Method for producing high-purity aluminum, production device, production system, and high-purity aluminum

Abstract

This method for producing high-purity aluminum has a step for disposing an Al crude metal anode and a cathode so as to face each other in a highly electroconductive molten salt, supplying electricity between the Al crude metal anode and the cathode at a temperature at which the Al crude metal anode is solid and the highly electroconductive molten salt is liquid to ionize and elute aluminum from the Al crude metal anode, and depositing an aluminum deposit on the cathode.

Description

High-purity aluminum manufacturing method, manufacturing equipment, manufacturing system, and high-purity aluminum

[0001] The present invention relates to a method for producing high-purity aluminum, an apparatus for producing high-purity aluminum, high-purity aluminum produced by the method or apparatus, and a system for producing high-purity aluminum configured to be operable as a single system. This application claims priority based on Japanese Patent Application No. 2022-073786, filed on April 27, 2022, the contents of which are incorporated herein by reference.

[0002] In the global aluminum market, the majority of aluminum products are typically cast aluminum, and pure aluminum is extremely rare; most is aluminum alloy containing fairly high concentrations of Cu, Fe, Mg, Mn, Si, Zn, etc. Meanwhile, the Aluminum Statistics Annual Report reports that the domestic shipment volume of high-purity aluminum used in electrolytic capacitors, magnetic disks, etc. as capacitor foil was just under 31,000 tons (2014).

[0003] The purity of virgin aluminum ingots (hereinafter, in this specification, such aluminum will be referred to as "virgin aluminum ingots") produced from bauxite, an aluminum raw material, by the Bayer process (alumina extraction) or the Hall-Hell process (molten salt electrolysis) varies depending on the production area and grade, but is generally in the range of 99.5 to 99.9%. There are two types of grades of virgin aluminum ingots: those that specify a minimum aluminum content, and those that specify the concentrations of individual components but do not specify a minimum aluminum content.

[0004] The chemical composition of high-purity aluminum used in electrolytic capacitors, magnetic disks, etc. is specified in the standard (JIS H4170) shown in Table 1.

[0005]

[0006] According to the standards in Table 1, in the case of A1N90H-H18, the total of Si, Fe, and Cu must be 1,300 ppm or less, and in the case of A1N99H-H18, the total of Si, Fe, and Cu must be 180 ppm or less. According to the component standards in Table 1, in the case of general-purpose new aluminum ingots with a purity of 99.6 to 99.7%, the total of Si, Fe, and Cu is 4,000 to 5,000 ppm, so it is necessary to remove these to about one-fifth to one-fiftieth.

[0007] Aluminum is a chemically active metal, and while the concentration of target elements is much lower than that of cast metal scrap, the difficulty of removal remains essentially the same. Due to the difficulty of aluminum refining, various specialized technologies have been used to industrially refine high-purity aluminum foil from virgin metal. Current technologies can be broadly divided into (i) fractional crystallization, (ii) directional solidification, and (iii) three-layer electrolysis, which utilize segregation phenomena during solidification. (i) Fractional crystallization and (ii) directional solidification utilize the difference in equilibrium solute concentrations between the solid (solidified phase) and liquid (unsolidified liquid phase) during solidification. That is, the equilibrium solute concentration in the solid is higher than that in the liquid, resulting in a solid-liquid equilibrium distribution ratio generally less than 1. (i) Fractional crystallization involves repeated crystallization and separation of primary crystals with high aluminum purity during the initial solidification stage. (ii) directional solidification involves applying an extremely slow cooling rate to control the direction of solidification and concentrate the solute in the unmelted portion, thereby achieving refinement.

[0008] (iii) As a specific example of the three-layer electrolysis method, for example, Non-Patent Document 1 discloses the structure of an electrolytic refining furnace, electrolytic bath (mass%), electrolysis temperature, etc., but it also discloses a method of using the difference in specific gravity to add a predetermined amount of copper to the aluminum virgin metal from the lower layer to increase its specific gravity, and a method of adding BaF, which has a higher specific gravity than pure aluminum, to the aluminum virgin metal from the lower layer. ２Electrolysis is carried out in a three-layer sandwich structure consisting of an aluminum-copper alloy molten salt phase and a pure aluminum phase. When electrolysis is carried out in this cell structure with the aluminum-copper alloy as the anode and the pure aluminum phase in the upper layer as the cathode, essentially only aluminum migrates from the anode side to the cathode side via the molten salt, so high-purity aluminum is obtained on the cathode side. The technical key is to control the specific gravity of the molten salt phase so that the anode and cathode phases do not mix, but the refining effect is great.

[0009] International Publication No. 2020 / 196013

[0010] Materia, Vol. 33, No. 1, 1994

[0011] (i) Fractional crystallization and (ii) unidirectional solidification are common methods for purifying substances, and although they can be expected to produce satisfactory purification results, they have two major drawbacks: low yield and high purification costs, due to the fact that concentrated solutes always remain and that temperature control at high temperatures is required for long periods of time.

[0012] Although the chemical composition of the raw material ingots varies, purity information for general-purpose aluminum virgin ingots with a purity of 99.6 to 99.7% refined by three methods (i) to (iii) is shown in Table 2.

[0013]

[0014] Compared to fractional crystallization and unidirectional solidification, the triple-layer electrolysis method produces high-purity aluminum with impurity concentrations about one order of magnitude lower. However, both methods suffer from low yields (segregation method) and high manufacturing costs. A key issue is how to reduce the energy costs, especially with the triple-layer electrolysis method, which has a significant refining effect.

