A method and system for treating fluorine-containing wastewater

Abstract

The application discloses a fluorine-containing wastewater treatment method and system. The fluorine-containing wastewater treatment method comprises the following steps: adding a calcium-aluminum complex into fluorine-containing wastewater, wherein the calcium-aluminum complex is formed by mixing a calcium salt and an aluminum-containing agent; and performing solid-liquid separation on the wastewater after the calcium-aluminum complex is added. The fluorine-containing wastewater treatment system comprises an adding device, a solid-liquid separation device and a control module. The adding device is used for adding the calcium-aluminum complex into the fluorine-containing wastewater, the solid-liquid separation device is used for performing solid-liquid separation on the wastewater after the calcium-aluminum complex is added, and the control module is used for adjusting the adding amount of the calcium-aluminum complex according to the fluoride ion concentration of the influent. By adding the calcium-aluminum complex formed by mixing the calcium salt and the aluminum-containing agent, the fluoride ions form stable precipitates, the fluoride ion removal rate and the solid-liquid separation effect are improved, and the effluent water quality is improved.

Description

Technical Field

[0001] This application relates to the field of wastewater treatment and environmental protection technology, specifically to a method and system for treating fluoride-containing wastewater. Background Technology

[0002] Fluoride-containing wastewater treatment via sedimentation is a method that uses chemical reagents to cause fluoride ions to form insoluble precipitates, followed by solid-liquid separation to purify the wastewater. Fluoride-containing wastewater treatment systems typically include units such as a fluoride equalization tank, pH reaction tank, coagulation tank, flocculation tank, sedimentation tank, and clarification tank. In conventional treatment processes, soluble metal salts are often used to react with fluoride ions to form insoluble fluoride precipitates; for example, calcium ions react with fluoride ions to form calcium fluoride precipitate. Simultaneously, to improve sludge settling, polyaluminum chloride is usually added as a coagulant, and polyacrylamide as a flocculant to flocculate particles formed in the wastewater.

[0003] In the treatment of fluoride-containing wastewater from semiconductor manufacturing, the aforementioned conventional methods have some shortcomings. For example, the removal rate of fluoride ions needs improvement; when using calcium chloride, the resulting calcium fluoride colloid often lacks stability, and the settling efficiency may be low when the particle size is large; polyaluminum chloride may undergo changes in its hydrolysis form under conditions of large pH fluctuations in the wastewater, resulting in a decrease in the proportion of effective components, which in turn affects sludge settling performance and effluent clarity; furthermore, the uniformity of mixing the reagents with the wastewater and the adequacy of the reaction need to be improved. These factors may affect the stability of effluent indicators and reduce the reliability of effluent quality. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this application aims to provide a method and system for treating fluoride-containing wastewater, applicable to semiconductor manufacturing, electronic chemical production, wafer cleaning, etching, photovoltaic material processing, and other industrial applications that generate fluoride-containing wastewater. This application improves the stability of fluoride ion precipitation by adding a calcium-aluminum complex formed from a mixture of calcium salts and aluminum-containing agents to the fluoride-containing wastewater, enabling the fluoride removal effect of calcium ions and the coagulation effect of aluminum-containing hydroxyl species to occur synergistically in the upstream reaction stage.

[0005] To achieve the above and other related objectives, this application adopts the following technical solution: In a first aspect, this application provides a method for treating fluoride-containing wastewater, comprising adding a calcium-aluminum complex to the fluoride-containing wastewater, wherein the calcium-aluminum complex is formed by mixing a calcium salt with an aluminum-containing agent; and subsequently performing solid-liquid separation on the wastewater after adding the calcium-aluminum complex.

[0006] In a preferred embodiment, the fluoride-containing wastewater treatment method includes the following steps: S1. Fluoride-containing wastewater is first introduced into a fluoride-containing equalization tank for homogenization and equalization to buffer fluctuations in influent volume and water quality.

[0007] S2. The effluent from the fluoride conditioning tank enters the first pH reaction tank, where sodium hydroxide, reducing agent, and calcium-aluminum complex are added simultaneously.

[0008] S3. The effluent from the first pH reaction tank enters the second pH reaction tank. In the second pH reaction tank, the pH is adjusted to 6-8 by adding at least one of sodium hydroxide and sulfuric acid, and defluoridating agent is added as needed for supplemental defluorination.

[0009] S4. The effluent from the second pH reaction tank enters the coagulation tank, where polyaluminum chloride (PAC) is added for coagulation.

[0010] S5. The effluent from the coagulation tank enters the flocculation tank, where polyacrylamide (PAM) is added for flocculation.

[0011] S6. The effluent from the flocculation tank enters the sedimentation tank for solid-liquid separation.

[0012] S7. The supernatant from the sedimentation tank enters the clarification tank, and the sludge produced in the sedimentation tank enters the sludge thickening tank. Part of the effluent from the clarification tank can be fed into at least one of the neutralization tank, the fluoride-containing equalization tank, or the inorganic emergency tank, depending on the operational needs.

[0013] Preferably, the calcium salt is calcium chloride, and the aluminum-containing agent is PAC. Calcium chloride provides calcium ions, and PAC provides aluminum-containing hydroxyl species. By controlling the molar ratio of calcium ions to aluminum ions within the range of 0.5:1 to 2:1, calcium ions and the aluminum-containing hydroxyl species formed by the hydrolysis of PAC form calcium-aluminum-hydroxyl complexes or calcium-aluminum-hydroxyl complex precursors. This calcium-aluminum-hydroxyl complex system can be understood as a complexed cation system containing calcium, aluminum, and hydroxyl groups with a positive charge. Its actual hydrolysis and complexation forms can vary with agent concentration, water quality, and reaction conditions. This description does not limit the existence of only a single fixed structure in the water treatment system, but is used to characterize the synergistic complexation state formed between calcium ions and aluminum-containing hydroxyl species.

[0014] In this application, the formation of the calcium-aluminum complex mainly depends on the control of the molar ratio of calcium ions to aluminum ions. The main function of adding sodium hydroxide to the first pH reaction tank is to adjust the pH of the wastewater, rather than simply to remove fluoride from calcium chloride under neutral or weakly alkaline conditions. In the prior art, calcium chloride defluorination usually requires adjusting the pH to around 7 to 8, resulting in high alkali consumption. This application, through the synergistic effect of the calcium-aluminum complex, can control the pH in the first pH reaction tank at 3 to 6, preferably 3 to 5, thereby reducing alkali consumption while maintaining the defluorination and nucleation effects.

