A method for treating indigo dye wastewater

Abstract

The application discloses a treatment method of indigo dye wastewater, and belongs to the technical field of industrial wastewater treatment. The method uses a chemical separating agent containing phenyl phosphonic acid mono (2-ethylhexyl) ester to selectively extract high-concentration aniline and methyl aniline in the wastewater. Compared with traditional phosphate ester extractants, the method significantly improves the extraction efficiency while reducing the amount of extractant. By accurately controlling the reaction time and temperature, the content of aniline and methyl aniline, toxic organic matter and COD index in the wastewater are greatly reduced, and the treatment cost is effectively reduced. The method can also reduce the microbial toxicity impact on the subsequent biological treatment unit, and the separated aniline and methyl aniline from the wastewater can be recycled as by-products for resource utilization, which has good environmental and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a method for treating indigo dye wastewater. Background Technology

[0002] Indigo is a blue dye with a long history and is widely used in the textile industry, especially for dyeing denim. Its structural formula is as follows:

[0003] Modern industrial production of indigo mainly employs chemical synthesis methods. Among them, the classic aniline process uses aniline (C6H5NH2) and chloroacetic acid (ClCH2COOH) as raw materials, and synthesizes the target product through condensation, alkali fusion and air oxidation in sequence.

[0004] However, this synthesis process generates a type of organic wastewater with high concentrations of characteristic pollutants and complex composition. The wastewater contains large amounts of unreacted aniline, methylaniline, and various intermediate derivatives, resulting in highly toxic pollutants, poor biodegradability, and difficulty in treatment. Direct discharge of such wastewater would pose a serious threat to the environment.

[0005] Currently, domestic technologies for treating indigo dye wastewater mainly include adsorption, advanced oxidation, membrane separation, microbial methods, and combinations thereof. However, these methods all have significant limitations when dealing with high concentrations and high toxicity of aniline pollutants. Adsorption method: Although it can effectively enrich pollutants, the adsorption material is easily saturated and needs to be frequently regenerated or replaced, resulting in high operating costs. In addition, it does not completely degrade pollutants and may cause secondary pollution. Advanced oxidation technologies (such as Fenton process, ozone oxidation, etc.): often do not completely degrade these complex macromolecular pollutants, have limited removal efficiency, and ozone oxidation has problems such as large equipment investment and high energy consumption. Membrane separation technology: Although it can physically retain pollutants, it faces serious problems of membrane fouling and concentrate treatment, and its long-term operational stability and economic efficiency are poor. Microbial treatment: It is not well adapted to the highly toxic wastewater from the synthesis of indigo, and is only suitable for low-concentration, low-toxicity wastewater. The microbial strains are easily inhibited or even killed, and the treatment effect is difficult to guarantee.

[0006] In summary, the core challenge of existing technologies lies in the difficulty of achieving efficient and selective separation and enrichment of high concentrations of aniline and methylaniline in wastewater. Large amounts of toxic pollutants directly enter subsequent biological or advanced treatment units, significantly increasing treatment load and costs, and resulting in the waste of useful chemical raw materials due to the failure to achieve resource recovery.

[0007] Therefore, developing a pretreatment technology that can efficiently separate and recover aniline pollutants from indigo wastewater, thereby significantly reducing the biotoxicity of the wastewater and creating conditions for resource utilization, has become the key to solving this type of wastewater pollution problem. Summary of the Invention

[0008] This invention aims to overcome the shortcomings of existing indigo dye wastewater treatment technologies and provide a highly efficient, low-cost, and resource-recovery treatment method. This method uses a chemical separating agent containing mono(2-ethylhexyl) phenylphosphonic acid ester to selectively extract high concentrations of aniline and methylaniline from the wastewater, significantly improving extraction efficiency while reducing the amount of extractant used. By precisely controlling the reaction time and temperature, the content of aniline and methylaniline, toxic organic matter, and COD in the wastewater are significantly reduced, and the resource recovery of aniline-based chemical raw materials is achieved.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for treating indigo dye wastewater, characterized by comprising the following steps performed sequentially: a. Chemical removal: Adjust the pH of the wastewater to 6-8, add polyaluminum chloride to carry out flocculation reaction, and then let it stand and filter after the reaction; b. Chemical separation: Adjust the pH of the wastewater treated in step a to 8-9, add a chemical separating agent to carry out an extraction reaction, the chemical separating agent comprising a complexing agent, a cosolvent, and a diluent, the complexing agent being phenylphosphonic acid mono(2-ethylhexyl) ester; after the reaction, perform phase separation to obtain a separated phase loaded with pollutants and a preliminarily purified aqueous phase; subsequently, add an acidic desorption agent to the separated phase for back-extraction to separate the regenerated chemical separating agent and the recovered phase; c. Aniline recovery: Add an alkaline solution to the recovered phase obtained in step b, react, and allow to stand for separation to obtain crude aniline oil phase; d. Aniline refining: The crude aniline oil phase obtained in step c is subjected to vacuum distillation to obtain refined aniline and refined methylaniline; The inventors discovered that the main pollutants in the wastewater generated during the production of indigo dye include unreacted aniline, methylaniline and their derivatives, as well as suspended solids such as sodium indigo and cotton fiber fragments. If these solids are directly treated, they will increase the loss of chemical separating agents, severely affecting separation efficiency and agent regeneration. Therefore, chemical purification must be performed first in the treatment process to remove solid impurities that interfere with subsequent steps.

