Process for the preparation of aluminum fluoride from regenerated cryolite

Abstract

This invention relates to a method for preparing aluminum fluoride from recycled cryolite, employing a short-process closed-loop process of "aluminum salt leaching – ammonium bifluoride precipitation – calcination conversion – water washing to remove sodium". Recycled cryolite is leached with an aluminum salt solution, followed by the direct addition of solid or solution of ammonium bifluoride to the resulting leachate, causing the aluminum to precipitate efficiently as a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate. Calcination converts the sodium component into a water-soluble form, followed by simple water washing to obtain high-purity aluminum fluoride. The mother liquor after precipitation can be recycled back to the leaching step after simple composition adjustment, thus achieving a closed-loop cycle of the leaching agent and reaction medium. This eliminates the generation of sodium fluoride-containing wastewater at the source and significantly reduces reagent consumption. This invention features an extremely short process flow, highly efficient closed-loop circulation, thorough aluminum-sodium separation, high product purity, high resource utilization rate, flexible process control, and significant environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and fluorochemistry in the aluminum industry, specifically involving a method for preparing high-purity aluminum fluoride using recycled cryolite as raw material through a closed-loop process of leaching-precipitation-calcination. Background Technology

[0002] Regenerated cryolite is a major product of the recycling and treatment of hazardous wastes such as aluminum electrolytic cell overhaul slag, waste cathode carbon blocks, and waste electrolytes. Its main component is cryolite (Na3AlF6). Further converting recycled cryolite into more valuable aluminum fluoride (AlF3) is key to achieving efficient recycling of fluorine resources in the aluminum industry.

[0003] Currently, there are two main technical bottlenecks in the preparation of aluminum fluoride from recycled cryolite. One is the high-temperature pyrolysis method, such as the method for producing aluminum fluoride from cryolite disclosed in CN 115836028 A. This method has extremely high energy consumption and is difficult to control product purity. The other is the wet treatment method, which, although carried out under mild conditions, generally suffers from lengthy process flows, high reagent consumption, and difficulty in treating sodium fluoride-containing wastewater. Furthermore, after leaching recycled cryolite with aluminum salts, the leachate has a complex composition (containing Al-F complex ions, Na⁺, SO₄²⁻). 2 Traditional processes require multiple separation steps (such as adjusting pH to precipitate hydroxyl aluminum fluoride, evaporation and crystallization to separate sodium salts), which are cumbersome and costly, hindering the large-scale application of this technology. Summary of the Invention

[0004] In view of the current state of the prior art, especially the problems of complex process flow, low resource utilization, and difficult wastewater treatment in the existing technology, the purpose of this invention is to provide a method for preparing aluminum fluoride from recycled cryolite. This invention creatively adopts a short-process closed-loop process of "aluminum salt leaching - ammonium bifluoride precipitation - calcination conversion - water washing to remove sodium". Its core lies in: leaching recycled cryolite with an aluminum salt solution, followed by the direct addition of solid or solution of ammonium bifluoride to the resulting leachate, causing aluminum and fluorine elements to precipitate as a mixture of sodium ammonium fluoroaluminate and ammonium fluoroaluminate; the mother liquor after precipitation can be returned to the leaching step for recycling, thereby achieving a closed-loop cycle of the leaching agent and reaction medium, and significantly reducing reagent consumption.

[0005] The technical solution for achieving the above-mentioned objectives can be summarized as follows:

[0006] A method for preparing aluminum fluoride from recycled cryolite includes the following steps:

[0007] S1. Aluminum salt solution leaching: The recycled cryolite powder is mixed with aluminum salt solution and stirred for leaching. After solid-liquid separation, a leachate and leaching residue rich in fluorine aluminum complex ions and sodium ions are obtained.

[0008] S2, ammonium bifluoride precipitation: Add ammonium bifluoride solid or solution to the leachate obtained in step S1 to carry out a crystallization reaction. After solid-liquid separation, a wet filter cake of sodium aluminate and ammonium fluoroaluminate mixture and circulating mother liquor are obtained.

