Caprolactam wastewater resourceful treatment method

Abstract

The invention belongs to the technical field of wastewater treatment, and particularly discloses a caprolactam wastewater resourceful treatment method which comprises the following steps: mixing caprolactam high-concentration wastewater with air / oxygen, and oxidizing in a subcritical water catalytic oxidation process; adding caustic soda liquid or caustic soda flakes into the oxidized liquid after the reaction to adjust the pH value, feeding the oxidized liquid into an ammonia distillation unit, distilling out 15-20% of ammonia water, and recycling the ammonia water for caprolactam production; further evaporating and crystallizing the wastewater after extracting the ammonia water to generate sodium sulfate salt; and after the subcritical water oxidation reaction, an energy recovery device is adopted to generate electricity, and the electricity is recycled to the system for use. By adopting the caprolactam wastewater resourceful treatment method disclosed by the invention, the problems of resource waste, high treatment cost, complex flow and the like in the existing caprolactam wastewater treatment technology are solved, efficient recycling of resources such as organic matters and ammonium sulfate in the caprolactam wastewater is realized, and meanwhile, the wastewater treatment cost is reduced; the treatment efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for the resource-based treatment of caprolactam wastewater. Background Technology

[0002] Caprolactam is an important organic chemical raw material, belonging to the nylon industry chain. It has a melting point of around 70℃ and is primarily a transparent liquid with a minty and acetone odor. Liquid caprolactam is mainly used to polymerize PA6 chips, with a small amount used in the manufacture of pharmaceutical intermediates. Caprolactam is an important chemical material, primarily used to polymerize polyurethane, which can be further processed into fibers, plastics, and films. The main domestic production methods for caprolactam are the toluene process and the benzene process, but both ultimately obtain caprolactam through the liquid-phase Beckmann rearrangement reaction of cyclohexanone oxime. This process has a fast reaction rate and high conversion rate, but the wastewater generated is difficult to treat.

[0003] The wastewater mainly contains caprolactam, ammonium sulfate, acetic acid, toluene, benzoic acid, cyclohexane carboxylic acid, cyclohexane carboxylic acid sulfonic acid, hexahydrobenzoic acid, cyclohexane, and other components. It contains a wide variety of organic matter, high chemical oxygen demand (COD), high salt content, and poor biodegradability. The production of 1 ton of caprolactam generates 4–7 tons of wastewater, with a COD typically exceeding 10,000, and containing high concentrations of sodium sulfate and ammonium sulfate.

[0004] Traditional caprolactam wastewater treatment methods have many drawbacks. For example, one method first removes 50% of the organic matter by adsorbing nitrogen-containing organic matter using absorbent materials, then removes 60% of the salt using an evaporation and crystallization device, and finally uses wet catalytic oxidation to efficiently degrade the organic matter in the evaporation condensate and crystallization mother liquor, with the final product undergoing biological treatment. This process uses specialized resins to adsorb organic matter, and after adsorption saturation, desorption is performed using N2, which easily causes air pollution; the adsorption method is merely a transfer of hazardous organic waste, not an effective degradation treatment. During evaporation and crystallization, the water still contains a high concentration of organic matter, and concentration leads to organic matter enrichment, resulting in the recovered salt not meeting reuse requirements. Another example is the use of a process combining hydrogen peroxide and catalytic wet oxidation to treat caprolactam wastewater. However, this requires adjusting the pH of the wastewater to 2 for the reaction, placing high demands on the acid corrosion resistance of the equipment, and consuming large amounts of expensive hydrogen peroxide, resulting in high operating costs. Therefore, developing a highly efficient, low-cost caprolactam wastewater treatment method that enables resource recovery is of significant practical importance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for the resource-based treatment of caprolactam wastewater, in order to solve the problems of resource waste, high treatment costs, and complex processes in existing caprolactam wastewater treatment technologies, and to achieve efficient recovery and utilization of organic matter and ammonium sulfate resources in caprolactam wastewater, while reducing wastewater treatment costs and improving treatment efficiency.

