaction step S1, known depolymerization reaction treatments may be carried out as appropriate, as long as the PET resin is subjected to a depolymerization reaction and a treatment solution containing terephthalic acid is obtained.

[0011] The first and second filtration and solid-liquid separation steps S2 and S3 are steps to prepare the treated liquid obtained in the depolymerization reaction step S1 for the neutralization and crystallization step S4. For example, in the first filtration and solid-liquid separation step S2, solid matter such as pigments, soil, and debris in the liquid is separated from the treated liquid by filtering the treated liquid after the depolymerization reaction. As a result, a treated liquid containing water, ethylene glycol (EG), terephthalic acid (TPA), and enzymes is obtained. In the second filtration and solid-liquid separation step S3, an alkaline solution, for example sodium hydroxide, is added to the treated liquid obtained in the first filtration and solid-liquid separation step S2 and filtered to remove flocs containing residues such as enzymes from the treated liquid.

[0012] The neutralization crystallization step S4 is a step in which an acid is added to the treatment solution obtained in the second filtration and solid-liquid separation step S3 to neutralize the treatment solution, thereby precipitating the terephthalate salt in the treatment solution as terephthalic acid. The acid used in the neutralization crystallization step S4 is preferably an acid whose acid dissociation constant pKa is about the same as or greater than that of terephthalic acid, that is, an acid equivalent to or weaker than terephthalic acid. As an example, acetic acid is used as the neutralizing acid. The addition of acetic acid is carried out continuously at a predetermined flow rate. Details of the acid used in step S4 will be described later.

[0013] In the solid-liquid separation step S5, terephthalic acid, which precipitated as a solid in the neutralization crystallization step S4, is separated from the treatment liquid. At this stage, the treatment liquid contains water, ethylene glycol, and sodium acetate. The terephthalic acid separated in the solid-liquid separation step S5 is washed with pure water in the washing step S6, and then dried in the drying step S7. This yields terephthalic acid crystals. On the other hand, the treatment liquid separated in the solid-liquid separation step S5 can be treated by removing water in the first distillation step S8 and separating it into ethylene glycol and sodium acetate in the second distillation step S9, as an example.

[0014] Next, we will explain the acid to be used in the neutralization crystallization step S4. As mentioned above, in step S4, an acid with an acid dissociation constant pKa of the same size as or larger than that of terephthalic acid, such as acetic acid, is used as the neutralizing acid. The acid dissociation constants of terephthalic acid are pKa1=3.5 and pKa2=4.46, while the acid dissociation constant of acetic acid is pKa=4.74. Therefore, acetic acid is a weaker acid than terephthalic acid.

[0015] Generally, it is recognized that when using an acid of the same strength or weaker than the acid to be neutralized, neutralization is difficult or, even if possible, inefficient, and such combinations of acids are not industrially viable. However, according to the inventors' findings, it is possible to neutralize and crystallize terephthalic acid using an acid of the same strength or weaker than terephthalic acid. In this case, the precipitation rate of terephthalic acid crystals and the recovery rate of terephthalic acid are within industrially acceptable limits. Moreover, when an acid of the same strength or weaker than terephthalic acid is used for neutralization, crystallization proceeds more gently compared to when strong acids such as sulfuric acid or hydrochloric acid are used, and the crystals grow sufficiently, increasing the particle size to a necessary and sufficient level.

[0016] Referring to Figures 2 and 3, one consideration of the cause of the above phenomenon will be explained. Figure 2 shows the results of investigating the relationship between the molar concentration of acetic acid in a solution and the pH of the solution when acetic acid is continuously added to a solution of terephthalic acid. Figure 3 shows the results of investigating the relationship between the amount of sulfuric acid added and the pH of the solution when the solution of terephthalic acid is neutralized under the same conditions as in Figure 2, except that sulfuric acid was used instead of acetic acid. Both examples in Figures 2 and 3 were carried out at room temperature. In a broad sense, room temperature is defined as 20°C ± 15°C in JIS Z 8703, and it is indicated that 20°C, 23°C, or 25°C can be used as standard temperatures depending on the purpose of the test. Here, the test was carried out by keeping the solution within the range of 24°C ± 2°C (excluding the temperature rise due to the heat of reaction).

