Treatment method of acrylonitrile wastewater

Abstract

According to the acrylonitrile wastewater treatment method provided by the invention, aiming at the problems that the content of tar in acrylonitrile wastewater is high, the evaporation is easy to block and mother liquor is difficult to utilize, chemical impurity removal is performed before the wastewater is evaporated, and granular active carbon with the pore diameter of 10-50nm is adopted as a selective impurity removal agent; acrylonitrile and other micromolecular organic matters can be excessively adsorbed by controlling the pore size range of activated carbon (the pore size is smaller than 10 nm), and the subsequent mother liquor recycling value is affected; the method has the advantages that tar macromolecular substances cannot be effectively removed if the pore size is larger than 50 nm), macromolecular tar substances formed in the evaporation process can be selectively removed, valuable small-molecular organic matters are reserved, the technical problems of tar blockage and low evaporation efficiency in acrylonitrile wastewater treatment are effectively solved, and mother liquor resource utilization is realized.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a method for treating acrylonitrile wastewater. Background Technology

[0002] Acrylonitrile is an important organic chemical raw material, widely used in the production of ABS resin, acrylic fiber, carbon fiber, and nitrile rubber. However, the wastewater generated during acrylonitrile production is characterized by complex pollutant composition, high chemical oxygen demand (COD), high color, and high tar content, making it difficult to treat, costly to invest in and operate, and presenting industry technological barriers. Traditional treatment technologies often employ single or combined pollutant removal methods. For example, incineration, while capable of thoroughly removing organic matter, suffers from high-salt clogging, severe corrosion, and high investment and operating costs. Oxidation processes, such as high-temperature wet oxidation, Fenton oxidation, and electrocatalytic oxidation, also have drawbacks including high investment, high operating costs, and significant safety hazards (such as high-temperature risks, large slag production, and hydrogen release), failing to meet the demands for efficient, safe, economical, and resource-efficient treatment. Therefore, there is an urgent need to develop a new, efficient, safe, reliable, and resource-efficient acrylonitrile wastewater treatment process to overcome the shortcomings of existing technologies and improve treatment efficiency and economics. Summary of the Invention

[0003] This invention addresses the problems of high tar content, high COD concentration, and high color in acrylonitrile wastewater. It proposes a comprehensive treatment method encompassing chemical impurity removal, evaporation and concentration, biochemical treatment, leaching, and reuse. The core of this method lies in the selection of granular activated carbon with a pore size of 10-50 nm. By precisely controlling the pore size range of the activated carbon, it avoids excessive adsorption of small-molecule organic matter due to excessively small pore sizes, which would affect the resource utilization of the mother liquor, and prevents ineffective retention of large tar molecules due to excessively large pore sizes. This achieves selective removal of large-molecule tar substances generated during evaporation while retaining valuable small-molecule organic matter, thereby effectively solving the problems of tar blockage and low evaporation efficiency, and realizing the resource utilization of the mother liquor. The technical solution provided by this invention is as follows: This invention provides a method for treating acrylonitrile wastewater, comprising the following steps: (1) Chemical impurity removal: Add an impurity removal agent to acrylonitrile wastewater, and after reaction, separate the solid and liquid to obtain filtrate; wherein, the impurity removal agent includes granular activated carbon with a pore size range of 10-50 nm; (2) Evaporation and concentration: The filtrate is evaporated and concentrated to obtain crude salt, distillate condensate and concentrated mother liquor respectively; (3) Biochemical treatment: The distillate condensate is subjected to biochemical treatment; (4) Leaching treatment: Add a reagent to the crude salt for leaching treatment to separate the salt from the organic matter attached to the surface of the salt and obtain leachate; (5) Reuse: The leachate is subjected to distillation to recover the reagent and distillation residue, and the recovered reagent is returned to step (4) for recycling.

[0004] In some specific embodiments, in step (1), the impurity remover further includes one or more of polyaluminum chloride, polyferric sulfate, and zinc sulfate, and the amount of the impurity remover added is 0.5%-1.5% of the acrylonitrile wastewater content.

[0005] In some specific embodiments, the impurity removal agent is prepared by compounding the granular activated carbon with any one of the flocculants selected from polyaluminum chloride, polyferric sulfate, and zinc sulfate at a mass ratio of (3–5):1.

[0006] In some specific implementations, in step (2), before the evaporation and concentration, the pH of the filtrate is adjusted to 3-5.

[0007] In some specific implementations, in step (2), the evaporation and concentration adopts multi-stage evaporation.

[0008] In some specific embodiments, the multi-stage evaporation is a triple-effect evaporation, and the control parameters are as follows: steam pressure greater than 0.5 MPa, vacuum pressure of 0.085-0.095 MPa, first-effect evaporation temperature of 85-95 ℃, second-effect evaporation temperature of 75-85 ℃, and third-effect evaporation temperature of 60-70 ℃.