[0006] To achieve the above objectives, the present invention provides a method for the resource-based treatment of caprolactam wastewater, comprising the following steps: Step 1: High-concentration caprolactam wastewater is mixed with an oxidant and then oxidized in a subcritical water catalytic oxidation process. Step 2: After the reaction, the oxidizing liquid is adjusted to pH by adding liquid alkali or flake alkali, and then sent to the ammonia distillation unit to distill off 15%~20% ammonia water, which is then reused in caprolactam production. Step 3: The wastewater after ammonia extraction is further evaporated and crystallized to produce sodium sulfate. Step 4: After the subcritical water oxidation reaction, the pressure is still relatively high. An energy recovery device is used to generate electricity, which is then reused in the system.

[0007] Preferably, in step 1, the subcritical water catalytic oxidation specifically involves: adjusting the chemical oxygen demand (COD) concentration of the high-concentration caprolactam wastewater to 30,000-40,000 mg / L using an oxidizing agent; initially diluting with soft water; adding an oxidant; pressurizing the solution to the reaction pressure using a compressor; mixing the mixture with the pressurized caprolactam preparation solution; heating the mixture within the subcritical water catalytic oxidation unit; and then introducing it into the reactor to remove organic matter from the water and mineralize organic matter (N and S to NH4). + SO4 2- After the reaction, the organic matter is decomposed into carbon dioxide, water and acetic acid. The exhaust gas is absorbed by dilute acid and then the remaining volatile organic compounds (VOCs) are removed by photocatalysis before being emitted.

[0008] Preferably, in step 1, the oxidant is either air or oxygen, and the amount of air added is 120-150 Nm. 3 / h, oxygen addition rate 25-30 Nm 3 / h.

[0009] Preferably, in step 1, the reaction temperature is 130~270℃ and the pressure is 1~8MPa.

[0010] Preferably, in step 2, the pH is adjusted to 10-13.

[0011] Preferably, in step 2, the ammonia stripping unit specifically involves: using the waste heat from the subcritical water catalytic oxidation unit to preheat the ammonia stripping tower to 70-75°C, introducing 110°C steam, heating to 80-95°C, and proceeding under a slightly positive pressure of 50-70 kPa to produce 15-20% ammonia water. The ammonia stripping tower is a packed tower with a theoretical number of 15-20 plates, and the tail gas is absorbed by 20% dilute sulfuric acid.

[0012] The advantages and beneficial effects of the above-mentioned caprolactam wastewater resource utilization method of the present invention are as follows: 1. This invention utilizes subcritical water catalytic oxidation for high organic matter removal efficiency and low energy consumption. Ammonia stripping is heated using the waste heat from the subcritical water catalytic oxidation process, effectively utilizing calorific value and reducing steam consumption. The wastewater after ammonia removal is directly evaporated, resulting in low energy consumption. An energy recovery device is used to recover the tail gas after oxidation, which can generate electricity.

[0013] 2. This invention can effectively recover organic matter and ammonium sulfate from wastewater, maximizing resource utilization and reducing resource waste.

[0014] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of the caprolactam wastewater resource utilization treatment method of the present invention. Detailed Implementation

[0016] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0018] Unless otherwise defined, all reagents, equipment and other materials used in this invention are commercially available.

[0019] A method for the resource-based treatment of caprolactam wastewater, such as Figure 1 As shown, it includes the following steps: Step 1: High-concentration caprolactam wastewater is mixed with an oxidant and then oxidized in a subcritical water catalytic oxidation process.

[0020] The subcritical water catalytic oxidation process specifically involves: adjusting the chemical oxygen demand (COD) of high-concentration caprolactam wastewater to 30,000-40,000 mg / L using an oxidizing agent; initially diluting with soft water; adding an oxidant; pressurizing the solution to the reaction pressure using a compressor; mixing the oxidant with the pressurized caprolactam solution; heating the mixture within the subcritical water catalytic oxidation unit before it enters the reactor; and removing organic matter from the water, as well as mineralizing organic matter (N and S to NH4).+ SO4 2- After the reaction, the organic matter is decomposed into carbon dioxide, water and acetic acid. The exhaust gas is absorbed by dilute acid and then the remaining VOCs are removed by photocatalysis before being emitted.

