Flue gas desulfurization wastewater defluorination system and method

Abstract

The invention relates to the field of sewage treatment, in particular to a flue gas desulfurization wastewater defluorination system and a flue gas desulfurization wastewater defluorination method. A wastewater collection unit, a chemical pretreatment unit, a solid-liquid separation unit, a clarification tank, a filtration and concentration unit, an evaporative crystallization unit, a sludge treatment unit, a fresh water tank and a fresh water recovery unit are integrated. The flue gas desulfurization wastewater defluorination method comprises the steps of aeration, dosing reaction, flocculation reaction, solid-liquid separation, sludge concentration, reverse osmosis treatment, concentration and evaporative crystallization. The system provided by the invention is stable in operation and high in wastewater treatment efficiency, and realizes complete recycling of purified water and inorganic salt after the flue gas desulfurization wastewater is treated.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a defluorination system and method for flue gas desulfurization wastewater. Background Technology

[0002] Thermal power plants generate a large amount of sulfur-containing byproducts during production. To reduce the sulfur dioxide content in flue gas emitted from burning fossil fuels such as coal and natural gas, flue gas desulfurization (FGD) technology is commonly used. In the commonly used limestone wet FGD process, the desulfurizing agent slurry fully contacts the flue gas to absorb sulfur dioxide. However, since fuels such as coal and natural gas themselves contain a certain amount of fluorine, fluorides enter the flue gas after combustion. During the FGD process, these fluorides are transferred to the FGD wastewater. Furthermore, limestone, used as the desulfurizing agent, also contains trace amounts of fluorine, which enters the FGD wastewater during the FGD reaction.

[0003] Fluoride-containing wastewater poses a significant hazard. Direct discharge of untreated fluoride-containing wastewater can have numerous negative impacts on the ecological environment. Firstly, it severely pollutes water bodies. High concentrations of fluoride ions alter the chemical properties of water, inhibiting the growth and reproduction of aquatic organisms, leading to an imbalance in the aquatic ecosystem. Many fish and plankton struggle to survive in high-fluoride environments. Secondly, when fluoride-containing wastewater seeps into the soil, it reacts with soil minerals, reducing soil fertility, affecting crop nutrient absorption, and threatening the sustainability of agricultural production. Long-term accumulation can also lead to fluorosis in humans through the food chain, harming human health and causing diseases such as dental fluorosis and skeletal fluorosis, affecting the normal function of bones and the nervous system. Therefore, defluorination of desulfurization wastewater is an urgent problem that thermal power plants need to solve.

[0004] The main components of flue gas desulfurization wastewater from thermal power plants include anions such as SO4²⁻, F⁻, Cl⁻, NO3⁻, NO2⁻, and S2O6²⁻, as well as cations such as Ca²⁺ and Mg²⁺, suspended solids such as gypsum particles, aluminum and iron oxides, and fly ash at concentrations of 6000~15000 mg / L, and trace amounts of heavy metal ions. Due to the complex composition and diverse forms of fluorides in flue gas desulfurization wastewater from thermal power plants, defluorination of this wastewater has always been a challenge in the industry.

[0005] Common methods for defluoridation of flue gas desulfurization wastewater include chemical precipitation, ion exchange, and adsorption. Chemical precipitation involves adding calcium chloride or calcium hydroxide to the wastewater, where calcium ions combine with fluoride ions to form calcium fluoride precipitate, reducing the fluoride content. However, due to the influence of sulfate ions in the wastewater, it consumes a large amount of defluoridation reagents, and some fluoride ions redissolve back into the water during the calcium fluoride precipitation process, resulting in unsatisfactory defluoridation. Ion exchange utilizes ion exchange resins with specific functional groups to selectively adsorb fluoride ions, achieving fluoride separation. However, this method is susceptible to interference from suspended solids in the wastewater, reducing the selective adsorption effect on fluoride ions. Furthermore, the resin is expensive, the regeneration process is cumbersome, and it requires large amounts of acid and alkali reagents. Moreover, it can only remove fluoride from the wastewater and cannot address other harmful substances, and the treated wastewater is difficult to reuse. The adsorption method uses adsorbents such as alumina and zeolite, which utilize their porous structure to adsorb fluoride ions onto the surface. However, due to limitations such as limited adsorption capacity and adsorbent regeneration costs, the treatment efficiency is not high when treating large amounts of flue gas desulfurization wastewater from thermal power plants.