[0015] On the other hand, Patent Document 1 describes a method in which an anode electrode containing 0.01 to 30 mass % of Si and Al and a cathode electrode are immersed in an electrolytic solution, and current is passed between the anode electrode and the cathode electrode to deposit aluminum on the cathode electrode (see claim 1), thereby obtaining a high-purity aluminum material from a low-purity aluminum material (anode electrode) (see paragraph 0010). Furthermore, the document lists an organic molten salt containing an alkylimidazolium halide and an aluminum halonide as a preferred electrolytic solution, with 1-ethyl-methylmidazolium chloride (EMIC) as a specific example of the alkylimidazolium halide and aluminum chloride (AlCl) as a specific example of the aluminum halonide. ３ ) is listed (see paragraph 0027). This method allows high-purity aluminum to be obtained from low-purity aluminum, but it is not easy to obtain high-purity aluminum of 99.9% or more. In addition, EMIC has a conductivity of 1 to 2 Sm －１ (25°C) and the conductivity of the LiCl-KCl eutectic salt used in the examples described later (187 Sm －１ (500°C)), which is two orders of magnitude smaller than that of AlCl. ３ Since AlCl has a very high vapor pressure at the electrolysis temperature, only a sealed electrolysis device can be used. ３ The bath composition is easily changed by evaporation of the metal, making it unsuitable for continuous electrolysis.

[0016] The present invention has been made in consideration of the above circumstances, and aims to provide a method, an apparatus, and a system for producing high purity aluminum of 99.9% or more, and high purity aluminum.

[0017] In order to solve the above problems, the present invention provides the following means.

[0018] A first aspect of the present invention is a method for producing high-purity aluminum, comprising the steps of: disposing an Al crude metal anode (hereinafter referred to as "Al crude metal anode") to be purified and a cathode facing each other in a highly conductive molten salt; and applying a current between the Al crude metal anode and the cathode at a temperature at which the Al crude metal anode is in a solid state and the highly conductive molten salt is in a liquid state, thereby ionizing and eluting aluminum from the Al crude metal anode and depositing an aluminum precipitate on the cathode.

[0019] Aspect 2 of the present invention is the method for producing high purity aluminum according to Aspect 1, wherein the highly conductive molten salt has a conductivity of 5 Sm －１ The above is preferably used.

[0020] A third aspect of the present invention is the method for producing high-purity aluminum according to the first or second aspect, wherein the high-purity aluminum is preferably used for electrolytic capacitors.

[0021] A fourth aspect of the present invention is the method for producing high-purity aluminum according to the first or second aspect, wherein the high-purity aluminum is preferably used for a magnetic recording medium substrate.

[0022] Aspect 5 of the present invention is preferably used in any one of the methods for producing high-purity aluminum according to Aspects 1 to 4, in which the Al crude metal anode and the cathode are flat, and the flat Al crude metal anode and the flat cathode are disposed opposite each other, or the cathode is rod-shaped, and a plate-shaped Al crude metal anode disposed concentrically around the rod-shaped cathode faces the cathode.

[0023] A sixth aspect of the present invention is characterized in that, in the method for producing high-purity aluminum according to any one of the first to fifth aspects, impurity elements that are not ionized from the crude Al metal anode remain in the crude Al metal anode.

[0024] Aspect 7 of the present invention is preferably used in any one of the methods for producing high purity aluminum according to Aspects 1 to 6, wherein the temperature is not less than room temperature and not more than 660°C.

[0025] Aspect 8 of the present invention is the method for producing high purity aluminum according to any one of Aspects 1 to 7, wherein the highly conductive molten salt is AlF３ The highly conductive molten salt preferably contains 1.5 to 35 mass % of the compound.

[0026] A ninth aspect of the present invention is the method for producing high-purity aluminum according to any one of the sixth to eighth aspects, further comprising the step of allowing anode slime containing non-ionized impurity elements to settle from the crude Al metal anode and removing the settled anode slime.

[0027] Aspect 10 of the present invention is the method for producing high purity aluminum according to any one of Aspects 1 to 9, wherein the direction in which the Al crude metal anode and the cathode are arranged to face each other is substantially the same direction as gravity.

[0028] An eleventh aspect of the present invention is an apparatus for producing high-purity aluminum, which comprises: an Al crude metal anode and a cathode disposed opposite each other in a highly conductive molten salt; and means for applying current between the Al crude metal anode and the cathode at a temperature at which the Al crude metal anode is in a solid state and the highly conductive molten salt is in a liquid state; and the apparatus ionizes aluminum from the Al crude metal anode to deposit aluminum on the cathode; and precipitates anode slime containing non-ionized impurity elements from the Al crude metal anode.

[0029] A twelfth aspect of the present invention is the high purity aluminum production apparatus of the eleventh aspect, wherein a plurality of anodes and a plurality of cathodes are disposed in the highly conductive molten salt, with the Al crude metal anodes and the cathodes being disposed opposite each other.

[0030] A thirteenth aspect of the present invention relates to the high purity aluminum production apparatus of the twelfth aspect, and further includes a means for wiring a plurality of anodes and cathodes, which are arranged opposite the crude Al metal anode and the cathode, and for energizing the anodes and cathodes in parallel or in series.

[0031] A fourteenth aspect of the present invention is configured such that a plurality of high purity aluminum production apparatuses according to the twelfth aspect can be wired together and operated as a single system.

