Method for producing aluminum

Abstract

This method for producing aluminum for obtaining aluminum having a higher purity than said aluminum material from an aluminum alloy material is characterized in that: a cathode electrode 9 and an anode electrode 7 comprising an aluminum alloy material containing 0.1 to 24 mass% Si are immersed in an electrolyte 11; applying an electric current at a current density of 0.1 to 25 mA / cm < 2 > on the surface of the anode electrode 7 where the electrolyte 11 and the aluminum alloy material are in contact with each other; and depositing aluminum on the cathode electrode.

Description

Technical Field

[0001] The present invention relates to a method for producing aluminum, wherein aluminum with higher purity can be obtained from, for example, casting alloys. Background Technology

[0002] Unlike steel, widely used aluminum cannot be smelted from ore using a thermal reduction reaction that utilizes carbon. Therefore, smelting methods typically require electricity, such as the Bayer process or the Hall-Hellenic process. For this reason, the production of raw aluminum emits large amounts of carbon dioxide, placing a heavy burden on the environment.

[0003] Therefore, it is preferable to reuse used aluminum products through recycling. For example, attempts have been made to easily produce high-purity aluminum materials from aluminum alloy scrap (e.g., Patent Document 1). Existing technology

[0004] Patent documents

[0005] Patent Document 1: International Patent Publication No. 2020 / 196013 Summary of the Invention

[0006] The problem to be solved by the present invention

[0007] The term "aluminum" encompasses a variety of alloys used depending on the application. For example, in the case of aluminum cans, wrought aluminum materials from the A3000 series (Al-Mn series) are used, and there is no problem if such materials are directly recycled back into the cans. However, when such materials are converted to other alloys, their composition can cause problems. In such cases, it is necessary to reduce the amount of recycled alloy and mix in a certain amount of the original metal. For this reason, recycling to the same alloy series is the most efficient.

[0008] However, despite the existence of a large amount of aluminum alloy scrap already used in automotive engine parts, the amount of recycled aluminum alloys is declining due to the widespread use of electric vehicles and other similar products in recent years. This makes it difficult to reuse aluminum alloys within the same field. In particular, the metals in the aforementioned fields are often castings (castings, die castings) containing a higher alloy content than wrought materials, making them difficult to convert into products for other fields. For example, AC2A alloy (JIS) used as a casting material contains more than 4.0% by mass and less than 6.0% by mass of silicon (Si), and the content of Si is much higher than that of wrought materials. This makes it extremely difficult to reuse such alloys in other wrought material applications.

[0009] This invention was made in view of the following problem. The object of this invention is to provide a method for producing aluminum, wherein aluminum-based metals or used aluminum alloys, particularly aluminum alloys with high Si content, are used as aluminum raw materials, so that aluminum with a higher purity than that of the aluminum raw materials can be produced.

[0010] Problem-solving methods

[0011] To achieve the above objectives, one aspect of the present invention is a method for producing aluminum from aluminum raw materials, wherein aluminum with a purity higher than that of the aluminum raw materials can be obtained. The method includes: immersing an anode electrode and a cathode electrode in an electrolyte, the anode electrode comprising an aluminum raw material containing 0.1% by mass and 24% by mass of Si, and at a concentration of 0.1 mA / cm². 2 Above and 25 mA / cm 2 The following current density is applied to the surface of the aluminum raw material in contact with the electrolyte in the anode electrode, thereby depositing aluminum on the cathode electrode.

[0012] The aluminum raw material for the anode electrode may further contain 0.1% by mass and 5% by mass of copper (Cu).

[0013] The aluminum raw material for the anode electrode may further contain more than 0.15% by mass and less than 1.8% by mass of iron (Fe).

[0014] Preferably, an electric current is applied to the anode electrode while the electrolyte around the anode electrode is stirred.

[0015] Preferably, the surface area of ​​the portion of the aluminum raw material in the anode electrode that contacts the electrolyte is greater than the surface area of ​​the portion of the cathode electrode that contacts the electrolyte.

[0016] Preferably, the electrolyte contains dialkyl sulfone and aluminum halide.

[0017] Preferably, the molar ratio of dialkyl sulfone to aluminum halide in the electrolyte is more than 1.5 moles and less than 5 moles of aluminum halide / 10 moles of dialkyl sulfone.

[0018] Preferably, the electrolyte further comprises at least one nitrogen-containing compound selected from the group consisting of: ammonium halides, hydrohalides of primary amines, hydrohalides of secondary amines, hydrohalides of tertiary amines, and compounds of the general formula [missing information]. The quaternary ammonium salt, where R represents R 1 To R 4 X is a counter anion of the same or different alkyl groups and is a quaternary ammonium cation.

[0019] Preferably, when current flows through the anode electrode, aluminum is deposited on the cathode electrode, such that the arithmetic mean height Sa of the anode electrode is 2.3 µm or more and 10 µm or less.

[0020] Preferably, the current density is 0.1 mA / cm². 2 Above and 20 mA / cm 2 the following.

[0021] Furthermore, the above features can be combined with each other in any combination.

[0022] Effects of the present invention

[0023] The present invention can provide a method for producing aluminum from aluminum raw materials, such as used aluminum alloys, especially aluminum alloys with high Si content, so that aluminum with higher purity can be produced. Attached Figure Description

[0024] Figure 1 This is a schematic diagram illustrating an aluminum production apparatus used in a method for producing aluminum according to an embodiment of the present invention.

