A method for preparing aluminum fluoride using fluorosilicic acid, a byproduct of phosphate fertilizer production.

Abstract

This invention relates to the field of phosphorus chemical by-product treatment technology, and discloses a method for preparing aluminum fluoride using fluorosilicic acid, a by-product of phosphate fertilizer production. The method includes the following sequential steps: S1. Three-stage deep purification of fluorosilicic acid from phosphate fertilizer production; S2. Preparation of composite modified aluminum source and medium-temperature reaction; S3. Crystallization and deep recovery of fluorine resources; S4. Segmented atmosphere calcination and finished product preparation. In the three-stage deep purification steps of fluorosilicic acid from phosphate fertilizer production in this invention, ammonium bicarbonate is first used to neutralize and adjust the pH to remove phosphorus, converting phosphate ions into soluble phosphates, thus removing phosphorus impurities from the source. Then, a stable water-soluble complex is formed between the metal ions and a complexing agent of disodium ethylenediaminetetraacetate-tartaric acid. Combined with the flocculation and filtration effects of polyacrylamide flocculant, metal ions such as iron, calcium, and magnesium are efficiently removed. The compounding ratio and addition amount of the complexing agent are precisely matched to the impurity content, ensuring a sufficient complexing reaction without the introduction of secondary impurities.

Description

Technical Field

[0001] This invention relates to the field of phosphorus chemical by-product treatment technology, and in particular to a method for preparing aluminum fluoride using fluorosilicic acid, a by-product of phosphate fertilizer production. Background Technology

[0002] Fluorine is a vital resource. In the wet-process phosphoric acid production of the phosphate chemical industry, fluorine from phosphate rock escapes with the tail gas. After being absorbed by water, it forms fluorosilicic acid as a byproduct. For every ton of phosphoric acid produced, 0.05 to 0.10 tons of fluorosilicic acid are generated as a byproduct, resulting in a considerable annual output. Fluorosilicic acid is highly corrosive and chemically unstable. Direct discharge would cause severe environmental pollution and lead to a significant waste of fluorine resources. Currently, high-value utilization technologies for fluorosilicic acid are not fully developed, and most fluorosilicic acid is only simply treated or used at low value, becoming an environmental burden and a resource waste problem for phosphate chemical enterprises.

[0003] Aluminum fluoride, a core product of the fluorochemical industry, is a key fluxing agent in electrolytic aluminum production and is also widely used in ceramics, specialty glass, catalysts, and other fields. Market demand for high-quality aluminum fluoride continues to grow. Existing processes for preparing aluminum fluoride using fluorosilicic acid, a byproduct of phosphate chemical production, generally suffer from incomplete purification of the fluorosilicic acid. Impurities such as phosphorus, iron, calcium, and magnesium in the crude fluorosilicic acid enter the reaction system, affecting not only the purity and performance of the aluminum fluoride product but also increasing side reactions and reducing raw material utilization. Furthermore, traditional processes often use unmodified industrial aluminum hydroxide as the aluminum source, which has low reactivity, resulting in a slow reaction rate with fluorosilicic acid, incomplete conversion, and a tendency for fluoride ion residues.

[0004] The existing process for preparing aluminum fluoride from fluorosilicic acid still suffers from problems such as severe equipment scaling, poor process continuity, and low batch stability of products. Impurities in crude fluorosilicic acid and silica produced during the reaction process are prone to deposit on the equipment surface, leading to a decrease in heat transfer and mixing efficiency, requiring frequent shutdowns for cleaning, increasing production and maintenance costs, and limiting the realization of industrial continuous production.

[0005] In summary, developing a stable aluminum fluoride preparation method that enables deep purification of fluorosilicic acid, efficient reaction of aluminum source, high-quality crystallization, recycling of fluorine resources, and high-value utilization of fluorosilicic acid byproducts from the phosphorus chemical industry is key to solving the problems of high-value utilization of fluorosilicic acid and high-quality aluminum fluoride production. Summary of the Invention

[0006] To address the problems in the prior art, this invention provides a method for preparing aluminum fluoride using fluorosilicic acid, a byproduct of phosphate fertilizer production.

[0007] The technical solution adopted by this invention to solve its technical problem is: a method for preparing aluminum fluoride using fluorosilicic acid, a byproduct of phosphate fertilizer production, comprising the following continuous steps: S1. Three-stage deep purification of fluorosilicic acid, a byproduct of phosphate fertilizer production: A neutralizing agent is added to the crude fluorosilicic acid to adjust the pH and neutralize and remove phosphorus, converting phosphate ions in the solution into soluble phosphates. A complexing agent is added to the phosphorus-removed solution to form stable water-soluble complexes with the metal ions. A flocculant is then added, followed by clarification and filtration to complete the complexation and impurity removal process. Concentrated hydrofluoric acid is added to the complexed and impurity-removed solution to adjust the fluorine-silica ratio and perform acidification and depolymerization to obtain soluble SiF6. 2- The fluorosilicic acid is depolymerized into SiO2 precipitate, and the SiO2 precipitate is removed by filtration to obtain a refined fluorosilicic acid solution; the refined fluorosilicic acid solution needs to pass the test before proceeding to step S2; S2. Preparation and medium-temperature reaction of composite modified aluminum source: Using aluminum hydroxide as raw material, a compound modifier is added to prepare a slurry and activate it at low temperature. After drying and pulverizing, a composite modified aluminum source is obtained. The composite modified aluminum source is mixed with the refined fluorosilicic acid solution obtained in step S1 and reacted. After the reaction is completed, the by-product silicon dioxide is removed by filtration to obtain a supersaturated aluminum fluoride solution. S3. Crystallization and deep recovery of fluorine resources: After surface modification treatment of the seed crystals, they are added to the supersaturated aluminum fluoride solution obtained in step S2 and crystallized at a constant temperature. After separation and washing, aluminum fluoride trihydrate crystals are obtained. After separation treatment of the crystallization mother liquor, the retentate is returned to the crystallization process for recrystallization, and the permeate is discharged after defluorination treatment to meet the standards. The adsorbed fluoride ions are desorbed and returned to step S1 for recycling. S4. Segmented calcination and finished product preparation: The aluminum trihydrate crystals obtained in step S3 are placed in a calcination furnace, a protective atmosphere is introduced, and the crystals are dehydrated and decrystallized by segmented heating and holding. After cooling, unqualified particles are removed by sieving to obtain anhydrous aluminum fluoride finished product.

[0008] As a further technical solution, the industrial standard indicators of the crude fluorosilicic acid by-product of phosphate fertilizer in step S1 are: P2O5 content 0.5-2.0 g / L, total metal ion content 0.1-0.3 g / L; the neutralizing agent is industrial-grade ammonium bicarbonate with a purity ≥98%; The specific conditions for neutralization and phosphorus removal in step S1 are as follows: the neutralizing agent is slowly added dropwise at a rate of 1-2 mL / min, the pH of the solution is adjusted to 3.0-3.5, and the reaction is carried out at 40-50℃ and a stirring speed of 200-250 r / min for 20-25 min; after the reaction is completed, the P2O5 content in the solution is ≤0.2 g / L. The complexing agent mentioned in step S1 is a compound complexing agent of disodium ethylenediaminetetraacetate and tartaric acid, with a mass ratio of 1:2; the amount of complexing agent added is the Fe in the solution. 3+ Ca 2+ Mg 2+ The complexation reaction is carried out at 20-25℃ for 25-30 minutes with the total mass being 10-12 times.

[0009] As a further technical solution, the flocculant in step S1 is polyacrylamide, and the addition amount is 0.01-0.02 g / L; the flocculation process is to stir for 5-10 min and then let it stand for 30-40 min to clarify, and then use plate and frame filtration to remove flocs; after complexation and impurity removal, the total metal ion content in the solution is ≤0.02 g / L; The concentrated hydrofluoric acid mentioned in step S1 is industrial grade, with a mass fraction of 40-48%; the specific conditions for acid-induced depolymerization are as follows: adjust the fluorine-silicon ratio of the solution to 6.5-7.0, and stir for 15-20 minutes at 30-40℃ and a stirring speed of 200-250 r / min; the SiO2 precipitate is removed by vacuum filtration with a vacuum degree of 0.07-0.08 MPa; The refined fluorosilicic acid solution in step S1 has the following specifications: purity ≥ 99.0%, P2O5 content ≤ 0.05 g / L, and total metal ion content ≤ 0.01 g / L. Each batch must be tested.

[0010] As a further technical solution, the aluminum hydroxide in step S2 is industrial grade with a purity ≥98.5%; the compound modifier is a kaolin-zirconia compound system with a mass ratio of 4:1, wherein the kaolin Al2O3 content is 45-60% and the zirconium dioxide purity is ≥99%; the amount of modifier added is 6-8% of the mass of aluminum hydroxide.

