Fluorine-free crystal seeds, preparation method and application thereof, and fluidized bed

Abstract

The application provides a defluorination seed crystal, which comprises a carrier material, OH ‑ a slow-release agent and a density regulator. The seed crystal is synthesized to be an alkaline slow-release defluorination seed crystal with acid resistance, and can directly induce calcium fluoride crystallization under the condition of pH≤1, thereby breaking through the crystallization difficulty in an acidic environment.

Description

Technical Field

[0001] This application relates to the field of water treatment technology, and in particular to a fluoride removal seed crystal, a method for preparing the fluoride removal seed crystal and its application, and a fluidized bed. Background Technology

[0002] Fluoride-containing wastewater mainly originates from industries such as metallurgy, photovoltaics, chemicals, electronics, electroplating, and glass manufacturing. It is typically acidic, with a pH value reaching 1. Hydrofluoric acid is a weak acid; under acidic conditions, F⁻ exists as HF molecules, making it difficult to react. Therefore, continuous addition of alkali is needed to convert fluoride into F⁻ ions, which then precipitate as CaF₂ for fluoride removal. Existing fluidized bed crystallization technologies primarily use quartz sand, sand, and calcium carbonate as seed crystals. However, these methods are inefficient under acidic conditions, as CaF₂ fails to nucleate and grow stably, preventing the direct generation of high-purity calcium fluoride. Frequent alkali additions are required to maintain the pH at 6-9, leading to high treatment costs. Therefore, there is an urgent need for an acid-resistant fluoride removal seed crystal to directly induce calcium fluoride crystallization under pH ≤ 1 conditions, overcoming the crystallization challenges in acidic environments. Summary of the Invention

[0003] This application provides defluorination seed crystals, their preparation method and application, and fluidized bed, to at least partially solve the above-mentioned problems. The first aspect of this application provides a defluorination seed crystal, comprising: a carrier material, OH... - Slow-release agent and density regulator. This seed crystal synthesis produces an acid-resistant, alkaline slow-release fluoride removal seed crystal, which can directly induce calcium fluoride crystallization under pH≤1 conditions, overcoming the challenge of crystallization in acidic environments.

[0004] In some embodiments of this application, the carrier material includes at least one of magnesium silicate, aluminum silicate, and zeolite; and / or, the OH... - The slow-release agent includes at least one of calcium hydroxyphosphate, carbon-based apatite, magnesium hydroxide, and calcium carbonate; and / or, the density regulator includes at least one of zirconium oxide and silicon carbide.

[0005] In some embodiments of this application, the fluoride-removing seed crystals comprise: 30.3%-90.9% carrier material and 7.3%-65.6% OH. - Slow-release agent, 0.9%-18.8% density modifier; preferably, 47%-83wt% carrier material, 14%-48% OH- slow-release agent, 2%-9% density modifier.

[0006] In some embodiments of this application, the particle size of the fluoride-removing seed crystals ranges from 100 to 200 μm; and / or, the density of the fluoride-removing seed crystals ranges from 1.5 to 2.5 g / cm³.

[0007] The second aspect of this application provides a method for preparing fluoride-removing seed crystals, comprising: mixing a carrier material, an OH- slow-release agent, and a density regulator, grinding and calcining the mixture, and then spray granulating it to obtain fluoride-removing seed crystals.

[0008] In some embodiments of this application, the carrier material includes at least one of magnesium silicate, aluminum silicate, and zeolite; and / or, the OH-relevant includes at least one of calcium hydroxyapatite, carbon-based apatite, magnesium hydroxide, and calcium carbonate; and / or, the density regulator includes at least one of zirconium oxide and silicon carbide.

[0009] In some embodiments of this application, the mixture of carrier material, OH-relevant agent, and density modifier comprises: 10-35 parts carrier material, 3-20 parts OH-relevant agent, and 0.5-3 parts density modifier; preferably, 15-30 parts carrier material, 5-15 parts OH-relevant agent, and 1-2 parts density modifier.

[0010] In some embodiments of this application, the particles need to be sieved after spray granulation. Preferably, the particle size range after sieving is 100-200 μm.

[0011] In some embodiments of this application, the grinding time is 60-120 min; and / or, the calcination temperature is 900-1200℃; and / or, the calcination time is 120-180 min; and / or, the spray granulation yields 100-mesh microspheres.

[0012] The third aspect of this application provides an application of fluoride-removing seed crystals in calcium fluoride recovery.

[0013] The fourth aspect of this application provides a fluidized bed with fluoride-removing seed crystals as described above.

[0014] Other features and advantages of this application will be described in detail in the following detailed description section. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] To gain a more complete understanding of this application and its beneficial effects, the following description will be provided in conjunction with the accompanying drawings, wherein the same reference numerals in the following description denote the same parts.

