An ammonium chloride wastewater treatment device and method

Abstract

This invention discloses an ammonium chloride wastewater treatment device and method, belonging to the field of wastewater treatment technology. The treatment device includes a pretreatment unit, an MVR unit, and a membrane distillation unit connected in sequence. The MVR unit includes a heat exchanger, an evaporator, a crystallizer, and a separator connected in series. The inlet of the heat exchanger is connected to the outlet of the pretreatment unit, and the outlet of the separator is connected to the inlet of the membrane distillation unit. It also includes an MVR compressor. The inlet of the MVR compressor is connected to the secondary steam outlet of the evaporator, and the outlet of the MVR compressor is simultaneously connected to the heat source inlets of the evaporator and the heat exchanger. The ammonium chloride wastewater treatment device and method provided by this invention, through the cooperation of the pretreatment unit, MVR unit, membrane distillation unit, and MVR compressor, can achieve efficient utilization of waste heat, reduce the probability of scaling, and reduce system energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to an ammonium chloride wastewater treatment device and method. Background Technology

[0002] In existing technologies, ammonium chloride wastewater is a typical high-salinity wastewater from industrial production. The ammonium chloride it contains not only leads to eutrophication of water bodies but also wastes nitrogen and chlorine resources. Efficient treatment and resource recovery have become core industry demands. Existing treatment technologies mainly include evaporation crystallization, membrane separation, and simple coupling processes. However, these methods suffer from insufficient waste heat utilization, easy scaling at high concentrations, and high energy consumption. Summary of the Invention

[0003] The purpose of this invention is to provide an ammonium chloride wastewater treatment device and method to achieve efficient utilization of waste heat, reduce the probability of scaling, and reduce system energy consumption.

[0004] According to one aspect, to achieve the above objectives, embodiments of the present invention provide an ammonium chloride wastewater treatment device, comprising a pretreatment unit, an MVR unit, and a membrane distillation unit connected in sequence. The MVR unit includes a heat exchanger, an evaporator, a crystallizer, and a separator connected in sequence. The inlet of the heat exchanger is connected to the outlet of the pretreatment unit, and the outlet of the separator is connected to the inlet of the membrane distillation unit. The device also includes an MVR compressor, the inlet of which is connected to the secondary steam outlet of the evaporator, and the outlet of the MVR compressor is simultaneously connected to the heat source inlets of the evaporator and the heat exchanger.

[0005] In one possible implementation, the pretreatment unit includes a grid filter, a ceramic membrane filter, and a softener connected in series. The outlet of the softener is connected to the inlet of the heat exchanger. The grid filter has a mesh size of 0.5~1mm, and the ceramic membrane filter has a filtration accuracy of 0.1~1μm.

[0006] In one possible implementation, the membrane distillation unit includes a permeate collection tank and a membrane assembly connected between the permeate collection tank and the separator, and also includes a feed liquid circulation pump connected between the permeate collection tank and the membrane assembly.

[0007] In one possible implementation, the evaporator is disposed within the permeate collection tank to absorb waste heat from the permeate.

[0008] In one possible implementation, an in-situ membrane protection unit is also included, which comprises a pulse backflushing assembly and an online cleaning assembly connected to the shell-side inlet of the membrane module.

[0009] In one possible implementation, a condensation recovery unit is also included, which comprises a condensation heat exchanger and a phase change mixer connected sequentially between the outlet of the membrane module and the crystallizer.

[0010] In one possible implementation, an anti-corrosion auxiliary unit is also included, which includes a nitrogen storage tank and a dosing device. The nitrogen storage tank is connected to the shell side of the membrane module via a pressure reducing valve, and the reagent outlet of the dosing device is connected to the discharge port of the pretreatment unit.

[0011] The present invention also provides a method for treating ammonium chloride wastewater using a treatment device, comprising the following steps: S1. Pretreatment: Ammonium chloride wastewater enters the pretreatment unit, is filtered and softened, and then mixed with a corrosion inhibitor to obtain pretreated wastewater. S2, MVR treatment: Pretreated wastewater enters the MVR unit, undergoes heat exchange and heating, evaporation, crystallization, and separation to obtain ammonium chloride crystals and ammonium chloride concentrate with a mass fraction of 25~30wt%. S3. Membrane distillation purification: Ammonium chloride concentrate enters the membrane distillation unit and is purified by distillation to obtain ammonium chloride crystals and permeate.

