Method for recycling caustic soda in straw pretreatment

Abstract

The invention belongs to the technical field of resource recovery and cyclic utilization, and particularly relates to a method for recycling caustic soda in straw pretreatment. The method comprises the following steps: S1, carrying out dust removal, cleaning, mechanical crushing and soaking on a crop straw raw material; fishing out, draining, and mixing with a caustic soda solution to obtain a mixture; putting the mixture into a twin-screw extruder to be kneaded; after kneading, preserving heat and standing, and replacing and washing in a twin-screw; waste liquid generated in the double screws is put into a sedimentation tank, and a precipitate and a supernatant are obtained through standing and layering; s2, taking the precipitate, and carrying out plate-frame pressure filtration to obtain filtrate and filter residues; s3, performing ultrafiltration treatment on the supernatant liquid and the filtrate by using an ultrafiltration membrane to obtain membrane clear liquid; filter residues generated in the ultrafiltration treatment process are returned to the plate frame to be filtered again; s4, selecting an alkali-resistant special membrane, and performing membrane passing operation on the membrane clear liquid; the selected alkali-resistant special membrane has selective permeability, so that caustic soda and water can pass through the membrane, and meanwhile, other metal salts and tiny organic matters are intercepted; and finally recovering to obtain a caustic soda solution.

Description

Technical Field

[0001] The present invention belongs to the technical field of resource recovery and recycling, and particularly relates to a method for recycling caustic soda in straw pretreatment. Background Art

[0002] In industrial production, especially in industries such as papermaking, printing and dyeing, and chemical engineering, alkali is an indispensable and important raw material. However, a large amount of alkali-containing wastewater is generated during the production process. Direct discharge not only causes serious environmental pollution but also wastes precious resources. Therefore, the application of alkali recovery technology from wastewater is particularly important.

[0003] Currently, the separation and recovery of lignin have become a major research hotspot in the field of biorefining technology. By converting lignin into high-value-added products, not only can the full utilization of crop straw be realized, but also the potential value of straw can be further explored, and the utilization rate of straw resources can be improved. At the same time, the production of by-products also helps to reduce the production cost of straw sugar fermentation and enhance the economic value in the whole production process. Currently, the main methods for separating lignin include acid precipitation method and membrane method. The acid precipitation method requires a large amount of acid consumption, which not only increases the production cost but also causes the ineffective recovery of caustic soda in the waste liquid, resulting in resource waste. While the membrane separation technology can recycle caustic soda from the waste liquid and obtain a high-lignin mixture at the same time. It avoids the consumption of a large amount of acid and realizes the recycling of caustic soda. In addition, the filter residue obtained after membrane separation and recovery is rich in a large amount of lignin, which can be used as a refined raw material for further purification and processing, or made into fulvic acid fertilizer to realize the reuse of resources. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention proposes a method for recycling caustic soda in straw pretreatment.

[0005] The object of the present invention can be achieved by the following technical solutions:

[0006] A method for recycling caustic soda in straw pretreatment, the method comprising the following steps:

[0007] S1. Dust removal, cleaning, mechanical crushing and soaking treatment are carried out on the crop straw raw material; then, after fishing out and draining the water, it is fully mixed with the caustic soda solution to obtain a mixture, controlling the solid-liquid ratio to be (5-8):15, and controlling the caustic soda concentration in the mixture to be 3-8%; the mixture is put into a twin-screw extruder for kneading treatment; after kneading, it is kept warm and static, and displacement washing is carried out in the twin-screw extruder to obtain the enzyme hydrolysis raw material holocellulose; the waste liquid generated in the twin-screw extruder is put into a sedimentation tank; a pretreatment agent is added, stirred and left static, and it is left to stand and layer to obtain a precipitate and a supernatant;

[0008] S2. Take the precipitate in the sedimentation tank, and use a plate and frame to carry out pressure filtration to obtain a filtrate and a filter residue containing a high-lignin mixture;

