A method for purifying high free amine NMP waste liquid based on composite treating agent

Abstract

This invention belongs to the field of solvent recovery and purification technology, specifically relating to a method for purifying high-free-amine NMP wastewater based on a composite treatment agent. The method targets NMP wastewater with a free amine content greater than 500 ppm. First, a primary treatment agent composed of an amino-functionalized ionic liquid and an organic carboxylate is sequentially added, followed by an auxiliary complexing agent composed of a thiol compound and porous nano-alumina for fractional pretreatment. Then, a two-stage pressure-controlled dehydration distillation is performed. Finally, under specific purity and impurity concentration conditions, a side stream is initiated for collection, with periodic reflux flow disturbances and closed-loop flow control based on real-time purity implemented during collection. This method can be efficiently operated on conventional batch distillation equipment, enabling deep purification of high-free-amine NMP wastewater and obtaining high-quality recovered NMP.

Description

Technical Field

[0001] This invention belongs to the field of solvent recovery and purification technology, specifically relating to a method for purifying high-free-amine NMP waste liquid based on a composite treatment agent. Background Technology

[0002] With the rapid development of the global new energy vehicle and energy storage industries, lithium batteries, as a core component, are experiencing continuous capacity expansion. N-methylpyrrolidone (NMP), due to its excellent solubility and stability, is widely used as a key solvent in the positive and negative electrode slurries of lithium batteries. A large amount of NMP waste liquid is generated during key processes such as coating and drying of battery electrodes. Efficiently recycling and purifying this waste liquid can not only significantly reduce the raw material costs of lithium battery production but is also an essential requirement for enterprises to achieve green manufacturing and reduce environmental pollution. In the strict quality control system of the lithium battery industry, the purity of recycled NMP, especially the content of free amines, is a core indicator related to battery performance and safety. Free amines, as an alkaline impurity, can cause electrolyte decomposition and accelerate side reactions at the electrode interface during battery cycling, and may corrode the current collector, ultimately leading to battery capacity decay, increased internal resistance, and even safety hazards such as thermal runaway. Therefore, battery manufacturers have imposed almost stringent requirements on the purity of recycled NMP, with the free amine content typically controlled below 10 ppm, or even lower.

[0003] Currently, the industry commonly uses distillation to purify conventional NMP waste liquid with low free amine content (e.g., below 150 ppm). This technology mainly relies on the difference in volatility between NMP and water, and light and heavy components, achieving separation through processes such as heating, condensation, and reflux. However, when faced with waste liquid generated from some battery production processes, where the initial concentration of free amine is abnormally high (even exceeding 500 ppm), existing distillation technology proves inadequate. Because the boiling points of free amine and NMP are relatively close, the difference in their relative volatility is small, making it difficult for conventional distillation processes relying solely on physical separation to achieve efficient separation. Even with enhancement measures such as significantly increasing the number of theoretical plates and improving the reflux ratio, the free amine content can often only be reduced to 20-30 ppm, which is insufficient to consistently meet the reuse standards of high-end battery production.

[0004] To overcome this challenge, the industry has explored various technical approaches, but all have significant drawbacks. One approach involves introducing pretreatment processes such as extraction, where the waste liquid is first washed with an acidic aqueous solution to neutralize and remove most of the free amine before distillation. While this method reduces the free amine load, it introduces a large amount of water, leading to a surge in energy consumption for subsequent dehydration. It also suffers from problems such as extractant entrainment, large wastewater volumes, complex processes, and high equipment investment and operating costs. Another approach employs a complex sequence of multi-tower continuous distillation, removing light and heavy components separately through multiple distillation columns, theoretically yielding high-purity products. However, this approach requires substantial equipment investment, has a lengthy process, and necessitates the control of numerous operating parameters such as temperature, pressure, and flow rate. It demands extremely high levels of automation and operator skills, and suffers from poor system flexibility, making it difficult to adapt to fluctuations in the composition of the feedstock, resulting in poor economic efficiency and operability. Furthermore, some have attempted to directly add inorganic acids to the waste liquid before distillation to neutralize the free amine. Although this method is simple, the introduction of strong acid can easily lead to equipment corrosion, and excessive acid or the generated salt may decompose or remain at the high temperature of distillation, which may introduce new inorganic impurities into the product, affecting the purity and quality stability of NMP, especially the control of metal ion content.

