CN1508076A - Method for Mitigation and Cleaning of Membrane Fouling When Synthesizing Polymer Flocculant with Membrane Reactor - Google Patents

Abstract

The invention belongs to the technical field of flocculant synthesis for water treatment, and in particular relates to a method for slowing down and cleaning membrane fouling when a membrane reactor is used to synthesize a polymer flocculant. The feature of this method is that by reverse negative pressure backwashing, the pollution of the inner cavity of the membrane filament is reduced, the amount of hydroxide formed is significantly reduced, and the drop rate of the flow rate of the aluminum salt solution is significantly reduced, which ensures the mixing strength of the fluid in the cavity and ensures the synthesis of inorganic materials. The quality of polymer flocculant. After the synthesis, three-step cleaning with soft water-acid solution-soft water can make the hydroxide layer on the membrane surface disappear and prolong the service life of the membrane module.

Description

Method for Mitigation and Cleaning of Membrane Fouling When Synthesizing Polymer Flocculant with Membrane Reactor

technical field

The invention belongs to the technical field of flocculant synthesis for water treatment, and in particular relates to a method for slowing down and cleaning membrane fouling when synthesizing a polymer flocculant with a membrane reactor.

Background technique

Inorganic polymer flocculants are a new type of water treatment agent developed on the basis of traditional aluminum salt flocculants. This type of agent has the advantages of high efficiency and low cost and is widely used.

The process of using a membrane reactor to synthesize flocculants is to use the membrane pores in the membrane structure as a liquid micro-distributor. Driven by pressure, the reactant solution on one side of the membrane is evenly and slowly pressed into the other side of the membrane. reactant solution and react with it. Since the droplet size of the permeate is equivalent to the pore size of the membrane, the size of the sodium hydroxide droplet can be reduced to the nanometer level while ensuring a certain sodium hydroxide addition rate, thereby effectively reducing the characteristic diffusion time of sodium hydroxide , to avoid local instantaneous generation of Al(OH) ₃ precipitation. It is generally believed that polythirteen aluminum (Al ₁₃ ) is the best coagulation and flocculation component in polyaluminium, and its content can reflect the effectiveness of the product. Using a membrane reactor to synthesize polyaluminum chloride, the content of Al _b in the product can be increased to 80 More than %, and easy to enlarge, has a good prospect of industrialization. At present, there are few methods of synthesizing polyaluminium chloride by membrane reactor, as disclosed in patent application No. 99109853.6, but this application does not propose a reasonable solution to the membrane fouling in the process of synthesizing polyaluminium chloride by membrane method.

Contents of the invention

The purpose of the present invention is to reduce membrane fouling and aluminum hydroxide precipitation in the inner cavity of hollow fiber membrane filaments during the synthesis of aluminum-based inorganic polymer flocculants, to ensure the membrane surface flow rate, and to provide a membrane reactor for avoiding the reduction of instantaneous mixing and stirring intensity. A method for slowing down and cleaning membrane fouling when synthesizing a polymer flocculant, in order to use this method to increase the Al _b content in the polyaluminum chloride inorganic polymer flocculant solution on the one hand, on the other hand to increase the degree of alkalization, and to increase the performance of the inorganic flocculant ; and after the reaction, by cleaning with acid solution, the flux of the membrane filament can be restored and the life of the membrane module can be extended.

