A method for defluoridation of fluoride-containing wastewater

Abstract

This application relates to the field of water treatment technology and discloses a method for defluoridating fluoride-containing wastewater. The method includes introducing fluoride-containing wastewater from the bottom into a crystalline granulation fluidized bed filled with natural garnet seed crystals, thus fluidizing the seed crystals. Simultaneously, calcium chloride solution and sodium hydroxide solution are pumped in from the side and below to adjust the Ca / F molar ratio and pH value of the reaction system, promoting an induced crystallization reaction between fluoride ions and calcium ions in the aqueous phase on the surface of the garnet seed crystals to precipitate calcium fluoride. After the reaction stabilizes, the treated water is discharged from the top of the bed to obtain defluorinated effluent. This method addresses the homogeneous nucleation phenomenon in the aqueous phase caused by high influent fluoride concentration shocks, and combines this with subsequent coagulation and sedimentation processes to remove suspended particles. This application overcomes the shortcomings of traditional sedimentation methods, which easily generate loose flocs, achieving a stable transfer of fluoride to a dense solid phase, effectively reducing the production of water-containing sludge, and ensuring the quality of the effluent from the defluoridation system under high load fluctuations.

Description

Technical Field

[0001] This application relates to the field of water treatment technology, specifically a method for defluoridating fluoride-containing wastewater. Background Technology

[0002] Fluoride ions are widely distributed in natural water bodies, and in recent years, fluoride-containing wastewater discharged from industrial manufacturing processes such as phosphate fertilizers and semiconductors has further exacerbated the risk of fluoride pollution in the aquatic environment. Currently, conventional treatment methods for industrial fluoride-containing wastewater mainly include membrane separation, adsorption, and chemical precipitation. Among these, membrane separation technology has high treatment efficiency, but it is prone to membrane module fouling during long-term operation, requires cumbersome daily maintenance, and generates secondary cleaning wastewater during backwashing. Adsorption methods are limited by the adsorption capacity of the materials themselves and are generally more suitable for treating low-concentration fluoride-containing water. Once the adsorbent reaches saturation, its regeneration process is not only complex but also generates new intermediate wastewater.

[0003] In contrast, chemical precipitation is more commonly used in practical engineering due to its relatively simple operation. This method mainly relies on the addition of alkaline reagents or inorganic flocculants to convert fluoride ions in the aqueous phase into precipitates for separation. However, traditional chemical precipitation often directly generates loosely structured amorphous flocs in the aqueous phase, resulting in sludge with high water content and large volume. This type of high-water-content sludge, as a typical industrial solid waste, increases the difficulty and economic cost for enterprises in subsequent sludge dewatering and off-site disposal.

[0004] To address the issue of high sludge production associated with traditional sedimentation methods, induced crystallization technology has begun to be applied to the treatment of fluoride-containing wastewater. This process utilizes a fluidized bed reactor, adding external seed crystals to guide fluoride ions and calcium sources to crystallize on the surface of a solid carrier, forming high-density calcium fluoride particles. While crystallization has shown advantages in reducing sludge volume, conventional crystallization fluidized beds still exhibit some operational limitations in complex industrial applications. Firstly, traditional reagent dosing methods can easily lead to excessively high reactant concentrations in localized areas, causing scaling at the inlet and bottom pipes, affecting the continuous and stable operation of the equipment. Secondly, the influent quality of industrial wastewater often fluctuates. When the influent fluoride concentration suddenly increases, the thermodynamic driving force for crystallization within the system increases accordingly, easily triggering homogeneous nucleation reactions in the aqueous phase. In this case, some tiny calcium fluoride nuclei do not have time to attach to the seed crystal surface but instead remain suspended in the aqueous phase and overflow the bed with the effluent, resulting in higher turbidity in the final effluent. For the problem of homogeneous nucleation and water quality deterioration caused by such high-concentration shock loads, existing single crystallization defluorination systems generally lack effective interception or compensation mechanisms, and the overall shock resistance of the system still needs to be further improved. Summary of the Invention

[0005] The technical problem addressed by this application is that existing fluoride-containing wastewater treatment processes suffer from high reagent consumption, high sludge production, difficulty in achieving stable transfer of fluoride to the solid phase, and the tendency for homogeneous nucleation of the aqueous phase to occur under high fluoride concentration shock loads, leading to deterioration of effluent quality.

