Method for treating hexamethylene diamine in nylon 66 wastewater based on fenton oxidation coupled with biological method

Abstract

The application discloses a method for treating hexamethylene diamine in nylon 66 wastewater based on Fenton oxidation coupling biological method, and the method is characterized by utilizing aldehyde substances in cyclohexanone process wastewater to reduce nitrate to generate nitrous acid, and adopting a calcium peroxide, trisodium citrate and ferrous salt slow-release catalytic system to realize stable supply of oxidizing agent and efficient generation of nitrous acid, so that a low and constant hydrogen peroxide concentration in the solution can be maintained, the hydrogen peroxide concentration reacts with nitrous acid to generate peroxynitrous acid with extremely strong oxidizing property, the substance cooperates with hydroxyl radicals to realize precise and efficient degradation of hexamethylene diamine molecules, not only the biological toxicity of hexamethylene diamine is completely broken, the wastewater biodegradability is significantly improved, but also organic nitrogen is converted into nitrogen forms which are easy to be utilized by microorganisms, so that efficient and stable reaction conditions are created for subsequent anaerobic biological treatment, and finally the advantages of chemical oxidation and biological degradation are complemented.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and more particularly to a method for treating hexamethylenediamine in nylon 66 wastewater based on the Fenton oxidation-coupled biological method. Background Technology

[0002] Nylon 66 wastewater refers to industrial wastewater generated during the production of nylon 66 resin, mainly originating from processes such as polymerization, slicing extraction, and equipment cleaning. It is characterized by high chemical oxygen demand (COD), high organic nitrogen content, and complex water quality, belonging to typical recalcitrant high-molecular-weight organic wastewater. Direct discharge of such wastewater would severely burden the aquatic ecosystem.

[0003] The wastewater from Nylon 66 mainly contains organic intermediates such as hexamethylenediamine, adipic acid, cyclohexane, cyclohexanone, and cyclohexanol, as well as reaction byproducts such as nitrates, ammonium salts, and trace amounts of catalysts. These pollutants are biotoxic and can inhibit microbial activity. Hexamethylenediamine is highly alkaline and corrosive, while adipic acid easily creates an acidic environment. The overall wastewater has a high oxygen demand, which can easily lead to eutrophication and harm aquatic organisms.

[0004] Hexamethylenediamine (HDMA) is a key pollutant that needs to be removed from Nylon 66 wastewater due to its high biotoxicity, strong alkalinity, and recalcitrant degradation. Removing HDMA can effectively reduce wastewater toxicity and prevent its long-term persistence in the environment, thus avoiding harm to the ecosystem.

[0005] In existing technologies, the treatment of hexamethylenediamine typically employs the Fenton process or biological treatment. However, using either the Fenton process alone can result in ineffective wastewater treatment. While the Fenton process can decompose large organic molecules like hexamethylenediamine through strong oxidation, it requires the addition of large amounts of hydrogen peroxide and ferrous salts to handle high concentrations of pollutants in wastewater, leading to high operating costs and uneconomical practices. Furthermore, the reaction often fails to completely mineralize the pollutants, leaving behind various small-molecule organic acids, preventing the effluent from meeting chemical oxygen demand (COD) standards and hindering comprehensive wastewater treatment. Using biological treatment alone to treat hexamethylenediamine in wastewater has a strong inhibitory and toxic effect on microorganisms, directly causing the biochemical system to shut down, weakening the biological treatment process, and rendering it ineffective in removing the pollutants.

[0006] However, the traditional homogeneous Fenton reaction system is unstable, and iron ions are prone to precipitation and deactivation. In order to ensure the effect of Fenton treatment, a large amount of iron salt is added. The excessive amount of iron salt added will leave a small amount of soluble ferrous ions or ferric complexes in the system.

[0007] These residual soluble ferrous ions or ferric complexes, when carried into subsequent biological treatment units along with the wastewater, react with substances such as hydrogen peroxide produced during biological metabolism in aerated or trace dissolved oxygen environments. This continuously generates highly oxidizing substances such as hydroxyl radicals. These substances indiscriminately attack the cell membranes, enzymes, and DNA of microorganisms, causing continuous oxidative stress damage to the bacterial flocs, inhibiting their activity, and even leading to bacterial death, thereby disrupting the stability of the biochemical system.

[0008] Therefore, existing technologies need to be improved to address the aforementioned problems. Summary of the Invention

[0009] This invention overcomes the shortcomings of the prior art and provides a method for treating hexamethylenediamine in nylon 66 wastewater based on Fenton oxidation coupled with biological methods, aiming to solve the defects of the poor treatment effect of Fenton oxidation and biological methods on hexamethylenediamine in nylon 66 wastewater in the prior art.

[0010] To achieve the above objectives, the technical solution adopted by this invention is as follows: a method for treating hexamethylenediamine in nylon 66 wastewater based on Fenton oxidation-coupled biological method, comprising the following steps:

[0011] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.8-1.4 under a stirring speed of 250-320 rpm to obtain mixed wastewater;

[0012] S2: Add sulfuric acid to the mixed wastewater to adjust the pH to 2.5-3.0, and stir at 28-35℃ for 15-25 minutes;

[0013] S3: Add composite oxidizing material to the mixed wastewater. The composite oxidizing material includes anhydrous ferrous sulfate, trisodium citrate and calcium peroxide. React for 45-60 minutes to obtain oxidized effluent.

[0014] S4: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 7.8-8.2, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation;

[0015] S5: The pretreated effluent is subjected to anaerobic biological treatment.

[0016] In a preferred embodiment of the present invention, the pH value of the nylon 66 polycondensation process wastewater is 1.8-2.2 and the nitrate concentration is 800-1200 mg / L, and the pH value of the cyclohexanone process wastewater is 3.5-4.5.

