Multistage treatment process for fluorine-containing wastewater

Abstract

The invention relates to the technical field of industrial wastewater treatment, in particular to a multistage treatment process for fluorine-containing wastewater. In order to solve the problems that in the prior art, a fluorine removal agent is low in reaction efficiency, fine sludge is difficult to settle, organic complexes cannot be synchronously degraded, and membrane pollution is serious, a liquid-phase multifunctional catalyst containing a polysilicic acid framework is prepared by adopting a high-shear in-situ copolymerization technology, and a multi-stage gradient reaction and a tubular ceramic membrane forced separation technology are coupled; according to the process, the Fenton-like catalytic activity and the rigid netting mechanism of the medicament are utilized, so that the deep synergistic removal of fluorine, COD and total phosphorus is realized, and the membrane flux attenuation rate and the sludge water content are remarkably reduced; the method has the advantages of short flow, stable and standard effluent and the like.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment technology, specifically a multi-stage treatment process for fluoride-containing wastewater. Background Technology

[0002] With the explosive growth of photovoltaic new energy, semiconductor integrated circuits, and fluorine fine chemical industries, the discharge of fluoride-containing wastewater is increasing dramatically. Hydrofluoric acid, ammonium fluoride, and fluoride-containing mixed acids are widely used as etching and cleaning agents in the texturing and etching processes of photovoltaic cells and the cleaning and polishing processes of semiconductor wafers. This results in wastewater with not only high concentrations of fluoride ions but also often high concentrations of organic pollutants, such as surfactants, alcohol solvents, and organic complexing agents. The complex water quality characteristics of the coexistence of fluorides and organic carbon greatly increase the difficulty of treatment.

[0003] Currently, existing fluoride-containing wastewater treatment technologies mainly rely on calcium salt precipitation, which involves adding lime or calcium chloride to generate calcium fluoride precipitate, supplemented by aluminum salt coagulation for deeper control. However, in practical engineering applications, this traditional process faces several bottlenecks. First, limited by the physical constraints of the calcium fluoride solubility product, the calcium method alone cannot stably reduce the effluent fluoride concentration to the stringent discharge standard of below 1 mg / L. Second, the large amount of organic complexing agents in the wastewater forms stable coordination compounds with fluoride or metal ions, inhibiting the precipitation reaction and resulting in incomplete fluoride removal. More seriously, organic matter coating the crystal nucleus surface hinders crystal growth, and the resulting sludge particles are small, lightweight, and highly hydrophilic, making them extremely difficult to settle in traditional gravity sedimentation tanks. This easily leads to excessive fluoride levels in the effluent suspended solids, the so-called "fluoride runoff."

[0004] Furthermore, existing advanced treatment processes, such as resin adsorption or fixed-bed adsorption, suffer from frequent regeneration, high requirements for influent suspended solids (SS), and susceptibility to organic poisoning and degradation. For membrane separation technology, traditional coagulated flocs are loose and sticky, easily causing severe membrane pore blockage and flux decline. Therefore, there is an urgent need to develop a synergistic treatment process that can simultaneously achieve advanced defluorination and organic matter degradation, significantly improve sludge settling and dewatering performance, and is compatible with high-efficiency membrane separation systems.

[0005] Therefore, a multi-stage treatment process for fluoride-containing wastewater is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-stage treatment process for fluoride-containing wastewater. A liquid-phase multifunctional catalyst containing a polysilicic acid framework, i.e., a fluoride removal catalyst, is prepared using high-shear in-situ copolymerization technology and coupled with multi-stage gradient reaction and tubular ceramic membrane forced separation technology. The process utilizes the Fenton-like catalytic activity and rigid mesh trapping mechanism of the reagent to achieve deep and synergistic removal of fluoride, COD, and total phosphorus, significantly reducing membrane flux decay rate and sludge moisture content. It has advantages such as a short process flow and stable effluent compliance.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a multi-stage treatment process for fluoride-containing wastewater, which is as follows: the raw fluoride-containing organic wastewater is pumped into a primary oxidation reactor, and ferrous sulfate solution is pumped in simultaneously to maintain the acidic environment of the system; then, Fenton oxidation is performed to break down the complex and obtain primary oxidized water. The primary oxidation treatment water is pumped into the secondary mineralization reactor, the system environment is adjusted to alkaline, and the mineralization reaction is carried out to obtain secondary mineralized water; The secondary mineralized water overflows into the tertiary advanced treatment vessel, where a defluorination catalyst is pumped in to obtain tertiary advanced treated water. The tertiary advanced treated water is then connected to a tubular ceramic membrane circulation system for cross-flow filtration, concentration, and reflux to obtain compliant water. The elements that make up the defluorination catalyst include iron, aluminum and silicon.

