Method for recycling aluminum electrolysis fluorine-containing waste to prepare aluminum fluoride

Abstract

The application discloses a method for preparing aluminum fluoride from aluminum electrolysis fluorine-containing waste, and relates to the technical field of aluminum electrolysis waste recovery. The method is aimed at solving the problems of relying on external substances to promote crystallization and insufficient product purity in the aluminum fluoride preparation process. The method comprises the following steps: mixing aluminum electrolysis fluorine-containing waste with an aluminum-containing acidic reaction agent to perform leaching reaction, and performing solid-liquid separation to obtain leaching liquid; performing evaporation concentration treatment on the leaching liquid, separating out sodium salt crystals, and obtaining concentrated liquid; performing hydrothermal reaction on the concentrated liquid to obtain hydrated aluminum fluoride; and performing calcination treatment on the hydrated aluminum fluoride to obtain anhydrous aluminum fluoride. The sodium salt separation step is arranged in advance to provide a low-sodium and high-purity reaction system for subsequent hydrothermal reaction. The key to the sodium removal is separation. Only when the sodium salt is physically removed, the interference of sodium on the crystallization of aluminum fluoride can be eliminated, and the system has the ability of spontaneous nucleation under hydrothermal conditions, so that no external seed and additional fluorinated salt are needed.

Description

Technical Field

[0001] This invention relates to the technical field of aluminum electrolysis waste recycling, specifically a method for recycling fluorine-containing aluminum electrolysis waste to prepare aluminum fluoride. Background Technology

[0002] Solid waste from aluminum electrolysis mainly consists of waste aluminum electrolyte, waste tank lining, and carbon slag. Its main components are carbon, cryolite, sub-cryolite, alumina, calcium fluoride, and small amounts of other fluoride additives. Each ton of aluminum produced generates 10-30 kg of waste aluminum electrolyte, 30-50 kg of waste tank lining, and 10-20 kg of carbon slag. This type of solid waste is classified as hazardous material, and currently there are no effective methods for large-scale treatment. Most of it is simply dumped, which inevitably leads to soluble fluoride salts seeping into groundwater, causing significant environmental damage.

[0003] In recent years, some researchers have used waste aluminum electrolytes as raw materials to prepare aluminum fluoride. Chinese invention patent applications CN111485252B, CN113149052B, CN115465876B, CN114804171A, and CN114314625B all disclose a method for the resource-based recycling of waste aluminum electrolytes to prepare aluminum fluoride. The core reaction in these methods involves the recombination of fluorides in the aluminum electrolyte with anhydrous aluminum salts under high-temperature conditions, resulting in the complete conversion of fluorides into aluminum fluoride. To ensure a high yield of aluminum fluoride, the calcination temperature is usually above 500℃. Even so, the influence of alumina in the waste aluminum electrolyte and calcium sulfate generated from calcium cannot be avoided, resulting in a calcination product that is a mixture of aluminum fluoride, alumina, and calcium sulfate, with low purity and low product value.

[0004] Chinese invention patents CN117383599B, ​​CN113249582B, and CN111690823A disclose a method for preparing aluminum fluoride from waste aluminum electrolyte using aluminum hydroxyfluoride as an intermediate product. The method involves dissolving fluoride from the aluminum electrolyte into a solution using aluminum salts / acids, adding an alkaline solution to obtain the intermediate product aluminum hydroxyfluoride, and then calcining it with calcining agents such as ammonium fluoride, hydrogen fluoride, and ammonium bifluoride to obtain anhydrous aluminum fluoride. Although this process produces a high-purity product, it involves multiple stages including leaching, precipitation, separation, and calcination with external calcining agents, resulting in a long process and high costs for calcining agents such as ammonium fluoride, leading to poor overall economic efficiency.

[0005] Chinese patent CN121202169A discloses a method for directly preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis. The method involves dissolving fluorides in a solution through alkaline leaching, then adding a large amount of caustic soda to precipitate fluoride as sodium fluoride. The sodium fluoride is then mixed with aluminum salt reactants in different proportions and calcined to obtain anhydrous aluminum fluoride and cryolite. However, the alkaline concentration of the solution used in the preparation of sodium fluoride is 10-20 mol / L, which causes severe corrosion to the equipment.