[0009] S3. Ultrafilter the supernatant of the sedimentation tank and the filtrate after plate-and-frame filtration using an ultrafiltration membrane to obtain a membrane filtrate; the filter residue generated during the ultrafiltration process is returned to the plate-and-frame for re-filtration; the pore size of the ultrafiltration membrane is 30 - 50 nm;

[0010] S4. Select an alkali-resistant special membrane and perform a membrane-passing operation on the membrane filtrate; the selected alkali-resistant special membrane has selective permeability, allowing caustic soda and water to pass through while retaining other metal salts and micro-organics; finally, a caustic soda solution is recovered; the pore size of the alkali-resistant special membrane is 0.5 - 5 nm;

[0011] Further, the crop straw raw material in step (1) includes one or more of rice straw, corn straw, and wheat straw.

[0012] Further, the length of the straw after mechanical crushing after cleaning in step (1) is 3 - 5 cm.

[0013] Further, the heat preservation temperature when the crop straw raw material is mixed with the caustic soda solution in step (1) is 80 - 90 °C, and the heat preservation and standing time is 0.5 - 1 h.

[0014] Further, the waste liquid in step (1) is the waste liquid generated by twin-screw kneading and displacement washing.

[0015] Further, the pretreatment agent in step (1) is a compound flocculant, specifically a combination of two or more of chitosan, diatomite, cationic starch, and activated carbon.

[0016] Further, the plate-and-frame in step (2) is a mechanical plate-and-frame, and the pressure is 8 - 12 MPa;

[0017] Further, the ultrafiltration membrane is a ceramic membrane, the pressure is 0.8 - 1.0 MPa, and the treatment temperature is 30 - 40 °C;

[0018] Further, the alkali-resistant special membrane is an alkali-resistant multi-stage nanofiltration membrane, the pressure is 0.8 - 2.0 MPa, and the working temperature is 25 - 35 °C.

[0019] Advantages of the present invention: The present invention treats the waste liquid generated during the pretreatment of straw, and successfully realizes the effective separation and recovery of the mixture of caustic soda and high-lignin in the waste liquid. The recovered dilute caustic soda can be mixed with fresh concentrated caustic soda and continue to be recycled, greatly reducing the unnecessary waste of dilute alkali, and at the same time significantly reducing environmental pollution. In addition, the separated high-lignin mixture has extremely high utilization value and can be used to produce by-products such as fulvic acid fertilizers, achieving the goal of turning waste into treasure. Description of the Drawings

[0020] To more clearly illustrate the technical solutions in the embodiments of the present disclosure or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 This is the preparation flow chart of the preparation method in the present invention. Detailed implementation manners

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0023] Embodiment 1

[0024] S1. Dust removal, cleaning, and mechanical crushing of wheat straw raw materials into 3-cm straw segments, and soaking the straw segments; controlling the solid-liquid ratio to be 6:15, fully mixing the water-controlled straw segments with a caustic soda solution to obtain a mixture, making the alkalinity in the mixture 3%; putting the mixture into a twin-screw extruder for kneading treatment; after kneading, keeping warm and standing for 1 hour, and performing displacement washing in the twin-screw extruder to obtain the enzymatic hydrolysis raw material holocellulose; putting the waste liquid generated in the twin-screw extruder into a sedimentation tank, adding chitosan, diatomite, and activated carbon, stirring and standing, and allowing it to stand and layer to obtain a precipitate and a supernatant;

[0025] S2. First pump out the supernatant in the sedimentation tank, and then use a mechanical plate and frame to press-filter the lower-layer precipitate. The plate and frame pressure is 8-12 MPa to obtain a filtrate and a filter residue containing a high-lignin mixture; the high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.;

[0026] S3. Ultrafiltrate the supernatant in the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain a membrane filtrate; the filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration; the pore size of the ceramic membrane for passing the membrane is 30-50 nm, the pressure is 0.8-1.0 MPa, and the treatment temperature is 30-40 °C;

[0027] S4. Select a multi-stage nanofiltration membrane and perform a membrane-passing operation on the membrane filtrate; the selected alkali-resistant special membrane has selective permeability, can allow caustic soda and water to pass through, and simultaneously retain other metal salts and tiny organic substances; finally, a caustic soda solution is recovered; the pore size of the multi-stage nanofiltration membrane is 0.5-5 nm, the pressure is 0.8-2.0 MPa, and the working temperature is 25-35 °C.