[0006] To address the above problems, this application provides the following technical solution:

[0007] This application provides a method for defluorinating fluoride-containing wastewater, employing the following technical solution: A method for defluorinating fluoride-containing wastewater includes: introducing fluoride-containing wastewater from the bottom into a crystallizing granulation fluidized bed filled with natural garnet seed crystals, the upward flow of water causing the natural garnet seed crystals to be in a fluidized state; simultaneously pumping in calcium chloride solution as a calcium agent and sodium hydroxide solution as a pH adjuster from the side and below, controlling the addition of the calcium chloride solution to ensure the Ca / F molar ratio is at a preset value, and controlling and adjusting the pH value of the reaction system, so that the fluoride ions in the fluoride-containing wastewater react with the added calcium ions on the surface of the natural garnet seed crystals to induce a crystallization reaction to generate calcium fluoride crystals; after the crystallization reaction stabilizes, the defluorinated water is discharged from the system through the top outlet pipe of the crystallizing granulation fluidized bed to obtain defluorinated effluent.

[0008] By adopting the above technical solution, the use of natural garnet particles as seed crystals and the crystallization and defluorination treatment in the fluidized bed can effectively solidify the fluoride in the water, reduce the generation of water-containing sediment sludge from the source, and ensure the long-term stable operation of the entire fluidized bed system.

[0009] Regarding the core crystallization and defluorination process, the fluoride-containing wastewater enters the equipment from bottom to top, lifting the natural garnet seed crystals inside and forming a fluidized bed. Considering the suitable specific gravity and physical strength of the garnet seed crystals, they can maintain a good suspension state under the scouring of water, thus creating a sufficient solid-liquid mass transfer environment for subsequent chemical reactions. Based on this, calcium chloride solution injected from the side and below replenishes the calcium source required for crystallization, and sodium hydroxide solution adjusts the system to a slightly alkaline environment suitable for precipitation. At this point, fluoride ions in the aqueous phase come into contact with calcium ions, and the local concentration rapidly increases, reaching a thermodynamic supersaturation state. The chemical equation for the crystallization reaction is: ; Once the aqueous phase is supersaturated, the natural cleavage planes and micro-roughness of the garnet seed crystals significantly reduce the surface free energy and activation energy required for the reaction. Therefore, fluoride and calcium ions in the aqueous phase do not combine blindly, but preferentially nucleate, attach, and grow on the garnet carrier surface. As the reaction progresses, the generated calcium fluoride gradually forms layered and granular porous coatings on the seed crystal surface, further expanding the specific surface area of ​​the reaction. This heterogeneous induced crystallization allows calcium fluoride to precipitate in a relatively dense crystalline form, not only avoiding the risk of reactants forming loose, amorphous flocs directly in the water, but also naturally achieving a stable transfer of fluorine from the aqueous phase to the solid phase.

[0010] Preferably, the addition of the calcium chloride solution is controlled to achieve a Ca / F molar ratio of 1.0–1.5, and the pH of the reaction system is controlled to be 6.5–7. By adopting the above technical solution, the pH of the reaction system is controlled within the range of 6.5–7, allowing the generated calcium fluoride product to remain stably in a low solubility state. Simultaneously, setting the Ca / F molar ratio within the range of 1.0–1.5, based on the principle of chemical equilibrium shift, appropriately increases the concentration of calcium ions in the system, which is beneficial for promoting the precipitation reaction to proceed continuously in the forward direction, thereby further reducing the residual fluoride ion concentration in the aqueous phase.

[0011] Preferably, the calcium chloride solution is added to achieve a Ca / F molar ratio of 1.0. By employing this technical solution, a better balance can be struck between the crystallization rate and reagent consumption costs at a given influent fluoride concentration. When the Ca / F molar ratio is controlled at 1.0, the entire system exhibits excellent fluoride removal kinetics; this avoids the problem of excessively high residual calcium ion levels in the effluent due to blindly increasing the calcium source dosage, and also prevents the adverse effect of excessive calcium ions on the kinetic inhibition of the fluoride removal crystallization rate.

[0012] Preferably, the natural garnet seed crystals have a particle size of 80 to 100 mesh. The selection of natural garnet seed crystals within this particle size range using the above-mentioned technical solution is primarily due to their suitable specific surface area and settling properties. Under the set influent load conditions, seed crystals of this size easily form and maintain a stable fluidized state, with very little particle loss. Furthermore, their inherent resistance to acid and alkali corrosion and their resistance to breakage fundamentally ensure the long service life of the crystal carrier in wastewater treatment environments.

[0013] Preferably, the calcium chloride solution and the sodium hydroxide solution are pumped in from the side and below, respectively, with the sodium hydroxide solution entering above the calcium chloride solution. By employing this technical solution, a reasonable concentration and pH gradient can be constructed from bottom to top within the crystallizing granulation fluidized bed. Specifically, because the calcium chloride solution enters first at the bottom, calcium ions are pre-mixed with the fluoride-containing aqueous phase; then, the sodium hydroxide solution, located above, enters to raise the local pH value of the water, thereby triggering the crystallization reaction. This spatially layered dosing sequence cleverly avoids areas of high ion concentration, effectively preventing severe scaling at the inlet and bottom pipes due to a sudden surge in supersaturation upon initial contact with the reagent, or the initiation of large-scale homogeneous nucleation.

