Aluminum electrodeposition method

Abstract

To provide a method for efficiently electrodepositing aluminum while suppressing evaporation of a molten salt as an electrolyte.SOLUTION: The present invention relates to an electrodeposition step for depositing aluminum on a deposit object by electrifying between the deposit object (e1) on the cathode side and an aluminum substrate (e2) on the anode side in a molten salt (m) including an aluminum chloride. The electrodeposition step includes coating the upper surface of the molten salt with a solution layer (a) separated two-layered from the molten salt. The solution layer, for example, comprises a nonpolar material such as a hydrocarbon having a lower melting point, higher boiling point, and higher flash point than the electrodeposition temperature. An embodiment of the solution layer is a squalane (C30H62) or a paraffin, one kind of saturated hydrocarbons. The electrodeposition step may be a plating step in which an aluminum plating layer is formed on the surface of the deposit object, or a refining step in which aluminum is extracted from a scrap material or a recycled material.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to an aluminum electrodeposition method and the like. [Background technology]

[0002] Nickel plating, zinc plating, tin plating, copper plating, etc. are applied to steel members to prevent corrosion, etc. However, such plating involves concerns about the supply of metal resources and environmental impacts when plating baths are disposed of.

[0003] As an alternative to metal plating, aluminum plating has been gaining attention. Aluminum sources are abundant, and aluminum plating can be performed without using harmful substances such as cyanide compounds. Aluminum plating can also be further anodized.

[0004] However, the standard electrode potential of Al (-1.67 V) is lower than the hydrogen evolution potential, making it a metal far more base than Zn, Ni, Sn, Cu, etc. In other words, the reductive deposition potential of Al is significantly lower than the potential window of water. For this reason, non-aqueous solvents (molten salts, ionic liquids with a wider potential window than water, etc.) are used for aluminum plating. For example, the following patent documents contain relevant information: [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2012 / 043129 [Patent Document 2] Patent Publication No. 2023-19681 Summary of the Invention [Problem to be solved by the invention]

[0006] Molten salts, which are obtained by mixing and heating aluminum halides (e.g., aluminum chloride) and metal halides (e.g., sodium chloride, potassium chloride), are used as inexpensive solvents (electrolytes) for aluminum plating. In particular, using molten salts containing 50 mol% or more of aluminum halide enables aluminum electrodeposition at low temperatures of approximately 100 to 200°C. However, aluminum halides (especially aluminum chloride) have a high vapor pressure and are prone to evaporation. Aluminum chloride and other substances released into the working environment can react with moisture in the air and corrode equipment. Even when using a mixed salt blended near the eutectic composition and the temperature of the molten salt is kept in the low range described above, the evaporation of aluminum chloride is significant, making equipment corrosion unavoidable.

[0007] No meaningful countermeasures against such evaporation (vapor) of aluminum chloride have been proposed up to now, including in the above-mentioned patent documents.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an electrodeposition method and the like that can efficiently deposit aluminum while suppressing the release of aluminum chloride vapor. [Means for solving the problem]

[0009] As a result of intensive research, the inventors came up with the idea of forming a stable liquid layer on the surface of the molten salt to prevent the molten salt from evaporating into the outside world, and have embodied this idea. By further developing this idea, the present invention, which will be described below, has been completed.

[0010] <Aluminum electrodeposition method> The present invention is an aluminum electrodeposition method comprising an electrodeposition step of applying an electric current between an object to be deposited on a cathode side and an aluminum substrate on an anode side in a molten salt containing aluminum chloride, thereby depositing aluminum on the object to be deposited, and the electrodeposition step is performed by covering the upper surface of the molten salt with a liquid layer that separates into two phases from the molten salt.

[0011] According to the aluminum electrodeposition method of the present invention (also simply referred to as the "electrodeposition method"), aluminum can be efficiently deposited while suppressing evaporation from the molten salt. The reason for this is believed to be as follows.

