A system for recycling waste lye based on nanofiltration membrane and a process thereof

Abstract

The application discloses a kind of based on nanofiltration membrane waste lye regeneration recovery system and process, belong to waste lye processing technical field.The present system includes: double-stage filtration module, pressurizing module, nanofiltration membrane separation module, circulating temperature control module, on-line cleaning module and concentrated liquid discharge module for the lack of accurate temperature control, on-line cleaning inconvenience, the problems such as low alkali recovery purity and efficiency.The nanofiltration membrane molecular weight cut-off is 200-500Da;Circulating temperature control module controls concentrated liquid backflow temperature at 35-45 DEG C by heat exchanger;On-line cleaning module is by special cleaning liquid tank and internal circulating pump constitutes independent closed loop cleaning circuit;Concentrated liquid is intermittently discharged when concentrated to 10%-20% of the volume of original liquid.The process includes pre-filtration, fine filtration pressurizing, nanofiltration separation, concentrated liquid temperature control circulation, up-to-standard discharge and on-line cleaning steps.The application realizes that alkali recovery rate is greater than or equal to 90%, protein removal rate is greater than 96%, membrane flux is stable, membrane service life is long, can be continuously and stably operated, and significantly reduces energy consumption and sewage station load.

Description

Technical Field

[0001] This invention relates to the field of waste alkali treatment and resource recycling technology, specifically to a waste alkali regeneration and recycling system and process based on nanofiltration membrane. Background Technology

[0002] Waste alkaline solutions are widely generated in the alkaline washing, decolorization, extraction, and regeneration processes of the chemical, pharmaceutical, and food processing industries. They contain high concentrations of sodium hydroxide (NaOH) or other alkaline substances, as well as proteins, pigments, suspended solids, and organic impurities. Traditional methods for treating waste alkaline solutions mainly include the following: Option 1: Neutralization and Precipitation Method. An acid (such as sulfuric acid or hydrochloric acid) is added to the waste alkaline solution for neutralization, adjusting the pH to neutral or weakly acidic, causing some organic matter to precipitate. Solid-liquid separation is then achieved through a sedimentation tank or plate and frame filter press, with the supernatant discharged into a wastewater treatment plant for further treatment. The drawbacks of this method are: it requires a large amount of acid, generating a large amount of neutralizing salts (such as sodium sulfate), increasing the burden of solid waste treatment; and the alkali resources are completely destroyed and cannot be recycled, resulting in poor economic efficiency.

[0003] Option 2: Conventional filtration + evaporation concentration method. The waste alkaline solution is first filtered through sand or microfiltration membranes to remove suspended particles, then enters a multi-effect evaporator or MVR evaporation system for concentration. The concentrated alkaline solution is reused, and the condensate is either discharged or reused. The drawbacks of this method are: extremely high energy consumption during evaporation and large equipment investment; conventional filtration cannot remove dissolved organic matter, and impurities in the alkaline solution are concentrated and enriched after evaporation, severely affecting product quality after reuse.

[0004] Option 3: Intermittent ultrafiltration. This method uses tubular or hollow fiber ultrafiltration membranes to filter the waste alkaline solution under ambient or heated conditions, retaining large organic molecules. The permeate is used as recycled alkaline solution. The drawbacks of this method are: the ultrafiltration membrane has a relatively large pore size (typically >10nm), limiting its ability to retain small soluble impurities (such as oligomers and pigments), resulting in low purity of the recycled alkaline solution; it lacks effective temperature control, leading to unstable membrane flux; and it lacks a systematic cleaning and regeneration mechanism, resulting in severe membrane fouling and a short service life.

[0005] In response to the shortcomings of the traditional methods mentioned above, membrane separation technology, especially nanofiltration membrane technology, has begun to be introduced into the field of waste alkali recovery.

[0006] Chinese utility model patent CN203498181U discloses a system for recycling waste acid and alkali. This system uses membrane separation technology to recover acid and alkali from resin regeneration waste acid and alkali solutions in their original form, offering advantages such as no phase change, ease of operation, low investment, and high efficiency. Chinese utility model patent CN207576141U discloses a nanofiltration membrane process system for deep recovery of waste alkali solutions. By setting the flow rate of the concentration reflux valve and circulation pipe, the system increases the membrane scouring volume, reduces the probability of impurity retention, and to some extent improves the service life of the nanofiltration membrane device. However, the aforementioned existing nanofiltration membrane recovery solutions still have the following shortcomings: lack of a precise circulation temperature control mechanism, inconvenient online cleaning or system complexity, and the absence of a systematic concentration ratio control and discharge mechanism.