[0005] Therefore, developing a new method for high-purity NMP recovery that can be operated on conventional and simple equipment, efficiently and deeply removes free amines and other impurities, and ensures that the process is economical, stable, and easy to implement has become a technical problem that urgently needs to be solved in the lithium battery industry and the solvent recovery field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for purifying high-free-amine NMP waste liquid based on a composite treatment agent, so as to overcome the defects of existing technologies in treating high-concentration free-amine waste liquid, such as low separation efficiency, complex process or easy introduction of secondary impurities.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A method for purifying high-free-amine NMP waste liquid based on a composite treatment agent includes the following steps:

[0009] (1) Add the ionic liquid main treatment agent and the auxiliary complexing agent to the high free amine NMP waste liquid to be treated in sequence, stir and react to obtain the pretreated material;

[0010] (2) The pretreated material is first heated and distilled at a pressure of 90-100 Torr and a top temperature of 55-65℃ to remove moisture; then the pressure is adjusted to 50-60 Torr and the top temperature is adjusted to 75-90℃, and the residual moisture is distilled off until the moisture content in the distillate at the top of the column is less than 0.05% by mass.

[0011] (3) When the purity of NMP in the gas phase of the column reaches 99.98% or higher and the concentration of nitrogen-containing organic impurities from the ionic liquid main treatment agent is less than 1 ppm, the material is started to be drawn from the side stream outlet of the distillation column. During the drawing process, the side stream drawing flow rate is dynamically adjusted according to the real-time value of product purity obtained by online monitoring.

[0012] (4) After step (3) is completed, drain the residual liquid at the bottom of the distillation unit.

[0013] Furthermore, the free amine content in the high free amine NMP waste liquid described in step (1) is greater than 500 ppm.

[0014] Further, the ionic liquid main treatment agent in step (1) is a compound of amino-functionalized ionic liquid and organic carboxylate; the auxiliary complexing agent is a suspension of organic compound containing thiol (-SH) and porous nano alumina.

[0015] Further, after adding the ionic liquid main treatment agent in step (1), the mixture is stirred and reacted at 50-65°C for 30-60 minutes, and then an auxiliary complexing agent is added, and the mixture is stirred and reacted at 40-50°C for 20-40 minutes to obtain the pretreated material.

[0016] Further, the ionic liquid main treatment agent in step (1) is composed of the following components in mass percentage: amino-functionalized ionic liquid: 70%-85%, wherein the amino-functionalized ionic liquid is selected from at least one of 1-aminoethyl-3-methylimidazolium chloride and 1-aminopropyl-3-butylimidazolium bromide; organic carboxylate: 15%-30%, wherein the organic carboxylate is selected from at least one of sodium citrate and potassium tartrate.

[0017] Further, in the auxiliary complexing agent of step (1), the thiol-containing organic compound is selected from at least one of 2-mercaptobenzothiazole and thioglycolic acid; the particle size of the porous nano-alumina is 20-100 nm; and the mass ratio of the thiol-containing organic compound to the porous nano-alumina is 1:(0.5-2).

[0018] Further, in step (1), the amount of ionic liquid main treatment agent added is 0.5%-3% of the mass of the high free amine NMP waste liquid; the amount of auxiliary complexing agent added is 0.1%-1% of the mass of the high free amine NMP waste liquid.

[0019] Furthermore, after the reaction in step (1) is completed, a short heating step is also included: the material temperature is raised to 75-90℃ and maintained for 5-15 minutes, and then cooled down to proceed with step (2).

