High-specific-conductivity and high-purity aluminum fluoride material for low-temperature aluminum electrolysis and preparation method of high-specific-conductivity and high-purity aluminum fluoride material

Abstract

The invention relates to the technical field of aluminum electrolysis, in particular to a high-specific-conductivity and high-purity aluminum fluoride material for low-temperature aluminum electrolysis and a preparation method of the high-specific-conductivity and high-purity aluminum fluoride material. According to the method, aluminum hydroxide is used as a raw material, an aluminum oxide precursor is obtained through ternary complex sol preparation and a hydrothermal reaction, then graded impurity removal and surface modification polydopamine nanoparticle treatment are performed, particle aggregation is inhibited, finally, the aluminum oxide precursor is mixed with lithium fluoride, and a gas-solid fluorination reaction is performed. The obtained aluminum fluoride material is high in purity, large in specific surface area and developed in porous structure, shows high specific conductivity and low liquidus temperature in low-temperature aluminum electrolysis, and can effectively reduce cell voltage, improve current efficiency and achieve energy conservation and consumption reduction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aluminum electrolysis, and in particular to a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis and a preparation method thereof. BACKGROUND

[0002] Traditional aluminum fluoride materials are mainly prepared by a wet neutralization-calcination process. In this process, aluminum fluoride precursor is generated by the reaction of hydrofluoric acid with aluminum hydroxide or aluminum oxide, and the final product is obtained by high-temperature dehydration. However, this process has obvious technical limitations. First, impurity elements such as sodium, silicon and iron in the raw materials are easily introduced into the product lattice during the reaction process, and are difficult to remove completely by conventional washing, resulting in that the purity of aluminum fluoride is difficult to meet the requirements of high chemical stability for low-temperature aluminum electrolysis. Second, the precursor generated by the wet process is prone to hard agglomeration of particles during drying and calcination, resulting in a significant reduction in specific surface area, uneven pore size distribution, and thus affecting the dissolution rate and dispersion uniformity in the low-temperature electrolyte melt. In addition, the means for controlling the lattice defects of existing aluminum fluoride materials is relatively single, and there is a lack of directional optimization of ion migration channels, making it difficult to achieve a synergistic improvement of high specific conductance and low liquidus temperature under low-temperature conditions.

[0003] In the process of low-temperature aluminum electrolysis, the conductivity of the electrolyte system is directly related to the energy consumption of the cell voltage and the current efficiency, and the purity, structural activity and interface characteristics of aluminum fluoride as a key additive have a decisive influence on the performance of the electrolyte. Due to the high impurity content, serious particle agglomeration and dense structure of existing aluminum fluoride materials, the electrolyte viscosity is easily increased and the ion migration resistance is increased during the low-temperature electrolysis process, resulting in an intensified polarization phenomenon and a decreased electrolysis efficiency. At the same time, the accumulation of impurity elements also accelerates the corrosion of the electrode and the aging of the electrolyte, shortening the life of the cell. Although some researches have tried to improve the performance of aluminum fluoride by optimizing the calcination temperature or introducing additives, most of them are limited to single parameter adjustment, and the synergistic problems of impurity migration control, porous structure construction and lattice modification cannot be fundamentally solved, making it difficult to achieve a breakthrough in the comprehensive performance of the material under low-temperature electrolysis conditions. SUMMARY

[0004] Therefore, the present application aims to provide a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis and a preparation method thereof, in order to solve the problem that the existing aluminum fluoride preparation process has insufficient impurity migration control, serious particle agglomeration and lack of lattice defect regulation, resulting in low specific conductance of the material in low-temperature aluminum electrolysis, intensified electrolyte polarization, and restricting the development of low-temperature energy-saving electrolysis technology.

[0005] In order to achieve the above purpose, the present application provides a preparation method of a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis, which comprises the following steps:

[0006] (1) dispersing aluminum hydroxide powder in ultrapure water, adding anhydrous citric acid and anhydrous oxalic acid at 78-82℃ in turn and stirring for 20-40min to make them dissolve, then adding ethylene glycol and continuing to stir at 78-82℃ for 45-75min; then adding ammonia water to adjust the pH of the system to 3.2-3.8, to obtain a complex sol;

[0007] (2) hydrothermally reacting the complex sol to obtain a colloidal crystalline slurry, filtering, washing and drying to obtain an alumina precursor powder;

[0008] (3) placing the alumina precursor powder in ultrapure water, heating to 45-55℃ and stirring, adding ammonium citrate and anhydrous citric acid in turn, filtering and washing after keeping at 45-55℃ for 20-40min to obtain a primary decontaminated wet filter cake;

[0009] (4) self-oxidizing and self-polymerizing dopamine hydrochloride in an alkaline aqueous solution to obtain polydopamine nanoparticles; dispersing the polydopamine nanoparticles in anhydrous ethanol, using 2,2-azobisisobutyronitrile as an initiator under nitrogen protection, grafting polymerization of methacrylic acid and tridecafluorooctyl methacrylate at 62-68℃ to obtain surface-modified polydopamine nanoparticles;

[0010] (5) placing the primary decontaminated wet filter cake in a mixture of anhydrous ethanol and ultrapure water to form a slurry, adding the surface-modified polydopamine nanoparticles at 25℃ and continuing to stir for 30-60min, then adding ammonium citrate and heating the system to 55-65℃ for 15-30min, and then filtering and washing to obtain a secondary decontaminated wet filter cake;

[0011] (6) vacuum drying the secondary decontaminated wet filter cake to obtain a dry powder, and then calcining to obtain a porous activated precursor;

[0012] (7) placing the porous activated precursor in anhydrous ethanol and stirring while adding lithium fluoride, continuing to stir for 20-40min and ultrasonicating for 5-15min, then rotary evaporating and vacuum drying to obtain a lithium fluoride pre-distribution solid-phase mixture;

[0013] (8) uniformly spreading the lithium fluoride pre-distribution solid-phase mixture in a high-purity alumina boat and placing it downstream of a tube furnace, placing ammonium bifluoride upstream of the same reaction tube, and performing a gas-solid fluorination reaction; after the reaction is completed, the product is removed under nitrogen protection when the temperature is lowered to below 100℃, and vacuum treatment is performed to obtain a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis.