[0017] Figure 1 This is a SEM image of a fluoride-removing seed crystal provided in an exemplary embodiment of this disclosure;

[0018] Figure 2 This is an XRD pattern of a fluoride-removing seed crystal provided in an exemplary embodiment of this disclosure;

[0019] Figure 3 This is a PSA diagram of a fluoride-removing seed crystal provided in an exemplary embodiment of this disclosure;

[0020] Figure 4 This is a simplified schematic diagram of a fluidized bed provided in an exemplary embodiment of this disclosure;

[0021] Figure 5 This is a SEM image of the recycled product calcium fluoride provided in an exemplary embodiment of this disclosure.

[0022] Figure label:

[0023] 1-Water inlet; 2-Chemical inlet; 3-Fluoride seed crystals; 4-Water outlet. Detailed Implementation

[0024] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the protection scope of this application.

[0025] Fluoride-containing wastewater mainly originates from industries such as metallurgy, photovoltaics, chemicals, electronics, electroplating, and glass manufacturing. It is typically acidic, with a pH value reaching 1. Existing fluidized bed crystallization technologies primarily use quartz sand, sand, and calcium carbonate as seed crystals. However, these methods are inefficient under acidic conditions and cannot directly generate high-purity calcium fluoride. Frequent addition of alkali is required to maintain the pH at 6-9, leading to high treatment costs. This invention synthesizes acid-resistant, alkaline, slow-release fluoride-removing seed crystals that directly induce calcium fluoride crystallization under pH ≤ 1 conditions, overcoming the challenge of crystallization in acidic environments.

[0026] Seed crystals provide nucleation sites for solutes in solution, thereby reducing the energy required for crystal growth, accelerating the crystallization process, and helping to improve crystallization efficiency and crystal quality.

[0027] The first aspect of this application provides a fluoride-removing seed crystal, which includes: a carrier material, OH... - Slow-release agent and density regulator. Because this defluorination seed crystal has the ability to release OH⁻, it can neutralize local H⁺, allowing the defluorinated product CaF₂ to directly and stably nucleate and grow. This seed crystal has high defluorination efficiency and high product purity. In addition, by applying the characteristics of this defluorination seed crystal, no additional alkali is needed during the defluorination process, the process is simplified, and reagents can be saved to reduce costs.

[0028] Specifically, the carrier material is the main material for removing fluoride seed crystals and can be used as an OH- - The matrix of sustained-release agents and density modifiers; OH - Sustained-release agents have the function of slowly releasing OH - The ability to continuously neutralize localized H⁺ allows fluoride in fluoride-containing wastewater to be produced as CaF₂ and stably nucleate and grow; the density regulator can control the compactness of the defluorination seed crystals, ensuring that the density of the defluorination seed crystals meets the application conditions of the defluorination process. In summary, it includes carrier materials, OH⁻, and other components. - The three components—a slow-release agent, a density regulator, and a defluorination seed crystal—achieve excellent defluorination performance.

[0029] In some embodiments of this application, the support material includes at least one of magnesium silicate, aluminum silicate, and zeolite. This type of support material has a large pore structure, suitable for adsorption, maintains structural stability at high temperatures, and possesses good mechanical strength, making it resistant to breakage in industrial use. Furthermore, the above-mentioned support materials are acid-resistant, insoluble under strongly acidic conditions (pH ≤ 1), and provide nucleation sites. The support material can be one of the above types, a combination of two different types, or a combination of three types; this is not limited here.

[0030] In some embodiments of this application, OH - The sustained-release agent includes at least one of calcium hydroxyapatite, carboapatite, magnesium hydroxide, and calcium carbonate. This type of OH... - Slow-release agents can achieve OH- in acidic fluoride-containing wastewater. - The slow release of OH⁻ locally neutralizes H⁺, creating a locally slightly alkaline environment (pH>7), which provides OH⁻ to work synergistically with Ca²⁺ and F⁻, lowering the nucleation energy barrier for CaF₂ crystallization, promoting CaF₂ crystallization, resulting in strong fluoride removal ability of the seed crystals and stable, pure product formation. - The sustained-release agent can be one of the above types, or a combination of two different types, or a combination of three types, which is not limited here.

[0031] In some embodiments of this application, the density modifier includes at least one of zirconium oxide and silicon carbide. This type of density modifier allows the seed crystals to have a suitable density, thus enabling them to function better under defluorination process conditions such as fluidized beds. The density modifier can be one or a combination of the above.