[0006] In summary, existing flue gas desulfurization wastewater defluorination technologies have their own limitations, and there is an urgent need for a new, more efficient, stable, and economical method for treating fluoride-containing wastewater. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a flue gas desulfurization wastewater defluorination system and method.

[0008] The present invention adopts the following technical solution:

[0009] A flue gas desulfurization wastewater defluorination system includes a wastewater collection unit, a chemical pretreatment unit, a solid-liquid separation unit, and a freshwater recovery unit connected in sequence. The wastewater collection unit includes an aeration tank, an aeration fan mounted on the aeration tank, and a flue gas desulfurization wastewater collection port. The chemical pretreatment unit includes a primary reaction tank and a secondary reaction tank. The primary reaction tank is connected to the wastewater collection unit, and the secondary reaction tank is connected to the primary reaction tank. Both the primary and secondary reaction tanks are equipped with automatic dosing devices and stirring devices. The outlet of the secondary reaction tank is connected to the solid-liquid separation unit. The solid-liquid separation unit includes a solid-liquid separation device, a solids treatment device, and a liquid treatment device. The flue gas desulfurization wastewater treated by the chemical pretreatment unit is separated by the solids-liquid separation device, and the separated solids and liquids enter the solids treatment device and the liquid treatment device, respectively. The freshwater recovery unit is connected to the freshwater outlet of the liquid treatment device.

[0010] Furthermore, it also includes a filtration and concentration unit and an evaporation and crystallization unit.

[0011] Furthermore, the filtration and concentration unit is disposed between the liquid treatment device and the freshwater recovery unit, and the filtration and concentration unit includes an algae ultrafiltration system and a concentration device.

[0012] Furthermore, the seaweed ultrafiltration system includes a tubular ultrafiltration device, a nanofiltration device, a conventional reverse osmosis device, and a high-pressure spiral wound membrane reverse osmosis device arranged in sequence; the concentration device is connected to the freshwater outlet of the high-pressure spiral wound membrane reverse osmosis device, and the freshwater treated by the concentration device enters the freshwater recovery unit for collection.

[0013] Furthermore, the nanofiltration device has a molecular weight cutoff of 200-500 Da and a pressure of 1.0-2.5 MPa.

[0014] Furthermore, the concentration device includes a multi-stage concentration device.