[0021] The oxidant is either air or oxygen, with the air addition amount being 120-150 Nm. 3 / h, oxygen addition rate 25-30 Nm 3 / h. Reaction temperature 130~270℃, pressure 1-8MPa.

[0022] Step 2: After the reaction, the oxidizing liquid is adjusted to pH 10-13 by adding liquid alkali or flake alkali, and then sent to the ammonia stripping unit to distill off 15%~20% ammonia water, which is then reused in caprolactam production.

[0023] The ammonia stripping unit is specifically designed as follows: the waste heat from the subcritical water catalytic oxidation unit is used to preheat the ammonia stripping tower to 70-75℃, and 110℃ steam is introduced to heat it to 80-95℃. The process is carried out under a slight positive pressure of 50-70kPa, producing 15-20% ammonia water. The ammonia stripping tower is a packed tower with a theoretical number of 15-20 plates, and the tail gas is absorbed by 20% dilute sulfuric acid.

[0024] Step 3: The wastewater after ammonia extraction is further evaporated and crystallized to produce sodium sulfate.

[0025] The wastewater after ammonia stripping does not need to be cooled. It can be directly evaporated and crystallized in one step by MVR to recover sodium sulfate, which can meet the industrial standard of anhydrous sodium sulfate GB / T6009-2023.

[0026] Step 4: After the subcritical water oxidation reaction, an energy recovery device is used to generate electricity, which is then reused in the system. The pressure remains high after the subcritical water oxidation reaction. The exhaust gas separated during air oxidation also has high pressure and a large amount of N2. This exhaust gas can be used to generate electricity using the energy recovery device and reused in the system.

[0027] Subcritical water catalytic oxidation technology for treating caprolactam wastewater utilizes the unique physicochemical properties of subcritical water, combined with the action of catalysts and oxidants, to achieve efficient degradation of complex organic matter in the wastewater, while simultaneously completing the conversion of elements such as nitrogen and sulfur.

[0028] In a subcritical water system, a catalyst is introduced to create a synergistic effect. The catalyst activates oxygen molecules through electron transfer, generating strong oxidizing species such as •OH (hydroxyl radical). Stable organic compounds in caprolactam wastewater, such as benzene ring compounds (benzoic acid, hexahydrobenzoic acid) and cyclic structures (cyclohexanecarboxylic acid), undergo C-C and CN bond breakage under the high-temperature environment of subcritical water and the selective catalysis of the catalyst, gradually decomposing into smaller organic molecules (such as acetic acid), and ultimately mineralizing into CO2 and H2O. Organic nitrogen in the wastewater (such as the amide groups in caprolactam) is converted into NH4 under oxidative conditions. + Organic sulfur compounds (such as sulfonic acids) are converted into SO4. 2- This achieves the harmless transformation of pollutants and lays the foundation for subsequent ammonia recovery by steaming and ammonia water recovery by evaporation and crystallization.

[0029] Example 1 A factory produces 300 tons of caprolactam wastewater per day with a COD concentration of 100,000 mg / L.

[0030] A method for the resource recovery treatment of caprolactam wastewater includes the following steps: Step 1: High-concentration caprolactam wastewater is mixed with air oxidant and then oxidized in a subcritical water catalytic oxidation process.

