Method for producing aluminum

Abstract

This method for producing aluminum is for obtaining, from an aluminum alloy material, aluminum having higher purity than said aluminum material, the method being characterized by: immersing, in an electrolytic solution 11, a cathode electrode 9 and an anode electrode 7 including an aluminum alloy material containing 0.1-24 mass% of Si; causing a current to flow at a current density of 0.1-25 mA / cm2 on the surface of the anode electrode 7 where the electrolytic solution 11 and the aluminum alloy material come in contact with each other; and depositing aluminum on the cathode electrode.<sp / >

Description

Aluminum manufacturing method

[0001] The present invention relates to a method for producing aluminum that can obtain aluminum of higher purity from, for example, a casting alloy.

[0002] Widely used aluminum cannot be refined from raw ore using a thermal reduction reaction with carbon, as is the case with steel, and therefore generally requires the use of electricity in the smelting process, as in the Bayer process or the Hall-Heroult process. For this reason, the production of virgin aluminum emits large amounts of carbon dioxide, placing a heavy burden on the environment.

[0003] Therefore, it is desirable to reuse used aluminum products through recycling. For example, attempts have been made to simply produce high-purity aluminum materials from aluminum alloy scrap (see, for example, Patent Document 1).

[0004] International Publication No. 2020 / 196013

[0005] Here, even though the word "aluminum" is used in general, a wide variety of alloys are used depending on the application. For example, in the case of aluminum cans, A3000 series (Al-Mn series) wrought material is used, and there is no problem if this is recycled as is into aluminum cans. However, when it is converted into other alloys, the components may become a problem. In this case, it is necessary to reduce the amount of recycled alloy used and mix it with a certain amount of virgin metal. For this reason, it is most efficient to recycle it into the same alloy system as much as possible.

[0006] However, although aluminum alloys used for automobile engine parts and the like have a large amount of scrap, their reuse has been decreasing due to the recent spread of electric vehicles and the like. This has made it difficult to reuse them in the same field. In particular, metals in these fields are often used for castings (castings, die-castings), which contain a higher amount of alloying elements than wrought materials, making their reuse in other fields difficult. For example, AC2A alloy (JIS), which is used as a casting material, contains 4.0% to 6.0% by mass of silicon (Si), which is significantly higher than the content of Si and other elements in wrought materials. This makes reuse in other wrought material applications extremely difficult.

[0007] The present invention has been made in view of the above problems, and aims to provide a method for producing aluminum that can use aluminum bullion or used aluminum alloy, particularly an aluminum alloy with a high Si content, as an aluminum material and produce aluminum of higher purity than the aluminum material.

[0008] In order to achieve the above-mentioned object, the present invention provides a method for producing aluminum from an aluminum material, for obtaining aluminum of higher purity than the aluminum material, the method comprising the steps of: immersing an anode electrode including an aluminum material containing 0.1 mass % to 24 mass % of Si, and a cathode electrode in an electrolyte; applying a current of 0.1 mA / cm to a surface of the anode electrode that is in contact with the aluminum material and the electrolyte; ２ 25mA / cm or more ２ This is a method for producing aluminum, characterized in that a current is passed through the cathode electrode at the following current density to deposit aluminum on the cathode electrode.

[0009] The aluminum material of the anode electrode may further contain 0.1% by mass to 5% by mass of copper (Cu).

[0010] The aluminum material of the anode electrode may further contain 0.15% by mass to 1.8% by mass of iron (Fe).

[0011] It is desirable to apply a current to the anode electrode while stirring the electrolyte around the anode electrode.

[0012] It is desirable that the surface area of ​​the portion of the anode electrode where the aluminum material is in contact with the electrolyte be larger than the surface area of ​​the portion of the cathode electrode where the aluminum material is in contact with the electrolyte.

[0013] The electrolyte preferably contains a dialkyl sulfone and an aluminum halide.

[0014] The molar ratio of the dialkyl sulfone to the aluminum halide in the electrolytic solution is preferably 1.5 to 5 moles of the aluminum halide per 10 moles of the dialkyl sulfone.

[0015] The electrolyte may comprise ammonium halide, a hydrogen halide salt of a primary amine, a hydrogen halide salt of a secondary amine, a hydrogen halide salt of a tertiary amine, or a compound of the general formula: R １ R ２ R ３ R ４ N.X. (R １ ~R ４ and X represents the same or different alkyl groups, and X represents a counter anion to the quaternary ammonium cation).

[0016] While a current is flowing through the anode electrode, it is desirable to deposit aluminum on the cathode electrode so that the arithmetic mean height Sa of the anode electrode is 2.3 μm or more and 10 μm or less.

[0017] The current density is 0.1 mA / cm ２ 20mA / cm or more ２ Preferably, the following: In addition, the above-mentioned features may be combined with each other in any combination.

[0018] According to the present invention, it is possible to provide a method for producing aluminum that can produce higher purity aluminum from aluminum materials such as used aluminum alloys, particularly aluminum alloys with a high Si content.

