A method for treating fluorine-containing wastewater

Abstract

This invention relates to the field of chemical wastewater treatment, specifically a method for treating fluoride-containing wastewater. The method includes the following steps: mixing light magnesium oxide with water and then subjecting it to a hydration reaction to prepare a magnesium-based reactive emulsion with nano-magnesium hydroxide as the main component; adding the prepared magnesium-based reactive emulsion to the fluoride-containing wastewater and stirring the reaction; adding alkali solution to the reacted system to adjust the pH to 3.5-7 and then reacting to generate magnesium fluoride crystals; after the reaction is complete, solid-liquid separation and drying are performed to obtain the magnesium fluoride product. This invention features an extremely simple process, convenient operation, mild influent and reaction conditions, no need for additional flocculants, high treatment efficiency, and simultaneous recovery of high-purity magnesium fluoride.

Description

Technical Field

[0001] This invention relates to the field of chemical wastewater treatment, specifically a method for treating fluoride-containing wastewater. Background Technology

[0002] Current methods for treating fluoride-containing wastewater mainly include electrodialysis, chemical precipitation, ion exchange, membrane separation, and adsorption. Among these, electrodialysis and membrane separation have high requirements for equipment and operating conditions, resulting in significant investment and maintenance costs. Ion exchange and adsorption are susceptible to interference from coexisting ions under complex water quality conditions, and the regeneration process of adsorbents or resins is complex, leading to high operating costs. In contrast, chemical precipitation remains one of the most widely used methods for treating industrial fluoride-containing wastewater due to its simple process flow, high treatment efficiency, stable operation, and suitability for large-scale industrial applications.

[0003] Existing chemical precipitation methods typically involve adding calcium hydroxide or soluble calcium salts (such as calcium chloride) to wastewater to precipitate fluoride ions as calcium fluoride. However, in actual industrial fluoride-containing wastewater, sulfate ions are often present. When sulfate and fluoride ions coexist, sulfate ions readily react with calcium ions to form calcium sulfate precipitate. This not only consumes the precipitant but also significantly reduces the efficiency of calcium fluoride formation and leads to poor precipitation performance, thereby affecting subsequent solid-liquid separation and stable system operation.

[0004] To address the aforementioned issues, existing technologies have attempted to circumvent the interference of sulfate ions on calcium precipitation reactions by introducing magnesium-based agents or composite agent systems. For example, patent CN113816480A discloses a defluorinating agent and method for a mixed solution containing sulfate and fluoride ions. This method removes fluoride ions and reduces the influence of sulfate ions by adding a defluorinating agent and catalyst under specific pH conditions. However, this method has strict requirements on the pH conditions of the influent, typically requiring the solution to be adjusted to a near-neutral range. Furthermore, the treatment process necessitates water bath heating, increasing energy consumption and operating costs, thus limiting its applicability to acidic fluoride-containing wastewater. In addition, in processes such as glass thinning, fluorinated chemical industry and hydrometallurgy, wastewater is usually acidic and high in salinity. When the sulfate concentration reaches 1 g / L or more, existing magnesium-based defluorination methods generally suffer from problems such as decreased reaction efficiency, fine precipitates, and difficulty in solid-liquid separation. They often need to add organic polymer flocculants such as polyacrylamide to complete the subsequent pressure filtration treatment, which leads to increased operating costs and also affects the purity and resource utilization of magnesium fluoride products. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problems of low treatment efficiency, difficult solid-liquid separation, poor purity of magnesium fluoride product, and high operating cost of acidic, high-salinity wastewater containing both fluoride and sulfate ions in the prior art. The invention provides a method for treating fluoride-containing wastewater that is extremely simple, easy to operate, has mild influent and reaction conditions, requires no additional flocculant, has high treatment efficiency, and can simultaneously recover high-purity magnesium fluoride. This method has low equipment requirements and low operating costs, and is suitable for industrial application.

[0006] The technical solution includes the following steps: A magnesium-based reactive emulsion with nano-magnesium hydroxide as the main component is prepared by mixing light magnesium oxide with water and then carrying out a hydration reaction. Preferably, the light magnesium oxide is added to water, and the magnesium oxide content in the mixture is controlled to be 3wt%–10wt%, with a hydration soaking time of 12–48 h; the stirring speed is controlled to be 300–700 r / min, while continuous bottom aeration is carried out at an aeration rate of 50–300 mL / min; the MgO content in the light magnesium oxide is 85–95wt%, and its loose bulk density is ≤0.5 g / cm³. 3 The magnesium-based reactive emulsion is a nano-dispersion system. The Z-average particle size measured by dynamic light scattering is 80-300 nm, and the polydispersity index (PDI) is ≤0.15.