[0012] In one possible implementation, in step S1, the amount of corrosion inhibitor added is 0.05~0.12% of the mass of the pretreated wastewater.

[0013] In one possible implementation, in step S3, the membrane flux of the membrane distillation unit is detected using a membrane flux sensor. When the membrane flux drops to a threshold, pulse backflushing is performed first. If the flux is less than 80% recovered, online cleaning is performed.

[0014] The significant technical advantage of this invention lies in the fact that the pretreatment unit removes suspended solids, colloids, and calcium and magnesium ions from wastewater, preventing scaling and clogging in the subsequent MVR evaporator and membrane distillation unit. The MVR unit is the core of the entire energy-consuming system. By using an MVR compressor to mechanically recompress the steam and then splitting the secondary steam for heating, efficient utilization of waste heat is achieved. No additional live steam is needed for feed preheating, reducing system energy consumption. Furthermore, the distribution ratio of the two steam streams can be adjusted according to the influent concentration.

[0015] Ammonium chloride wastewater is preheated to near its boiling point before entering the evaporator. This eliminates the temperature difference between the wastewater and the evaporation temperature, preventing localized supersaturation and scaling caused by cold feed. It also increases the effective heat transfer temperature difference in the evaporator, making its heat load more stable. This avoids the decrease in evaporation efficiency caused by feed temperature fluctuations in traditional processes, thereby improving the evaporator's processing capacity. Pre-precipitating some ammonium chloride crystals in the crystallizer reduces the feed concentration to the subsequent membrane distillation unit, lessening its load and increasing the membrane module's processing capacity. It also reduces the risk of salt crystal deposition on the membrane surface, extending the membrane's lifespan. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the 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.

[0017] Figure 1 This is a schematic diagram of the principle of the present invention.

[0018] In the diagram: 1. Pretreatment unit, 101. Grille filter, 102. Ceramic membrane filter, 103. Softener; 2. MVR unit, 201. Heat exchanger, 202. Evaporator, 203. Crystallizer, 204. Separator, 205. MVR compressor; 3. Membrane distillation unit, 301. Permeate collection tank, 302. Membrane module, 303. Feed liquid circulation pump; 4. Membrane in-situ protection unit, 401. Pulse backflushing assembly, 402. Online cleaning assembly; 5. Condensation recovery unit, 501. Condensation heat exchanger, 502. Phase change mixer; 6. Corrosion protection auxiliary unit, 601. Nitrogen storage tank, 602. Dosing device. Detailed Implementation The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0020] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0021] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0022] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship. In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple groups" refers to two or more groups (including two groups), and "multiple pieces" refers to two or more pieces (including two pieces).

[0023] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0024] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation", "connection", "linking", and "fixing" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components.

[0025] Please see Figure 1This invention illustrates an ammonium chloride wastewater treatment device according to an embodiment of the present invention, comprising a pretreatment unit 1, an MVR unit 2, and a membrane distillation unit 3 connected in sequence. The MVR unit 2 includes a heat exchanger 201, an evaporator 202, a crystallizer 203, and a separator 204 connected in sequence. The inlet of the heat exchanger 201 is connected to the outlet of the pretreatment unit 1, and the outlet of the separator 204 is connected to the inlet of the membrane distillation unit 3. The device also includes an MVR compressor 205, the inlet of which is connected to the secondary steam outlet of the evaporator 202, and the outlet of the MVR compressor 205 is simultaneously connected to the heat source inlets of the evaporator 202 and the heat exchanger 201.

[0026] Compared with existing technologies, this embodiment provides an ammonium chloride wastewater treatment device. The pretreatment unit 1 removes suspended solids, colloids, and calcium and magnesium ions from the wastewater, preventing scaling and clogging of the subsequent MVR evaporator 202 and membrane distillation unit 3. The MVR unit 2 is the core of the entire device's energy consumption. Through mechanical vapor recompression via the MVR compressor 205 and the diversion of secondary steam for heating, efficient utilization of waste heat is achieved. No additional live steam is needed for feed preheating, reducing system energy consumption. Furthermore, the distribution ratio of the two steam streams can be adjusted according to the influent concentration.