[0010] Before using polyaluminum chloride (PAC) for chemical impurity removal, the inventors discovered that adjusting the pH of the wastewater to the range of 6–8 was crucial because: when pH < 6 (too acidic), excessively high levels of H+ in the water... +Concentration will inhibit the hydrolysis equilibrium of PAC from moving to the right, hindering the formation of polynuclear hydroxyaluminum complexes with highly efficient flocculation effects. At this point, aluminum mainly exists as free Al. 3+ The presence of ionic impurities results in a weak ability to neutralize negatively charged colloidal impurities, making it difficult to form "floc"-like flocs and significantly reducing flocculation efficiency. When pH > 8 (excessively alkaline), the excessively high OH- ions... - Concentration will promote excessive hydrolysis reaction, causing Al 3+ It is converted into aluminum hydroxide (Al(OH)3) precipitate, and may even further form soluble aluminate (AlO2). - The former produces small, loose flocs that are difficult to aggregate and settle; the latter completely lacks adsorption capacity, causing PAC to lose its flocculation function. Within a pH range of 6–8 (neutral to weakly acidic / alkaline), PAC hydrolysis is in optimal equilibrium, efficiently generating positively charged polynuclear aluminum hydroxyl complexes. These active substances can rapidly neutralize the negative charge on the surface of colloidal particles in wastewater and, through "adsorption bridging," connect dispersed colloids into dense, coarse flocs. This removes suspended solids such as sodium indigo naturalis while simultaneously reducing COD and color, thus improving overall treatment efficiency.

[0011] In the chemical impurity removal step, the dosage of polyaluminum chloride (PAC) must be strictly controlled, preferably at 3-5 wt%. Insufficient or excessive dosage will seriously affect the flocculation effect and subsequent treatment process. When the dosage is insufficient, the number of effective flocculants generated by hydrolysis (such as polynuclear aluminum hydroxyl ions and Al(OH)3 colloids) is insufficient, resulting in inadequate destabilization and aggregation of pollutants. Specifically, this manifests as: incomplete charge neutralization, making it difficult for colloidal particles to aggregate; weak adsorption bridging, failing to form dense and easily settled flocs. As a result, the removal rate of suspended solids, COD, and color is low, the effluent is turbid, and residual pollutants may inhibit the activity of microorganisms in subsequent biological treatment. When the dosage is excessive, the excess positive charge will cause the destabilized particles to "restabilize" and redisperse in the water, leading to an increase in effluent turbidity. At the same time, excess Al(OH)3 colloids will form fine, loose flocs with poor settling performance, which can easily cause "floc escape" and block subsequent filters or membrane modules. In addition, the cost of chemical consumption will increase significantly, the residual aluminum ions in the effluent may exceed the standard, causing secondary pollution, and may also affect the pH stability of the system and interfere with subsequent chemical separation.

[0012] Preferably, the reaction temperature in step a should be controlled within the range of 20–30°C. If the temperature is too low, the molecular thermal motion weakens, the reaction kinetics slows down, and the hydrolysis process of polyaluminum chloride (PAC) is inhibited, resulting in insufficient formation of effective flocculants. Simultaneously, the probability of colloidal particle collision decreases, and the viscosity of water increases, making floc formation difficult and increasing sedimentation resistance. While excessively high temperatures can accelerate the reaction, they easily disrupt the balance between PAC hydrolysis and colloidal stability, causing excessive hydrolysis and ineffective precipitation, exacerbating the thermal motion of the flocs and leading to their destruction, while also increasing energy consumption and operating costs.

[0013] Preferably, in step b, before the extraction reaction, the pH of the wastewater is controlled within a weakly alkaline range of 8–9. Under these conditions, aniline compounds mainly exist in the form of free molecules (C6H5NH2), which have low water solubility and are easily extracted from the aqueous phase through subsequent chemical separation processes. Conversely, at pH < 8, aniline molecules are easily protonated to aniline cations (C6H5NH3). + This form exhibits significantly enhanced water solubility, making it difficult to achieve effective separation through conventional extraction or flocculation methods; furthermore, if the pH > 9, it can lead to the release of free OH groups. - The concentration increased sharply, and the system became too alkaline, causing a significant decrease in extraction selectivity and partition coefficient, and a large amount of OH- in the water. - Competition for phosphoryl group active sites; in addition, excessive alkalinity makes it difficult for the extractant to saponify and the organic phase to emulsify and separate, because: phenylphosphonic acid mono(2-ethylhexyl) ester has a weakly acidic P-OH group, which will undergo neutralization and saponification under strong alkaline conditions of pH>9, causing non-selective co-extraction of impurities and resulting in loss of separation selectivity; at the same time, the high concentration of aromatic amines in the system is prone to oxidative polymerization side reactions, aggravating organic phase pollution and significantly increasing the difficulty of subsequent back-extraction and regeneration.

