Method and device for removing fluorine from ammonium chloride mother liquor

Abstract

The application discloses a method and device for removing fluorine from ammonium chloride mother liquor, and the method comprises the following steps: adjusting the pH of the ammonium chloride mother liquor to be weakly acidic to obtain a precipitation system, the components of the ammonium chloride mother liquor include 5-15 g / L of Mg 2+ , 30-50 g / L of NH4 + , 70-130 g / L of Cl ‑ and 5-15 mg / L of F ‑ ; and adding aluminum chloride solution to the precipitation system to promote the precipitation of magnesium fluoride grains to remove fluorine. The method for removing fluorine from ammonium chloride mother liquor provided by the application does not need to add fluorine-removing agents, and innovatively uses the original Mg 2+ in the ammonium chloride mother liquor, adjusts the pH of the ammonium chloride mother liquor to be weakly acidic to inhibit the dissolution of MgF2 in high ammonium salt, and makes the fine MgF2 grains grow together after the addition of the aluminum chloride solution, so that stable MgF2 microcrystal precipitates are generated to effectively remove F ‑ .

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and resource recovery technology, specifically to a method and apparatus for defluorinating ammonium chloride mother liquor. Background Technology

[0002] With increasingly stringent environmental regulations and the deepening of zero-emission initiatives, the resource-based treatment of high-salinity wastewater has become a core issue in the field of industrial wastewater treatment. MVR (Mechanical Vapor Recompression) evaporation crystallization technology, due to its significant energy-saving advantages—saving approximately 60% of operating costs compared to traditional multi-effect evaporators, with power consumption per ton of water evaporation controlled at 25-35 kWh—has been widely applied to the treatment and resource recovery of ammonium chloride wastewater. Using MVR evaporation to recover ammonium chloride is currently the mainstream technical route for treating high-concentration ammonium chloride wastewater such as rare earth ammonia saponification wastewater and new energy material production wastewater.

[0003] Ammonium chloride wastewater is characterized by high salinity and high corrosiveness. When treating ammonium chloride mother liquor using MVR or triple-effect evaporation equipment, the equipment materials face severe corrosion challenges. Evaporation equipment in chloride-containing systems typically requires TA2-grade titanium, but the corrosion problem is even more pronounced when fluoride ions are also present in the wastewater. Fluoride ions have a small radius and high electronegativity, allowing them to penetrate the dense oxide film on the titanium surface and react directly with the substrate, initiating pitting corrosion. As evaporation and crystallization proceed, fluoride ions continuously accumulate in the mother liquor, with concentrations rising from an initial 10-20 ppm to 100-500 ppm, causing extremely severe corrosion to titanium materials in acidic environments. Titanium materials are only suitable for neutral or alkaline fluoride environments and are strictly prohibited from use in free HF environments. Therefore, to ensure long-term stable operation of the evaporation equipment and extend its service life, strict defluorination pretreatment of the ammonium chloride mother liquor before it enters the MVR triple-effect equipment is essential to control the fluoride ion concentration within a safe range that the equipment materials can tolerate.

[0004] Currently, magnesium or calcium salt precipitation is the most widely used method for fluoride removal. However, it involves high reagent consumption and operating costs. For some specific components, the ammonium chloride mother liquor fails to fully utilize the original ionic components in the system, resulting in resource waste and increased reagent costs. Furthermore, excessive addition of precipitant may introduce new impurities. Summary of the Invention

[0005] The purpose of this invention is to develop a defluorination pretreatment method that can fully utilize the inherent magnesium ions in ammonium chloride mother liquor, avoid introducing external harmful impurities, achieve high defluorination depth, and be well integrated with the MVR triple-effect evaporation system. This method has significant industrial application value for ensuring long-term stable operation of evaporation equipment, improving the quality of ammonium chloride products, and reducing wastewater treatment costs.

[0006] To achieve the above-mentioned technical objectives, the present invention provides a method for defluorinating ammonium chloride mother liquor, comprising: The pH of the ammonium chloride mother liquor was adjusted to a slightly acidic state to obtain a precipitation system. The components of the ammonium chloride mother liquor included 5-15 g / L of Mg. 2+ NH4 30-50g / L + 70-130g / L Cl - and 5-15 mg / L of F - ; Add aluminum chloride solution to the precipitation system to promote the precipitation of magnesium fluoride crystals and remove fluoride.