[0008] Preferably, the defluorination catalyst is obtained by injecting a metal salt modified solution into a polysilicic acid stock solution to obtain a precursor mixture; sodium bicarbonate powder is added to the precursor mixture, the basicity of the solution is adjusted, and then the temperature is raised, stirred and matured, and cooled to obtain the catalyst.

[0009] Preferably, the metal salt modified solution is obtained by dissolving polyaluminum chloride, ferrous sulfate heptahydrate, and titanium oxysulfate in deionized water; the polysilicic acid stock solution is obtained by adding sulfuric acid solution dropwise to diluted sodium silicate solution to adjust the system to a strongly acidic environment and stirring the reaction.

[0010] Preferably, the three-stage deep treatment vessel includes a rapid mixing zone and a slow flocculation zone; the residence time in the rapid mixing zone is 1.5-2.5 min; the residence time in the slow flocculation zone is 13.5-18 min; the hydraulic residence time in the primary oxidation reactor is 42-75 min; the hydraulic residence time in the secondary mineralization reactor is 20-40 min; the cross-flow velocity on the membrane surface in the membrane tube of the membrane separation is 3.5-4.5 m / s, and the operating pressure is 0.12-0.18 MPa.

[0011] Taking a cross-flow velocity of 4 m / s as an example, the high velocity means that the fluid is flushed at high speed inside the membrane tube. The high-speed flow generates strong surface shear force, which continuously scrapes away CaF2 particles and colloids that attempt to deposit on the surface of the membrane pores, preventing the rapid formation of a dense filter cake layer. This is the fundamental reason why the ceramic membrane can operate continuously for 24 hours without clogging. At the same time, a separation mode of concentration and recirculation is set. The recirculation ratio of 10:1 means that for every 11 parts of sludge entering the membrane tube, only 1 part becomes clear permeate and flows out through the membrane pores; the remaining 10 parts do not permeate the membrane, but instead act as a sweeper, carrying away the high-concentration sludge that has been flushed down, flowing out from the outlet and returning to the front-end circulation tank, i.e., the secondary mineralization zone. This ensures that there is always enough water volume and flow rate inside the membrane tube to maintain the flushing effect.

[0012] Cross-flow filtration breaks the high-concentration boundary layer on the membrane surface through turbulent agitation, forcing the sludge back into the bulk liquid flow, thereby maintaining a high membrane flux.

[0013] Preferably, the pumping flow rate of ferrous sulfate solution is 90-110 mL / h; and the pumping flow rate of hydrogen peroxide solution is 250-350 mL / h.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A multifunctional aluminum-iron-silicon reagent with Fenton-like catalytic activity was prepared by liquid-phase in-situ copolymerization technology. The iron-silicon bonding structure was used to stimulate heterogeneous catalytic oxidation potential under slightly acidic conditions, and the high charge density of aluminum and iron was used for coordination complexation. This solved the problem of incomplete fluoride ion removal caused by organic complex encapsulation in traditional processes. It achieved a dual-effect synergistic effect of deep fluoride removal and oxidative degradation of organic pollutants in the same reaction system, which significantly improved the purification depth and compliance stability of wastewater.

[0015] 2. A rigid polysilicic acid framework with a high degree of polymerization is introduced into the defluorination agent. Through micro-interface shear regulation, dense and incompressible granular flocs are induced, thus matching the cross-flow filtration process of tubular ceramic membranes. The rigid particles effectively avoid the formation of a dense filter cake layer on the membrane surface, which is a consequence of the soft colloids formed by traditional aluminum salts. This weakens the membrane pore clogging effect and concentration polarization, significantly reducing the membrane flux decay rate during operation and achieving efficient, long-term stable operation of the membrane separation system.

[0016] 3. By utilizing the netting, sweeping, and lattice modification effects of polysilicic acid components, the gel-like microstructure of traditional fluorinated sludge, which is characterized by strong hydrophilicity and large specific surface area, was altered. This resulted in the construction of micron-sized crystal nuclei aggregates with abundant hydrophobic channels, thereby improving the mechanical dewatering performance of the sludge. Under conventional plate and frame filtration conditions, a large amount of water can be discharged by disrupting capillary forces, reducing the final water content of the sludge and achieving significant reduction in solid waste volume and facilitating subsequent resource-based disposal.

[0017] 4. Utilizing the high positive charge density of the multinuclear aluminum-iron complex and the three-dimensional adsorption bridging ability of the polysilicate network, a treatment agent was prepared based on a strong competitive adsorption mechanism targeting multi-anion coexisting systems. While treating fluoride-containing wastewater, this agent can efficiently capture coexisting pollutants such as phosphate using dedicated adsorption sites, and forcibly retain fine phosphate colloids that are difficult to remove by traditional precipitation methods through ceramic membrane separation technology. This solves the problem of low phosphorus removal efficiency caused by ion competition in complex water quality, achieving simultaneous deep purification of multiple pollutants.