[0007] (ii) Add the magnesium-based reactive emulsion obtained in step (i) to the fluoride-containing wastewater and stir to react; preferably, in step (ii), the magnesium-based reactive emulsion is added at a magnesium to fluoride molar ratio of 0.3 to 0.8. The stirring reaction time is 30 to 60 minutes; (iii) Add alkali solution to the system after the reaction in step (ii) to adjust the pH to 3.5-7, and then proceed with the reaction to generate magnesium fluoride crystals; Preferably, in step three), the concentration of the alkali solution is 20%~30wt%, and the reaction time after adding the alkali solution is 20~40min; the alkali in the alkali solution can be sodium hydroxide, potassium hydroxide, or sodium carbonate; soluble magnesium salt is added simultaneously or after the above reaction for further reaction, the soluble magnesium salt being magnesium chloride or magnesium sulfate; the amount of soluble magnesium salt added is controlled at a molar ratio of magnesium ions to fluoride ions in the original fluoride-containing wastewater of 0.1~0.3; (iv) After the reaction is complete, the resulting slurry undergoes solid-liquid separation and drying without the addition of additional organic polymer flocculants to obtain magnesium fluoride. Solid-liquid separation can be performed using a plate and frame filter press, followed by filtration to obtain a stable, shaped magnesium fluoride cake. The cake is dried naturally or placed in an electrically heated forced-air dryer at a specific temperature. Dry at 40~60℃ for 12~24 hours.

[0008] The inventors discovered that one of the key reasons affecting the treatment efficiency of chemical precipitation for acidic fluoride-containing wastewater is the generally large crystal size, slow hydration, and easy passivation of magnesium oxide used in the past, resulting in low activity of the magnesium-based reaction system. Therefore, this application makes the following improvements: In step one), light magnesium oxide is specifically selected as the raw material for the hydration reaction. It possesses characteristics of high specific surface area, low bulk density, fine grains, and numerous pores. During the hydration reaction, it can significantly accelerate the hydration rate and promote the growth of Mg. 2+ The leaching / effective exposure of Mg–OH active sites effectively improves the neutralization and defluorination efficiency of fluoride-containing wastewater; the MgO content in the light magnesium oxide is 85~95wt%, and its loose density is ≤0.5 g / cm³. 3 (Definition parameters for lightweight).

[0009] Furthermore, under low solid content conditions, the coupling of stirring and bottom aeration achieves full hydration of light magnesium oxide, effectively suppressing the collision, adhesion, and secondary agglomeration of magnesium oxide particles that are prone to occur during hydration under non-aeration conditions. This results in a magnesium-based reactive emulsion with a narrow particle size distribution, a nanoscale single-peak dispersion system (Z-average particle size 80-300 nm, PDI ≤ 0.15), which can provide a highly active and uniform reaction interface in acidic fluorine-containing systems, promoting the controllable nucleation and growth of magnesium fluoride. The generated magnesium fluoride precipitate particles have a stable structure and a certain degree of self-support, thus enabling rapid solid-liquid separation of magnesium fluoride slurry and improving filtration stability without the need for additional organic polymer flocculants, resulting in magnesium fluoride cake with stable filtration performance.

[0010] The stirring speed is preferably controlled at 300–700 r / min, and the aeration rate at 50–300 mL / min. Variable frequency stirring is preferred. Stirring provides shear force, while bottom aeration achieves continuous dispersion. The synergistic effect of these two methods inhibits collision adhesion and secondary agglomeration between magnesium oxide particles, ensuring complete hydration of magnesium oxide to generate nano-magnesium hydroxide. Compared to unhydrated light magnesium oxide, the nano-magnesium oxide in this nano-dispersion system has a larger specific surface area and higher reactivity and dispersibility. The nano-magnesium hydroxide prepared in this way possesses excellent fluoride ion adsorption capacity, laying the foundation for subsequent efficient fluoride removal and solving the problems of low reactivity and fine precipitation of existing magnesium-based agents in acidic sulfate-containing systems. Simultaneously, the alkaline characteristics of nano-magnesium hydroxide can neutralize the acidity of wastewater in subsequent steps, reducing alkali consumption. The magnesium oxide mixture ratio is preferably controlled at 3–10 wt%, ensuring sufficient hydration while reducing the solid content of the hydration system, decreasing particle collision frequency, and improving the dispersion stability of the emulsion. The hydration time is 12 to 48 hours. If it is too long, it will cause maturation and growth, as well as flocculation and sedimentation, resulting in a decrease in specific surface area and reduced activity. If it is too short, the hydration will be incomplete, resulting in insufficient formation of nano-magnesium hydroxide, and insufficient dispersibility and reactivity.