[0009] In some specific embodiments, in step (3), the biochemical treatment adopts a combination of hydrolysis acidification, anoxic and aerobic processes, wherein the hydrolysis acidification residence time is 1-2 days, the pH is 6-9, the dissolved oxygen is 0-0.2 mg / L, and the temperature is 20-40 ℃; the anoxic residence time is 0.5-1.5 days, the dissolved oxygen is less than 0.5 mg / L, and the nitrification liquid reflux ratio is 1-3; the aerobic residence time is 1-2 days, the dissolved oxygen is 2-4 mg / L, the pH is 7-8, and the temperature is 20-35 ℃.

[0010] In some specific embodiments, in step (4), the agent used in the leaching treatment is methanol.

[0011] In some specific embodiments, in step (4), the leaching treatment is carried out at room temperature, the leaching time is 10-30 min, and the mass ratio of the leaching agent to the crude salt is (1-2):1.

[0012] In some specific implementations, in step (5), the top temperature of the distillation column in the distillation recovery process is 64-66℃, the bottom temperature is >70℃, and the reflux ratio is 1-3.

[0013] By adopting the above technical solution, the acrylonitrile wastewater treatment method provided by the present invention has the following beneficial effects: 1. This invention effectively removes tar-like substances and other impurities from wastewater through chemical purification treatment, thereby improving the evaporation efficiency of the subsequent evaporation and concentration process. It avoids problems such as pipeline blockage and reduced heat transfer efficiency caused by the accumulation of tar-like substances during evaporation, and ensures the stable operation of the evaporation system.

[0014] 2. The preferred impurity remover in this invention is granular activated carbon with a pore size of 10-50 nm. By precisely controlling the pore size range of the activated carbon, the problem of excessive adsorption of small molecule organic matter such as acrylonitrile by granular activated carbon with too small a pore size, which reduces the organic matter content in the evaporation mother liquor, is avoided. At the same time, the problem of incomplete removal of large tar-like organic matter is also prevented when the pore size is too large. This achieves selective removal of large tar-like substances while retaining small molecule organic matter with recycling value, thereby improving the resource utilization value of the evaporation mother liquor and subsequent products.

[0015] 3. This invention achieves component separation and resource recovery through multi-stage evaporation and concentration. After multi-stage evaporation treatment, acrylonitrile wastewater yields crude salt, distillate condensate, and concentrated mother liquor. The concentrated mother liquor has a high organic content and possesses certain product recovery value, which is beneficial for subsequent resource utilization and improves the overall economic benefits of the wastewater treatment process.

[0016] 4. The crude salt obtained by evaporation and concentration in this invention is leached by adding a reagent. The leachate can be recycled after the reagent is recovered by distillation. The distillation residue has a high organic content, similar to the evaporation mother liquor, and also has high product value. This not only reduces the generation of solid waste, but also achieves effective separation and resource recovery of organic components and salt substances in the waste liquid, reduces the emission of harmful substances in the sewage treatment process, and achieves the goals of wastewater treatment reduction, resource recovery and harmlessness, with good environmental and economic benefits. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a process flow diagram of an acrylonitrile wastewater treatment method provided in an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0020] The term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. In the description of the invention, it should be understood that the terms "upper," "lower," "top," "bottom," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Moreover, the terms "first," "second," etc., are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein.

[0021] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included. For example, a specified range from “1 to 10” should be considered to include any and all subranges between the minimum value 1 and the maximum value 10. Exemplary subranges of the range 1 to 10 include, but are not limited to, 1 to 6.1, 3.5 to 7.8, 5.5 to 10, etc.

[0022] Please see Figure 1 This invention provides a method for treating acrylonitrile wastewater, comprising the following steps: (1) Chemical removal: Add a removal agent to acrylonitrile wastewater, and after reaction, separate solid and liquid to obtain filtrate; wherein, the removal agent includes granular activated carbon with a pore size range of 10-50 nm; (2) Evaporation and concentration: The filtrate is evaporated and concentrated to obtain crude salt, distillate condensate and concentrated mother liquor respectively; (3) Biochemical treatment: The distillate condensate is subjected to biochemical treatment; (4) Leaching treatment: Add a reagent to the crude salt for leaching treatment to separate the salt from the organic matter attached to the surface of the salt and obtain leachate; (5) Reuse: The leachate is subjected to distillation to recover the reagent and distillation residue, and the recovered reagent is returned to step (4) for recycling.