[0017] In a preferred embodiment of the present invention, the total aldehyde content in the cyclohexanone process wastewater is 450-600 mg / L, and the aldehydes include formaldehyde, acetaldehyde, and cyclohexenal, with a molar ratio of formaldehyde:acetaldehyde:cyclohexenal = 1:0.6-0.9:0.3-0.5, and the total reducing equivalent is 2.4-3.1. .

[0018] In a preferred embodiment of the present invention, the calcium peroxide in the composite oxide material is a powder with an average particle size of 40-60 μm, which is generated by hydrolysis in water. The release rate is 0.18-0.28 mmol / min·L.

[0019] In a preferred embodiment of the present invention, the mass ratio of the composite oxide material added is: : : =1:0.4-0.6:1.6-3.6, where The dosage is 1.6-2.4 mmol / L. The dosage is 8.0-12.0 mmol / L.

[0020] In a preferred embodiment of the present invention, the composite oxide material adopts a segmented addition strategy: 60% of the total addition is added all at once in the initial stage of the reaction, and the remaining 40% is added in two equal amounts at 15 minutes and 30 minutes after the start of the reaction.

[0021] In a preferred embodiment of the present invention, the anhydrous ferrous sulfate, the trisodium citrate, and the calcium peroxide are added as follows: the anhydrous ferrous sulfate and the trisodium citrate are mixed evenly and then added to the mixed wastewater and stirred for 3-5 minutes, and then calcium peroxide is added to the mixed wastewater.

[0022] In a preferred embodiment of the present invention, the S4 sodium hydroxide solution is a 25-35% (w / w) sodium hydroxide aqueous solution.

[0023] In a preferred embodiment of the present invention, the concentration of flocculant added in S4 is 5-10 mg / L.

[0024] In a preferred embodiment of the present invention, the anaerobic biological treatment conditions in S5 are a temperature of 33-37°C and a hydraulic retention time of 24-36 h.

[0025] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0026] (1) This invention provides a method for treating hexamethylenediamine in nylon 66 wastewater based on Fenton oxidation-coupled biological method. It utilizes aldehydes in cyclohexanone process wastewater to reduce nitrates to generate nitrites and uses a slow-release catalytic system of calcium peroxide, trisodium citrate and ferrous salt as a feature to achieve stable supply of oxidant and efficient generation of nitrites. It can maintain a low and constant concentration of hydrogen peroxide in the solution, so that it reacts preferentially with nitrites to generate highly oxidizing peroxynitrites, which can work synergistically with hydroxyl radicals to achieve precise and efficient degradation of hexamethylenediamine molecules. Compared with existing hexamethylenediamine treatment methods, it not only completely eliminates the biotoxicity of hexamethylenediamine and significantly improves the biodegradability of wastewater, but also converts organic nitrogen into nitrogen forms that are easily utilized by microorganisms, thereby creating efficient and stable reaction conditions for subsequent anaerobic biological treatment, and finally achieving the complementary advantages of chemical oxidation and biodegradation.

[0027] (2) In this invention, the two types of wastewater are mixed and then treated. The nitrate-rich nylon 66 wastewater and the cyclohexanone process wastewater containing aldehydes are mixed in proportion to achieve water quality homogenization and introduce key reactants. Under subsequent strong acid conditions, the aldehydes in the cyclohexanone process wastewater act as reducing agents to efficiently reduce the nitrates in the nylon 66 wastewater to the key intermediate nitrous acid. This nitrous acid can react with hydrogen peroxide generated by the slow-release oxidant to generate highly oxidizing peroxynitrous acid, thereby achieving efficient and specific degradation of recalcitrant organic compounds such as hexamethylenediamine in synergy with hydroxyl radicals. This process not only reduces the cost of individual treatment but also enhances the targeting and thoroughness of the oxidation reaction from the source. By utilizing the synergistic reaction of its specific chemical components, waste treatment is achieved and the treatment efficiency is significantly improved.

[0028] (3) In this invention, calcium peroxide powder is used and a specific hydrolysis release rate is provided as a feature to achieve slow and continuous release of hydrogen peroxide. This avoids the instantaneous peak concentration of hydrogen peroxide and the ineffective decomposition reaction caused by it under the traditional addition method. This makes the concentration of hydroxyl radicals generated by ferrous ion catalysis in the system more stable. At the same time, it ensures that nitrous acid can fully react with the continuously low concentration of hydrogen peroxide to generate peroxynitrous acid. The synergistic effect of the two free radicals prolongs the effective oxidation time, thereby significantly improving the oxidation efficiency of recalcitrant organic matter and reducing reagent consumption. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart illustrating the method steps of a preferred embodiment of the present invention. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0033] Application Overview:

[0034] Currently, the traditional Fenton process faces significant challenges in treating Nylon 66 wastewater. The high concentrations of nitrates and recalcitrant organic matter in this wastewater inhibit subsequent biological treatment. Traditional methods, such as directly adding high-concentration hydrogen peroxide, are not only costly but also result in violent reactions, significant ineffective decomposition, and a lack of sustained and efficient generation of strong oxidizing agents capable of breaking down these stubborn pollutants. Simultaneously, the total nitrogen pollution problem in the wastewater remains unresolved; existing technologies suffer from low oxidation efficiency, high reagent consumption, operational instability, and an inability to synergistically remove nitrogen.

[0035] An unexpected discovery was made: by mixing cyclohexanone process wastewater containing aldehydes with nylon 66 wastewater containing nitrates under strongly acidic conditions, the reducing properties of aldehydes can convert nitrates in situ into the key intermediate nitrous acid, laying the foundation for subsequent reactions. Abandoning traditional liquid hydrogen peroxide, innovatively using solid calcium peroxide as a slow-release oxidant precursor, and supplemented with citric acid to complex iron ions to form a stable catalyst, the slow and continuous release of hydrogen peroxide and iron ions was achieved. This allows nitrous acid to react efficiently with in-situ, low-concentration hydrogen peroxide, directionally generating highly oxidizing peroxynitrous acid, which then synergistically acts with hydroxyl radicals to achieve precise and efficient decomposition of recalcitrant organic compounds such as hexamethylenediamine.