[0007] In summary, existing technologies struggle to achieve precise temperature control, convenient online cleaning, and reasonable concentration discharge control while ensuring high alkali recovery rates and high purity. Therefore, developing a waste alkali solution nanofiltration membrane regeneration and recovery system and process that combines these functions and can operate continuously and stably for extended periods has significant practical value.

[0008] The purpose of this invention is to provide a waste alkali solution regeneration and recycling system and its process to solve the problems of low alkali recovery rate, poor purity of recycled alkali solution, high energy consumption, and inability to operate continuously and stably in the prior art. It enables the direct reuse of high-purity alkali solution (without removing macromolecular impurities such as proteins and pigments), controllable system operating temperature, high membrane separation efficiency, online cleaning and regeneration function, extended membrane life, centralized discharge of concentrate, and reduced wastewater treatment load.

[0009] The technical solution of this invention: a waste alkaline solution regeneration and recovery system based on nanofiltration membrane, comprising, Dual-stage filtration module: includes a pre-filtration device and a security filter connected in series. The pre-filtration device is used to coarsely filter the waste alkaline solution to remove large particulate impurities, and the security filter is used to finely filter the liquid entering the pressurization module to remove micron-sized particles. The booster module includes a high-pressure pump and a booster pump connected in series, the inlet of which is connected to the outlet of the security filter, for boosting the liquid pressure to 0.8-1.5 MPa; The nanofiltration membrane separation module is connected to the outlet of the pressurization module. This module uses a spiral wound nanofiltration membrane assembly with a molecular weight cutoff of 200-500 Da to allow alkali and water to pass through to form a purified alkali solution, and to retain proteins and pigments to form a concentrated solution. The alkaline solution collection module includes a permeate tank, which is connected to the permeate outlet of the nanofiltration membrane separation module; The circulating temperature control module includes a heat exchanger installed on the return pipeline between the concentrate outlet of the nanofiltration membrane separation module and the circulating tank, for adjusting the temperature of the return concentrate to 35-45°C. The online cleaning module includes a cleaning liquid tank. In the cleaning mode, the cleaning liquid in the cleaning liquid tank passes through the feed pump, security filter, booster module, and internal circulation pump before entering the nanofiltration membrane separation module and flowing through the heat exchanger back to the cleaning liquid tank, forming a closed-loop cleaning circuit. The concentrate discharge module is connected to the circulation tank and is used to intermittently discharge the concentrate when the liquid volume in the circulation tank is concentrated to 10% to 20% of the initial waste alkali liquid volume. The system also includes a circulation tank, whose inlet is connected to the outlet of the pre-filtration device, for receiving the coarsely filtered waste alkaline solution and the concentrate returned from the nanofiltration membrane separation module. A feed pump is connected to the outlet of the circulation tank, whose outlet is connected to the inlet of the security filter, for conveying the mixture in the circulation tank to the security filter.

[0010] Furthermore, the pre-filtration device is a bag filter with a filtration accuracy of 100–200 μm; the security filter has a filtration accuracy of 5–50 μm.

[0011] Furthermore, the outlet of the feed pump is connected to the concentrate discharge module via a pipeline equipped with a valve, which is used to switch and realize the discharge of concentrate.

[0012] Furthermore, the specific path for the concentrated liquid discharge is as follows: the liquid outlet at the bottom of the circulation tank is pumped to the concentrated liquid discharge outlet.

[0013] Furthermore, a valve is installed between the booster pump and the heat exchanger to adjust the filtration rate. When the valve is closed, all the liquid passing through the booster pump flows into the spiral wound nanofiltration membrane module through the internal circulation pump. When the valve is open, part of the liquid passing through the booster pump flows into the internal circulation pump, and the other part flows from the valve to the heat exchanger, thus adjusting the filtration rate.

[0014] Furthermore, the spiral wound nanofiltration membrane module is provided with a permeate outlet and a concentrate outlet, the permeate outlet being connected to the permeate tank and the concentrate outlet being connected to the heat exchanger.