[0023] Specifically, for high-concentration organic wastewater from acrylonitrile production workshops, which contains a high content of tar-like substances, most existing technologies do not perform targeted pretreatment of tar-like substances, causing them to easily adhere to the heat transfer surface of the evaporator in the subsequent evaporation and concentration process, resulting in equipment scaling, a significant decrease in heat transfer efficiency, and even blockage of pipes and heat exchangers.Therefore, before entering the evaporation and concentration process, this invention innovatively employs a chemical impurity removal process to pretreat the wastewater. An impurity removal agent is added to the acrylonitrile wastewater, and through the synergistic effect of chemical reaction and physical adsorption, tar-like substances, suspended solids, and other large molecular impurities in the wastewater are effectively removed, thereby improving the treatment efficiency and operational stability of the subsequent evaporation and concentration process. This impurity removal agent includes granular activated carbon with a pore size precisely controlled within the range of 10–50 nm (e.g., 15 nm, 25 nm, 35 nm, 45 nm, etc.). If the pore size is less than 10 nm, the activated carbon will preferentially adsorb small molecular organic matter such as acrylonitrile and propylene, resulting in a reduction in recoverable organic matter in the evaporation mother liquor and lowering its subsequent resource recovery value. If the pore size is large... A pore size of 50 nm is insufficient to effectively trap large tar molecules, failing to address the issues of tar accumulation and pipeline blockage during evaporation. Granular activated carbon with a pore size range of 10–50 nm is preferred, as it can efficiently remove large tar impurities while retaining economically valuable small-molecule organic matter in the distillate condensate, effectively preventing tar accumulation and blockage during evaporation and ensuring long-term stable operation of the evaporation system. The filtrate after impurity removal is then evaporated and concentrated. Under multi-stage evaporation conditions, water is evaporated, and soluble salts and high-concentration organic matter in the wastewater are gradually concentrated and precipitated, yielding three main products: first, crude salt mainly composed of inorganic salts with often attached organic matter on the surface; second, distillate condensate with low organic matter content; and third, distillate condensate with high organic matter content. The concentrated mother liquor, containing crude salt requiring further treatment, and the distillate condensate with low organic content, is suitable for subsequent biochemical treatment. The concentrated mother liquor, due to its high concentration of organic matter, possesses potential product recovery value. The distillate condensate then enters the biochemical treatment unit, where microbial metabolism degrades easily biodegradable organic matter (such as alcohols, aldehydes, and some organic acids), effectively reducing COD, BOD, and other pollution indicators, ensuring the effluent meets discharge or reuse standards. For the organic matter adhering to the surface of the crude salt, a chemical leaching process is used to effectively separate the salt from the organic matter, yielding a leachate rich in high-concentration organic components. This leachate is then subjected to distillation, utilizing the boiling point differences of different components to separate the chemical reagents from the organic residue, thus recovering and recycling the organic matter. This method uses reagents to reduce treatment costs while obtaining distillation residues with high organic content, comparable to the concentrated mother liquor, thus possessing product or energy recovery value and achieving high-value utilization of organic matter. Through an integrated process route of "chemical impurity removal—evaporation concentration—biochemical treatment—leaching treatment—reuse," this method effectively solves problems such as tar blockage, low evaporation efficiency, difficult mother liquor treatment, and resource waste inherent in traditional processes. It achieves efficient removal of tar-like substances from wastewater, separation and recovery of valuable substances, and recycling. It boasts advantages such as high treatment efficiency, stable operation, high resource utilization, and environmental friendliness, making it suitable for the comprehensive treatment and green, low-carbon recycling of high-concentration organic wastewater from acrylonitrile and related chemical industries.

[0024] In some specific embodiments, in step (1), the impurity remover further includes one or more of polyaluminum chloride, polyferric sulfate, and zinc sulfate, and the amount of impurity remover added is 0.5%-1.5% of the acrylonitrile wastewater content. Specifically, the impurity remover is composed of granular activated carbon and one or more of polyaluminum chloride (PAC), polyferric sulfate (PFS), and zinc sulfate. The pore size of the granular activated carbon is controlled at 10-50 nm, which is used to efficiently remove tar-like macromolecular substances and suspended impurities in the wastewater through the synergistic effect of chemical adsorption and physical interception. Auxiliary flocculants or adsorbents such as polyaluminum chloride, polyferric sulfate, and zinc sulfate can further enhance the coagulation, adsorption, and sedimentation effect on fine suspended solids, colloids, and some organic matter, thereby improving the overall impurity removal efficiency. The dosage of the impurity remover is controlled at 0.5%-1.5% of the total wastewater volume (e.g., 0.5%, 0.7%, 1.0%, 1.3%, 1.5%, etc.). If the dosage is less than 0.5%, the removal of tar and impurities in the wastewater will be insufficient, leading to tar accumulation, pipeline blockage, and decreased heat transfer efficiency during evaporation and concentration, which will seriously affect the stable operation and treatment effect of subsequent evaporation units. If the dosage is higher than 1.5%, although it can improve the initial impurity removal rate, it will cause a large amount of non-target substances to be adsorbed or flocculated and precipitated, generating excessive waste residue and increasing the burden of solid waste treatment. At the same time, excessive impurity remover will non-selectively adsorb acrylonitrile, propylene and its polymers and other small molecule organic compounds with recycling value, reducing the organic content in the distillate condensate and concentrated mother liquor, thereby affecting the effect of subsequent biochemical treatment and the productization and resource utilization value of the mother liquor. Therefore, choosing a dosage ratio of 0.5%–1.5% can ensure the effective removal of tar and impurities, ensure the efficient and stable operation of the evaporation system, and maximize the retention of economically valuable organic components in the wastewater, achieving synergistic optimization of treatment efficiency, system stability and resource utilization benefits.