[0011] As a further technical solution, the solid content of the slurry in step S2 is 30-40%, the low-temperature activation conditions are 60-70℃, the stirring speed is 350-400r / min, and the activation time is 40-50min; the drying is spray drying, the pulverization is air jet pulverization, and the particle size of the composite modified aluminum source after pulverization is ≤40μm; The specific parameters for the spray drying are: inlet air temperature 180-200℃, outlet air temperature 80-90℃, and feed rate 20-30mL / min; the inlet air pressure for the airflow pulverizer is 0.7-0.9MPa.

[0012] As a further technical solution, the composite modified aluminum source and the Al in the refined fluorosilicic acid solution mentioned in step S2 3+ With F - The molar ratio is 1:3.0-3.1; the reaction conditions are: temperature 65-75℃, stirring speed 400-450 r / min, reaction time 30-40 min; nitrogen gas is introduced during the reaction. In step S2, a ceramic membrane with a pore size of 0.1-0.2 μm is used for filtration; the aluminum fluoride entrainment in the by-product silica filter residue is ≤0.5%, and the SiO2 content in the supersaturated aluminum fluoride solution after filtration is ≤0.01 g / L.

[0013] As a further technical solution, the seed crystal in step S3 is a β-type aluminum fluoride trihydrate seed crystal with a particle size of 30-40 μm and a purity of ≥99.5%. The surface modification treatment of the seed crystal includes: first, vacuum drying at 60-70℃ and a vacuum degree of 0.06-0.07 MPa for 15-20 min, after which the water content of the seed crystal is ≤0.5%; then, immersing it in a dilute solution of silane coupling agent for 5-10 min, and then drying it at 80-90℃ for 10-15 min, after which the hydroxyl content on the surface of the seed crystal is reduced by ≥30%. The silane coupling agent is KH550, and the mass fraction of the dilute solution is 0.5%. The solvent is a mixture of deionized water and ethanol in a volume ratio of 3:1. The dilute solution is prepared by dissolving KH550 in the mixed solvent, stirring evenly, and then letting it stand for 5 minutes. The amount of seed crystals added in step S3 is 4-5% of the theoretical yield of aluminum fluoride; the crystallization conditions are: adjust the solution pH to 2.0-2.2, raise the temperature to 80-85℃, and use a gradient stirring process to crystallize at a constant temperature for 3-4 hours. The gradient stirring process is: the stirring speed is 450-500 r / min for the first hour of crystallization, and then the stirring speed is reduced to 300-350 r / min for the next 2-3 hours.

[0014] As a further technical solution, the separation in step S3 is carried out by centrifugal filtration with the following parameters: rotation speed 3500-4000 r / min, centrifugation time 15-20 min; washing is carried out by countercurrent washing 2-3 times, washing water temperature 40-50℃, and the amount of washing water is 1-1.5 times the mass of crystals; after washing, the residual fluoride ion content on the crystal surface is ≤0.1%, and the water content is ≤4%; The separation and treatment of the crystallization mother liquor in step S3 is carried out by nanofiltration membrane separation. The nanofiltration membrane has a molecular weight cutoff of 200-300 and an operating pressure of 0.3-0.5MPa. The aluminum fluoride concentration in the retentate is enriched to 150-200g / L. After the permeate is adsorbed by a strong basic anion exchange resin, the fluoride ion content is ≤10mg / L before it can be discharged. The fluoride ions adsorbed by the strongly basic anion exchange resin are desorbed by a 4-6% sodium hydroxide solution. The fluoride ion concentration in the desorbed solution is ≥50g / L. The desorbed solution is then returned to the purified fluorosilicic acid solution in step S1 for recycling.

[0015] As a further technical solution, the specific conditions for the segmented calcination in step S4 are as follows: (1) Low-temperature dehydration: Dry nitrogen gas is introduced as a protective atmosphere, and the temperature is raised to 220-240℃ at a heating rate of 5-8℃ / min, and held for 2.5-3h. After dehydration, the water content of the crystals is ≤0.5%; (2) High-temperature decrystallization: Continue heating at a rate of 3-5℃ / min to 600-620℃, switch to a nitrogen-argon mixed protective atmosphere, and hold for 3-4 hours to completely convert β-type aluminum fluoride trihydrate into anhydrous aluminum fluoride. After decrystallization, the water content of the crystals is ≤0.2%. The flow rate of the dry nitrogen is 150-200 mL / min; the volume ratio of the nitrogen-argon mixed protective atmosphere is 1:1, and the total flow rate is maintained at 150-200 mL / min; during the atmosphere switching process, a slight positive pressure of 0.001-0.002 MPa is maintained inside the calcining furnace.

[0016] As a further technical solution, the cooling conditions in step S4 are as follows: after calcination, maintain the protective atmosphere and cool to below 100℃ at a cooling rate of 4-6℃ / min, then allow to cool naturally to room temperature; the sieving adopts ultrasonic-assisted vibration sieving, with a screen mesh size of 80-100 mesh, ultrasonic power of 50-80W, sieving time of 5-10min, and sieving efficiency ≥98%; The specifications of the anhydrous aluminum fluoride product in step S4 are: particle size distribution range of 40-80μm and sphericity ≥0.95.

[0017] The beneficial effects of this invention are: In the three-stage deep purification process of fluorosilicic acid, a byproduct of phosphate fertilizer production, the present invention first uses ammonium bicarbonate to neutralize and adjust the pH to remove phosphorus, converting phosphate ions into soluble phosphates and removing phosphorus impurities at the source. Next, a complexing agent consisting of disodium ethylenediaminetetraacetate and tartaric acid forms a stable water-soluble complex with metal ions. Combined with the flocculation and filtration effects of polyacrylamide flocculant, iron, calcium, magnesium, and other metal ions are efficiently removed. The complexing agent's compounding ratio and addition amount are precisely matched to the impurity content, ensuring a complete complexing reaction without the introduction of secondary impurities. Finally, concentrated hydrofluoric acid is used to adjust the fluorine-silica ratio to achieve acid-induced depolymerization, converting soluble fluorosilicate ions into silica precipitates, which are then filtered out. This step, by controlling the fluorine-silica ratio and reaction conditions, achieves deep removal of silicon impurities, ultimately yielding a high-purity refined fluorosilicic acid solution with a purity exceeding 99.0% and impurity content reduced to extremely low levels, laying a raw material foundation for the subsequent preparation of high-quality aluminum fluoride. In the preparation of the composite modified aluminum source, kaolin-zirconia is used as a compound modifying agent. The aluminum oxide in kaolin can form a synergistic aluminum source structure with aluminum hydroxide, while the zirconium dioxide can refine the aluminum source particles and improve surface activity. After low-temperature activation, spray drying, and air jet milling, the particle size of the composite modified aluminum source is ≤40μm, the specific surface area is significantly increased, and the contact area with fluorosilicic acid is significantly increased, effectively solving the problem of low reactivity of traditional aluminum sources. In the crystallization process, β-type aluminum trihydrate fluoride crystals are vacuum dried and modified with silane coupling agent KH550. Vacuum drying makes the water content of the crystals ≤0.5%, and the silane coupling agent reduces the surface energy by replacing the hydroxyl groups on the crystal surface. After modification, the hydroxyl content on the crystal surface is reduced by ≥30%, which fundamentally inhibits the occurrence of crystal agglomeration. The gradient stirring crystallization process achieves uniform dispersion of crystals through high-speed stirring in the early stage and ensures orderly crystal growth through low-speed stirring in the later stage. Combined with the control of the amount of crystal added and the crystallization temperature, the aluminum trihydrate fluoride crystals have regular morphology and excellent powder properties. The segmented atmosphere calcination process utilizes different heating rates and protective atmospheres based on the dehydration and decrystallization characteristics of aluminum trihydrate. In the low-temperature dehydration stage, nitrogen is used as the protective gas, and a slower heating rate is employed to gently remove surface free water and prevent crystal cracking. In the high-temperature decrystallization stage, a nitrogen-argon mixed protective gas is used to completely remove the water of crystallization, while preventing the aluminum fluoride from being oxidized or decomposed at high temperatures, thus ensuring the purity and structural stability of the anhydrous aluminum fluoride product.