[0007] Furthermore, the term "weakly acidic" refers to a pH of 5.2-5.8.

[0008] Furthermore, the concentration of the aluminum chloride solution is 5-15 mg / L.

[0009] Furthermore, the volume of the aluminum chloride solution is 0.1%-0.5% of the volume of the precipitation system.

[0010] Furthermore, the precipitation process includes stirring at 150-350 rpm for 30-45 minutes at a temperature of 25-35°C, followed by settling for ≥3 hours.

[0011] Furthermore, the ammonium chloride mother liquor also contains 20-50 g / L of SO4. 2- and 10-20 g / L of Na + .

[0012] Furthermore, the initial pH of the ammonium chloride mother liquor is 2.5-3.5.

[0013] Furthermore, ammonia water was used to adjust the pH of the ammonium chloride mother liquor.

[0014] Further, after precipitation, solid-liquid separation is performed, and the resulting liquid is transported to an MVR evaporator.

[0015] The present invention also provides a defluorination device for ammonium chloride mother liquor, for implementing the above-mentioned method, including a pH control unit, a defluorination reaction unit and a filtration unit; The pH control unit is used to adjust the pH of the ammonium chloride mother liquor to obtain a precipitation system; The defluorination reaction unit is used to mix aluminum chloride solution with precipitation system to precipitate magnesium fluoride crystals and remove fluoride. The filtration unit is used to separate the solid and liquid components of the defluorinated system. The defluorination reaction unit includes a stirred flocculation tank and a settling tank. The filtration unit includes a bag filter and a tubular filter.

[0016] Compared with the prior art, the beneficial effects of the present invention include: The method for defluorinating ammonium chloride mother liquor provided by this invention eliminates the need for external defluorinating agents and innovatively utilizes the Mg naturally present in the ammonium chloride mother liquor. 2+ The pH of the ammonium chloride mother liquor was adjusted to be weakly acidic to inhibit the dissolution of MgF2 in high ammonium salts. The addition of aluminum chloride solution caused the fine MgF2 crystals to agglomerate and grow, forming a stable MgF2 microcrystalline precipitate, thus effectively removing F. - .

[0017] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0019] Figure 1 A schematic diagram of the ammonium chloride mother liquor defluorination device of the present invention is shown; Figure 2 A flowchart of the method for defluorinating ammonium chloride mother liquor according to the present invention is shown; Explanation of reference numerals in the attached figures: 1. Mother liquor collection tank; 2. pH control unit; 3. Defluoridation reaction unit; 31. Stirred flocculation tank; 32. Settling tank; 4. Filtration unit; 41. Bag filter; 42. Tube filter. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0022] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] With increasingly stringent treatment requirements, more and more processes are adopting combined pretreatment routes. For example, some engineering cases use a combined process of lime slurry reaction-fluoride removal reaction-coagulation sedimentation-flocculation sedimentation-quartz sand filtration-activated carbon filtration to pretreat high-salinity wastewater before it enters the MVR evaporation system. Other studies have proposed a process flow of defluoridation reaction tank-plate and frame filter press-hardening high-density tank-multimedia filter-MVR evaporation system. While these combined processes can achieve a certain defluoridation effect, they are often lengthy, require large areas, and have high operating costs. Furthermore, they introduce exogenous ions such as calcium and aluminum, which may affect the purity of the final ammonium chloride product. Using external calcium or magnesium salts as precipitants results in high reagent consumption and operating costs. For ammonium chloride mother liquor that already contains a high concentration of magnesium ions, the existing magnesium resources within the system are not fully utilized, leading to resource waste and increased reagent costs.

[0024] Therefore, this invention is proposed.

[0025] A method for defluorinating ammonium chloride mother liquor, comprising: The pH of the ammonium chloride mother liquor was adjusted to a slightly acidic state to obtain a precipitation system. The components of the ammonium chloride mother liquor included 5-15 g / L of Mg. 2+ NH4 30-50g / L + 70-130g / L Cl - and 5-15 mg / L of F - It also includes 20-50 g / L of SO4. 2- and 10-20 g / L of Na + The initial pH of the ammonium chloride mother liquor is 2.5-3.5.

