Electrolytic aluminum melt purification and impurity removal method

Abstract

The invention discloses an electrolytic aluminum melt purification and impurity removal method, which relates to the technical field of electrolytic aluminum and comprises the following steps: (1) melt pretreatment: after electrolytic aluminum liquid is injected into a smelting furnace, a powdery deslagging agent is scattered on the surface of the electrolytic aluminum liquid, and slagging-off is performed after stirring and standing; (2) refining and deslagging by using a composite flux method: introducing the pretreated melt into a standing furnace, blowing a composite refining solvent into the melt by using argon, and standing and slagging off after the spraying, stirring and refining are finished; the composite refining solvent comprises a villiaumite system, nano-porous Al2O3 / MgO composite particles and graphene coated activated carbon in a mass ratio of 7: 2: 1; and (3) inert gas purging and refining degassing and deslagging: in a standing furnace, uniformly blowing mixed gas of argon (Ar) and CO2 into the melt to refine the melt, enhancing degassing by adopting multi-frequency ultrasound, and after refining is finished, standing and slagging off. According to the invention, efficient impurity removal is realized through the synergistic effect of multifunctional flux design and ultrasonic enhanced degassing in inert gas purging and refining, the degassing efficiency is improved, and impurity residues are reduced.

Description

Technical Field

[0001] This invention relates to the field of electrolytic aluminum technology, and more specifically to a method for purifying and removing impurities from electrolytic aluminum melt. Background Technology

[0002] Electrolytic aluminum melt is the foundation for the production of aluminum and aluminum alloys. The temperature of electrolytic aluminum melt typically reaches as high as 950℃, making the melt highly susceptible to gas absorption and slag formation. It contains numerous metallic and non-metallic inclusions and has a high gas content. If the electrolytic aluminum melt is not adequately purified, it can easily lead to defects such as porosity, looseness, and slag inclusions in the ingots, thus affecting the microstructure and properties of the processed aluminum products. Therefore, when directly using electrolytic aluminum melt to produce aluminum alloy flat ingots, it is essential to strengthen the purification process of the electrolytic aluminum melt to obtain an aluminum alloy melt with the required purity.

[0003] During the electrolysis process, due to the use of production materials such as alumina, cryolite, sodium fluoride, calcium fluoride, and carbon anodes, the electrolytic aluminum liquid contains a certain amount of alkali metal impurities such as sodium, calcium, lithium, Fe, and Si, which mainly come from raw materials, electrolytes, and tools such as anode steel claws. It also contains non-metallic oxide inclusions such as alumina (Al2O3) and aluminum fluoride (AlF3). Alumina (Al2O3) and aluminum fluoride (AlF3) mainly come from alumina and fluoride raw materials, which are prone to forming defects such as pores and looseness. The electrolytic aluminum liquid also contains gaseous impurities such as H2, CO, and CO2, of which H2 accounts for more than 85%.

[0004] Existing impurity removal methods mainly employ sodium and potassium salt-based chloride and fluoride refining agents, along with inert gas purging and refining. However, these methods have limited efficiency and unsatisfactory results, and more effective impurity removal methods are still needed. Summary of the Invention

[0005] To address the above problems, the technical problem to be solved by the present invention is to provide a method for purifying and removing impurities from electrolytic aluminum melt. This method achieves efficient impurity removal through the synergistic effect of multifunctional flux design and ultrasonic-enhanced degassing, thereby improving degassing efficiency and reducing impurity residue.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for purifying and removing impurities from electrolytic aluminum melt includes the following steps: (1) Melt pretreatment: After the electrolytic aluminum liquid is injected into the melting furnace, 0.1%-0.2% of the weight of the powdered slag remover is sprinkled onto the surface of the melt at 750°C, stirred for 5-10 minutes, and then allowed to stand for 20 minutes before removing the slag. (2) Slag removal by composite flux method The pretreated melt is introduced into a settling furnace. Argon gas is used to inject 0.3-0.5% of the composite refining solvent by weight into the melt at 750°C. The injection pressure is 0.2-0.3 MPa. The melt is stirred and refined for 5 minutes. After the injection is completed, the melt is settling for 20 minutes and then the slag is removed. The composite refining solvent includes a NaSiF6 / KAlF4 fluoride salt system with a mass ratio of 7:2:1, nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. (3) Inert gas purging, refining, degassing and slag removal In the settling furnace, a mixture of argon (Ar) and CO2 gas is uniformly blown into the melt through the permeable bricks at the bottom of the furnace to refine the melt. Ultrasonic waves are activated to enhance degassing using multi-frequency ultrasonic waves. The mixed gas is added at a rate of 60 L / min, and the refining time is 20 min. After refining, the melt is settling for 20 min before slag is removed.