[0020] The amino-functionalized ionic liquid in the ionic liquid-type main treatment agent can efficiently bind with free amines in the waste liquid through hydrogen bonds, electrostatics, and intermolecular forces, transforming them from volatile neutral molecules into non-volatile ion-associated aggregates or macromolecules. This completely alters their volatility characteristics during subsequent distillation, fixing them in the distillation vessel. The compounded organic carboxylate acts as a buffer and auxiliary complexing agent, stabilizing the system pH, preventing local over-acidity or over-alkalinity, and initially complexing some metal ions with its carboxylate ions. The thiol compound loaded on high specific surface area porous nano-alumina in the auxiliary complexing agent can selectively chemically capture and physically adsorb various metal ion impurities through thiol groups, significantly reducing their levels. The pretreatment is carried out in two steps: first, the main treatment agent stabilizes the free amines, and then the auxiliary complexing agent deeply purifies the metal impurities. The short-term heating step is beneficial for the final stabilization of the complex structure and the full activation of the nanomaterials, thus creating a material system with stable impurity morphology and simplified composition for subsequent distillation separation.

[0021] Furthermore, in step (3), during the side-line extraction, the top return flow rate is periodically increased by 10%-30% on the basis of the original stable value. Each increase operation lasts for 2-5 minutes and is performed every 15-30 minutes.

[0022] Periodically increasing the reflux flow rate can actively disrupt the gas-liquid equilibrium achieved within the distillation column. During side-stream sampling, the column composition gradually changes, and the separation factor between trace impurities and NMP may be extremely low, easily forming a "tailing" effect. Briefly increasing the reflux flow rate can instantly increase flooding and liquid-phase reflux within the column, enhancing the washing and mass transfer of rising vapor. This forces trace impurities that are difficult to separate due to their similar volatility or adsorbed on column components to be captured by the additional liquid phase and carried back to the bottom of the column. As a result, after the system returns to steady state, the purity of the gas phase in the middle section of the column is periodically improved.

[0023] Further, in step (3), the specific method for dynamically adjusting the side-line extraction flow rate is as follows: when the real-time value of the product purity monitored online is higher than 99.99%, the extraction flow rate is increased at a rate of 1%-5% per minute; when the real-time value of the product purity is between 99.98% and 99.99%, the current extraction flow rate is kept unchanged; when the real-time value of the product purity is lower than 99.98%, the extraction flow rate is reduced at a rate of 5%-10% per minute or extraction is suspended.

[0024] This dynamic control strategy establishes an asymmetric flow regulation mechanism based on quality feedback. When the purity is above 99.99%, indicating excellent separation within the column, the system tentatively increases output at a low rate, aiming to steadily increase capacity without disrupting the current ideal balance. When the purity is within the acceptable range of 99.98%-99.99%, the flow rate is maintained constant, providing a stable adjustment window for the system, allowing the composition within the column to adjust naturally without disturbance. Once the purity falls below the 99.98% safeguard threshold, the system reduces or stops output at a faster rate. This asymmetric response reflects the principle of prioritizing quality and safety, rapidly reducing the output of non-conforming products and forcing the system to self-correct through methods such as increasing reflux, thereby ensuring the stability and consistency of product purity.

[0025] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0026] 1. Deep removal of impurities, resulting in superior product quality: This invention utilizes the chemical conversion and efficient adsorption of a specialized composite treatment agent, combined with a coupled distillation process, to stably reduce high-concentration free amines (>500 ppm) to below 5 ppm, while simultaneously removing metal impurities to the ppb level. The moisture content is far below the conventional reuse standard, significantly improving product purity.

[0027] 2. Strong process adaptability and wide processing range: Specifically designed for high free amine waste liquid that is difficult to treat effectively with existing technologies, it can adapt to waste liquid raw materials with different initial concentrations through adjustable pretreatment and distillation parameters, and achieve stable compliance.

[0028] 3. High efficiency and energy saving, simple operation: The single-tower intermittent process is adopted, the process is simple and there are few key control points; the two-stage pressure-controlled dehydration strategy and dynamic optimization purification reduce the overall energy consumption; the introduction of automated control logic reduces the intensity of manual intervention and improves the stability and repeatability of operation. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise specified, all raw materials used in the embodiments are commercially available products. The following sources are for illustrative purposes only:

[0031] The porous nano-alumina was purchased from Sinopharm Chemical Reagent Co., Ltd., with a particle size of 20-50 nm.

