Method for preparing aluminum through electrolysis by using cryolite hollow microspheres

Abstract

The invention discloses a method for preparing aluminum through electrolysis by using cryolite hollow microspheres. The method comprises the following steps: (1) heating and melting aluminum oxide in the cryolite hollow microspheres; the cryolite hollow microspheres are prepared by hydrothermal reaction of a mixed solution comprising inorganic aluminum salt, organic acid sodium salt and a fluorinating reagent, and the molar ratio of aluminum ions to sodium ions in the mixed solution is 1: 2-1: 10; the molar ratio of aluminum ions to fluorine ions is 1: 0.5-1: 4, and the pH value of the mixed solution is 1-6; (2) adding an electrolyte into the molten aluminum oxide, and jointly heating to a molten state to obtain an electrolyte melt; and (3) the electrolyte melt is placed in an electrolytic cell to be electrolyzed, and molten aluminum is obtained. The cryolite microsphere with the hollow structure is prepared by adopting a hydrothermal method, and the cryolite microsphere is used for a fluxing agent in the electrolytic aluminum industry and has better mechanical stability and better fluxing performance.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytic aluminum production, and in particular to a method for electrolytic aluminum production using cryolite hollow microspheres. Background Art

[0002] Alumina is an important raw material for electrolytic aluminum production, but its melting point is very high (2054℃), which brings difficulties to electrolytic aluminum production. Artificial cryolite is a key flux for electrolytic aluminum, which can greatly reduce the melting temperature of alumina. Traditional artificial cryolite is produced by wet method, and the main methods include hydrofluoric acid method (dry method and wet method), fluosilicic acid method, carbonation method, aluminum industry recycling method, etc. The cryolite obtained by these methods has a large particle size (the particle size is generally around 15μm), and the microstructure is a regular octahedron or disordered polyhedron or irregular grain agglomeration; the apparent particle size of cryolite sold on the market is generally 1 to 10mm; these cryolites are used in the electrolytic aluminum industry, and the mechanical stability and flux are poor. At present, there is little consideration on how to produce cryolite products with new structures such as small particle size, high dispersion, and regular structure, which limits the application of cryolite in the electrolytic aluminum industry.

[0003] Chinese patent CN108975369A discloses a production process of pure cryolite without wastewater discharge, including the following steps: 1) pulping; 2) reaction; 3) centrifugal separation; 4) purification; 5) precipitation; 6) alkalization; 7) wastewater treatment. Although this process can produce pure cryolite, the preparation process is long and still needs to be operated under alkaline conditions, which consumes a large amount of alkali and is costly. If cryolite with a high molecular weight ratio can be synthesized under acidic conditions, it will not only open up a new path for the preparation of cryolite, but also can widely use waste liquid containing aluminum and fluorine-containing waste acid to prepare cryolite.

[0004] Chinese patent CN101264918A discloses a method for producing ultrafine cryolite, which uses Bayer process concentrate, caustic soda and hydrofluoric acid as raw materials to produce cryolite. Although the particle size can be controlled at 1 to 5 μm, the particle morphology is irregular and exists in the form of polyhedral grains of various sizes. In addition, the product purity is not high and contains some impurities such as AlF3, silicon oxide, and iron oxide. Summary of the invention

[0005] The present invention aims to overcome the above-mentioned problems existing in cryolite used as flux for electrolytic aluminum in the prior art, and provides a method for electrolytic aluminum production using cryolite hollow microspheres. Cryolite microspheres with hollow structures are prepared by a hydrothermal method and used as flux in the electrolytic aluminum industry, having better mechanical stability and better fluxing performance.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A method for electrolytically producing aluminum using cryolite hollow microspheres comprises the following steps: (1) adding alumina and cryolite hollow microspheres to an electrolytic cell for heating and melting; the cryolite hollow microspheres are prepared by hydrothermal reaction of a mixed solution comprising an inorganic aluminum salt, an organic acid sodium salt and a fluoridation agent, wherein the molar ratio of aluminum ions to sodium ions in the mixed solution is 1:2 to 1:10; the molar ratio of aluminum ions to fluoride ions is 1:0.5 to 1:4, and the pH value of the mixed solution is 1 to 6; (2) adding an electrolyte additive to the melted alumina and heating them together until they are molten to obtain an electrolyte melt; (3) Connecting the electrodes of the electrolytic cell to a power source and performing electrolysis to obtain aluminum liquid.