[0036] This scheme utilizes aldehydes in cyclohexanone process wastewater to reduce nitrates and generate nitrite, and employs a calcium peroxide-ferric citrate slow-release catalytic system to achieve a stable supply of oxidant and effective inhibition of ineffective decomposition. This slow-release system maintains a low and constant concentration of hydrogen peroxide in the solution, prioritizing its reaction with nitrite over the self-decomposition side reaction of nitrite. This results in the continuous and efficient generation of peroxynitrite, a strong oxidant, which, in synergistic action with hydroxyl radicals, achieves the complete degradation of hexamethylenediamine molecules. This deep oxidation process not only significantly reduces the chemical oxygen demand and biotoxicity of the wastewater, but also creates easily degradable low-molecular-weight organic matter and suitable nitrogen forms for subsequent biological treatment, ensuring the stable operation of the biological system and the efficient removal of total nitrogen.

[0037] like Figure 1 As shown, a method for treating hexamethylenediamine in nylon 66 wastewater based on the Fenton oxidation-coupled biological process includes the following steps:

[0038] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.8-1.4 under a stirring speed of 250-320 rpm to obtain mixed wastewater;

[0039] Nylon 66 wastewater contains high concentrations of nitrates, which are relatively inert in traditional Fenton oxidation and contribute to total nitrogen pollution. Cyclohexanone process wastewater, on the other hand, contains reducing organic compounds such as aldehydes. When the two are mixed in a specific ratio, under subsequent strongly acidic conditions, the aldehydes act as effective reducing agents, reducing the nitrates in the nylon 66 wastewater to nitrites. Without the aldehydes provided by the cyclohexanone process wastewater, this crucial source of nitrites cannot be effectively established.

[0040] The nitrous acid generated reacts with the hydrogen peroxide slowly released by the subsequently added calcium peroxide to produce peroxynitrous acid, a highly oxidizing agent with specific degradation effects on nitrogen-containing organic matter. This achieves waste-to-waste treatment, providing key reactants for advanced oxidation processes and simultaneously initiating the conversion and removal of total nitrogen in the wastewater, creating favorable conditions for subsequent biological denitrification. Therefore, the mixing of the two wastewaters is not a simple dilution or homogenization, but rather the chemical basis for constructing a highly efficient oxidative degradation system.

[0041] S2: Add sulfuric acid to the mixed wastewater to adjust the pH to 2.5-3.0, and stir at 28-35℃ for 15-25 minutes;

[0042] The purpose of adding sulfuric acid is to create the necessary strongly acidic environment for subsequent key chemical reactions. Acidic conditions are thermodynamically necessary for the conversion of nitrates to nitrous acid, significantly enhancing the reducing power of aldehydes in the cyclohexanone process wastewater, thereby efficiently generating nitrous acid that initiates a chain oxidation reaction. Simultaneously, a low pH value is a prerequisite for maintaining the dissolved state of iron ions and ensuring the efficient conduct of the Fenton reaction, and it also effectively prevents iron ions from prematurely forming hydroxide precipitates and becoming deactivated.

[0043] The purpose of mixing the two types of wastewater thoroughly before adding sulfuric acid is to ensure that the subsequent key chemical reactions can proceed uniformly and efficiently throughout the system, avoiding side reactions and reduced reaction efficiency caused by local overconcentration or pH unevenness.

[0044] Thorough mixing of the wastewater beforehand with mechanical stirring ensures that the aldehydes in the cyclohexanone process wastewater and the nitrates in the nylon 66 wastewater are evenly distributed in the solution. When sulfuric acid is added to adjust the pH to strongly acidic, the entire reaction system can simultaneously reach the preset acidity conditions.

[0045] This homogenized environment creates optimal kinetic conditions for the reduction of nitrates by aldehydes to produce nitrites, ensuring a rapid and thorough reaction and avoiding problems such as excessively high local pH leading to nitrate decomposition failure or insufficient local pH causing slow initiation of the reduction reaction. This lays a stable and reliable foundation for the entire oxidation chain.

[0046] S3: Add composite oxidizing material to the mixed wastewater. The composite oxidizing material includes anhydrous ferrous sulfate, trisodium citrate and calcium peroxide. React for 45-60 minutes to obtain oxidized effluent.

[0047] A strongly acidic environment is the most efficient pH condition for Fenton-like reactions. Pre-acidifying to a low pH ensures that the subsequently added iron salt remains in a soluble, active state in the solution, preventing it from rapidly forming ferric hydroxide precipitate and losing its catalytic activity at neutral pH. Simultaneously, calcium peroxide exhibits optimal stability and slow-release properties under acidic conditions. If the order is reversed, with the oxidant and catalyst added to the neutral wastewater before pH adjustment, it will lead to instantaneous precipitation of iron ions and excessively rapid decomposition of hydrogen peroxide, resulting in significant waste of reagents and a sharp decline in reaction efficiency.

[0048] S4: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 7.8-8.2, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation;

[0049] The effluent after strong acid Fenton oxidation contains a large amount of iron ions, unreacted small-molecule organic matter and intermediate products, and has a low pH. By adding sodium hydroxide to adjust the pH to near neutral, the direct toxicity of the acidic environment to microorganisms can be eliminated, and the dissolved iron ions can be converted into ferric hydroxide precipitate. The addition of cationic polyacrylamide can coagulate these precipitates, colloids and some hydrophobic organic matter into easily settling flocs through adsorption bridging. Finally, these flocs are removed through solid-liquid separation, thereby significantly reducing the toxicity, suspended solids and color of the wastewater, and providing anaerobic biological treatment with moderate load and stable water quality influent conditions.

[0050] S5: The pretreated effluent will undergo anaerobic biological treatment.

[0051] The steps will be described in detail below:

[0052] In step S1,

[0053] The pH value of the wastewater from the nylon 66 polycondensation process is 1.8-2.2, and the nitrate concentration is 800-1200 mg / L. The pH value of the wastewater from the cyclohexanone process is 3.5-4.5.

