Depolymerization recovery method of PET / PC mixed polyester waste

Abstract

The invention discloses a PET / PC mixed polyester waste depolymerization recovery method, which comprises: mixing PET / PC mixed polyester waste, a modified magnesium hydroxide catalyst and an alcohol compound, carrying out a heating reaction to obtain a reaction liquid, carrying out solid-liquid separation on the reaction liquid while the reaction liquid is hot to obtain a first solid and a first liquid, cooling the first liquid to a room temperature, and carrying out solid-liquid separation to obtain a second solid and a second liquid; adding water into the second liquid, and carrying out cooling crystallization, so as to obtain bisphenol A crystals; and mixing the second solid, a modified magnesium hydroxide catalyst and an alcohol compound, carrying out a heating reaction to obtain a reaction liquid, carrying out solid-liquid separation on the reaction liquid while the reaction liquid is hot to obtain a third liquid, cooling the third liquid to room temperature, carrying out solid-liquid separation to obtain a fourth liquid, adding water into the fourth liquid, and carrying out cooling crystallization to obtain the BHET crystal. The yields of BPA and BHET obtained by the method are up to 99.1% and 93.2% respectively, the problems that the mixed polyester waste is difficult to separate and low in recovery value are solved, the environmental influence and the production cost are reduced, and a large-scale technical scheme is provided for plastic circular economy.

Description

Technical Field

[0001] This invention belongs to the field of polyester waste recycling technology, specifically relating to a method for depolymerization and recycling of PET / PC mixed polyester waste. Background Technology

[0002] Polyethylene terephthalate (PET) and polycarbonate (PC) are currently the most widely used commercially available polyester plastics containing ester bonds, and are extensively used in packaging, textiles, electronic product casings, and automotive interiors. However, these plastics are often carelessly discarded after consumption, causing serious environmental problems; at the same time, PC waste gradually releases bisphenol A (BPA) into the natural environment, posing a long-term threat to human health.

[0003] Currently, PET and PC recycling still primarily relies on mechanical recycling. However, this method is only suitable for high-purity waste, and the thermal and mechanical properties of the materials gradually degrade during multiple recycling cycles, making it difficult to meet the quality requirements for reuse. To address these issues, the industry has explored various chemical recycling pathways, including pyrolysis, hydrolysis, methanololysis, hydrogenolysis, ammonolysis, and glycololysis. However, existing chemical recycling technologies have significant drawbacks: on the one hand, when applied to plastic waste with complex compositions, they generally suffer from poor product selectivity and numerous byproducts; on the other hand, some technologies require the introduction of co-solvents or complex catalytic systems, leading to new problems such as difficulties in co-solvent recovery and cross-contamination of residual monomers. While traditional metal oxide catalysts offer the advantage of easy recycling, their catalytic activity is insufficient, requiring harsh conditions such as high temperature and high pressure to achieve effective depolymerization. Furthermore, their selective catalytic ability for mixed waste is weak, making it difficult to achieve directional depolymerization of PET and PC. Therefore, developing a novel catalyst with high catalytic activity, high selectivity, easy recycling, and recyclability is crucial to overcoming the bottleneck of efficient recycling of mixed polyester waste.

[0004] Based on this, the present invention has developed a modified magnesium hydroxide catalyst, which can achieve stepwise selective depolymerization of mixed PET / PC waste by introducing high-density active sites, solving the problems of low catalytic efficiency, poor selectivity and harsh reaction conditions in the existing technology, and providing a new technical path for the large-scale and high-value recycling of mixed polyester waste. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for depolymerization and recycling of PC and PET blended polyester, which addresses the shortcomings of the prior art.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0007] A method for depolymerizing and recycling PET / PC mixed polyester waste, wherein the PET / PC mixed polyester waste is a mixture of waste polyester made of PET and waste polyester made of PC, includes the following steps:

[0008] Step 1: The PET / PC mixed polyester waste, modified magnesium hydroxide catalyst and alcohol compound are mixed and subjected to a first heating reaction to obtain a first reaction solution.

[0009] Step 2: While the first reaction solution is still hot, a first solid-liquid separation is performed to obtain a first solid and a first liquid. After the first liquid is cooled to room temperature, a second solid-liquid separation is performed to obtain a second solid and a second liquid. Water is added to the second liquid, and the mixture is cooled to crystallize, thus obtaining bisphenol A crystals.