[0015] A waste alkaline solution regeneration and recovery process based on nanofiltration membranes includes the following steps: Step 1: Send the waste alkaline solution into a pre-filtration device to remove large particulate impurities, and then it enters the circulation tank; Step 2: The liquid in the circulation tank is sent to the security filter for fine filtration by the feed pump. The finely filtered liquid is pressurized to 0.8-1.5MPa by the high pressure pump and the booster pump in sequence, and then enters the spiral wound nanofiltration membrane module through the internal circulation pump to separate the permeate and concentrate. The permeate is collected in the permeate tank. Step 3: Flow the concentrated liquid through the heat exchanger, adjust the temperature to 35-45°C, and then return it to the circulation tank to mix with the liquid therein. Repeat step 2 to achieve circulation concentration. Step 4: When the volume of the concentrate in the circulation tank is concentrated to 10% to 20% of the initial waste alkali liquid volume, stop the circulation and discharge the concentrate through the concentrate discharge pipeline using the feed pump. Step 5: When the membrane flux drops below the predetermined value, perform online cleaning: close the valve between the booster pump and the heat exchanger, allowing the cleaning solution to flow from the cleaning solution tank through the feed pump, security filter, high-pressure pump, booster pump, internal circulation pump, spiral wound nanofiltration membrane module, and then sequentially through the heat exchanger and circulation tank before returning to the cleaning solution tank. After the specified cleaning time, the solution is discharged and rinsed.

[0016] Furthermore, in step 2, the operating pressure is 1.0 MPa; in step 3, the concentrate is adjusted to 40°C and then returned to the circulation tank.

[0017] Furthermore, in step 4, the concentrated liquid is discharged when its volume is concentrated to 10% to 15% of the initial waste alkali liquid volume; Further, in step 5, the cleaning solution is a 0.5% to 1% citric acid solution or a 2% sodium hydroxide solution, and the circulation cleaning time is 30 to 60 minutes.

[0018] The beneficial effects of this invention are: 1. This invention uses a spiral wound nanofiltration membrane with a molecular weight cutoff of 200-500 Da, which can efficiently retain large organic molecules such as proteins and pigments while retaining the alkaline solution, thus obtaining a high-purity recovered alkaline solution.

[0019] 2. The present invention features a dual-stage filtration protection design, with pre-filtration (coarse filtration) and security filter (fine filtration) connected in series. This design can intercept particles ranging from large to micron-sized particles, effectively protecting the high-pressure pump and membrane modules and extending the system life.

[0020] 3. This invention uses a high-pressure pump and a booster pump connected in series for pressurization, with the two pumps working together to ensure that the inlet pressure of the membrane module is stable and adjustable, and can adapt to the treatment needs of waste alkaline solutions with different concentrations and viscosities.

[0021] 4. The heat exchanger of the present invention realizes closed-loop temperature control. The heat exchanger is installed in the concentrate return pipeline to accurately control the material temperature at about 40°C, so as to ensure the membrane flux and avoid high temperature damage to the membrane material.

[0022] 5. This invention has an independent cleaning circuit, equipped with a dedicated cleaning liquid tank and an internal circulation pump, which can complete chemical cleaning online without disassembling the membrane module, significantly reducing maintenance costs and extending the membrane service life.

[0023] 6. The batch concentration and discharge mechanism of this invention achieves intermittent discharge of concentrated liquid through liquid level control of the circulating tank, which not only ensures a high alkali recovery rate (≥90%), but also avoids continuous high-load operation of the wastewater treatment plant. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a process flow diagram for the regeneration and recycling of waste alkali solution.

[0026] Attached reference numerals: 1-Waste alkaline solution; 2-Pre-filtration device; 3-Circulation tank; 4-Feed pump; 5-Security filter; 6-High pressure pump; 7-Booster pump; 8-Internal circulation pump; 9-Screw-wound nanofiltration membrane module; 10-Permeate tank; 11-Cleaning tank; 12-Heat exchanger; 13-Concentrate discharge pipeline. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should not be construed that the scope of the subject matter of the present invention is limited to the following embodiments. All modifications, substitutions and alterations made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention are included within the scope of the present invention.

[0028] Reference Figure 1 This invention provides a waste alkali regeneration and recovery system based on nanofiltration membrane. The system consists of a two-stage filtration module, a pressurization module, a nanofiltration membrane separation module, a purified alkali collection module, a circulating temperature control module, an online cleaning module, and a concentrate discharge module.

