A method for treating and recycling alkali washing wastewater

Abstract

The present invention relates to a method for treating and recycling alkali-washing wastewater, and belongs to the technical field of wastewater treatment. The present invention effectively reduces the complexity of wastewater and improves the recovery efficiency of sodium hydroxide through finely controlled process steps such as pretreatment, ultrafiltration separation and nanofiltration separation. Among them, for the pretreatment step of alkali-washing wastewater, an adsorbent prepared from bagasse is combined with copper ion loading and calcium acrylate introduction, and sodium alginate is further added, which significantly enhances the adsorption capacity of organic matter and oils, realizes waste resource utilization, and creates favorable conditions for subsequent membrane separation. In addition, a special membrane cleaning agent is also prepared, which, through optimized formula design, can efficiently clean membrane components, maintain membrane permeability, extend service life, ensure stable operation of the membrane separation system, and improve the reliability and economy of the wastewater treatment process.

Description

Technical Field

[0001] The invention belongs to the technical field of wastewater treatment and relates to a method for treating and recycling alkali washing wastewater. Background Art

[0002] When using methanol-to-olefins technology to produce products like propylene and ethylene, the olefin separation unit typically uses sodium hydroxide solution to scrub the acidic gases present in the reaction gases. This process generates alkaline wash wastewater containing a large number of pollutants. Alkaline wash wastewater has high pH, ​​high oil content, and high COD, making it difficult to treat. Direct discharge into the biochemical system of a sewage treatment plant would severely impact the wastewater treatment process, disrupting normal operation.

[0003] The oil in the alkaline washing wastewater can be either condensed into unsaturated ketones or aldehydes under the action of alkali, or polymerized into butter. The other is that the unsaturated olefins or dienes dissolved in the alkali solution will produce free radicals and undergo coupling reactions.

[0004] Oils can be classified primarily into dissolved oil, emulsified oil, dispersed oil, and floating oil, depending on their specific form in water and the source of the oily wastewater. Different oil-water separation methods are suitable for different oil types. Floating oil and dispersed oil are generally separated using physical methods, exploiting the difference in oil and water density. However, emulsified oil is too small to form a stable oil-water interface with water molecules, necessitating specialized demulsification techniques for two-phase separation. Current demulsification and oil removal technologies include physical, chemical, and biological demulsification. Physical demulsification works by physically disrupting the stable oil-water interface in wastewater, thereby separating the oil and water. Commonly used physical techniques include ultrasound, adsorption, flotation, membrane separation, and microwaves. Chemical demulsification involves adding a demulsifier to the stable emulsified oil-water interface, reducing its strength and causing it to rupture, thereby separating the oil and water. While chemical demulsification is highly efficient and rapid, the added demulsifier is generally non-recyclable, resulting in high costs and potential for new pollution, making wastewater treatment more difficult.

[0005] There have been many studies on the treatment of waste alkali liquor both domestically and internationally, using flotation + triple-effect evaporation and crystallization to treat alkaline wastewater. The waste alkali liquor enters an oil-water separator and flotation integrated machine for oil removal, or is temporarily stored in a waste alkali liquor accident pool before being lifted to an oil-water separator and flotation integrated machine for oil removal. After oil removal, the waste alkali liquor is lifted to a triple-effect evaporator for evaporation and crystallization treatment. The evaporated condensate enters the wastewater treatment biochemical unit, and the crystallized waste alkali residue is treated as solid hazardous waste outsourced. The most typical technology for waste alkali liquor is wet air oxidation, which can thoroughly treat the sulfides and organic matter in the waste alkali liquor, but the investment and operating costs are high, making it unsuitable for treating waste alkali liquor from methanol to olefins units. Incineration is widely used in the industry, but the cost of direct incineration treatment is high, and the particulate matter content in the flue gas is easily exceeded, posing a major environmental risk.

