Method for treating production wastewater of pendimethalin

Abstract

The application discloses a treatment method of pendimethalin production wastewater, and relates to the technical field of pesticide production wastewater treatment; the method comprises the following steps: firstly, adjusting the pH of the pendimethalin production wastewater to 2-3 to perform acid separation, so that part of organic intermediates are separated out in the form of precipitation and recovered; secondly, countercurrently extracting the acid separation filtrate by using a mixed solvent of cyclohexane and kerosene, so as to recover residual pendimethalin and nitrobenzene intermediates; thirdly, removing residual organic matters by deep adsorption of the raffinate through macroporous adsorption resin; finally, evaporating and concentrating the adsorbed water to recover sulfuric acid and return the sulfuric acid to a nitration process, so that the organic matters and sulfuric acid in the wastewater are recovered in a gradient resource utilization mode, and the COD of the wastewater and the amount of hazardous waste are greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of pesticide production wastewater treatment technology, specifically a method for treating wastewater from pendimethalin production. Background Technology

[0002] The nitration stage in the production of pendimethalin generates a large amount of nitration mother liquor. This wastewater typically contains 8%–18% sulfuric acid and has a COD as high as 30,000–90,000 mg / L. The main organic matter consists of nitrobenzene intermediates, unreacted pendimethalin, and small amounts of byproducts. The wastewater is dark in color and has a pH less than 2, classifying it as high-concentration, strongly acidic, and difficult-to-degrade organic wastewater. Taking actual production data from a certain company as an example, producing 1 ton of pendimethalin generates approximately 3–5 tons of nitration mother liquor, with a sulfuric acid content of about 120 kg / ton and an organic intermediate content of about 15–25 kg / ton. Direct discharge or introduction into a conventional biological treatment system is not feasible.

[0003] Currently, the main methods for treating this type of wastewater are neutralization precipitation, Fenton oxidation, and evaporation concentration. Neutralization precipitation requires the addition of large amounts of lime or liquid alkali, resulting in high costs for neutralizing agents per ton of wastewater. It also generates large amounts of hazardous sulfate waste containing organic matter, the disposal costs of which far exceed the cost of the agents. Furthermore, sulfuric acid and organic intermediates in the wastewater cannot be recovered, leading to serious resource waste. While Fenton oxidation can degrade some organic matter, it consumes large amounts of hydrogen peroxide and ferrous sulfate, produces a large amount of sludge, and is not effective at removing high concentrations of COD, resulting in high operating costs. Evaporation concentration can recover some salts, but high concentrations of organic matter in the wastewater easily cause coking and blockage in the evaporator. High concentrations of sulfuric acid severely corrode the equipment, and the concentrated mother liquor becomes even more difficult to treat as hazardous waste, resulting in extremely high actual operating and maintenance costs.

[0004] The methods described above have failed to effectively recover sulfuric acid and organic matter, resulting in incomplete treatment and high costs. Enterprises are facing increasing resource waste and hazardous waste disposal pressures. Therefore, a method for treating dimethomorph production wastewater that can simultaneously recover sulfuric acid and organic intermediates from wastewater, reduce subsequent treatment load, decrease hazardous waste generation, and has low operating costs is needed. Existing technologies cannot meet these requirements, making this a pressing technical problem for enterprises to solve. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for treating wastewater from dimethylpentylamine production. This method involves first adjusting the pH of the wastewater to 2-3 for acid precipitation, causing some organic intermediates to precipitate and be recovered. Then, the acid precipitation filtrate is subjected to countercurrent extraction with a mixed solvent of cyclohexane and kerosene to recover residual dimethylpentylamine and nitrobenzene intermediates. The raffinate is then subjected to deep adsorption using macroporous adsorption resin to remove residual organic matter. Finally, the adsorbed effluent is evaporated and concentrated to recover sulfuric acid, which is then reused in the nitration process. This achieves a tiered resource recovery of organic matter and sulfuric acid from the wastewater, significantly reducing COD and hazardous waste generation.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for treating wastewater from the production of pendimethalin, the method comprising the following components:

[0007] Acid precipitation step: Adjust the pH of the dimethylpentylamine production wastewater to 2-3, stir and let it stand to allow some organic intermediates to precipitate out in the form of precipitate. Separate the solid and liquid to obtain acid precipitation precipitate and acid precipitation filtrate.

[0008] Extraction steps: The acid precipitation filtrate was subjected to countercurrent extraction using a mixed solvent of cyclohexane and kerosene as the extractant. After standing and phase separation, the extract phase and raffinate were obtained. The extract phase was then distilled to recover the extractant and recover the organic matter.

[0009] Resin adsorption steps: The raffinate is passed into an adsorption column packed with macroporous adsorption resin for adsorption treatment to obtain adsorbed water and saturated resin. The saturated resin is desorbed and regenerated with a desorbent, and the organic matter is recovered from the desorbed liquid.

[0010] Sulfuric acid concentration and reuse steps: The adsorbed water is evaporated and concentrated to obtain concentrated sulfuric acid, which is then reused in the nitration process.

[0011] Furthermore, in the acid precipitation step, the wastewater from the dimethylpentylamine production process is a nitration mother liquor, with a sulfuric acid mass fraction of 5%–20%, a COD of 20,000–100,000 mg / L, a nitrobenzene intermediate concentration of 1,000–5,000 mg / L, and an initial pH less than 2.