[0007] As a further improvement of the present invention, the mixing volume ratio of the argon (Ar) gas to the CO2 gas mixture is 7:3.

[0008] As a further improvement of the present invention, the multi-frequency ultrasonic enhanced degassing includes first treating with low-frequency 20-30kHz ultrasound for 3-5 minutes, and then treating with high-frequency 80-100kHz ultrasound for 3-5 minutes.

[0009] As a further improvement of the present invention, the powdered slag remover comprises NaCl and KCl in a mass ratio of 1:1.

[0010] As a further improvement of the present invention, the preparation method of the fluoride salt system is as follows: Crushing: Na2SiF6 and KAlF4 were crushed and sieved separately to obtain Na2SiF6 powder and KAlF4 powder with a particle size <50 μm. The powders were dried at 150°C for 2 hours, and the moisture content was controlled to be <0.1%. Melting and mixing: Na2SiF6 powder and KAlF4 powder are mixed at a mass ratio of 6:4. In a graphite crucible, under argon protection, the temperature is increased to 700°C at 10°C / min and held for 30 minutes. Stirring is carried out to make the melt uniformly mixed. Cooling and pulverizing: Pour the melt into a stainless steel mold and allow it to cool naturally to room temperature. After coarse crushing with a jaw crusher, pulverize it to 50-100 μm with an air jet mill to obtain fluoride salt system particles.

[0011] As a further improvement of the present invention, the method for preparing the nanoporous Al2O3 / MgO composite particles is as follows: Sol preparation: Al(NO3)3·9H2O and Mg(NO3)2·6H2O were dissolved in deionized water at an Al:Mg molar ratio of 3:1 to prepare a 0.5 mol / L mixed solution. 10% of the total volume of the template agent PEG-2000 was added to the mixed solution and stirred at 500 rpm until completely dissolved. Coprecipitation reaction: Ammonia water was added dropwise at a rate of 2 mL / min to pH=9.0 in a 60°C constant temperature water bath, and stirring was continued for 2 hours to generate Al(OH)3 / Mg(OH)2 gel; Aging and drying: The gel was aged at 60°C for 12 hours, centrifuged at 8000 rpm for 10 minutes, washed three times with ethanol to remove impurity ions, and vacuum dried at 80°C for 24 hours to obtain the Al(OH)3 / Mg(OH)2 precursor. Calcination: In a tube furnace, the temperature is increased to 600°C at 3°C / min and held in air for 2 hours to obtain nanoporous Al2O3 / MgO composite particles.

[0012] As a further improvement of the present invention, the method for preparing the graphene-coated activated carbon is as follows: Acid washing: Immerse activated carbon in 3 mol / L HCl and sonicate it for 30 minutes at 400W and 40 kHz to remove ash, then dry it at 120°C for 6 hours. Dispersion preparation: Graphene oxide was dispersed in deionized water to prepare a dispersion with a concentration of 1 mg / mL. The dispersion was then subjected to ultrasonic treatment at a power of 400 W and a frequency of 40 kHz for 1 hour to obtain a uniform graphene oxide dispersion. Impregnation: The acid-washed activated carbon was impregnated in the graphene oxide dispersion, with a mass ratio of activated carbon to graphene oxide of 1:5, and stirred and dispersed for 12 hours. Reduction coating: Add hydrazine hydrate as a reducing agent, with a mass ratio of hydrazine hydrate to graphene oxide of 5:1. Recirculate at 95°C for 6 hours to reduce graphene oxide to graphene. Dry under vacuum at 80°C for 12 hours to obtain graphene-coated activated carbon.

[0013] As a further improvement of the present invention, the preparation method of the composite refining solvent is as follows: Dry mixing: The NaSiF6 / KAlF4 fluoride salt system, nanoporous Al2O3 / MgO composite particles and graphene-coated activated carbon are mixed evenly in a mixer after being proportioned in a certain ratio. Melt blending: Under argon protection, the mixture is heated to 650°C and held for 15 minutes. Then, it is rapidly cooled with a water-cooled copper roller at a cooling rate of more than 100°C / s to suppress component segregation. Surface activation treatment: The cooled blend was surface activated for 10 minutes using a radio frequency plasma instrument to obtain a composite refining solvent.