[0010] Step 3: Mix the second solid, the modified magnesium hydroxide catalyst, and the alcohol compound, and carry out a second heating reaction to obtain a second reaction solution.

[0011] Step 4: While the second reaction solution is still hot, a third solid-liquid separation is performed to obtain a third liquid and a third solid. After the third liquid is cooled to room temperature, a fourth solid-liquid separation is performed to obtain a fourth liquid and a fourth solid. Water is added to the fourth liquid, and the mixture is cooled to crystallize, thus obtaining BHET crystals.

[0012] In step 2, the first solid is the modified magnesium hydroxide catalyst, which can be reused after being washed sequentially with deionized water and ethanol and then vacuum dried; the second solid is PET, which did not participate in the first heating reaction, and was carried out in the second heating reaction after being washed sequentially with ultrapure water and ethanol and then vacuum dried.

[0013] In step 4, the third solid is the modified magnesium hydroxide catalyst, which can be reused after being washed sequentially with deionized water and ethanol and then vacuum dried; the fourth solid is undepolymerized PET.

[0014] In step 4, the obtained BHET crystals contained trace amounts of oligomers. To obtain BHET with higher purity, the BHET crystals were dissolved in an appropriate amount of ethylene glycol by heating to 80°C. Deionized water was added while hot to promote the precipitation of oligomers, and the precipitate was removed by filtration. The filtrate was concentrated by rotary evaporation at 60°C to remove excess water. The concentrated solution was crystallized at 4°C for 12 hours, and the white needle-like BHET crystals were collected by filtration and vacuum dried at room temperature to obtain BHET crystals with higher purity.

[0015] In step 1, the initial mass ratio of waste PC polyester to the modified magnesium hydroxide catalyst in the PET / PC mixed polyester waste is 1:0.001~0.01, preferably 1:0.002~0.01, more preferably 1:0.005~0.01, and most preferably 1:0.01; the mass ratio of the alcohol compound to the PET / PC mixed polyester waste is 1~10:1, preferably 2~4:1; the temperature of the first heating reaction is 130~160℃, preferably 150~160℃, and most preferably 150℃; the time of the first heating reaction is 10 min~6 h, preferably 30~60 min, and most preferably 60 min; preferably, the stirring speed during the first heating reaction is 300~1000 rpm, preferably 300 rpm.

[0016] In step 2, the amount of water used is 10 to 100 times the volume of the second liquid, preferably 20 times.

[0017] In step 3, the initial mass ratio of the second solid to the modified magnesium hydroxide catalyst is 1:0.001~0.01, preferably 1:0.002~0.01, more preferably 1:0.005~0.01, and most preferably 1:0.01; the mass ratio of the alcohol compound to the second solid is 1~10:1, preferably 4:1; the second heating reaction is carried out in an air atmosphere at a temperature of 170~200℃ and a stirring speed of 300~1000 rpm for 10 min~6 h; preferably, the second heating reaction is carried out in an air atmosphere at a temperature of 180℃ and a stirring speed of 300 rpm for 1 h.

[0018] In step 4, the amount of water used is 10 to 100 times the volume of the fourth liquid, preferably 20 times.

[0019] In steps 2 and 4, the cooling crystallization is performed by overnight cooling crystallization at 0 to 4°C.

[0020] In steps 1 and 3, the alcohol compound is any one or more combinations of ethylene glycol, methanol, ethanol, diethylene glycol, propylene glycol, butanediol, hexanediol, n-octanol, and isooctanol, preferably ethylene glycol.

[0021] Preferably, the modified magnesium hydroxide catalyst is prepared by: adding an aqueous sodium hydroxide solution to an aqueous magnesium chloride solution to carry out a first reaction to obtain a suspension; adding an aqueous L-ascorbic acid solution to carry out a second reaction; allowing the solution to stand; performing solid-liquid separation on the resulting reaction solution; and washing and vacuum drying the solid to obtain the final product.

[0022] The concentration of the sodium hydroxide aqueous solution is 0.01 ~ 1 mol / L, preferably 0.05 ~ 0.5 mol / L, more preferably 0.2 ~ 0.5 mol / L, and most preferably 0.2 mol / L; the concentration of the magnesium chloride aqueous solution is 0.01 ~ 1 mol / L, preferably 0.01 ~ 0.1 mol / L, more preferably 0.05 ~ 0.1 mol / L, and most preferably 0.1 mol / L; the amounts of sodium hydroxide aqueous solution and magnesium chloride aqueous solution are controlled such that the molar ratio of sodium hydroxide to magnesium chloride is 0.2 ~ 20 : 1, preferably 0.2 ~ 10 : 1, more preferably 0.2 ~ 5 : 1, and most preferably 0.5 : 1.