[0031] The subcritical water catalytic oxidation process specifically involves: preparing caprolactam wastewater to a capacity of 900 t / d using an oxidizing solution. The high-concentration caprolactam wastewater has a chemical oxygen demand (COD) of 35,000 mg / L, ammonia nitrogen of 2,500 mg / L, and total nitrogen of 4,000 mg / L. Initially, soft water is used for dilution, and air oxidant is added, with 130 Nm³ of air added per ton of prepared solution. 3 The solution is pressurized to the reaction pressure using a compressor and then mixed with a pressurized caprolactam preparation solution. The mixture is then heated in a subcritical water catalytic oxidation unit before entering the reactor to remove organic matter from the water and mineralize organic matter (N and S are converted to NH4). + SO4 2- After the reaction, the organic matter is decomposed into carbon dioxide, water, and a small amount of acetic acid. The reaction temperature is 260℃ and the reaction pressure is 6MPa. The ammonia nitrogen concentration in the product water is 3700mg / L, and the COD concentration is 5500mg / L. The exhaust gas is absorbed by dilute acid and then photocatalytically removed to remove the remaining volatile organic compounds (VOCs) before being discharged.

[0032] Step 2: After the reaction, the oxidizing liquid is adjusted to pH 11.5 by adding liquid alkali or flake alkali, and then sent to the ammonia distillation unit to distill off 15%~20% ammonia water, which is then reused in caprolactam production.

[0033] The ammonia stripping unit specifically involves: preheating the ammonia stripping tower to 75°C using waste heat from the subcritical water catalytic oxidation unit; introducing 110°C steam to raise the temperature to 85°C; and operating under a slightly positive pressure of 55 kPa, resulting in an ammonia nitrogen concentration of 100 mg / L in the product water. This produces 15% ammonia water. The ammonia stripping tower is a packed tower with 18 theoretical plates, and the tail gas is absorbed using 20% ​​dilute sulfuric acid.

[0034] Step 3: The wastewater after ammonia extraction is further evaporated and crystallized to produce sodium sulfate.

[0035] The wastewater after ammonia stripping does not need to be cooled. It can be directly evaporated and crystallized in one step by MVR to recover sodium sulfate. The resulting anhydrous sodium sulfate has a purity of ≥98.5%, which meets the first-class standard of GB / T6009-2023.

[0036] Step 4: After the subcritical water oxidation reaction, an energy recovery device is used to generate electricity, which is then reused in the system. The pressure remains high after the subcritical water oxidation reaction. The exhaust gas separated during air oxidation also has high pressure and a large amount of N2. This exhaust gas can be used to generate electricity after the energy recovery device and reused in the system. The exhaust gas volume after oxidation is 4500 Nm³. 3 / h, generating 900kW·h of electricity per day.

[0037] Example 2 A factory produces 500 tons of caprolactam wastewater per day with a COD concentration of 120,000 mg / L.

[0038] A method for the resource recovery treatment of caprolactam wastewater includes the following steps: Step 1: High-concentration caprolactam wastewater is mixed with air oxidant and then oxidized in a subcritical water catalytic oxidation process.

[0039] The subcritical water catalytic oxidation process specifically involves: preparing caprolactam wastewater to a capacity of 1500 t / d using an oxidizing solution. The high-concentration caprolactam wastewater has a chemical oxygen demand (COD) concentration of 40,000 mg / L, ammonia nitrogen of 3,200 mg / L, and total nitrogen of 5,700 mg / L. Initially, soft water is used for dilution, and oxygen oxidant is added at a rate of 30 Nm³ of oxygen per ton of prepared solution. 3 The solution is pressurized to the reaction pressure using a compressor and then mixed with a pressurized caprolactam preparation solution. The mixture is then heated in a subcritical water catalytic oxidation unit before entering the reactor to remove organic matter from the water and mineralize organic matter (N and S are converted to NH4). + SO4 2- After the reaction, organic matter is decomposed into carbon dioxide, water, and a small amount of acetic acid. The reaction temperature is 210℃, and the reaction pressure is 5MPa. The ammonia nitrogen concentration in the product water is 4800mg / L, and the COD concentration is 6200mg / L. The tail gas is absorbed by dilute acid and then the remaining VOCs are removed by photocatalysis before being discharged.

[0040] Step 2: After the reaction, the oxidizing liquid is adjusted to pH 12.5 by adding liquid alkali or flake alkali, and then sent to the ammonia distillation unit to distill off 20% ammonia water, which is then reused in caprolactam production.

