A method for treating caprolactam wastewater

Abstract

The present invention discloses a method for treating caprolactam wastewater. By separately mixing and hierarchically treating the wastewater from multiple process sections in the production of caprolactam by the toluene method, the wastewater from multiple process sections can be integrally treated, reducing the content of heavy metals and organic substances while saving the resources required for wastewater treatment, meeting the influent requirements of the biochemical system, improving the biodegradability of the wastewater, and solving the defects in the prior art that the biochemical treatment effect of caprolactam wastewater by the toluene method is weak and harmful to the biochemical treatment system; the residual cobalt salt catalyst and palladium-carbon catalyst in the wastewater have a promoting effect on the oxidation-reduction reaction of ferrous salt and hydrogen peroxide. Various salts have different catalytic mechanisms and active sites, providing multiple catalytic pathways for the decomposition of hydrogen peroxide, increasing the number of active centers of the reaction, and thus significantly improving the decomposition rate of hydrogen peroxide.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment, and particularly relates to a method for treating caprolactam wastewater by multi-stage wastewater treatment. Background Art

[0002] Caprolactam is an organic compound. The main use of caprolactam is to produce nylon 6 fiber and nylon 6 engineering plastics. Nylon 6 fiber has the characteristics of high strength, good wear resistance, excellent elasticity, etc., and is widely used in the textile industry, and can be made into various clothes, ropes, fishing nets, etc. For example, sports clothing often uses nylon 6 fiber because it can provide good elasticity and breathability, making the wearer more comfortable during exercise.

[0003] In the existing caprolactam production process, a large amount of caprolactam wastewater is generated. The wastewater contains heavy metal ions and a large amount of organic matter, which will harm the subsequent biochemical treatment system, reduce or even damage the effect of the biochemical treatment system, resulting in unsatisfactory wastewater treatment effect.

[0004] Therefore, it is necessary to improve the method for treating caprolactam wastewater in the prior art to solve the above problems. Summary of the Invention

[0005] The present invention overcomes the deficiencies of the prior art and provides a method for treating caprolactam wastewater, aiming to solve the defects of weak biochemical treatment effect of toluene-process caprolactam wastewater and harm to the biochemical treatment system in the prior art.

[0006] To achieve the above object, the technical solution adopted by the present invention is: A method for treating caprolactam wastewater, comprising the following steps:

[0007] S1: Mix the toluene oxidation process wastewater, cyclohexane carboxylic acid preparation process wastewater and oximation reaction process wastewater, and add ferrous salt and hydrogen peroxide to react to obtain the first treated wastewater;

[0008] S2: Mix the amidation reaction process wastewater and rearrangement reaction process wastewater, and heat to 80 - 90 °C to obtain the second treated wastewater;

[0009] S3: After the first treated wastewater in S1 is extracted with an extractant, it is respectively mixed with the second treated wastewater in S2 and the purification process wastewater, and after flocculation treatment, the third treated wastewater is obtained;

[0010] S4: Add the third treated wastewater in S4 into the biochemical treatment system.

[0011] In a preferred embodiment of the present invention, the ferrous salt in S1 is FeSO4•7H2O, and the dosage is 1 / 20 - 1 / 15 of the COD value of the mixed wastewater.

[0012] In a preferred embodiment of the present invention, the mass concentration of hydrogen peroxide in S1 is 30%-35%, and the mass ratio of hydrogen peroxide to ferrous salt is 3-5:1.

[0013] In a preferred embodiment of the present invention, the heating rate during the S2 process is 2-4 °C / min. After reaching the specified heating temperature, it is kept warm for 0.5-1 h and then cooled to room temperature.

[0014] In a preferred embodiment of the present invention, the wastewater is subjected to acidolysis reduction treatment before heating in S2. The technological steps of the acidolysis reduction treatment include pH adjustment, neutralization precipitation, and addition of a reducing agent.