[0006] In summary, various treatment methods for alkaline washing wastewater have problems such as no recycling of alkali solution, large investment in treatment equipment, high operating costs, low efficiency, and environmental risks. Summary of the Invention

[0007] The present invention aims to provide a method for treating and recycling alkali wash wastewater. The olefin separation unit in the methanol-to-olefins process uses sodium hydroxide solution to wash the acidic reaction gases. This process produces alkali wash wastewater containing a large amount of pollutants, including high alkali concentration, high COD, and high oil content. This method removes suspended solids and organic matter from the wastewater through pretreatment, and then uses membrane technology to recover the sodium hydroxide.

[0008] The purpose of the present invention can be achieved through the following technical solutions:

[0009] A method for treating and recycling alkali washing wastewater, wherein the specific steps of the recycling method are as follows:

[0010] S1. Pretreatment: Add the adsorbent to the alkali washing wastewater, stir at 550-650 r / min at 40-45 °C for 30-60 min to obtain a pretreated mixed solution;

[0011] S2, ultrafiltration membrane separation: The pretreated mixed liquid is subjected to plate and frame filter pressing. The plate and frame filtrate is passed through the ultrafiltration membrane to further remove suspended matter and macromolecular substances in the liquid. The filtration temperature is 35-70 °C, the filtration precision is 0.05-0.5 μm, and the recovery rate of the ultrafiltration membrane is controlled at 90%-95%. After the operation, the ultrafiltration membrane is cleaned with a membrane cleaning agent. The mass concentration of the membrane cleaning agent is 0.5%-2%, the cleaning temperature is set at 40-80 °C, and the cleaning time is 30-60 min. After cleaning, the flux of the ultrafiltration membrane is restored;

[0012] S3. Nanofiltration membrane separation: The ultrafiltration membrane filtrate is filtered through a spiral nanofiltration membrane to remove residual organic matter in the liquid and recover sodium hydroxide. When cleaning the spiral nanofiltration membrane, first use a membrane cleaning agent with a mass concentration of 0.1% to 0.5% for 30 to 60 minutes, and the cleaning temperature is set to 30 to 40 °C. After cleaning, the flux of the spiral nanofiltration membrane is restored.

[0013] As a preferred technical solution of the present invention, the suspended matter content in the alkali washing wastewater in S1 is 200~300 mg / L, the oil content is 800~1300 mg / L, the COD content is 10000~12000 mg / L, and the sodium hydroxide content is 0.5wt%~0.8wt%.

[0014] As a preferred technical solution of the present invention, the preparation method of the adsorbent in S1 is as follows:

[0015] S3-1: Add 10-20 parts by weight of bagasse powder to 80-90 parts by weight of deionized water, ultrasonicate for 20-40 minutes, then add 1-2 parts by weight of copper chloride, stir at 200-300 r / min for 6-10 hours, dry in a vacuum drying oven at 60°C for 12 hours, transfer to a crucible, and calcine in a muffle furnace under nitrogen for 1-2 hours to obtain powder A;

[0016] S3-2: Add 15-25 parts by weight of powder A to 100 parts by weight of deionized water, sonicate for 10 min, add 15-25 parts by weight of a calcium source, stir at 500-600 r / min for 30 min, then add acrylic acid in an equal molar ratio to the calcium source, stir at 700-800 r / min for 1-2 h, and dry in a vacuum drying oven at 60-80°C for 12-24 h to obtain powder B;

[0017] S3-3: Powder B and sodium alginate were mixed in a mass ratio of (3-5):1, and the mixture was ball-milled in a ball mill for 2-4 h to obtain the adsorbent.

[0018] As a preferred technical solution of the present invention, the dosage of the adsorbent in S1 is 0.5% to 5% of the weight of the alkali washing wastewater.

[0019] As a preferred technical solution of the present invention, the calcination temperature of the muffle furnace in S3-1 is 500-700°C.

[0020] As a preferred technical solution of the present invention, the calcium source in S3-2 is one of calcium oxide or calcium hydroxide.

[0021] As a preferred technical solution of the present invention, the ball milling speed in S3-3 is 200~300 r / min.

