Method for producing an insoluble electrode and metallic magnesium

Abstract

To provide an insoluble electrode having excellent durability, employing MoSi2 as a main component, and capable of improving purity of obtainable metal Mg.SOLUTION: An electrode according to an embodiment of the present invention employs MoSi2 as a main component. An oxide film containing oxygen and silicon is formed on a surface of the electrode.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing insoluble electrodes and metallic magnesium. [Background technology]

[0002] Two main methods for producing magnesium (hereinafter also simply referred to as Mg) are the thermal reduction method (Si reduction method) and the molten salt electrolysis method. Currently, approximately 80% of Mg is produced by the thermal reduction method. While the thermal reduction method requires simpler equipment and operation compared to the molten salt electrolysis method, it emits significantly more CO2. Therefore, attention is being drawn to the molten salt electrolysis method in order to reduce the CO2 emissions.

[0003] In molten salt electrolysis, carbon is generally used as the anode. When producing metallic Mg by molten salt electrolysis, magnesium chloride (MgCl2), the raw material, is highly hygroscopic, requiring extensive dehydration of the MgCl2. If the dehydration of MgCl2 is insufficient, the carbon electrode reacts with dissolved oxygen in the molten salt through an electrochemical reaction, generating CO2 and causing the carbon electrode to wear down (for example, Non-Patent Literature 1). When the carbon electrode wears down, it needs to be replaced, and furthermore, carbon may be mixed into the molten salt, leading to a decrease in electrolysis efficiency.

[0004] Therefore, insoluble electrodes were desired to avoid anode wear. Patent Document 1 discloses such an insoluble electrode, in which wear can be suppressed by coating the surface of molybdenum carbide with diamond crystal (DLC). Non-Patent Document 2 discloses La 0.7 Sr 0.3 FeO 3-δ This document describes a non-consumable oxygen-evolving anode to which Ni has been added. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2014-224282 [Non-patent literature]

[0006] [Non-Patent Document 1] Journal of the Mining Society of Japan / 85 977 ('69-9) 802-810 [Non-Patent Document 2] Abstracts of the 2020 Joint Web Symposium ~ 52nd Symposium on Molten Salt Chemistry, 44th Symposium on Electrolysis Technology - Soda Industry Technology Symposium - and 40th Annual Meeting of the Hydrogen Energy Association, 2B10, pp. 112-113. [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the electrode described in Patent Document 1 has a thin DLC protective film, and once damaged, it is difficult to repair, making it unsuitable for long-term use. On the other hand, it is technically difficult to make the protective film thick enough to provide excellent durability. Therefore, there was room for improvement from the standpoint of durability.

[0008] In the insoluble electrode described in Non-Patent Literature 2, when electrolysis of the molten salt is performed, Fe or Ni, which are the raw materials for the electrode, dissolve into the molten salt and become mixed with the precipitated Mg as impurities. Since Fe and Ni are materials that Mg dislikes, even a small amount of their presence significantly reduces the quality of the resulting magnesium.

[0009] One aspect of the present invention aims to provide an insoluble electrode mainly composed of MoSi2 that has excellent durability and can improve the purity of the resulting metallic Mg. [Means for solving the problem]

[0010] In order to solve the aforementioned problems, the inventors conducted diligent research and found that the problems could be solved by using an electrode whose main component is MoSi2 and which has an oxide film containing oxygen and silicon formed on its surface, thus completing the present invention.

[0011] That is, the present invention includes the following configurations. <1>An electrode having MoSi2 as a main component and an oxide film containing oxygen and silicon formed on the surface. <2>The electrode according to <1>, wherein the oxide film contains magnesium. <3>The electrode according to <1> or <2>, wherein the oxide film contains MgSiO3 and / or SiO2. <4>A method for producing metallic magnesium, comprising the step of electrolyzing a magnesium salt by a molten salt electrolysis method using the electrode according to any one of <1> to <3>. <5>The method for producing metallic magnesium according to <4>, wherein the electrolytic solution contains molten MgCl2 and oxide ions. <6>The method for producing metallic magnesium according to <5>, wherein the oxide ions are derived from MgO. <7>The method for producing metallic magnesium according to any one of <4> to <6>, wherein in the step of performing the electrolysis, the voltage of the anode is less than 6.3V.

Advantages of the Invention

[0012] According to one aspect of the present invention, an insoluble electrode mainly composed of MoSi2 with excellent durability and capable of improving the purity of the obtained metallic Mg can be provided.

Brief Description of the Drawings

[0013] [Figure 1] It is a graph showing the cyclic voltammogram of the MoSi2 electrode prepared in the example. [Figure 2] It is a diagram showing the results of observing and analyzing the cross section of the MoSi2 electrode prepared in the example by SEM-EDX. [Figure 3] It is a graph showing the results of analyzing the MoSi2 electrode prepared in the example by X-ray diffraction. [Figure 4] It is a graph showing the weight change of the electrode when the voltage during electrolysis is changed using the MoSi2 electrode prepared in the example as the anode. [Figure 5] It is a graph showing the purity of metallic magnesium when the addition amount of MgO is changed regarding the precipitation of metallic magnesium carried out in the examples.

Mode for Carrying Out the Invention

[0014] [1. Electrode] An electrode according to an embodiment of the present invention (hereinafter also referred to as the present electrode) contains MoSi2 as a main component, and an oxide film containing oxygen and silicon is formed on the surface.

