High-efficiency rectification purification and recovery process of low-concentration NMP waste liquid in lithium battery production

Abstract

This invention relates to the field of lithium battery production waste liquid treatment and solvent recovery technology, and discloses a high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production. The process includes: preheating low-concentration NMP waste liquid to 60-80°C, and feeding it into a two-stage series mechanical vapor recompression falling film evaporator for pre-concentration to obtain a first intermediate liquid; mixing the first intermediate liquid with an ionic liquid extractant at a mass ratio of 1:0.2-0.5 and feeding it into the middle of an extractive distillation column, where separation is performed at 0.1-0.2 bar and 45-55 theoretical plates, collecting water and the ionic liquid-extractant mixture at the top of the column, and obtaining an NMP-ionic liquid mixture at the bottom; then feeding the NMP-ionic liquid mixture into a flash regeneration column, where the ionic liquid is separated at 150-180°C and 0.05-0.1 bar to obtain crude NMP; finally, continuously feeding the crude NMP into a three-stage distillation column, collecting high-purity NMP products from the side streams of the upper rectification section, the middle side stream, and the lower stripping section, and discharging heavy component residues from the bottom of the column. This invention achieves low-energy consumption and high-purity continuous recovery of low-concentration NMP waste liquid.

Description

Technical Field

[0001] This invention relates to the field of lithium battery production waste liquid treatment and solvent recovery technology, specifically a high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid from lithium battery production. Background Technology

[0002] N-Methylpyrrolidone (NMP) is an excellent polar aprotic solvent, widely used as a dispersion medium for polyvinylidene fluoride (PVDF) in the manufacturing process of lithium-ion battery electrodes. After the coating process, NMP evaporates into the waste gas during drying and is collected by a rotary or condenser system to form a low-concentration NMP waste liquid. Typically, this waste liquid contains 5%-20% NMP by mass, with the remainder being water and small amounts of high-boiling-point impurities, particulate matter, and decomposition products. Direct discharge not only wastes expensive NMP resources but also poses environmental risks due to the potential toxicity of NMP to the reproductive system. Therefore, recycling and purifying the low-concentration NMP waste liquid to achieve battery-grade purity (≥99.9%) and reusing it is a crucial step in cost reduction, efficiency improvement, and green manufacturing in the lithium battery industry.

[0003] Existing technologies for recovering low-concentration NMP waste liquid mainly include conventional distillation, pervaporation, membrane distillation, and adsorption. Conventional distillation is difficult to directly obtain high-purity NMP because NMP's boiling point (202℃) is much higher than water's (100℃), and NMP forms an azeotrope with water (azeotropic point approximately 170℃, azeotropic composition approximately 23% NMP). Industrially, multi-effect evaporation or vacuum distillation is often used for dehydration, but this is extremely energy-intensive—recovering 1 ton of NMP requires 3-5 tons of steam, and the energy consumption per unit product increases exponentially as the waste liquid concentration decreases. While pervaporation and membrane distillation technologies theoretically have lower energy consumption, the membrane materials have poor stability in high-temperature NMP-containing environments, rapid flux decay, and cannot effectively remove impurities with boiling points close to NMP (such as degradation products like methylamine and butyrolactone), making it difficult to guarantee product purity. Adsorption methods require frequent regeneration, generating secondary solid waste, and are unsuitable for large-scale continuous production.

[0004] In recent years, mechanical vapor recompression (MVR) technology has been attempted for the pre-concentration of NMP waste liquid. By compressing secondary steam to improve the heat grade, evaporation energy consumption can be reduced. However, single-stage MVR falling film evaporation still suffers from low heat transfer coefficients and excessively high compressor power consumption due to excessively high compression ratios when dealing with low-concentration feeds. Furthermore, in conventional vacuum distillation dehydration processes, because the relative volatility of NMP-water does not change significantly with pressure, a large number of theoretical plates and a high reflux ratio are required, resulting in a relatively high reboiler temperature that easily leads to NMP thermal decomposition. Ionic liquids, as green extractants, can significantly alter the gas-liquid equilibrium behavior of the NMP-water system, but existing research has largely focused on atmospheric pressure extractive distillation, without systematic integration with MVR pre-concentration and subsequent flash regeneration. On the other hand, traditional distillation columns use a single reboiler discharge method, leading to the accumulation of heavy component impurities within the column, which eventually mix into the product, resulting in higher product color and decreased purity.

[0005] Therefore, there is an urgent practical need to develop a continuous recovery process for low-concentration NMP waste liquid with low energy consumption, high yield, and high product purity. Summary of the Invention

[0006] The purpose of this invention is to provide a high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides a high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production, the process comprising the following sequentially executed operation units: First operating unit: After preheating the low-concentration NMP waste liquid to 60-80℃, it is sent to a two-stage series mechanical vapor recompression falling film evaporator for pre-concentration to obtain the first intermediate liquid; Second operating unit: The first intermediate liquid and the ionic liquid extractant are mixed at a mass ratio of 1:0.2-0.5 and then fed into the middle of the extractive distillation column. Water and the ionic liquid-extractant mixture are collected from the top of the column, and NMP-ionic liquid mixture is collected from the bottom of the column. The operating pressure of the extractive distillation column is 0.1-0.2 bar, and the theoretical plate number is 45-55. Third operating unit: The NMP-ionic liquid mixture is fed into a flash regeneration tower, where the ionic liquid is separated at 150-180℃ and 0.05-0.1 bar to obtain crude NMP; Fourth operating unit: The crude NMP is continuously fed into a three-stage distillation column, which includes an upper distillation section, a middle side-stream extraction section and a lower stripping section. High-purity NMP product is extracted from the side stream, and heavy component residue is discharged from the bottom of the column.