[0006] Chinese patent CN120622520A discloses a method for preparing aluminum fluoride from waste aluminum electrolyte. The method involves directly mixing waste aluminum electrolyte, aluminum salt, and acid in a closed container and reacting. After the reaction, β-aluminum fluoride can be obtained by direct separation. However, this method requires a hydrogen ion concentration of 3-24 mol / L in the acid solution, placing high demands on the corrosion resistance of the equipment. Furthermore, because the reaction is completed rapidly in a closed container, unreacted cryolite and impurities such as alumina and calcium fluoride in the raw materials cannot be effectively separated and remain directly in the product, resulting in low purity and low product value of the aluminum fluoride. In addition, the product is β-crystalline aluminum fluoride, which lacks sufficient high-temperature thermal stability and cannot meet the stable crystal structure requirements of aluminum electrolysis cells.

[0007] Chinese patent CN122520105A discloses a method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis. This method dissolves the fluorine-containing components in the waste and prepares a supersaturated aluminum fluoride solution by adding an auxiliary agent. High-temperature induced crystallization then grows small-diameter aluminum fluoride seed crystals into large-particle aluminum fluoride. However, this method has the following drawbacks: First, the preparation of the supersaturated solution requires the addition of fluoride salts as an auxiliary material. This material is only used to adjust the aluminum-fluorine ratio and only a small amount enters the final product, making it a purely consumable addition and increasing raw material input. Second, the crystallization process is highly dependent on the addition of activated seed crystals, which require pretreatment such as grinding and soaking in an activator, making the process cumbersome. Furthermore, the mother liquor has a high residual fluoride ion content, resulting in a low primary fluoride resource recovery rate. Improper pH control can easily generate hydroxyl aluminum fluoride impurities, affecting product purity. These multiple factors combined limit the economic benefits of this technical solution and make it difficult to meet the requirements for industrial-scale promotion.

[0008] In summary, it is of great importance to propose a process that can directly produce high-purity aluminum fluoride from fluorine-containing waste from aluminum electrolysis and reduce the consumption of auxiliary materials. Summary of the Invention

[0009] To address the aforementioned problems, namely the issues raised in the background section, this invention proposes a method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis.

[0010] Technical solution The first aspect of this invention provides a method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis, comprising: S1. Fluorine-containing waste from aluminum electrolysis is mixed with an aluminum-containing acidic reactant for leaching reaction, and the leachate is obtained by solid-liquid separation; S2. The leachate is evaporated and concentrated to separate the precipitated sodium salt crystals and obtain a concentrated solution. The sodium salt crystals separated and precipitated in S2 are the acidic sodium salts corresponding to the acids used in the aluminum-containing acidic reactants. For example, the sodium nitrate crystals obtained from the nitric acid-aluminum nitrate system are sodium nitrate crystals. The principle is as follows: During the evaporation and concentration process of S2, due to the presence of a large number of anions (sulfate, chloride, or nitrate) introduced by the aluminum-containing acidic reactants in the leachate, the evaporation and concentration bring the system to a supersaturated state. Utilizing the common ion effect, the solubility of the corresponding sodium salt (sodium sulfate, sodium chloride, or sodium nitrate) decreases, leading to preferential crystallization. Simultaneously, the solubility of the aluminum-fluoride complex formed by aluminum and fluoride ions is not saturated under these concentration conditions, and it remains in the concentrate, thus achieving the selective separation of sodium from fluorine and aluminum.

[0011] S3. The concentrated solution is subjected to a hydrothermal reaction to obtain hydrated aluminum fluoride; S4. The hydrated aluminum fluoride is calcined to obtain anhydrous aluminum fluoride.