[0015] Furthermore, the filtration and concentration unit also includes an inlet high-pressure pump, an inlet low-pressure pump, an inlet valve, an inlet pipe, a concentrate pipe, an outlet pipe, and a support frame. The inlet pipe is connected to the inlet low-pressure pump at its front end and is equipped with an inlet valve. A tubular ultrafiltration device, a nanofiltration device, a reverse osmosis device, and a high-pressure spiral wound membrane reverse osmosis device are sequentially fixed on the support frame. The inlet pipe is connected to a first branch pipe at its rear end, which is connected to several ultrafiltration inlet branch pipes. Each inlet branch pipe is connected to a tubular ultrafiltration membrane structure. The rear end of each tubular ultrafiltration membrane structure is connected to an ultrafiltration concentrate branch pipe and an ultrafiltration intermediate connecting pipe. The rear end of the ultrafiltration intermediate connecting pipe is connected to a second branch pipe, which is connected to several nanofiltration inlet branch pipes. The rear end of each nanofiltration inlet branch pipe is connected to a nanofiltration concentrate branch pipe and a nanofiltration intermediate connecting pipe. The rear end of the nanofiltration intermediate connecting pipe is connected to several conventional reverse osmosis structures. The rear end of each conventional reverse osmosis structure is connected to a conventional reverse osmosis concentrate pipe and a conventional reverse osmosis intermediate connecting pipe. A conventional reverse osmosis intermediate connecting pipe; the rear end of the conventional reverse osmosis intermediate connecting pipe is connected to the inlet of a high-pressure pump, and the outlet of the high-pressure pump is connected to a high-pressure spiral wound membrane reverse osmosis device via a pipe. The high-pressure spiral wound membrane reverse osmosis device includes a frame, a high-pressure spiral wound membrane reverse osmosis assembly mounted on the frame, and a pressure vessel. Multiple stages of spiral wound reverse osmosis membrane tubes are vertically arranged within the frame. Each stage includes a central tube and layers of polyamide material, cellulose triacetate, polysulfone material, and non-woven fabric sequentially laid from the inside out around the surface of the central tube. A permeate mesh is laid between the polyamide material layer and the central tube. An inlet cap is provided at the front end of the central tube, and an outlet cap is provided at the rear end. The inlet and outlet caps are used to fix the membrane layers and the central tube. The outlet cap connects to the outlet pipe and the concentrate pipe. The pressure vessel...

[0016] Furthermore, the evaporation crystallization unit includes a steam compressor, a plate heat exchanger, a crystallization heater, a screw conveyor, a deaerator, and a centrifugal dehydrator; the steam outlet of the steam compressor is connected to the hot-side inlet of the plate heat exchanger, the hot-side outlet of the plate heat exchanger is connected to the heating medium inlet of the crystallization heater, and the steam outlet of the steam compressor is connected to the heating medium outlet of the crystallization heater, forming a heating medium circulation loop; the discharge port of the crystallization heater is connected to the feed inlet of the screw conveyor, and the discharge port of the screw conveyor is connected to the feed inlet of the centrifugal dehydrator through a connecting pipeline equipped with a solid-liquid separation pump, and the centrifugal dehydrator... The liquid phase outlet of the dehydrator is connected to the return water port of the deoxygenation device to form a liquid phase reflux loop; the outlet of the deoxygenation device is connected to the cold side inlet of the plate heat exchanger and the raw material inlet of the crystallization heater respectively; a first flow control valve is provided on the pipeline between the deoxygenation device and the plate heat exchanger, and a second flow control valve is provided on the pipeline between the deoxygenation device and the crystallization heater; the cold side outlet of the plate heat exchanger is connected to the raw material replenishment port of the crystallization heater; the top steam outlet of the crystallization heater is connected to the cold side preheating inlet of the plate heat exchanger, and the cold side preheating outlet of the plate heat exchanger is connected to the auxiliary air inlet of the steam compressor.

[0017] The present invention also provides a method for defluorination of flue gas desulfurization wastewater, comprising the following steps:

[0018] 1) Aeration: Flue gas desulfurization wastewater flows into the aeration tank through the flue gas desulfurization wastewater collection port, and the aeration blower performs aeration until the dissolved oxygen in the flue gas desulfurization wastewater is maintained at 2-4 mg / L;

[0019] 2) Chemical dosing reaction: The aerated flue gas desulfurization wastewater is sent into the primary reaction tank. The automatic dosing device adds a compound defluorinating agent to the primary reaction tank and adds acid or alkali solution to adjust the pH value. The reaction time is 2-3 hours.

[0020] 3) Flocculation reaction: After the chemical reaction, the desulfurization wastewater flows into the secondary reaction tank, where flocculant is added, stirred, and allowed to settle;

[0021] 4) Solid-liquid separation: The desulfurization wastewater after flocculation reaction is passed through a solid-liquid separation unit for solid-liquid separation, and the supernatant is recovered.

[0022] Furthermore, it also includes the following steps: performing reverse osmosis treatment and evaporation crystallization on the liquid after solid-liquid separation; and performing sludge concentration treatment on the sludge after solid-liquid separation.

