A method for preparing new membrane using end-of-life membrane

Abstract

The present invention provides a method for preparing a new membrane using an end-of-life membrane, belonging to the field of membrane preparation technology. The method comprises the following steps: (1) pre-cleaning the end-of-life microfiltration / ultrafiltration membrane to remove large contaminants on the membrane surface and in the membrane pores; (2) cutting the pre-cleaned end-of-life microfiltration / ultrafiltration membrane into small pieces / segments and drying them; (3) after drying, mixing the membrane fiber pieces / segments, a green solvent, and a porogen in a certain proportion at a certain temperature, using a non-woven fabric as a substrate, and preparing a new microfiltration / ultrafiltration membrane by a phase inversion method. The present invention links the treatment and disposal of the end-of-life membrane with the green preparation of a new membrane. It innovatively dissolves the end-of-life membrane in a green solvent, and can controllably prepare a new microfiltration / ultrafiltration membrane by a phase inversion method, thereby achieving effective recycling of the end-of-life membrane and significantly improving the sustainability of membrane separation technology.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane preparation, and in particular to a method for recovering a membrane at the end of its life by adopting green dissolution to prepare a new membrane. Background Art

[0002] Membrane separation technology boasts advantages such as high separation efficiency, no chemical addition, high equipment integration, and ease of automated control. It is increasingly being used in water treatment applications such as wastewater treatment and reuse, water treatment, and seawater desalination. However, the sustainability of commercial separation membranes is currently poor. After reaching the end of their lifespan (typically 5-7 years), they must be replaced to maintain the normal operation of the water treatment system, resulting in a large amount of membrane waste.

[0003] Currently, end-of-life membranes are mostly disposed of by landfill or incineration, which inevitably has long-term negative environmental impacts. Proper disposal of end-of-life membranes is a key and challenging issue in improving the sustainability of membrane separation technology.

[0004] In recent years, academia and industry have begun exploring the recycling of end-of-life membranes, developing a range of membrane regeneration technologies, including flat-level regeneration, downgrading, and upgrading. However, these technologies are only applicable to end-of-life membranes without significant physical damage. For membranes that have sustained significant physical damage, while the polymer components of the membranes themselves may still be worth recycling, traditional membrane regeneration methods are no longer applicable. Therefore, new technologies are urgently needed to recycle these end-of-life membranes.

[0005] One of the mainstream methods for preparing traditional (polymer) membrane materials is the phase inversion method. This method involves dissolving an organic polymer powder (such as polyvinylidene fluoride) and a porogen in an organic solvent, followed by a phase inversion in water, to produce a separation membrane with microfiltration / ultrafiltration precision. However, the organic polymer powder used in this process often originates from the petrochemical industry, resulting in high carbon emissions and environmental concerns. Furthermore, the organic solvents used are biotoxic, posing potential health and environmental risks. Summary of the Invention

[0006] To address the aforementioned issues with existing technologies, the present invention provides a method for preparing new membranes from end-of-life membranes. This method links the treatment and disposal of end-of-life membranes with the green preparation of new membranes. Innovatively, the end-of-life membranes are dissolved in green solvents, and new microfiltration / ultrafiltration membranes can be prepared through a phase inversion process. This method effectively recycles end-of-life membranes and significantly enhances the sustainability of membrane separation technology.

[0007] The technical solutions of the present invention are as follows:

[0008] A method for preparing a new membrane using an end-of-life membrane comprises the following steps:

[0009] (1) Pre-cleaning of microfiltration / ultrafiltration membranes at the end of their life: Pre-clean the microfiltration / ultrafiltration membranes that have reached the end of their life to remove large contaminants on the membrane surface and in the membrane pores;

[0010] (2) Pretreatment of the end-of-life microfiltration / ultrafiltration membrane: Cut the pre-cleaned end-of-life microfiltration / ultrafiltration membrane into small pieces or segments to obtain membrane blocks / segments, and place them in an oven until completely dried;

[0011] (3) Preparation of new membrane: The dried membrane filament blocks / segments, green solvent and porogen are mixed evenly, allowed to stand for degassing, and then a new microfiltration / ultrafiltration membrane is prepared using a non-woven fabric as a substrate by a phase inversion method.

