Method for impurity removal and recovery of ionic liquid after cellulose spinning

Abstract

The invention relates to the field of wastewater treatment, in particular to a method for impurity removal and recovery of an ionic liquid after cellulose spinning, which comprises the following steps: (1) biodegradation: feeding the ionic liquid to be treated into a membrane bioreactor, and removing organic impurities in the ionic liquid through microbial degradation, the organic matter impurities are one or more of small molecular sugar, cellulose and hemicellulose; (2) multi-stage filtration and impurity removal: flocculent precipitation, air floatation, filtration, resin adsorption, nanofiltration, reverse osmosis and multi-effect evaporation; the COD (Chemical Oxygen Demand) inlet water of the membrane bioreactor is 200 to 1200 mg / L; the water inlet flow velocity is 4-6.3 m < 3 > / h; the water inlet temperature is 30-35 DEG C; the retention time is 5-12 hours; and the membrane aperture of the membrane bioreactor is 0.02-0.05 micron. Impurities in the ionic liquid are effectively removed through a low-energy-consumption and low-cost method, the obtained purified ionic liquid aqueous solution meets the recycling requirement, and repeated cyclic utilization of the ionic liquid and the aqueous solution can be achieved.

Description

Technical Field

[0001] The invention relates to the field of wastewater treatment, and in particular to a method for removing impurities and recovering ionic liquid after cellulose spinning. Background Art

[0002] Ionic liquid is a new type of green solvent that is liquid at room temperature. It has the advantages of low volatility, high thermal stability, and strong solubility. It plays an important role in dissolving cellulose in spinning. Compared with the traditional viscose process for producing regenerated cellulose materials, the process of using ionic liquid as a solvent to dissolve cellulose for spinning is more environmentally friendly. In the preparation of regenerated fibers using the ionic liquid method, the ionic liquid can be recycled as a solvent, and then after concentration, it can be used again in the cellulose dissolution process. The ionic liquid can be recycled.

[0003] In the process of dissolving cellulose in ionic liquid spinning, various trace impurities will be introduced because the raw materials used in production are natural products and additives in the spinning process flow. As the ionic liquid continuously and repeatedly dissolves cellulose and the ionic liquid is recovered, various impurities will continue to accumulate in the recovered ionic liquid. As impurities continue to accumulate in the ionic liquid, the purity of the ionic liquid continues to decrease, causing the ability of the ionic liquid to dissolve cellulose to continue to decrease, and then causing fluctuations in large-scale dissolution processes, inconsistent product performance, too dark color of regenerated cellulose materials, and instability in the recovery process. Moreover, since ionic liquids are expensive, discarding these recovered ionic liquids or replacing new ionic liquids in batches will have a very adverse effect on production costs and the environment. CN103147169A is a method for recovering ionic liquids for spinning using activated carbon, which removes impurities in the ionic liquid by absorption by activated carbon. The activated carbon will also absorb a large amount of ionic liquid while adsorbing the impurities in the ionic liquid, causing a large loss of ionic liquid. CN116712787A is a method and device for recovering solvents for preparing regenerated cellulose materials. It can remove inorganic substances in ionic liquids through resin adsorption and membrane filtration, but it is difficult to remove impurities such as cellulose, hemicellulose, and small molecule sugars in the ionic liquid that are close to the molecular weight of the ionic liquid. It increases the number of ionic liquid cycles to a certain extent, but does not completely solve the problem of ionic liquid recovery. CN113620393A is a method for removing impurity ions in an ionic liquid aqueous solution system. It recovers ionic liquids through a multi-stage electrodialysis method. However, during the spinning process of the ionic liquid, inorganic salts exist in the ionic liquid, which will also migrate with the ionic liquid under the driving force of the electric field, causing the concentration of inorganic salts, and does not completely solve the problem of ionic liquid recovery.

[0004] In summary, the problem of ionic liquid recovery in cellulose spinning needs to be considered from the perspective of both inorganic and organic impurity removal. Based on current research progress, it is necessary to develop a new process for ionic liquid impurity removal in cellulose spinning to achieve the reuse of ionic liquids, which is the prerequisite for the large-scale industrial application of ionic liquids. Summary of the invention

[0005] In view of the problems and shortcomings in the prior art, the purpose of the present invention is to provide a method for removing impurities and recovering ionic liquid after cellulose spinning. The present invention uses a membrane bioreactor to remove impurities such as cellulose, hemicellulose, and small molecule sugars with a molecular weight similar to that of the ionic liquid by microbial degradation to achieve low-cost ionic liquid impurity removal and recovery.