[0014] During the research and development of indigo dye wastewater treatment, the inventors discovered that indigo dye wastewater has a complex composition, mainly containing three types of characteristic pollutants: indigo molecules themselves, which have a conjugated aromatic structure and are highly hydrophobic; aniline derivatives containing polar amino groups (-NH2), which are toxic and biodegradable; and other polar or weakly polar organic compounds that may contain phenols, sulfonic acid groups, etc. Traditional phosphate ester separating agents (general formula R3PO4) contain only P=O groups, lack free hydroxyl groups, have weak hydrogen bond donor capacity, and have insufficient affinity for polar aniline pollutants, while also having limited adsorption capacity for hydrophobic indigo molecules. In comparison, phenylphosphonic acid mono(2-ethylhexyl) ester has significant structural advantages: its P=O double bond and partially present P–OH groups can act as strongly polar sites, forming double hydrogen bonds with the -NH2 of aniline molecules to achieve efficient extraction; at the same time, the phenyl group in the molecule binds tightly to aromatic pollutants such as indigo through π-π stacking, and the hydrophobic aromatic ring further promotes the transfer of pollutants from the aqueous phase to the organic phase; in addition, the structure has both polar and hydrophobic segments, which can act as a "bridge" between polar aniline molecules and non-polar indigo molecules to achieve synergistic removal; furthermore, the introduction of the phenyl group enhances the chemical stability and hydrolysis resistance of the complexing agent, and its low water solubility is conducive to phase separation and recycling, thus giving phenylphosphonic acid mono(2-ethylhexyl) ester better engineering applicability and economy, making it more suitable for treating complex indigo dye wastewater.

[0015] Preferably, in step b, the co-solvent in the chemical separating agent is an isomeric alcohol with 10-25 carbon atoms or a mixture thereof, and the diluent is at least one of aviation kerosene or diesel oil. The reason for choosing isomeric alcohols with 10-25 carbon atoms is their excellent solubilizing effect; their core function is "bridging," allowing originally incompatible components to mix stably together. Long-chain isomeric alcohols can perform this task well. Preferably, the isomeric alcohol is tridecyl alcohol. Preferably, by volume percentage, the complexing agent is 10%-50%, the co-solvent is 5%-25%, and the diluent is the balance. Excessive proportion of the complexing agent will lead to poor system stability, wasted costs, difficulty in subsequent back-extraction, and reduced separation purity; excessively low proportion will lead to insufficient extraction efficiency, incomplete complexation, and decreased separation selectivity. Excessively high proportion of the co-solvent will weaken the diluent's effect, decrease selectivity, and increase costs; excessively low proportion will lead to insufficient solubility of the complexing agent, system heterogeneity, and hindered complexation reaction.

[0016] Preferably, in step b, the volume ratio of the chemical separating agent to the wastewater is 1:5 to 1:10. If too little separating agent is added, it will be difficult to fully extract aniline from the aqueous phase, resulting in excessive aniline concentration in the effluent and low treatment efficiency. If too much is added, it will not only significantly increase the cost of the reagents, but may also cause secondary pollution due to the residue of the separating agent, such as increased COD or oil content, inhibition of subsequent biochemical treatment, or even the formation of difficult-to-treat scum or sludge due to intensified emulsification, while increasing the equipment processing load and solvent loss risk.

[0017] Preferably, in the extraction reaction in step b, the extraction reaction is divided into two stages in time, wherein the control temperature of the later stage is lower than the control temperature of the earlier stage. In the chemical separation process of step b, the inventors discovered a dual influence mechanism of temperature on the extraction process: on the one hand, increasing the temperature can accelerate molecular diffusion and chemical reactions, but on the other hand, it can also exacerbate side reactions such as hydrolysis of the extractant and interfacial emulsification. To balance extraction efficiency and process stability, preferably, the extraction reaction is divided into two stages in terms of time, with the temperature of the later stage being lower than that of the earlier stage. The earlier stage uses a higher temperature to accelerate the rate of molecular diffusion and interfacial reaction, and promote the rapid achievement of preliminary equilibrium in chemical adsorption. The temperature is appropriately reduced in the later stage to help stabilize the formed extraction complex and suppress side reactions such as hydrolysis of the extractant and interfacial emulsification.

[0018] Regarding the specific staged temperature control, preferably, the temperature is controlled at 35-40℃ for 0-30 minutes after the start of the reaction; and at 30-35℃ when the reaction has proceeded for 30-60 minutes. The inventors discovered that in the first stage, by controlling the temperature at 35–40℃, the chemisorption rate can be effectively increased without significantly aggravating solvent hydrolysis (cumulative hydrolysis rate <1.5% within 30 minutes). This increases the hydrogen bond formation rate between aniline and phenylphosphonic acid mono(2-ethylhexyl) ester by approximately 1.8 times compared to 30℃, thus rapidly establishing adsorption equilibrium in the initial stage. In the second stage, controlling the temperature at 30–35℃ maintains the system's diffusion mass transfer capability (total extraction efficiency loss <2% within 60 minutes) while significantly suppressing side reactions. The hydrolysis rate of phosphonate ester is reduced by approximately 50% compared to 40℃, the emulsification rate decreases from 8% to 3%, and the low-temperature stability of the hydrogen-bonded complex is enhanced, preventing the desorption of captured contaminants. This results in an overall efficient, stable, and recyclable extraction and separation effect.