[0012] Furthermore, the aluminum-containing acidic reactant includes an aluminum salt and an acid. The aluminum salt is selected from at least one of aluminum sulfate, aluminum chloride, and aluminum nitrate, and the acid is the acid corresponding to the aluminum salt, namely sulfuric acid, hydrochloric acid, or nitric acid. In step S1, the aluminum salt and acid are added to the fluorine-containing waste from aluminum electrolysis. In the mixture of the fluorine-containing waste from aluminum electrolysis and the aluminum-containing acidic reactant, the molar ratio of aluminum to fluorine is 1:1-4. Within this range, fluoride ions can fully coordinate with aluminum ions to form a highly soluble complex that exists stably in the solution. During the evaporation and concentration process, it will not precipitate with the sodium salt, thus achieving no aluminum-fluorine loss during the selective removal of sodium salt. At the same time, it provides a component-matched precursor system for subsequent hydrothermal crystallization, enabling directional crystallization to form hydrated aluminum fluoride under hydrothermal conditions.

[0013] Furthermore, the aluminum-containing acidic reactant has an aluminum ion concentration of 0.1-3 mol / L and a hydrogen ion concentration of 0.1-10 mol / L, preferably a hydrogen ion concentration of 0.1-2 mol / L.

[0014] Furthermore, the leaching reaction in S1 is carried out at a temperature of 20-90°C for a time of 0.5-6 h.

[0015] Furthermore, the evaporation and concentration process in S2 is vacuum low-temperature evaporation, the temperature of which is 30-90℃, the evaporation pressure is 0.1-10 Pa, and the evaporation concentration factor is 2-50 times.

[0016] By controlling the system's temperature and vacuum level to regulate the saturated vapor pressure of water, water can be continuously evaporated and removed at lower temperatures. Fluorine, in the form of aluminum fluoride complex ions, remains stably present in the concentrate and is not carried away. Specifically, the vacuum condition lowers the boiling point of water in the system, allowing evaporation to occur at a mild temperature of 30-90℃. This avoids the risk of fluoride ions combining with hydrogen ions to form HF and volatilizing under high-temperature evaporation conditions. Simultaneously, this temperature range ensures that the evaporation rate of water meets the concentration requirements without compromising the stability of the aluminum fluoride complex or promoting aluminum ion hydrolysis. Through this low-temperature vacuum evaporation operation, water is selectively removed from the leachate, reducing the solution volume (concentration factor of 2-50 times). The sodium ion concentration subsequently increases to a supersaturated state, causing sodium salt crystallization. Fluorine and aluminum, however, remain entirely in the concentrate as aluminum fluoride complex ions, achieving zero loss of fluorine and aluminum during the dehydration and concentration process.

[0017] Furthermore, the temperature of the hydrothermal reaction in S3 is 90-200℃, and the time of the hydrothermal reaction is 0.5-48h.

[0018] Furthermore, after the hydrothermal reaction in step S3 is completed, the product is dispersed in water and filtered to obtain the hydrated aluminum fluoride.

[0019] Furthermore, the roasting process is a multi-stage roasting process with a roasting temperature of 200-800℃.

[0020] Furthermore, in the multi-stage roasting, the holding time for each stage of roasting is not less than 2 hours. As a preferred embodiment, the multi-stage roasting is as follows: holding at 200°C for 2 hours, then raising the temperature to 600°C and holding for 2 hours.

[0021] Furthermore, the leaching residue obtained by solid-liquid separation in S1 is returned to S1 for secondary leaching; the condensate generated by evaporation and concentration in S2 is returned to S1 as a leaching medium; and the filtrate obtained by solid-liquid separation after hydrothermal reaction in S3 is returned to S1 for secondary leaching.