[0008] Preferably, the mechanical vapor recompression falling film evaporator in the first operating unit has a compressor with a compression ratio of 1.5-2.0; The heat exchange tubes of the falling film evaporator are 4-6 meters long, and the Reynolds number inside the tubes is controlled between 2000 and 4000. The evaporation temperatures of each stage of the falling film evaporator are 65-70℃ and 55-60℃, respectively. The NMP mass concentration of the first intermediate liquid at the outlet of the second-stage falling film evaporator is maintained within the range of 45-52% by adjusting the compressor speed and feed flow rate.

[0009] Preferably, the preheating step in the first operating unit is achieved using a shell-and-tube heat exchanger; The shell side of the shell-and-tube heat exchanger is fed with the top steam of the three-stage distillation column in the fourth operating unit. The inlet temperature of the top steam is 85-95℃, and the outlet condensate temperature is 40-45℃. The ratio of the flow rate of low-concentration NMP waste liquid in the tube side of the shell-and-tube heat exchanger to the flow rate of the top steam of the tower is controlled at 1:1.2-1.5; The temperature of the preheated low-concentration NMP waste liquid is stabilized at 68-72℃ by adjusting the ratio.

[0010] Preferably, the ionic liquid extractant in the second operating unit is 1-ethyl-3-methylimidazolium tetrafluoroborate, with a mass fraction of 95-99%. The ionic liquid extractant is premixed with the first intermediate liquid in a static mixer before entering the extractive distillation column; The static mixer has 20-30 mixing units, and the mixing temperature is controlled at 50-60℃. The mass ratio of the first intermediate liquid to the ionic liquid extractant is adjusted by feedback from an online concentration meter to maintain the molar ratio of NMP to ionic liquid in the mixture entering the extractive distillation column at 1:0.3-0.4.

[0011] Preferably, the extractive distillation column in the second operating unit has a reflux ratio of 0.8-1.2; The mixing and feeding position of the first intermediate liquid and the ionic liquid extractant is at the 25th-30th theoretical plate; The water and ionic liquid-extractant mixture collected from the top of the column are sent to a liquid-liquid separator and allowed to stand at 25-35°C to separate into layers. The upper aqueous phase is discharged from the system, and the lower ionic liquid-extractant phase is returned to the 5th-8th theoretical plates at the top of the extractive distillation column. The bottom temperature of the extractive distillation column is controlled at 110-125°C by a reboiler heat source, and the top pressure is maintained at 0.12-0.15 bar by a vacuum pump.

[0012] Preferably, the flash regeneration tower in the third operating unit uses a falling film reboiler as the heating element; The heating medium of the falling film reboiler is heat transfer oil, with an inlet temperature of 190-200℃ and an outlet temperature of 170-180℃. The operating pressure of the flash regeneration tower is controlled at 0.06-0.08 bar by a vacuum regulating valve, and the operating temperature is correspondingly controlled at 160-170℃. The ionic liquid separated from the top of the flash regeneration tower enters the storage tank by gravity flow, and after being cooled to 50-60°C, it is pumped back to the static mixer inlet of the second operating unit.

[0013] Preferably, the flash regeneration tower in the third operating unit has its top vapor phase outlet connected to a primary condenser and a secondary condenser; The cooling water inlet temperature of the first-stage condenser is 25-30℃, and the outlet temperature is 35-40℃, which cools the gas phase to 80-90℃, and the condensate is crude NMP. The cooling medium of the secondary condenser is 5-10°C chilled water, which further cools the uncondensed gas to 20-25°C. The condensate is then collected in the crude NMP storage tank. The liquid level in the crude NMP storage tank is interlocked with the feed flow rate of the flash regeneration tower. When the liquid level reaches the set value, the delivery pump is started to send the crude NMP to the fourth operating unit.

[0014] Preferably, in the three-stage distillation column of the fourth operating unit, the upper rectification section has 15-20 theoretical plates, the middle side-stream extraction section has 10-15 theoretical plates, and the lower stripping section has 20-25 theoretical plates. The feed location for the crude NMP is at the junction of the upper rectification section and the middle side-stream extraction section, that is, between the 15th and 20th theoretical plates. The operating pressure at the top of the column is 0.02-0.03 bar, and the operating pressure at the bottom of the column is 0.05-0.06 bar. The temperature at the top of the column is controlled at 45-55℃, and the temperature at the bottom of the column is controlled at 130-145℃.

[0015] Preferably, in the three-stage distillation column of the fourth operating unit, the middle section side stream is drawn off at the 35th-40th theoretical plate. Side-stream extraction rate is 70-80% of the feed quality; The high-purity NMP product obtained from the side stream is first cooled to 30-35°C by a cooler before being sent to the product storage tank. The heavy component residue discharged from the tower bottom accounts for 2-5% of the feed mass and is discharged intermittently by a gear pump, with a discharge interval of 8-12 hours. The purity of the high-purity NMP product is monitored by an online gas chromatograph. When the purity is below 99.9%, the side-stream extraction rate is reduced by 2-5%.

[0016] Preferably, the process further includes an ionic liquid circulation and purification branch, which is led out from the ionic liquid pipeline returning to the second operating unit from the flash regeneration tower. The split ratio is 5-10% of the total ionic liquid flow rate; The diverted ionic liquid is fed into an adsorption tower, which is filled with a mixture of activated alumina and molecular sieves in a mass ratio of 1:1, with a particle size of 0.5-1.0 mm. The operating temperature of the adsorption tower is 40-50℃, and the space velocity is 1-2 h⁻¹; The ionic liquid purified by adsorption is then combined with the undiverted main ionic liquid. The total concentration of the combined ionic liquid is maintained at the initial set value of the process by adding fresh ionic liquid.