[0032] Example 1

[0033] This embodiment provides a method for purifying high-free-amine NMP waste liquid based on a composite treatment agent, including the following steps:

[0034] 2.0 kg of NMP waste liquid generated from a lithium battery coating process was used as raw material. After testing, its free amine content was 650 ppm, the moisture content was 2.1%, and it contained a variety of metal ion impurities, among which the contents of iron (Fe), copper (Cu), sodium (Na) ions were relatively high, totaling about 550 ppm.

[0035] (1) Add 30.0 g of ionic liquid main treatment agent to the waste liquid. The main treatment agent is composed of 1-aminopropyl-3-butylimidazolium bromide (75% of the main treatment agent mass) and sodium citrate (25% of the main treatment agent mass). Stir the reaction at 300 rpm at 60°C for 40 minutes.

[0036] Subsequently, 6.0 g of auxiliary complexing agent was added to the system. The auxiliary complexing agent was a suspension prepared by 2-mercaptobenzothiazole and porous nano-alumina in a mass ratio of 1:1. The system temperature was adjusted to 45°C and the reaction was continued for 30 minutes.

[0037] After the reaction is complete, the material temperature is raised to 75°C and maintained for 10 minutes, then cooled to about 50°C to obtain the pretreated material.

[0038] (2) Transfer all the pretreated material into a distillation vessel equipped with a side-stream outlet, a top condenser, a reflux ratio controller, and a vacuum system. Start the first stage of dehydration: turn on the vacuum system, stabilize the top pressure at 100 Torr, start heating, and control the top temperature at around 55°C. At this time, a large amount of azeotropic material will distill out. Collect the distillate from this stage.

[0039] Initiate the second stage of dehydration: When the top temperature of the column begins to rise and the distillation rate decreases significantly, adjust the system pressure to 60 Torr and adjust the heating power accordingly to stabilize the top temperature at 75°C. Continue to collect the distillate and monitor it using an online moisture analyzer. Stop the dehydration stage when the moisture content of the collected distillate is less than 0.05% for three consecutive measurements.

[0040] (3) After dehydration, continue to slowly increase the temperature of the vessel to enter the purification stage. The gas phase composition is monitored in real time by gas chromatography. When the monitoring shows that the NMP purity reaches 99.983% and the concentration corresponding to the characteristic ion fragment signal of 1-aminopropyl-3-butylimidazole is less than 0.8 ppm, the side-stream sampling line is opened.

[0041] During extraction, the top return flow rate is increased by 20% every 20 minutes based on the original stable value. This high return flow rate is maintained for 3 minutes before returning to the original value.

[0042] The frequency of the extraction pump is dynamically adjusted to change the extraction flow rate (F) based on the real-time purity (P) of the side-stream product. The control logic is set as follows: if P > 99.99%, the extraction flow rate is slowly increased at a rate of 3% per minute; if 99.98% ≤ P ≤ 99.99%, F remains unchanged; if P < 99.98%, the extraction flow rate is rapidly decreased at a rate of 8% per minute. During this process, the product purity P is always maintained above 99.98%.

[0043] Extraction continued until the temperature at the bottom of the column rose significantly and online monitoring showed that the NMP purity began to decline trend, at which point extraction was stopped. Approximately 1.82 kg of qualified NMP product was extracted during this stage.

[0044] (4) Stop heating, turn off the vacuum, and wait for the system to return to normal pressure and cool down before discharging the viscous, dark-colored residual liquid from the bottom of the distillation vessel.

[0045] Example 2

[0046] This embodiment provides a method for purifying high-free-amine NMP waste liquid based on a composite treatment agent, including the following steps:

[0047] 1.5 kg of NMP waste liquid generated from a batch of lithium battery production was taken as raw material. After testing, its free amine content was 850 ppm, the moisture content was 1.8%, and it contained a variety of metal ion impurities, among which the contents of iron (Fe), copper (Cu), sodium (Na) ions were relatively high, totaling about 780 ppm.