[0054] The total aldehyde content in the cyclohexanone process wastewater is 450-600 mg / L, including formaldehyde, acetaldehyde, and cyclohexenal, with a molar ratio of formaldehyde:acetaldehyde:cyclohexenal = 1:0.6-0.9:0.3-0.5, and a total reducing equivalent of 2.4-3.1. .

[0055] The strong acidity of nylon 66 wastewater mixed with the weak acidity of cyclohexanone process wastewater means that the initial pH value after mixing will naturally be in a lower acidic range. This reduces the amount of sulfuric acid required to adjust the system to the target pH of 2.5-3.0 in step S2, lowers the reagent costs of subsequent treatment, and demonstrates the economic efficiency of the process.

[0056] The high concentration of nitrates clarifies the crucial basis for the mixing ratio. The high nitrate concentration of 800-1200 mg / L in nylon 66 wastewater is precisely the reactant that needs to be reduced to nitrite under acidic conditions, while the aldehydes in the cyclohexanone process wastewater act as a reducing agent. The core calculation basis for mixing the two at a volume ratio of 1:0.8-1.4 is to ensure that the total amount of aldehydes provided by the cyclohexanone process wastewater achieves an optimal stoichiometric ratio with the high concentration of nitrates carried by the nylon 66 wastewater. A ratio that is too high or too low will result in an excess of reducing agent or nitrate, thereby affecting the efficiency of nitrite formation and producing unnecessary byproducts.

[0057] Different aldehydes exhibit varying reduction characteristics and reaction rates: formaldehyde has strong reducing properties and reacts rapidly, quickly initiating the reduction process of nitrates; acetaldehyde and cyclohexenal provide continuous and stable reducing capabilities. This multi-aldehyde composite system avoids the problems of violent reactions or insufficient subsequent kinetics that may occur with single aldehydes, ensuring a stable and efficient nitrite formation stage.

[0058] The total reducing equivalent of 2.4-3.1 ensures the total reducing capacity provided by the cyclohexanone process wastewater, which is perfectly matched with the high concentration of nitrates (800-1200 mg / L) in the Nylon 66 wastewater. This keeps the ratio of reducing agent to oxidant in the optimal reaction range, aiming to convert nitrates into nitrites. This avoids waste of reducing agent and prevents nitrate residues from burdening subsequent processes.

[0059] In step S2,

[0060] The sulfuric acid used is industrial-grade concentrated sulfuric acid with a mass concentration ranging from 92.5% to 98.0%. The dosage is 0.35-0.5 ml per liter of mixed wastewater, and this variable is ultimately used to stabilize the pH value of the system within the target range of 2.5 to 3.0.

[0061] The optimal reaction temperature is maintained within the range of 28 to 35°C. At this temperature, the reaction is stirred for 15 to 25 minutes. This duration is intended to ensure that the temperature and pH of the mixed wastewater system reach uniformity and stability, and to provide sufficient reaction time for the chemical reaction of aldehydes in the cyclohexanone process wastewater reducing nitrates in the nylon 66 wastewater, in order to efficiently generate the target intermediate, nitrite.

[0062] The stirring rate was maintained at 250 to 320 r / min, corresponding to a power input per unit volume of approximately 0.8 to 1.2 kW / m³. This intensity of stirring aims to achieve a fully turbulent state in the reaction system, ensuring rapid and uniform dispersion of sulfuric acid, avoiding localized over-acidity, while simultaneously enhancing the mass transfer process and promoting full contact and reaction between aldehydes and nitrate ions.

[0063] In step S3,

[0064] The calcium peroxide in the composite oxide material is a powder with an average particle size of 40-60 μm, which hydrolyzes in water to form... The release rate is 0.18-0.28 mmol / min·L.

[0065] The mass ratio of composite oxide material added is : : =1:0.4-0.6:1.6-3.6, where The dosage is 1.6-2.4 mmol / L. The dosage is 8.0-12.0 mmol / L.

[0066] The composite oxide material adopts a staged addition strategy: 60% of the total addition is added all at once in the initial stage of the reaction, and the remaining 40% is added in two equal amounts at 15 minutes and 30 minutes after the start of the reaction.

[0067] The specific steps for adding anhydrous ferrous sulfate, trisodium citrate, and calcium peroxide are as follows: Mix anhydrous ferrous sulfate and trisodium citrate evenly, add them to the mixed wastewater and stir for 3-5 minutes, then add calcium peroxide to the mixed wastewater.

[0068] The effective content of calcium peroxide used is not less than 75.0% (w / w). The material is a white powder with a particle size distribution that meets the requirement of D50 of 40-60μm and a particle size range controlled below 1.2 to ensure good dispersibility and suspension in water and avoid rapid sedimentation and agglomeration.

[0069] In the aqueous reaction system, the hydrolysis of calcium peroxide is the core function. Its release rate, defined as the rate of hydrogen peroxide formation per unit volume of reaction solution at a temperature of 25±1℃, initial pH=3.0, and stirring speed of 300 rpm, was measured to be 0.18-0.28 mmol / (min·L). This sustained-release characteristic is central to the process design, aiming to maintain a stable and moderate hydrogen peroxide concentration in the system.

[0070] Based on the organic pollution load in the target wastewater, the dosage of calcium peroxide is controlled to maintain the mass ratio of the theoretical dosage of effective hydrogen peroxide to COD within the range of 0.8:1 to 1.2:1. Its slow hydrolysis characteristic effectively avoids the ineffective decomposition caused by localized overconcentration when adding high concentrations of hydrogen peroxide instantaneously, thus ensuring a stable, efficient, and thorough oxidation reaction.