[0017] Compared with the prior art, the beneficial effects of the present invention are: A two-stage series-connected MVR falling film evaporator pre-concentrates low-concentration NMP waste liquid. Compared to single-stage MVR or multi-effect evaporation, the two-stage series connection allows for flexible allocation of the compression ratio, preventing the compressor from operating at excessively high pressure ratios and reducing compressor power consumption. Simultaneously, the falling film evaporation method ensures the feed liquid forms a uniform liquid film on the inner wall of the heating tubes, resulting in a high heat transfer coefficient and short residence time, effectively suppressing hydrolysis and heat-sensitive side reactions of NMP during preheating and evaporation. The feed is preheated to 60-80°C before entering the MVR system, further reducing heating steam consumption. Through pre-concentration, the NMP concentration of the first intermediate liquid can be increased to 40%-60%, significantly reducing the liquid phase load and steam consumption of the subsequent extractive distillation unit.

[0018] An ionic liquid extractant is introduced into an extractive distillation column. Utilizing the difference in solubility of NMP and water and its significant alteration of the relative volatility of NMP-water, the azeotropic behavior of NMP and water is broken. The actual operating pressure is controlled at a low pressure of 0.1-0.2 bar, reducing the column bottom operating temperature (approximately 90-120°C) and preventing NMP thermal polymerization. The ionic liquid and the first intermediate liquid are mixed at a mass ratio of 0.2-0.5 and efficiently separated in a column with 45-55 theoretical plates. The top water carries trace amounts of ionic liquid and extractant, which can be recovered by simple flash evaporation before being discharged in compliance with standards or reused. The NMP-ionic liquid mixture obtained in the bottom of the column has an NMP concentration close to 99%, containing only a small amount of ionic liquid and heavy components. Compared to traditional vacuum distillation dehydration, this extractive distillation process reduces the reflux ratio by approximately 50% and significantly decreases energy consumption.

[0019] The flash regeneration column separates the ionic liquid from the NMP-ionic liquid mixture under mild conditions of 150-180℃ and 0.05-0.1 bar. The resulting ionic liquid can be directly recycled back to the extractive distillation column. Due to the low-pressure operation, the regeneration temperature is much lower than the thermal decomposition temperature of the ionic liquid (typically >300℃), resulting in extremely low ionic liquid loss. Simultaneously, the flash process does not introduce additional chemical reagents, avoiding secondary pollution. The regenerated crude NMP is essentially anhydrous, with the main impurities being small amounts of high-boiling-point polymers and trace amounts of residual ionic liquid. The three-stage distillation column consists of an upper rectification section, a middle side-stream collection section, and a lower stripping section. Crude NMP is continuously fed into the column. Lighter components (such as residual water or low-boiling-point substances) are discharged from the top of the upper section, while heavier components (including polymers, oxidative degradation products, and a very small amount of entrained ionic liquid) settle to the bottom and are continuously discharged. High-purity NMP is collected from the side-stream liquid phase. This side-stream extraction method avoids the accumulation of heavy components within the tower, which can then be re-introduced into the product. It also prevents prolonged high-temperature heating in the tower reboiler from causing NMP color to darken. Compared to conventional three-stage towers with reboiler discharge, the side-stream extraction location in this process can be flexibly adjusted according to the feed composition and product purity requirements, ensuring a stable supply of battery-grade NMP (color ≤10 Hazen, purity ≥99.9%). The amount of heavy component residue discharged from the reboiler is small (approximately 1%-3% of the total feed), which can be centrally incinerated or further processed. Attached Figure Description

[0020] Figure 1 This diagram illustrates the working steps of the high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production as described in this invention. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to specific embodiments.

[0022] This invention presents a high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid from lithium battery production. The core process employs a coupled "MVR pre-concentration - ionic liquid extraction distillation - flash regeneration - three-stage precision distillation" process, complemented by an ionic liquid circulation purification branch, achieving efficient, low-energy, and high-purity recovery of low-concentration NMP waste liquid. The overall process consists of four core operating units and auxiliary purification branches. Each unit is precisely matched with process parameters, solving the technical challenges of high energy consumption, low purity, large extractant loss, and enrichment of residues and impurities in traditional low-concentration NMP waste liquid recovery methods.

[0023] The low-concentration NMP waste liquid used in this invention is waste liquid generated during the lithium battery cell coating process. The initial NMP mass concentration is 8%-15%, and the impurities in the waste liquid mainly consist of trace amounts of lithium battery slurry dust, moisture, trace amounts of alcohol impurities, and high molecular weight heavy components. The ionic liquid extractant used is 1-ethyl-3-methylimidazolium tetrafluoroborate. Through gradient parameter control and closed-loop circulation purification, high-purity NMP products with a purity ≥99.9% can be stably recovered, meeting the lithium battery production reuse standards.

[0024] Three progressive implementation examples were set up, corresponding to the lower limit, median value, and upper limit of the optimal range of process parameters, respectively, covering the entire parameter range of the claims of this invention, to verify the universality and stability of the process parameter range. The overall process steps of the three implementation examples are completely identical, with only the core operating parameters of each unit showing gradient changes.

[0025] Three sets of comparative examples were set up, corresponding to three conventional process defect conditions: missing two-stage pre-concentration process of MVR, no use of ionic liquid extraction and distillation, and no ionic liquid circulation purification branch. The remaining basic parameters were kept the same as those in Example 2 (intermediate optimal parameters). By comparing the performance data, the technical advantages of the coupled process and parameter optimization of the present invention were highlighted.