[0015] Preferably, in the first pH reaction tank, the flow rate of the calcium salt dosing pipe is monitored by a first flow meter, and the flow rate of the aluminum-containing agent dosing pipe is monitored by a second flow meter. The control module calculates the molar ratio of calcium ions to aluminum ions based on the flow signals from the first and second flow meters, combined with the concentration of the calcium salt solution and the aluminum content in the aluminum-containing agent. The control module automatically adjusts the frequencies of the calcium salt dosing pump and the aluminum-containing agent dosing pump based on the calculation result, maintaining the molar ratio of calcium ions to aluminum ions within the range of 0.25:1 to 2:1.

[0016] Preferably, an online fluoride ion concentration monitor is installed before the fluoride-containing wastewater enters the first pH reaction tank. The control module adjusts the dosage of calcium-aluminum complex and PAC based on the influent fluoride ion concentration detected by the online monitor. When the influent fluoride ion concentration increases, the control module increases the dosage of calcium-aluminum complex and correspondingly increases the dosage of PAC in the coagulation tank; when the influent fluoride ion concentration decreases, the control module reduces the corresponding dosage to avoid overdose. Through this gradient dosing control, the size of the generated calcium fluoride crystal nuclei is controlled between 50 nm and 80 nm.

[0017] The gradient dosing method used in this application refers to the separate addition of agents with different but synergistic effects at different treatment stages. In the first stage, a calcium-aluminum complex is added to the first pH reaction tank, focusing on the formation of stable crystal nuclei and initial precipitation. In the second stage, the pH is adjusted and a defluorinating agent is added to the second pH reaction tank, focusing on supplemental defluorination. In the third stage, PAC is added to the coagulation tank, focusing on further agglomerating the crystal nuclei and colloidal particles formed at the front end. Through this gradient dosing, the formation of calcium fluoride crystal nuclei, particle growth, coagulation and agglomeration, and flocculation sedimentation can occur gradually, avoiding the problems of excessive particle growth or difficulty in settling fine colloids caused by traditional single-point high-concentration dosing.

[0018] Preferably, the reducing agent added to the first pH reaction tank is at least one of sodium bisulfite, ferrous sulfate, or sodium thiosulfate. The reducing agent is used to treat any oxidizing substances that may be present in the wastewater, reducing the impact of oxidizing substances on the subsequent coagulation and flocculation effect.

[0019] Preferably, the stirring speed in the first pH reaction tank is between 300 rpm and 800 rpm to ensure thorough mixing of the calcium-aluminum complex, sodium hydroxide, reducing agent, and fluoride-containing wastewater. If the stirring speed is too low, the reagents will not disperse sufficiently; if the stirring speed is too high, the initial crystal nuclei and micro-floc structure may be damaged. Therefore, this stirring range balances mixing effectiveness and crystal nucleus stability.

[0020] Preferably, the calcium-aluminum complex is sprayed into the first pH reaction tank through multiple nozzles. The nozzles can be arranged in a ring or radial pattern, allowing the calcium-aluminum complex to enter the wastewater in a fountain-like or multi-point jet-like manner. A pressure gauge is installed on the outlet pipe of the calcium-aluminum complex dispensing pump, and the outlet pressure is controlled between 49 kPa and 98 kPa. When the pressure is below 49 kPa, the control module increases the pump frequency; when the pressure is above 98 kPa, the control module decreases the pump frequency. This pressure control ensures uniform diffusion of the calcium-aluminum complex within the first pH reaction tank, improving reagent utilization.

[0021] Preferably, a portion of the sludge-water mixture in the coagulation tank is recycled to the first pH reaction tank, with the recycling ratio controlled at 1:0.2 to 1:0.4. Here, the recycling ratio refers to a recycling sludge-water mixture flow rate of 0.2 to 0.4 parts when the main influent flow rate is 1 part. The recycled sludge-water mixture contains pre-formed calcium fluoride crystal nuclei, calcium-aluminum fluoride particles, PAC hydrolysis products, and primary flocs. After being recycled to the first pH reaction tank, this sludge-water mixture provides a heterogeneous nucleation surface for newly entering fluoride and calcium ions, allowing the defluorination reaction to occur more frequently on the surface of existing seed crystals or flocs, thereby reducing the random formation of fine colloids and improving subsequent sludge settling performance. Simultaneously, the recycled sludge-water mixture typically has a certain alkalinity after passing through the second pH reaction tank and the coagulation tank; recycling it to the first pH reaction tank increases the pH of the first pH reaction tank, reducing the amount of sodium hydroxide required.

[0022] Secondly, this application provides a fluoride-containing wastewater treatment system, including a dosing device, a solid-liquid separation device, and a control module. The dosing device is used to add a calcium-aluminum complex to the fluoride-containing wastewater, the calcium-aluminum complex being formed by mixing a calcium salt with an aluminum-containing agent. The solid-liquid separation device is used to perform solid-liquid separation on the wastewater after the addition of the calcium-aluminum complex. The control module is used to adjust the dosage of the calcium-aluminum complex according to the influent fluoride ion concentration.

[0023] Preferably, the dosing device includes a first pH reaction tank and a second pH reaction tank. The first pH reaction tank is equipped with a sodium hydroxide dosing port, a reducing agent dosing port, and a calcium-aluminum complex dosing port. The second pH reaction tank is equipped with a sodium hydroxide dosing port, a sulfuric acid dosing port, and a defluorinating agent dosing port. The calcium-aluminum complex dosing port is connected to a calcium salt dosing pipe and an aluminum-containing agent dosing pipe, respectively. A first flow meter is installed on the calcium salt dosing pipe, and a second flow meter is installed on the aluminum-containing agent dosing pipe. The control module adjusts the frequency of the calcium salt dosing pump and the aluminum-containing agent dosing pump according to the flow ratio of the first and second flow meters to maintain the molar ratio of calcium ions to aluminum ions between 0.5:1 and 2:1.

[0024] Preferably, the fluoride-containing wastewater treatment system further includes a coagulation tank, a flocculation tank, a sedimentation tank, and a clarification tank connected in sequence. The coagulation tank is connected to a polyaluminum chloride dosing pipe. The flocculation tank is connected to a polymeric flocculant dosing pipe, preferably polyacrylamide (PAM), but polyacrylic acid-based polymeric flocculants can also be used. To accommodate different on-site naming conventions, the polymeric flocculant dosing pipe can be called a polyacrylamide dosing pipe or a polyacrylic acid dosing pipe. The sludge outlet of the sedimentation tank is connected to a sludge thickening tank. The effluent pipe of the clarification tank is connected to a neutralization tank, a fluoride-containing equalization tank, and an inorganic emergency tank, respectively.

[0025] Preferably, the coagulation tank is connected to the first pH reaction tank via a return pipeline, and a return pump is installed on the return pipeline. The return pump is controlled by the control module to maintain the return ratio at 1:0.2 to 1:0.4. The calcium-aluminum complex dosing port is equipped with multiple nozzles within the first pH reaction tank, arranged in a ring or radial pattern. A pressure gauge is installed on the outlet pipeline of the calcium-aluminum complex dispensing pump, with the pressure set to 49 kPa to 98 kPa. A pH meter is installed within the first pH reaction tank, electrically connected to the control module. The control module adjusts the sodium hydroxide dosage based on the pH meter signal.