[0041] The ammonia stripping unit specifically involves: preheating the ammonia stripping tower to 70°C using waste heat from the subcritical water catalytic oxidation unit; introducing 110°C steam to raise the temperature to 90°C; and operating under a slightly positive pressure of 65 kPa, resulting in an ammonia nitrogen concentration of 120 mg / L in the product water. This produces 18% ammonia water. The ammonia stripping tower is a packed tower with 20 theoretical plates, and the tail gas is absorbed using 20% ​​dilute sulfuric acid.

[0042] Step 3: The wastewater after ammonia extraction is further evaporated and crystallized to produce sodium sulfate.

[0043] The wastewater after ammonia stripping does not need to be cooled. It can be directly evaporated and crystallized in one step by MVR to recover sodium sulfate. The resulting anhydrous sodium sulfate has a purity of ≥98.5%, which meets the first-class standard of GB / T6009-2023.

[0044] Step 4: After the subcritical water oxidation reaction, an energy recovery device is used to generate electricity, which is then reused in the system. The pressure remains high after the subcritical water oxidation reaction. The exhaust gas separated during air oxidation also has high pressure and a large amount of N2. This exhaust gas can be used to generate electricity after the energy recovery device and reused in the system. The exhaust gas volume after oxidation is 5500 Nm³. 3 / h, generating 1000kW·h of electricity per day.

[0045] Comparative Example 1 The caprolactam wastewater is the same as in Example 1, with the following initial water quality parameters: treatment scale 300 t / d, COD concentration 100,000 mg / L, ammonia nitrogen 2,500 mg / L, total nitrogen 4,000 mg / L. The wastewater contains high concentrations of recalcitrant organic matter and sulfates. The treatment steps are as follows: 1. Introduce the wastewater into the equalization tank and adjust the pH value to 3.0 by adding 98% concentrated sulfuric acid.

[0046] 2. Core treatment: Fenton oxidation degrades organic matter.

[0047] The adjusted wastewater is pumped into a plug-flow Fenton reactor, and the reagents are added in two stages: Catalyst addition: Ferrous sulfate heptahydrate was selected as the catalyst, prepared into a 20% aqueous solution, and continuously added via a metering pump. The dosage was based on Fe... 2+ The ratio of ferrous sulfate heptahydrate to wastewater COD is controlled at 1:15, meaning approximately 6.7 kg of ferrous sulfate heptahydrate is added per cubic meter of wastewater to ensure sufficient Fe content in the reaction system. 2+ Sufficient concentration.

[0048] Oxidant addition: 27.5% industrial-grade hydrogen peroxide is used as the oxidant, added to the reaction tank in three points (dosage ratio of 3:3:4). The total dosage is controlled according to a hydrogen peroxide to COD mass ratio of 1.5:1, that is, approximately 15 kg of hydrogen peroxide is added per cubic meter of wastewater. A turbine agitator installed in the tank ensures thorough mixing of the reagent and wastewater.

[0049] Reaction conditions control: The hydraulic retention time in the reaction tank is controlled at 4 hours, and the reaction temperature is maintained at 25-30℃ throughout the process.

[0050] After the reaction, the COD concentration of the wastewater dropped to 35,000 mg / L. The recalcitrant organic matter was oxidized by hydroxyl radicals into small molecule organic acids (such as acetic acid and formic acid), but the concentrations of ammonia nitrogen and total nitrogen remained basically unchanged (ammonia nitrogen 2,480 mg / L, total nitrogen 3,950 mg / L). The sulfate concentration increased slightly due to the reaction.

[0051] 3. Post-treatment: Neutralization, sedimentation, and sludge removal.

[0052] The oxidized wastewater was introduced into a neutralization sedimentation tank, the pH was adjusted to 7.5, and polyaluminum chloride (PAC, concentration 10%) was added as a coagulant (dosage 50 mg / L) and 0.1% polyacrylamide (PAM) as a coagulant aid (dosage 5 mg / L). The mixture was stirred and allowed to settle.