[0026] Example 1 (lower limit of parameter range operating condition) See appendix Figure 1 This embodiment uses the lower limit of the process parameter range of the present invention, and the specific operation steps are as follows: S1. Two-stage preheating and MVR pre-concentration: Low-concentration NMP waste liquid from lithium battery production is selected, with an initial NMP mass concentration of 10.2%. A shell-and-tube heat exchanger is used to preheat the waste liquid. Steam from the top of a three-stage distillation column is introduced into the shell side of the heat exchanger, with an inlet temperature of 85℃ and an outlet condensate temperature of 40℃. The ratio of the low-concentration NMP waste liquid flow rate to the top steam flow rate is controlled at 1:1.2 in the tube side, preheating the waste liquid to 68℃. The preheated waste liquid is then fed into a two-stage series mechanical vapor recompression falling film evaporator. The evaporator compressor has a compression ratio of 1.5, a heat exchange tube length of 4m, and a Reynolds number controlled at 2000. The first-stage falling film evaporator reaches an evaporation temperature of 65℃, and the second-stage falling film evaporator reaches an evaporation temperature of 55℃. By adjusting the compressor speed and feed flow rate, the NMP mass concentration of the first intermediate liquid at the second-stage outlet is stably maintained at 45%, completing the waste liquid pre-concentration.

[0027] S2. Ionic Liquid Extractive Distillation: A 95% (w / w) 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid extractant is selected. The first intermediate liquid and the ionic liquid extractant are fed into a static mixer with 20 mixing units at a mass ratio of 1:0.2. The mixture is thoroughly premixed at 50°C. The ratio is adjusted via an online concentration meter to control the molar ratio of NMP to ionic liquid in the mixture to be 1:0.3. The premixed material is then fed to the 25th theoretical plate of the extractive distillation column. The extractive distillation column has 45 theoretical plates, an operating pressure of 0.1 bar, and a reflux ratio of 0.8. The temperature of the reboiler is controlled at 110°C using a reboiler heat source, and the pressure at the top of the column is maintained at 0.12 bar by a vacuum pump. A mixture of water and ionic liquid is collected from the top of the column, and a mixture of NMP and ionic liquid is collected from the reboiler. The mixture collected from the top is sent to a liquid-liquid separator and allowed to settle and separate at 25°C. The upper aqueous phase is directly discharged from the system, while the lower ionic liquid phase is returned to the 5th theoretical plate at the top of the extractive distillation column for recycling.

[0028] S3. Ionic Liquid Flash Regeneration: The NMP-ionic liquid mixture collected from the reboiler is fed into a flash regeneration tower. The regeneration tower is heated using a falling film reboiler with heat transfer oil as the heating medium. The heat transfer oil inlet temperature is 190℃, and the outlet temperature is 170℃. The operating pressure of the regeneration tower is controlled at 0.06 bar, corresponding to an operating temperature of 160℃, through a vacuum regulating valve. The vapor phase at the top of the regeneration tower passes sequentially through a primary condenser and a secondary condenser. The primary condenser uses cooling water with an inlet temperature of 25℃ and an outlet temperature of 35℃, cooling the vapor phase to 80℃ and condensing it to obtain crude NMP. The secondary condenser uses 5℃ chilled water to cool the uncondensed gas to 20℃, and the condensate is collected in the crude NMP storage tank. The ionic liquid separated from the top of the regeneration tower flows into the storage tank by gravity, is cooled to 50℃, and then pumped back to the static mixer inlet for recycling. The crude NMP storage tank level is interlocked with the flash feed flow rate; once the level reaches the target, the transfer pump is started to deliver the feed.

[0029] S4. Three-stage precision distillation purification: Crude NMP is continuously fed into a three-stage distillation column. The upper rectification section has 15 theoretical plates, the middle side-stream collection section has 10 plates, and the lower stripping section has 20 plates. The crude NMP feed point is the 15th theoretical plate (the boundary between the rectification and collection sections). The column top operating pressure is 0.02 bar and the temperature is 45°C, while the column bottom operating pressure is 0.05 bar and the temperature is 130°C. The middle side-stream collection point is the 35th theoretical plate, with a side-stream recovery rate of 70% of the feed mass. The collected high-purity NMP is cooled to 30°C by a cooler before being sent to the product storage tank. The heavy component residue in the column bottom accounts for 2% of the feed mass and is intermittently discharged using a gear pump at 8-hour intervals. The product purity is monitored in real time using an online gas chromatograph to stably control the collection parameters.

[0030] S5. Ionic Liquid Circulation and Purification: A 5% flow rate diversion of the ionic liquid returning from the flash regeneration tower is sent to the adsorption tower for purification. The adsorption tower is filled with a 1:1 mass ratio mixture of activated alumina and molecular sieve, with a packing particle size of 0.5 mm. The adsorption tower operates at a temperature of 40℃ and a space velocity controlled at 1 h⁻¹. The purified ionic liquid is combined with the main ionic liquid, and a small amount of fresh ionic liquid is added to maintain a stable circulating ionic liquid concentration.

[0031] Example 2 (Intermediate operating condition within the parameter range) This embodiment uses the intermediate value of the process parameter range of the present invention, and the specific operation steps are as follows: S1. Two-stage preheating and MVR pre-concentration: Low-concentration NMP waste liquid from lithium battery production is selected, with an initial NMP mass concentration of 10.5%. A shell-and-tube heat exchanger is used to preheat the waste liquid. Steam from the top of a three-stage distillation column is introduced into the shell side of the heat exchanger, with an inlet temperature of 90℃ and an outlet condensate temperature of 42℃. The ratio of waste liquid flow rate to top steam flow rate is 1:1.35 in the tube side, preheating the waste liquid to 70℃. After preheating, the waste liquid is sent to a two-stage series MVR falling film evaporator. The compressor compression ratio is 1.75, the heat exchange tube length is 5m, and the Reynolds number inside the tube is 3000. The first-stage evaporation temperature is 68℃, and the second-stage evaporation temperature is 58℃. The equipment operating parameters are precisely adjusted to maintain the NMP mass concentration of the first intermediate liquid at the second-stage outlet at a stable 48%.