The principle of the technical solution of the present invention is: in the process of synthesizing the polymer inorganic flocculant by using a membrane reactor, as the reaction proceeds, the viscosity and alkalization degree of the aluminum salt solution increase, and at the moment the alkali is added, because OH ^- If it cannot be diffused rapidly, local high alkalinity will inevitably occur, which will greatly increase the chance of contact between Al ³⁺ and OH ^- , and the number and speed of crystal nuclei forming hydroxide will increase. If under the condition of low alkalization degree, through vigorous stirring, the crystal nuclei of the hydroxide formed in the aluminum salt solution are less, and the newly formed polymer form will gradually transform into a stable polymer without forming hydroxide precipitation. If under the condition of high alkalinity, the local high alkalinity will last for a longer time, increasing the number of hydroxide crystal nuclei, and the generated polymer will be converted into hydroxide precipitates, and these crystals will be adsorbed on the inner wall of the membrane. , Gradually aging and growing up, blocking the membrane lumen. The present invention is to change the flow state of the fluid in the cavity by negative pressure to change the direction of feeding, so as to enhance the mixing intensity. This scheme uses negative pressure, because sodium hydroxide flows from the outer surface of the membrane filament to the inner surface. If positive pressure is used to reverse the direction, it is possible to make the aluminum salt solution and sodium hydroxide solution contact and react in the membrane channel, and the generated hydrogen The alumina crystal nuclei may be adsorbed in the membrane pores and block the membrane pores, resulting in a decrease in the membrane flux and causing irreversible pollution of the flux.

The aluminum hydroxide precipitate, which is generally metal, is formed on the inner surface of the membrane filament, which is dissolved in a strong acid solution. Therefore, after the synthesis is completed, it is dissolved from the membrane surface by using a strong acid to restore the membrane flux and the inner diameter of the membrane filament.

The equipment used in the present invention utilizes existing combined equipment for preparing inorganic polymer flocculants, and the components are connected by pipelines, as shown in Figure 1 . The method to slow down and clean membrane fouling when synthesizing polymer flocculant with membrane reactor is:

1. Back flushing during synthesis:

In the process of synthesizing the polymer inorganic flocculant, the three-way valve 4-1 is in the positive state, the aluminum salt solution in the reaction tank 1-1 flows from the branch pipe W to the branch pipe U, the three-way valve 4-6 is in the positive state, and the aluminum salt solution From the branch pipe M to the branch pipe T, the three-way valve 4-7 is in a positive state, the aluminum salt solution flows from the branch pipe R to the branch pipe S; the constant flow pump 8 pumps the sodium hydroxide solution in the sodium hydroxide tank 9, and passes through the three-way The branch pipe M of the valve 4-6 flows to the branch pipe T, joins the membrane module shell, and reacts with the aluminum salt solution in the inner cavity of the membrane filament through the membrane wall;

(1). When the membrane module is not recoiled with aluminum salt solution, open valve 3-4 and valve 3-5, and vent the aluminum salt solution in pipelines BaG and FbK. The aluminum salt solution is sucked out from the circulating reaction tank 1-1 under the suction of the circulating pump 6, and passes through the three-way valve 4-1 and the rotameter 2 in the positive state, and the flow of the aluminum salt solution is passed through the control valve 3- 1 to adjust; the three-way valve 4-2 is in a positive state, the aluminum salt solution flowing out from the rotameter 2 flows from the branch pipe A of the three-way valve 4-2 to the branch pipe C, and the three-way valve 4-3 is in a positive state, from The aluminum salt solution flowing out of the branch pipe C flows to the branch pipe E by the branch pipe D of the three-way valve 4-3, and the aluminum salt solution flowing out from the branch pipe E enters the hollow fiber membrane module 5 from one end of the hollow fiber membrane module 5; The sodium hydroxide solution in the sodium oxide solution tank 9 is pumped out and flows to the branch pipe T through the branch pipe M of the three-way valve 4-6, and is added to the membrane module shell. After mixing with the aluminum salt solution entering the membrane module, it flows out from the other end of the hollow fiber membrane module 5 through the inner cavity of the membrane filament, and the three-way valve 4-4 is in a forward state, and the aluminum salt solution flowing out from the membrane module 5 The salt solution flows from the branch pipe H of the three-way valve 4-4 to the branch pipe I, and the three-way valve 4-5 is in a positive state, and the aluminum salt solution flowing out from the branch pipe I flows from the branch pipe J of the three-way valve 4-5 to the branch pipe L, and finally, The aluminum salt solution added with sodium hydroxide is sent to the circulation reaction tank 1-1 through the circulation pump 6 through the three-way valve 4-7. The aluminum salt solution containing sodium hydroxide is continuously circulated until the required alkalinity is reached.