[0009] S3. Calcination and Sodium Removal of Intermediate: The intermediate mixture of sodium aluminate and ammonium fluoroaluminate obtained in step S2 is dried and calcined. After the calcined product is washed with water to remove sodium and dried, high-purity aluminum fluoride product is obtained.

[0010] According to the present invention, it further includes S4, mother liquor recycling and sodium salt enrichment and recovery steps: the recycled mother liquor obtained in step S2 is returned to step S1 for preparation or replenishment of leachate for recycling.

[0011] According to the present invention, preferably, the concentration of the aluminum salt solution in S1 is 0.5-2.0 mol / L (in Al... 3+ (Calculated); preferably, the liquid-solid ratio of the leachate to the regenerated cryolite in S1 is (3-20):1 mL / g, more preferably (3-10):1 mL / g; preferably, the leaching temperature in S1 is 70-95℃, and the leaching time is 0.5-4 hours; preferably, the particle size of the regenerated cryolite in S1 is less than 100 mesh.

[0012] According to the present invention, preferably, the amount of ammonium bifluoride solid or aqueous solution added in S2 is controlled at (3.8-6.2):1, based on the ratio of the number of moles of fluorine in the ammonium bifluoride to the total number of moles of aluminum in the leachate (F / Al molar ratio); preferably, the precipitation reaction temperature in S2 is 20-60°C, and the reaction time is 0.5-2 hours. The obtained intermediate is a mixture of sodium ammonium fluoroaluminate ((NH4)2NaAlF6) and ammonium tetrafluoroaluminate (NH4AlF4) or ammonium hexafluoroaluminate ((NH4)3AlF6), with a sodium content of 2-5 wt%.

[0013] According to the present invention, preferably, the drying temperature in S3 is 80-120°C; the calcination temperature is 350-450°C; and the calcination time is 1-3 hours. Preferably, the calcined product is washed with deionized water or dilute acid solution for 5-15 minutes to remove sodium impurities. Preferably, the ammonia and hydrogen fluoride released during calcination in S3 are recovered and used to prepare ammonium hydrogen fluoride, thereby realizing the recycling of nitrogen and fluorine elements.

[0014] According to the present invention, preferably, before the mother liquor is circulated in S4, aluminum salt is added to maintain the aluminum ion concentration required for leaching; preferably, when the Na⁺ concentration in the circulating mother liquor is enriched to 50-150 g / L in S4, the extracted portion is used for sodium salt recovery.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] 1. Extremely short process flow: The conversion of recycled cryolite into aluminum fluoride products is achieved through a short process of "leaching-precipitation-calcination-washing". The process flow is short and the economic benefits are high.

[0017] 2. High product purity: Utilizing the calcination phase transformation-water washing sodium removal mechanism, sodium impurities in the intermediate are efficiently removed, and the resulting aluminum fluoride product has a sodium content of less than 0.2wt%, exhibiting excellent quality.

[0018] 3. Closed-loop circulation with no wastewater: The mother liquor is recycled, the washing liquid is returned to the leaching system, and sodium is ultimately recovered as a byproduct. The entire process produces no fluoride- or sodium-containing wastewater. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of the method for preparing aluminum fluoride from recycled cryolite according to the present invention.

[0020] Figure 2 The image shows the XRD pattern of the recycled cryolite raw material used in Example 1.

[0021] Figure 3 The image shows the XRD pattern of the mixture of sodium ammonium fluoroaluminate and ammonium hexafluoroaluminate obtained in Example 1.

[0022] Figure 4 The image shows the XRD pattern of the aluminum fluoride product obtained in Example 1. Detailed Implementation

[0023] This invention provides a method for preparing aluminum fluoride from regenerated cryolite. It creatively employs a short-process closed-loop technology: "aluminum salt leaching - ammonium bifluoride precipitation - calcination preparation - water washing to remove sodium." The core of this method lies in: leaching regenerated cryolite with an aluminum salt solution, followed by the direct addition of solid or solution of ammonium bifluoride to the resulting leachate, causing the aluminum to precipitate efficiently as an intermediate mixture of sodium aluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate. The mother liquor after precipitation can be recycled back to the leaching step after simple composition adjustments (such as adding aluminum salt), thus achieving a closed-loop cycle of the leaching agent and reaction medium. This eliminates the generation of sodium fluoride-containing wastewater at the source and significantly reduces reagent consumption.