[0029] The dual-stage filtration module includes a pre-filtration device 2 and a security filter 5 connected in series. The pre-filtration device 2 is a bag filter with a filtration accuracy of 100–200 μm, used for coarse filtration of the waste alkaline solution 1 to remove large particulate impurities. The security filter 5 has a filtration accuracy of 5–50 μm and is used for fine filtration of the liquid entering the pressurization module to remove micron-sized particles.

[0030] The inlet of the circulation tank 3 is connected to the outlet of the pre-filtration device 2, and is used to receive the waste alkaline solution after coarse filtration and the concentrate returned from the nanofiltration membrane separation module. The outlet of the circulation tank 3 is connected to a feed pump 4, and the outlet of the feed pump 4 is connected to the inlet of the security filter 5, and is used to transport the mixture in the circulation tank 3 to the security filter 5. At the same time, the outlet of the feed pump 4 is also connected to the concentrate discharge port 13 through a pipeline equipped with a valve, and is used to switch to realize the discharge of concentrate.

[0031] The booster module includes a high-pressure pump 6 and a booster pump 7 connected in series. Its inlet is connected to the outlet of the security filter 5, and it is used to boost the liquid pressure to 0.8MPa to 1.5MPa.

[0032] The outlet of the booster module is connected to an internal circulation pump 8, which is connected to a nanofiltration membrane separation module. This module uses a spiral wound nanofiltration membrane assembly 9 with a molecular weight cutoff of 200–500 Da. The spiral wound nanofiltration membrane assembly 9 is provided with a permeate outlet and a concentrate outlet, which are used to allow alkali and water to permeate to form a purified alkali solution, and to retain proteins and pigments to form a concentrate.

[0033] The alkaline solution collection module includes a permeate tank 10, which is connected to the permeate outlet of the spiral wound nanofiltration membrane module 9.

[0034] The circulating temperature control module includes a heat exchanger 12 installed on the return pipeline between the concentrate outlet of the spiral wound nanofiltration membrane module 9 and the circulating tank 3, which is used to adjust the temperature of the return concentrate to 35℃~45℃.

[0035] The online cleaning module includes a cleaning fluid tank 11 and an internal circulation pump 8. A valve is provided between the outlet of the booster pump 7 and the heat exchanger 12.

[0036] The concentrate discharge module is connected to the circulation tank 3 and is used to discharge the concentrate intermittently through the concentrate discharge port 13 via the feed pump 4 when the liquid volume in the circulation tank 3 is concentrated to 10% to 20% of the initial waste alkali liquid volume.

[0037] The system's operating procedure is as follows: Waste alkaline solution is discharged into the system from the workshop and first passes through a pre-filtration device 2 to intercept large suspended solids, fibers, and other impurities, preventing damage to downstream equipment. The pre-filtered waste alkaline solution then enters a circulation tank 3, where it is mixed with the concentrate returned from the membrane module. The mixture is then pumped by a feed pump 4 to a security filter 5 to remove residual particles, ensuring the safe operation of the high-pressure pump and membrane module.

[0038] The effluent from the security filter is sequentially boosted to the operating pressure (0.8–1.5 MPa) by a high-pressure pump 6 and a booster pump 7, and then enters the spiral wound membrane module 9 through an internal circulation pump 8. Under pressure, small molecules (alkali, water) permeate through the membrane to form permeate (purified alkali solution), while large molecules (proteins, pigments, colloids) are retained to form concentrate.

[0039] The permeate (purified alkali solution) then flows into the permeate tank 10, and can be sent back to the production process (such as alkali washing, extraction, etc.) for direct reuse at any time via the conveying pipeline. The concentrate discharged from the membrane module 9 first passes through the heat exchanger 12 to adjust the temperature to 35-45℃ (40℃ recommended), and then returns to the circulation tank 3. After mixing with fresh waste alkali solution, it re-enters the membrane module to achieve cyclic concentration.

[0040] When the concentrated liquid in the circulating tank 3 accumulates to 10% to 20% of the initial volume (i.e., the concentration ratio reaches 5 to 10 times), the batch processing is completed, and the concentrated liquid is discharged to the sewage treatment station through the concentrated liquid discharge (13) pipeline.