[0017] In Figure 2, preliminary study 1 shows an example where acetic acid with a molar concentration of 4 mol / L was added, while preliminary studies 2 and 3 show examples where undiluted acetic acid was added. As shown in Figure 2, when neutralization is performed using acetic acid, the solution remains in the alkaline range of pH 13-14 for a while after the start of acetic acid addition, and as the addition continues, the solution shifts to the acidic range of pH 6. After that, even if acetic acid is added, the pH of the solution changes slowly within a range where it does not fall below 4. This confirms that the solution becomes an acetic acid-sodium acetate buffer. In the example in Figure 2, the average particle size of the precipitated terephthalic acid was 52-127 μm, and the recovery rate was 98.8%. On the other hand, in the example in Figure 3 using sulfuric acid, the solution remains in the weakly acidic range of pH 6 for a while after the start of addition, but as the addition progresses, the solution rapidly changes to the strongly acidic range of pH 2. Along with this change, the precipitation of terephthalic acid proceeds rapidly. In the example shown in Figure 3, the average particle size of the precipitated terephthalic acid was approximately 2 μm, and the recovery rate was 99.0%.

[0018] Based on the results in Figures 2 and 3, the following explanation can be given for why relatively large particle sizes of terephthalic acid are obtained in the neutralization crystallization step S4 of Figure 1. When neutralization is performed with acetic acid, the solution becomes an acetic acid-sodium acetate buffer in the acidic range. If acetic acid is continued to be added in this state, hydrogen ions slightly dissociated from acetic acid combine with terephthalate ions in the solution to produce terephthalic acid. Since terephthalic acid has a low saturation solubility, the terephthalic acid that could not be dissolved precipitates. This precipitation shifts the chemical equilibrium in the solution, causing dissociation of hydrogen ions, production of terephthalic acid, and subsequent precipitation. Through the repetition of such reactions, terephthalic acid is gradually and continuously produced. This reaction is schematically shown in Figure 4. When acetic acid is added dropwise, the buffering action of the solution suppresses the excessive generation of hydrogen ions. Therefore, a large number of terephthalic acid crystal nuclei are not formed rapidly. As a result, the crystals grow slowly, and crystals with large particle sizes are obtained.

[0019] According to the above considerations, the acid to be used in the neutralization crystallization step S4 is not necessarily limited to acetic acid. It is inferred that any acid that can produce a buffering effect in the solution obtained by the depolymerization reaction, suppress the excessive generation of hydrogen ions, and thereby gently grow the crystal nuclei of terephthalic acid can be used as the acid to be added in the neutralization crystallization step S4.

[0020] The results of tests for attempting to recover terephthalic acid while appropriately changing the conditions in the neutralization crystallization step S4 of FIG. 1 are shown in FIG. 5. The acids used for neutralization are acetic acid for conditions 1 to 10, formic acid for condition 11, phosphoric acid for condition 12, and citric acid for condition 13. In addition, "room temperature" in FIG. 5 was in the range of 24°C ± 2°C, similar to the examples in FIGS. 2 and 3.

[0021] Condition 1 is an example in which neutralization crystallization was carried out under standard conditions to clarify the difference from the case of neutralization using sulfuric acid. The concentration of terephthalate (TPA-2Na salt concentration) in the treatment liquid (the treatment liquid obtained in step S3 of FIG. 1) in condition 1 was 6.5% by weight, the concentration of acetic acid to be added was 17.5 mol / L, the acid addition amount was 7.2 times equivalent, the temperature of the treatment liquid was room temperature, the acetic acid addition time was 2 minutes, and the holding time was 10 minutes, respectively. The acid addition amount is the value obtained by excluding the acid addition amount required for neutralizing sodium hydroxide contained in the treatment liquid. The addition time is the time required to add the total amount of the acid addition amount shown in the table, and the acid addition amount / addition time is the addition amount (addition rate) per unit time. The holding time is the time after the total amount of acid has been added and the state is maintained. The average particle diameter of the terephthalic acid obtained under condition 1 was 52 μm, and the recovery rate was 100.4%. Comparing with the average particle diameter of 2 μm when neutralized using sulfuric acid, it can be confirmed that the average particle diameter has increased significantly.

[0022] Condition 2 is an example in which the temperature of the treatment liquid was changed to 80°C with respect to condition 1 to confirm the effect of temperature. Other conditions are the same as those in condition 1. In condition 2, the average particle diameter of terephthalic acid was 110 μm, and the recovery rate was 97.5%. From the results, it can be seen that further increase in the particle diameter can be expected by heating the treatment liquid.