[0018] 5. An integrated process design with reaction-separation coupling is adopted. The high-precision physical retention of inorganic ceramic membranes replaces the traditional gravity sedimentation tank, which has a large footprint and whose settling performance is significantly affected by the environment. Through precise pH gradient control and fluid dynamics optimization, the process flow and hydraulic retention time are significantly shortened. While ensuring excellent effluent quality, the investment in infrastructure and the amount of redundant reagents added during operation are greatly reduced. Attached Figure Description

[0019] Figure 1 This is a detailed flow chart of the multi-stage treatment process for fluoride-containing wastewater according to the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1 This invention provides a multi-stage treatment process for fluoride-containing wastewater, the technical solution of which is as follows: Example 1 The raw fluoride-containing organic wastewater before treatment in this example has the following parameters: fluoride ion concentration (F⁻) = 2200 mg / L; chemical oxygen demand (COD) = 3500 mg / L; pH value = 3.5; total phosphorus (TP) = 15 mg / L. The above raw fluoride-containing organic wastewater is water that has been concentrated by mixing industrial cleaning water and domestic sewage.

[0022] 1000g of liquid sodium silicate with a modulus of 3.2 was added to 15L of deionized water and stirred at 300rpm at 25℃ to obtain a diluted sodium silicate solution. A 20% sulfuric acid solution was added dropwise to the diluted sodium silicate solution at a rate of 50mL / min, and the pH value was monitored in real time until the pH was adjusted to 2.5. The solution was then stirred at 150rpm at 25℃ for 50min to activate it until the solution showed a slightly blue opalescent color and the viscosity at 25℃ was 2.0-5.0mPa·s, thus obtaining a highly active polysilicic acid stock solution.

[0023] 500g of solid polyaluminum chloride (Al2O3 content ≥29%), 200g of ferrous sulfate heptahydrate and 50g of titanium oxysulfate were dissolved in 2000mL of deionized water. After complete dissolution by stirring, a metal salt modified solution was obtained. A high-shear emulsifier was turned on, and the metal salt modified solution was rapidly injected into the highly active polysilicic acid stock solution within 2 minutes. The mixture was stirred at a high speed of 3500rpm for 5 minutes to obtain a precursor mixture.

[0024] Sodium bicarbonate powder was slowly added to the precursor mixture to adjust the basicity of the solution to 60%, and then the temperature was raised to 45°C. The mixture was then matured for 4 hours under low-speed stirring at 100 rpm and cooled to 25°C to obtain a reddish-brown viscous liquid-phase multifunctional defluorination catalyst. The molar ratio of Al:Fe:Si in the catalyst was controlled at 5:2:3, and the final amount of sodium bicarbonate powder added was 125.2 g.

[0025] Fluorine-containing organic wastewater was continuously pumped into the primary oxidation reactor at a flow rate of 100 L / h using an inlet pump. Simultaneously, a 10% ferrous sulfate heptahydrate solution was added to the primary oxidation reactor at a flow rate of 100 mL / h, and an automatic acid addition device (adding 0.1 M hydrochloric acid) was used to maintain the pH at 3.5. Subsequently, a 30% hydrogen peroxide solution was continuously added dropwise to the reactor at a flow rate of 300 mL / h. The hydraulic residence time in the primary oxidation reactor was controlled at 60 min, and the stirring speed was 200 rpm for Fenton oxidation complex breaking treatment.

[0026] The overflow from the primary oxidation reactor enters the secondary mineralization reactor. The dosage of calcium hydroxide emulsion with a mass concentration of 10% is controlled by an online pH controller to stabilize the pH of the reaction system at 10.0. The rapid mineralization reaction is carried out at a speed of 300 rpm, and the hydraulic residence time is controlled at 30 min. The mud-water mixture overflowing from the secondary mineralization reactor enters the tertiary deep treatment reactor. The prepared liquid-phase multifunctional defluorination catalyst is added at a flow rate of 80 mL / h via a metering pump. At the same time, hydrogen peroxide with a mass concentration of 30% is added at a flow rate of 10 mL / h, and the pH is automatically adjusted to 6.5. The tertiary deep treatment reactor is equipped with a rapid mixing zone and a slow flocculation zone. The rapid mixing zone has a stirring speed of 400 rpm and a residence time of 2 min, while the slow flocculation zone has a stirring speed of 60 rpm and a residence time of 15 min.

[0027] The outlet of the three-stage deep treatment vessel is connected to a tubular ceramic membrane circulation system. Under an operating pressure of 0.15 MPa, the reaction mixture is pumped into a tubular zirconia ceramic membrane filter with a pore size of 50 nm for cross-flow filtration. The membrane surface flow rate is controlled at 4 m / s, the concentrate reflux ratio is set at 10:1, and the permeate is the treated water that meets the standards.

