Method for recovering alkali using modified chitosan anion exchange membrane

Abstract

This invention provides a method for recovering alkali using a modified chitosan anion exchange membrane, comprising the following steps: Step S1: Alternatingly arranging several cation exchange membranes and modified chitosan anion exchange membranes in an electrodialysis device, with partitions placed between adjacent cation exchange membranes and modified chitosan anion exchange membranes, forming a concentration chamber and a desalination chamber; Step S2: Circulating sodium sulfate solution into the anode chamber and cathode chamber respectively, circulating sodium nitrate solution into the concentration chamber, and circulating alkaline industrial wastewater to be recovered into the desalination chamber; Step S3: Starting the device in constant current mode, with temperature controlled throughout the electrodialysis process. This method for recovering alkali eliminates the need for adding acidic neutralizing agents, flocculants, precipitants, or other chemical agents, thus eliminating the cost of reagent consumption at the source. The recovery method of this application exhibits high selectivity for hydroxide ions, achieving high-selectivity alkali recovery by separating alkaline substances from the waste liquid, without generating harmful gases, sludge, or other byproducts throughout the process.

Description

Technical Field

[0001] This invention belongs to the field of alkaline wastewater separation and recycling technology, and specifically refers to a method for recovering alkali using a modified chitosan anion exchange membrane. Background Technology

[0002] Alkali, a key raw material in modern industry, plays an irreplaceable role in core sectors such as metallurgy, textiles, electroplating, and papermaking. However, the large-scale discharge and improper treatment of alkaline wastewater not only cause serious waste of alkali resources but also trigger a series of environmental problems. In industries such as tungsten smelting and textile printing and dyeing, the complex composition of tungsten ore leaching wastewater and mercerizing dilute alkali wastewater poses a significant challenge to the efficient and selective recovery of alkali.

[0003] Traditional alkaline treatment technologies include acid-base neutralization, electrolysis, biological methods, crystallization, flocculation, and chemical precipitation. Each method has significant drawbacks, exhibiting overall problems such as high cost, high risk of secondary pollution, poor adaptability, and low resource utilization, as detailed below:

[0004] 1. Problems with reagent and equipment consumption: Acid-base neutralization, flocculation, and chemical precipitation methods require the addition of large amounts of chemical reagents, resulting in high reagent costs.

[0005] 2. Secondary pollution and process complexity issues: Electrolysis generates harmful gases during the process, requiring additional specialized gas treatment procedures, which increases process complexity and equipment investment; flocculation produces a large amount of sludge, which requires special subsequent disposal and is prone to secondary pollution; the chemical residues from acid-base neutralization may also have a secondary impact on water bodies, making it less environmentally friendly.

[0006] 3. Limitations in treatment stability and applicability: Biological methods rely on microbial activity and are easily affected by environmental factors such as temperature and water quality, resulting in poor treatment stability. They also require the construction of a large number of structures, occupying a large area and incurring high maintenance costs. Crystallization methods are only applicable to high-concentration alkaline waste liquids, limiting their applicability. Furthermore, evaporation and crystallization consume extremely high energy, making it difficult to promote on a large scale.

[0007] 4. Low resource utilization and overall benefits: Most traditional methods take "achieving emission standards" as their core objective, only completing the purification of waste liquid and failing to recover alkali resources, resulting in resource waste; moreover, most methods focus on environmental protection effects and ignore the economic value of resource recovery, resulting in poor overall benefits.

[0008] Therefore, there is an urgent need to develop an alkali recovery method that is low-cost, has a simplified process, provides stable treatment results, and is highly adaptable. Summary of the Invention

[0009] To solve the above-mentioned technical problems, the present invention provides a method for recovering alkali using a modified chitosan anion exchange membrane, comprising the following steps:

[0010] Step S1: Several cation exchange membranes and modified chitosan anion exchange membranes are alternately arranged in the electrodialysis device. A partition is placed between adjacent cation exchange membranes and modified chitosan anion exchange membranes, and the alternating arrangement forms a concentration chamber and a desalination chamber. Several cation exchange membranes and modified chitosan anion exchange membranes form a membrane stack. An anode plate and a cathode plate are respectively set at both ends of the membrane stack to form an independent anode chamber and a cathode chamber, thereby completing the assembly of the electrodialysis membrane stack.

[0011] Furthermore, the anode plate uses a corrosion-resistant titanium electrode with a surface coating of noble metal oxides.