[0032] In some embodiments of this application, the defluorination seed crystals comprise: 30.3%-90.9% carrier material, 7.3%-65.6% OH⁻ slow-release agent, and 0.9%-18.8% density regulator; further, they comprise: 47%-83wt% carrier material, 14%-48% OH⁻ slow-release agent, and 2%-9% density regulator. A suitable weight ratio of the carrier material ensures a sufficient amount of carrier to form a continuous and robust skeletal network, encapsulating and supporting other functional components, resulting in a complete overall seed crystal structure. This prevents pulverization under high-speed collisions and friction in a fluidized bed, reducing mechanical loss and minimizing the generation of fine powder that is difficult to separate. Simultaneously, this weight ratio range allows necessary space for the functional components (OH⁻ slow-release agent and density regulator), ensuring defluorination efficiency. A suitable amount of slow-release agent precisely balances the slow-release capacity and release duration, ensuring a stable and moderately alkaline microenvironment on the seed crystal surface throughout the entire operating cycle, efficiently inducing selective crystallization of high-purity CaF₂. The density regulator is added precisely in a relatively small proportion, achieving ideal fluidization performance of the seed crystal particles, ensuring sufficient solid-liquid contact and efficient mass transfer within the reactor, thereby obtaining high defluorination efficiency and large particle products.

[0033] In some embodiments of this application, the particle size of the fluoride-free seed crystals ranges from 100 to 200 μm. Theoretically, during heterogeneous crystallization in a supersaturated solution, when the total Gibbs free energy (ΔG) at the crystal-liquid surface... S ) and the Gibbs free energy (ΔG) of new phase formation V The difference (ΔG) between the two is greater than the nucleation energy barrier (ΔG). homo When ΔG is reached, the precipitate will crystallize heterogeneously on the seed crystal. However, the formation of a new phase in the solution is ΔG. V During the reduction process, as the size of the new phase increases, the surface area also gradually increases, leading to a continuous increase in interfacial energy, i.e., ΔG. S The Gibbs free energy increases, therefore the change in Gibbs free energy during the entire nucleation process is closely related to the crystal nucleus size. In the study of fluoride removal seeds in this application, it was found that the fluoride removal crystallization effect was best when the particle size was 100-200 μm. Within this particle size range, the particle size can be uniformly concentrated at a single point, such as 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm, or any point within the range without limitation; the particle size can also be within this range but in a more discrete distribution, for example, some particles are 150 μm, some are 180 μm or 130 μm, but overall they conform to the above particle size range. The particle size can be obtained using SEM testing.

[0034] In some embodiments of this application, the density range of the fluoride-free seed crystals is 1.5-2.5 g / cm³. When the crystallization product of this application is calcium fluoride, within this density range, the seed crystals can better exert their inducing crystallization effect in a fluidized bed.

[0035] Furthermore, the particle size and density meet the conditions for inducing calcium fluoride crystallization, enabling direct induction of CaF2 crystallization in strongly acidic environments (pH≤1) without the need for additional alkali, thus overcoming the technical bottleneck of inefficient fluoride removal from acidic wastewater.

[0036] The second aspect of this application provides a method for preparing fluoride-removing seed crystals, the method comprising: mixing a carrier material, OH... - The slow-release agent and density regulator are mixed, ground, calcined, and then spray-granulated to obtain defluorinated seed crystals. This method is simple to prepare, and the carrier material and OH... - The addition of slow-release agents and density regulators enables the prepared defluorination seed crystals to achieve efficient and stable induced crystallization, resulting in excellent defluorination performance.

[0037] In some embodiments of this application, the support material includes at least one of magnesium silicate, aluminum silicate, and zeolite. This type of support material has a large pore structure, suitable for adsorption, maintains structural stability at high temperatures, and possesses good mechanical strength, making it resistant to breakage in industrial use. Furthermore, the above-mentioned support materials are acid-resistant, insoluble under strongly acidic conditions (pH ≤ 1), and provide nucleation sites. The support material can be one of the above types, a combination of two different types, or a combination of three types; this is not limited here.

[0038] In some embodiments of this application, OH - The sustained-release agent includes at least one of calcium hydroxyapatite, carboapatite, magnesium hydroxide, and calcium carbonate. This type of OH... - Slow-release agents can achieve OH- in acidic fluoride-containing wastewater. - The slow release of OH locally neutralizes H⁺, promoting CaF₂ crystallization, resulting in strong fluoride removal ability of the seed crystals and stable, pure product formation. - The sustained-release agent can be one of the above types, or a combination of two different types, or a combination of three types, which is not limited here.

[0039] In some embodiments of this application, the density modifier includes at least one of zirconium oxide and silicon carbide. This type of density modifier allows the seed crystals to have a suitable density, thus enabling them to function better under defluorination process conditions such as fluidized beds. The density modifier can be one or a combination of the above.