[0014] As a further improvement of the present invention, the radio frequency plasma instrument operates at a frequency of 13.56 MHz, uses Ar / O2 working gas at a ratio of 9:1, has a power of 200 W, and a vacuum of 5 Pa.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention first pre-treats the electrolytic aluminum liquid, using NaCl-KCl binary eutectic as a powdered slag remover, which can adsorb inclusions such as alumina and separate them from the aluminum liquid, thereby initially removing the slag containing non-metallic inclusions such as alumina. This invention employs a two-stage in-furnace refining process—a composite flux refining method and an inert gas purging refining method—to further remove impurities. The composite refining solvent comprises a NaSiF6 / KAlF4 fluoride salt system (mass ratio 7:2:1), nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. The fluoride salt system uses NaSiF6 and KAlF4. Na2SiF6 and KAlF4 undergo ion exchange reactions with free Na and Li ions in the molten aluminum to generate volatile fluorides (NaF, LiF). These fluorides selectively adsorb Na and Li from the melt through ion exchange reactions. Furthermore, the fluoride salts react with Ca to form insoluble fluorides (CaF2), while simultaneously adsorbing oxide inclusions (such as Al2O3) that float to the slag layer. This slag is then skimmed off, removing Na, Li, Ca, and Al from the melt. The nanoporous Al2O3 / MgO composite particles capture suspended SiO2 and Fe2O3 in the molten aluminum through surface hydroxyl adsorption sites. Tiny inclusions such as Fe and Si are deeply removed. Graphene-coated activated carbon reacts with O2 in the melt to generate CO2 / CO gas, forming a gas nucleation effect that promotes H2 aggregation and flotation, improving dehydrogenation efficiency. In the inert gas purging refining process, ultrasonic waves are used to enhance the synergistic effect of degassing and inert gas purging. Low frequency pushes large bubbles to the surface, while high frequency cavitation breaks up microbubbles, shortening the degassing time and further improving degassing efficiency and reducing impurity residue. Detailed Implementation

[0016] The specific embodiments of the present invention will be further described below with reference to examples.

[0017] Example 1

[0018] A method for purifying and removing impurities from electrolytic aluminum melt includes the following steps: (1) Melt pretreatment: After the electrolytic aluminum liquid is injected into the melting furnace, 0.1% of the weight of the powdered slag remover is sprinkled onto the surface of the melt at 750°C, stirred for 5-10 minutes, and allowed to stand for 20 minutes before removing the slag; the powdered slag remover consists of NaCl and KCl in a mass ratio of 1:1. (2) Slag removal by composite flux method The pretreated melt was introduced into a settling furnace. Argon gas was used to inject 0.3% of the composite refining solvent by weight into the melt at 750°C. The injection pressure was 0.2 MPa. The melt was stirred and refined for 5 minutes. After refining, the melt was settling for 20 minutes and then the slag was removed. The composite refining solvent includes a NaSiF6 and KAlF4 fluoride salt system with a mass ratio of 7:2:1, nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. (3) Inert gas purging, refining, degassing and slag removal Inside the settling furnace, a mixture of argon (Ar) and CO2 gas is uniformly blown into the melt through the permeable bricks at the bottom of the furnace to refine the melt. The volume ratio of the argon (Ar) to CO2 gas mixture is 7:3. Ultrasonic degassing is enhanced by multi-frequency ultrasound. First, low-frequency 20kHz ultrasound is used for 5 minutes, followed by high-frequency 80kHz ultrasound for 5 minutes. The amount of mixed gas added is 60L / min, and the refining time is 20 minutes. After refining, the melt is settling for 20 minutes before slag is removed.

[0019] The preparation method of the fluoride salt system is as follows: Crushing: Na2SiF6 and KAlF4 were crushed separately and passed through a 200-mesh sieve. The sieved powders were then dried at 150°C for 2 hours, with the moisture content controlled to be <1%. Melting and mixing: Na2SiF6 powder and KAlF4 powder are mixed at a mass ratio of 6:4. In a graphite crucible, under argon protection, the temperature is increased to 700°C at 10°C / min, and stirred and kept at the temperature for 30 minutes to make the melt uniformly mixed. Cooling and pulverizing: Pour the melt into a stainless steel mold and allow it to cool naturally to room temperature. After coarse crushing with a jaw crusher, pulverize it to 50-100 μm with an air jet mill to obtain a fluoride salt system.

