Recovery method and application of catalyst triethylamine in polycarbonate production process

Abstract

The invention discloses a recovery method and application of a catalyst triethylamine in a polycarbonate production process. The recovery method comprises the following steps: S1, adding an inorganic acid aqueous solution into a PC oil-phase reaction solution, converting triethylamine into ammonium salt, dissolving the ammonium salt in a water phase, carrying out oil-water separation, and collecting the water phase; s2, alkali liquor is added into the obtained water phase, and ammonium salt is converted into triethylamine again; s3, carrying out rectification / steam stripping on the solution obtained in the step S2, collecting a tower top gas phase, and condensing to obtain a triethylamine solution; s4, cooling the triethylamine solution in the step S3 to 20 DEG C or below, and filtering to remove solid impurities; s5, adding alkali liquor into the filtrate, performing high-speed mixing to obtain emulsion, performing centrifugal separation, and collecting an oil phase, namely a recycled triethylamine solution. After the recycled triethylamine solution obtained according to the recycling method is recycled in the preparation process of polycarbonate, the heat resistance, the powder color, the transmittance and the like of polycarbonate can be effectively improved.

Description

Technical Field

[0001] The invention relates to a recovery method, in particular to a recovery method and application of triethylamine catalyst in a polycarbonate production process. Background Art

[0002] Polycarbonate (PC) is an engineering plastic with good comprehensive performance, which is widely used in electronics, automobiles, construction and other industries. High-end polycarbonate is also widely used in the optical field, which requires it to have good mechanical properties as well as excellent optical properties and base color. In the processing of polycarbonate, it needs to be injection molded at a high temperature above 300°C. During this process, polycarbonate products usually turn yellow due to heat, which affects the optical properties of polycarbonate. Therefore, it is of great research significance to improve the heat resistance stability and base color of polycarbonate. On the other hand, for polycarbonate products that are widely used in the optical field, especially in the field of anti-ultraviolet radiation lenses, the transmittance index requirements are very strict. However, in the production process of polycarbonate, a very small amount of impurities will have a great impact on its transmittance. Therefore, it is very important to develop advanced polycarbonate production processes and improve the base color, heat resistance and transmittance and other indicators.

[0003] In the production process of polycarbonate, especially in the phosgene synthesis process, triethylamine is usually used as a catalyst to accelerate the reaction process of interfacial polycondensation, and the triethylamine in the PC reaction solution is recycled after the interfacial polycondensation reaction, which, on the one hand, improves the resource utilization of triethylamine, and on the other hand, reduces the adverse effects of its residue in PC on the product. Among them, the recovery method of triethylamine is relatively conventional in the field, generally adding an inorganic acid solution to the PC reaction solution to convert the amine into an ammonium salt that dissolves in the water phase, then adding an alkali solution to the water phase to convert the ammonium salt back into amine, and finally distilling and separating to obtain purified triethylamine.

[0004] However, the inventors have found in their continued research on the above reaction system that the polycarbonate prepared after recycling the recovered triethylamine generally has a significantly reduced product quality, especially the thermal stability and base color of the polycarbonate are greatly affected. It is speculated that a certain amount of impurities that affect the quality of the above products will be generated during the production of polycarbonate or the recycling of triethylamine.

[0005] Patent CN117801254A conducted an in-depth study on the factors affecting the heat stability of PC and found that by adding thiourea dioxide, impurities including the substance of formula I can be removed, thereby advantageously improving the heat stability of PC and improving the product color and transmittance. However, the inventors verified through a small-scale test that this method can only improve the performance of PC powder to a certain extent, and the introduction of thiourea dioxide will remain in the polycarbonate in large quantities, resulting in deterioration of indicators such as the crystal point and transmittance of the product.

[0006]

[0007] Therefore, how to obtain recycled triethylamine with high purity and low impurity content, and thus improve the product quality of polycarbonate, is still one of the key projects that need to be continuously studied in this field. Summary of the invention

[0008] In order to solve the above technical problems, the present invention provides a method for recovering triethylamine, a catalyst, in a polycarbonate production process and its application.

[0009] The inventors have found in their research that the triethylamine recovery process is usually carried out under high temperature conditions of distillation / stripping. Even if the impurities affecting product performance such as those shown in Formula I are removed in advance of the distillation / stripping, the industrial polycarbonate produced using the recovered triethylamine as a catalyst still has less than ideal heat resistance, powder color and transmittance.

