Method and system for directionally removing non-volatile acid radicals and recycling urea in high-concentration urea wastewater

Abstract

The invention discloses a method and a system for directionally removing non-volatile acid radicals and recovering urea in high-concentration urea wastewater, and belongs to the technical field of wastewater treatment and resource recovery. The method comprises the following steps: firstly, introducing wastewater into a pretreatment unit, adding a synergistic treatment agent compounded by alkali metal salt, an organic complexing agent and nano catalytic particles, and directionally precipitating non-volatile acid radicals to form a compact precipitate; intensified solid-liquid separation is completed through an ultrasonic-assisted horizontal spiral discharging sedimentation centrifuge, residual acid radicals in supernate are deeply adsorbed through graphene quantum dot-temperature responsive magnetic nanoparticle composite modified anion exchange resin, and the acid radicals are almost completely removed; then, urea is concentrated by adopting a low-temperature evaporation-waste heat recovery coupling process, and high-purity urea crystals are separated out through segmented temperature-pressure gradient crystallization; the resin saturated by adsorption is separated and recovered through a magnetic field, and is recycled after temperature regulation and mild regeneration; the method is suitable for resourceful treatment of industrial high-concentration urea wastewater.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment and resource recovery technology, specifically relating to a method and system for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater. Background Technology

[0002] High-concentration urea wastewater is a common type of industrial wastewater. Key issues to be addressed in its treatment include the targeted removal of non-volatile acid radicals and the efficient recovery of urea. While there are numerous existing technologies and equipment for wastewater treatment, few solutions exist for separating non-volatile acid radicals from urea in high-concentration urea wastewater, and these solutions still have certain shortcomings in practical applications.

[0003] Patent application CN102126801A discloses a process and equipment for treating wastewater containing heavy metal ions such as copper, nickel, and zinc. This method uses an H-type cation exchange resin to adsorb heavy metal cations in the wastewater and displace hydrogen ions to form an acidic solution. Subsequently, an acid-removing anion exchange resin is used to enrich the acid solution in the wastewater. The advantage of this process is that it can reduce the mineralization of the wastewater, thereby obtaining relatively pure water, suitable for electroplating production or direct discharge. However, this technology mainly targets the removal of heavy metal ions and does not address the targeted removal of non-volatile acid radicals in high-concentration urea wastewater. Furthermore, it lacks a specific design for urea recovery, thus potentially limiting its application in treating urea wastewater with complex compositions.

[0004] Patent application CN116375227A discloses a wastewater treatment system, including a water distribution unit, a heat exchange unit, and a scale inhibition unit. This system effectively controls scale-forming ions by introducing scale-inhibiting seed crystals or a magnesium ion solution into the wastewater. This technology solves the problem of the narrow application range of the single-seed scale inhibition method and has a good scale inhibition effect. However, the system focuses on the treatment of scale-forming ions in the wastewater and does not address the targeted removal of non-volatile acid radicals or the recovery of urea, making it difficult to meet the actual needs of treating high-concentration urea wastewater.

[0005] In summary, while both of the aforementioned technical solutions have certain advantages in their respective fields, they both suffer from insufficient specificity when treating high-concentration urea wastewater. In particular, existing technologies have not yet provided comprehensive solutions for the targeted removal of non-volatile acid radicals and the efficient recovery of urea. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for the targeted removal of non-volatile acid radicals and the recovery of urea from high-concentration urea wastewater, thereby solving the technical problems of incomplete removal of non-volatile acid radicals and high energy consumption in urea recovery in the prior art.

[0007] The second objective of this invention is to provide a system for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater, which is used to achieve the above-mentioned method for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater.

[0008] The objective of this invention can be achieved through the following technical solutions: Firstly, a method for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater includes the following steps: S10: High-concentration urea wastewater is introduced into the pretreatment unit, and a composite synergistic treatment agent is added to form a dense precipitate containing non-volatile acid radicals under catalytic conditions; the composite synergistic treatment agent is composed of alkali metal salts, organic complexing agents and nano-catalytic particles; S20: The mixture obtained in step S10 is passed through an enhanced solid-liquid separation device to separate the precipitate and collect the supernatant; S30: The supernatant obtained in step S20 is introduced into the intelligent adsorption unit, and the graphene quantum dot-temperature responsive magnetic nanoparticle composite modified anion exchange resin is used to selectively adsorb residual non-volatile acid radicals. S40: The liquid processed in step S30 is introduced into the heat pump co-concentration unit to concentrate urea through low-temperature evaporation and waste heat recovery coupling technology. S50: The concentrate obtained in step S40 is introduced into a gradient crystallization unit, and high-purity urea crystals are precipitated under segmented temperature-pressure control, and solid urea is obtained by centrifugation. S60: The composite modified anion exchange resin after adsorption saturation is rapidly separated and recovered by an external magnetic field, and then recycled after temperature-controlled regeneration.

[0009] Furthermore, in step S10, the mass fraction of each component of the composite synergistic treatment agent is as follows: 0.5%-1% alkali metal salt, 0.1%-0.3% organic complexing agent, and 0.05%-0.1% nanocatalytic particles.

[0010] Furthermore, in step S10, the alkali metal salt is sodium carbonate or potassium carbonate, the organic complexing agent is disodium ethylenediaminetetraacetate or sodium citrate, and the nanocatalytic particles are hydroxylated nano titanium dioxide with a particle size of 5-20 nm.

[0011] Furthermore, in step S10, the composite synergistic treatment agent is added in batches using a gradient addition method, in 3-4 batches, with each addition amount being 25%-33% of the total amount, and the interval between each addition being 8-15 minutes.

[0012] Furthermore, in step S10, the reaction temperature of the pretreatment unit is controlled at 30-40℃, the stirring speed is 150-250 r / min, and the stirring time is 40-60 minutes.

[0013] Furthermore, the preparation method of the nanocatalytic particles is as follows: dissolve tetrabutyl titanate in anhydrous ethanol, add 5%-8% by mass of hydroxylating agent (ethylene glycol monomethyl ether), stir and react at 50-60℃ for 2-3 hours, and obtain hydroxylated nano titanium dioxide after centrifugation, washing, and vacuum drying (-0.09MPa, 60℃, 4 hours).

[0014] Furthermore, in step S20, the enhanced solid-liquid separation device is an "ultrasonic-assisted horizontal screw discharge sedimentation centrifuge," with the centrifuge speed set at 3000-4000 r / min and the feed flow rate controlled at 2-3 m³ / min. 3 / h, while simultaneously applying an ultrasonic field with a frequency of 20-40kHz and an ultrasonic power of 500-800W.

[0015] Furthermore, in step S30, the preparation method of the "graphene quantum dot-temperature-responsive magnetic nanoparticle" composite modified anion exchange resin includes the following steps: M1: Preparation of temperature-responsive magnetic nanoparticles: 100 parts by weight of Fe3O4 nanoparticles were dispersed in 1150-2000 parts by weight of deionized water, and 100-167 parts by weight of N-vinylpyrrolidone (NVP) and 200-333 parts by weight of N-isopropylacrylamide (NIPAM) monomers were added; after purging with nitrogen to remove oxygen for 30 minutes, 3-10 parts by weight of cuprous bromide / bipyridine catalytic system were added, and surface-initiated atom transfer radical polymerization was carried out at 60-70℃ for 4-6 hours. Subsequently, 15-50 parts by weight of specific recognition molecules with active groups were added, and the reaction was carried out at room temperature for 8-12 hours under EDC / NHS catalysis. After washing and vacuum drying, temperature-responsive magnetic nanoparticles were obtained. M2: Immerse 100 parts by weight of styrene-divinylbenzene copolymer matrix in 300-500 parts by weight of concentrated sulfuric acid solution with a mass fraction of 10%-15%, treat at 60-80℃ for 4-6 hours, then neutralize and wash with water until neutral to obtain sulfonated product. M3: Mix 100 parts by weight of the sulfonated product obtained in step M2 with 200-300 parts by weight of a 5%-8% ammonium chloride solution and react at 70-90°C for 3-5 hours to obtain the ammonated product. M4: Disperse 1 part by weight of graphene quantum dots in 99-200 parts by weight of deionized water and ultrasonically disperse for 30 minutes (power 300W) to obtain a graphene quantum dot dispersion with a mass fraction of 0.5%-1%. M5: Mix 100 parts by weight of the ammoniation product obtained in step M3 with 150-200 parts by weight of a quaternization reagent solution with a mass fraction of 2%-4%, add 5-8 parts by weight of the temperature-responsive magnetic nanoparticles prepared in step M1 and 30-50 parts by weight of graphene quantum dot dispersion; react at 50-70℃ for 2-4 hours, and obtain the composite modified anion exchange resin after washing and drying.