[0028] Under this condition, the light transmittance of the dilute alkali solution obtained is 99%, the recovery rate of caustic soda is 90.2%, and the membrane equipment operates normally.

[0029] Example 2

[0030] S1. Dust removal, cleaning, and mechanical crushing of wheat straw raw materials are carried out to cut them into 3-cm straw segments, and the straw segments are soaked; the solid-liquid ratio is controlled to be 7:15, and the water-controlled straw segments are fully mixed with the caustic soda solution to obtain a mixture, so that the alkalinity in the mixture is 5%; the mixture is put into a twin-screw extruder for kneading treatment; after kneading, it is kept warm and static for 1 hour, and displacement washing is carried out in the twin-screw extruder to obtain the enzyme hydrolysis raw material holocellulose; the waste liquid generated in the twin-screw extruder is put into a sedimentation tank, chitosan and activated carbon are added, stirred and left static, and it is left to stand and layer to obtain a precipitate and a supernatant;

[0031] S2. First pump out the supernatant in the sedimentation tank, and then use a mechanical plate-and-frame filter press to filter the lower precipitate, and the plate-and-frame pressure is 8-12 MPa to obtain a filtrate and a filter residue containing a high-lignin mixture; the high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.;

[0032] S3. The supernatant in the sedimentation tank and the filtrate after plate-and-frame filtration are subjected to ultrafiltration treatment using a ceramic membrane to obtain a membrane filtrate; the filter residue generated during the ultrafiltration treatment is returned to the plate-and-frame for re-filtration; the pore size of the ceramic membrane for passing through the membrane is ≥50 nm, the pressure is 0.8-1.0 MPa, and the treatment temperature is 30-40 °C

[0033] S4. A multi-stage nanofiltration membrane is selected to perform a membrane-passing operation on the membrane filtrate; the selected alkali-resistant special membrane has selective permeability, can allow caustic soda and water to pass through, and simultaneously intercept other metal salts and tiny organic substances; finally, a caustic soda solution is recovered; the pore size of the multi-stage nanofiltration membrane is 0.5-5 nm, the pressure is 0.8-2.0 MPa, and the working temperature is 25-35 °C.

[0034] Under this condition, the light transmittance of the dilute alkali solution obtained is 98%, the recovery rate of caustic soda is 68.3%, the flux of the ceramic membrane increases, but the flux of the nanofiltration membrane decays rapidly, and the operation energy consumption increases.

[0035] Example 3

[0036] S1. Dust removal, cleaning, and mechanical crushing of wheat straw raw materials are carried out to cut them into 3-cm straw segments, and the straw segments are soaked; the solid-liquid ratio is controlled to be 8:15, and the water-controlled straw segments are fully mixed with the caustic soda solution to obtain a mixture, so that the alkalinity in the mixture is 4%; the mixture is put into a twin-screw extruder for kneading treatment; after kneading, it is kept warm and static for 1 hour, and displacement washing is carried out in the twin-screw extruder to obtain the enzyme hydrolysis raw material holocellulose; the waste liquid generated in the twin-screw extruder is put into a sedimentation tank, chitosan and diatomaceous earth are added, stirred and left static, and it is left to stand and layer to obtain a precipitate and a supernatant;

[0037] S2. First, pump out the supernatant of the sedimentation tank, and then use a mechanical plate and frame to press-filter the sediment at the lower layer. The plate and frame pressure is 8 - 12 MPa to obtain filtrate and filter residue containing a high-lignin mixture. The high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.