[0010] The inventors have conducted in-depth research and analysis on the reaction process of triethylamine recovery and recycling, and finally found a crucial indicator that affects the performance of industrial polycarbonate products, namely, the impurity content shown in Formula II. The impurity is generated in large quantities during the conventional distillation / stripping process of the triethylamine recovery process. By strictly controlling the content of the impurity, industrial polycarbonate that meets higher use requirements in terms of heat resistance, powder color and transmittance can be advantageously prepared, and an effective solution for controlling the impurity content to 10 ppm or less is proposed, thereby completing the present invention.

[0011]

[0012] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0013] A method for recovering triethylamine catalyst in a polycarbonate production process comprises the following steps:

[0014] S1. Adding an aqueous solution of inorganic acid to the PC oil phase reaction liquid to convert triethylamine into an ammonium salt which dissolves in the water phase, separating the oil from the water and collecting the water phase;

[0015] S2, adding alkali solution to the obtained aqueous phase to convert the ammonium salt into triethylamine again;

[0016] S3, distilling / stripping the solution obtained in step S2, collecting the gas phase at the top of the tower and condensing it to obtain a triethylamine solution;

[0017] S4, cooling the triethylamine solution in step S3 to 20°C or below and filtering to remove solid impurities;

[0018] S5. Add alkali solution to the filtrate, mix at high speed to obtain an emulsion, and then centrifuge to separate the emulsion. Collect the oil phase to obtain the recycled triethylamine solution.

[0019] As some examples of the present invention, in step S1, the inorganic acid is one or both of hydrochloric acid and sulfuric acid;

[0020] Preferably, the concentration of the inorganic acid aqueous solution is 0.5-10 wt %, and the amount of the inorganic acid aqueous solution is 8-10 times the mass of the PC oil phase reaction liquid.

[0021] Preferably, the oil-water separation in step S1 adopts one or more of a coalescing separator, an inclined plate separator, and a centrifugal extraction separator.

[0022] As some examples of the present invention, the PC oil phase reaction liquid is an oil phase product obtained by separating oil from water from a polycarbonate reaction liquid prepared by an interfacial polycondensation method using bisphenol A and phosgene as raw materials in the presence of a catalyst, triethylamine;

[0023] Preferably, the polycarbonate content in the PC oil phase reaction liquid is 10-25wt%, and the catalyst triethylamine content is 100-500ppm.

[0024] As some examples of the present invention, in step S2, the alkali solution is sodium hydroxide and / or potassium hydroxide solution;

[0025] Preferably, the amount of alkali solution added is such that the pH of the aqueous phase is greater than 12;

[0026] Preferably, the concentration of the alkali solution is 20-40 wt%.

[0027] As some examples of the present invention, in step S3, the distillation / stripping conditions are: tower bottom temperature 100-140° C., theoretical plate number 20-40, and tower bottom steam usage is 3-10% of the feed amount.

[0028] As some examples of the present invention, the triethylamine solution in step S4 is cooled to 10-15° C. and then filtered;

[0029] Preferably, step S4 is performed by filtering through a filter press or a candle filter.

[0030] As some examples of the present invention, in step S5, the alkali solution is sodium hydroxide and / or potassium hydroxide solution;

[0031] Preferably, the amount of alkali solution added is 3-5% of the mass of the filtrate;

[0032] Preferably, the concentration of the alkali solution is 5-12 wt%.

[0033] As some examples of the present invention, the high-speed mixing in step S5 uses a homogenizer or a high-speed mixer, and the stirring speed is preferably 1000-3500 rpm.

[0034] Preferably, the centrifugal speed in step S5 is 1500-2800 rpm, and the water content of the oil phase after centrifugal separation is 100-300 ppm.

[0035] The present invention also provides a recycled triethylamine solution obtained by the method described above, comprising 2-4 wt % of triethylamine, impurities of the structure of formula II ≤ 10 ppm, and the remainder being an oil phase solvent;

[0036]

[0037] Preferably, the oil phase solvent is selected from one or more of dichloromethane, chloroform, tetrachloroethane, chlorobenzene, tetrahydrofuran, dioxane, and pyridine.