[0040] In some embodiments of this application, the raw materials for the fluoride-removing seed crystals include 10-35 parts of carrier material and 3-20 parts of OH. -Sustained-release agent, 0.5-3 parts density modifier. Specifically, the carrier material can be 10 parts, 12 parts, 14 parts, 15 parts, 16 parts, 18 parts, 20 parts, 22 parts, 24 parts, 26 parts, 28 parts, 30 parts, 31 parts, 32 parts, 33 parts, 34 parts, 35 parts, or non-integer parts, which are not limited here; OH - The slow-release agent can be 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 19, 20 parts or non-integer parts, without limitation; the density adjuster can be 0.5, 0.6, 0.7, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3 parts or non-integer parts, without limitation. Under this optimized ratio, the seed crystals maintain excellent mechanical strength and fluidization performance while achieving the best kinetic match between the OH⁻ slow-release rate and the acid neutralization requirement, thus obtaining higher calcium fluoride crystal purity (>90%) and longer seed crystal cycle life with extremely low raw material usage. Furthermore, the raw materials for the fluoride-removing seed crystals include 15-30 parts of carrier material, 5-15 parts of OH- slow-release agent, and 1-2 parts of density regulator.

[0041] In some embodiments of this application, the particles need to be sieved after spray granulation. Preferably, the particle size range after sieving is 100-200 μm. By screening for a suitable particle size range, the particle size of the seed crystals is limited to the optimal range, which can further improve the seed crystallization induction effect and obtain a product with higher purity.

[0042] In some embodiments of this application, the grinding time is 60-120 minutes, which allows the raw material components to be fully mixed, resulting in better grain performance.

[0043] In some embodiments of this application, the calcination temperature is 900-1200℃. At this calcination temperature, sufficient solid-state reaction and sintering can occur between the raw material components to form a stable composite with a dense structure, high mechanical strength, and ideal microporous structure, ensuring the long-term structural integrity of the seed crystals in a strong acid environment and the controllable slow-release performance of OH⁻.

[0044] In some embodiments of this application, the calcination time is 120-180 min. This calcination time is sufficient to allow the components to fully react and transform their crystal form, forming a composite carrier with a stable structure and suitable strength. At the same time, it avoids the deactivation of active components due to sintering caused by excessive time, ensuring the durability and stability of the OH⁻ sustained-release function.

[0045] In some embodiments of this application, the spray granulation yields 100-mesh microspheres. By controlling this mesh size, the particle size range of the granulated microspheres is approximately 150 μm. Combined with subsequent screening to obtain crystals with a particle size range of 100-200 μm, seed crystals with better fluoride removal crystallization effect are obtained.

[0046] The third aspect of this application provides the application of the fluoride-removing seed crystals prepared by the above-mentioned method or the second aspect method in calcium fluoride recovery. In calcium chloride recovery, the above-mentioned seed crystals can eliminate the need for conventional pH adjustment using alkaline solutions. These seed crystals can spontaneously adjust the pH value within the range of 6-8, which is beneficial to the crystallization reaction of fluoride ions and calcium ions in the fluidized bed crystallizer and improves the reaction efficiency.

[0047] The fourth aspect of this application provides a fluidized bed containing fluoride-removing seed crystals prepared by the method of preparing fluoride-removing seed crystals of the first aspect or the second aspect. Figure 4 This is a simplified schematic diagram of a fluidized bed provided in an exemplary embodiment. Wastewater enters the fluidized bed through 1-inlet; calcium salt, which can be calcium chloride, is injected through 2-inlet. The calcium salt reacts with fluoride in the wastewater to form calcium fluoride; 3-fluoride seed crystals are placed inside the fluidized bed to induce crystallization; the defluorinated wastewater is discharged from 4-outlet. The calcium fluoride inside the fluidized bed is removed from above 4-outlet, and after draining, the calcium fluoride product is obtained.

[0048] The present application will be specifically described below through specific embodiments. These embodiments are only some embodiments of the present application and are not intended to limit the present application. Unless otherwise specified, the raw materials used in the following embodiments are all commercially available products.

[0049] Example 1:

[0050] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts magnesium silicate, 15 parts hydroxyapatite, and 2 parts zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.8 g / cm³. Its main components are 64% magnesium silicate, 32% hydroxyapatite, and 4% zirconium oxide.

[0051] Add the above seed crystals Figure 4In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 18 mg / L. The crystalline product, CaF2, has an average particle size of 2 mm, a water content of 15%, a purity of 92%, and a fluoride recovery rate of 94.8%.