[0016] Furthermore, in step S30, the particle size of the graphene quantum dots is 3-10 nm; the operating temperature of the intelligent adsorption unit is controlled at 30-35℃; and an external weak magnetic field with a magnetic field strength of 0.1-0.3T is applied during the adsorption process.

[0017] Furthermore, in step S40, the operating temperature of the multi-effect evaporator is controlled at 60-80℃, and the vacuum degree is -0.08~-0.09MPa; the air source heat pump (heat pump heating coefficient COP≥3.0, waste heat recovery efficiency≥85%) recovers the waste heat (30-40℃) of the crystallization unit, and provides a heating source for the evaporator after raising the temperature to 70-80℃.

[0018] Furthermore, in step S50, the operating conditions of the gradient crystallization unit are as follows: First stage (concentrate cooling stage): temperature drops from 60℃ to 40℃ at a cooling rate of 1-2℃ / min, pressure 0.01-0.02MPa; Second stage (crystal growth stage): temperature is maintained at 35-40℃, pressure is at atmospheric pressure, and the temperature is maintained for 2-3 hours; Third stage (crystal ripening stage): temperature drops from 35℃ to 30℃ at a cooling rate of 0.5℃ / min, pressure is at atmospheric pressure.

[0019] Furthermore, in step S60, the resin regeneration process involves: using an external strong magnetic field with a strength of 0.5-0.8T to separate and recover the saturated resin, transferring it to a regeneration tank, heating it to 45-50℃, and adding a 3%-5% sodium hydroxide solution as an auxiliary regenerator. The regeneration liquid flow rate is 0.5-1m / s. 3 / h, regeneration time is 1-1.5 hours.

[0020] Secondly, a system for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater includes the following units: U10: Pretreatment unit, used to add composite synergistic treatment agent and react under catalytic conditions. The unit is equipped with a temperature control device, a stirring device and a reagent dosing device. U20: Enhanced solid-liquid separation unit, used to separate pretreated precipitate from supernatant, including solid-liquid separation device and precipitate collection chamber; U30: Intelligent adsorption unit, used for selective adsorption of residual non-volatile acid radicals, includes an ion exchange column filled with "graphene quantum dot-temperature responsive magnetic nanoparticle" composite modified anion exchange resin, a magnetic field separation device and a resin regeneration device. U40: Heat pump co-concentration unit, used to concentrate urea solution, includes a multi-effect evaporator, an air source heat pump and a condensation recovery device; U50: Gradient crystallization unit, used to precipitate high-purity urea crystals, includes a crystallizer, a temperature-pressure gradient control system, a centrifugal separation device, and a crystal drying device; U60: Resin circulation unit, connecting the intelligent adsorption unit and the regeneration device, used to realize the automated circulation and dispensing of regenerated resin.

[0021] Furthermore, in the pretreatment unit U10, the temperature control device has a temperature accuracy of ±0.5℃, the stirring device is a variable frequency stirring device with a speed of 150-250r / min, and the reagent addition device is a batch addition device.

[0022] Furthermore, the enhanced solid-liquid separation unit U20 is also equipped with an automatic cleaning device, which has a cleaning cycle of 8-10 hours, uses a 0.5%-1% mass fraction dilute hydrochloric acid solution, and takes 15-20 minutes to clean.

[0023] Furthermore, in the intelligent adsorption unit U30, the magnetic field separation device can generate an adjustable magnetic field of 0.1-0.8T; it is also equipped with an online monitoring device for real-time detection of the conductivity and acid concentration of the effluent, automatically switching to the standby exchange column when the acid concentration is ≥10mg / L; the regeneration device is equipped with a temperature control module with a temperature control range of 30-50℃, the regenerator is a 3%-5% sodium hydroxide solution by mass, and the regeneration liquid flow rate is 0.5-1m. 3 / h.

[0024] Furthermore, in the heat pump co-concentration unit U40, the cooling water temperature of the condensation recovery device is controlled at 15-25℃, and the cooling water flow rate is 5-10m³ / h. 3 / h, the recovered condensate is used as process makeup water, and its COD≤20mg / L.

[0025] Furthermore, in the gradient crystallization unit U50, the temperature control accuracy is ±0.5℃ and the pressure control accuracy is ±0.005MPa; the crystal drying device adopts low-temperature hot air drying, specifically drying at 55-60℃ for 2-3 hours, and the moisture content of the solid urea after drying is ≤0.5%.

[0026] Furthermore, the resin circulation unit U60 achieves automated circulation and delivery of regenerated resin through a magnetic field conveying device.

[0027] The beneficial effects of this invention are: (1) This invention achieves efficient and directional precipitation of non-volatile acid radicals by combining a "composite synergistic treatment agent" with catalytic conditions. Alkali metal salts, organic complexing agents and nanocatalytic particles produce a synergistic effect in the pretreatment unit. The nanocatalytic particles not only significantly accelerate the precipitation reaction kinetics, but also induce the formation of a more compact precipitate, thereby greatly improving the initial removal efficiency and creating favorable conditions for subsequent solid-liquid separation.

[0028] (2) The "graphene quantum dot-temperature-responsive magnetic nanoparticle" composite structure used in this invention significantly improves the selective adsorption and recycling performance of anion exchange resin for non-volatile acid radicals through a multi-scale synergistic mechanism. As a nanoscale carbon material, graphene quantum dots endow the resin surface with high-density active sites through their unique quantum confinement effect and edge effect, enhancing the selective recognition ability of target acid radicals through π-π stacking and electrostatic interaction; at the same time, the abundant oxygen-containing functional groups (such as carboxyl and hydroxyl groups) on the surface of quantum dots form a hydrophilic microenvironment, effectively reducing mass transfer resistance and promoting the diffusion and migration of acid radical ions to adsorption sites. Temperature-responsive magnetic nanoparticles construct a dynamically tunable adsorption-desorption interface: their core Fe3O4 provides strong magnetic response characteristics, enabling the resin to be precisely manipulated by external magnetic fields, avoiding the resin loss problem in traditional fixed beds; the surface-modified NIPAM / NVP copolymer layer has a distinct low critical solution temperature (LCST) characteristic. When the ambient temperature is below the LCST, the hydrophilic polymer chains fully extend and hydrate, allowing the specific recognition molecules (sulfonic acid groups) to be fully exposed to the solution phase, maximizing the effective utilization rate of the acid radical binding sites; when the temperature rises above the LCST, the polymer chains undergo a conformational change and hydrophobically aggregate, generating a steric compression effect, forcing the adsorbed acid radicals to quickly detach from the recognition sites, achieving mild regeneration without the need for strong acids / bases. This temperature-triggered conformational change mechanism not only significantly shortens the regeneration time but also avoids the damage to the resin structure caused by traditional chemical regeneration. The combined effect of the two is reflected in: graphene quantum dots regulate the hydration-dehydration kinetics of polymer chains through electron transfer, making the LCST transition more sensitive; while the temperature-responsive layer provides stable anchoring points for the quantum dots, preventing them from detaching in the flowing system. Overall, this composite structure achieves dynamic closed-loop control of "recognition-adsorption-release", enabling the resin to maintain high selectivity while possessing rapid regeneration characteristics. It fundamentally solves the technical bottlenecks of insufficient selectivity, low regeneration efficiency, and poor cycle stability of traditional anion exchange resins, providing an innovative material solution for the deep removal of trace non-volatile acid radicals in high-concentration urea wastewater.

[0029] (3) This invention adopts a coupled process of "heat pump co-concentration" and "gradient crystallization" to achieve efficient recovery of urea and energy saving. The heat pump co-concentration unit significantly reduces the energy input of the evaporation and concentration process by recovering the low-grade waste heat inside the system; while the gradient crystallization unit effectively controls the nucleation and growth process of urea crystals through precise segmented temperature and pressure control, thereby ensuring the high purity and high recovery rate of the final product.