[0038] The present invention also provides a use of the recycled triethylamine solution recovered by the method described above or the recycled triethylamine solution described above as a catalyst for preparing polycarbonate through an interfacial polycondensation method.

[0039] Specific application methods include:

[0040] The oil phase solvent containing phosgene is subjected to interfacial polycondensation reaction with the water phase containing bisphenol A and alkali metal hydroxide under the action of recycled triethylamine solution and end-capping agent to prepare polycarbonate;

[0041] Preferably, after the reaction is completed, the polycarbonate reaction liquid is subjected to oil-water separation to obtain an oil phase product, and the catalyst triethylamine is recycled according to the above-mentioned recycling method.

[0042] Preferably, the amount of phosgene added is 1-1.3 times the molar amount of bisphenol A;

[0043] Preferably, the concentration of bisphenol A in the aqueous phase is 10-25 wt %;

[0044] Preferably, the alkali metal hydroxide is sodium hydroxide / potassium hydroxide; more preferably, the concentration of the alkali metal hydroxide in the aqueous phase is 3-10 wt%;

[0045] Preferably, the amount of the recycled triethylamine solution added is 0.2-0.6 wt % based on the mass of triethylamine relative to bisphenol A;

[0046] Preferably, the end-capping agent is a monohydroxyphenol compound, preferably one or more of phenol, p-tert-butylphenol, and p-cumylphenol;

[0047] Preferably, the amount of the end-capping agent added is 1-3 wt% of the mass of bisphenol A;

[0048] Preferably, the reaction temperature of the interfacial polycondensation reaction is 20-50°C.

[0049] Preferably, the oil phase solvent is selected from one or more of dichloromethane, chloroform, tetrachloroethane, chlorobenzene, tetrahydrofuran, dioxane, and pyridine.

[0050] Preferably, after the reaction is completed, oil-water separation is performed by a static separator and / or a dynamic separator, preferably one or more of an inclined plate separator, a coalescing separator, and a centrifugal extraction separator, to obtain a polycarbonate oil phase reaction liquid.

[0051] According to the recovery method of the present invention, a recycled triethylamine solution with extremely low impurity content shown in formula II can be obtained, and after being recycled in the preparation process of polycarbonate, the heat resistance, powder color and transmittance of polycarbonate can be effectively improved. DETAILED DESCRIPTION

[0052] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present invention.

[0053] The main raw material information used in the following examples of the present invention is as follows:

[0054] Bisphenol A: industrial grade, Wanhua Chemical;

[0055] Phosgene: industrial grade, Wanhua Chemical;

[0056] Triethylamine: analytical grade, purchased from Tianjin Komiou Technology Co., Ltd.;

[0057] p-tert-Butylphenol: analytical grade, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0058] Dichloromethane: analytical grade, purchased from Beijing Inokai Chemical Reagent Co., Ltd.;

[0059] Pure water: industrial grade, Wanhua Chemical;

[0060] Sodium hydroxide: analytical grade, purchased from Tianjin Kemeiou Chemical Reagent Co., Ltd.;

[0061] Hydrochloric acid: 31%, Wanhua Chemical

[0062] Sulfuric acid: 98%, purchased from Beijing Inokai Chemical Reagent Co., Ltd.

[0063] The main equipment information used in the following embodiments of the present invention is as follows:

[0064] Inclined plate separator: purchased from Sulzer

[0065] Stripper: purchased from Sulzer

[0066] Water-oil separation coalescer: purchased from Franken Company

[0067] High-speed mixer: purchased from Laining

[0068] Homogenizer: purchased from Lai Ning

[0069] Centrifuge: purchased from Chongqing Jiangbei

[0070] Filter: purchased from Shangyang Filtration.

[0071] The performance testing methods involved in the following embodiments of the present invention are:

[0072] Determination of substance content: GC-2030 gas chromatograph, HS-10 equipped with hydrogen flame ionization detector (FID) and headspace autosampler (Shimadzu Corporation), chromatographic column (DB-624 column, 30m×0.53mm×3.0μm, Agilent Corporation) was used for determination.