[0022] The beneficial technical effects of this invention are as follows: By placing the sodium salt precipitation and separation steps beforehand, the sodium salt is separated out by evaporation and concentration in S2 after leaching in S1 and before the hydrothermal reaction in S3. This transforms the sodium salt from a "byproduct recovered at the end" into an "impurity removed at the front end." By controlling the leaching conditions, no aluminum or fluorine loss occurs during sodium salt removal, providing a low-sodium, high-purity reaction system for the subsequent hydrothermal reaction. It is important to emphasize that the key to pre-treatment sodium removal lies in "separation" rather than merely "precipitation." If sodium salt is only crystallized without being physically separated from the system, the precipitated sodium salt will still redissolve and release sodium ions under hydrothermal conditions, competing with fluorine and aluminum to form a more stable cryolite structure. The final product is only a mixture of aluminum hydroxyfluoride and hydrated aluminum fluoride. Only by physically removing the sodium salt through solid-liquid separation after precipitation can the hydrothermal reaction be ensured to proceed in a pure system. Based on this pre-treatment sodium removal architecture, the sodium ion concentration in the solution entering the hydrothermal reaction is significantly reduced, eliminating the interference of sodium on aluminum fluoride crystallization and enabling the system to spontaneously nucleate under hydrothermal conditions, thus eliminating the need for external seed crystals. Simultaneously, this application requires no additional fluoride salt additives; aluminum fluoride can be directly prepared from fluorine-containing waste simply through the control of leaching conditions and the synergistic effect of pre-treatment sodium removal. A close synergistic relationship exists between pre-treatment sodium removal and seedless hydrothermal crystallization: pre-treatment sodium removal creates the preconditions for seedless crystallization, while seedless crystallization is a direct benefit of pre-treatment sodium removal. Based on this synergistic mechanism, this application achieves the direct preparation of high-purity aluminum fluoride from fluorine-containing waste from aluminum electrolysis under conditions without the addition of fluoride salts or external seed crystals. The product purity meets industry standards, while simplifying the process flow, reducing raw material consumption and waste liquid treatment burden, and demonstrating promising industrial application prospects. Attached Figure Description

[0023] Figure 1 A schematic diagram of the process for preparing aluminum fluoride according to the present invention is shown.

[0024] Figure 2 The XRD pattern of the sodium salt separated and precipitated in Example 1 of the present invention is shown.

[0025] Figure 3 The XRD pattern of the hydrated aluminum fluoride of the present invention is shown.

[0026] Figure 4 The XRD pattern of anhydrous α-aluminum fluoride of the present invention is shown.

[0027] Figure 5 The XRD pattern of the hydrothermal products of Comparative Example 1 of the present invention is shown.

[0028] Figure 6 The XRD pattern of the hydrothermal products of Comparative Example 2 of the present invention is shown. Detailed Implementation

[0029] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0030] This invention proposes a method for recovering fluorine-containing waste from aluminum electrolysis to prepare aluminum fluoride, the process of which is as follows: Figure 1 As shown.

[0031] Example 1: S1. Mix 100 g of waste aluminum electrolyte, 200 g of aluminum nitrate nonahydrate and 2 L of 0.1 mol / L dilute nitric acid evenly. The initial hydrogen ion concentration in the reaction system is 0.1 mol / L. The molar ratio of aluminum to fluorine in the mixture is 1:3.0. Leach at 60℃ for 2 h to obtain leachate and leach residue. S2. The leachate was concentrated by vacuum low-temperature evaporation at 90℃ for 3 h, with a pressure of 5 Pa and a concentration factor of 5 times. After separation, 400 g of sodium salt product and the concentrated solution were obtained. The XRD diffraction pattern of the obtained sodium salt is as follows. Figure 2 As shown, the product is a pure sodium nitrate phase, with no aluminum or fluorine-related phases precipitated. S3. The concentrate was hydrothermally heated at 150℃ for 12 h to obtain 85.3 g of hydrated aluminum fluoride. The phase composition results are as follows. Figure 3 The product shown is a high-purity aluminum fluoride monohydrate phase, without other impurity phases; S4. The hydrated aluminum fluoride was calcined in multiple stages at 200℃ for 2 h and 600℃ for 2 h to obtain 70.7 g of anhydrous α-aluminum fluoride product.

[0032] It also includes returning the leaching residue obtained from solid-liquid separation in S1 to S1 for secondary leaching; returning the condensate generated from evaporation and concentration in S2 to S1 as a leaching medium; and returning the filtrate obtained from solid-liquid separation after hydrothermal reaction in S3 to S1 for secondary leaching.

[0033] Tested according to YS / T 581-2024 standard, the aluminum fluoride purity reaches 96.5%, achieving AF-0 grade aluminum fluoride. Figure 4 XRD phase analysis showed that the product was pure α-aluminum fluoride with no other impurity peaks.