[0028] The treatments in Examples 2-5 were based on Example 1, with some parameters adjusted as shown in Table 1. The elemental molar ratio was the Al:Fe:Si molar ratio of the metal salt modified solution. In Example 2, the rapid mixing zone time was 1.5 min, and the slow flocculation zone time was 13.5 min; in Example 3, the rapid mixing zone time was 2 min, and the slow flocculation zone time was 18 min; in Example 4, the rapid mixing zone time was 1.5 min, and the slow flocculation zone time was 14.5 min; and in Example 5, the rapid mixing zone time was 2.5 min, and the slow flocculation zone time was 15.5 min.

[0029] Table 1. Parameters for Multistage Treatment of Fluoride-Containing Wastewater

[0030] Comparative Examples 1-4 are all based on Example 1, with the following parameters adjusted.

[0031] Comparative Example 1: In the catalyst preparation stage, instead of activating polysilicic acid and high-shear emulsification copolymerization, commercially available polyaluminum chloride, ferric sulfate, and sodium silicate liquids of equal mass fractions were directly mixed and stirred physically for 20 minutes to obtain a mixed agent for use as a defluorination agent.

[0032] Comparative Example 2 follows the same steps as Example 1, except that the reaction backoff pH is set to 9.0 in the three-stage deep treatment vessel. This pH range is the standard pH range for calcium-based defluorination, not the pH 6.5 in Example 1 of this invention.

[0033] Comparative Example 3: The effluent from the tertiary deep treatment vessel does not enter the tubular ceramic membrane system, but instead enters a traditional inclined plate sedimentation tank. The surface load is controlled at 0.8 m³ / m²·h, and 2 ppm of polyacrylamide is added to aid coagulation. After settling, the supernatant is taken as the effluent.

[0034] Comparative Example 4: No sodium silicate and sulfuric acid activation steps were used in the catalyst preparation stage. 500g of solid polyaluminum chloride, 200g of ferrous sulfate heptahydrate, and 50g of titanium oxysulfate were directly dissolved in water and mixed under the same high-shear conditions of 3500 rpm. The basicity was then adjusted and the mixture was matured to obtain a silicon-free polyaluminum-iron reagent. An equal mass of this polyaluminum-iron reagent was added in the three-stage deep treatment step.

[0035] Test Example 1 The fluoride ion concentration (F⁻) and chemical oxygen demand (COD) removal rates of the treated water from the examples and comparative examples were tested. The specific test methods are as follows: F⁻ was determined strictly in accordance with the Chinese national standard GB / T 7484-1987 "Determination of Fluoride in Water - Ion Selective Electrode Method". To eliminate the interference of high salinity and metal ions, a total ionic strength adjustment buffer was added before the test, and the standard addition method was used for calibration.

[0036] COD was strictly determined according to the Chinese National Standard HJ 828-2017 "Determination of Chemical Oxygen Demand in Water - Dichromate Method". For water samples with high chloride ion content, a low-concentration potassium dichromate solution was used with the addition of mercuric sulfate masking agent to eliminate interference. The average effluent data of the examples and comparative examples after 24 hours of stable operation are recorded in Table 2.

[0037] Table 2 Comparison of performance test results of Examples 1-5 and Comparative Examples 1-4 Group Fluoride ion concentration in effluent (mg / L) COD removal rate (%) Example 1 0.6 98.5 Example 2 0.8 97.2 Example 3 0.5 98.9 Example 4 0.7 97.8 Example 5 0.9 96.4 Comparative Example 1 4.5 75.3 Comparative Example 2 1.4 68.1 Comparative Example 3 8.7 82.4 Comparative Example 4 5.4 88.2 As shown in Table 2, as illustrated in Examples 1-5, the multifunctional catalyst-coupled tubular ceramic membrane process prepared using the liquid-phase micro-interface control technology of this invention can achieve deep purification of high-concentration fluoride-containing organic wastewater within the specified component molar ratio, pH value, and flow rate range. The wastewater treatment method of this invention utilizes the entrapment effect of the silica skeleton and the strong coordination ability of aluminum-iron active sites to stably reduce the residual fluoride ion concentration to below 1.0 mg / L. Simultaneously, it utilizes the Fenton-like catalytic activity of the Fe-O-Si structure formed in situ in the reagent to achieve highly efficient COD removal with a removal rate >96%, demonstrating excellent treatment effect. Although the wastewater treatment method involves multi-stage reactions, the process is compact, and it solves the problem of difficult settling of fine fluoride-containing sludge in traditional processes. Example 3 showed the best performance because it adopted a higher Al / Fe / Si ratio and a higher basicity. At this point, the long-chain structure of polysilicic acid was most developed, and the high basicity provided a large number of hydroxylation active sites, maximizing the synergistic effect of adsorption bridging and catalytic oxidation.