[0023] The first reaction is carried out at 25-90°C and a stirring speed of 300-1000 rpm for 1-4 hours; the settling is carried out at room temperature for 1-24 hours.

[0024] The concentration of the L-ascorbic acid aqueous solution is 0.01 ~ 0.1 mol / L, preferably 0.02 ~ 0.1 mol / L, more preferably 0.02 ~ 0.05 mol / L, and most preferably 0.025 mol / L. The amount of L-ascorbic acid aqueous solution added is controlled such that the amount of L-ascorbic acid added is 0.05 ~ 10 times the amount of magnesium chloride, preferably 0.05 ~ 5 times, more preferably 0.05 ~ 2 times, and most preferably 0.5 times. The conditions for the second reaction are 25 ~ 90°C and a stirring speed of 300 ~ 1000 rpm for 1 ~ 24 h.

[0025] The washing process involves sequentially washing with ethanol and water; the vacuum drying process is carried out at a temperature of 60-80°C for a time of 6-24 hours.

[0026] Preferably, the washing process involves first washing with ethanol 2 to 4 times, and then washing with water 2 to 4 times.

[0027] In the PET / PC mixed polyester waste of the present invention, waste polyester of PET material and waste polyester of PC material are mixed in any proportion.

[0028] Beneficial effects:

[0029] (1) The modified magnesium hydroxide catalyst used in this invention has the advantages of high efficiency, easy recovery and multiple recycling, which can promote the alcoholysis reaction of PET and PC and the recovery of high-purity BPA and BHET crystals in a short time.

[0030] (2) The PET / PC mixed polyester waste alcoholysis polymerization strategy developed in this invention has the advantages of sequential depolymerization and high catalytic efficiency. Under standard reaction conditions, it can complete the alcoholysis-directed conversion process from PC to BPA with a yield of 98.4% and the alcoholysis-directed conversion process from PET to BHET with a yield of 93.2%. Moreover, the PC depolymerization process does not involve the depolymerization of PET. Therefore, no PET depolymerization products are mixed into the PC depolymerization product BPA, realizing the recovery of high-purity BPA and BHET crystals. This strategy is more in line with practical applications. It does not require physical processing and separation of commonly used mixed PET / PC waste. It can directly generate high-value products through sequential depolymerization by chemical methods, which greatly reduces the cost of recycling mixed PET / PC waste and improves economic benefits.

[0031] (3) The technical solution of the present invention has a wide range of applications. It can be used for waste mixed PET / PC waste with complex composition. Moreover, the types of PET / PC raw materials are varied. Various types of PET / PC waste, such as packaging, textiles, beverage containers, electronic product shells and automotive interiors, can be efficiently upgraded and transformed in a short time. Attached Figure Description

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer.

[0033] Figure 1 This is a flowchart of the depolymerization and recycling process of PET / PC mixed polyester waste in Example 3. Detailed Implementation

[0034] The present invention will be further described below with reference to the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0035] In the following examples, the conversion rate of PC or PET and the yield of the product were calculated according to formulas (1) and (2), respectively:

[0036]

[0037]

[0038] Example 1: Preparation of Modified Magnesium Hydroxide Catalyst

[0039] Preparation method of modified magnesium hydroxide catalyst (p-Mg(OH)2): 2.033 g of MgCl2·6H2O (10 mmol) was dissolved in 100 mL of deionized water to prepare a 0.1 M MgCl2 aqueous solution. 25 mL of 0.2 M sodium hydroxide aqueous solution was added dropwise, and the reaction was carried out at 25 °C and a stirring speed of 600 rpm for 2 h. Then, 200 mL of 0.025 M L-ascorbic acid aqueous solution was added dropwise, and the reaction was carried out at 25 °C and a stirring speed of 600 rpm for 8 h. The resulting solution was then allowed to stand for 12 hours to age. The solid product was then collected by centrifugation, washed several times with ethanol and water, and dried overnight at 60 °C.