[0019] In the back-extraction process of step b, the inventors discovered that the type, concentration, dosage, and reaction conditions of the acidic eluent jointly determine the regeneration efficiency of the extractant and the overall economic efficiency of the process. Preferably, in step b, the acidic eluent is a 10-20% hydrochloric acid solution by mass, with a volume ratio of 1:6 to 1:10 to the separation phase, and the pH value of the back-extraction reaction is controlled to be ≤0.5, and the temperature is controlled at 40±3℃. The inventors found that using a 10%–20% hydrochloric acid solution by mass, and controlling its volume ratio to the separation phase to be between 1:6 and 1:10, can ensure sufficient H+. + The concentration was optimized to promote efficient dissociation of the aniline-extractant complex while avoiding excessive acid concentration that could exacerbate equipment corrosion or cause unnecessary side reactions. This volume ratio range effectively balanced the mass transfer interface and reaction efficiency, achieving thorough desorption while reducing the cost of subsequent neutralization treatment. The inventors further discovered that controlling the pH of the back-extraction system under strongly acidic conditions (≤0.5) effectively inhibited the redissolution of aniline compounds, shifting the chemical equilibrium towards dissociation, thereby significantly improving the recovery rate of pollutants and achieving complete regeneration of the extractant, providing a thermodynamic basis for subsequent recycling. In designing the temperature parameters, the inventors found that: when the temperature was too low, the reverse reaction rate decreased significantly, leading to incomplete extractant regeneration, reducing recovery efficiency, and the residual solvent could cause secondary pollution; while when the temperature was too high, although the initial reaction rate could be increased, the inventors observed significant volatilization loss of the extractant, and irreversible structural changes such as thermal decomposition, polymerization, or oxidation were prone to occur at high temperatures, leading to decreased extraction performance and loss of recycling capacity; based on these findings, the inventors optimized the temperature setting to 40±3℃.

[0020] In the aniline recovery process of step c, the inventors discovered that insufficient alkalinity was the main reason for the low aniline recovery rate. Alkali is a reactant; if the amount added is insufficient, the pH will be too low, and a large amount of aniline will remain in the aqueous phase as salt, unable to be released and separated. Furthermore, it will inhibit the formation of free aniline, resulting in poor separation. Preferably, in step c, the pH of the reaction system is controlled at 12-14. Too high a pH will lead to reagent waste and increased costs. The alkaline solution is a sodium hydroxide solution with a mass fraction of 10%-48%, and the reaction temperature is controlled at 40±3℃. The inventors discovered that, under the pH conditions of the above reaction system, excessively low temperatures significantly slow down the neutralization reaction rate between the alkali and aniline salt, while also increasing the solubility of aniline in the aqueous phase and raising the viscosity of the two phases, thereby delaying stratification and reducing recovery efficiency. On the other hand, excessively high temperatures can accelerate the reaction, but they can exacerbate aniline volatilization, posing a risk of toxicity to the operating environment, and may also promote side reactions such as oxidation and polymerization of aniline, generating tar-like impurities that are difficult to handle. By setting the temperature to 40±3℃, it is possible to effectively suppress volatilization and side reaction risks while ensuring a sufficient reaction rate and good separation of the two phases, thus achieving a safe, efficient, and stable aniline recovery process.

[0021] In the vacuum distillation process of step d, the inventors discovered that the coordinated control of system pressure and heating temperature has a crucial impact on distillation efficiency, product purity, and process stability. Preferably, the absolute pressure is controlled at 15–25 mmHg, and the distillation temperature is controlled at 70–80℃. The inventors found that when the system pressure is too low, the distillation rate of aniline slows down significantly, reducing production efficiency; it may also alter the relative volatility of impurities and aniline, forming azeotropes or causing bumping entrainment, affecting product purity. Conversely, excessively high pressure leads to an increase in the boiling point of aniline, requiring an increase in distillation temperature, which in turn triggers side reactions such as oxidation and polymerization of aniline, generating tar-like substances, resulting in darker product color, decreased purity, and yield loss. Regarding temperature control, the inventors further discovered that when the temperature is too low, aniline is difficult to distill effectively, leading to low distillation efficiency, prolonged production cycle, and severe retention of the target product; excessively high temperatures cause localized overheating, exacerbating aniline decomposition and impurity co-distillation, similarly leading to decreased product purity and increased side reactions.

[0022] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in the following aspects: 1. Synergistic effect of composite chemical separating agents The phenylphosphonic acid mono(2-ethylhexyl) ester complexing agent used in this invention has both a polar P=O / P–OH group and a hydrophobic phenyl structure. It achieves specific capture of aniline pollutants through hydrogen bonding, and at the same time enhances the adsorption of hydrophobic indigo molecules by means of π-π stacking effect. This solves the technical bottleneck of traditional phosphate ester extractants having insufficient affinity for polar organics and limited adsorption capacity for hydrophobic components, and achieves synergistic and efficient removal of polar and non-polar pollutants.

[0023] 2. Process optimization based on multi-parameter collaborative control 1) By optimizing key process parameters including pH, temperature, pressure, and reagent ratio, efficient and stable operation of the entire process was achieved: 2) During the chemical impurity removal stage, by controlling the pH 6–8 and optimizing the PAC dosage, the density and settling performance of the flocs were significantly improved; 3) In the chemical separation stage, a staged temperature control strategy (35–40℃ in the first stage and 30–35℃ in the second stage) is adopted to effectively suppress solvent hydrolysis and interfacial emulsification while ensuring the extraction rate. 4) During the back-extraction regeneration stage, by controlling the strongly acidic environment (pH≤0.5) and the moderate temperature (40±3℃), efficient regeneration of the extractant and high-yield recovery of aniline are achieved; 5) In the aniline refining stage, vacuum distillation (15–25 mmHg, 70–80℃) is used to achieve high-purity separation of aniline under low-temperature conditions, avoiding heat-sensitive side reactions.