That is, the process of forward synthesis of inorganic polymer flocculant is: 1-1→4-1→2→3-1→4-2→4-3→5→4-4→4-5→6→4-7 →1-1.

(2). When the membrane module needs to be backwashed with aluminum salt solution, firstly close the vent valves 3-4 and 3-5. The aluminum salt solution containing sodium hydroxide is sucked out from the circulation reaction tank 1-1 under the suction action of the circulation pump 6, and passes through the three-way valve 4-1 and the rotameter 2 in the positive state; the three-way valve 4-2 In the state of reversing, the aluminum salt solution flowing out from the rotameter 2 flows from the branch pipe A of the three-way valve 4-2 to the branch pipe B, the three-way valve 4-4 is in the state of reversing, and the solution flowing out of the branch The branch pipe G of 4-4 flows to the branch pipe H, and the aluminum salt solution flowing out from the branch pipe H enters the hollow fiber membrane module 5 from one end of the hollow fiber membrane module 5; The other end of the component 5 flows out; the three-way valve 4-3 is in a reversing state, the aluminum salt solution flows from the branch pipe E of the three-way valve 4-3 to the branch pipe F, and the three-way valve 4-5 is in a reversing state, and flows out from the branch pipe F The aluminum salt solution flows from the branch pipe K of the three-way valve 4-5 to the branch pipe L, and finally the aluminum salt solution is sent back to the circulation reaction tank 1-1 through the circulation pump 6.

The flow direction of the aluminum salt solution during recoil is: 1-1→4-1→2→3-1→4-2→4-4→5→4-3→4-5→6→4-7→1-1 .

After the reverse flushing is completed, the three-way valve is adjusted from the reversing state to the forward state, and the whole system returns to the state when the membrane module is not flushed with aluminum salt, open valve 3-4 and valve 3-5, and vent the pipeline BaG and aluminum salt solution in FbK.

During the whole synthesis reaction process, backwashing should be carried out many times until the required degree of alkalinity is reached, and the finally synthesized inorganic polymer flocculant solution is a slightly turbid solution containing a small amount of fine particles.

After negative pressure backwashing, the pollution of the inner cavity of the membrane filament is reduced, the amount of fouling in the inner cavity is significantly reduced, and the decrease in the flow rate of the aluminum salt solution is smaller, which ensures the quality of the synthesized inorganic polymer flocculant.

2. The cleaning after the synthesis is to empty the flocculant aluminum salt solution in the pipeline: stop the constant flow pump 8, close the valve 3-11, open the valve 3-10, and empty the sodium hydroxide in the membrane module 5. The circulation pump 6 continues to run until the flocculant aluminum salt solution in the pipeline is emptied, and then the circulation pump 6 is stopped. Change the direction of the three-way valve 4-1 so that soft water or acid can flow from the branch pipe V of the three-way valve 4-1 to the branch pipe U, and the direction of the three-way valve 4-6 is reversed, and the aluminum salt solution flows from the branch pipe N of the three-way valve 4-6 to The branch pipe T and the three-way valve 4-7 change direction, and the aluminum salt solution flows from the three-way valve 4-7 branch pipe R to the branch pipe Q. The three-way valves 4-2, 4-3, 4-4 and 4-5 are in forward state.

The specific implementation method is:

(1). Clean with soft water. After the synthesis, open the valve 3-10 to vent the sodium hydroxide solution in the membrane module, and then close the valve 3-10. Open the valves 3-2, 3-7, 3-9 and 3-11, and close the valves 3-3, 3-6 and 3-8; vent the sodium hydroxide in the membrane module shell, then close the valve 3-10, Open the valve 3-11, the soft water in the soft water tank 10-1 flows into the shell of the membrane module by gravity, and first rinse the inner shell of the membrane module with soft water; turn on the circulating pump 6, and the soft water in the soft water tank 1-3 depends on the circulation pump 6 To flush the membrane surface, adjust the valve 3-1 to maintain a high flow rate to flush the membrane surface for 5 minutes, then the three-way valves 4-2, 4-3, 4-4 and 4-5 are in the reverse state, and then flush for 5 minutes. Minutes; close valves 3-7 and 3-2, empty the soft water in the pipeline and components, close the circulating pump 6, and adjust the three-way valves 4-2, 4-3, 4-4 and 4-5 to be in a positive state.