[0012] Furthermore, a cation exchange membrane, a desalination chamber partition, a modified chitosan anion exchange membrane, and a concentration chamber partition are stacked together to form a membrane pair. A membrane pair contains a desalination chamber and a concentration chamber. Multiple membrane pairs are arranged together to form a membrane stack, forming alternating concentration chambers and desalination chambers.

[0013] Step S2: Sodium sulfate solution is circulated into the anode and cathode chambers respectively, sodium nitrate solution is circulated into the concentration chamber, and alkaline industrial wastewater to be recovered is circulated into the desalination chamber. Sodium sulfate and sodium nitrate solutions provide good conductivity, and the circulation pumping ensures uniform mixing of the solutions and reduces concentration polarization.

[0014] Step S3: Start the device in constant current mode and control the temperature throughout the electrodialysis process.

[0015] The modified chitosan anion exchange membrane is prepared by casting a chitosan solution with a mass concentration of 4-5% to obtain a pure membrane, and then immersing the pure membrane in a sodium tetrahydroxycubic acid solution for cross-linking.

[0016] Furthermore, after starting the device in step S3, the anions in the drive system migrate in a directional manner; with the help of the high selectivity of the modified chitosan anion exchange membrane for hydroxide ions, the hydroxide ions preferentially permeate through the membrane into the concentration chamber, thereby achieving selective separation and recovery of alkali in the concentration chamber.

[0017] Furthermore, the cation exchange membrane is a strong alkali-resistant cation exchange membrane; the separator is a strong alkali-resistant polymer material; the separator thickness is the membrane spacing, ranging from 0.8 to 1.2 mm.

[0018] Furthermore, the concentration of sodium sulfate solution in the electrode chamber is 0.3-0.5 mol / L; the concentration of sodium nitrate solution in the concentration chamber is 0.01-0.1 mol / L; the concentration of alkaline wastewater to be treated in the desalination chamber is 0.1-3 mol / L; and the circulation linear velocity is 8-12 cm / s.

[0019] Furthermore, the current density during the electrodialysis process in step S3 is controlled at 60 mA / cm². 2The temperature should be controlled between 25-35℃.

[0020] Furthermore, a plate heat exchanger resistant to strong alkali is used to control the system temperature range within 25-35℃.

[0021] Furthermore, the preparation method of the modified chitosan anion exchange membrane includes the following steps:

[0022] (1) Dissolve chitosan in a weak acid solution and stir until dissolved. The mass concentration of the chitosan solution is 4-5%. After sealing and standing to remove foam, the chitosan solution is prepared.

[0023] (2) The chitosan solution prepared in (1) is cast onto a glass plate, spread evenly, and then dried to form a pure chitosan film.

[0024] Furthermore, in step (2), the chitosan solution is spread evenly using an adjustable wet film coater, placed in a constant temperature and humidity test chamber to dry and form, and sealed and stored after drying to obtain a pure chitosan film.

[0025] (3) Add copper sulfate solution to excess sodium hydroxide solution. The volume ratio of copper sulfate solution to sodium hydroxide solution is 1:4-1:5. Stir at constant temperature to allow the two to react fully. After the reaction is completed, seal and let stand to separate the layers. Take the supernatant, which is sodium tetrahydroxycubic acid solution.

[0026] (4) The pure chitosan membrane prepared in (2) was immersed in the sodium tetrahydroxycubic acid solution prepared in (3) for 4-6 days to complete cross-linking. Then, the unreacted substances remaining on the membrane surface were removed with deionized water, and the membrane was placed in a vacuum drying oven for constant temperature drying to finally obtain the modified chitosan anion exchange membrane.

[0027] Furthermore, in step S3 of the electrodialysis process, the amount of copper ions dissolved from the modified chitosan anion exchange membrane in the desalination chamber effluent must be monitored. When the amount of copper ions dissolved reaches 20%-30% of the initial copper ion content per unit area, the process is stopped and the membrane is replaced.

[0028] Furthermore, samples were taken every 4-8 hours to monitor the amount of copper ions dissolved per unit area of ​​the modified chitosan anion exchange membrane.

[0029] Furthermore, in step (1), the mass concentration of the chitosan solution is 4-5%; the dissolution temperature is 25-30℃; the stirring time is 4-6h; and the standing defoaming time is 1-1.5 days.

[0030] Furthermore, the weak acid is selected from one of acetic acid, formic acid, citric acid, and lactic acid.

[0031] Furthermore, in step (2), the thickness of the chitosan solution spread on the glass plate is 1.2-1.5 mm; the drying temperature is 23-27℃ and the humidity is 10-15%; the thickness of the pure chitosan film after drying is 64-66 μm.