[0024] The present invention provides a method for preparing aluminum fluoride from recycled cryolite, comprising the following steps:

[0025] S1. Aluminum salt solution leaching: The recycled cryolite powder is mixed with aluminum salt solution and leached by stirring at a certain temperature. After solid-liquid separation, a leachate and leaching residue rich in fluorine aluminum complex ions and sodium ions are obtained.

[0026] S2, ammonium bifluoride precipitation: Add ammonium bifluoride solid or solution to the leachate obtained in step S1, carry out precipitation reaction under suitable conditions, and after solid-liquid separation, obtain a wet filter cake and circulating mother liquor of a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate.

[0027] S3. Calcination of intermediates and washing to remove sodium: The sodium aluminate obtained in step S2 is calcined with a mixture of ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate. The calcined product is then washed with water to remove sodium and dried to obtain high-purity aluminum fluoride.

[0028] According to the present invention, after the intermediate S3 is calcined, the process further includes S4, a mother liquor recycling and sodium salt enrichment and recovery step: the recycled mother liquor obtained in step S2 is returned to step S1 for use in preparing or replenishing the leachate for recycling. After multiple cycles, sodium ions are enriched in the system, and a portion of the mother liquor can be extracted for sodium salt product recovery via crystallization.

[0029] According to the present invention, during the S1 leaching process, aluminum salt reacts with cryolite to generate soluble aluminum fluoride complex ions, and sodium enters the solution. In one or more preferred embodiments, the concentration of the aluminum salt solution in S1 is 0.5-2.0 mol / L (in Al...). 3+ (Calculation). Preferably, the liquid-solid ratio of the leachate to the regenerated cryolite in S1 is (3-20):1mL / g, more preferably (3-10):1mL / g.

[0030] According to the present invention, leaching at a certain temperature is more advantageous during the S1 leaching process. In one or more preferred embodiments, the leaching temperature in S1 is 70-95°C, and the leaching time is 0.5-4 hours. Preferably, the particle size of the regenerated cryolite in S1 is less than 100 mesh.

[0031] According to the present invention, in the S2 precipitation process, ammonium bifluoride provides a high concentration of F⁻ and NH₄⁺, which combine with aluminum fluoride complex ions to form a precipitate of sodium ammonium fluoroaluminate and a mixture of ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate. In one or more preferred embodiments, the amount of ammonium bifluoride added in S2 is controlled at (3.8-6.2):1, based on the ratio of the moles of fluorine in the ammonium bifluoride to the total moles of aluminum in the leachate (F / Al molar ratio). The resulting intermediate is a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate, with a sodium content of 2-5 wt%.

[0032] In one or more preferred embodiments, the precipitation reaction temperature in S2 is 20-60°C, and the reaction time is 0.5-2 hours.

[0033] According to the present invention, in the S3 calcination and water washing process for sodium removal, a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate undergoes thermal decomposition during calcination to generate aluminum fluoride, releasing NH3 and HF. The sodium component undergoes phase reconstruction under an HF atmosphere, transforming into water-soluble NaF or NaHF2 and accumulating on the particle surface. After cooling, the calcined product is washed with deionized water or dilute acid to reduce the sodium content to below 0.2 wt%.

[0034] In one or more preferred embodiments, the drying temperature of S3 is 80-120°C; the calcination temperature is 350-450°C; the calcination time is 1-3 hours; the washing time is 5-15 minutes; and the washing liquid is incorporated into the S4 circulation system.

[0035] According to the present invention, the ammonia and hydrogen fluoride released in S3 can be recycled for the preparation of ammonium hydrogen fluoride, thereby realizing the recycling of nitrogen and fluorine elements.