[0026] As described above, the fluoride-containing wastewater treatment method and system provided in this application have the following beneficial effects: This application replaces the traditional addition of calcium chloride alone with a calcium-aluminum complex, enabling the calcium ion fluoride removal reaction and the coagulation and adsorption of aluminum-containing hydroxyl species to occur synergistically in the front-end reaction stage, thereby improving the stability of fluoride ion precipitation.

[0027] This application controls the molar ratio of calcium ions to aluminum ions within the range of 0.5:1 to 2:1, enabling calcium salts and aluminum-containing agents to form a stable calcium-aluminum hydroxyl complex system, reducing the uncontrolled hydrolysis of PAC due to pH fluctuations, and improving the system's shock resistance.

[0028] This application can control the pH of the first pH reaction tank at 3 to 6, preferably 3 to 5. Compared with the traditional calcium salt defluorination method that requires raising the pH to 7 to 8, it can reduce the amount of sodium hydroxide added and reduce operating costs.

[0029] This application uses a gradient dosing of calcium-aluminum complex and PAC to trigger heterogeneous nucleation of pollutants and controls the size of calcium fluoride crystal nuclei to 50 nm to 80 nm, making the crystal nuclei an effective framework for subsequent coagulation and flocculation, thereby improving sludge settling efficiency.

[0030] This application uses multiple nozzles to spray and add calcium-aluminum complexes, and controls the nozzle pressure to make the agent more evenly distributed in the first pH reaction tank, reducing the problems of local overdose, incomplete reaction and low agent utilization caused by single outlet addition.

[0031] This application introduces existing seed crystals, primary flocs, and alkalinity by recirculating the sludge-water mixture from the coagulation tank to the first pH reaction tank. This not only promotes heterogeneous nucleation and accelerates the front-end defluorination reaction, but also reduces alkali consumption and improves subsequent sludge settling performance.

[0032] This application uses an online fluoride ion concentration monitor, a first flow meter, a second flow meter, a pH meter, and a pressure gauge to work in conjunction with a control module to achieve dynamic control of multiple parameters, including calcium-aluminum molar ratio, pH, spraying pressure, PAC dosage, and reflux ratio, thereby improving the system's adaptability to fluctuations in the quality of semiconductor fluoride-containing wastewater. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of a fluoride-containing wastewater treatment system according to one embodiment of this application.

[0034] Figure 2 This is a schematic diagram of the structure in which the calcium-aluminum complex of this application is dispersed and added in the first pH reaction tank.

[0035] Component Number Description 10. Calcium-aluminum complex dosing port; 11. Spray nozzle. Detailed Implementation

[0036] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present application.

[0037] The following specific examples illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification. This application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this application.

[0038] like Figure 1 As shown, the fluoride-containing wastewater treatment system of this application includes a fluoride-containing equalization tank, a first pH reaction tank, a second pH reaction tank, a coagulation tank, a flocculation tank, a sedimentation tank, and a clarification tank connected in sequence. The fluoride-containing equalization tank is used to receive fluoride-containing wastewater discharged during semiconductor manufacturing and to homogenize and equalize the wastewater. A stirring device or an aeration homogenization device can be installed in the fluoride-containing equalization tank to reduce instantaneous fluctuations in the influent fluoride ion concentration and pH.

[0039] An online fluoride ion concentration monitor is installed on the effluent pipeline of the fluoride equalization tank. This monitor is used to detect the fluoride ion concentration in real time before the effluent enters the first pH reaction tank and sends the detection signal to the control module. The control module can be a PLC controller, an AI control module with data learning and prediction functions, or a control system formed by combining a PLC and a host computer.

[0040] The first pH reaction tank is equipped with a sodium hydroxide dosing port, a reducing agent dosing port, and a calcium-aluminum complex dosing port. The sodium hydroxide dosing port is connected to a sodium hydroxide storage tank via a sodium hydroxide dosing pipe, which is equipped with a sodium hydroxide dosing pump. The reducing agent dosing port is connected to a reducing agent storage tank via a reducing agent dosing pipe, which is equipped with a reducing agent dosing pump. The calcium-aluminum complex dosing port is connected to both the calcium salt dosing pipe and the aluminum-containing agent dosing pipe.

[0041] In this embodiment, calcium chloride is used as the calcium salt, and PAC is used as the aluminum-containing agent. The calcium chloride solution is stored in a calcium salt storage tank, and the PAC solution is stored in an aluminum-containing agent storage tank. The calcium salt storage tank is connected to a calcium salt dosing pipe via a calcium salt dosing pump, and the aluminum-containing agent storage tank is connected to an aluminum-containing agent dosing pipe via an aluminum-containing agent dosing pump. A first flow meter is installed on the calcium salt dosing pipe, and a second flow meter is installed on the aluminum-containing agent dosing pipe. The calcium salt dosing pipe and the aluminum-containing agent dosing pipe converge into a mixing section before entering the first pH reaction tank. The mixing section can be a static mixer or a pipe section with a baffle structure or a turbulence structure. The calcium chloride solution and the PAC solution mix rapidly in the mixing section to form a calcium-aluminum complex or a calcium-aluminum complex precursor, which is then pumped into the first pH reaction tank via a discharge pump.

[0042] The control module receives detection signals from the first and second flow meters, and calculates the molar ratio of calcium ions to aluminum ions based on the concentration of the calcium chloride solution, the aluminum content in the PAC solution, and the actual flow rates of the two reagents. The control module maintains the molar ratio of calcium ions to aluminum ions within the range of 0.5:1 to 2:1 by adjusting the frequencies of the calcium salt dosing pump and the aluminum-containing reagent dosing pump. If the molar ratio is lower than 0.5:1, the frequency of the calcium salt dosing pump is increased or the frequency of the aluminum-containing reagent dosing pump is decreased. If the molar ratio is higher than 2:1, the frequency of the calcium salt dosing pump is decreased or the frequency of the aluminum-containing reagent dosing pump is increased.

[0043] Within this molar ratio range, calcium ions can form a calcium-aluminum-hydroxyl complex system with aluminum-containing hydroxyl species generated from PAC hydrolysis. This complex system enables a more uniform distribution of calcium ions in wastewater, reducing the risk of rapid coarsening of calcium fluoride particles caused by excessively high local calcium ion concentrations. Simultaneously, the aluminum-containing hydroxyl species can neutralize, adsorb, and bridge the charge of calcium fluoride microcrystals and colloidal particles, making it easier for fluoride-containing particles to form dense flocs in subsequent coagulation and flocculation stages.