[0019] 1 is a conceptual diagram showing an aluminum production apparatus used in an aluminum production method according to one embodiment of the present invention, in which the current density of the anode electrode is 10 mA / cm ２ This is a scanning electron microscope (SEM) photograph of a cross section near the surface of the anode electrode after electrolysis when the current density of the anode electrode was 20 mA / cm. ２ SEM photograph of a cross section near the surface of the anode electrode after electrolysis when the current density of the anode electrode was 40 mA / cm ２ SEM photograph of a cross section near the surface of the anode electrode after electrolysis when the current density of the anode electrode was 80 mA / cm ２ 1. SEM photographs of a cross section near the anode electrode surface after electrolysis at 1000 kJ / s. 2. A diagram showing the correlation between the current density of the anode electrode and the surface roughness of the anode electrode surface. 3. SEM photographs of the surface of a film electrodeposited on a cathode electrode when high-purity aluminum was used as the anode electrode. 4. SEM photographs of the cross section of a film electrodeposited on a cathode electrode when high-purity aluminum was used as the anode electrode. 5. SEM photographs of the surface of a film electrodeposited on a cathode electrode when electrolysis was performed at a high current density using an AC2A alloy anode electrode. 6. SEM photographs of the cross section of a film electrodeposited on a cathode electrode when electrolysis was performed at a high current density using an AC2A alloy anode electrode. 7. SEM photographs of the surface of a film electrodeposited on a cathode electrode when electrolysis was performed at a low current density using an AC2A alloy anode electrode. 8. SEM photographs of the cross section of a film electrodeposited on a cathode electrode when electrolysis was performed at a low current density using an AC2A alloy anode electrode. 9. A diagram showing the impurity concentrations of films electrodeposited on a cathode electrode.

[0020] In one embodiment of the present invention, a method for producing high-purity aluminum from an aluminum alloy includes immersing an anode electrode including an aluminum alloy containing 0.1 mass % to 24 mass % of Si and a cathode electrode in an electrolyte, and applying a current of 0.1 mA / cm to a surface of the anode electrode that is in contact with the aluminum alloy and the electrolyte. ２ 25mA / cm or more ２ The method is characterized in that a current is passed at the following current density to deposit aluminum on the cathode electrode.

[0021] Hereinafter, a method for producing aluminum according to one embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram of an aluminum production apparatus 1 used in the method for producing aluminum according to one embodiment of the present invention. In the following description, the concentration of each element can be measured by ICP atomic emission spectrometry.

[0022] The aluminum production apparatus 1 used in the aluminum production method of this embodiment includes an electrolytic cell 3, a DC power supply 5, an anode electrode 7, a cathode electrode 9, and an electrolytic solution 11. The anode electrode 7 contains an aluminum alloy extracted from scrap or the like. The anode electrode 7 and the cathode electrode 9, on which highly purified aluminum is deposited, are immersed in the electrolytic solution 11, and a DC voltage is applied between the anode electrode 7 and the cathode electrode 9 by the DC power supply 5. While the DC voltage is applied between the anode electrode 7 and the cathode electrode 9, the current flowing between the anode electrode 7 and the cathode electrode 9 is controlled, or the surface area of ​​the aluminum alloy contained in the anode electrode 7 that contacts the electrolytic solution 11 is adjusted, so that the current density at the portion of the aluminum alloy surface of the anode electrode 7 that contacts the electrolytic solution 11 falls within a predetermined range. By passing a current through the anode electrode 7 in this manner, a highly pure aluminum film is deposited on the cathode electrode 9.

[0023] In this specification, the current density at the portion of the surface of the aluminum alloy contained in the anode electrode that is in contact with the electrolyte is referred to as the “current density of the anode electrode.” In addition, in this specification, the current density at the portion of the surface of the aluminum alloy contained in the anode electrode that is in contact with the electrolyte is the value obtained by dividing the current between the anode electrode and a DC power source by the area of ​​the portion of the surface of the aluminum alloy contained in the anode electrode that is in contact with the electrolyte.

[0024] An electrolytic cell 3 provided in the aluminum production apparatus 1 stores an electrolyte 11 capable of dissolving aluminum, and an anode electrode 7 and a cathode electrode 9 are immersed in the electrolyte 11. The anode electrode 7 and the cathode electrode 9 are connected via a DC power supply 5. The anode electrode 7 is made of an aluminum alloy having an aluminum purity lower than the target purity, and contains Si: 0.1% by mass to 24% by mass. The anode electrode 7 may further contain Cu: 0.1% by mass to 5% by mass and Fe: 0.15% by mass to 1.8% by mass. If the Si content exceeds the upper limit, it may hinder the dissolution of aluminum during electrolysis, slowing the deposition rate of the desired high-purity aluminum and reducing throughput. On the other hand, if the Si content exceeds the upper limit, Cu and Fe are more likely to dissolve into the electrolyte 11. This makes it difficult to obtain the desired high-purity aluminum. The anode electrode 7 can be obtained, for example, by forming used AC2A alloy material into an electrode shape by casting or the like. The anode electrode 7 may contain at least 0.1 mass % to 24 mass % of Si, and may further contain an element other than magnesium (Mg) that is more noble than aluminum.

[0025] The cathode electrode 9 is made of, for example, aluminum, and pure aluminum or an aluminum alloy specified by the JIS standard for wrought aluminum can be used. That is, the cathode electrode 9 has a lower Si and Cu content than the aluminum alloy contained in the anode electrode 7. By using aluminum for the cathode electrode 9, the cathode electrode on which aluminum is electrodeposited can be used as high-purity aluminum ingot. Therefore, the cathode electrode 9 is preferably made of aluminum having an aluminum purity equal to or higher than the target purity. Other materials that can be used for the cathode electrode 9 include titanium and stainless steel. Titanium and stainless steel have dense oxide films on their surfaces, so when aluminum is electrodeposited on the surface using a titanium or stainless steel cathode electrode, the electrodeposited aluminum can be easily peeled off from the cathode electrode 9.