[0038] Comparative Example 1 employed simple physical mixing instead of the high-shear micro-interface copolymerization technology of this invention. Physical mixing failed to form stable chemical bonds, leading to rapid dissociation of the reagent in water and the inability to form a catalytically active crystal structure. Consequently, its ability to destroy organic complexes was significantly reduced, and an effective sieve-sweeping effect was not achieved, resulting in poor defluorination efficiency. Comparative Example 2, with the tertiary reaction pH set in the conventional alkaline range, resulted in an effluent fluoride concentration of 3.2 mg / L and a significant decrease in COD removal rate. In the defluorination catalyst system described in this invention, the aluminum and iron components rapidly underwent hydrolysis under these conditions to generate amorphous hydroxide precipitates, losing their coordination exchange capacity for F⁻. Simultaneously, Fenton and Fenton-like reactions were essentially ineffective under alkaline conditions, preventing the oxidative degradation of organic matter. This demonstrates the necessity of deep treatment under slightly acidic to neutral conditions as described in this invention. Comparative Example 3, which eliminated the ceramic membrane and used gravity sedimentation, resulted in severely excessive fluoride concentrations in the effluent. This clearly demonstrates that for complex wastewater containing organic matter and colloids, the generated flocs are often lightweight and highly hydrophilic, making them extremely difficult to separate by gravity sedimentation. A large amount of fine suspended solids carried the adsorbed fluoride out with the water, causing fluoride runoff and verifying the irreplaceable nature of membrane separation technology in this solution. Comparative Example 4, lacking the skeletal support of polysilicic acid, generated loose aluminum-iron flocs with poor shear resistance. During pumping and stirring, the flocs easily broke, releasing the adsorbed pollutants. Furthermore, the lack of silica flocs easily clogged the membrane pores, leading to unstable treatment efficiency.

[0039] It should be noted that although pH 9.0-10.0 is the thermodynamically optimal range for calcium fluoride precipitation, when the fluoride concentration is below 20 mg / L, the calcium method alone is insufficient to further remove dissolved fluoride due to precipitation crystallization kinetics. At this point, deep fluoride removal mainly relies on the complexation adsorption on the reagent surface. The prepared fluoride removal catalyst exhibits a positive Zeta potential (+20 to 30 mV) at pH 6.5, demonstrating strong electrostatic adsorption and coordination exchange for F⁻. However, in Comparative Example 2 at pH 9.0, the reagent surface potential becomes negative, resulting in electrostatic repulsion with F⁻ and causing adsorption failure. Therefore, although Comparative Example 2 retains some calcium precipitation effect, it is far less effective than the synergistic adsorption effect of Example 1.

[0040] Test Example 2 To evaluate the impact of the liquid-phase multifunctional defluorination catalyst in the process of this invention on the operational stability of the subsequent ceramic membrane filtration system, the focus was on investigating the resistance of the flocs generated by the catalyst to membrane fouling. Examples 1-5 and Comparative Examples 1, 2, and 4 were selected for testing.

[0041] The flux testing method in industry standard HY / T 063-2002 "Hollow Fiber Microporous Filter Membrane Modules" was modified and applied to tubular ceramic membranes. Under a constant transmembrane pressure difference of 0.15 MPa, the initial permeate flux (J0) of the ceramic membrane was determined using pure water. The process was then switched to a mixture of effluent and mud from the tertiary reactors of each group. Continuous filtration was performed for 24 hours while maintaining a cross-flow velocity of 4 m / s and a transmembrane pressure difference of 0.15 MPa. After 24 hours, the permeate flux was recorded. The membrane flux decay rate (R) = (J0 - J0) / ( ... t The lower the decay rate, the less fouling the membrane and the better the permeability of the resulting filter cake layer. The membrane flux decay rates of each group after 24 hours of operation are shown in Table 3.