[0011] In step two), the pH of the fluoride-containing wastewater is not pre-adjusted. Instead, the magnesium-based reactive emulsion from step one is added first. Since the added magnesium-based reactive emulsion is alkaline, it can neutralize the acid in the wastewater, thus reducing the amount of liquid alkali used, lowering alkali consumption and operating costs. Furthermore, the magnesium ions released during the neutralization reaction promote the formation of magnesium fluoride. A suitable amount of magnesium ions can be released to react with fluoride ions in a preliminary crystallization reaction. Simultaneously, nano-magnesium hydroxide adsorbs fluoride ions, forming a localized concentration enrichment, which is beneficial for the combination of fluoride and magnesium ions, accelerating the precipitation of magnesium fluoride. This process is free from sulfate interference, solving the problem of sulfate preferentially consuming the precipitant in traditional calcium-based precipitation methods. The alkali in the solution can be selected from sodium hydroxide or potassium hydroxide, etc. The emulsion is added at a magnesium-fluoride molar ratio of 0.3~0.8. Adding too much will increase the cost of chemical dosing and reduce the purity of the recovered magnesium fluoride; adding too little will result in insufficient acid neutralization and low fluoride removal efficiency.

[0012] In step three, adjusting the pH with liquid alkali provides the optimal reaction environment for magnesium fluoride crystallization, enabling rapid crystal formation. Adding soluble magnesium salts solves the problem of incomplete fluoride ion removal due to insufficient magnesium ions. The synergistic effect of these two methods increases the fluoride removal rate to over 90%, while also promoting magnesium fluoride crystal growth, resulting in a more stable precipitate particle structure and excellent solid-liquid separation performance. Preferably, the solution pH is adjusted to 3.5–7.0, and the reaction continues for 20–40 minutes after pH adjustment. This pH range is the optimal acidity / alkalinity for magnesium fluoride crystallization, promoting rapid combination of magnesium and fluoride ions to form magnesium fluoride crystals and preventing excessively high pH levels from causing precipitation of other metal ions and affecting the purity of the magnesium fluoride product. A liquid alkali concentration of 20%–30% balances pH adjustment efficiency with operational safety, avoiding sudden local pH changes due to excessively high concentrations. The dosage of magnesium salt was controlled according to a molar ratio of magnesium ions to fluoride ions in the original fluoride-containing wastewater of 0.1. After addition, the reaction was continuously stirred for 20-40 minutes. Adding magnesium salt significantly increased the concentration of reactive magnesium ions in the system, maintaining the Mg... 2+ When the activity is within a suitable range, it promotes the continued growth of magnesium fluoride crystals, achieving further deep removal of fluoride ions, reducing the residual fluoride ion concentration in the system, and further increasing the size of magnesium fluoride precipitate particles, thereby improving its solid-liquid separation performance.

[0013] In step four), after the reaction is complete, the resulting magnesium fluoride slurry does not require the addition of additional organic polymer flocculants and can be directly separated into solid and liquid phases using a plate and frame filter press. Because the generated magnesium fluoride precipitate particles have a stable structure and self-supporting properties, the filtration process proceeds smoothly, the filtrate is clear and transparent, the filter cake forms well, and there are no issues such as mud runoff or filtration difficulties. The separation efficiency is high, reducing operating costs and preventing the introduction of impurities, thus ensuring the high purity of the magnesium fluoride product. The drying method can be natural air drying or drying with an electric heating blower. If drying is used, the drying temperature should be controlled at 40~60℃ and the drying time at 12~24h. This mild drying condition avoids structural changes in the magnesium fluoride cake due to high-temperature drying, ensuring stable product performance.

