阅读说明：本技术 一种用于染料脱盐的双层中空纤维疏松纳滤膜的制备方法 (Preparation method of double-layer hollow fiber loose nanofiltration membrane for dye desalination ) 是由 陈英波 王传风 于 2021-10-26 设计创作，主要内容包括：本发明提供了一种用于染料脱盐的双层中空纤维疏松纳滤膜的制备方法,包括如下步骤：1)将成膜聚合物、致孔剂、挥发性共溶剂、溶剂混合均匀；2)配置凝固浴,其中,所述凝固浴为水或浓度低于50wt％的溶剂I水溶液,所述溶剂I为乙醇、二甲基乙酰胺或二甲基甲酰胺；3)通过精确调控涂覆液配方和纤维形成(聚合物浓度、添加剂含量、共溶剂配比及空气浴长度(蒸发时间)),采用在线涂覆装置利用相转化制备分离孔径在纳滤范围的高强度双层中空纤维膜。本发明制备过程简单、经济、无需后处理、易于在连续工业生产中实现,同时对染料和盐有高的选择性,优异的渗透性,具有广阔的市场应用前景。(The invention provides a preparation method of a double-layer hollow fiber loose nanofiltration membrane for dye desalination, which comprises the following steps: 1) uniformly mixing a film-forming polymer, a pore-foaming agent, a volatile cosolvent and a solvent; 2) preparing a coagulation bath, wherein the coagulation bath is water or a solvent I water solution with the concentration of less than 50 wt%, and the solvent I is ethanol, dimethylacetamide or dimethylformamide; 3) by precisely controlling the coating solution formulation and fiber formation (polymer concentration, additive content, co-solvent ratio, and air bath length (evaporation time)), a high-strength double-layer hollow fiber membrane having a separation pore size in the nanofiltration range was prepared using an on-line coating apparatus using phase inversion. The preparation method is simple and economic in preparation process, does not need post-treatment, is easy to realize in continuous industrial production, and has high selectivity on dye and salt, excellent permeability and wide market application prospect.)

1. A preparation method of a double-layer hollow fiber loose nanofiltration membrane for dye desalination is characterized by comprising the following steps:

1) preparing a coating solution: mixing a polymer, a pore-foaming agent, a volatile cosolvent and a solvent according to the following mass percentages: 16-28% of polymer, 0-5% of pore-forming agent, 0-30% of volatile cosolvent and 50-80% of solvent, wherein the sum of the components is 100%; stirring the mixture at room temperature for 5-24 hours to obtain a homogeneous coating liquid;

2) preparing a coagulating bath: wherein the coagulating bath is water or solvent I water solution with concentration of less than 50 wt%; the solvent I is ethanol, dimethylacetamide or dimethylformamide;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the hollow fiber membrane vertically penetrates through a coating device at a constant speed of 1-3 cm/s under the action of traction force, the coating liquid is uniformly coated on the outer surface of the hollow fiber membrane, the coated hollow fiber enters a coagulating bath through an air bath, and a double-layer hollow fiber membrane with nanofiltration separation capacity is formed by controlling the length of the air bath, namely the evaporation time of a volatile cosolvent and utilizing phase inversion. The obtained fiber is soaked in clear water bath for 1 day, then transferred into 30 wt% glycerol aqueous solution for soaking for 2 days, dried at room temperature and stored in a sealed bag.

2. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the polymer in the step 1) is one of polyvinylidene fluoride (PVDF), Polyethersulfone (PES), Polysulfone (PSF), Polyacrylonitrile (PAN) or Sulfonated Polyethersulfone (SPES) or a mixture of the polyvinylidene fluoride (PVDF), the Polyethersulfone (PES), the Polysulfone (PSF), the Polyacrylonitrile (PAN) or the Sulfonated Polyethersulfone (SPES) in any proportion.

3. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: in the step 1), the pore-forming agent is one or a compound of lithium chloride (LiCl), polyvinyl pyridine (PVP), calcium chloride (CaCl2) or polyethylene glycol (PEG).

4. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the volatile cosolvent in the step 1) is one of Tetrahydrofuran (THF), Acetone (Acetone), n-hexane (n-hexane) or Ethanol (Ethanol) or a mixture of the solvents in any proportion.

5. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the solvent in the step 1) is one of N, N-dimethylacetamide (DMAc), N, dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF) or N-methylpyrrolidone (NMP).

6. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the hollow fiber membrane in the step 3) is one of a porous reinforced hollow fiber membrane, a woven reinforced hollow fiber membrane and a continuous fiber reinforced hollow fiber membrane.

7. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the coating device in the step 3) contains coating liquid, and the outlet diameter of the coating liquid is 0.05-0.7 mm larger than the outer diameter of the hollow fiber membrane.

8. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: the length of the air bath in the step 3) is 0-200 cm.

9. The method of preparing a double-layered hollow fiber bulk nanofiltration membrane for dye desalination as claimed in claim 1, wherein the method comprises the following steps: and in the step 3), the evaporation time is 0-60 s.

Technical Field

The invention relates to the technical field of nanofiltration membranes, in particular to a preparation method of a double-layer hollow fiber loose nanofiltration membrane for dye desalination.

Background

Textile wastewater contains a large amount of synthetic dyes, is complex and difficult to degrade, and millions of tons of valuable dyes are discharged into surface water, which increases environmental burden and threatensIs good for human health. In addition, a large amount of inorganic salts (mainly NaCl or Na)₂SO₄) The presence of (a) increases the difficulty of dye recovery and purification. The conventional techniques (such as biodegradation, adsorption, coagulation and advanced oxidation) are limited by their high energy consumption, low separation efficiency and complex separation apparatus.

Nanofiltration (NF) is a membrane separation technology with separation effects between reverse osmosis and ultrafiltration, and shows great potential in many separation fields with the advantages of high separation efficiency, low energy consumption, environmental friendliness, and the like. However, conventional NF membranes have a high rejection of divalent salts, which increases the difficulty of dye and salt recovery. Recently, the preparation of loose NF membranes for the separation of dye/salt mixtures has received increasing attention due to their relatively large pore size, allowing high permeation of salt and water while maintaining high retention of dye molecules, and has become an important purification technique for the treatment of textile wastewater. Currently, many methods (interfacial polymerization, in situ self-assembly, co-deposition, grafting, etc.) have been proposed to fabricate loose NF membranes. However, these reported modified membranes are dominated by flat sheet membranes, requiring complex preparation procedures and expensive raw materials to achieve dye desalting performance, which largely limits their large scale application in textile wastewater. In addition, compared with a flat membrane, the hollow fiber membrane has the unique advantages of large specific surface area, high packing density, self-supporting property, less pretreatment and the like.

The simple substance hollow fiber membrane prepared by the traditional solution phase inversion method has poor mechanical property, and in the application of treating actual wastewater, the fiber is subjected to pulsation or impact disturbance of long-time high-pressure water flow, high-speed water flow and frequent cleaning generate great damage to the fiber, and the fiber breakage is a common phenomenon in the use process of the hollow fiber. The defect not only influences the stability of the effluent quality in the operation of the equipment, but also increases the cost of engineering operation. Therefore, the development of high strength, high selectivity hollow fiber bulk nanofiltration membranes by simple, inexpensive procedures remains a challenge.

Disclosure of Invention

In order to solve the technical problems, the invention provides a preparation method of a double-layer hollow fiber loose nanofiltration membrane for dye desalination, which is used for preparing a high-strength double-layer hollow fiber membrane with a separation pore diameter in the NF range by adopting an online coating device and utilizing phase inversion through accurately regulating and controlling a coating liquid formula and fiber formation (polymer concentration, additive content, cosolvent proportion and air bath length (evaporation time)). The preparation method is simple and economic in preparation process, does not need post-treatment, is easy to realize in continuous industrial production, and has high selectivity on dye and salt, excellent permeability and wide market application prospect.