[0006] Based on the above purpose, the present invention adopts the following technical solution: The present invention provides a method for removing impurities and recovering ionic liquid after cellulose spinning, which mainly comprises the following steps: (1) Biodegradation: The ionic liquid to be treated is introduced into a membrane bioreactor, and organic impurities with a molecular weight close to that of the ionic liquid are removed by microbial degradation. The organic impurities are one or more of small molecule sugars, cellulose and hemicellulose. (2) Flocculation and precipitation: The ionic liquid treated by the membrane bioreactor is subjected to flocculation and precipitation treatment. After adjusting the pH value of the ionic liquid to be treated, PAC and PAM are added to the water to condense the colloids and fine suspended matter in the ionic liquid into larger flocs that are more conducive to sedimentation. (3) Flotation: The ionic liquid after flocculation and precipitation is subjected to flotation treatment to remove fine impurities. Highly dispersed tiny bubbles are released into the ionic liquid to be treated, so that they adhere to the impurities in the ionic liquid to be treated, and the microbubbles carry the fine impurities to the surface of the ionic liquid to be treated. The impurities carried by the microbubbles float on the surface of the ionic liquid after flotation treatment, and the floating objects are scraped out by an automatic scraper, thereby achieving the purpose of removing impurities from the ionic liquid to be treated; (4) Filtration: The ionic liquid after flotation treatment is filtered. Sand filtration can intercept most of the unprecipitated suspended matter and reduce the subsequent membrane pollution. Ultrafiltration can effectively intercept pollutants with smaller diameters in membrane bioreactors, flocculation sedimentation, flotation, and sand filtration units. (5) Resin adsorption: The filtered ionic liquid flows into an ion exchange resin composed of anion and cation resin columns in series for resin adsorption treatment. The cation exchange resin adsorbs high-valent cations (e.g., Ca 2+ Mg 2+ , Fe 2+ , Fe 3+), through the anion exchange resin to adsorb the anions in the ionic liquid to be treated (for example: PO4 3- 、SO4 2- 、CO3 2- 、SO3 2- ), resin adsorption can reduce the anions and cations in the ionic liquid to below 5 ppm, effectively removing the influence of inorganic substances on the recovery of ionic liquid; (6) Nanofiltration: The ionic liquid treated with ion exchange resin is subjected to nanofiltration to intercept smaller broken resin adsorption particles and organic matter with a molecular weight of 200-1000, and to concentrate the ionic liquid; (7) Reverse osmosis: The ionic liquid after nanofiltration treatment is further concentrated by reverse osmosis, and the concentrated ionic liquid enters the subsequent evaporation unit; (8) Multiple-effect evaporation: The ionic liquid concentrated by reverse osmosis is evaporated to the required concentration. In multiple-effect evaporation, the secondary steam is used as heating steam and introduced into another evaporator as a heating heat source. The new secondary steam generated by the second evaporator can be used as heating steam for the third evaporator.

[0007] Furthermore, the membrane bioreactor is a biochemical reaction system that combines membrane separation technology with a bioreactor. In the traditional activated sludge method, mud-water separation is accomplished by gravity sedimentation in the secondary sedimentation tank, and its efficiency depends on the sedimentation characteristics of the activated sludge. The better the sludge sedimentation, the higher the mud-water separation efficiency. Since the sludge is subject to the impact of sewage load, it is easy to cause unstable effluent. In addition, due to economic costs, the volume of the secondary sedimentation tank is not very large, so the concentration of activated sludge in the aeration tank is not high, which in turn limits the efficiency of biochemical treatment. The membrane bioreactor achieves solid-liquid separation through membrane components, and the retained sludge is returned to the biochemical system, and the permeate enters the next step of treatment.

[0008] Furthermore, the membrane bioreactor has a processing capacity of 100 to 150 m 3 / d, membrane area is 200~400 m 2 , COD influent 200~1200 mg / L, BOD5 influent 50~500 mg / L, pH value 6~9, influent flow rate 4~6.3 m 3 / h. The concentration of activated sludge is 6000-12000 mg / L, which enhances the removal of organic matter. The membrane filtration pore size of the membrane bioreactor is 0.02-0.05 microns, which effectively intercepts sludge and forms a backflow.

[0009] Furthermore, the inlet water temperature of the membrane bioreactor is 30-35° C., and the residence time is 5-12 hours.