[0040] Example 2: Depolymerization of polycarbonate using a modified magnesium hydroxide catalyst

[0041] First, p-Mg(OH)₂, 1.0 g of polycarbonate (PC), and 4 g of ethylene glycol were added to a 10 mL reactor. The reactor was then stirred at 300 rpm and heated to 130–160 °C for 10–60 min. Specific p-Mg(OH)₂ usage, reaction temperature, and reaction time are shown in Table 1. After the reaction, a first filtration was performed while hot to remove the catalyst. The resulting filtrate was cooled to room temperature and then filtered again to remove unreacted PC. 80 mL of deionized water was added to the filtrate from the first filtration, and the mixture was then refrigerated overnight to promote crystallization. BPA crystals were collected through a second filtration and dried to obtain the BPA product. Ethylene glycol (EG) and ethylene carbonate (EC) in the filtrate from the second filtration were recovered by vacuum distillation. The PC conversion and BPA yield were calculated.

[0042] The experimental results under different reaction conditions are shown in Table 1. The results show that the conversion rate of PC and the yield of BPA are both high when the mass ratio of PC to catalyst is 1:0.005~0.01, the reaction temperature is 150~160℃, and the reaction time is not less than 30 min.

[0043] Table 1. Effect of p-Mg(OH)2 on the catalytic alcoholysis of waste PC under different reaction conditions

[0044]

[0045] Example 3: Depolymerization of PC / PET mixed polyester waste using a modified magnesium hydroxide catalyst

[0046] Step 1: 10 mg of p-Mg(OH)₂ catalyst, 1.0 g of PC powder, 1.0 g of PET powder, and 4.0 g of ethylene glycol were loaded into a 10 mL reactor. The reactor was stirred at 300 rpm, and the mixture was heated to 150 °C for 1 hour. After the reaction was complete, the mixture was filtered while hot to recover the solid catalyst. It was washed successively with deionized water and ethanol, vacuum dried, and reused. The filtrate from the first filtration was cooled to room temperature and then filtered a second time to separate the unreacted solid PET. It was washed with ultrapure water and ethanol, vacuum dried, weighed, and used for the subsequent depolymerization step. 80 mL of ultrapure water was added to the filtrate from the second filtration to precipitate bisphenol A. The mixture was then cooled overnight at 4 °C to promote bisphenol A crystallization. The bisphenol A crystals were collected by vacuum filtration, dried, and the bisphenol A product was obtained. Ethylene glycol and ethylene carbonate in the filtrate from the vacuum filtration were recovered by vacuum distillation.

[0047] Step 2: The 10 mg p-Mg(OH)₂ catalyst recovered in Step 1, 1.0 g PET powder, and 4.0 g ethylene glycol were loaded into a 10 mL reactor. The reactor was stirred at 300 rpm, and the mixture was heated to 180°C for 1 hour. After the reaction was complete, a first filtration was performed while hot to recover the solid catalyst. The catalyst was washed successively with deionized water and ethanol, vacuum dried, and reused. The filtrate from the first filtration was cooled to room temperature and then subjected to a second filtration to separate the unreacted solid PET. The PET was washed with ultrapure water and ethanol, vacuum dried, and weighed. 80 mL of ultrapure water was added to the filtrate from the second filtration to precipitate BHET. The precipitate was then cooled overnight at 4°C to promote BHET crystallization. The BHET crystals were collected by vacuum filtration and dried to obtain the BHET product.

[0048] Step 3: The BHET product obtained in Step 2 contained trace amounts of oligomers. To improve its purity, it was purified. The BHET product obtained in Step 2 was heated to 80°C and dissolved in 4.0 g of ethylene glycol. Then, 80 mL of deionized water was added to induce the oligomers to precipitate. The precipitate was removed by filtration. The resulting filtrate was concentrated by rotary evaporation at 60°C to remove excess water. The concentrated solution was crystallized at 4°C for 12 hours. After crystallization, white needle-like BHET crystals were collected by filtration and vacuum dried at room temperature to obtain BHET with higher purity.

[0049] The flowchart for the depolymerization and recycling of PET / PC mixed polyester waste in this embodiment is as follows: Figure 1 As shown.

[0050] The conversion rates of PC and PET, as well as the yields of BPA and BHET, were calculated. The conversion rates of PC and PET were both 100%, and the yields of BPA and BHET were 99.1% and 93.2%, respectively.

[0051] Example 4: Depolymerization Application of Post-Consumer Blended Polyester Waste

[0052] Different post-consumption polyester waste materials were selected and depolymerized according to the method in Example 3. The results are shown in Table 2.