[0014] Preferably, when the influent fluoride concentration of the fluoride-containing wastewater is greater than or equal to 300 mg / L, the fluoride ions in the highly supersaturated state in the aqueous phase not only induce crystallization on the surface of the natural garnet seed crystals, but also trigger a large number of homogeneous nucleation reactions in the aqueous phase. The defluorinated water is discharged from the top effluent pipe of the crystallizing granulation fluidized bed to obtain high-turbidity fluidized bed effluent. Coagulants and flocculants are added to the high-turbidity fluidized bed effluent for coagulation and sedimentation treatment. After solid-liquid separation to remove suspended particles in the aqueous phase, the defluorinated effluent is obtained. The main purpose of adopting the above technical solution is to deal with the risk of water quality fluctuations that may be caused when the fluidized bed encounters a high concentration of fluoride load. When the influent fluoride concentration increases significantly, the thermodynamic driving force for crystallization in the system will increase sharply. This inevitably leads to some tiny calcium fluoride crystal nuclei not having time to attach to the solid carrier, but instead being generated directly inside the water body and overflowing with the water flow. Therefore, by introducing a coagulation and sedimentation process at the back end in a timely manner, these overflowing suspended particles can be separated and intercepted in a targeted manner, thus making up for the technical shortcomings of single fluidized bed equipment that is prone to increased effluent turbidity when treating ultra-supersaturated wastewater.

[0015] Preferably, the influent fluoride concentration of the fluoride-containing wastewater is 600 mg / L.

[0016] Preferably, the coagulant is polyaluminum chloride (PAC), and the flocculant is polyacrylamide (PAM). Utilizing the charge neutralization and destabilization capabilities of PAC and the adsorption and bridging effects of PAM, suspended calcium fluoride particles in the aqueous phase are coagulated and precipitated, maintaining the turbidity of the defluorinated effluent below 30 NTU. By employing the above technical solution, relying on the polynuclear hydroxyl complexes generated by the hydrolysis of inorganic coagulants like PAC in water, the surface charge of suspended calcium fluoride particles in the water can be rapidly neutralized and their double layer compressed, forcing the particles to destabilize and begin to aggregate. The added organic polymeric flocculant, polyacrylamide, utilizes its long-chain structure in water, employing physical adsorption and bridging properties to further aggregate the already destabilized fine particles into large, dense flocs. The synergistic effect of these two agents not only significantly accelerates the solid-liquid separation process but also maintains the final turbidity of the effluent below 30 NTU even under high-load operating conditions, providing reliable shock resistance for the entire defluorination system.

[0017] Preferably, the influent flow rate of the fluoride-containing wastewater is controlled at 0.63 m³ / h. 3 The natural garnet seed crystals are filled to a static bed height of 80 cm within the crystallization granulation fluidized bed. By employing the above technical solution, a specific influent flow rate matched with the equipment cross-sectional area provides sufficient and suitable hydraulic residence time for the crystallization defluorination reaction. Simultaneously, controlling the static bed height to 80 cm ensures that after the seed crystal particles expand and enter the fluidized state under the action of water flow, their main body remains stably stationary within the core reaction zone, preventing excessive elevation with the water flow, thereby maintaining efficient mass transfer between the aqueous and solid phases over a long period.

[0018] This application provides a method for defluoridation of fluoride-containing wastewater. It has the following beneficial effects: 1. This application introduces natural garnet particles of a specific size as seed crystals inside a crystallizing fluidized bed. Combined with a calcium source and pH adjuster pumped in from the side and below, the reaction site of fluoride ions and calcium ions in the aqueous phase is primarily guided to the surface of the seed crystals. This heterogeneous induced crystallization process causes calcium fluoride to precipitate in a dense crystalline form and coat the garnet carrier, changing the traditional chemical precipitation method that directly generates loose, amorphous flocs in water. This technique achieves a stable transfer of fluoride to the solid phase, significantly reducing the amount of high-water-content sludge generated at the source, and lowering the difficulty and cost of subsequent solid-liquid separation and sludge disposal.

[0019] 2. To address the issue that high-concentration fluoride wastewater impacts can easily lead to homogeneous nucleation in the aqueous phase, thus deteriorating effluent quality, this application establishes a synergistic treatment mechanism based on influent load response. When dealing with the overflow of suspended micro-calcium fluoride crystals under highly supersaturated conditions, polyaluminum chloride and polyacrylamide are added at the downstream end of the fluidized bed effluent. Their destabilizing and adsorption bridging effects are utilized to intercept suspended solids in the aqueous phase. This scheme effectively compensates for the shortcomings of single fluidized bed crystallization processes in intercepting fine particulate matter under extreme high-load conditions, ensuring that the system can still output stable defluorinated effluent with low turbidity even when encountering water quality shocks.