[0032] Furthermore, in step (3), the molar concentration of copper sulfate solution is 0.7-1 mol / L; the molar concentration of sodium hydroxide solution is 10-12 mol / L; the volume ratio of copper sulfate solution to sodium hydroxide solution is 1:4-1:5; the reaction temperature range is 25-30℃; the stirring time is 10-30s; and the standing time is 24-30h.

[0033] Furthermore, the cross-linking reaction temperature range in step (4) is 25-30℃; the cross-linked chitosan membrane is rinsed with deionized water for 1-2 minutes and then dried in a vacuum drying oven at 23-27℃ for 4-5 hours to obtain the modified chitosan anion exchange membrane.

[0034] Compared with the prior art, the present invention has the following outstanding features and advantages:

[0035] 1. The method for recovering alkali using a modified chitosan anion exchange membrane in this application does not require the addition of acidic neutralizing agents, flocculants, precipitants, or other chemical agents, thus eliminating the cost of agent consumption at the source. This application has high selectivity for hydroxide ions, achieving high selective recovery of alkali by separating alkaline substances from waste liquid, with no harmful gases, sludge, or other byproducts generated throughout the process.

[0036] 2. This application relies on the directional migration characteristics of ions in electrodialysis to achieve only the separation and recovery of alkaline substances and waste liquid, without generating harmful gases, sludge or other byproducts throughout the process, thus eliminating the risk of secondary pollution from the source; there is no need to add an additional byproduct treatment process, the process flow is simple, and the investment in equipment infrastructure and operation and maintenance is reduced.

[0037] 3. The electrodialysis equipment of this application is less affected by temperature and water quality fluctuations, and the treatment effect is consistently stable; the device has a compact structure, requiring no complex structures, thus reducing infrastructure and maintenance costs; it can be adapted to alkaline waste liquids of low, medium and high concentrations, breaking the concentration limitations of traditional crystallization methods, and its energy consumption is significantly reduced compared to evaporation crystallization, making it suitable for large-scale promotion.

[0038] 4. While completing the treatment of alkaline waste liquid to meet the standards, this application can efficiently concentrate and recover the alkali resources in the waste liquid, realize the recycling of alkali, and upgrade "end-of-pipe treatment" to "resource utilization". It not only solves the pollution problem, but also reduces the enterprise's alkali raw material procurement cost, and has significant ecological and environmental protection value and economic value. Its comprehensive benefits far exceed those of traditional single purification technology. Attached Figure Description

[0039] Figure 1This is a schematic diagram of the device for recovering alkali using a modified chitosan anion exchange membrane according to the present invention;

[0040] Figure 2 The OH groups of the chitosan membrane before and after copper ion crosslinking in Example 2 were measured in the electrodialysis test. - and A comparison chart of separation performance;

[0041] Figure 3 This is a diagram showing the effect of the modified chitosan anion exchange membrane in Example 3 on the electrodialysis alkali recovery test of mercerized dilute alkaline wastewater;

[0042] Wherein: 1-Anode plate, 2-Anode chamber, 3-First alkali-resistant commercial cation exchange membrane, 4-Concentration chamber, 5-Modified chitosan anion exchange membrane, 6-Desalination chamber, 7-Second alkali-resistant commercial cation exchange membrane, 8-Cathode chamber, 9-Cathode plate. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The technical solutions of the present invention will be further described below with reference to implementation examples.

[0044] In the following examples, the first alkali-resistant commercial cation exchange membrane 3 and the second alkali-resistant commercial cation exchange membrane 7 are both commercial Fumasep FKS-30 cation exchange membranes.

[0045] Example 1

[0046] This embodiment provides a method for preparing a modified chitosan anion exchange membrane, comprising the following steps:

[0047] (1) Dissolve 10g of chitosan solution in 190g of 4wt% dilute acetic acid solution, stir continuously at 25℃ for 4h, let the chitosan solution stand to defoam for 1.5 days, and then seal and store for later use.

[0048] (2) The chitosan solution prepared in (1) is uniformly cast onto a clean glass plate, and the solution is spread to a thickness of 1.5 mm using a scraper. The glass plate is placed in a constant temperature and humidity test chamber at 25°C and 15% humidity for 24 hours to dry. After the film is formed, it is peeled off from the glass plate and sealed for preservation to obtain a pure chitosan film (denoted as CS).