[0023] The reverse osmosis process involves sequentially passing the supernatant after solid-liquid separation through tubular ultrafiltration, nanofiltration, conventional reverse osmosis, and high-pressure spiral wound reverse osmosis before recovery.

[0024] The evaporation crystallization involves evaporating and crystallizing the concentrated water after treatment by the seaweed ultrafiltration system, and recovering the inorganic salts after crystallization.

[0025] The sludge thickening process involves a dewatering machine dewatering the sludge in the sludge tank to form thickened sludge, which is then transported out by a screw conveyor.

[0026] Furthermore, in step 2), the composite defluorinating agent includes an aluminum-calcium composite defluorinating agent and an iron-rare earth composite defluorinating agent. The composite defluorinating agent is added in stages. Specifically, the first stage involves adding the iron-rare earth composite defluorinating agent to the reaction tank and adjusting the pH value to 5-6 with hydrochloric acid, with a reaction time of 0.5-1 hour. The second stage involves adding sodium hydroxide to the primary reaction tank to adjust the pH value to 10-11, adding the aluminum-calcium composite defluorinating agent, stirring, and reacting for 1-1.5 hours.

[0027] Furthermore, the iron-rare earth composite defluorinating agent is an iron-lanthanum composite oxide, and the aluminum-calcium composite defluorinating agent is one of polyaluminum calcium chloride and aluminum calcium sulfate.

[0028] Furthermore, the mass ratio of the rare earth composite defluorinating agent to the aluminum-calcium composite defluorinating agent is 1:2 to 1:3.

[0029] Furthermore, the flocculant is polyacrylamide and sodium carbonate.

[0030] The flue gas desulfurization wastewater defluorination system provided by this invention has the following advantages:

[0031] (1) The present invention adopts a combined process of high efficiency solid-liquid separation unit and innovative seaweed ultrafiltration device to achieve efficient removal of fluoride in flue gas desulfurization wastewater and simultaneous removal of other harmful substances. The treated desulfurization wastewater meets the discharge requirements.

[0032] (2) The present invention adopts integrated high-efficiency membrane concentration technology, which provides a basis for inorganic salt recovery through multi-stage cyclic concentration.

[0033] (3) The present invention further evaporates and crystallizes the concentrated water after high-efficiency membrane concentration, and finally realizes the recovery of inorganic salts. Attached Figure Description

[0034] Figure 1 This is a flowchart of the main process flow of the present invention;

[0035] Figure 2 This is a flowchart of the evaporation and crystallization unit of the present invention.

[0036] Among them, 11-aeration tank, 21-primary reaction tank, 22-secondary reaction tank, 3-solid-liquid separation unit, 4-clarification tank, 5-algae ultrafiltration system, 51-tubular ultrafiltration device, 52-nanofiltration device, 53-conventional reverse osmosis device, 54-high pressure spiral wound membrane reverse osmosis device, 55-multi-stage thickening device, 6-freshwater tank, 7-sludge treatment unit, 71-dewatering machine, 73-screw conveyor, 8-evaporation crystallization unit, 81-deoxygenation device, 82-plate heat exchanger, 83-crystallization heater, 84-steam compressor, 85-screw conveyor, 86-centrifugal dewatering machine. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0038] It should be noted that all directional indications (such as up, down, left, right, front, back, front end, rear end, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship between the components in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0039] Furthermore, the use of terms such as "first," "second," etc., in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first" and "second" may explicitly or implicitly specify the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of those features.