[0052] Example 2:

[0053] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 15 parts magnesium silicate, 5 parts hydroxyapatite, and 1 part zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.9 g / cm³. Its main components are 71% magnesium silicate, 24% hydroxyapatite, and 5% zirconium oxide.

[0054] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 0.9 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 17 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 22 mg / L. The average particle size of the crystalline product CaF2 is 1.5 mm, with a water content of 12%, a purity of 94%, and a fluoride recovery rate of 93.7%.

[0055] Example 3:

[0056] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 20 parts magnesium silicate, 10 parts hydroxyapatite, and 1 part zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.7 g / cm³. Its main components are 65% magnesium silicate, 32% hydroxyapatite, and 3% zirconium oxide.

[0057] Add the above seed crystals Figure 4In a fluidized bed reactor, fluoride-containing wastewater with a pH of 0.6 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 15 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 15 mg / L. The average particle size of the crystalline product CaF2 is 1.8 mm, with a water content of 11%, a purity of 95%, and a fluoride recovery rate of 95.7%.

[0058] Example 4:

[0059] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 15 parts magnesium silicate, 5 parts hydroxyapatite, and 2 parts zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 2.5 g / cm³. Its main components are 68% magnesium silicate, 23% hydroxyapatite, and 9% zirconium oxide.

[0060] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 0.8 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 22 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 16 mg / L. The average particle size of the crystalline product CaF2 is 1.6 mm, with a water content of 12.5%, a purity of 95%, and a fluoride recovery rate of 95.4%.

[0061] Example 5:

[0062] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts magnesium silicate, 15 parts hydroxyapatite, and 1 part zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.5 g / cm³. Its main components are 65% magnesium silicate, 33% hydroxyapatite, and 2% zirconium oxide.

[0063] Add the above seed crystals Figure 4In a fluidized bed reactor, fluoride-containing wastewater with a pH of 0.7 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 12 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 20 mg / L. The crystalline product, CaF2, has an average particle size of 1.4 mm, a water content of 14%, a purity of 93%, and a fluoride recovery rate of 94.3%.

[0064] Example 6:

[0065] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts aluminum silicate, 15 parts carbon-based apatite pretreated by calcination at 900℃ for 120 min in a muffle furnace, and 2 parts silicon carbide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined at 1200℃ in a muffle furnace for 180 min, and spray-granulated into microspheres of approximately 100 mesh. These microspheres are then passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm). After thorough shaking, the intermediate layer particles are collected and sieved again to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.8 g / cm³. Its main components are 64% aluminum silicate, 32% carbon-based apatite, and 4% silicon carbide.

[0066] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (inlet 2). Calcium chloride solution is introduced through inlet 2 (chemical inlet 2), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 17 mg / L. The average particle size of the crystalline product CaF2 is 1.9 mm, with a water content of 12%, a purity of 93%, and a fluoride recovery rate of 95.1%.

[0067] Example 7:

[0068] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts zeolite, 15 parts magnesium hydroxide pretreated by calcination at 900℃ for 120 min in a muffle furnace, and 2 parts silicon carbide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined at 1200℃ in a muffle furnace for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.9 g / cm³. Its main components are 64% zeolite, 32% magnesium hydroxide, and 4% silicon carbide.

[0069] Add the above seed crystals Figure 4In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (water inlet). Calcium chloride solution is introduced through inlet 2 (chemical inlet), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 16 mg / L. The average particle size of the crystalline product CaF2 is 1.8 mm, with a water content of 13%, a purity of 94%, and a fluoride recovery rate of 95.4%.

[0070] Example 8:

[0071] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts zeolite, 15 parts calcium carbonate pretreated by calcination at 900℃ for 120 min in a muffle furnace, and 2 parts silicon carbide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined at 1200℃ in a muffle furnace for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 2.1 g / cm³. Its main components are 64% zeolite, 32% calcium carbonate, and 4% silicon carbide.

[0072] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (inlet 2). Calcium chloride solution is introduced through inlet 2 (chemical inlet 2), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 20 mg / L. The average particle size of the crystalline product CaF2 is 1.8 mm, with a water content of 13%, a purity of 94%, and a fluoride recovery rate of 94.3%.

[0073] Example 9:

[0074] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 15 parts magnesium silicate, 15 parts hydroxyapatite, and 1.92 parts zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a coarse (>200 μm) sieve and a fine (<100 μm) sieve, and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 2.1 g / cm³. Its main components are 47% magnesium silicate, 47% hydroxyapatite, and 6% zirconium oxide.

[0075] Add the above seed crystals Figure 4In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (inlet 2). Calcium chloride solution is introduced through inlet 2 (chemical inlet 2), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 17 mg / L. The average particle size of the crystalline product CaF2 is 2.0 mm, with a water content of 14%, a purity of 93%, and a fluoride recovery rate of 95.1%.