[0073] Powder a, b, L value: detected by colorimeter

[0074] PC test piece YI value: detected by colorimeter

[0075] Crystal point test: The crystal point was tested using the method specified in ASTM D3351. The polycarbonate film was processed using the TTC-1200 casting machine from Richard E. Mistler, and the test instrument was the CPJ-3030A overhead projector from Suzhou Shifeng Precision Instrument Co., Ltd.

[0076] 380nm transmittance: LS182 optical transmittance measuring instrument.

[0077]

Preparation example

[0078] An aqueous solution with a bisphenol A concentration of 12wt% and a sodium hydroxide concentration of 5wt% was prepared and introduced into a reactor at a flow rate of 15t / h. At the same time, phosgene was introduced into the reactor at a flow rate of 0.9t / h, dichloromethane was introduced at a flow rate of 12t / h, a dichloromethane solution of 3% triethylamine was introduced at a flow rate of 120kg / h, and p-tert-butylphenol was introduced at a flow rate of 27kg / h, and an interfacial polymerization reaction was carried out at 25°C. After the reaction was completed, oil-water separation was carried out by an inclined plate separator, the aqueous phase was treated as wastewater, and the oil phase composition was: 15% polycarbonate, 200ppm triethylamine, and 85% dichloromethane, which was used as the PC oil phase reaction liquid raw material of each embodiment and comparative example.

[0079] [Example 1]

[0080] A method for recovering triethylamine catalyst in a polycarbonate production process comprises the following steps:

[0081] S1. Add 1% hydrochloric acid solution to the PC oil phase reaction liquid, the latter being 10 times the mass of the former, and stir at a speed of 70r / min for 10min. After acid washing, use a coalescing separator to separate oil and water, and collect the water phase; after desolvation of the oil phase, dry to obtain PC powder;

[0082] S2, add 22% sodium hydroxide solution to the obtained aqueous phase until pH = 12.8;

[0083] S3, passing the solution obtained in step S2 into a stripping tower, controlling the bottom temperature of the tower to be 110° C., the theoretical plate number of the stripping tower to be 23, the amount of steam used in the bottom of the tower to be 5% of the feed amount, condensing and collecting the gas phase obtained by stripping to obtain a triethylamine solution;

[0084] S4, cooling the triethylamine solution in step S3 to 10°C, and filtering with a filter press to remove solid impurities;

[0085] S5. Add 6% sodium hydroxide solution to the filtrate in an amount of 5% of the mass of the filtrate, fully stir and mix in a high-speed mixer (rotation speed is 1500 rpm) to obtain an emulsion, and then centrifuge at 1750 rpm to collect the oil phase, which is the recycled triethylamine solution, wherein the water content is 283.8 ppm.

[0086] The recycled triethylamine solution provided in this embodiment comprises 3.1 wt % of triethylamine, 8.4 ppm of impurities of the structure of formula II, and 96.9 wt % of dichloromethane;

[0087] Wherein, the substance of formula II is enriched by the following method and qualitatively analyzed by nuclear magnetic hydrogen spectrum:

[0088] Take 10 kg of the oil phase obtained by centrifugation (i.e., the recycled triethylamine solution) and place it in a 20 L jacketed stirring kettle. Set the jacket temperature to 90°C for evaporation and concentration until there is no obvious liquid phase. Add pure dichloromethane to dissolve the residual heavy components, and analyze the solution to determine the NMR structure.

[0089] 1H NMR(CDCl3,500MHz): δ7.36-7.58(m,2H),6.32-6.59(m,2H),6.53-6.69(m,2H),6.12-6.33(m,2H),5.78-6.01(m,2H ),4.89-5.29(m,1H),4.02(q,J=7.12Hz,2H),3.78(q,J=7.35Hz,2H),1.99(t,J=6.79Hz,3H),1.78(t,J=6.91Hz,3H)

[0090]

[0091] [Example 2]

[0092] A method for recovering triethylamine catalyst in a polycarbonate production process comprises the following steps:

[0093] S1. Add 5% hydrochloric acid solution to the PC oil phase reaction liquid, the latter being 8 times the mass of the former, and stirring at a speed of 70r / min for 10min. After acid washing, use a coalescing separator to separate oil and water, and collect the water phase; after desolvation of the oil phase, dry to obtain PC powder;

[0094] S2, add 30% sodium hydroxide solution to the obtained aqueous phase until pH = 13;