[0023] Condition 3 is an example where the addition time is changed to 60 minutes and the acid addition rate is set lower than in Condition 1, while the rest is the same as in Condition 1. Condition 4 is an example where the addition time is changed to 60 minutes and the acid addition rate is set lower than in Condition 2, while the rest is the same as in Condition 2. In all examples, the average particle size is further increased compared to Conditions 1 and 2. It is presumed that this is because reducing the acid addition rate makes it less likely for the acid to become supersaturated, and the crystals grow more slowly. Furthermore, Condition 5 is an example where the concentration of terephthalate and acetic acid is reduced compared to Condition 3, while the rest is the same as in Condition 3. Condition 6 is an example where the concentration of terephthalate and acetic acid is reduced compared to Condition 4, while the rest is the same as in Condition 4. In Condition 5, the average particle size is increased compared to Condition 3, while in Condition 6, the average particle size is decreased compared to Condition 4. These results suggest that, under room temperature conditions, suppressing supersaturation can lead to a further increase in average particle size, while under heating conditions, a further increase in average particle size may not necessarily be expected. In any case, it can be understood that by appropriately selecting the concentration of the neutralizing acid and the amount of acid added according to the concentration of terephthalate, the average particle size will clearly increase compared to when sulfuric acid is used.

[0024] Condition 7 is an example in which the amount of acid added is changed to three times the equivalent amount and the addition time to 60 minutes compared to Condition 1, while the rest is the same as Condition 1. Furthermore, Condition 8 is an example in which the addition time is changed to 60 minutes and the holding time to 720 minutes (12 hours) compared to Condition 7, Condition 9 is an example in which the temperature of the treatment solution is changed to 80°C compared to Condition 7, and Condition 10 is an example in which the amount of acid added is changed to twice the equivalent amount compared to Condition 7. Comparing these examples, Condition 7 can be evaluated as a relatively preferable condition among Conditions 1 to 10, as it is possible to significantly increase the particle size with an average particle diameter of 56 μm compared to when sulfuric acid is used, the recovery rate is 96.4%, which is sufficient, and the treatment solution can be used at room temperature. However, in practical terms, it is preferable to keep the acid addition time as short as possible, and from this viewpoint, Condition 1, which yields an average particle diameter almost the same as Condition 7 and requires only a short acid addition time of 2 minutes, can be evaluated as the best. However, if a 60-minute acid addition time is acceptable, condition 7 may be selected as a suitable condition, and conditions 3 or 5 may also be selected. If average particle size is prioritized, conditions 2, 4, 5, or 9 may also be selected. However, conditions 2, 4, and 9 require heating of the treatment solution. Condition 8 is more advantageous than condition 7 in terms of average particle size, but the addition time is significantly longer. Also, conditions 9 and 10 are less advantageous than conditions 1 to 8 in terms of recovery rate.

[0025] For conditions 11-13, where the acid used for neutralization was changed, the average particle size was smaller than when acetic acid was used. However, compared to the average particle size of 2 μm obtained using sulfuric acid, a sufficient increase was observed in all conditions. The acid addition time and the recovery rate of terephthalic acid were the same as in condition 1, and the fact that the treatment solution could be used at room temperature was also the same as in condition 1. On the other hand, the acid dissociation constants of formic acid used in condition 11 were pKa=3.54, the acid dissociation constants of phosphoric acid used in condition 12 were pKa1=1.83, pKa2=6.43, pKa3=11.46, and the acid dissociation constants of citric acid used in condition 13 were pKa1=2.90, pKa2=4.35, pKa3=5.69. Considering that the acid dissociation constant of terephthalic acid is pKa1 = 3.54 when monovalent, and that the acid dissociation constant of sulfuric acid, which was conventionally used for neutralization, is pKa = 1.96, it is understood that using an acid with an acid dissociation constant of 2.9 or higher for neutralization crystallization will allow crystallization to proceed gently, enabling sufficient crystal growth and thereby a sufficient increase in particle size. Such acids are not limited to acetic acid, formic acid, and citric acid shown in Figure 5, but can be used as appropriate as long as their acid dissociation constant is 2.9 or higher. For example, organic carboxylic acids that can be represented by formula 1 or formula 2 below and have an acid dissociation constant of 2.9 or higher, specifically propionic acid, fumaric acid, acrylic acid, butyric acid, valeric acid, lauric acid, palmitic acid, succinic acid, adipic acid, benzoic acid, acetylacetic acid, o-anisic acid, m-anisic acid, p-anisic acid, isocaproic acid, isovaleric acid, isocitric acid, isophthalic acid, isobutyric acid, caproic acid, glycolic acid, glutaric acid, crotonic acid, trans-cinnamic acid, cyclohexanecarboxylic acid, cyclohexane-1,1-dicarboxylic acid, trimethylacetic acid, lactic acid, 2-hydroxy-2-methylpropionic acid, 2-hydroxybutyric acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, phenylacetic acid, phenylpropionic acid, mandelic acid, methylacrylic acid, malic acid, levulinic acid, uric acid, and barbituric acid can be cited as examples, but are not limited to these examples.