[0012] The liquid layer, which has a lower specific gravity than the molten salt, stably covers the upper surface of the molten salt containing aluminum chloride (referred to as "electrolyte" as appropriate). This liquid layer suppresses evaporation from the electrolyte and reduces the release of molten salt vapor into the environment. As a result, the adverse effects of molten salt vapor (chloride gas, etc.) on the working environment and equipment are also reduced.

[0013] Furthermore, since evaporation of the molten salt is suppressed, the composition and amount of the electrolyte can be stabilized, and the cost of preparing the electrolyte can be reduced. Furthermore, the conductivity of the electrolyte can be improved by increasing the temperature of the electrolyte, which in turn reduces the power consumption and speeds up electrodeposition. Thus, according to the present invention, aluminum can be deposited efficiently at lower cost.

[0014] Result The present invention can also be understood as a resultant product obtained by electrodeposition. For example, the present invention may be an aluminum-plated member. In this case, the electrodeposition step can be understood as a plating step of forming an aluminum plating layer on the surface of a substrate (substrate, core material, etc.).

[0015] In the present invention, the deposit itself (aluminum material) may also be used. In this case, the electrodeposition step can be understood as, for example, a refining step of extracting aluminum from an aluminum substrate.

[0016] "Device" The present invention can also be understood as an electrodeposition apparatus. For example, the present invention may be an aluminum electrodeposition apparatus including an electrolytic cell containing a molten salt containing aluminum chloride and a liquid layer that separates into two phases from the molten salt and covers the upper surface of the molten salt. This electrodeposition apparatus may further include a power supply device that applies electricity to a cathode-side substrate to be deposited and an anode-side aluminum substrate, both of which are at least partially immersed in the molten salt.

[0017] "others" (1) As used herein, "electrodeposition" refers to the deposition of aluminum by passing an electric current through the material, regardless of the purpose, treatment conditions, etc. Before electrodeposition, the material to be deposited may be subjected to a surface treatment or the like.

[0018] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Also, "x to y mm" in this specification means x mm to y mm. The same applies to other unit systems. [Brief explanation of the drawings]

[0019] [Figure 1A] FIG. 1 is a schematic diagram illustrating an example of an aluminum electrodeposition apparatus. [Figure 1B] 1 is a photograph showing the state in which the upper surface of the molten salt is covered with a liquid layer of squalane. [Figure 1C] This is a scanning electron microscope (SEM) image of deposits (electrodeposits) on the cathode. [Figure 2] FIG. 2 is a schematic diagram showing another example of an aluminum electrodeposition apparatus. DETAILED DESCRIPTION OF THE INVENTION

[0020] The above-described components of the present invention may be supplemented with one or more components selected from the present specification. The contents described in the present specification may be method components or product components.

[0021] 《Electrolyte》 The electrolyte is made of a molten salt containing aluminum chloride. The molten salt may be prepared using, for example, a stable metal halide (chloride, bromide, etc.). Metal elements constituting the halide include Na, K, Li, etc.

[0022] The molten salt may be prepared using, for example, sodium chloride, potassium chloride, aluminum chloride, or the like. These metal halides are inexpensive and stable, making them suitable raw materials for the molten salt. By using a mixed salt in which multiple types of metal halides are blended in a desired composition, it becomes possible to adjust the melting point, density (specific gravity), and other properties of the molten salt. The molten salt is not limited to a single layer, and may be a multi-layer.

[0023] Aluminum chloride may not be contained in the initial molten salt and may be generated after the start of the electrodeposition process. However, using an electrolyte (molten salt) that initially contains aluminum chloride promotes the deposition of aluminum on the cathode side.