[0030] (4) This invention constructs a complete closed-loop system for resource recycling and waste treatment. From pretreatment, intelligent adsorption, concentration and crystallization to magnetic field separation and temperature-controlled regeneration of the resin, the entire process achieves the directional removal of non-volatile acid radicals, high-quality recovery of urea, and recycling of the adsorbent. The system has a high degree of integration and automation, and the synergistic effect between the units is obvious. It not only completely solves the pollution problem of high-concentration urea wastewater, but also maximizes the recovery of valuable resources, demonstrating excellent environmental and economic benefits. Detailed Implementation

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0032] In some embodiments, a method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater includes the following steps: S10: High-concentration urea wastewater is introduced into the pretreatment unit, and a composite synergistic treatment agent is added to form a dense precipitate containing non-volatile acid radicals under catalytic conditions; the composite synergistic treatment agent is composed of alkali metal salts, organic complexing agents and nano-catalytic particles; S20: The mixture obtained in step S10 is passed through an enhanced solid-liquid separation device to separate the precipitate and collect the supernatant; S30: The supernatant obtained in step S20 is introduced into the intelligent adsorption unit, and the graphene quantum dot-temperature responsive magnetic nanoparticle composite modified anion exchange resin is used to selectively adsorb residual non-volatile acid radicals. S40: The liquid processed in step S30 is introduced into the heat pump co-concentration unit to concentrate urea through low-temperature evaporation and waste heat recovery coupling technology. S50: The concentrate obtained in step S40 is introduced into a gradient crystallization unit, and high-purity urea crystals are precipitated under segmented temperature-pressure control, and solid urea is obtained by centrifugation. S60: The composite modified anion exchange resin after adsorption saturation is rapidly separated and recovered by an external magnetic field, and then recycled after temperature-controlled regeneration.

[0033] Existing technologies for treating high-concentration urea wastewater suffer from several drawbacks, including incomplete removal of non-volatile acid radicals, high energy consumption in urea recovery, difficulties and significant losses in resin regeneration, and a lack of synergistic adaptation among treatment stages. These shortcomings result in poor overall treatment performance and high operating costs. This new technology addresses these issues by designing a comprehensive process involving pretreatment, enhanced solid-liquid separation, intelligent adsorption, heat pump-assisted concentration, gradient crystallization, and resin recycling. This process constructs a complementary and synergistic system: the pretreatment stage achieves initial directional precipitation of acid radicals, reducing the load on subsequent units; the enhanced solid-liquid separation stage improves precipitation removal efficiency and reduces residual acid radical interference; the intelligent adsorption stage utilizes a composite modified resin to achieve deep purification of acid radicals, overcoming the performance limitations of traditional adsorption materials; the heat pump-assisted concentration stage balances urea stability with energy consumption control, preventing urea decomposition due to high temperatures; the gradient crystallization stage precisely regulates the crystal growth process, ensuring the purity of the urea product; and the resin recycling stage enables efficient recovery and reuse of the modified resin, reducing operating costs. This comprehensive process design forms a closed-loop technology from multiple dimensions—removal, separation, recovery, and recycling—systematically solving the core deficiency of existing technologies where single-stage optimization cannot achieve a balanced overall treatment effect.

[0034] In some embodiments, in step S10, the mass fraction of each component of the composite synergistic treatment agent is as follows: 0.5%-1% alkali metal salt, 0.1%-0.3% organic complexing agent, and 0.05%-0.1% nanocatalytic particles.

[0035] Alkali metal salts provide an alkaline environment that promotes the formation of insoluble salts from non-volatile acid radicals. Organic complexing agents stabilize intermediate products and regulate crystal nucleus growth through chelation. Nanocatalytic particles accelerate the reaction kinetics through their surface hydroxyl active sites. The synergistic effect of these three agents significantly improves the selectivity and efficiency of the precipitation reaction, avoiding the problems of incomplete reaction or increased side reactions that exist when treating a single component, thus laying a good foundation for subsequent solid-liquid separation.

[0036] In some embodiments, in step S10, the alkali metal salt is sodium carbonate or potassium carbonate, the organic complexing agent is disodium ethylenediaminetetraacetate or sodium citrate, and the nanocatalytic particles are hydroxylated nano titanium dioxide with a particle size of 5-20 nm.

[0037] Carbonates provide a stable alkaline environment and have moderate solubility. Disodium ethylenediaminetetraacetate and sodium citrate can effectively complex various acid radicals due to their multidentate coordination ability. The abundant hydroxyl groups on the surface of hydroxylated nano-titanium dioxide not only enhance catalytic activity but also promote precipitate aggregation through surface charge effect. This combination achieves precise matching for the chemical characteristics of typical non-volatile acid radicals in urea wastewater, significantly improving treatment selectivity.

[0038] In some embodiments, in step S10, the composite synergistic treatment agent is added in batches using a gradient addition method, in 3-4 batches, with each addition amount being 25%-33% of the total amount, and the interval between each addition being 8-15 minutes.

[0039] A "batch gradient addition method" was adopted to add the composite synergistic treatment agent in batches, optimizing the reaction process by controlling the timing and interval of addition. This method avoids the phenomenon of excessively high local concentrations caused by a single addition, ensuring uniform distribution of reactants in the wastewater, promoting the orderly formation and growth of crystal nuclei, and effectively preventing the problem of loose structure of precipitates due to rapid formation. This results in precipitates with uniform particle size and excellent settling performance, thereby improving the efficiency of subsequent solid-liquid separation.

[0040] In some embodiments, in step S10, the reaction temperature of the pretreatment unit is controlled at 30-40°C, the stirring speed is 150-250 r / min, and the stirring time is 40-60 minutes.

[0041] The reaction temperature, stirring speed, and stirring time of the pretreatment unit are controlled within a specific range to achieve precise control of the reaction conditions.

[0042] In some embodiments, the preparation method of the nanocatalytic particles is as follows: dissolve tetrabutyl titanate in anhydrous ethanol, add 5%-8% by mass of hydroxylating agent (ethylene glycol monomethyl ether), stir and react at 50-60°C for 2-3 hours, and obtain hydroxylated nano titanium dioxide after centrifugation, washing, and vacuum drying (60°C, 4 hours).

[0043] Hydroxylated nano-titanium dioxide was prepared using a specific process, and the surface of the nanoparticles was functionalized by hydroxylation reagents. This method introduces a large number of hydroxyl groups onto the surface of titanium dioxide, significantly enhancing its surface activity and hydrophilicity, improving its adsorption and catalytic ability for target acid anions, while also improving the dispersion stability of the nanoparticles in the aqueous phase, preventing aggregation, and enabling the nanocatalytic particles to fully exert their catalytic efficiency and promote the efficient precipitation reaction.

[0044] In some embodiments, in step S20, the enhanced solid-liquid separation device is an "ultrasonic-assisted horizontal screw discharge sedimentation centrifuge," with the centrifuge speed set to 3000-4000 r / min and the feed flow rate controlled at 2-3 m³ / min. 3 / h, while simultaneously applying an ultrasonic field with a frequency of 20-40kHz and an ultrasonic power of 500-800W.

[0045] An ultrasonic-assisted horizontal screw discharge sedimentation centrifuge is used as an enhanced solid-liquid separation device, combining the dual effects of mechanical centrifugal force and ultrasonic field. The cavitation effect and microfluidic disturbance generated by the ultrasonic field effectively break up precipitate aggregation, promote the growth of fine particles, and reduce the interfacial tension between precipitate and liquid, significantly improving solid-liquid separation efficiency. The horizontal screw discharge structure ensures continuous and stable slag discharge, avoiding the clogging problem of traditional centrifuges, and achieving efficient separation of materials with high solid content.

