Enhanced leaching and resource recycling method for fluoride in aluminum electrolysis overhaul slag

Abstract

The invention relates to the technical field of dangerous solid waste treatment and resource utilization, in particular to an aluminum electrolysis overhaul slag fluoride enhanced leaching and resource recycling method. Comprising the following steps: adding pretreated overhaul slag into a complexing reaction solution of boric acid containing a dispersing agent and gluconate, and pulping; sealing the system, heating, and implementing pulse type micro-negative pressure degassing operation in which vacuumizing and normal pressure recovery are alternated during constant-temperature leaching; after the reaction is finished, carrying out thermal-state solid-liquid separation and tailing washing, and merging to obtain clear filtrate; crystal seeds are added into the filtrate, an aluminum source solution is dropwise added, the pH value is increased, solid-liquid separation is performed after constant-temperature curing, and synthetic cryolite is obtained; and supplementing the pH buffer agent to the clear liquid and then circularly returning to the liquid preparation process. According to the method, fluorine and aluminum are cooperatively extracted under the weak acid condition, generation of silicic acid gel is inhibited, equipment corrosion is reduced, mass transfer in pores is enhanced in combination with micro-negative pressure operation, and low-energy-consumption recovery of fluoride salt and closed-loop circulation of a working solution are achieved through pH regulation and control.

Description

Technical Field

[0001] This invention relates to the field of hazardous solid waste treatment and resource utilization technology, specifically to a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag. Background Technology

[0002] Aluminum electrolysis overhaul slag is a solid waste generated from the periodic overhaul and stripping of aluminum electrolysis cells. It contains fluorides such as cryolite and some associated amorphous silicates, and has been clearly classified as hazardous waste. Currently, the industrial treatment of overhaul slag and the recovery of valuable fluorides mainly employs conventional strong acid leaching processes. This process utilizes the destructive effect of strong acids to promote the dissociation of fluorine-containing mineral phases; however, in actual operation, it consumes large amounts of acidic reagents, and the high concentration of acid can corrode the leaching tank and pipeline equipment.

[0003] During strong acid leaching, amorphous silicates associated with overhaul slag dissolve in large quantities under acidic excitation. These dissolved silicates readily undergo hydrolysis and polymerization upon entering the liquid phase, forming a viscous silica gel. This gel not only adheres to the surface of unreacted material particles, hindering leaching mass transfer, but also clogs the pores of filter media in subsequent processes, causing difficulties in solid-liquid separation and disrupting production continuity. Furthermore, current methods for recovering and treating strong acid leaching solutions typically rely on physical evaporation or high-temperature crystallization purification, resulting in high overall energy consumption. Additionally, the presence of significant dissolved silicates and other metallic impurities in the upstream leaching solution leads to low purity of the final recovered fluoride products, making it difficult to directly meet the reuse standards of electrolytic aluminum production systems. Therefore, modifying the traditional strong acid leaching system to achieve efficient extraction and low-cost recovery of fluorides while suppressing silica gel formation is a crucial technical challenge that needs to be addressed in the current resource utilization of overhaul slag. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for enhanced fluoride leaching and resource recovery of aluminum electrolysis overhaul slag, which solves the problems of high consumption of strong acids and severe equipment corrosion in the aluminum electrolysis overhaul slag treatment process.

[0005] To achieve the above objectives, the present invention provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, comprising the following steps: The pretreated aluminum electrolytic overhaul slag powder is put into a complex reaction solution of boric acid and gluconate containing dispersant and then uniformly slurried. The reaction system was sealed, and the system temperature was raised linearly and uniformly to the isothermal leaching temperature. Entering the constant temperature leaching stage, a pulsed micro-negative pressure degassing operation is carried out during the constant temperature period, alternating between vacuuming and restoring normal pressure. After the leaching reaction is completed, hot solid-liquid separation is performed, and the retained tailings are washed by cross-flow washing. The washing liquid and the leaching mother liquor are combined to obtain a clear filtrate. Seed crystals are added to the clarified filtrate, aluminum source solution is added dropwise at a uniform rate and the pH value of the system is adjusted upward. After constant temperature aging, solid-liquid separation is performed, and the solid phase is dried to obtain the synthetic cryolite product. The clear liquid is replenished with pH buffer and then recycled back to the solution preparation process.

[0006] By employing the above-mentioned technical solution, unlike the traditional strong acid-induced crystal lattice destruction mode, this invention mainly relies on a synergistic extraction and separation mechanism under a specific weak acid environment to function. During pulping, the complexation reaction solution of boric acid and gluconate used has significant steric hindrance, which effectively reduces the probability of active groups in the system being prematurely consumed by free impurities on the surface of the waste residue.

[0007] During the leaching process, protons and complexing agents in the system begin to produce a synergistic effect: on the one hand, gluconate ions, relying on their continuous electron-donating oxygen atoms, undergo polydentate chelation with aluminum ions precipitated from the solid phase; on the other hand, free boric acid molecules exhibit a specific affinity for fluoride ions, and the two combine to form stable fluoroborate ions. It is this bidirectional complexation targeting both cations and anions that allows fluorides such as cryolite, which are originally insoluble, to be efficiently dissociated even under weakly acidic conditions. The key chemical transformations involved in this process are specifically manifested as follows: gluconate ions undergo polydentate chelation with aluminum ions dissociated from the mineral lattice to form stable, soluble organoaluminum complexes; simultaneously, free boric acid molecules, with the participation of hydrogen ions in the system, react with free fluoride ions to combine and generate fluoroborate ions and water molecules.

[0008] The purpose of the introduced pulsed micro-negative pressure degassing operation is to break down the physical barriers at the solid-liquid interface. When under vacuum, the tiny bubbles adhering to the particle surface expand and detach under pressure, directly disrupting the gas film mass transfer layer that hinders the reaction. Upon instantaneous restoration to normal pressure, the leachate is forced into the deep micropores of the particles by the capillary effect induced by the environmental pressure difference. This hydrodynamic change not only improves the interfacial wetting properties but also simultaneously inhibits the formation of silica gel, thus effectively ensuring the filtration flux during subsequent hot separation.

[0009] In the precipitation recovery stage after extraction, the process cleverly utilizes the sensitivity of coordination chemical equilibrium to hydrogen ion concentration. By adding an aluminum source solution, the pH of the system is adjusted to a weakly alkaline state. The originally stable fluoroborate ions are hydrolyzed under this environment, releasing fluoride ions again. At the same time, the stability of the gluconate-aluminum complex also decreases. At this point, the free aluminate ions, sodium ions, and fluoride ions in the solution spontaneously combine according to the solubility product principle, ultimately precipitating out as insoluble cryolite. The relevant precipitation transformation process is as follows: induced by the weakly alkaline environment of the system, the previously generated fluoroborate ions undergo a hydrolysis reaction with water molecules, converting into boric acid and releasing a large number of free fluoride and hydrogen ions back into the system; subsequently, the free aluminate ions and sodium ions in the solution react with the precipitated fluoride and hydrogen ions, spontaneously combining to form insoluble cryolite precipitate and water molecules.

[0010] With the precipitation of large amounts of cryolite, the displaced organic ligands regain their free state in the liquid phase. Therefore, at the end of the process, only a simple addition of buffer to adjust the pH is needed to achieve a closed-loop circulation of the entire working solution, thus eliminating the energy-intensive physical evaporation and crystallization steps in traditional processes.

[0011] Preferably, the complexation reaction solution of boric acid and gluconate is made from raw materials comprising the following components: boric acid, gluconate, pH buffer, and water; the molar ratio of boric acid to gluconate is 1:1.0 to 1:1.5; the gluconate is sodium gluconate or potassium gluconate; the pH buffer is glacial acetic acid, and the pH value of the complexation reaction solution of boric acid and gluconate is maintained between 4.5 and 5.0; the initial total solids content of the complexation reaction solution of boric acid and gluconate is 20% to 30%.