[0032] S2. Ionic Liquid Extractive Distillation: A 97% (w / w) 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid extractant is used. The first intermediate liquid to extractant mass ratio is 1:0.35. The mixture is fed into a static mixer with 25 mixing units and premixed at 55°C. The molar ratio of NMP to ionic liquid in the mixture is adjusted to 1:0.35 using an online concentration meter. The mixture is then fed into the 28th theoretical plate of the extractive distillation column. The extractive distillation column has 50 theoretical plates, an operating pressure of 0.15 bar, and a reflux ratio of 1.0. The reboiler temperature is controlled at 118°C, and the top pressure is stabilized at 0.14 bar. The top mixture is allowed to settle and separate at 30°C. The upper aqueous phase is discharged, and the lower ionic liquid phase is returned to the 6th theoretical plate at the top of the extractive distillation column for circulation.

[0033] S3. Ionic Liquid Flash Regeneration: The NMP-ionic liquid mixture is fed into a flash regeneration tower. The inlet temperature of the heat transfer oil in the falling film reboiler is 195℃, and the outlet temperature is 175℃. The operating pressure of the regeneration tower is 0.07 bar, and the operating temperature is 165℃. The vapor phase at the top of the tower is cooled to 85℃ by a primary condenser (cooling water inlet 28℃, outlet 38℃) to obtain crude NMP. The uncondensed gas is condensed to 22℃ by secondary condensation with 8℃ chilled water, and the condensate is collected in the crude NMP storage tank. The separated ionic liquid is cooled to 55℃ and refluxed. The tank level and feed flow rate are interlocked and controlled.

[0034] S4. Three-stage precision distillation purification: The three-stage distillation column has 18 theoretical plates in the upper section, 12 in the middle section, and 22 in the lower section; the crude NMP feed is located at the 18th theoretical plate. The top pressure is 0.025 bar and the temperature is 50°C; the bottom pressure is 0.055 bar and the temperature is 138°C. The side stream is collected at the 38th theoretical plate, with a recovery rate of 75%. High-purity NMP is cooled to 32°C before being fed into the tank. The bottom residue accounts for 3% of the feed, and residue is discharged intermittently every 10 hours. Online chromatography is used to monitor product purity in real time.

[0035] S5. Ionic Liquid Circulation and Purification: 7.5% of the total ionic liquid flow rate is diverted into the adsorption tower. The adsorption tower packing consists of equal masses of activated alumina and molecular sieves with a particle size of 0.8 mm. The operating temperature is 45℃, and the space velocity is 1.5 h⁻¹. After purification, the ionic liquid is merged with the main flow, and fresh reagent is added to maintain a stable circulating concentration.

[0036] Example 3 (Upper Limit of Parameter Range) This embodiment uses the upper limit of the process parameter range of the present invention, and the specific operation steps are as follows: S1. Primary Preheating and Two-Stage MVR Pre-Concentration: Low-concentration NMP waste liquid from lithium battery production is selected, with an initial NMP concentration of 10.8%. The inlet temperature of the shell-side top steam in the shell-and-tube heat exchanger is 95℃, and the outlet condensate temperature is 45℃. The ratio of waste liquid to steam flow rate in the tube side is 1:1.5, and the waste liquid is preheated to 72℃. The preheated waste liquid is sent to a two-stage MVR falling film evaporator with a compressor compression ratio of 2.0, a heat exchange tube length of 6m, and a Reynolds number of 4000. The first-stage evaporation temperature is 70℃, and the second-stage evaporation temperature is 60℃. The equipment parameters are adjusted to maintain the NMP concentration of the first intermediate liquid at a stable 52%.

[0037] S2. Ionic Liquid Extractive Distillation: A 99% (w / w) 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid extractant is used. The first intermediate liquid to extractant mass ratio is 1:0.5. The mixture is fed into a static mixer with 30 mixing units and premixed at 60°C. The molar ratio of NMP to ionic liquid in the mixture is adjusted to 1:0.4. The mixture is then fed into the 30th theoretical plate of the extractive distillation column. The extractive distillation column has 55 theoretical plates, an operating pressure of 0.2 bar, a reflux ratio of 1.2, a reboiler temperature of 125°C, and a top pressure of 0.15 bar. The top mixture is allowed to settle and separate at 35°C. The upper aqueous phase is discharged, and the lower ionic liquid phase is returned to the 8th theoretical plate at the top of the extractive distillation column for recycling.

[0038] S3. Ionic Liquid Flash Regeneration: The NMP-ionic liquid mixture is fed into the flash regeneration tower. The inlet temperature of the heat transfer oil in the falling film reboiler is 200℃, and the outlet temperature is 180℃. The operating pressure of the regeneration tower is 0.08 bar, and the operating temperature is 170℃. The primary condenser uses cooling water with an inlet temperature of 30℃ and an outlet temperature of 40℃ to cool the gas phase to 90℃. The secondary condenser uses 10℃ chilled water to cool the uncondensed gas to 25℃. The condensates from the two stages are combined to form crude NMP. The ionic liquid is cooled to 60℃ and then recirculated. The tank level and feed flow rate are interlocked.

[0039] S4. Three-stage precision distillation purification: The three-stage distillation column has 20 theoretical plates in the upper section, 15 in the middle section, and 25 in the lower section; the crude NMP feed is located at the 20th theoretical plate. The top pressure is 0.03 bar and the temperature is 55°C; the bottom pressure is 0.06 bar and the temperature is 145°C. The side stream is collected at the 40th theoretical plate, with a recovery rate of 80%. The product is cooled to 35°C before being placed in a tank. The bottom residue accounts for 5% of the feed, and residue is discharged every 12 hours. The recovery rate is dynamically adjusted based on online chromatographic purity data.