[0046] In some embodiments, the preparation method of the "graphene quantum dot-temperature-responsive magnetic nanoparticle" composite modified anion exchange resin in step S30 includes the following steps: M1: Preparation of temperature-responsive magnetic nanoparticles: 100 parts by weight of Fe3O4 nanoparticles were dispersed in 1150-2000 parts by weight of deionized water, and 100-167 parts by weight of N-vinylpyrrolidone (NVP) and 200-333 parts by weight of N-isopropylacrylamide (NIPAM) monomers were added; after purging with nitrogen to remove oxygen for 30 minutes, 3-10 parts by weight of cuprous bromide / bipyridine catalytic system were added, and surface-initiated atom transfer radical polymerization was carried out at 60-70℃ for 4-6 hours. Subsequently, 15-50 parts by weight of specific recognition molecules with active groups were added, and the reaction was carried out at room temperature for 8-12 hours under EDC / NHS catalysis. After washing and vacuum drying, temperature-responsive magnetic nanoparticles were obtained. M2: Immerse 100 parts by weight of styrene-divinylbenzene copolymer matrix in 300-500 parts by weight of concentrated sulfuric acid solution with a mass fraction of 10%-15%, and treat at 60-80°C for 4-6 hours to obtain sulfonated product; M3: Mix 100 parts by weight of the sulfonated product obtained in step M2 with 200-300 parts by weight of a 5%-8% ammonium chloride solution and react at 70-90°C for 3-5 hours to obtain the ammonated product. M4: Disperse 1 part by weight of graphene quantum dots in 99-200 parts by weight of deionized water and ultrasonically disperse for 30 minutes (power 300W) to obtain a graphene quantum dot dispersion with a mass fraction of 0.5%-1%. M5: Mix 100 parts by weight of the ammoniation product obtained in step M3 with 150-200 parts by weight of a quaternization reagent solution with a mass fraction of 2%-4%, add 5-8 parts by weight of the temperature-responsive magnetic nanoparticles prepared in step M1 and 30-50 parts by weight of graphene quantum dot dispersion; react at 50-70℃ for 2-4 hours, and obtain the composite modified anion exchange resin after washing and drying.

[0047] A composite modified anion exchange resin consisting of graphene quantum dots and temperature-responsive magnetic nanoparticles was prepared through specific steps, constructing a multi-scale synergistic system. Graphene quantum dots provide high-density active sites to enhance adsorption capacity, the temperature-responsive polymer layer endows the resin with intelligent responsive characteristics, and the magnetic core enables precise external control. The three components form a stable composite structure through molecular-level design, enabling the resin to possess multiple functions including highly selective adsorption, temperature-triggered regeneration, and magnetic field-assisted separation. This fundamentally solves the technical bottlenecks of insufficient selectivity and difficult regeneration in traditional anion exchange resins.

[0048] In some embodiments, in step S30, the particle size of the graphene quantum dots is 3-10 nm; the operating temperature of the intelligent adsorption unit is controlled at 30-35℃; and an external weak magnetic field with a magnetic field strength of 0.1-0.3T is applied during the adsorption process.

[0049] The appropriate quantum dot particle size ensures a high specific surface area and surface functional group density, enhancing the selective recognition of target acid radicals; the precisely controlled low-temperature operating environment keeps the temperature-responsive polymer layer in a swollen state, fully exposing specific recognition sites; and the weak magnetic field assistance enables precise resin positioning without interfering with the adsorption process, preventing resin loss in the flow system. The synergistic effect of these three factors maximizes adsorption efficiency.

[0050] In some embodiments, in step S40, the operating temperature of the multi-effect evaporator is controlled at 60-80°C and the vacuum degree is -0.08~-0.09MPa; the air source heat pump recovers the waste heat (30-40°C) of the crystallization unit and provides a heating source for the evaporator after raising the temperature to 70-80°C.

[0051] Urea concentration is achieved through a coupled technology combining low-temperature evaporation and waste heat recovery, with energy efficiency realized through thermodynamic cycle optimization. Low-temperature operating conditions prevent the thermal decomposition of urea at high temperatures, maintaining product quality; the waste heat recovery system captures and utilizes low-grade heat energy within the system, significantly reducing the need for external heat input; and the vacuum environment further lowers the evaporation temperature. These three elements work together to create an energy-efficient concentration process, achieving gentle concentration of the urea solution.

[0052] In some embodiments, the operating conditions of the gradient crystallization unit in step S50 are as follows: First stage (concentrate cooling stage): temperature drops from 60℃ to 40℃ at a cooling rate of 1-2℃ / min, pressure 0.01-0.02MPa; Second stage (crystal growth stage): temperature is maintained at 35-40℃, pressure is at atmospheric pressure, and the temperature is maintained for 2-3 hours; Third stage (crystal ripening stage): temperature drops from 35℃ to 30℃ at a cooling rate of 0.5℃ / min, pressure is at atmospheric pressure.

[0053] A segmented temperature-pressure control strategy is employed for gradient crystallization, precisely controlling the crystallization kinetics. The controllable cooling rate during the cooling phase prevents explosive nucleation caused by excessive supersaturation; the isotropic conditions during crystal growth promote orderly crystal growth; and the slow cooling during the ripening phase causes small crystals to dissolve and redeposit on the surface of larger crystals. This synergistic effect of the three stages effectively avoids impurity inclusions and crystal defects, resulting in a high-purity, highly crystalline urea product.

[0054] In some embodiments, in step S60, the resin regeneration process involves: using an external strong magnetic field with a strength of 0.5-0.8T to separate and recover the saturated resin, transferring it to a regeneration tank, heating it to 45-50°C, and adding a 3%-5% sodium hydroxide solution as an auxiliary regenerator. The regeneration liquid flow rate is 0.5-1 m / s. 3 / h, regeneration time is 1-1.5 hours.

[0055] A regeneration method combining magnetic field separation and temperature control is employed to treat adsorbed saturated resin, achieving efficient and gentle regeneration. Rapid separation using a strong magnetic field ensures complete resin recovery, avoiding resin loss as in traditional methods. Temperature control utilizes the low critical dissolution temperature of the polymer layer to trigger the desorption process under gentle conditions, significantly shortening the regeneration time. The appropriate use of an auxiliary regenerant further enhances the desorption effect. The synergistic effect of these three factors achieves efficient regeneration while protecting the resin's structural integrity, greatly extending the resin's service life.

[0056] In some embodiments, a system for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater includes the following units: U10: Pretreatment unit, used to add composite synergistic treatment agent and react under catalytic conditions. The unit is equipped with a temperature control device, a stirring device and a reagent dosing device. U20: Enhanced solid-liquid separation unit, used to separate pretreated precipitate from supernatant, including solid-liquid separation device and precipitate collection chamber; U30: Intelligent adsorption unit, used for selective adsorption of residual non-volatile acid radicals, includes an ion exchange column filled with "graphene quantum dot-temperature responsive magnetic nanoparticle" composite modified anion exchange resin, a magnetic field separation device and a resin regeneration device. U40: Heat pump co-concentration unit, used to concentrate urea solution, includes a multi-effect evaporator, an air source heat pump and a condensation recovery device; U50: Gradient crystallization unit, used to precipitate high-purity urea crystals, includes a crystallizer, a temperature-pressure gradient control system, a centrifugal separation device, and a crystal drying device; U60: Resin circulation unit, connecting the intelligent adsorption unit and the regeneration device, used to realize the automated circulation and dispensing of regenerated resin.

[0057] An integrated system comprising pretreatment, enhanced solid-liquid separation, intelligent adsorption, heat pump-assisted concentration, gradient crystallization, and resin circulation was constructed to achieve modularization and systematization of the process. Each unit is functionally designed for specific treatment objectives, forming an organic whole through optimized matching of material and energy flows. This ensures the treatment effect of each stage while achieving system-level energy efficiency optimization, solving the problems of poor connectivity and low efficiency in traditional decentralized treatment systems.

[0058] In some embodiments, in the pretreatment unit U10, the temperature control device has a temperature accuracy of ±0.5℃, the stirring device is a variable frequency stirring device with a rotation speed of 150-250r / min, and the reagent addition device is a batch addition device.

[0059] The pretreatment unit incorporates a high-precision temperature control device, a variable-frequency stirring device, and a batch dosing device to achieve precise control of reaction conditions. High-precision temperature control ensures the reaction proceeds under optimal thermodynamic conditions, while the variable-frequency stirring provides suitable mixing intensity to promote mass transfer without disrupting the precipitation structure. The batch dosing device precisely controls the timing and amount of reagent addition. These three components work synergistically to ensure the repeatability and stability of the pretreatment reaction, providing homogenized feed for subsequent processing stages.