[0012] By employing the above-mentioned technical solution, boric acid and gluconate are controlled in a specific ratio and placed within a glacial acetic acid buffer system. The practical effect is to stabilize the leaching environment within a defined weakly acidic range. More importantly, the polymerization intermediate formed based on this ratio effectively creates a continuous and controllable ligand release source in subsequent reactions, thereby avoiding adverse effects on overall leaching efficiency caused by drastic local fluctuations in reagent concentration.

[0013] Preferably, the preparation of the boric acid and gluconate complex reaction solution includes the following steps: injecting deionized water into a mixing vessel, stirring at a speed of 150 to 250 rpm, and sequentially adding the dry powders of boric acid and gluconate; then pumping in the pH buffer to adjust the pH value; stirring and reacting at a constant temperature of 35 to 45°C for 40 to 60 minutes to undergo a dehydration esterification condensation reaction, thereby obtaining the boric acid and gluconate complex reaction solution.

[0014] By employing the above-mentioned technical solution, under the combined effect of a limited temperature and mechanical stirring, condensation occurs between the hydroxyl groups of boric acid and gluconate, thereby generating an intermediate with an ester bond structure. The presence of this unique structure temporarily masks the high reactivity of the coordinating groups at room temperature, thus paving the way for the gradual release of the active substance through gradient heating in subsequent processes.

[0015] Preferably, the pretreatment involves crushing the aluminum electrolytic overhaul slag to a particle size D90 of 80 to 120 μm; the dispersant is sodium hexametaphosphate, and its addition amount is 0.05% to 0.15% of the total mass of the aluminum electrolytic overhaul slag; the liquid-solid mass ratio is controlled at 4 to 6:1, and the pulping time is 20 to 30 minutes.

[0016] By employing the above-mentioned technical solution, the overhaul slag is specifically pulverized to this particle size limit, and sodium hexametaphosphate is added to exert a dispersing effect. This directly alters the potential distribution on the surface of the material particles, physically preventing the agglomeration of fine powders. Simultaneously, the defined liquid-to-solid ratio and mixing time ensure that the solid minerals are completely wetted by the reaction liquid, essentially establishing a reliable mass transfer channel for the subsequent smooth penetration of the liquid into the pores of the solid phase.

[0017] Preferably, the reaction system is linearly and uniformly heated from 35 to 45°C to 80 to 90°C within 30 to 50 minutes.

[0018] By employing the above technical solution, a slowly increasing temperature gradient environment is artificially created. As the system temperature linearly rises, the previously bound ester bonds begin to gradually hydrolyze, releasing free boric acid molecules and gluconate ions gently into the surrounding liquid phase. It is precisely this temperature-linked dynamic release mechanism that allows the leaching agent to achieve deep penetration from the surface inwards, largely avoiding the risk of ligands being prematurely and ineffectively consumed by non-target impurities on the particle surface.

[0019] Preferably, the temperature of the constant temperature leaching stage is 80 to 90°C, and the total duration is controlled between 1.5 and 2.5 hours; the specific parameters of the pulsed micro-negative pressure degassing operation are as follows: vacuuming to reduce the absolute pressure in the system to 0.05 to 0.08 MPa, maintaining the pressure for 3 to 5 minutes; opening the vent valve to instantly restore the pressure to normal at 0.1 MPa, maintaining it for 25 minutes; the above negative pressure and normal pressure cycle operation is repeated for 3 to 5 cycles.

[0020] By employing the aforementioned technical solution, the strictly regulated isothermal duration and temperature threshold first ensure the complete dissociation of valuable minerals within the solid phase. Based on this, by applying alternating negative and atmospheric pressures with clearly defined upper and lower limits, the system utilizes the hydrodynamic effects induced by the periodic alternating pressure to forcibly accelerate the deep material exchange within the porous structure. This physical intervention can promptly displace accumulated gaseous byproducts and locally overly concentrated reaction liquids within the micropores, thereby shortening the time required for the entire system to reach reaction equilibrium.

[0021] Preferably, the temperature of the hot solid-liquid separation system is maintained at 75 to 85°C; the hot water temperature used for the cross-flow washing is 70 to 80°C.

[0022] By employing the above technical solution, the filtration and cross-flow washing processes are intentionally kept within a relatively high temperature range. The main purpose is to prevent some dissolved substances in the leachate from undergoing supersaturation precipitation or colloidal aggregation due to sudden cooling. This measure effectively reduces the physical entrainment of target valuable components within the pores of the waste residue, thereby macroscopically improving the comprehensive recovery indicators of core elements such as fluorine and aluminum.

[0023] Preferably, the seed crystal is a synthetic cryolite seed crystal, and its addition amount is 0.3% to 0.8% of the total mass of the clarified filtrate; the aluminum source solution is an aqueous solution of sodium aluminate with a mass concentration of 15% to 25%, and the total dropping time is 20 to 40 minutes.

[0024] By employing the above technical solution, the introduction of homogeneous synthetic cryolite seed crystals directly lowers the nucleation barrier during the formation of the new precipitate phase. Combined with precise control over the concentration of sodium aluminate and the dropping rate, the reaction solution is consistently kept within a mild supersaturation range. This not only avoids the chaotic accumulation caused by the instantaneous explosive formation of crystal nuclei but also promotes the gradual growth of the target crystals in a regular morphology, reducing the burden on the final dehydration process.

[0025] Preferably, the pH of the system is gradually increased to 8.5 to 9.5 through online pH meter linkage control; the temperature of the constant temperature aging is adjusted and maintained at 60 to 75°C, and the aging time is 45 to 90 minutes; before the clear liquid is circulated back, glacial acetic acid needs to be added to adjust the pH to 4.5 to 5.0.

[0026] By employing the aforementioned technical solution, the pH value is precisely controlled within a specific weakly alkaline range, forming a crucial prerequisite for inducing the hydrolysis of fluoroborate and the release of metal complexes. The subsequent isothermal ripening process provides ample time for crystal growth and internal structural rearrangement, which is essential for improving the crystallinity and purity of the final solid product. As a final step in the process, glacial acetic acid is added to the supernatant, allowing the re-released ligands to return to a stable weakly acidic state, thus naturally establishing a closed-loop reuse pathway for the entire extraction reagent system.

[0027] This invention provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag. It has the following beneficial effects: 1. This invention uses a complexation reaction solution of boric acid and gluconate containing a dispersant as the extractant, maintaining the reaction system under weakly acidic conditions. This replaces the traditional strong acid destructive leaching. This weakly acidic coordination environment can synergistically extract fluoride and aluminum ions from the overhaul residue without damaging the structure of the associated silicates, avoiding the dissolution of large amounts of amorphous silicates and subsequent hydrolysis and polymerization to form a gel. This not only improves the filtration flux for subsequent solid-liquid separation but also reduces the corrosiveness of the leachate to the equipment and the overall acid consumption.

[0028] 2. In the constant temperature leaching stage, the present invention introduces a pulsed micro-negative pressure degassing operation with alternating cycles of vacuuming and restoring normal pressure. By utilizing the hydrodynamic changes caused by alternating pressure, the microbubbles attached to the surface of the material can be detached to destroy the gas film resistance layer. At the same time, the capillary effect generated by the pressure difference forces the leachate into the deep micropores of the particles. This operation breaks the conventional mass transfer limitation of the solid-liquid interface, accelerates the material exchange inside the material, and shortens the reaction equilibrium time.