[0040] S5. Ionic Liquid Circulation and Purification: 10% of the total ionic liquid flow rate is diverted into the adsorption tower. The adsorption tower packing material is of equal mass ratio and has a particle size of 1.0 mm. The operating temperature is 50℃, and the space velocity is 2h⁻¹. The purified ionic liquid is combined with the main flow, and fresh reagent is added to maintain the stability of the process system concentration.

[0041] All comparative examples used the same initial waste liquid raw materials, basic equipment specifications, and operating time as Example 2, except that the core innovative process was omitted or the key parameters were deviated from. The specific steps are as follows: Comparative Example 1 (eliminating the two-stage pre-concentration process of MVR) This comparative example omits the two-stage pre-concentration process of the first operating unit MVR. The low-concentration NMP waste liquid is directly preheated before entering the extractive distillation process. The remaining process parameters are completely consistent with those of Example 2. The specific steps are as follows: The low-concentration NMP waste liquid with an initial concentration of 10.5% is preheated to 70°C through a shell-and-tube heat exchanger. It is then directly mixed with 97% pure ionic liquid extractant at a mass ratio of 1:0.35 without pre-concentration. After pre-mixing in a static mixer at 55°C, it is sent to the extractive distillation column. The subsequent extractive distillation, flash regeneration, three-stage distillation, and ionic liquid purification processes all operate with the parameters of Example 2, without any waste liquid pre-concentration process.

[0042] Comparative Example 2 (using conventional distillation instead of ionic liquid extractive distillation) This comparative example omits the ionic liquid extraction distillation step and replaces it with traditional pure water azeotropic distillation. The remaining process parameters are completely consistent with those of Example 2. The specific steps are as follows: the low-concentration NMP waste liquid is pre-concentrated in two stages by MVR to obtain a first intermediate liquid with a concentration of 48%. The intermediate liquid is directly fed into a conventional distillation column for azeotropic dehydration distillation. The ionic liquid extraction, static mixing, and ionic liquid circulation processes are eliminated. The basic parameters of the distillation column, such as the theoretical plate number, pressure, and temperature, are consistent with those of the extraction distillation column in Example 2. The subsequent flash evaporation and three-stage distillation processes are operated using the parameters of Example 2.

[0043] Comparative Example 3 (Ionic liquid circulation purification branch removed) This comparative example completely eliminates the ionic liquid circulation and purification branch. The ionic liquid is simply filtered and then directly recycled without adsorption purification process. All other process parameters and operating steps are completely consistent with Example 2. Specifically, the operating process of each unit is synchronized with Example 2. The ionic liquid separated by the flash regeneration tower is only filtered through a simple filter screen to remove impurities. There is no split adsorption, no alumina and molecular sieve purification process. It is directly returned to the static mixer for recycling without adding fresh ionic liquid, thus maintaining the operation of the original circulation system.

[0044] The core performance indicators of each embodiment and comparative example were uniformly tested, including five key parameters: purity of high-purity NMP product, total NMP recovery rate, energy consumption per unit product, loss rate of ionic liquid per cycle, and water content of product. All tests were conducted using national standard testing methods. Samples were taken and tested after each group of experiments had been running stably for 72 hours, and the average value was taken as the final data. The specific results and analysis are as follows.

[0045] Table 1: Data on core process parameters and performance indicators for each embodiment As shown in Table 1, the three sets of embodiments operated within the process parameter range defined by the present invention, and their overall performance met the lithium battery-grade high-purity NMP recycling standard. The product purity was ≥99.9%, the recovery rate was ≥96%, and the energy consumption and loss indicators were all at the industrial-grade high-quality level, which verified the overall stability and feasibility of the process parameter range of the present invention.

[0046] Analysis of the gradient parameter variation patterns shows that Example 1, operating under the lower limit of parameters, had the pre-concentration concentration, extractant ratio, and purity at their lowest values. This resulted in weaker removal of water and impurities from the waste liquid, leading to the highest product water content, lowest recovery rate, and highest energy consumption. Ionic liquid loss was relatively high, but still met basic industrial production requirements. Example 3, operating under the upper limit of parameters, had high extractant dosage, operating temperature, and recovery rate, improving impurity removal. However, the excessively high extractant ratio and operating parameters slightly increased system energy consumption and extractant loss. Compared to the intermediate operating conditions, the recovery rate and purity decreased slightly.

[0047] Example 2 employs the intermediate optimal parameter ratio, achieving the best synergistic matching of process parameters in each unit. The MVR pre-concentration concentration is moderate, and the purity, ratio, and mixing conditions of the ionic liquid extractant reach optimal balance. The separation efficiency of each process—extractive distillation, flash regeneration, and precision distillation—is maximized, ultimately achieving the optimal indicators of product purity of 99.95% and recovery rate of 98.7%. Simultaneously, unit energy consumption, ionic liquid loss, and product moisture content are all optimal across the three groups, proving that the median value of the parameter range in this invention represents the optimal industrial operating condition. The upper and lower limits of the parameter range can meet the adaptation requirements of different waste liquid concentrations and different production loads, demonstrating strong process versatility.

[0048] Table 2: Comparison of Core Performance Indicators between Example 2 and Comparative Examples As shown in Table 2, compared with Example 2 of the complete process of the present invention, the performance indicators of the three comparative examples all deteriorated to varying degrees, which fully verified the synergistic effect of the three core technologies of the present invention: MVR pre-concentration, ionic liquid extraction and distillation, and circulating purification branch. The technical necessity and innovation of each core process were highlighted.