(2). Acid cleaning, close the valves 3-11 and 3-9, open the valves 3-3, 3-6 and 3-8, the acid in the acid tank 10-2 flows into the inner casing of the membrane module by gravity ; Turn on the circulation pump 6, the acid solution in the acid tank 1-2 circulates in the pipeline through the circulation pump 6, after cleaning for 30 minutes, stop the circulation pump 6; adjust the three-way valve 4-2, 4-3, 4- 4 and 4-5 are in reverse state, then turn on circulation pump 6, and rinse with acid for 5 minutes. Close the valves 3-3 and 3-6, open the valve 3-11, empty the acid liquid in the components and pipelines, and adjust the three-way valves 4-2, 4-3, 4-4 and 4-5 to be in the positive state.

(3). Cleaning with soft water, open the valves 3-2 and 3-11, first flush the inner shell of the membrane module with soft water until the pH value of the soft water flowing out of the valve 3-11 is neutral. Open the valves 3-7 and 3-9, close the valves 3-6 and 3-8, open the circulating pump 6 until the water outlet of the valve 3-9 is neutral; stop the operation of the pump 6, and change the three-way valves 4-2, 4- 3. 4-4 and 4-5 are in the reversing state, turn on the pump 6 to flush the pipeline BaG and pipeline FbK until the water outlet from the valve 3-9 is neutral.

The membrane filaments are selected from all high temperature, acid and alkali resistant hollow fiber ultrafiltration membranes with cut molecular weight less than 10,000 Daltons, such as polysulfone, polyethersulfone or sulfonated polysulfone.

The initial concentration of the sodium hydroxide is 1-2 mol/l, the initial concentration of the aluminum salt is 0.2-2 mol/l, and the aluminum salt solution used includes aluminum trichloride or aluminum sulfate and the like.

The transmembrane pressure required for the reverse negative pressure cleaning is controlled at 0.015-0.025 MPa.

In each synthesis process, before the degree of alkalinity is less than 1.5, and the membrane surface flow rate is less than 90% of the initial flow rate, reverse flushing is required, and each flushing lasts 1 to 2 minutes.

In each synthesis process, when the degree of alkalinity is greater than 1.5 and the membrane surface flow rate is less than 80% of the initial flow rate, backwashing is required, and each flushing lasts 1 to 2 minutes.

The effluent from the back flushing needs to be filtered with a 400-mesh stainless steel filter, and the filter is immersed in the aluminum salt solution in the circulating reaction tank.

The reverse cleaning aluminum salt solution is the metal salt solution in the reaction tank after being filtered through a 400-mesh stainless steel filter screen.

The hollow fiber membrane module, pipeline BaG or pipeline FbK are installed horizontally.

The installation position of the circulating reaction tank is lower than that of the pipeline BaG and the pipeline FbK.

The pipeline BaG and the pipeline DbI must be equipped with a vent valve, and the vent valve is connected with the circulating reaction tank.

The operating pressure of the soft water cleaning is 0.015-0.02MPa.

The permeation pressure of the hollow fiber membrane module is controlled at 0.015-0.02 MPa with acid solution, the pickling time is kept at 40-60 minutes, and the pH value of the acid used is 1.5-3, which can be hydrochloric acid, sulfuric acid or nitric acid.

The pickling solution in the inner cavity of the module flows into the shell of the membrane module by gravity, and passes through the ultrafiltration membrane under negative pressure to clean the pores of the ultrafiltration membrane.