[0040] Example

[0041] like Figure 1 The flue gas desulfurization wastewater defluorination system shown includes, in sequence, a wastewater collection unit, a chemical pretreatment unit, a solid-liquid separation unit, a clarifier 4, a filtration and concentration unit 5, an evaporation and crystallization unit 6, a sludge treatment unit 7, a freshwater tank 8, and a freshwater recovery unit 9, connected by pipelines. Figure 1 As shown, the specific structure is:

[0042] The wastewater collection unit includes multiple aeration tanks 11 connected in parallel. Each aeration tank is equipped with a flue gas desulfurization wastewater collection port. The desulfurization wastewater from the thermal power plant enters the aeration tank 11 through the flue gas desulfurization wastewater collection port. Each aeration tank is equipped with three aeration blowers, with two blowers in use and one in standby.

[0043] The chemical pretreatment unit has two reaction tanks: a primary reaction tank 21 and a secondary reaction tank 22. The primary reaction tank is connected to the aeration tank via a pipeline equipped with a booster pump. The primary reaction tank 21 is equipped with a bactericide dosing device, an alkali dosing device, a sodium carbonate dosing system, an acid dosing device, and a stirring device. The secondary reaction tank is equipped with an automatic dosing device and a stirring device. The outlet of the secondary reaction tank 22 is connected to the inlet of the solid-liquid separation unit 3, which is equipped with a plate filter press and a compressed air tank for solid-liquid separation of the settled wastewater. The sludge treatment unit 7 is equipped with a sludge tank 71, a dewatering machine 72, and a spiral welder. The conveyor 73 and sludge tank 71 are used to hold the sludge after solid-liquid separation. The dewatering machine 72 dewaters the sludge in the sludge tank and transports it to the dewatered sludge storage area by the screw conveyor 73. The liquid after solid-liquid separation enters the clarification tank 4 and then passes through the tubular ultrafiltration device 51, nanofiltration device 52, conventional reverse osmosis device 53 and high-pressure spiral wound membrane reverse osmosis device 54 of the seaweed ultrafiltration system in the filtration and concentration unit 5 for filtration. Then it passes through multiple multi-stage concentration devices 55 connected in series in the concentration unit 5. The freshwater tank 6 is connected to the freshwater outlet of the concentration device. The evaporation and crystallization unit 8 is connected to the concentrated water outlet of the concentration device.

[0044] like Figure 2The evaporation and crystallization unit 8 shown includes a steam compressor 84, a plate heat exchanger 82, a crystallization heater 83, a screw conveyor 85, a deaerator 81, and a centrifugal dehydrator 86. The steam outlet of the steam compressor 84 is connected to the hot-side inlet of the plate heat exchanger 82, and the hot-side outlet of the plate heat exchanger 82 is connected to the heating medium inlet of the crystallization heater 83. The steam outlet of the steam compressor 84 is also connected to the heating medium outlet of the crystallization heater 83, forming a heating medium circulation loop. The discharge port of the crystallization heater 83 is connected to the feed inlet of the screw conveyor 85, and the discharge port of the screw conveyor 85 is connected to the feed inlet of the centrifugal dehydrator via a connecting pipe equipped with a solid-liquid separation pump. The liquid phase outlet of the dehydrator 86 is connected to the return water port of the deoxygenation device 81 to form a liquid phase reflux loop. The outlet of the deoxygenation device 81 is connected to the cold side inlet of the plate heat exchanger 82 and the raw material inlet of the crystallization heater 83. A first flow control valve is installed on the pipeline between the deoxygenation device 81 and the plate heat exchanger 82, and a second flow control valve is installed on the pipeline between the deoxygenation device 81 and the crystallization heater 83. The cold side outlet of the plate heat exchanger 82 is connected to the raw material replenishment port of the crystallization heater 83. The top steam outlet of the crystallization heater 83 is connected to the cold side preheating inlet of the plate heat exchanger 82, and the cold side preheating outlet of the plate heat exchanger 82 is connected to the auxiliary air inlet of the steam compressor 84. The concentrated water is heated, crystallized, and concentrated by the steam compressor 84, plate heat exchanger 82, crystallization heater 83, and screw conveyor 85 in the evaporation crystallization unit 8 to form inorganic salts. The freshwater tank 6 contains treated water and is connected to the freshwater reuse unit 7 for the reuse of treated water.