[0018] The various steps in this invention work synergistically, with the entire process chain from raw materials, reaction, crystallization to calcination cooperating and progressing step by step, enhancing the technical effect. Deep purification of fluorosilicic acid provides a clean reaction system for the subsequent efficient reaction of the composite modified aluminum source, avoiding the complexation and adsorption of impurities with the aluminum source or fluoride ions, allowing the high reactivity of the composite modified aluminum source to be fully utilized. Combined with medium-temperature reaction conditions and nitrogen protection, rapid and complete reaction is achieved, with the amount of by-product silica ≤0.5%, significantly improving the conversion efficiency of aluminum fluoride. The efficient reaction of refined fluorosilicic acid with the composite modified aluminum source yields a low-impurity, high-concentration supersaturated aluminum fluoride solution, providing a high-quality solution system for the crystallization of modified seed crystals, avoiding interference from impurities on crystal growth, and fully realizing the modification effect of the seed crystals and the regulatory effect of the gradient stirring crystallization process. The prepared aluminum fluoride trihydrate crystals exhibit high purity and regular morphology, with controllable fluoride ion content in the crystallization mother liquor, creating conditions for deep recovery of fluoride resources. After staged atmospheric calcination, the high-quality aluminum fluoride trihydrate crystals undergo more thorough dehydration and decrystallization, resulting in a more uniform crystal size distribution. The anhydrous aluminum fluoride finished product after sieving has a particle size of 40-80 μm. Simultaneously, the deep recovery of fluoride resources during the crystallization process returns the nanofiltration membrane retentate to the crystallization process and the desorption solution to the fluorosilicic acid purification process, achieving the recycling of fluoride resources. The recycled fluoride resources then provide raw material replenishment for the fluorosilicic acid purification and reaction processes, further improving raw material utilization and forming a closed-loop process of raw material recycling. Furthermore, the deep impurity removal during fluorosilicic acid purification, the composite modification of the aluminum source, and the surface modification of the seed crystals collectively reduce impurity deposition and crystal adhesion during the reaction and crystallization processes, significantly reducing equipment scaling rates and significantly improving the continuity and stability of each process step, providing a guarantee for continuous industrial production.

[0019] This invention transforms environmentally friendly waste from the phosphate chemical industry into high-quality fluorochemical products, realizing the recycling and high-value utilization of fluorine resources. At the same time, it promotes the coordinated and sustainable development of the phosphate and fluorochemical industries, and has significant economic, environmental and industrial benefits. Attached Figure Description

[0020] Figure 1 This is a comparison chart showing the total utilization rate of fluorine resources between an example and a comparative example of a method for preparing aluminum fluoride from fluorosilicic acid, a byproduct of phosphate fertilizer production. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a method for preparing aluminum fluoride from fluorosilicic acid, a byproduct of phosphate fertilizer production. The method involves a three-stage deep purification process for the fluorosilicic acid, combined with the preparation of a composite modified aluminum source and a medium-temperature reaction, isothermal crystallization of modified seed crystals, and deep recovery of fluorine resources. Finally, anhydrous aluminum fluoride is prepared by staged atmospheric calcination. This method achieves high-value utilization of fluorosilicic acid, a byproduct of phosphate chemical production. The prepared anhydrous aluminum fluoride product has high purity and uniform particle size distribution, while simultaneously achieving the recycling of fluorine resources and reducing environmental pollution. The method of this invention specifically includes the following sequential steps: S1. Three-stage deep purification of fluorosilicic acid by-product of phosphate fertilizer: A neutralizing agent is added to the crude fluorosilicic acid by-product of phosphate fertilizer to adjust the pH and neutralize and remove phosphorus, so that the phosphate ions in the solution are converted into soluble phosphates; a complexing agent is added to the solution after phosphorus removal, so that the metal ions and the complexing agent form a stable water-soluble complex, and then a flocculant is added, followed by clarification and filtration to complete the complexing and impurity removal; concentrated hydrofluoric acid is added to the solution after complexing and impurity removal to adjust the fluorine-silica ratio, and acidification and depolymerization are carried out to depolymerize the soluble fluorosilicate ions into silica precipitate, and the silica precipitate is removed by filtration to obtain a refined fluorosilicic acid solution; the refined fluorosilicic acid solution needs to pass the test before proceeding to step S2.

[0023] In this invention, the preferred industrial parameters of the crude fluorosilicic acid produced as a byproduct of phosphate fertilizer production are a phosphorus pentoxide content of 0.5-2.0 g / L and a total metal ion content of 0.1-0.3 g / L; the preferred neutralizing agent is industrial-grade ammonium bicarbonate with a purity of not less than 98%. The preferred conditions for neutralization and phosphorus removal are: the neutralizing agent is slowly added dropwise at a rate of 1-2 mL / min, the pH of the solution is adjusted to 3.0-3.5, and the reaction is carried out at 40-50°C and a stirring speed of 200-250 r / min for 20-25 min; after the reaction, the phosphorus pentoxide content in the solution is not higher than 0.2 g / L.

[0024] The complexing agent is preferably a compound complexing agent of disodium ethylenediaminetetraacetate and tartaric acid, with a mass ratio of 1:2. The amount of complexing agent added is preferably 10-12 times the total mass of ferric ions, divalent calcium ions, and divalent magnesium ions in the solution. The complexation reaction is preferably carried out by stirring at room temperature (20-25℃) for 25-30 minutes. The flocculant is preferably polyacrylamide, with an addition amount of 0.01-0.02 g / L. The flocculation process is preferably stirred for 5-10 minutes, then allowed to stand for clarification for 30-40 minutes, followed by plate and frame filtration to remove flocs. The total metal ion content in the solution after complexation and impurity removal is preferably not higher than 0.02 g / L.

[0025] The concentrated hydrofluoric acid is preferably industrial grade, with a mass fraction of 40-48%. The specific conditions for acidification and depolymerization are preferably: adjusting the fluorine-silica ratio of the solution to 6.5-7.0, stirring at 30-40℃ and a stirring speed of 200-250 r / min for 15-20 min; silica precipitate is removed by vacuum filtration, with a vacuum degree preferably of 0.07-0.08 MPa. The purified fluorosilicic acid solution preferably has the following specifications: purity not less than 99.0%, phosphorus pentoxide content not more than 0.05 g / L, and total metal ion content not more than 0.01 g / L. Each batch must meet these standards before proceeding to subsequent steps.

[0026] S2. Preparation and medium-temperature reaction of composite modified aluminum source: Using aluminum hydroxide as raw material, a compound modifier is added to prepare a slurry and activate it at low temperature. After drying and pulverizing, a composite modified aluminum source is obtained. The composite modified aluminum source is mixed with the refined fluorosilicic acid solution obtained in step S1 and reacted. After the reaction is completed, the by-product silicon dioxide is removed by filtration to obtain a supersaturated aluminum fluoride solution.

[0027] In this invention, the aluminum hydroxide is preferably industrial grade with a purity of not less than 98.5%; the compound modifier is preferably a kaolin-zirconia compound system with a mass ratio of 4:1, wherein the kaolin aluminum oxide content is 45-60% and the zirconium dioxide purity is not less than 99%; the amount of modifier added is preferably 6-8% of the mass of aluminum hydroxide.

[0028] The slurry preferably has a solid content of 30-40%, and the low-temperature activation conditions are preferably 60-70℃, a stirring speed of 350-400 r / min, and an activation time of 40-50 min. Drying is performed by spray drying, and pulverization is performed by air jet milling. The particle size of the composite modified aluminum source after pulverization is preferably no higher than 40 μm. The specific parameters for spray drying are preferably: inlet air temperature 180-200℃, outlet air temperature 80-90℃, and feed rate 20-30 mL / min; the inlet air pressure for air jet milling is preferably 0.7-0.9 MPa.

[0029] The preferred molar ratio of trivalent aluminum ions to fluoride ions in the composite modified aluminum source and the refined fluorosilicic acid solution is 1:3.0-3.1. The preferred reaction conditions are: temperature 65-75℃, stirring speed 400-450 r / min, and reaction time 30-40 min. Nitrogen gas is introduced during the reaction to create an inert reaction environment. In step S2, a ceramic membrane is used for filtration, with a preferred pore size of 0.1-0.2 μm. The aluminum fluoride entrainment in the byproduct silica filter residue is preferably no more than 0.5%, and the silica content in the supersaturated aluminum fluoride solution after filtration is preferably no more than 0.01 g / L.

[0030] S3. Crystallization and deep recovery of fluorine resources: After surface modification treatment of the seed crystals, they are added to the supersaturated aluminum fluoride solution obtained in step S2 and crystallized at a constant temperature. After separation and washing, aluminum fluoride trihydrate crystals are obtained. After separation treatment of the crystallization mother liquor, the retentate is returned to the crystallization process for recrystallization, and the permeate is discharged after defluorination treatment to meet the standards. The adsorbed fluoride ions are desorbed and returned to step S1 for recycling.

[0031] In this invention, the seed crystal is preferably a β-type aluminum fluoride trihydrate seed crystal with a particle size of 30-40 μm and a purity of not less than 99.5%. The surface modification treatment of the seed crystal preferably includes: firstly, vacuum drying at 60-70℃ and a vacuum degree of 0.06-0.07 MPa for 15-20 min, after which the water content of the seed crystal is not higher than 0.5%; then, immersing it in a dilute solution of silane coupling agent for 5-10 min, and then drying it at 80-90℃ for 10-15 min, after which the hydroxyl content on the surface of the seed crystal is reduced by not less than 30%.