[0014] Preferably, in the step (1), the amount of ultrapure water is 550-650 mL, the amount of anhydrous citric acid is 90-110 g, the amount of anhydrous oxalic acid is 25-35 g, and the amount of ethylene glycol is 70-90 g, based on 100 g of aluminum hydroxide powder, and the mechanical stirring speed is 550-650 rpm.

[0015] Preferably, in the step (2), the hydrothermal reaction is carried out at 175-185 ℃ for 30-60 min.

[0016] Preferably, in the step (2), a polytetrafluoroethylene microporous membrane with a pore size of 1 μm is used for filtration.

[0017] Preferably, in the step (3), the amount of ultrapure water added is 900-1100 mL, the amount of ammonium citrate is 8-12 g, and the amount of anhydrous citric acid is 8-12 g, based on 100 g of aluminum oxide precursor powder, and the stirring speed is 350-450 rpm.

[0018] Preferably, in the step (4), the specific preparation steps of the polydopamine nanoparticles are as follows: tris(hydroxymethyl)aminomethane is dissolved in ultrapure water and stirred, ammonia is added to adjust the pH of the solution to 8.6-9.0, then dopamine hydrochloride is added, and the mixture is stirred at 25 ℃ for 4-8 h to obtain a polydopamine nanoparticle suspension; the precipitate is then collected by centrifugation, washed, and vacuum dried to obtain a polydopamine nanoparticle powder.

[0019] Preferably, in the step (4), the mass ratio of polydopamine nanoparticles, 2,2-azobisisobutyronitrile, methacrylic acid, and tridecafluoro-1-octyl methacrylate is 1.5-2.5:0.08-0.12:5-7:3-5.

[0020] Preferably, in the step (5), the amount of surface-modified polydopamine nanoparticles is 0.3-0.7 g, and the amount of ammonium citrate is 15-25 g, based on 100 g of aluminum oxide precursor powder.

[0021] Preferably, in the step (6), the vacuum drying is carried out at 110-130 ℃ for 8-12 h.

[0022] Preferably, in the step (6), the calcination is carried out by heating at 2 ℃ / min to 230-270 ℃ for 30-120 min, then heating at 2 ℃ / min to 380-420 ℃ for 30-120 min, and finally cooling.

[0023] Preferably, in the step (7), 40-60 mL of anhydrous ethanol is added, and the amount of lithium fluoride added is 0.20-0.45 g, based on 100 g of porous activated precursor, and the stirring speed is 250-350 rpm.

[0024] Preferably, the amount of ammonium hydrogen fluoride used in step (8) is 90-150 g, based on 100 g of the porous activated precursor.

[0025] Preferably, the distance between the two boats upstream and downstream in step (8) is 170-230 mm.

[0026] Preferably, in step (8), the gas-solid fluorination reaction is: after replacement with 180-220 mL / min of nitrogen, the temperature is raised to 210-230 DEG C for 1-3 h, then raised to 500-540 DEG C for 3-5 h, and finally raised to 760-800 DEG C for 20-60 min.

[0027] Preferably, in step (8), the vacuum treatment is: 190-210 DEG C for 1-3 h.

[0028] Further, the present application also provides a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis, which is obtained by the preparation method of the high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis.

[0029] The present application has the following advantages:

[0030] The present application realizes the simultaneous optimization of the purity and structural activity of the aluminum fluoride material by regulating the precursor crystallization process through a ternary complex system, combining the staged impurity removal and surface modification strategies. The complex sol forms uniformly distributed nanocrystalline nuclei under hydrothermal conditions, effectively inhibiting the solid solution of impurity elements in the crystal lattice, laying a foundation for the preparation of high-purity aluminum fluoride; in the staged impurity removal process, the synergistic effect of citric acid and ammonium citrate promotes the deep migration of residual impurities in the particles, and the introduction of polydopamine nanoparticles further strengthens the impurity removal effect through the selective adsorption of the surface functional groups, making the material have lower metal impurity content.

[0031] Through the steric hindrance and hydrophobic effect of the surface modified polydopamine nanoparticles, the particle agglomeration tendency of the precursor in the drying and heat treatment process is effectively inhibited, so that the aluminum fluoride material maintains a high specific surface area and a suitable pore size distribution. This porous structure is beneficial to the uniform distribution of lithium fluoride and its defect regulation in the crystal lattice, thereby enhancing the solubility and ion conduction ability of the aluminum fluoride in the low-temperature electrolyte. The porous activated precursor realizes the partial pressure control and deep conversion of ammonium hydrogen fluoride through a staged temperature rising program in the gas-solid fluorination process, avoiding the collapse of the pore channel caused by local over-reaction, so that the final product has high crystallinity and structural openness.

[0032] The pre-distribution of lithium fluoride and the synergy with the gas-solid fluorination process enable the formation of stable fluorine vacancies and lithium doping defects in the aluminum fluoride lattice, which significantly improves the specific conductivity of the material under low-temperature electrolysis conditions. At the same time, this lattice modification effect can reduce the liquidus temperature of the electrolyte melt, and promote the stability and flowability of the molten salt system at low temperature. The final aluminum fluoride material exhibits lower steady-state cell voltage and higher current efficiency in the low-temperature aluminum electrolysis cell, providing key material support for energy saving and consumption reduction in the aluminum electrolysis industry. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with specific examples.

[0034] In the specific embodiments of the present application, the aluminum hydroxide powder is a commercially available high-purity aluminum hydroxide (purity not less than 99.99%, particle size 2-10 μm); the anhydrous citric acid, anhydrous oxalic acid, ethylene glycol, anhydrous ethanol, ammonia water (mass fraction about 28%), ammonium citrate, tris(hydroxymethyl) aminomethane, dopamine hydrochloride, methacrylic acid, methacrylic acid tridecafluoro octyl ester, 2,2-azobis isobutyronitrile, lithium fluoride, ammonium bifluoride are all commercially available analytical pure or above grade reagents, wherein the lithium fluoride is preferably a high-purity reagent with a purity not less than 99.99%; the ultrapure water is ultrapure water with a resistivity not less than 18.2 MΩ·cm; the nitrogen gas is high-purity nitrogen gas with a volume fraction not less than 99.999%.