[0024] 3. The unity of resource utilization and cost control This invention not only significantly reduces the concentration of aniline and COD in the effluent, thus alleviating the load on subsequent biological treatment, but also achieves efficient recovery of aniline and methylaniline. The refined product can be sold as a by-product, thereby improving the overall economic efficiency of the process while achieving pollutant reduction.

[0025] 4. Improved engineering applicability and environmental friendliness The selected phenylphosphonic acid mono(2-ethylhexyl) ester complexing agent has good chemical stability, low water solubility and high regeneration efficiency. It can maintain excellent performance in multiple cycles, reducing operating costs and the risk of secondary pollution, and has strong industrial promotion value. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of an indigo wastewater treatment method according to the present invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0028] <Example 1> The method of this invention is used to treat indigo dye wastewater discharged during the production process of a chemical enterprise. The wastewater quality is shown in the table below:

[0029] a. Chemical purification: Take 1000L of workshop wastewater and add a 12% hydrochloric acid solution to adjust the pH to 7.0. Then add polyaluminum chloride at a mass of 3% of the wastewater mass. React at 25℃ for 60 minutes, then let stand for 20 minutes to allow the flocs to settle completely. Finally, filter and collect the filtrate. The resulting waste residue should be disposed of according to hazardous waste regulations.

[0030] b. Chemical separation: The pH of the filtrate obtained in step a is adjusted to 8.0 with 30 wt% liquid alkali, followed by the addition of a chemical separating agent, which consists of: complexing agent phenylphosphonic acid mono(2-ethylhexyl) ester, cosolvent isotridecyl alcohol, and diluent kerosene, in a mass ratio of 1:1:8. The volume ratio of the chemical separating agent to the wastewater is 1:8.

[0031] The extraction process employs a staged temperature control strategy: First stage (0–30 minutes): Control the temperature at 38±2℃; Later stage (30–60 minutes): Reduce the temperature to 32±2℃.

[0032] After the reaction was completed, the system was allowed to stand for 20 minutes. The system separated into an upper layer of pollutant-loaded phase and a lower layer of pre-purified aqueous phase.

[0033] The separated phase enters the back-extraction regeneration process: a 12% hydrochloric acid solution is added, with a volume ratio of 1:8 to the separated phase. The reaction system is controlled at pH ≤ 0.5, temperature 40℃, and reacted for 60 minutes, followed by standing for 20 minutes. After separation, the regenerated chemical separating agent (upper layer) and the recovered phase rich in aniline compounds (lower layer) are obtained.

[0034] c. Aniline Recovery: Add 32% sodium hydroxide solution to the recovered phase obtained in step b, adjust the pH of the system to 14, control the reaction temperature at 40℃, and the reaction time at 60 minutes. After the reaction is complete, allow the mixture to stand and separate into layers, obtaining an upper crude aniline oil phase and a lower alkaline waste liquid. The alkaline waste liquid is sent to a subsequent wastewater treatment system for further treatment.

[0035] d. Aniline refining: The crude aniline oil phase obtained in step c is transferred to a vacuum distillation unit. The absolute pressure of the control system is set at 20 mmHg, and the distillation temperature is 80℃. The distillate is collected to obtain refined aniline product. The small amount of residual water in the distillate is returned to the upstream wastewater equalization tank for reprocessing.

[0036] Testing revealed that in step b, the COD of the waste liquid decreased to 400 mg / L, aniline decreased to 150 mg / L, and the concentration of methylaniline was below 50 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 65%; in step d, the purity of aniline after purification reached 98.5%, and the purity of methylaniline reached 97.2%; the overall recovery rate of aniline reached 97%, and the recovery rate of methylaniline reached 92%; the chemical separating agent was regenerated and recycled 15 times, and its extraction efficiency still remained above 70% of the initial value.

[0037] <Example 2> Compared to Example 1, the difference is that in step b, the extraction process did not use segmented temperature control, the extraction temperature was 35°C, and the time was 60 min.

[0038] Testing revealed the following: In step b, the COD of the waste liquid decreased to 1000 mg / L, aniline decreased to 500 mg / L, and the concentration of methylaniline was below 150 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 53%; in step d, the purity of aniline after purification reached 92%, and the purity of methylaniline reached 91%; the overall recovery rate of aniline reached 83%, and the recovery rate of methylaniline reached 78%; the chemical separating agent, after regeneration and recycling 11 times, maintained an extraction efficiency of over 65% of its initial value. The reason for this discrepancy is that segmented temperature control was not employed; excessively low temperatures in the initial stage disrupted the extraction equilibrium and mass transfer efficiency, leading to a decrease in separation performance.

[0039] <Example 3> Compared to Example 1, the difference is that in step b, the extraction process uses a low temperature followed by a high temperature. First stage (0–30 minutes): Control the temperature at 32±2℃; Later stage (30–60 minutes): Raise the temperature to 38±2℃.

[0040] Testing revealed the following: In step b, the COD of the waste liquid decreased to 3000 mg / L, aniline decreased to 1500 mg / L, and the concentration of methylaniline was below 600 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 41%; in step d, the purity of aniline after purification reached 91%, and the purity of methylaniline reached 87%; the overall recovery rate of aniline reached 77%, and the recovery rate of methylaniline reached 71%; the chemical separating agent, after regeneration and recycling eight times, maintained an extraction efficiency of over 50% of its initial value. The reason for this discrepancy is that reversing the temperature control sequence directly reverses the extraction equilibrium and mass transfer patterns, leading to a significant decline in separation efficiency.