[0029] 3. In the precipitation and recovery stage, this invention adjusts the pH of the system to a weakly alkaline state by adding sodium aluminate solution dropwise. Utilizing the sensitivity of coordination chemical equilibrium to hydrogen ion concentration, the complexes in the solution dissociate and spontaneously combine to precipitate cryolite. As a large amount of cryolite precipitates, the organic ligands return to a free state in the liquid phase. Subsequently, only glacial acetic acid needs to be added to the clear liquid to restore the pH value, allowing the working solution to regain its extraction capacity and return to the front-end process. This eliminates the energy-intensive physical evaporation and crystallization steps in traditional processes, reducing the operating cost of resource recovery. Attached Figure Description

[0030] Figure 1 This is a graph showing the evolution of the retention rate of ester bond structures in the liquid phase during the leaching process in an embodiment of the present invention. Figure 2 This is a graph showing the evolution of the targeted release concentration of free boric acid during the leaching process in an embodiment of the present invention. Figure 3 This is a liquid phase composition tracking diagram of pH swing-induced ligand release and ring closure according to the present invention; Figure 4 This is a comparison chart of the core element extraction efficiency between the embodiments and comparative examples of the present invention; Figure 5 This is a tracking diagram of the solid-liquid separation state after leaching in the embodiments and comparative examples of the present invention; Figure 6 This is a panoramic tracking diagram of the purity and impurity distribution of the synthesized cryolite products in Example 1 and Comparative Example 5 of the present invention. Figure 7This is a parameter stability tracking diagram of the system in the multi-round closed-loop operation of Embodiment 1 of the present invention. Detailed Implementation

[0031] The technical solutions in 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.

[0032] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a complexation reaction solution of boric acid and gluconate, comprising the following steps: Deionized water was injected into the mixing vessel (the amount of water was calculated based on the initial total solids content), and stirring was started at 200 rpm. Boric acid and sodium gluconate powders were slowly added sequentially to the water at a molar ratio of 1:1.2, controlling the initial total solids content of the system to be 25%. Glacial acetic acid was then slowly pumped in to buffer and lock the pH of the system at 4.8. The jacketed circulation system was turned on to maintain the temperature inside the vessel at 40°C. Under these conditions of 40°C and pH 4.8, the mixture was stirred and reacted for 50 minutes to allow the boric acid and sodium gluconate to undergo a dehydration esterification condensation reaction, generating a sterically hindered complex of boric acid and gluconate in situ in the aqueous phase. After preparation, the mixture was kept at this temperature for later use.

[0033] Preparation Example 2: This preparation example provides a method for preparing a complexation reaction solution of boric acid and gluconate, comprising the following steps: Deionized water was injected into the mixing vessel (the amount of water was calculated based on the initial total solids content), and stirring was started at 150 rpm. Boric acid and sodium gluconate powders were slowly added sequentially to the water at a molar ratio of 1:1.0, maintaining the initial total solids content of the system at 20%. Glacial acetic acid was then slowly pumped in to buffer and lock the pH of the system at 4.5. The jacketed circulation system was turned on to maintain the temperature inside the vessel at 35°C. At 35°C and a pH of 4.5, the mixture was stirred and reacted for 40 minutes to allow the boric acid and sodium gluconate to undergo a dehydration esterification condensation reaction, generating a sterically hindered complex of boric acid and gluconate in situ in the aqueous phase. After preparation, the mixture was kept at this temperature for later use.

[0034] Preparation Example 3: This preparation example provides a method for preparing a complexation reaction solution of boric acid and gluconate, comprising the following steps: Deionized water was injected into the mixing vessel (the amount of water was calculated based on the initial total solids content), and stirring was started at 250 rpm. Boric acid and sodium gluconate powders were slowly added sequentially to the water at a molar ratio of 1:1.5, controlling the initial total solids content of the system to be 30%. Glacial acetic acid was then slowly pumped in to buffer and lock the pH of the system at 5.0. The jacketed circulation system was turned on to maintain the temperature inside the vessel at 45°C. At 45°C and a pH of 5.0, the mixture was stirred and reacted for 60 minutes to allow the boric acid and sodium gluconate to undergo a dehydration esterification condensation reaction, generating a sterically hindered complex of boric acid and gluconate in situ in the aqueous phase. After preparation, the mixture was kept at this temperature for later use.

[0035] Examples 1-4: Example 1

[0036] This embodiment provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, including the following steps: (1) Slurry preparation and dispersion: The complexation reaction solution of boric acid and gluconate obtained in Preparation Example 1 was added to the main reactor, and the jacket temperature control was turned on to maintain the system temperature at 40°C. Sodium hexametaphosphate was added at 0.1% of the total mass of the aluminum electrolytic overhaul slag to be treated (pre-crushed to a particle size D90 of 100 μm). Then the above aluminum electrolytic overhaul slag powder was added, and the liquid-solid mass ratio was controlled at 5:1. The mixture was slurried at a uniform speed for 25 minutes to obtain a mixed slurry.

[0037] (2) Gradient temperature rise triggering: The closed reaction system is achieved by a closed reaction vessel. The heating system is adjusted so that the temperature of the mixed slurry in the reaction vessel rises linearly and uniformly from 40℃ to 85℃ within 40 minutes.

[0038] (3) Pulsed micro-negative pressure coupling reaction: When the temperature reaches 85℃, the constant temperature leaching stage is entered, and the total duration is controlled at 2 hours. During the constant temperature period, the vacuum pump is started to perform pulsed micro-negative pressure degassing operation on the mixed slurry: the vacuum is drawn to reduce the absolute pressure in the reactor to 0.06MPa and the pressure is maintained for 4 minutes; the vent valve is opened to restore the pressure to normal (0.1MPa) instantly and maintain it for 25 minutes; the above negative pressure-normal pressure cycle operation is repeated for 4 cycles, and the leaching solution is obtained after the leaching reaction is completed.

[0039] (4) Solid-liquid separation and tailings washing: After the leaching reaction is completed, the system temperature is maintained at 80°C while it is still hot. The leaching liquid is pumped into a plate and frame filter press for solid-liquid separation and the primary leaching mother liquor is collected. The retained filter cake is washed with hot water at 75°C in a cross-flow manner. The washing liquid is combined with the primary mother liquor to obtain a clear filtrate. The tailings after washing are discharged from the system.

[0040] (5) Reactive precipitation and ligand closed-loop regeneration: The combined clarified filtrate was transferred to a precipitation crystallization vessel, and the liquid temperature was adjusted and maintained at 65°C using a heat exchanger. Synthetic cryolite seed crystals were pre-added at 0.5% of the total mass of the clarified filtrate. Stirring was started, and a 20% sodium aluminate aqueous solution was slowly and uniformly added to the vessel, with the total addition time controlled to be 30 minutes. The pH value of the system was gradually adjusted to 9.0 using an online pH meter, and the system was kept at a constant temperature and stirred for 60 minutes under this pH condition to guide the growth and precipitation of cryolite on the seed crystal surface. The matured liquid was centrifuged, and the solid phase was dried to obtain the synthetic cryolite (Na3AlF6) product. The clear liquid obtained from centrifugation was pumped back to the mixing vessel, and glacial acetic acid was added to adjust the pH to 4.8 for recycling. Example 2

[0041] This embodiment provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, including the following steps: (1) Slurry preparation and dispersion: The boric acid and gluconate complexation reaction solution obtained in Preparation Example 2 was added to the main reactor, and the jacket temperature control was turned on to maintain the system temperature at 35°C. Sodium hexametaphosphate was added at 0.05% of the total mass of the aluminum electrolytic overhaul slag to be treated (pre-crushed to a particle size D90 of 80 μm). Then the above aluminum electrolytic overhaul slag powder was added, and the liquid-solid mass ratio was controlled at 4:1. The mixture was slurried at a uniform speed for 20 minutes to obtain a mixed slurry.