[0035] Example 1:

[0036] Step S1: weigh 100 g of aluminum hydroxide powder into a beaker, add 550 mL of ultrapure water and start mechanical stirring at 550 rpm, and heat to 78℃; at 78℃, add 90 g of anhydrous citric acid and 25 g of anhydrous oxalic acid in turn and maintain stirring at 550 rpm for 20 min to make the acid completely dissolved, then add 70 g of ethylene glycol and continue stirring at 78℃ for 45 min; then add ammonia water to adjust the pH of the system to 3.2, to obtain a complex sol;

[0037] Step S2: transfer the complex sol while hot to a polytetrafluoroethylene-lined hydrothermal reactor, with a liquid loading rate ≤70%, and react at 175℃ for 30 min in a closed condition; after the reaction is completed, cool to below 50℃ and then open the reactor to obtain a colloidal crystallization slurry; use a 1 μm polytetrafluoroethylene microporous membrane to perform suction filtration, wash the filter cake twice with ultrapure water, and then wash once with anhydrous ethanol; dry the filter cake at 80℃ under vacuum for 10 h to obtain an aluminum oxide precursor powder;

[0038] Step S3: 100 g of alumina precursor powder was weighed into a reaction bucket, 900 mL of ultrapure water was added and heated to 45℃, and stirred at 350 rpm; 8 g of ammonium citrate and 8 g of anhydrous citric acid were added in turn, and after maintaining at 45℃ for 20 min, the filter cake was washed twice with ultrapure water to obtain a primary decontaminated wet filter cake;

[0039] Step S4: 1.5 g of tris(hydroxymethyl)aminomethane was dissolved in 900 mL of ultrapure water and stirred at 550 rpm, and after adjusting the pH of the solution to 8.6 by adding ammonia water, 1.5 g of dopamine hydrochloride was added and stirred at 25℃ for 4 h to form a polydopamine nanoparticle suspension by self-oxidation and self-polymerization; then the precipitate was collected by centrifugation at 9000 rpm for 15 min, washed twice with ultrapure water and once with anhydrous ethanol, and finally dried at 50℃ under vacuum for 10 h to obtain polydopamine nanoparticle powder;

[0040] Step S5: 1.5 g of polydopamine nanoparticles was weighed into a three-necked flask, 180 mL of anhydrous ethanol was added and ultrasonically dispersed for 8 min, then 5 g of methacrylic acid and 3 g of tridecafluoro-1-octyl methacrylate were added under nitrogen protection, and 80 mg of 2,2-azobis(isobutyronitrile) was added as an initiator; the reaction was stirred at 450 rpm at 62℃ for 3 h, and after the reaction, the solid was collected by centrifugation at 9000 rpm for 15 min and washed twice with anhydrous ethanol, twice with ultrapure water, and dried at 50℃ under vacuum for 10 h to obtain surface-modified polydopamine nanoparticles;

[0041] Step S6: The wet filter cake of the primary decontaminated precursor obtained in step S3 was immediately transferred into a polypropylene reaction bucket, 320 mL of anhydrous ethanol and 140 mL of ultrapure water were added to form a slurry, and stirred at 350 rpm, 300 mg of surface-modified polydopamine nanoparticles were added at 25℃ and continued to stir for 30 min, then 400 mL of ultrapure water and 15 g of ammonium citrate were added, the system was heated to 55℃ and maintained for 15 min, then filtered, washed three times with ultrapure water and once with anhydrous ethanol to obtain a secondary decontaminated wet filter cake;

[0042] Step S7: The secondary decontaminated wet filter cake obtained in step S6 was spread on a high-purity alumina crucible, first dried at 110℃ under vacuum for 8 h to obtain a dry powder, then heated to 230℃ at a rate of 2℃ / min in an air atmosphere muffle furnace and maintained for 30 min, and then heated to 380℃ at a rate of 2℃ / min and maintained for 30 min, and then naturally cooled to obtain a porous activated precursor;

[0043] Step S8: 100 g of the porous activated precursor was weighed into a beaker, 40 mL of anhydrous ethanol was added and stirred at 250 rpm, 200 mg of lithium fluoride was added, stirring was continued for 20 min and ultrasonic treatment was performed for 5 min, then rotary evaporation was performed at 55℃ for 35 min and vacuum drying was performed at 115℃ for 1 h to obtain a lithium fluoride pre-distribution solid-phase mixture; the mixture was uniformly spread in a high-purity alumina boat and placed downstream of a tube furnace, 90 g of ammonium bifluoride was placed upstream of the same reaction tube, and the distance between the two boats was controlled at 170 mm; after nitrogen replacement at a flow rate of 180 mL / min, the temperature was raised: to 210℃ for 1 h, then to 500℃ for 3 h, and finally to 760℃ for 20 min, the nitrogen flow rate was maintained at 180 mL / min throughout the process, and an alkaline solution absorption device was connected to the tail gas end; after the reaction was completed, the product was removed under nitrogen protection when the temperature was below 100℃, and vacuum treatment was performed at 190℃ for 1 h to obtain a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis;

[0044] Example 2:

[0045] Step S1: 100 g of aluminum hydroxide powder was weighed into a beaker, 600 mL of ultrapure water was added, and mechanical stirring was started at 600 rpm; the temperature was raised to 80℃; 100 g of anhydrous citric acid and 30 g of anhydrous oxalic acid were added at 80℃, and stirring was maintained at 600 rpm for 30 min to ensure complete dissolution of the acids; then 80 g of ethylene glycol was added and stirring was continued at 80℃ for 60 min; then ammonia water was added to adjust the pH of the system to 3.5, and a complex sol was obtained;