[0015] In a preferred embodiment of the present invention, the pH adjustment process is to adjust the pH of the wastewater to 2-3; the neutralization precipitation process is to add one or more of NaOH and Ca(OH)2, and also add PAC and PAM; the addition of a reducing agent is to add one or more of iron powder and aluminum powder.

[0016] In a preferred embodiment of the present invention, after the acidolysis reduction treatment, the wastewater is introduced into an upflow anaerobic sludge bed for treatment.

[0017] In a preferred embodiment of the present invention, the extractant in S3 is selected from one of n-octanol and n-butanol.

[0018] In a preferred embodiment of the present invention, the treatment process in the biochemical treatment system is an anaerobic-aerobic-anaerobic process.

[0019] In a preferred embodiment of the present invention, the volume ratio among the wastewater from the toluene oxidation process, the wastewater from the cyclohexanecarboxylic acid preparation process, the wastewater from the oximation reaction process, the wastewater from the amidation reaction process, the wastewater from the rearrangement reaction process, and the wastewater from the purification process is 10-15:8-12:20-30:20-30:25-35:5-10.

[0020] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0021] (1) The present invention provides a method for treating caprolactam wastewater. By mixing the wastewater from multiple process sections of caprolactam production by the toluene method and treating them separately in stages, the wastewater from multiple process sections can be integrally treated. Compared with the existing caprolactam wastewater treatment methods, it reduces the content of heavy metals and organic substances while saving the resources required for wastewater treatment, meets the inlet requirements of the biochemical system, helps the subsequent treatment of the biochemical system, improves the biodegradability of the wastewater, and solves the defects that the biochemical treatment effect of toluene method caprolactam wastewater in the prior art is weak and harmful to the biochemical treatment system.

[0022] (2) In the present invention, the added ferrous salt is FeSO4•7H2O. The ferrous salt can chelate heavy metals in the wastewater to form coprecipitates. Compared with the prior art, it can effectively prevent the redissolution of heavy metal ions in subsequent treatment processes or interference with other treatment steps, ensuring the stability and reliability of the wastewater treatment process. Moreover, the free radicals in the reaction can destroy the aromaticity of benzene compounds, effectively reducing the content of organic matter in the wastewater and alleviating the burden of subsequent treatment processes.

[0023] (3) In the present invention, the residual cobalt salt catalyst and palladium-carbon catalyst in the wastewater have a promoting effect on the redox reaction of ferrous salt and hydrogen peroxide. Various salts have different catalytic mechanisms and active sites. Compared with the prior art, it provides multiple catalytic pathways for the decomposition of hydrogen peroxide, increasing the number of active centers of the reaction, thereby significantly improving the decomposition rate of hydrogen peroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0025] Figure 1 It is a flowchart of the method steps of the preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0027] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0028] In the description of the present application, it should be understood that the terms "first", "second", etc. are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise stated, the meaning of "a plurality" is two or more.

[0029] The present invention is directed to caprolactam wastewater generated in the preparation of caprolactam using toluene as a raw material. The main process steps for preparing caprolactam using toluene as a raw material are as follows: 1) oxidizing toluene to form benzoic acid; 2) hydrogenating benzoic acid to form cyclohexanecarboxylic acid; 3) subjecting to amidation reaction to obtain an amide solution; 4) subjecting to oximation reaction to form cyclohexanone oxime; 5) subjecting to rearrangement reaction to obtain a preliminary caprolactam product; and 6) purification process.

[0030] As Figure 1 shown, a method for treating caprolactam wastewater includes the following steps:

[0031] S1: Mixing toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, and oximation reaction process wastewater, and adding ferrous salt and hydrogen peroxide for reaction to obtain first treated wastewater;

[0032] S2: Mixing amidation reaction process wastewater and rearrangement reaction process wastewater, and heating to 80 - 90 °C to obtain second treated wastewater;

[0033] S3: After extracting the first treated wastewater in S1 with an extractant, mixing it with the second treated wastewater in S2 and purification process wastewater respectively, and performing flocculation treatment to obtain third treated wastewater;

[0034] S4: Adding the third treated wastewater in S4 into a biochemical treatment system.