[0048] (1) Add 7.5 g of ionic liquid main treatment agent to the waste liquid. The main treatment agent is composed of 1-aminoethyl-3-methylimidazolium chloride (80% of the main treatment agent mass) and potassium tartrate (20% of the main treatment agent mass). Stir and react at 55°C for 30 minutes.

[0049] Subsequently, 1.5 g of auxiliary complexing agent was added to the system. The auxiliary complexing agent was a suspension prepared by mercaptoacetic acid and porous nano alumina at a mass ratio of 1:1.5. The system temperature was adjusted to 40°C and the reaction was continued for 20 minutes.

[0050] After the reaction is complete, the material temperature is raised to 80°C and maintained for 8 minutes, then cooled to about 50°C to obtain the pretreated material.

[0051] (2) Transfer all pretreated materials into the distillation vessel. Start the first stage of dehydration: turn on the vacuum system, stabilize the pressure at the top of the column at 90 Torr, control the temperature at the top of the column at 65°C for dehydration, and collect the distillate.

[0052] Initiate the second stage of dehydration: When the distillation rate decreases significantly, adjust the system pressure to 60 Torr and stabilize the top temperature at 80°C to continue dehydration. Stop dehydration when the water content in the distillate is less than 0.05% for three consecutive measurements using an online moisture analyzer.

[0053] (3) After dehydration, the purification stage begins. The side stream is activated when the NMP purity reaches 99.985% and the concentration of characteristic ion fragments is below 0.5 ppm, as monitored by an online gas chromatograph.

[0054] During extraction, every 25 minutes, the reflux flow rate at the top of the tower is increased by 15% from the stable value, maintained for 4 minutes, and then restored.

[0055] The extraction flow rate (F) is dynamically adjusted based on the real-time product purity value (P). The control logic is as follows: if P > 99.99%, the extraction rate is increased by 2% per minute based on the current F value; if 99.98% ≤ P ≤ 99.99%, F remains unchanged; if P < 99.98%, the extraction rate is decreased by 7% per minute based on the current F value. Extraction continues until the reboiler temperature rises significantly and the purity shows a downward trend, at which point extraction stops. Approximately 1.38 kg of qualified product was extracted during this stage.

[0056] (4) Stop the operation and wait for the system to cool down before draining the residual liquid at the bottom of the distillation vessel.

[0057] Example 3

[0058] This embodiment provides a method for purifying high-free-amine NMP waste liquid based on a composite treatment agent, including the following steps:

[0059] 3.0 kg of NMP waste liquid generated from a lithium battery coating process was used as raw material. After testing, its free amine content was 800 ppm, the moisture content was 2.5%, and it contained a variety of metal ion impurities, among which the contents of iron (Fe), copper (Cu), sodium (Na) ions were relatively high, totaling about 420 ppm.

[0060] (1) Add 60.0 g of ionic liquid main treatment agent to the waste liquid. The main treatment agent is composed of 1-aminopropyl-3-butylimidazolium bromide (70% of the main treatment agent mass) and sodium citrate (30% of the main treatment agent mass). Stir and react at 65°C for 50 minutes.

[0061] Subsequently, 15.0 g of auxiliary complexing agent was added to the system. The auxiliary complexing agent was a suspension prepared by 2-mercaptobenzothiazole and porous nano-alumina at a mass ratio of 1:0.8. The system temperature was adjusted to 48°C and the reaction was continued for 35 minutes.

[0062] After the reaction is complete, the material temperature is raised to 90°C and maintained for 12 minutes, then cooled to about 50°C to obtain the pretreated material.

[0063] (2) Transfer all pretreated materials into the distillation vessel. Start the first stage of dehydration: turn on the vacuum system, stabilize the pressure at the top of the column at 95 Torr, control the temperature at the top of the column at 58°C for dehydration, and collect the distillate.

[0064] Initiate the second stage of dehydration: When the distillation rate decreases significantly, adjust the system pressure to 55 Torr and stabilize the top temperature at 88°C to continue dehydration. Stop dehydration when the water content in the distillate is less than 0.05% for three consecutive measurements using an online moisture analyzer.