[0026] The DC power supply 5 that applies a DC voltage between the anode electrode 7 and the cathode electrode 9 may be any known DC power supply device. However, it is preferable that the DC power supply 5 be provided with a current control mechanism that controls the output current in order to reduce the amount of impurities eluted into the electrolyte 11, as described below. Furthermore, to control the current density of the anode electrode 7, a current detector (not shown) that measures the current flowing between the anode electrode 7 and the DC power supply 5 may be provided near the anode electrode 7. The current density of the anode electrode can be calculated based on the surface area of ​​the anode electrode 7 in contact with the electrolyte 11 and the current detected by the current detector. Therefore, it is preferable that the output current of the DC power supply 5 be adjustable by the current control mechanism of the DC power supply 5 so that the current density of the anode electrode falls within a predetermined range. Furthermore, the contact area of ​​the aluminum alloy used for the anode electrode 7 with the electrolyte 11 may be changed depending on the current flowing through the anode electrode 7. For example, the current density of the anode electrode may be adjusted by changing the area of ​​the anode electrode exposed above the surface of the electrolyte 11, thereby changing the contact area of ​​the anode electrode 7 with the electrolyte 11.

[0027] The electrolyte 11 in which the anode electrode 7 and the cathode electrode 9 are immersed is a non-aqueous system (non-aqueous electrolyte) containing, for example, (1) dialkyl sulfone and (2) aluminum halide. The use of this electrolyte 11 allows high-purity aluminum to be formed on the surface of the cathode electrode 9 at a high film formation rate. Furthermore, the electrolyte 11 may contain, for example, (3) ammonium halide, hydrogen halide salt of a primary amine, hydrogen halide salt of a secondary amine, hydrogen halide salt of a tertiary amine, or a compound represented by the general formula: R １ R ２ R ３ R ４ N.X. (R １ ~R ４ and X represents the same or different alkyl groups, and X represents a counter anion to the quaternary ammonium cation. The above-mentioned electrolyte solution 11 is industrially advantageous in terms of the availability of reagents and low cost compared to electrolyte solutions using ionic liquids.

[0028] Examples of dialkyl sulfones contained in the electrolytic solution 11 include those (which may be linear or branched) having an alkyl group with 1 to 6 carbon atoms, such as dimethyl sulfone, diethyl sulfone, dipropyl sulfone, dihexyl sulfone, and methyl ethyl sulfone. Dimethyl sulfone is preferably used from the viewpoints of good electrical conductivity and ease of availability.

[0029] Examples of aluminum halides include aluminum chloride and aluminum bromide. It is desirable for the aluminum halide to be anhydrous. Anhydrous aluminum halides do not contain water molecules, which can cause a decrease in electrodeposition efficiency, and therefore the electrodeposition efficiency is not reduced.

[0030] Examples of ammonium halides that can be used as the nitrogen-containing compound include ammonium chloride and ammonium bromide. Furthermore, examples of the primary to tertiary amines in the hydrogen halide salts of primary to tertiary amines include those in which the alkyl group has 1 to 6 carbon atoms (either linear or branched), such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, tripropylamine, hexylamine, and methylethylamine. Examples of hydrogen halides include hydrogen chloride and hydrogen bromide. General formula: R １ R ２ R ３ R ４ N.X. (R １ ~R ４ are the same or different alkyl groups, and X represents a counter anion to the quaternary ammonium cation), １ ~R ４ Examples of the alkyl group represented by the formula (I) include those having 1 to 6 carbon atoms (which may be linear or branched), such as methyl, ethyl, propyl, and hexyl groups. X may be a halide ion such as a chloride ion, a bromide ion, or an iodide ion, as well as BF ４ － and PF ６ －Examples of the compound include tetramethylammonium chloride, tetramethylammonium bromide, tetramethylammonium iodide, tetraethylammonium boron tetrafluoride, etc. Examples of a suitable nitrogen-containing compound include a tertiary amine hydrochloride, such as trimethylamine hydrochloride, which facilitates the formation of high-purity aluminum at a high film formation rate.

[0031] Regarding the blending ratio of dialkyl sulfone and aluminum halide, for example, the aluminum halide is preferably 1.5 mol or more and 5.0 mol or less per 10 mol of dialkyl sulfone. Furthermore, the upper limit is more preferably 4.0 mol or less, 3.0 mol or less, and 2.5 mol or less, in that order, and even more preferably 1.5 mol or more and 2.0 mol or less. If the blending amount of aluminum halide is less than 1.5 mol per 10 mol of dialkyl sulfone, there is a risk of the aluminum being blackened (a phenomenon called "burning") or a decrease in film formation efficiency. On the other hand, if the blending amount of aluminum halide is more than 5.0 mol per 10 mol of dialkyl sulfone, the solution resistance of the electrolytic solution 11 becomes too high, which may cause the electrolytic solution 11 to generate heat and decompose.

[0032] Furthermore, if the composition ratio of aluminum halide to dialkyl sulfone is too high, the concentration of ions serving as an aluminum source becomes too high, which may hinder the migration of aluminum ions from the anode electrode.On the other hand, if the composition ratio of aluminum halide to dialkyl sulfone is too low, the amount of ions contributing to electrical conduction decreases, and the efficiency of electrolysis decreases.