[0026] Add aluminum chloride solution to the precipitation system to promote the precipitation of magnesium fluoride crystals and remove fluoride.

[0027] In some preferred embodiments, the weak acidity refers to a pH of 5.2-5.8. Precise pH control can effectively inhibit the dissolution of MgF2 in high ammonium salts, laying the foundation for subsequent precipitation.

[0028] Under strongly acidic conditions with an initial pH of 3, fluorine exists primarily in molecular states HF and HF2. - It exists in the form of fluoride ions (F). - H +(Protonization). Molecular HF is uncharged and difficult to react with Mg. 2+ It combines with other substances and is extremely corrosive to titanium. Precisely adjusting the pH to 5.2-5.8 consumes H+ in the solution. + Deprotonation occurs. At pH less than 5.2, the degree of dissociation is insufficient, and a large amount of fluorine remains as HF, unable to precipitate. At pH 5.2-5.8, free F... - The significantly increased concentration provides ample reactant ions for subsequent precipitation; simultaneously, the pH does not exceed 6.0, avoiding the formation of Mg. 2+ Hydrolysis produces Mg(OH)2 precipitate, preventing the consumption of magnesium source and the generation of colloidal byproducts.

[0029] The solubility product constant of MgF2 is approximately 6.4 × 10⁻⁶. -9 The precipitation reaction is: Mg 2+ +2F - →MgF2(s)↓.

[0030] The magnesium ion concentration in the mother liquor is as high as 5-15 g / L, while the fluoride ion concentration is only 5-15 mg / L. The magnesium ion concentration is several hundred times higher than the fluoride ion concentration. Therefore, it is possible to forcibly expel fluoride from the liquid phase without the need for external magnesium salts, by utilizing the inherent high magnesium content of the system.

[0031] In a weakly acidic pH range of 5.2-5.8, a trace amount of Al was added. 3+ (5-15 mg / L) will not immediately generate a large amount of Al(OH)3 main flocs, but will hydrolyze to form a series of highly positively charged polynuclear hydroxy complexes such as Al2(OH)2 4+ and Al3(OH)4 5+ The newly formed MgF2 precipitate has an extremely small particle size and a negatively charged surface, causing mutual repulsion and making it difficult to settle naturally. Positively charged aluminum hydroxide complexes adsorb onto the negatively charged MgF2 microcrystals, neutralizing their surface charge and eliminating the electrostatic repulsion barrier; simultaneously, utilizing the adsorption bridging effect of their polymer chains, they "bridge" the dispersed fine MgF2 particles into loose, large flocs. The ingenuity of this invention lies in the fact that traditional processes rely on external reagents for the reaction, while this invention adjusts the environment (pH) to allow the existing elements (Mg) within the system to react spontaneously, followed by the use of trace amounts of reagents to assist separation. This synergistic mechanism of "treating waste with waste and internal transformation" achieves deep defluorination while maximizing the purity of the ammonium chloride product.

[0032] In some preferred embodiments, the concentration of the aluminum chloride solution is 5-15 mg / L. The extremely low concentration of the added aluminum chloride solution avoids the introduction of large amounts of aluminum salts that could lead to a decrease in product purity, serving only as a catalytic / bridging agent for "coagulation and sedimentation."

[0033] In some preferred embodiments, the volume of the aluminum chloride solution is 0.1%-0.5% of the volume of the precipitation system.

[0034] In some preferred embodiments, the precipitation includes stirring at 150-350 rpm for 30-45 minutes at a temperature of 25-35°C, followed by settling for ≥3 hours.

[0035] In some preferred embodiments, ammonia is used to adjust the pH of the ammonium chloride mother liquor.

[0036] The present invention will be further described in detail below through specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0037] Example 1 A defluorination device for ammonium chloride mother liquor, such as Figure 1 As shown, the system includes a mother liquor collection tank 1, a pH adjustment unit 2, a defluorination reaction unit 3, and a filtration unit 4. The mother liquor collection tank 1 stores ammonium chloride mother liquor; the pH adjustment unit 2 adjusts the pH of the ammonium chloride mother liquor to obtain a precipitation system; the defluorination reaction unit 3 mixes aluminum chloride solution with the precipitation system to precipitate magnesium fluoride crystals and remove fluoride; and the filtration unit 4 separates the solid and liquid components of the defluorinated system. The defluorination reaction unit 3 includes a stirred flocculation tank 31 and a settling tank 32; the filtration unit 4 includes a bag filter 41 and a tubular filter 42.