[0026] {R1-COOH} …Formula 1 [HOOC-R1-COOH] …Formula 2 However, in formulas 1 and 2, R1 is a hydrogen atom, a linear or branched alkyl group having one or more carbon atoms, or an aromatic group.

[0027] The phosphoric acid used for neutralization in condition 12 has an acid dissociation constant equivalent to that of sulfuric acid, and is clearly a stronger acid than terephthalic acid. However, phosphoric acid is a trivalent acid, and while its monovalent acid dissociation constant pKa1 is equivalent to that of sulfuric acid, its divalent and trivalent acid dissociation constants pKa2 and pKa3 are significantly larger than not only the monovalent acid dissociation constant pKa1 = 3.5 for terephthalic acid, but also the divalent acid dissociation constant pKa2 = 4.46. In condition 12, coupled with the fact that the temperature of the treatment solution is set to room temperature, it is presumed that the rapid progress of the neutralization crystallization reaction after the start of addition is suppressed, and the crystal nuclei of terephthalic acid grow relatively slowly. Therefore, the objective of the present invention can also be achieved by a method of neutralization crystallization using phosphoric acid at room temperature.

[0028] In Figure 5, "room temperature" is set to 24°C ± 2°C as described above. However, considering that the average particle size increases significantly in this temperature range compared to conventional examples using sulfuric acid, it is presumed that the range of "room temperature" may include a wider range. In its broadest sense, the range of room temperature defined in the JIS standard (20°C ± 15°C) can be assumed, but preferably, a temperature range of ±10°C centered on 25°C may be set as the range of "room temperature". The range of 25°C ± 10°C is a range that can be commonly used as the temperature of the environment in which the processing equipment relating to this embodiment is installed, and is a temperature range in which it is assumed that it is not necessary to adjust the temperature by heating or cooling the processing liquid. If processing is performed in such a temperature range, it is possible to increase the average particle size while suppressing energy consumption. Furthermore, even when using phosphoric acid, it is possible to reliably mitigate the progress of the neutralization crystallization reaction compared to heating the processing liquid to a clearly high temperature range such as 80°C.

[0029] The average particle size of terephthalic acid recovered under each condition was determined as the median value based on volume from the results of laser diffraction particle size distribution measurements. A Malvern LMS-2000e laser diffraction particle size distribution analyzer was used as the measuring instrument. Water was used as the dispersion medium, and ultrasonic irradiation was used as appropriate to ensure that the terephthalic acid powder was sufficiently dispersed. The measurement range was 0.02 to 2000 μm.

[0030] As explained above, this recovery method allows terephthalic acid to be precipitated in the neutralization crystallization step S4 with a significantly larger particle size than conventional methods. Therefore, it is possible to omit steps such as applying a heating and melting treatment, such as autoclaving, after the neutralization crystallization step S4, in the subsequent steps of the solid-liquid separation step S5, in order to further grow the terephthalic acid crystals. Thus, terephthalic acid with a sufficient particle size can be recovered using a simpler procedure than conventional methods. Furthermore, when using strong acids such as sulfuric acid as the neutralizing acid, it is necessary to take corrosion countermeasures such as applying glass lining to parts of the equipment that come into contact with the acid. In contrast, by using relatively weak acids such as acetic acid, it is possible to eliminate or reduce the need for such measures, and consequently, equipment costs can be reduced.

[0031] The combination of steps S1 to S9 in the above configuration, and their details, are merely examples. The group of steps may be modified or changed as appropriate, as long as an acid equivalent to or weaker than terephthalic acid is used for neutralization in the neutralization crystallization step S4, or phosphoric acid is used for neutralization at room temperature. For example, the first and second filtration and solid-liquid separation steps S2 and S3 may be changed as appropriate depending on the state of the treatment solution obtained in the depolymerization reaction step S1. The steps S5 to S9 following the neutralization crystallization step S4 may also be changed as appropriate, as long as the precipitated terephthalic acid can be separated and obtained, and the remaining treatment solution can be treated appropriately.

[0032] Various aspects of the present invention derived from the embodiments and test examples described above are described below. In the following description, corresponding components shown in the accompanying drawings are indicated in parentheses to facilitate understanding of each aspect of the present invention, but this does not mean that the present invention is limited to the illustrated forms.