[0025] Figure 2A This is when the current density at the anode electrode is 10 mA / cm². 2 Scanning electron microscope (SEM) image of a cross section near the surface of the anode electrode after electrolysis.

[0026] Figure 2B This is when the current density at the anode electrode is 20 mA / cm². 2 SEM image of a cross section near the surface of the anode electrode after electrolysis.

[0027] Figure 2C When the current density at the anode electrode is 40 mA / cm 2 SEM image of a cross section near the surface of the anode electrode after electrolysis.

[0028] Figure 2D When the current density at the anode electrode is 80 mA / cm 2 SEM image of a cross section near the surface of the anode electrode after electrolysis.

[0029] Figure 3 This is a view showing the correlation between the current density at the anode electrode and the surface roughness of the anode electrode surface.

[0030] Figure 4A This is a SEM image of the surface of the electrodeposited film deposited on the cathode electrode when high-purity aluminum is used as the anode electrode.

[0031] Figure 4B This is a SEM image of the cross-section of the electrodeposited film deposited on the cathode electrode when high-purity aluminum is used as the anode electrode.

[0032] Figure 5A This is a SEM image of the surface of the electrodeposited film deposited on the cathode electrode when AC2A alloy is used as the anode electrode and electrolyzed at a high current density.

[0033] Figure 5B This is a SEM image of the cross-section of the electrodeposited film deposited on the cathode electrode when AC2A alloy is used as the anode electrode and electrolyzed at a high current density.

[0034] Figure 6A This is a SEM image of the surface of the electrodeposited film deposited on the cathode electrode when AC2A alloy is used as the anode electrode and electrolysis is performed at a low current density.

[0035] Figure 6B This is a SEM image of the cross-section of the electrodeposited film deposited on the cathode electrode when AC2A alloy is used as the anode electrode and electrolysis is performed at a low current density.

[0036] Figure 7 This is a view showing the concentration of impurities in the electrodeposited film deposited on the cathode electrode. Detailed Implementation

[0037] A method for producing high-purity aluminum from aluminum alloys according to an embodiment of the present invention includes: immersing an anode electrode and a cathode electrode in an electrolyte, the anode electrode comprising an aluminum alloy containing 0.1% by mass and 24% by mass of Si, and at a concentration of 0.1 mA / cm². 2 Above and 25 mA / cm 2 The following current density is applied to the surface of the aluminum material in contact with the electrolyte in the anode electrode, thereby depositing aluminum on the cathode electrode.

[0038] In the following description, a method for producing aluminum according to an embodiment of the present invention will be described with reference to the accompanying drawings. Figure 1 This is a schematic diagram of aluminum production equipment 1 used in a method for producing aluminum according to an embodiment of the present invention. In the following description, the concentration of each element can be measured by ICP emission spectroscopy.

[0039] The aluminum production apparatus 1 used in the method for producing aluminum according to an embodiment of the present invention includes an electrolytic cell 3, a DC power supply 5, an anode electrode 7, a cathode electrode 9, and an electrolyte 11. The anode electrode 7 comprises an aluminum alloy extracted from waste materials, etc. The anode electrode 7 and the cathode electrode 9, on which high-purity aluminum is deposited, are then immersed in the electrolyte 11, and the DC power supply 5 applies a DC voltage between the anode electrode 7 and the cathode electrode 9. At this time, when 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 portion of the aluminum alloy contained in the anode electrode 7 that contacts the electrolyte 11 is adjusted, such that the current density at the portion of the aluminum alloy surface of the anode electrode 7 that contacts the electrolyte 11 falls within a predetermined range. By applying current in this manner to allow it to flow through the anode electrode 7, an electrodeposition film of high-purity aluminum is deposited on the cathode electrode 9.

[0040] 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 will be referred to as the "current density at the anode electrode". Furthermore, 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 a value obtained by dividing the current between the anode electrode and the DC power supply 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.

[0041] An electrolytic cell 3, located within the aluminum production equipment 1, stores an electrolyte 11 in which aluminum can be dissolved, and an anode electrode 7 and a cathode electrode 9 are immersed in the electrolyte 11. The anode electrode 7 and cathode electrode 9 are connected to each other via a DC power supply 5. The anode electrode 7 is made of an aluminum alloy with an aluminum purity lower than the target purity and contains 0.1% by mass and 24% by mass of Si. Furthermore, the aluminum alloy may contain 0.1% by mass and 5% by mass of Cu and 0.15% by mass and 1.8% by mass of Fe. If the Si content exceeds the aforementioned upper limit, Si may become a factor hindering the dissolution of aluminum during electrolysis, slowing down the deposition rate of the target high-purity aluminum and leading to a decrease in yield. On the other hand, if the Cu and Fe contents exceed the aforementioned upper limits, Cu and Fe tend to dissolve into the electrolyte 11. This makes it more difficult to obtain the target high-purity aluminum. The anode electrode 7 can be obtained by molding used materials (such as AC2A alloy) into electrode shapes using casting or the like. The anode electrode 7 may contain at least 0.1% by mass and less than 24% by mass of Si, and may further contain elements more valuable than aluminum, other than magnesium (Mg).