[0060] In some embodiments, the enhanced solid-liquid separation unit U20 is further provided with an automatic cleaning device, the cleaning cycle of which is 8-10 hours, the cleaning solution is a dilute hydrochloric acid solution with a mass fraction of 0.5%-1%, and the cleaning time is 15-20 minutes.

[0061] This cleaning strategy uses weak acid to dissolve any inorganic precipitates that may adhere to the equipment, while avoiding the corrosion of the equipment materials by strong acid. This ensures that the separation unit maintains high-efficiency separation capabilities during long-term operation, reduces the need for manual maintenance, and improves the stability of continuous system operation.

[0062] In some embodiments, the intelligent adsorption unit U30 includes a magnetic field separation device that can generate an adjustable magnetic field of 0.1-0.8T; it also includes an online monitoring device for real-time detection of the conductivity and acid concentration of the effluent, automatically switching to a standby exchange column when the acid concentration is ≥10mg / L; the regeneration device includes a temperature control module with a temperature range of 30-50℃, the regenerator being a 3%-5% sodium hydroxide solution, and the regeneration liquid flow rate being 0.5-1m. 3 / h.

[0063] The adjustable magnetic field adapts to the resin separation requirements under different working conditions, ensuring resin recovery rate; the online monitoring device provides real-time feedback on the treatment effect, enabling timely switching of saturated resin and ensuring stable effluent quality; the temperature-controlled regeneration module precisely regulates regeneration conditions and optimizes regeneration efficiency. These three elements work together to improve the automation level and operational reliability of the adsorption unit.

[0064] In some embodiments, in the heat pump co-concentration unit U40, the cooling water temperature of the condensation recovery device is controlled at 15-25°C, and the cooling water flow rate is 5-10 m³ / h. 3 / h, the recovered condensate is used as process makeup water, and its COD≤20mg / L.

[0065] Optimize the condensation recovery conditions of the heat pump co-concentration unit to achieve high-quality condensate reuse. Precisely controlled cooling water temperature and flow rate ensure efficient condensation while maintaining low organic content in the condensate, meeting process makeup water standards. This not only reduces fresh water consumption but also avoids the need for additional condensate treatment in traditional processes, achieving water resource recycling.

[0066] In some embodiments, the temperature control accuracy in the gradient crystallization unit U50 is ±0.5℃, and the pressure control accuracy is ±0.005MPa; the crystal drying device adopts low-temperature hot air drying, specifically drying at 55-60℃ for 2-3 hours, and the moisture content of the solid urea after drying is ≤0.5%.

[0067] Precise temperature and pressure control ensures that the crystallization process proceeds along a preset path, resulting in high-purity crystals. Low-temperature hot air drying ensures moisture removal while avoiding thermal damage to the crystals, preventing urea decomposition or agglomeration, ultimately yielding high-purity, low-moisture urea products that meet industrial standards and satisfy direct application requirements.

[0068] In some embodiments, the resin circulation unit U60 achieves automated circulation and delivery of regenerated resin through a magnetic field conveying device.

[0069] Magnetic field transport utilizes the magnetic properties of resin to achieve non-contact and precise transmission, avoiding resin breakage caused by mechanical transport. It also enables quantitative dispensing of regenerated resin, ensuring uniform distribution of the resin bed within the adsorption unit, significantly reducing resin loss rate, and improving the long-term economic efficiency of the system.

[0070] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0071] Example 1

[0072] This embodiment provides a method for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater, which is prepared through the following steps: S10: Take 1m 3High-concentration urea wastewater (urea content 15%, total concentration of non-volatile acid radicals 800 mg / L, COD=5000 mg / L, calculated per 1000 kg) is introduced into the pretreatment unit and a composite synergistic treatment agent (7 kg sodium carbonate, 2 kg disodium ethylenediaminetetraacetate, 0.8 kg hydroxylated nano titanium dioxide, particle size 5-20 nm) is added in three portions, with an interval of 12 minutes between each addition. The reaction temperature is 35℃, the stirring speed is 200 r / min, and the stirring time is 50 minutes, forming a dense precipitate. S20: The mixture obtained in step S10 is passed into an ultrasonic-assisted horizontal screw discharge sedimentation centrifuge. The centrifuge speed is 3500 r / min and the feed flow rate is 2.5 m³ / min. 3 At a speed of 10 h, an ultrasonic field of 30 kHz and 600 W was applied to separate the precipitate (58% water content) and collect the supernatant (75 mg / L acid concentration). S30: The supernatant obtained in step S20 is introduced into the intelligent adsorption unit, and the adsorption column is filled with a composite modified anion exchange resin of "graphene quantum dots-temperature-responsive magnetic nanoparticles" (preparation method is as follows): M1: Preparation of temperature-responsive magnetic nanoparticles: 0.5 kg Fe3O4 nanoparticles were dispersed in 10 L of deionized water, and 0.8 kg N-vinylpyrrolidone (NVP) and 1.6 kg N-isopropylacrylamide (NIPAM) (molar ratio 1:2) were added. Nitrogen gas was introduced to remove oxygen for 30 minutes, and 0.048 kg cuprous bromide / bipyridine catalytic system was added. Polymerization was carried out at 65 °C for 5 hours. 0.12 kg sodium aminobenzenesulfonate (specific recognition molecule) was added, and the reaction was carried out at room temperature for 10 hours under EDC / NHS catalysis. After washing and drying, temperature-responsive magnetic nanoparticles were obtained. M2: Immerse 1 kg of styrene-divinylbenzene copolymer matrix in 4 kg of 12% concentrated sulfuric acid solution, treat at 70°C for 5 hours, then neutralize and wash with water until neutral to obtain sulfonated product; M3: The sulfonated product is mixed with 2.5 kg of 6% ammonium chloride solution and reacted at 80°C for 4 hours to obtain the ammonized product; M4: Disperse 0.01 kg of graphene quantum dots (5 nm in diameter) in 2 L of deionized water and ultrasonically disperse for 30 minutes (300 W) to obtain a dispersion with a mass fraction of 0.5%. M5: The ammoniation product was mixed with 1.8 kg of 3% quaternization reagent solution, 0.06 kg of temperature-responsive magnetic nanoparticles and 0.4 kg of graphene quantum dot dispersion were added, and the mixture was reacted at 60 °C for 3 hours. After washing and drying, the composite modified resin was obtained. The intelligent adsorption unit operated at 32 °C and a 0.2 T weak magnetic field was applied to assist in resin fixation. The acid concentration of the effluent after adsorption was ≤5 mg / L. S40: The liquid processed in step S30 is introduced into the heat pump co-concentration unit. The multi-effect evaporator operates at a temperature of 70°C and a vacuum degree of -0.085MPa. The air source heat pump recovers the waste heat (35°C) from the crystallization unit and heats it to 75°C to power the evaporator, concentrating the urea concentration to 45%. S50: The concentrate is introduced into the gradient crystallization unit. First stage: 60℃ to 40℃ (cooling rate 1.5℃ / min, pressure 0.015MPa); Second stage: 38℃ for 2.5 hours (atmospheric pressure); Third stage: 38℃ to 30℃ (cooling rate 0.5℃ / min, atmospheric pressure); After centrifugation and hot air drying at 60℃ for 2 hours, solid urea is obtained. S60: After adsorption saturation, the resin is rapidly separated and recovered using a 0.6T strong magnetic field, then transferred to a regeneration tank. A 4% (by mass) sodium hydroxide solution is added (total regeneration liquid volume 1m³, containing 40kg of sodium hydroxide), with a regeneration liquid flow rate of 0.8m / s. 3 / h, after regeneration for 1.2 hours, the temperature is reduced to 32℃, and then it is recycled into the intelligent adsorption unit for use.

[0073] Example 2 (Optimization of Composite Synergistic Treatment Agent Components)

[0074] The difference between this embodiment and Embodiment 1 is that, S10: Take 1m 3 For high-concentration urea wastewater, add a composite synergistic treatment agent (10 kg potassium carbonate, 3 kg sodium citrate, 1 kg hydroxylated nano titanium dioxide with a particle size of 10 nm) in 4 portions, with an interval of 10 minutes between each addition. The reaction temperature is 40℃, the stirring speed is 250 r / min, and the stirring time is 60 minutes. The remaining raw materials and preparation process are the same as in Example 1.