[0032] A fifteenth aspect of the present invention is high-purity aluminum produced by any one of the methods for producing high-purity aluminum according to the first to tenth aspects.

[0033] Aspect 16 of the present invention is preferably used in which the Si concentration is 0.05 mass% or less and the Cu concentration is 0.05 mass% or less in the high purity aluminum of Aspect 15. Aspect 17 of the present invention may also be applied to the method for producing high purity aluminum according to any one of Aspects 1 to 10, in which the crude Al metal anode is an aluminum alloy anode, and the crude Al metal obtained by the method for producing high purity aluminum according to any one of Aspects 1 to 10 is used for the crude Al metal anode.

[0034] According to the method for producing high-purity aluminum of the present invention, the production time can be significantly reduced compared to conventionally known methods for purifying Al, such as fractional crystallization, and a method for producing high-purity aluminum with a purity of 99.9% or more can be provided effectively in industrial processes.

[0035] FIG. 1 is an example of a manufacturing process flow diagram showing a method for producing an Al crude metal anode used in the manufacturing method of the present invention from aluminum alloy scrap. FIG. 2 is an SEM image of a cross section of an aluminum alloy anode after a solid electrolysis process. FIG. 3 is an example of a vertical cross-sectional view of a solid electrolytic device energized in a method for producing an Al crude metal anode or high-purity aluminum. FIG. 4 is an example of a vertical cross-sectional view of another example of a solid electrolytic device energized in a method for producing an Al crude metal anode or high-purity aluminum. FIG. 5 is an example of a vertical cross-sectional view of yet another example of a solid electrolytic device energized in a method for producing an Al crude metal anode or high-purity aluminum. FIG. 6 is an example of a vertical cross-sectional view of a solid electrolytic device energized in a method for producing an Al crude metal anode or high-purity aluminum, in which plate-shaped aluminum alloy anodes are concentrically arranged around a rod-shaped cathode and face the cathode. 1A is an example of a graph showing the composition and concentration analysis results (Al, Si, Cu) of the raw aluminum alloy anode used in the method for producing an Al crude metal anode, the anode slime (anode slime) by-produced by energization, and the aluminum deposited on the cathode side by energization; (b) is a photograph of the raw aluminum alloy anode; (c) is a photograph of the aluminum deposit peeled from the cathode; (d) is an example of a photograph of high-purity aluminum obtained by remelting the aluminum deposit; and (e) is an example of a photograph of anode slime.

[0033] FIG. 1B is an example of a graph showing the results of XRD analysis of anode slime.

[0034] FIG. 1C is an example of a vertical cross-sectional schematic diagram showing yet another example of a solid electrolytic device energized in the method for producing an Al crude metal anode.

[0035] FIG. 1D is a graph showing the relationship between the apparent standard electrode potential (500°C) in LiCl-KCl and the standard electrode potential (room temperature) in an aqueous solution.

[0036] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that in the following embodiments, identical or equivalent parts may be designated by the same reference numerals in the drawings. Furthermore, the drawings used in the following description may show characteristic parts enlarged for the sake of clarity, and the dimensional ratios of each component may not necessarily be the same as in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto. Appropriate modifications can be made within the scope of the effects of the present invention. A configuration shown in one embodiment may also be applied to other embodiments.

[0037] [Method for Producing High-Purity Aluminum] A method for producing high-purity aluminum according to one embodiment of the present invention includes the steps of placing an Al crude metal anode and a cathode facing each other in a highly conductive molten salt, and passing a current between the Al crude metal anode and the cathode at a temperature at which the Al crude metal anode is in a solid state and the highly conductive molten salt is in a liquid state, thereby ionizing and dissolving aluminum from the Al crude metal anode and depositing an aluminum precipitate on the cathode.

[0038] A method for producing high-purity aluminum according to one embodiment of the present invention may include a step of preparing an anode made of crude Al metal to be refined. Fig. 1 shows an example of a flow chart of the process for preparing a crude Al metal anode. Fig. 3 is a schematic vertical cross-sectional view showing an example of a solid electrolysis apparatus for carrying out the solid electrolysis step of the method for producing high-purity aluminum.

[0039] In this specification, "high-purity aluminum" in the "method for producing high-purity aluminum" refers to aluminum with a purity of 99.9% (three nines) or more. Furthermore, the "high-purity aluminum" in the "method for producing high-purity aluminum" is not limited in shape, and any form made of high-purity aluminum, such as an ingot, plate, rod, foil, irregular lump, or fine particle, is included in the "high-purity aluminum." The "high-purity aluminum" in the "method for producing high-purity aluminum" preferably has a purity of 99.99% (four nines) or more, more preferably 99.999% (five nines) or more, and even more preferably 99.9999% (six nines) or more. Although there is no upper limit to the purity range of "high-purity aluminum," in this specification, due to constraints such as production costs, the purity of high-purity aluminum is preferably less than 99.99999% (seven nines).

[0040] The method for producing high-purity aluminum of the present invention is a method for producing high-purity aluminum with a purity of 99.9% or higher by removing alloy elements from an Al crude metal anode having a purity of 99% or higher. The "Al crude metal anode" used in the method for producing high-purity aluminum of the present invention means an anode made of an Al alloy having a purity of 99% or higher. The Al crude metal anode can be produced (manufactured) by various methods, and there is no limitation on the production (manufacturing) method. The Al crude metal anode with a purity of 99% or higher may be produced, for example, from the above-mentioned virgin aluminum or may be produced by a method of producing it from aluminum casting scrap, as described in detail below.