[0027] The heat exchanger 201 includes a primary heat exchanger, a secondary heat exchanger, and a tertiary heat exchanger connected in series, dividing the heat exchange process into three stages of gradient heat exchange. Compared with a single heat exchanger in a single stage, this results in more complete heat exchange, lower energy consumption, gentler temperature changes, stronger adaptability to operating conditions, and less scaling and corrosion. The heat pump unit provides a heat source for the heat exchanger 201 and the evaporator 202.

[0028] Ammonium chloride wastewater is preheated to near its boiling point before entering evaporator 202. This eliminates the temperature difference between the wastewater and the evaporation temperature, preventing localized supersaturation and scaling caused by cold feed. It also increases the effective heat transfer temperature difference in evaporator 202, making its heat load more stable. This avoids the decrease in evaporation efficiency caused by feed temperature fluctuations in traditional processes, thereby improving the processing capacity of evaporator 202. Pre-precipitating some ammonium chloride crystals in crystallizer 203 reduces the feed concentration to subsequent membrane distillation unit 3, lessening the load on membrane distillation unit 3, increasing the processing capacity of membrane module 302, reducing the risk of salt crystal deposition on the membrane surface, and extending membrane lifespan.

[0029] Please see Figure 1 In some embodiments, the pretreatment unit 1 includes a grid filter 101, a ceramic membrane filter 102 and a softener 103 connected in series. The outlet of the softener 103 is connected to the inlet of the heat exchanger 201. The grid gap of the grid filter 101 is 0.5~1mm and the filtration accuracy of the ceramic membrane filter 102 is 0.1~1μm.

[0030] If the screen opening of the bar filter 101 is <0.5mm, the bar will be quickly clogged by fine suspended solids, requiring frequent manual cleaning. If the screen opening is >1mm, it cannot effectively intercept fibrous impurities, which will entangle the membrane module of the membrane distillation unit 3, leading to a decrease in membrane flux. The bar filter 101 is used for primary coarse filtration of ammonium chloride wastewater to intercept large particles of organic residue, fibers, silt, and other solid impurities ≥0.5mm in the ammonium chloride wastewater, preventing blockage of subsequent pipes and the membrane module 302 of the membrane distillation unit 3. The ceramic membrane filter 102 is used for secondary fine filtration of ammonium chloride wastewater to remove colloids, fine suspended solids, and emulsified oil. The suspended solids in the treated effluent are ≤10mg / L, which can prevent clogging of the heating tubes of the MVR evaporator 202 and the membrane pores of the membrane module 302. A softener 103 is used to perform three-stage hardness removal on ammonium chloride wastewater. The softener is filled with 001×7 series strong acid cation exchange resin, which removes calcium and magnesium ions from the water through ion exchange. The total hardness after treatment is ≤5 mg / L, preventing carbonate and sulfate scaling on the heating surface and membrane surface at the source. This invention employs a three-stage progressive pretreatment process of coarse filtration, fine filtration, and softening, removing impurities from the wastewater step by step from large to small, significantly improving the cleanliness of ammonium chloride wastewater and creating stable influent conditions for subsequent MVR evaporation and membrane distillation.

[0031] Please see Figure 1 In some embodiments, the membrane distillation unit 3 includes a permeate collection tank 301 and a membrane assembly 302 connected between the permeate collection tank 301 and the separator 204, and also includes a feed liquid circulation pump 303 connected between the permeate collection tank 301 and the membrane assembly 302.

[0032] Membrane module 302 is the core separation component, employing a hydrophobically modified PVDF hollow fiber membrane. It utilizes the vapor pressure difference across the membrane to achieve selective separation of water and ammonium chloride. The 25-30 wt% ammonium chloride concentrate output from the separator of MVR unit 2 enters the tube side of membrane module 302 at high speed. Due to the temperature difference across the membrane (70-80°C for the tube side and room temperature for the shell side), water molecules permeate through the hydrophobic membrane as vapor, while ammonium chloride ions are retained. Feed liquid circulation pump 303 is a variable frequency centrifugal pump that maintains the circulation velocity of the concentrate in the tube side of membrane module 302 at 0.5-1 m / s, preventing concentration polarization and salt crystal deposition on the membrane surface. Permeate collection tank 301, made of polyethylene, collects the high-purity distilled water produced by membrane distillation. The concentrate after MVR pre-concentration undergoes deep purification, achieving an ammonium chloride rejection rate ≥99.9%, and the produced distilled water has an ammonium chloride content ≤100 mg / L, which can be directly reused in production. High-flow-rate circulation design can effectively suppress concentration polarization.