[0042] Table 3. Comparison of membrane flux attenuation rate test results between Examples 1-5 and Comparative Examples 1, 2, and 4 Group Membrane flux decay rate (%) Remark Example 1 5.2 Extremely low attenuation, stable operation Example 2 6.8 Low attenuation Example 3 4.5 Best performance and strongest resistance to pollution Example 4 5.9 Low attenuation Example 5 7.1 The attenuation is slightly higher, but better than the comparative example. Comparative Example 1 15.3 Significant attenuation, requiring frequent backwashing. Comparative Example 2 18.6 Severe blockage, rapid decrease in flux Comparative Example 4 25.4 Extremely severe blockage, almost paralyzed As shown in Table 3, as illustrated in Examples 1-5, the use of the polysilicic acid-containing reagent prepared according to this invention, combined with the tubular ceramic membrane process, significantly reduces flux decay during membrane filtration within the shear copolymerization conditions and proportions specified in this application, achieving a flux decay rate controlled below 8% during 24-hour continuous operation. The core of this invention lies in utilizing the rigid inorganic polymer framework formed by polysilicic acid to induce the formation of a dense but highly porosity granular filter cake layer from aluminum-iron flocs, rather than the gelled filter cake formed by traditional reagents. The resulting rigid filter cake is incompressible and maintains good permeability channels even under pressure, thereby significantly delaying membrane pore clogging, reducing backwashing frequency, and extending the service life of the membrane module. Example 3 exhibited the lowest membrane flux attenuation rate and the best performance. Based on the parameters in Table 1, Example 3 utilized a higher silicon content and the highest basicity. The highly polymerized polysilicate network acted as a support structure, supporting the floc structure and preventing it from breaking or deforming under high-velocity cross-flow shear. Simultaneously, the high basicity indicated a high degree of hydroxylation, resulting in a stable hydration film on the floc surface and making it less prone to adsorption and deposition on the ceramic membrane surface. Example 5 showed a slightly higher attenuation rate, mainly due to its excessively high Si content, leading to excessively large floc particles. Although the rigidity was sufficient, it was prone to localized accumulation at the membrane tube inlet during cross-flow filtration, slightly increasing fluid resistance. However, the overall performance remained within the excellent range.

[0043] Comparative Example 4, with the sodium silicate component removed, exhibited the worst membrane flux decline rate of 25.4%, the highest among all groups. This was primarily because the hydrolyzed flocs generated from pure aluminum-iron salts are loose and compressible soft flocs. Under a transmembrane pressure of 0.15 MPa, these soft flocs were compacted on the membrane surface, effectively sealing the membrane pores like mud, forming a dense and impermeable filter cake layer. This fully demonstrates that the introduction of a rigid polysilicic acid framework in this invention is crucial and indispensable for the membrane coupling process, and is a key technical means to solve the pain point of membrane fouling. Comparative Example 1 suffered from unstable reagent structure due to physical mixing, causing the flocs to break into fine particles during pumping, which directly embedded in the membrane pores, leading to flux decline. Comparative Example 2, with its alkaline pH, generated a gel-like hydroxide precipitate that covered the membrane surface, forming a high-resistance layer, resulting in a 18.7% flux decline. These two sets of results further confirm the importance of in-situ copolymer structure stability and precise pH control for maintaining membrane flux.

[0044] Test Example 3 To evaluate the reduction and dewatering performance of the final waste sludge produced by the process of this invention, the effects of the reagent components on the sludge microstructure and water-holding capacity were investigated. Examples 1-5 and Comparative Examples 1, 2, and 4 were selected for testing.

[0045] The determination of sludge moisture content was strictly carried out according to the gravimetric method in the Chinese Urban Construction Industry Standard CJ / T 221-2023. After 24 hours of continuous operation, the bottom sludge from the sedimentation tanks of each group was collected and uniformly filtered using a plate and frame filter press at 1.0 MPa for 30 minutes. The filtered sludge cake was taken as the test sample, and approximately 50g of the sludge cake sample was accurately weighed and recorded as W1, accurate to 0.01g. The sample was then dried in an oven at 105℃ until constant weight (4h), removed, cooled in a desiccator, and weighed, with the weight recorded as W2. The sludge moisture content was calculated as (W1-W2) / W1×100%. The sludge moisture content data for each group after treatment are shown in Table 4.

[0046] Table 4. Comparison of mud moisture content test results between Examples 1-5 and Comparative Examples 1, 2, and 4 Group Sludge moisture content (%) Remark Example 1 58.2 Mud cookies are refreshing and easy to shape. Example 2 59.7 The mud cake is relatively dry and easy to peel off. Example 3 56.5 Lowest moisture content, best volume reduction effect Example 4 58.9 The mud cakes were well shaped Example 5 60.1 The mud cakes were well shaped Comparative Example 1 75.3 The mud cake was sticky and difficult to press dry. Comparative Example 2 82.8 It has a colloidal form and is extremely difficult to dehydrate. Comparative Example 4 85.6 Typical hydroxide putty, with extremely high water content As shown in Table 4, as illustrated in Examples 1-5, the defluorination catalyst prepared using the liquid-phase in-situ copolymerization technology of this invention for treating fluoride-containing wastewater achieves a fundamental improvement in sludge properties under the process conditions specified in this application. The wastewater treatment method of this invention utilizes the rigid framework of polysilicic acid to induce the generated precipitate to transform from a traditional flocculent gel into a porous, channel-rich particle cluster structure. This structure is not easily closed even under high pressure during mechanical filtration, allowing free water and some capillary water to be smoothly discharged, thereby stably reducing the sludge moisture content to approximately 60%, significantly reducing sludge volume and lowering subsequent hazardous waste disposal costs.