[0014] Beneficial effects: This invention constructs a magnesium-based reaction system with good reactivity by stirring and hydrating light magnesium oxide. This system is then added to acidic wastewater containing both fluoride and sulfate ions. The pH of the system is adjusted using liquid alkali, and magnesium salts are subsequently added. This achieves efficient removal of fluoride ions from the wastewater and simultaneous recovery of magnesium fluoride. Through the coordinated control of the reaction system construction and process parameters, magnesium fluoride can be stably generated and effectively separated into solid and liquid components even under acidic and sulfate-containing conditions. This avoids the dependence on organic polymer flocculants in traditional processes. The system operates stably, the reaction conditions are mild, the process flow is simple, and it is suitable for continuous industrial operation. Simultaneously, it realizes the recovery and utilization of fluoride resources from fluoride-containing wastewater. Furthermore, in this invention, a highly dispersed magnesium hydroxide reaction emulsion is generated and maintained in situ in an acidic fluoride-containing system to stabilize the nucleation and growth process of magnesium fluoride. Therefore, the generation and recovery of magnesium fluoride can still be stably achieved in acidic fluoride-containing wastewater with a sulfate ion concentration of not less than 1 g / L. This overcomes the problem of significantly reduced reaction efficiency and separation performance under acidic conditions with the coexistence of sulfate ions in the prior art. It can treat various fluoride-containing wastewaters, especially acidic wastewater with the coexistence of fluoride ions and sulfate ions (pH≤2.4, sulfate concentration not less than 1 g / L). Attached Figure Description

[0015] Figure 1 is a process flow diagram of wastewater defluorination and magnesium fluoride recovery in Examples 5, 6 and 7; Figure 2 shows SEM images of the light magnesium oxide before and after hydration in Example 6, where (a) is before magnesium oxide hydration and (b) is after magnesium oxide hydration. Figure 3 shows the dynamic light scattering (DLS) particle size distribution of the magnesium-based reactive emulsion prepared in Example 6; Figure 4 shows the results of fluoride ion treatment in wastewater in Examples 1-7 and Comparative Examples 1 and 2; Figure 5 shows the XRD patterns of the magnesium fluoride products recovered in Examples 1-7; Figure 6 is a picture of the sludge cake obtained by pressure filtration in Example 6. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings. However, the implementation of the present invention is not limited to the following embodiments. Unless otherwise specified, the percentages in the following embodiments are all mass percentages.

[0017] The water quality of the fluoride-containing wastewater used in the following examples is shown in the table below. The fluoride-containing wastewater is acidic wastewater containing both fluoride and sulfate ions, with a pH of 1.7 and fluoride and sulfate concentrations of 17240 ppm and 1169.86 ppm, respectively. The light magnesium oxide described below was purchased from Yingkou Ruichen Magnesium Industry Co., Ltd., and its bulk density is ≤0.5 g / cm³. 3 All other raw materials and equipment were purchased commercially.

[0018] Table 1. Water quality of fluoride-containing wastewater Example 1 Light magnesium oxide with a MgO content of 90wt% was added to a hydration tank, and water was added to prepare a magnesium oxide emulsion with a mass fraction of 10%. Under stirring and aeration conditions (using mechanical variable frequency stirring at a speed of 500r / min, while continuously aerating at the bottom at an aeration rate of 200mL / min), hydration and soaking were carried out for 12 hours to ensure full hydration and to prepare a magnesium-based reactive emulsion with nano-magnesium hydroxide as the main component.

[0019] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 267.8 nm, and the polydispersity index (PDI) was 0.128, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0020] Magnesium-based reactive emulsion and fluoride-containing wastewater (acidic wastewater containing both fluoride and sulfate ions) were injected into the reaction vessel, and stirring was started simultaneously. The reaction was carried out for 30 minutes. The injection ratio of magnesium-based reactive emulsion to fluoride-containing wastewater was controlled at a molar ratio of magnesium ions to fluoride ions of 0.6.

[0021] Add 30wt% liquid alkali to the reaction vessel to adjust the pH to 4.0, and react for 30 minutes to allow magnesium ions and fluoride ions in the solution to react and form magnesium fluoride crystals. After the reaction is complete, discharge the solution into a plate and frame filter press for mud-water separation. Dry the sludge pressed from the filter press at 50℃ for 24 hours to obtain magnesium fluoride product.