[0044] A pH meter and a stirrer are installed in the first pH reaction tank. The pH meter is electrically connected to the control module, which adjusts the frequency of the sodium hydroxide dosing pump based on the pH meter feedback signal to maintain the pH in the first pH reaction tank between 3 and 6. Preferably, the pH in the first pH reaction tank is controlled between 3 and 5. This pH range satisfies the initial reaction and nucleation requirements of calcium-aluminum complexes with fluoride ions, while avoiding the increased operating costs caused by raising the pH to 7 to 8 with large amounts of alkali solution in traditional calcium salt defluorination processes.

[0045] The reducing agent in the first pH reaction tank can be at least one of sodium bisulfite, ferrous sulfate, or sodium thiosulfate. The reducing agent is used to consume any oxidizing components that may be present in the wastewater, preventing them from affecting the effectiveness of subsequent coagulants and flocculants. The stirring speed of the agitator in the first pH reaction tank is controlled between 300 and 800 revolutions per minute. When the influent water quality fluctuates significantly or when the reagents are first added, the stirring speed can be appropriately increased to promote rapid dispersion of the calcium-aluminum complex. Once the system is operating stably and crystal nuclei have formed stably, the stirring speed can be appropriately reduced to avoid excessive shearing of the initial crystal nuclei and micro-flocs.

[0046] like Figure 2 As shown, the calcium-aluminum complex dosing port 10 is connected to multiple nozzles 11 within the first pH reaction tank. These nozzles 11 can be mounted on an annular dosing pipe, which is arranged around the central area of ​​the first pH reaction tank. Alternatively, the nozzles 11 can be mounted on a radial dosing pipe, which extends outward from the central main pipe to form multiple branch pipes, each with one or more nozzles 11. The outlet direction of the nozzles 11 can be towards the center of the tank, diagonally downwards, or towards the main water flow direction within the tank, allowing the calcium-aluminum complex to enter the wastewater in a sprayed state.

[0047] A pressure gauge is installed on the outlet pipeline of the calcium-aluminum complex dispensing pump. The pressure gauge can be a mechanical gauge or an electronic pressure transmitter. The pressure gauge detects the pipeline pressure upstream of the nozzle and sends the pressure signal to the control module. The control module adjusts the pump frequency based on the pressure signal to maintain the nozzle outlet pressure between 49 kPa and 98 kPa. If the pressure is below 49 kPa, the spray range is insufficient, and the chemical diffusion effect decreases, so the control module increases the pump frequency. If the pressure is above 98 kPa, the spray impact is too strong or the pipeline load is too high, so the control module decreases the pump frequency. Through this method, the calcium-aluminum complex can enter the first pH reaction tank in a multi-point, dispersed, and stable state, thereby improving the contact efficiency between the chemical and the wastewater.

[0048] In the first pH reaction tank, after the calcium-aluminum complex comes into contact with the fluoride-containing wastewater, calcium ions and fluoride ions form the initial crystal nuclei of calcium fluoride, while aluminum-containing hydroxyl species participate in colloidal electrochemical regulation and microcrystalline surface adsorption. Because calcium and aluminum ions are mixed in a predetermined molar ratio before addition, and the reagent is dispersed and added through multiple nozzles, the localized rapid reaction caused by single-point addition is reduced, resulting in a more uniform calcium fluoride crystal nucleus formation process. The crystal nuclei formed in this stage are not solely aimed at rapid sedimentation, but rather serve as the foundation particles for subsequent coagulation and flocculation, allowing them to continue growing and aggregating in subsequent units.

[0049] The effluent from the first pH reaction tank enters the second pH reaction tank. The second pH reaction tank is equipped with sodium hydroxide, sulfuric acid, and defluoridant dosing ports. A pH meter is installed inside the second pH reaction tank and is electrically connected to the control module. The control module controls the sodium hydroxide or sulfuric acid dosing pump based on the pH reading from the second pH reaction tank, maintaining the pH between 6 and 8. If the pH is below 6, the sodium hydroxide dosage is increased. If the pH is above 8, the sulfuric acid dosage is increased or the sodium hydroxide dosage is decreased.

[0050] The defluoridating agent in the second pH reaction tank can be at least one of calcium chloride, PAC, or calcium-aluminum complex. When the fluoride ion concentration in the effluent from the first pH reaction tank remains high, the control module can increase the dosage of the defluoridating agent in the second pH reaction tank for supplementary defluoridation. When the defluoridation effect of the first pH reaction tank is stable, the dosage of the defluoridating agent in the second pH reaction tank can be reduced to maintain only the necessary water quality stabilization. As a supplementary reaction unit, the second pH reaction tank can improve the system's resistance to shocks when the influent fluoride ion concentration suddenly increases.

[0051] The effluent from the second pH reaction tank enters the coagulation tank. The coagulation tank is connected to a PAC dosing pipe, which is equipped with a PAC dosing pump. The control module adjusts the PAC dosage in the coagulation tank based on the influent fluoride ion concentration, the dosage of the first pH reaction tank, the dosage of the defluoridating agent in the second pH reaction tank, and the trend of effluent turbidity changes. In the coagulation tank, PAC further performs charge neutralization, adsorption bridging, and sweeping functions, causing the calcium fluoride crystal nuclei, calcium-aluminum fluoride-containing particles, and other suspended particles formed at the front end to further aggregate.

[0052] In this application, the addition of calcium-aluminum complex in the first pH reaction tank, the supplemental defluorination in the second pH reaction tank, and the addition of PAC in the coagulation tank constitute a gradient dosing process. Gradient dosing allows the treatment of fluoride-containing pollutants to proceed in stages. The first stage focuses on forming stable crystal nuclei, the second stage focuses on supplementing and removing residual fluoride ions, and the third stage focuses on promoting the aggregation and growth of microcrystals and colloidal particles. Compared to concentrating the reagent at a single location, gradient dosing can reduce abnormal particle growth caused by excessively high instantaneous reagent concentrations, and also reduce the difficulty in settling fine colloids caused by insufficient reagent concentrations.

[0053] By adjusting the dosage of calcium-aluminum complex and PAC in conjunction with an online fluoride ion concentration monitor and control module, the target size of calcium fluoride crystal nuclei can be controlled within the range of 50 nm to 80 nm. During system commissioning, samples can be taken from the outlet of the first or second pH reaction tank, and the crystal nuclei particle size distribution can be detected using a particle size analyzer. If the average crystal nuclei particle size is less than 50 nm, it indicates that the crystal nuclei are too fine, and subsequent sedimentation may be insufficient. In this case, the PAC dosage, reflux ratio, or calcium-aluminum molar ratio can be appropriately increased. If the average crystal nuclei particle size is greater than 80 nm and a front-end deposition trend is observed, the local calcium salt dosage can be reduced, the spray uniformity improved, or the stirring speed adjusted to make the crystal nuclei formation more uniform.