[0020] The preparation method of nanoporous Al2O3 / MgO composite particles is as follows: Sol preparation: Aluminum nitrate (Al(NO3)3·9H2O, purity ≥99%) and magnesium nitrate (Mg(NO3)2·6H2O, purity ≥99%) were dissolved in deionized water at an Al:Mg molar ratio of 3:1 to prepare a 0.5 mol / L mixed solution. 10% of the total volume of the template agent PEG-2000 was added to the mixed solution and the mixture was magnetically stirred until completely dissolved. Coprecipitation reaction: In a 60°C constant temperature water bath, ammonia water (NH3·H2O, 25%) was added dropwise at a rate of 2 mL / min until pH=9.0, and the mixture was stirred continuously for 2 hours to generate Al(OH)3 / Mg(OH)2 gel. Aging and drying: After aging the gel at 60°C for 12 hours, centrifuge at 8000 rpm for 10 minutes, wash with ethanol three times to remove impurity ions, and vacuum dry at 80°C for 24 hours to obtain Al(OH)3 / Mg(OH)2 precursor; Calcination: The temperature was increased to 600°C in a tube furnace at 3°C / min, and calcined in air for 2 hours to obtain nanoporous Al2O3 / MgO composite particles.

[0021] The preparation method of graphene-coated activated carbon is as follows: Acid washing: Immerse activated carbon in 3 mol / L HCl and sonicate it for 30 minutes at 400W and 40 kHz to remove ash, then dry it at 120°C for 6 hours. Dispersion preparation: Graphene oxide was dispersed in deionized water to prepare a dispersion with a concentration of 1 mg / mL. The dispersion was then subjected to ultrasonic treatment at a power of 400 W and a frequency of 40 kHz for 1 hour to obtain a uniform graphene oxide dispersion. Impregnation: The acid-washed activated carbon was impregnated in the graphene oxide dispersion, with a mass ratio of activated carbon to graphene oxide of 1:5, and stirred and dispersed for 12 hours. Reduction coating: Add hydrazine hydrate as a reducing agent, with a mass ratio of hydrazine hydrate to graphene oxide of 5:1. Recirculate at 95°C for 6 hours to reduce graphene oxide to graphene. Dry under vacuum at 80°C for 12 hours to obtain graphene-coated activated carbon.

[0022] The preparation method of the composite refining solvent is as follows: Dry mixing: The NaSiF6 / KAlF4 fluoride salt system, nanoporous Al2O3 / MgO composite particles and graphene-coated activated carbon are mixed evenly in a mixer after being proportioned in a certain ratio. Melt blending: Under argon protection, the mixture is heated to 650°C and held for 15 minutes. Then, it is rapidly cooled with a water-cooled copper roller at a cooling rate of more than 100°C / s to suppress component segregation. Surface activation treatment: The cooled blend was surface activated for 10 minutes using a radio frequency plasma instrument with a working frequency of 13.56 MHz, working gas Ar / O2=9:1, power of 200 W, and vacuum degree of 5 Pa to obtain a composite refining solvent.

[0023] Example 2

[0024] A method for purifying and removing impurities from electrolytic aluminum melt includes the following steps: (1) Melt pretreatment: After the electrolytic aluminum liquid is injected into the melting furnace, 0.1% of the weight of the powdered slag remover is sprinkled onto the surface of the melt at 750°C, stirred for 10 min, and allowed to stand for 20 min before slag removal; the slag remover consists of NaCl and KCl in a mass ratio of 1:1. (2) Slag removal by composite flux method The pretreated melt was introduced into a settling furnace. Argon gas was used to inject 0.3% of the composite refining solvent by weight into the melt at 750°C. The injection pressure was 0.2 MPa. The melt was stirred and refined for 5 minutes. After the injection was completed, the melt was settling for 20 minutes and then the slag was removed. The composite refining solvent includes a NaSiF6 / KAlF4 fluoride salt system with a mass ratio of 7:2:1, nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. (3) Inert gas purging, refining, degassing and slag removal Inside the settling furnace, a mixture of argon (Ar) and CO2 gas is uniformly blown into the melt through the permeable bricks at the bottom of the furnace to refine the melt. The volume ratio of the argon (Ar) to CO2 gas mixture is 7:3. Ultrasonic degassing is enhanced by multi-frequency ultrasonic treatment. First, low-frequency 30kHz ultrasonic treatment is used for 5 minutes, followed by high-frequency 100kHz ultrasonic treatment for 5 minutes. The mixed gas addition rate is 60L / min, and the refining time is 20 minutes. After refining, the melt is settling for 20 minutes before slag is removed.