[0024] 《Liquid layer》 The liquid layer covering the molten salt is stable with respect to the molten salt and preferably has a melting point, a boiling point, and a flash point lower than the set temperature (referred to as the "electrodeposition temperature") when the electrodeposition step is carried out. The liquid layer covering the molten salt in a two-phase separated state is naturally made of a substance that has a lower density (specific gravity) and a smaller polarity (for example, difference in electronegativity) than the molten salt, or more specifically, a substantially non-polar substance. Such a substance may be inorganic or organic. Examples of organic substances include hydrocarbons (for example, those having carbon atoms of 20 to 50 or 25 to 40), and saturated hydrocarbons with low polarity are preferably used. Saturated hydrocarbons may be linear or branched. Furthermore, saturated hydrocarbons may be chain alkanes or cyclic cycloalkanes.

[0025] A typical example of saturated hydrocarbons is squalane (C 30 H 62 ) and paraffin (a mixture of saturated hydrocarbons). Paraffin is a saturated hydrocarbon with 20 or more carbon atoms. Although a single type of paraffin may be used, it is usually a mixture (composition) of multiple types of hydrocarbons. As long as the paraffin (as a whole mixture) is fluid (liquid) at least on the molten salt, it may be solid paraffin that is solid at room temperature (for example, 15 to 25°C) or liquid paraffin that is liquid at room temperature. Liquid paraffin is easy to inject into the treatment tank (on the molten salt) and is easy to handle.

[0026] 《Deposited body》 The cathode-side substrate may be a conductor at least partially immersed in a molten salt. The substrate may be a substrate to be aluminum plated or a core material (e.g., an aluminum substrate) for depositing aluminum. The surface of the substrate may be previously subjected to a treatment to promote aluminum deposition or to the formation of a base layer (e.g., an aluminum layer).

[0027] <Aluminum substrate> The aluminum substrate on the anode side serves as a supply source of aluminum ions (including complex ions, etc.). The aluminum substrate does not necessarily have to be metallic, as long as it can supply aluminum ions through an electrochemical reaction. The aluminum substrate can have any composition or form (shape, size). Examples of aluminum substrates include ingots (wrought materials, castings), compacts, and sintered materials made of pure aluminum or aluminum alloys. Using scrap or recycled materials for the aluminum substrate allows for the effective use of aluminum resources, contributing to the realization of the SDGs (Sustainable Development Goals) and a circular economy.

[0028] In this specification, the term "X substrate (material)" refers to a material containing X as a component (for example, the content of X element relative to the total is 10 mass % or more, 30 mass % or more, 50 mass % or more, 70 mass % or more), and may be any of a pure metal, an alloy, a compound, or the like, regardless of its structure.

[0029] 《Electrodeposition process》 The electrodeposition conditions are appropriately determined taking into consideration the purpose of the treatment, the treatment speed, etc. For example, the concentration of aluminum chloride in the electrolytic solution is, for example, 50 to 90 mol %, 52 to 70 mol %, or 55 to 65 mol %.

[0030] The electrolyte is preferably set to a temperature below the melting point of Al and at which a two-phase separation state with the liquid layer is maintained. This temperature is, for example, 100 to 200°C, 110 to 180°C, or 120 to 150°C. Lowering the electrolyte temperature can decrease its conductivity and increase the power consumption required for electrodeposition. Furthermore, setting the electrolyte temperature too high can hinder energy conservation. Incidentally, to maintain such an electrolyte temperature, the electrodeposition temperature is preferably set to about 130°C.

[0031] The current density is, for example, 1 to 50 mA / cm 2 or 5 to 40 mA / cm 2 , 10~30mA / cm 2 The current application time is, for example, 0.1 to 10 hours, 0.3 to 5 hours, or 0.5 to 3 hours. [Example]

[0032] Aluminum was electrodeposited using a molten salt (electrolyte) containing aluminum chloride. The present invention will be described in more detail based on this specific example.

[0033] [First Example] The following experiment confirmed that the release of chloride gas (halogen-based gas) can be suppressed by providing a liquid layer on the electrolyte. In addition, the deposits (electrodeposits) obtained using such an electrolytic bath were observed and analyzed.