The characteristics of the method of the present invention are that through reverse negative pressure backwashing, the pollution of the inner cavity of the membrane filament is reduced, the amount of hydroxide formed is significantly reduced, and the rate of decline in the flow rate of the aluminum salt solution is significantly reduced, which ensures the mixing strength of the fluid in the cavity and ensures the synthesis The quality of inorganic polymer flocculant. After the synthesis, three-step cleaning with soft water-acid solution-soft water can make the hydroxide layer on the membrane surface disappear and prolong the service life of the membrane module.

Description of drawings

Figure 1. Schematic diagram of the equipment used in the present invention.

Fig. 2. Example 1 of the present invention utilizes NaOH and _AlCl The flow rate-time curve diagram when the membrane module is reversely cleaned and not reversely cleaned in the process of synthesizing polyaluminum chloride PAC.

reference sign

1-1. Reaction tank 1-2. Acid tank 1-3. Soft water tank 2. Rotameter

3-1, 3-2, 3-3, 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, 3-11. Valve

4-1, 4-2, 4-3, 4-4, 4-5, 4-6, 4-7. Three-way valve

5. Membrane module 6. Circulation pump 7-1, 7-2. Negative pressure gauge

8. Constant flow pump 9. Sodium hydroxide tank 10-1. Soft water tank 10-2. Acid tank

11-1, 11-2, 11-3, 11-4, 11-5. Three-way branch pipe BaG, FbK. Pipeline

A, B, C, D, E, F, G, H, I, J, K, L, M, N, Q, R, S, T, U, V,

W. Tee Branch

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with examples and accompanying drawings.

Embodiment 1: Utilize NaOH and AlCl _Synthetic polyaluminum chloride PAC (referring to Fig. 1)

(1). Add 150ml of 0.2mol/l _AlCl solution in the reaction tank 1-1, and add 1mol/l NaOH solution in the sodium hydroxide tank 9. Adjust the three-way valves 4-1, 4-2, 4-3, 4-4, 4-5, 4-6 and 4-7 to the positive state, start the pump 6, and adjust the valve 3-1 so that the _AlCl3 The flow is controlled at 30 l/h, the sodium hydroxide pump 8 is turned on, the control flow is 0.3 ml/min, and the synthesis begins. When the AlCl ₃ circulation flow rate is 26.5l/h, backflushing is performed. Close the valves 3-4 and 3-5, stop the operation of the circulation pumps 6 and 8, and the reversing three-way valves 4-2, 4-3, 4-4, 4-5 are in the reverse state, that is, the AlCl ₃ solution is supplied by the branch pipe A Flow to branch pipe B, AlCl ₃ solution flows from branch pipe G to branch pipe H, AlCl ₃ solution flows from branch pipe E to branch pipe F, AlCl ₃ solution flows from branch pipe K to branch pipe L, all three-way valves must complete the reversing operation in a short time, and start the cycle Pump 6, adjust the circulation flow rate of AlCl ₃ to 45l/h, high-speed recoil for 1 minute. Stop the circulation pump 6 from running, change the three-way valve 4-2, 4-3, 4-4 and 4-5 to be in a positive state, that is, the _AlCl3 solution flows from the branch pipe A to the branch pipe C, and the _AlCl3 solution flows from the branch pipe D to the branch pipe E, _AlCl3 solution flows from branch pipe H to branch pipe I, _AlCl3 solution flows from branch pipe J to branch pipe L, open valves 3-4 and 3-5, and empty the residual liquid in the pipeline BaG and pipeline FbK sections. Open circulation pump 6 and constant flow pump 8, continue to synthesize, AlCl _Circulation amount rises to 27.5l/h. According to this method, until the required degree of alkalization is reached, the synthesis of polyaluminum chloride is completed. The results of backwashing and non-backwashing are shown in Figure 2.