[0033] Please see Figure 1 In some embodiments, the evaporator 202 is disposed in the permeate collection tank 301 to absorb the waste heat of the permeate.

[0034] Evaporator 202 is immersed in the high-temperature permeate in permeate collection tank 301, which can absorb the waste heat of permeate. This allows permeate collection tank 301 to simultaneously perform the dual functions of permeate storage and waste heat recovery. The temperature inside the tank is stabilized at 70~80℃, which can reduce heat loss and improve heat utilization.

[0035] Please see Figure 1 In some embodiments, the membrane in-situ protection unit 4 is also included, which includes a pulse backflush assembly 401 and an online cleaning assembly 402 connected to the shell-side inlet of the membrane assembly 302.

[0036] The pulse backflushing assembly 401 includes a nitrogen pulse valve and a gas storage tank. It uses high-pressure nitrogen pulses (0.1~0.15 MPa) to backflush the membrane surface, removing loose salt crystal deposits and colloidal adsorption. The online cleaning assembly 402 includes a cleaning fluid storage tank, a transfer pump, and a three-way valve. When pulse backflushing fails to restore membrane flux, it automatically switches to cleaning mode, using a compounded cleaning fluid to circulate and flush the membrane module, removing stubborn fouling. This reduces membrane flux decay, extends membrane module cleaning cycles and service life, and lowers maintenance costs.

[0037] Please see Figure 1 In some embodiments, a condensation recovery unit 5 is also included. The condensation recovery unit 5 includes a condensation heat exchanger 501 and a phase change mixer 502 connected sequentially between the outlet of the membrane module 302 and the crystallizer 203. The condensation heat exchanger 501 is a shell-and-tube heat exchanger, with industrial cold water flowing through the shell side. The steam discharged from the membrane module 302 is condensed into liquid through the condensation heat exchanger 501, and then returned to the crystallizer 203 for recrystallization through the phase change mixer 502, which can improve the ammonium chloride recovery rate.

[0038] Please see Figure 1 In some embodiments, an anti-corrosion auxiliary unit 6 is also included. The anti-corrosion auxiliary unit 6 includes a nitrogen storage tank 601 and a dosing device 602. The nitrogen storage tank 601 is connected to the shell side of the membrane module 302 via a pressure reducing valve, and the reagent outlet of the dosing device 602 is connected to the discharge port of the pretreatment unit 1. The nitrogen storage tank 601 provides high-purity nitrogen, and the pressure reducing valve maintains a slight positive pressure of 0.02~0.05 MPa within the shell side of the membrane module 302. This prevents air from entering the oxide film material and the inner wall of the equipment, thus preventing oxidative degradation of the membrane material and oxygen corrosion of the inner wall of the equipment. The dosing device 602 adds corrosion inhibitors to the ammonium chloride wastewater, which can reduce the corrosion rate of the equipment.

[0039] The pipes used in this invention are made of 316L stainless steel, with an inner wall coated with 0.5mm thick polytetrafluoroethylene (PTFE). 316L stainless steel is a low-carbon austenitic stainless steel, possessing excellent resistance to chloride ion corrosion and is a commonly used material for chemical wastewater treatment. PTFE is an inert polymer material, resistant to strong acids, strong alkalis, and strong corrosion. Its smooth surface prevents scaling, and it can form a physical isolation layer, blocking contact between corrosive media and the metal. This reduces the pipe corrosion rate, extends pipe service life, and lowers maintenance costs.

[0040] The present invention also provides a method for treating ammonium chloride wastewater using a treatment device, comprising the following steps: S1. Pretreatment: Ammonium chloride wastewater enters pretreatment unit 1 at a feed rate of 1-3 m³ / h. It sequentially passes through a bar screen filter to remove large particles, a ceramic membrane filter to remove suspended solids (SS ≤ 10 mg / L), and an ion exchange softener to remove calcium and magnesium ions (total hardness ≤ 5 mg / L). Then, it is mixed with a corrosion inhibitor to obtain pretreated wastewater. The corrosion inhibitor is a mixture of benzotriazole and hexamethylenetetramine in a mass ratio of 1:(1-4), and the amount of inhibitor added is 0.05-0.12% of the pretreated wastewater mass. Benzotriazole can form a dense passivation film on the inner wall of the equipment, inhibiting anodic corrosion. Hexamethylenetetramine hydrolyzes to produce formaldehyde and ammonia, which can increase the pH value of the wastewater and inhibit cathodic corrosion. The anti-corrosion effect of the benzotriazole and hexamethylenetetramine mixture is 2-3 times that of a single corrosion inhibitor.