[0032] The preferred silane coupling agent is KH550, with a dilute solution mass fraction of 0.5%, and the solvent is a mixture of deionized water and ethanol in a volume ratio of 3:1. The dilute solution is prepared by dissolving KH550 in the mixed solvent, stirring evenly, and then letting it stand for 5 minutes. The preferred amount of seed crystals added is 4-5% of the theoretical yield of aluminum fluoride. The preferred crystallization conditions are: adjusting the solution pH to 2.0-2.2, raising the temperature to 80-85℃, and using a gradient stirring process for constant temperature crystallization for 3-4 hours. The gradient stirring process is as follows: the stirring speed is 450-500 r / min for the first hour of crystallization, and then the stirring speed is reduced to 300-350 r / min for the next 2-3 hours.

[0033] The separation is performed by centrifugal filtration, with the following parameters: rotation speed 3500-4000 r / min, centrifugation time 15-20 min; washing is performed by countercurrent washing 2-3 times, washing water temperature 40-50℃, and the amount of washing water is 1-1.5 times the mass of the crystal; after washing, the residual fluoride ion content on the crystal surface is preferably not higher than 0.1%, and the water content is preferably not higher than 4%.

[0034] The crystallization mother liquor is separated using nanofiltration membrane separation. The nanofiltration membrane has a molecular weight cutoff of 200-300 and an operating pressure of 0.3-0.5 MPa. The aluminum fluoride concentration in the retentate is enriched to 150-200 g / L. After the permeate is adsorbed by a strongly basic anion exchange resin to remove fluoride ions, the fluoride ion content can be discharged only if it does not exceed 10 mg / L. The fluoride ions adsorbed by the strongly basic anion exchange resin are desorbed by a 4-6% sodium hydroxide solution. The fluoride ion concentration in the desorbed solution is not less than 50 g / L. The desorbed solution is returned to the purified fluorosilicic acid solution in step S1 for recycling.

[0035] S4. Segmented calcination and finished product preparation: The aluminum trihydrate crystals obtained in step S3 are placed in a calcination furnace, a protective atmosphere is introduced, and the crystals are dehydrated and decrystallized by segmented heating and holding. After cooling, unqualified particles are removed by sieving to obtain anhydrous aluminum fluoride finished product.

[0036] In this invention, the specific conditions for the segmented calcination are preferably as follows: (1) Low-temperature dehydration: dry nitrogen is introduced as a protective atmosphere, and the temperature is raised to 220-240°C at a heating rate of 5-8°C / min, and held for 2.5-3 hours. After dehydration, the water content of the crystal is not higher than 0.5%; (2) High-temperature decrystallization: the temperature is raised to 600-620°C at a heating rate of 3-5°C / min, and the nitrogen-argon mixed protective atmosphere is switched. The temperature is held for 3-4 hours to completely convert β-type aluminum trihydrate into anhydrous aluminum fluoride. After decrystallization, the water content of the crystal is not higher than 0.2%.

[0037] The preferred flow rate of the dry nitrogen is 150-200 mL / min; the volume ratio of the nitrogen-argon mixed protective atmosphere is 1:1, and the total flow rate is maintained at 150-200 mL / min; during the atmosphere switching process, a slight positive pressure of 0.001-0.002 MPa is maintained inside the calcining furnace.

[0038] The preferred cooling conditions are: after calcination, maintaining a constant protective atmosphere, cooling to below 100°C at a cooling rate of 4-6°C / min, and then naturally cooling to room temperature; the sieving is performed using ultrasonic-assisted vibration sieving with a screen mesh size of 80-100 mesh, ultrasonic power of 50-80 watts, sieving time of 5-10 min, and a sieving efficiency of not less than 98%. The preferred specifications for the anhydrous aluminum fluoride product are: particle size distribution range of 40-80 μm.

[0039] This invention provides a method for preparing aluminum fluoride from fluorosilicic acid, a byproduct of phosphate fertilizer production. Through a three-stage deep purification process, high-purity refining of fluorosilicic acid is achieved, effectively removing impurities such as phosphorus and metal ions. The preparation of a composite modified aluminum source enhances its reactivity, while medium-temperature reaction conditions effectively control side reactions. Surface modification of the seed crystals and a gradient stirring crystallization process significantly improve the crystallization yield and morphological regularity of aluminum fluoride trihydrate crystals. Deep recovery of fluorine resources enables the recycling of raw materials, reducing production costs. The segmented atmosphere calcination process effectively avoids the decomposition and agglomeration of aluminum fluoride during calcination, resulting in anhydrous aluminum fluoride with high purity and uniform particle size distribution, meeting the needs of high-end industrial applications. Furthermore, the entire process generates no harmful waste, complying with the requirements of green and environmentally friendly industrial development.

[0040] To further illustrate the present invention, the following embodiments are provided for detailed description. The raw materials used in the following embodiments of the present invention are all conventional industrial raw materials, which can be obtained through commercial channels unless otherwise specified. The equipment used are all conventional chemical production equipment, without any special limitations.

[0041] Example 1: This embodiment utilizes fluorosilicic acid, a byproduct of phosphate fertilizer production, to prepare aluminum fluoride. The specific steps are as follows: 1. Three-stage deep purification of fluorosilicic acid by-product of phosphate fertilizer: Crude fluorosilicic acid by-product of phosphate fertilizer was selected, with a phosphorus pentoxide content of 0.5 g / L and a total metal ion content of 0.1 g / L. 98% pure industrial-grade ammonium bicarbonate was added to the crude fluorosilicic acid as a neutralizing agent. The neutralizing agent was slowly added dropwise at a rate of 1 mL / min to adjust the pH of the solution to 3.0. The reaction was carried out at 40℃ and a stirring speed of 200 r / min for 20 min. After the reaction, the phosphorus pentoxide content in the solution was 0.15 g / L.

[0042] Add a complexing agent of disodium ethylenediaminetetraacetate-tartaric acid in a mass ratio of 1:2 to the phosphorus-removed solution. The amount of complexing agent added is 10 times the total mass of ferric ions, calcium ions, and magnesium ions in the solution. Stir at 20°C for 25 minutes to carry out the complexation reaction. Then add 0.01 g / L of polyacrylamide as a flocculant, stir for 5 minutes, and let stand for 30 minutes to clarify. Remove the flocs by plate and frame filtration. After complexation and impurity removal, the total metal ion content in the solution is 0.015 g / L.

[0043] Add 40% (w / w) of industrial-grade concentrated hydrofluoric acid to the complexed and purified solution to adjust the fluorine-silica ratio to 6.5. Acid-induced depolymerization is carried out by stirring at 30℃ and 200 r / min for 15 min. Silica precipitate is removed by vacuum filtration at a vacuum degree of 0.07 MPa to obtain a purified fluorosilicic acid solution. The purified fluorosilicic acid solution was found to have a purity of 99.2%, a phosphorus pentoxide content of 0.04 g / L, and a total metal ion content of 0.008 g / L, meeting the standards for proceeding to the next step.

[0044] 2. Preparation of composite modified aluminum source and medium-temperature reaction: Industrial-grade aluminum hydroxide with a purity of 98.5% was selected as raw material, and kaolin-zirconia composite modifier was added with a mass ratio of 4:1. The kaolin contained 45% aluminum oxide and the zirconia had a purity of 99%. The amount of modifier added was 6% of the mass of aluminum hydroxide. A slurry with a solid content of 30% was prepared and activated at low temperature for 40 min at 60℃ and a stirring speed of 350 r / min.

[0045] The activated slurry was spray-dried at an inlet air temperature of 180℃, an outlet air temperature of 80℃, and a feed rate of 20mL / min; then it was pulverized by airflow at an inlet air pressure of 0.7MPa to obtain a composite modified aluminum source with a particle size of 35μm.

[0046] The composite modified aluminum source was mixed with the above-mentioned refined fluorosilicic acid solution, and the molar ratio of trivalent aluminum ions to fluoride ions was controlled at 1:3.0. Nitrogen gas was introduced and the reaction was carried out at 65℃ and a stirring speed of 400 r / min for 30 min. After the reaction, the by-product silica was removed by filtration with a ceramic membrane with a pore size of 0.1 μm. The aluminum fluoride entrainment in the by-product silica filter residue was 0.4%, and the silica content in the supersaturated aluminum fluoride solution after filtration was 0.008 g / L.

[0047] 3. Crystallization and Deep Recovery of Fluorine Resources: β-type aluminum fluoride trihydrate seed crystals with a particle size of 30 μm and a purity of 99.5% were selected for surface modification: First, they were vacuum dried at 60℃ and 0.06 MPa for 15 min, after which the water content of the seed crystals was 0.4%; then, they were immersed in a 0.5% KH550 silane coupling agent dilute solution for 5 min. The dilute solution solvent was a mixture of deionized water and ethanol with a volume ratio of 3:1. After removal, they were dried at 80℃ for 10 min. After modification, the hydroxyl content on the surface of the seed crystals decreased by 32%.