[0035] By separately mixing and hierarchically treating the wastewater from multiple process sections in the production of caprolactam by the toluene method, the wastewater from multiple process sections can be integrally treated, reducing the content of heavy metals and organic substances while saving resource treatment, meeting the inlet water requirements of the biochemical system, facilitating the treatment of the subsequent biochemical system, improving the biodegradability of the wastewater, and solving the defects of weak biochemical treatment effect of caprolactam wastewater by the toluene method and harm to the biochemical treatment system in the prior art.

[0036] Further, the ferrous salt in S1 is FeSO4•7H2O, and the dosage is 1 / 20 - 1 / 15 of the COD value of the mixed wastewater. Fe 2+ In the process of being oxidized to Fe 3+ it can chelate heavy metals in the wastewater to form coprecipitates, preventing heavy metals from entering the subsequent biochemical system. This chelation precipitation effect can effectively prevent the heavy metal ions from redissolving in the subsequent treatment process or interfering with other treatment links, ensuring the stability and reliability of the wastewater treatment process. The free radicals in the reaction can destroy the aromaticity of benzene compounds, causing the benzene ring to be decomposed, and the carbon - carbon bonds and carbon - hydrogen bonds on the benzene ring will be oxidized and broken to generate small - molecule organic substances or carbon dioxide and water. This oxidation and decomposition effect can effectively reduce the content of organic substances in the wastewater and reduce the burden on the subsequent treatment process.

[0037] Chelation to remove heavy metals reduces the toxicity of wastewater. These heavy metal ions have an inhibitory effect on microorganisms and can affect the activity and growth of microorganisms in the biochemical treatment system. After removing heavy metals, microorganisms can survive and reproduce in a more suitable environment, improving the treatment effect of the biochemical treatment system.

[0038] The residual cobalt salt catalyst and palladium-carbon catalyst in the wastewater have a promoting effect on the above oxidation-reduction reaction. These various salts have different catalytic mechanisms and active sites, providing multiple catalytic pathways for the decomposition of hydrogen peroxide, increasing the number of active centers of the reaction, and thus significantly increasing the decomposition rate of hydrogen peroxide. In addition to generating hydroxyl radicals, the cobalt salt and palladium salt catalyze the decomposition of hydrogen peroxide to produce other reactive oxygen species, such as superoxide radicals. These different reactive oxygen species have different oxidation abilities and reaction characteristics, and can oxidize and decompose organic substances through multiple pathways. And it can further react with residual raw materials such as toluene to generate more products in the next step, such as benzoic acid, which is convenient for subsequent removal and recovery.

[0039] Further, the mass concentration of hydrogen peroxide in S1 is 30%-35%, and the mass ratio of hydrogen peroxide to ferrous salt is 3-5:1. This can ensure that there is enough hydrogen peroxide in the reaction system to participate in the reaction and generate sufficient hydroxyl radicals. Hydroxyl radicals can attack the chemical bonds of these organic substances, decomposing them into small-molecule organic substances or carbon dioxide and water.

[0040] Further, the heating rate in S2 is 2-4 °C / min. After reaching the specified heating temperature, it is kept warm for 0.5-1 h and then cooled to room temperature. Wastewater often contains some colloidal substances, such as clay colloids and organic colloids. These colloidal substances have stability and are difficult to settle naturally. Heating can destroy the stability of the colloid, causing the colloidal particles to coagulate. Colloidal particles usually carry charges on their surfaces. Heating can change the charge properties on the surfaces of colloidal particles, reducing the electrostatic repulsion between particles, so that the colloidal particles aggregate with each other to form larger particles and settle down, which is beneficial to the clarification and subsequent treatment of wastewater. Heating can make metastable substances transform into stable states, making them easier to remove. There are some volatile pollutants in the wastewater. Heating can cause these volatile substances to volatilize from the wastewater. By collecting and treating the volatilized gas, the purpose of removing pollutants is achieved.