[0012] Furthermore, in the acid precipitation step, the alkaline substance used to adjust the pH is one or more of ammonia, sodium hydroxide solution, and lime milk. The mass fraction of ammonia is 20%–25%, the mass fraction of sodium hydroxide solution is 20%–30%, and the mass fraction of lime milk is 10%–20%. The final pH is adjusted to 2.0–3.0, the acid precipitation temperature is 15–38°C, the standing time is 0.5–4 hours, and the solid-liquid separation is performed by filtration or centrifugation.

[0013] Furthermore, in the acid precipitation step, the acid precipitate is dried and then recovered as an organic intermediate. The drying temperature is 60-100℃, the drying time is 2-6 hours, and the water content of the dried acid precipitate is less than 5%.

[0014] Furthermore, in the extraction step, the volume ratio of cyclohexane to kerosene in the extractant is 1.5:1 to 3:1, the volume ratio of oil phase to water phase in the extraction phase is 1:2 to 1:4, the number of countercurrent extraction stages is 3 to 5, the extraction temperature is 20 to 40°C, the mixing time for each stage is 10 to 30 min, and the settling and phase separation time is 10 to 60 min.

[0015] Furthermore, in the extraction step, the distillation temperature for recovering the extractant by distillation of the extractant phase is 60-85°C. The extractant recovered at the top of the column is recycled, and the recovered organic matter is obtained at the bottom of the column. The recovered organic matter is returned to the front-end process of dimethylpentylene production.

[0016] Furthermore, in the resin adsorption step, the macroporous adsorption resin is one of XAD-4, H-103, NKA-9, and DA-201, the adsorption flow rate is 1–5 BV / h, the adsorption temperature is 20–40℃, and the desorbent is one of 80%–95% (v / v) ethanol aqueous solution, 80%–95% (v / v) methanol aqueous solution, 80%–95% (v / v) acetone aqueous solution, or 5%–10% (w / w) sodium hydroxide solution, with a desorption flow rate of 0.5–2 BV / h.

[0017] Furthermore, in the resin adsorption step, the diameter-to-height ratio of the adsorption column is 1:3 to 1:8, and the resin loading is 60% to 80% of the effective volume of the adsorption column. When the COD of the adsorbed water is less than 2000 mg / L, adsorption continues, and when the COD of the adsorbed water is greater than or equal to 2000 mg / L, the system switches to regeneration mode.

[0018] Furthermore, in the sulfuric acid concentration and reuse step, the adsorbed water is softened before evaporation and concentration. Sodium carbonate or sodium hydroxide is added to adjust the pH to 8-10, causing calcium and magnesium ions to precipitate. After filtration, the water is evaporated and concentrated. The evaporation and concentration adopts multi-effect evaporation or MVR evaporation. Multi-effect evaporation is three-effect or four-effect co-current evaporation. The water is evaporated and concentrated to a sulfuric acid mass fraction of 50%-70%. The concentrated sulfuric acid is reused in the nitrification process, and the evaporation condensate enters the plant's biochemical treatment system.

[0019] Compared with existing technologies, this method for treating dimethoate production wastewater has the following advantages:

[0020] I. This invention utilizes a tiered treatment process involving acid precipitation, countercurrent extraction with mixed solvents, and deep adsorption with macroporous adsorption resins to recover most of the dimethylpentylene and nitrobenzene intermediates from wastewater. The recovered organic intermediates can be directly returned to the production process, significantly reducing the wastewater's COD and color. Simultaneously, sulfuric acid, after evaporation and concentration, is reused in the nitration process, achieving resource recycling of sulfuric acid and organic matter in the wastewater. This significantly reduces the generation and discharge of hazardous waste. Statistical analysis of measured data from three embodiments shows that the total COD removal rate reaches 97.5%–99.2%, and the total organic matter recovery rate is approximately 82%–91%, with acid precipitation recovering approximately 45%–55%, extraction recovering approximately 25%–30%, resin desorption recovering approximately 8%–12%, and sulfuric acid recovering approximately 78%–85%. The hazardous waste generation per ton of wastewater is reduced from approximately 80–120 kg using traditional neutralization methods to approximately 5–15 kg.

[0021] II. This invention utilizes the wastewater itself under acidic conditions for acid precipitation, which significantly reduces alkali consumption and avoids the drawbacks of generating hazardous sulfate waste compared to traditional neutralization methods. The extractant is recycled after distillation, and the resin is reused after desorption and regeneration. This results in low reagent consumption and low treatment costs. The adsorbed water is softened before evaporation and concentration, effectively preventing scaling and clogging of the evaporator. The process operates stably and is suitable for actual production conditions, showing promising industrial application prospects. Preliminary economic calculations indicate that the comprehensive treatment cost per ton of wastewater in this invention is approximately 55% to 70% of that of traditional neutralization and precipitation methods, demonstrating good economic feasibility.

[0022] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0024] Figure 1 This is a main flow chart of a method for treating wastewater from the production of dimethoate;

[0025] Figure 2 A detailed flow chart of the acid precipitation steps in a method for treating wastewater from dimethoate production;

[0026] Figure 3A detailed flow chart of the extraction steps in a method for treating wastewater from dimethoate production. Detailed Implementation

[0027] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0028] It should be noted in advance that the wastewater quality parameters, treatment scale and operating conditions used in the following three embodiments were all obtained on-site at the enterprise. The process parameters of each embodiment were adjusted and optimized according to the actual on-site working conditions, and are not simply a repetition of the same template.