[0022] Example 2 Light magnesium oxide with a MgO content of 90wt% was added to a hydration tank, and water was added to prepare a magnesium oxide emulsion with a mass fraction of 10%. Under stirring and aeration conditions (using mechanical variable frequency stirring at a speed of 700r / min, while continuously aerating at the bottom at an aeration rate of 50mL / min), hydration and soaking were carried out for 36 hours to ensure full hydration and to prepare a magnesium-based reactive emulsion with nano-magnesium hydroxide as the main component.

[0023] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 206.2 nm, and the polydispersity index (PDI) was 0.112, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0024] Magnesium-based reactive emulsion and fluoride-containing wastewater (acidic wastewater containing both fluoride and sulfate ions) were injected into the reaction vessel, and stirring was started simultaneously. The reaction was carried out for 60 minutes. The injection ratio of magnesium-based reactive emulsion to fluoride-containing wastewater was controlled at a molar ratio of magnesium ions to fluoride ions of 0.8.

[0025] 20wt% liquid alkali was added to the reaction vessel to adjust the pH to 5.0. The reaction was allowed to proceed for 40 minutes, allowing magnesium ions in the solution to react with fluoride ions to form magnesium fluoride crystals. After the reaction was complete, the solution was discharged into a plate and frame filter press for mud-water separation. The mud pressed out of the filter press was dried at 50℃ for 24 hours to obtain magnesium fluoride product.

[0026] Example 3 Light magnesium oxide with a MgO content of 90wt% was added to a hydration tank, and water was added to prepare a magnesium oxide emulsion with a mass fraction of 10%. Under stirring and aeration conditions (using mechanical variable frequency stirring at a speed of 300r / min, while continuously aerating at the bottom at an aeration rate of 300mL / min), hydration and soaking were carried out for 48h to ensure full hydration and to prepare a magnesium-based reactive emulsion with nano-magnesium hydroxide as the main component.

[0027] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 188.6 nm, and the polydispersity index (PDI) was 0.092, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0028] Magnesium-based reactive emulsion and fluoride-containing wastewater (acidic wastewater containing both fluoride and sulfate ions) were injected into the reaction vessel, and stirring was started simultaneously. The reaction was carried out for 50 minutes. The injection ratio of magnesium-based reactive emulsion to fluoride-containing wastewater was controlled at a molar ratio of magnesium ions to fluoride ions of 0.3.

[0029] 25wt% liquid alkali was added to the reaction vessel to adjust the pH to 7.0. The reaction was allowed to proceed for 20 minutes, allowing magnesium ions in the solution to react with fluoride ions to form magnesium fluoride crystals. After the reaction was complete, the solution was discharged into a plate and frame filter press for mud-water separation. The mud pressed out of the filter press was dried at 50℃ for 24 hours to obtain the magnesium fluoride product.

[0030] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 188.6 nm, and the polydispersity index (PDI) was 0.092, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0031] Example 4 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 3, yielding magnesium fluoride product. The difference lies in the MgO content of the light magnesium oxide being 95%.

[0032] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 188.9 nm, and the polydispersity index (PDI) was 0.089, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0033] Example 5 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 4, and magnesium fluoride is finally obtained. The difference is that after adjusting the pH of the solution with liquid alkali, 30% magnesium chloride solution is added to the reaction tank. The amount added is based on a molar ratio of magnesium ions to fluoride ions in the original fluoride-containing wastewater of 0.1. The reaction is stirred for 30 minutes, and after the reaction is completed, it is discharged into a plate and frame filter press.

[0034] Example 6 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 5, yielding magnesium fluoride product. The difference lies in the fact that during the hydration of light magnesium oxide, the mass fraction of magnesium oxide in the mixture of light magnesium oxide and water is 5%, and the amount of magnesium chloride solution added is based on a molar ratio of magnesium ions to fluoride ions in the original fluoride-containing wastewater of 0.2. The mixture is stirred and reacted for 20 minutes, and after the reaction is completed, it is discharged into a plate and frame filter press.

[0035] Example 7 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 5, yielding magnesium fluoride product. The difference lies in the fact that during the hydration of light magnesium oxide, the mass fraction of magnesium oxide in the mixture of light magnesium oxide and water is 3%, and the amount of magnesium chloride solution added is based on a molar ratio of magnesium ions to fluoride ions in the original fluoride-containing wastewater of 0.3. The mixture is stirred and reacted for 20 minutes, and after the reaction is completed, it is discharged into a plate and frame filter press.