[0042] The cathode electrode 9 is made of, for example, aluminum, and can be pure aluminum or an aluminum alloy specified by JIS standards for wrought materials. That is, the cathode electrode 9 has a lower content of at least Si and Cu than the aluminum alloy contained in the anode electrode 7. By making the cathode electrode 9 from aluminum, the entire cathode electrode on which aluminum is electrodeposited can be used as a high-purity aluminum-based metal. Therefore, it is preferred that the cathode electrode 9 be made of aluminum with a purity equal to or higher than the target purity. Besides aluminum, metals such as titanium or stainless steel can also be used for the cathode electrode 9. Titanium or stainless steel has a dense oxide film on its surface. Therefore, when a cathode electrode made of titanium or stainless steel is used and aluminum is electrodeposited on its surface, the electrodeposited aluminum can be easily peeled off from the cathode electrode 9.

[0043] The DC power supply 5, which applies a DC voltage between the anode electrode 7 and the cathode electrode 9, can be any known DC power supply device. However, to reduce the amount of impurities dissolved into the electrolyte 11, as will be described below, it is preferable that the DC power supply is equipped with a current control mechanism for controlling the output current. Additionally, to control the current density at the anode electrode 7, a current detector (not shown) for measuring the current flowing between the anode electrode 7 and the DC power supply 5 can be placed near the anode electrode 7. The current density at the anode electrode can be calculated from the surface area of ​​the anode electrode 7 in contact with the electrolyte 11 and the current detected by the current detector. For this reason, it is preferable that the output current of the DC power supply 5 can be adjusted by the current control mechanism of the DC power supply 5 so that the current density at the anode electrode falls within a predetermined range. Furthermore, the area of ​​the aluminum alloy used for the anode electrode 7 in contact with the electrolyte 11 can be changed according to the current flowing through the anode electrode 7. For example, the current density at the anode electrode can be adjusted by changing the area of ​​the anode electrode exposed above the surface of the electrolyte 11, thereby changing the contact area between the anode electrode 7 and the electrolyte 11.

[0044] The electrolyte 11 into which the anode electrode 7 and cathode electrode 9 are to be immersed is a non-aqueous system (non-aqueous electrolyte) containing, for example, (1) dialkyl sulfone and (2) aluminum halide. By using such an electrolyte 11, high-purity aluminum can be formed on the surface of the cathode electrode 9 at a high film-forming rate. Furthermore, it is preferred that the electrolyte 11 further contains (3) at least one nitrogen-containing compound selected from the group consisting of: ammonium halides, hydrohalates of primary amines, hydrohalates of secondary amines, hydrohalates of tertiary amines, and compounds of the general formula [missing information]. The quaternary ammonium salt, where R represents R 1 To R 4 The alkyl groups are the same or different, and X is a counter anion of a quaternary ammonium cation. Compared with electrolytes using ionic liquids, the above-described electrolyte 11 has industrial advantages in terms of reagent availability and low cost.

[0045] Examples of dialkyl sulfones included in the electrolyte 11 described above include those dialkyl sulfones having 1 to 6 carbon atoms (which may be straight-chain or branched), such as dimethyl sulfone, diethyl sulfone, dipropyl sulfone, dihexyl sulfone, and methyl ethyl sulfone. Dimethyl sulfone is preferably used from the perspective of good conductivity and availability.

[0046] Examples of aluminum halides include aluminum chloride and aluminum bromide. Aluminum halides are preferably anhydrous. Anhydrous compounds do not contain water molecules that reduce electrodeposition efficiency, and therefore the electrodeposition efficiency is not reduced.

[0047] Examples of ammonium halides that can be used as nitrogen-containing compounds include ammonium chloride and ammonium bromide. Additionally, examples of primary to tertiary amines in the hydrohalides of primary to tertiary amines include those amines in which the alkyl group has 1 to 6 carbon atoms (which can be straight-chain 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. (The last sentence appears to be incomplete and possibly refers to a different context.) The quaternary ammonium salt, where R represents R 1 To R 4 The counter anion of R is either the same or different alkyl group and X is a quaternary ammonium cation. 1 To R 4 Examples of alkyl groups are those having 1 to 6 carbon atoms (which can be straight-chain or branched), such as methyl, ethyl, propyl, and hexyl. Examples of X include halide ions, such as chloride, bromide, and iodide ions, as well as BF4. - and PF6 - Specific examples of compounds include tetramethylammonium chloride, tetramethylammonium bromide, tetramethylammonium iodide, and tetramethylammonium tetrafluoroborate. Preferred examples of nitrogen-containing compounds, from the perspective of facilitating the formation of high-purity aluminum at high film-forming rates, include tertiary amine hydrochlorides, such as trimethylamine hydrochloride.

[0048] For example, the mixing ratio of dialkyl sulfone and aluminum halide is preferably 1.5 mol or more and 5.0 mol or less aluminum halide / 10 mol dialkyl sulfone. Furthermore, the upper limit of this mixing ratio is more preferably 4.0 mol or less, 3.0 mol or less, or 2.5 mol or less, and most preferably 1.5 mol or more and 2.0 mol or less. If the amount of aluminum halide mixed is less than 1.5 mol / 10 mol dialkyl sulfone, the formed aluminum may darken (a phenomenon known as "burning"), or the film-forming efficiency may deteriorate. On the other hand, if the amount of aluminum halide mixed exceeds 5.0 mol / 10 mol dialkyl sulfone, the liquid resistance of the electrolyte 11 becomes too high, which may cause the electrolyte 11 to generate heat and decompose.