[0038] The following examples and comparative examples are all implemented using the apparatus of Example 1.

[0039] The pH of the ammonium chloride mother liquor in the following examples and comparative examples was 3, and it was prepared with 8 g / L Mg. 2+ 35g / L of NH4 + 90g / L Cl - 10 mg / L of F - 30g / L SO4 2- and 15g / L Na + constitute.

[0040] Example 2 like Figure 2 As shown, a method for defluorinating ammonium chloride mother liquor includes the following steps: S1. Adjust pH: Use ammonia (28.wt) in the pH control unit to adjust the pH of the ammonium chloride mother liquor to 5.2 to obtain the precipitation system.

[0041] S2. Flocculation and sedimentation: The sedimentation system is transported to a stirred flocculation tank, and then 10 mg / L aluminum chloride aqueous solution is added, with the volume of the aluminum chloride aqueous solution being 0.3% of the sedimentation system; at a temperature of 30℃, it is stirred at a speed of 150 rpm for 30 min to allow the fine magnesium fluoride crystals to clump together and grow, reducing the difficulty of sedimentation; then it is transferred to a settling tank and allowed to settle for 5 h.

[0042] S3. Separation and Reuse: After sedimentation, the solid-liquid mixture is passed through a bag filter and a tubular filter in sequence for solid-liquid separation. The collected liquid is then directly transported to the MVR evaporator.

[0043] Example 3 A method for defluorinating ammonium chloride mother liquor includes the following steps: S1. Adjust pH: Use ammonia (28.wt) in the pH control unit to adjust the pH of the ammonium chloride mother liquor to 5.5 to obtain the precipitation system.

[0044] S2. Flocculation and sedimentation: The sedimentation system is transported to a stirred flocculation tank, and then 10 mg / L aluminum chloride aqueous solution is added, with the volume of the aluminum chloride aqueous solution being 0.3% of the sedimentation system; at a temperature of 30℃, it is stirred at a speed of 150 rpm for 30 min to allow the fine magnesium fluoride crystals to clump together and grow, reducing the difficulty of sedimentation; then it is transferred to a settling tank and allowed to settle for 5 h.

[0045] S3. Separation and Reuse: After sedimentation, the solid-liquid mixture is passed through a bag filter and a tubular filter in sequence for solid-liquid separation. The collected liquid is then directly transported to the MVR evaporator.

[0046] Example 4 A method for defluorinating ammonium chloride mother liquor includes the following steps: S1. Adjust pH: Use ammonia (28.wt) in the pH control unit to adjust the pH of the ammonium chloride mother liquor to 5.2 to obtain the precipitation system.

[0047] S2. Flocculation and sedimentation: The sedimentation system is transported to a stirred flocculation tank, and then 10 mg / L aluminum chloride aqueous solution is added, with the volume of the aluminum chloride aqueous solution being 0.3% of the sedimentation system; at a temperature of 30℃, it is stirred at a speed of 150 rpm for 30 min to allow the fine magnesium fluoride crystals to clump together and grow, reducing the difficulty of sedimentation; then it is transferred to a settling tank and allowed to settle for 5 h.

[0048] S3. Separation and Reuse: After sedimentation, the solid-liquid mixture is passed through a bag filter and a tubular filter in sequence for solid-liquid separation. The collected liquid is then directly transported to the MVR evaporator.

[0049] Comparative Example 1 A method for defluorinating ammonium chloride mother liquor includes the following steps: S1. Adjust pH: Use ammonia water (28.wt) in the pH control unit to adjust the pH of the ammonium chloride mother liquor to 5.0 to obtain the precipitation system.