[0036] Comparative Example 1 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 1, yielding magnesium fluoride product. The difference lies in the stirring and soaking time during the hydration process, which is 0.5 hours.

[0037] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25℃ with water as the dispersion medium. The results showed that the Z-average particle size of the obtained magnesium-based reactive emulsion was 289.6 nm, and the polydispersity index (PDI) was 0.163, exhibiting a unimodal distribution, indicating that the emulsion has good nanodispersibility and uniformity.

[0038] Comparative Example 2 The method for treating fluoride-containing wastewater and recovering magnesium fluoride is basically the same as in Example 3, yielding magnesium fluoride product. The difference lies in the use of heavy magnesium oxide (loose bulk density > 5 g / cm³). 3 ).

[0039] Figure 2 shows the SEM images of the light magnesium oxide before and after hydration in Example 6. Before hydration, MgO appears as short rod-shaped or columnar nanocrystalline polymers. After hydration, the rod-shaped MgO transforms into stacked nanopetal-shaped nanoparticles, exhibiting a typical nano-Mg(OH)2 morphology. The generated nano-Mg(OH)2 has a larger specific surface area than MgO, further enhancing its adsorption capacity for fluoride ions. - The localized concentration enrichment further accelerated the Mg... 2+ With F - The precipitation reaction.

[0040] The obtained magnesium-based reactive emulsion sample was subjected to particle size analysis using dynamic light scattering (DLS) at 25°C with water as the dispersion medium. The results are shown in Figure 3. The emulsion particles were mainly concentrated in the range of approximately 80–220 nm, exhibiting a unimodal distribution, indicating that the hydration products were uniformly dispersed as nanoscale particles. Combined with the DLS test results, the Z-average particle size of the obtained magnesium-based reactive emulsion was 147.9 nm, and the polydispersity index (PDI) was 0.049, also showing a unimodal distribution. This indicates that the emulsion possesses good nanodispersibility and uniformity, thus providing a uniformly dispersed and highly active reaction interface in acidic fluorine-containing systems. This is beneficial for the subsequent nucleation and growth of magnesium fluoride precipitates and improves the stability of solid-liquid separation.

[0041] The above embodiments were all pilot-scale engineering demonstrations conducted at the pilot plant. The pilot-scale processes for embodiments 5-7 are described below. Figure 1 First, light magnesium oxide is fully hydrated to obtain an emulsion containing nano-magnesium hydroxide. This emulsion is then added to acidic fluoride-containing wastewater and mixed thoroughly. During this process, acid-base neutralization and magnesium fluoride crystallization occur, thus generating magnesium fluoride precipitate while neutralizing some of the acid. Next, liquid alkali is added to adjust the pH, and magnesium salts are added to further promote the precipitation of magnesium fluoride. Finally, the sludge is filtered out using a filter press to achieve sludge-water separation.

[0042] Figure 4 shows the results of fluoride ion treatment in wastewater from Examples 1-7 and Comparative Examples 1 and 2. Examples 1 and 3 show that extending the hydration time of magnesium oxide from 24 hours to 48 hours increased the fluoride removal rate from 74.99% to 84.21%. Examples 3 and 4 show that increasing the magnesium oxide content from 90% to 95% improved the fluoride removal rate. Examples 5, 6, and 7 showed improvements in fluoride removal rates when the magnesium oxide hydration concentration was 10%, 5%, and 3%, respectively. This indicates that the magnesium oxide content, hydration concentration, and hydration time all significantly affected the fluoride removal rate. Examples 5, 6, and 7 show that the fluoride removal rate exceeded 90% after adding magnesium salts. Furthermore, comparing the fluoride removal rate of Example 1 (74.99%) with that of Comparative Example 1 (65.33%) shows that appropriately extending the hydration time can significantly improve the fluoride removal rate. Compared to Example 3, Comparative Example 2 used heavy magnesium oxide for defluorination, and the defluorination rate was significantly reduced from 80.13% to 50.56%.