[0095] S3, passing the solution obtained in step S2 into a stripping tower, controlling the bottom temperature of the tower to be 120° C., the theoretical plate number of the stripping tower to be 32, the steam dosage of the bottom of the tower to be 7% of the feed amount, condensing and collecting the gas phase obtained by stripping to obtain a triethylamine solution;

[0096] S4, cooling the triethylamine solution in step S3 to 12°C, and filtering with a candle filter to remove solid impurities;

[0097] S5. Add 8% potassium hydroxide solution to the filtrate in an amount of 3.5% of the mass of the filtrate, fully stir and mix in a high-speed mixer (rotation speed is 2500 rpm) to obtain an emulsion, and then centrifuge at 2200 rpm to collect the oil phase, which is the recycled triethylamine solution, wherein the water content is 174.6 ppm.

[0098] The recycled triethylamine solution provided in this embodiment includes 3.6 wt % of triethylamine, 6.7 ppm of impurities with the structure of formula II, and 96.4 wt % of dichloromethane.

[0099] [Example 3]

[0100] A method for recovering triethylamine catalyst in a polycarbonate production process comprises the following steps:

[0101] S1. Add 10% hydrochloric acid solution to the PC oil phase reaction liquid, the latter being 9 times the mass of the former, and stirring at a speed of 70r / min for 10min. After acid washing, use a coalescing separator to separate oil and water, and collect the water phase; after desolvation of the oil phase, dry to obtain PC powder;

[0102] S2, adding 40% potassium hydroxide solution to the obtained aqueous phase until the pH is 13.5;

[0103] S3, passing the solution obtained in step S2 into a stripping tower, controlling the bottom temperature of the tower to be 135° C., the theoretical plate number of the stripping tower to be 37, the amount of steam used in the bottom of the tower to be 10% of the feed amount, condensing and collecting the gas phase obtained by stripping to obtain a triethylamine solution;

[0104] S4, cooling the triethylamine solution in step S3 to 15°C, and filtering with a candle filter to remove solid impurities;

[0105] S5. Add 12% potassium hydroxide solution to the filtrate in an amount of 3% of the mass of the filtrate, fully stir and mix in a high-speed mixer (rotation speed is 3250 rpm) to obtain an emulsion, and then centrifuge at 2750 rpm to collect the oil phase, which is the recycled triethylamine solution, wherein the water content is 102.5 ppm.

[0106] The recycled triethylamine solution provided in this embodiment includes 2.2 wt % of triethylamine, 2.4 ppm of impurities with the structure of formula II, and 97.8 wt % of dichloromethane.

[0107] [Comparative Example 1]

[0108] The PC oil phase reaction solution provided in the preparation example of the present invention is used as a raw material, and the triethylamine catalyst is recovered by referring to the method in Example 1 of Patent CN117801254A. After analysis, the recycled triethylamine solution contains 2.8wt% triethylamine, 322.4ppm of impurities of formula II structure, and 97.2wt% of dichloromethane.

[0109] [Comparative Example 2]

[0110] The triethylamine solution prepared in steps S1-S3 of Example 1 was directly used as the recycled triethylamine solution, wherein the triethylamine was 2.9 wt %, the impurity of the structure of formula II was 637 ppm, and the dichloromethane was 97.1 wt %.

[0111]

Application examples

[0112] The recycled triethylamine solution prepared in each embodiment and comparative example was adjusted to a triethylamine mass concentration of 3% (evaporation or solvent addition), and recycled to the preparation of PC according to the following methods:

[0113] An aqueous solution with a bisphenol A concentration of 12 wt% and a sodium hydroxide concentration of 5 wt% is prepared, and introduced into a reactor at a flow rate of 15 t / h. At the same time, phosgene is introduced into the reactor at a flow rate of 0.9 t / h, dichloromethane is introduced at a flow rate of 12 t / h, a dichloromethane solution of 3% triethylamine is introduced at a flow rate of 120 kg / h, and p-tert-butylphenol is introduced at a flow rate of 27 kg / h, and an interfacial polymerization reaction is carried out at 25° C. After the reaction is completed, oil-water separation is carried out through an inclined plate separator, the aqueous phase is treated as wastewater, a 10% hydrochloric acid solution is added to the oil phase, and the amount of hydrochloric acid added is 10% of the mass of the oil phase, and the mixture is stirred at a speed of 70 r / min for 10 minutes. After pickling, a coalescing separator is used to separate oil and water, and the oil phase is collected, desolventized, and dried to obtain PC powder.