[0054] After stable operation, the control module can automatically adjust based on the correlation established during the commissioning phase between fluoride ion concentration, calcium-aluminum complex dosage, PAC dosage, and crystal nucleus size. When the influent fluoride ion concentration increases, the control module increases the calcium-aluminum complex dosage and correspondingly increases the PAC dosage in the coagulation tank. When the influent fluoride ion concentration decreases, the control module reduces the dosage of the above-mentioned agents to avoid excessive agents leading to increased sludge volume or abnormal crystal nucleus growth.

[0055] The effluent from the coagulation tank enters the flocculation tank. The flocculation tank is connected to a polymeric flocculant dosing pipe, with PAM being the preferred polymeric flocculant. The polymeric flocculant can also be a polyacrylic acid flocculant, or anionic, cationic, or nonionic polymeric flocculant can be selected depending on the wastewater properties. Relatively low-intensity stirring is used in the flocculation tank to allow the coagulated particles to form larger flocs under the action of polymeric chain bridging. The stirring intensity in the flocculation tank should be such that it does not break the already formed flocs.

[0056] The effluent from the flocculation tank enters the sedimentation tank. The sedimentation tank can be a horizontal flow sedimentation tank, a vertical flow sedimentation tank, an inclined tube sedimentation tank, or other sedimentation equipment capable of solid-liquid separation of fluoride-containing flocs. In the sedimentation tank, the fluoride-containing flocs settle under gravity to form sludge, and the supernatant flows out from the top of the sedimentation tank and enters the clarification tank. A sludge outlet is located at the bottom of the sedimentation tank, connected to a sludge sedimentation tank, which is used to receive and temporarily store the fluoride-containing sludge. Subsequent treatment options include sludge thickening, filtration, or compliant disposal, depending on on-site requirements.

[0057] The clarifier tank receives the supernatant from the sedimentation tank and further stabilizes the effluent quality. The effluent from the clarifier tank is connected to the neutralization tank, the fluoride equalization tank, and the inorganic emergency tank. If the effluent quality from the clarifier tank meets the requirements for subsequent treatment or discharge, it can enter the neutralization tank or subsequent discharge treatment unit. If online monitoring detects abnormalities in the effluent fluoride concentration, pH, or turbidity, the effluent from the clarifier tank can be temporarily stored in the inorganic emergency tank to prevent direct discharge of abnormal effluent. A portion of the effluent from the clarifier tank can also be returned to the fluoride equalization tank for circulation and adjustment under specific operating conditions.

[0058] The coagulation tank is connected to the first pH reaction tank via a return pipeline, on which a return pump is installed. The inlet of the return pipeline is preferably located in a position within the coagulation tank where the sludge and water are sufficiently mixed, ensuring that the return liquid contains appropriate amounts of calcium fluoride crystal nuclei, calcium-aluminum fluoride particles, and PAC hydrolysis products. The outlet of the return pipeline is preferably located at the inlet of the first pH reaction tank or in an area with strong stirring, allowing the return sludge-water mixture to quickly mix with the newly entering fluoride-containing wastewater and calcium-aluminum complexes.

[0059] The control module regulates the recirculation ratio based on the main influent flow rate and the recirculation pump flow rate, maintaining the recirculation ratio between 1:0.2 and 1:0.4. This recirculation ratio indicates that when the main influent flow rate is 1 part, the recirculated sludge-water mixture flow rate is 0.2 to 0.4 parts. If the recirculation ratio is lower than 1:0.2, the recirculated seed crystals and alkalinity replenishment are insufficient, resulting in a weaker improvement effect on heterogeneous nucleation and alkali consumption savings. If the recirculation ratio is higher than 1:0.4, the suspended solids concentration and hydraulic load in the first pH reaction tank are too high, which may lead to increased stirring energy consumption or sludge accumulation in the tank. Therefore, controlling the recirculation ratio within the above range can balance nucleation promotion, pH buffering, and stable system operation.

[0060] The functions of the recirculated sludge-water mixture mainly include three aspects. First, the recirculated sludge-water mixture contains pre-formed calcium fluoride crystal nuclei and primary flocs, which provide heterogeneous nucleation surfaces for newly entering fluoride and calcium ions, making it easier for calcium fluoride to deposit on the surface of existing particles, rather than randomly forming a large number of fine colloids in the water. Second, the recirculated sludge-water mixture usually has a certain alkalinity after passing through the second pH reaction tank and coagulation tank. Returning it to the first pH reaction tank can increase the pH of the first pH reaction tank, thereby reducing the amount of sodium hydroxide added. Third, the recirculated sludge-water mixture ensures that a certain amount of seed crystals and flocculants are continuously present in the upstream reaction tank, which is beneficial to improving the subsequent sludge settling performance and sludge-water separation stability.

[0061] The control module of this application can realize multi-parameter linkage control. The control module receives fluoride ion concentration signals from the online fluoride ion concentration monitor, reagent flow signals from the first and second flow meters, pH signals from the first and second pH reaction tanks, discharge pressure signals from the pressure gauge, and main inlet flow and return flow signals. Based on the above signals, the control module adjusts the operating frequency or start / stop status of the calcium salt dosing pump, aluminum-containing reagent dosing pump, sodium hydroxide dosing pump, sulfuric acid dosing pump, defluoridant dosing pump, PAC dosing pump, PAM dosing pump, discharge pump, and return pump.

[0062] In the specific control process, the control module first calculates the fluoride ion load entering the system per unit time based on the influent fluoride ion concentration and influent flow rate. Secondly, the control module determines the target dosage of the calcium-aluminum complex based on a preset dosage coefficient. Next, the control module determines the initial dosage flow rates of the two reagents based on the calcium salt solution concentration and the aluminum content in the PAC solution, and drives the calcium salt dosing pump and the aluminum-containing reagent dosing pump to operate. Subsequently, the control module adjusts the dosage of the two reagents based on the actual flow rates of the first and second flow meters, maintaining the molar ratio of calcium ions to aluminum ions within the range of 0.5:1 to 2:1.

[0063] The control module also controls the sodium hydroxide dosing pump based on the pH meter signal from the first pH reaction tank. If the pH of the first pH reaction tank is lower than the set lower limit, the sodium hydroxide dosage is increased. If the pH of the first pH reaction tank is higher than the set upper limit, the sodium hydroxide dosage is decreased. For preferred operating conditions, the set lower limit can be 3 and the set upper limit can be 5. For general operating conditions, the set lower limit can be 3 and the set upper limit can be 6. Through this control, the first pH reaction tank can stably perform defluorination and nucleation under acidic or weakly acidic conditions.

[0064] The control module controls the calcium-aluminum complex dispensing pump based on pressure gauge signals. If the pressure is below 49 kPa, the pump frequency is increased to ensure sufficient spray intensity. If the pressure is above 98 kPa, the pump frequency is decreased to stabilize pipeline pressure and spray intensity. Through closed-loop pressure control, the calcium-aluminum complex consistently enters the first pH reaction tank in a relatively uniform spray pattern.