[0041] For example, when aluminum alloy anodes are made by melting aluminum casting scrap and processing it into an anode shape, it becomes possible to produce Al crude metal anodes using high-purity aluminum with a reduced concentration of alloying elements compared to the original aluminum casting scrap. Hereinafter, with reference to FIGS. 1 and 3 , an example of a method for producing an Al crude metal anode using aluminum casting scrap as a raw material will be described. As shown in FIG. 1 , the method for producing an Al crude metal anode can be divided into an aluminum alloy scrap melting step (101), an aluminum alloy anode production step (102), a solid electrolysis step (103), an aluminum precipitate recovery step (104), a recovered aluminum precipitate melting step (105), and an Al crude metal anode production step (not shown). The Al crude metal anode production step (not shown) can be performed in the same manner as the aluminum alloy anode production step (102). After the recovered aluminum precipitate melting step (105), the melted aluminum precipitate may be collected, for example, as an aluminum ingot (106). In addition, if the steps of the method for producing an Al crude metal anode described below are performed only once and an "Al crude metal anode" is not obtained, the steps can be performed two or three or more times until an "Al crude metal anode" is obtained.

[0042] Furthermore, the method for producing an Al crude metal anode described below can be adapted to the method for producing high-purity aluminum of the present invention by replacing the "aluminum alloy anode" with an "Al crude metal anode." In addition, in the method for producing high-purity aluminum of the present invention, the steps may be repeated two or three or more times until high-purity aluminum of 99.9% or higher is produced.

[0043] <Step of Melting Aluminum Alloy Scrap> As shown in Fig. 1 , step 101 of melting aluminum alloy scrap is performed as needed in the production of an Al crude metal anode, and can be performed arbitrarily by a known method for melting aluminum alloys. Note that step 102 of producing an aluminum alloy anode is not an essential step when the raw aluminum alloy material to be highly purified can be used as an anode as is.

[0044] In this specification, "aluminum alloy scrap" typically refers to scrap of used aluminum products, but is not limited to this and includes all aluminum alloy scrap that is desired to be made highly pure. The "aluminum alloy scrap" to be melted may be one or more pieces. Examples of "aluminum alloy scrap" include scrap of cast material, scrap of wrought material, and mixed scrap of cast material and wrought material.

[0045] Furthermore, the alloying elements (impurity elements) contained in "aluminum alloy scrap" include more than 50 types specified in the JIS standard, and representative ones include Mg, Cu, Si, Fe, Zn, Mn, etc. When it is said that an aluminum alloy has been purified to a high purity by the method for producing high-purity aluminum of the present invention, it means that the concentrations of one or more of these alloying elements have been reduced.

[0046] Furthermore, the concentration of alloying elements contained in "aluminum alloy scrap" is not limited in principle, but is, for example, 30% or less when scrap of used aluminum products is used. In this specification, the "concentration" and "purity" of alloying elements and aluminum refer to mass % unless otherwise specified. Furthermore, when, for example, scrap of a casting material conforming to JIS standards other than Al-Si-Cu-Mg-Ni alloy is used as "aluminum alloy scrap," the concentration of alloying elements contained is 20% or less. Furthermore, when, for example, scrap of a wrought material conforming to JIS standards other than the 4000 series is used as "aluminum alloy scrap," the concentration of alloying elements contained is 10% or less.

[0047] <Step of Producing Aluminum Alloy Anode> The step 102 of producing an aluminum alloy anode typically includes processing into an anode shape (for example, processing into a plate shape), but also includes all other steps performed to produce an aluminum alloy anode. Note that the step 102 of producing an aluminum alloy anode is not an essential step when the aluminum alloy material to be made highly pure can be used as an anode as is.

[0048] <Solid Electrolysis Step> The solid electrolysis step 103 involves immersing an aluminum alloy anode and a cathode in a molten salt while they are facing each other, and passing current between the aluminum alloy anode and the cathode at a temperature at which the aluminum alloy anode is in a solid state and the molten salt is in a liquid state, thereby dissolving the aluminum alloy anode. At the same time, an aluminum precipitate is deposited on the cathode. Examples of the "facing aluminum alloy anode and cathode" configuration include a configuration in which a flat aluminum alloy anode and a flat cathode are facing each other (see FIG. 4, etc.), and a configuration in which a plate-shaped aluminum alloy anode is concentrically arranged around a rod-shaped cathode. In addition to these configurations, any known configuration commonly used in electrolysis can be used. When a flat aluminum alloy anode and a flat cathode are facing each other, the aluminum alloy anode and the cathode are preferably positioned substantially parallel to each other. When the electrodes are nearly parallel, the distance between the electrodes becomes constant over the entire surface of the electrodes, and aluminum ions are uniformly released from the surface of the aluminum alloy anode.

[0049] In this specification, "solid electrolysis" means that the aluminum alloy anode is electrolyzed while remaining in a solid state. FIG. 2 shows an SEM image of the cross section of the aluminum alloy anode after the solid electrolysis process. The SEM image was obtained using an FE-SEM (JXA-8530F (manufactured by JEOL Ltd.)) at an accelerating voltage of 15 kV. The electric field conditions were LiCl-KCl-5 mol% AlF at 500°C. ３ In an electrolytic bath, aluminum casting alloy AC2A was used as the aluminum alloy anode and a pure aluminum plate was used as the cathode, and electrolysis was carried out at an anode current density of 200 mAcm for 2 hours. SEM images show that the surface side of the aluminum alloy anode has a porous structure.