[0045] The above-mentioned device of the present invention adopts a conventional device in the prior art.

[0046] In some embodiments of the present invention, the nanofiltration device has a molecular weight cutoff of 200-500 Da and a pressure of 1.0-2.5 MPa.

[0047] In some embodiments of the present invention, the high-pressure spiral wound membrane reverse osmosis device includes a frame, and a multi-stage spiral wound reverse osmosis membrane tube is vertically arranged inside the frame. The multi-stage spiral wound reverse osmosis membrane tube includes a central tube and a polyamide material layer, a triacetate cellulose layer, a polysulfone material layer, and a non-woven fabric layer sequentially laid from the inside to the outside around the surface of the central tube. A permeate mesh is laid between the polyamide material layer and the central tube. An inlet end cap is provided at the front end of the central tube, and an outlet end cap is provided at the rear end of the central tube. The inlet end cap and the outlet end cap are used to fix each membrane layer and the central tube. The outlet end cap connects the outlet pipe and the concentrate pipe.

[0048] The method for removing fluoride from flue gas desulfurization wastewater using the above-mentioned flue gas desulfurization wastewater defluorination system includes the following steps:

[0049] 1) Aeration: Flue gas desulfurization wastewater flows into the aeration tank through the flue gas desulfurization wastewater collection port, and the aeration blower performs aeration until the dissolved oxygen in the flue gas desulfurization wastewater is maintained at 2-4 mg / L;

[0050] 2) Chemical dosing reaction: The aerated flue gas desulfurization wastewater is sent into the primary reaction tank. The automatic dosing device adds a compound defluorinating agent to the primary reaction tank and adds acid or alkali solution to adjust the pH value. The reaction time is 2-3 hours.

[0051] 3) Flocculation reaction: After the chemical reaction, the desulfurization wastewater flows into the secondary reaction tank, where flocculant is added, stirred, and allowed to settle;

[0052] 4) Solid-liquid separation: The desulfurization wastewater after flocculation reaction is passed through the solid-liquid separation unit for solid-liquid separation, and the clear liquid in the upper layer flows into the clear water tank.

[0053] 5) Sludge thickening: The dewatering machine dewaters the sludge in the sludge tank to form thickened sludge, and the screw conveyor transports the thickened sludge out.

[0054] 6) Reverse osmosis treatment: The clarified liquid in the clarifier is sequentially treated by tubular ultrafiltration, nanofiltration, conventional reverse osmosis, and high-pressure spiral wound reverse osmosis.

[0055] 7) Evaporation and crystallization: The concentrated water after treatment by the seaweed ultrafiltration system is evaporated and crystallized to recover the inorganic salts after crystallization.

[0056] In some embodiments of the present invention, the composite defluorinating agent in step 2) includes an aluminum-calcium composite defluorinating agent and an iron-rare earth composite defluorinating agent. The composite defluorinating agent is added in stages. Specifically, the first stage involves adding the iron-rare earth composite defluorinating agent to the reaction tank and adjusting the pH value to 5-6 with hydrochloric acid, with a reaction time of 0.5-1 hour. The second stage involves adding sodium hydroxide to the primary reaction tank to adjust the pH value to 10-11, adding the aluminum-calcium composite defluorinating agent, stirring, and reacting for 1-1.5 hours.

[0057] In some embodiments of the present invention, the iron-rare earth composite defluorinating agent is an iron-lanthanum composite oxide, and the aluminum-calcium composite defluorinating agent is one of polyaluminum calcium chloride and aluminum calcium sulfate.

[0058] In some embodiments of the present invention, the mass ratio of the rare earth composite defluorinating agent to the aluminum-calcium composite defluorinating agent is 1:2 to 1:3.