[0029] Example 1

[0030] In the wastewater treatment scenario of the nitration stage of a dimethyl pendimethalin production plant, the nitration mother liquor, as the main wastewater generated during the dimethyl pendimethalin production process, is characterized by high sulfuric acid content, high organic matter concentration, deep color, and strong acidity. Taking a dimethyl pendimethalin production plant with an annual output of 3,000 tons as an example, the nitration stage generates approximately 40 tons of nitration mother liquor daily. This wastewater has a sulfuric acid content of 12%–15%, a COD of 45,000–65,000 mg / L, a nitrobenzene intermediate concentration of 2,000–3,500 mg / L, an initial pH of 0.5–1.2, and a dark brown to black color. The original treatment method for this plant involved neutralization and precipitation followed by treatment at the plant's wastewater treatment station, generating approximately 180 tons of hazardous calcium sulfate waste monthly. Hazardous waste disposal costs were high, and a large amount of organic matter and sulfuric acid in the wastewater could not be recovered. This embodiment uses the method of this invention to treat the nitration mother liquor at the plant's production site. The specific implementation process is as follows.

[0031] In the acid precipitation step, the nitration mother liquor is introduced into the acid precipitation reactor through the bottom valve of the nitration reactor. The acid precipitation reactor is made of glass-lined material, with an effective volume of 20 m³, and is equipped with a stirring device and an online pH monitor. Since the initial pH value of the wastewater is below 2, an alkaline substance is used to adjust the pH to the target range. In this embodiment, 20% ammonia water by mass is selected as the neutralizing agent. The reason for choosing ammonia water is that the company's subsequent nitration reaction has a high tolerance for ammonium ions in concentrated sulfuric acid, and the price of ammonia water is about 12% lower than that of sodium hydroxide. Ammonia water is slowly added to the acid precipitation reactor through a metering pump. During the addition process, stirring is turned on, and the stirring speed is controlled at 60-80 r / min to ensure that the ammonia water and wastewater are fully mixed. The online pH monitor provides real-time feedback on the pH value changes of the system. When the pH value approaches 2.0, the rate of ammonia water addition is slowed down to avoid local over-alkalinity that would cause the precipitated precipitate to redissolve. The final pH value is adjusted to be controlled between 2.0 and 2.5. The acid precipitation temperature is the natural temperature at which the wastewater is discharged. In this embodiment, the measured temperature is 28–32°C, slightly higher in summer and slightly lower in winter, requiring no additional heating or cooling. After adding ammonia, continue stirring for 15 minutes to ensure the system reacts fully, then stop stirring and let it stand for 1.5 hours. During the standing process, the solubility of some nitrobenzene intermediates and unreacted dimethylpentylene in the wastewater decreases under acidic conditions, gradually precipitating out as solid precipitates. The precipitate is dark brown and flocculent, settling at the bottom of the reactor. Sampling and testing showed that after 1.5 hours of standing, the suspended solids content in the supernatant had decreased to below approximately 200 mg / L. Extending the standing time to 3 hours resulted in an increase of less than 5% in the amount of precipitate, indicating that precipitation equilibrium was essentially reached after 1.5 hours. After standing, the discharge valve at the bottom of the reactor was opened, and the acid precipitation mixture was sent to a plate and frame filter press for solid-liquid separation. The plate and frame filter press has a filtration area of ​​60 m², uses polyester filter cloth, and has a filtration precision of 200 mesh. The filtration operation pressure is controlled at 0.3–0.5 MPa, and the filtration cycle is approximately 40 minutes. After filtration, acid precipitate and acid filtrate are obtained. The acid precipitate, after being discharged from the plate and frame filter press, is sent to a vacuum drying oven for drying at 80℃ for 4 hours, reducing the moisture content of the precipitate to below 3.5%. The dried acid precipitate is recovered in solid form, with a recovery rate of approximately 12–15 kg per ton of wastewater. The main components of this precipitate are nitrobenzene intermediates and a small amount of dimethylpentylene. Liquid chromatography analysis shows that the effective component (dimethylpentylene + nitrobenzene intermediates) content is approximately 78%–85%, the moisture content is approximately 3.5%, and the remainder consists of small amounts of inorganic salts and resinous byproducts, which can be returned to the pre-production stage of dimethylpentylene as raw materials for reuse. The acid filtrate is collected in an acid filtrate storage tank and proceeds to the next processing stage. After acid precipitation, some organic matter in the wastewater was removed in the form of precipitation, and the COD was reduced from about 55,000 mg / L before treatment to about 38,000 mg / L, with a COD removal rate of about 31%.

[0032] In the extraction step, the acid precipitate is pumped from the storage tank into a countercurrent extraction tower for treatment. The countercurrent extraction tower is a packed tower with a diameter of 800 mm and a height of 8 m, filled with structured packing material, achieving an effective separation height of 6 m, and a designed processing capacity of 2 t / h. The extractant is a mixed solvent of cyclohexane and kerosene, prepared at a volume ratio of 2:1, and thoroughly mixed in an extractant preparation tank before use. During the extraction operation, the acid precipitate, as the heavy phase, enters from the top of the extraction tower, while the extractant, as the light phase, enters from the bottom. The two phases flow countercurrently within the tower, achieving a three-stage countercurrent extraction effect. The extraction ratio is controlled at 1:3 (oil phase to water phase volume ratio), meaning 1 volume of mixed extractant is used for every 3 volumes of acid precipitate processed. The extraction temperature is room temperature, controlled in this embodiment at 25–35 °C. During the contact between the two phases within the extraction tower, residual dimethylpentylene and nitrobenzene intermediates in the acid precipitate transfer from the aqueous phase to the organic phase, completing the extraction mass transfer. After extraction, the extract phase is discharged from the top of the column, and the raffinate is discharged from the bottom. Analysis showed that the total concentration of dimethylpentylene and nitrobenzene intermediates in the extract phase was approximately 8–12 g / L. The extract phase was sent to a distillation kettle for distillation recovery at a temperature of 70–80℃. The cyclohexane-kerosene mixed solvent distilled from the top of the column was condensed and refluxed back to the extractant preparation tank for recycling. Recovered organic matter was obtained from the bottom of the column, mainly dimethylpentylene and nitrobenzene intermediates. Approximately 6–8 kg of this organic matter could be recovered per ton of wastewater and returned to the upstream process of dimethylpentylene production. The COD of the raffinate decreased from approximately 38,000 mg / L after acid precipitation to approximately 12,000–15,000 mg / L, with a COD removal rate of approximately 68%–72%. The raffinate was collected in a raffinate storage tank and then entered the resin adsorption process.