[0022] As a preferred technical solution of the present invention, the ultrafiltration membrane structure in S2 is one of tubular, plate, curtain or roll ultrafiltration membranes, and the ultrafiltration membrane material is one of ceramic, stainless steel and PTFE.

[0023] As a preferred technical solution of the present invention, the components and mass percentages of the membrane cleaning agent are 5%~15% sodium hydroxide, 10%~25% tetrasodium ethylenediaminetetraacetic acid, 0.5%~1.5% propanol, 0.5%~1% ethylene glycol, 0.5%~1% polyvinyl alcohol, 0.5%~1.5% sodium dodecylbenzenesulfonate, 0.3%~0.5% fatty alcohol polyoxyethylene ether, 0.05%~0.5% ether, and the balance deionized water.

[0024] As a preferred technical solution of the present invention, the filtrate of the rolled nanofiltration membrane in S3 can be subjected to secondary nanofiltration membrane filtration to further improve the quality of the recovered sodium hydroxide.

[0025] The characteristics of alkaline wash wastewater include high suspended solids concentration, high oil content, high COD value, and the presence of sodium hydroxide. The oil in the wastewater is primarily in an emulsified state and has a complex composition, including olefins, alkanes, aldehydes, ketones, and other organic compounds. These characteristics make traditional flotation methods difficult to effectively remove the oil, making the removal of oil and suspended solids a major challenge in the pretreatment stage.

[0026] Bagasse, a byproduct of the sugarcane industry, is widely available and inexpensive, providing a rich raw material for the preparation of highly effective adsorbents. Using bagasse to prepare adsorbents not only recycles waste but also reduces production costs. The porous structure and high surface area of ​​bagasse provide an excellent physical foundation for its use as an adsorbent, enabling the adsorbent to fully interact with organic matter and oil molecules in alkaline wastewater.

[0027] Copper ions and sugarcane bagasse are placed in a solution and mixed for a certain period of time, and then high-temperature roasting is used to stably load the copper ions on the surface and pores of sugarcane bagasse-based biochar. After high-temperature roasting and ball milling, the porous structure and specific surface area of ​​the sugarcane bagasse-based biochar are further optimized, improving the adsorption performance. At the same time, copper ions, as an effective adsorption site, can further enhance the adsorbent's adsorption capacity for organic matter and oils. Copper ions can interact with heterocyclic nitrogen, sulfur and other atoms in organic compounds by forming π complex bonds, thereby effectively separating and adsorbing these organic compounds. In addition, copper-loaded biochar also has the ability to activate persulfate substances in alkaline wastewater, generating active species such as singlet oxygen and superoxide radicals. These active species can promote the degradation of organic pollution in alkaline wastewater, further improving wastewater treatment efficiency.

[0028] Calcium acrylate is generated by reacting a calcium source with acrylic acid under certain conditions. The carboxyl groups in the calcium acrylate molecules can react with the hydroxyl and carboxyl groups on the surface of biochar to form covalent bonds or ionic bonds, thereby enhancing the stability and adsorption performance of the adsorbent. The adsorbent is added to the alkaline washing wastewater. Since there is a certain amount of organic matter in the alkaline washing wastewater, the organic matter can act as a cross-linking agent or initiator for calcium acrylate, causing the calcium acrylate to undergo a polymerization reaction, stimulating the double chain in the acrylate molecular structure to form monomer free radicals. When the monomer free radicals meet with new monomers, they easily combine and transfer this excitation effect, which is repeated in turn. The molecules are connected to each other to form a planar chain or three-dimensional network structure. The formation of this structure further increases the adsorption surface area and adsorption sites of the adsorbent, thereby significantly improving its adsorption capacity for organic matter and oil molecules in alkaline wastewater.

[0029] By mixing sodium alginate with powder B at a specific mass ratio, the sodium alginate and calcium acrylate work together to form a more stable cross-linked structure. This structure not only improves the mechanical strength of the adsorbent but also facilitates the adsorption and fixation of target pollutants. Furthermore, the addition of sodium alginate further enhances the bonding and synergistic effects between the raw materials, further stabilizing the copper loading on the bagasse-based biochar, making the entire adsorbent system more stable and efficient.