[0020] 3. This application features a rational spatial design of the flow field and concentration gradient within the reaction system. By positioning the inlet point of the sodium hydroxide solution above the inlet point of the calcium chloride solution, a gentle supersaturation transition environment is created in the bottom inlet zone of the bed. This spatially staggered dosing structure effectively prevents pipe scaling and large-scale homogeneous nucleation caused by excessively high instantaneous mixing concentrations of reactants. Combined with predetermined fluidized bed hydraulic parameters and optimized calcium-fluoride molar ratio control, a high crystallization reaction kinetic rate is maintained while ineffective consumption of calcium agent is avoided, ensuring long-term continuous and stable operation of the entire defluorination system. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the crystallization granulation fluidized bed defluorination test system of this application; Figure 2 This is a correlation diagram showing the effect of influent fluoride ion content on effluent fluoride content and removal rate in this application. Figure 3 This is a correlation diagram showing the effect of influent fluoride ion content on effluent calcium ion content and turbidity in this application. Figure 4 This is a correlation diagram showing the effect of the Ca / F molar ratio on the effluent fluoride content in this application; Figure 5 This is a comparison of fluoride removal rates and linear fitting under different Ca / F molar ratios in this application. Figure 6 This is a comparison image of the scanning electron microscope morphology of the induced seed crystals before and after the operation of Example 1 of this application; Figure 7 This is an energy dispersive spectroscopy (EDS) image of the untreated seed crystals before operation in Example 1 of this application; Figure 8 This is an energy dispersive spectroscopy (EDS) image of the seed crystals after operation in Example 1 of this application, showing the elemental distribution and content. Detailed Implementation

[0022] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0023] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0024] Raw materials and equipment section: The raw water for this experiment was taken from a groundwater source in Xi'an City. During the experiment, the fluoride ion content in the raw water was adjusted by adding analytical grade sodium fluoride to simulate high-concentration industrial wastewater containing fluoride. The initial water quality indicators of the raw water are shown in Table 1.

[0025] Table 1. Water quality indicators of the raw water used in the experiment

[0026] The chemical reagents and materials used in the embodiments and comparative examples of this application specifically include sodium fluoride (CAS No.: 7681-49-4) for controlling the fluoride ion content in the raw water, calcium chloride (CAS No.: 10043-52-4) as a calcium precipitant for inducing crystallization, sodium hydroxide (CAS No.: 1310-73-2) and hydrochloric acid (CAS No.: 7647-01-0) for adjusting the pH value of the reaction system, polyaluminum chloride (CAS No.: 1327-41-9) as an inorganic polymeric coagulant, and polyacrylamide (CAS No.: 9003-05-8) as an organic polymeric flocculant. All of the above chemical reagents are commercially available conventional products commonly used in the field. The seed crystals used in the crystallization reaction are natural garnet crystal particles with a particle size of 80 to 100 mesh. These natural garnet seed crystals are chemically stable, have good resistance to acid and alkali corrosion, are easy to settle, are not easily broken, and have a long service life, making them suitable for various wastewater environments.

[0027] This application employs a pilot-scale crystallization granulation fluidized bed for the induced crystallization removal of fluoride ions in water. The pilot-scale crystallization granulation fluidized bed equipment has a diameter of 0.2 m, a height of 3.7 m, a maximum throughput of 2 m³ / h, and is made of plexiglass. It uses a bottom-inlet, top-outlet design and has five sampling ports along the flow path, all of which can be used for sampling and chemical dosing. The static bed layer of the fluidized bed has a filling height of 80 cm.

[0028] like Figure 1As shown, the experimental system of this application consists of three main parts: a water inlet device, a reagent dosing device, and a crystallization granulation fluidized bed. The water inlet device includes a water storage tank and a main inlet pipeline connected to its outlet. Along the water flow direction, the main inlet pipeline is sequentially equipped with a valve, a centrifugal pump, and a static mixer. The end of the main inlet pipeline is connected to the inlet at the bottom of the crystallization granulation fluidized bed. The reagent dosing device includes a sodium fluoride dosing unit, a calcium chloride dosing unit, and a sodium hydroxide dosing unit. The outlet of the sodium fluoride dosing unit is connected to the main inlet pipeline via a peristaltic pump, and the confluence point is located between the centrifugal pump and the static mixer. The outlet of the calcium chloride dosing unit is directly connected to the lower part of the crystallization granulation fluidized bed via a peristaltic pump. The outlet of the sodium hydroxide dosing unit is also directly connected to the lower part of the crystallization granulation fluidized bed via a peristaltic pump. A water outlet pipeline is located at the top of the crystallization granulation fluidized bed, and an intake port is provided on the water outlet pipeline.