[0012] Furthermore, the microfiltration / ultrafiltration membrane that has reached the end of its life refers to a microfiltration / ultrafiltration membrane that has, after 5-7 years of long-term operation, had its membrane flux dropped to a low level and could not meet the water production needs of the project. It is difficult to recover through conventional chemical cleaning and is severely damaged, making it difficult to apply traditional membrane regeneration methods.

[0013] Furthermore, the component form of the microfiltration / ultrafiltration membrane includes but is not limited to a flat membrane, a hollow fiber membrane or a spiral membrane.

[0014] Furthermore, the material of the microfiltration / ultrafiltration membrane includes but is not limited to polyvinylidene fluoride, polyethersulfone, polysulfone, sulfonated polyethersulfone, sulfonated polysulfone, polyacrylonitrile, polypropylene or polytetrafluoroethylene.

[0015] Preferably, the pre-cleaning method in step (1) is: first soaking and cleaning with a sodium hypochlorite solution with a concentration of 0.5-1.0wt% for 1.0-2.0h, and then soaking and cleaning with a citric acid solution with a concentration of 0.5-3.0wt% for 1.0-2.0h.

[0016] Preferably, the area of ​​the small piece in step (2) is 1-4 cm 2 ; The length of the small segment is 1-3 cm.

[0017] Preferably, the green solvent in step (3) is any one of 5-(dimethylamino)-2-methyl-5-oxopentanoic acid methyl ester, isosorbide dimethyl ether, dihydro-levulinone or triethyl phosphate.

[0018] Preferably, the porogen in step (3) is one of polyvinyl pyrrolidone, polyethylene glycol or lithium chloride.

[0019] Preferably, the mass ratio of the membrane filament block / segment, green solvent and porogen in step (3) is 12-24:71-85:3-5.

[0020] Preferably, the temperature during the mixing process in step (3) is 100-200°C.

[0021] The beneficial technical effects of the present invention are:

[0022] 1. The present invention proposes to link the treatment and disposal of end-of-life membranes with the green preparation of new membranes. It innovatively dissolves the end-of-life membranes based on green solvents, uses the pollutants in the end-of-life membranes as in-situ additives, couples the addition of porogens, and then uses the phase inversion method to controllably prepare new microfiltration / ultrafiltration membranes, which are suitable for large-scale production.

[0023] 2. The present invention realizes the effective recycling of end-of-life membranes, replacing traditional polymer powders to prepare new membranes. It not only reduces the generation of difficult-to-degrade polymer solid waste / effectively reduces the negative environmental impact of membrane technology, but also reduces the use of petroleum-based polymer materials, thereby significantly improving the sustainability of membrane separation technology.

[0024] 3. The present invention provides a new idea for the preparation of green membrane materials. The end-of-life membrane produced during water treatment is used to replace traditional petroleum-based polymer powder. At the same time, environmentally friendly green solvents are used to replace traditional highly toxic organic solvents to prepare new membranes, thereby realizing the recycling of membrane materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the inorganic contamination content of the membrane after pre-cleaning obtained in Example 1.

[0026] Figure 2 This is a three-dimensional fluorescence image of the pre-cleaned membrane obtained in Example 1.

[0027] Figure 3 This is a scanning electron microscope characterization image of the new microfiltration membrane obtained in Example 2.

[0028] Figure 4 This is a scanning electron microscope characterization image of the new ultrafiltration membrane obtained in Example 3.

[0029] Figure 5 The pore size distribution diagram of the fresh microfiltration and ultrafiltration membranes obtained in Examples 2 and 3.

[0030] Figure 6 This is the pore size distribution diagram of the microfiltration membrane obtained in Comparative Example 1. DETAILED DESCRIPTION

[0031] The present invention is described in detail below with reference to the accompanying drawings and embodiments. It is apparent that the embodiments described are only a portion of the embodiments of the present invention, rather than all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0032] Example 1:

[0033] End-of-life polyvinylidene fluoride hollow fiber membrane fibers were recovered from a sewage treatment plant. They were taken from ultrafiltration membrane modules that had been in operation for 7 years. The membrane flux had dropped to below 30LMH / bar, unable to meet the water production needs of the project. Conventional chemical cleaning (combination cleaning with sodium hypochlorite and citric acid) was difficult to restore, and obvious fiber breakage occurred.