[0071] In the initial stage of the reaction, 60% of the total dosage was added to the reaction system at once; the remaining 40% was divided into two equal portions and added in equal amounts at 15 min and 30 min of the reaction, respectively. The dosages at each stage, based on the effective components, were as follows: initial dosage of ferrous ions 0.96-1.44 mmol / L, citrate 4.8-7.2 mmol / L, and calcium peroxide 7.2-10.8 mmol / L; each subsequent addition was of the same dosage: ferrous ions 0.32-0.48 mmol / L, citrate 1.6-2.4 mmol / L, and calcium peroxide 2.4-3.6 mmol / L.

[0072] Before adding anhydrous ferrous sulfate and trisodium citrate, they must first be mixed in a dedicated mixer at a speed of 20-30 rpm for 3-5 minutes under dry conditions to ensure that the two are physically homogeneous.

[0073] Add the homogeneously mixed solid material to mixed wastewater with a pH adjusted to 2.5-3.0. Under stirring conditions of 28-35℃ and 250-320 rpm, continue the reaction for 3-5 minutes. The purpose of this stage is to allow the trisodium citrate to dissolve and immediately undergo a complexation reaction with ferrous ions to form a water-soluble ferrous citrate complex. This complex exhibits high stability and effectively prevents premature oxidation or hydrolysis precipitation of ferrous ions in acidic systems.

[0074] After the complexation reaction is complete, a specified amount of calcium peroxide is added to the system. The addition of calcium peroxide is also carried out under continuous stirring to ensure that it is uniformly dispersed in the aqueous phase and begins to slowly hydrolyze and release hydrogen peroxide.

[0075] In step S4,

[0076] The sodium hydroxide solution is a 25-35% (w / w) aqueous solution of sodium hydroxide, and the flocculant concentration is 5-10 mg / L.

[0077] The sodium hydroxide solution is an industrial-grade product, with its concentration strictly controlled within the range of 25% to 35%. This concentration range is based on an optimized balance between operational safety and efficiency: too low a concentration will result in an excessively large volume of liquid added, increasing the subsequent sludge production; too high a concentration can easily cause localized over-alkalinity, leading to the redissolution of ferric hydroxide precipitate or the formation of fine flocs that are difficult to settle, thus posing operational safety risks. During addition, the pH of the system should ultimately be stabilized within the target range of 7.8-8.2.

[0078] The flocculant is cationic polyacrylamide with a molecular weight ranging from 8 million to 12 million Daltons and an ionicity of 20%-40%. The dosage refers to the final concentration in the mixing reaction tank, precisely controlled at 5 to 10 ml / L. Before addition, it needs to be prepared as a 0.1%-0.2% dilute solution. This low-dose addition aims to destabilize and aggregate the formed ferric hydroxide and other colloidal particles into dense, easily sedimenting flocs through charge neutralization and adsorption bridging. Excessive addition will cause the flocs to re-stabilize and disperse.

[0079] In step S5,

[0080] The conditions for anaerobic biological treatment are a temperature of 33-37℃ and a hydraulic retention time of 24-36h.

[0081] The anaerobic biological treatment uses one of the following bacterial species: methanogens, lactobacilli, and glucosibibrio.

[0082] The activated sludge inoculated in the anaerobic biological treatment process is a specially acclimated complex of bacteria. Its main functional bacteria include, but are not limited to: methanogens, such as *Methanococcus* and *Methanotherium*, which are responsible for converting acetic acid, hydrogen, etc. into methane; fermentative bacteria, such as *Lactobacillus fermentum*, whose main function is to hydrolyze and ferment complex organic matter into lactic acid, ethanol, etc.; and homo-acetic bacteria such as *Glucosobacterium oxysporum*, which can convert fermentation products into acetic acid.

[0083] The initial concentration of the microbial community in the inoculated sludge was controlled at 1.0 × 10^6 CFU / mL. After inoculation, the microbial community underwent intensive acclimatization for 15-25 days at 33-37℃ and pH 7.0-7.5, using pretreated effluent as the substrate, to fully adapt to the wastewater characteristics. Under these optimized conditions, the composite microbial agent achieved a COD removal load of 5.0-8.0 kgCOD / (m³·d) and a degradation efficiency of over 95% for typical small-molecule organic acids.

[0084] Example 1:

[0085] This invention provides a method for treating hexamethylenediamine in nylon 66 wastewater based on the Fenton oxidation-coupled biological process, comprising the following steps:

[0086] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.5 under a stirring speed of 320 rpm to obtain a mixed wastewater.

[0087] The nylon 66 polycondensation process wastewater has a pH of 2.0 and a nitrate concentration of 1000 mg / L, while the cyclohexanone process wastewater has a pH of 4.0. The total aldehyde content in the cyclohexanone process wastewater is 550 mg / L, including formaldehyde, acetaldehyde, and cyclohexenal, with a molar ratio of formaldehyde:acetaldehyde:cyclohexenal = 1:0.7:0.4, and a total reducing equivalent of 2.8. .

[0088] S2: Add sulfuric acid to the mixed wastewater to adjust the pH to 3.0, and stir at 30°C for 20 minutes;

[0089] S3: Add composite oxidizing material to the mixed wastewater. The composite oxidizing material includes anhydrous ferrous sulfate, trisodium citrate and calcium peroxide. React for 50 minutes to obtain oxidized effluent.

[0090] The calcium peroxide in the composite oxide material is a powder with an average particle size of 50 μm, which is hydrolyzed in water to produce... The release rate was 0.25 mmol / min·L.

[0091] The dosage of anhydrous ferrous sulfate in the composite oxide material is 2.0 mmol / L, and the dosage of trisodium citrate is 1.0 mmol / L. The dosage was 12.0 mmol / L.

[0092] The composite oxide material adopts a staged addition strategy: 60% of the total addition is added all at once in the initial stage of the reaction, and the remaining 40% is added in two equal amounts at 15 minutes and 30 minutes after the start of the reaction.

[0093] The specific steps for adding anhydrous ferrous sulfate, trisodium citrate, and calcium peroxide are as follows: after mixing anhydrous ferrous sulfate and trisodium citrate evenly, add them to the mixed wastewater and stir for 4 minutes, then add calcium peroxide to the mixed wastewater.