[0029] Combination Figure 1 The working process and principle of the crystallization granulation fluidized bed defluorination system of this application are as follows: Raw water in the storage tank enters the main inlet pipeline under the power of a centrifugal pump. During this process, a quantitatively added sodium fluoride solution flows into the pipeline, and the two are fully mixed under the action of a static mixer to form a simulated high-concentration fluoride-containing wastewater mixture. The mixture is fed from the bottom into a crystallization granulation fluidized bed filled with natural garnet seed crystals. At the same time, calcium chloride solution as a calcium agent and sodium hydroxide solution as a pH adjuster are pumped in from the lower side of the crystallization granulation fluidized bed. Inside the crystallization granulation fluidized bed, the upward water flow keeps the natural garnet seed crystals in a fluidized state. Fluoride ions in the aqueous phase and the added calcium ions undergo an induced crystallization reaction in an environment adjusted to the set pH, generating calcium fluoride crystals with the natural garnet seed crystals as the core and attaching them to their surface, thereby realizing the transfer and removal of fluoride from the aqueous phase to the solid phase. The defluorinated water is discharged from the system through the outlet pipeline at the top of the crystallization granulation fluidized bed.

[0030] Examples 1-3: Example 1: This embodiment provides a method for defluoridating fluoride-containing wastewater, including the following steps: The raw water in the storage tank is pumped into the main inlet pipeline. During this process, sodium fluoride solution is added to the raw water in a quantitative manner and mixed under the action of a static mixer to form fluoride-containing wastewater with an inlet fluoride ion content of 75 mg / L. Fluoride-containing wastewater was treated at a concentration of 0.63m³. 3A flow rate of / h is introduced from the bottom into a crystallizing fluidized bed filled with natural garnet seed crystals. At the same time, calcium chloride solution and sodium hydroxide solution are pumped in from the side and below. The addition of calcium chloride is controlled to make the Ca / F molar ratio 1.0, and the pH of the reaction system is controlled and adjusted to 6.5~7. This allows the fluoride ions in the fluoride-containing wastewater to react with the added calcium ions on the surface of the natural garnet seed crystals to induce crystallization and generate calcium fluoride crystals. After the crystallization reaction stabilizes, the defluorinated water is discharged from the system through the top outlet pipe of the crystallization granulation fluidized bed, resulting in defluorinated effluent.

[0031] Example 2: This embodiment provides a method for defluoridating fluoride-containing wastewater, including the following steps: The raw water in the storage tank is pumped into the main inlet pipeline. During this process, sodium fluoride solution is added to the raw water in a quantitative manner and mixed under the action of a static mixer to form high-load fluoride-containing wastewater with an inlet fluoride ion content of 600 mg / L. High-load fluoride-containing wastewater was treated at a rate of 0.63m³. 3 A flow rate of / h is introduced from the bottom into a crystallizing fluidized bed filled with natural garnet seed crystals. At the same time, calcium chloride solution and sodium hydroxide solution are pumped in from the side and below. The addition of calcium chloride is controlled to make the Ca / F molar ratio 1.0, and the pH of the reaction system is controlled and adjusted to 6.5~7. This allows the fluoride ions in the high-load fluoride-containing wastewater, which are in a highly supersaturated state, to not only induce crystallization on the surface of the natural garnet seed crystals, but also to trigger a large number of homogeneous nucleation reactions in the aqueous phase. The high-turbidity fluidized bed effluent is discharged from the top. Polyaluminum chloride and polyacrylamide are added to the effluent of a high-turbidity fluidized bed for coagulation and sedimentation treatment. After solid-liquid separation to remove suspended particles from the aqueous phase, defluorinated effluent is obtained.

[0032] Example 3: This embodiment provides a method for defluoridating fluoride-containing wastewater, including the following steps: The raw water in the storage tank is pumped into the main inlet pipeline. During this process, sodium fluoride solution is added to the raw water in a quantitative manner and mixed under the action of a static mixer to form fluoride-containing wastewater with an inlet fluoride ion content of 75 mg / L. Fluoride-containing wastewater was treated at a concentration of 0.63m³. 3 A flow rate of / h is introduced from the bottom into a crystallizing fluidized bed filled with natural garnet seed crystals. At the same time, calcium chloride solution and sodium hydroxide solution are pumped in from the side and below. The addition of calcium chloride is controlled to make the Ca / F molar ratio 1.5, and the pH of the reaction system is controlled and adjusted to 6.5~7. This allows the fluoride ions in the fluoride-containing wastewater to react with the added calcium ions on the surface of the natural garnet seed crystals to induce crystallization and generate calcium fluoride crystals. After the crystallization reaction stabilizes, the defluorinated water is discharged from the system through the top outlet pipe of the crystallization granulation fluidized bed, resulting in defluorinated effluent.