[0050] S2. Flocculation and sedimentation: The sedimentation system is transported to a stirred flocculation tank, and then 10 mg / L aluminum chloride aqueous solution is added, with the volume of the aluminum chloride aqueous solution being 0.3% of the sedimentation system; at a temperature of 30℃, it is stirred at a speed of 150 rpm for 30 min to allow the fine magnesium fluoride crystals to clump together and grow, reducing the difficulty of sedimentation; then it is transferred to a settling tank and allowed to settle for 5 h.

[0051] S3. Separation: After sedimentation, the solids are separated by passing the mixture through a bag filter and a tubular filter in sequence.

[0052] Comparative Example 2 A method for defluorinating ammonium chloride mother liquor includes the following steps: S1. Adjust pH: Use ammonia (28.wt) in the pH control unit to adjust the pH of the ammonium chloride mother liquor to 6.0 to obtain the precipitation system.

[0053] S2. Flocculation and sedimentation: The sedimentation system is transported to a stirred flocculation tank, and then 10 mg / L aluminum chloride aqueous solution is added, with the volume of the aluminum chloride aqueous solution being 0.3% of the sedimentation system; at a temperature of 30℃, it is stirred at a speed of 150 rpm for 30 min to allow the fine magnesium fluoride crystals to clump together and grow, reducing the difficulty of sedimentation; then it is transferred to a settling tank and allowed to settle for 5 h.

[0054] S3. Separation: After sedimentation, the solids are separated by passing the mixture through a bag filter and a tubular filter in sequence.

[0055] Comparative Example 3 The difference from Example 3 is that aluminum chloride aqueous solution is not added in step S2.

[0056] Comparative Example 4 The difference compared to Example 3 is that the concentration of the aluminum chloride aqueous solution in step S2 is 3 mg / L.

[0057] Comparative Example 5 The difference compared to Example 3 is that the concentration of the aluminum chloride aqueous solution in step S2 is 20 mg / L.

[0058] Comparative Example 6 Compared with Example 3, the difference is that the stirring time in step S2 is shortened from 30 min to 20 min.

[0059] Comparative Example 7 Compared with Example 3, the difference is that the stirring time in step S2 is extended from 30 min to 60 min.

[0060] Comparative Example 8 Compared with Example 3, the difference is that the settling time in step S2 is shortened to 2 hours.

[0061] The F content in the liquid obtained after solid-liquid separation by bag filter and tubular filter was tested in the examples and comparative examples. - The concentrations were determined, and the results are shown in Table 1.

[0062] Table 1

[0063] As can be seen from the test results in Table 1, only Examples 2, 3, and 4 were able to stably achieve liquid output F. - The concentration is <3.0 mg / L, meeting the corrosion prevention requirements for titanium MVR equipment feed, and the separated liquid can be directly reused. This is because the pH is within the optimal dissociation window (HF→F). - The 8 g / L magnesium source produces a strong common ion effect, which will cause dissolved F to... - The pressure is reduced to extremely low levels; 10 mg / L aluminum salt effectively neutralizes the negative charge of MgF2 microcrystals, resulting in coarse bridging flocs; 5 hours of settling allows for complete solid-liquid separation, physical filtration catches the bottom, and the filtered clear liquid meets the corrosion protection requirements for feeding titanium MVR equipment.

[0064] Compared to Examples 2-4, Comparative Examples 1 and 2 had lower and higher pH values, respectively, and the effluent F... - The concentration of HF increased significantly. This is because at pH below 5.2, the degree of HF dissociation is significantly insufficient (large amounts of F...). - H + Protonization), free F - Insufficient concentration, unable to be absorbed by Mg 2+ After sufficient precipitation, the residual dissolved fluorine level rises significantly. When the pH reaches 6.0, Mg... 2+ The system begins to hydrolyze extensively to generate Mg(OH)2 colloid. This colloid not only consumes the precious magnesium source but also encapsulates some MgF2 precipitate. Furthermore, it is flocculent and difficult to settle, resulting in the presence of fine suspended fluorides and complexed fluoride in the filtered clear liquid.