[0034] 《Electrodeposition device》 An aluminum electrodeposition apparatus D1 was fabricated as shown in Figure 1A. The electrodeposition apparatus D1 includes a cylindrical electrolytic cell C1 with a bottom, a lid C2 that airtightly closes the top opening of the electrolytic cell C1, an inlet pipe P1 that introduces gas g into the space above the electrolytic cell C1, an outlet pipe P2 that discharges gas g from the space above (collectively referred to as the "gas pipe P"), a cathode e1 and an anode e2 (collectively referred to as the "electrodes e"), a DC power supply s that supplies power to the cathode e1 and the anode e2, and a heater (not shown) that is disposed around the outside of the electrolytic cell C1 and can heat or maintain the temperature inside the electrolytic cell C1 at a desired temperature.

[0035] The gas pipe P and the electrode e are detachable from the cover C2. The gas pipe P and the electrode e are inserted through the cover C2 from top to bottom and are fixed to the cover C2 in an airtight manner. For convenience of explanation, the direction indicated by the arrow in the upper right of Fig. 1A is referred to as the up-down direction or the left-right direction (the same applies hereinafter).

[0036] 《Electrolyte》 Commercially available aluminum chloride, sodium chloride, and potassium chloride were weighed and mixed to prepare a mixed salt (61% AlCl3-26% NaCl-13% KCl). The mixing ratio (%) is expressed as a molar ratio (mol%) relative to the total mixed salt.

[0037] The mixed salt was dissolved as follows to prepare electrolyte m. The mixed salt was placed in an electrolytic cell C1 (inner diameter φ50 mm) and sealed with a lid C2 without an electrode e inserted therein. Ar gas was introduced into the sealed upper space of the electrolytic cell m through an inlet pipe P1 and exhausted through an outlet pipe P2. After purging the upper space with Ar gas, the mixed salt in the electrolytic cell C1 was heated with a heater to dissolve it in an Ar atmosphere with Ar gas flowing.

[0038] "measurement" (1) No liquid layer With the molten salt maintained at 130°C, the introduction of Ar gas was stopped, the lid C2 was opened, and the gas discharged was inhaled using an inhalation-type gas detector tube at the center of the opening of the electrolytic cell C1 to measure the concentration of chloride gas (HCl gas), which was 3100 ppm (volume percentage / same below).

[0039] (2) With liquid layer The lid C2 was opened, and 10 mL of squalane was added to 120 g of electrolyte m (molten salt). At this time, as shown in Figure 1B, the top of the molten salt m was covered with a liquid layer a. The top opening of the approximately cylindrical electrolytic cell C1 (inner diameter φ50 mm) with a bottom was again sealed with the lid C2, and the space above it was filled with the above-mentioned Ar atmosphere and maintained for 1 hour. The molten salt was maintained at 130°C (electrodeposition temperature).

[0040] Squalane does not react with the molten salt and is stable (non-polar), and its physical properties are as follows: For this reason, it is believed that the liquid layer a above the molten salt consists essentially of only squalane. Molten salt Squalane Density 1.67 g / cm 3 0.81 g / cm 3 Melting point 91℃ -38℃ Boiling point - ℃ 350 ℃ Flash point - ℃ 217 ℃

[0041] The HCl gas concentration in the electrolytic cell C1 to which squalane had been added was measured in the same manner as described above, and was found to be 350 ppm.

[0042] It was confirmed that the addition of squalane reduced the amount of chloride gas released from the electrolyte (molten salt) to the outside world by approximately one-tenth.

[0043] 《Electrodeposition process》 (1) Conditions The electrode e was airtightly inserted into the lid C2. The lower end of the electrode e was immersed in the electrolyte m containing squalane, and the upper opening of the electrolytic cell C1 was sealed with the lid C2. The space above it was filled with the above-mentioned Ar atmosphere and maintained for 0.5 hours. Then, a direct current was supplied between the cathode e1 and the anode e2 (plating process).