[0033] Comparative Examples 1-3: Comparative Example 1: Compared with Example 1, the difference is that the crystallization granulation fluidized bed is not filled with natural garnet seed crystals, but all other aspects are the same.

[0034] Comparative Example 2: Compared with Example 1, the difference is that the addition of calcium chloride is controlled so that the Ca / F molar ratio is 0.6, while all other aspects are the same.

[0035] Comparative Example 3: Compared with Example 2, the difference is that polyaluminum chloride and polyacrylamide are not added to the high-turbidity fluidized bed effluent for coagulation and sedimentation treatment, but all other aspects are the same.

[0036] Test Example 1-3: Test Example 1: The Influence of Influent Fluoride Ion Concentration on Treatment Efficiency and Shock Resistance Test The fluoride-containing wastewater with a fluoride ion content of 75 mg / L in Example 1, the fluoride-containing wastewater with a fluoride ion content of 600 mg / L in Example 2 and Comparative Example 3, and multiple groups of fluoride-containing wastewater with concentrations ranging from 100 mg / L to 500 mg / L were used as experimental subjects.

[0037] The influent flow rate for each experimental group was controlled at 0.63 m³ / s. 3 The reaction system pH is in the range of 6.5 to 7, and the continuous operation of the crystallization granulation fluidized bed is maintained under the condition that the calcium dosage remains unchanged.

[0038] After the system crystallization reaction reaches a stable state, samples are taken. The fluoride ion content in the fluidized bed effluent is determined using an ion meter and the fluoride ion removal rate is calculated. The residual calcium ion content in the effluent is determined using EDTA titration. The turbidity of each water sample is analyzed using a turbidity meter.

[0039] The residual fluoride ion content and system turbidity of the defluorinated effluent after coagulation and sedimentation treatment with polyaluminum chloride and polyacrylamide in Example 2 were measured to obtain comparative data.

[0040] Figure 2 The broken line with hollow circles represents the effluent fluoride content, and the column represents the fluoride ion removal rate. The data point with an influent fluoride content of 75 mg / L on the horizontal axis corresponds to the test results of Example 1, and the data point with an influent fluoride content of 600 mg / L on the horizontal axis corresponds to the test results of Example 3 in the fluidized bed effluent stage.

[0041] Figure 3The broken line with hollow circles represents the calcium ion content of the effluent, and the column represents the turbidity of the effluent. The test data with an influent fluoride content of 75 mg / L on the horizontal axis corresponds to Example 1, and the test data with an influent fluoride content of 600 mg / L on the horizontal axis corresponds to Example 3.

[0042] according to Figure 2 and Figure 3 Data shows that within the influent fluoride ion concentration range of 75 mg / L to 600 mg / L, the fluidized bed achieved an average fluoride ion removal rate of 89%. With increasing influent load, the calcium source consumed for calcium fluoride crystal formation increases, leading to a decrease in the residual calcium ion concentration in the effluent. When the influent fluoride ion concentration exceeds 500 mg / L, the effluent fluoride content shows an upward trend.

[0043] When the influent fluoride ion concentration is greater than or equal to 300 mg / L, the supersaturation inside the reaction system increases, and homogeneous nucleation occurs in the aqueous phase. Some calcium fluoride micronuclei are generated directly in the aqueous phase rather than attached to the surface of garnet seed crystals. These particles overflow the system with the rising water flow, leading to an increase in effluent turbidity, exhibiting the high turbidity phenomenon shown in Example 3 when the influent concentration is 600 mg / L. In Example 2, polyaluminum chloride and polyacrylamide were added at the downstream end of the fluidized bed. The charge neutralization and destabilization ability of polyaluminum chloride and the adsorption bridging effect of polyacrylamide were used to coagulate and separate suspended calcium fluoride particles in the aqueous phase. After adding coagulants, the effluent turbidity of the system remained below 30 NTU, and the overall fluoride ion removal rate reached 94.1%, compensating for the water quality fluctuation problem caused by a single fluidized bed under high supersaturation conditions.

[0044] Test Example 2: Effect of Ca / F molar ratio on effluent quality and crystallization reaction kinetics The fluoride-containing wastewater treatment processes of Examples 1, 3 and Comparative Example 2 were used as experimental subjects.