[0048] The modified seed crystals were added to a supersaturated aluminum fluoride solution at a rate of 4% of the theoretical yield of aluminum fluoride. The pH of the solution was adjusted to 2.0, and the temperature was raised to 80°C. Gradient stirring was used for constant temperature crystallization for 3 hours. The stirring speed was 450 r / min for the first hour of crystallization and then reduced to 300 r / min for the next 2 hours.

[0049] After crystallization, the crystals were separated by centrifugation and filtration at a speed of 3500 r / min for 15 min. The crystals were then washed twice countercurrently with washing water at 40℃, with the amount of washing water being equal to the mass of the crystals. After washing, the residual fluoride ions on the crystal surface were 0.08%, and the water content was 3.5%, yielding aluminum fluoride trihydrate crystals.

[0050] The mother liquor from crystallization was separated using a nanofiltration membrane with a molecular weight cutoff of 200 at an operating pressure of 0.3 MPa. The aluminum fluoride concentration in the retentate was enriched to 150 g / L and returned to the crystallization process for recrystallization. The permeate was subjected to adsorption of fluoride ions by a strongly basic anion exchange resin, and the fluoride ion content was 8 mg / L, which met the discharge standard. The adsorbed fluoride ions were desorbed by a 4% sodium hydroxide solution, and the fluoride ion concentration in the desorbed solution was 55 g / L, which was returned to the purified fluorosilicic acid solution from step 1 for recycling.

[0051] 4. Segmented atmosphere calcination and finished product preparation: Aluminum fluoride trihydrate crystals were placed in a calcination furnace, and dry nitrogen gas with a flow rate of 150 mL / min was introduced as a protective atmosphere. The temperature was increased to 220°C at a heating rate of 5°C / min, and held at that temperature for 2.5 hours for low-temperature dehydration. After dehydration, the water content of the crystals was 0.4%.

[0052] Continue heating at a rate of 3℃ / min to 600℃, switch to a nitrogen-argon mixed protective atmosphere with a volume ratio of 1:1, maintain a total flow rate of 150mL / min, keep a slight positive pressure of 0.001MPa in the calcination furnace, and hold for 3 hours to carry out high-temperature decrystallization. After decrystallization, the water content of the crystals is 0.15%.

[0053] After calcination, the mixed protective atmosphere is kept unchanged, and the temperature is cooled to below 100℃ at a cooling rate of 4℃ / min, and then naturally cooled to room temperature. Ultrasonic-assisted vibration sieving is used with a screen mesh size of 80 mesh, ultrasonic power of 50 watts, sieving time of 5 min, and sieving efficiency of 98.2%. After removing unqualified particles, anhydrous aluminum fluoride product is obtained with a particle size distribution range of 40-60μm.

[0054] Example 2: This embodiment utilizes fluorosilicic acid, a byproduct of phosphate fertilizer production, to prepare aluminum fluoride. The specific steps are as follows: 1. Three-stage deep purification of fluorosilicic acid by-product of phosphate fertilizer: Crude fluorosilicic acid by-product of phosphate fertilizer was selected, with a phosphorus pentoxide content of 2.0 g / L and a total metal ion content of 0.3 g / L. 98% pure industrial-grade ammonium bicarbonate was added to the crude fluorosilicic acid as a neutralizing agent. The neutralizing agent was slowly added dropwise at a rate of 2 mL / min to adjust the pH of the solution to 3.5. The reaction was carried out at 50℃ and a stirring speed of 250 r / min for 25 min. After the reaction, the phosphorus pentoxide content in the solution was 0.18 g / L.

[0055] Add a complexing agent of disodium ethylenediaminetetraacetate-tartaric acid in a mass ratio of 1:2 to the phosphorus-removed solution. The amount of complexing agent added is 12 times the total mass of ferric ions, calcium ions, and magnesium ions in the solution. Stir at 25°C for 30 min to carry out the complexation reaction. Then add 0.02 g / L of polyacrylamide as a flocculant, stir for 10 min, and let stand for 40 min to clarify. Remove the flocs by plate and frame filtration. After complexation and impurity removal, the total metal ion content in the solution is 0.018 g / L.

[0056] Add 48% (w / w) industrial-grade concentrated hydrofluoric acid to the complexed and purified solution to adjust the fluorine-silica ratio to 7.0. Acid-induced depolymerization is carried out by stirring at 40℃ and 250 r / min for 20 min. Silica precipitate is removed by vacuum filtration at a vacuum degree of 0.08 MPa to obtain a purified fluorosilicic acid solution. The purified fluorosilicic acid solution was found to have a purity of 99.5%, a phosphorus pentoxide content of 0.03 g / L, and a total metal ion content of 0.009 g / L, meeting the standards for proceeding to the next step.

[0057] 2. Preparation of Composite Modified Aluminum Source and Medium-Temperature Reaction: Industrial-grade aluminum hydroxide with a purity of 98.5% was selected as raw material, and kaolin-zirconia composite modifier was added at a mass ratio of 4:1. The kaolin contained 60% aluminum oxide and the zirconia had a purity of 99%. The amount of modifier added was 8% of the mass of aluminum hydroxide. A slurry with a solid content of 40% was prepared and activated at a low temperature of 70℃ and a stirring speed of 400r / min for 50min.

[0058] The activated slurry was spray-dried at an inlet air temperature of 200℃, an outlet air temperature of 90℃, and a feed rate of 30mL / min; then it was pulverized by airflow at an inlet air pressure of 0.9MPa to obtain a composite modified aluminum source with a particle size of 38μm.

[0059] The composite modified aluminum source was mixed with the above-mentioned refined fluorosilicic acid solution, and the molar ratio of trivalent aluminum ions to fluoride ions was controlled at 1:3.1. Nitrogen gas was introduced and the reaction was carried out at 75℃ and stirring speed of 450 r / min for 40 min. After the reaction, the by-product silica was removed by filtration with a ceramic membrane with a pore size of 0.2 μm. The aluminum fluoride entrainment in the by-product silica filter residue was 0.45%, and the silica content in the supersaturated aluminum fluoride solution after filtration was 0.009 g / L.

[0060] 3. Crystallization and Deep Recovery of Fluorine Resources: β-type aluminum fluoride trihydrate seed crystals with a particle size of 40 μm and a purity of 99.5% were selected for surface modification: First, they were vacuum dried at 70℃ and 0.07 MPa for 20 min, after which the water content of the seed crystals was 0.35%; then, they were immersed in a 0.5% KH550 silane coupling agent dilute solution for 10 min. The dilute solution solvent was a mixture of deionized water and ethanol with a volume ratio of 3:1. After removal, they were dried at 90℃ for 15 min. After modification, the hydroxyl content on the surface of the seed crystals decreased by 35%.

[0061] The modified seed crystals were added to a supersaturated aluminum fluoride solution at a rate of 5% of the theoretical yield of aluminum fluoride. The pH of the solution was adjusted to 2.2, and the temperature was raised to 85°C. Gradient stirring was used for constant temperature crystallization for 4 hours. The stirring speed was 500 r / min for the first hour of crystallization and then reduced to 350 r / min for the next 3 hours.

[0062] After crystallization, the crystals were separated by centrifugation and filtration at a speed of 4000 r / min for 20 min. The crystals were then washed three times countercurrently with 50°C washing water at a volume of 1.5 times the mass of the crystals. After washing, the residual fluoride ions on the crystal surface were 0.09% and the water content was 3.8%, yielding aluminum fluoride trihydrate crystals.

[0063] The mother liquor from crystallization was separated using a nanofiltration membrane with a molecular weight cutoff of 300 at an operating pressure of 0.5 MPa. The aluminum fluoride concentration in the retentate was enriched to 200 g / L and returned to the crystallization process for recrystallization. The permeate was subjected to adsorption of fluoride ions by a strongly basic anion exchange resin, and the fluoride ion content was 7 mg / L, which met the discharge standard. The adsorbed fluoride ions were desorbed by a 6% sodium hydroxide solution, and the fluoride ion concentration in the desorbed solution was 60 g / L, which was returned to the purified fluorosilicic acid solution from step 1 for recycling.

[0064] 4. Segmented atmosphere calcination and finished product preparation: Aluminum fluoride trihydrate crystals were placed in a calcination furnace, and dry nitrogen gas with a flow rate of 200 mL / min was introduced as a protective atmosphere. The temperature was increased to 240°C at a heating rate of 8°C / min, and held at that temperature for 3 hours for low-temperature dehydration. After dehydration, the water content of the crystals was 0.3%.

[0065] Continue heating at a rate of 5℃ / min to 620℃, switch to a nitrogen-argon mixed protective atmosphere with a volume ratio of 1:1, maintain a total flow rate of 200mL / min, keep a slight positive pressure of 0.002MPa in the calcination furnace, and hold for 4 hours to carry out high-temperature decrystallization. After decrystallization, the crystal water content is 0.1%.