[0042] (2) Gradient temperature rise triggering: The closed reaction system is achieved through a closed reaction vessel. The heating system is adjusted so that the temperature of the mixed slurry in the vessel rises linearly and uniformly from 35°C to 80°C within 30 minutes.

[0043] (3) Pulsed micro-negative pressure coupling reaction: When the temperature reaches 80℃, the constant temperature leaching stage is entered, and the total duration is controlled at 1.5 hours. During the constant temperature period, the vacuum pump is started to perform pulsed micro-negative pressure degassing operation on the mixed slurry: the vacuum is drawn to reduce the absolute pressure in the reactor to 0.08MPa and the pressure is maintained for 3 minutes; the vent valve is opened to instantly restore the pressure to normal (0.09MPa) and maintain it for 20 minutes; the above negative pressure-normal pressure cycle operation is repeated for 3 cycles, and the leaching solution is obtained after the leaching reaction is completed.

[0044] (4) Solid-liquid separation and tailings washing: After the leaching reaction is completed, the system temperature is maintained at 75°C while it is still hot. The slurry is pumped into a plate and frame filter press for solid-liquid separation and the primary leaching mother liquor is collected. The retained filter cake is washed with hot water at 70°C in a cross-flow manner. The washing liquid is combined with the primary mother liquor to obtain a clear filtrate. The washed tailings are discharged from the system.

[0045] (5) Reactive precipitation and ligand closed-loop regeneration: The combined clarified filtrate was transferred to a precipitation crystallization vessel, and the liquid temperature was adjusted and maintained at 60°C using a heat exchanger. Synthetic cryolite seed crystals were pre-added at 0.3% of the total mass of the clarified filtrate. Stirring was started, and a 15% sodium aluminate aqueous solution was slowly and uniformly added to the vessel, with a total addition time of 20 minutes. The pH of the system was gradually increased to 8.5 using an online pH meter, and the mixture was kept at a constant temperature and stirred for 45 minutes under this pH condition to induce precipitation. The matured liquid was centrifuged, and the solid phase was dried to obtain synthetic cryolite (Na3AlF). 6 The product is centrifuged; the clear liquid obtained is pumped back into the mixing tank, and glacial acetic acid is added to adjust the pH to 4.5 for recycling. Example 3

[0046] This embodiment provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, including the following steps: (1) Slurry preparation and dispersion: The complexation reaction solution of boric acid and gluconate obtained in Preparation Example 3 was added to the main reactor, and the jacket temperature control was turned on to maintain the system temperature at 45°C. Sodium hexametaphosphate was added at 0.15% of the total mass of the aluminum electrolytic overhaul slag to be treated (pre-crushed to a particle size D90 of 120 μm). Then the above aluminum electrolytic overhaul slag powder was added, and the liquid-solid mass ratio was controlled at 6:1. The mixture was slurried at a uniform speed for 30 minutes to obtain a mixed slurry.

[0047] (2) Gradient temperature rise triggering: The closed reaction system is achieved through a closed reaction vessel. The heating system is adjusted so that the temperature of the mixed slurry in the vessel rises linearly and uniformly from 45℃ to 90℃ within 50 minutes.

[0048] (3) Pulsed micro-negative pressure coupling reaction: When the temperature reaches 90℃, the constant temperature leaching stage is entered, and the total duration is controlled at 2.5 hours. During the constant temperature period, the vacuum pump is started to perform pulsed micro-negative pressure degassing operation on the mixed slurry: the vacuum is drawn to reduce the absolute pressure in the reactor to 0.05MPa and the pressure is maintained for 5 minutes; the vent valve is opened to restore the pressure to normal (0.11MPa) instantly and maintain it for 30 minutes; the above negative pressure and normal pressure cycle operation is repeated for 5 cycles, and the leaching solution is obtained after the leaching reaction is completed.

[0049] (4) Solid-liquid separation and tailings washing: After the leaching reaction is completed, the system temperature is maintained at 85°C while it is still hot. The leaching liquid is pumped into a plate and frame filter press for solid-liquid separation and the primary leaching mother liquor is collected. The retained filter cake is washed with hot water at 80°C in a cross-flow manner. The washing liquid is combined with the primary mother liquor to obtain a clear filtrate. The tailings after washing are discharged from the system.

[0050] (5) Reactive precipitation and ligand closed-loop regeneration: The combined clarified filtrate was transferred to a precipitation crystallization vessel, and the liquid temperature was adjusted and maintained at 75°C using a heat exchanger. Synthetic cryolite seed crystals were pre-added at 0.8% of the total mass of the clarified filtrate. Stirring was started, and a 25% sodium aluminate aqueous solution was slowly and uniformly added to the vessel, with the total addition time controlled to be 40 minutes. The pH value of the system was gradually adjusted to 9.5 using an online pH meter, and the mixture was kept at a constant temperature and stirred for 90 minutes under this pH condition to induce precipitation. The matured liquid was centrifuged, and the solid phase was dried to obtain synthetic cryolite (Na3AlF6) product. The clear liquid obtained from centrifugation was pumped back to the mixing vessel, and glacial acetic acid was added to adjust the pH to 5.0 for recycling. Example 4

[0051] This embodiment provides a method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, including the following steps: (1) Slurry preparation and dispersion: The complexation reaction solution of boric acid and gluconate was prepared using the same steps and parameters as in Preparation Example 1, except that sodium gluconate was replaced with commercially available industrial-grade potassium gluconate in equal molar amounts. The resulting complex solution was added to the main reactor, and the jacket temperature control was turned on to maintain the system temperature at 40°C. Sodium hexametaphosphate was added at 0.1% of the total mass of the aluminum electrolytic overhaul slag to be treated (pre-crushed to a particle size D90 of 100 μm). Then the above aluminum electrolytic overhaul slag powder was added, and the liquid-solid mass ratio was controlled at 5:1. The mixture was slurried at a uniform speed for 25 minutes to obtain a mixed slurry.

[0052] (2) Gradient temperature rise triggering: The closed reaction system is achieved through a closed reaction vessel. The heating system is adjusted so that the temperature of the mixed slurry in the vessel rises linearly and uniformly from 40℃ to 85℃ within 40 minutes.

[0053] (3) Pulsed micro-negative pressure coupling reaction: When the temperature reaches 85℃, the constant temperature leaching stage is entered, and the total duration is controlled at 2 hours. During the constant temperature period, the vacuum pump is started to perform pulsed micro-negative pressure degassing operation on the mixed slurry: the vacuum is drawn to reduce the absolute pressure in the reactor to 0.06MPa and the pressure is maintained for 4 minutes; the vent valve is opened to restore the pressure to normal (0.1MPa) instantly and maintain it for 25 minutes; the above negative pressure-normal pressure cycle operation is repeated for 4 cycles, and the leaching solution is obtained after the leaching reaction is completed.

[0054] (4) Solid-liquid separation and tailings washing: After the leaching reaction is completed, the system temperature is maintained at 80°C while it is still hot. The leaching liquid is pumped into a plate and frame filter press for solid-liquid separation and the primary leaching mother liquor is collected. The retained filter cake is washed with hot water at 75°C in a cross-flow manner. The washing liquid is combined with the primary mother liquor to obtain a clear filtrate. The tailings after washing are discharged from the system.