[0045] The fluoride ion content in the influent was controlled at 75 mg / L, the pH of the reaction system was adjusted to be between 6.5 and 7, and the influent flow rate was 0.63 m³ / h.

[0046] The dosage of calcium chloride was adjusted to allow the system to operate under Ca / F molar ratios of 1.0, 1.5, and 0.6, respectively.

[0047] During fluidized bed operation, samples were taken from the outlet at time intervals, and the fluoride ion content in the effluent was measured using an ion meter at different operating times.

[0048] Based on the effluent fluoride content measurement results at various time points, the corresponding fluoride removal rate was calculated, and a linear fitting analysis was performed on the trend of crystallization removal rate under different Ca / F ratio conditions.

[0049] Figure 4The light red broken-line area with squares represents the test data when the Ca / F molar ratio is 0.6, corresponding to Example 2; the light green broken-line area with circles represents the test data when the Ca / F molar ratio is 1.0, corresponding to Example 1; the light purple broken-line area with triangles represents the test data when the Ca / F molar ratio is 1.5, corresponding to Example 3.

[0050] Figure 5 The light red broken line with squares and the corresponding fitted line represent data with a Ca / F molar ratio of 0.6, corresponding to Example 2; the light green broken line with circles and the corresponding fitted line represent data with a Ca / F molar ratio of 1.0, corresponding to Example 1; the light purple broken line with triangles and the corresponding fitted line represent data with a Ca / F molar ratio of 1.5, corresponding to Example 3.

[0051] according to Figure 4 and Figure 5 Data shows that during the initial startup phase of the fluidized bed system, the system undergoes an induced process of crystal nucleation and crystal growth, resulting in a decreasing trend in effluent fluoride content across all groups. After 3 hours of operation, the crystallization reaction within the system stabilizes, and the change in effluent fluoride content over time becomes more gradual. Comparison of data with different dosing ratios reveals that increasing the concentration of calcium ions in the system promotes the forward shift of the precipitation reaction. When the Ca / F ratio is 0.6, the stable effluent fluoride content is 24.25 mg / L; increasing the Ca / F ratio to 1.0 and 1.5 reduces the stable effluent fluoride content to 16.34 mg / L and 14.04 mg / L, respectively. This phenomenon of decreasing residual fluoride ions in the aqueous phase by increasing calcium ion concentration is consistent with the thermodynamic laws of crystallization.

[0052] When examining the kinetic characteristics of the reaction system, the decrease in effluent fluoride concentration was not positively correlated with the crystallization removal rate. Linear fitting comparison of the fluoride removal rate revealed that the slope of the fitted line in Representative Example 1 (Ca / F = 1.0) was higher, indicating a higher fluoride removal rate under these conditions. Increasing the calcium source dosage to achieve a Ca / F ratio of 1.5 resulted in an increase in residual calcium ions in the effluent, inhibiting the overall fluoride removal rate. Based on the effluent quality and reaction kinetic data, controlling the Ca / F ratio at 1.0 under the given influent fluoride concentration conditions is the suitable process condition that balances crystallization rate and reagent consumption.

[0053] Test Example 3: Test of Surface Morphology and Composition of Induced Seed Crystals The garnet seed crystals filled in the fluidized bed before the operation of Example 1, and the seed crystals taken out from the fluidized bed after the crystallization reaction of Example 1 was stable, were used as experimental subjects.

[0054] After the reaction, the seed crystals were taken out from the fluidized bed of crystallization granulation in Example 1, washed with deionized water to remove surface residues, and placed in a drying oven for drying treatment. Untreated garnet seed crystals were prepared as a control.

[0055] The dried, untreated seed crystals and the reacted seed crystals were respectively fixed onto conductive adhesive and sputtered with gold. The surface morphology of the seed crystals was observed using a scanning electron microscope.

[0056] The equipped energy dispersive spectrometer was used to perform elemental surface scanning and quantitative analysis on the seed crystal surface to obtain the elemental types and mass fractions of the material on the seed crystal surface and to analyze the chemical composition of the crystal deposits.

[0057] Figure 6 These are scanning electron microscope (SEM) morphology comparison images of the induced seed crystals before and after operation in Example 1 of this application. Figure 6 Subgraph of (a) and Figure 6 The sub-image in (b) corresponds to the untreated seed crystal surface morphology before operation in Example 1. Figure 6 Subgraph of (c) and Figure 6 The sub-image in (d) corresponds to the surface morphology of the seed crystal after the operation of Example 1.