[0010] Furthermore, the membrane bioreactor decomposes the organic small molecule sugars, cellulose, and hemicellulose produced during the spinning process into carbon dioxide and water; the membrane bioreactor is a split membrane bioreactor, the bioreactor and the membrane unit are independent of each other, and the ionic liquid to be treated passes through the membrane assembly and enters the next treatment unit through an external mixing circulation pump. The online detection system can detect the organic content in the ionic liquid to be treated entering the system in real time, adjust the water intake in time, and ensure the efficient operation of the membrane bioreactor.

[0011] Furthermore, the flocculation precipitation process adopts a flocculation precipitation reactor with mechanical stirring, and the flocculation precipitation reactor is provided with three water tanks connected by overflow; the reagent is added into the water tank by a metering pump, and sodium hydroxide or calcium oxide (concentration of 10-30%) is added to the first water tank, and self-feedback is achieved by an online pH meter to control the pH value to 8-9, and the addition amount is 1×10 4 ~1×10 7 ppm; the second water tank uses polyaluminium chloride or polyferric chloride (concentration 10%) as flocculant, and the addition amount is 1×10 4 ~1×10 7 ppm; the third tank uses a water-soluble long-chain compound as a flocculant, with an addition amount of 1×10 3 ~1×10 6 ppm; the water-soluble long-chain compound is polyacrylamide or polydiallylmethylammonium chloride (concentration is 0.1%).

[0012] Furthermore, three mixers are provided in the three water tanks of the flocculation sedimentation reactor, and the mixers adopt vertical stirring.

[0013] Furthermore, the diameter or side length of the flocculation sedimentation tank is generally no more than 8 meters, and the material is made of 304 stainless steel to prevent acid and alkali corrosion. An artificial channel and sampling point are set on one side of the flocculation sedimentation tank to facilitate sampling and observation. The control system can monitor the liquid level, temperature, pH value, and dosage of the reagent in real time, and realize dynamic adjustment.

[0014] Furthermore, the flotation reactor used in the flotation process is to release highly dispersed tiny bubbles into the ionic liquid to be treated, so that the bubbles adhere to impurities such as cellulose, hemicellulose, small molecule sugars, etc. in the ionic liquid to be treated, so that the microbubbles float to the surface of the ionic liquid to be treated with tiny impurities. The impurities on the microbubbles float on the surface of the ionic liquid after flotation treatment, and the floating objects are scraped out by an automatic scraper, thereby achieving the purpose of removing impurities from the ionic liquid to be treated. The flotation reactor is an impeller flotation reactor, which forms a negative pressure under the cover plate through a high-speed rotating impeller and then inhales air. The gas mixed in the ionic liquid to be treated forms microbubbles under the high-speed shearing action of the impeller.

[0015] Furthermore, during the flotation treatment process: the size of the microbubbles is 20 to 60 microns, and the water flow rate is 4 to 8 m 3 / h, the water depth of the ionic liquid to be treated is 2 to 4 m, and the flotation time is 20 to 60 min.

[0016] Furthermore, the filtration includes sand filtration and ultrafiltration, and the ionic liquid to be treated is first coarsely filtered by sand filtration, and then finely filtered by ultrafiltration. The sand filtration is carried out in a sand filter tank, the diameter of the sand filter tank does not exceed 4 m, the sand filtration rate is 3-10 m / s, and there is a water distributor at the water inlet on the top of the sand filter tank to improve the dispersion effect of water. The sand filter tank is filled with three specifications of quartz sand, from bottom to top, quartz sand with a diameter of 0.5-1 mm, quartz sand with a diameter of 1-4 mm, and quartz sand with a diameter of 4-8 mm. The overall quartz sand filling height is not higher than three-quarters of the sand filter tank. After sand filtration treatment, suspended matter and macromolecular organic matter are intercepted, and the ionic liquid to be treated enters the ultrafiltration system, and the ratio of concentrated water to fresh water is controlled at 1-2. Pay attention to the pressure difference before and after the ultrafiltration system membrane, control membrane pollution, and control the pressure difference within 0.2 MPa.

[0017] Furthermore, the operating temperature of the hollow fiber membrane in the ultrafiltration system is 5 to 50° C. When the water inlet pressure exceeds 0.45 MPa, the ultrafiltration system needs to be backwashed.