[0076] Example 10:

[0077] A defluorination seed crystal for recovering calcium fluoride under acidic conditions comprises the following components by weight: 30 parts magnesium silicate, 5 parts hydroxyapatite, and 1.2 parts zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a coarse (>200 μm) sieve and a fine (<100 μm) sieve, and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a defluorination seed crystal with a particle size of 100-200 μm and a density of 1.9 g / cm³. Its main components are 82.9% magnesium silicate, 13.8% hydroxyapatite, and 3.3% zirconium oxide.

[0078] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1 (inlet 2). Calcium chloride solution is introduced through inlet 2 (chemical inlet 2), with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity within the fluidized bed is 18 m / h, and the fluoride-removing seed crystals are in a vigorous, uniformly suspended, pseudo-fluid state. After the reaction stabilizes, the effluent flowing out through outlet 4 has a fluoride concentration of 16 mg / L. The average particle size of the crystalline product CaF2 is 1.9 mm, with a water content of 14%, a purity of 91%, and a fluoride recovery rate of 95.4%.

[0079] Comparative Example 1:

[0080] A fluoride-removing seed crystal comprises the following components by weight: 30 parts magnesium silicate and 15 parts calcium hydroxyphosphate. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a coarse (>200 μm) sieve and a fine (<100 μm) sieve, and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a fluoride-removing seed crystal with a particle size of 100-200 μm and a density of 1.1 g / cm³. Its main components are 67% magnesium silicate and 33% calcium hydroxyphosphate.

[0081] Add the above seed crystals Figure 4In the fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L is introduced through inlet 1, and calcium chloride solution is introduced through inlet 2, with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity in the fluidized bed is 15 m / h. Because no calcium oxide material is added to adjust the density, the seed crystal density is low, not reaching the range of 1.5-2.5 g / cm³. The excessively high flow velocity in the fluidized bed leads to seed crystal loss, making it impossible to recover CaF2.

[0082] Comparative Example 2:

[0083] A fluoride-removing seed crystal comprises the following components by weight: 30 parts magnesium silicate and 2 parts zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a fluoride-removing seed crystal with a particle size of 100-200 μm and a density of 2.3 g / cm³. Its main components are 94% magnesium silicate and 6% zirconium oxide.

[0084] Add the above seed crystals Figure 4 In a fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L was introduced through inlet 1 (water inlet), while calcium chloride solution was introduced through inlet 2 (chemical inlet), controlling the calcium-fluoride molar ratio at 1.1. The upward flow velocity within the fluidized bed was 15 m / h, and the fluoride-removing seed crystals were in a vigorous, uniformly suspended pseudo-fluid state. Due to the absence of added hydroxyapatite, OH- slow-release capacity could not be provided, resulting in reduced crystallization efficiency. The effluent fluoride concentration was 350 mg / L, and the particle size of the crystalline product CaF2 showed no significant change, thus CaF2 could not be recovered.

[0085] Comparative Example 3:

[0086] A fluoride-removing seed crystal comprises the following components by weight: 15 parts of calcium hydroxyphosphate and 2 parts of zirconium oxide. Preparation method: The materials are mixed and ball-milled for 120 min, calcined in a muffle furnace at 1200℃ for 180 min, spray-granulated into microspheres of approximately 100 mesh, passed through a sieve with a coarse upper layer (>200 μm) and a fine lower layer (<100 μm), and after thorough shaking, the intermediate layer particles are collected and sieved to obtain a fluoride-removing seed crystal with a particle size of 100-200 μm and a density of 1.6 g / cm³. Its main components are 88% calcium hydroxyphosphate and 12% zirconium oxide.

[0087] Add the above seed crystals Figure 4In the fluidized bed reactor, fluoride-containing wastewater with a pH of 1 and a concentration of 350 mg / L was introduced through inlet 1, and calcium chloride solution was introduced through inlet 2, with the calcium-fluoride molar ratio controlled at 1.1. The upward flow velocity in the fluidized bed was 15 m / h. Due to the lack of magnesium silicate material, the acid resistance provided by the carrier was insufficient. After the seed crystals came into contact with the acidic fluoride-containing wastewater, they gradually dissolved and were lost, failing to provide sites for crystallization and thus failing to recover CaF2.

[0088] Test methods and conditions:

[0089] 1. Seed size

[0090] The PSA (partial particle size distribution) of calcium fluoride crystals was measured using a laser particle size analyzer. The principle is that when a laser beam irradiates a particle sample, scattering occurs. The intensity and angle of the scattered light are related to the particle size; therefore, by measuring the intensity of scattered light at different angles, the particle size distribution can be inferred. Before the experiment, a certain amount of sample needs to be placed in water or ethanol and dispersed using an ultrasonic vibrator to avoid particle agglomeration or sedimentation affecting the test results.