[0007] The present invention adopts a hydrothermal synthesis method to synthesize cryolite hollow microspheres, which is carried out in a kettle reaction without stirring, is easy to operate and control, and has mild reaction conditions; and the size and wall thickness of the microspheres can be effectively controlled by controlling the amount of reactants, pH and other parameters: the organic acid sodium salt in the present invention is used as a morphology regulator, which can adjust the growth rate of each crystal face, control the wall thickness, size and surface of the cryolite hollow microspheres, and play an important role in the morphology of cryolite; the fluorination agent can effectively adjust the crystal phase composition and structural morphology of cryolite; adjusting the pH value can regulate the morphology and structure of cryolite. The cryolite hollow microsphere sample prepared by the present invention has high dispersibility and regular size and morphology.

[0008] The cryolite hollow microspheres of the present invention are used as flux in the electrolytic aluminum production process, and have better mechanical stability. Moreover, due to the regular structure and high dispersibility of the cryolite hollow microspheres, they are more conducive to heat conduction and have a larger contact area with alumina powder, which lowers the melting temperature of alumina. Therefore, the cryolite hollow microspheres of the present invention are used as solvents for electrolytes in the electrolytic aluminum production process, which accelerates the melting of alumina and is more conducive to the migration of electrons during the electrolysis process, making the density of the electrolytic current lower, which can solve the problem of high energy consumption in the electrolytic aluminum industry and has good economic and social benefits.

[0009] Preferably, the hydrothermal reaction temperature for preparing cryolite hollow microspheres is 100-200° C., and the hydrothermal reaction time is 12-24 h.

[0010] Preferably, the inorganic aluminum salt is one of aluminum nitrate, aluminum chloride, aluminum sulfate and aluminum silicate.

[0011] Preferably, the organic acid sodium salt is one or two of sodium edetate, sodium acetate, sodium citrate, sodium benzoate, sodium malate and sodium gluconate.

[0012] Preferably, the fluorination agent is one or two of sodium tetrafluoroborate, hydrogen fluoride, sodium fluoride and ammonium fluoride.

[0013] Preferably, in step (1), the mass ratio of alumina to cryolite hollow microspheres is 2-7:68-88.

[0014] Preferably, the mass ratio of the electrolyte additive added in step (2) to alumina is 2-7:10-25.

[0015] Preferably, the electrolyte additive in step (2) comprises, by weight: 1 to 6 parts of calcium fluoride, 1 to 6 parts of magnesium fluoride, 2 to 3 parts of lithium fluoride, and 6 to 10 parts of potassium fluoride.

[0016] Preferably, the heating temperature during electrolysis in step (3) is 860-890°C.

[0017] Therefore, the present invention has the following beneficial effects: (1) The cryolite hollow microspheres are synthesized by hydrothermal synthesis, which is carried out in a kettle reaction without stirring, is easy to operate and control, and has mild reaction conditions; (2) Cryolite hollow microspheres are added to the electrolyte of the electrolytic aluminum production process. Due to the regular structure and high dispersibility of cryolite hollow microspheres, they are more conducive to heat conduction and have a larger contact area with alumina powder, which lowers the melting temperature of alumina. (3) Using cryolite hollow microspheres as the solvent of the electrolyte in the electrolytic aluminum production process accelerates the melting of alumina and is more conducive to the migration of electrons during the electrolysis process, making the density of the electrolytic current lower, which can solve the problem of high energy consumption in the electrolytic aluminum industry and has good economic and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the X-ray diffraction pattern of cryolite obtained in Example 1 of the present invention (standard PDF card comparison).

[0019] Figure 2 This is a SEM image of cryolite obtained in Example 3 of the present invention.

[0020] Figure 3 It is a SEM picture of cryolite obtained in Comparative Example 1 of the present invention.

[0021] Figure 4 It is a SEM picture of cryolite obtained in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0023] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are conventional methods in the art.