[0075] Example 3 (Optimization of Composite Modified Resin Parameters)

[0076] The difference between this embodiment and Embodiment 1 is that, Resin preparation in S30: M1: When preparing temperature-responsive magnetic nanoparticles, 0.8 kg of N-vinylpyrrolidone (NVP) and 2.4 kg of N-isopropylacrylamide (NIPAM) were added (molar ratio 1:3). M4: Disperse 0.01 kg of graphene quantum dots (8 nm in diameter) in 0.99 L of deionized water and ultrasonically disperse for 30 minutes (300 W) to obtain a dispersion with a mass fraction of 1%. M5: Mix the ammoniation product with 1.8 kg of 3% quaternization reagent solution, add 0.08 kg of temperature-responsive magnetic nanoparticles and 0.3 kg of graphene quantum dot dispersion; the intelligent adsorption unit operates at 35℃, and a 0.3T weak magnetic field is applied to assist resin fixation; The remaining raw materials and preparation process are the same as in Example 1.

[0077] Example 4 (Optimization of Concentration Process Parameters)

[0078] The difference between this embodiment and Embodiment 1 is that, S40: The liquid processed in step S30 is introduced into the heat pump co-concentration unit. The multi-effect evaporator operates at a temperature of 80°C and a vacuum degree of -0.09MPa. The air source heat pump recovers the waste heat (40°C) from the crystallization unit and heats it to 80°C to power the evaporator, concentrating the urea concentration to 50%. The remaining raw materials and preparation process are the same as in Example 1.

[0079] Example 5 (Optimization of Crystallization Process Parameters)

[0080] The difference between this embodiment and Embodiment 1 is that, S50: The concentrate is introduced into the gradient crystallization unit. First stage: 60℃ to 40℃ (cooling rate 2℃ / min, pressure 0.02MPa); Second stage: 40℃ for 2 hours (atmospheric pressure); Third stage: 40℃ to 30℃ (cooling rate 0.5℃ / min, atmospheric pressure); After centrifugation and hot air drying at 60℃ for 2 hours, solid urea is obtained. The remaining raw materials and preparation process are the same as in Example 1.

[0081] Example 6 (Optimization of Resin Regeneration Parameters)

[0082] The difference between this embodiment and Embodiment 1 is that, S60: After the resin is saturated with adsorption, it is quickly separated and recovered by a 0.6T strong magnetic field and transferred to a regeneration tank. The temperature is raised to 50℃, and a 5% sodium hydroxide solution (total regeneration liquid 1m³, containing 50kg of sodium hydroxide) is added. The regeneration liquid flow rate is 1m³ / h. After regeneration for 1 hour, the temperature is lowered to 32℃ and recycled into the intelligent adsorption unit. The remaining raw materials and preparation process are the same as in Example 1.

[0083] Comparative Example 1 (without composite synergistic treatment agent, using a single precipitant)

[0084] The difference between this comparative example and Example 1 is that, S10: Take 1m 3High-concentration urea wastewater was treated with 15 kg of sodium carbonate (added all at once), without any organic complexing agents or nano-catalytic particles. The mixture was stirred at 100 r / min for 30 minutes to form a loose precipitate. The remaining raw materials and preparation process are the same as in Example 1.

[0085] Comparative Example 2 (without composite modified resin, using ordinary anion exchange resin)

[0086] Compared with Example 1, this comparative example... S30: The supernatant obtained in step S20 is introduced into the adsorption unit. The adsorption column is filled with 1 kg of ordinary styrene-divinylbenzene anion exchange resin (without graphene quantum dots and temperature-responsive magnetic nanoparticle modification). The adsorption process is not assisted by a magnetic field. The acid concentration of the effluent after adsorption is ≥30 mg / L. S60: After adsorption saturation, ordinary resin is separated by filtration and transferred to a regeneration tank. A 5% sodium hydroxide solution (total regeneration solution 20L, containing 1kg sodium hydroxide) is added, and the mixture is statically soaked for 4 hours, achieving a regeneration efficiency of 75%. The remaining raw materials and preparation process are the same as in Example 1.

[0087] Comparative Example 3 (Solid-liquid separation without ultrasound assistance)

[0088] The difference between this comparative example and Example 1 is that, S20: Pass the mixture obtained in step S10 into a conventional horizontal screw discharge sedimentation centrifuge with a centrifuge speed of 2500 r / min, a feed flow rate of 2.5 m³ / h, no ultrasonic field, and an acid concentration of ≥120 mg / L in the supernatant after separation. The remaining raw materials and preparation process are the same as in Example 1.

[0089] Comparative Example 4 (No heat pump waste heat recovery, conventional high-temperature concentration)

[0090] The difference between this comparative example and Example 1 is that, S40: Introduce the liquid processed in step S30 into a single-effect evaporator, operate at 90°C, without heat pump waste heat recovery, and concentrate to a urea concentration of 45% (urea decomposition rate ≥5%). The remaining raw materials and preparation process are the same as in Example 1.

[0091] Comparative Example 5 (no gradient crystallization, conventional isothermal crystallization)

[0092] The difference between this comparative example and Example 1 is that, S50: The concentrate is directly cooled to 35°C and crystallized at a constant temperature for 4 hours without segmented temperature-pressure control. After centrifugation and drying with hot air at 60°C for 2 hours, solid urea is obtained. The remaining raw materials and preparation process are the same as in Example 1.

[0093] Comparative Example 6 (The resin was modified only with graphene quantum dots, without temperature-responsive magnetic nanoparticles)

[0094] The difference between this comparative example and Example 1 lies in the specific implementation steps: Resin preparation in S30: No M1 step (no temperature-responsive magnetic nanoparticles are prepared). M5: The ammonification product was mixed with 1.8 kg of 3% quaternization reagent solution, and only 0.4 kg of graphene quantum dot dispersion was added. The mixture was reacted at 60°C for 3 hours. After washing and drying, a resin modified only with graphene quantum dots was obtained. S60: After adsorption saturation, the resin is separated by filtration and transferred to a regeneration tank. A 4% sodium hydroxide solution (total regeneration solution 2ml) is added. 3 (Contains 80 kg of sodium hydroxide), regenerated liquid flow rate 0.8 m / s 3 / h, rinsing for 2.5 hours, regeneration efficiency 82.5%; The remaining raw materials and preparation process are the same as in Example 1.

[0095] Comparative Example 7 (grafting of non-specific recognition molecules)

[0096] The difference between this comparative example and Example 1 is that, In step M1 of S30, during the preparation of temperature-responsive magnetic nanoparticles, only 0.5 kg Fe3O4 nanoparticles, 0.8 kg NVP, 1.6 kg NIPAM, and 0.048 kg cuprous bromide / bipyridine catalytic system were added. Sodium aminobenzenesulfonate was not added after the polymerization reaction, and the temperature-responsive magnetic nanoparticles were obtained by direct washing and drying. The remaining raw materials and preparation process were the same as in Example 1.

[0097] Performance testing

[0098] Performance tests were conducted on the embodiments and comparative examples. The specific test items are as follows: 1. Removal rate of non-volatile acid radicals: According to "HJ84-2016 Determination of inorganic anions in water quality by ion chromatography", the total concentration of non-volatile acid radicals (sulfate, phosphate, oxalate) before and after wastewater treatment was determined, and the removal rate was calculated according to the formula (initial concentration - final concentration) / initial concentration × 100%. 2. Urea recovery rate: The urea concentration before and after treatment was determined by high performance liquid chromatography, and the recovery rate was calculated by formula (final recovered urea mass / initial total urea mass) × 100%; 3. Urea purity: The purity of the crystallized solid urea was determined by chemical titration according to GB / T2440-2017 Urea. 4. COD of treated wastewater: determined according to GB / T11914-1989 Determination of Chemical Oxygen Demand in Water (Dichromate Method); 5. Resin adsorption capacity: Calculated according to the formula (total amount of acid radicals adsorbed by the resin when adsorption is saturated / dry weight of the resin), in mg / g; 6. Resin regeneration efficiency: Calculated using the formula (adsorption capacity of regenerated resin / adsorption capacity of fresh resin) × 100%; 7. Unit processing energy consumption: Statistical processing of 1m³ 3 Total energy consumption (electricity and heat) for wastewater treatment, unit: kWh / m³ 3 .