[0025] In some specific embodiments, the impurity removal agent is composed of granular activated carbon and any one of the flocculants selected from polyaluminum chloride, polyferric sulfate, and zinc sulfate at a mass ratio of (3–5):1. The selected flocculant can promote the formation of flocculent aggregates of large molecular tar substances originally existing in a dispersed state in the wastewater through charge neutralization, bridging, or entrapment, thereby increasing their particle size, improving settling properties, or enhancing adsorption properties. This facilitates efficient removal by granular activated carbon through physical retention and surface adsorption, effectively preventing the accumulation of tar substances during evaporation and concentration, which could lead to pipeline blockage and reduced heat transfer efficiency. The impurity removal agent uses granular activated carbon and the aforementioned flocculant at a mass ratio of (3–5):1 (e.g., 3:1, 3.5:1, 4:1, 4.5:1, 5:1). When compounding granular activated carbon (PEC) with flocculant, if the proportion of granular activated carbon is too small (i.e., insufficient granular activated carbon), the adsorption of tar agglomerates formed after flocculation will be incomplete, resulting in a high residual amount of tar in the wastewater, affecting the stable operation and treatment effect of subsequent evaporation units. If the proportion of granular activated carbon is too large (i.e., excessive granular activated carbon), it will non-selectively adsorb small molecule organic matter with recovery value, such as acrylonitrile, propylene and its polymers, in the wastewater, reducing the content of organic components in the distillate condensate and concentrated mother liquor, thereby affecting the effect of subsequent biochemical treatment and the productization and resource utilization value of the mother liquor. Therefore, selecting a mass ratio of (3–5):1 of granular activated carbon to flocculant can fully utilize the synergistic effect of the flocculant on the coagulation of large molecule tar and the adsorption and removal capacity of granular activated carbon, achieving efficient removal of tar while minimizing the non-selective adsorption of small molecule organic matter, ensuring the smooth progress of subsequent evaporation concentration, biochemical treatment and resource utilization of mother liquor.

[0026] In some specific implementations, in step (2), before evaporation and concentration, the pH of the filtrate is adjusted to 3-5. By precisely controlling the pH of the filtrate within the range of 3-5, the pH of the distillate condensate generated during the subsequent evaporation and concentration process can be kept neutral, allowing it to directly enter the biochemical treatment in step (3) without additional adjustment, thus simplifying the process and reducing costs; if the pH is adjusted too low (less than 3), a large amount of H+ will be released during the evaporation process. +Ammonia nitrogen migrates into the distillate with the water vapor, making the distillate acidic. This necessitates an alkali neutralization process before entering the biological treatment, increasing not only the consumption of acid and alkali reagents and operational steps but also raising the overall treatment cost. If the pH is adjusted too high (greater than 5), the ammonia nitrogen in the wastewater, originally in ionic or bound state, will convert to free ammonia and transfer to the gas phase as the pH increases, accumulating in the evaporated distillate. This leads to an increase in the ammonia nitrogen concentration in the distillate, requiring pH adjustment before biological treatment as well. Furthermore, the high ammonia nitrogen content inhibits microbial activity, reducing the efficiency of biological treatment and further increasing operational risks and treatment difficulty. Therefore, controlling the pH of the filtrate within the suitable range of 3–5 can ensure efficient operation of the evaporation and concentration process while reducing the impact of subsequent acid-base adjustments and ammonia nitrogen load.

[0027] In some specific implementations, in step (2), the evaporation concentration adopts multi-stage evaporation. Multi-stage evaporation improves the evaporation intensity and separation efficiency, achieving efficient separation and crystallization of soluble salts in wastewater, while gradually enriching and increasing the concentration of high-value-added organic matter such as acrylonitrile, propylene and its polymers in the mother liquor; multi-stage evaporation controls the evaporation temperature, vacuum degree and material residence time at each stage, so that water evaporates gradually in the multi-stage system, and salts preferentially reach a supersaturated state due to solubility limitations and precipitate out in the form of crude salt, thereby maximizing the separation of salt and organic phase; at the same time, as water continues to evaporate, the originally low concentration of acrylonitrile, propylene and its polymers and other organic matter in the wastewater is concentrated in the mother liquor, forming a concentrated mother liquor with high organic matter content, providing a material basis for subsequent resource utilization (such as extraction of organic components or energy recovery).

[0028] In some specific implementations, multi-stage evaporation is triple-effect evaporation, and the control parameters are as follows: steam pressure greater than 0.5 MPa, vacuum pressure of 0.085-0.095 MPa, first-effect evaporation temperature of 85-95 ℃, second-effect evaporation temperature of 75-85 ℃, and third-effect evaporation temperature of 60-70 ℃. Specifically, the steam pressure needs to be greater than 0.5 MPa. If the pressure is too low, the heating capacity will be insufficient, resulting in a reduction in the feed rate per unit time, affecting the overall processing efficiency and system capacity. The condenser vacuum pressure should be controlled within the range of 0.085–0.095 MPa. If the vacuum is too low (less than 0.085 MPa), the evaporation temperature in the system will rise, the liquid level will rise abnormally, and there is a risk of evaporator overflow, affecting operational safety. The evaporation temperatures of the first, second, and third effects should be controlled at 85–95℃, 75–85℃, and 60–70℃, respectively. This gradient temperature setting meets the process requirements of multi-stage evaporation and gradual cooling, ensuring efficient evaporation of water while avoiding excessively high temperatures that could cause organic matter decomposition, rapid crystallization of supersaturated salts, or scaling and blockage on the inner wall of the evaporator, thus ensuring long-term stable operation of the system. If the evaporation temperature at each stage is too low, the heat transfer driving force will be insufficient, the evaporation rate will decrease, and the processing efficiency will be reduced. Through precise control of the above parameters, the acrylonitrile wastewater will ultimately be concentrated to 1 / 10 of its original volume. The concentration of acrylonitrile, propylene and its polymers in the mother liquor is increased to ensure that the concentrated mother liquor has a high product recovery value or resource utilization potential, thereby achieving efficient water separation, effective salt precipitation and efficient enrichment of target organic matter.