[0034] This embodiment employs a low hydrogen ion concentration system, completing the leaching under precise aluminum-fluorine molar ratio conditions. The process places the sodium salt precipitation-physical separation step after leaching and before hydrothermal treatment, removing sodium salt as a front-end impurity rather than a final byproduct. After solid-liquid separation to remove sodium salt, a low-sodium concentrate is obtained, providing a pure reaction environment for subsequent hydrothermal treatment. Relying on the synergistic effect of pre-removal sodium and hydrothermal crystallization, the system achieves crystallization without added seed crystals and eliminates the need for additional fluoride salt additives. Combined with hydrothermal crystallization and segmented calcination, high-purity α-aluminum fluoride is successfully obtained, while simultaneously recovering sodium salt byproducts and avoiding the problem of strong corrosion of equipment by a high-acid system.

[0035] Example 2: Example 1 was repeated, except that the waste aluminum electrolyte in S1 was replaced with carbon slag and the mass of aluminum nitrate nonahydrate was 150 g; the final product was 40.5 g of aluminum fluoride with a purity of 97.3%.

[0036] In this embodiment, the raw material is replaced with aluminum electrolysis carbon slag, and the amount of aluminum salt fed is adjusted to match the fluorine content of the raw material. Even with the change of waste source, the core process of pre-sodium removal and physical separation of sodium salt after leaching is still performed to eliminate the interference of sodium ions on crystallization and achieve seedless hydrothermal crystallization. The product still maintains high purity, indicating that this "pre-sodium removal-seedless hydrothermal" process architecture has good raw material adaptability to different types of aluminum electrolysis fluorine-containing waste and can be extended to the resource utilization treatment of various solid wastes in the electrolytic aluminum industry.

[0037] Example 3: Example 1 was repeated, except that the concentration factor in S2 was 20 times; the final product was 74.3g of aluminum fluoride with a purity of 98.9%.

[0038] This embodiment enhances the pre-treatment sodium removal effect by using a higher concentration factor combined with solid-liquid separation, thereby eliminating the interference of sodium ions on aluminum fluoride crystallization to a greater extent, ensuring the smooth progress of the seedless hydrothermal reaction, and simultaneously improving the yield and purity of aluminum fluoride. This demonstrates that increasing the depth of sodium removal can further optimize product quality.

[0039] Example 4: Repeat Example 1, except that the initial hydrogen ion concentration in the reaction system is 5 mol / L.

[0040] The final product yielded 71.1g of aluminum fluoride with a purity of 97.9%.

[0041] Increasing the hydrogen ion concentration in the system to 5 mol / L accelerated the leaching reaction rate and improved product purity. In this embodiment, the pre-sodium removal mechanism of leaching-concentration-sodium salt physical separation was still relied upon to obtain a low-sodium system, ensuring the normal progress of subsequent seedless hydrothermal crystallization. However, high acidity would increase the pressure on equipment corrosion prevention. The crystal form of the product did not change, indicating that α-aluminum fluoride can still be stably obtained in this acidity range with the pre-sodium removal architecture.

[0042] Example 5: Example 1 was repeated, except that the initial hydrogen ion concentration in the reaction system was 10 mol / L.

[0043] The final product obtained was aluminum fluoride: 71.5g, with a purity of 98.5%.

[0044] As the hydrogen ion concentration continues to increase, the product purity improves slightly, but excessively high acidity increases the risk of generating byproducts such as aluminum hydroxyfluoride, while also exacerbating acid consumption and equipment corrosion. In this embodiment, pre-treatment to remove sodium and physically remove sodium salts remains a necessary prerequisite for achieving seedless hydrothermal preparation of high-purity aluminum fluoride. However, this increases the overall cost, so it is not recommended to increase the acidity of the system indefinitely. The preferred hydrogen ion concentration is 0.1-2 mol / L.

[0045] Example 6: Repeat Example 1, except that: S1. Mix 100 g of waste aluminum electrolyte, 177 g of aluminum sulfate octadechydrate (or 100 g of anhydrous aluminum sulfate) with 2 L of 1 mol / L dilute sulfuric acid until homogeneous. The initial hydrogen ion concentration in the reaction system is 2 mol / L. Control the molar ratio of aluminum to fluorine in the mixture to be 1:3.0. Leach at 60°C for 2 h to obtain leachate and leach residue.

[0046] The final product yielded 71.9 g of aluminum fluoride with a purity of 95.4%.