[0033] On the other hand, the amount of the nitrogen-containing compound blended together with the dialkyl sulfone and aluminum halide is preferably 0.01 mol to 2.0 mol, more preferably 0.05 mol to 1.5 mol, per 10 mol of dialkyl sulfone. If the amount of the nitrogen-containing compound blended is less than 0.01 mol per 10 mol of dialkyl sulfone, the effects of blending the compound, i.e., the effect of improving the film formation rate based on the improved electrical conductivity of the electrolyte solution 11, and the effects of increasing the purity and ductility of the aluminum, may be difficult to obtain. Furthermore, if the amount of the nitrogen-containing compound blended exceeds 2.0 mol per 10 mol of dialkyl sulfone, the composition of the electrolyte solution 11 may be essentially changed, resulting in the risk of aluminum not being precipitated.

[0034] The electrolysis of aluminum using the above-mentioned electrolytic solution 11 is carried out, for example, at a temperature of the electrolytic solution 11 of 80° C. or higher and 120° C. or lower, and at an applied current density of 0.1 mA / cm ２ 25mA / cm or more ２ The test can be performed under the following conditions: the applied current density is 0.1 mA / cm ２ 20mA / cm or more ２It is more preferable to perform the electrolytic plating under the following conditions. The lower limit of the temperature of the electrolytic solution 11 should be determined taking into account the melting point of the electrolytic solution 11, and is preferably 85°C or higher, more preferably 95°C or higher (below the melting point of the electrolytic solution 11, the plating solution solidifies, making plating no longer possible). If the temperature of the electrolytic solution 11 is 120°C or lower, the activation of the reaction between the aluminum and the electrolytic solution can be suppressed, making it difficult for impurities to be incorporated into the aluminum. To efficiently obtain an electrodeposited aluminum film with high purity through electrolysis, it is necessary to efficiently transfer the metal ions to be deposited at the cathode electrode from the anode electrode to the surface of the cathode electrode. Here, increasing the temperature reduces the viscosity of the electrolytic solution, thereby increasing the transport efficiency of materials and improving electrical conductivity. Therefore, considering electrical conductivity, a higher electrolytic solution temperature is preferable. On the other hand, if the electrolytic solution temperature becomes too high, the surface of the cathode electrode becomes activated, facilitating the deposition of impurities. This may result in a decrease in purity. For this reason, the temperature of the electrolytic solution 11 during electrolysis is preferably 85° C. or higher, more preferably 95° C., and the upper limit is preferably 120° C. or lower. In addition, when the anode current density is 0.1 mA / cm ２ If the current is 25 mA / cm or more, the decrease in film formation efficiency can be suppressed. ２ If the current is less than 20 mA / cm, it is possible to easily achieve high purity of aluminum, which will be described later. ２ If the content of the electrolyte solution 11 is less than 100%, it is possible to more easily achieve a high level of purity.

[0035] Next, the principle of aluminum purification in an aluminum production apparatus 1 according to one embodiment of the present invention will be described with reference to Figure 1. When an anode electrode 7 and a cathode electrode 9 are immersed in an electrolytic solution and a direct current is passed through them, aluminum loses electrons on the surface of the anode electrode 7 and dissolves into the electrolytic solution 11 as aluminum ions (A in Figure 1). On the surface of the cathode electrode 9, electrons are supplied to the aluminum ions, causing them to precipitate as aluminum metal.

[0036] In addition, for efficient supply of aluminum ions to the cathode electrode 9, it is preferable to stir the electrolytic solution 11 while a current is flowing through the anode electrode 7. Therefore, it is desirable to provide an outlet for releasing an inert gas such as nitrogen gas at the bottom of the electrolytic cell 3 and release bubbles of the inert gas from the outlet into the electrolytic solution 11 (cathode electrode 9) (so-called bubbling). The flow of bubbles in the electrolytic solution 11 also generates convection in the electrolytic solution 11, thereby stirring the electrolytic solution 11. Alternatively, a liquid flow may be imparted using a pump or a stirring device, or the electrodes themselves may be swung. In the illustrated example, a pair of anode electrodes 7 and cathode electrodes 9 is disposed, each of which is a single electrode, but multiple anode electrodes 7 and cathode electrodes 9 may be disposed alternately.

[0037] Here, the inventors have found that (1) when metal ions are eluted from the anode electrode 7, the eluted metal ions vary depending on the current density at the anode electrode 7. For example, because Cu has a higher standard electrode potential than aluminum, aluminum is preferentially ionized at the anode electrode 7 in an equilibrium state. However, in reality, ionization of Cu also proceeds under conditions such as when Cu is dissolved in aluminum, or when the current density is greater than or equal to a predetermined value, resulting in a high aluminum ion concentration on the surface of the anode electrode 7. In contrast, when the current density at the anode electrode 7 is set to 25 mA / cm ２ It has been found that the elution of elements more noble than aluminum, such as Cu, can be suppressed by setting the following. Therefore, the aluminum production apparatus 1 of one embodiment of the present invention uses a DC power supply 5 having a current adjustment mechanism. The current adjustment mechanism of the DC power supply 5 changes the current flowing from the DC power supply 5 to the anode electrode 7, so that the current of the anode electrode 7 can be adjusted to achieve the above-mentioned current density even if the surface shape of the anode electrode 7 changes and the surface area in contact with the electrolytic solution 11 changes.