[0047] Comparative Example 4, with the removal of silicon components, resulted in a sludge moisture content as high as 85.6%. The Al(OH)3 and Fe(OH)3 generated from the hydrolysis of pure aluminum and iron salts are typical amphoteric hydroxide colloids with strong hydrophilicity and a large specific surface area, binding a large amount of water. The resulting colloidal sludge exhibits high compressibility during filtration, forming a dense skin layer on the surface under pressure, trapping internal moisture in the center of the sludge cake and preventing it from drying completely. This result conversely demonstrates that introducing a silicon source to construct a rigid framework is the decisive factor in achieving deep dewatering in this invention. Comparative Example 2, under alkaline pH conditions, primarily precipitated metal ions as amorphous precipitates with incomplete crystal development, resulting in a muddy sludge cake with a moisture content as high as 82.8%, making it unsuitable for landfill or resource recovery. Comparative Example 1, lacking in-situ polymerized cross-linking structures, formed loose flocs with unstable internal pore structures, easily collapsing during filtration, resulting in mechanical dewatering performance far inferior to the modified agent of this invention.

[0048] In summary, this invention not only solves the problem of effluent compliance but also improves the physical properties of solid waste through materials science. The significant differences between the examples and comparative examples demonstrate that the polysilicic acid component in the defluorination catalyst is not only a coagulant aid but also a sludge structure modifier, effectively addressing the challenges of difficult dewatering and expensive disposal of fluoride-containing sludge.

[0049] Test Example 4 To verify the synergistic removal capacity of the process of this invention for total phosphorus (TP), a coexisting pollutant, while treating fluoride-containing organic wastewater, the competitive adsorption and chemical precipitation effects of the fluoride removal catalyst on phosphate ions were investigated. All examples and comparative examples were selected for testing.

[0050] The determination of total phosphorus in water was strictly carried out in accordance with the Chinese National Standard GB 11893-89, "Determination of Total Phosphorus in Water - Ammonium Molybdate Spectrophotometric Method". First, digestion was performed. A suitable amount of well-mixed water sample was taken, and potassium persulfate solution was added. Digestion was carried out at 120℃ for 30 min to convert all phosphorus forms, including organic phosphorus and polyphosphates, into orthophosphates. Then, a colorimetric reaction was performed. The pH of the sample was adjusted to neutral, and a mixed colorimetric reagent of ammonium molybdate-potassium antimony tartrate-ascorbic acid was added. The sample was then left to stand at room temperature for 15 min. The absorbance was measured at 700 nm using a spectrophotometer. The total phosphorus concentration was calculated based on the standard curve. Finally, the total phosphorus removal rate was calculated using the following formula: (Influent TP - Effluent TP) / Influent TP × 100%. The total phosphorus removal rate data for each group after 24 hours of stable operation are shown in Table 5.

[0051] Table 5 Comparison of Total Phosphorus Removal Rate Test Results Group Total phosphorus removal rate (%) Total phosphorus concentration in effluent (mg / L) Example 1 98.2 ＜0.3 Example 2 97.5 ＜0.4 Example 3 99.1 ＜0.2 Example 4 97.9 ＜0.4 Example 5 96.8 ＜0.5 Comparative Example 1 78.4 3.2 Comparative Example 2 88.7 1.7 Comparative Example 3 82.1 2.7 Comparative Example 4 80.5 2.9 As shown in Table 5, as illustrated in Examples 1-5, the combined process of the liquid-phase in-situ copolymerization defluorination catalyst and ceramic membrane of this invention can achieve highly efficient removal of coexisting total phosphorus within the parameter range specified in this application. The removal rate is consistently above 96%, and the total phosphorus concentration in the effluent is far below 0.5 mg / L, meeting the Class A standard. The wastewater treatment method of this invention utilizes the strong chemical affinity between aluminum and iron ions and phosphate ions to generate insoluble AlPO4 and FePO4. Simultaneously, the sieve-sweeping effect of polysilicic acid thoroughly traps the fine phosphate colloids, resulting in good treatment effect and a simple wastewater treatment method. While removing fluoride and COD, it significantly reduces the total phosphorus value in the wastewater, achieving synergistic deep purification of multiple pollutants. Example 3 showed the highest total phosphorus removal rate because this group had the optimal Al / Fe ratio and the highest basicity. Aluminum ions were the main force in phosphorus removal, while an appropriate amount of iron ions could increase the specific gravity of the flocs and the number of adsorption sites. Under slightly acidic conditions of pH 6.5-7.0, the polynuclear hydroxy complexes produced by the hydrolysis of PSAF-C had a high density of positive charges on their surface, which could undergo strong charge neutralization and coordination adsorption reactions with negatively charged phosphate ions.