[0091] 2. Seed density

[0092] The seed crystal density was determined using the volumetric method. Procedure: 10 mL of distilled water was placed in a 25 mL graduated cylinder, and the total mass of the cylinder was recorded as m1. Approximately 5 g of seed crystals was then placed in the graduated cylinder, and the total mass of the cylinder was recorded as m2. The volume V in the graduated cylinder was then read. The seed crystal density was calculated using the following formula:

[0093]

[0094] 3. Fluoride concentration in effluent (Determination of fluoride in water by ion-selective electrode method (GB 7484-87))

[0095] In the experiment, the fluoride ion concentration was determined using the ion-selective electrode method. According to the Nernst equation, the electromotive force of the battery changes with the activity of fluoride ions, and the fluoride ion concentration can be calculated through the linear relationship between the electromotive force and the negative logarithm of the fluoride ion activity. Procedure: Adjust the pH of the sample to the range of 5-8 with HCl or NaOH. Take a certain volume of sample into a 50 mL volumetric flask, add 10 mL of total ionic strength adjustment buffer, dilute to the mark with deionized water, and mix well. Place the solution in a 100 mL beaker, with the fluoride ion electrode and reference electrode below the liquid surface, and stir continuously at 300 rpm. Read the voltage after it stabilizes.

[0096] 4. Average particle size of CaF2

[0097] The particle size distribution of calcium fluoride crystals was measured using a laser particle size analyzer. The average particle size of the seed crystals was characterized by D50 (the particle size value corresponding to 50% of the cumulative particle size distribution of the sample, indicating that the particles with a size greater than and less than this value each account for 50%).

[0098] 5. CaF2 moisture content (Calcium Fluoride (GBT 27804-2011))

[0099] The obtained calcium fluoride product was filtered, and the wet weight of the product was obtained. It was then placed in an oven at 105~110℃ and dried until the mass was constant. The product was then removed, placed in a desiccator, cooled for 30 minutes, and weighed to obtain the dry weight. The moisture content of the calcium fluoride product was calculated according to the following formula.

[0100]

[0101] 6. CaF2 purity (Calcium Fluoride (GBT 27804-2011))

[0102] The product was decomposed by heating with a mixture of hydrochloric acid, boric acid, and sulfuric acid. The calcium fluoride content in the solution was determined by EDTA titration. The purity was calculated by dividing the mass of the tested calcium fluoride by the total mass of the sample. Procedure: Weigh 0.50 g of the sample (accurate to 0.0001 g) into a 250 mL beaker, moisten with anhydrous ethanol, then add 50 mL of the hydrochloric acid-boric acid-sulfuric acid mixture. Heat to a gentle boil for 30 min. Remove from heat, cool, and dilute to approximately 80 mL. Continue heating to a gentle boil, then filter through rapid filter paper into a 100 mL volumetric flask. After cooling, dilute with water to the mark and mix well. Pipette 0.20 mL of the test solution into a 50 mL beaker, dilute with water to 10 mL, add 0.5 mL of triethanolamine solution and 0.4 mL of potassium hydroxide solution, and titrate the calcium fluoride content with EDTA standard solution.

[0103] 7. Fluorine recovery rate

[0104] When treating fluoride-containing wastewater with a fluidized bed, fluoride ions are removed and recovered in the form of calcium fluoride crystals. Therefore, the fluoride recovery rate is expressed as the fluoride ion removal rate, and the calculation formula is as follows:

[0105]

[0106] Table 1

[0107]

[0108] The data above shows that the seed crystal density in Examples 1-10 was maintained within the ideal range of 1.5-2.5 g / cm³, achieving stable fluidization, high defluorination efficiency, and yielding high-purity CaF2 products with large particles of 1-2 mm. Comparative Example 1, lacking a density regulator, had a low density and was flushed out of the reactor, resulting in seed crystal loss. Comparative Example 2, without the addition of calcium hydroxyphosphate, could not provide OH- slow-release capacity, leading to reduced crystallization efficiency, an effluent fluoride concentration of 350 mg / L, and no significant change in the CaF2 particle size, thus failing to recover CaF2. Comparative Example 3, lacking the addition of magnesium silicate, lacked the acid resistance provided by a carrier; the seed crystals gradually dissolved and were lost after contact with acidic fluoride-containing wastewater, failing to provide crystallization sites and resulting in CaF2 recovery.