[0046] Step S2: The complex sol was hot transferred to a polytetrafluoroethylene-lined hydrothermal reactor, and the liquid loading rate was ≤70%, and the reaction was carried out at 180℃ for 40 min; after the reaction was completed, the temperature was lowered to below 50℃, and the reactor was opened to obtain a colloidal crystalline slurry; a polytetrafluoroethylene microporous membrane with a pore size of 1 μm was used for suction filtration, and the filter cake was washed twice with ultrapure water and once with anhydrous ethanol; the filter cake was vacuum dried at 80℃ for 12 h to obtain an alumina precursor powder;

[0047] Step S3: 100 g of the alumina precursor powder was weighed into a reaction bucket, 1000 mL of ultrapure water was added, and the temperature was raised to 50℃, and stirring was performed at 400 rpm; 10 g of triammonium citrate and 10 g of anhydrous citric acid were added in sequence, and after maintaining at 50℃ for 30 min, suction filtration was performed, and the wet filter cake was washed twice with ultrapure water to obtain a primary decontamination wet filter cake;

[0048] Step S4: 2 g of tris(hydroxymethyl)aminomethane was weighed into 1000 mL of ultrapure water and stirred at 600 rpm. Ammonia water was added to adjust the pH of the solution to 8.8, then 2 g of dopamine hydrochloride was added and stirred at 25°C for 6 h to form a polydopamine nanoparticle suspension by self-oxidation and self-polymerization. Then, the precipitate was collected by centrifugation at 10000 rpm for 20 min, washed twice with ultrapure water and once with anhydrous ethanol, and finally dried at 50°C under vacuum for 12 h to obtain polydopamine nanoparticle powder;

[0049] Step S5: 2 g of polydopamine nanoparticles were weighed into a three-necked flask, ultrasonically dispersed in 200 mL of anhydrous ethanol for 10 min, and then 6 g of methacrylic acid and 4 g of tridecafluoro-octyl methacrylate were added under nitrogen protection. 100 mg of 2,2-azobis-isobutyronitrile was added as an initiator. The reaction was stirred at 500 rpm at 65°C for 4 h. After the reaction, the solid was collected by centrifugation at 10000 rpm for 20 min and washed twice with anhydrous ethanol and twice with ultrapure water. The surface-modified polydopamine nanoparticles were obtained by drying at 50°C under vacuum for 12 h;

[0050] Step S6: The wet cake of the primary decontamination precursor obtained in step S3 was immediately transferred into a polypropylene reaction barrel, 350 mL of anhydrous ethanol and 150 mL of ultrapure water were added to form a slurry, and stirred at 400 rpm. 500 mg of surface-modified polydopamine nanoparticles were added at 25°C and stirred for 45 min. Then, 500 mL of ultrapure water and 20 g of ammonium citrate were added. The system was heated to 60°C and maintained for 20 min, then filtered. The wet cake was washed three times with ultrapure water and once with anhydrous ethanol to obtain a secondary decontamination wet cake;

[0051] Step S7: The secondary decontamination wet cake obtained in step S6 was spread on a high-purity alumina crucible, dried at 120°C under vacuum for 10 h to obtain a dry powder, then heated to 250°C at a rate of 2°C / min in an air atmosphere muffle furnace for 1 h, and then heated to 400°C at a rate of 2°C / min for 1 h. The natural cooling obtained a porous activated precursor;

[0052] Step S8: 100 g of the porous activated precursor was weighed into a beaker, 50 mL of anhydrous ethanol was added and stirred at 300 rpm, 300 mg of lithium fluoride was added, and stirring was continued for 30 min and ultrasonic treatment was performed for 10 min, then rotary evaporation was performed at 60°C for 45 min and vacuum drying was performed at 120°C for 2 h to obtain a lithium fluoride pre-distribution solid-phase mixture; the mixture was uniformly spread in a high-purity alumina boat and placed downstream of a tube furnace, 120 g of ammonium bifluoride was placed upstream of the same reaction tube, and the distance between the two boats was controlled to be 200 mm; after replacement with 200 mL / min of nitrogen, heating was started: the temperature was raised to 220°C for 2 h, then to 520°C for 4 h, and finally to 780°C for 30 min, and 200 mL / min of nitrogen was maintained throughout the process, and an alkaline solution absorption device was connected to the tail gas end; after the reaction was completed, the product was removed under the protection of nitrogen when the temperature was below 100°C, and vacuum treatment was performed at 200°C for 2 h to obtain a high-specific-conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis.

[0053] Example 3:

[0054] Step S1: 100 g of aluminum hydroxide powder was weighed into a beaker, 650 mL of ultrapure water was added, and mechanical stirring was started at 650 rpm; the temperature was raised to 82°C; 110 g of anhydrous citric acid and 35 g of anhydrous oxalic acid were added in sequence at 82°C and stirring was maintained at 650 rpm for 40 min to ensure complete dissolution of the acids; then 90 g of ethylene glycol was added and stirring was continued at 82°C for 75 min; then ammonia water was added to adjust the pH of the system to 3.8 to obtain a complex sol;

[0055] Step S2: The complex sol was transferred hot to a polytetrafluoroethylene-lined hydrothermal reactor, and the liquid loading rate was ≤70%; the reactor was sealed and reacted at 185°C for 60 min; after the reaction was completed, the temperature was lowered to below 50°C, and the reactor was opened to obtain a colloidal crystalline slurry; a polytetrafluoroethylene microporous membrane with a pore size of 1 μm was used for suction filtration, and the filter cake was washed twice with ultrapure water and once with anhydrous ethanol; the filter cake was vacuum dried at 80°C for 14 h to obtain an aluminum oxide precursor powder;

[0056] Step S3: 100 g of the aluminum oxide precursor powder was weighed into a reaction bucket, 1100 mL of ultrapure water was added, and the temperature was raised to 55°C with stirring at 450 rpm; 12 g of ammonium citrate and 12 g of anhydrous citric acid were added in sequence, and after maintaining at 55°C for 40 min, suction filtration was performed, and the wet filter cake was washed twice with ultrapure water to obtain a primary decontamination wet filter cake;