Therefore, the technical scheme of the invention is as follows:

1) preparing a coating solution: mixing a polymer, a pore-foaming agent, a volatile cosolvent and a solvent according to the following mass percentages: 16-28% of polymer, 0-5% of pore-forming agent, 0-30% of volatile cosolvent and 50-80% of solvent, wherein the sum of the components is 100%; stirring the mixture at room temperature for 5-24 hours to obtain a homogeneous coating liquid;

2) preparing a coagulating bath: wherein the coagulating bath is water or solvent I water solution with concentration of less than 50 wt%; the solvent I is ethanol, dimethylacetamide or dimethylformamide;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the hollow fiber membrane vertically penetrates through a coating device at a constant speed of 1-3 cm/s under the action of traction force, the coating liquid is uniformly coated on the outer surface of the hollow fiber membrane, the coated hollow fiber enters a coagulating bath through an air bath, and a double-layer hollow fiber membrane with nanofiltration separation capacity is formed by controlling the length of the air bath, namely the evaporation time of a volatile cosolvent and utilizing phase inversion. The obtained fiber is soaked in clear water bath for 1 day, then transferred into 30 wt% glycerol aqueous solution for soaking for 2 days, dried at room temperature and stored in a sealed bag.

Further, the polymer in the step 1) is one of polyvinylidene fluoride (PVDF), Polyethersulfone (PES), Polysulfone (PSF), Polyacrylonitrile (PAN) or Sulfonated Polyethersulfone (SPES) or a mixture of the polyvinylidene fluoride (PVDF), the Polyethersulfone (PES), the Polysulfone (PSF), the Polyacrylonitrile (PAN) or the Sulfonated Polyethersulfone (SPES) in any proportion.

Further, the pore-forming agent in the step 1) is lithium chloride (LiCl), polyvinyl pyridine (PVP) and calcium chloride (CaCl)₂) Or one of polyethylene glycol (PEG)Or a combination thereof.

Further, the volatile cosolvent in the step 1) is one of Tetrahydrofuran (THF), Acetone (Acetone), n-hexane (n-hexane) or Ethanol (Ethanol) or a mixture thereof in any proportion.

Further, in the step 1), the solvent is one of N, N-dimethylacetamide (DMAc), N, dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF) or N-methylpyrrolidone (NMP).

Further, the hollow fiber membrane in the step 3) is one of a porous reinforced hollow fiber membrane, a woven reinforced hollow fiber membrane and a continuous fiber reinforced hollow fiber membrane.

Further, the coating device in the step 3) contains a coating liquid, and the outlet diameter of the coating liquid is 0.05-0.7 mm larger than the outer diameter of the hollow fiber membrane.

Further, the length of the air bath in the step 3) is 0-200 cm.

Further, the evaporation time in the step 3) is 0-60 s.

The invention has the following advantages and beneficial effects:

1. the double-layer hollow fiber loose nanofiltration membrane for dye desalination has the advantages of simple preparation method, low preparation cost, no need of post-treatment, high repeatability and easy realization of industrial production.

2. The double-layer hollow fiber loose nanofiltration membrane for dye desalination has high selectivity on dye and salt, excellent permeability and wide market application prospect.

Drawings

FIG. 1 is a schematic diagram of the preparation of a double-layered hollow fiber bulk nanofiltration membrane for dye desalination obtained in example 1;

FIG. 2 is a scanning electron microscope picture of the whole cross section of the double-layer hollow fiber loose nanofiltration membrane for dye desalination obtained in example 1;

FIG. 3 is a partial enlarged cross-sectional scanning electron microscope picture of the double-layer hollow fiber loose nanofiltration membrane for dye desalination obtained in example 1;

FIG. 4 is a molecular weight cut-off and pore size distribution diagram of the double-layer hollow fiber loose nanofiltration membrane for dye desalination obtained in example 1;

FIG. 5 is an SEM image of the whole cross section of the double-layered hollow fiber ultrafiltration membrane for dye desalting obtained in comparative example 2;

FIG. 6 is a scanning electron microscope picture of a partially enlarged cross section of the double-layered hollow fiber ultrafiltration membrane for dye desalination obtained in comparative example 2;

FIG. 7 is a molecular weight cut-off and pore size distribution diagram of the double-layer hollow fiber ultrafiltration membrane for dye desalination obtained in comparative example 2;

FIG. 8 is a scanning electron microscope picture of the whole cross section of the double-layer hollow fiber ultrafiltration membrane for dye desalination obtained in example 4;

FIG. 9 is a partially enlarged cross-sectional SEM image of a double-layered hollow fiber ultrafiltration membrane for dye desalting obtained in example 4.

Detailed Description

The invention is further described below with reference to the accompanying drawings and examples.