[0094] S4: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 8.0, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation; the sodium hydroxide solution is a 30% mass concentration sodium hydroxide aqueous solution, and the flocculant concentration is 8 mg / L.

[0095] S5: The pretreated effluent will undergo anaerobic biological treatment. The anaerobic biological treatment conditions are: temperature 35℃, hydraulic retention time 30h, and the bacterial species is methanogens. The initial concentration of the bacterial species in the inoculated sludge is controlled at 1.0×10⁻⁶. 6 CFU / mL.

[0096] Example 2:

[0097] The difference between this embodiment and Embodiment 1 is that the volume ratio of the nylon 66 polycondensation process wastewater to the cyclohexanone oxidation process wastewater is 1:0.8, while the rest are the same.

[0098] Example 3:

[0099] The difference between this embodiment and Embodiment 1 is that the volume ratio of the nylon 66 polycondensation process wastewater to the cyclohexanone oxidation process wastewater is 1:1.1, while the rest are the same.

[0100] Example 4:

[0101] The difference between this embodiment and Embodiment 1 is that the volume ratio of the nylon 66 polycondensation process wastewater to the cyclohexanone oxidation process wastewater is 1:1.4, while the rest are the same.

[0102] Example 5:

[0103] The difference between this embodiment and Embodiment 1 is that the volume ratio of the nylon 66 polycondensation process wastewater to the cyclohexanone oxidation process wastewater is 1:1.7, while the rest are the same.

[0104] Example 6:

[0105] The difference between this embodiment and embodiment 3 is that, The dosage was 9.0 mmol / L, and the rest were the same.

[0106] Example 7:

[0107] The difference between this embodiment and embodiment 3 is that, The dosage was 15.0 mmol / L, and the rest were the same.

[0108] Example 8:

[0109] The difference between this embodiment and embodiment 3 is that, The dosage was 18.0 mmol / L, and the rest were the same.

[0110] Example 9:

[0111] The difference between this embodiment and embodiment 3 is that, The dosage was 21.0 mmol / L, and the rest were the same.

[0112] Comparative Example 1:

[0113] This comparative example provides a nylon 66 wastewater treatment process, and the treatment steps are as follows:

[0114] S1: Adjust the pH of the nylon 66 polycondensation process wastewater to 3.0;

[0115] S2: Add composite oxidant material to the wastewater from the nylon 66 polycondensation process. The composite oxidant material includes anhydrous ferrous sulfate and hydrogen peroxide. React for 50 minutes to obtain oxidized effluent. The dosage of anhydrous ferrous sulfate in the composite oxidant material is 2.0 mmol / L, the dosage of trisodium citrate is 10 mmol / L, and the dosage of hydrogen peroxide is 15.0 mmol / L.

[0116] S3: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 8.0, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation; the sodium hydroxide solution is a 30% mass concentration sodium hydroxide aqueous solution, and the flocculant concentration is 8 mg / L.

[0117] S4: The pretreated effluent will be subjected to anaerobic biological treatment. The conditions for anaerobic biological treatment are: temperature 35℃, hydraulic retention time 30h, bacterial species: methanogens, and the initial concentration of bacterial species in the inoculated sludge is controlled at 1.0×10^6 CFU / mL.

[0118] Comparative Example 2:

[0119] This comparative example provides a nylon 66 wastewater treatment process, and the treatment steps are as follows:

[0120] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.5 under a stirring speed of 320 rpm to obtain a mixed wastewater.

[0121] S2: Add composite oxidant material to the wastewater from the nylon 66 polycondensation process. The composite oxidant material includes anhydrous ferrous sulfate and hydrogen peroxide. React for 50 minutes to obtain oxidized effluent. The dosage of anhydrous ferrous sulfate in the composite oxidant material is 2.0 mmol / L, the dosage of trisodium citrate is 10 mmol / L, and the dosage of hydrogen peroxide is 15.0 mmol / L.

[0122] S3: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 8.0, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation; the sodium hydroxide solution is a 30% mass concentration sodium hydroxide aqueous solution, and the flocculant concentration is 8 mg / L.

[0123] S4: The pretreated effluent will undergo anaerobic biological treatment. The anaerobic biological treatment conditions are: temperature 35℃, hydraulic retention time 30h, and the bacterial species is methanogens. The initial concentration of the bacterial species in the inoculated sludge is controlled at 1.0×10⁻⁶. 6 CFU / mL.

[0124] Comparative Example 3:

[0125] This comparative example provides a nylon 66 wastewater treatment process, and the treatment steps are as follows:

[0126] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.5 under a stirring speed of 320 rpm to obtain a mixed wastewater.

[0127] S2: Add composite oxidant material to the wastewater from the nylon 66 polycondensation process. The composite oxidant material includes anhydrous ferrous sulfate and hydrogen peroxide. React for 50 minutes to obtain oxidized effluent. The dosage of anhydrous ferrous sulfate in the composite oxidant material is 2.0 mmol / L, the dosage of trisodium citrate is 10 mmol / L, and the dosage of hydrogen peroxide is 15.0 mmol / L.

[0128] S3: Add sodium hydroxide solution to the oxidized effluent to adjust the pH to 8.0, add cationic polyacrylamide as a flocculant, and obtain the pretreated effluent after solid-liquid separation; the sodium hydroxide solution is a 30% mass concentration sodium hydroxide aqueous solution, and the flocculant concentration is 8 mg / L.

[0129] Comparative Example 4:

[0130] This comparative example provides a nylon 66 wastewater treatment process, the specific steps of which are as follows:

[0131] Wastewater from the nylon 66 polymerization process was treated anaerobicly using biological methods. The anaerobic biological treatment conditions were: temperature 35℃, hydraulic retention time 30 h, and methanogenic bacteria as the bacterial species. The initial concentration of the bacteria in the inoculated sludge was controlled at 1.0 × 10⁻⁶. 6 CFU / mL.