[0041] S2, MVR treatment: Pretreated wastewater enters MVR unit 2 and first enters heat exchanger 201, where it exchanges heat with high-temperature secondary steam output from MVR compressor 205, preheating it to 90-105℃, close to its evaporation boiling point. The preheated wastewater then enters evaporator 202, where it is heated to boiling point by high-temperature steam in the shell side of the heating tubes, forming a gas-liquid mixture. This mixture undergoes gas-liquid separation in the evaporation chamber at the top of evaporator 202, with the concentrated liquid discharged from the bottom and entering crystallizer 203. In crystallizer 203, the concentrated liquid reaches a supersaturated state, and ammonium chloride crystals begin to precipitate, forming a crystal slurry. The crystal slurry is sent to separator 204 for solid-liquid separation, yielding ammonium chloride crystals and a 25-30 wt% concentrated ammonium chloride solution. The ammonium chloride crystals can be collected directly.

[0042] The low-temperature, low-pressure secondary steam generated in the evaporation chamber of evaporator 202, with a temperature of 75~90℃ and a pressure of 0.08~0.1MPa, is sent to MVR compressor 205. MVR compressor 205 adiabatically compresses the secondary steam, raising its temperature to 105~120℃ and its pressure to 0.12~0.2MPa, making it a reusable high-temperature heat source. The compressed high-temperature steam is divided into two streams. One stream enters the heating tubes of evaporator 202, serving as a heat source to heat the pretreated wastewater, causing it to continuously evaporate. After releasing heat, the steam condenses into distilled water, which can be directly reused. The other stream enters the shell side of heat exchanger 201, exchanging heat with the room-temperature pretreated wastewater to preheat it to near its boiling point. After releasing heat, the steam also condenses into distilled water for reuse. The forced circulation evaporator and high-flow-rate design make it difficult for salt crystals to deposit on the inner wall of the heating tubes, extending the scaling cycle.

[0043] S3. Membrane distillation purification: Ammonium chloride concentrate enters the tube side of membrane distillation unit 3 at a flow rate of 0.5~1.0 m / s, a flow rate of 1~5 m³ / h, and a head of 15~20 m. The feed temperature of the ammonium chloride concentrate is 70~80℃. Nitrogen gas is purged through the shell side of membrane module 302 to maintain a positive pressure of 0.02~0.05 MPa. The ammonium chloride concentrate circulates in the tube side of membrane module 302. Under the action of the vapor pressure difference across the membrane, water molecules permeate through the hydrophobic membrane in the form of vapor into the shell side. After condensation, distilled water with an ammonium chloride content ≤100 mg / L is obtained. Ammonium chloride is retained by the membrane, and the concentrate is further concentrated to 35~40 wt%, yielding ammonium chloride crystals and permeate. The vapor is returned to crystallizer 203 for further crystallization after condensation and heat exchange.

[0044] The membrane flux of membrane module 302 is detected using a membrane flux sensor. A threshold for membrane flux is set at 90% of the initial flux. When the membrane flux drops to the threshold, the pulse backflush module 401 is triggered for 5 minutes of pulse backflush. If the flux recovers to less than 80%, the online cleaning module 402 performs online cleaning for 30 minutes. The backflush pressure of the pulse backflush module 401 is 0.1~0.15 MPa, and the frequency is 30 seconds / cycle. The cleaning solution in the online cleaning module 402 is a compound solution of 0.5wt%~2wt% sodium bicarbonate, 0.2wt%~0.8wt% citric acid, and 0.2wt%~1wt% sodium dodecylbenzenesulfonate. The circulation flow rate of the cleaning solution is 1.2~1.5 m / s.