[0029] In some specific implementations, in step (3), the biochemical treatment adopts a combination of hydrolysis acidification, anoxic and aerobic processes, wherein the hydrolysis acidification residence time is 1-2 days, the pH is 6-9, the dissolved oxygen is 0-0.2 mg / L, and the temperature is 20-40 ℃; the anoxic residence time is 0.5-1.5 days, the dissolved oxygen is less than 0.5 mg / L, and the nitrification liquid reflux ratio is 1-3; the aerobic residence time is 1-2 days, the dissolved oxygen is 2-4 mg / L, the pH is 7-8, and the temperature is 20-35 ℃.

[0030] Specifically, in step (3), the biochemical treatment adopts a combination of hydrolysis acidification, anoxic and aerobic processes to treat the distillate condensate produced after multi-stage evaporation and concentration. This distillate condensate has an imbalance between the ratio of organic matter and ammonia nitrogen after evaporation of the original wastewater, exhibiting a COD (Chemical Oxygen Demand) to ammonia nitrogen ratio greater than 10:1 (i.e., carbon-nitrogen ratio > 10:1). It needs to achieve chain-breaking degradation of macromolecular organic matter, denitrification of ammonia nitrogen, and deep removal of COD through this combination process, so that the effluent meets the receiving standards of the park's wastewater treatment plant. Among them, the retention time of the hydrolysis acidification stage is controlled at 1–2 days, the pH range is 6–9, and the dissolved oxygen is maintained at 0–0.2 mg / L (preferably 0 mg / L). The anaerobic stage, with a concentration of nitrifying liquid (mg / L) and a temperature of 20–40℃, utilizes anaerobic microorganisms to break down and hydrolyze large, recalcitrant organic molecules in the condensate into smaller, easily biodegradable organic molecules. This provides a high-quality substrate for subsequent aerobic microorganisms, improving overall organic matter removal efficiency. A pH of 6–9 maintains a suitable environment for the enzyme activity of hydrolytic bacteria, while dissolved oxygen is maintained at 0–0.2 mg / L to ensure strict anaerobic conditions. A temperature of 20–40℃ is the optimal growth range for most hydrolytic microorganisms. The anoxic stage has a residence time of 0.5–1.5 days (typically 1 day), with dissolved oxygen strictly controlled below 0.5 mg / L and a nitrification liquor reflux ratio of 1–3. Its main function is to utilize organic matter as a carbon source through denitrifying bacteria to denitrify nitrate nitrogen (NO3) from the aerobic stage or remaining parts of the system. - / NO2 - The nitrogen gas is reduced to nitrogen (N2) to remove the nitrogen, thereby reducing the total nitrogen content. Dissolved oxygen is less than 0.5 mg / L to ensure an anaerobic environment. A suitable reflux ratio can provide sufficient nitrate nitrogen and optimize carbon source utilization efficiency. The aerobic stage has a residence time of 1–2 days, with dissolved oxygen controlled at 2–4 mg / L, pH at 7–8, and temperature at 20–35℃. In this stage, through the metabolic activities of aerobic microorganisms, the remaining small molecule organic matter is completely oxidized and decomposed to reduce COD, and nitrification is carried out simultaneously to convert ammonia nitrogen into nitrate nitrogen, creating conditions for subsequent anoxic denitrification. Dissolved oxygen of 2–4 mg / L is the optimal metabolic condition for aerobic bacteria. 7–8 is the optimal range for nitrification and COD degradation bacteria, and a temperature of 20–35℃ ensures microbial activity and treatment efficiency. Through the synergistic effect of hydrolysis acidification, anoxic and aerobic stages, this combined process achieves the stepwise transformation of macromolecular organic matter → small molecule organic matter → COD degradation, ammonia nitrogen → nitrate nitrogen → nitrogen gas, and finally effectively removes the main pollutants in the condensate, ensuring that the effluent meets the acceptance standards of the park's wastewater treatment plant.