[0053] Table 2. Alcohololysis effect of different polyester materials on consumer products catalyzed by catalysts.

[0054]

[0055] This invention provides a concept and method for the depolymerization and recycling of PET / PC mixed polyester waste. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method for depolymerizing and recycling PET / PC mixed polyester waste, wherein the PET / PC mixed polyester waste is a mixture of waste polyester made of PET and waste polyester made of PC, characterized in that, Includes the following steps: Step 1: The PET / PC mixed polyester waste, modified magnesium hydroxide catalyst and alcohol compound are mixed and subjected to a first heating reaction to obtain a first reaction solution; Step 2: While the first reaction solution is hot, a first solid-liquid separation is performed to obtain a first solid and a first liquid. After the first liquid is cooled to room temperature, a second solid-liquid separation is performed to obtain a second solid and a second liquid. Water is added to the second liquid, and the mixture is cooled to crystallize, thus obtaining bisphenol A crystals. Step 3: Mix the second solid, the modified magnesium hydroxide catalyst, and the alcohol compound, and carry out a second heating reaction to obtain a second reaction solution; Step 4: While the second reaction solution is still hot, perform a third solid-liquid separation to obtain a third liquid. Cool the liquid to room temperature and perform a fourth solid-liquid separation to obtain a fourth liquid. Add water to the liquid and cool it to crystallize, thus obtaining BHET crystals.

2. The method according to claim 1, characterized in that, In step 1, the mass ratio of waste polyester (PC material) in the PET / PC mixed polyester waste to the initial mass ratio of the modified magnesium hydroxide catalyst is 1:0.001~0.01; the mass ratio of the alcohol compound to the PET / PC mixed polyester waste is 1~10:1; the first heating reaction is carried out in an air atmosphere at a temperature of 130~160℃ with stirring for 10 min~6 h.

3. The method according to claim 1, characterized in that, In step 3, the initial mass ratio of the second solid to the modified magnesium hydroxide catalyst is 1:0.001 to 0.01; the mass ratio of the alcohol compound to the second solid is 1 to 10:1; the second heating reaction is carried out in an air atmosphere at a temperature of 170 to 200°C with stirring for 10 min to 6 h.

4. The method according to claim 1, characterized in that, In steps 2 and 4, the cooling crystallization is performed by overnight cooling crystallization at 0 to 4°C.

5. The method according to any one of claims 1 to 3, characterized in that, In steps 1 and 3, the alcohol compound is any one or more combinations of ethylene glycol, methanol, ethanol, diethylene glycol, propylene glycol, butanediol, hexanediol, n-octanol, and isooctanol, preferably ethylene glycol.

6. The method according to claim 1, characterized in that, The preparation method of the modified magnesium hydroxide catalyst includes the following steps: adding an aqueous sodium hydroxide solution to an aqueous magnesium chloride solution to carry out a first reaction to obtain a suspension, adding an aqueous L-ascorbic acid solution to carry out a second reaction, allowing it to stand, performing solid-liquid separation on the resulting reaction solution, and washing and vacuum drying the solid part to obtain the catalyst.

7. The method according to claim 6, characterized in that, The concentration of the sodium hydroxide aqueous solution is 0.01 ~ 1 mol / L; the concentration of the magnesium chloride aqueous solution is 0.01 ~ 1 mol / L; the amount of sodium hydroxide aqueous solution and magnesium chloride aqueous solution used is controlled so that the molar ratio of sodium hydroxide to magnesium chloride is 0.2 ~ 20 :

1.

8. The method according to claim 6, characterized in that, The first reaction is carried out at 25-90°C and a stirring speed of 300-1000 rpm for 1-4 hours; the standing is carried out at room temperature for 1-24 hours.

9. The method according to claim 6, characterized in that, The concentration of the L-ascorbic acid aqueous solution is 0.01 ~ 0.1 mol / L; the amount of L-ascorbic acid added is controlled so that the amount of L-ascorbic acid added is 0.05 ~ 10 times the amount of magnesium chloride; the conditions for the second reaction are stirring at 25 ~ 90°C for 1 ~ 24 h.

10. The method according to claim 6, characterized in that, The washing process involves sequentially washing with ethanol and water; the vacuum drying process is carried out at a temperature of 60-80°C for 6-24 hours.

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