[0045] The weakly alkaline environment of baking soda can saponify and decompose organic pollutants on the membrane surface, especially residual benzotriazole and hexamethylenetetramine corrosion inhibitors introduced during pretreatment. This prevents the formation of a dense hydrophobic barrier on the membrane surface by the organic layer, which would lead to a decrease in flux. The reaction with citric acid generates a large number of micro- and nano-sized carbon dioxide bubbles. The localized impact force generated when these bubbles burst on the membrane surface can loosen and peel off the dense ammonium chloride crystal shell adhering to the membrane surface (although ammonium chloride is easily soluble in water, it forms a hard crystalline layer at the membrane pores during concentration, which is difficult to remove with simple water rinsing). The weak alkalinity of baking soda will not damage the fluorocarbon coating of the hydrophobic membrane, avoiding the risks of membrane hydrophilization and leakage caused by strong alkaline cleaning agents such as NaOH.

[0046] Citric acid, as a weak organic acid, possesses a strong chelating ability for calcium and magnesium ions, dissolving trace amounts of residual calcium carbonate and magnesium carbonate scale after pretreatment and preventing scale blockage of membrane pores. The weakly acidic environment alters the surface charge of ammonium chloride crystals, causing the crystals to desorb from the membrane surface, while simultaneously preventing corrosion of membrane modules and stainless steel pipelines by strong acids such as hydrochloric acid and sulfuric acid. Sodium citrate, produced by the reaction of citric acid and baking soda, is an excellent dispersant, preventing the dissolved salts from redepositing on the membrane surface.

[0047] Sodium dodecylbenzenesulfonate, as an anionic surfactant, can significantly reduce the surface tension of the cleaning solution, allowing it to penetrate into the membrane pores and the gap between the crystalline layer and the membrane. This removes deep-seated contaminants at their source. It can emulsify and disperse organic contaminants and fine crystalline particles on the membrane surface, preventing them from re-aggregating and depositing during the cleaning process. It can also improve the wettability of the cleaning solution on the hydrophobic membrane surface, avoiding incomplete cleaning caused by water droplets forming on the membrane surface.

[0048] The greatest value of the compound solution lies in the synergistic effect of its three components. Baking soda and citric acid continuously react during the cleaning process to generate microbubbles. The bursting force of these bubbles creates a gentle physical scouring effect. Combined with a circulation flow rate of 1.2~1.5 m / s, this thoroughly removes chemically dissolved contaminant particles, resulting in higher efficiency than single-chemical cleaning. It can simultaneously address three types of problems: inorganic salt crystallization, calcium and magnesium scaling, and organic fouling, avoiding the cumbersome step-by-step cleaning process. A single online cleaning cycle can restore membrane flux to over 95% of its initial value. Sodium dodecylbenzenesulfonate makes the generated bubbles smaller and more stable, prolonging their residence time on the membrane surface, enhancing the physical scouring effect, and preventing large bubbles from impacting and damaging the membrane fibers.

[0049] Hydrophobic membranes used in membrane distillation are highly sensitive to strong oxidizing agents and strong acids or alkalis. Improper cleaning agents can damage the hydrophobic layer, causing membrane leakage and rendering the membrane unusable. The compound solution of this invention does not contain strong oxidizing agents such as sodium hypochlorite or hydrogen peroxide, and will not oxidize or damage the hydrophobic coating of the membrane. The pH value is controlled between 6 and 9, which is well within the tolerance range of the hydrophobic membrane, and will not cause membrane material aging with long-term use. Rinsing with clean water for 10 minutes after cleaning is sufficient to completely remove residues, and will not affect subsequent ammonium chloride concentration and crystallization processes.

[0050] The main components of the cleaning wastewater are sodium citrate, sodium chloride, and trace amounts of sodium dodecylbenzene sulfonate. It has a low COD content and can be directly discharged into pretreatment unit 1 for treatment, eliminating the need for additional wastewater treatment facilities. All three components are commonly used industrial chemicals, readily available, and inexpensive.

[0051] This invention increases the ammonium chloride recovery rate from the current 96.8% to 99.3%, resulting in more thorough resource recovery and directly improving economic benefits. The purity of ammonium chloride crystals increases from the current 98.3% to 99.6%, improving product quality. The energy consumption for treating each ton of ammonium chloride wastewater decreases from the current 188 kWh to 148 kWh, significantly reducing operating costs and demonstrating outstanding energy-saving benefits. The membrane flux decay rate decreases from the current 11.8% per month to 1.9% per month, improving membrane flux stability. The equipment corrosion rate decreases from the current 0.16 mm / a to 0.065 mm / a, enhancing equipment durability and extending its service life. The cleaning cycle is extended from the current 2.5 months to 7.5 months, significantly reducing maintenance frequency and saving maintenance costs and downtime losses. Energy consumption fluctuations under different concentrations decrease from the current 21.3% to 4.1%, demonstrating strong system adaptability, stable operation, and excellent resistance to influent fluctuations.