[0038] The inventors also discovered (2) conditions under which segregated portions of Cu, Si, and the like (crystallized portions of Cu and Si) present in the material constituting the anode electrode 7 are exposed to the surface of the anode electrode 7 and can be removed during the elution of aluminum ions from the anode electrode 7. The inventors discovered that high-purity aluminum can be efficiently produced by preventing Cu, Si, and the like from eluting into the electrolyte 11 and allowing the crystallized foreign element components such as Cu and Si on the surface of the anode electrode 7 to be removed in a solid phase (see B in FIG. 1 ). Specifically, the inventors discovered that when Cu and Si are not completely uniformly dispersed in the material constituting the anode electrode 7 but partially contain a Cu phase, a Si phase, or an intermetallic compound phase containing these (e.g., a phase with a maximum diameter of 10 μm or more), these phases can be removed from the anode electrode 7 without elution by setting the current density of the anode electrode 7 to a predetermined value or less.

[0039] In this way, in order to efficiently remove impurities contained in the aluminum of the anode electrode 7 from the surface of the anode electrode 7 without dissolving them into the electrolytic solution 11, it is preferable to apply a DC voltage between the anode electrode 7 and the cathode electrode 9 while stirring the electrolytic solution 11 around the anode electrode 7. For example, instead of or in addition to the above-mentioned bubbling toward the cathode electrode 9, bubbling near the anode electrode 7 or stirring the electrolytic solution using a stirrer or the like may be performed, or the anode electrode 7 may be rocked or rotated. Furthermore, the surface of the anode electrode 7 may be physically rubbed with another insulating member during electrolysis to remove impurities.

[0040] Furthermore, in order to efficiently remove impurities from the anode 7, a process for coarsening the impurity phase may be performed during the manufacture of the anode 7, thereby making it easier for the impurities to be removed. For example, when an aluminum material, such as an aluminum alloy or aluminum bullion containing Si or Cu extracted from scrap, is melted and cast to form an anode, the cooling rate after solidification can be reduced to form a Cu phase, Si phase, or an intermetallic compound phase containing these, each having a size of 10 μm or more (or even 20 μm or more).

[0041] In order to reduce the current density of the anode electrode 7, it is desirable to make the surface area of ​​the anode electrode (the area in contact with the electrolytic solution 11; the same applies hereinafter) sufficiently larger (for example, 1.5 times or more) than the surface area of ​​the portion of the cathode electrode 9 in contact with the electrolytic solution 11. While there is an upper limit to the current density of the anode electrode 7 as described above, the deposition rate of aluminum deposited on the cathode electrode 9 increases if the current flowing through the anode electrode 7 is large. Therefore, by making the surface area of ​​the portion of the anode electrode 7 in contact with the electrolytic solution larger than the surface area of ​​the portion of the cathode electrode 9 in contact with the electrolytic solution 11, it is possible to improve the efficiency of electrodeposition of high-purity aluminum. Furthermore, the anode electrode 7 of this embodiment may be composed of a mesh-like cage made of a metallic material of an element more noble than aluminum, and granulated aluminum alloy extracted from scrap, placed inside the cage. By using such an anode electrode 7, it is possible to increase the current flowing through the anode electrode 7 while keeping the current density of the anode electrode 7 at or below the upper limit value described above, and it is possible to increase the deposition rate of aluminum deposited on the cathode electrode 9.

[0042] Furthermore, impurity particles (sludge) that fall off from the anode electrode 7 settle below the anode electrode 7 in the electrolytic solution 11. For this reason, a step of recovering this sludge may be included simultaneously with or after the electrolysis step. For example, the sludge can be recovered by filtration using filter paper, a filter, an anode bag, or a cathode bag, and the electrolytic solution 11 can be reused. The nominal mesh size of the filter paper, filter, anode bag, or cathode bag used may be 0.1 μm to 100 μm. The lower limit is preferably 1 μm or more. Furthermore, for aluminum materials in which the crystal grains of Cu phases, Si phases, or intermetallic compound phases containing these phases have been increased, a mesh size of 10 μm or more, more preferably 20 μm or more, is preferred.

[0043] As described above, according to this embodiment, high-purity aluminum can be efficiently obtained by reducing the impurity elements, particularly for aluminum alloys containing large amounts of Si and Cu. For example, the high-purity aluminum obtained by this embodiment can have a total content of Si, Fe, and Cu of 0.1 mass % or less. Furthermore, the CO emitted when 1 kg of high-purity aluminum is obtained by electrolysis according to this embodiment can be ２ The amount of CO emitted when obtaining 1 kg of new metal from ore using conventional smelting methods ２ The amount can be reduced by about 45%.

[0044] The three-layer electrolysis method is commonly used to purify aluminum. However, because foundry alloys have particularly high impurity concentrations, even if used foundry alloys are purified using the three-layer electrolysis method, the anode composition cannot be maintained, making it difficult to apply. Furthermore, silicon, a typical alloying element, has a density similar to that of aluminum and is lighter than the molten salt used, so silicon is mixed into the refined aluminum and is difficult to separate.

[0045] In contrast, the aluminum production method of the present embodiment can efficiently remove impurity elements including Si, and therefore the obtained aluminum can be further purified by a three-layer electrolysis process. That is, the aluminum obtained by the present embodiment is effective as an aluminum raw material (anode-side raw material) in a three-layer electrolysis process.