[0041] Further, before heating in S2, the wastewater is subjected to acidolysis reduction treatment. The acidolysis reduction treatment process includes pH adjustment, neutralization precipitation, and application of a reducing agent. The pH adjustment process is to adjust the pH of the wastewater to 2-3; the neutralization precipitation process is to add one or more of NaOH and Ca(OH)2, and add PAC and PAM; the application of a reducing agent is to add one or more of iron powder or aluminum powder.

[0042] Acidic conditions can change the existence form of organic pollutants, making them more easily reduced and decomposed by subsequent reducing agents. Adding NaOH and Ca(OH)2 can neutralize the acidity of the wastewater and simultaneously cause heavy metal ions to form hydroxide precipitates. As flocculants, PAC (poly aluminum chloride) and PAM (polyacrylamide) can adsorb these precipitate particles to form larger flocs, facilitating subsequent solid-liquid separation, thereby effectively removing heavy metal ions from the wastewater. Iron powder and aluminum powder have strong reducibility and can reduce some heavy metal ions in high valence states to low valence states, thereby reducing their toxicity and solubility, and reacting with some organic pollutants to convert the organic pollutants into a stable state.

[0043] Further, after acidolysis and reduction treatment, the wastewater is introduced into an upflow anaerobic sludge bed for treatment. The upflow anaerobic sludge bed treatment has a high organic load treatment capacity and can treat a large amount of wastewater in a relatively small reactor volume. This makes it an efficient organic matter removal unit in wastewater treatment and reduces the load on subsequent treatment processes. The upflow anaerobic sludge bed treatment can remove some toxic substances in the wastewater, such as some organic acids and sulfides, reducing the toxicity of the wastewater.

[0044] Further, the extractant in S3 is selected from one of n-octanol and n-butanol. N-octanol and n-butanol have good extraction ability for benzoic acid and cyclohexanecarboxylic acid. Through the extraction operation, benzoic acid and cyclohexanecarboxylic acid in the wastewater can be transferred from the aqueous phase to the organic phase, achieving efficient separation of target pollutants. Benzoic acid and cyclohexanecarboxylic acid are important organic chemical raw materials with high economic value. Through extraction and recovery, these substances can be separated from the wastewater and, after further purification and processing, can be reused in production, realizing resource recycling and reducing production costs. Benzoic acid and cyclohexanecarboxylic acid have certain toxicity to microorganisms and will inhibit the growth and metabolism of microorganisms. By extracting and removing these organic acids, the toxicity of the wastewater can be reduced, and the inhibition of microorganisms in subsequent biochemical treatment can be alleviated, enabling microorganisms to better play their role in degrading organic matter.

[0045] The flocculation treatment in S3 is to adjust the pH of the mixed wastewater to alkaline for the first static precipitation and then to neutral, and add PAC and PAM as flocculants for the second static precipitation. Adjusting the pH to alkaline for the first static precipitation is to cause the dissolved cobalt ions in the wastewater to precipitate and remove them through the first static precipitation. Adjusting to neutral and adding PAC and PAM as flocculants is to adsorb and precipitate the remaining palladium-carbon catalyst to remove it.

[0046] Furthermore, the treatment process in the biochemical treatment system is an anaerobic-aerobic-anaerobic process. Under anaerobic conditions, anaerobic microorganisms can decompose macromolecular organic matter in wastewater into small-molecular organic matter. For example, proteins can be decomposed into amino acids, and fats can be decomposed into fatty acids and glycerol. These small-molecular organic matter are more easily degraded by subsequent aerobic microorganisms. Under aerobic conditions, aerobic microorganisms can use small-molecular organic matter for metabolism and completely decompose them into carbon dioxide and water. The anaerobic-aerobic-anaerobic process provides a suitable living environment for different types of microorganisms, making the microbial community more diverse. Anaerobic microorganisms, aerobic microorganisms, denitrifying bacteria, etc. play their respective roles at different stages and cooperate with each other, improving the stability and shock resistance of the entire treatment system.