[0065] The control module adjusts the PAC dosage in the coagulation tank based on the trends in influent fluoride ion concentration and effluent turbidity. If the influent fluoride ion concentration increases, the dosage of calcium aluminum complex and PAC is increased simultaneously to allow more fluoride ions to form crystal nuclei and complete coagulation at the downstream end. If the influent fluoride ion concentration decreases, the dosage of calcium aluminum complex and PAC is reduced to avoid overdose. If the turbidity of the supernatant in the sedimentation tank or the effluent from the clarifier increases, but the influent fluoride ion concentration does not increase significantly, the dosage of PAC or PAM can be appropriately increased to improve particle aggregation and sedimentation.

[0066] The control module controls the reflux pump based on the main influent flow rate. If the main influent flow rate is Q, the reflux pump flow rate is controlled between 0.2Q and 0.4Q. If the influent fluoride ion concentration suddenly increases or the front-end crystal nucleation effect is insufficient, the control module can increase the reflux ratio within the above range to return more seed crystals and primary flocs to the first pH reaction tank. If the suspended solids concentration in the first pH reaction tank is too high or the stirring load increases, the control module can decrease the reflux ratio within the above range.

[0067] When using a PLC as the control module, the above control can be achieved through preset thresholds, proportional control, and closed-loop feedback. When using an AI control module, historical operating data can be further collected, including influent fluoride concentration, influent pH, chemical dosage, effluent fluoride concentration, effluent turbidity, sludge settling status, and reflux ratio, and a predictive model can be established. The AI ​​control module can predict influent shocks in advance based on historical patterns, and pre-adjust the dosage of calcium-aluminum complex, PAC, and reflux ratio, thereby reducing control lag and improving effluent stability. Example 1

[0068] Fluorine-containing wastewater from a semiconductor production line first enters a fluoride equalization tank. The equalization tank is equipped with a stirring device to homogenize the fluoride ion concentration and pH of the wastewater. The effluent from the equalization tank is then pumped to a first pH reaction tank. An online fluoride ion concentration monitor is installed on the effluent pipeline to continuously monitor the fluoride ion concentration before it enters the first pH reaction tank.

[0069] Calcium chloride solution is stored in a calcium salt storage tank, and PAC solution is stored in an aluminum-containing reagent storage tank. The calcium chloride solution enters the calcium salt dosing line via a calcium salt dosing pump, and the PAC solution enters the aluminum-containing reagent dosing line via an aluminum-containing reagent dosing pump. A first flow meter is installed on the calcium salt dosing line, and a second flow meter is installed on the aluminum-containing reagent dosing line. The two reagents mix in a mixing section to form a calcium-aluminum complex, which is then pumped into multiple nozzles within the first pH reaction tank.

[0070] The control module controls the frequency of the calcium salt dosing pump and the aluminum-containing agent dosing pump based on the flow signals fed back from the first and second flow meters, maintaining the molar ratio of calcium ions to aluminum ions between 0.5:1 and 2:1. In this embodiment, the molar ratio of calcium ions to aluminum ions is preferably controlled at around 1:1. When the concentration of fluoride ions in the influent increases, the total dosage of the calcium-aluminum complex can be appropriately increased, but the molar ratio of calcium ions to aluminum ions should still not exceed the above range.

[0071] Sodium hydroxide, sodium bisulfite, and a calcium-aluminum complex are simultaneously added to the first pH reaction tank. Sodium hydroxide is used to adjust the pH, sodium bisulfite acts as a reducing agent, and the calcium-aluminum complex reacts with fluoride ions to form initial crystal nuclei. The control module adjusts the amount of sodium hydroxide added based on the pH meter signal in the first pH reaction tank to maintain the pH in the tank between 3 and 6, preferably between 3 and 5. The stirring speed in the first pH reaction tank is controlled between 300 and 800 revolutions per minute.

[0072] The calcium-aluminum complex is sprayed into the first pH reaction tank through multiple nozzles. These nozzles are arranged in a ring, covering the main reaction area within the tank. A pressure gauge on the outlet pipe of the dispensing pump detects the pressure before the nozzles, and the control module maintains the outlet pressure between 49 kPa and 98 kPa based on the gauge signal. This multi-nozzle spraying method allows the calcium-aluminum complex to mix rapidly and uniformly with the fluoride-containing wastewater, reducing the risk of overly rapid or incomplete reactions caused by single-point dosing.

[0073] The effluent from the first pH reaction tank enters the second pH reaction tank. In the second pH reaction tank, the pH is adjusted to 6-8 using sodium hydroxide or sulfuric acid, and defluoridating agent is added as needed. If the fluoride ion concentration in the effluent from the first pH reaction tank is still too high, calcium chloride, PAC, or calcium-aluminum complex is added to the second pH reaction tank for supplemental defluoridation. If the fluoride ion concentration in the effluent from the first pH reaction tank is consistently low, the amount of defluoridating agent added to the second pH reaction tank is reduced.

[0074] The effluent from the second pH reaction tank enters the coagulation tank. PAC is added to the coagulation tank to further aggregate the calcium fluoride crystal nuclei and fluoride-containing calcium-aluminum particles formed at the front end. The control module adjusts the dosage of calcium-aluminum complex and PAC according to the influent fluoride ion concentration, keeping the target crystal nucleus size between 50 nm and 80 nm. Crystal nuclei within this size range can enter the subsequent flocculation unit with the water flow and form relatively dense flocs under the action of PAM.

[0075] The effluent from the coagulation tank enters the flocculation tank. PAM is added to the flocculation tank, and slow stirring causes the coagulated particles to form larger flocs through polymer bridging. The effluent from the flocculation tank enters the sedimentation tank, where the flocs settle to form sludge. The supernatant enters the clarification tank. The sludge at the bottom of the sedimentation tank is discharged into the sludge sedimentation tank.

[0076] In this embodiment, a portion of the sludge-water mixture in the coagulation tank is returned to the first pH reaction tank via a reflux pump. The reflux ratio is controlled between 1:0.2 and 1:0.4, preferably between 1:0.3. The returned sludge-water mixture contains calcium fluoride nuclei, calcium-aluminum fluoride particles, PAC hydrolysis products, and primary flocs, which can provide a heterogeneous nucleation surface for the first pH reaction tank and bring some alkalinity back to the first pH reaction tank. Through this reflux, the formation of calcium fluoride nuclei in the first pH reaction tank is more stable, the consumption of sodium hydroxide is reduced, and the subsequent sludge settling effect is improved. Example 2

[0077] This embodiment illustrates the control method under fluctuating water quality conditions. The fluoride ion concentration in the effluent from the fluoride equalization tank is monitored in real time by an online fluoride ion concentration monitor. The control module calculates the fluoride ion load based on the influent fluoride ion concentration and influent flow rate. When the influent fluoride ion concentration increases, the control module increases the frequency of the calcium salt dosing pump and the aluminum-containing agent dosing pump to increase the dosage of calcium-aluminum complex, while simultaneously increasing the PAC dosage in the coagulation tank. The control module still controls the molar ratio of calcium ions to aluminum ions within the range of 0.5:1 to 2:1 based on feedback signals from the first and second flow meters.