[0030] In view of the characteristics of alkaline washing wastewater, the membrane cleaning agent prepared by the present invention can effectively remove organic pollutants such as grease and protein and inorganic pollutants such as metal ions remaining on the membrane through reasonable component ratio and concentration setting, prevent membrane pollution and clogging, and restore membrane flux.

[0031] Among them, sodium hydroxide has strong detergency, effectively breaking down oil, protein, and certain organic contaminants. It converts oil into soap and glycerin through saponification. Combined with tetrasodium EDTA, it enhances the chelation of metal ions, preventing them from damaging membranes. Combining tetrasodium EDTA with propanol improves the permeability and solubility of the cleaning agent. Propanol has excellent solubility and volatility, dissolving oil, grease, and other oily substances and thoroughly removing them. Ethylene glycol has good solubility and stability, and also aids in oil-water separation, reducing wastewater treatment costs. Polyvinyl alcohol, as a high-molecular-weight polymer, has lubricating and moisturizing properties, helping to reduce mechanical damage to membranes caused by the cleaning agent. It synergizes with surfactants to enhance the lubricity and protective properties of the cleaning agent. Fatty alcohol polyoxyethylene ether, as a nonionic surfactant, possesses excellent emulsifying, dispersing, and detergency capabilities. It synergizes with the anionic surfactant sodium dodecylbenzenesulfonate to significantly enhance the emulsification and detergency of the cleaning agent. Ether helps to accelerate the volatilization and drying process of the cleaning agent, prevents the cleaning agent from remaining on the membrane surface, and works with other ingredients to improve the volatility and drying performance of the cleaning agent.

[0032] Beneficial effects of the present invention:

[0033] (1) The treatment and recycling method of the present invention introduces a series of process steps such as pretreatment, ultrafiltration separation and nanofiltration separation, and finely controls the parameters of each step. Through this comprehensive treatment process, the complexity and treatment difficulty of wastewater are effectively reduced, while the recovery efficiency of sodium hydroxide is improved.

[0034] (2) Based on the characteristics of alkaline washing wastewater, an adsorbent was prepared for pretreatment. The porous structure and high specific surface area of ​​sugarcane bagasse were utilized, combined with the loading of copper ions and the introduction of calcium acrylate to significantly optimize the adsorption performance. Copper ions, as effective adsorption sites, interact with heterocyclic atoms in organic matter through π complex bonds, enhancing the adsorption capacity for organic matter and oils. At the same time, copper-loaded biochar activates persulfate to produce active species, promoting the degradation of organic pollution. The polymerization reaction of calcium acrylate forms a planar chain or three-dimensional network structure, further increasing the adsorption surface area and sites. The introduction of sodium alginate further enhances the stability of the adsorption system, significantly improving the adsorption capacity for organic matter and oil molecules in alkaline wastewater, realizing waste resource utilization and improving wastewater treatment efficiency, and providing more favorable conditions for subsequent membrane separation processes.

[0035] (3) In view of the special water quality of the alkaline washing waste liquid, the present invention prepares a membrane cleaning agent. The cleaning agent has an optimized formula design for the pollutant components in the alkaline washing waste liquid and has an excellent cleaning effect. By using the membrane cleaning agent to regularly and efficiently clean the membrane assembly, the permeability of the membrane is maintained, the service life of the membrane is extended, the stable operation of the membrane separation system is ensured, and the reliability and economy of the entire wastewater treatment process are improved. DETAILED DESCRIPTION

[0036] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, the specific implementation methods, structures, features and effects of the present invention are described in detail below in combination with the embodiments.