[0057] Step S4: 2.5 g of tris(hydroxymethyl)aminomethane was weighed into 1100 mL of ultrapure water and stirred at 650 rpm, and ammonia water was added to adjust the pH of the solution to 9.0, then 2.5 g of dopamine hydrochloride was added and stirred at 25°C for 8h to form a polydopamine nanoparticle suspension by self-oxidation and self-polymerization; then centrifuged at 11000 rpm for 25 min to collect the precipitate, washed twice with ultrapure water and once with anhydrous ethanol, and finally dried at 50°C under vacuum for 14h to obtain polydopamine nanoparticle powder;

[0058] Step S5: 2.5 g of polydopamine nanoparticles were weighed into a three-necked flask, ultrasonically dispersed in 220 mL of anhydrous ethanol for 12 min, and then 7 g of methacrylic acid and 5 g of tridecafluorooctyl methacrylate were added under nitrogen protection, followed by the addition of 120 mg of 2,2-azobis isobutyronitrile as an initiator; stirred at 68°C and 550 rpm for 6h, and then centrifuged at 11000 rpm for 25 min to collect the solid and washed twice with anhydrous ethanol and twice with ultrapure water, and dried at 50°C under vacuum for 14h to obtain surface-modified polydopamine nanoparticles;

[0059] Step S6: The wet cake of the primary decontamination precursor obtained in step S3 was immediately transferred into a polypropylene reaction barrel, 380 mL of anhydrous ethanol and 160 mL of ultrapure water were added to form a slurry, and stirred at 450 rpm, 700 mg of surface-modified polydopamine nanoparticles were added at 25°C and continued to stir for 60 min, then 600 mL of ultrapure water and 25 g of ammonium citrate were added, the system was heated to 65°C and kept for 30 min, then filtered, washed three times with ultrapure water and once with anhydrous ethanol, to obtain a secondary decontamination wet cake;

[0060] Step S7: The secondary decontamination wet cake obtained in step S6 was spread on a high-purity alumina crucible, first dried at 130°C under vacuum for 12h to obtain a dry powder, then heated to 270°C at 2°C / min in an air atmosphere muffle furnace for 2h, and then heated to 420°C at 2°C / min for 2h, and then naturally cooled to obtain a porous activated precursor;

[0061] Step S8: Take 100 g of porous activated precursor and place it in a beaker, add 60 mL of anhydrous ethanol and stir at 350 rpm, while adding 450 mg of lithium fluoride, continue stirring for 40 min and ultrasonic for 15 min, then rotary evaporation at 65°C for 55 min and vacuum drying at 125°C for 3 h to obtain a lithium fluoride pre-distributed solid phase mixture; then evenly spread it in a high-purity alumina boat and place it downstream of the tube furnace, place 150 g of ammonium bifluoride upstream of the same reaction tube, the distance between the two boats is controlled at 230 mm; after nitrogen replacement at 220 mL / min, start heating: rise to 230°C for 3 h, then rise to 540°C for 5 h, and finally rise to 800°C for 60 min, maintain 220 mL / min of nitrogen gas throughout the process and connect a lye absorption device at the tail gas end; after the reaction is completed, cool to below 100°C under nitrogen protection and take out the product, then vacuum treat at 210°C for 3 h to obtain a high specific conductance high-purity aluminum fluoride material for low-temperature aluminum electrolysis.

[0062] Comparative Example 1:

[0063] The difference between Comparative Example 1 and Example 2 is that in Step S1, the anhydrous oxalic acid used for complexation is replaced by anhydrous citric acid, i.e. 130 g of anhydrous citric acid is added at 80°C; the rest of the conditions are consistent with Example 2.

[0064] Comparative Example 2:

[0065] The difference between Comparative Example 2 and Example 2 is that in Step S2, the closed reaction time of the reaction kettle at 180°C is changed from 40 min to 4 h; the rest of the conditions are consistent with Example 2.

[0066] Comparative Example 3:

[0067] The difference between Comparative Example 3 and Example 2 is that in Step S3, the first-stage impurity removal formula of adding 10 g of ammonium citrate and 10 g of anhydrous citric acid is changed to adding 20 g of ammonium citrate; the rest of the conditions are consistent with Example 2.

[0068] Comparative Example 4:

[0069] The difference between Comparative Example 4 and Example 2 is that in Step S5, the monomer system used for surface modification is changed from 6 g of methacrylic acid and 4 g of tridecafluoro octyl methacrylate to 10 g of methacrylic acid; the rest of the conditions are consistent with Example 2.

[0070] Comparative Example 5:

[0071] The difference between Comparative Example 5 and Example 2 is that in Step S6, the surface-modified polydopamine nanoparticles added are replaced by equal mass of polydopamine nanoparticles; the rest of the conditions are consistent with Example 2.

[0072] Comparative Example 6:

[0073] Comparative Example 6 differs from Example 2 in that no lithium fluoride is added in the lithium fluoride pre-distribution stage of step S8; the rest of the conditions are consistent with Example 2.

[0074] Comparative Example 7:

[0075] Comparative Example 7 differs from Example 2 in that in the gas-solid fluorination arrangement of step S8, the spatial separation of fluorine release upstream and fluorine reception downstream is cancelled, and the ammonium bifluoride and the lithium fluoride pre-distribution solid-phase mixture are placed in the downstream of the tube furnace; the rest of the conditions are consistent with Example 2.

[0076] Comparative Example 8:

[0077] Comparative Example 8 differs from Example 2 in that in the temperature rising and holding procedure of step S8, the segmented procedure of rising to 220°C for 2h, then rising to 520°C for 4h, and finally rising to 780°C for 30min is changed to directly rising to 780°C for 390min; the rest of the conditions are consistent with Example 2.