[0050] Table 3 also shows the results of compositional analysis (ICP-AES) of the aluminum alloy anode before electrolysis and the porous structure after electrolysis. It can be seen that the proportion of Al is significantly reduced in the porous structure after electrolysis. From the SEM images and compositional analysis results, it is believed that the porous structure formed on the surface side of the aluminum alloy anode is due to the removal of aluminum ions from the surface of the aluminum alloy anode in the solid state. Furthermore, the concentrations of impurities such as Si and Cu are significantly increased in the porous structure after electrolysis. This indicates that impurities such as Si and Cu have become the main elements constituting the skeleton of the porous structure. This is thought to be due to the relatively high concentrations of Si and Cu before electrolysis, at 5.1% and 3.8%, respectively. It is believed that the original high concentrations of these impurities led to their becoming the main elements constituting the skeleton of the porous structure.

[0051]

[0052] In this process, aluminum precipitates are deposited on the cathode in addition to solid electrolysis, making it a solid electrolysis / precipitate deposition process. Furthermore, in addition to solid electrolysis, impurity elements in the aluminum alloy anode may settle as anode slime depending on their concentration. As described above, when the impurity concentration is relatively high, the impurity elements remain in the porous structure on the surface, but part of this porous structure may fall off and become slime. Alternatively, when the impurity concentration is low, the anode slime settles as slime, as is known in copper electrolysis. The anode slime settles in the direction of gravity between the aluminum alloy anode and the cathode, allowing it to be easily separated and extracted. Therefore, the aluminum alloy anode and the cathode are positioned facing each other in a direction substantially parallel to gravity. As shown in FIG. 1 , aluminum precipitate (corresponding to the purity of the crude Al metal in this specification) can be recovered through the aluminum alloy scrap melting process 101 → process 102 → process 103 → process 104. Furthermore, the method for producing high purity aluminum of the present invention is achieved by using the aluminum precipitate or crude aluminum metal as the Al raw material and applying the steps (101 to 106) shown in FIG.

[0053] 3 is a schematic vertical cross-sectional view of an example of a solid electrolytic device that can be used in the method for producing crude Al metal, and is an example of a solid electrolytic device used in step 103 of FIG.

[0054] 3 includes an anode holder 2 for holding an aluminum alloy anode 1, a cathode 3, an electrolytic cell 5 for accommodating a molten salt 4, a heating device 6 capable of maintaining a temperature at which the aluminum alloy anode 1 is in a solid state and the molten salt 4 is in a liquid state, and a power source 7 for applying current between the aluminum alloy anode 1 and the cathode 3. When the solid electrolysis process is performed, the aluminum alloy anode 1 is attached to the anode holder 2, and the molten salt 4 is placed in the electrolytic cell 5.

[0055] The molten salt may be a chloride-based molten salt, a fluoride-based molten salt, a bromide-based molten salt, or a mixture thereof. Examples of the chloride-based molten salt include KCl, NaCl, CaCl, LiCl, RbCl, CsCl, SrCl, BaCl, MgCl, or a mixture thereof. Examples of the fluoride-based molten salt include LiF, NaF, KF, RbF, CsF, MgF, CaF, SrF, BaF, and AlF. ３ In the case of fluoride-based compounds, AlF ３ is preferably contained in the molten salt. For example, AlF ３ is contained in the molten salt in a range of preferably 1.5 to 35 mass %, more preferably 3 to 25 mass %, and even more preferably 6 to 15 mass %. ３ is preferably contained in the molten salt in a range of 1 to 28 mol %, more preferably in a range of 2 to 14 mol %, and even more preferably in a range of 4 to 10 mol %. From the viewpoint of lowering the melting point, it is effective to positively contain an aluminum halide in the molten salt. Examples of aluminum halides include aluminum fluoride (AlF ３ ), aluminum chloride (AlCl ３ ), aluminum bromide (AlBr３ ) and the like. From the viewpoint of reducing changes in the composition of the electrolytic bath due to evaporation and from the viewpoint of easily performing continuous electrolysis, the use of aluminum fluoride is preferable to aluminum chloride. When aluminum chloride is used, electrolysis must be performed in a sealed environment due to its high vapor pressure at the electrolysis temperature, whereas when aluminum fluoride is used, the vapor pressure of the electrolytic bath is reduced, making continuous electrolysis possible even in an open environment. Furthermore, the molten salt may contain other components as unavoidable impurities or may intentionally contain other components within the scope of the effects of the present invention. In particular, aluminum alloy scrap may usually contain Mg element, and therefore Mg is mixed into the molten salt in the solid electrolysis process. For example, when a chloride-based molten salt such as LiCl-KCl eutectic salt (reference example) or NaCl-KCl is used, MgCl ２ -LiCl-KCl and MgCl ２ - MgCl such as NaCl-KCl ２ In some cases, the eutectic salt of the base is used, but even in this case, the effects of the present invention are achieved. For this reason, inexpensive MgCl ２ The NaCl-KCl system is preferably used. By adjusting each component, the applicable temperature range of the molten salt can be adjusted, which is advantageous. For example, MgCl ２ is MgCl ２ In the -NaCl-KCl molten salt, the content of the aluminum compound is preferably in the range of 1 to 70 mass %, more preferably in the range of 10 to 60 mass %, and even more preferably in the range of 20 to 50 mass %. In the present invention, an ionic liquid made of a known organoaluminum compound can be used as the molten salt. Specific examples of such organoaluminum compounds include aluminum chloride (AlCl ３ An example of an ionic liquid is one composed of 1-ethyl-3-methylimidazolium chloride ([EtMeIm]Cl) and 1-ethyl-3-methylimidazolium chloride ([EtMeIm]Cl) (Light Metals, Vol. 69, No. 1 (2019), 15-21).