[0018] Furthermore, the ion exchange resin used in the resin adsorption process is composed of anion and cation exchange resins connected in series. The ionic liquid to be treated flows into the resin column from the bottom of the resin column, and flows evenly into the resin column after being distributed by the water distributor. The diameter of the resin column is 0.4 to 0.7 m, and the filling height accounts for two-thirds to three-quarters of the height of the entire resin column, ensuring that the resin fills the entire resin column when water flows through. The high-valent cations (e.g., Ca 2+ Mg 2+ , Fe 2+ , Fe 3+ ), through the anion exchange resin to adsorb the anions in the ionic liquid to be treated (for example: PO4 3- 、SO4 2- 、CO3 2- 、SO3 2- ).

[0019] Furthermore, the nanofiltration treatment process uses a 300-1000 Dalton high-pressure nanofiltration membrane, a membrane pressure of 5-6 MPa, and an inlet water temperature of 25-45°C.

[0020] Furthermore, nanofiltration is a method to further intercept high-valent ions (e.g., Ca 2+ Mg2+ , Fe 2+ , Fe 3+ ) and remove organic matter with a molecular weight of 200 to 1000. The concentration of the ionic liquid to be treated is controlled at 20% to 30%, and the ratio of fresh water to concentrated water in the nanofiltration system is controlled at 1 to 2.

[0021] Furthermore, the reverse osmosis process uses a cellulose acetate membrane or an aromatic polyamide membrane, the membrane uses a spiral winding device, a porous supporting material is sandwiched between two layers of reverse osmosis membranes, three sides of the membrane are sealed to form a membrane bag, and the other open is sealed and connected to a porous water collecting pipe, so that the inflowing ionic liquid to be treated is completely separated from the permeated ionic liquid; the inlet water temperature is 20-40°C, the inlet water pH value is 2-11, and the ratio of fresh water to concentrated water is 1-3; the ionic liquid concentrated by reverse osmosis enters the subsequent evaporation unit, which can effectively reduce the energy consumption of the entire system, and the fresh water passing through the reverse osmosis system can be reused in the system as production water.

[0022] Furthermore, the reverse osmosis process uses a spiral membrane, with an operating temperature of 25 to 45°C, an operating pH range of 3 to 10, and a maximum water inlet pressure of 3 to 4 MPa. When the water inlet pressure exceeds the normal operating pressure, the machine needs to be shut down for cleaning.

[0023] Furthermore, the temperature of the multiple-effect evaporation process is 70-100° C., and the temperature difference between the heating steam and the ionic liquid to be treated is 5-6° C.

[0024] Furthermore, the evaporation system used in the multiple-effect evaporation is to use the secondary steam as heating steam, introduce another evaporator as a heating heat source, and the newly generated secondary steam of the second evaporator can be used as heating steam for the third evaporator. In this way, each evaporator is called one effect, and multiple evaporators are connected and operated together to form a multiple-effect evaporation system. The evaporator to which raw steam is added is called the first effect, and the evaporator heated by the secondary steam of the first effect is called the second effect, and so on. Repeated use of heat energy can significantly reduce heat energy consumption, greatly reduce costs and increase efficiency. The load range of the multiple-effect evaporator is from 40% to 110%, the steam source is 0.1-0.5MPa steam, and the outlet ionic liquid concentration is 80%-90%.

[0025] Furthermore, during the ultrafiltration, nanofiltration and reverse osmosis treatment, when the membrane pressure exceeds the normal working pressure, the membrane needs to be cleaned with an acidic cleaning solution or an alkaline cleaning solution; the acidic cleaning solution is one or more of dilute hydrochloric acid, acetic acid, dilute sulfuric acid, carbonic acid and phosphoric acid, with a pH value of 2 to 6 and a temperature of 30 to 50°C; the alkaline cleaning solution is one or more of sodium hydroxide, potassium hydroxide, barium hydroxide and ammonia water, with a pH value of 9 to 10 and a temperature of 40 to 50°C. After acid and alkali cleaning, rinse with clean water until the water inlet pressure returns to the working pressure.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) The small molecular weight of organic impurities such as small sugars, cellulose, and hemicellulose in the ionic liquid to be treated is very close to that of the ionic liquid, and it is difficult to remove the impurities in the ionic liquid to be treated by membrane separation. This application utilizes the property of ionic liquids that are difficult to degrade by microorganisms to achieve the removal of organic impurities in the ionic liquid to be treated by microorganisms.

[0027] (2) Through multi-stage treatment of organic and inorganic substances in ionic liquids, the organic and inorganic substances in the ionic liquids to be treated are completely removed, effectively ensuring the recovery of the ionic liquids to be treated.