Example 1

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of tetrahydrofuran, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating solution was injected into an on-line coating apparatus, and the porous reinforced hollow fiber membrane vertically passed through the coating apparatus at a rate of 3cm/s, passed through an air bath having a length of 90cm (evaporation time of 30s), and then entered into a coagulation bath, to prepare a double-layered hollow fiber membrane having a separation pore size in the NF range by phase inversion, a preparation diagram of which is shown in fig. 1. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

The scanning electron microscope morphology of the double-layer hollow fiber loose nanofiltration membrane for dye desalination obtained in the embodiment is shown in fig. 2-3, the molecular weight cut-off and the pore size distribution are shown in fig. 4, and it can be observed from the cross-sectional views of fig. 2 and 3 that the dense PVDF selective layer is uniformly coated on the reinforced hollow fiber support membrane, and the coating thickness is about 7.6 μm. As shown in fig. 4, the dense PVDF separation layer reduces the molecular weight cut-off (MWCO) and pore size of the double-layer hollow fiber membrane, which is advantageous for improving selectivity.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

comparative example 1

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 78g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber ultrafiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm, and preparing the double-layer hollow fiber ultrafiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution, the performance of the double-layer hollow fiber ultrafiltration membrane is tested under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

comparative example 2

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride and 76g N, N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber ultrafiltration membrane: and injecting the coating solution into online coating equipment, enabling the porous reinforced hollow fiber membrane to vertically penetrate through a coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm, and preparing the double-layer hollow fiber ultrafiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

The morphology of the double-layer hollow fiber ultrafiltration membrane obtained in this comparative example by scanning electron microscopy is shown in FIGS. 5 and 6, the molecular weight cut-off and pore size distribution are shown in FIG. 7, and it can be observed from the cross-sectional views of FIGS. 5 and 6 that the PVDF-selective layer consists of finger-like pores and a sponge-like structure, and the coating thickness is about 22 μm. The relatively loose structure of the separation layer gives a large MWCO and pore size for the double layer hollow fiber membrane, as shown in fig. 7. It can be seen by comparison with the comparative example that the addition of tetrahydrofuran and the control of the evaporation time contribute to a significant reduction in the thickness of the PVDF selection layer.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution, the performance of the double-layer hollow fiber ultrafiltration membrane is tested under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

comparative example 3

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 4g of anhydrous lithium chloride, 74g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber ultrafiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm, and preparing the double-layer hollow fiber ultrafiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution, the performance of the double-layer hollow fiber ultrafiltration membrane is tested under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

comparative example 4

1) Preparing a coating solution: taking 18g of polyvinylidene fluoride, 2g of anhydrous lithium chloride and 80g N, N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber ultrafiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm, and preparing the double-layer hollow fiber ultrafiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution, the performance of the double-layer hollow fiber ultrafiltration membrane is tested under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

comparative example 5

l) preparing a coating solution: taking 26g of polyvinylidene fluoride, 2g of anhydrous lithium chloride and 72g N, N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber ultrafiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm, and preparing the double-layer hollow fiber ultrafiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution, the performance of the double-layer hollow fiber ultrafiltration membrane is tested under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

comparative example 6

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 11.7g of tetrahydrofuran, 66.3g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparation of a double-layer hollow fiber membrane: the coating liquid was injected into an in-line coating apparatus, and the porous reinforced hollow fiber membrane vertically passed through the coating apparatus at a rate of 3cm/s, passed through an air bath having a length of 90cm (evaporation time of 30s), and then entered into a coagulation bath, to prepare a double-layered hollow fiber membrane by phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 2

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 3.8g of tetrahydrofuran, 72.2g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm (the evaporation time is 30s), and preparing the double-layer hollow fiber loose nanofiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

example 3

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 19g of tetrahydrofuran, 57g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane: and injecting the coating solution into an online coating device, enabling the porous reinforced hollow fiber membrane to vertically penetrate through the coating device at the speed of 3cm/s, enabling the porous reinforced hollow fiber membrane to enter a coagulating bath after passing through an air bath with the length of 90cm (the evaporation time is 30s), and preparing the double-layer hollow fiber loose nanofiltration membrane through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2h continuous test, the obtained results are as follows:

example 4

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of tetrahydrofuran, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparation of a double-layer hollow fiber membrane: the coating solution was injected into an on-line coating apparatus, and the porous reinforced hollow fiber membrane vertically passed through the coating apparatus at a rate of 3cm/s, passed through an air bath having a length of 0cm (evaporation time of 0s), and then entered into a coagulation bath, to prepare a double-layered hollow fiber membrane by phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