[0132] Comparative Example 5:

[0133] This comparative example provides a nylon 66 wastewater treatment process, and the treatment steps are as follows:

[0134] S1: Mix the wastewater from the nylon 66 polycondensation process and the wastewater from the cyclohexanone oxidation process at a volume ratio of 1:0.5 under a stirring speed of 320 rpm to obtain a mixed wastewater.

[0135] S2: The mixed wastewater is subjected to anaerobic biological treatment. The conditions for anaerobic biological treatment are: temperature 35℃, hydraulic retention time 30h, bacterial species are methanogens, and the initial concentration of bacterial species in the inoculated sludge is controlled at 1.0×10^6 CFU / mL.

[0136] In the above examples and comparative examples, anhydrous ferrous sulfate was FAS-01, sourced from Tianjin Botian Chemical Co., Ltd.; trisodium citrate was TSC-ID, sourced from Shandong Lemon Biochemical Co., Ltd.; calcium peroxide was CPO-75, sourced from Jiangsu Tianyuan Chemical Co., Ltd.; cationic polyacrylamide was FO4190SB, sourced from Essen, France; methanogen was GNK-AN01, sourced from Beijing Grank Environmental Technology Co., Ltd.; and hydrogen peroxide was Peroxal™ 35, sourced from Solvay.

[0137] The concentration of hexamethylenediamine (HDD) in the wastewater before treatment was measured. The same volume of wastewater from Examples 1-9 and Comparative Examples 1-5 after treatment was also measured, and the reduction rate of HDD content before and after treatment was calculated. The chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) of the wastewater before anaerobic biological treatment in Examples 1-9 and Comparative Examples 1, 2, 4, and 5 were tested, respectively. The COD and BOD5 of the wastewater after treatment in Comparative Example 3 were also tested. COD was tested according to GB 11914-89 "Determination of Chemical Oxygen Demand in Water - Potassium Dichromate Method," and BOD5 was tested according to GB7488-87 "Determination of Five-Day Biochemical Oxygen Demand (BOD5) in Water - Dilution and Inoculation Method." The BOD5 / COD ratio was calculated, and the data are shown in Table 1.

[0138] Table 1. Reduction rate of hexamethylenediamine content in Examples 1-9 and Comparative Examples 1-5

[0139]

[0140] As shown in Table 1, the reduction rate of hexamethylenediamine content and the BOD5 / COD ratio of Examples 1-9 are greater than those of Comparative Examples 1-5. That is, the treatment effect and biodegradability of hexamethylenediamine in Examples 1-9 are better than those in Comparative Examples 1-5, and this application has advantages.

[0141] In Examples 1-5, as the proportion of cyclohexanone oxidation wastewater in the volume ratio of nylon 66 polycondensation wastewater to cyclohexanone oxidation wastewater increased, the reduction rate of hexamethylenediamine content and the BOD5 / COD ratio both first increased and then decreased. This is because as the proportion of cyclohexanone oxidation wastewater increased, the amount of aldehydes it contained increased. Under strongly acidic conditions, it could more fully reduce nitrates in nylon 66 wastewater to nitrites. This characteristic initially enhanced the generation efficiency of peroxynitrites, thereby improving the oxidative degradation effect on hexamethylenediamine. At the same time, it converted large organic molecules into easily biodegradable small molecule acids, increasing the BOD5 / COD ratio. However, when the proportion of cyclohexanone wastewater was too high, the excessive aldehydes in the wastewater would compete for oxidants such as hydroxyl radicals, resulting in insufficient specific oxidation reaction of hexamethylenediamine and a decrease in degradation efficiency. At the same time, the excessive organic matter might increase the COD load of the effluent, and incomplete oxidation would lead to the accumulation of recalcitrant intermediate products, ultimately causing a decrease in both the hexamethylenediamine removal rate and the BOD5 / COD ratio. The preferred embodiment is Embodiment 3.

[0142] In Examples 3 and 6-9, as the dosage of calcium peroxide gradually increased, the reduction rate of hexamethylenediamine content and the BOD5 / COD ratio both first increased and then decreased. This is because as the dosage of calcium peroxide increased, the concentration of hydrogen peroxide produced by its hydrolysis increased accordingly, allowing for a more complete reaction with ferrous ions and nitrous acid to generate hydroxyl radicals and peroxynitrous acid. This characteristic enhanced the oxidative degradation efficiency of hexamethylenediamine, converting it into easily degradable substances such as small-molecule organic acids, thereby increasing the BOD5 / COD ratio. However, when excessive calcium peroxide was added, the excessively high concentration of hydrogen peroxide acted as a scavenger of hydroxyl radicals, resulting in a self-consumption reaction. Simultaneously, the pH of the system increased due to the alkalinity of the calcium peroxide hydrolysis products, which destroyed the catalytic activity of ferrous ions and led to iron sludge deposition, thus inhibiting the oxidation reaction efficiency. This resulted in incomplete degradation of hexamethylenediamine, accumulation of intermediate products, and a simultaneous decrease in removal rate and biodegradability. The preferred embodiment is Example 7.

[0143] The reason why the reduction rate of hexamethylenediamine content in Comparative Examples 1-5 is less than that in Examples 1-9 is as follows:

[0144] Comparative Example 1 cannot generate nitrous acid by reducing nitrate with aldehydes, thus failing to form highly oxidizing peroxynitrous acid, which has a specific attack effect on nitrogen-containing organic matter. Its oxidation pathway relies solely on the traditional Fenton reaction, mainly using hydroxyl radicals for indiscriminate attack. Although subsequent anaerobic biological treatment can degrade some small molecule products, due to incomplete initial oxidation, a large number of recalcitrant intermediates may remain, and the utilization rate of the oxidant is low, resulting in an overall removal rate and biodegradability significantly lower than in Examples 1-9.