[0043] As shown in Figure 5, the XRD analysis results indicate that the diffraction characteristic peaks of the products obtained in Examples 1-7 are basically consistent with those of standard MgF2 (PDF#41-1443). However, a small number of impurity peaks are still visible in the spectra of Examples 1, 2, and 3. When the magnesium oxide content is increased from 90% to 95%, the impurity peaks in Examples 4-7 disappear significantly, and the product purity is significantly improved. Furthermore, as shown in Figure 6, after the defluorination reaction, no organic polymeric flocculant was added to the resulting reaction slurry, which was directly fed into a plate and frame filter press for solid-liquid separation. The filtration process proceeded smoothly, the filtrate was clear and transparent, the filter cake formed well, and there was no significant mud runoff or filtration difficulty. The test results of the obtained magnesium fluoride products are shown in Table 2. The F content is greater than 45%, the Mg content is greater than 28%, and the mass fractions of the remaining components also meet the requirements of grade MF-2. The overall performance conforms to the YS / T691-2009 standard. In Example 4, magnesium oxide was added according to the Mg / F molar ratio. Experimental results showed that the best overall effect was achieved when Mg / F = 0.6: it could neutralize some of the acidity and release Mg²⁺ to participate in the defluorination reaction. As the dosage increased from 0.6 to 0.8, the pH of the system did not increase significantly and remained at approximately 2; only when Mg / F > 0.8 could the pH rise above 3, while the residual fluoride ions could be reduced to approximately 200 ppm. However, under these addition conditions, the purity of the recovered magnesium fluoride product could not meet the requirements (Mg ≥ 23%, F ≥ 45%).

[0044] In summary, this invention, through the construction of a magnesium-based reaction system with specific reaction kinetics and the coordinated control of process parameters, has achieved an engineered treatment method for the efficient recovery of magnesium fluoride in acidic fluoride-containing wastewater with a sulfate ion concentration of not less than 1 g / L without the addition of external organic polymer flocculants, which has significant practical value.

Claims

1. A method for treating fluorine-containing wastewater, characterized by, The method comprises the following steps: I) mixing light magnesium oxide with water and carrying out hydration reaction to prepare magnesium-based reaction emulsion with nano magnesium hydroxide as main body; II) adding the magnesium-based reaction emulsion prepared in step I) into fluorine-containing wastewater and stirring to react; III) adding alkali liquor to the system after step II) to adjust the pH value to 3.5-7 and then carrying out reaction to generate magnesium fluoride crystals; IV) carrying out solid-liquid separation and drying treatment after the reaction to obtain magnesium fluoride product.

2. The fluorine-containing wastewater treatment method according to claim 1, wherein In step I), the light magnesium oxide is added into water, the proportion of magnesium oxide in the mixed solution is controlled to be 3-10wt%, and the hydration soaking time is 12-48h.

3. The method for treating fluorochemical wastewater according to claim 1, wherein In step I), the stirring speed is controlled to be 300-700r / min, and the bottom is continuously aerated with an aeration amount of 50-300mL / min.

4. The method of claim 1 wherein the fluorine-containing waste water is a waste water from a semiconductor manufacturing process. In the step one), the MgO content in the light magnesium oxide is 85-95wt%, and the loose bulk density thereof is ≤0.5 g / cm 3 .

5. The fluorine-containing wastewater treatment method according to any one of claims 1 to 4, characterized by, In step I), the magnesium-based reaction emulsion is a nano dispersion system, the Z-average particle size measured by dynamic light scattering is controlled to be 80-300nm, and the polydispersity index PDI is ≤0.

15.

6. The method of claim 1 wherein the fluorine-containing waste water is a waste water from a semiconductor manufacturing process. In step II), the magnesium-based reaction emulsion is added in a molar ratio of magnesium to fluorine of 0.3-0.8, and the stirring reaction time is 30-60min.

7. The method of claim 1 wherein the fluorine-containing waste water is a waste water from a semiconductor manufacturing process. In step III), the concentration of alkali liquor is 20wt%-30wt%, and the reaction time after adding alkali liquor is 20-40min.

8. The fluorine-containing wastewater treatment method according to claim 1 or 7, wherein In step III), soluble magnesium salt is additionally added during or after the pH value adjustment reaction.

9. The method of claim 8 wherein the fluorine-containing waste water is treated by a method comprising: In step III), the magnesium salt is magnesium chloride or magnesium sulfate.

10. The method of claim 8 wherein the fluorine-containing waste water is a waste water from a semiconductor manufacturing process. In step III), the magnesium salt is added in a molar ratio of magnesium ions to fluorine ions in the original fluorine-containing wastewater of 0.1-0.3.

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