[0066] After calcination, the mixed protective atmosphere is kept unchanged, and the temperature is cooled to below 100°C at a cooling rate of 6°C / min, and then naturally cooled to room temperature. Ultrasonic-assisted vibration sieving is used with a screen mesh size of 100 mesh, ultrasonic power of 80 watts, sieving time of 10 min, and sieving efficiency of 99%. After removing unqualified particles, anhydrous aluminum fluoride product is obtained with a particle size distribution range of 60-80 μm.

[0067] Example 3: This embodiment utilizes fluorosilicic acid, a byproduct of phosphate fertilizer production, to prepare aluminum fluoride. The specific steps are as follows: 1. Three-stage deep purification of fluorosilicic acid by-product of phosphate fertilizer: Crude fluorosilicic acid by-product of phosphate fertilizer was selected, with a phosphorus pentoxide content of 1.2 g / L and a total metal ion content of 0.2 g / L. 98% pure industrial-grade ammonium bicarbonate was added to the crude fluorosilicic acid as a neutralizing agent. The neutralizing agent was slowly added dropwise at a rate of 1.5 mL / min to adjust the pH of the solution to 3.2. The reaction was carried out at 45℃ and a stirring speed of 220 r / min for 22 min. After the reaction, the phosphorus pentoxide content in the solution was 0.16 g / L.

[0068] Add a complexing agent of disodium ethylenediaminetetraacetate-tartaric acid in a mass ratio of 1:2 to the phosphorus-removed solution. The amount of complexing agent added is 11 times the total mass of ferric ions, calcium ions, and magnesium ions in the solution. Stir at 22℃ for 28 min to carry out the complexation reaction. Then add 0.015 g / L of polyacrylamide as a flocculant, stir for 8 min, and let stand for 35 min to clarify. Remove the flocs by plate and frame filtration. After complexation and impurity removal, the total metal ion content in the solution is 0.01 g / L.

[0069] Add 44% (w / w) industrial-grade concentrated hydrofluoric acid to the complexed and purified solution to adjust the fluorine-silica ratio to 6.8. Acid-induced depolymerization is carried out by stirring at 35℃ and 220 r / min for 18 min. Silica precipitate is removed by vacuum filtration at a vacuum degree of 0.075 MPa to obtain a purified fluorosilicic acid solution. The purified fluorosilicic acid solution was found to have a purity of 99.6%, a phosphorus pentoxide content of 0.02 g / L, and a total metal ion content of 0.005 g / L, meeting the standards for proceeding to the next step.

[0070] 2. Preparation of Composite Modified Aluminum Source and Medium-Temperature Reaction: Industrial-grade aluminum hydroxide with a purity of 98.5% was selected as raw material, and kaolin-zirconia composite modifier was added at a mass ratio of 4:1. The kaolin contained 52% aluminum oxide and the zirconia had a purity of 99%. The amount of modifier added was 7% of the mass of aluminum hydroxide. A slurry with a solid content of 35% was prepared and activated at low temperature for 45 min at 65℃ and a stirring speed of 380 r / min.

[0071] The activated slurry was spray-dried at an inlet air temperature of 190℃, an outlet air temperature of 85℃, and a feed rate of 25mL / min; then it was pulverized by airflow at an inlet air pressure of 0.8MPa to obtain a composite modified aluminum source with a particle size of 30μm.

[0072] The composite modified aluminum source was mixed with the above-mentioned refined fluorosilicic acid solution, and the molar ratio of trivalent aluminum ions to fluoride ions was controlled at 1:3.05. Nitrogen gas was introduced and the reaction was carried out at 70℃ and a stirring speed of 420 r / min for 35 min. After the reaction, the by-product silica was removed by filtration with a ceramic membrane with a pore size of 0.15 μm. The aluminum fluoride entrainment in the by-product silica filter residue was 0.3%, and the silica content in the supersaturated aluminum fluoride solution after filtration was 0.005 g / L.

[0073] 3. Crystallization and Deep Recovery of Fluorine Resources: β-type aluminum fluoride trihydrate seed crystals with a particle size of 35 μm and a purity of 99.5% were selected for surface modification: First, they were vacuum dried at 65℃ and a vacuum degree of 0.065 MPa for 18 min, after which the water content of the seed crystals was 0.3%; then, they were immersed in a 0.5% KH550 silane coupling agent dilute solution for 8 min. The dilute solution solvent was a mixture of deionized water and ethanol with a volume ratio of 3:1. After removal, they were dried at 85℃ for 12 min. After modification, the hydroxyl content on the surface of the seed crystals decreased by 38%.

[0074] The modified seed crystals were added to a supersaturated aluminum fluoride solution at a rate of 4.5% of the theoretical yield of aluminum fluoride. The pH of the solution was adjusted to 2.1, and the temperature was raised to 82°C. The solution was then crystallized at a constant temperature for 3.5 hours using a gradient stirring process. The stirring speed was 480 r / min for the first hour of crystallization and then reduced to 320 r / min for the next 2.5 hours.

[0075] After crystallization, the crystals were separated by centrifugation and filtration at 3800 r / min for 18 min. The crystals were then washed twice countercurrently with 45°C washing water at a volume of 1.2 times the mass of the crystals. After washing, the residual fluoride ions on the crystal surface were 0.05%, and the water content was 3%, yielding aluminum fluoride trihydrate crystals.

[0076] The mother liquor from crystallization was separated using a nanofiltration membrane with a molecular weight cutoff of 250 at an operating pressure of 0.4 MPa. The aluminum fluoride concentration in the retentate was enriched to 180 g / L and returned to the crystallization process for recrystallization. The permeate was subjected to adsorption of fluoride ions by a strongly basic anion exchange resin, and the fluoride ion content was 5 mg / L, meeting the discharge standard. The adsorbed fluoride ions were desorbed by a 5% sodium hydroxide solution, and the fluoride ion concentration in the desorbed solution was 65 g / L, which was returned to the purified fluorosilicic acid solution from step 1 for recycling.

[0077] 4. Segmented atmosphere calcination and finished product preparation: Aluminum fluoride trihydrate crystals were placed in a calcination furnace, and dry nitrogen gas with a flow rate of 180 mL / min was introduced as a protective atmosphere. The temperature was increased to 230℃ at a heating rate of 6℃ / min and held at that temperature for 2.8 hours for low-temperature dehydration. After dehydration, the water content of the crystals was 0.25%.

[0078] Continue heating at a rate of 4℃ / min to 610℃, switch to a nitrogen-argon mixed protective atmosphere with a volume ratio of 1:1, maintain a total flow rate of 180mL / min, keep a slight positive pressure of 0.0015MPa in the calcination furnace, and hold for 3.5 hours to carry out high-temperature decrystallization. After decrystallization, the water content of the crystals is 0.08%.

[0079] After calcination, the mixed protective atmosphere is kept unchanged, and the temperature is cooled to below 100°C at a cooling rate of 5°C / min, and then naturally cooled to room temperature. Ultrasonic-assisted vibration sieving is used with a screen mesh size of 90 mesh, ultrasonic power of 65 watts, sieving time of 8 min, and sieving efficiency of 98.8%. After removing unqualified particles, anhydrous aluminum fluoride product is obtained with a particle size distribution range of 50-70 μm.

[0080] Comparative Example 1: This comparative example did not perform a three-stage deep purification of the fluorosilicic acid by-product of phosphate fertilizer. Instead, crude fluorosilicic acid was directly reacted with aluminum hydroxide. The remaining operations were the same as in Example 3. Specifically, the neutralization and phosphorus removal, complexation and impurity removal, and acidification and depolymerization steps of fluorosilicic acid were omitted. The crude fluorosilicic acid by-product of phosphate fertilizer was directly mixed and reacted with the composite modified aluminum source. The subsequent crystallization, calcination and other steps were the same as in Example 3.

[0081] Comparative Example 2: This comparative example did not prepare a composite modified aluminum source, but directly used industrial-grade aluminum hydroxide as the aluminum source. The remaining operations were the same as in Example 3, specifically: the steps of adding modifying agents, slurry activation, drying and pulverizing were omitted, and industrial-grade aluminum hydroxide with a purity of 98.5% was directly mixed and reacted with refined fluorosilicic acid solution. The subsequent crystallization, calcination and other steps were the same as in Example 3.

[0082] Comparative Example 3: This comparative example did not perform surface modification treatment on the seed crystals. Unmodified seed crystals were used directly for crystallization. The remaining operations were the same as in Example 3. Specifically, β-type aluminum fluoride trihydrate seed crystals that had not undergone vacuum drying and silane coupling agent soaking were directly added to a supersaturated aluminum fluoride solution for crystallization. Subsequent separation, washing, fluorine resource recovery, and calcination steps were the same as in Example 3.