[0024] Overall embodiment: A method for electrolytically producing aluminum using cryolite hollow microspheres comprises the following steps: (1) adding alumina and cryolite hollow microspheres to an electrolytic cell for heating and melting; the cryolite hollow microspheres are prepared by hydrothermal reaction of a mixed solution comprising an inorganic aluminum salt, an organic acid sodium salt and a fluoridation agent, wherein the molar ratio of aluminum ions to sodium ions in the mixed solution is 1:2 to 1:10; the molar ratio of aluminum ions to fluoride ions is 1:0.5 to 1:4, and the pH value of the mixed solution is 1 to 6; (2) adding an electrolyte additive to the melted alumina and heating them together until they are molten to obtain an electrolyte melt; (3) Connecting the electrodes of the electrolytic cell to a power source and performing electrolysis to obtain aluminum liquid.

[0025] As a specific implementation method, the preparation method of cryolite hollow microspheres comprises the following steps: A) mixing and dissolving an inorganic aluminum salt and an organic acid sodium salt in deionized water to form a clear solution A; B) adding a fluorination agent into deionized water, stirring and dissolving to obtain a fluorination liquid B; C) adding fluorinated liquid B to the clear solution A and adjusting the pH to obtain a mixed solution; D) transferring the mixed solution to a reactor for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution is naturally cooled to room temperature, and the precipitate is obtained by vacuum filtration, and the precipitate is washed, centrifuged and dried to obtain the cryolite microspheres.

[0026] As a specific implementation manner, the hydrothermal reaction temperature for preparing cryolite hollow microspheres is 100-200° C., and the hydrothermal reaction time is 12-24 hours.

[0027] As a specific implementation, the inorganic aluminum salt is one of aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum silicate.

[0028] As a specific implementation, the organic acid sodium salt is one or two of sodium edetate, sodium acetate, sodium citrate, sodium benzoate, sodium malate, and sodium gluconate.

[0029] As a specific implementation, the fluorination agent is one or two of sodium tetrafluoroborate, hydrogen fluoride, sodium fluoride, and ammonium fluoride.

[0030] As a specific implementation, in step (1), the mass ratio of alumina to cryolite hollow microspheres is 2-7:68-88.

[0031] As a specific implementation, the mass ratio of the electrolyte additive added in step (2) to alumina is 2-7:10-25.

[0032] As a specific embodiment, the electrolyte additive in step (2) includes, by weight: 1 to 6 parts of calcium fluoride, 1 to 6 parts of magnesium fluoride, 2 to 3 parts of lithium fluoride, and 6 to 10 parts of potassium fluoride.

[0033] As a specific implementation, the heating temperature during electrolysis in step (3) is 860-890°C.

[0034] Embodiment 1: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium citrate were weighed in a molar ratio of 1:4 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:2 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 3 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged and precipitated, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 5.5 μm and a microsphere wall thickness of 2.7 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 880° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 78 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 10.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 890°C to obtain aluminum liquid.

[0035] Embodiment 2: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium citrate were weighed in a molar ratio of 1:5 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:2 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 1 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged to separate the precipitate, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 3-4 μm and a microsphere wall thickness of 1.2 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 870° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 78 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 10.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 880°C to obtain aluminum liquid.

[0036] Embodiment 3: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium citrate were weighed in a molar ratio of 1:8 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:4 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 3 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged to separate the precipitate, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 2-3 μm and a microsphere wall thickness of 0.5 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 860° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 80 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 8.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 861°C to obtain aluminum liquid.

[0037] Embodiment 4: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium ethylenediaminetetraacetate were weighed in a molar ratio of 1:4 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:3 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 1 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged to separate the precipitate, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 3.2 μm and a microsphere wall thickness of 1.5 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 865° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 78 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 10.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 875°C to obtain aluminum liquid.

[0038] Embodiment 5: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium citrate were weighed in a molar ratio of 1:6 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:2 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 3 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged to separate the precipitate, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 3.6 μm and a microsphere wall thickness of 1 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 860° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 79 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 9.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 865°C to obtain aluminum liquid.