[0041] <Example 4> Compared to Example 1, the difference is that in step b, the extraction process uses a high temperature at the beginning and a low temperature at the end, but the overall temperature is lower than the range. First stage (0–30 minutes): Control the temperature at 32±2℃; Later stage (30–60 minutes): Raise the temperature to 28±2℃.

[0042] Testing revealed the following: In step b, the COD of the waste liquid decreased to 5000 mg / L, aniline decreased to 2400 mg / L, and the concentration of methylaniline was below 900 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 33%; in step d, the purity of aniline after purification reached 77%, and the purity of methylaniline reached 75%; the overall recovery rate of aniline reached 70%, and the recovery rate of methylaniline reached 66%; the chemical separating agent, after regeneration and recycling five times, maintained an extraction efficiency of over 45% of its initial value. The reason for this discrepancy is that the entire process was conducted at temperatures below the standard, resulting in a significant decrease in extraction efficiency, extremely poor system fluidity and stability, passively reduced selectivity, impurity accumulation, and low extraction utilization.

[0043] <Example 5> Compared to Example 1, the difference is that in step a, the amount of polyaluminum chloride added is 10%.

[0044] Testing revealed the following: In step b, the COD of the wastewater decreased to 1300 mg / L, aniline decreased to 530 mg / L, and the concentration of methylaniline was below 220 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 55%; in step d, the purity of aniline after purification reached 75%, and the purity of methylaniline reached 73%; the overall recovery rate of aniline reached 85%, and the recovery rate of methylaniline reached 80%; the chemical separating agent, after regeneration and recycling 12 times, maintained an extraction efficiency of over 55% of its initial value. The reason for this was excessive polyaluminum chloride, leading to an excess of positive charge in the water. The previously destabilized flocculent particles became positively charged again, causing the colloid to re-stabilize. The particles repelled each other and could not settle, resulting in water quality deterioration, secondary pollution, and affecting subsequent treatment effects.

[0045] <Example 6> Compared to Example 1, the difference is that in step b, the amount of separating agent added is increased, and the volume ratio of chemical separating agent to wastewater is 1:4.

[0046] Testing revealed the following: In step b, the COD of the waste liquid decreased to 500 mg / L, aniline decreased to 180 mg / L, and the concentration of methylaniline was below 88 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 61%; in step d, the purity of aniline after purification reached 75%, and the purity of methylaniline reached 71%; the overall recovery rate of aniline reached 76%, and the recovery rate of methylaniline reached 79%; the chemical separating agent, after regeneration and recycling 12 times, maintained an extraction efficiency of over 62% of its initial value. The reason for this discrepancy is that excessive addition of the chemical separating agent led to emulsification of the oil phase. The emulsion layer contained a large number of oil droplets, and aniline and methylaniline were trapped in the aqueous phase and the intermediate emulsion layer, being discharged with the wastewater during separation, resulting in data distortion. Simultaneously, excessive alkalinity promoted the water solubility and loss of aniline, shifting the distribution ratio and disrupting the gas-liquid / liquid-liquid balance, leading to decreased extraction efficiency and increased burden on subsequent processing.

[0047] <Example 7> Compared to Example 1, the difference lies in step b, the back-extraction regeneration process: adding a 36% hydrochloric acid solution at a volume ratio of 1:20 to the separated phase. The pH of the reaction system is controlled to be greater than 1.

[0048] Testing revealed the following: In step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 45%; in step d, the purity of aniline after refining reached 76%, and the purity of methylaniline reached 73%; the overall recovery rate of aniline reached 77%, and the recovery rate of methylaniline reached 73%; the chemical separating agent, after regeneration and recycling four times, had an extraction efficiency reduced to between 20% and 30%. The reasons for this were: insufficient hydrochloric acid dosage, a system pH greater than 0.5, and a lack of hydrogen ion concentration, which prevented the protonation of aniline and methylaniline complexed on the extractant to form water-soluble ammonium salts, limiting the forward reaction of the back-extraction and leaving a large amount of amines remaining in the organic phase; incomplete regeneration of the extractant, with residual aromatic amines continuously accumulating and occupying active sites, leading to a decline in the extractant's recycling performance, making it difficult to effectively extract and enrich the target amine components in the upstream wastewater, ultimately resulting in low aniline and methylaniline content in the crude aniline product and a decrease in the overall material recovery rate.

[0049] <Example 8> Compared to Example 1, the difference is that in step b, the back-extraction and regeneration process controls the reaction temperature at 35°C.

[0050] Testing revealed the following: In step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 47%; in step d, the purity of aniline after refining reached 78%, and the purity of methylaniline reached 75%; the overall recovery rate of aniline reached 79%, and the recovery rate of methylaniline reached 76%; the chemical separating agent, after regeneration and recycling 8 times, had its extraction efficiency reduced to 30%-45% of its initial value. The reasons for this are: the low temperature in the back-extraction section inhibited the forward reversible equilibrium of back-extraction, making it difficult for aniline and methylaniline complexed in the extractant to dissociate and elute into the aqueous phase; furthermore, the molecular diffusion mass transfer rate decreased, the back-extraction kinetics deteriorated, resulting in a large residual amount of amines in the organic phase and incomplete regeneration of the extractant. The continuous accumulation of residual aromatic amines occupied the active extraction sites, leading to a decrease in the extractant's recycling capacity. Consequently, aniline and methylaniline in the upstream feed solution could not be fully extracted and enriched, resulting in a significant reduction in their recovered content and overall recovery rate.