[0047] In this embodiment, the aluminum source and acid system are changed to aluminum sulfate-dilute sulfuric acid system. The core process of pre-concentration after leaching and physical separation of sodium salt is still used. Seedless hydrothermal crystallization is achieved by relying on the low sodium environment, and there is no need to add fluoride salt auxiliary materials. The purity of the product is slightly reduced, mainly due to a small amount of sulfate residue, which proves that the sulfuric acid system is feasible, but the risk of impurity entrainment is higher than that of the nitric acid system.

[0048] Example 7: Repeat Example 1, except that: S1. Mix 100 g of waste aluminum electrolyte, 128 g of aluminum chloride hexahydrate (or 70.6 g of anhydrous aluminum chloride) with 2 L of 1 mol / L dilute hydrochloric acid until homogeneous. The initial hydrogen ion concentration in the reaction system is 1 mol / L. Control the molar ratio of aluminum to fluorine in the mixture to be 1:3.0. Leach at 60°C for 2 h to obtain leachate and leach residue.

[0049] The final product yielded 70.9 g of aluminum fluoride with a purity of 97.8%.

[0050] Using an aluminum chloride-hydrochloric acid system, after performing pre-treatment sodium removal and physical separation of sodium salts, a low-sodium reaction solution is obtained. Hydrothermal conversion can be completed without seed crystals or fluoride salt additives, resulting in high product purity. However, chloride ions pose a risk of volatilization and corrosion to equipment. The appropriate aluminum salt-acid combination can be selected according to the actual working conditions.

[0051] Comparative Example 1: Example 1 is repeated, except that the concentration factor in S2 is 0, that is, the first leachate is not evaporated and concentrated, but is directly used in the subsequent hydrothermal reaction in S3.

[0052] The XRD pattern of the obtained product is as follows Figure 5 As shown. By Figure 5 It was observed that the product exhibited the characteristic diffraction peaks of aluminum hydroxyfluoride, rather than those of hydrated aluminum fluoride. The results indicate that without evaporation and concentration of the leachate, a large number of sodium ions remain in the solution and participate in the hydrothermal reaction because the sodium ion concentration in the solution does not reach the saturation precipitation condition. Under the interference of sodium ions, aluminum ions preferentially undergo hydrolysis to form aluminum hydroxyfluoride, thus failing to yield the target product, hydrated aluminum fluoride. This demonstrates that evaporation and concentration in step S2 is a necessary prerequisite for subsequent hydrothermal crystallization to produce hydrated aluminum fluoride.

[0053] Comparative Example 2: Repeat Example 1, except that the sodium salt product obtained by evaporation and concentration in S2 is not separated and directly enters the hydrothermal reaction in S3 along with the concentrate.

[0054] Depend on Figure 6 It was observed that the product exhibited a distinct characteristic peak of hydroxyl aluminum fluoride, rather than that of pure hydrated aluminum fluoride. The results indicate that although the evaporation and concentration of S2 brought the sodium salt to a saturated precipitation state, the precipitated sodium salt crystals were not physically separated and remained in the reaction system. Under hydrothermal conditions, some of the sodium salt redissolved and released sodium ions. The residual sodium ions competed with fluorine and aluminum, preferentially forming a stable cryolite structure or inducing aluminum ion hydrolysis, interfering with the normal crystallization process of aluminum fluoride. This resulted in the product being contaminated with hydroxyl aluminum fluoride, making it impossible to obtain pure hydrated aluminum fluoride. This demonstrates that pre-treatment for sodium removal not only requires evaporation and concentration to crystallize the sodium salt but also necessitates physical removal through solid-liquid separation to ensure the purity of the subsequent hydrothermal crystallization product.

[0055] Comparative Example 3: Example 1 was repeated, except that the molar ratio of aluminum to fluorine in the mixture in S1 was 1:0.5. The XRD results of the product were the same as those in Example 1. Figure 5The diffraction peaks were similar to those of aluminum hydroxyfluoride, but no characteristic peaks of hydrated aluminum fluoride were detected. The results indicate that when the aluminum-fluoride molar ratio is 1:0.5, the relative content of fluoride ions in the solution is insufficient to provide enough coordinating anions for aluminum ions. Under hydrothermal conditions, aluminum ions preferentially undergo hydrolysis to form aluminum hydroxyfluoride, rather than coordinating with fluoride ions to crystallize and form hydrated aluminum fluoride. This demonstrates that the aluminum-fluoride ratio is a key parameter determining the hydrothermal crystallization pathway—when fluoride is insufficient, the crystallization pathway shifts from "fluorination" to "hydrolysis," and even with a pre-treatment sodium removal and seedless hydrothermal crystallization process, the target product cannot be obtained.