[0033] In the resin adsorption step, the raffinate is pumped from the storage tank into an adsorption column filled with macroporous adsorption resin for deep adsorption treatment. The adsorption column is made of stainless steel, with an inner diameter of 600 mm, a column height of 3 m, a diameter-to-height ratio of 1:5, a resin packing height of 2.1 m, and a packing volume of approximately 70% of the effective volume. In this embodiment, H-103 type macroporous adsorption resin was selected. This resin showed a dynamic adsorption capacity of approximately 118 mg / g (calculated as pendimethalin) in the pilot test at the company's site, and after 50 adsorption-desorption cycles, the adsorption capacity decreased by approximately 7.2%, yet it could still maintain stable operation, meeting the requirements for long-term industrial operation. Two sets of adsorption columns were set up, one for use and one for standby. When one set of adsorption columns became saturated, the other set was switched to continue operation, and the saturated column underwent desorption regeneration. During the adsorption operation, the raffinate flowed from top to bottom through the adsorption column at a flow rate of 2 BV / h, and the adsorption temperature was room temperature (25–35 °C). A COD online monitoring instrument was installed at the outlet of the adsorption column to monitor the COD value of the effluent in real time. When the COD of the effluent is below 2000 mg / L, the adsorption column continues to operate; when the COD reaches or exceeds 2000 mg / L, it indicates that the resin adsorption is approaching saturation, and the adsorption column switches to regeneration mode. In this embodiment, a single adsorption cycle is approximately 6–8 hours, and the treated water volume is approximately 8–10 m³. The COD of the effluent is approximately 1500–1700 mg / L, the COD removal rate is approximately 86%–89%, and the color changes from dark brown to light yellow. For the desorption and regeneration of the saturated resin, a 90% (v / v) ethanol aqueous solution is used as the desorbent. The desorbent flows from top to bottom through the adsorption column at a flow rate of 1 BV / h, and the desorption temperature is room temperature. The initial desorption efficiency was measured to be approximately 94%, and after 5 cycles, the desorption efficiency stabilized between 89% and 91%. During desorption, the organic matter adsorbed by the resin flows out along with the desorbent, resulting in an desorbed liquid containing organic matter. After collection, the desorbed liquid is used to recover ethanol and organic matter through distillation. The ethanol is recycled, and the organic matter is returned to the production process. After desorption is complete, the resin column is rinsed with deionized water at a flow rate of 2 BV / h to remove residual desorbent. The rinse water is then recycled into the raffinate storage tank. The resin column is then put back into use after its adsorption capacity is restored.

[0034] In the sulfuric acid concentration and reuse step, the adsorption effluent enters the evaporation concentration system for sulfuric acid concentration and recovery. Since the adsorption effluent may contain small amounts of calcium and magnesium ions, softening treatment is performed before evaporation concentration to prevent scaling in the evaporator. The adsorption effluent is introduced into a softening reaction tank, where sodium carbonate solution is added to adjust the pH to 8.5–9.0, causing calcium and magnesium ions to precipitate as carbonates. After softening treatment, the calcium ion concentration in the effluent decreases from approximately 120 mg / L to approximately 8 mg / L, and the magnesium ion concentration decreases from approximately 45 mg / L to approximately 5 mg / L, meeting the evaporator influent water quality requirements. After filtration by a plate and frame filter press, the clarified liquid is sent to the evaporation system. Evaporation concentration adopts a triple-effect co-current evaporation process. The evaporation temperature of the first effect is approximately 85–90℃, the second effect is approximately 75–80℃, and the third effect is approximately 60–65℃, with the system vacuum maintained at -0.08–-0.09 MPa. After triple-effect evaporation concentration, the sulfuric acid mass fraction of the adsorption effluent is concentrated from the initial approximately 12%–15% to 55%–65%. After cooling, concentrated sulfuric acid is pumped into a sulfuric acid storage tank and reused in the nitration section to prepare mixed acid. Feedback from the company's nitration workshop indicates that the conversion rate of the mixed acid prepared with recycled concentrated sulfuric acid in the nitration reaction differs from that prepared with fresh sulfuric acid by no more than 1.5 percentage points, and the product purity meets the company's standards. The condensate generated during the evaporation process has a COD below 500 mg / L and undergoes further treatment in the plant's biochemical treatment system before being discharged in compliance with standards.