[0046] As described above, the aluminum production method of this embodiment can produce aluminum with higher purity than aluminum alloys or aluminum bullion containing 0.1% by mass to 24% by mass of Si. Therefore, for example, recycled alloys from castings can be highly purified and reused for other applications. Furthermore, recycled alloys, particularly those from automobile engine parts containing a large amount of Cu, can be easily reused for other applications, enabling more effective utilization of used alloys. High-purity aluminum can also be produced from aluminum bullion. Furthermore, by stirring the liquid near the anode electrode 7, sludge can be efficiently removed from the anode. Furthermore, by making the surface area of ​​the anode electrode 7 in contact with the electrolyte larger than the surface area of ​​the cathode electrode, high-purity aluminum can be efficiently deposited on the cathode while suppressing the current density of the anode.

[0047] Furthermore, by using an electrolyte containing dialkyl sulfone, aluminum halide, and a nitrogen-containing compound, it is possible to carry out the process at a lower temperature than when using a molten salt, and the process is safe and easy to handle. Furthermore, by collecting the sludge that falls off the anode electrode 7, contamination and other issues can be suppressed, and the aluminum alloy can be efficiently purified. In this case, the sludge collection rate can be increased by collecting the sludge using filter paper or the like with appropriate mesh size.

[0048] Electrolysis of an aluminum alloy used as an anode electrode and deposition of high-purity aluminum on a cathode electrode were carried out using an apparatus with a configuration similar to that of the aluminum production apparatus shown in Figure 1. An AC2A aluminum alloy (Si: 4.0% by mass or more and 6.0% by mass or less, Cu: 3.0% by mass or more and 4.5% by mass or less, Fe: 0.8% by mass or less) was used as the anode electrode. Figures 2A to 2D are SEM photographs of cross sections near the anode electrode surface, showing the difference in the anode electrode surface after electrolysis when the current density of the anode electrode is changed. Note that in each SEM photograph, the current density of the anode electrode is 10 mA / cm in Figure 2A. ２ , 20 mA / cm in FIG. 2B ２ , 40 mA / cm in Fig. 2C ２, 80 mA / cm in FIG. 2D. ２ The cumulative current per unit volume of the electrolyte is in the range of 9.2 to 10.6 Ah / L.

[0049] As described above, when a current is passed through the anode, aluminum in the base material of the AC2A aluminum alloy is dissolved. Here, segregations (crystallized products) of Cu, Si, Fe, etc. and intermetallic compounds (hereinafter simply referred to as impurities) are partially present in the structure of the AC2A aluminum alloy of each anode. When the current density of the anode is 10 mA / cm, ２ (Fig. 2A), 20 mA / cm ２ In the case of (FIG. 2B), when aluminum is dissolved, these impurities remain on the surface. Therefore, as the aluminum dissolution progresses, these impurities eventually fall off the surface of the anode electrode and become sludge.

[0050] On the other hand, the current density of the anode electrode is 40 mA / cm ２ (Fig. 2C) and 80 mA / cm ２ In the case of (Figure 2D), the aluminum surface and the impurity surface are almost the same. This indicates that the impurities are partially dissolved together with the aluminum. In this way, for example, Cu, which has a higher standard electrode potential than aluminum, also ionizes together with the aluminum when the current density increases.

[0051] 3 shows the results of measuring the surface roughness of the anode electrode surface after electrolysis. Both the arithmetic mean height Sa and the maximum height Sz were measured when the current density of the anode electrode was 25 mA / cm ２ 10 mA / cm or less ２ and 20 mA / cm ２ On the other hand, when the current density is 25 mA / cm, the value is relatively high and inversely proportional to the current density of the anode electrode. ２ Greater than 40 mA / cm ２ , 80mA / cm ２ This is because the surface roughness is small when the current density is 25 mA / cm ２In the following cases, impurities contained in the aluminum alloy used for the anode electrode remain on the surface of the anode electrode during the purification of aluminum. ２ By setting the above values, the surface roughness (arithmetic mean height Sa and maximum height Sz) of the anode electrode surface can be controlled, and the elution of impurities can be suppressed.

[0052] It is desirable to deposit aluminum on the cathode electrode so that the arithmetic mean height Sa of the anode electrode during electrolysis (any time from the start of electrolysis to the planned end time of electrolysis after a predetermined time has elapsed) or at the planned end time of electrolysis is 2.3 μm to 10 μm. For example, while a current is flowing through the anode electrode, elements contained in the aluminum alloy are eluted from the anode electrode into the electrolytic solution, so that during this time, it is desirable to deposit aluminum on the cathode electrode by controlling the current to the anode electrode or adjusting the area of ​​the anode electrode exposed above the liquid surface so that the arithmetic mean height Sa of the anode electrode is 2.3 μm to 10 μm.

[0053] When the arithmetic mean height Sa of the portion of the anode electrode surface where the aluminum alloy contacts the electrolyte is 2.3 μm or more, the elution of Cu and other elements is reduced. Furthermore, when the arithmetic mean height Sa is 10 μm or less, the current concentration at the tip of the fine irregularities appearing on the surface of the anode electrode, which would otherwise result in an increase in anode current density, is suppressed, thereby reducing the elution of Cu and other elements. The arithmetic mean height Sa and maximum height Sz were measured using a laser microscope at five locations in a 100 μm × 100 μm square area, with n = 5 for each location, and the average values ​​were calculated. A laser microscope (Keyence VK-X100) with a pinhole confocal optical system was used, using a wavelength of 658 nm, a 50x objective lens, a vertical resolution of 5 nm, and a horizontal resolution of 10 nm.