[0038] S3. Ultrafilter the supernatant of the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain membrane permeate. The filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration. The pore size of the ceramic membrane for passing through the membrane is ≤30 nm, the pressure is 0.8 - 1.0 MPa, and the treatment temperature is 30 - 40 °C.

[0039] S4. Select a multi-stage nanofiltration membrane and perform a membrane passing operation on the membrane permeate. The selected alkali-resistant special membrane has selective permeability, allowing caustic soda and water to pass through while retaining other metal salts and micro-organics. Finally, a caustic soda solution is recovered. The pore size of the multi-stage nanofiltration membrane is 0.5 - 5 nm, the pressure is 0.8 - 2.0 MPa, and the working temperature is 25 - 35 °C.

[0040] Under these conditions, the obtained dilute alkali solution has a light transmittance of 98%, the recovery rate of caustic soda is 86.9%, the flux of the ceramic membrane gradually decreases, and returns to normal after cleaning.

[0041] Example 4

[0042] S1. Dust, wash, and mechanically crush the wheat straw raw material into 3 cm straw segments, and soak the straw segments. Control the solid-liquid ratio to be 7:15, fully mix the water-controlled straw segments with the caustic soda solution to obtain a mixture, and make the alkalinity in the mixture 5%. Put the mixture into a twin-screw extruder for kneading treatment. After kneading, keep it warm and static for 1 hour, and perform displacement washing in the twin-screw extruder to obtain the enzymatic hydrolysis raw material holocellulose. Put the waste liquid generated in the twin-screw extruder into a sedimentation tank, add chitosan and activated carbon, stir and let it stand, and make it stand and layer to obtain sediment and supernatant.

[0043] S2. First, pump out the supernatant of the sedimentation tank, and then use a mechanical plate and frame to press-filter the sediment at the lower layer. The plate and frame pressure is 8 - 12 MPa to obtain filtrate and filter residue containing a high-lignin mixture. The high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.

[0044] S3. Ultrafilter the supernatant of the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain membrane permeate. The filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration. The pore size of the ceramic membrane for passing through the membrane is 30 - 50 nm, the pressure is ≤0.8 MPa, and the treatment temperature is 25 - 35 °C.

[0045] S4. Select a multi-stage nanofiltration membrane and perform a membrane filtration operation on the membrane permeate. The selected alkali-resistant special membrane has selective permeability, allowing caustic soda and water to pass through while retaining other metal salts and micro-organics. Finally, a caustic soda solution is recovered. The aperture of the multi-stage nanofiltration membrane is 0.5 - 5 nm, the pressure is 0.8 - 2.0 MPa, and the working temperature is 25 - 35 °C.

[0046] Under these conditions, the obtained dilute alkali solution has a light transmittance of 98% and a caustic soda recovery rate of 88.1%, with an increase in the membrane filtration time.

[0047] Example 5

[0048] S1. Dust, wash, and mechanically crush wheat straw raw materials into 3 cm straw segments, and soak the straw segments. Control the solid-liquid ratio to be 6:15, fully mix the water-controlled straw segments with the caustic soda solution to obtain a mixture, and make the alkalinity in the mixture 4%. Put the mixture into a twin-screw extruder for kneading treatment. After kneading, keep it warm and static for 1 hour, and perform displacement washing in the twin-screw extruder to obtain the enzyme hydrolysis raw material holocellulose. Put the waste liquid generated in the twin-screw extruder into a sedimentation tank, add chitosan and activated carbon, stir and let it stand, and let it stand and layer to obtain a precipitate and a supernatant.

[0049] S2. First pump out the upper clear liquid of the sedimentation tank, and then use a mechanical plate and frame to press-filter the lower precipitate. The plate and frame pressure is 8 - 12 MPa to obtain a filtrate and a filter residue containing a high-lignin mixture. The high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.