[0059] The above method was used to treat desulfurization wastewater from a coal mine in Shandong Province. The fluoride ion content in the wastewater was detected using the GB7484-87 national standard method. The fluoride ion content before treatment was 19.2 mg / L, and the fluoride ion content after treatment was 1.8 mg / L. After treatment using the method of this invention, the fluoride ion removal rate reached 93%.

[0060] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A flue gas desulfurization wastewater defluorination system, characterized in that, It includes a wastewater collection unit, a chemical pretreatment unit, a solid-liquid separation unit, and a freshwater recovery unit connected in sequence; The wastewater collection unit includes an aeration tank, an aeration fan installed on the aeration tank, and a flue gas desulfurization wastewater collection port. The chemical pretreatment unit includes a primary reaction tank and a secondary reaction tank. The primary reaction tank is connected to the wastewater collection unit, and the secondary reaction tank is connected to the primary reaction tank. Both the primary and secondary reaction tanks are equipped with automatic dosing devices and stirring devices. The outlet of the secondary reaction tank is connected to the solid-liquid separation unit. The solid-liquid separation unit includes a solid-liquid separation device, a solid treatment device, and a liquid treatment device. The flue gas desulfurization wastewater treated by the chemical pretreatment unit is separated by the solid-liquid separation device, and the separated solids and liquids enter the solid treatment device and the liquid treatment device, respectively. The freshwater recovery unit is connected to the freshwater outlet of the liquid treatment device.

2. The flue gas desulfurization wastewater defluorination system according to claim 1, characterized in that, It also includes a filtration and concentration unit and an evaporation and crystallization unit; The filtration and concentration unit is located between the liquid treatment device and the freshwater recovery unit. The filtration and concentration unit includes an algae ultrafiltration system and a concentration device. The algae ultrafiltration system includes a tubular ultrafiltration device, a nanofiltration device, a conventional reverse osmosis device, and a high-pressure spiral wound membrane reverse osmosis device arranged in sequence. The concentration device is connected to the freshwater outlet of the high-pressure spiral wound membrane reverse osmosis device, and the freshwater treated by the concentration device enters the freshwater recovery unit for collection. The evaporation and crystallization unit is connected to the concentrate outlet of the high-pressure spiral wound membrane reverse osmosis device.

3. The flue gas desulfurization wastewater defluorination system according to claim 2, characterized in that, The concentration device includes a multi-stage concentration unit; the evaporation crystallization unit includes a steam compressor, a plate heat exchanger, a crystallization heater, a screw conveyor, a deoxygenation device, and a centrifugal dehydrator; the steam outlet of the steam compressor is connected to the hot-side inlet of the plate heat exchanger, the hot-side outlet of the plate heat exchanger is connected to the heating medium inlet of the crystallization heater, and the steam outlet of the steam compressor is connected to the heating medium outlet of the crystallization heater, forming a heating medium circulation loop; the discharge port of the crystallization heater is connected to the feed inlet of the screw conveyor, and the discharge port of the screw conveyor is connected to the feed inlet of the centrifugal dehydrator through a connecting pipeline equipped with a solid-liquid separation pump. The centrifugal dehydrator's liquid phase outlet is connected to the deoxygenation device's return water inlet to form a liquid phase reflux loop; the deoxygenation device's outlet is connected to the cold side inlet of the plate heat exchanger and the raw material inlet of the crystallization heater, respectively; a first flow control valve is installed on the pipeline between the deoxygenation device and the plate heat exchanger, and a second flow control valve is installed on the pipeline between the deoxygenation device and the crystallization heater; the plate heat exchanger's cold side outlet is connected to the crystallization heater's raw material replenishment port; the crystallization heater's top steam outlet is connected to the plate heat exchanger's cold side preheating inlet, and the plate heat exchanger's cold side preheating outlet is connected to the steam compressor's auxiliary air inlet.