[0051] <Example 9> Compared to Example 1, the difference is that in step b, the back-extraction and regeneration process controls the reaction temperature at 45°C.

[0052] Testing revealed the following: In step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 49%; in step d, the purity of aniline after purification reached 82%, and the purity of methylaniline reached 78%; the overall recovery rate of aniline reached 83%, and the recovery rate of methylaniline reached 81%; the chemical separating agent, after regeneration and recycling 9 times, saw its extraction efficiency decrease to 35%-47%. The reason for this was that the back-extraction temperature was too high, disrupting the back-extraction equilibrium and system stability, leading to a decrease in the recovery rate of the target substance and increased extractant loss.

[0053] <Example 10> Compared to Example 1, the difference is that in step c, the reaction temperature is controlled at 30°C.

[0054] Testing revealed that in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 44%; in step d, the purity of aniline after purification reached 73%, and the purity of methylaniline reached 70%; the overall recovery rate of aniline reached 81%, and the recovery rate of methylaniline reached 75%. The reason for this discrepancy was that the temperature was too low, leading to a severe imbalance in the extraction equilibrium and a significant deterioration in the separation effect of the target substance (toluene), making it impossible to meet the separation requirements.

[0055] <Example 11> Compared to Example 1, the difference is that in step c, the reaction temperature is controlled at 45°C.

[0056] Testing revealed that in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 45%; in step d, the purity of aniline after purification reached 72%, and the purity of methylaniline reached 71%; the overall recovery rate of aniline reached 83%, and the recovery rate of methylaniline reached 77%. The reason for this was that the excessively high temperature disrupted the extraction equilibrium and system stability, leading to a decrease in both extraction efficiency and selectivity.

[0057] <Example 12> Compared to Example 1, the difference is that in step d, the absolute pressure is controlled at 10 mmHg and the distillation temperature is controlled at 50°C.

[0058] Testing revealed that after purification in step d, the purity of aniline reached 80%, and the purity of methylaniline reached 75%; the overall recovery rate of aniline reached 85%, and the recovery rate of methylaniline reached 77%. The reason for this was that the low temperature led to a significant decrease in distillation efficiency and poor separation, resulting in insufficient recovery of the target substances.

[0059] <Example 13> Compared to Example 1, the difference is that in step d, the absolute pressure is controlled at 30 mmHg and the distillation temperature is controlled at 90°C.

[0060] Testing revealed that coking occurred in step d, making recovery impossible. The reason is that excessively high temperatures contradict the core objective of separating high-boiling-point / heat-sensitive substances at low temperatures, leading to product deterioration, reduced separation efficiency, and coking of the equipment.

[0061] <Comparative Example 1> Compared to Example 1, the difference is that the separating agent in step b is replaced with monobutyl phosphate.

[0062] Testing revealed the following: In step b, the COD of the waste liquid decreased to 25000 mg / L, aniline decreased to 9000 mg / L, and the concentration of methylaniline was below 3000 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 37%; in step d, the purity of aniline after purification reached 35%, and the purity of methylaniline reached 41%; the overall recovery rate of aniline reached 38%, and the recovery rate of methylaniline reached 25%. The reasons for this are: decreased complexation strength, insufficient selectivity, reduced system compatibility and mass transfer efficiency, and a shift in the extraction equilibrium constant. These factors led to a decrease in the extraction removal rate of the target substance, difficulty in two-phase emulsification and stratification, increased solvent loss, poorer regeneration of the extractant, increased reagent costs, and reduced equipment processing efficiency.

[0063] <Comparative Example 2> Compared to Example 1, the difference is that step a (chemical purification) was not performed; instead, step b (chemical separation) and step c (crude benzene recovery) were directly adopted.

[0064] Testing revealed the following: In step b, the COD of the wastewater decreased to 20,000 mg / L, aniline decreased to 9,000 mg / L, and the concentration of methylaniline was below 2,000 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 15%; in step d, the purity of aniline after purification reached 35%, and the purity of methylaniline reached 33%; the overall recovery rate of aniline reached 23%, and the recovery rate of methylaniline reached 21%; the chemical separating agent, after regeneration and recycling twice, maintained an extraction efficiency of over 13% of its initial value. The reason for this discrepancy is that the suspended solids in the wastewater were not removed, leading to significant loss of the separating agent, which in turn affected the chemical separation and the regeneration process.

[0065] <Comparative Example 3> Compared to Example 1, the difference is that in step b, no alkali solution was added to adjust the pH before chemical separation, and the pH of the waste liquid was about 6-7 at this time.

[0066] Testing revealed the following: In step b, the COD of the wastewater decreased to 28,000 mg / L, aniline decreased to 8,000 mg / L, and the concentration of methylaniline was below 3,000 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 23%; in step d, the purity of aniline after purification reached 45%, and the purity of methylaniline reached 43%; the overall recovery rate of aniline reached 25%, and the recovery rate of methylaniline reached 20%; the chemical separating agent, after regeneration and recycling five times, maintained an extraction efficiency of over 39% from its initial value. The reason for this discrepancy was the excessively low reaction pH, which caused some of the separating agent to combine with alkaline substances in the wastewater during the chemical separation process, resulting in a reduced pollutant removal rate, poor solid-liquid separation, and substandard effluent quality.