[0041] System cleaning: Perform the cleaning procedure after completing several processing batches or when the membrane flux drops by more than 20%. Add deionized water to cleaning tank 11 and add acidic cleaning agent (such as 0.5% to 1% citric acid) in proportion.

[0042] Start the internal circulation pump 8 to circulate the cleaning solution within the system (cleaning solution tank 11 → feed pump 4 → security filter 5 → high pressure pump 6 → booster pump 7 → internal circulation pump 8 → spiral wound nanofiltration membrane module 9 → heat exchanger 12 → circulation tank 3 → return to cleaning solution tank 11) for 30-60 minutes.

[0043] Drain the cleaning solution, rinse with deionized water until neutral, and restore membrane flux.

[0044] The performance of this system in treating waste alkaline solution from the decolorization tower regeneration is as follows: Feed alkaline solution: NaOH concentration 4.8%~5.2%, COD about 8000mg / L, color 300 times.

[0045] Permeate: NaOH concentration 4.5%~4.9% (recovery rate about 93%), COD reduced to <500mg / L, color <20 times, protein removal rate >96%.

[0046] The volume of concentrated liquid discharged is only 10% to 15% of the original liquid volume, which greatly reduces the treatment load of downstream sewage treatment plants.

[0047] The membrane flux remained stable at 35~45 L / (m²). 2 After cleaning, the flux is restored to more than 95% of the initial flux (·h).

[0048] The system ran continuously for 6 months, and no significant performance degradation was observed in the membrane modules.

[0049] This system enables high-purity, low-energy, and continuous recovery and reuse of alkali in waste alkaline solutions.

[0050] Main process flow: Waste alkaline solution inlet 1 → pre-filtration 2 → circulation tank 3 → feed pump 4 → security filter 5 → high pressure pump 6 → booster pump 7 → internal circulation pump 8 → spiral wound nanofiltration membrane module 9.

[0051] Permeate outlet of membrane module 9 → permeate tank 10.

[0052] The concentrate outlet of membrane module 9 → heat exchanger 12 → circulation tank 3, forming a circulating concentration loop.

[0053] Concentrate discharge: Bottom drain of circulating tank 3 → feed pump 4 → concentrated liquid discharge outlet 13 → sewage station.

[0054] Cleaning circuit: Cleaning liquid tank 11 → feed pump 4 → security filter 5 → high pressure pump 6 → booster pump 7 → internal circulation pump 8 → spiral wound nanofiltration membrane module 9 → heat exchanger 12 → circulation tank 3 → return to cleaning liquid tank 11, forming a closed-loop cleaning process.

[0055] Example 1: Processing 30m using the system of the present invention 3 Waste alkaline solution Raw material: Waste alkaline liquid taken from the decolorization tower after regeneration on the production line, with a NaOH concentration of 5.1% (mass fraction) and a protein content of about 0.3%, and is dark brown in color.

[0056] System configuration: Spiral wound nanofiltration membrane module (molecular weight cutoff 300 Da, effective membrane area 40 m²) 2 The high-pressure pump outlet pressure is set to 1.0 MPa; the heat exchanger return temperature is set to 40℃.

[0057] Operation process: 30m 3 The waste alkaline solution is pre-filtered through a 200μm bag filter 2 and then sent to the circulation tank 3; Start the feed pump 4. After the liquid passes through the security filter 5 and reaches an accuracy of 10μm, it is pressurized to 1.0MPa by the high pressure pump 6 and the booster pump 7 and then enters the membrane module 9. During normal production, the internal circulation pump 8 is turned off, and the concentrate discharged from the membrane module 9 is cooled to 40°C by the heat exchanger 12 and then returned to the circulation tank 3. The system runs continuously for 6 hours (average processing capacity is approximately 5m). 3 / h), sampling and testing the concentration of alkali and COD in the permeate solution every hour. When the liquid level in circulation tank 3 drops to approximately 3.6m 3 When the concentration reaches approximately 8.3 times, stop feeding and discharge the concentrated liquid to the wastewater treatment plant through outlet 13. The total amount of permeate collected was approximately 25.5 m³. 3 The average NaOH concentration is 4.7%, COD is 420 mg / L, and it is colorless and transparent. The concentrate is approximately 4.2–4.5 ml. 3 It has a COD as high as 58,000 mg / L and is black and viscous.