[0050] S3. Ultrafilter the upper clear liquid of the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain a membrane permeate. The filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration. The membrane aperture of the ceramic membrane is 0.5 - 5 nm, the pressure is 0.8 - 1.0 MPa, and the treatment temperature is 30 - 40 °C.

[0051] S4. Select a multi-stage nanofiltration membrane and perform a membrane filtration operation on the membrane permeate. The selected alkali-resistant special membrane has selective permeability, allowing caustic soda and water to pass through while retaining other metal salts and micro-organics. Finally, a caustic soda solution is recovered. The aperture of the multi-stage nanofiltration membrane is 0.5 - 5 nm, the pressure is ≤0.8 MPa, and the working temperature is 25 - 35 °C.

[0052] Under these conditions, the obtained dilute alkali solution has a light transmittance of 98% and a caustic soda recovery rate of 63.9%. The membrane flux of the multi-stage nanofiltration membrane is significantly reduced, and the passing rate is slow.

[0053] Example 6

[0054] S1. Dust, wash, and mechanically crush wheat straw raw materials into 3-cm straw segments, and soak the straw segments; control the solid-liquid ratio to be 8:15, fully mix the water-controlled straw segments with sodium hydroxide solution to obtain a mixture, and make the alkalinity in the mixture 3%; put the mixture into a twin-screw extruder for kneading treatment; after kneading, keep warm and stand for 1 hour, and carry out replacement washing in the twin-screw extruder to obtain the enzymatic hydrolysis raw material holocellulose; put the waste liquid generated in the twin-screw extruder into a sedimentation tank, add chitosan and cationic starch, stir and stand, and let it stand and layer to obtain a precipitate and supernatant;

[0055] S2. First pump out the supernatant in the sedimentation tank, and then use a mechanical plate and frame to press-filter the lower-layer precipitate. The plate and frame pressure is 8 - 12 MPa to obtain a filtrate and a filter residue containing a high-lignin mixture; the high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.;

[0056] S3. Ultrafilter the supernatant in the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain a membrane filtrate; the filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration; the pore size of the ceramic membrane is 30 - 50 nm, the pressure is 0.8 - 1.0 MPa, and the treatment temperature is 30 - 40 °C;

[0057] S4. Select a multi-stage nanofiltration membrane and perform a membrane passing operation on the membrane filtrate; the selected alkali-resistant special membrane has selective permeability, can allow sodium hydroxide and water to pass through, and simultaneously intercept other metal salts and micro-organics; finally, recover the sodium hydroxide solution; the pore size of the multi-stage nanofiltration membrane is 0.5 - 5 nm, the pressure is 0.8 - 2.0 MPa, and the working temperature is ≤25 °C.

[0058] Under this condition, the obtained dilute alkali solution has a light transmittance of 97%, the sodium hydroxide recovery rate is 75.2%, the flux of the multi-stage nanofiltration membrane gradually decreases, and the membrane passing time is prolonged.

[0059] Example 7

[0060] S1. Dust, wash, and mechanically crush wheat straw raw materials into 3-cm straw segments, and soak the straw segments; control the solid-liquid ratio to be 6:15, fully mix the water-controlled straw segments with sodium hydroxide solution to obtain a mixture, and make the alkalinity in the mixture 3%; put the mixture into a twin-screw extruder for kneading treatment; after kneading, keep warm and stand for 1 hour, and carry out replacement washing in the twin-screw extruder to obtain the enzymatic hydrolysis raw material holocellulose; put the waste liquid generated in the twin-screw extruder into a sedimentation tank, add diatomaceous earth and cationic starch, stir and stand, and let it stand and layer to obtain a precipitate and supernatant;

[0061] S2. First, pump out the supernatant liquid of the sedimentation tank, and then use a mechanical plate and frame to press-filter the sediment in the lower layer. The plate and frame pressure is 8 - 12 MPa to obtain filtrate and filter residue containing a high-lignin mixture. The high-lignin mixture can be used for the production of lignin-related products, such as fulvic acid, etc.