[0065] (3) After dehydration, the purification stage begins. When the NMP purity reaches 99.98% and the concentration of characteristic ion fragment signals is below 0.3 ppm, the side stream sampling is activated by monitoring with an online gas chromatograph.

[0066] During extraction, every 15 minutes, the top return flow rate is increased by 25% from the stable value, maintained for 2.5 minutes, and then restored.

[0067] The extraction flow rate (F) is dynamically adjusted based on the real-time product purity value (P). The control logic is as follows: if P > 99.99%, the extraction rate is increased by 4% per minute based on the current F value; if 99.98% ≤ P ≤ 99.99%, F remains unchanged; if P < 99.98%, the extraction rate is decreased by 10% per minute based on the current F value, or extraction is paused. Extraction continues until the reboiler temperature rises significantly and the purity shows a downward trend, at which point extraction stops. Approximately 2.78 kg of qualified product was extracted during this stage.

[0068] (4) Stop the operation and wait for the system to cool down before draining the residual liquid at the bottom of the distillation vessel.

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that in step (1), the ionic liquid main treatment agent is replaced with 13.5g of a 10 wt% dilute hydrochloric acid solution.

[0071] Comparative Example 2

[0072] The difference between this comparative example and Example 1 is that in step (1), the auxiliary complexing agent is replaced with an equal mass of disodium ethylenediaminetetraacetate.

[0073] Comparative Example 3

[0074] The difference between this comparative example and Example 1 is that step (1) is replaced by adding 30.0 g of ionic liquid main treatment agent and 6.0 g of auxiliary complexing agent to the waste liquid at the same time, and stirring at 300 rpm for 70 minutes at 60°C; wherein the composition of the ionic liquid main treatment agent and the auxiliary complexing agent is the same as that of Example 1.

[0075] After the reaction is complete, the material temperature is raised to 75°C and maintained for 10 minutes, then cooled to about 50°C to obtain the pretreated material.

[0076] Comparative Example 4

[0077] The difference between this comparative example and Example 1 is that the short heating step of “raising the material temperature to 75°C, maintaining it for 10 minutes, and then cooling it to about 50°C” in step (1) is omitted.

[0078] Comparative Example 5

[0079] The difference between this comparative example and Example 1 is that the periodic return flow disturbance operation in step (3) is omitted, which involves "increasing the top return flow rate by 20% every 20 minutes during the extraction period, maintaining this high return flow rate for 3 minutes before returning to the original value." During sideline extraction, the top return flow rate remains constant.

[0080] Performance testing

[0081] To objectively evaluate the effectiveness of the process of the present invention, the final products obtained in Examples 1-3 and Comparative Examples 1-5 were tested. The key quality indicators included NMP purity, free amine content, moisture content and total metal ion content.

[0082] The test results are shown in Table 1.

[0083] Table 1 Performance Test Results

[0084] sample NMP purity (%) Free amines (ppm) Moisture (%) Total metal ions (ppb) Example 1 99.993 1.2 0.0015 3 Example 2 99.991 1.8 0.0013 2 Example 3 99.995 0.8 0.001 1 Comparative Example 1 99.97 18.5 0.0082 38 Comparative Example 2 99.982 2.5 0.015 28 Comparative Example 3 99.985 8.1 0.005 15 Comparative Example 4 99.987 6.3 0.0032 9 Comparative Example 5 99.988 1.5 0.0016 4

[0085] The test results above show that the technical solution provided by this invention achieves the best balance in terms of product purity and the depth of removal of key impurities.

[0086] Comparative Example 1 showed a significantly higher level of free amine because hydrochloric acid only performs simple neutralization, and the resulting amine salt may partially decompose during subsequent heating, without the stabilizing effect of ionic liquids. Comparative Example 2 showed a higher level of moisture, mainly because disodium ethylenediaminetetraacetate needed to be pre-prepared as an aqueous solution before addition, thus introducing a large amount of additional moisture. Comparative Example 3 showed significantly higher levels of free amine and metal residue than the examples, demonstrating that staged pretreatment is crucial for reaction efficiency and adsorption sequence, while the addition of feed leads to component competition and interference. Comparative Example 4 demonstrated that the short-heating step has a detectable improvement effect on stabilizing the pretreated product and preventing the release of trace impurities during distillation. Comparative Example 5 had no reflux disturbance, and the impurity content was similar to the examples, with an NMP purity of 99.988%, demonstrating that the "periodic reflux disturbance" operation directly contributes to improving the product purity limit (from 99.98% to 99.99%).