The appearance of the double-layer hollow fiber membrane obtained by the comparative example in a scanning electron microscope is shown in FIGS. 8-9. It can be observed from the cross-sectional views of fig. 8 and 9 that the PVDF selection layer is relatively dense, with a coating thickness of about 17 μm. A significant effect of evaporation time on separation layer thickness can be seen by comparison with the comparative example.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 5

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of tetrahydrofuran, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating liquid is injected into an online coating device, the porous reinforced hollow fiber membrane vertically passes through the coating device at the speed of 3cm/s, enters a coagulating bath after passing through an air bath with the length of 45cm (the evaporation time is 15s), and a double-layer hollow fiber membrane with the separation pore diameter in the NF range is prepared through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 6

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of tetrahydrofuran, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating liquid was injected into an on-line coating apparatus, and the porous reinforced hollow fiber membrane vertically passed through the coating apparatus at a rate of 3cm/s, passed through an air bath having a length of 135cm (evaporation time of 45s), and then entered into a coagulation bath, to prepare a double-layered hollow fiber membrane having a separation pore size in the NF range by phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm ofRose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 7

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of tetrahydrofuran, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating liquid is injected into an online coating device, the porous reinforced hollow fiber membrane vertically passes through the coating device at the speed of 3cm/s, enters a coagulating bath after passing through an air bath with the length of 180cm (the evaporation time is 60s), and a double-layer hollow fiber membrane with the separation pore diameter in the NF range is prepared through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 8

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of acetone, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating liquid is injected into an online coating device, the porous reinforced hollow fiber membrane vertically passes through the coating device at the speed of 3cm/s, enters a coagulating bath after passing through an air bath with the length of 90cm (the evaporation time is 30s), and a double-layer hollow fiber membrane with the separation pore diameter in the NF range is prepared through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

example 9

1) Preparing a coating solution: taking 22g of polyvinylidene fluoride, 2g of anhydrous lithium chloride, 11.4g of N-hexane, 64.6g N and N-dimethylacetamide, placing the components in a three-necked bottle, mechanically stirring for 12 hours at normal temperature, standing and defoaming for 6 hours to obtain a homogeneous coating solution;

2) preparing a coagulating bath: the coagulating bath is water;

3) preparing a double-layer hollow fiber loose nanofiltration membrane for dye desalination: the coating liquid is injected into an online coating device, the porous reinforced hollow fiber membrane vertically passes through the coating device at the speed of 3cm/s, enters a coagulating bath after passing through an air bath with the length of 90cm (the evaporation time is 30s), and a double-layer hollow fiber membrane with the separation pore diameter in the NF range is prepared through phase inversion. The obtained fiber membrane was immersed in a clear water bath for 1 day, then transferred to a 30 wt% glycerin aqueous solution for immersion for 2 days, dried at room temperature, and stored in a sealed bag.

Respectively using 50ppm rose bengal (molecular weight: 1017.64 Dalton), chrome black T (molecular weight: 461.38 Dalton), methylene blue (molecular weight: 319.85 Dalton), 1000ppm NaCl solution, Na₂SO₄The solution is tested for the performance of the double-layer hollow fiber loose nanofiltration membrane under the operation pressure of 0.2MPa, and after 2 hours of continuous test, the obtained results are as follows:

remarking: the following table lists the parameters of examples 1-9 and comparative examples 1-6 with changes in the manufacturing process to clearly show the differences.

8	Polyvinylidene fluoride (g)	Anhydrous lithium chloride (g)	Acetone (g)	N, N-Dimethylacetamide (g)	Air gap (cm)
						Examples	22	2	11.4	64.6	90

9	Polyvinylidene fluoride (g)	Anhydrous lithium chloride (g)	N-hexane (g)	N, N-Dimethylacetamide (g)	Air gap (cm)
						Examples	22	2	11.4	64.6	90

Although the present invention has been described in connection with the accompanying drawings and the accompanying tables, the present invention is not limited thereto, and various modifications made by the method concept and technical solution of the present invention are within the scope of the present invention.