[0078] Performance test:

[0079] Sample preparation: the final products obtained in the examples and comparative examples are respectively taken as test samples, and the samples are uniformly treated at 200°C under vacuum conditions for 2h and cooled to room temperature in a desiccator before being sealed for storage; among them, the intrinsic characterization samples are directly taken from the above powders; the low-temperature aluminum electrolyte molten salt samples used in the application performance test are prepared according to the same reference formula: 950g of industrial cryolite and 50g of aluminum oxide are mixed and placed in a high-purity aluminum oxide crucible, heated to 930°C under nitrogen protection, and melted and held for 60min to form a reference molten salt, and then 100g of each sample aluminum fluoride powder is added to the reference molten salt in the same mass fraction, stirred for 20min and placed for 10min as the corresponding electrolyte molten salt test sample.

[0080] Chemical composition and impurity content determination: moisture content and loss on ignition were determined according to YS / T 581.1-2024 "Chemical analysis methods and physical property determination methods for aluminum fluoride Part 1: determination of moisture content and loss on ignition gravimetric method", specifically, 2.0000 sample was weighed in a constant weight dish, dried at 150℃ for 2h, cooled for 30min, then reweighed and repeated until the mass difference was not more than 0.0003g to calculate the moisture content; then the dried sample was placed in a muffle furnace, heated to 1000℃ at 10℃ / min and kept for 60min, cooled to room temperature and weighed to calculate the loss on ignition; the fluorine content was determined according to YS / T 581.3-2021 "Chemical analysis methods and physical property determination methods for aluminum fluoride Part 3: determination of fluorine content", the sample size was fixed at 0.2000g and the fluorine content was obtained by distillation and titration according to the standard procedure; the metal impurity elements (including sodium, silicon, iron) were prepared into test solution and determined according to YS / T 581.19-2023 "Chemical analysis methods and physical property determination methods for aluminum fluoride Part 19: determination of element content inductively coupled plasma atomic emission spectrometry", the external standard method was used for quantification, each sample was determined in triplicate and the arithmetic mean value was taken;

[0081] Specific surface area and pore structure parameters: nitrogen adsorption-desorption test was carried out according to GB / T 19587-2017 "Gas adsorption BET method determination of specific surface area", 0.200g of sample was placed in a sample tube, vacuum degassed at 200℃ for 6h, then isothermal adsorption test was carried out at 77K; BET linear fitting was carried out in the relative pressure range of 0.05 to 0.3 to obtain the specific surface area;

[0082] Particle size distribution and agglomeration tendency: particle size test was carried out according to GB / T 19077-2024 "Particle size analysis laser diffraction method", the dispersion medium was fixed as anhydrous ethanol, 0.100g of sample was weighed into 100mL of anhydrous ethanol, ultrasonic dispersion was carried out for 3min, then the machine was immediately tested; the circulating pump speed was fixed at 2000rpm during testing, and D50 under the output volume distribution was obtained;

[0083] Low temperature aluminum electrolyte molten salt specific conductance: the specific conductance of molten salt was determined by resistivity conversion, the high temperature resistivity test idea of GB / T 41708-2022 "Glass melt resistivity test method" was referred to construct a fluorine-resistant salt conductance cell and calibrate the cell constant; specifically, the aforementioned reference molten salt and the molten salt supplemented with each sample of aluminum fluoride were placed in a graphite crucible, after being kept at 930℃ for 30min, a coaxial electrode type conductance cell was inserted, an alternating current signal with a frequency of 1000Hz was applied, and the steady-state resistance was recorded, the resistivity was converted according to the cell constant, and the reciprocal of the resistivity was taken to obtain the specific conductance; each molten salt sample was continuously sampled five times to obtain the average value, and the test temperature fluctuation was recorded as not more than ±1℃;

[0084] Liquidus temperature: The differential scanning calorimetry test procedure was established according to YS / T 1257-2018 "Non-ferrous metal materials Melting and crystallization enthalpy test Differential scanning calorimetry method" and used for phase transition temperature characterization of molten salt samples. Specifically, about 20 mg of molten salt after adding aluminum fluoride to each sample was placed in a high-purity alumina crucible with a cover. Under argon protection, the temperature was raised from 30 to 1050 °C at a rate of 10 °C / min and held for 10 min, and then cooled to 30 °C at a rate of 10 °C / min. The starting temperature of the main endothermic peak in the heating curve was taken as the liquidus temperature. Each sample was tested in triplicate and the average value was taken;

[0085] Small test cell steady-state cell voltage and current efficiency: The parameters of the small test cell were measured and calculated according to YS / T 784-2012 "Aluminum electrolysis cell technical parameter measurement method". A graphite crucible type small test cell was used, 800 g of the corresponding molten salt sample was loaded and kept at 930 °C, the anode was made of graphite material and the cathode was made of aluminum liquid cathode. The constant current was 100 A for electrolysis for 2 h. The cell voltage was recorded every minute during the electrolysis process, and the average value of the last 30 min after entering the steady state was taken as the steady-state cell voltage. The current efficiency was calculated as the ratio of the actual aluminum production mass to the theoretical aluminum production mass, wherein the aluminum liquid was cast and weighed after electrolysis and the residual molten salt entrainment mass was deducted.

[0086] Table 1: Performance test results summary table of examples and comparative examples

[0087]

[0088] Data analysis:

[0089] As can be seen from the data of examples 1-3 in table 1, the aluminum fluoride samples prepared by the present application are at a low impurity level in terms of chemical composition and impurity control, and at the same time have a certain specific surface area and suitable particle size distribution, so that they can form more stable molten salt electrical response and phase transition temperature performance in the low temperature aluminum electrolyte system. Combined with the preparation mechanism, the ternary complex network constructed by anhydrous citric acid, anhydrous oxalic acid and ethylene glycol is beneficial to uniform nucleation and controlled growth during the hydrothermal crystallization stage, thereby providing a migratable impurity channel for subsequent fractional impurity removal; at the same time, the polydopamine nanoparticles formed by tris(hydroxymethyl) aminomethane and hydrochloric acid dopamine, and further modified by grafting of methacrylic acid and tridecafluoro octyl methacrylate, introduce a strong contrast surface chemistry of hydrophilic complexing / capturing sites and hydrophobic low surface energy sites, so that impurity removal enhancement and drying aggregation inhibition can be realized simultaneously in the same system; after porous activation and gas-solid fluorination process, the organic action disappears but the pore and dispersed skeleton are retained, finally providing a comprehensive basis of easy dissolution, low aggregation and low impurity for low temperature electrolysis application.