[0087] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for purifying high-free-amine NMP waste liquid based on a composite treatment agent, characterized in that, Includes the following steps: (1) Add the ionic liquid main treatment agent and the auxiliary complexing agent to the high free amine NMP waste liquid to be treated in sequence, stir and react to obtain the pretreated material; (2) The pretreated material is first heated and distilled at a pressure of 90-100 Torr and a top temperature of 55-65℃ to remove moisture; then the pressure is adjusted to 50-60 Torr and the top temperature is adjusted to 75-90℃, and the residual moisture is distilled off until the moisture content in the distillate at the top of the column is less than 0.05% by mass. (3) When the purity of NMP in the gas phase of the column reaches 99.98% or higher and the concentration of nitrogen-containing organic impurities from the ionic liquid main treatment agent is less than 1 ppm, the material is started to be drawn from the side stream outlet of the distillation column. During the drawing process, the side stream drawing flow rate is dynamically adjusted according to the real-time value of product purity obtained by online monitoring. (4) After step (3) is completed, drain the residual liquid at the bottom of the distillation unit.

2. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, The free amine content in the high free amine NMP waste liquid in step (1) is greater than 500 ppm.

3. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, The ionic liquid main treatment agent in step (1) is a compound of amino-functionalized ionic liquid and organic carboxylate; the auxiliary complexing agent is a suspension of organic compound containing thiol group and porous nano alumina.

4. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, After adding the ionic liquid main treatment agent in step (1), stir and react at 50-65℃ for 30-60 minutes, then add the auxiliary complexing agent, and continue to stir and react at 40-50℃ for 20-40 minutes to obtain the pretreated material.

5. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, The ionic liquid main treatment agent in step (1) is composed of the following components by mass percentage: amino-functionalized ionic liquid: 70%-85%, wherein the amino-functionalized ionic liquid is selected from at least one of 1-aminoethyl-3-methylimidazolium chloride and 1-aminopropyl-3-butylimidazolium bromide; organic carboxylate: 15%-30%, wherein the organic carboxylate is selected from at least one of sodium citrate and potassium tartrate.

6. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, In step (1), the auxiliary complexing agent contains a thiol-containing organic compound selected from at least one of 2-mercaptobenzothiazole and thioglycolic acid; the porous nano-alumina has a particle size of 20-100 nm; and the mass ratio of the thiol-containing organic compound to the porous nano-alumina is 1:(0.5-2).

7. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, In step (1), the amount of the ionic liquid main treatment agent added is 0.5%-3% of the mass of the high free amine NMP waste liquid; the amount of the auxiliary complexing agent added is 0.1%-1% of the mass of the high free amine NMP waste liquid.

8. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, After the reaction in step (1) is completed, a short heating step is also included: the material temperature is raised to 75-90℃ and maintained for 5-15 minutes, and then cooled down to proceed with step (2).

9. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, In step (3), during the side-line extraction, the top return flow rate is periodically increased by 10%-30% based on the original stable value. Each increase operation lasts for 2-5 minutes and is performed every 15-30 minutes.

10. The method for purifying high-free-amine NMP waste liquid based on a composite treatment agent according to claim 1, characterized in that, In step (3), the specific method for dynamically adjusting the side-line extraction flow rate is as follows: when the real-time value of the product purity monitored online is higher than 99.99%, the extraction flow rate is increased at a rate of 1%-5% per minute; when the real-time value of the product purity is between 99.98% and 99.99%, the current extraction flow rate is kept unchanged; when the real-time value of the product purity is lower than 99.98%, the extraction flow rate is reduced at a rate of 5%-10% per minute or extraction is suspended.