[0052] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of protection of the claims of the present invention.

Claims

1. An ammonium chloride wastewater treatment device, characterized in that, The device includes a pretreatment unit (1), an MVR unit (2), and a membrane distillation unit (3) connected in sequence. The MVR unit (2) includes a heat exchanger (201), an evaporator (202), a crystallizer (203), and a separator (204) connected in sequence. The inlet of the heat exchanger (201) is connected to the outlet of the pretreatment unit (1), and the outlet of the separator (204) is connected to the inlet of the membrane distillation unit (3). The device also includes an MVR compressor (205). The inlet of the MVR compressor (205) is connected to the secondary steam outlet of the evaporator (202), and the outlet of the MVR compressor (205) is connected to the heat source inlets of both the evaporator (202) and the heat exchanger (201).

2. The ammonium chloride wastewater treatment device according to claim 1, characterized in that, The pretreatment unit (1) includes a grid filter (101), a ceramic membrane filter (102) and a softener (103) connected in series. The outlet of the softener (103) is connected to the inlet of the heat exchanger (201). The grid gap of the grid filter (101) is 0.5~1mm, and the filtration accuracy of the ceramic membrane filter (102) is 0.1~1μm.

3. The ammonium chloride wastewater treatment device according to claim 1, characterized in that, The membrane distillation unit (3) includes a permeate collection tank (301) and a membrane module (302) connected between the permeate collection tank (301) and the separator (204), and also includes a feed liquid circulation pump (303) connected between the permeate collection tank (301) and the membrane module (302).

4. The ammonium chloride wastewater treatment device according to claim 3, characterized in that, The evaporator (202) is installed inside the permeate collection tank (301) to absorb the waste heat of the permeate.

5. The ammonium chloride wastewater treatment device according to claim 3, characterized in that, It also includes a membrane in-situ protection unit (4), which includes a pulse backflush assembly (401) and an online cleaning assembly (402) connected to the shell-side inlet of the membrane module (302).

6. The ammonium chloride wastewater treatment device according to claim 3, characterized in that, It also includes a condensation recovery unit (5), which includes a condensation heat exchanger (501) and a phase change mixer (502) connected in sequence between the outlet of the membrane module (302) and the crystallizer (203).

7. The ammonium chloride wastewater treatment device according to claim 3, characterized in that, It also includes an anti-corrosion auxiliary unit (6), which includes a nitrogen storage tank (601) and a dosing device (602). The nitrogen storage tank (601) is connected to the shell side of the membrane module (302) through a pressure reducing valve, and the reagent outlet of the dosing device (602) is connected to the discharge port of the pretreatment unit (1).

8. The treatment method of the ammonium chloride wastewater treatment device according to any one of claims 1-7, characterized in that, Includes the following steps: S1. Pretreatment: Ammonium chloride wastewater enters the pretreatment unit (1), and after filtration and softening, it is mixed with corrosion inhibitor to obtain pretreated wastewater. S2, MVR treatment: Pretreated wastewater enters the MVR unit (2), and after heat exchange, heating, evaporation, crystallization, and separation, ammonium chloride crystals and ammonium chloride concentrate with a mass fraction of 25~30wt% are obtained; S3. Membrane distillation purification: Ammonium chloride concentrate enters the membrane distillation unit (3) and is purified by distillation to obtain ammonium chloride crystals and permeate.

9. The treatment method of the ammonium chloride wastewater treatment device according to claim 8, characterized in that, In step S1, the amount of corrosion inhibitor added is 0.05~0.12% of the mass of the pretreated wastewater.

10. The treatment method of the ammonium chloride wastewater treatment device according to claim 8, characterized in that, In step S3, the membrane flux of the membrane distillation unit (3) is detected by a membrane flux sensor. When the membrane flux drops to the threshold, pulse backflushing is performed first. If the recovery is less than 80%, online cleaning is performed.

Images