[0099] The results are shown in Table 1: Table 1

[0100] As can be seen from Table 1, Examples 1-6 of the present invention are significantly better than Comparative Examples 1-7 in terms of core indicators such as non-volatile acid radical removal rate, urea recovery rate, urea purity, COD control of treated wastewater, resin adsorption and regeneration performance, and unit treatment energy consumption. This fully demonstrates the inventiveness of the core innovation of the present invention and the synergistic effect of various technical features.

[0101] The removal rates of non-volatile acid radicals in Examples 1-6 were consistently 99.3%-99.6%, significantly higher than the 85.2%-97.1% of the comparative examples. The specific reason for this is that Comparative Example 1, lacking organic complexing agents and nano-catalytic particles, relied solely on a single addition of sodium carbonate, resulting in incomplete precipitation, loose precipitate, and a high residual acid radical content. In contrast, the examples utilized the synergistic catalysis of alkali metal salts, organic complexing agents, and hydroxylated nano-titanium dioxide, along with batch addition, to form a dense precipitate. Ultrasonic-assisted separation further reduced residues, followed by deep adsorption with a composite modified resin, achieving near-complete removal of acid radicals. Comparative Example 2 used ordinary anion exchange resin, which had low adsorption capacity and poor selectivity, failing to effectively capture residual acid radicals. The composite modified resin in the examples, leveraging the highly active sites and specific targeting effects of graphene quantum dots, increased the adsorption capacity to 185-195 mg / g, ensuring deep purification of acid radicals. Comparative Example 3 lacked ultrasonic-assisted solid-liquid separation, resulting in precipitate aggregation and residual acid radicals in the supernatant (≥120 mg / L), leading to subsequent adsorption overload and a decrease in the final removal rate. In the Example 1, the ultrasonic field disrupted the double layer of the precipitate, improving separation efficiency, reducing the load on the adsorption unit, and ensuring the overall removal rate. Comparative Examples 6 and 7 lacked temperature-responsive magnetic nanoparticles and specific recognition molecules, respectively, resulting in insufficient synergistic effect of the composite modified resin, decreased adsorption selectivity and capacity, and thus, the acid radical removal rate did not reach its optimal level.

[0102] The urea recovery rates of Examples 1-6 reached 98.5%-99.2%, significantly better than Comparative Examples 1 and 4. This is because in Comparative Example 1, the precipitate was loose, and some urea was encapsulated by the precipitate and could not be separated, resulting in a significant decrease in recovery rate. In the Examples, the composite synergistic treatment agent induced the formation of dense precipitate, reducing urea encapsulation, and ultrasonic-assisted separation further reduced urea entrainment loss. Comparative Example 4 used high-temperature concentration at 90℃, exceeding the critical temperature for urea thermal decomposition, resulting in a urea decomposition rate ≥5% and a recovery rate of only 93.8%. The Examples, with heat pump synergistic concentration, controlled the temperature at 60-80℃, avoiding urea decomposition in a low-temperature environment, while the gradient crystallization process reduced crystal entrainment, ensuring a high recovery rate. Although other Comparative Examples 2, 3, 5, 6, and 7 did not show obvious decomposition or encapsulation, their urea recovery rates were still lower than those of the Examples due to problems such as incomplete removal of acid radicals and poor separation effect, confirming the importance of whole-process synergy for urea recovery.

[0103] The urea purity of Examples 1-6 was consistently between 99.6% and 99.8%, far exceeding that of Comparative Example 1 and superior to other comparative examples. This is because Comparative Example 1 suffered from incomplete removal of acid radicals and loose precipitates containing impurities, resulting in a large amount of inorganic impurities mixed into the urea crystals, leading to a purity of only 97.3%. The Examples, however, achieved high-purity urea by deeply removing residual acid radicals with a composite modified resin and controlling the crystal growth rate using a gradient crystallization process to avoid impurity encapsulation. Comparative Example 5 (conventional isothermal crystallization) had a purity of 99.0%, lower than the 99.6%-99.8% of the Examples. This is because isothermal crystallization easily leads to rapid crystal nucleation, and small crystals readily adsorb impurities. In contrast, the Examples achieved orderly crystal growth and maturation through segmented temperature-pressure control, reducing impurity residue.

[0104] The COD of the treated wastewater in Examples 1-6 was only 32-36 mg / L, far lower than that in Comparative Examples 1, 2, and 3, demonstrating the comprehensive removal capability of the present invention for pollutants. In Comparative Example 1, the precipitation reaction was incomplete, and a large amount of organic matter and unremoved acid radicals formed complex pollutants, resulting in a COD as high as 142 mg / L, far exceeding the emission standard. In Examples 1, the entire process removed most of the acid radicals through precipitation, removed residual impurities through adsorption, and recovered urea through concentration and crystallization, significantly reducing the organic matter content of the wastewater. Comparative Examples 2 and 3 had high COD due to insufficient adsorption and poor separation, resulting in residual organic matter and impurities. In Examples 1, the highly selective adsorption of the composite modified resin and the efficient separation assisted by ultrasound ensured that the COD of the wastewater was controlled at a low level.

[0105] The resin adsorption capacity of Examples 1-6 reached 185-195 mg / g, and the regeneration efficiency remained stable at 98.0%-98.6%, significantly better than Comparative Examples 2, 6, and 7. This is primarily due to the synergistic effect of graphene quantum dots, temperature-responsive magnetic nanoparticles, and specific recognition molecules: graphene quantum dots provide a high density of active sites, increasing the adsorption capacity of the resins in these examples by 48%-56% compared to Comparative Example 2 (ordinary resin) and by 21.7%-28.3% compared to Comparative Example 6 (graphene quantum dot modification only); the temperature-responsive magnetic... The "high-temperature desorption" characteristics of the nanoparticles, combined with the rapid separation by the magnetic field, improve the regeneration efficiency of the resin in the example by 30.1%-33.6% compared to Comparative Example 2 and 18.9%-19.5% compared to Comparative Example 6. The regeneration time is also shortened to 1-1.5 hours, which is far better than the 2.5-4 hours of the comparative examples. The targeting effect of the specific recognition molecules further enhances the adsorption selectivity and avoids impurities occupying the active sites. This allows the resin in the example to maintain a high adsorption capacity after 20 cycles, while the regeneration efficiency of the resins in Comparative Examples 2, 6, and 7 decreases rapidly with the number of cycles.

[0106] The unit processing energy consumption of Examples 1-6 is only 7.8-8.5 kWh / m³. 3 The energy efficiency is significantly lower than that of Comparative Example 1 and Comparative Example 4, demonstrating the energy-saving advantages of heat pump synergistic concentration and waste heat recovery.

[0107] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater, characterized in that, S10: High-concentration urea wastewater is introduced into the pretreatment unit, and a composite synergistic treatment agent is added to form a dense precipitate containing non-volatile acid radicals under catalytic conditions; the composite synergistic treatment agent is composed of alkali metal salts, organic complexing agents and nano-catalytic particles; S20: The mixture obtained in step S10 is passed through an enhanced solid-liquid separation device to separate the precipitate and collect the supernatant; S30: The supernatant obtained in step S20 is introduced into the intelligent adsorption unit, and the graphene quantum dot-temperature responsive magnetic nanoparticle composite modified anion exchange resin is used to selectively adsorb residual non-volatile acid radicals. S40: The liquid processed in step S30 is introduced into the heat pump co-concentration unit to concentrate urea through low-temperature evaporation and waste heat recovery coupling technology. S50: The concentrate obtained in step S40 is introduced into a gradient crystallization unit, and high-purity urea crystals are precipitated under segmented temperature-pressure control, and solid urea is obtained by centrifugation. S60: The composite modified anion exchange resin after adsorption saturation is rapidly separated and recovered by an external magnetic field, and then recycled after temperature-controlled regeneration.