[0035] In summary, this embodiment achieves the graded recovery and utilization of nitrobenzene intermediates and sulfuric acid from dimethylpentylamine nitration mother liquor through a step-by-step combination of four processes: acid precipitation, countercurrent extraction with mixed solvents, deep adsorption with macroporous adsorption resin, and evaporation concentration. Most of the organic matter in the wastewater is returned to the production process in the form of precipitates, extracts, and desorption liquids. The sulfuric acid, after evaporation and concentration, is reused in the nitration reaction, thus realizing the recycling of resources from the wastewater. The treatment process does not use large amounts of neutralizing alkali, avoiding the generation of hazardous wastes such as calcium sulfate, significantly reducing the company's hazardous waste disposal costs and raw material consumption costs, and demonstrating good economic and environmental benefits.

[0036] Example 2

[0037] In a scenario where a dimethyl pendimethalin production plant has a large volume of nitration mother liquor and the wastewater is mainly composed of nitrobenzene intermediates, the concentration of nitrobenzene intermediates in the nitration mother liquor is relatively high (approximately 4000–5000 mg / L), while the suspended solids content in the wastewater is low, which is conducive to acid precipitation and solid-liquid separation. This embodiment was implemented in a dimethyl pendimethalin production plant with an annual output of 5000 tons. The plant's nitration section produces approximately 60 tons of nitration mother liquor daily, with a sulfuric acid mass fraction of approximately 8%–10%, a COD of approximately 70000–90000 mg / L, a nitrobenzene intermediate concentration of 4000–5000 mg / L, an initial pH of 0.8–1.0, and a dark brown color. The organic matter concentration in the wastewater of this plant is significantly higher than in Example 1, and the challenge lies in ensuring extraction and adsorption efficiency under a high organic matter load. This embodiment uses the method of the present invention to treat this wastewater, and the specific implementation process is as follows.

[0038] In the acid precipitation step, the nitration mother liquor is introduced into the acid precipitation reactor from the nitration system. The acid precipitation reactor is made of glass-lined material with an effective volume of 30 m³, equipped with a stirring device and a pH meter. In this embodiment, a 25% sodium hydroxide solution is selected as the neutralizing agent. The reason for choosing sodium hydroxide is that the subsequent nitration reaction of this enterprise has a high tolerance for sodium ions in concentrated sulfuric acid, and the sodium hydroxide solution has a fast neutralization reaction rate and produces less organic matter entrained in the precipitate. It is calculated that approximately 18-22 L of sodium hydroxide solution needs to be added per ton of wastewater to adjust the pH from 1.0 to about 2.5. The sodium hydroxide solution is added slowly through a metering pump, and stirring is started simultaneously with the addition, with a stirring speed of 50-70 r / min. The pH meter is used for real-time monitoring, and alkali addition is stopped when the pH value reaches 2.3-2.8. The acid precipitation temperature is the wastewater discharge temperature, which is controlled at 30-35℃ in this embodiment. After the alkali addition is completed, the mixture is stirred for 20 minutes, and then allowed to stand for 2 hours. During the standing process, nitrobenzene intermediates gradually precipitate out, forming a dark brown precipitate. It is worth noting that the amount of precipitate after 2 hours of settling in this embodiment is approximately 1.15 times that after 1.5 hours, indicating that higher concentrations of organic matter require a longer settling time. Therefore, the settling time was extended to 2 hours. After settling, a horizontal screw centrifuge was used for solid-liquid separation. The centrifuge drum diameter was 450 mm, the rotation speed was 3000 r / min, and the separation factor was approximately 1500. After centrifugation, acid precipitate and acid filtrate were obtained. The acid precipitate was sent to a disc dryer for drying at 90°C for 3 hours. After drying, the moisture content was less than 4%. Approximately 16-20 kg of acid precipitate was recovered per ton of wastewater. The recovered products were mainly nitrobenzene intermediates, which were returned to the production process for reuse. The acid filtrate was collected and then entered the extraction process.

[0039] In the extraction step, the extractant was a mixed solvent of cyclohexane and kerosene at a volume ratio of 2.5:1. The reason for increasing the cyclohexane ratio here is that the concentration of nitrobenzene organic compounds in the wastewater in this embodiment is higher, requiring stronger extraction capacity. Laboratory tests confirmed that increasing the cyclohexane ratio from 2:1 to 2.5:1 increased the extraction rate from approximately 71% to approximately 78%. The extractant was mixed evenly in the extractant preparation tank and then set aside. The countercurrent extraction employed a four-stage extraction tower series operation, with each stage equipped with a stirring device and a settling / separation section. The acid precipitate was sequentially passed through the four extraction towers, with each stage having an oil-to-water phase volume ratio of 1:2.5, an extraction temperature of 30–38°C, a mixing time of 20 min per stage, and a settling / separation time of 30 min. After four stages of countercurrent extraction, the organic matter content in the raffinate was significantly reduced. After collection, the extract phase is sent to a distillation column for distillation at a temperature of 75–80°C. The cyclohexane-kerosene mixed solvent recovered at the top of the column is condensed and returned to the extractant preparation tank for recycling. At the bottom of the column, approximately 7–10 kg of organic matter can be recovered per ton of wastewater and returned to the pre-processing stage of dimethyl pendimethalin production. The COD of the raffinate decreases from approximately 50,000–60,000 mg / L after acid precipitation to approximately 15,000–20,000 mg / L.