[0047] Furthermore, the volume ratio of toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater and purification process wastewater is 10-15:8-12:20-30:20-30:25-35:5-10. The types of organic matter contained in different process wastewaters vary. Toluene oxidation process wastewater may contain toluene and its oxidation products, such as benzoic acid, etc.; cyclohexanecarboxylic acid preparation process wastewater mainly contains cyclohexanecarboxylic acid and related intermediates; oximation reaction process wastewater contains oxime compounds; amidation reaction process wastewater contains amide substances; rearrangement reaction process wastewater has rearrangement products; and purification process wastewater may contain various impurities.

[0048] Toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater and oximation reaction process wastewater are mixed for treatment. They have similar pH values, are rich in heavy metals and contain a large amount of toxic organic matter; amidation reaction process wastewater and rearrangement reaction process wastewater contain a large amount of temperature-sensitive substances and volatile substances; the COD in purification process wastewater is small and has better biodegradability. Summing up each classification can make the organic matter composition more diverse and avoid the problem of excessive accumulation of some refractory organic matter that may occur during the treatment of single wastewater.

[0049] When the first treated wastewater, the second treated wastewater and the purification process wastewater are mixed, iron ions can promote flocculation as a flocculation crystal nucleus, and NaOH and Ca(OH)2 will remain after the acidolysis reduction process of the second treated wastewater, which can neutralize the residual acid in the first treated wastewater, making the pH of the final wastewater tend to be neutral.

[0050] Example 1

[0051] In this example, the volume ratio of toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater and purification process wastewater before treatment is 10:8:25:20:30:8;

[0052] This embodiment provides a method for treating caprolactam wastewater, comprising the following steps:

[0053] S1: Mix the toluene oxidation process wastewater, cyclohexane carboxylic acid preparation process wastewater, and oximation reaction process wastewater, and add ferrous salt and hydrogen peroxide for reaction. The ferrous salt is FeSO4•7H2O, and the dosage is 1 / 15 of the COD value of the mixed wastewater. The concentration of hydrogen peroxide is 30%, and the mass ratio of hydrogen peroxide to ferrous salt is 2:1 to obtain the first treated wastewater;

[0054] S2: Mix the amidation reaction process wastewater and rearrangement reaction process wastewater, and perform acidolysis reduction treatment on the wastewater. The acidolysis reduction treatment process includes pH adjustment, neutralization precipitation, and addition of a reducing agent. The pH adjustment process is to adjust the pH of the wastewater to 2.5; the neutralization precipitation process is to add NaOH, PAC, and PAM; the addition of the reducing agent is to add aluminum powder. After the acidolysis reduction treatment, the wastewater is introduced into an upflow anaerobic sludge bed for treatment; then it is heated to 90 °C at a heating rate of 3 °C / min, kept warm for 0.5 h after reaching 90 °C, and cooled to room temperature to obtain the second treated wastewater;

[0055] S3: Extract the first treated wastewater in S1 with an extractant. The extractant is selected as n-octanol, and it is mixed with the second treated wastewater in S2 and the purification process wastewater, and then flocculation treatment is carried out. The flocculation treatment is to adjust the pH to alkaline after mixing the wastewater, perform primary static precipitation, then adjust it to neutral, and add PAC and PAM as flocculants for secondary static precipitation to obtain the third treated wastewater;

[0056] S4: Add the third treated wastewater in S4 to the biochemical treatment system, and the treatment process in the biochemical treatment system is anaerobic-aerobic-anoxic process.

[0057] Example Two

[0058] The difference between this embodiment and Example One is that in S1, the mass ratio of hydrogen peroxide to ferrous salt is 3:1, and the rest is the same.