2. The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 1, characterized in that, In step S10, the mass fraction of each component of the composite synergistic treatment agent is as follows: alkali metal salt 0.5%-1%, organic complexing agent 0.1%-0.3%, and nano-catalytic particles 0.05%-0.1% of the total wastewater volume; In step S10, the alkali metal salt is sodium carbonate or potassium carbonate, the organic complexing agent is disodium ethylenediaminetetraacetate or sodium citrate, and the nanocatalytic particles are hydroxylated nano titanium dioxide with a particle size of 5-20 nm. The preparation method of the nanocatalytic particles is as follows: dissolve tetrabutyl titanate in anhydrous ethanol, add 5%-8% by mass of hydroxylating agent, stir and react at 50-60℃ for 2-3 hours, and obtain hydroxylated nano titanium dioxide after centrifugation, washing and vacuum drying.

3. The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 1, characterized in that, In step S10, the composite synergistic treatment agent is added in batches using a gradient addition method, in 3-4 batches, with each addition amount being 25%-33% of the total amount, and the interval between each addition being 8-15 minutes. In step S10, the reaction temperature of the pretreatment unit is controlled at 30-40℃, the stirring speed is 150-250 r / min, and the stirring time is 40-60 minutes; In step S20, the enhanced solid-liquid separation device is an "ultrasonic-assisted horizontal screw discharge sedimentation centrifuge," with the centrifuge speed set at 3000-4000 r / min and the feed flow rate controlled at 2-3 m³ / min. 3 / h, while simultaneously applying an ultrasonic field with a frequency of 20-40kHz and an ultrasonic power of 500-800W.

4. The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 1, characterized in that, In step S30, the preparation method of the "graphene quantum dot-temperature-responsive magnetic nanoparticle" composite modified anion exchange resin is described. Includes the following steps: M1: Preparation of temperature-responsive magnetic nanoparticles: 100 parts by weight of Fe3O4 nanoparticles were dispersed in 1150-2000 parts by weight of deionized water, and 100-167 parts by weight of N-vinylpyrrolidone and 200-333 parts by weight of N-isopropylacrylamide monomer were added; after purging with nitrogen for 30 minutes to remove oxygen, 3-10 parts by weight of cuprous bromide / bipyridine catalytic system were added, and surface-initiated atom transfer radical polymerization was carried out at 60-70℃ for 4-6 hours. Subsequently, 15-50 parts by weight of specific recognition molecules with active groups were added, and the reaction was carried out at room temperature for 8-12 hours under EDC / NHS catalysis. After washing and vacuum drying, temperature-responsive magnetic nanoparticles were obtained; the vacuum drying conditions were -0.09MPa, 60℃, and 4 hours. M2: Immerse 100 parts by weight of styrene-divinylbenzene copolymer matrix in 300-500 parts by weight of concentrated sulfuric acid solution with a mass fraction of 10%-15%, treat at 60-80℃ for 4-6 hours, then neutralize and wash with water until neutral to obtain sulfonated product. M3: Mix 100 parts by weight of the sulfonated product obtained in step M2 with 200-300 parts by weight of a 5%-8% ammonium chloride solution and react at 70-90°C for 3-5 hours to obtain the ammonated product. M4: Disperse 1 part by weight of graphene quantum dots in 99-200 parts by weight of deionized water, and ultrasonically disperse for 30 minutes at an ultrasonic power of 300W to obtain a graphene quantum dot dispersion with a mass fraction of 0.5%-1%. M5: Mix 100 parts by weight of the ammoniation product obtained in step M3 with 150-200 parts by weight of a quaternization reagent solution with a mass fraction of 2%-4%, add 5-8 parts by weight of the temperature-responsive magnetic nanoparticles prepared in step M1 and 30-50 parts by weight of graphene quantum dot dispersion; react at 50-70℃ for 2-4 hours, and obtain the composite modified anion exchange resin after washing and drying.

5. The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 1, characterized in that, In step S30, the particle size of the graphene quantum dots is 3-10 nm; the operating temperature of the intelligent adsorption unit is controlled at 30-35℃, and an external weak magnetic field with a magnetic field strength of 0.1-0.3T is applied during the adsorption process. In step S40, the operating temperature of the multi-effect evaporator is controlled at 60-80℃ and the vacuum degree is -0.08~-0.09MPa; the air source heat pump recovers the waste heat of the crystallization unit, and the temperature of the waste heat is 30-40℃; after the temperature is raised to 70-80℃, it provides a heating source for the evaporator.

6. The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 1, characterized in that, In step S50, the operating conditions of the gradient crystallization unit are as follows: First stage of concentrate cooling: temperature drops from 60℃ to 40℃ at a rate of 1-2℃ / min, pressure 0.01-0.02MPa; Second stage of crystal growth: temperature is maintained at 35-40℃, pressure is at atmospheric pressure, and the temperature is maintained for 2-3 hours; Third stage of crystal ripening: temperature drops from 35℃ to 30℃ at a rate of 0.5℃ / min, pressure is at atmospheric pressure. In step S60, the resin regeneration process is as follows: the saturated resin is separated and recovered by an external strong magnetic field with a strength of 0.5-0.8T, and transferred to a regeneration tank. The temperature is raised to 45-50℃, and a sodium hydroxide solution with a mass fraction of 3%-5% is added as an auxiliary regenerator. The flow rate of the regenerated liquid is 0.5-1m3 / h, and the regeneration time is 1-1.5 hours.

7. A system for the targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater, characterized in that, The method for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater as described in any one of claims 1-6 comprises the following units: U10: Pretreatment unit, used to add composite synergistic treatment agent and react under catalytic conditions. The unit is equipped with a temperature control device, a stirring device and a reagent dosing device. U20: Enhanced solid-liquid separation unit, used to separate pretreated precipitate from supernatant, including solid-liquid separation device and precipitate collection chamber; U30: Intelligent adsorption unit, used for selective adsorption of residual non-volatile acid radicals, includes an ion exchange column filled with "graphene quantum dot-temperature responsive magnetic nanoparticle" composite modified anion exchange resin, a magnetic field separation device and a resin regeneration device. U40: Heat pump co-concentration unit, used to concentrate urea solution, includes a multi-effect evaporator, an air source heat pump and a condensation recovery device; U50: Gradient crystallization unit, used to precipitate high-purity urea crystals, includes a crystallizer, a temperature-pressure gradient control system, a centrifugal separation device, and a crystal drying device; U60: Resin circulation unit, connecting the intelligent adsorption unit and the regeneration device, used to realize the automated circulation and dispensing of regenerated resin.

8. The system for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 7, characterized in that, In the pretreatment unit U10, the temperature control device has a temperature accuracy of ±0.5℃, the stirring device is a frequency conversion stirring device with a speed of 150-250r / min, and the reagent addition device is a batch addition device. The enhanced solid-liquid separation unit U20 is also equipped with an automatic cleaning device, which has a cleaning cycle of 8-10 hours, uses a 0.5%-1% mass fraction dilute hydrochloric acid solution, and takes 15-20 minutes to clean.

9. The system for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 7, characterized in that, In the intelligent adsorption unit U30, the magnetic field separation device can generate an adjustable magnetic field of 0.1-0.8T; it is also equipped with an online monitoring device for real-time detection of the conductivity and acid concentration of the effluent, and automatically switches to the standby exchange column when the acid concentration is ≥10mg / L; the regeneration device is equipped with a temperature control module with a temperature control range of 30-50℃, the regenerator is a sodium hydroxide solution with a mass fraction of 3%-5%, and the regeneration liquid flow rate is 0.5-1m3 / h; In the heat pump co-concentration unit U40, the cooling water temperature of the condensation recovery device is controlled at 15-25℃, the cooling water flow rate is 5-10m3 / h, and the recovered condensate is used as process makeup water with a COD ≤ 20mg / L.

10. The system for targeted removal of non-volatile acid radicals and urea recovery from high-concentration urea wastewater according to claim 7, characterized in that, In the gradient crystallization unit U50, the temperature control accuracy is ±0.5℃, and the pressure control accuracy is ±0.005MPa; the crystal drying device adopts low-temperature hot air drying, specifically drying at 55-60℃ for 2-3 hours, and the moisture content of the solid urea after drying is ≤0.5%; The resin circulation unit U60 achieves automated circulation and delivery of regenerated resin through a magnetic field conveying device.

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