[0040] In the resin adsorption step, the raffinate is passed through an adsorption column filled with XAD-4 macroporous adsorption resin for treatment. XAD-4 resin was chosen in this embodiment because the nitrobenzene intermediates in the wastewater have a relatively large molecular weight. The pore size distribution of XAD-4 is more conducive to the diffusion adsorption of macromolecular organic matter compared to H-103. Static adsorption experiments show that the equilibrium adsorption capacity of XAD-4 for this wastewater is about 8% higher than that of H-103. The adsorption column is made of stainless steel, with an inner diameter of 700 mm, a column height of 3.5 m, a diameter-to-height ratio of 1:5, and a resin packing height of 2.5 m, with a packing volume of approximately 72% of the effective volume. During adsorption, the raffinate flows through the adsorption column at a flow rate of 3 BV / h, and the adsorption temperature is 30–35 °C. The effluent COD is monitored online. Adsorption continues when the effluent COD is below 1800 mg / L, and regeneration is switched when it reaches or exceeds 1800 mg / L. In this embodiment, a single adsorption cycle is approximately 5–7 hours, and the treated water volume is approximately 10–12 m³. The COD of the adsorbed water is approximately 1600–1800 mg / L. The saturated resin is regenerated using an 85% (v / v) methanol-water solution as the desorbent. Methanol-water solution is chosen instead of ethanol-water solution because methanol is slightly less polar than ethanol and has a stronger elution capacity for non-polar nitrobenzene organic compounds. However, methanol is more toxic, requiring operation in a closed system equipped with a gas recovery device. The desorption flow rate is 1.5 BV / h, and the desorption temperature is room temperature. After collection, the eluent is used to recover methanol and organic matter through distillation, and the methanol is recycled. After desorption, the resin is rinsed with deionized water until neutral and then set aside for later use.

[0041] In the sulfuric acid concentration and reuse step, the adsorption effluent is first introduced into a softening reaction tank, where sodium hydroxide solution is added to adjust the pH to 9.0–9.5, causing calcium and magnesium ions to form hydroxide precipitates. After filtration through a precision filter, the clarified liquid is sent to the MVR evaporation system for concentration. The MVR evaporator uses mechanical vapor recompression technology, with the evaporation temperature controlled at 70–80℃, resulting in low system energy consumption. After MVR evaporation and concentration, the sulfuric acid mass fraction of the adsorption effluent is concentrated from approximately 8%–10% to 60%–70%. The concentrated sulfuric acid is cooled and reused in the nitration section for the preparation of mixed acid. The COD of the evaporation condensate is approximately 400–600 mg / L, which is then treated in the plant's biological treatment system.

[0042] In summary, this embodiment addresses the high concentration of dimethylpentylene nitration mother liquor containing nitrobenzene intermediates. High-concentration nitrobenzene intermediates are recovered in solid form through acid precipitation. Residual organic matter is further extracted using countercurrent extraction with a mixed solvent, and finally, residual organic matter is removed by deep adsorption with macroporous adsorption resin, achieving a tiered recovery of organic matter. Sulfuric acid is concentrated and reused via MVR evaporation, significantly reducing the company's sulfuric acid procurement and waste acid disposal costs. Simultaneously, it avoids the problem of generating large amounts of hazardous sulfate waste using traditional neutralization methods, and the process operates stably and reliably.

[0043] Example 3

[0044] In scenarios where dimethomorph production enterprises have wastewater with high sulfuric acid content and relatively low COD, the wastewater is highly acidic and saline, requiring high corrosion resistance from treatment equipment. However, the relatively low organic matter concentration is beneficial for improving the efficiency of acid precipitation and extraction. This embodiment is unique in that the wastewater has a high sulfuric acid content, resulting in high energy consumption for direct evaporation and concentration, and a greater risk of equipment corrosion from high-concentration sulfuric acid. Therefore, lime slurry is used in the acid precipitation stage to pre-separate some of the sulfuric acid as calcium sulfate. This embodiment is implemented in a dimethomorph production enterprise with an annual output of 2000 tons. The enterprise's nitration section produces approximately 25 tons of nitration mother liquor daily. The wastewater has a sulfuric acid content of approximately 16%–18%, a COD of approximately 25000–35000 mg / L, a nitrobenzene intermediate concentration of 1000–2000 mg / L, and an initial pH value less than 1. This embodiment uses the method of the present invention to treat this wastewater, and the specific implementation process is as follows.

[0045] In the acid precipitation step, the nitration mother liquor is introduced from the nitration reactor into the acid precipitation reactor. The acid precipitation reactor is made of titanium composite plate. The reason for choosing titanium composite plate here is that the sulfuric acid mass fraction in the wastewater of this enterprise is as high as 16% to 18%. Conventional glass-lined equipment is at risk of the glass layer peeling off under high temperature and high flow rate. Titanium has better corrosion resistance to high-temperature sulfuric acid, but the cost is higher. Therefore, titanium composite plate is used to balance corrosion resistance and economy. The effective volume is 15m³, equipped with a stirring and pH monitoring system. Since the initial pH of the wastewater is extremely low (about 0.5 to 0.8), this embodiment uses lime milk with a mass fraction of 15% as a neutralizing agent. The addition of lime milk can convert part of the sulfuric acid in the wastewater into calcium sulfate precipitate, while adjusting the pH to the target range. Another consideration for choosing lime milk here is that the enterprise has low requirements for the calcium ion content in the recovered sulfuric acid. Therefore, removing part of the sulfuric acid in the acid precipitation stage can reduce the sulfuric acid load of evaporation and concentration, and also prevent calcium ions from entering the concentrated sulfuric acid product. However, it is important to control the amount of lime slurry added to avoid excessive pH increase, which could lead to an increase in the amount of organic matter encapsulated in the precipitate. The lime slurry is slowly added to the acid precipitation reactor using a screw pump, with stirring started during the addition process at a speed of 40–60 r / min. Since the lime slurry neutralization reaction is exothermic, the addition rate must be controlled to prevent the system temperature from becoming too high; in this embodiment, the acid precipitation temperature is controlled at 25–35℃. Addition is stopped when the pH reaches 2.5–3.0, and stirring continues for 20 minutes, followed by standing for 1 hour. During standing, some organic intermediates and calcium sulfate co-precipitate. Solid-liquid separation is performed using a plate and frame filter press with a filtration area of ​​40 m², using acid-resistant polypropylene filter cloth. After filtration, acid precipitation precipitate and acid precipitation filtrate are obtained. In addition to organic matter, the acid precipitation precipitate also contains a small amount of calcium sulfate. Analysis shows that the precipitate contains approximately 15%–20% calcium sulfate. This precipitate can still be recovered as an organic intermediate after drying, but the production workshop should be informed that this batch of recovered material contains a small amount of inert calcium sulfate packing material, which has no significant negative impact on subsequent reactions. Approximately 8–12 kg of acid precipitate is recovered per ton of wastewater. Due to the high concentration of sulfuric acid in the wastewater, the sulfuric acid mass fraction in the acid precipitate remains at approximately 14%–16%, and it then proceeds to the extraction process.