[0033] A method for recovering terephthalic acid according to one aspect of the present invention includes a depolymerization step (S1) in which crushed polyethylene terephthalate resin is subjected to a depolymerization reaction in an alkaline solution, and a neutralization crystallization step (S4) in which terephthalic acid is precipitated by adding an acid with an acid dissociation constant of 2.9 or higher as a neutralizing acid to the treatment solution obtained in the depolymerization reaction and neutralizing and crystallizing the treatment solution.

[0034] According to the above embodiment, if neutralization crystallization is performed using an acid with an acid dissociation constant of 2.9 or higher, the treatment solution becomes buffered, and terephthalic acid crystals grow slowly, resulting in crystals with a large particle size. Therefore, the need to further grow crystals using heating and melting treatments such as autoclaving in the subsequent stages of the neutralization crystallization process can be eliminated or reduced. As a result, terephthalic acid can be recovered using a simpler procedure than in the conventional method.

[0035] In the above embodiment, the neutralizing acid may be an acid that can be expressed by formula 1 or formula 2 above and has an acid dissociation constant of 2.9 or higher. By using such acids, the effects of the above embodiment can be reliably achieved.

[0036] In the neutralization crystallization step, the treatment solution may be kept at room temperature. In this case, heating of the treatment solution in the neutralization crystallization step becomes unnecessary, reducing the energy consumption required for the recovery of terephthalic acid, simplifying the equipment, and consequently lowering equipment costs.

[0037] When the treatment solution is kept at room temperature during the neutralization crystallization step, acetic acid may be used as the neutralizing acid, and acetic acid in an amount equivalent to three times the amount of terephthalate in the treatment solution may be added over a period of two minutes or more. This ensures that crystals of terephthalic acid with a large particle size can be reliably obtained even at room temperature.

[0038] Another embodiment of the present invention relates to a method for recovering terephthalic acid, comprising: a depolymerization reaction step (S1) in which crushed polyethylene terephthalate resin is subjected to a depolymerization reaction in an alkaline solution; and a neutralization crystallization step (S4) in which, while maintaining the treatment solution obtained in the depolymerization reaction at room temperature, phosphoric acid is added to the treatment solution as a neutralizing acid to neutralize and crystallize the treatment solution, thereby precipitating terephthalic acid.

[0039] According to the above embodiment, it is possible to buffer the processing solution and slowly grow terephthalic acid crystals to obtain crystals with a large particle size. This eliminates or reduces the need to further grow the crystals using heating and melting treatments such as autoclaving in the subsequent stage of the neutralization crystallization process, thereby allowing terephthalic acid to be recovered using a simpler procedure than conventional methods. [Explanation of Symbols]

[0040] S1 Depolymerization reaction step S4 Neutralization crystallization process

Claims

1. A depolymerization reaction step in which crushed polyethylene terephthalate resin is depolymerized in an alkaline solution, A neutralization crystallization step is performed in which an acid with an acid dissociation constant of 2.9 or higher is added to the treatment solution obtained in the depolymerization reaction as a neutralizing acid, and the treatment solution is neutralized and crystallized to precipitate terephthalic acid. Includes, A method for recovering terephthalic acid, comprising the neutralization crystallization step, in which the neutralizing acid is continuously added at a predetermined rate until a predetermined amount is reached, and after the entire amount of the added acid has been added, the treatment solution is held for a predetermined time.

2. The method for recovering terephthalic acid according to claim 1, wherein the neutralizing acid is an acid that can be represented by the following formula 1 or formula 2 and has an acid dissociation constant of 2.9 or higher. {R 1 -COOH} …Formula 1 [HOOC-R] 1 -COOH] …Equation 2 However, in equations 1 and 2, R １ This is a hydrogen atom, or a linear or branched alkyl group having one or more carbon atoms, or an aromatic group.

3. The method for recovering terephthalic acid according to claim 2, wherein the neutralization crystallization step is performed by keeping the treatment solution at room temperature.

4. The method for recovering terephthalic acid according to claim 3, wherein in the neutralization crystallization step, acetic acid is used as the acid for neutralization, and 3 times the amount of acetic acid relative to the terephthalate in the treatment solution is added over a period of 2 minutes or more.

5. A depolymerization reaction step in which crushed polyethylene terephthalate resin is depolymerized in an alkaline solution, A neutralization crystallization step is performed in which, while maintaining the treatment solution obtained in the depolymerization reaction at room temperature, phosphoric acid is added to the treatment solution as a neutralizing acid to neutralize and crystallize the treatment solution, thereby precipitating terephthalic acid. Includes, A method for recovering terephthalic acid, comprising the neutralization crystallization step, in which phosphoric acid is continuously added at a predetermined rate until a predetermined amount is reached, and after the entire amount of phosphoric acid has been added, the treatment solution is held for a predetermined time.

Images