[0036] According to the present invention, before the mother liquor is circulated in S4, aluminum salt needs to be added to maintain the aluminum ion concentration required for leaching. Preferably, when the Na⁺ concentration in the circulating mother liquor in S4 is enriched to 50-150 g / L, a portion can be drawn off for sodium salt recovery.

[0037] The core reaction mechanism of this invention is as follows:

[0038] During the S1 leaching process, aluminum salts react with cryolite to form soluble aluminum fluoride complex ions, and sodium enters the solution.

[0039]

[0040] During the S2 precipitation process, ammonium bifluoride provides a high concentration of F⁻ and NH⁺, which combine with aluminum fluoride complex ions to form a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate:

[0041]

[0042] or

[0043]

[0044] During the S3 calcination process, sodium ammonium fluoroaluminate and a mixture of ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate undergo homothermal decomposition to aluminum fluoride, while sodium is converted to a water-soluble form.

[0045]

[0046] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0047] The recycled cryolite raw material used in this embodiment comes from a waste electrolyte recovery line of an aluminum plant. Its main phase is Na3AlF6, and its XRD pattern is shown below. Figure 2 As shown.

[0048] Example 1

[0049] A method for preparing aluminum fluoride from recycled cryolite includes the following steps:

[0050] S1, Aluminum Salt Leaching: Weigh 100g of recycled cryolite powder with a particle size of -200 mesh, and place it in 500mL of a solution with a concentration of 1.0mol / L (Al).3 The solution of aluminum salt (calculated as ⁺) was leached at 85°C with stirring at 300 rpm for 2 hours. After the reaction was completed, the solution was filtered while hot to separate the leachate and a small amount of leaching residue. The leaching residue was washed with a small amount of hot water, and the washings were added to the leachate.

[0051] S2, Ammonium fluoride precipitation: The above leaching solution was cooled to 30°C. While stirring, 183g of solid ammonium fluoride (NH4HF2, purity ≥98%) was added in batches. At this point, the molar ratio of fluorine in the ammonium fluoride to the total aluminum in the leaching solution (F / Al) was approximately 6.0:1. After the addition was complete, the reaction was continued at 30°C for 1.5 hours with stirring. After the reaction was complete, vacuum filtration was performed to obtain a white wet filter cake. Chemical analysis showed that the sodium content of the mixture was 3.5wt%; XRD analysis (see...) Figure 3 The phase composition of the sample is sodium ammonium fluoroaluminate and ammonium hexafluoroaluminate.

[0052] S3. Calcination of the intermediate: The obtained wet filter cake was dried at 100℃ for 4 hours, then transferred to a crucible and placed in a muffle furnace. Under air atmosphere, the temperature was increased to 400℃ at a rate of 5℃ / min, and calcined at this temperature for 2 hours. The gas generated during calcination was introduced into a dilute sulfuric acid absorption device to recover ammonia. After calcination, the mixture was naturally cooled to room temperature, and 100 mL of deionized water was added and stirred and washed for 10 minutes. The mixture was then filtered, and the filter cake was dried at 100℃ for 2 hours to obtain 43.8 g of white powdered aluminum fluoride product.

[0053] Product Analysis: Chemical analysis revealed that the product contains 65.0% fluorine (F), 32.0% aluminum (Al), 0.08% sodium (Na), 0.05% silicon dioxide (SiO2), and a loss on ignition of 0.7% by mass. XRD analysis of the aluminum fluoride product (e.g.) Figure 4 As shown in the figure, the product is pure phase AlF3.

[0054] Example 2:

[0055] A method for preparing aluminum fluoride from recycled cryolite includes the following steps:

[0056] S1. The leaching process is exactly the same as in Example 1.

[0057] S2, direct precipitation with ammonium bifluoride: The leachate was cooled to 40°C, and 127g of solid ammonium bifluoride was added with stirring, controlling the F / Al molar ratio to be approximately 4.1:1. After stirring and reacting at 40°C for 1 hour, the mixture was filtered to obtain a wet filter cake of a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate.