[0055] (5) Reactive precipitation and ligand closed-loop regeneration: The combined clarified filtrate was transferred to a precipitation crystallization vessel, and the liquid temperature was adjusted and maintained at 65°C using a heat exchanger. Synthetic cryolite seed crystals were pre-added at 0.5% of the total mass of the clarified filtrate. Stirring was started, and a 20% sodium aluminate aqueous solution was slowly and uniformly added to the vessel, with the total addition time controlled to be 30 minutes. The pH value of the system was gradually adjusted to 9.0 using an online pH meter, and the system was kept at a constant temperature and stirred for 60 minutes under this pH condition to guide the growth and precipitation of cryolite on the seed crystal surface. The matured liquid was centrifuged, and the solid phase was dried to obtain synthetic cryolite (K3AlF6 / Na3AlF6 mixed crystals) product. The clear liquid obtained from centrifugation was pumped back to the mixing vessel, and glacial acetic acid was added to adjust the pH to 4.8 for recycling.

[0056] Comparative Examples 1-6: Comparative Example 1: Compared with Example 1, the difference is that the conventional strong acid leaching method is used, without adding the complexing solution and sodium hexametaphosphate obtained in Example 1, and the sulfuric acid solution with pH less than 2.0 is directly used for constant temperature leaching at 85°C, while the other macroscopic parameters such as solid-liquid ratio and reaction time are the same.

[0057] Comparative Example 2: Compared with Example 1, the difference is that only an equal molar amount of sodium gluconate was added in the preparation of the base solution, and boric acid was not added, that is, the dual-ligand system was missing, while the rest were the same.

[0058] Comparative Example 3: Compared with Example 1, the difference is that: instead of going through the 40°C pulping and masking stage, all reagents are directly mixed with aluminum electrolysis overhaul slag at 85°C and leached at a constant temperature, while the rest are the same.

[0059] Comparative Example 4: Compared with Example 1, the difference is that the entire process is carried out under normal pressure at a constant temperature of 85°C for leaching reaction, and the vacuum pump is not started to implement pulsed micro-negative pressure degassing operation. All other aspects are the same.

[0060] Comparative Example 5: Compared with Example 1, the difference is that after leaching and separation, sodium aluminate aqueous solution is not added dropwise to the clarified filtrate to adjust the alkali, but the filtrate is directly sent to a multi-effect evaporator for conventional reduced pressure evaporation and cooling crystallization. All other aspects are the same.

[0061] Comparative Example 6: Compared with Example 1, the difference is that sodium hexametaphosphate is not added at all during the pulping stage, while everything else is the same.

[0062] Test Cases 1-6 Test Example 1: This test case provides a feasibility verification test for the thermodynamically responsive macromolecular masking-dissociation mechanism of the present invention. The specific experimental description is as follows: The experimental subject was the solid-liquid mixture slurry system in the main reactor of Example 1. During the stage of adding the complexation reaction solution of boric acid and gluconate and sodium hexametaphosphate and maintaining a constant temperature of 40°C for slurrying, the liquid phase was continuously introduced into the flow cell of the in-situ Fourier transform infrared spectrometer through a corrosion-resistant bypass circulation pipeline to continuously track the dynamic evolution of the characteristic absorption peak of BOC ester bond in the 1080 cm⁻¹ band, which characterizes the boron-polyhydroxy structure.

[0063] When the slurry in the reactor finishes pulping and begins to linearly heat up to 85°C, and then enters the subsequent pulsed micro-negative pressure constant temperature leaching stage, about 20 mL of slurry sample is extracted at preset time nodes using a pneumatic sampling valve, and the actual measured temperature of the fluid in the reactor is recorded at the moment of sample extraction.

[0064] The extracted slurry samples were immediately subjected to ice-water bath thermodynamic quenching to freeze the current chemical equilibrium, followed by high-speed centrifugation to obtain the supernatant. The ester bond integrated peak area data at the corresponding time point were retrieved from the spectrometer control system and normalized based on the initial value. Simultaneously, all quenched supernatant samples were analyzed using mannitol-sodium hydroxide standard solution titration to quantify the real-time molar concentration of free boric acid in the system.

[0065] Table 1. Changes in relative peak area of ​​ester bonds and free boric acid concentration in the liquid phase at different process stages.

[0066] according to Figure 1 , Figure 2As shown in Table 1, during the initial 40℃ pulping stage (0-25 minutes), the area of ​​the BOC characteristic peak representing the boron-polyhydroxyl masked structure in the infrared spectrum remained consistently above 98%, indicating that the free boric acid concentration was consistently suppressed at around 0.05 mol / L. In engineering practice, the surface of overhaul slag typically contains a large number of free water-soluble impurity ions. This highly stable test result confirms that the dual-ligand system constructed from boric acid and sodium gluconate spontaneously maintains a sterically hindered esterified complex at low temperatures, effectively blocking the coordination active sites that readily react with impurities. As the process transitions to a linear heating stage at 25 minutes, and the feed temperature exceeds 50℃ and continues to climb towards 85℃, the ester bond characteristic peak area begins to decline, and the free boric acid concentration exhibits a highly symmetrical nonlinear release characteristic. After 65 minutes of reaction and the onset of the 85°C pulsed isothermal leaching period, the concentration of free boric acid rapidly increased and stabilized at a plateau of around 0.65 mol / L. The amount of residual ester bonds in the system was less than 10% of its initial value. This temperature-driven chemothermodynamic response clearly demonstrates the in-situ dissociation behavior of the masked complex deep within the micropores of the solid particles. The large release of highly active gluconate and boric acid precisely achieves targeted push-pull action of the two ligands against the insoluble cryolite lattice. Dynamic data validates the feasibility of this invention in avoiding ineffective ligand consumption and ensuring the driving force for deep leaching.

[0067] Test Example 2: This test example provides a verification test for the feasibility of pH swing-induced ligand release and mother liquor loop closure of the present invention. The specific experimental description is as follows: The experimental subject was the clarified filtrate obtained after solid-liquid separation and merging of wash water using a plate and frame filter press in Example 1. This weakly acidic mother liquor with an initial pH value was transferred to a precipitation and crystallization vessel, and the jacketed circulation heating was activated. A 20% sodium aluminate aqueous solution was continuously and slowly added dropwise to the vessel using a high-precision peristaltic pump. Simultaneously, the pH of the liquid phase in the vessel was monitored in real-time throughout the entire process using an online pH meter with a digital output interface.

[0068] When the system pH value displayed on the online instrument rises sequentially to 6.0, 7.5, 8.5 and the final maturation endpoint of 9.0, about 30 mL of solid suspension slurry sample is quickly extracted from the main circulation pipeline using a pneumatic sampling valve, and the corresponding pH value at the moment of sample extraction is recorded.

[0069] High-speed centrifugation was used to separate trace amounts of the precipitated synthetic cryolite precursor solid phase from the solid slurry collected at different sampling points. The clarified liquid phase obtained from centrifugation was then collected and brought to a final volume. The total organic carbon (TOC) concentration of all clarified liquid phase samples was determined using a total organic carbon analyzer. Simultaneously, high-performance liquid chromatography (HPLC) equipped with a differential refractive index detector was used to test the same batch of liquid phase samples to quantitatively track the evolution of the true concentration of free gluconate in the system. The overall liquid phase retention rate of the multidentate ligand during the closed-loop process of the mother liquor was calculated based on the initial liquid phase data.

[0070] Table 2. Distribution data of organic ligands in the liquid phase at different pH values ​​during the precipitation reaction stage in Example 1.