[0049] Comparative Example 1 eliminated the two-stage pre-concentration process of MVR, and the low-concentration waste liquid directly entered the extractive distillation system. The excessively high water content in the system significantly increased the load on subsequent distillation separation, and the water could not be removed efficiently, directly causing the product water content to soar to 216 ppm and the purity to drop to 99.52%. At the same time, the large amount of low-concentration material increased the operating energy consumption of the distillation column and reboiler, with the unit energy consumption increasing by 53.1%. A large amount of NMP in the unconcentrated waste liquid was lost with the aqueous phase and residue, and the recovery rate dropped sharply to 89.4%, with the residue output doubling. This fully demonstrates that the MVR pre-concentration process can effectively reduce the load on subsequent separation and is the basic core process for achieving efficient recovery of low-concentration waste liquid.

[0050] Comparative Example 2 uses conventional azeotropic distillation instead of ionic liquid extractive distillation. Lacking the specific complexing and separation effect of ionic liquids on NMP, the azeotropic system of NMP and water cannot be effectively broken down, making it difficult to remove moisture and trace organic impurities. This is the operating condition with the lowest product purity and recovery rate, and the highest energy consumption and residue among the three comparative examples. The product purity is only 99.36%, which cannot meet the high-end lithium battery recycling standards, and the recovery rate is only 85.2%. A large amount of NMP is discarded with the residue, resulting in serious resource waste. Energy consumption is 70.2% higher than the process of this invention. This directly verifies the disruptive advantage of ionic liquid extractive distillation technology compared to traditional distillation, and it is the core key technology to ensure high product purity and high recovery rate.

[0051] Comparative Example 3, by eliminating the ionic liquid circulation and purification branch, shows that after long-term recycling of the ionic liquid, trace amounts of slurry dust, high-molecular impurities, and degradation products continuously accumulate in the waste liquid. This leads to a decrease in the activity and complexing capacity of the ionic liquid, resulting in poorer extraction and separation effects, a slight decrease in product purity and recovery rate, and an increase in water content. Simultaneously, impurity accumulation accelerates ionic liquid failure and loss, with the daily loss rate soaring from 0.08% to 0.35%, significantly increasing consumable costs and production and maintenance costs. This data demonstrates that the ionic liquid circulation and purification branch can effectively maintain the activity of the extractant, reduce loss, and ensure long-term stable operation of the process.

[0052] Table 3: Comparison of Long-Term Stability of the Process (Continuous Operation for 30 Days) Table 3 shows the verification under long-term continuous industrial operation conditions. In Example 2, equipped with an ionic liquid circulation purification branch, after 30 days of continuous operation, the product purity only decreased from 99.95% to 99.92%, with a decrease of only 0.03%, and almost no performance degradation; the cumulative loss of ionic liquid was only 2.1%, and the average recovery rate remained stable at 98.5% over 30 days. At the same time, adsorption purification can effectively remove solid dust and colloidal impurities in the circulation system, significantly reducing the scaling rate of the equipment. The equipment operates stably, without the need for frequent shutdowns for cleaning, and is suitable for continuous industrial production.

[0053] In Comparative Example 3, which lacks a purification branch, impurities continuously accumulate in the ionic liquid and the entire process system as the operating time increases. The activity of the ionic liquid continues to deteriorate, and the extraction and separation efficiency drops significantly. After 30 days, the product purity decreases by 0.57% to 99.15%, which can no longer meet the high-purity NMP standards for lithium battery production. The cumulative loss of ionic liquid reaches 10.5%, which is 5 times that of the process in this invention, resulting in a significant increase in consumable costs. Impurities adhere to the surfaces of heat exchangers, distillation trays, and reboilers, leading to a significant increase in the scaling rate of the equipment, a continuous decrease in heat exchange and separation efficiency, a significant reduction in recovery rate, and extremely poor production stability.

[0054] This set of data fully demonstrates that the ionic liquid circulation purification branch added in this invention can maintain the stability of the process system for a long time, reduce consumable consumption and equipment operation and maintenance costs, and solve the industry pain points of traditional NMP recovery processes such as long-term performance degradation, large consumable consumption, and easy scaling of equipment.

[0055] Based on the analysis of three sets of examples, three sets of comparative examples, and multiple sets of data, it can be seen that this invention forms a complete and efficient recovery system for low-concentration NMP waste liquid through a coupled process design of MVR two-stage pre-concentration, precise parameterized ionic liquid extraction distillation, gradient flash regeneration, three-stage segmented precision distillation, and ionic liquid closed-loop purification. The precise matching and synergistic effect of parameters in each process unit effectively solves the shortcomings of traditional processes in treating low-concentration NMP waste liquid, such as high energy consumption, low recovery rate, substandard product purity, large extractant loss, and long-term unstable operation. The process of this invention can stably recover lithium-ion battery-grade high-purity NMP products with a purity ≥99.9%, a total NMP recovery rate ≥96%, significantly reduced energy consumption per unit product, extremely low ionic liquid loss rate, and stable long-term equipment operation. It is suitable for the large-scale and continuous recovery production of various low-concentration NMP waste liquids in the lithium battery industry.

[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production, characterized in that, The process includes the following sequentially executed operation units: First operating unit: After preheating the low-concentration NMP waste liquid to 60-80℃, it is sent to a two-stage series mechanical vapor recompression falling film evaporator for pre-concentration to obtain the first intermediate liquid; Second operating unit: The first intermediate liquid and the ionic liquid extractant are mixed at a mass ratio of 1:0.2-0.5 and then fed into the middle of the extractive distillation column. Water and the ionic liquid-extractant mixture are collected from the top of the column, and NMP-ionic liquid mixture is collected from the bottom of the column. The operating pressure of the extractive distillation column is 0.1-0.2 bar, and the theoretical plate number is 45-55. The third operating unit: The NMP-ionic liquid mixture is fed into a flash regeneration tower, where the ionic liquid is separated at 150-180℃ and 0.05-0.1 bar to obtain crude NMP; Fourth operating unit: The crude NMP is continuously fed into a three-stage distillation column, which includes an upper distillation section, a middle side-stream extraction section and a lower stripping section. High-purity NMP product is extracted from the side stream, and heavy component residue is discharged from the bottom of the column.

2. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The mechanical vapor recompression falling film evaporator in the first operating unit has a compressor with a compression ratio of 1.5-2.0; The heat exchange tubes of the falling film evaporator are 4-6 meters long, and the Reynolds number inside the tubes is controlled between 2000 and 4000. The evaporation temperatures of each stage of the falling film evaporator are 65-70℃ and 55-60℃, respectively. The NMP mass concentration of the first intermediate liquid at the outlet of the second-stage falling film evaporator is maintained within the range of 45-52% by adjusting the compressor speed and feed flow rate.

3. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The preheating step in the first operating unit is achieved using a shell-and-tube heat exchanger; The shell side of the shell-and-tube heat exchanger is fed with the top steam of the three-stage distillation column in the fourth operating unit. The inlet temperature of the top steam is 85-95℃, and the outlet condensate temperature is 40-45℃. The ratio of the flow rate of low-concentration NMP waste liquid in the tube side of the shell-and-tube heat exchanger to the flow rate of the top steam of the tower is controlled at 1:1.2-1.5; The temperature of the preheated low-concentration NMP waste liquid is stabilized at 68-72℃ by adjusting the ratio.

4. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The ionic liquid extractant in the second operating unit is 1-ethyl-3-methylimidazolium tetrafluoroborate, with a mass fraction of 95-99%. The ionic liquid extractant is premixed with the first intermediate liquid in a static mixer before entering the extractive distillation column; The static mixer has 20-30 mixing units, and the mixing temperature is controlled at 50-60℃. The mass ratio of the first intermediate liquid to the ionic liquid extractant is adjusted by feedback from an online concentration meter to maintain the molar ratio of NMP to ionic liquid in the mixture entering the extractive distillation column at 1:0.3-0.

4.

5. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The extractive distillation column in the second operating unit has a reflux ratio of 0.8-1.2; The mixing and feeding position of the first intermediate liquid and the ionic liquid extractant is at the 25th-30th theoretical plate; The water and ionic liquid-extractant mixture collected from the top of the column are sent to a liquid-liquid separator and allowed to stand at 25-35°C to separate into layers. The upper aqueous phase is discharged from the system, and the lower ionic liquid-extractant phase is returned to the 5th-8th theoretical plates at the top of the extractive distillation column. The bottom temperature of the extractive distillation column is controlled at 110-125°C by a reboiler heat source, and the top pressure is maintained at 0.12-0.15 bar by a vacuum pump.

6. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The flash regeneration tower in the third operating unit uses a falling film reboiler as the heating element. The heating medium of the falling film reboiler is heat transfer oil, with an inlet temperature of 190-200℃ and an outlet temperature of 170-180℃. The operating pressure of the flash regeneration tower is controlled at 0.06-0.08 bar by a vacuum regulating valve, and the operating temperature is correspondingly controlled at 160-170℃. The ionic liquid separated from the top of the flash regeneration tower enters the storage tank by gravity flow, and after being cooled to 50-60°C, it is pumped back to the static mixer inlet of the second operating unit.

7. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The flash regeneration tower in the third operating unit has its top gas phase outlet connected to the primary condenser and the secondary condenser. The cooling water inlet temperature of the first-stage condenser is 25-30℃, and the outlet temperature is 35-40℃, which cools the gas phase to 80-90℃, and the condensate is crude NMP. The cooling medium of the secondary condenser is 5-10°C chilled water, which further cools the uncondensed gas to 20-25°C. The condensate is then collected in the crude NMP storage tank. The liquid level in the crude NMP storage tank is interlocked with the feed flow rate of the flash regeneration tower. When the liquid level reaches the set value, the delivery pump is started to send the crude NMP to the fourth operating unit.

8. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The three-stage distillation column in the fourth operating unit has a theoretical plate number of 15-20 in the upper rectification section, 10-15 in the middle side-stream extraction section, and 20-25 in the lower stripping section. The feed location for the crude NMP is at the junction of the upper rectification section and the middle side-stream extraction section, that is, between the 15th and 20th theoretical plates. The operating pressure at the top of the column is 0.02-0.03 bar, and the operating pressure at the bottom of the column is 0.05-0.06 bar. The temperature at the top of the column is controlled at 45-55℃, and the temperature at the bottom of the column is controlled at 130-145℃.

9. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The three-stage distillation column in the fourth operating unit has its middle section side stream sampling location at theoretical plates 35-40. Side-stream extraction rate is 70-80% of the feed quality; The high-purity NMP product obtained from the side stream is first cooled to 30-35°C by a cooler before being sent to the product storage tank. The heavy component residue discharged from the tower bottom accounts for 2-5% of the feed mass and is discharged intermittently by a gear pump, with a discharge interval of 8-12 hours. The purity of the high-purity NMP product is monitored by an online gas chromatograph. When the purity is below 99.9%, the side-stream extraction rate is reduced by 2-5%.

10. The high-efficiency distillation purification and recovery process for low-concentration NMP waste liquid in lithium battery production according to claim 1, characterized in that: The process also includes an ionic liquid circulation purification branch, which is a branch that is led out from the ionic liquid pipeline returning to the second operating unit from the flash regeneration tower. The split ratio is 5-10% of the total ionic liquid flow rate; The diverted ionic liquid is fed into an adsorption tower, which is filled with a mixture of activated alumina and molecular sieves in a mass ratio of 1:1, with a particle size of 0.5-1.0 mm. The adsorption tower operates at a temperature of 40-50℃ and a space velocity of 1-2 h⁻¹. The ionic liquid purified by adsorption is then combined with the undiverted main ionic liquid. The total concentration of the combined ionic liquid is maintained at the initial set value of the process by adding fresh ionic liquid.

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