[0090] As can be seen from the data of Example 2 and Comparative Example 1 in Table 1, when anhydrous oxalic acid is replaced by anhydrous citric acid in step S1, the impurity control of the sample and the structural activity appear adverse changes, and then the electrical and phase change indicators of the low-temperature molten salt system are weakened as a whole. The main reason is that anhydrous oxalic acid is more conducive to providing short-range strong complexation and bridging effect in the ternary complex network, which can constrain nucleation-growth in more uniform microzones and form more continuous mesoporous channels; after replacement, the complex structure is more inclined to long-chain multi-site complexation, and the crystallization and subsequent impurity removal migration are more prone to local differences, finally the synergistic effect of pore retention and impurity removal migration is reduced.

[0091] As can be seen from the data of Example 2 and Comparative Example 2 in Table 1, after significantly prolonging the closed reaction time of step S2, the particle size and pore structure of the material are more inclined to crystal growth and densification, thereby weakening the dissolution and mass transfer advantage in the low-temperature molten salt, and the key indicators of electrolytic application are followed. The main reason is that the crystallization time is too long, which makes the crystal change from multi-point nucleation and restricted growth to preferential growth and perfect crystal face, although it may bring local point improvement in purity, but the pore connectivity and the number of external surface active sites are reduced, which limits the lithium fluoride pre-distribution and gas-solid fluorination depth, resulting in that defect regulation and structural accessibility cannot be fully superimposed, which shows that there is a significant coupling window between the crystallization time and the subsequent structure retention.

[0092] As can be seen from the data of Example 2 and Comparative Example 3 in Table 1, when anhydrous citric acid is cancelled in step S3 and only ammonium citrate is used for primary impurity removal, the impurity migration and structural stability are out of sync, which makes it difficult to maintain the optimal low-temperature electrolytic performance. The main reason is that the combination of anhydrous citric acid and ammonium citrate not only provides complexation, but also promotes the outward migration of residual impurities inside the particles under a certain acidity gradient; when ammonium citrate is used alone, the system is more inclined to mild complexation, and the internal migration driving force is insufficient although the surface removal is relatively easy; the subsequent secondary impurity removal and surface modification can partially make up for it, but it is difficult to achieve the superposition effect of first constructing a local complex interface and then performing solution chelation migration in Example 2, which shows that the synergy of staged impurity removal formula cannot be simplified.

[0093] As can be seen from the data of Example 2 and Comparative Example 4 in Table 1, when the monomer system of step S5 is changed from methacrylic acid and tridecafluoro octyl methacrylate to only methacrylic acid, although the impurity control may appear point improvement, but the low-temperature electrolytic application related indicators do not reach the level of Example 2. The main reason is that the carboxyl provided by methacrylic acid is conducive to hydrophilic complexing / capturing impurities, but lacks the hydrophobic low surface energy / anti-capillary bridge effect brought by tridecafluoro octyl methacrylate, which makes it easier to produce capillary bridge induced agglomeration during the drying process, resulting in that the pore and dispersed skeleton are difficult to be completely retained; therefore, the impurity removal enhancement and the agglomeration inhibition are not a simple linear relationship.

[0094] As can be seen from the data of Example 2 and Comparative Example 5 in Table 1, when the surface-modified polydopamine nanoparticles are replaced by polydopamine nanoparticles in step S6, the coupling between the de-impurity and the structural stability of the material is weakened, which is manifested as the overall downward trend of the low-temperature electrolysis performance related indicators. The main reason is that the surface-modified polydopamine nanoparticles of Example 2 introduce strong contrast surface chemistry of carboxyl and perfluoroalkyl through grafting, on the one hand, enhancing the interface trapping and migration, on the other hand, reducing the agglomeration caused by dry capillary bridge; after being replaced by polydopamine nanoparticles, a certain adsorption capacity is still retained, but the hydrophobic low surface energy support is lacking, and the pore and dispersed skeleton are more prone to collapse during drying and heat treatment.

[0095] As can be seen from the data of Example 2 and Comparative Example 6 in Table 1, when lithium fluoride is not added in step S8, the chemical impurities and structural parameters are close as a whole, but the key indicators of electrolytic application appear systematic decline, indicating that lithium fluoride is not dispensable. The main reason is that the pre-distribution of lithium fluoride in solid phase and the embedding in the pore of Example 2 make it easier to form uniform defect regulation and ion migration channel in the subsequent gas-solid fluorination and crystal phase fixation process; when lithium fluoride is lacking, even if it still has continuous pores and lower agglomeration, it is difficult to obtain the same conductivity contribution and liquidus line synergistic reduction effect in low-temperature molten salt.

[0096] As can be seen from the data of Example 2 and Comparative Example 7 in Table 1, when the spatial separation of upstream fluorine release and downstream fluorine receiving is cancelled in step S8, the low-temperature molten salt specific conductivity and liquidus temperature show obvious deterioration, the steady-state cell voltage increases and the current efficiency decreases. The main reason is that spatial separation can control the partial pressure and diffusion depth of ammonium hydrogen fluoride release fluorine, and can inhibit the internal diffusion bottleneck caused by surface instantaneous densification through staged heating; after cancellation, ammonium hydrogen fluoride is mixed with solid phase, which is more likely to cause local high partial pressure and local reaction too fast, resulting in outer layer densification and inner layer limitation, which not only affects the uniformity of transformation but also induces agglomeration sintering, so that the pore advantage and lithium fluoride pre-distribution advantage cannot be realized.