[0059] Example Three

[0060] The difference between this embodiment and Example One is that in S1, the mass ratio of hydrogen peroxide to ferrous salt is 4:1, and the rest is the same.

[0061] Example Four

[0062] The difference between this embodiment and Example One is that in S1, the mass ratio of hydrogen peroxide to ferrous salt is 5:1, and the rest is the same.

[0063] Example Five

[0064] The difference between this embodiment and Embodiment 1 is that in S1, the mass ratio of hydrogen peroxide to ferrous salt is 6:1, and the rest is the same.

[0065] Example 6

[0066] The difference between this embodiment and Embodiment 3 is that when the toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater are not treated, the volume ratio is 7.5:6:25:15:30:8, and the rest is the same.

[0067] Example 7

[0068] The difference between this embodiment and Embodiment 3 is that when the toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater are not treated, the volume ratio is 12.5:10:25:25:30:8, and the rest is the same.

[0069] Example 8

[0070] The difference between this embodiment and Embodiment 3 is that when the toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater are not treated, the volume ratio is 15:12:25:30:30:8, and the rest is the same.

[0071] Example 9

[0072] The difference between this embodiment and Embodiment 3 is that when the toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater are not treated, the volume ratio is 17.5:14:25:35:30:8, and the rest is the same.

[0073] Comparative Example 1

[0074] This comparative example provides a method for treating caprolactam wastewater, including the following steps:

[0075] Mix the toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater according to an equal volume ratio respectively, and then directly introduce them into the biochemical treatment system.

[0076] In Examples 1 to 9 and Comparative Example 1, the COD value ranges in the wastewater from toluene oxidation process, cyclohexanecarboxylic acid preparation process, oximation reaction process, amidation reaction process, rearrangement reaction process, and purification process are 14,000 - 15,500, 9,500 - 10,500, 33,000 - 37,000, 19,000 - 21,000, 11,500 - 12,500, and 8,000 - 9,000 respectively. The volume ratios between the wastewater from each process section in Examples 1 to 9 are the original volume ratios before treating each process wastewater with the wastewater treatment method in this application.

[0077] Take equal masses of the wastewater samples before entering the biochemical treatment system in Examples 1 to 9 and Comparative Example 1, and measure the chemical oxygen demand COD and biochemical oxygen demand BOD5 respectively. The chemical oxygen demand is measured according to GB 11914 - 89 "Water Quality - Determination of Chemical Oxygen Demand - Potassium Dichromate Method", and the biochemical oxygen demand is measured according to GB 7488 - 87 "Water Quality - Determination of Five-Day Biochemical Oxygen Demand (BOD5) - Dilution and Inoculation Method", and calculate the BOD5 / COD ratio. The test data are shown in Table 1.

[0078] Table 1 Chemical oxygen demand and biochemical oxygen demand data of Examples 1 to 9 and Comparative Example 1

[0079]

[0080] As can be seen from Table 1, the BOD5 / COD values of Examples 1 to 9 are all greater than those of Comparative Example 1, and the COD values of Examples 1 to 9 are all less than those of Comparative Example 1. The organic matter content in the wastewater of this application is lower and the biodegradability is better, indicating the superiority of this application.

[0081] In Examples 1 to 5, as the mass ratio of hydrogen peroxide to ferrous salt gradually increases, the COD value first decreases and then increases, and the BOD5 / COD first increases and then decreases. This is because when the mass ratio of hydrogen peroxide to ferrous salt gradually increases within a relatively low and appropriate range, it means that the relative amount of hydrogen peroxide increases. In the Fenton reaction, ferrous salt reacts with hydrogen peroxide to generate hydroxyl radicals with strong oxidation ability. As the amount of hydrogen peroxide increases, the number of generated hydroxyl radicals increases. These radicals can effectively attack the organic molecules in the wastewater. Long-chain hydrocarbons or aromatic compounds that were originally difficult to be decomposed by microorganisms are oxidized and decomposed into short-chain fatty acids, alcohols, etc., increasing the proportion of biodegradable organic matter in the wastewater, making it more easily utilized by microorganisms, enhancing the biodegradability, decreasing the COD value, and increasing the BOD5. Excessive hydrogen peroxide is relatively stable in the reaction system itself, and the substances such as oxygen generated by its decomposition may, to a certain extent, change the reaction environment, which is not conducive to the continuous and effective degradation of organic matter, resulting in a decrease in the degradation efficiency of some organic matter and an increase in the total amount of organic matter in the wastewater, increasing the COD value and decreasing the BOD5. The preferred example is Example 3.