[0056] The density of the molten salt is preferably as small as possible to facilitate the precipitation and separation of alloy components such as Cu and Si from the aluminum alloy anode as slime. From this viewpoint, the density is preferably, for example, equal to or less than the density of pure aluminum (2.70 g / cm3 at around room temperature). ３ , 2.375 g / cm near the melting point (660 ° C) ３ ) molten salt can be used.

[0057] From the viewpoint of reducing energy consumption and productivity of high purity aluminum, the molten salt is used with a conductivity of 1 Sm －１ It is preferable to use a material having a viscosity of 10 Sm or more. －１ It is more preferable to use a material with a viscosity of 100 Sm or more. －１ It is more preferable to use a molten salt having a conductivity of 1 Sm or more. －１ If the conductivity is less than 500 Sm, the efficiency of electrolytic oxidation will be poor, and the productivity of high-purity aluminum will decrease. －１ The electrical conductivity can be 500 Sm or less. －１ This is because anything exceeding this value is metallic and cannot be used as a molten salt. For example, the conductivity of the LiCl-KCl eutectic salt shown in the Reference Example is 187 Sm －１ (500°C).

[0058] The cathode may be made of aluminum or an aluminum alloy, but is not limited to these and may be made of any electrode material that can remove deposited aluminum, such as stainless steel, carbon, nickel, or iron.

[0059] The temperature at which the solid electrolysis process is carried out is a temperature at which the aluminum alloy anode is in a solid state and the molten salt is in a liquid state. Specifically, for example, the temperature can be in the range of room temperature or higher and 660°C or lower. The temperature at which the solid electrolysis process is carried out is preferably in the range of 150 to 600°C, more preferably in the range of 300 to 550°C, and even more preferably in the range of 450 to 550°C. This temperature will not exceed 660°C because the melting point of pure aluminum is 660°C, and the addition of alloy components lowers the melting point. For example, when pure aluminum contains 10% Si, the melting point drops to about 570°C. Note that "the molten salt is in a liquid state" can also be rephrased as "the molten salt is in a molten state."

[0060] 4, 6, and 9 are schematic vertical cross-sectional views showing other examples of solid electrolysis apparatuses for carrying out the solid electrolysis process. In Fig. 4 and Fig. 9, figures, letters, and symbols are also included for conceptually explaining phenomena occurring in the solid electrolysis process.

[0061] In the example of the solid electrolytic device shown in FIGS. 4 and 5 , aluminum alloy anodes 11 and cathodes 13 are alternately arranged in parallel and facing each other in molten salt 14 in an electrolytic cell 15. That is, a plurality of pairs of anodes and cathodes are arranged in the molten salt, with the aluminum alloy anodes 11 and cathodes 13 arranged in parallel and facing each other. Furthermore, the anodes and cathodes do not necessarily have to be arranged in pairs ( FIG. 5 ); the anodes and cathodes may be arranged alternately facing each other. In FIG. 4 , reference numeral 20 denotes anode slime, and the illustration shows an example in which the anode slime has settled. In FIG. 4 , an enlarged view of the circled portion where the aluminum alloy anodes 11, cathodes 13, and anode slime 20 are close to each other may be provided below the aluminum alloy anodes 11. A basket for collecting anode slime that falls from the aluminum alloy anodes 11 may be provided below the aluminum alloy anodes 11. In the example of the solid electrolytic device shown in FIG. 6 , four plate-shaped aluminum alloy anodes 21 are concentrically arranged around a rod-shaped cathode 23 in molten salt 14 in a cylindrical electrolytic cell 25, facing the cathode 23. Each plate-shaped aluminum alloy anode 21 is arranged in an arc shape when viewed from above. In the example shown in FIG. 6 , the number of plate-shaped aluminum alloy anodes 21 is not limited to four, but may be two, six, eight, etc. Furthermore, the anodes may be in the shape of a single, continuous cylinder. Furthermore, the arrangement of each plate-shaped aluminum alloy anode 21 does not necessarily have to be arc-shaped when viewed from above, and may be four flat plates arranged in a square circumferential shape, six flat plates arranged in a regular hexagonal circumferential shape, eight flat plates arranged in a regular octagonal circumferential shape, or the like. Furthermore, in the solid electrolytic device, if necessary, a known porous body (having a separator function) having a porous structure that allows molten salt ions to pass through may be used between the aluminum alloy anode 11 and the cathode 13. Examples of the porous body include, but are not limited to, glass cloth and ceramic fiber molded products with a high alumina content.