[0058] S3. Calcination of intermediates: After drying the wet filter cake, calcine it at 380℃ for 2.5h. After calcination, allow it to cool naturally to room temperature, add 100mL of deionized water, stir and wash for 10min, filter, and dry the filter cake at 100℃ for 2h to obtain aluminum fluoride product. Product analysis shows that the F content is 65.1%, the Al content is 32.0%, the Na content is 0.09%, the silicon dioxide (SiO2) content is 0.05%, and the loss on ignition is 0.7% by mass.

[0059] Example 3: Closed-loop circulation of mother liquor

[0060] As shown in Example 1, the difference is that after the intermediate S3 is calcined, the process includes S4, mother liquor recycling, and sodium salt enrichment and recovery steps: the recycled mother liquor obtained in step S2 is returned to step S1 for use in preparing or replenishing the leaching solution for recycling. Before recycling the mother liquor in S4, aluminum salt is added to maintain the aluminum ion concentration required for leaching. After multiple cycles, sodium ions are enriched in the system. When the Na⁺ concentration in the recycled mother liquor reaches 110 g / L, a portion of the mother liquor is drawn off and the sodium salt product is recovered through crystallization.

[0061] To verify the closed-loop stability and comprehensive resource recovery effect of the process of this invention, five consecutive batches of cyclic experiments were conducted according to the process flow and conditions of Example 3. Each batch processed 100g of the same recycled cryolite raw material. The first cycle (Cycle 1) used freshly prepared aluminum salt leachate. From the second cycle (Cycle 2) onwards, the mother liquor generated from the S2 step of the previous cycle was used as the base liquid in the S1 leaching step of each cycle. Before each cycle, the chemical composition of the mother liquor was analyzed, and solid aluminum salt was added to restore its aluminum ion concentration to 1.0 mol / L, and then it was directly used for leaching. The amount of ammonium bifluoride added in the S2 step was calculated based on the measured total aluminum content in the batch of leachate at a ratio of F / Al = 4.1:1. The results of the cyclic experiments are shown in Table 1.

[0062] Table 1: Results of five consecutive batches of closed-loop cycle experiments

[0063]

[0064] As shown in Table 1, the key quality indicators (F, Al, Na content) of the obtained aluminum fluoride product remained highly stable in five consecutive cycles, and the aluminum recovery rate (from leaching to precipitation) was always higher than 92%, which proves the reliability of the process and the stability of the product.

[0065] As the cycle continues, sodium ions accumulate in the mother liquor system (see the last column of Table 1). After the fifth batch, the sodium ion concentration in the mother liquor reaches approximately 110 g / L. At this point, one-third of the mother liquor volume is withdrawn from the system and subjected to freeze crystallization at 5°C for 12 hours. After filtration and drying, approximately 38 g of high-purity anhydrous sodium sulfate (Na₂SO₄) is obtained as a byproduct. The entire cycle process requires no discharge of process wastewater, only a small amount of water for filter cake washing, achieving intensive utilization of water resources.

[0066] Comparative Example: Traditional Two-Step Process for Aluminum Hydroxide Fluoride

[0067] To illustrate the advantages of this invention, 100g of regenerated cryolite was treated using the exact same raw materials and leaching conditions as in Example 1, resulting in a leachate with similar composition. Subsequent processing was then carried out using common industry-standard processes.

[0068] 1. Slowly add a 10% sodium hydroxide solution to the leachate, precisely adjust the pH to 5.5 while stirring, and maintain the reaction at 80°C for 1 hour to precipitate aluminum as aluminum hydroxyfluoride. Filter to obtain aluminum hydroxyfluoride filter cake.

[0069] 2. To reduce impurity entrainment, wash the filter cake twice with 500mL of 60℃ hot water, and filter after each wash.

[0070] 3. The washed aluminum fluorohydride filter cake is re-slurryed with deionized water, and then ammonium bifluoride is added stoichiometrically. The mixture is reacted at 80°C for 2 hours to convert it into ammonium fluoroaluminate. The mixture is then filtered again to obtain wet ammonium fluoroaluminate.