[0046] In the extraction step, the extractant is a mixed solvent of cyclohexane and kerosene prepared at a volume ratio of 1.8:1. The countercurrent extraction employs a four-stage countercurrent extraction method, achieving multi-stage contact within the extraction tower. The extraction phase has a volume ratio of oil to water of 1:3.5, the extraction temperature is 20–30℃, the mixing time for each stage is 15 min, and the settling time for phase separation is 20 min. Residual dimethylpentylene and nitrobenzene intermediates in the acid precipitation filtrate are fully extracted to the organic phase during the four-stage countercurrent extraction process. The extractant is recovered by distillation of the extract phase, and recovered organic matter is obtained at the bottom of the tower, with approximately 4–6 kg recovered per ton of wastewater, which is returned to the production process. The COD of the raffinate decreases from approximately 18,000–25,000 mg / L after acid precipitation to approximately 6,000–10,000 mg / L.

[0047] In the resin adsorption step, the raffinate is passed through an adsorption column filled with NKA-9 macroporous adsorption resin for treatment. The reason for choosing NKA-9 instead of H-103 in this embodiment is that the residual organic matter in the raffinate after extraction has relatively high polarity. NKA-9, as a polar adsorption resin, has a better adsorption effect on polar residues than the non-polar resin H-103. Static adsorption experiments show that the equilibrium adsorption capacity of NKA-9 for the extracted wastewater is about 15% higher than that of H-103. The adsorption column has an inner diameter of 500 mm, a column height of 2.5 m, a diameter-to-height ratio of 1:5, a resin packing height of 1.8 m, and a packing volume of approximately 72% of the effective volume. The adsorption flow rate is 1.5 BV / h, and the adsorption temperature is 25–30 °C. Effluent COD is monitored online. Adsorption continues when the effluent COD is below 1800 mg / L, and regeneration is switched when it reaches or exceeds 1800 mg / L. In this embodiment, a single adsorption cycle is approximately 8–10 hours, and the treated water volume is approximately 6–8 m³. The COD of the adsorbed water is approximately 1400–1700 mg / L. The saturated resin is regenerated using a 5% (v / v) sodium hydroxide solution as the desorbent. An alkaline desorbent is chosen because the polar organic compounds adsorbed by NKA-9 contain acidic groups such as phenolic hydroxyl groups; alkaline conditions ionize these groups, enhancing the desorption effect, achieving a desorption rate of 93%–95%. The desorption flow rate is 0.8 BV / h, and the desorption temperature is 30–35℃. The alkaline desorbent desorbs and partially hydrolyzes adsorbed nitrobenzene organic compounds. The collected eluent is neutralized and filtered to recover the organic matter.

[0048] In the sulfuric acid concentration and reuse step, the adsorption effluent is first softened, and sodium carbonate is added to adjust the pH to 8.0–8.5. After precipitating calcium and magnesium ions, it is filtered. The clarified liquid is sent to a four-effect co-current evaporation system for evaporation and concentration. The evaporation temperature of the first effect of the four-effect evaporator is approximately 90℃, the second effect approximately 80℃, the third effect approximately 70℃, and the fourth effect approximately 60℃. The system vacuum is maintained at -0.085 to -0.095 MPa. After the adsorption effluent is concentrated by the four-effect evaporation, the sulfuric acid mass fraction is concentrated from the initial approximately 14%–16% to 60%–65%. The concentrated sulfuric acid is cooled and reused in the nitration process. The COD of the evaporation condensate is approximately 300–500 mg / L and is discharged into the plant's biological treatment system.

[0049] In summary, this embodiment addresses the issue of high sulfuric acid concentration and relatively low organic matter concentration in the nitration mother liquor of pendimethalin. Lime slurry is used as an acid precipitation neutralizing agent, adjusting the pH while simultaneously causing some sulfuric acid to co-precipitate with the organic matter in the form of calcium sulfate, reducing the sulfuric acid load for subsequent evaporation and concentration. After further removal of organic matter through extraction and resin adsorption, the acid precipitation filtrate is evaporated and concentrated to obtain a higher concentration of sulfuric acid for reuse in production. The entire process boasts high resource recovery rate and low operating costs, making it suitable for treating wastewater from pendimethalin production with high sulfuric acid content.