[0083] Comparative Example 4: This comparative example uses a one-step calcination process instead of segmented atmosphere calcination. The remaining operations are the same as in Example 3. Specifically, aluminum trihydrate fluoride crystals are placed in a calcination furnace, a nitrogen protective atmosphere is introduced, and the temperature is directly raised to 610°C at a heating rate of 6°C / min. The temperature is held for 6 hours, and the cooling and sieving steps are the same as in Example 3.

[0084] test: Experiment 1: Core performance testing of anhydrous aluminum fluoride finished products: Purity testing: The aluminum fluoride content was determined using the fluoride ion selective electrode method, and the purity of the finished product was calculated. Particle size distribution uniformity: The particle size of the finished product is measured using a laser particle size analyzer, and the coefficient of variation of particle size distribution is calculated. The smaller the coefficient of variation, the more uniform the particle size distribution. Moisture content testing: The moisture content of the finished product was determined using a Karl Fischer moisture analyzer; Impurity content detection: The total content of impurities such as phosphorus, iron, calcium, and magnesium in the finished product was determined using inductively coupled plasma atomic emission spectrometry. Bulk density testing: The bulk density of the finished product is measured using a bulk density meter, which reflects the physical properties of the product.

[0085] Experimental results: Table 1. Test Results of Core Performance of Anhydrous Aluminum Fluoride Finished Products

[0086] The anhydrous aluminum fluoride products prepared in Examples 1, 2, and 3 all exhibited high purity, low water content, and low impurity content, and had uniform particle size distribution and high bulk density.

[0087] Comparative Example 1, lacking a three-stage deep purification process for fluorosilicic acid, allowed impurities such as phosphorus and metal ions from the crude fluorosilicic acid to directly enter the reaction system, resulting in a significant decrease in the purity of the finished product and a substantial increase in impurity content. Furthermore, the presence of these impurities affected crystal growth and agglomeration, leading to poorer particle size distribution uniformity and reduced bulk density. Comparative Example 2, without using a composite modified aluminum source, relied on ordinary aluminum hydroxide, which exhibited low reactivity and was prone to localized uneven reaction during the reaction process. This resulted in the presence of a small amount of impurities in the finished product, and the particle size distribution uniformity and bulk density were also affected. Comparative Example 3… Without surface modification of the seed crystals, the unmodified seed crystals have a high hydroxyl content on their surface, which makes them prone to crystal agglomeration during crystallization, increasing the coefficient of variation in particle size distribution. At the same time, the agglomerates are prone to trapping moisture and a small amount of impurities, resulting in a slight increase in the moisture content and impurity content of the finished product. Comparative Example 4 uses a one-step calcination process, but the heating rate is too fast and the protective atmosphere is not switched. The dehydration process of aluminum trihydrate is uneven, and some crystals produce micro-agglomerations due to excessive heating, resulting in a decrease in the uniformity of particle size distribution. In addition, incomplete dehydration leads to a higher moisture content and a lower bulk density in the finished product.

[0088] Experiment 2: Testing of Fluorine Resource Utilization Rate and Environmental Friendliness of Processes Total fluorine resource utilization rate: The total fluorine resource utilization rate is calculated based on the total mass of fluorine in the input fluorosilicic acid and the total mass of fluorine in the final anhydrous aluminum fluoride product and the recycled desorption solution. Fluoride ion recovery rate of crystallization mother liquor: The fluoride ion recovery rate of crystallization mother liquor is calculated based on the mass of fluoride in the crystallization mother liquor and the mass of fluoride recovered from the retentate and desorption solutions. Fluoride ion content in permeate: The fluoride ion content in the permeate after defluorination treatment was determined by the fluoride ion selective electrode method. Aluminum fluoride entrainment in silica filter residue: The amount of aluminum fluoride entrained in silica filter residue was determined by gravimetric method, reflecting the efficiency of the reaction and filtration process.

[0089] Experimental results: Table 2. Results of Fluorine Resource Utilization Rate and Environmental Friendliness Testing

[0090] The total utilization rate of fluorine resources and the fluoride ion recovery rate of the crystallization mother liquor in Examples 1, 2, and 3 are all at a high level. The fluoride ion content in the permeate is far below the emission standard, and the amount of aluminum fluoride entrained in the silica filter residue is low, which demonstrates the excellent fluorine resource recycling capacity and environmental friendliness of the process of this invention.

[0091] In Comparative Example 1, due to the lack of purification of fluorosilicic acid, impurities formed complexes with fluoride ions, preventing some fluoride ions from participating in the reaction. Simultaneously, the impurities affected the separation efficiency of the nanofiltration membrane, reducing the fluoride ion recovery rate of the crystallization mother liquor and significantly decreasing the overall fluoride resource utilization rate. Furthermore, the unreacted fluoride ions resulted in excessive fluoride ion content in the permeate. In Comparative Example 2, using ordinary aluminum hydroxide as the aluminum source, the low reactivity led to insufficient fluoride ion conversion, leaving some fluoride ions in the mother liquor. Uneven reaction also resulted in the silica filter residue carrying more aluminum fluoride, further reducing the overall fluoride resource utilization rate. In Comparative Example 3, the unmodified seed crystals had slightly lower crystallization efficiency, with a small amount of fluoride ions not participating in crystallization and entering the mother liquor, resulting in a slight decrease in the fluoride ion recovery rate of the crystallization mother liquor. However, the impact on the overall fluoride resource utilization rate was minimal, and the fluoride ion content in the permeate still met the standard. In Comparative Example 4, the calcination process had no significant impact on fluoride resource utilization. The overall fluoride resource utilization rate and the fluoride ion recovery rate of the crystallization mother liquor were close to those of the examples, demonstrating the stability of the fluoride resource recovery process of this invention.

[0092] Experiment 3: Process repeatability and product batch stability testing: Five batches of continuous production were carried out on each test object. The purity, particle size distribution coefficient of variation, and water content of each batch of anhydrous aluminum fluoride were measured. The batch coefficient of variation of each indicator was calculated. The smaller the batch coefficient of variation, the better the process repeatability and product batch stability. At the same time, the equipment scaling rate during the five batches of production was statistically analyzed to reflect the industrial applicability of the process.

[0093] Experimental results: Table 3 Results of process repeatability and product batch stability tests

[0094] After five consecutive production runs of Examples 1, 2, and 3, the batch variation coefficients of the core indicators of the finished products were all small, and the equipment scaling rate was extremely low, indicating that the process of the present invention has good repeatability and high batch stability, making it suitable for industrial continuous production.

[0095] In Comparative Example 1, impurities in unpurified fluorosilicic acid easily deposit on the equipment surface, leading to a significant increase in equipment scaling rate. Simultaneously, fluctuations in impurity content affected the stability of the reaction and crystallization processes, resulting in a significant increase in the batch-to-batch coefficient of variation for various indicators of the finished product and poor process repeatability. In Comparative Example 2, the reactivity of ordinary aluminum hydroxide was greatly affected by the batch of raw materials, leading to poor consistency in the reaction process, increased batch-to-batch coefficient of variation for the finished product indicators, and byproducts generated from uneven reaction easily caused slight equipment scaling. In Comparative Example 3, the crystallization process of unmodified crystals was affected by the surface state of the crystals, resulting in slight differences in crystallization efficiency between batches. This resulted in a slightly higher batch-to-batch coefficient of variation for the finished product indicators compared to the Example, but the equipment scaling rate was lower, and the overall process stability was better. In Comparative Example 4, the one-step calcination process involved uneven heating, resulting in slight differences in the degree of crystal agglomeration between batches. This led to increased batch-to-batch coefficients of variation for particle size distribution and water content, and the process stability was slightly lower than that of the Example.