[0049] (3) Take 20 mL of 0.78 mol / L sodium sulfate solution and slowly add it to 80 mL of 12 mol / L sodium hydroxide solution while stirring continuously for 19 s. The entire reaction process is carried out at 25 °C. After the reaction is completed, let the solution stand for 26 h and take the supernatant liquid, which is the sodium tetrahydroxycubic acid crosslinking solution. Seal and store it for later use.

[0050] (4) The pure chitosan membrane prepared in (2) was immersed in the sodium tetrahydroxycubic acid solution synthesized in (3), and the immersion time was controlled to be 2, 4 and 6 days respectively to obtain modified chitosan anion exchange membranes with different degrees of crosslinking (denoted as CS-Cu2d, CS-Cu4d and CS-Cu6d respectively). After immersion, the membrane surface was rinsed with deionized water for 2 min to remove residual unreacted substances. The membrane was placed in a vacuum drying oven at 26℃ for 4 h. After the membrane was completely dried, the modified chitosan anion exchange membrane was obtained. According to the different crosslinking times, the copper ion content per unit membrane area of ​​CS-Cu2d, CS-Cu4d and CS-Cu6d was 0.53 mg / cm³. 2 0.64 mg / cm 2 and 0.72 mg / cm 2 (The modified chitosan anion exchange membrane was immersed in 50 mL of 0.04 wt% nitric acid solution. After complete dissolution, the copper ion content in the dilute nitric acid solution containing copper ions was measured using an inductively coupled plasma atomic emission spectrometer (Prodigy Plus, Lehman Brothers, USA). The ratio of the copper ion content to the effective area of ​​the modified chitosan anion exchange membrane determined that the copper ion content per unit area of ​​the modified chitosan anion exchange membrane was 0.53 mg / cm².) 2 0.64 mg / cm 2 and 0.72 mg / cm 2 ).

[0051] Example 2

[0052] This embodiment provides a method for recovering alkali using a chitosan ion exchange membrane, including the following steps:

[0053] Step S1: In the electrodialysis apparatus, a first alkali-resistant commercial cation exchange membrane 3, a chitosan anion exchange membrane (one of the pure chitosan membrane CS prepared in Example 1, and CS-Cu2d, CS-Cu4d, and CS-Cu6d modified chitosan anion exchange membranes 5) and a second alkali-resistant commercial cation exchange membrane 7 are arranged sequentially from the anode plate 1 to the cathode plate 9. Figure 1 As shown, the effective membrane area is 1.77 cm². 2The membrane spacing is 1.0 mm. A partition (not shown in the figure) is placed between the adjacent first alkali-resistant commercial cation exchange membrane 3 and the chitosan anion exchange membrane, and a partition (not shown in the figure) is also placed between the chitosan anion exchange membrane and the second alkali-resistant commercial cation exchange membrane 7. The first alkali-resistant commercial cation exchange membrane 3 and the chitosan anion exchange membrane form a concentration chamber 4, and the chitosan anion exchange membrane and the second alkali-resistant commercial cation exchange membrane 7 form a desalination chamber 6. An anode plate 1 and a cathode plate 9 are respectively set at both ends of the membrane stack to form independent anode chambers 2 and cathode chambers 8, thereby completing the assembly of the electrodialysis membrane stack.

[0054] Step S2: Pump 100 mL of 0.3 mol / L sodium sulfate solution into anode chamber 2 and cathode chamber 8; pump 100 mL of 0.1 mol / L sodium nitrate solution into concentration chamber 4; pump 100 mL of a mixed solution of 0.1 mol / L sodium hydroxide and sodium tungstate as tungsten ore alkaline leaching wastewater (prepared by adding 40 g of sodium hydroxide and 32.985 g of sodium tungstate to one liter of pure water) into desalination chamber 6. The circulation linear velocity of each pump is 10 cm / s.

[0055] Step S3: After the power is turned on, run at a constant current of 5 mA·cm -2 The electrodialysis experiment was conducted at a constant current density for 30 minutes at a constant temperature of 25°C. After the experiment, the OH- concentration was determined by acid-base titration with a standard hydrochloric acid solution. - The concentration was determined by inductively coupled plasma atomic emission spectrometry. The concentration.

[0056] like Figure 2 The results showed that pure chitosan membranes were effective against OH-. - and The selectivity of the modified chitosan anion exchange membrane was only 13, while the selectivities of the modified chitosan anion exchange membranes CS-Cu2d, CS-Cu4d, and CS-Cu6d were 64, 98, and 237, respectively, indicating that the modified chitosan anion exchange membrane CS-Cu6d has excellent and stable selective separation performance of hydroxide ions. The copper ion content per unit area of ​​CS-Cu2d is 26% lower than that of CS-Cu6d, but the selective separation performance is 73% lower. Therefore, the function of the membrane can be monitored by monitoring the amount of copper ions dissolved per unit area of ​​the modified chitosan anion exchange membrane.