[0028] (3) Through multi-stage filtration and impurity removal, the membrane contamination of the reverse osmosis membrane is controlled. The concentration of the ionic liquid through the reverse osmosis membrane reduces the subsequent removal of water from the ionic liquid to be treated through multi-effect evaporation, thereby reducing the energy consumption of the entire system.

[0029] (4) The original process of removing impurities from ionic liquids by adsorption materials such as activated carbon has been changed, further reducing the loss of ionic liquids due to adsorption, improving recovery efficiency and reducing recovery costs. After the entire system is processed, both organic and inorganic impurities in the ionic liquid are removed, while the loss of ionic liquid is reduced, and no new impurities are introduced due to the addition of new reagents.

[0030] (5) The treatment of organic and inorganic matter in the entire treatment system realizes the removal of impurities in the fresh water of reverse osmosis and the condensed water of multi-effect evaporation, realizes the water circulation of the entire ionic liquid recovery system, and achieves zero wastewater discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of the present invention for removing impurities and recovering ionic liquid after cellulose spinning; Figure 2 Dissolving a small sample of cellulose in the ionic liquid to be treated; Figure 3 A sample of cellulose dissolved in ionic liquid after treatment in Example 1; Figure 4 A sample of cellulose dissolved in ionic liquid after treatment in Example 2; Figure 5 This is a small sample of cellulose dissolved in ionic liquid after the treatment in Example 3. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below through embodiments in combination with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0033] Example 1 A method for removing impurities and recovering ionic liquid after cellulose spinning, the flow chart of which is as follows Figure 1 As shown, the following steps are included: (1) The ionic liquid to be treated is introduced into the membrane bioreactor. The content of small molecular sugars, cellulose, and hemicellulose in the liquid to be treated is detected in real time through an online detection system. The influent COD is detected to be 300 mg / L, and the influent flow rate of the membrane bioreactor is controlled to be 5 m / s. 3 / h, the inlet water temperature was controlled at 30°C, the membrane pore size of the membrane bioreactor was 0.02 μm, and the residence time was 5 hours.

[0034] (2) The ionic liquid treated by the membrane bioreactor enters the flocculation sedimentation unit for treatment, and the inlet flow rate is controlled at 5m 3 / h, connected by continuous self-overflow, configured with 30% sodium hydroxide solution, and the pH value of the first water inlet tank was controlled to 8 by a metering pump and an online pH monitoring device. 10% PAC was added to the second water inlet tank, and the dosage was 1×10 4 ppm PAC26, add 0.1% polyacrylamide to the third water inlet tank connected by self-overflow, and the dosage is 1×10 4 ppm.

[0035] (3) After flocculation and precipitation, the ionic liquid enters the flotation unit, and the size of the nano-bubbles is controlled to be 20 to 60 microns. The water inlet flow rate is 5 m 3 / h, water depth 2 m, flotation time 30 min.

[0036] (4) After flotation treatment, the ionic liquid enters the sand filter tank. The diameter of the sand filter tank is 2 m, and the filtration rate is 5 m / s. There is a water distributor at the water inlet on the top of the sand filter tank to improve the water dispersion effect. The sand filter tank is filled with quartz sand of different specifications, from bottom to top, quartz sand with a diameter of 0.5 mm, quartz sand with a diameter of 1 mm, and quartz sand with a diameter of 4 mm. The overall quartz sand filling height is three-quarters of the sand filter tank. After sand filtration treatment, suspended solids and macromolecular organic matter are intercepted, and the ionic liquid to be treated enters the ultrafiltration system. The ratio of concentrated water to fresh water is controlled at 1. Pay attention to the pressure difference before and after the ultrafiltration system membrane, and control the pressure difference within 0.2 MPa.

[0037] (5) The filtered ionic liquid flows into the ion exchange resin, which is composed of anion and cation resin columns connected in series. The diameter of the resin column is 0.5 m, and the filling height is about three quarters of the height of the entire resin column, ensuring that the resin fills the entire resin column when water flows through.

[0038] (6) After being treated with ion exchange resin, the ionic liquid to be treated enters the nanofiltration treatment unit. The nanofiltration membrane uses a 300 Dalton high-pressure nanofiltration membrane, the membrane pressure is controlled at 5 MPa, and the inlet water temperature is maintained at 25°C.