[0053] The supernatant enters the subsequent treatment unit, where the iron sludge generated at the bottom is removed. Supernatant water quality parameters: COD 32000 mg / L, ammonia nitrogen 2450 mg / L, total nitrogen 3900 mg / L, suspended solids ≤50 mg / L.

[0054] The supernatant after neutralization and precipitation is sent to a forced circulation evaporator crystallizer for cooling and crystallization, and crude salt product is obtained by centrifugal separation.

[0055] After centrifugation, the crude salt has a moisture content of ≤5% and a purity of 82%, which does not meet the first-class standard of GB / T6009-2023. It also contains trace amounts of organic nitrogen and must be disposed of as hazardous waste.

[0056] 4. Biochemical treatment to achieve nitrogen removal standards.

[0057] The condensate after evaporation and crystallization is introduced into the A / O biological treatment unit. Nitrifying bacteria are added to enhance ammonia nitrification, converting ammonia nitrogen into nitrate nitrogen.

[0058] After biological treatment, the effluent enters the secondary sedimentation tank for sedimentation. The supernatant has COD ≤ 500 mg / L, ammonia nitrogen ≤ 80 mg / L, and total nitrogen ≤ 150 mg / L, which meets the discharge standards.

[0059] The exhaust gas passes through a packed absorption tower and is then treated by an activated carbon adsorption device before being discharged. The secondary steam generated by evaporation and crystallization is condensed and recovered as process makeup water, while the non-condensable gas is discharged directly. All solid waste (iron sludge, coarse salt, and biochemical sludge) is entrusted to a qualified unit for safe disposal in accordance with hazardous waste disposal regulations.

[0060] Comparative Example 2 The caprolactam wastewater, identical to that in Example 1, had an initial COD concentration of 100,000 mg / L, ammonia nitrogen of 2,500 mg / L, and total nitrogen of 4,000 mg / L. The treatment steps are as follows: 1. pH adjustment.

[0061] The raw wastewater was sent to the equalization tank, and 98% concentrated sulfuric acid was added to adjust the pH to 2. The mixture was stirred evenly, and the water temperature was controlled at 25℃ with a retention time of 30 minutes.

[0062] 2. Hydrogen peroxide and low-temperature catalytic wet oxidation.

[0063] The pH-adjusted wastewater is fed into an oxidation reactor, and a titanium dioxide-supported activated carbon catalyst is added. 30% hydrogen peroxide is added at a molar ratio of 1.5:1 to COD.

[0064] The reaction temperature was controlled at 130℃, the operating pressure at 1MPa, the stirring rate at 200r / min, and the reaction time at 3h. COD changes were monitored during the process, and the reaction was stopped when COD dropped to 10000mg / L.

[0065] 3. Neutralization and solid-liquid separation.

[0066] After the reaction, the effluent is sent to a neutralization tank, where 40% liquid alkali is added to adjust the pH to 7, generating sodium sulfate precipitate. After a residence time of 1 hour, the effluent is sent to a plate and frame filter press for solid-liquid separation to obtain filter cake (mainly sodium sulfate, with a water content ≤30%). The filtrate is collected for later use.

[0067] 4. Try steaming ammonia.

[0068] Add 40% liquid alkali to the filtrate to adjust the pH to 12, and send it to the ammonia stripping tower (packed tower, theoretical number of trays 10). Introduce 110℃ steam, control the bottom temperature of the tower to 90℃ and the top pressure of the tower to 0.05MPa, and attempt to recover ammonia water.

[0069] Because the COD in the filtrate is still high (about 8000 mg / L), and the distilled ammonia water contains organic impurities (concentration ≥1000 mg / L), it cannot be reused in caprolactam production and needs to be further purified by distillation (adding an extra 500 kW·h / t ammonia water energy consumption).

[0070] 5. Final processing.

[0071] The COD of the wastewater after ammonia stripping is about 6000 mg / L. It is sent to the biological treatment unit (A / O process) and the final effluent COD is ≤500 mg / L. The filter cake obtained by plate and frame filter press is dried and has a sodium sulfate purity of about 90%. Due to the presence of catalyst residue, it cannot meet the industrial reuse standard.