[0054] To maintain the surface roughness within a predetermined range during electrolysis, an electrolysis test can be performed in advance using an anode electrode manufactured with the same composition as the anode electrode to be actually used (e.g., aluminum alloy or aluminum bullion obtained from scrap), varying the conditions, and the electrolysis conditions can be set using the correlation information between the conditions and the surface roughness obtained from the electrolysis test. For example, electrolysis can be performed in the same electrolyte by varying the current flowing through the anode electrode while applying a DC voltage between the anode and cathode electrodes and measuring the roughness over time. This allows for the correlation information between the change in surface roughness over time for each current to be obtained. Furthermore, by varying the current and the surface area of ​​the anode electrode in contact with the electrolyte, the correlation information between the current density and the surface roughness can be obtained. For example, as described above, after a predetermined time has elapsed since the start of electrolysis, coarse impurities remain on the aluminum surface, increasing the surface roughness (Sa, Sz), and then the coarse impurities are repeatedly removed and exposed. Therefore, it is desirable to perform electrolysis while maintaining this condition.

[0055] These electrolysis conditions are set based on the correlation information. Furthermore, if the correlation information determines that the surface roughness at any time between the start of electrolysis and the end of electrolysis after a predetermined time has elapsed or at the estimated end of electrolysis will exceed the predetermined range, the current density may be reduced before the surface roughness exceeds the predetermined range to prevent further increase in surface roughness. That is, the current may be reduced after a predetermined time has elapsed from the start of electrolysis based on the correlation information obtained in advance. When the electrolysis information is set based on the surface roughness after a predetermined time has elapsed from the start of electrolysis, the correlation information between the surface roughness and the current density when the current density is changed after a predetermined time has elapsed from the start of electrolysis may also be measured in advance, thereby enabling the timing of changing the current value and the amount of reduction. Conversely, since the surface roughness increases as the electrolysis progresses, the surface area of ​​the anode electrode effectively increases. Therefore, the current may be increased to fall within the set current density range, taking into account the increase in the surface area of ​​the anode electrode. That is, the current may be increased after a predetermined time has elapsed from the start of electrolysis based on the correlation information described above.

[0056] Next, the electrodeposited film on the surface of the cathode electrode was observed. Figure 4A is an SEM photograph of the surface of the electrodeposited film, and Figure 4B is an SEM photograph of the cross section of the electrodeposited film. High-purity aluminum (99.99%) was used as the anode electrode, and the anode current density and cathode current density were 80 mA / cm. ２ If high-purity aluminum is used as the anode, there is almost no contamination with impurities, and high-purity electrodeposited films can be obtained even at high current densities.

[0057] On the other hand, Figures 5A and 5B show the results of the AC2A alloy used as the anode electrode, with the anode current density and cathode current density set to 10-80 mA / cm ２ 1 shows SEM photographs of the surface of an electrodeposited film and a cross section of the electrodeposited film when the current density was changed to 40 mA / cm. ２ When the temperature exceeded 100°C, particles that appeared to be Cu were observed in some parts of the cross section, which indicates that Cu ions were reduced on the cathode electrode surface.

[0058] In contrast, Figure 6A is an SEM photograph of the surface of the electrodeposit, and Figure 6B is an SEM photograph of the cross section of the electrodeposit. AC2A alloy was used as the anode electrode, and the anode current density and cathode current density were 10 mA / cm. ２ The results are for a case where the current density was constant at 25 mA / cm. ２ In the following, no Cu was observed in the cross section, and an electrodeposited film having a morphology substantially similar to that obtained when high-purity aluminum was used as the anode electrode (FIGS. 4A and 4B) was obtained.

[0059] FIG. 7 shows the results of component analysis of the electrodeposited film. ２ ~80mA / cm ２ When the current is changed at 10 mA / cm ２ By setting the current density of the anode electrode at a predetermined value or less, the impurity concentration in the deposited film could be reduced.

[0060] The electrolysis conditions were also changed to evaluate the impurity concentrations in the electrodeposited films in more detail. The various conditions and evaluation results for each example are shown in Table 1.

[0061]

[0062] The electrolytes used in Examples No. 1 to No. 6 all contained dialkyl sulfone, aluminum chloride, and additives of ammonium chloride and tetramethylammonium chloride. The electrolytes used in Examples No. 1 to No. 3 and No. 6 contained 0.2 mol of ammonium chloride and 1.0 mol of tetramethylammonium chloride per 10 mol of dialkyl sulfone. On the other hand, the electrolytes used in Examples No. 4 and No. 5 contained 0.1 mol of ammonium chloride and 0.5 mol of tetramethylammonium chloride per 10 mol of dialkyl sulfone.

[0063] In the table, the "composition ratio" of the electrolyte is the molar ratio of dialkyl sulfone to aluminum halide (aluminum chloride). The electrical conductivity varies mainly depending on the composition ratio of dialkyl sulfone to aluminum halide and the temperature. The current density of the anode electrodes of each example from No. 1 to No. 6 is as shown in the table, with the current density of No. 6 being 10 to 80 mA / cm. ２ The impurity concentration in the electrodeposited film was evaluated as follows.