[0039] (7) The ionic liquid after nanofiltration treatment enters the reverse osmosis unit, and the inlet water temperature is controlled at 25°C, the inlet water pH value is 3-4, and the ratio of fresh water to concentrated water is controlled at 1. The generated fresh water is used for enterprise production water.

[0040] (8) The ionic liquid after reverse osmosis treatment enters the multi-effect evaporation unit, and the evaporation process temperature is controlled at 100°C, and the temperature difference between the heating steam and the ionic liquid to be treated is maintained at 5°C.

[0041] After the ionic liquid was treated by the above method, the concentration of the ionic liquid reached 85%, which reached the recycling condition, of which 95% of the soluble impurities were removed, and the platinum-cobalt color of the ionic liquid dropped from 192.36 to 28.67. The viscosity of the ionic liquid aqueous solution after treatment dropped from 528.1 to 419.2 mpa·s. The dissolved fiber sample could not completely dissolve the cellulose before treatment, and a large amount of undissolved fibers were observed under a polarizing microscope, such as Figure 2 As shown in FIG. 1 , after 2.1 hours of dissolving cellulose in the treated ionic liquid, the cellulose was almost completely dissolved under a polarizing microscope, as shown in FIG. Figure 3 The comparison data before and after ionic liquid treatment are shown in Table 1.

[0042] Example 2 A method for removing impurities and recovering ionic liquid after cellulose spinning, comprising the following steps: (1) The ionic liquid to be treated is introduced into the membrane bioreactor. The content of small molecular sugars, cellulose, and hemicellulose in the liquid to be treated is detected in real time through the online detection system. The influent COD is detected to be 500 mg / L, and the influent flow rate of the membrane bioreactor is controlled to be 5 m / s. 3 / h, the inlet water temperature was controlled at 35°C, the membrane pore size of the membrane bioreactor was 0.02 μm, and the residence time was 8 hours.

[0043] (2) The ionic liquid treated by the membrane bioreactor enters the flocculation sedimentation unit for treatment, and the inlet flow rate is controlled at 5m 3 / h, connected by continuous self-flow, configured with 30% sodium hydroxide solution, and the pH value of the first water inlet tank was controlled to 9 by a metering pump and an online pH monitoring device. 10% PAC was added to the second water inlet tank, and the dosage was 1×10 5 ppm PAC26, add 0.1% polyacrylamide to the third water inlet tank connected by self-overflow, and the dosage is 1×10 4 ppm.

[0044] (3) After flocculation and precipitation, the ionic liquid enters the flotation unit, and the size of the nano-bubbles is controlled to be 20 to 60 microns. The water inlet flow rate is 5 m 3 / h, water depth 2 m, flotation time 40 min.

[0045] (4) After flotation treatment, the ionic liquid enters the sand filter tank. The diameter of the sand filter tank is 3 m, and the filtration rate is 6 m / s. There is a water distributor at the water inlet on the top of the sand filter tank to improve the water dispersion effect. The sand filter tank is filled with quartz sand of different specifications, from bottom to top, quartz sand with a diameter of 0.8 mm, quartz sand with a diameter of 1.5 mm, and quartz sand with a diameter of 6 mm. The overall quartz sand filling height is three-quarters of the sand filter tank. After sand filtration treatment, suspended solids and macromolecular organic matter are intercepted, and the ionic liquid to be treated enters the ultrafiltration system. The ratio of concentrated water to fresh water is controlled at 1. Pay attention to the pressure difference before and after the ultrafiltration system membrane, and control the pressure difference within 0.2 MPa.

[0046] (5) The filtered ionic liquid flows into the ion exchange resin, which is composed of anion and cation resin columns connected in series. The diameter of the resin column is 0.5 m, and the filling height is about three quarters of the height of the entire resin column, ensuring that the resin fills the entire resin column when water flows through.

[0047] (6) After being treated with ion exchange resin, the ionic liquid to be treated enters the nanofiltration treatment unit. The nanofiltration membrane uses a 400 Dalton high-pressure nanofiltration membrane, the membrane pressure is controlled at 3 MPa, and the inlet water temperature is maintained at 30°C.

[0048] (7) The ionic liquid after nanofiltration treatment enters the reverse osmosis unit, and the inlet water temperature is controlled at 30°C, the inlet water pH value is 6-7, and the ratio of fresh water to concentrated water is controlled at 2. The generated fresh water is used for enterprise production water.

[0049] (8) The ionic liquid after reverse osmosis treatment enters the multi-effect evaporation unit, and the evaporation process temperature is controlled at 100 °C, and the temperature difference between the heating steam and the ionic liquid to be treated is maintained at 6 °C.