[0049] Furthermore, if the ratio of aluminum halide to dialkyl sulfone is too high, the concentration of ions serving as the aluminum source may become too high, potentially hindering the transfer of aluminum ions from the anode side. On the other hand, if the ratio of aluminum halide to dialkyl sulfone is too low, it contributes to a reduction in the number of conductive ions, which may degrade the efficiency of electrolysis.

[0050] On the other hand, the nitrogen-containing compound to be blended with dialkyl sulfone and aluminum halide is preferably 0.01 mol or more and 2.0 mol or less per 10 mol of dialkyl sulfone, or more preferably 0.05 mol or more and 1.5 mol or less per 10 mol of dialkyl sulfone. If the amount of nitrogen-containing compound blended is less than 0.01 mol per 10 mol of dialkyl sulfone, it may be difficult to obtain the blending effect, such as the effect of increasing the film formation rate by increasing the conductivity of electrolyte 11, and the effect of improving the purity and ductility of aluminum. In addition, if the amount of nitrogen-containing compound blended exceeds 2.0 mol per 10 mol of dialkyl sulfone, the composition of electrolyte 11 may be substantially changed, which may hinder the deposition of aluminum.

[0051] Electrolysis of aluminum using the above-described electrolyte 11 can be performed in an electrolyte 11 at a temperature of 80°C or higher and 120°C or lower, with an applied current density of 0.1 mA / cm². 2 Above and 25 mA / cm 2 The following conditions are preferred. More preferably, the applied current density is 0.1 mA / cm². 2 Above and 20 mA / cm 2 The following conditions apply. The lower limit of the temperature of the electrolyte 11 is determined by considering its melting point, and is preferably above 85°C, or more preferably above 95°C (if the temperature is below the melting point of the electrolyte 11, the electroplating solution solidifies and electroplating can no longer be performed). When the temperature of the electrolyte 11 is below 120°C, the active reaction between aluminum and the electrolyte can be suppressed, making it more difficult for a large number of impurities to be incorporated into the aluminum. In order to efficiently obtain a high-purity aluminum electrodeposition film by electrolysis, it is necessary to efficiently transfer the metal ions to be deposited on the cathode electrode from the anode electrode to the surface of the cathode electrode. Here, the viscosity of the electrolyte decreases by increasing the temperature, and therefore, the mass transfer efficiency increases, thereby improving the conductivity. Therefore, considering the conductivity, the temperature of the electrolyte should be high. However, if the temperature of the electrolyte becomes too high, the surface of the cathode electrode becomes activated, and the deposition of impurities is more likely to occur. This may lead to a deterioration in purity. For this reason, the temperature of the electrolyte 11 during electrolysis is preferably 85°C or higher, or more preferably 95°C or higher, and its upper limit is preferably 120°C or lower.

[0052] Additionally, 0.1 mA / cm 2The above-mentioned anodic current density can suppress the decrease in film formation efficiency. On the other hand, 25 mA / cm 2 The following anodic current densities can contribute to the achievement of high-purity aluminum, as described below. Furthermore, preferably, 20 mA / cm². 2 The following anolyte current density allows for easier achievement of high purity. There are no particular restrictions on the electrolyte 11 used.

[0053] Next, we will refer to Figure 1 This describes the theory for obtaining high-purity aluminum in an aluminum production apparatus 1 according to an embodiment of the present invention. When the anode electrode 7 and the cathode electrode 9 are immersed in the electrolyte and a DC current is applied to make them flow, aluminum loses its electrons on the surface of the anode electrode 7 and dissolves into the electrolyte 11 as aluminum ions (…). Figure 1 (A) On the surface of cathode electrode 9, electrons are provided to aluminum ions, and aluminum metal is deposited.

[0054] To efficiently supply aluminum ions to the cathode electrode 9, it is preferable to stir the electrolyte 11 while current flows through the anode electrode 7. Therefore, it is preferable to provide a release outlet for releasing an inert gas (such as nitrogen) at the bottom of the electrolytic cell 3, and to release bubbles of the inert gas from such a release outlet into the electrolyte 11 (cathode electrode 9) (so-called bubbling). Due to the flow of bubbles in the electrolyte 11, convection also occurs in the electrolyte 11, thereby stirring the electrolyte 11. Alternatively, a pump or stirring device can be used to provide the liquid flow, or the electrodes themselves can be swung. Furthermore, although a pair of anode electrodes 7 and cathode electrodes 9 are provided in the illustrated example, multiple anode electrodes 7 and cathode electrodes 9 can also be provided alternately.

[0055] Here, the inventors of this invention have discovered that: (1) when metal ions dissolve from the anode electrode 7, the amount of dissolved metal ions varies depending on the current density at the anode electrode 7. For example, since Cu has a higher standard electrode potential than aluminum, aluminum preferentially ionizes at the anode electrode 7 under equilibrium conditions. However, in practice, Cu ionization also occurs when Cu dissolves in aluminum, or when the current density is equal to or greater than a predetermined value and the concentration of aluminum ions on the surface of the anode electrode 7 increases. In contrast, it has been found that maintaining the current density at the anode electrode 7 at 25 mA / cm² has been effective. 2The following measures can suppress the leaching of elements more valuable than aluminum (such as Cu). Therefore, in the aluminum production apparatus 1 according to an embodiment of the present invention, a DC power supply 5 with a current adjustment mechanism is used. The current adjustment mechanism of the DC power supply 5 can change the current flowing from the DC power supply 5 to the anode electrode 7, so that even when the surface shape of the anode electrode 7 changes and the area of ​​the surface in contact with the electrolyte 11 changes, the current through the anode electrode 7 can be adjusted to achieve the aforementioned current density.