[0071] according to Figure 3 According to the data in Table 2, under weakly acidic conditions with an initial pH of 4.8, the total organic carbon concentration in the liquid phase system of the clarified filtrate was at the baseline level of 38.45 g / L, while the free gluconate concentration monitored by high-performance liquid chromatography was only 12.31 g / L. This data reflects that at the end of the strong leaching stage, most of the polyhydroxy ligands still existed in the form of stable chelated complexes with aluminum ions, and did not accumulate in large quantities in the free state. In conventional industrial crystallization precipitation operations, if such a strongly coordinated system is directly subjected to physical concentration and evaporation, the high concentration of organic macromolecules often undergoes irreversible encapsulation and co-precipitation with inorganic salts, resulting in a serious excess of carbon content in the final product and loss of valuable leaching agents. As sodium aluminate solution was continuously added dropwise to the reactor, gradually shifting the overall system's pH to the weakly alkaline range, the total organic carbon concentration showed only a very slight decreasing trend, remaining at a high level of 37.18 g / L even at the final ripening pH of 9.0. The total liquid-phase retention rate of organic ligands reached as high as 96.69%. This minimal decrease directly ruled out the presence of severe organic impurities in the spontaneously precipitated solid products. The concentration of free gluconate in the liquid phase exhibited a reverse upward trend with increasing pH, eventually approaching a peak of 59.87 g / L near the alkaline endpoint. This competitive decomplexing behavior, driven by the common ion effect induced by the newly introduced aluminate and local supersaturation, caused the originally large sterically hindered macromolecular structure to lose its anchoring ability to target metal ions under a weakly alkaline environment. The desorbed inorganic ions rapidly recombine into the lattice array of the extremely insoluble cryolite solid phase, while the organic ligands that were competitively squeezed out are almost undamaged and restored to their small molecule free state, remaining in the centrifuged supernatant. Relying on this pH-sensitive chemical state transition, the chemical coordination potential of the entire cycle system can be reshaped by simply adding acid at the end of the process. This demonstrates the absolute feasibility of this process in achieving a low-cost, closed-loop regeneration pathway using expensive reagents from the perspective of coordination chemistry microkinetics.

[0072] Test Example 3: This test example provides a comparative evaluation test of the leaching rates of the core elements fluorine and aluminum in the embodiment and the comparative example. The specific experimental description is as follows: The experimental subjects were selected as raw aluminum electrolysis overhaul slag powder and the final clarified filtrate samples collected after the full-process leaching separation of Examples 1 to 4 and Comparative Examples 2 to 4. Before testing, the raw aluminum electrolysis overhaul slag powder initially added was thoroughly digested using the alkaline melting method, and after volume adjustment, the theoretical total molar amount of fluorine and aluminum in the whole system was established as a baseline.

[0073] 5.00 mL of the clarified filtrate from each set of examples and comparative examples was accurately measured using a pipette and placed in a 250 mL volumetric flask. The filtrate was then diluted to volume with 2% nitric acid solution to bring the ion concentration of the test system back to the instrument's optimal linear response range. Any trace suspended solids generated during dilution were filtered a second time through a 0.22 μm polytetrafluoroethylene syringe filter.

[0074] The total aluminum ion concentration in each dilution solution was determined using inductively coupled plasma optical emission spectrometry (ICP-OES), with the test wavelength set at 396.152 nm to avoid matrix spectral interference. Simultaneously, the total fluoride concentration was determined using a fluoride ion selective electrode method equipped with a reference electrode, with the addition of a total ionic strength adjustment buffer. The true apparent leaching rates of each core element were calculated by combining the final total volume of each liquid phase with the initial amount of raw ore input.

[0075] Table 3 shows the leaching rate data of the core elements fluorine and aluminum in the examples and comparative examples.

[0076] Figure 4 This is a comparison chart of the extraction efficiency of core elements between the embodiments and comparative examples of the present invention. Figure (a) shows the apparent leaching rate distribution of fluorine in the liquid phase products of each experimental group, and Figure (b) shows the apparent leaching rate distribution of aluminum in the liquid phase products of each experimental group. The horizontal axis represents Examples 1 to 4, which operated under different process parameter variations, while Figures 5 to 7 correspond to Comparative Examples 2 to 4, which used the controlled variable method to remove specific core steps.

[0077] According to the data in Table 3, the fluorine and aluminum leaching rates in the systems of Examples 1 to 4 remained consistently high at over 89%, with the optimal parameter combination in Example 1 even driving the fluorine leaching rate close to the theoretical limit of 96.47%. In previous field observations of treating this type of dense and highly toxic overhaul slag, it was extremely difficult to increase the extraction rate of bound fluorides to 40% by simply relying on conventional weak acid systems. This precipitous efficiency breakthrough of the proposed method presents a stark contrast. Comparative Example 2, with the boric acid ligand stripped, provides a reverse verification sample, where the fluorine and aluminum leaching rates plummeted to 22.36% and 31.74%, respectively. This data collapse directly reveals the stubbornness of insoluble cryolite (Na3AlF6) in terms of lattice energy. Although the single gluconate ion has multidentate chelating ability, it cannot overcome the dissociation thermodynamic barrier of the solid surface in the absence of the specific affinity pull of boric acid on fluoride ions. This thoroughly confirms that the dual-ligand push-pull mechanism is the absolute key to initiating deep leaching.

[0078] Comparative Example 3, which involved modifying the process temperature control logic, also exhibited a severe performance decline, with the fluorine leaching rate hovering around 64.28%. Under conditions where the temperature was directly raised to 85°C without low-temperature pulping and masking, the free hydroxyl ligands and boric acid in the system were severely consumed by the large amount of exposed water-soluble iron compounds or non-target aluminum phases on the surface before penetrating into the micropores of the overhaul slag. These side reactions deprived the chemical ammunition intended for deep cryolite extraction, and this imbalance in ligand distribution directly underscores the necessity of setting a 40°C macromolecular esterification masking stage in the examples. Shifting our focus to Comparative Example 4, the 78.45% fluorine extraction limit under atmospheric pressure leaching mode exposed the shortcomings of microfluidics in porous media reactions. The precipitation of trace amounts of aluminum carbide or aluminum nitride microbubbles at the acidic interface within the overhaul slag forms a hydrophobic gas film resistance layer on the micropore walls that is extremely difficult to dissipate with macroscopic stirring. The pulsed micro-negative pressure alternating operation introduced in the embodiment is essentially a physical pump constructed at the micrometer scale. It uses the periodic expansion and collapse of bubbles to tear apart this mass transfer barrier, forcing the highly active unmasking agent into the depth of the blind hole, thereby completely extracting the solid phase fluoride in the dead zone into the liquid phase circulation system.

[0079] Test Example 4: This test case provides a comparative evaluation of the macroscopic fluid dynamics and solid-liquid separation performance of the embodiment and the comparative example. The specific experimental description is as follows: The experimental subjects were the hot mixed slurries from Examples 1, 1, and 6, before they entered the plate and frame filter press stage after the isothermal leaching reaction. To maintain the consistency of fluid properties with those in the industrial setting, the slurries were immediately placed in a storage tank with a constant-temperature water bath jacket after extraction, maintaining the system temperature at 80°C.

[0080] A standard laboratory constant-pressure dead-end filtration test system was constructed, using industrial-grade polypropylene filter cloth with an average porosity of 30 μm as the filter medium. The air compressor was turned on and the buffer tank pressure was adjusted to precisely set the constant filtration drive differential pressure to 0.2 MPa. Each group of hot slurry was rapidly introduced into the pressurized filtration chamber through the bottom valve. A recorder was activated to collect and weigh the filtrate mass permeating the filter cloth at four discrete time points: 5 minutes, 15 minutes, 30 minutes, and 60 minutes. Combining the liquid density with the effective cross-sectional area of ​​the filter cloth, the dynamic filtration flux data of the system was calculated.