[0025] The preparation methods for the NaSiF6 / KAlF4 fluoride salt system, nanoporous Al2O3 / MgO composite particles, graphene-coated activated carbon, and composite refining solvent are the same as in Example 1.

[0026] Example 3

[0027] A method for purifying and removing impurities from electrolytic aluminum melt includes the following steps: (1) Melt pretreatment: After the electrolytic aluminum liquid is injected into the melting furnace, 0.2% of the weight of the powdered slag remover is sprinkled onto the surface of the melt at 750°C, stirred for 10 min, and allowed to stand for 20 min before slag removal; the slag remover consists of NaCl and KCl in a mass ratio of 1:1. (2) Slag removal by composite flux method The pretreated melt was introduced into a settling furnace. Argon gas was used to inject 0.5% of the composite refining solvent by weight into the melt at 750°C. The injection pressure was 0.3 MPa. The melt was stirred and refined for 10 minutes. After the injection was completed, the melt was settling for 20 minutes and then the slag was removed. The composite refining solvent includes a fluoride salt system with a mass ratio of 7:2:1, nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. (3) Inert gas purging, refining, degassing and slag removal In the settling furnace, a mixture of argon (Ar) and CO2 gas is uniformly blown into the melt through the permeable bricks at the bottom of the furnace to refine the melt. Ultrasonic waves are activated to enhance degassing using multi-frequency ultrasonic waves. The mixed gas is added at a rate of 60 L / min, and the refining time is 20 min. After refining, the melt is settling for 20 min before slag is removed.

[0028] The preparation methods for the NaSiF6 / KAlF4 fluoride salt system, nanoporous Al2O3 / MgO composite particles, graphene-coated activated carbon, and composite refining solvent are the same as in Example 1.

[0029] Comparative Example 1 Unlike Example 1, step (3) of the composite flux refining and slag removal only uses the Na2SiF6 / KAlF4 fluoride salt system, and the preparation method of the Na2SiF6 / KAlF4 fluoride salt system is the same as that in Example 1.

[0030] Comparative Example 2 Unlike Example 1, multi-frequency ultrasonic enhanced degassing was not used in step (3) of inert gas purging, refining, degassing and slag removal.

[0031] After impurities were removed from the electrolytic aluminum liquid, samples were taken to test the content of impurity elements. Na, Li, Ca, Fe, and Si impurities were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES / MS). Al2O3 was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). H2 content was tested using a LECO oxygen, nitrogen, and hydrogen analyzer. The test results for Examples 1-3 and Comparative Examples 1-2 are shown in Table 1.

[0032] Table 1 Test Results

[0033] The data in the table show that after impurity removal treatment, the Na and Li impurity content in the electrolytic aluminum liquid is less than 10 ppm, the Ca content is less than 20 ppm, the Si and Fe content is less than 50 ppm, the Al2O3 impurity content is less than 20 ppm, and the hydrogen content is also low, indicating a low impurity content.

[0034] The above description is a detailed description of the preferred embodiments of the present invention. However, the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modifications made under the technical spirit of the present invention should be covered by the patent of the present invention.

Claims

1. A method for purifying and removing impurities from electrolytic aluminum melt, characterized in that, It includes the following steps: (1) Melt pretreatment: After the electrolytic aluminum liquid is injected into the melting furnace, 0.1%-0.2% of the weight of the molten metal powder is sprinkled onto the surface of the molten metal at 750℃, stirred for 5-10 minutes, and then allowed to stand for 20 minutes before removing the slag. (2) Slag removal by composite flux method The pretreated melt is introduced into a settling furnace. Argon gas is used to inject 0.3-0.5% of the composite refining solvent by weight into the melt at 750°C. The injection pressure is 0.2-0.3 MPa. The melt is stirred and refined for 5-10 minutes. After refining, the melt is settling for 20 minutes and then the slag is removed. The composite refining solvent includes a NaSiF6 / KAlF4 fluoride salt system with a mass ratio of 7:2:1, nanoporous Al2O3 / MgO composite particles, and graphene-coated activated carbon. (3) Inert gas purging, refining, degassing and slag removal In the settling furnace, a mixture of argon (Ar) and CO2 gas is evenly blown into the melt through the permeable bricks at the bottom of the furnace to refine the melt. Ultrasonic waves are activated to enhance degassing using multi-frequency ultrasonic waves. The mixed gas is added at a rate of 60 L / min, and the refining time is 20 min. After refining, the melt is settling for 20 min before slag is removed.

2. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The volume ratio of the argon (Ar) to CO2 mixture is 7:

3.

3. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The multi-frequency ultrasonic enhanced degassing process includes first treating with low-frequency 20-30kHz ultrasound for 3-5 minutes, and then treating with high-frequency 80-100kHz ultrasound for 3-5 minutes.

4. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The powdered slag remover comprises NaCl and KCl in a mass ratio of 1:

1.

5. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The preparation method of the fluoride salt system is as follows: Crushing: NaSiF6 and KAlF4 were crushed separately and passed through a 200-mesh sieve. The sieved powders were then dried at 150°C for 2 hours, with the moisture content controlled to be <0.1%. Melting and mixing: Na2SiF6 powder and KAlF4 powder are mixed at a mass ratio of 6:

4. In a graphite crucible, under argon protection, the temperature is increased to 700°C at 10°C / min and held for 30 minutes. Stirring is carried out to make the melt uniformly mixed. Cooling and pulverizing: Pour the melt into a stainless steel mold, allow it to cool naturally to room temperature, coarsely crush it using a jaw crusher, and then pulverize it to 50-100 μm using an air jet mill to obtain fluoride salt system particles.

6. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The method for preparing the nanoporous Al2O3 / MgO composite particles is as follows: Sol preparation: Al(NO3)3·9H2O and Mg(NO3)2·6H2O were dissolved in deionized water at an Al:Mg molar ratio of 3:1 to prepare a 0.5 mol / L mixed solution. 10% of the total volume of the template agent PEG-2000 was added to the mixed solution and stirred at 500 rpm until completely dissolved. Coprecipitation reaction: Ammonia water was added dropwise at a rate of 2 mL / min to pH=9.0 in a 60°C constant temperature water bath, and stirring was continued for 2 hours to generate Al(OH)3 / Mg(OH)2 gel; Aging and drying: The gel was aged at 60°C for 12 hours, centrifuged at 8000 rpm for 10 minutes, washed three times with ethanol to remove impurity ions, and vacuum dried at 80°C for 24 hours to obtain the Al(OH)3 / Mg(OH)2 precursor. Calcination: In a tube furnace, the temperature is increased to 600°C at 3°C / min and held in air for 2 hours to obtain nanoporous Al2O3 / MgO composite particles.

7. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The method for preparing the graphene-coated activated carbon is as follows: Acid washing: Immerse activated carbon in 3 mol / L HCl and sonicate it at 400W and 40 kHz for 30 minutes to remove ash, then dry it at 120°C for 6 hours. Dispersion preparation: Graphene oxide was dispersed in deionized water to prepare a dispersion with a concentration of 1 mg / mL. The dispersion was then subjected to ultrasonic treatment at a power of 400W and a frequency of 40kHz for 1 hour to obtain a uniform graphene oxide dispersion. Impregnation: The acid-washed activated carbon was impregnated in the graphene oxide dispersion, with a mass ratio of activated carbon to graphene oxide of 1:5, and stirred and dispersed for 12 hours. Reduction coating: Add hydrazine hydrate as a reducing agent, with a mass ratio of hydrazine hydrate to graphene oxide of 5:

1. Recirculate at 95°C for 6 hours to reduce graphene oxide to graphene. Dry under vacuum at 80°C for 12 hours to obtain graphene-coated activated carbon.

8. The method for purifying and removing impurities from electrolytic aluminum melt according to claim 1, characterized in that, The preparation method of the composite refining solvent is as follows: Dry mixing: The NaSiF6 / KAlF4 fluoride salt system, nanoporous Al2O3 / MgO composite particles and graphene-coated activated carbon are mixed evenly in a mixer after being proportioned in a certain ratio. Melt blending: Under argon protection, the mixture is heated to 650°C and held for 15 minutes. Then, it is rapidly cooled with a water-cooled copper roller at a cooling rate of more than 100°C / s to suppress component segregation. Surface activation treatment: The cooled blend was surface activated for 10 minutes using a radio frequency plasma instrument to obtain a composite refining solvent.

9. A method for purifying and removing impurities from electrolytic aluminum melt according to claim 8, characterized in that, The radio frequency plasma instrument operates at a frequency of 13.56 MHz, with a working gas Ar / O2 ratio of 9:1, a power of 200 W, and a vacuum level of 5 Pa.

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