[0056] The inventors have also discovered (2) the conditions under which segregated portions of Cu, Si, etc. (crystallized portions of Cu and Si) already present in the material constituting the anode electrode 7 appear on the surface of the anode electrode 7 and detach when aluminum ions dissolve from the anode electrode 7. It has been found that if Cu and Si do not dissolve into the electrolyte 11, and the different elemental components (such as Cu and Si) crystallized on the surface of the anode electrode 7 as described above detach in solid form ( Figure 1 In the case of B), high-purity aluminum can be produced efficiently. That is, by setting the current density at the anode electrode 7 to be equal to or less than a predetermined value, it has been found that when Cu and Si are not completely uniformly dispersed in the material constituting the anode electrode 7, but rather Cu phase, Si phase, or intermetallic compound phase containing them (e.g., phase with a maximum diameter of 10 µm or more) are present, such phases can detach from the anode electrode 7 and can be removed without being dissolved.

[0057] To allow impurities contained in the aluminum of the anode electrode 7 to be efficiently removed from the anode electrode 7 without dissolving into the electrolyte 11, it is preferable to apply a DC voltage between the anode electrode 7 and the cathode electrode 9 while, for example, stirring the electrolyte 11 around the anode electrode 7. For example, instead of the bubbling directed towards the cathode electrode 9 as described above, stirring the electrolyte can be done by bubbling near the anode electrode 7 or by using a stirring device, or by oscillating or rotating the anode electrode 7. Additionally, the surface of the anode electrode 7 can be physically rubbed with another insulating member during electrolysis to allow impurities to be removed.

[0058] Furthermore, to allow impurities to be efficiently removed from the anode electrode 7, a process for coarsening the impurity phase can be performed during the manufacturing of the anode electrode 7 to facilitate impurity removal. This makes the impurities easier to remove. For example, when forming the anode electrode by melting and casting aluminum materials containing Si or Cu (such as aluminum alloys or aluminum-based metals extracted from scrap), Cu phases, Si phases, or intermetallic compound phases containing them with a size of 10 µm or more (or even 20 µm or more) can be formed by reducing the cooling rate after solidification.

[0059] To reduce the current density at the anode electrode 7, it is preferable that the surface area of ​​the anode electrode (the area in contact with the electrolyte 11, which also applies hereinafter) is sufficiently larger (e.g., more than 1.5 times) than the surface area of ​​the portion of the cathode electrode 9 in contact with the electrolyte 11. Although there is an upper limit to the current density at the anode electrode 7 as mentioned above, the deposition rate of aluminum deposited on the cathode electrode 9 increases with the increase of the current flowing through the anode electrode 7. Therefore, by making the surface area of ​​the portion of the aluminum alloy of the anode electrode 7 in contact with the electrolyte larger than the surface area of ​​the portion of the cathode electrode 9 in contact with the electrolyte 11, the efficiency of electrodeposition of high-purity aluminum can be improved.

[0060] Furthermore, in embodiments of the present invention, the anode electrode 7 can be constructed from a mesh cage made of a metallic material of an element more precious than aluminum, wherein aluminum alloy is extracted from waste and processed into particles contained within the cage. By using such an anode electrode 7, the current flowing through the anode electrode 7 can be increased while maintaining the current density at the anode electrode 7 at or below the aforementioned upper limit, thereby increasing the deposition rate of aluminum deposited on the cathode electrode 9.

[0061] Additionally, in the electrolyte 11, impurity (sludge) particles detached from the anode electrode 7 settle below the anode electrode 7. Therefore, a step of collecting this sludge during or after electrolysis can be included. For example, the sludge can be collected by filtration using filter paper, a filter, an anode bag, or a cathode bag, allowing the electrolyte 11 to be reused. The filter paper, filter, anode bag, or cathode bag used can have a nominal pore size from 0.1 µm to 100 µm. The lower limit is preferably 1 µm or more. In the case of an aluminum material with increased grain size containing Cu phase, Si phase, or intermetallic compound phases comprising them, the lower limit is preferably 10 µm or more, or more preferably 20 µm or more.

[0062] As described above, according to embodiments of the present invention, high-purity aluminum can be obtained efficiently, particularly from aluminum alloys containing large amounts of Si and Cu, wherein the amount of such impurity elements is reduced. For example, the total content of Si, Fe, and Cu in the high-purity aluminum obtained from embodiments of the present invention can be less than 0.1% by mass. Furthermore, compared to the CO2 emissions from obtaining 1 kg of primary metal from ore using conventional smelting methods, the CO2 emissions from obtaining 1 kg of high-purity aluminum by electrolysis according to embodiments of the present invention can be reduced by approximately 45%.

[0063] Three-layer electrolysis is typically known for its use in highly purified aluminum. However, it is difficult to improve the purity of used materials in cast alloys using three-layer electrolysis because cast alloys have particularly high impurity concentrations and cannot maintain the anodic composition. Therefore, three-layer electrolysis is difficult to apply. Furthermore, the typical alloying element Si has a density similar to that of aluminum and is lighter than the molten salt used. Therefore, Si may be mixed into the refined aluminum, making its separation difficult.