[0081] Once no more continuous droplets are filtered out from the bottom of each pressurized filtration chamber and gas permeation has occurred, the gas supply is cut off and the pressure is released. The retained filter cake is completely peeled off the surface of the filter cloth and its wet weight is measured using a high-precision electronic balance. The resulting wet filter cake is then transferred to a 105°C electric heating drying oven and baked to constant weight. The final microscopic moisture content of the filter cake is calculated and recorded based on the mass difference.

[0082] Table 4. Constant pressure filtration hydrodynamic measurement data of the leachate slurries from the examples and comparative examples.

[0083] according to Figure 5 As shown in Table 4, when handling complex overhaul slag, the conventional strong acid method and the multi-ligand targeted system of this process exhibit a drastic difference in hydrodynamics. Reviewing the field filtration behavior of Comparative Example 1, the flux experienced a catastrophic drop less than 5 minutes after valve opening, barely maintaining at 142.6 L / (m³). 2 (·h), and then at the 60-minute mark, it plummeted to less than 5.0L / (m). 2The extremely low limit of h). The intensely acidic environment not only violently tears apart the cryolite lattice, but also simultaneously strips away the various amorphous silicate networks associated with the overhaul slag. A large number of silicon atoms enter the liquid phase and hydrolyze and polymerize in a very short time, forming a transparent silica gel layer with extremely strong viscoelasticity. This gel structure tightly binds the fine solid particles to the surface of the filter cloth and seals the pores of the entire slag bed, making it difficult for water molecules to diffuse by osmotic pressure. The resulting high water holding capacity of 59.84% confirms that a large amount of the target fluorine and aluminum mother liquor is trapped deep in the waste slag. This silica gelation phenomenon is enough to completely paralyze the dehydration line of an actual plant. In Comparative Example 6, which does not add sodium hexametaphosphate as a dispersant, even though strong silica gelation is avoided, the lack of an effective Zeta potential regulator causes severe flocculation and sedimentation of micron-sized carbon particles and aluminum fluoride debris, which are compressed to form a dense filter cake layer with extremely low porosity. The flux decline is also very obvious. Example 1, employing a complete system, perfectly avoids the large-scale dissolution of background free impurities by utilizing the targeted weak acid release mechanism of boron-polyhydroxyl macromolecules. Under constant pressure, fine particles containing moderately negatively charged repulsive groups stack to form an extremely stable and highly permeable bridging microporous structure. Within this healthy fluid channel, the initial flux of Example 1 approaches 487.3 L / (m³). 2 The system can maintain a considerable washing flow rate even after an hour of accumulated interception resistance. The final moisture content of 21.63% means that the heavy metals and effective leachate attached to the capillary pores of the tailings have been efficiently replaced by the system.

[0084] Test Example 5: This test example provides a comparative evaluation test between the embodiments of the present invention and the comparative examples on the control of the purity and microscopic impurity entrainment of the resource-based products. The specific experimental description is as follows: The experimental subjects were the synthetic solid products collected at the end of the process of Example 1 (precipitated by pH swing reaction) and Comparative Example 5 (precipitated by multi-effect evaporation and vacuum concentration) and dried at constant weight at 105℃. The two groups of solid products were physically ground in agate mortars until all the powder passed through a 200-mesh standard analytical sieve. They were then placed in self-sealing bags and placed in a desiccator for 24 hours to eliminate the interference of environmental humidity.

[0085] X-ray diffractometer was used to scan the crystal structure of two sets of sieved powder samples. The test target was Cu-Kα rays, the tube voltage was set to 40 kV, the tube current to 40 mA, and the scanning angle 2θ range was set to 10° to 80°. The instrument's built-in PDF standard card database (JCPDS25-0772) was retrieved to refine and fit the main characteristic peaks of the diffraction patterns (such as the peak positions of 2θ near 32° and 46°) to obtain the absolute matching degree parameters of the Na3AlF6 phase in the product.

[0086] 0.5000g of the ground solid powder was weighed and added to a special ceramic crucible for a high-frequency infrared carbon-sulfur analyzer. A tungsten-tin flux was added, and the mixture was subjected to high-temperature induction combustion in an oxygen-rich flow environment to accurately determine the trace total carbon and total sulfur content entrained in the solid lattice. Subsequently, a separate quantitative powder sample was subjected to microwave digestion. Inductively coupled plasma mass spectrometry (ICP-MS) coupled with ion chromatography was used to quantitatively scan the ppm abundance of characteristic microscopic impurity elements such as chlorine and iron retained in the interstitial spaces of the product lattice.

[0087] Table 5. Physicochemical properties and impurity content data of the synthetic cryolite products obtained in Example 1 and Comparative Example 5.

[0088] according to Figure 6 As shown in Table 5, the aluminum industry has stringent standards for lattice integrity and impurity limits for synthetic cryolite used as a flux in electrolytic cells. In Example 1, the XRD master-slave matching degree of the precipitated product was as high as 98.74%, and the crystal configuration closely adhered to the characteristic diffraction plane of standard cryolite. In Comparative Example 5, after treating the same enriched mother liquor using a conventional thermodynamic multi-effect evaporation crystallization process, the master-slave matching degree severely deteriorated, dropping to 82.31%. The physical root cause of the crystal form deterioration lies in the large-scale escape of solvent water molecules during the evaporation phase transition stage. Various solute groups in the liquid phase system, which were originally independent, were forced to simultaneously cross the supersaturated critical surface. This non-selective forced crystallization environment induced violent primary burst nucleation, with aluminates, fluorides, and complex impurity phases competing and adhering disorderly to the crystal nucleus surface, ultimately manifesting as severe distortion of epitaxial growth and the incorporation of a large amount of amorphous phases. Compared to the macroscopic deviation in phase fit, the microscopic impurity migration data at different interfaces further revealed the differences in chemical selectivity during the precipitation process. The carbon entrainment in the solid phase of Comparative Example 5 was abnormally high at 15,842 ppm. This catastrophic organic enrichment signal revealed that the boron-gluconic acid complex ligand underwent structural collapse under long-term concentrated thermal stress. The resulting large molecular organic salt was firmly wrapped inside the inorganic salt aggregate, completely cutting off the regeneration loop of expensive leaching agent flowing back to the bottom liquid.

[0089] By comparing the impurity spectra of Example 1, it can be observed that within the reactive precipitation space constructed by the common ion effect and the gradual fluctuation of the system pH, the retention of solid carbon is strongly suppressed to the trace limit of 215 ppm. At the weakly alkaline node of 9.0, the free fluoride and aluminum ions that recombine into the new phase lattice precisely exclude the decomposed gluconate ions from the growth interface, allowing large organic molecules to achieve high-purity retention in the liquid phase. High-risk permeability impurities carried in the background of the overhaul slag ore, such as sulfur, chlorine, and active free iron, exhibit extremely low background states of 142 ppm, 87 ppm, and 45 ppm, respectively, in the cryolite product of Example 1. The solubility product parameters of these low-valence and easily soluble ions in the weakly alkaline aging solution are far from touching the nucleation and precipitation threshold, and they always maintain a highly active free state. By avoiding the concentration compression effect caused by water evaporation, the targeted recombination path thoroughly reshapes the separation boundary between the target metal element and the complex impurity group, directly severing the co-evolution chain of impurity elements without relying on complex deep purification processes.

[0090] Test Example 6: This test example provides a comparative evaluation test of the system stability of the process system of the present invention under multiple rounds of closed-loop operation. The specific experimental description is as follows: The experimental material was the clarified liquid obtained after reactive precipitation and centrifugation in Example 1. Glacial acetic acid was added to this clarified liquid to precisely adjust the pH to 4.8, serving as the regeneration base solution for the next batch of leaching reactions. A brand-new batch of homologous aluminum electrolysis overhaul slag powder was added, and the closed-loop operation was continuously carried out for five complete cycles, strictly adhering to the macroscopic operating parameters set in Example 1, including temperature gradient, pulse negative pressure, and liquid-solid ratio.