[0057] OH - and Selective testing: ① The procedure for determining ion flux is as follows: After the electrodialysis experiment, the OH- ions can be obtained by titration with a 0.01 mol / L hydrochloric acid standard solution using an acid-base titration method. -Concentration. Determined by inductively coupled plasma atomic emission spectrometry. The concentration of OH. - and The ion flux can be calculated using equation (1):

[0058] Equation (1);

[0059] In the formula: J i—— OH in the membrane - and Ion transport rate (mol m) -2 h -1 );

[0060] C i—— Indicates OH in the concentration chamber - and Concentration (M);

[0061] A— This represents the effective area of ​​the membrane during the testing process;

[0062] V— This represents the volume of the mixture added to the concentration chamber.

[0063] ② OH in the membrane - and The selectivity can be calculated by the following formula (2).

[0064] Equation (2);

[0065] In the formula: — OH in the membrane - Compared to ion selectivity;

[0066] —OH in the desalination chamber - Concentration (M);

[0067] —in the desalination chamber The concentration (M).

[0068] Example 3

[0069] This embodiment provides a method for recovering alkali using the CS-Cu4d and CS-Cu6d modified chitosan anion exchange membranes prepared in Example 1, comprising the following steps:

[0070] Step S1: In the electrodialysis apparatus, a first alkali-resistant commercial cation exchange membrane 3, a modified chitosan anion exchange membrane 5 (one of the CS-Cu4d and CS-Cu6d modified chitosan anion exchange membranes prepared in Example 1), and a second alkali-resistant commercial cation exchange membrane 7 are arranged sequentially from the anode plate 1 to the cathode plate 9, as follows: Figure 1 As shown, the effective membrane area is 7.08 cm². 2 The membrane spacing is 1.0 mm. A partition (not shown in the figure) is placed between the adjacent first alkali-resistant commercial cation exchange membrane 3 and the modified chitosan anion exchange membrane 5, and a partition (not shown in the figure) is also placed between the modified chitosan anion exchange membrane 5 and the second alkali-resistant commercial cation exchange membrane 7. The first alkali-resistant commercial cation exchange membrane 3 and the modified chitosan anion exchange membrane 5 form a concentration chamber 4, and the modified chitosan anion exchange membrane 5 and the second alkali-resistant commercial cation exchange membrane 7 form a desalination chamber 6. An anode plate 1 and a cathode plate 9 are respectively set at both ends of the membrane stack to form an independent anode chamber 2 and a cathode chamber 8, thereby completing the assembly of the electrodialysis membrane stack.

[0071] Step S2: 200 mL of 0.5 mol / L sodium sulfate solution is pumped into anode chamber 2 and cathode chamber 8; 150 mL of 0.1 mol / L sodium nitrate solution is pumped into concentration chamber 4; 200 mL of pretreated mercerizing dilute alkaline wastewater solution is pumped into desalination chamber 6. The circulation linear velocity of the pumps is 10 cm / s. (The mercerizing dilute alkaline wastewater from a certain enterprise is first allowed to stand for oil separation, then pumped through activated carbon for further removal of dissolved organic matter, pigments, and surfactants to reduce COD. Finally, an ultrafiltration device is used for terminal fine filtration. The suspended particulate matter in the wastewater is <1 mg / L, turbidity is <0.3 NTU, COD is <124 mg / L, and sodium hydroxide concentration is 1.5 mol / L.)

[0072] Step S3: After the power is turned on, maintain a constant current of 36 mA·cm⁻¹ -2 The current density was maintained during the electrodialysis test, which lasted 180 minutes, with samples taken every 30 minutes. The temperature was kept constant at 25°C. The method for testing the alkali concentration was the same as in Example 2. The results are as follows: Figure 3 As shown, the modified chitosan anion exchange membrane 5 can efficiently enrich and recover alkali from actual wastewater. During 180 minutes of operation, the amount of copper ions dissolved in the desalination chamber outlet liquid was far lower than 20% of the copper ion content per unit area of ​​the membrane in its initial state, indicating that the membrane can operate stably with structural integrity in complex wastewater.