[0037] Example 1

[0038] A method for treating and recycling alkali washing wastewater, wherein the specific steps of the recycling method are as follows:

[0039] S1. Pretreatment: adding an adsorbent to alkali-wash wastewater, wherein the alkali-wash wastewater has a suspended solid content of 218 mg / L, an oil content of 1235 mg / L, a COD content of 11800 mg / L, and a sodium hydroxide content of 0.6 wt %, and the adsorbent is added in an amount of 0.5% of the weight of the alkali-wash wastewater. The mixture is stirred at 40°C and 600 r / min for 30 min to obtain a pretreated mixed solution.

[0040] S2, ultrafiltration membrane separation: The pretreated mixed liquid was subjected to plate and frame filter press. The plate and frame filtrate was passed through an ultrafiltration membrane to further remove suspended solids and macromolecular substances in the liquid. The ultrafiltration membrane was a ceramic spiral membrane. The filtration temperature was 40°C, the filtration precision was 0.2 μm, and the recovery rate of the ultrafiltration membrane was controlled at 95%. After the operation, the ultrafiltration membrane was cleaned with a membrane cleaning agent. The mass concentration of the membrane cleaning agent was 1.5%, the cleaning temperature was set to 80°C, and the cleaning time was 30 minutes. After cleaning, the flux of the ceramic ultrafiltration membrane was restored;

[0041] S3. Nanofiltration membrane separation: The ultrafiltration membrane filtrate was filtered through a spiral nanofiltration membrane to remove residual organic matter in the liquid and recover sodium hydroxide. When cleaning the spiral nanofiltration membrane, a membrane cleaning agent with a mass concentration of 0.4% was first used for 60 minutes. The cleaning temperature was set to 40°C. After cleaning, the flux of the spiral nanofiltration membrane was restored.

[0042] The preparation method of the adsorbent in S1 is as follows:

[0043] S3-1: 15 parts by weight of bagasse powder were added to 85 parts by weight of deionized water, ultrasonicated for 30 min, and then 1.5 parts by weight of copper chloride was added. The mixture was stirred at 250 r / min for 8 h, dried in a vacuum drying oven at 60 °C for 12 h, and then transferred to a crucible. The mixture was then calcined in a muffle furnace under nitrogen protection for 1.5 h at 600 °C to obtain powder A.

[0044] S3-2: 20 parts by weight of powder A were added to 100 parts by weight of deionized water, ultrasonicated for 10 min, 20 parts by weight of calcium oxide were added, and the mixture was stirred at 550 r / min for 30 min. Acrylic acid in an equal molar ratio to the calcium oxide was then added, and the mixture was stirred at 750 r / min for 1.5 h. The mixture was then dried in a vacuum drying oven at 70°C for 16 h to obtain powder B.

[0045] S3-3: Powder B and sodium alginate were mixed in a mass ratio of 4:1, and the mixture was ball-milled in a ball mill for 3 h at a ball mill speed of 250 r / min to obtain the adsorbent.

[0046] The components and mass percentages of the membrane cleaning agent are 10% sodium hydroxide, 17% tetrasodium ethylenediaminetetraacetic acid, 1% propanol, 0.8% ethylene glycol, 0.8% polyvinyl alcohol, 1% sodium dodecylbenzenesulfonate, 0.4% fatty alcohol polyoxyethylene ether, 0.3% ethyl ether, and the balance deionized water.

[0047] Test results: The recovered nanofiltration membrane filtrate contained 5 mg / L of suspended solids, 65 mg / L of oil, 230 mg / L of COD, and 0.73 wt% of sodium hydroxide. The recovery rate of sodium hydroxide was greater than 100%.

[0048] Example 2

[0049] A method for treating and recycling alkali washing wastewater, wherein the specific steps of the recycling method are as follows:

[0050] S1. Pretreatment: adding an adsorbent to alkali-wash wastewater, wherein the alkali-wash wastewater has a suspended solid content of 285 mg / L, an oil content of 825 mg / L, a COD content of 10030 mg / L, and a sodium hydroxide content of 0.8 wt %, and the adsorbent is added in an amount of 4.5% of the weight of the alkali-wash wastewater. The mixture is stirred at 550 r / min at 42°C for 40 min to obtain a pretreated mixed solution.