[0114] The following performance tests were conducted on the PC powders prepared by catalysis using the recycled triethylamine solution in each embodiment and comparative example. The results are shown in Table 1.

[0115] Table 1. PC performance test

[0116]

[0117]

[0118] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for recovering triethylamine catalyst in a polycarbonate production process, characterized in that: The following steps are involved: S1. Adding an aqueous solution of inorganic acid to the PC oil phase reaction liquid to convert triethylamine into an ammonium salt which dissolves in the water phase, separating the oil from the water and collecting the water phase; S2, adding alkali solution to the obtained aqueous phase to convert the ammonium salt into triethylamine again; S3, distilling / stripping the solution obtained in step S2, collecting the gas phase at the top of the tower and condensing it to obtain a triethylamine solution; S4, cooling the triethylamine solution in step S3 to 20°C or below and filtering to remove solid impurities; S5. Add alkali solution to the filtrate, mix at high speed to obtain an emulsion, and then centrifuge to separate the emulsion. Collect the oil phase to obtain the recycled triethylamine solution.

2. The method for recovering the catalyst triethylamine in the polycarbonate production process according to claim 1, characterized in that: In step S1, the inorganic acid is one or both of hydrochloric acid and sulfuric acid; Preferably, the concentration of the inorganic acid aqueous solution is 0.5-10 wt %, and the amount of the inorganic acid aqueous solution is 8-10 times the mass of the PC oil phase reaction liquid.

3. The method for recovering the catalyst triethylamine in the polycarbonate production process according to claim 1, characterized in that: The PC oil phase reaction liquid is an oil phase product obtained by separating the oil and water of a polycarbonate reaction liquid prepared by an interfacial polycondensation method using bisphenol A and phosgene as raw materials in the presence of a catalyst, triethylamine; Preferably, the polycarbonate content in the PC oil phase reaction liquid is 10-25wt%, and the catalyst triethylamine content is 100-500ppm.

4. The method for recovering the catalyst triethylamine in the polycarbonate production process according to any one of claims 1 to 3, characterized in that: In step S2, the alkali solution is sodium hydroxide and / or potassium hydroxide solution; Preferably, the amount of alkali solution added is such that the pH of the aqueous phase is greater than 12; Preferably, the concentration of the alkali solution is 20-40 wt%.

5. The method for recovering the catalyst triethylamine in the polycarbonate production process according to any one of claims 1 to 4, characterized in that: In step S3, the distillation / stripping conditions are: the bottom temperature is 100-140° C., the number of theoretical plates is 20-40, and the amount of steam used in the bottom is 3-10% of the feed amount.

6. The method for recovering the catalyst triethylamine in the polycarbonate production process according to any one of claims 1 to 5, characterized in that: In step S4, the triethylamine solution is cooled to 10-15° C. and then filtered; Preferably, step S4 is filtered through a filter press or a candle filter.

7. The method for recovering the catalyst triethylamine in the polycarbonate production process according to any one of claims 1 to 6, characterized in that: In step S5, the alkali solution is sodium hydroxide and / or potassium hydroxide solution; Preferably, the amount of alkali solution added is 3-5% of the mass of the filtrate; Preferably, the concentration of the alkali solution is 5-12 wt%.

8. The method for recovering the catalyst triethylamine in the polycarbonate production process according to any one of claims 1 to 7, characterized in that: The high-speed mixing in step S5 is performed by a homogenizer or a high-speed mixer, and preferably the stirring speed is 1000-3500 rpm; preferably, the centrifugal speed in step S5 is 1500-2800 rpm, and the water content of the oil phase after centrifugal separation is 100-300 ppm.

9. A triethylamine solution recovered by the method according to any one of claims 1 to 8, characterized in that: It comprises 2-4 wt% of triethylamine, impurities of formula II structure ≤10 ppm, and the balance is an oil phase solvent; 10. Use of the recycled triethylamine solution recovered by the method according to any one of claims 1 to 8 or the recycled triethylamine solution according to claim 9 as a catalyst for preparing polycarbonate by interfacial polycondensation.

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