[0058] The overall alkali recovery rate of the system (alkali content in the permeate / alkali content in the feed) is approximately 91%.

[0059] Reuse effect: The recycled alkali solution was directly used in the alkali washing process of the original section, and the product qualification rate was not statistically different from that of using fresh alkali solution.

[0060] Example 2: Effect of different temperatures on separation performance Raw materials: The same batch of waste alkaline solution (NaOH 4.9%, COD 8200mg / L) as in Example 1.

[0061] Method: Keep the operating pressure constant at 1.0 MPa, and control the return liquid temperature to 30℃, 40℃ and 50℃ respectively through the heat exchanger. After running for 2 hours each time, the membrane flux and COD of the permeate were measured.

[0062] result: 30℃: Membrane flux 28 L / (m 2 ·h), permeate COD 580mg / L.

[0063] 40℃: Membrane flux 42 L / (m 2 ·h), COD of the permeate was 450 mg / L.

[0064] 50℃: Membrane flux 45 L / (m 2 ·h), but the COD of the permeate increased to 680 mg / L (membrane retention efficiency decreased).

[0065] Conclusion: 40℃ is the optimal operating temperature, which can achieve the best impurity removal effect at higher throughput.

[0066] Example 3: Verification of Online Cleaning Effect Post-operation status: After the system continuously treated waste alkaline solution for 120 hours, the membrane flux increased from the initial 42 L / (m²) 2 ·h) decreased to 26L / (m 2 ·h).

[0067] Cleaning procedure: Add 500L of deionized water to cleaning tank 11, add NaOH solid to prepare a 2% cleaning solution, and heat to 40℃.

[0068] Start the internal circulation pump 8 to circulate the cleaning solution within the system for 45 minutes.

[0069] Drain the cleaning solution and then rinse with deionized water for 15 minutes until the pH of the effluent is neutral.

[0070] Cleaning results: Membrane flux recovered to 39 L / (m²) 2 •h), recovery rate 92.9%.

[0071] Long-term stability: After three months of continuous operation with cleaning every 12 hours, the membrane flux remained consistently between 38 and 42 L / (m²). 2 Between ·h), no visible damage was observed in the membrane module.

[0072] Referring to the table below, the same raw materials were used, but different processing methods were employed for each process, as detailed below:

[0073] The above provides a detailed description of the waste alkali liquid regeneration and recovery system and its process based on nanofiltration membrane provided by the present invention. Specific examples have been used to illustrate the structure and working principle of the invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A waste alkaline solution regeneration and recovery system based on nanofiltration membrane, characterized in that: include, Dual-stage filtration module: includes a pre-filtration device (2) and a security filter (5) connected in series. The pre-filtration device (2) is used to coarsely filter the waste alkaline solution to remove large particulate impurities, and the security filter (5) is used to finely filter the liquid entering the pressurization module to remove micron-sized particles. The booster module includes a high-pressure pump (6) and a booster pump (7) connected in series, the inlet of which is connected to the outlet of the security filter (5) for boosting the liquid pressure to 0.8 to 1.5 MPa; The nanofiltration membrane separation module is connected to the outlet of the pressurization module. This module uses a spiral wound nanofiltration membrane assembly (9) with a molecular weight cutoff of 200-500 Da to allow alkali and water to pass through to form a pure alkali solution, and to retain proteins and pigments to form a concentrated solution. The alkaline solution collection module includes a permeate tank (10) connected to the permeate outlet of the nanofiltration membrane separation module; The circulating temperature control module includes a heat exchanger (12) installed on the return pipeline between the concentrate outlet of the nanofiltration membrane separation module and the circulating tank (3), which is used to adjust the temperature of the return concentrate to 35-45°C. The online cleaning module includes a cleaning liquid tank (11). In the cleaning mode, the cleaning liquid in the cleaning liquid tank (11) passes through the feed pump (4), security filter (5), pressurization module, and internal circulation pump (8) before entering the nanofiltration membrane separation module and flowing through the heat exchanger (12) back to the cleaning liquid tank (11), forming a closed-loop cleaning circuit. The concentrate discharge module is connected to the circulation tank (3) and is used to intermittently discharge the concentrate when the liquid volume in the circulation tank (3) is concentrated to 10% to 20% of the initial waste alkali liquid volume; The system also includes a circulation tank (3), whose inlet is connected to the outlet of the pre-filtration device (2) for receiving the waste alkaline liquid after coarse filtration and the concentrated liquid returned from the nanofiltration membrane separation module. A feed pump (4) is connected to the outlet of the circulation tank (3), whose outlet is connected to the inlet of the security filter (5) for transporting the mixed liquid in the circulation tank (3) to the security filter (5).