[0051] S2, ultrafiltration membrane separation: The pretreated mixed liquid was subjected to plate and frame filter press. The plate and frame filtrate was passed through a stainless steel ultrafiltration membrane to further remove suspended matter and macromolecular substances in the liquid. The filtration temperature was 60°C, the filtration precision was 0.05 μm, and the recovery rate of the stainless steel ultrafiltration membrane was controlled at 95%. After the operation, the stainless steel ultrafiltration membrane was cleaned with a membrane cleaning agent at a mass concentration of 2%, the cleaning temperature was set to 55°C, and the cleaning time was 45 minutes. After cleaning, the flux of the stainless steel ultrafiltration membrane was restored;

[0052] S3. Nanofiltration membrane separation: The filtrate from the stainless steel ultrafiltration membrane is filtered through a rolled nanofiltration membrane. The filtrate from the stainless steel membrane enters the nanofiltration membrane for the first separation, and then the first filtrate is filtered through a second nanofiltration membrane to remove residual organic matter in the liquid and recover sodium hydroxide. When cleaning the rolled nanofiltration membrane, a membrane cleaning agent with a mass concentration of 0.5% is first used for 40 minutes, and the cleaning temperature is set to 45°C. After cleaning, the flux of the rolled nanofiltration membrane is restored.

[0053] The preparation method of the adsorbent in S1 is as follows:

[0054] S3-1: 10 parts by weight of bagasse powder was added to 90 parts by weight of deionized water, and ultrasonicated for 20 min. 1 part by weight of copper chloride was then added, and the mixture was stirred at 200 r / min for 6 h. The mixture was dried in a vacuum drying oven at 60 °C for 12 h. The mixture was then transferred to a crucible and calcined in a muffle furnace under nitrogen protection for 1 h at 500 °C to obtain powder A.

[0055] S3-2: 15 parts by weight of powder A were added to 100 parts by weight of deionized water, ultrasonicated for 10 min, 15 parts by weight of calcium hydroxide were added, and the mixture was stirred at 500 r / min for 30 min. Acrylic acid in an equal molar ratio to the calcium hydroxide was then added, and the mixture was stirred at 700 r / min for 1 h. The mixture was then dried in a vacuum drying oven at 60°C for 12 h to obtain powder B.

[0056] S3-3: Powder B and sodium alginate were mixed in a mass ratio of 3:1, and the mixture was placed in a ball mill and ball milled for 2 h at a ball mill speed of 200 r / min to obtain the adsorbent.

[0057] The components and mass percentages of the membrane cleaning agent are 5% sodium hydroxide, 10% tetrasodium ethylenediaminetetraacetic acid, 0.5% propanol, 0.5% ethylene glycol, 0.5% polyvinyl alcohol, 0.5% sodium dodecylbenzenesulfonate, 0.3% fatty alcohol polyoxyethylene ether, 0.05% ethyl ether, and the balance deionized water.

[0058] Test results: No suspended matter was detected in the recovered secondary nanofiltration membrane filtrate, the oil content was 15 mg / L, the COD content was 23 mg / L, the sodium hydroxide content was 0.92 wt%, and the recovery rate of sodium hydroxide was greater than 100%.

[0059] Example 3

[0060] A method for treating and recycling alkali washing wastewater, wherein the specific steps of the recycling method are as follows:

[0061] S1. Pretreatment: adding an adsorbent to alkali-wash wastewater, wherein the alkali-wash wastewater has a suspended solid content of 205 mg / L, an oil content of 1025 mg / L, a COD content of 11280 mg / L, and a sodium hydroxide content of 0.5 wt %, and the adsorbent is added in an amount of 5% of the weight of the alkali-wash wastewater. The mixture is stirred at 45°C and 650 r / min for 60 min to obtain a pretreated mixed solution.