[0058] according to Figure 6 The image shows that the surface of the untreated garnet seed crystals before operation was smooth and had cleavage planes. After the fluidized bed crystallization induction process, the surface morphology of the seed crystals changed, with layered and granular attachments covering the surface, exhibiting a porous morphology. This morphological change indicates that the reactants in the aqueous phase underwent a crystallization reaction on the seed crystal surface. The nucleation and growth of crystals on the garnet carrier surface, and the increased surface roughness, provided crystallization attachment sites for the reactant ions in the aqueous phase, maintaining the crystallization removal capacity of the fluidized bed system.

[0059] according to Figure 7 and Figure 8 Data, Figure 7 The untreated garnet seed crystals showed the presence of silicon, oxygen, aluminum, iron, and calcium on their surface, but no fluorine was detected. After crystallization and fluorine removal, Figure 8 The results indicate that the mass fraction of fluorine on the seed crystal surface after the reaction is 41.65%, the mass fraction of calcium is 22.67%, and the total mass ratio of fluorine and calcium is 64.32%. Converting the mass fractions of fluorine and calcium, their molar ratio on the seed crystal surface is approximately 2:1, consistent with the stoichiometric ratio of calcium fluoride. Morphological characteristics and elemental analysis show that fluoride ions in the system react with the added calcium agent on the surface of the garnet seed crystal to form a crystallization precipitation reaction, generating a product containing calcium fluoride.

Claims

1. A method for defluoridating fluoride-containing wastewater, characterized in that, include: Fluorine-containing wastewater is passed from the bottom into a crystallization and granulation fluidized bed filled with natural garnet seeds. The upward flow of water keeps the natural garnet seeds in a fluidized state. Simultaneously, calcium chloride solution as a calcium agent and sodium hydroxide solution as a pH adjuster are pumped in from the side and below, respectively. The addition of the calcium chloride solution is controlled to make the Ca / F molar ratio a preset value, and the pH value of the reaction system is controlled and adjusted so that the fluoride ions in the fluoride-containing wastewater and the added calcium ions undergo an induced crystallization reaction on the surface of the natural garnet seed crystal to generate calcium fluoride crystals. After the crystallization reaction stabilizes, the defluorinated water is discharged from the system through the top outlet pipe of the crystallization granulation fluidized bed to obtain defluorinated effluent.

2. The method for defluoridation of fluoride-containing wastewater according to claim 1, characterized in that, The addition of the calcium chloride solution is controlled to maintain a Ca / F molar ratio of 1.0 to 1.5, and the pH of the reaction system is controlled to be 6.5 to 7.

3. The method for defluoridation of fluoride-containing wastewater according to claim 2, characterized in that, The addition of the calcium chloride solution is controlled so that the Ca / F molar ratio is 1.

0.

4. The method for defluoridation of fluoride-containing wastewater according to claim 3, characterized in that, The natural garnet seed crystals have a grain size of 80 to 100 mesh.

5. The method for defluoridation of fluoride-containing wastewater according to claim 1, characterized in that, The specific method for pumping the calcium chloride solution and the sodium hydroxide solution from the side and below is as follows: the inlet point of the sodium hydroxide solution is located above the inlet point of the calcium chloride solution.

6. The method for defluoridation of fluoride-containing wastewater according to claim 1, characterized in that, When the influent fluoride concentration of the fluoride-containing wastewater is greater than or equal to 300 mg / L, the fluoride ions in the aqueous phase, which are in a highly supersaturated state, not only induce crystallization on the surface of the natural garnet seed crystals, but also trigger a large number of homogeneous nucleation reactions in the aqueous phase. The defluorinated water is discharged from the top outlet pipe of the crystallizing granulation fluidized bed to obtain high turbidity fluidized bed effluent. Coagulant and flocculant are added to the high-turbidity fluidized bed effluent for coagulation and sedimentation treatment. After solid-liquid separation to remove suspended particles from the aqueous phase, the defluorinated effluent is obtained.

7. The method for defluoridation of fluoride-containing wastewater according to claim 6, characterized in that, The influent fluoride concentration of the fluoride-containing wastewater is 600 mg / L.

8. The method for defluoridation of fluoride-containing wastewater according to claim 6, characterized in that, The coagulant is polyaluminum chloride, and the flocculant is polyacrylamide.

9. A method for defluoridating fluoride-containing wastewater according to claim 8, characterized in that, By utilizing the charge neutralization and destabilization capabilities of the polyaluminum chloride and the adsorption bridging effect of the polyacrylamide, suspended calcium fluoride particles in the aqueous phase are precipitated and separated, thereby maintaining the turbidity of the defluorinated effluent below 30 NTU.

10. The method for defluoridation of fluoride-containing wastewater according to claim 1, characterized in that, The influent flow rate of the fluoride-containing wastewater is controlled at 0.63 m³ / h. 3 / h, the natural garnet seed crystals fill the static bed layer in the crystallization granulation fluidized bed to a height of 80cm.

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