[0064] In contrast, the method for producing aluminum according to embodiments of the present invention can efficiently remove impurity elements including Si, and therefore the obtained aluminum can be further purified to a higher degree by a three-layer electrolysis process. That is, the aluminum obtained by embodiments of the present invention is effectively used as an aluminum raw material (anode-side raw material) in a three-layer electrolysis process.

[0065] As described above, the method for producing aluminum according to embodiments of the present invention can obtain aluminum of higher purity from aluminum alloys or aluminum-based metals containing more than 0.1% by mass and less than 24% by mass of Si. Therefore, recycled casting alloys can, for example, be highly purified for conversion to other purposes. Furthermore, it is easier to convert recycled alloys (such as automotive engine parts, particularly those containing large amounts of Cu) into products for other purposes, thereby making more efficient use of used alloys. Additionally, high-purity aluminum can be produced from aluminum-based metals.

[0066] Furthermore, by stirring the liquid near the anode electrode 7, sludge can be efficiently removed from the anode. Additionally, by making the surface area of ​​the anode electrode 7 in contact with the electrolyte larger than the surface area of ​​the cathode electrode, the current density at the anode can be suppressed, while simultaneously allowing high-purity aluminum to be efficiently deposited onto the cathode.

[0067] In addition, by using an electrolyte containing dialkyl sulfones, aluminum halides, and nitrogen-containing compounds, operation can be carried out at a lower temperature than when using molten salts, and the process is safe and easy.

[0068] In addition, by collecting the sludge that falls off from the anode electrode 7, pollution can be suppressed, and aluminum alloys can be purified efficiently.

[0069] At this point, the sludge collection rate can be increased by using filter paper or similar materials with appropriate pore sizes to collect the sludge.

[0070] Example

[0071] Use with Figure 1The equipment shown is configured similarly to the aluminum production equipment, performing electrolysis of the aluminum alloy used as the anode electrode and deposition of high-purity aluminum on the cathode electrode. As the anode electrode, 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) is used. Figures 2A to 2D These are SEM images of a cross-section near the surface of the anode electrode, showing the differences in the anode electrode surface after electrolysis as the current density at the anode electrode changes. In each of the SEM images, the current density at the anode electrode is... Figure 2A The value is 10 mA / cm. 2 ,exist Figure 2B The value is 20 mA / cm. 2 ,exist Figure 2C The value is 40 mA / cm 2 ,exist Figure 2D The value is 80 mA / cm 2 The cumulative current per unit volume of electrolyte is in the range of 9.2 to 10.6 Ah / L.

[0072] As mentioned above, by applying a current flowing through the anode electrode, the aluminum of the AC2A aluminum alloy matrix material is dissolved. Here, segregation (crystallization products) or intermetallic compounds (hereinafter referred to as impurities) of Cu, Si, Fe, etc., are partially present in each structure of the AC2A aluminum alloy at the anode electrode. When the current density at the anode electrode is 10 mA / cm²... 2 ( Figure 2A ) or 20 mA / cm 2 ( Figure 2B When aluminum is dissolved, such impurities remain on the surface. Therefore, as aluminum dissolution proceeds, these impurities eventually detach from the surface of the anode electrode, becoming sludge.

[0073] On the other hand, when the current density at the anode electrode is 40 mA / cm 2 ( Figure 2C ) or 80 mA / cm 2 ( Figure 2D At this point, the aluminum surface and the impurity surface almost overlap each other. This indicates that the impurities are partially dissolved along with the aluminum. As mentioned above, when the current density increases, Cu, which has a higher standard electrode potential than aluminum, is ionized, for example, along with aluminum.

[0074] Figure 3 The results of measuring the surface roughness of the anode electrode surface after electrolysis are shown. This was done when the current density at the anode electrode was 25 mA / cm². 2 The following applies, where the current density is 10 mA / cm². 2 and 20 mA / cm2 In the case of [specific event], both the arithmetic mean height Sa and the maximum height Sz are relatively high, and are inversely proportional to the current density at the anode electrode. On the other hand, when the current density at the anode electrode is greater than 25 mA / cm², [the following occurs]. 2 When the current density is 40 mA / cm 2 or 80 mA / cm 2 At this time, the surface roughness has a relatively small value. This is because when the current density is 25 mA / cm², the surface roughness is relatively small. 2 In the following cases, impurities contained in the aluminum alloy used as the anode electrode remain on the anode electrode surface during the high purification of aluminum. The current density is thus set to 25 mA / cm². 2 The surface roughness (arithmetic mean height Sa and maximum height Sz) of the anode electrode surface can be controlled to suppress the leaching of impurities.

[0075] Preferably, aluminum is deposited on the cathode electrode during electrolysis (any time from the start of electrolysis until the planned end time of the electrolysis program after a predetermined period) or at the end of the electrolysis program, such that the arithmetic mean height Sa is 2.3 µm or more and 10 µm or less. For example, when current flows through the anode electrode, elements contained in the aluminum alloy dissolve from the anode electrode into the electrolyte, and therefore, during such a period, it is preferable to deposit aluminum on the cathode electrode while controlling the current through the anode electrode or adjusting the area of ​​the anode electrode exposed above the liquid surface, such that the arithmetic mean height Sa is 2.3 µm or more and 10 µm or less.