[0062] S3. Ultrafilter the supernatant liquid of the sedimentation tank and the filtrate after plate and frame filtration using a ceramic membrane to obtain membrane permeate. The filter residue generated during the ultrafiltration process is returned to the plate and frame for re-filtration. The pore size of the ceramic membrane for filtration is 30 - 50 nm, the pressure is 0.8 - 1.0 MPa, and the treatment temperature is 30 - 40 °C.

[0063] S4. Select a multi-stage nanofiltration membrane and perform a membrane filtration operation on the membrane permeate. The selected alkali-resistant special membrane has selective permeability, allowing caustic soda and water to pass through while retaining other metal salts and micro-organics. Finally, a caustic soda solution is recovered. The pore size of the multi-stage nanofiltration membrane is 0.5 - 5 nm, the pressure is 0.8 - 2.0 MPa, and the working temperature is ≥ 35 °C.

[0064] Under these conditions, the obtained dilute alkali solution has a light transmittance of 95% and a caustic soda recovery rate of 91.5%. The flux of the multi-stage nanofiltration membrane increases, but the membrane separation effect decreases.

[0065] In the description of this specification, the description with reference to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0066] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A method for recycling caustic soda in straw pretreatment, characterized in that: The method comprises the following steps: S1. Dust-removing, cleaning, mechanically crushing and soaking the crop straw raw material; then taking out and controlling the water, fully mixing with caustic soda solution to obtain a mixture, controlling the solid-liquid ratio to (5-8):15, and controlling the caustic soda concentration in the mixture to be 3-8%; putting the mixture into a twin-screw extruder for kneading; after kneading, keeping warm and standing, replacing and washing in the twin-screw extruder to obtain enzymatic hydrolysis raw material holocellulose; putting the waste liquid generated in the twin-screw extruder into a sedimentation tank; adding a pretreatment agent, stirring and standing, allowing it to stand and separate to obtain a precipitate and a supernatant; S2, taking the sediment from the sedimentation tank, and filtering it using a plate and frame to obtain a filtrate and a filter residue containing a high-lignin mixture; S3, the supernatant in the sedimentation tank and the filtrate after plate and frame filtration are subjected to ultrafiltration treatment using an ultrafiltration membrane to obtain a membrane clear liquid; the filter residue produced during the ultrafiltration treatment is returned to the plate and frame for re-filtration; the pore size of the ultrafiltration membrane is 30-50nm; S4. Use an alkali-resistant special membrane to perform membrane operation on the membrane clear liquid; the selected alkali-resistant special membrane has selective permeability, which allows caustic soda and water to pass through, while intercepting other metal salts and tiny organic matter; finally, the caustic soda solution is recovered; the pore size of the alkali-resistant special membrane is 0.5-5nm.

2. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The crop straw raw material in step (1) comprises one or more of rice straw, corn straw and wheat straw.

3. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: In step (1), the length of the straw after cleaning and mechanical crushing is 3-5 cm.

4. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: In step (1), the holding temperature when the crop straw raw material is mixed with the caustic soda solution is 80-90° C., and the holding time is 0.5-1 h.

5. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The waste liquid in step (1) is the waste liquid generated by twin-screw kneading and displacement washing.

6. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The pretreatment agent in step (1) is a composite flocculant, specifically a combination of two or more of chitosan, diatomaceous earth, cationic starch and activated carbon.

7. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The plate frame in step (2) is a mechanical plate frame with a pressure of 8-12 MPa.

8. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The ultrafiltration membrane is a ceramic membrane, the pressure is 0.8-1.0 MPa, and the processing temperature is 30-40°C.

9. A method for recycling caustic soda in straw pretreatment according to claim 1, characterized in that: The alkali-resistant special membrane is an alkali-resistant multi-stage nanofiltration membrane with a pressure of 0.8-2.0 MPa and a working temperature of 25-35°C.

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