[0034] The end-of-life membrane was soaked and cleaned with a 1.0 wt% sodium hypochlorite solution for 2.0 h, and then soaked and cleaned with a 3.0 wt% citric acid solution for 2.0 h to remove large contaminants on the membrane surface and in the membrane pores.

[0035] The surface element analysis and three-dimensional fluorescence characterization of the pre-cleaned membrane obtained in Example 1 showed that after pre-cleaning, inorganic pollutants such as Al and Ca still existed on the membrane surface ( Figure 1 ) and organic pollutants such as humic acid and protein ( Figure 2 Inorganic pollutants such as Al and Ca can serve as natural inorganic additives in the membrane preparation process, improving the mechanical strength and stability of the membrane; while organic pollutants such as humic acid and protein can improve the hydrophilicity of the membrane and enhance its anti-pollution ability. In other words, the presence of these two types of pollutants can, to a certain extent, assist in the preparation of new membranes with excellent performance.

[0036] Example 2:

[0037] The end-of-life membrane pre-cleaned in Example 1 was cut into 2 cm segments and dried in a 40°C oven. After drying, the membrane segments, dimethyl isosorbide, and polyvinyl pyrrolidone were mixed in a mass ratio of 12:83:5, magnetically stirred at 140°C for 12 hours, and allowed to stand for 12 hours to degas, thereby obtaining a casting solution.

[0038] The non-woven fabric support layer is then placed on a clean glass plate, and a scraper is pulled at a constant speed to evenly coat the casting liquid on the non-woven fabric to form a flat liquid membrane; the glass plate with the liquid membrane attached is quickly immersed in a non-solvent gel bath (deionized water), and after a phase inversion process, it becomes a solid membrane; finally, the membrane is immersed in deionized water for 24 hours to remove the residual organic solvent in the membrane to obtain a new microfiltration membrane.

[0039] through Figure 3 Scanning electron microscopy observations show that the new microfiltration membrane has the dense cortex and finger-like pore structure of a general microfiltration membrane.

[0040] Example 3:

[0041] The end-of-life membrane described in Example 1 was used and was soaked and cleaned in a 0.5 wt % sodium hypochlorite solution for 1.0 h, and then soaked and cleaned in a 0.5 wt % citric acid solution for 1.0 h to remove large contaminants on the membrane surface and in the membrane pores.

[0042] The pre-cleaned end-of-life membrane was cut into 2 cm segments and dried in a 60°C oven. After drying, the membrane segments, dihydro-levulinone, and polyvinylpyrrolidone were mixed in a mass ratio of 24:71:5, magnetically stirred at 100°C for 10 hours, and allowed to stand for 10 hours to degas. This mixture produced a casting solution.

[0043] The non-woven fabric support layer is then placed on a clean glass plate, and a scraper is pulled at a constant speed to evenly coat the casting liquid on the non-woven fabric to form a flat liquid membrane; the glass plate with the liquid membrane attached is quickly immersed in a non-solvent gel bath (deionized water), and after a phase inversion process, it becomes a solid membrane; finally, the membrane is immersed in deionized water for 24 hours to remove the residual organic solvent in the membrane to obtain a new ultrafiltration membrane.

[0044] through Figure 4 Scanning electron microscope observations show that the new ultrafiltration membrane has the dense cortex and finger-like pore structure of a general ultrafiltration membrane.

[0045] Comparative Example 1:

[0046] Polyvinylidene fluoride powder, dimethyl isosorbide, and polyvinyl pyrrolidone were mixed in a mass ratio of 12:83:5, magnetically stirred at 140°C for 12 hours, and allowed to stand for 12 hours to degas. A non-woven fabric support layer was placed on a clean glass plate, and a scraper was pulled horizontally at a constant speed to evenly coat the non-woven fabric with the casting solution, forming a flat liquid membrane. The glass plate with the liquid membrane was quickly immersed in a non-solvent gel bath (deionized water), where it transformed into a solid membrane after a phase inversion process. Finally, the membrane was immersed in deionized water for 24 hours to remove any residual organic solvent, resulting in an ultrafiltration membrane.