[0091] During the bottom liquid slurry preparation stage of each cycle batch, a small amount of water lost due to solid-liquid separation and evaporation is replenished through linkage between a level gauge and a high-precision flow meter. Based on the small concentration difference fed back by liquid chromatography, a very small amount of free ligand is added to maintain the absolute constant of the initial design molar ratio of the entire system.

[0092] Samples of the leaching mother liquor were collected batch by batch from the hot slurry after pressure filtration separation, and synthetic cryolite powder produced after drying in the precipitation crystallization vessel was collected simultaneously. The apparent leaching rate of fluorine in a single batch was calculated using the same inductively coupled plasma atomic emission spectrometry (ICP-AES) and fluoride ion selective electrode method as in the previous test examples, and the crystallinity matching degree of the solid phase of the corresponding batch was refined using X-ray diffraction. At the same time, the regeneration bottom liquid at the end of each batch's circulation was extracted, and the cumulative concentration of inert impurity salts, represented by sodium sulfate, in the liquid phase was determined by a combination of titration analysis and gravimetric method.

[0093] Table 6 Core performance tracking data for five consecutive batches under the process parameters of Example 1.

[0094] according to Figure 7 According to the data in Table 6, the process system of Example 1 exhibited an extremely high engineering interference resistance threshold after undergoing prolonged physical flow and chemical reset. Under conventional hydrometallurgical cycle conditions, the aqueous solvent phase inevitably accumulates various soluble inert impurities introduced from the bottom ore. This salt accumulation drastically increases the apparent viscosity of the fluid and compresses the activity coefficient of the effective solvent. In the past, similar pickling lines for overhaul slag often experienced a direct collapse of leaching kinetics due to extreme deterioration of ionic strength when operating to the third or fourth batch. Observing the dynamic feedback of this experiment, the concentration of impurity salts in the circulating mother liquor did indeed show an uncontrolled linear increase, accumulating from 12.4 g / L in the first batch to 61.3 g / L in the fifth batch. The solution environment inside the system had deviated from the initial pure water baseline. Despite the severe stress of high salinity, the core process indicators did not experience the performance decline predicted in process theory. The fluorine leaching rate of the fifth batch only slightly decreased from the initial extreme value of 96.47% to 94.88%, while the XRD fit of the cryolite main phase remained firmly within the industrial-grade range of 97.92%. The liquid phase components were able to maintain smooth reaction under a high salting-out effect, which indirectly confirms the specific coordination stability of the sterically hindered structure of the boric acid and gluconate complex reaction solution. Free impurity ions could not intervene and block the targeted dissociation channel due to spatial configuration repulsion. The multidentate ligands, which were repeatedly squeezed out and reprotonated after pH swing precipitation, did not undergo fatigue degradation at the chemical bond level. This long-lasting, high-value reagent non-destructive closed-loop system not only cut off the environmental risk of wastewater discharge, but also provided a controllable operating cost boundary for large-scale production.

Claims

1. A method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag, characterized in that, Includes the following steps: The pretreated aluminum electrolytic overhaul slag powder was added to the complexation reaction solution of boric acid and gluconate, and a dispersant was added to make a uniform pulp to obtain a mixed slurry. The reaction system is closed, and the temperature of the mixed slurry is raised linearly and uniformly to the isothermal leaching temperature. After the constant temperature leaching temperature is reached, the constant temperature leaching stage is entered. During the constant temperature period, the mixed slurry is subjected to a pulsed micro-negative pressure degassing operation that alternates between vacuuming and restoring normal pressure. After the leaching reaction is completed, the leaching liquid is obtained. While the leaching liquid is still hot, solid-liquid separation is performed to collect the leaching mother liquor and the retained tailings. The retained tailings are then subjected to cross-flow washing to obtain a washing liquid. The washing liquid and the leaching mother liquor are combined to obtain a clear filtrate. Seed crystals are added to the clarified filtrate, aluminum source solution is added dropwise at a uniform rate and the pH value of the system is adjusted upward. After constant temperature aging, solid-liquid separation is performed. The separated solid phase is dried to obtain synthetic cryolite product. The separated clear liquid is replenished with pH buffer and then recycled back to the solution preparation process.

2. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, The complexation reaction solution of boric acid and gluconate is made from raw materials containing the following components: Boric acid, gluconate, pH buffer, and water; The complexation reaction solution of boric acid and gluconate generates a structure with large steric hindrance in situ in the aqueous phase, which is used to complex and extract bound fluorides from overhaul residue under weakly acidic conditions.

3. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 2, characterized in that, The molar ratio of boric acid to gluconate is 1:(1.0-1.5); The gluconate is sodium gluconate or potassium gluconate; The pH buffer is glacial acetic acid, and the pH value of the complexation reaction solution of boric acid and gluconate is locked between 4.5 and 5.

0. The initial total solids content of the complexation reaction solution of boric acid and gluconate is 20% to 30%.

4. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 2, characterized in that, The preparation of the complexation reaction solution of boric acid and gluconate includes the following steps: Deionized water was injected into the mixing vessel and stirred at 150-250 rpm. The dry powders of boric acid and gluconate were added in sequence and mixed evenly to obtain the initial solution. The pH buffer is then pumped into the initial solution to adjust the pH value, resulting in an acid-adjusted mixture. The acid-adjusting mixture is stirred at a constant temperature of 35-45°C for 40-60 minutes to undergo a dehydration esterification condensation reaction, thereby obtaining a complex reaction solution of boric acid and gluconate.

5. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, The pretreatment involves crushing the aluminum electrolysis overhaul slag to a particle size D90 of 80–120 μm. The dispersant is sodium hexametaphosphate, and its addition amount is 0.05% to 0.15% of the total mass of aluminum electrolysis overhaul slag; the liquid-solid mass ratio is controlled at (4 to 6):1, and the pulping time is 20 to 30 minutes.

6. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, During the heating stage, the mixed slurry is linearly and uniformly heated from 35-45°C to 80-90°C within 30-50 minutes.

7. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, The temperature of the constant temperature leaching stage is 80-90℃, and the total duration is controlled between 1.5 and 2.5 hours; The specific parameters for the pulsed micro-negative pressure degassing operation are as follows: The system is evacuated to reduce the absolute pressure to 0.05–0.08 MPa, and the pressure is maintained for 3–5 minutes. Open the vent valve to instantly restore the pressure to normal (0.09–0.11 MPa) and maintain it for 20–30 minutes. The above negative pressure and normal pressure cycle operation is repeated for 3 to 5 cycles.

8. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, The solid-liquid separation temperature is 75-85℃; the cross-flow washing is performed using hot water at a temperature of 70-80℃.

9. The method for enhanced leaching and resource recovery of fluoride from aluminum electrolysis overhaul slag according to claim 1, characterized in that, The seed crystal is a synthetic cryolite seed crystal, and the amount of cryolite seed crystal added is 0.3% to 0.8% of the total mass of the clarified filtrate; the aluminum source solution is an aqueous solution of sodium aluminate with a mass concentration of 15% to 25%, and the total dropwise addition time is 20 to 40 minutes.

10. A method for enhanced fluoride leaching and resource recovery of aluminum electrolysis overhaul slag according to claim 9, characterized in that, During the process of adding the aluminum source solution and adjusting the pH of the system, the pH of the system is gradually increased to 8.5-9.5 through online pH meter linkage control. During the constant temperature curing stage, the temperature is adjusted and maintained at 60-75°C, and the curing time is 45-90 minutes. Before the separated clear liquid is recycled back, glacial acetic acid needs to be added to adjust the pH to 4.5-5.0.

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