[0050] Comparative Example

[0051] To verify the overall synergistic effect of the technical solution of this invention, the applicant set up the following comparative experiment using the wastewater quality of Example 1 as the object:

[0052] (1) Using only acid precipitation + extraction (without resin adsorption): the COD of the effluent is about 8000-10000 mg / L, which cannot meet the water quality requirements for entering the evaporation and concentration system (COD < 2000 mg / L). During the evaporation process, organic matter is severely coked, and the evaporator needs to be shut down for cleaning after only about 72 hours of operation.

[0053] (2) Using only acid precipitation + resin adsorption (without extraction): The resin adsorption load is large, the single adsorption cycle is shortened to about 2.5 hours, the regeneration frequency is too high, and the COD of the adsorbed water is about 3500~5000mg / L, which still does not meet the requirements of evaporation influent.

[0054] (3) Using only extraction + resin adsorption (without acid precipitation): the consumption of extractant increases by about 40% (due to severe emulsification caused by suspended solids and colloidal substances in the wastewater), the pressure drop of the resin adsorption column rises faster, and the operation stability is poor.

[0055] The above comparative experiments show that there is a synergistic effect among the three steps of acid precipitation-extraction-resin adsorption in this invention, and none of them can be omitted; it is not a simple superposition of the steps.

[0056] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for treating wastewater from the production of dimethoate, characterized in that, The method comprises the following components: Acid precipitation step: Adjust the pH of the dimethylpentylamine production wastewater to 2-3, stir and let it stand to allow some organic intermediates to precipitate out in the form of precipitate. Separate the solid and liquid to obtain acid precipitation precipitate and acid precipitation filtrate. Extraction steps: The acid precipitation filtrate was subjected to countercurrent extraction using a mixed solvent of cyclohexane and kerosene as the extractant. After standing and phase separation, the extract phase and raffinate were obtained. The extract phase was then distilled to recover the extractant and recover the organic matter. Resin adsorption steps: The raffinate is passed into an adsorption column packed with macroporous adsorption resin for adsorption treatment to obtain adsorbed water and saturated resin. The saturated resin is desorbed and regenerated with a desorbent, and the organic matter is recovered from the desorbed liquid. Sulfuric acid concentration and reuse steps: The adsorbed water is evaporated and concentrated to obtain concentrated sulfuric acid, which is then reused in the nitration process.

2. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the acid precipitation step, the wastewater from the production of dimethylpentylamine is a nitration mother liquor with a sulfuric acid mass fraction of 5% to 20%, a COD of 20,000 to 100,000 mg / L, a nitrobenzene intermediate concentration of 1,000 to 5,000 mg / L, and an initial pH of less than 2.

3. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the acid precipitation step, the alkaline substance used to adjust the pH is one or more of ammonia, sodium hydroxide solution, and lime milk. The mass fraction of ammonia is 20%–25%, the mass fraction of sodium hydroxide solution is 20%–30%, and the mass fraction of lime milk is 10%–20%. The final pH is adjusted to 2.0–3.

0. The acid precipitation temperature is 15–38°C, the standing time is 0.5–4 hours, and the solid-liquid separation is performed by filtration or centrifugation.

4. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the acid precipitation step, the acid precipitate is dried and then recovered as an organic intermediate. The drying temperature is 60-100℃ and the drying time is 2-6 hours. After drying, the water content of the acid precipitate is less than 5%.

5. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the extraction step, the volume ratio of cyclohexane to kerosene in the extractant is 1.5:1 to 3:1, the volume ratio of oil phase to water phase in the extraction phase is 1:2 to 1:4, the number of countercurrent extraction stages is 3 to 5, the extraction temperature is 20 to 40°C, the mixing time for each stage is 10 to 30 min, and the settling and phase separation time is 10 to 60 min.

6. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the extraction step, the distillation temperature for recovering the extractant by distillation of the extractant phase is 60-85℃. The extractant recovered at the top of the column is recycled, and the recovered organic matter is obtained at the bottom of the column. The recovered organic matter is returned to the front-end process of dimethylpentylene production.

7. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the resin adsorption step, the macroporous adsorption resin is one of XAD-4, H-103, NKA-9, and DA-201, the adsorption flow rate is 1–5 BV / h, the adsorption temperature is 20–40℃, and the desorbent is one of 80%–95% (v / v) ethanol aqueous solution, 80%–95% (v / v) methanol aqueous solution, 80%–95% (v / v) acetone aqueous solution, or 5%–10% (w / w) sodium hydroxide solution, with a desorption flow rate of 0.5–2 BV / h.

8. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the resin adsorption step, the diameter-to-height ratio of the adsorption column is 1:3 to 1:8, and the resin loading is 60% to 80% of the effective volume of the adsorption column. When the COD of the adsorbed water is less than 2000 mg / L, adsorption continues, and when the COD of the adsorbed water is greater than or equal to 2000 mg / L, the system switches to regeneration mode.

9. The method for treating dimethoate production wastewater according to claim 1, characterized in that, In the sulfuric acid concentration and reuse step, the adsorbed water is softened before evaporation and concentration. Sodium carbonate or sodium hydroxide is added to adjust the pH to 8-10, so that calcium and magnesium ions form precipitates. After filtration, the water is evaporated and concentrated. The evaporation and concentration adopts multi-effect evaporation or MVR evaporation. Multi-effect evaporation is three-effect or four-effect co-current evaporation. The water is evaporated and concentrated to a sulfuric acid mass fraction of 50%-70%. The concentrated sulfuric acid is reused in the nitrification process, and the evaporation condensate enters the plant's biochemical treatment system.

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