[0078] When the influent fluoride concentration decreases, the control module reduces the dosage of calcium-aluminum complex and PAC to avoid excessive sludge production. If the turbidity of the clarifier effluent increases but the fluoride concentration does not increase significantly, the control module can appropriately increase the dosage of PAC or PAM to enhance coagulation and flocculation. If the pH of the first pH reaction tank remains low, the control module can increase the dosage of sodium hydroxide or, within permissible limits, increase the reflux ratio to allow the sludge-water mixture from the coagulation tank, which has a certain alkalinity, to be refluxed back to the first pH reaction tank.

[0079] Under shock conditions where the influent fluoride ion concentration suddenly increases, the control module can increase the frequency of the reflux pump within a reflux ratio range of 1:0.2 to 1:0.4, allowing more seed crystals and primary flocs to return to the first pH reaction tank. The refluxed seed crystals can shorten the induction period for calcium fluoride crystal nucleus formation, enabling newly introduced fluoride ions to be converted into coagulable particles more quickly, thereby reducing fluctuations in the effluent fluoride ion concentration. Comparative Example

[0080] The comparative example uses a traditional fluoride-containing wastewater treatment process. Wastewater enters the first pH reaction tank after passing through a fluoride equalization tank. Calcium chloride is added separately in the first pH reaction tank to adjust the pH to 7-8. Subsequently, the wastewater enters the second pH reaction tank, coagulation tank, flocculation tank, sedimentation tank, and clarification tank. PAC is added separately in the coagulation tank, and PAM is added in the flocculation tank. Both calcium chloride and PAC are added through a single outlet, and the sludge-water mixture is not returned from the coagulation tank to the first pH reaction tank.

[0081] In the comparative example, after calcium chloride enters the first pH reaction tank through a single outlet, the calcium ion concentration near the dosing port is high, making it easy for it to react rapidly with fluoride ions to form calcium fluoride particles with uneven particle size distribution. Some particles are prematurely deposited in the upstream reaction tank, while some fine colloids are difficult to settle in the sedimentation tank. PAC is only added in the coagulation tank and cannot form a synergistic effect with calcium ions in the upstream nucleation stage. When the influent pH and fluoride ion concentration fluctuate, the hydrolysis form of PAC is unstable, the coagulation effect decreases, and the turbidity of the supernatant in the sedimentation tank and the effluent indicators of the clarifier are prone to fluctuation.

[0082] In the embodiments of this application, calcium salts and aluminum-containing agents form a calcium-aluminum complex at a predetermined molar ratio, which is then dispersed and added in a first pH reaction tank through multiple nozzles. The calcium-aluminum complex enables calcium ions and aluminum-containing hydroxyl species to synergistically interact in the initial reaction stage, promoting the uniform formation of calcium fluoride crystal nuclei. The PAC (polyacrylamide) coagulation tank further coagulates the crystal nuclei and colloidal particles formed in the initial stage, while PAM (polyacrylamide precipitate) in the flocculation tank causes the particles to form larger flocs. After the sludge-water mixture from the coagulation tank is returned to the first pH reaction tank, it provides seed crystals and alkalinity, promoting heterogeneous nucleation and reducing alkali consumption.

[0083] Therefore, this application can improve the uniformity and utilization rate of reagent mixing, reduce local overdosing, improve the particle size distribution of calcium fluoride crystals, improve sludge settling effect, reduce effluent turbidity and fluoride ion concentration fluctuations, and achieve stable defluorination under the condition that the pH of the first pH reaction tank is 3 to 6, thereby reducing alkali consumption.

[0084] It should be noted that the calcium-aluminum complex in this application does not require complete pre-preparation in a separate reactor. The purpose of this application can be achieved as long as the calcium salt and the aluminum-containing agent are mixed before or during the introduction of the fluoride-containing wastewater, and a calcium-aluminum hydroxyl complex system with synergistic defluorination effect is formed within a molar ratio of calcium ions to aluminum ions of 0.5:1 to 2:1. Calcium chloride is preferred as the calcium salt, but other soluble calcium salts may be used if they can provide calcium ions without introducing undesirable impurities. PAC is preferred as the aluminum-containing agent, but other aluminum-containing coagulants may be used if they can form aluminum-containing hydroxyl species and participate in complexation and coagulation.

[0085] It should also be noted that the sedimentation tank can also be called a sedimentation vessel, and the clarification tank can also be called a clarification pool. The first pH reaction tank and the second pH reaction tank can be independent tanks or integrated compartmentalized reaction tanks. Multiple nozzles can be arranged in a ring, radial, or multiple rows depending on the shape of the tank. As long as the dispersed spraying and addition of calcium-aluminum complex, segmented pH control, gradient dosing, solid-liquid separation, and sludge return from the coagulation tank can be achieved, it falls under the technical concept disclosed in this application.

[0086] Under continuous operation, when using this application to treat fluoride-containing wastewater, the distribution of calcium fluoride crystal nuclei formed in the first pH reaction tank is more uniform, the floc formation rate in the subsequent coagulation and flocculation tanks is faster, the sludge interface in the sedimentation tank is clearer, and the turbidity fluctuation of the supernatant is smaller. Compared with the existing process of separately adding calcium chloride and PAC at single points, this application adds calcium-aluminum complex at the front end, so that calcium ions, aluminum-containing hydroxyl species, and fluoride ions form a synergistic effect in the early stage of the reaction, reducing the coexistence of coarse particles and fine colloids caused by local supersaturation.

[0087] When the influent fluoride ion concentration fluctuates, the control module can adjust the dosage of calcium-aluminum complex based on the detection results of the online fluoride ion concentration monitor, and simultaneously adjust the PAC dosage and reflux ratio, so that the system maintains good defluorination stability under water quality shock conditions. Since the sludge-water mixture from the coagulation tank is refluxed to the first pH reaction tank, the calcium fluoride crystal nuclei and primary flocs in the reflux liquid can act as heterogeneous nucleation sites, reducing the randomness of new calcium fluoride crystal formation and improving the efficiency of fluoride ion conversion into settleable particles.