[0072] In Comparative Example 1, the COD was reduced to 500 mg / L through biochemical treatment, with no ammonia recovery, the crude salt was classified as hazardous waste, no energy recovery, and the desorption tail gas was directly discharged. In Comparative Example 2, the COD was reduced to 500 mg / L through oxidative biochemical treatment, with an ammonia recovery rate of 60% and a sodium sulfate purity of 90%, which did not meet the reuse standard. In Example 1, the COD after oxidation was 500 mg / L, requiring no additional biochemical treatment, with an ammonia recovery rate of 100%, a sodium sulfate purity ≥98.5% (for reuse), and the tail gas could be used to generate electricity.

[0073] Therefore, this invention employs the aforementioned method for the resource-based treatment of caprolactam wastewater. It utilizes subcritical water catalytic oxidation for high organic matter removal efficiency and low energy consumption. Ammonia stripping is heated using the waste heat from the subcritical water catalytic oxidation process, effectively utilizing calorific value and reducing steam consumption. The wastewater after ammonia removal is directly evaporated, resulting in low energy consumption. An energy recovery device is used to recover the tail gas after oxidation, generating electricity. This effectively recovers organic matter and ammonium sulfate from the wastewater, maximizing resource utilization and reducing waste.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for the resource-based treatment of caprolactam wastewater, characterized in that, Includes the following steps: Step 1: High-concentration caprolactam wastewater is mixed with an oxidant and then oxidized in a subcritical water catalytic oxidation process. Step 2: After the reaction, the oxidizing liquid is adjusted to pH by adding liquid alkali or flake alkali, and then sent to the ammonia distillation unit to distill off 15%~20% ammonia water, which is then reused in caprolactam production. Step 3: The wastewater after ammonia extraction is further evaporated and crystallized to produce sodium sulfate. Step 4: After the subcritical water oxidation reaction, the pressure is still relatively high. An energy recovery device is used to generate electricity, which is then reused in the system.

2. The method for resource-based treatment of caprolactam wastewater according to claim 1, characterized in that, In step 1, the subcritical water catalytic oxidation specifically involves: adjusting the chemical oxygen demand (COD) of the high-concentration caprolactam wastewater to 30,000-40,000 mg / L using an oxidizing agent; initially diluting with soft water; adding an oxidant; pressurizing the solution to the reaction pressure using a compressor; mixing the mixture with the pressurized caprolactam solution; heating the mixture within the subcritical water catalytic oxidation unit; and then introducing it into the reactor to remove organic matter from the water and mineralize organic matter (N and S are converted to NH4). + SO4 2- After the reaction, the organic matter is decomposed into carbon dioxide, water and acetic acid. The exhaust gas is absorbed by dilute acid and then the remaining volatile organic compounds are removed by photocatalysis before being emitted.

3. The method for resource-based treatment of caprolactam wastewater according to claim 1, characterized in that: In step 1, the oxidant is either air or oxygen, and the amount of air added is 120-150 Nm. 3 / h, oxygen addition rate 25-30 Nm 3 / h.

4. The method for resource recovery treatment of caprolactam wastewater according to claim 1, characterized in that: In step 1, the reaction temperature is 130~270℃ and the pressure is 1~8MPa.

5. The method for resource recovery treatment of caprolactam wastewater according to claim 1, characterized in that: In step 2, adjust the pH to 10-13.

6. The method for resource-based treatment of caprolactam wastewater according to claim 1, characterized in that, In step 2, the ammonia stripping unit specifically involves: using the waste heat from the subcritical water catalytic oxidation unit to preheat the ammonia stripping tower to 70-75℃, introducing 110℃ steam, heating to 80-95℃, and proceeding under a slightly positive pressure of 50-70kPa to produce 15-20% ammonia water. The ammonia stripping tower is a packed tower with a theoretical number of 15-20 plates, and the tail gas is absorbed by 20% dilute sulfuric acid.

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