[0145] Although Comparative Example 2 introduced cyclohexanone process wastewater, generating nitrous acid and creating conditions for the formation of peroxynitrous acid, its core drawback lies in the use of liquid hydrogen peroxide. Directly adding hydrogen peroxide leads to an excessively high concentration in the initial stages of the reaction. On one hand, it reacts violently with ferrous ions, causing a large number of hydroxyl radicals to be ineffectively quenched in a short time; on the other hand, the reaction with nitrous acid to form peroxynitrous acid is too rapid, causing this unstable strong oxidant to decompose and become ineffective far from the target pollutant. This oxidation mode with an excessively fast reaction rate is inefficient. Although superior to Comparative Example 1, the oxidation effect is still unstable and incomplete. Therefore, the removal rate and biodegradability are not as good as those of Examples 1-9.

[0146] The simplified scheme in Comparative Example 3, which only involves chemical oxidation without biological treatment, suffers from the same two shortcomings as Comparative Example 1: it lacks co-oxidation with cyclohexanone process wastewater and utilizes inefficient hydrogen peroxide. Fenton oxidation typically only degrades large organic molecules into smaller intermediates, making complete mineralization difficult. Therefore, a considerable concentration of small-molecule organic acids and other COD components remain after the reaction. The degradation of hexamethylenediamine is also limited to the extent achievable by the oxidation step, making subsequent economical biological methods for deep purification impossible. Consequently, its removal rate and biodegradability are lower than those of Comparative Examples 1 and 2.

[0147] Comparative Example 4 underwent direct biological treatment, and its low removal rate directly confirms the necessity of pretreatment to eliminate toxicity. Hexamethylenediamine in nylon 66 wastewater has a strong inhibitory and toxic effect on anaerobic microorganisms, severely suppressing their metabolic activities and even preventing normal growth, thus making effective degradation of hexamethylenediamine almost impossible. Limited removal likely comes only from trace adsorption or co-metabolism, and the reduction rate of hexamethylenediamine content and the BOD5 / COD ratio are lower than in Comparative Example 3.

[0148] Although Comparative Example 5 diluted the wastewater through mixing, and the readily degradable organic matter in the cyclohexanone process wastewater may have provided some carbon source for the microorganisms, the fundamental problem remained unresolved: the biotoxicity of hexamethylenediamine persisted. While mixing and dilution may have slightly reduced the inhibition, it failed to fundamentally destroy the molecular structure of hexamethylenediamine and completely eliminate its toxicity, unlike advanced oxidation, resulting in poor biodegradability. Therefore, the microbial system remained inhibited, with degradation efficiency only slightly improved compared to direct treatment, but the overall effect was still very poor.

[0149] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for treating hexamethylene diamine in nylon 66 wastewater based on Fenton oxidation coupled biological process, characterized in that, The method comprises the following steps: S1: mixing nylon 66 polycondensation process wastewater and cyclohexanone oxidation process wastewater uniformly at a volume ratio of 1:0.8-1.4 under the condition of a stirring speed of 250-320 rpm to obtain mixed wastewater; the nitrate concentration of the nylon 66 polycondensation process wastewater is 800-1200 mg / L, and the total aldehyde content in the cyclohexanone oxidation process wastewater is 450-600 mg / L, and the aldehydes include formaldehyde, acetaldehyde and cyclohexenyl aldehyde; S2: adding sulfuric acid to the mixed wastewater to adjust the pH to 2.5-3.0 and stirring at 28-35 ℃ for 15-25 min; S3: adding a composite oxidation material to the mixed wastewater, the composite oxidation material comprising anhydrous ferrous sulfate, trisodium citrate and calcium peroxide, and reacting for 45-60 min to obtain oxidized effluent; S4: adding sodium hydroxide solution to the oxidized effluent to adjust the pH to 7.8-8.2, adding cationic polyacrylamide as a flocculant, and obtaining pretreated effluent after solid-liquid separation; S5: subjecting the pretreated effluent to anaerobic biological treatment.

2. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The pH of the nylon 66 polycondensation process wastewater is 1.8-2.2, and the pH of the cyclohexanone oxidation process wastewater is 3.5-4.

5.

3. The method for treating hexamethylene diamine in nylon 66 wastewater based on Fenton oxidation coupled biological method according to claim 1, characterized in that: The molar ratio of aldehydes in the cyclohexanone oxidation process wastewater is formaldehyde: acetaldehyde: cyclohexenal = 1: 0.6-0.9: 0.3-0.5, and the total reducing equivalent is 2.4-3.1 .

4. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The calcium peroxide in the composite oxidizing material is a powder having an average particle size of 40-60 μm, which hydrolyzes in water to generate a release rate of 0.18-0.28 mmol / (min-L).

5. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The composite oxidation material is added in a mass ratio of : : =1:0.4-0.6:1.6-3.6, wherein The dosage is 1.6-2.4 mmol / L, The dosage is 8.0-12.0 mmol / L, based on the volume of the mixed wastewater.

6. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The composite oxidation material adopts a segmented addition strategy: 60% of the total addition amount is added at one time at the initial stage of the reaction, and the remaining 40% is added in two equal amounts at the 15th min and the 30th min after the start of the reaction.

7. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The anhydrous ferrous sulfate, the trisodium citrate and the calcium peroxide are added as follows: the anhydrous ferrous sulfate and the trisodium citrate are mixed uniformly and then added to the mixed wastewater and stirred for 3-5 min, and then the calcium peroxide is added to the mixed wastewater.

8. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The sodium hydroxide solution in S4 is a 25-35% mass concentration sodium hydroxide aqueous solution.

9. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The flocculant in S4 is added at a concentration of 5-10 mg / L.

10. The method for treating hexamethylene diamine in nylon 66 wastewater by Fenton oxidation coupled biological method according to claim 1, characterized in that: The conditions of the anaerobic biological treatment in S5 are a temperature of 33-37 ℃ and a hydraulic retention time of 24-36 h.

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