2. The waste alkaline solution regeneration and recovery system based on nanofiltration membrane according to claim 1, characterized in that: The pre-filtration device (2) is a bag filter with a filtration accuracy of 100-200μm; the security filter (5) has a filtration accuracy of 5-50μm.

3. The waste alkaline solution regeneration and recovery system based on nanofiltration membrane according to claim 1, characterized in that: The outlet of the feed pump (4) is also connected to the concentrate discharge module through a pipeline equipped with a valve, which is used to switch to realize the discharge of concentrate.

4. The waste alkaline solution regeneration and recovery system based on nanofiltration membrane according to claim 3, characterized in that: The specific path for the concentrated liquid to be discharged is as follows: the liquid outlet at the bottom of the circulation tank (3) is connected to the concentrated liquid outlet (13) via the feed pump (4).

5. The waste alkaline solution regeneration and recovery system based on nanofiltration membrane according to claim 1, characterized in that: A valve is provided between the booster pump (7) and the heat exchanger (12) to adjust the filtration rate. When the valve is closed, all the liquid passing through the booster pump (7) flows into the spiral wound nanofiltration membrane module (9) through the internal circulation pump (8). When the valve is opened, part of the liquid passing through the booster pump (7) flows into the internal circulation pump (8), and the other part flows from the valve to the heat exchanger (12) to adjust the filtration rate.

6. The waste alkaline solution regeneration and recovery system based on nanofiltration membrane according to claim 1, characterized in that: The spiral wound nanofiltration membrane module (9) is provided with a permeate outlet and a concentrate outlet. The permeate outlet is connected to the permeate tank (10), and the concentrate outlet is connected to the heat exchanger (12).

7. A waste alkaline solution regeneration and recovery process based on nanofiltration membrane, characterized in that: Using the system as described in any one of claims 1 to 6, the process includes the following steps: Step 1: Send the waste alkaline solution into the pre-filtration device (2) to remove large particulate impurities, and then enter the circulation tank (3). Step 2: The liquid in the circulation tank (3) is sent to the security filter (5) for fine filtration by the feed pump (4). The finely filtered liquid is pressurized to 0.8-1.5MPa by the high pressure pump (6) and the booster pump (7) in sequence, and then enters the spiral wound nanofiltration membrane module (9) through the internal circulation pump (8) to separate the permeate and concentrate. The permeate is collected in the permeate tank (10). Step 3: The concentrated liquid is passed through the heat exchanger (12), and the temperature is adjusted to 35-45°C. Then it is returned to the circulation tank (3) to mix with the liquid therein. Step 2 is executed again to achieve circulation concentration. Step 4: When the volume of the concentrate in the circulation tank (3) is concentrated to 10% to 20% of the initial waste alkali volume, stop the circulation and discharge the concentrate through the concentrate discharge pipeline (13) via the feed pump (4). Step 5: When the membrane flux drops below a predetermined value, perform online cleaning: close the valve between the booster pump (7) and the heat exchanger (12), so that the cleaning solution flows from the cleaning solution tank (11) through the feed pump (4), security filter (5), high pressure pump (6), booster pump (7), internal circulation pump (8), spiral wound nanofiltration membrane module (9), and then through the heat exchanger (12) and circulation tank (3) before returning to the cleaning solution tank (11). After the specified cleaning time, the solution is discharged and rinsed.

8. The process according to claim 7, characterized in that: In step 2, the operating pressure is 1.0 MPa; in step 3, the concentrate is adjusted to 40°C and then returned to the circulation tank (3).

9. The process according to claim 7, characterized in that: In step 4, the concentrated liquid is discharged when its volume is concentrated to 10% to 15% of the initial waste alkali liquid volume.

10. The process according to claim 7, characterized in that: In step 5, the cleaning solution is a 0.5% to 1% citric acid solution or a 2% sodium hydroxide solution, and the circulation cleaning time is 30 to 60 minutes.

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