[0064] For Si, 50 ppm or less was rated A (excellent), more than 50 ppm and 150 ppm or less was rated B (good), and more than 150 ppm was rated C (bad). For Cu, 1000 ppm or less was rated A (excellent), more than 1000 ppm and 1500 ppm or less was rated B (good), and more than 1500 ppm was rated C (bad). For Fe, 100 ppm or less was rated A (excellent), more than 100 ppm and 200 ppm or less was rated B (good), and more than 200 ppm was rated C (bad). Regarding the total impurity concentration of Si+Cu+Fe, a value of 1000 ppm or less was rated as A (excellent), a value of more than 1000 ppm and 2000 ppm or less was rated as B (good), and a value of more than 2000 ppm was rated as C (bad). Note that the impurity contents are all expressed as mass ratios.

[0065] From the results, when only Si was considered, all tests were passed (rating B or higher). On the other hand, when the current density was 80 mA / cm ２ reached 25 mA / cm ２ No. 6, which exceeded 100 ppm, had a Si content of 150 ppm or less and received a good evaluation, but the Cu and Fe contents exceeded 1500 ppm and 200 ppm, respectively. Therefore, the evaluations of Cu and Fe were unacceptable (evaluation C), and as a result, the total impurity concentration was unacceptable. In other words, it can be seen that the current density has a greater effect on Cu and Fe than on Si.

[0066] Furthermore, it was found that, for the same composition ratio, the electrical conductivity tends to increase as the temperature increases.

[0067] Considering the results in Table 1 based on the above, a comparison of No. 1, No. 2, and No. 6 shows that the impurity concentration can be reduced by lowering the current density. Furthermore, a comparison of No. 2 and No. 3 and a comparison of No. 4 and No. 5 shows that the impurity concentration tends to increase when the temperature is too high. Furthermore, a comparison of No. 2 and No. 5 and a comparison of No. 3 and No. 4 shows that the impurity concentration tends to increase when the aluminum halide composition ratio is too high.

[0068] The temperature of the electrolytic solution may be 80°C or higher and 120°C or lower, more preferably 85°C or higher and 110°C or lower, and even more preferably 95°C or higher and 110°C or lower.

[0069] While the embodiments of the present invention have been described above with reference to the accompanying drawings, the technical scope of the present invention is not limited to the above-described embodiments. It is clear that those skilled in the art can conceive of various modifications and alterations within the scope of the technical ideas described in the claims, and these modifications and alterations also naturally fall within the technical scope of the present invention.

[0070] DESCRIPTION OF SYMBOLS 1: Aluminum manufacturing apparatus 3: Electrolytic cell 5: DC power supply 7: Anode electrode 9: Cathode electrode 11: Electrolyte

Claims

1. A method for producing aluminum from an aluminum material, for obtaining aluminum of higher purity than the aluminum material, comprising: immersing an anode electrode containing an aluminum material containing 0.1% by mass to 24% by mass of Si and a cathode electrode in an electrolyte; applying a current of 0.1 mA / cm to the surface of the anode electrode where the aluminum material and the electrolyte are in contact with each other; ２ 25mA / cm or more ２ A method for producing aluminum, comprising: passing a current at the following current density to deposit aluminum on the cathode electrode.

2. The method for producing aluminum according to claim 1, wherein the aluminum material of the anode electrode further contains 0.1% by mass or more and 5% by mass or less of Cu.

3. The method for producing aluminum according to claim 1, wherein the aluminum material of the anode electrode further contains 0.15 mass % or more and 1.8 mass % or less of Fe.

4. The method for producing aluminum according to claim 1, characterized in that a current is passed through said anode electrode while stirring the electrolyte around said anode electrode.

5. A method for producing aluminum as claimed in claim 1, characterized in that the surface area of ​​the portion of the anode electrode where the aluminum material and the electrolyte are in contact is made larger than the surface area of ​​the portion of the cathode electrode where the electrolyte is in contact.

6. The method for producing aluminum according to claim 1, wherein the electrolytic solution contains dialkylsulfone and aluminum halide.

7. The method for producing aluminum according to claim 6, characterized in that the molar ratio of said dialkyl sulfone to said aluminum halide in said electrolytic solution is 1.5 to 5 moles of said aluminum halide per 10 moles of said dialkyl sulfone.

8. The electrolyte may comprise an ammonium halide, a hydrogen halide salt of a primary amine, a hydrogen halide salt of a secondary amine, a hydrogen halide salt of a tertiary amine, or a cationically unsaturated amine having the general formula: １ R ２ R ３ R ４ N.X. (R １ ~R ４ The method for producing aluminum according to claim 7, further comprising at least one nitrogen-containing compound selected from the group consisting of quaternary ammonium salts represented by the following formulas: (a) quaternary ammonium salt represented by the formula (I) (wherein X is an alkyl group which may be the same or different, and X is a counter anion for the quaternary ammonium cation).

9. A method for producing aluminum as described in claim 1, characterized in that aluminum is deposited on the cathode electrode so that the arithmetic mean height Sa of the anode electrode is 2.3 μm or more and 10 μm or less while a current is flowing through the anode electrode.

10. The current density is 0.1 mA / cm ２ 20mA / cm or more ２ The method for producing aluminum according to any one of claims 1 to 9, characterized in that:

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