(2). After the synthesis is finished, stop the operation of pumps 6 and 8, and change the three-way valves 4-1, 4-6 and 4-7 to be in a reversing state, that is, the _AlCl solution flows from the branch pipe V to the branch pipe U, and from the branch pipe R to the branch pipe R. Branch Q flows from branch N to branch T.

a. Wash with soft water. Keep the three-way valves 4-2, 4-3, 4-4 and 4-5 in the positive state, open the valves 3-7 and 3-9, turn on the circulating pump 6, adjust the valve 3-1, and keep the rotameter 2 Flush for 5 minutes at a flow rate of 45 l/h, and adjust the three-way valves 4-2, 4-3, 4-4 and 4-5 to the reversing state for 5 minutes. Close the valve 3-7, empty the soft water in the pipeline, then stop the pump 6, and close the valve 3-9. At the same time, open 3-10, vent the 1mol/l sodium hydroxide solution in the membrane module, close valve 3-10, open valves 3-2 and 3-11, rinse the inner shell of the membrane module with soft water until the pH value of the effluent is neutral Sex, close 3-2 and 3-11.

b. Acid cleaning. Inject the hydrochloric acid solution of pH=1～1.5 into the acid solution tanks 1-2 and 10-2. Open the valves 3-6 and 3-8, turn on the circulating pump 6, adjust the valve 3-1, keep the flow rate of the rotameter 2 at 30 l/h, flush for 45 minutes, stop the operation of the circulating pump 6, adjust the three-way valve 4-2, 4-3, 4-4 and 4-5 are flushing in positive state for 10 minutes, close valve 3-6 to drain the hydrochloric acid in the pipeline, close valve 3-8; open valve 3-3 at the same time to fill the membrane module with hydrochloric acid After the circulating pump 6 stops running, open the valve 3-11 to vent the hydrochloric acid in the membrane module.

c. Wash with soft water. Open the valves 3-7 and 3-9, turn on the circulation pump 6, adjust the valve 3-1, keep the flow rate of the rotameter 2 at 30l/h, until the pH value of the water outlet from the valve 3-9 remains neutral, adjust the three-way valve 4-2, 4-3, 4-4 and 4-5 flush the membrane assembly in the reverse state until the pH value of the outlet water from valve 3-9 remains neutral. Close 3-7 to empty the soft water in the pipeline, then stop the pump 6 from running, and close 3-9. At the same time, open the valve 3-2, rinse the inner cavity of the membrane module with soft water, until the pH value of the outlet water from the valve 3-11 is neutral, and close the valve 3-2.

After washing with soft water-acid solution-soft water, the flux was restored, and the results are shown in the table below. Inlet pressure (MPa) Outlet pressure (MPa) Pure water flow(l/h) Pure water flux (ml/min) before use 0.005 0.08 30 0.795 Not pickled 0.004 0.0015 twenty four 0.245 After pickling 0.005 0.010 28 0.539

Claims (10)