[0050] After the ionic liquid was treated by the above method, the concentration of the ionic liquid reached 86%, which was suitable for recycling. 94% of the soluble impurities were removed, and the platinum-cobalt color of the ionic liquid was reduced from 192.36 to 27.62. The viscosity of the ionic liquid aqueous solution after treatment was reduced from 528.1 to 410.1 mpa·s. The dissolved fiber sample could not completely dissolve the cellulose before treatment. A large amount of undissolved fibers were observed under a polarizing microscope, such as Figure 2 As shown in FIG. 1 , after 1.9 hours of dissolving cellulose in the treated ionic liquid, the cellulose was almost completely dissolved under a polarizing microscope, as shown in FIG. Figure 4 The comparison data before and after ionic liquid treatment are shown in Table 1.

[0051] Example 3 A method for removing impurities and recovering ionic liquid after cellulose spinning, comprising the following steps: (1) The ionic liquid to be treated is introduced into the membrane bioreactor. The content of small molecular sugars, cellulose, and hemicellulose in the liquid to be treated is detected in real time through the online detection system. The influent COD is detected to be 1500 mg / L, and the influent flow rate of the membrane bioreactor is controlled to be 5 m / s. 3 / h, the inlet water temperature was controlled at 35°C, the membrane pore size of the membrane bioreactor was 0.02 μm, and the residence time was 12 hours.

[0052] (2) The ionic liquid treated by the membrane bioreactor enters the flocculation sedimentation unit for treatment, and the inlet flow rate is controlled at 5m 3 / h, connected by continuous self-flow, configured with a 30% calcium oxide solution, and the pH value of the first water inlet tank was controlled at 9 by a metering pump and an online pH monitoring device. 10% PAC was added to the second water inlet tank, and the dosage was 1×10 6 ppm PAC26, add 0.1% polyacrylamide to the third water inlet tank connected by self-overflow, and the dosage is 1×10 5 ppm.

[0053] (3) After flocculation and precipitation, the ionic liquid enters the flotation unit, and the size of the nano-bubbles is controlled to be 20 to 60 microns. The water inlet flow rate is 5 m 3 / h, water depth 2 m, flotation time 50 min.

[0054] (4) After flotation treatment, the ionic liquid enters the sand filter tank. The diameter of the sand filter tank is 5 m, and the filtration rate is 5 m / s. There is a water distributor at the water inlet on the top of the sand filter tank to improve the water dispersion effect. The sand filter tank is filled with quartz sand of different specifications, from bottom to top, quartz sand with a diameter of 1 mm, quartz sand with a diameter of 1.5 mm, and quartz sand with a diameter of 6 mm. The overall quartz sand filling height is three-quarters of the sand filter tank. After sand filtration treatment, suspended solids and macromolecular organic matter are intercepted, and the ionic liquid to be treated enters the ultrafiltration system. The ratio of concentrated water to fresh water is controlled at 1. Pay attention to the pressure difference before and after the ultrafiltration system membrane, and control the pressure difference within 0.2 MPa.

[0055] (5) The filtered ionic liquid flows into the ion exchange resin, which is composed of anion and cation resin columns connected in series. The diameter of the resin column is 0.5 m, and the filling height is about three quarters of the height of the entire resin column, ensuring that the resin fills the entire resin column when water flows through.

[0056] (6) After being treated with ion exchange resin, the ionic liquid to be treated enters the nanofiltration treatment unit. The nanofiltration membrane uses a 1000 Dalton high-pressure nanofiltration membrane, the membrane pressure is controlled at 2.5 MPa, and the inlet water temperature is maintained at 35°C.

[0057] (7) The ionic liquid after nanofiltration treatment enters the reverse osmosis unit, and the inlet water temperature is controlled at 35°C, the inlet water pH value is 8-9, and the ratio of fresh water to concentrated water is controlled at 3. The generated fresh water is used for enterprise production water.

[0058] (8) The ionic liquid after reverse osmosis treatment enters the multi-effect evaporation unit, and the evaporation process temperature is controlled at 70°C, and the temperature difference between the heating steam and the ionic liquid to be treated is maintained at 5°C.