[0062] S2, ultrafiltration membrane separation: The pretreated mixed liquid was subjected to plate and frame filter press. The plate and frame filtrate was passed through an ultrafiltration membrane to further remove suspended solids and macromolecular substances in the liquid. The ultrafiltration membrane was a PTFE curtain membrane. The filtration temperature was 35°C, the filtration precision was 0.5 μm, and the recovery rate of the ultrafiltration membrane was controlled at 90%. After the operation, the ultrafiltration membrane was cleaned with a membrane cleaning agent. The mass concentration of the membrane cleaning agent was 0.5%, the cleaning temperature was set to 40°C, and the cleaning time was 60 min. After cleaning, the flux of the ultrafiltration membrane was restored;

[0063] S3. Nanofiltration membrane separation: The ultrafiltration membrane filtrate was filtered through a spiral nanofiltration membrane to remove residual organic matter in the liquid and recover sodium hydroxide. When cleaning the spiral nanofiltration membrane, a membrane cleaning agent with a mass concentration of 0.3% was first used for 60 minutes. The cleaning temperature was set to 40°C. After cleaning, the flux of the spiral nanofiltration membrane was restored.

[0064] The preparation method of the adsorbent in S1 is as follows:

[0065] S3-1: 20 parts by weight of bagasse powder were added to 90 parts by weight of deionized water, ultrasonicated for 40 min, and then 2 parts by weight of copper chloride were added. The mixture was stirred at 300 r / min for 10 h, dried in a vacuum drying oven at 60 °C for 12 h, and then transferred to a crucible. The mixture was then calcined in a muffle furnace under nitrogen protection for 2 h at 700 °C to obtain powder A.

[0066] S3-2: 25 parts by weight of powder A were added to 100 parts by weight of deionized water, ultrasonicated for 10 min, 25 parts by weight of calcium oxide were added, and the mixture was stirred at 600 r / min for 30 min. Acrylic acid in an equal molar ratio to the calcium oxide was then added, and the mixture was stirred at 800 r / min for 2 h. The mixture was then dried in a vacuum drying oven at 80°C for 24 h to obtain powder B.

[0067] S3-3: Powder B and sodium alginate were mixed in a mass ratio of 5:1, and the mixture was placed in a ball mill and ball milled for 4 h at a ball mill speed of 300 r / min to obtain the adsorbent.

[0068] The components and mass percentages of the membrane cleaning agent are 15% sodium hydroxide, 25% tetrasodium ethylenediaminetetraacetic acid, 1.5% propanol, 1% ethylene glycol, 1% polyvinyl alcohol, 1.5% sodium dodecylbenzenesulfonate, 0.5% fatty alcohol polyoxyethylene ether, 0.5% ether, and the balance deionized water.

[0069] Test results: The recovered nanofiltration membrane filtrate contained 2 mg / L of suspended solids, 35 mg / L of oil, 85 mg / L of COD, and 0.63 wt% of sodium hydroxide. The sodium hydroxide recovery rate was greater than 100%.

[0070] Comparative Example 1

[0071] No copper chloride was added during the preparation of the adsorbent, and the remaining steps were the same as in Example 1.

[0072] The recovered nanofiltration membrane filtrate contained 16 mg / L of suspended solids, 114 mg / L of oil, 415 mg / L of COD, and 0.46 wt% of sodium hydroxide.

[0073] Comparative Example 2

[0074] The preparation of the adsorbent does not involve step S3-2, and the remaining steps are the same as those in Example 1.

[0075] The recovered nanofiltration membrane filtrate contained 21 mg / L of suspended solids, 136 mg / L of oil, 302 mg / L of COD, and 0.52 wt% of sodium hydroxide.

[0076] Comparative Example 3

[0077] Sodium alginate was not added during the preparation of the adsorbent, and the remaining steps were the same as those in Example 1.

[0078] The recovered nanofiltration membrane filtrate contained 19 mg / L of suspended solids, 97 mg / L of oil, 289 mg / L of COD, and 0.56 wt% of sodium hydroxide.