[0076] When the arithmetic mean height Sa of the portion of the aluminum alloy on the anode electrode surface in contact with the electrolyte is greater than 2.3 µm, the dissolution of Cu and other substances decreases. When the arithmetic mean height Sa is less than 10 µm, the current concentrates on the tip of the small roughness on the surface of the anode electrode, which suppresses the increase in anode current density, thereby reducing the dissolution of Cu and other substances. Both the arithmetic mean height Sa and the maximum height Sz were measured using a laser microscope at five locations within a 100 µm × 100 µm square area, where n = 5 for each location, and their average values ​​were calculated. The laser microscope used was a pinhole confocal optical system (using a VK-X100 manufactured by Keyence), with a wavelength of 658 nm, a 50x objective lens, a vertical resolution of 5 nm, and a horizontal resolution of 10 nm.

[0077] To perform electrolysis while maintaining surface roughness within a predetermined range, an anode electrode (e.g., an aluminum alloy or aluminum-based metal obtained from scrap) manufactured with the same composition as the anode electrode to be used can be pre-tested under various conditions. Information obtained from these electrolysis tests regarding the correlation between various conditions and surface roughness can be used to set the electrolysis conditions. For example, by applying a DC voltage between the anode and cathode electrodes in the same electrolyte while varying the current flowing through the anode electrode to perform electrolysis, and measuring the roughness over an elapsed time, information regarding the correlation between elapsed time and changes in surface roughness for each current can be obtained. Furthermore, by varying the current and the surface area of ​​the anode electrode in contact with the electrolyte, information regarding the correlation between current density and surface roughness can be obtained. For example, as mentioned above, after a predetermined time has elapsed since the start of electrolysis, coarse impurities remain on the aluminum surface, increasing surface roughness (Sa and Sz), and the shedding and exposure of these coarse impurities then repeats. Therefore, it is preferable to maintain this state during electrolysis.

[0078] These electrolysis conditions are set based on the aforementioned correlation information. Furthermore, if the correlation information determines that the surface roughness will exceed the predetermined range at any time from the start of electrolysis until the end of electrolysis after a predetermined time, or at the planned end of the electrolysis, the current density can be reduced before the surface roughness exceeds this range to suppress further increase in surface roughness. In other words, the current can be reduced based on the pre-obtained correlation information after a predetermined time period since the start of electrolysis. It is important to note that when electrolysis information is set based on correlation information obtained from surface roughness after a predetermined time period since the start of electrolysis, the timing and amount of current reduction for changing the current value can be determined by pre-measuring the correlation information between surface roughness and current density when the current density changes after a predetermined time since the start of electrolysis. Conversely, as electrolysis proceeds, surface roughness increases, and therefore the surface area of ​​the anode electrode increases significantly. For this reason, considering the increase in the surface area of ​​the anode electrode, the current can be increased to keep it within the set current density range. In other words, the current can be increased after a predetermined time since the start of electrolysis based on the aforementioned correlation information.

[0079] Next, observe the electrodeposited film deposited on the surface of the cathode electrode. Figure 4A This is a SEM image of the surface of the electrodeposited film. Figure 4B These are SEM images of cross-sections of the electrodeposited films, in which high-purity aluminum (99.99%) is used as the anode electrode and the anode and cathode current densities are set to 80 mA / cm². 2The result is as follows. If high-purity aluminum is used as the anode electrode, there is almost no contamination from impurities, and high-purity electrodeposited films can be obtained even at high current densities.

[0080] On the other hand, Figure 5A and Figure 5B These are when AC2A alloy is used as the anode electrode and the anode current density and cathode current density are from 10 mA / cm². 2 Change to 80 mA / cm 2 SEM images of the surface and cross-section of the electrodeposited film. When AC2A alloy is used as the anode electrode and the current density exceeds 40 mA / cm². 2 At that time, what appeared to be Cu particles were observed on the cross-section. This indicates that Cu ions were reduced on the surface of the cathode electrode.

[0081] In comparison, Figure 6A This is a SEM image of the surface of the electrodeposited film. Figure 6B These are SEM images of cross-sections of the electrodeposited films, where AC2A alloy is used as the anode electrode and the anode and cathode current densities are kept constant at 10 mA / cm². 2 The results under the condition that AC2A alloy is used as the anode electrode and the current density is 25 mA / cm². 2 In the following cases, no Cu was observed in the cross-section, and high-purity aluminum could be used as the anode electrode. Figure 4A and Figure 4B The electrodeposited film has a roughly the same morphology as the previous one.

[0082] Figure 7 The results of the compositional analysis of the electrodeposited film are shown, illustrating the current ranging from 10 mA / cm². 2 Change to 80mA / cm 2 The situation and the current is kept constant at 10 mA / cm 2 The difference lies in the conditions between the two. By setting the current density at the anode electrode below a predetermined value, the concentration of impurities in the obtained electrodeposited film can be reduced.

[0083] Furthermore, the concentration of impurities in the electrodeposited film was evaluated in more detail by changing the electrolysis conditions. Table 1 shows the various conditions and evaluation results for each working example.

[0084] [Table 1]

[0085]

[0086] The electrolytes used in Examples 1 through 6 all contain dialkyl sulfones, aluminum chloride, and the additives ammonium chloride and tetramethylammonium chloride. The electrolytes used in Examples 1 through 3 and 6 contain 0.2 moles of aluminum chloride and 1.0 mole of tetramethylammonium chloride per 10 moles of dialkyl sulfones. On the other hand, the electrolytes used in Examples 4 and 5 contain 0.1 moles of aluminum chloride and 0.5 moles of tetramethylammonium chloride per 10 moles of dialkyl sulfones.