[0096] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0097] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing aluminum fluoride using fluorosilicic acid, a byproduct of phosphate fertilizer production, characterized in that, Includes the following sequential steps: S1. Three-stage deep purification of fluorosilicic acid, a byproduct of phosphate fertilizer production: A neutralizing agent is added to the crude fluorosilicic acid to adjust the pH and neutralize and remove phosphorus, converting phosphate ions in the solution into soluble phosphates. A complexing agent is added to the phosphorus-removed solution to form stable water-soluble complexes with the metal ions. A flocculant is then added, followed by clarification and filtration to complete the complexation and impurity removal process. Concentrated hydrofluoric acid is added to the complexed and impurity-removed solution to adjust the fluorine-silica ratio and perform acidification and depolymerization to obtain soluble SiF6. 2- The fluorosilicic acid is depolymerized into SiO2 precipitate, and the SiO2 precipitate is removed by filtration to obtain a refined fluorosilicic acid solution; the refined fluorosilicic acid solution needs to pass the test before proceeding to step S2; S2. Preparation and medium-temperature reaction of composite modified aluminum source: Aluminum hydroxide is used as raw material, compound modifiers are added, a slurry is prepared and activated at low temperature, and then dried and pulverized to obtain composite modified aluminum source; The composite modified aluminum source is mixed with the refined fluorosilicic acid solution obtained in step S1 and reacted. After the reaction is completed, the by-product silicon dioxide is removed by filtration to obtain a supersaturated aluminum fluoride solution. S3. Crystallization and deep recovery of fluorine resources: After surface modification treatment of the seed crystals, they are added to the supersaturated aluminum fluoride solution obtained in step S2 and crystallized at a constant temperature. After separation and washing, aluminum fluoride trihydrate crystals are obtained. After separation treatment of the crystallization mother liquor, the retentate is returned to the crystallization process for recrystallization, and the permeate is discharged after defluorination treatment to meet the standards. The adsorbed fluoride ions are desorbed and returned to step S1 for recycling. S4. Segmented calcination and finished product preparation: The aluminum trihydrate crystals obtained in step S3 are placed in a calcination furnace, a protective atmosphere is introduced, and the crystals are dehydrated and decrystallized by segmented heating and holding. After cooling, unqualified particles are removed by sieving to obtain anhydrous aluminum fluoride finished product.

2. The method according to claim 1, characterized in that, The industrial standard indicators for the crude fluorosilicic acid byproduct of phosphate fertilizer production in step S1 are: P2O5 content 0.5-2.0 g / L, total metal ion content 0.1-0.3 g / L; the neutralizing agent is industrial-grade ammonium bicarbonate with a purity ≥98%. The specific conditions for neutralization and phosphorus removal in step S1 are as follows: the neutralizing agent is slowly added dropwise at a rate of 1-2 mL / min, the pH of the solution is adjusted to 3.0-3.5, and the reaction is carried out at 40-50℃ and a stirring speed of 200-250 r / min for 20-25 min; after the reaction is completed, the P2O5 content in the solution is ≤0.2 g / L. The complexing agent mentioned in step S1 is a compound complexing agent of disodium ethylenediaminetetraacetate and tartaric acid, with a mass ratio of 1:2; the amount of complexing agent added is the Fe in the solution. 3+ Ca 2+ Mg 2+ The complexation reaction is carried out at 20-25℃ for 25-30 minutes with the total mass being 10-12 times.

3. The method according to claim 1, characterized in that, The flocculant mentioned in step S1 is polyacrylamide, and the addition amount is 0.01-0.02 g / L; the flocculation process is to stir for 5-10 min and then let it stand for 30-40 min to clarify, and then use plate and frame filtration to remove flocs; after complexation and impurity removal, the total metal ion content in the solution is ≤0.02 g / L; The concentrated hydrofluoric acid mentioned in step S1 is industrial grade, with a mass fraction of 40-48%; the specific conditions for acid-induced depolymerization are as follows: adjust the fluorine-silicon ratio of the solution to 6.5-7.0, and stir for 15-20 minutes at 30-40℃ and a stirring speed of 200-250 r / min; the SiO2 precipitate is removed by vacuum filtration with a vacuum degree of 0.07-0.08 MPa; The refined fluorosilicic acid solution in step S1 has the following specifications: purity ≥ 99.0%, P2O5 content ≤ 0.05 g / L, and total metal ion content ≤ 0.01 g / L. Each batch must be tested.

4. The method according to claim 1, characterized in that, The aluminum hydroxide mentioned in step S2 is industrial grade with a purity ≥98.5%; the compound modifier is a kaolin-zirconia compound system with a mass ratio of 4:1, wherein the kaolin Al2O3 content is 45-60% and the zirconium dioxide purity is ≥99%; the amount of modifier added is 6-8% of the mass of aluminum hydroxide.

5. The method according to claim 1, characterized in that, The solid content of the slurry in step S2 is 30-40%, the low-temperature activation conditions are 60-70℃, the stirring speed is 350-400r / min, and the activation time is 40-50min; the drying is spray drying, the pulverization is air jet milling, and the particle size of the composite modified aluminum source after pulverization is ≤40μm; The specific parameters for the spray drying are: inlet air temperature 180-200℃, outlet air temperature 80-90℃, and feed rate 20-30mL / min; the inlet air pressure for the airflow pulverizer is 0.7-0.9MPa.

6. The method according to claim 1, characterized in that, The composite modified aluminum source and the Al in the refined fluorosilicic acid solution mentioned in step S2 3+ With F - The molar ratio is 1:3.0-3.1; the reaction conditions are: temperature 65-75℃, stirring speed 400-450 r / min, reaction time 30-40 min; nitrogen gas is introduced during the reaction. In step S2, a ceramic membrane with a pore size of 0.1-0.2 μm is used for filtration; the aluminum fluoride entrainment in the by-product silica filter residue is ≤0.5%, and the SiO2 content in the supersaturated aluminum fluoride solution after filtration is ≤0.01 g / L.

7. The method according to claim 1, characterized in that, The seed crystals mentioned in step S3 are β-type aluminum fluoride trihydrate seed crystals with a particle size of 30-40 μm and a purity of ≥99.5%. The surface modification treatment of the seed crystals includes: first, vacuum drying at 60-70℃ and 0.06-0.07MPa for 15-20 minutes, after which the moisture content of the seed crystals is ≤0.5%; then, immersing in a dilute solution of silane coupling agent for 5-10 minutes, and then drying at 80-90℃ for 10-15 minutes, after which the hydroxyl content on the surface of the seed crystals is reduced by ≥30%; The silane coupling agent is KH550, and the mass fraction of the dilute solution is 0.5%. The solvent is a mixture of deionized water and ethanol in a volume ratio of 3:

1. The dilute solution is prepared by dissolving KH550 in the mixed solvent, stirring evenly, and then letting it stand for 5 minutes. The amount of seed crystals added in step S3 is 4-5% of the theoretical yield of aluminum fluoride; the crystallization conditions are: adjust the solution pH to 2.0-2.2, raise the temperature to 80-85℃, and use a gradient stirring process to crystallize at a constant temperature for 3-4 hours. The gradient stirring process is: the stirring speed is 450-500 r / min for the first hour of crystallization, and then the stirring speed is reduced to 300-350 r / min for the next 2-3 hours.

8. The method according to claim 1, characterized in that, The separation in step S3 is performed by centrifugal filtration with the following parameters: rotation speed 3500-4000 r / min, centrifugation time 15-20 min; washing is performed by countercurrent washing 2-3 times, washing water temperature 40-50℃, and the amount of washing water is 1-1.5 times the mass of the crystals; after washing, the residual fluoride ions on the crystal surface are ≤0.1%, and the water content is ≤4%; The separation and treatment of the crystallization mother liquor in step S3 is carried out by nanofiltration membrane separation. The nanofiltration membrane has a molecular weight cutoff of 200-300 and an operating pressure of 0.3-0.5MPa. The aluminum fluoride concentration in the retentate is enriched to 150-200g / L. After the permeate is adsorbed by a strong basic anion exchange resin, the fluoride ion content is ≤10mg / L before it can be discharged. The fluoride ions adsorbed by the strongly basic anion exchange resin are desorbed by a 4-6% sodium hydroxide solution. The fluoride ion concentration in the desorbed solution is ≥50g / L. The desorbed solution is then returned to the purified fluorosilicic acid solution in step S1 for recycling.

9. The method according to claim 1, characterized in that, The specific conditions for the segmented calcination described in step S4 are as follows: (1) Low-temperature dehydration: Dry nitrogen gas is introduced as a protective atmosphere, and the temperature is raised to 220-240℃ at a heating rate of 5-8℃ / min, and held for 2.5-3h. After dehydration, the water content of the crystals is ≤0.5%; (2) High-temperature decrystallization: Continue heating at a rate of 3-5℃ / min to 600-620℃, switch to a nitrogen-argon mixed protective atmosphere, and hold for 3-4 hours to completely convert β-type aluminum fluoride trihydrate into anhydrous aluminum fluoride. After decrystallization, the water content of the crystals is ≤0.2%. The flow rate of the dry nitrogen is 150-200 mL / min; the volume ratio of the nitrogen-argon mixed protective atmosphere is 1:1, and the total flow rate is maintained at 150-200 mL / min; during the atmosphere switching process, a slight positive pressure of 0.001-0.002 MPa is maintained inside the calcining furnace.

10. The method according to claim 1, characterized in that, The cooling conditions in step S4 are as follows: after calcination, maintain the protective atmosphere and cool to below 100℃ at a cooling rate of 4-6℃ / min, then allow to cool naturally to room temperature; the sieving is performed using ultrasonic-assisted vibration sieving with a screen mesh size of 80-100 mesh, ultrasonic power of 50-80W, sieving time of 5-10min, and sieving efficiency ≥98%; The specifications of the anhydrous aluminum fluoride product in step S4 are: particle size distribution range of 40-80 μm.

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