[0039] Embodiment 6: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium ethylenediaminetetraacetate were weighed in a molar ratio of 1:4 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:4 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 1 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged to separate the precipitate, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 2.0 μm and a microsphere wall thickness of 1 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 860° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 80 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 8.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 862°C to obtain aluminum liquid.

[0040] Embodiment 7: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium gluconate were weighed in a molar ratio of 1:2 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:4 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 3 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged and precipitated, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 4.2 μm and a microsphere wall thickness of 2.3 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 884° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 80 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 3 parts of magnesium fluoride, 3.5 parts of lithium fluoride, and 8.5 parts of potassium fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 898°C to obtain aluminum liquid.

[0041] Embodiment 8: A method for electrolytically producing aluminum using cryolite hollow microspheres, comprising the following steps: (1) Preparation of cryolite hollow microspheres: A) Al(NO3)3·9H2O and sodium gluconate were weighed in a molar ratio of 1:4 and dissolved in deionized water to form a clear solution A; B) weighing NaBF4 at an Al / F molar ratio of 1:2 and dissolving it in deionized water to form fluorinated liquid B; C) After vigorous stirring for 30 min, slowly drop fluoride solution B into clear solution A, and add concentrated HNO3 to adjust the pH value to 1 (measured with a pH meter) to obtain a mixed solution; D) After continuing to stir for 30 min, the mixture was transferred to a Teflon bottle in a stainless steel autoclave, sealed, and kept at 180° C. for 24 h for hydrothermal reaction; E) After the hydrothermal reaction, the reaction solution was naturally cooled to room temperature, and the precipitate was obtained by vacuum filtration, which was washed with deionized water and ethanol in sequence, and finally centrifuged and precipitated, and dried at 100° C. for 12 h to obtain cryolite hollow microspheres with a particle size of 3.8 μm and a microsphere wall thickness of 2.4 μm; (2) adding alumina and prepared cryolite hollow microspheres into an electrolytic cell, heating and melting the alumina, and heating the alumina to 867° C. to form a high-temperature alumina melt; (3) adding an electrolyte additive to the melted alumina, and heating them together until they are molten to obtain an electrolyte melt; the electrolyte melt comprises, by weight, 3 parts of cryolite hollow microspheres, 3 parts of alumina, 2 parts of calcium fluoride, 2.5 parts of magnesium fluoride, 1.5 parts of lithium fluoride, 29 parts of sodium fluoride, and 59 parts of aluminum fluoride; (4) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis at 870°C to obtain aluminum liquid.

[0042] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that sodium nitrate is used instead of sodium citrate when preparing cryolite, and the rest is the same as in Example 1. The cryolite prepared in Comparative Example 1 is irregular large-grain cryolite with a particle size of 10-25 μm.

[0043] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that the pH value of cryolite is adjusted to 10 with aqueous ammonia in step C), and the rest is the same as in Example 1. The cryolite prepared in Comparative Example 2 is irregular olive-shaped long crystals with a particle size of 5×15 μm.

[0044] Comparative Example 3: The difference between Comparative Example 3 and Example 4 is that sodium nitrate is used instead of sodium ethylenediaminetetraacetate when preparing cryolite, and the rest is the same as in Example 4. The cryolite prepared in Comparative Example 3 has an irregular solid crystal structure and a particle size of 15 μm.

[0045] Comparative Example 4: The difference between Comparative Example 4 and Example 7 is that sodium nitrate is used instead of sodium gluconate when preparing cryolite, and the rest is the same as in Example 7. The cryolite prepared in Comparative Example 4 has an irregular solid crystal structure and a particle size of 18-26 μm.

[0046] The size, electrolysis temperature and alumina melting rate of the cryolite products obtained in the above examples and comparative examples were tested, and the results are shown in Table 1.

[0047] Table 1: Cryolite performance test results.