[0044] Pure aluminum pieces (purity 99% / width 20 mm × length 70 mm × thickness 2 mm) were used for the cathode e1 (substrate to be deposited) and the anode e2 (aluminum substrate). At this time, part of the cathode e1 was masked with polytetrafluoroethylene (PTFE) tape to make the reaction area 10 mm × 20 mm. The electrolysis conditions were: current density: 20 mA / cm 2 The power supply time was 1 hour.

[0045] (2) Observation The cross section of cathode e1 removed from electrolytic cell C1 was observed using an SEM. This is shown in Figure 1C. As can be seen from Figure 1C, a deposit (plating layer) was confirmed on the surface of cathode e1. The deposit was confirmed to be Al (99% purity) using an EDX (energy dispersive X-ray analyzer) attached to the SEM. The deposit in Figure 1C appears to have two layers due to the abrasive used.

[0046] [Second Example] Based on the results of the first example, the refinement to obtain high purity aluminum from a low purity aluminum substrate was carried out as follows (refining step).

[0047] 《Electrodeposition device》 An aluminum electrodeposition apparatus D2 shown in Figure 2 was fabricated. The electrodeposition apparatus D2 is configured by adding a reference electrode e3 to the electrodeposition apparatus D1. The reference electrode e3 is also detachably attached to the lid C2. The lower end of the reference electrode e3, which is airtightly inserted into the lid C2, is immersed in the electrolyte m. The same components of the electrodeposition apparatus D2 (Figure 2) as those of the electrodeposition apparatus D1 (Figure 1A) are designated by the same reference numerals, and detailed descriptions thereof are omitted.

[0048] 《Refining process》 (1) Electrode The cathode e1 (substrate) was a pure aluminum piece (20 mm wide x 70 mm long x 2 mm thick / 99% purity). The anode e2 (aluminum substrate) was an aluminum alloy piece (20 mm wide x 70 mm long x 2 mm thick / ADC12 equivalent: Al purity approximately 84.5%). The reference electrode e3 was a pure aluminum rod (φ2 mm / 99.99% purity).

[0049] (2) Electrolyte The electrolyte solution m was prepared by dissolving the solid salt obtained after preliminary electrolysis to remove impurities from the reagent. The preliminary electrolysis was carried out by heating and dissolving the mixed salt used in Example 1 in the electrodeposition apparatus D1. Specifically, a current of 200 mA was applied between the two electrodes made of pure aluminum in the above-mentioned Ar atmosphere for 6 hours. The molten salt was kept at a temperature of 120 to 180°C.

[0050] The molten salt after preliminary electrolysis was poured out of the electrolytic cell C1 and cooled and solidified. A part of the solidified molten salt was used in the refining step.

[0051] 120 g of the weighed solid salt was placed in the electrolytic cell C1 of the electrodeposition apparatus D2 and dissolved in the above-mentioned Ar atmosphere. The molten salt was maintained at 130° C. This dissolution was carried out without inserting each electrode into the lid C2.

[0052] (3) Refining Approximately 10 ml of squalane was added to the electrolyte solution m consisting of the molten salt. The cathode e1, anode e2, and reference electrode e3 were each inserted into the lid C2, and the bottom end of each electrode was immersed in the electrolyte solution m. At this time, part of the cathode e1 was masked with polytetrafluoroethylene (PTFE) tape to limit the reaction area to 10 mm × 20 mm.

[0053] After sealing the electrolytic cell C1 with the lid C2, each electrode was connected to a DC power supply s (potentiostat). The space above the electrolytic cell C1 was filled with the above-mentioned Ar atmosphere, and a current was applied between the cathode e1 (pure Al) and the anode e2 (ADC12). At this time, the anode e2 was set to +30 mV (constant) relative to the reference electrode e3. This state was maintained for 3 hours. During this current application, the electrolyte m was at 130 to 145°C.