1. a method for slowing down and cleaning membrane fouling when synthesizing macromolecule flocculant with membrane reactor, the cleaning process of this method is divided into two stages, it is characterized in that: described cleaning process is: 1. Back flushing during synthesis, 2. Cleaning after synthesis, including: (1). Soft water cleaning; (2). Acid cleaning; (3). Clean with soft water. 2. The method according to claim 1, characterized in that: the back flushing in the synthesis process is in the process of synthesizing the macromolecular inorganic flocculant, the three-way valve (4-1) is in a forward state, and the aluminum salt The solution flows from the branch pipe W to the branch pipe U, the three-way valve (4-6) is in a positive state, the aluminum salt solution flows from the branch pipe M to the branch pipe T, the three-way valve (4-7) is in a positive state, and the aluminum salt solution flows from the branch pipe R to the branch pipe S; the constant flow pump (8) pumps out the sodium hydroxide solution in the sodium hydroxide tank (9), flows through the branch pipe M of the three-way valve (4-6) to the branch pipe T, and joins it in the membrane module shell, The membrane wall reacts with the aluminum salt solution in the inner cavity of the membrane filament; (1). When the aluminum salt solution is not used to recoil the membrane module, first open the valve (3-4) and the valve (3-5), and the aluminum salt solution is drawn from the circulating reaction tank ( 1-1), through the three-way valve (4-1) and the rotameter (2) in the positive state, the flow of the aluminum salt solution is regulated by the control valve (3-1); the three-way valve ( 4-2) is in a positive state, the aluminum salt solution flowing out from the rotameter (2) flows from the branch pipe A of the three-way valve (4-2) to the branch pipe C, and the three-way valve (4-3) is in a positive state, and flows from the branch pipe C The aluminum salt solution flowing out of C flows to the branch pipe E through the branch pipe D of the three-way valve (4-3), and the aluminum salt solution flowing out from the branch pipe E enters the hollow fiber membrane module (5) from one end of the hollow fiber membrane module (5): constant The flow pump (8) pumps out the sodium hydroxide solution in the sodium hydroxide solution tank (9) and flows through the branch pipe M of the three-way valve (4-6) to the branch pipe T, and adds it into the membrane module shell, and the added hydrogen is oxidized The volume of sodium is based on the degree of alkalinity required for each synthesis; and after mixing with the aluminum salt solution entering the membrane module, it flows out from the other end of the hollow fiber membrane module (5) through the membrane filament lumen, and the three-way valve ( 4-4) is in a positive state, the aluminum salt solution flowing out from the membrane module (5) flows from the branch pipe H of the three-way valve (4-4) to the branch pipe I, and the three-way valve (4-5) is in a positive state, and flows from the branch pipe I The aluminum salt solution flowing out flows from the branch pipe J of the three-way valve (4-5) to the branch pipe L, and finally, through the circulation pump (6), the aluminum salt solution added with sodium hydroxide is sent to the In the circulating reaction tank (1-1); the aluminum salt solution containing sodium hydroxide is continuously circulated until reaching the required degree of alkalinization; (2). When the membrane module needs to be backwashed with aluminum salt solution, the vent valves (3-4) and (3-5) are closed; the suction effect of the aluminum salt solution containing sodium hydroxide on the circulating pump (6) suction from the circulating reaction tank (1-1), and pass through the three-way valve (4-1) and the rotameter (2) in the positive state; the three-way valve (4-2) is in a reversing state, and the The aluminum salt solution that meter (2) flows out flows to branch pipe B by branch pipe A of three-way valve (4-2), and three-way valve (4-4) is in the reversing state, and the solution that flows out from branch pipe B is passed by three-way valve (4-4) -4) the branch pipe G flows to the branch pipe H, and the aluminum salt solution flowing out from the branch pipe H enters the hollow fiber membrane module (5) from one end of the hollow fiber membrane module (5); Flow out from the other end of the hollow fiber membrane module (5); the three-way valve (4-3) is in a reversing state, and the aluminum salt solution flows to the branch pipe F by the branch pipe E of the three-way valve (4-3), and the three-way valve (4-3) -5) In the reversing state, the aluminum salt solution flowing out from the branch pipe F flows from the branch pipe K of the three-way valve (4-5) to the branch pipe L, and finally the aluminum salt solution is sent back to the circulation through the circulation pump (6) In the reaction tank (1-1); After the reverse flushing is completed, the three-way valve is adjusted from the reversing state to the forward state, and the whole system returns to the state when the membrane module is not flushed with aluminum salt, open the valve (3-4) and the valve (3-5), and discharge Aluminum salt solution in empty lines BaG and FbK. 3. The method according to claim 1, characterized in that: the cleaning after the completion of the synthesis is to empty the flocculant aluminum salt solution in the pipeline: stop the constant flow pump (8) operation, close the valve (3- 11), open the valve (3-10), and empty the sodium hydroxide in the membrane module (5); the circulation pump (6) continues to run until the flocculant aluminum salt solution in the pipeline is emptied, and the circulation pump (6 ) operation; then change the direction of the three-way valve (4-1), so that soft water or acid can flow from the branch pipe V of the three-way valve (4-1) to the branch pipe U, and the three-way valve (4-6) is reversed, and the aluminum salt The solution flows from the branch pipe N of the three-way valve (4-6) to the branch pipe T, and the three-way valve (4-7) changes direction, and the aluminum salt solution flows from the branch pipe R of the three-way valve (4-7) to the branch pipe Q; -2), (4-3), (4-4) and (4-5) are positive; The specific implementation method is: (1). Cleaning with soft water, after the synthesis is finished, open the valve (3-10), the sodium hydroxide solution in the air defense membrane assembly, then close the valve (3-10); the valves (3-2), (3-7 ), (3-9) and (3-11) are opened, valves (3-3), (3-6) and (3-8) are closed; empty the sodium hydroxide in the membrane module shell, then close the valve (3 -10), open the valve (3-11), the soft water in the soft water tank (10-1) flows into the shell of the membrane module by gravity, and first wash the inner shell of the membrane module with soft water; open the circulation pump (6), the soft water tank ( The soft water in 1-3) flushes the membrane surface by means of the circulation pump (6), and the valve (3-1) is adjusted to maintain a high flow rate to flush the membrane surface, and then the three-way valve (4-2), (4- 3), (4-4) and (4-5) are in the state of reversing, and then flush; close the valves (3-7) and (3-2), drain the soft water in the pipeline and components, and turn off the circulating pump ( 6), adjust the three-way valves (4-2), (4-3), (4-4) and (4-5) to be in a positive state; (2). Acid cleaning, close valves (3-11) and (3-9), open valves (3-3), (3-6) and (3-8), acid tank (10-2) The acid liquid inside flows into the inner casing of the membrane module by gravity; the circulating pump (6) is turned on, the acid liquid in the acid liquid tank (1-2) is circulated and cleaned in the pipeline through the circulating pump (6), and the circulating pump (6) is stopped ); adjust the three-way valves (4-2), (4-3), (4-4) and (4-5) to be in the reversing state, then open the circulation pump (6), and rinse with acid; close the valve (3 -3) and (3-6), open the valve (3-11), empty the acid liquid in the components and pipelines, adjust the three-way valves (4-2), (4-3), (4-4) and (4-5) are positive; (3). Cleaning with soft water, open the valve (3-2) and valve (3-11), first rinse the inner shell of the membrane module with soft water until the pH value of the soft water flowing out of the valve (3-11) is neutral; open the valve (3-7) and valve (3-9), close valve (3-6) and valve (3-8), open circulation pump (6), until the water outlet of valve (3-9) is neutral; stop the pump ( 6) Run, change the three-way valves (4-2), (4-3), (4-4) and (4-5) to the reversing state, turn on the pump (6) to flush the pipeline BaG and pipeline FbK, It is neutral until the water outlet of valve (3-9). 4. The method according to claim 2, characterized in that: the membrane filaments are selected from high temperature and acid and alkali resistant polysulfone, polyethersulfone or sulfonated polysulfone hollow fiber ultra- filter membrane. 5. The method according to claim 2 or 3, characterized in that: said aluminum salt solution comprises aluminum trichloride or aluminum sulfate solution, and the initial concentration of aluminum salt solution is 0.2～2mol/l; The initial concentration of sodium is 1-2 mol/l. 6. The method according to claim 2, characterized in that: the transmembrane pressure required for the back flushing is controlled at 0.015-0.025 MPa. 7. the method as claimed in claim 1 or 2 is characterized in that: in each synthetic process, before alkalization degree is less than 1.5 and membrane surface flow velocity is less than initial flow velocity 90%, needs back flushing; During the synthesis process, when the alkalization degree is greater than 1.5 and the membrane surface flow rate is less than 80% of the initial flow rate, backwashing is required. 8. The method according to claim 3, characterized in that: the operating pressure of the soft water cleaning is 0.015-0.02 MPa. 9. The method according to claim 3, characterized in that: the membrane penetration pressure of the acid solution cleaning hollow fiber membrane module is controlled at 0.015-0.02 MPa, the pickling time is kept at 40-60 minutes, and the pH value of the acid used is 1.5-3. 10. The method according to claim 1, 3 or 9, characterized in that: said acid is hydrochloric acid, sulfuric acid or nitric acid.

Images