[0031] In some specific embodiments, in step (4), the leaching agent used is methanol. This choice is based on the good solubility of the small amount of organic matter (mainly acrylonitrile, propylene, and their polymers, which are similar to the mother liquor components) adhering to the surface of the crude salt obtained after evaporation and concentration. Methanol, as a polar organic solvent, can form a homogeneous miscible system with these organic substances, effectively washing away the organic matter adhering to the surface of the salt particles. At the same time, methanol has almost no dissolving effect on salt substances (such as inorganic salt crystals), preventing salt loss or the introduction of impurities, thus ensuring efficient separation of salt and organic matter. In step (5), the used leachate is distilled... The process utilizes the significant boiling point difference between methanol and organic matter (methanol has a boiling point of about 64.7℃, while organic matter such as acrylonitrile generally has a higher boiling point) to achieve effective separation of the two: the low-boiling-point methanol is distilled off and condensed for recovery, and after purification, it can be recycled back to step (4) as a leaching agent, which greatly reduces the consumption of agents and processing costs; while the high-boiling-point distillation kettle residue (i.e. the organic matter concentrated phase) is rich in high-value components such as acrylonitrile, propylene and its polymers washed off from the surface of crude salt. Its organic matter content is comparable to that of the concentrated mother liquor, and it has similar potential for productization or energy utilization as the mother liquor, thereby realizing the efficient recovery and resource utilization of solvent and valuable organic components in the leachate.

[0032] In some specific embodiments, in step (4), the leaching treatment is carried out at room temperature for 10-30 minutes, and the mass ratio of the leaching agent to the crude salt is (1-2):1. Specifically, when leaching is carried out at room temperature, the preferred mass ratio of the leaching agent to the crude salt is (1-2):1. When the mass ratio is less than 1:1, the amount of leaching agent is insufficient, making it difficult to effectively remove organic matter from the surface of the crude salt, thus affecting the leaching effect. When the mass ratio is greater than 2:1, although it helps to improve the impurity removal efficiency, it will lead to the excessive use of leaching agent, increasing the load and cost of subsequent recovery and treatment processes, resulting in poor economic efficiency. Therefore, controlling the mass ratio of the leaching agent to the crude salt within the range of (1-2):1 can ensure the leaching effect while taking into account the economy and rationality of the process.

[0033] In some specific implementations, in step (5), the top temperature of the distillation column in the distillation recovery process is 64-66℃, the bottom temperature is >70℃, and the reflux ratio is 1-3. The reflux ratio is a key parameter affecting methanol recovery efficiency and operating costs. When the reflux ratio is less than 1, insufficient gas-liquid mass transfer and reflux in the distillation column lead to incomplete methanol recovery and reduced recovery rate. When the reflux ratio is greater than 3, although it helps improve the separation effect, it increases the heat load of the reboiler and the load of the condenser, resulting in increased energy consumption and processing costs. Therefore, controlling the reflux ratio within the range of 1-3 ensures efficient methanol recovery while also considering the stability and economy of the process.

[0034] The following detailed description of examples of the present invention is exemplary and only used to explain the present invention, and should not be construed as limiting the present invention. The wastewater was taken from the acrylonitrile production workshop of a chemical plant in Jilin Province, and the water quality was as follows: COD = 86765 mg / L, pH = 5, ammonia nitrogen = 58048 mg / L.

[0035] Example 1 A method for treating acrylonitrile wastewater, the specific treatment steps are as follows: (1) Chemical removal: Take 500 mL of wastewater sample from the acrylonitrile production workshop and add 5 g of removal agent. The removal agent is composed of granular activated carbon and polyaluminum chloride (PAC) in a mass ratio of 4:1. The average pore size of the granular activated carbon is 35 nm. After mixing the wastewater and the removal agent, stir for 30 min to allow tar-like substances and macromolecular organic matter to be fully adsorbed and removed. Then, filter and separate to obtain the filtrate. (2) Evaporation and concentration: The pH value of the filtrate obtained in step (1) is adjusted to 4, and then put into the evaporation system for multi-stage evaporation and concentration. The triple-effect evaporation process is adopted, and the specific control parameters are: steam pressure is 0.8 MPa, vacuum pressure is 0.095 MPa, first-effect evaporation temperature is 90 ℃, second-effect evaporation temperature is 80 ℃, and third-effect evaporation temperature is 70 ℃; crude salt, distillate condensate and concentrated mother liquor are obtained. (3) Biochemical treatment: The condensate of the distillate obtained in step (2) is subjected to biochemical treatment using a combination of hydrolysis acidification + anoxic + aerobic process. The specific parameters are as follows: Hydrolysis and acidification stage: residence time 2 days, pH = 8, dissolved oxygen 0 mg / L, temperature 28 ℃; Anoxic phase: residence time 1 day, dissolved oxygen 0.3 mg / L, nitrification liquor reflux ratio 2; Aerobic phase: residence time 2 days, dissolved oxygen 3 mg / L, pH = 8, temperature 30 ℃; (4) Leaching treatment: Place the crude salt obtained in step (2) into a small beaker, add methanol, soak at room temperature for 30 min, and then filter to separate, and obtain leachate and pure salt solid; (5) Distillation recovery: The leachate obtained in step (4) is subjected to distillation recovery treatment, wherein the top temperature of the distillation column is 65°C, the bottom temperature is 75°C, and the reflux ratio is 2.5, and the reagent and distillation residue are recovered respectively, wherein the recovered reagent is returned to step (4) for recycling.

[0036] Example 2 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that polyaluminum chloride is not added to the impurity removal agent (i.e., the impurity removal agent only contains granular activated carbon).