[0088] This application also reduces alkali consumption. Traditional processes typically require raising the pH of the upstream reaction to 7-8 to promote calcium fluoride precipitation. This application, through the synergistic effect of calcium-aluminum complexes, reflux seed crystals, and gradient dosing, enables the first pH reaction tank to achieve stable nucleation and preliminary defluorination at pH 3-6, thus reducing the amount of sodium hydroxide required. Simultaneously, the downstream second pH reaction tank adjusts the pH to a range suitable for coagulation and sedimentation, avoiding the waste of reagents caused by a large-scale alkali adjustment at the upstream stage.

[0089] The above embodiments are merely illustrative of the principles and effects of this application and are not intended to limit this application. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of this application. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in this application should still be covered by the claims of this application.

Claims

1. A method for treating fluoride-containing wastewater, characterized in that, Includes the following steps: A calcium-aluminum complex is added to fluoride-containing wastewater, wherein the calcium-aluminum complex is formed by mixing calcium salts with aluminum-containing agents; Solid-liquid separation was performed on the wastewater after the addition of the calcium-aluminum complex.

2. The method for treating fluoride-containing wastewater according to claim 1, characterized in that, Specifically, the following steps are included: S1. Fluorine-containing wastewater is fed into a fluorine-containing equalization tank for homogenization and equalization. S2. The effluent from the fluoride conditioning tank is fed into the first pH reaction tank, and sodium hydroxide, reducing agent and the calcium-aluminum complex are added to the first pH reaction tank at the same time. S3. Pass the effluent from the first pH reaction tank into the second pH reaction tank; S4. Pass the effluent from the second pH reaction tank into the coagulation tank, and add polyaluminum chloride into the coagulation tank. S5. Pass the effluent from the coagulation tank into the flocculation tank, and add polyacrylamide into the flocculation tank for flocculation. S6. The effluent from the flocculation tank is fed into a sedimentation tank for solid-liquid separation; S7. Pass the supernatant in the sedimentation tank into the clarification tank.

3. The method for treating fluoride-containing wastewater according to claim 1 or 2, characterized in that, The calcium salt is calcium chloride, and the aluminum-containing agent is polyaluminum chloride; The calcium-aluminum complex is a calcium-aluminum hydroxyl complex with the chemical formula [CaAl(OH)]. n ] (m+) , where n is a positive integer and m is a positive integer; The pH in the first pH reaction tank is controlled between 3 and 6.

4. The method for treating fluoride-containing wastewater according to claim 2, characterized in that, In step S3, sodium hydroxide and / or sulfuric acid are added to the second pH reaction tank to adjust the pH to 6-8, and a defluorinating agent is added to supplement the defluorination process; the defluorinating agent is at least one of calcium chloride, polyaluminum chloride, or calcium-aluminum complex.

5. The method for treating fluoride-containing wastewater according to claim 2, characterized in that, In the first pH reaction tank, the flow rate of the calcium salt dosing pipe is monitored by the first flow meter, and the flow rate of the aluminum-containing agent dosing pipe is monitored by the second flow meter. The control module automatically adjusts the frequency of the calcium salt dosing pump and the aluminum-containing agent dosing pump according to the flow signals of the first and second flow meters, so that the molar ratio of calcium ions to aluminum ions is maintained at 0.5:1 to 2:

1.

6. The method for treating fluoride-containing wastewater according to claim 2, characterized in that, An online fluoride ion concentration monitor is installed before the fluoride-containing wastewater enters the first pH reaction tank. The dosage of the calcium-aluminum complex and the polyaluminum chloride is adjusted according to the fluoride ion concentration so that the size of the generated calcium fluoride crystal nuclei is controlled between 50 nm and 80 nm.

7. The method for treating fluoride-containing wastewater according to claim 2, characterized in that, The reducing agent is at least one of sodium bisulfite, ferrous sulfate, or sodium thiosulfate. The mixture was stirred at a speed of 300 rpm to 800 rpm in the first pH reaction tank. The calcium-aluminum complex is applied by spraying through multiple nozzles, with the nozzle pressure ranging from 49 kPa to 98 kPa. A portion of the mud-water mixture in the coagulation tank is returned to the first pH reaction tank, with the return ratio controlled at 1:0.2 to 1:0.

4. The sludge produced in the sedimentation tank is discharged into the sludge thickening tank. A portion of the effluent from the clarification tank is fed into at least one of a neutralization tank, a fluoride-containing conditioning tank, or an inorganic emergency tank.

8. A fluoride-containing wastewater treatment system, characterized in that, include: A dosing device for adding calcium-aluminum complexes to fluoride-containing wastewater, wherein the calcium-aluminum complexes are formed by mixing calcium salts with aluminum-containing agents; A solid-liquid separation device is used for solid-liquid separation of wastewater after the addition of the calcium-aluminum complex; The control module is used to adjust the dosage of the calcium-aluminum complex according to the fluoride ion concentration in the influent.

9. The fluoride-containing wastewater treatment system according to claim 8, characterized in that, The dosing device includes a first pH reaction tank and a second pH reaction tank. The first pH reaction tank is provided with a sodium hydroxide dosing port, a reducing agent dosing port and a calcium-aluminum complex dosing port. The second pH reaction tank is equipped with a sodium hydroxide dosing port, a sulfuric acid dosing port, and a defluorinating agent dosing port; The calcium-aluminum complex dosing port is connected to both the calcium salt dosing tube and the aluminum-containing agent dosing tube. The calcium salt dosing tube is equipped with a first flow meter, and the aluminum-containing agent dosing tube is equipped with a second flow meter; The control module adjusts the frequency of the calcium salt dosing pump and the aluminum-containing agent dosing pump according to the flow ratio of the first flow meter and the second flow meter, so that the molar ratio of calcium ions to aluminum ions is maintained at 0.5:1 to 2:

1.

10. The fluoride-containing wastewater treatment system according to claim 9, characterized in that, It also includes a coagulation tank, a flocculation tank, a sedimentation tank, and a clarification tank connected in sequence; The coagulation tank is connected to a polyaluminum chloride dosing pipe; The flocculation tank is connected to a polyacrylamide dosing pipe; The sludge outlet of the sedimentation tank is connected to the sludge thickening tank. The effluent pipe of the clarification tank is connected to the neutralization tank, the fluoride-containing conditioning tank and the inorganic emergency tank, respectively. The coagulation tank is connected to the first pH reaction tank through a return pipeline, and a return pump is installed on the return pipeline, with the return ratio controlled at 1:0.2 to 1:0.

4. The calcium-aluminum complex dosing port is provided with multiple nozzles in the first pH reaction tank, and the multiple nozzles are arranged in a ring or radial pattern in the first pH reaction tank. A pressure gauge is installed on the outlet pipeline of the calcium-aluminum complex dispensing pump, with the pressure set to 49 kPa to 98 kPa. The first pH reaction tank is equipped with a pH meter, which is electrically connected to the control module. The control module adjusts the amount of sodium hydroxide added based on the pH meter signal.

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