[0097] As can be seen from the data of Example 2 and Comparative Example 8 in Table 1, after changing the staged heating program of step S8 to direct high-temperature long-time holding, although the burn-off and fluorine content may present a closer point-like advantage of anhydrous, the specific surface area and particle size distribution are significantly deteriorated, resulting in that the low-temperature electrolysis application related indicators are still obviously not superior. The main reason is that the activation, complete fluorination and crystal phase fixation of the staged program can control the hydrogen fluoride partial pressure and diffusion while inhibiting sintering; while long-time high-temperature can strengthen the grain growth and pore collapse, so that the structural accessibility formed by surface modification and porous activation is weakened, and then the dissolution and mass transfer limitation at low temperature is magnified, which is manifested as an anomaly that the chemical is more like anhydrous but the structure is more inactivated, thereby proving the synergistic optimality of Example 2 in structure and chemistry.

[0098] Those skilled in the art should understand that the above discussion of any embodiment is only intended to be exemplary in nature and is not intended to suggest that the present application is limited to these examples; under the concept of the present application, the above embodiments or technical features among different embodiments can be combined, steps can be implemented in any order, and there are many other variations of different aspects of the present application as described above, which are not provided in details for the sake of brevity.

Claims

1. A method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis, characterized in that, Includes the following steps: (1) Disperse aluminum hydroxide powder in ultrapure water, add anhydrous citric acid and anhydrous oxalic acid in sequence at 78-82℃ and stir for 20-40 min to dissolve them, then add ethylene glycol and continue stirring at 78-82℃ for 45-75 min; then add ammonia water to adjust the pH of the system to 3.2-3.8 to obtain complex sol; (2) The complexed sol was subjected to a hydrothermal reaction to obtain a colloidal crystallized slurry, which was then filtered, washed and dried to obtain alumina precursor powder. (3) Place the alumina precursor powder in ultrapure water, heat it to 45-55℃ and stir, add triammonium citrate and anhydrous citric acid in sequence, keep it at 45-55℃ for 20-40 min and then filter and wash to obtain a wet filter cake with primary impurity removal. (4) Dopamine hydrochloride is self-oxidized and self-polymerized in an alkaline aqueous solution to obtain polydopamine nanoparticles; the polydopamine nanoparticles are dispersed in anhydrous ethanol, and methacrylic acid and tridecafluorooctyl methacrylate are grafted and polymerized at 62-68°C under nitrogen protection using 2,2-azobisisobutyronitrile as an initiator to obtain surface-modified polydopamine nanoparticles; (5) The wet filter cake after primary impurity removal is placed in a mixture of anhydrous ethanol and ultrapure water to form a slurry. The surface-modified polydopamine nanoparticles are added at 25°C and the mixture is stirred for 30-60 min. Then, triammonium citrate is added and the system is heated to 55-65°C and maintained for 15-30 min. After filtration and washing, the wet filter cake after secondary impurity removal is obtained. (6) The wet filter cake after secondary impurity removal is vacuum dried to obtain dry powder, and then calcined to obtain a porous activated precursor; (7) The porous activated precursor was placed in anhydrous ethanol and stirred while lithium fluoride was added. The stirring was continued for 20-40 min and ultrasonicated for 5-15 min. Then, it was rotary evaporated and vacuum dried to obtain a lithium fluoride pre-distributed solid mixture. (8) The lithium fluoride pre-distributed solid mixture is evenly spread in a high-purity alumina boat and placed downstream of a tube furnace. Ammonium bifluoride is placed upstream of the same reaction tube to carry out a gas-solid fluorination reaction. After the reaction is completed, the product is taken out under nitrogen protection and cooled to below 100°C. The product is then vacuum treated to obtain a high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis. In step (4), the mass ratio of polydopamine nanoparticles, 2,2-azobisisobutyronitrile, methacrylic acid and tridecafluorooctyl methacrylate is 1.5-2.5:0.08-0.12:5-7:3-5; Based on 100g of alumina precursor powder, in step (5), the amount of surface-modified polydopamine nanoparticles is 0.3-0.7g, and the amount of triammonium citrate is 15-25g.

2. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (1), based on 100g of aluminum hydroxide powder, the amount of ultrapure water is 550-650mL, the amount of anhydrous citric acid is 90-110g, the amount of anhydrous oxalic acid is 25-35g, the amount of ethylene glycol is 70-90g, and the mechanical stirring speed is 550-650rpm.

3. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (2), the hydrothermal reaction is carried out at 175-185℃ in a closed environment for 30-60 minutes.

4. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (3), based on 100g of alumina precursor powder, 900-1100mL of ultrapure water is added, the amount of triammonium citrate is 8-12g, the amount of anhydrous citric acid is 8-12g, and the stirring speed is 350-450rpm.

5. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (4), the specific preparation steps of polydopamine nanoparticles are as follows: tris(hydroxymethyl)aminomethane is dissolved in ultrapure water and stirred. Ammonia water is added to adjust the pH of the solution to 8.6-9.0, and then dopamine hydrochloride is added. The mixture is stirred in an open container at 25°C for 4-8 hours to obtain a polydopamine nanoparticle suspension. The precipitate is then collected by centrifugation, washed, and dried under vacuum to obtain polydopamine nanoparticle powder.

6. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (6), calcination is performed by heating to 230-270℃ at 2℃ / min in air atmosphere and holding for 30-120min, then heating to 380-420℃ at 2℃ / min and holding for 30-120min before cooling.

7. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (7), 40-60 mL of anhydrous ethanol is added based on 100 g of porous activated precursor, and the amount of lithium fluoride added is 0.20-0.45 g. The stirring speed is 250-350 rpm.

8. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, Based on 100g of porous activated precursor, the amount of ammonium bifluoride used in step (8) is 90-150g.

9. The method for preparing high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis according to claim 1, characterized in that, In step (8), the gas-solid fluorination reaction is as follows: after purging with 180-220 mL / min of nitrogen, the temperature is raised to 210-230℃ and kept for 1-3 hours, then raised to 500-540℃ and kept for 3-5 hours, and finally raised to 760-800℃ and kept for 20-60 minutes.

10. A high-specific-conductivity, high-purity aluminum fluoride material for low-temperature aluminum electrolysis, characterized in that, It is obtained by the preparation method of high specific conductivity and high purity aluminum fluoride material for low temperature aluminum electrolysis as described in any one of claims 1-9.

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