[0037] Example 3 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that the amount of impurity remover added is 0.5g (i.e., the amount added is 0.1% of the acrylonitrile wastewater content).

[0038] Example 4 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that granular activated carbon and polyaluminum chloride are mixed in a mass ratio of 1:1.

[0039] Example 5 The treatment method for acrylonitrile wastewater in Example 1 is different in that after obtaining the filtrate in step (1), it is directly sent to a triple-effect evaporator without adjusting the pH with acid.

[0040] Example 6 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that a single-effect evaporator is used for evaporation and concentration at atmospheric pressure and an evaporation temperature of 105 °C, without multiple stages.

[0041] Example 7 Referring to the treatment method of acrylonitrile wastewater in Example 1, the difference is that in step (2), the evaporation and concentration adopts triple-effect evaporation, and the parameters are changed to: the first-effect evaporation temperature is 110 ℃, the second-effect evaporation temperature is 95 ℃, the third-effect evaporation temperature is 80 ℃, the steam pressure is 0.4 MPa, and the vacuum degree is 0.080 MPa.

[0042] Example 8 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that the pH of the hydrolysis acidification in the biochemical treatment is 5.

[0043] Example 9 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that the dissolved oxygen in the aerobic stage of the biochemical treatment is controlled at 6 mg / L.

[0044] Example 10 Referring to the acrylonitrile wastewater treatment method in Example 1, the difference is that the leaching agent used is ethanol instead of methanol; Comparative Example 1 The treatment method for acrylonitrile wastewater in Example 1 differs in that the acrylonitrile wastewater is directly pumped into a triple-effect evaporator for evaporation and concentration without any chemical removal.

[0045] Comparative Example 2 Referring to the treatment method of acrylonitrile wastewater in Example 1, the difference is that the impurity removal agent in step (1) is granular activated carbon, but with an average pore size of 80 nm.

[0046] Comparative Example 3 Referring to the treatment method of acrylonitrile wastewater in Example 1, the difference is that the impurity removal agent in step (1) is granular activated carbon, but with an average pore size of 5 nm.

[0047] Comparative Example 4 The treatment method for acrylonitrile wastewater in Example 1 is similar, except that granular activated carbon was not added to the impurity removal agent; only polyaluminum chloride flocculant was used.

[0048] Test case Typical products generated in each step of the acrylonitrile wastewater treatment methods in Examples 1-10 and Comparative Examples 1-4, including the filtrate obtained from the chemical impurity removal step, the distillate condensate from the evaporation and concentration step, the concentrated mother liquor and crude salt, the final effluent from the biochemical treatment step, the leachate obtained from the leaching treatment step, and the reagents and distillation residues recovered from the rectification and recovery step, were tested and analyzed. The effectiveness of chemical impurity removal was evaluated by measuring the COD (dichromate method) and ammonia nitrogen (Nessler's reagent colorimetric method or salicylic acid spectrophotometric method) of the original acrylonitrile wastewater and filtrate. The adsorption and removal efficiency of the activated carbon-flocculator compound impurity removal agent on tar and macromolecular organic matter was assessed, and the clarity of solid-liquid separation after filtration was also considered. Comprehensive assessment (characterization results are shown in Table 1); Characteristics of evaporation and concentration products were analyzed by detecting COD (dichromate method), pH (glass electrode method), and ammonia nitrogen (Nessler's reagent method or salicylic acid method) of the distillate condensate to assess the concentration and separation effect of the evaporation process on organic matter and ammonia nitrogen. Simultaneously, the appearance of the crude salt and the composition of the concentrated mother liquor were observed (characterization results are shown in Table 2); Biochemical treatment performance was verified by measuring the COD (dichromate method), ammonia nitrogen (Nessler's reagent method), and pH (glass electrode method) of the final effluent after biochemical treatment of the distillate condensate to verify its removal efficiency for recalcitrant organic pollutants and ammonia nitrogen (characterization results are shown in Table 3); Leaching treatment effect was assessed by examining the effect of crude salt soaked in chemicals for 30 minutes. The organic matter content (such as COD or specific organic matter concentration) and the purity of the salt solid (visual observation or COD measurement after dissolution) in the leachate obtained after filtration after min were used to evaluate the desorption capacity of the reagent for organic matter attached to the salt surface (relevant data are integrated in Table 4); the distillation recovery efficiency was verified by measuring the purity of the reagent after distillation recovery of the leachate (such as reagent content analyzed by gas chromatography GC) and the properties of the distillation residue (including COD and composition) to verify the recycling efficiency of the reagent and the feasibility of residue disposal, thereby ensuring the closed-loop operation and resource utilization level of the process (characterization results are also shown in Table 4).