[0082] In Examples 6 to 9, as the proportion of toluene oxidation process wastewater, cyclohexanecarboxylic acid preparation process wastewater, and amidation reaction process wastewater in the wastewater increases, the COD value first decreases and then increases, and the BOD5 / COD first increases and then decreases. This is because the residual catalysts in these wastewaters increase the Fenton reaction, promoting the decomposition efficiency of organic matter, decreasing the COD value and increasing the BOD5. However, when the proportion is too high, the relatively more refractory organic matter generates toxic intermediate products, which have an inhibitory effect on the subsequent treatment process, increasing the COD value and decreasing the BOD5. The preferred example is Example 7.

[0083] Based on the ideal examples of the present invention as inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A method for treating caprolactam wastewater, characterized in that, It includes the following steps: S1: Mix the toluene oxidation process wastewater, cyclohexane carboxylic acid preparation process wastewater, and oximation reaction process wastewater, and add ferrous salt and hydrogen peroxide for reaction. The ferrous salt is FeSO4•7H2O, and the dosage is 1 / 20 - 1 / 15 of the COD value of the mixed wastewater. The mass concentration of hydrogen peroxide is 30% - 35%, and the mass ratio of hydrogen peroxide to ferrous salt is 3 - 5:1 to obtain the first treated wastewater. S2: Mix the amidation reaction process wastewater and rearrangement reaction process wastewater, and perform acidolysis reduction treatment on the wastewater. The technological steps of the acidolysis reduction treatment include pH adjustment, neutralization precipitation, and addition of a reducing agent, and then heat to 80 - 90 °C to obtain the second treated wastewater. S3: After the first treated wastewater in S1 is extracted with an extractant, it is mixed with the second treated wastewater in S2 and the purification process wastewater respectively, and then subjected to flocculation treatment to obtain the third treated wastewater. S4: Add the third treated wastewater in S3 into the biochemical treatment system. The volume ratio of the toluene oxidation process wastewater, cyclohexane carboxylic acid preparation process wastewater, oximation reaction process wastewater, amidation reaction process wastewater, rearrangement reaction process wastewater, and purification process wastewater before treatment is 10 - 15:8 - 12:20 - 30:20 - 30:25 - 35:5 - 10.

2. The method for treating caprolactam wastewater according to claim 1, characterized in that: In the process of S2, the heating rate is 2 - 4 °C / min, keep warm for 0.5 - 1 h after reaching the specified heating temperature, and then cool to room temperature.

3. The method for treating caprolactam wastewater according to claim 1, characterized in that: The technological steps of pH adjustment are to adjust the pH of the wastewater to 2 - 3; the technological steps of neutralization precipitation are to add one or more of NaOH and Ca(OH)2, and add PAC and PAM; the addition of the reducing agent is to add one or more of iron powder and aluminum powder.

4. A method for treating caprolactam wastewater according to claim 1, characterized in that: After the acidolysis reduction treatment, the wastewater is passed into an upflow anaerobic sludge bed for treatment.

5. The method for treating caprolactam wastewater according to claim 1, characterized in that: The extractant in S3 is selected from one of n-octanol and n-butanol.

6. The method for treating caprolactam wastewater according to claim 1, characterized in that: The treatment process in the biochemical treatment system is an anaerobic-aerobic-anaerobic process.

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