[0065] Compared to Example 3, Comparative Example 3 did not add aluminum chloride. The initial MgF2 crystals were only tens of nanometers in size, and their strong negative surface charges repelled each other. Although the dissolved fluorine was extremely low, a large amount of colloidal / suspended MgF2 could not aggregate due to the repulsive force, directly penetrating the microfiltration membrane (or causing filtration failure after membrane blockage), resulting in a serious exceedance of the total fluorine in the effluent. Compared to Example 3, Comparative Examples 4 and 5 had relatively low and high amounts of aluminum chloride, respectively. With less aluminum chloride, the concentration of aluminum hydroxide complex was insufficient to completely neutralize the surface charge of the MgF2 particles; the flocculation "bridging" effect was weak, and a large number of tiny crystals remained suspended in the liquid phase, resulting in incomplete retention. With a higher amount of aluminum chloride, excessive Al... 3+ At pH 5.5, it forms a large number of positively charged colloids, which have a strong flocculation effect, but high concentrations of Al... 3+ With F - Formation of soluble complex ions (such as AlF₂) 2+ AlF2 + This process "pulls" the precipitated fluorine back into the liquid phase, causing the dissolved fluorine to rebound and introducing aluminum impurities that affect the quality of ammonium chloride.

[0066] Comparative Examples 6 and 7 had shorter and longer stirring times compared to Example 3, respectively. Insufficient reaction and flocculation time meant that the lattice growth of MgF2 microcrystals and interparticle collision bridging were not completed; the particle size was too small, resulting in slow settling speed. After 5 hours of settling, the supernatant still contained a large number of unagglomerated submicron particles, leading to high filtration pressure and a high risk of breakthrough. Excessive slow stirring time caused the already formed coarse flocs to be sheared and broken under high shear force, redispersing into fine particles; these fine particles were difficult to flocculate effectively again, leading to increased effluent turbidity and suspended fluoride. Comparative Example 8 had a shorter settling time of 2 hours compared to Example 2, which was insufficient. A large number of fine flocs entered the filter with the effluent before settling to the bottom; due to the surge in suspended solids load, the bag filter and tubular microfiltration system quickly became clogged or their retention efficiency decreased, resulting in a large amount of suspended fluoride penetrating into the clarified liquid.

[0067] These results also demonstrate the irreplaceable nature of the synergistic control of multiple parameters in this invention, including precise pH range for weakly acidic conditions, trace aluminum salt coagulation, medium-speed short-time stirring, and sufficient time for static settling.

[0068] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for defluorinating ammonium chloride mother liquor, characterized in that, include, The pH of the ammonium chloride mother liquor was adjusted to a slightly acidic state to obtain a precipitation system. The components of the ammonium chloride mother liquor included 5-15 g / L of Mg. 2+ NH4 30-50g / L + 70-130g / L Cl - and 5-15 mg / L of F - ; Add aluminum chloride solution to the precipitation system to promote the precipitation of magnesium fluoride crystals and remove fluoride.

2. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The term "weakly acidic" refers to a pH value of 5.2-5.

8.

3. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The concentration of the aluminum chloride solution is 5-15 mg / L.

4. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The volume of the aluminum chloride solution is 0.1%-0.5% of the volume of the precipitation system.

5. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The precipitation process involves stirring at 150-350 rpm for 30-45 minutes at a temperature of 25-35°C, followed by settling for ≥3 hours.

6. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The ammonium chloride mother liquor also contains 20-50 g / L of SO4. 2- and 10-20 g / L of Na + .

7. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The initial pH of the ammonium chloride mother liquor is 2.5-3.

5.

8. The method for defluorinating ammonium chloride mother liquor according to claim 1, characterized in that, The pH of the ammonium chloride mother liquor was adjusted using ammonia water.

9. The method for defluorinating ammonium chloride mother liquor according to any one of claims 1-8, characterized in that, After precipitation, solid-liquid separation is performed, and the resulting liquid is sent to an MVR evaporator.

10. A defluorination device for ammonium chloride mother liquor, used to implement the method according to any one of claims 1-9, characterized in that, Includes a pH control unit, a fluoride removal reaction unit, and a filtration unit; The pH control unit is used to adjust the pH of the ammonium chloride mother liquor to obtain a precipitation system; The defluorination reaction unit is used to mix aluminum chloride solution with precipitation system to precipitate magnesium fluoride crystals and remove fluoride. The filtration unit is used to separate the solid and liquid components of the defluorinated system. The defluorination reaction unit includes a stirred flocculation tank and a settling tank. The filtration unit includes a bag filter and a tubular filter.

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