[0049] Table 1. Effect of chemical impurity removal

[0050] Table 2 Characteristics of Evaporation and Concentration Products

[0051] Table 3. Biochemical treatment performance (final effluent)

[0052] Table 4 Distillation Recovery Efficiency

[0053] Based on the test data from Examples 1-10 and Comparative Examples 1-4, it can be seen that in the entire process of treating saline wastewater, the parameters of each process step in Example 1 are optimally coordinated: the chemical impurity removal using a mixture of granular activated carbon and polyaluminum chloride (PAC) at a mass ratio of 4:1 achieves the best removal effect for tar and macromolecular organic matter, with a COD removal rate of 17.2% and significantly improved filtrate clarity (SS < 150), which is far superior to Example 2 without PAC, Comparative Examples 2-3 with unsuitable activated carbon pore size, and Comparative Example 4 using only PAC. Furthermore, it avoids the problems of evaporation system blockage, high COD (6320 mg / L) in the distillate condensate, and tar entrainment in coarse salt, which are caused by direct evaporation without chemical impurity removal in Comparative Example 1. The resulting distillate condensate has a lower COD (4000 mg / L) and moderate ammonia nitrogen (375 mg / L). The crude salt had good appearance (mg / L); in the evaporation and concentration stage, single-effect atmospheric pressure evaporation (Example 6) and high-temperature triple-effect evaporation (Example 7) did not show significant advantages and may increase energy consumption; the biological treatment adopted a combination process of "hydrolysis acidification + anoxic + aerobic", and the final effluent COD was as low as 289 mg / L, with a COD removal rate of 99.7%, and ammonia nitrogen was reduced to 14 mg / L, with a removal rate of 99.9%, which was significantly better than the treatment effect under conditions such as abnormal pH (Example 8) or high dissolved oxygen (Example 9); in the leaching treatment, methanol was used as the leaching agent and soaked at room temperature for 30 minutes. The process effectively removes organic matter from the surface of crude salt, yielding salt solids with a purity of 99.0%, which is superior to using ethanol (Example 10) or the indirect effects of not optimizing impurity removal (Comparative Example 1). In the distillation recovery stage, under optimized conditions of a distillation column top temperature of 65°C, a column bottom temperature of 75°C, and a reflux ratio of 2.5, efficient recovery of the leaching agent is achieved (purity 98.0%, recovery rate 98.0%). Therefore, Example 1 demonstrates the best pollutant removal efficiency, system operational stability, and resource utilization performance, showing significant technical advancement and economic rationality compared to the comparative examples and examples with parameter deviations.

[0054] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for treating acrylonitrile wastewater, characterized in that, Includes the following steps: (1) Chemical impurity removal: Add an impurity removal agent to acrylonitrile wastewater, and after reaction, separate the solid and liquid to obtain filtrate; wherein, the impurity removal agent includes granular activated carbon with a pore size range of 10-50 nm; (2) Evaporation and concentration: The filtrate is evaporated and concentrated to obtain crude salt, distillate condensate and concentrated mother liquor respectively; (3) Biochemical treatment: The distillate condensate is subjected to biochemical treatment; (4) Leaching treatment: Add a reagent to the crude salt for leaching treatment to separate the salt from the organic matter attached to the surface of the salt and obtain leachate; (5) Reuse: The leachate is subjected to distillation to recover the reagent and distillation residue, and the recovered reagent is returned to step (4) for recycling.

2. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (1), the impurity remover further includes one or more of polyaluminum chloride, polyferric sulfate, and zinc sulfate, and the amount of the impurity remover added is 0.5%-1.5% of the acrylonitrile wastewater content.

3. The method for treating acrylonitrile wastewater according to claim 2, characterized in that, The impurity removal agent is prepared by compounding the granular activated carbon with any one of the flocculants selected from polyaluminum chloride, polyferric sulfate, and zinc sulfate at a mass ratio of (3–5):

1.

4. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (2), before evaporation and concentration, the pH of the filtrate is adjusted to 3-5.

5. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (2), the evaporation and concentration adopts multi-stage evaporation.

6. The method for treating acrylonitrile wastewater according to claim 5, characterized in that, The multi-stage evaporation is a triple-effect evaporation, and the control parameters are as follows: steam pressure greater than 0.5 MPa, vacuum pressure of 0.085-0.095 MPa, first-effect evaporation temperature of 85-95 ℃, second-effect evaporation temperature of 75-85 ℃, and third-effect evaporation temperature of 60-70 ℃.

7. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (3), the biochemical treatment adopts a combination of hydrolysis acidification, anoxic and aerobic processes, wherein the hydrolysis acidification residence time is 1-2 days, the pH is 6-9, the dissolved oxygen is 0-0.2 mg / L, and the temperature is 20-40 ℃; the anoxic residence time is 0.5-1.5 days, the dissolved oxygen is less than 0.5 mg / L, and the nitrification liquor reflux ratio is 1-3; the aerobic residence time is 1-2 days, the dissolved oxygen is 2-4 mg / L, the pH is 7-8, and the temperature is 20-35 ℃.

8. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (4), the agent used in the leaching treatment is methanol.

9. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (4), the leaching treatment is carried out at room temperature for 10-30 minutes, and the mass ratio of the leaching agent to the crude salt is (1-2):

1.

10. The method for treating acrylonitrile wastewater according to claim 1, characterized in that, In step (5), the top temperature of the distillation column in the distillation recovery process is 64-66℃, the bottom temperature is >70℃, and the reflux ratio is 1-3.

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