[0109] Figure 1 The image shown is an SEM image of the defluorinated seed crystals prepared in Example 1. It can be seen that the microspheres have a porous and rough surface, and their active components are uniformly distributed in the support framework, forming an ideal surface morphology that is conducive to induced crystallization and slow release of OH⁻.

[0110] Figure 2 The image shows the XRD patterns of four seed crystals. The first curve, "composite seed crystal," represents the fluorine-removing seed crystal prepared in Example 1 of this application. The second, third, and fourth curves represent the XRD peaks of magnesium silicate, calcium hydroxyphosphate, and zirconium oxide, respectively. Figure 2 It can be seen that the fluoride-removing seed crystals prepared in this application are the result of the synergistic effect of the three component raw materials in removing fluoride.

[0111] Figure 3 The particle size analysis (PSA) of the defluorination seed crystals obtained in Example 1 shows that the particle size of the prepared defluorination seed crystals is concentrated in the target range of 100-200 micrometers, and the distribution curve is single-peaked and narrow, which proves its good particle size uniformity and fully meets the requirements of fluidized bed process for seed crystal hydraulic classification characteristics.

[0112] Figure 5 The SEM image of the recovered calcium fluoride product shows that the recovered calcium fluoride product is composed of numerous micron-sized cubic crystals tightly aggregated together. Its rough surface morphology increases the specific surface area of ​​the product, confirming the crystallization mechanism of in-situ heterogeneous nucleation and growth of calcium fluoride on the seed surface.

[0113] In the description of this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more features. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0114] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0115] The embodiments, implementation methods, and related technical features of this application can be combined and substituted for each other without conflict.

[0116] The above are merely preferred embodiments of this application and are not intended to limit this application in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of this application without departing from the scope of the technical solution of this application shall still fall within the scope of the technical solution of this application.

Claims

1. A fluoride-removing seed crystal, characterized in that, include: Carrier material, OH - Sustained-release agents and density modifiers.

2. The fluoride-removing seed crystal according to claim 1 or 2, characterized in that, The carrier material includes at least one of magnesium silicate, aluminum silicate, and zeolite; and / or, the OH... - The slow-release agent includes at least one of calcium hydroxyphosphate, carbon-based apatite, magnesium hydroxide, and calcium carbonate; and / or, the density regulator includes at least one of zirconium oxide and silicon carbide.

3. The fluoride-removing seed crystal according to claim 1 or 2, characterized in that, The defluorinated seed crystals comprise: 30.3%-90.9% carrier material and 7.3%-65.6% OH. - Sustained-release agent, 0.9%-18.8% density modifier; preferably, comprising: 47%-83wt% carrier material, 14%-48% OH- - Sustained-release agent, 2%-9% density regulator.

4. The fluoride-removing seed crystal according to any one of claims 1-3, characterized in that, The particle size range of the fluoride-removing seed crystals is 100-200 μm; and / or the density range of the fluoride-removing seed crystals is 1.5-2.5 g / cm³.

5. A method for preparing fluoride-removing seed crystals according to any one of claims 1-4, characterized in that, Carrier material, OH - The slow-release agent and density regulator are mixed, ground and calcined, and then spray-granulated to obtain fluoride-free seed crystals.

6. The method for preparing fluoride-removing seed crystals according to claim 5, characterized in that, The carrier material includes at least one of magnesium silicate, aluminum silicate, and zeolite; and / or, the OH... - The slow-release agent includes at least one of calcium hydroxyphosphate, carbon-based apatite, magnesium hydroxide, and calcium carbonate; and / or, the density regulator includes at least one of zirconium oxide and silicon carbide.

7. The method for preparing fluoride-removing seed crystals according to claim 5 or 6, characterized in that, The carrier material, OH - The mixture of slow-release agent and density modifier includes: 10-35 parts carrier material, 3-20 parts OH- - Sustained-release agent, 0.5-3 parts density modifier; preferably, 15-30 parts carrier material, 5-15 parts OH- - Sustained-release agent, 1-2 parts density adjuster.

8. The method for preparing fluoride-removing seed crystals according to any one of claims 5-7, characterized in that, After spray granulation, the particles need to be sieved. Preferably, the particle size range after sieving is 100-200μm.

9. The method for preparing fluoride-removing seed crystals according to any one of claims 5-8, characterized in that, The grinding time is 60-120 min; and / or the calcination temperature is 900-1200℃; and / or the calcination time is 120-180 min; and / or the spray granulation yields 100-mesh microspheres.

10. The application of a fluoride-removing seed crystal prepared by any one of claims 1-4 or any one of claims 5-9 in calcium fluoride recovery.

11. A fluidized bed, characterized in that, It includes defluorination seed crystals prepared by any one of the methods described in claims 1-4 or claims 5-9.

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