[0071] 4. After drying the wet material, calcine it at 400℃ for 2 hours to obtain aluminum fluoride product.

[0072] A comprehensive comparison between the process of this invention and the traditional process is shown in Table 2.

[0073] Table 2: Comprehensive Comparison of the Invention Process and Traditional Processes

[0074] The above examples and comparative data fully demonstrate that the "aluminum salt leaching-ammonium fluoride precipitation-calcination conversion-water washing sodium removal" process provided by the present invention is significantly superior to the traditional aluminum hydroxy fluoride process in terms of process simplicity, resource recovery rate, environmental friendliness, water conservation and product purity, realizing efficient, clean and high-value utilization of recycled cryolite.

Claims

1. A method for preparing aluminum fluoride from recycled cryolite, comprising the following steps: S1. Aluminum salt solution leaching: The recycled cryolite powder is mixed with aluminum salt solution and stirred for leaching. After solid-liquid separation, a leachate and leaching residue rich in fluorine aluminum complex ions and sodium ions are obtained. S2, ammonium bifluoride precipitation: Add ammonium bifluoride solid or solution to the leachate obtained in step S1 to carry out precipitation reaction. After solid-liquid separation, a wet filter cake of sodium ammonium fluoroaluminate and ammonium fluoroaluminate mixture and circulating mother liquor are obtained. S3. Calcination of intermediate and washing to remove sodium: The sodium fluoroaluminate obtained in step S2 is calcined with a mixture of ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate. After washing to remove sodium and drying, the calcined product is used to obtain high-purity aluminum fluoride.

2. The method of claim 1, wherein the method is characterized by, S3 is followed by S4, a mother liquor recycling and sodium salt enrichment and recovery step, in which the recycled mother liquor obtained in step S2 is returned to step S1 for use in preparing or replenishing the leachate for recycling.

3. The method of claim 1 or 2, wherein the method is characterized by, The concentration of the aluminum salt solution in S1 is 0.5-2.0 mol / L, calculated as Al 3+ Preferably, the liquid-solid ratio of the leaching solution to regenerated cryolite in S1 is (3-20): 1 mL / g.

4. The method of claim 1 or 2, wherein the method is characterized by, The leaching temperature in S1 is 70-95℃, and the leaching time is 0.5-4 hours.

5. The method of claim 1 or 2, wherein the method is characterized by, The amount of ammonium bifluoride added in S2 is controlled at (3.8-6.2):1, based on the ratio of the number of moles of fluorine in the ammonium bifluoride to the total number of moles of aluminum in the leachate.

6. The method for preparing aluminum fluoride from recycled cryolite according to claim 5, characterized in that, The intermediate is a mixture of sodium ammonium fluoroaluminate and ammonium tetrafluoroaluminate or ammonium hexafluoroaluminate, and the sodium content in the intermediate is 2-5 wt%.

7. The method for preparing aluminum fluoride from recycled cryolite according to claim 1 or 2, characterized in that, The precipitation reaction temperature in S2 is 20-60℃, and the reaction time is 0.5-2 hours.

8. The method for preparing aluminum fluoride from recycled cryolite according to claim 1 or 2, characterized in that, The drying temperature in S3 is 80-120℃.

9. The method for preparing aluminum fluoride from recycled cryolite according to claim 1 or 2, characterized in that, The calcination temperature is 350-450℃, and the calcination time is 1-3 hours. The calcined product is washed with deionized water or dilute acid solution for 5-15 minutes to remove sodium impurities. Preferably, the ammonia and hydrogen fluoride released during calcination in S3 are recovered and used to prepare ammonium hydrogen fluoride, thereby realizing the recycling of nitrogen and fluorine elements.

10. The method for preparing aluminum fluoride from recycled cryolite according to claim 2, characterized in that, Before the mother liquor is circulated in S4, aluminum salt is added to maintain the aluminum ion concentration required for leaching; preferably, when the Na⁺ concentration in the circulating mother liquor is enriched to 50-150 g / L in S4, a portion is drawn off for sodium salt recovery.

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