[0059] After the ionic liquid was treated by the above method, the concentration of the ionic liquid reached 84%, which was suitable for recycling. 96% of the soluble impurities were removed, and the platinum-cobalt color of the ionic liquid was reduced from 192.36 to 28.51. The viscosity of the ionic liquid aqueous solution after treatment was reduced from 528.1 to 412.3 mpa·s. The cellulose in the dissolved fiber sample could not be completely dissolved before treatment. A large amount of undissolved fibers were observed under a polarizing microscope, such as Figure 2 As shown, after 2.0 hours of dissolving cellulose in the treated ionic liquid, the cellulose was almost completely dissolved under a polarizing microscope, as shown in FIG. Figure 5 The comparison data before and after ionic liquid treatment are shown in Table 1.

[0060] Table 1: Detection indexes of ionic liquids in the examples and before treatment

Claims

1. A method for removing impurities and recovering ionic liquid after cellulose spinning, characterized in that: The main steps are as follows: (1) Biodegradation: The ionic liquid to be treated is introduced into a membrane bioreactor, and organic impurities in the ionic liquid are removed by microbial degradation. The organic impurities are one or more of small molecule sugars, cellulose and hemicellulose; (2) Multi-stage filtration and impurity removal: flocculation sedimentation, flotation, filtration, resin adsorption, nanofiltration, reverse osmosis, and multi-effect evaporation; The COD influent of the membrane bioreactor is 200-1200 mg / L; the influent flow rate is 4-6.3 m 3 / h; the inlet water temperature is 30-35°C; the residence time is 5-12 hours; the membrane pore size of the membrane bioreactor is 0.02-0.05 microns.

2. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The flocculation and precipitation process adopts a flocculation and precipitation reactor with mechanical stirring. The flocculation and precipitation reactor is equipped with three water tanks connected by overflow. The pH value of the first water tank is controlled to 8-9 by adding sodium hydroxide or calcium oxide, and the addition amount is 1×10 4 ~1×10 7 ppm; the second water tank uses polyaluminium chloride or polyferric chloride as flocculant, and the addition amount is 1×10 4 ~1×10 7 ppm; the third tank uses a water-soluble long-chain compound as a flocculant, with an addition amount of 1×10 3 ~1×10 6 ppm; the water-soluble long-chain compound is polyacrylamide or polydiallylmethylammonium chloride.

3. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: During the flotation treatment process, the microbubble size is 20 to 60 microns, and the water flow rate is 4 to 8 m 3 / h, the water depth of the ionic liquid to be treated is 2 to 4 m, and the flotation time is 20 to 60 min.

4. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The filtration includes sand filtration and ultrafiltration; the sand filtration is carried out in a sand filter tank with a diameter of no more than 4 m and a sand filtration rate of 3 to 10 m / s. The sand filter tank is filled with three specifications of quartz sand with diameters of 0.5 to 1 mm, 1 to 4 mm, and 4 to 8 mm from bottom to top; during the ultrafiltration treatment process, the ratio of concentrated water to fresh water is 1 to 2, and the pressure difference before and after the ultrafiltration system membrane is within 0.2 MPa.

5. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The ion exchange resin used in the resin adsorption process is composed of anion and cation exchange resin stages connected in series, the resin column diameter is 0.4-0.7 m, and the resin filling height is two-thirds to three-quarters of the entire resin column.

6. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The nanofiltration treatment process uses a 300-1000 Dalton high-pressure nanofiltration membrane, a membrane pressure of 5-6 MPa, and an inlet water temperature of 25-45°C.

7. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The reverse osmosis process uses cellulose acetate membrane or aromatic polyamide membrane, the inlet water temperature is 20-40°C, the inlet water pH value is 2-11, and the ratio of fresh water to concentrated water is 1-3.

8. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 1, characterized in that: The temperature of the multiple-effect evaporation process is 70-100° C., and the temperature difference between the heating steam and the ionic liquid to be treated is 5-6° C.

9. The method for removing impurities and recovering ionic liquid after cellulose spinning according to claim 5, 7 or 8, characterized in that: During the ultrafiltration, nanofiltration and reverse osmosis treatment process, when the pressure of the membrane exceeds the normal working pressure, the membrane needs to be cleaned with an acidic cleaning solution or an alkaline cleaning solution; the acidic cleaning solution is one or more of dilute hydrochloric acid, acetic acid, dilute sulfuric acid, carbonic acid, and phosphoric acid, the pH value of the acidic cleaning solution is 2-6, and the temperature is 30-50°C; the alkaline cleaning solution is one or more of sodium hydroxide, potassium hydroxide, barium hydroxide, and ammonia water, the pH value of the alkaline cleaning solution is 9-10, and the temperature is 40-50°C.

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