[0048] As can be seen from the above table, in the process of hydrothermal synthesis of cryolite, the morphology, size and wall thickness of cryolite can be adjusted by adjusting the amount of organic acid sodium salt, the amount of NaBF4 (Al / F molar ratio) and different organic acid sodium salts. The hollow spherical morphology of cryolite has an important influence on the performance of aluminum electrolysis by alumina. It can be seen from Comparative Examples 1-4 that the non-hollow spherical cryolite prepared by inorganic sodium salt sodium nitrate has poor performance on electrolytic aluminum, its initial crystal temperature is greater than 900°C, and the alumina melting rate is significantly lower than 0.3g / min. Organic acid sodium salt has an important influence on the structural morphology of cryolite. Organic acid sodium salt mainly plays a role in the structural assembly of cryolite microcrystals. In Comparative Examples 1, 3, and 4, since no organic acid sodium salt is added, irregular large-grain cryolite is generated. In Examples 1-8, the amount of different organic acid sodium salts added has a great influence on the wall thickness and microsphere diameter of the hollow microspheres. The amount of NaBF4 (Al / F molar ratio) also has a very important influence on the cryolite sample. In the acidic environment range of pH 1 and 3, cryolite with hollow microsphere morphology is mainly generated, while in the alkaline environment of pH = 10 (Comparative Example 2), irregular olive-shaped long grains are mainly generated, and its electrolytic aluminum performance is also poor. The method for synthesizing cryolite by this method is simple, the operating conditions are easy to control, the structural morphology of cryolite is easy to control, and cryolite microspheres with novel structures can be synthesized. The size and wall thickness of the microspheres can be adjusted. Compared with the traditional ordinary structure cryolite, the hollow microsphere cryolite can reduce the initial crystallization temperature of electrolytic aluminum and accelerate the melting of alumina.

[0049] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for electrolytic aluminum production using cryolite hollow microspheres, Its characteristics are: The steps include: (1) adding alumina and cryolite hollow microspheres to an electrolytic cell for heating and melting; the cryolite hollow microspheres are prepared by hydrothermal reaction of a mixed solution comprising an inorganic aluminum salt, an organic acid sodium salt and a fluoridation agent, wherein the molar ratio of aluminum ions to sodium ions in the mixed solution is 1:2-1:10; the molar ratio of aluminum ions to fluoride ions is 1:0.5-1:4, and the pH value of the mixed solution is 1-6; (2) adding an electrolyte additive to the melted alumina and heating them together until they are molten to obtain an electrolyte melt; (3) Connect the electrodes of the electrolytic cell to a power source and perform electrolysis to obtain aluminum liquid.

2. The method for electrolytically producing aluminum using cryolite hollow microspheres according to claim 1, Its characteristics are: The hydrothermal reaction temperature for preparing cryolite hollow microspheres is 100-200°C, and the hydrothermal reaction time is 12-24h.

3. The method for electrolytically producing aluminum using cryolite hollow microspheres according to claim 1, Its characteristics are: The inorganic aluminum salt is one of aluminum nitrate, aluminum chloride, aluminum sulfate and aluminum silicate.

4. The method for electrolytically producing aluminum using cryolite hollow microspheres according to claim 1, Its characteristics are: The organic acid sodium salt is one or two of sodium ethylenediaminetetraacetate, sodium acetate, sodium citrate, sodium benzoate, sodium malate and sodium gluconate.

5. The method for electrolytic aluminum production using cryolite hollow microspheres according to claim 1, Its characteristics are: The fluorination agent is one or two of sodium tetrafluoroborate, hydrogen fluoride, sodium fluoride and ammonium fluoride.

6. The method for electrolytic aluminum production using cryolite hollow microspheres according to claim 1, Its characteristics are: In step (1), the mass ratio of alumina to cryolite hollow microspheres is 2-7:68-88.

7. The method for electrolytic aluminum production using cryolite hollow microspheres according to claim 1, Its characteristics are: The mass ratio of the electrolyte additive added in step (2) to alumina is 2-7:10-25.

8. The method for electrolytic aluminum production using cryolite hollow microspheres according to claim 1 or 7, Its characteristics are: The electrolyte additive in step (2) includes, by weight, 1 to 6 parts of calcium fluoride, 1 to 6 parts of magnesium fluoride, 2 to 3 parts of lithium fluoride, and 6 to 10 parts of potassium fluoride.

9. The method for electrolytic aluminum production using cryolite hollow microspheres according to claim 1, Its characteristics are: The heating temperature during electrolysis in step (3) is 860-890°C.

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