[0062] In the solid electrolysis process, when a current is passed between the aluminum alloy anode 11 and the cathode 13, aluminum dissolves on the surface of the aluminum alloy anode 11 to form aluminum Al ３+The aluminum alloy anode 11 moves to the surface of the cathode 13 and deposits there. At the same time, alloy components such as Cu and Si contained in the aluminum alloy anode 11 are precipitated and separated as slime. Such a solid electrolytic device may be a large-scale device (system) equipped with a means for passing current in parallel or in series to each of a plurality of pairs of anodes and cathodes, each pair being arranged opposite an aluminum alloy anode and a cathode. A high current density is desirable from the viewpoint of increasing the aluminum deposition rate and improving productivity. For example, a current density of 5 to 2000 mA / cm is used. ２ In the present invention, the state of formation of deposited aluminum (for example, non-uniformity of the aluminum film, dendrite formation, etc.) is not an issue, so the current density can be determined mainly from the viewpoint of production efficiency.

[0063] The current flowing between the aluminum alloy anode 11 and the cathode 13 can be constant. As solid electrolysis progresses, the aluminum alloy anode becomes porous from the surface side, as shown in Figure 2. The porous structure increases resistance, and as the porous structure expands, the voltage increases in order to maintain a constant current. For this reason, it is preferable to provide a voltage monitoring device and monitor the voltage during the production of high-purity aluminum.

[0064] In the method for producing high purity aluminum of the present invention, the crude Al metal anode can be subjected to solid electrolysis in the same manner as in the <Solid Electrolysis Step>, and the solid electrolysis apparatus shown in Figures 3 to 6 can be used. The "highly conductive molten salt" used in the method for producing high purity aluminum of the present invention can be selected from the molten salts listed in the explanation of the <Solid Electrolysis Step>, and has a conductivity of 5 Sm －１ It is preferable that this is equal to or greater than this.

[0065] <Aluminum Deposit Recovery Step> The aluminum deposit recovery step 104 is a step of recovering the aluminum deposit deposited on the cathode in the solid electrolysis step.

[0066] The aluminum deposit deposited on the cathode can be recovered by any known method, for example, by mechanically scraping off the aluminum deposit, or, if the cathode is made of aluminum or an aluminum alloy, by melting the aluminum deposit together with the aluminum deposit, and using it for industrial alloy applications.

[0067] In the method for producing high purity aluminum of the present invention, the aluminum deposit deposited on the cathode can be recovered in the same manner as in the <Step of recovering aluminum deposit>.

[0068] <Step of Dissolving Recovered Aluminum Precipitate> Step 105 of dissolving recovered aluminum precipitate is a step of dissolving the aluminum precipitate recovered in the above-described aluminum precipitate recovery step.

[0069] The process of dissolving the aluminum precipitate can be carried out, for example, by a method similar to that used in the process of dissolving aluminum alloy scrap. Adding alloying components also makes it possible to produce high-purity aluminum alloy materials with desired concentrations. Furthermore, this method for producing crude Al metal anodes can significantly remove aluminum alloy elements such as silicon (Si) and copper (Cu), which are commonly found in aluminum casting scrap and the like, thereby enabling the recovery of high-purity aluminum. In Reference Example 1, the aluminum purity was 99.9%, and in Reference Example 2, the purity was 99.88%. In the present invention, the silicon (Si) concentration can be reduced to, for example, a range of 1000 to 10 ppm. Furthermore, by repeating the aforementioned electrolysis process from such crude Al metal-level aluminum, the high-purity aluminum of the present invention can be obtained, and then aluminum processed products of various shapes and sizes can be obtained using known processing techniques.

[0070] <Step of Producing a Crude Al Metal Anode> A crude Al metal anode is produced by a step similar to the step (102) of producing an aluminum alloy anode in FIG. 1 , and the high purity aluminum of the present invention can be produced using the obtained crude Al metal anode.

[0071] <Aluminum Alloy Collecting Step> In the aluminum alloy collecting step 106, the aluminum precipitate dissolved in the above-described step of melting the recovered aluminum precipitate may be collected as, for example, an aluminum ingot.

[0072] <Others> As solid electrolysis progresses, the aluminum alloy anode develops a porous structure from the surface side, but the core side remains the aluminum alloy of the original composition, as shown in Figure 2. Therefore, after or during the production of aluminum with a higher purity than the raw material, the aluminum alloy anode may be removed, and the core side may be used as a raw material for producing an aluminum alloy anode.

[0073] Furthermore, the porous electrolytic residue formed on the surface of the aluminum alloy anode can be used as a raw material for producing any impurity metals (for example, copper) contained therein.

[0074] The results of experiments showing the effect of refining by the solid electrolytic method used in the method for producing high purity aluminum of the present invention are shown below as Reference Examples 1 and 2.

[0075] Reference Example 1 Potassium chloride (KCl, >99.5%) and lithium chloride (LiCl, >99.0%) were weighed out to form a eutectic composition (LiCl-41 mol% KCl) and thoroughly mixed. ３ ) was added and mixed. ３ After drying in an oven at 200°C for 24 hours, the mixture was transferred to a graphite crucible and dried in a vacuum at 300°C for 2 hours. The mixture was then heated to 550°C in an Ar atmosphere and held at that temperature for 1 hour to dissolve the molten salt to a uniform composition, and used as an electrolytic bath.

[0076] In the electrolysis step, approximately 300 g of the pre-dissolved molten salt for the electrolytic bath was weighed and placed in a graphite crucible. The temperature was raised to 500°C in an Ar atmosphere and maintained at that temperature. A general-purpose aluminum die-cast alloy AD12.1 plate (see Figure 7(b)) was used as the anode, and an aluminum plate was used as the cathode. As shown in Figure 5, to ensure stable electrolysis, the anode was placed in the center, and two cathodes were placed on either side of the anode facing each other. The anode current density was 200 mAcm. －２ , cathode current density 100 mA cm －２ The solution was electrolyzed for 2 hours.