[0087] In the table, the "composition ratio" of the electrolyte is the molar ratio of dialkyl sulfone to aluminum halide (aluminum chloride). The conductivity varies mainly depending on the composition ratio of dialkyl sulfone to aluminum halide and the temperature.

[0088] The current density at the anode electrode of each of Examples 1 to 6 is shown in the table, wherein the current density in Example 6 is 10 mA / cm². 2 Up to 80 mA / cm 2 It varies within a certain range.

[0089] The concentration of impurities in the electrodeposited film was evaluated as follows.

[0090] For Si, concentrations below 50 ppm are rated A (Excellent), concentrations between 50 ppm and 150 ppm are rated B (Good), and concentrations above 150 ppm are rated C (Poor). Similarly, for Cu, concentrations below 1000 ppm are rated A (Excellent), concentrations between 1000 ppm and 1500 ppm are rated B (Good), and concentrations above 1500 ppm are rated C (Poor). For Fe, concentrations below 100 ppm are rated A (Excellent), concentrations between 100 ppm and 200 ppm are rated B (Good), and concentrations above 200 ppm are rated C (Poor). Finally, for the total impurity concentration of Si+Cu+Fe, concentrations below 1000 ppm are rated A (Excellent), concentrations between 1000 ppm and 2000 ppm are rated B (Good), and concentrations above 2000 ppm are rated C (Poor). In all cases, the impurity content is expressed as a mass ratio.

[0091] The results show that all tests passed (achieving an evaluation of B or higher) when only Si was considered. On the other hand, the current density reached 80 mA / cm². 2 More than 25 mA / cm 2In item 6, although the Si content was below 150 ppm and rated as good, the Cu and Fe contents exceeded 1500 ppm and 200 ppm, respectively. Therefore, Cu and Fe were rated as unqualified (Rating C), and thus, the overall impurity concentration was unqualified. In other words, it was found that current density had a greater effect on Cu and Fe than on Si.

[0092] Furthermore, it was found that for the same composition ratio, conductivity tends to increase with increasing temperature.

[0093] Based on the results in Table 1, comparing items 1, 2, and 6, it appears that reducing the current density can decrease the impurity concentration. Furthermore, comparing items 2 and 3, and 4 and 5, it was found that the impurity concentration tends to increase with increasing temperature. Additionally, comparing items 2 and 5, and 3 and 4, it was found that the impurity concentration tends to increase as the aluminum halide composition becomes too high.

[0094] Preferably, the temperature of the electrolyte is above 80°C and below 120°C, more preferably above 85°C and below 110°C, or even more preferably above 95°C and below 110°C.

[0095] Although embodiments of the invention have been described with reference to the accompanying drawings, the scope of the invention is not limited to the embodiments described above. It will be apparent to those skilled in the art that various changes or modifications will occur within the scope of the technical concept disclosed in the claims, and it should be understood that they naturally fall within the scope of the invention.

[0096] Explanation of symbols

[0097] 1 ...... Aluminum production equipment

[0098] 3 ...... Electrolytic cell

[0099] 5 ...... DC power supply

[0100] 7 ...... Anode electrode

[0101] 9 ...... Cathode electrode

[0102] 11 ...... Electrolyte

Claims

1. A method for producing aluminum from aluminum raw materials, wherein aluminum with a purity higher than that of the aluminum raw materials can be obtained, the method comprising: The anode electrode and the cathode electrode are immersed in an electrolyte, wherein the anode electrode comprises the aluminum raw material containing more than 0.1% by mass and less than 24% by mass of Si; as well as At 0.1 mA / cm 2 Above and 25 mA / cm 2 The following current density is applied to the surface of the anode electrode where the aluminum raw material is in contact with the electrolyte, thereby depositing aluminum on the cathode electrode.

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

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

4. The method for producing aluminum according to claim 1, wherein... The current is applied to the anode electrode while the electrolyte around the anode electrode is stirred.

5. The method for producing aluminum according to claim 1, wherein... The surface area of ​​the portion of the aluminum material in the anode electrode that contacts the electrolyte is greater than the surface area of ​​the portion of the cathode electrode that contacts the electrolyte.

6. The method for producing aluminum according to claim 1, wherein... The electrolyte contains dialkyl sulfone and aluminum halide.

7. The method for producing aluminum according to claim 6, wherein... The molar ratio of the dialkyl sulfone to the aluminum halide in the electrolyte is more than 1.5 moles and less than 5 moles of the aluminum halide / 10 moles of the dialkyl sulfone.

8. The method for producing aluminum according to claim 7, wherein The electrolyte further comprises at least one nitrogen-containing compound selected from the group consisting of: ammonium halides, hydrohalates of primary amines, hydrohalates of secondary amines, hydrohalates of tertiary amines, and compounds of the general formula [missing information]. The quaternary ammonium salt, where R represents R 1 To R 4 X is a counter anion of the same or different alkyl groups and is a quaternary ammonium cation.

9. The method for producing aluminum according to claim 1, wherein... As the current flows through the anode electrode, the aluminum is deposited on the cathode electrode such that the arithmetic mean height Sa of the anode electrode is greater than 2.3 µm and less than 10 µm.

10. The method for producing aluminum according to any one of claims 1 to 9, wherein The current density is 0.1 mA / cm². 2 Above and 20 mA / cm 2 the following.

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