[0047] Test example:

[0048] The newly formed membrane sheets obtained in Example 2, Example 3 and Comparative Example 1 were placed in an ultrafiltration cup and pre-pressed for 30 minutes at a certain pressure using high-pressure nitrogen as the driving force until the membrane flux was basically stable, and then the pure water flux was measured.

[0049] The membrane flux of the newly formed microfiltration membrane and ultrafiltration membrane obtained in Examples 1 and 2 were 1509.8 L / (m 2 h bar) and 206.9L / (m 2 h bar), the microfiltration membrane flux obtained in Comparative Example 1 was 1215.5 L / (m 2 h bar).

[0050] The pore size distribution of the three new membranes was determined by using a membrane pore size analyzer ( Figure 5 and Figure 6), the average pore sizes of the new microfiltration membrane and ultrafiltration membrane prepared in Example 1 and Example 2 were 0.51 μm and 0.02 μm, respectively, and the average pore size of the membrane prepared in Comparative Example 1 was 0.18 μm. Compared with the new microfiltration membrane obtained under the same formula, the pore size distribution was relatively dispersed.

[0051] By comparison, it can be seen that under the same formula, the new membrane made from the contaminated membrane has better water permeability and more concentrated pore size distribution, thanks to the positive role played by the organic and inorganic pollutants in the contaminated membrane as in-situ additives in the pore formation process.

[0052] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the description and implementation methods. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A method for preparing a new membrane using an end-of-life membrane, characterized in that: The following steps are involved: (1) Pre-cleaning of microfiltration / ultrafiltration membranes at the end of their life: Pre-clean the microfiltration / ultrafiltration membranes that have reached the end of their life to remove large contaminants on the membrane surface and in the membrane pores; (2) Pretreatment of end-of-life microfiltration / ultrafiltration membrane: Cut the pre-cleaned end-of-life microfiltration / ultrafiltration membrane into small pieces or segments to obtain membrane blocks / segments, and place them in an oven until completely dried; (3) Preparation of new membrane: Mix the dried membrane filament blocks / segments, green solvent and porogen evenly, let them stand for degassing, and then use non-woven fabric as the substrate to prepare new microfiltration / ultrafiltration membrane by phase inversion method; The green solvent in step (3) is any one of 5-(dimethylamino)-2-methyl-5-oxopentanoate, dimethyl isosorbide, and dihydro-levulinone; The mass ratio of the membrane filament block / segment, green solvent and porogen in step (3) is 12-24:71-85:3-5.

2. The method according to claim 1, characterized in that The microfiltration / ultrafiltration membrane that has reached the end of its life refers to a microfiltration / ultrafiltration membrane that has, after 5-7 years of long-term operation, had its membrane flux dropped to a low level and could not meet the water production needs of the project. It is difficult to recover through conventional chemical cleaning and is severely damaged, making it difficult to apply traditional membrane regeneration methods.

3. The method according to claim 1, characterized in that The component forms of the microfiltration / ultrafiltration membrane include flat membrane, hollow fiber membrane or spiral membrane.

4. The method according to claim 1, wherein The material of the microfiltration / ultrafiltration membrane includes polyvinylidene fluoride, polyethersulfone, polysulfone, sulfonated polyethersulfone, sulfonated polysulfone, polyacrylonitrile, polypropylene or polytetrafluoroethylene.

5. The method according to claim 1, wherein The pre-cleaning method in step (1) is: first soaking and cleaning with a sodium hypochlorite solution with a concentration of 0.5-1.0 wt% for 1.0-2.0 h, and then soaking and cleaning with a citric acid solution with a concentration of 0.5-3.0 wt% for 1.0-2.0 h.

6. The method according to claim 1, characterized in that The area of ​​the small piece in step (2) is 1-4 cm 2 ; The length of the small segment is 1-3 cm.

7. The method according to claim 1, characterized in that The porogen in step (3) is one of polyvinyl pyrrolidone, polyethylene glycol or lithium chloride.

8. The method according to claim 1, characterized in that The temperature during the mixing process in step (3) is 100-200°C.

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