[0052] In Comparative Example 2, the total phosphorus removal rate was 88.7% when the tertiary treatment maintained a pH of 9.0. Although calcium ions can react with phosphorus to form hydroxyapatite precipitate under alkaline conditions, achieving deep phosphorus removal below 0.5 mg / L requires the adsorption of aluminum / iron salts. At pH 9.0, OH... - High ion concentration, with PO4 3-Intense competitive adsorption occurs, and aluminum and iron readily form negatively charged complexes, significantly weakening their electrostatic adsorption capacity for phosphate, thus hindering deep phosphorus removal. In Comparative Example 3, gravity sedimentation resulted in low removal rates. The precipitated particles of aluminum phosphate and iron phosphate were often very fine colloidal particles, extremely difficult to separate by natural sedimentation. A large number of fine phosphorus particles were lost with the supernatant in Comparative Example 3. In contrast, this invention utilizes the physical sieving effect of a ceramic membrane to forcibly retain these colloidal phosphorus particles, demonstrating the absolute advantage of membrane coupling technology in phosphorus removal. The removal rates of Comparative Examples 1 and 4 were both unsatisfactory. The former failed to form an effective three-dimensional network structure for capturing phosphates because the reagents were not polymerized; the latter lacked a silica framework, resulting in fine and fragile phosphate flocs that were easily desorbed or penetrated under water shear, failing to achieve stable removal effects.

[0053] In summary, the technical solution of this invention has significant advantages in solving the problem of treating wastewater with coexisting fluoride and phosphorus. Through the triple synergy of Al / Fe chemical precipitation, Si adsorption bridging, and membrane physical retention, it ensures that the total phosphorus index, fluoride, and COD indexes simultaneously and excellently meet the standards.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A multi-stage treatment process for fluoride-containing wastewater, characterized in that, The multi-stage treatment process is as follows: the raw fluoride-containing organic wastewater is pumped into the primary oxidation reactor, and ferrous sulfate solution is pumped in at the same time to maintain the acidic environment of the system; then Fenton oxidation complex-breaking treatment is performed to obtain primary oxidized water. The primary oxidation treatment water is pumped into the secondary mineralization reactor, the system environment is adjusted to alkaline, and a mineralization reaction is carried out to obtain secondary mineralized water; The secondary mineralized water overflows into the tertiary deep treatment vessel, and a defluorination catalyst is pumped in to obtain tertiary deep treated water; the tertiary deep treated water is connected to a tubular ceramic membrane circulation system, and cross-flow filtration, concentration and reflux are performed to obtain qualified water; The defluorination catalyst is composed of iron, aluminum and silicon.

2. The multi-stage treatment process for fluoride-containing wastewater according to claim 1, characterized in that, The defluorination catalyst is obtained by injecting a metal salt modified solution into a polysilicic acid stock solution to obtain a precursor mixture; sodium bicarbonate powder is added to the precursor mixture, the basicity of the solution is adjusted, the temperature is raised, stirred and matured, and then cooled.

3. The multi-stage treatment process for fluoride-containing wastewater according to claim 2, characterized in that, The metal salt modified solution is obtained by dissolving polyaluminum chloride, ferrous sulfate heptahydrate, and titanium oxysulfate in deionized water; the polysilicic acid stock solution is obtained by adding sulfuric acid solution dropwise to diluted sodium silicate solution to adjust the system to a strongly acidic environment and stirring the reaction; the pH corresponding to the strongly acidic environment is 2.0-3.

0.

4. The multi-stage treatment process for fluoride-containing wastewater according to claim 1, characterized in that, The Fenton oxidation complex-breaking treatment is a reaction involving the dropwise addition of hydrogen peroxide solution; the hydrogen peroxide solution is pumped in simultaneously with the defluorination catalyst, and the system environment is adjusted to acidity.

5. The multi-stage treatment process for fluoride-containing wastewater according to claim 1, characterized in that, The three-stage deep treatment vessel includes a rapid mixing zone and a slow flocculation zone; the residence time in the rapid mixing zone is 1.5-2.5 min; the residence time in the slow flocculation zone is 13.5-18 min; the hydraulic residence time in the primary oxidation reactor is 42-75 min; and the hydraulic residence time in the secondary mineralization reactor is 20-40 min.

6. The multi-stage treatment process for fluoride-containing wastewater according to claim 1, characterized in that, The cross-flow filtration process involves a portion of the tertiary deep-treated water carrying the trapped suspended solids as a concentrate flowing out of the outlet and back to the front-end reaction system; the remaining portion of the tertiary deep-treated water is discharged as a permeate through a tubular ceramic membrane to obtain the qualified water.

7. The multi-stage treatment process for fluoride-containing wastewater according to claim 1, characterized in that, The pumping flow rate of the ferrous sulfate solution is 90-110 mL / h; the pumping flow rate of the hydrogen peroxide solution is 250-350 mL / h; and the cross-flow velocity on the membrane surface inside the membrane tube is 3.5-4.5 m / s.

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