4. The flue gas desulfurization wastewater defluorination system according to claim 2, characterized in that, The high-pressure spiral wound membrane reverse osmosis device includes a frame, a high-pressure spiral wound membrane reverse osmosis assembly mounted on the frame, and a pressure vessel. The high-pressure spiral wound membrane reverse osmosis assembly includes multi-stage spiral wound reverse osmosis membrane tubes. Each multi-stage spiral wound reverse osmosis membrane tube includes a central tube and layers of polyamide material, cellulose triacetate, polysulfone material, and non-woven fabric sequentially laid from the inside to the outside around the surface of the central tube. A permeate mesh is laid between the polyamide material layer and the central tube. An inlet end cap is provided at the front end of the central tube, and an outlet end cap is provided at the rear end of the central tube. The inlet and outlet end caps are used to fix each membrane layer and the central tube. The outlet end cap connects the outlet pipe and the concentrate pipe.

5. A method for defluorinating flue gas desulfurization wastewater, characterized in that, The flue gas desulfurization wastewater defluorination system according to any one of claims 1-4 includes the following steps: 1) Aeration: Flue gas desulfurization wastewater flows into the aeration tank through the flue gas desulfurization wastewater collection port, and the aeration blower performs aeration until the dissolved oxygen in the flue gas desulfurization wastewater is maintained at 2-4 mg / L; 2) Chemical dosing reaction: The aerated flue gas desulfurization wastewater is sent into the primary reaction tank. The automatic dosing device adds a compound defluorinating agent to the primary reaction tank and adds acid or alkali solution to adjust the pH value. The reaction time is 2-3 hours. 3) Flocculation reaction: After the chemical reaction, the desulfurization wastewater flows into the secondary reaction tank, where flocculant is added, stirred, and allowed to settle; 4) Solid-liquid separation: The desulfurization wastewater after flocculation reaction is passed through a solid-liquid separation unit for solid-liquid separation, and the supernatant is recovered.

6. The method for defluorination of flue gas desulfurization wastewater according to claim 5, characterized in that, It also includes the following steps: The liquid after solid-liquid separation undergoes reverse osmosis treatment and evaporation crystallization steps; the sludge after solid-liquid separation undergoes sludge concentration treatment. The reverse osmosis process involves sequentially passing the supernatant after solid-liquid separation through tubular ultrafiltration, nanofiltration, conventional reverse osmosis, and high-pressure spiral wound reverse osmosis before recovery. The evaporation crystallization involves evaporating and crystallizing the concentrated water after treatment by the seaweed ultrafiltration system, and recovering the inorganic salts after crystallization. The sludge thickening process involves a dewatering machine dewatering the sludge in the sludge tank to form thickened sludge, which is then transported out by a screw conveyor.

7. The method for defluorinating flue gas desulfurization wastewater according to claim 5, characterized in that, Step 2) involves adding a composite defluorinating agent, which includes an aluminum-calcium composite defluorinating agent and an iron-rare earth composite defluorinating agent. The composite defluorinating agent is added in stages. Specifically, the first stage involves adding the iron-rare earth composite defluorinating agent to the primary reaction tank and adjusting the pH value to 5-6 with hydrochloric acid, with a reaction time of 0.5-1 hour. The second stage involves adding sodium hydroxide to the primary reaction tank to adjust the pH value to 10-11 and adding the aluminum-calcium composite defluorinating agent, stirring, and reacting for 1-1.5 hours.

8. The method for defluorinating flue gas desulfurization wastewater according to claim 7, characterized in that, The iron-rare earth composite defluorinating agent is an iron-lanthanum composite oxide, and the aluminum-calcium composite defluorinating agent is one of polyaluminum calcium chloride and aluminum calcium sulfate.

9. A method for defluorinating flue gas desulfurization wastewater according to claim 8, characterized in that, The mass ratio of the rare earth composite defluorinating agent to the aluminum-calcium composite defluorinating agent is 1:2 to 1:

3.

10. A method for defluorinating flue gas desulfurization wastewater according to claim 5, characterized in that, The flocculant is polyacrylamide and sodium carbonate.

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