[0054] "analysis" The deposit on the cathode e1 removed from the electrolytic cell C1 was scraped off. After thoroughly washing with pure water, the deposit was analyzed by EDX and found to be Al with a purity of 99.44%.

[0055] Thus, it was found that high-purity aluminum could be obtained by refining a low-purity aluminum substrate (Al content: 84.49% / ADC12) while suppressing the generation of chloride gases.

[0056] [Third Example] Experiments similar to those in Examples 1 and 2 were carried out, except that squalane was replaced with liquid paraffin. A commercially available liquid paraffin (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used. This liquid paraffin was a mixture of multiple saturated hydrocarbons, and its physical properties were as follows: Density 0.86~089g / cm 3 Melting point: - (liquid at room temperature) Boiling point 300℃ Flash point 224℃

[0057] (1) When an experiment similar to that in the first example was carried out, the HCl gas concentration in the electrolytic cell C1 to which liquid paraffin was added was 500 ppm. It was confirmed that the addition of liquid paraffin reduced the amount of chloride gas released from the electrolytic solution m (molten salt) to the outside to about one-sixth.

[0058] (2) When an experiment similar to that in Example 2 was conducted, the deposit (electrodeposit) on the cathode e1 removed from the electrolytic cell C1 was found to be Al with a purity of 99.6%. It was confirmed that aluminum can be refined while suppressing the generation of chloride gas even when liquid paraffin is used.

[0059] [supplement] Squalane (C 30 H 62 ) instead of the unsaturated hydrocarbon squalene (C 30 A similar experiment was conducted using H60. In this case, the generation of chloride gas was suppressed, but aluminum could not be efficiently purified. Although the mechanism is unclear, it is thought that it is preferable for the liquid layer formed on the molten salt to be mainly composed of one or more saturated hydrocarbons.

[0060] From the above, it was confirmed that according to the present invention, aluminum can be electrodeposited while the evaporation of the molten salt that serves as the electrolyte is suppressed (shielded) by the liquid layer. [Explanation of symbols]

[0061] m Electrolyte (molten salt) a liquid layer e1 cathode (deposited object) e2 anode (aluminum substrate)

Claims

1. an electrodeposition step of applying current between a cathode-side substrate and an anode-side aluminum substrate in a molten salt containing aluminum chloride to deposit aluminum on the substrate; The electrodeposition step is an aluminum electrodeposition method in which the upper surface of the molten salt is covered with a liquid layer that separates into two phases from the molten salt.

2. 2. The aluminum electrodeposition method according to claim 1, wherein the liquid layer is made of a substantially non-polar substance having a lower melting point, a higher boiling point, and a higher flash point than the set temperature at which the electrodeposition step is carried out.

3. 3. The method for electrodepositing aluminum according to claim 1, wherein the liquid layer comprises a saturated hydrocarbon.

4. The saturated hydrocarbon is squalane (C ３０ H ６２ 4. The method for electrodepositing aluminum according to claim 3, wherein the solvent is olefin, propylene glycol, or paraffin.

5. 2. The method for electrodepositing aluminum according to claim 1, wherein the aluminum substrate is a scrap material or a recycled material.

6. 2. The aluminum electrodeposition method according to claim 1, wherein the electrodeposition step is performed by heating the molten salt to a temperature of 100 to 200°C.

7. 2. The method for electrodepositing aluminum according to claim 1, wherein the molten salt is prepared using sodium chloride, potassium chloride, and aluminum chloride.

8. 2. The aluminum electrodeposition method according to claim 1, wherein the electrodeposition step is a plating step of forming an aluminum plating layer on the surface of the substrate.

9. 2. The aluminum electrodeposition method according to claim 1, wherein the electrodeposition step is a purification step of extracting aluminum from the aluminum substrate.

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