[0067] <Comparative Example 4> Compared to Example 1, the difference is that in step a, the pH was not adjusted to 7.0.

[0068] Testing revealed the following: In step b, the COD of the wastewater decreased to 27000 mg / L, aniline decreased to 8000 mg / L, and the concentration of methylaniline was below 3500 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 20%; in step d, the purity of aniline after purification reached 47%, and the purity of methylaniline reached 44%; the overall recovery rate of aniline reached 17%, and the recovery rate of methylaniline reached 21%; the chemical separating agent, after regeneration and recycling three times, maintained an extraction efficiency of over 40% of its initial value. The reason for this is that at this pH level, PAC will generate aluminum hydroxide precipitate, making it difficult to form large, settleable flocs. Furthermore, the precipitate easily disperses in the water, leading to a sharp drop in turbidity and suspended solids removal rates. This results in a significant decrease in the removal rates of suspended solids and turbidity, and the effluent quality fails to meet standards.

[0069] <Comparative Example 5> Compared to Example 1, the difference is that in step b, before chemical separation, an excess of alkali solution is added, at which point the pH of the waste liquid is approximately 9.5.

[0070] Testing revealed the following: In step b, the COD of the wastewater decreased to 2900 mg / L, aniline decreased to 7500 mg / L, and the concentration of methylaniline was below 2300 mg / L; in step c, the total content of aniline and methylaniline in the crude aniline oil phase was approximately 21%; in step d, the purity of aniline after purification reached 48%, and the purity of methylaniline reached 41%; the overall recovery rate of aniline reached 18%, and the recovery rate of methylaniline reached 21%; the chemical separation agent, after regeneration and recycling five times, maintained an extraction efficiency of over 60% of its initial value. The reason for this is that excessive alkali caused the wastewater pH to reach a strongly alkaline environment of 9.5, where the amphoteric heavy metal hydroxide precipitate combined with excess hydroxide ions to form soluble hydroxyl complexes, causing the precipitate to redissolve; simultaneously, the extraction / complexing separation agents were hydrolyzed and deactivated under strong alkali conditions, the water-oil system easily emulsified, the two phases could not separate, the chemical separation reaction proceeded in reverse, and the removal effect deteriorated sharply. This resulted in a decrease in the removal rate of the target pollutants.

[0071] The above are merely specific embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for treating indigo dye wastewater, characterized in that... Includes the following steps: a. Chemical removal: Adjust the pH of the wastewater to 6-8, add polyaluminum chloride to carry out flocculation reaction, and then let it stand and filter after the reaction; b. Chemical separation: Adjust the pH of the wastewater treated in step a to 8-9, add a chemical separating agent to carry out an extraction reaction, the chemical separating agent comprising a complexing agent, a cosolvent, and a diluent, the complexing agent being phenylphosphonic acid mono(2-ethylhexyl) ester; after the reaction, perform phase separation to obtain a separated phase loaded with pollutants and a preliminarily purified aqueous phase; subsequently, add an acidic desorption agent to the separated phase for back-extraction to separate the regenerated chemical separating agent and the recovered phase; c. Aniline recovery: Add an alkaline solution to the recovered phase obtained in step b, react, and allow to stand for separation to obtain crude aniline oil phase; d. Aniline refining: The crude aniline oil phase obtained in step c is subjected to vacuum distillation to obtain refined aniline and refined methylaniline.

2. The method according to claim 1, characterized in that, In the chemical separating agent, the co-solvent is an isomeric alcohol or a mixture thereof, and the diluent is at least one of aviation kerosene or diesel oil; by volume percentage, the complexing agent: 10%-50%, the co-solvent: 5%-25%, and the diluent: balance.

3. The method according to claim 2, characterized in that, In step b, the volume ratio of the chemical separating agent to the wastewater is between 1:5 and 1:

10.

4. The method according to claim 1 or 2, characterized in that, In the extraction reaction described in step b, the extraction reaction is divided into two phases in time, with the control temperature of the later phase being lower than that of the earlier phase.

5. The method according to claim 4, characterized in that, In the extraction reaction described in step b, staged temperature control is adopted: the temperature is controlled at 35-40℃ for 0-30 minutes after the start of the reaction; the temperature is controlled at 30-35℃ when the reaction proceeds to 30-60 minutes.

6. The method according to claim 1, characterized in that, In step b, the acidic eluent is a hydrochloric acid solution with a mass fraction of 10%-20%, and its volume ratio with the separation phase is 1:6 to 1:

10. The pH value of the back-extraction reaction is controlled to be ≤0.5, and the temperature is controlled at 40±3℃.

7. The method according to claim 1, characterized in that, In step c, the alkaline solution is a sodium hydroxide solution with a mass fraction of 10%-48%, the pH of the reaction system is controlled at 13-14, and the reaction temperature is controlled at 40±3℃.

8. The method according to claim 1, characterized in that, In step d, the absolute pressure of the vacuum distillation is controlled at 15-25 mmHg, and the distillation temperature is controlled at 70-80℃.

9. The method according to claim 1, characterized in that, In step a, the amount of polyaluminum chloride added is 3-5 wt%.

10. The method according to claim 1, characterized in that, In step a, the temperature of the flocculation reaction is controlled at 20-30℃.

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