[0079] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for treating and recycling alkali washing wastewater, characterized in that: The specific steps of the recycling method are as follows: S1. Pretreatment: Add the adsorbent to the alkali washing wastewater, stir at 550-650 r / min at 40-45 °C for 30-60 min to obtain a pretreated mixed solution; S2, ultrafiltration membrane separation: The pretreated mixed liquid is subjected to plate and frame filter pressing. The plate and frame filtrate is passed through the ultrafiltration membrane to further remove suspended matter and macromolecular substances in the liquid. The filtration temperature is 35-70 °C, the filtration precision is 0.05-0.5 μm, and the recovery rate of the ultrafiltration membrane is controlled at 90%-95%. After the operation, the ultrafiltration membrane is cleaned with a membrane cleaning agent. The mass concentration of the membrane cleaning agent is 0.5%-2%, the cleaning temperature is set at 40-80 °C, and the cleaning time is 30-60 min. After cleaning, the flux of the ultrafiltration membrane is restored; S3, nanofiltration membrane separation: The ultrafiltration membrane filtrate is filtered through a spiral nanofiltration membrane to remove residual organic matter in the liquid and recover sodium hydroxide. The spiral nanofiltration membrane is first cleaned with a membrane cleaning agent with a mass concentration of 0.1% to 0.5% for 30 to 60 minutes at a cleaning temperature of 30 to 40°C. After cleaning, the flux of the spiral nanofiltration membrane is restored; The preparation method of the adsorbent in S1 is as follows: S3-1: Add 10-20 parts by weight of bagasse powder to 80-90 parts by weight of deionized water, ultrasonicate for 20-40 min, add 1-2 parts by weight of copper chloride, stir at 200-300 r / min for 6-10 h, dry in a vacuum drying oven at 60 °C for 12 h, transfer to a crucible, and calcine in a muffle furnace under nitrogen for 1-2 h to obtain powder A; S3-2: Add 15-25 parts by weight of powder A to 100 parts by weight of deionized water, sonicate for 10 min, add 15-25 parts by weight of a calcium source, stir at 500-600 r / min for 30 min, then add acrylic acid in an equal molar ratio to the calcium source, stir at 700-800 r / min for 1-2 h, and dry in a vacuum drying oven at 60-80°C for 12-24 h to obtain powder B; S3-3: Powder B and sodium alginate were mixed in a mass ratio of (3-5):1, and the mixture was ball-milled in a ball mill for 2-4 h to obtain the adsorbent.

2. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The alkali washing wastewater in S1 has a suspended solid content of 200-300 mg / L, an oil content of 800-1300 mg / L, a COD content of 10000-12000 mg / L, and a sodium hydroxide content of 0.5 wt%-0.8 wt%.

3. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The dosage of the adsorbent in S1 is 0.5% to 5% of the weight of the alkali washing wastewater.

4. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The calcination temperature of the muffle furnace in S3-1 is 500-700°C.

5. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The calcium source in S3-2 is one of calcium oxide and calcium hydroxide.

6. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The ball milling speed in S3-3 is 200~300 r / min.

7. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The ultrafiltration membrane structure in S2 is one of tubular, plate, curtain or roll ultrafiltration membranes, and the ultrafiltration membrane material is one of ceramic, stainless steel and PTFE.

8. The method for treating and recycling alkali washing wastewater according to claim 1, characterized in that: The components and mass percentages of the membrane cleaning agent are 5%-15% sodium hydroxide, 10%-25% tetrasodium ethylenediaminetetraacetic acid, 0.5%-1.5% propanol, 0.5%-1% ethylene glycol, 0.5%-1% polyvinyl alcohol, 0.5%-1.5% sodium dodecylbenzenesulfonate, 0.3%-0.5% fatty alcohol polyoxyethylene ether, 0.05%-0.5% diethyl ether, and the balance deionized water.

9. The method for treating and recycling alkali washing wastewater according to claim 1, wherein: The filtrate of the rolled nanofiltration membrane in S3 is subjected to secondary nanofiltration membrane filtration to further improve the quality of the recovered sodium hydroxide.