[0073] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for recovering alkali using a modified chitosan anion exchange membrane, characterized in that, Includes the following steps: Step S1: Several cation exchange membranes and modified chitosan anion exchange membranes are alternately arranged in the electrodialysis device. A partition is placed between adjacent cation exchange membranes and modified chitosan anion exchange membranes, and the alternating arrangement forms a concentration chamber and a desalination chamber. Several cation exchange membranes and modified chitosan anion exchange membranes form a membrane stack. An anode plate and a cathode plate are respectively set at both ends of the membrane stack to form an independent anode chamber and a cathode chamber, thereby completing the assembly of the electrodialysis membrane stack. Step S2: Sodium sulfate solution is pumped into the anode chamber and cathode chamber respectively, sodium nitrate solution is pumped into the concentration chamber, and alkaline industrial wastewater to be recovered is pumped into the desalination chamber. Step S3: Start the device in constant current mode, and control the temperature throughout the electrodialysis process; The modified chitosan anion exchange membrane is prepared by casting a chitosan solution with a mass concentration of 4-5% to obtain a pure membrane, and then immersing the pure membrane in a sodium tetrahydroxycubic acid solution for cross-linking.

2. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 1, characterized in that, The cation exchange membrane is a strong alkali resistant cation exchange membrane; the separator is a strong alkali resistant polymer material; the thickness of the separator is the membrane spacing, which ranges from 0.8 to 1.2 mm.

3. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 1, characterized in that, The concentration of sodium sulfate solution in the electrode chamber is 0.3-0.5 mol / L; the concentration of sodium nitrate solution in the concentration chamber is 0.01-0.1 mol / L; the concentration of alkaline wastewater to be treated in the desalination chamber is 0.1-3 mol / L; and the circulation linear velocity is 8-12 cm / s.

4. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 1, characterized in that, In step S3, the current density during the electrodialysis process is controlled at 60 mA / cm². 2 The temperature should be controlled between 25-35℃.

5. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 1, characterized in that, The preparation method of the modified chitosan anion exchange membrane includes the following steps: (1) Dissolve chitosan in a weak acid solution and stir until dissolved. The mass concentration of the chitosan solution is 4-5%. After sealing and standing to remove foam, the solution is ready for use. (2) The chitosan solution prepared in (1) is cast onto a glass plate, spread evenly, and then dried to form a pure chitosan film. (3) Add copper sulfate solution to excess sodium hydroxide solution, with a volume ratio of copper sulfate solution to sodium hydroxide solution of 1:4-1:

5. Stir at constant temperature to allow the two to react fully. After the reaction is complete, seal and let stand to separate the layers. Take the supernatant, which is sodium tetrahydroxycubic acid solution. (4) The pure chitosan membrane prepared in (2) was immersed in the sodium tetrahydroxycubic acid solution prepared in (3) for 4-6 days to complete cross-linking. Then, the unreacted substances remaining on the membrane surface were removed with deionized water, and the membrane was placed in a vacuum drying oven for constant temperature drying to finally obtain the modified chitosan anion exchange membrane.

6. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 5, characterized in that, In step S3, the electrodialysis process also requires monitoring the amount of copper ions dissolved from the modified chitosan anion exchange membrane in the desalination chamber effluent. When the amount of copper ions dissolved reaches 20%-30% of the initial copper ion content per unit area, the process is stopped and the membrane is replaced.

7. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 5, characterized in that, In step (1), the mass concentration of the chitosan solution is 4-5%; the dissolution temperature is 25-30℃; the stirring time is 4-6h; and the standing defoaming time is 1-1.5 days.

8. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 5, characterized in that, In step (2), the thickness of the chitosan solution spread on the glass plate is 1.2-1.5 mm; the drying temperature is 23-27℃ and the humidity is 10-15%; the thickness of the pure chitosan film after drying is 64-66 μm.

9. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 5, characterized in that, In step (3), the molar concentration of copper sulfate solution is 0.7-1 mol / L; the molar concentration of sodium hydroxide solution is 10-12 mol / L; the volume ratio of copper sulfate solution to sodium hydroxide solution is 1:4-1:5; the reaction temperature range is 25-30℃; the stirring time is 10-30s; and the standing time is 24-30h.

10. The method for recovering alkali using a modified chitosan anion exchange membrane according to claim 5, characterized in that, Step (4) The cross-linking reaction temperature range is 25-30℃; the cross-linked chitosan membrane is rinsed with deionized water for 1-2 minutes and then dried in a vacuum drying oven at 23-27℃ for 4-5 hours to obtain the modified chitosan anion exchange membrane.

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