[0056] Conversely, when the aluminum-fluorine molar ratio exceeds 1:4 (e.g., 1:5, 1:6), aluminum ions are in excess relative to fluorine ions. Excess aluminum salt increases the aluminum ion concentration in the solution. During leaching, aluminum ions hydrolyze to produce hydrogen ions, inhibiting the formation of aluminum-fluorine complexes. This results in insufficient leaching of fluorine from the waste, significantly reducing the leaching rate. Simultaneously, the low fluorine ion ratio and high aluminum ion ratio in the leachate disrupt the coordination structure of the aluminum-fluorine complex in the concentrate. Free aluminum ions also preferentially undergo hydrolysis under hydrothermal conditions, producing hydroxyaluminum fluoride instead of hydrated aluminum fluoride. The XRD results of the comparative example (e.g., aluminum-fluorine ratio 1:5) are compared with... Figure 5 The results show only the characteristic diffraction peaks of aluminum hydroxyfluoride. Therefore, controlling the aluminum-fluorine ratio within the range of 1:1 to 1:4 is a necessary condition to ensure the directional formation of hydrated aluminum fluoride through hydrothermal crystallization.

[0057] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A method for recovering fluorine-containing waste from aluminum electrolysis to prepare aluminum fluoride, characterized in that, include: S1. Fluorine-containing waste from aluminum electrolysis is mixed with an aluminum-containing acidic reactant for leaching reaction, and the leachate is obtained by solid-liquid separation; S2. The leachate is evaporated and concentrated to separate the precipitated sodium salt crystals and obtain a concentrated solution. S3. The concentrated solution is subjected to a hydrothermal reaction to obtain hydrated aluminum fluoride; S4. The hydrated aluminum fluoride is calcined to obtain anhydrous aluminum fluoride.

2. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, The aluminum-containing acidic reactant includes an aluminum salt and an acid. The aluminum salt is selected from at least one of aluminum sulfate, aluminum chloride, and aluminum nitrate, and the acid is sulfuric acid, hydrochloric acid, or nitric acid. The aluminum salt and acid are added to the aluminum electrolysis fluorine-containing waste, and the molar ratio of aluminum to fluorine in the mixture of the aluminum electrolysis fluorine-containing waste and the aluminum-containing acidic reactant is 1:1-4.

3. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 2, characterized in that, The aluminum-containing acidic reactant has an aluminum ion concentration of 0.1-3 mol / L and a hydrogen ion concentration of 0.1-10 mol / L.

4. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, The leaching reaction in S1 is carried out at a temperature of 20-90℃ for a time of 0.5-6 h.

5. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, The evaporation and concentration process in S2 is vacuum low-temperature evaporation, with a temperature of 30-90℃, an evaporation pressure of 0.1-10 Pa, and an evaporation concentration ratio of 2-50 times.

6. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, The hydrothermal reaction in S3 is carried out at a temperature of 90-200℃ for a duration of 0.5-48 h.

7. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, After the hydrothermal reaction in step S3 is completed, the product is dispersed in water and filtered to obtain the hydrated aluminum fluoride.

8. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, The roasting process is a multi-stage roasting process with a roasting temperature of 200-800℃.

9. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 8, characterized in that, In the multi-stage roasting process, the holding time for a single stage roasting is not less than 2 hours.

10. The method for preparing aluminum fluoride from fluorine-containing waste from aluminum electrolysis according to claim 1, characterized in that, It also includes returning the leaching residue obtained from solid-liquid separation in S1 to S1 for secondary leaching; returning the condensate generated from evaporation and concentration in S2 to S1 as a leaching medium; and returning the filtrate obtained from solid-liquid separation after hydrothermal reaction in S3 to S1 for secondary leaching.

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