[0077] The composition of the anode used is shown in Table 4.

[0078]

[0079] The anode and cathode potentials were stable during the electrolysis process, and the cell voltage was 0.35 V. In particular, the stability of the anode and cathode potentials was confirmed by electrochemical experiments using the conventional cyclic voltammetry (CV) method. Figure 10 shows the cyclic voltammogram curve of aluminum deposited on the cathode, which shows the ideal redox oxidation-reduction wave of aluminum. Regarding the potentials in Figure 8, the equilibrium potential of chlorine evolution was measured using the same Ag / AgCl reference electrode, and the potential of the Ag / AgCl reference electrode used was found to be -1.18 V (vs. Cl). ２ / Cl － ), the reference potential (Cl ２ / Cl － The CV potential was calculated by converting the aluminum deposited on the cathode into the aluminum of LiCl-KCl-0.5 mol% AlF ３Under molten salt conditions (T = 400°C), no redox reactions of impurity elements were detected, demonstrating the ideal Nernstian response of aluminum. After the electrolysis process, the anode and cathode were removed and their compositions were analyzed using X-ray fluorescence (XRF). The results are shown in Figure 7(a). Nearly pure aluminum was deposited on the cathode, while the Si and other elements in the AD12.1 alloy used as the anode remained as anode slime. The aluminum deposit (Figure 7(c)) removed from the cathode was remelted and collected as an ingot (Figure 7(d)). The composition analysis of the resulting ingot (high-purity aluminum) was shown in Table 4. The aluminum purity was 99.9%, with approximately 0.005% Si and 0.002% Cu. The aluminum yield before and after the electrolysis process was 95.6%, with only a small portion migrating into the anode slime (see Figure 7(e)).

[0080] [Reference Example 2] The preparation of the electrolytic bath and the electrolysis experiment were carried out in the same manner as in Reference Example 1. A typical aluminum casting alloy, AC2A, was used as the anode. A pure aluminum plate was also used as the cathode. The anode current density was 200 mA cm －２ , cathode current density 100 mA cm －２ The solution was electrolyzed for 2 hours.

[0081] The composition of the aluminum casting alloy AC2A used as the anode is also shown in Table 4.

[0082] The anode potential and cathode potential were stable during the electrolysis process, and the cell voltage was about 0.3 V. After electrolysis, the anode and cathode were removed and their compositions were analyzed using XRF. The results are also shown in Table 4. Almost pure aluminum was deposited on the cathode. In addition, the anode slime after electrolysis was collected and its components were identified by XRD. The results are shown in Figure 9. The XRD analysis results showed that the anode slime remaining after electrolysis was mainly composed of Si and Al. ２ It was found that Al, which is composed of Cu and is the main component of the aluminum casting alloy AC2A, was almost completely dissolved.

[0083] Fig. 11 is a diagram for explaining the principle of the solid electrolysis method used in the method for producing high-purity aluminum of the present invention, showing the relationship between the apparent standard electrode potential (500°C) in LiCl-KCl and the standard electrode potential (room temperature) in an aqueous solution. Even in an environment of the apparent standard electrode potential (500°C) in LiCl-KCl, the ionization potential of Al is lower than the ionization potentials of Co, Cr, Ga, Ni, Pb, etc., indicating that Al is preferentially ionized by electrolysis. The horizontal axis represents Cl. ２ / Cl － Standard, vertical axis is H ＋ / H ２ Although the potential standards are different, the same standards are used in the molten salt and aqueous solution solvents, so it is believed that the dissolution potentials of each element can be simply compared. Figure 11 shows a clear correlation between the two. Based on this trend, Co, Cr, Ga, Ni, and Pb in the Al crude metal anode can also be removed using the same principle. Furthermore, Mg and Ti dissolve in the molten salt and their concentrations increase, but when the Al crude metal anode is electrolytically refined, the amounts of Mg and Ti transferred are small compared to the amount of aluminum transferred, so their impact can be ignored. Furthermore, MgCl, which was listed as an example of a conductive molten salt system, ２ In the -KCl-NaCl system, the dissolution of Mg into the molten salt is not affected. By preparing an Al crude metal anode from the high-purity aluminum deposited on the cathode in Reference Examples 1 and 2 and applying the method for producing high-purity aluminum of the present invention, it is possible to obtain aluminum of even higher purity. Furthermore, if high-purity aluminum of 99.9% or higher is not obtained by performing the steps in the method for producing high-purity aluminum of the present invention once, or if high-purity aluminum of the desired high purity of 99.9% or higher is not obtained, the steps may be performed two, three, or more times until high-purity aluminum of 99.9% or higher is obtained.

[0084] REFERENCE SIGNS LIST 1, 11 Aluminum alloy anode or crude Al metal anode 2 Anode holder 3, 13 Cathode 4, 14 Molten salt 5, 15 Electrolytic cell 6 Heating device 7 Power source 10 Solid electrolytic device

Claims

[Claim 1] A method for producing high-purity aluminum, comprising the steps of: placing an Al crude metal anode and a cathode opposite each other in a highly conductive molten salt; applying an electric current between the Al crude metal anode and the cathode at a temperature in which the Al crude metal anode is in a solid state and the highly conductive molten salt is in a liquid state; ionizing and dissolving aluminum from the Al crude metal anode; and depositing an aluminum precipitate on the cathode.