[0015] The present inventor took advantage of the presence of moisture or dissolved oxygen in the molten chloride used in the molten salt electrolysis method, and by performing an anode in the molten salt, it was found that a stable oxide can be formed on the surface of MoSi2, which is an electrode material. Thereby, consumption of the electrode due to dissolved oxygen can be prevented. Further, it was found that even if the oxide film is damaged during electrolysis, self-repair is possible by using the oxide as a protective film. Furthermore, it was found that neither Mo nor Si will cause a fatal deterioration of material properties even if they are both contained in metallic Mg, and it was also found that the purity of the obtained metallic Mg can be improved.

[0016] (1-1. Components of the Electrode) The present electrode contains MoSi2 as a main component. In this specification, "containing MoSi2 as a main component" means that the ratio of the weight of MoSi2 to the weight of all substances contained in the present electrode exceeds 50%. The all substances include the oxide film. The ratio is preferably 60% by weight or more, more preferably 80% by weight or more, further preferably 99% by weight or more, and most preferably 100% by weight. In addition, examples of components other than MoSi2 that can be contained in the electrode include Mg or Ca.

[0017] Since MoSi2 is industrially manufactured and used as a heating element, using MoSi2 as the main component of this electrode allows for the inexpensive manufacture of the electrode. MoSi2 is suitable as an electrode material because its electrical resistance decreases at high temperatures.

[0018] Furthermore, unlike the components of conventional electrodes, the MoSi2 can react not only with added oxide ions but also with oxide ions derived from water in the molten chloride to form an oxide film. This simplifies the dehydration method for the molten chloride, improving the productivity of metallic Mg.

[0019] Furthermore, the constituent elements Mo and Si of MoSi2 do not affect the material properties of metallic Mg even when present in small amounts together. In other words, unlike the electrode described in Non-Patent Literature 2, there are no undesirable elements present together with Mg.

[0020] Therefore, because the main component of this electrode is MoSi2, it is possible to obtain metallic Mg that fully retains the inherent material properties of Mg.

[0021] The main component of this electrode, MoSi2, may be chemically synthesized, for example, or a commercially available product (e.g., rod-shaped MoSi2) may be used. From the viewpoint of availability, it is preferable to use a commercially available product.

[0022] In addition to the components described above, this electrode may further contain binders, conductive materials, etc., that are typically found in electrodes. These components can be fixed to the electrode by known electrode manufacturing techniques.

[0023] (1-2. Oxide film on electrodes) The oxide film formed on the surface of this electrode contains oxygen and silicon. The formation of this oxide film on the surface of the electrode protects the electrode and suppresses the elution of molybdenum ions derived from MoSi2 into the molten salt. Therefore, even during prolonged electrolysis, most of the current is consumed in the generation of chlorine gas, thus suppressing electrode wear.

[0024] In other words, the present invention does not have the problem of anode wear, as occurs when a carbon electrode is used as the anode. When a carbon electrode is used, magnesium hydroxide, magnesium oxide, etc. are produced from water when MgCl2 is dehydrated. The oxygen produced when this magnesium hydroxide, etc. is electrolyzed reacts with carbon to produce CO or CO2, which causes the carbon electrode to wear down. Therefore, in conventional molten salt electrolysis methods using a carbon electrode, it is necessary to strictly dehydrate the MgCl2.

[0025] On the other hand, according to the present invention, electrode wear is suppressed by the formation of the oxide film. Therefore, electrode wear caused by the presence of magnesium hydroxide, etc., does not occur, as in the case when carbon electrodes are used. For this reason, it is not necessary to strictly dehydrate MgCl2.

[0026] Molten salt electrolysis is more advantageous than thermal reduction in terms of energy consumption and CO2 emission reduction, and given its environmental impact, it is desirable that it be expanded in the future. However, thermal reduction is currently the mainstream method for producing metallic magnesium. One reason for this is that molten salt electrolysis requires a complicated process of preparing high-purity MgCl2 through rigorous dehydration.

[0027] As described above, according to the present invention, it is not necessary to strictly dehydrate MgCl2. Therefore, the aforementioned complicated steps can be avoided. As a result, it is possible to provide a method that has the advantage of molten salt electrolysis, which has little impact on the environment, and that can easily produce metallic Mg.

[0028] Furthermore, when producing metallic Mg from a molten salt containing MgCl2 using the molten salt electrolysis method, high-temperature (approximately 700°C) chlorine gas is continuously generated from the anode. However, because this electrode has an oxide film, corrosion of the electrode due to chlorine gas can be suppressed.

[0029] The oxide film is preferably formed, for example, by performing electrolysis (e.g., constant potential electrolysis) in a molten salt bath containing MgCl2 and oxide ions using the rod-shaped MoSi2 described above, thereby causing the MoSi2 to become passivated on the surface. By forming the oxide film in this manner, the oxide film on the surface becomes dense, thus adequately protecting the electrode.

[0030] In addition, even if the oxide film on the electrode surface is damaged for any reason during electrolysis, the oxide film can be reformed. In other words, because the oxide film is self-repairing, the electrode can be used stably for a long period of time.

[0031] The aforementioned oxide ion has the molecular formula O 2- It is a divalent anion represented by . The presence of the oxide ion in the molten salt bath allows for the formation of an oxide film on MoSi2. The method for obtaining the oxide ion in the molten salt bath is not particularly limited, but for example, it may be obtained from the oxide produced when MgCl2 hydrate is dehydrated. When MgCl2 hydrate is dehydrated, Mg(OH)2 is produced by hydrolysis, and if the reaction proceeds further, MgO is produced, so an oxide ion derived from MgO can be obtained. In addition, if necessary, oxides such as MgO may be added to the molten salt bath.

[0032] From the viewpoint of minimizing peeling of the oxide film, the upper limit of the voltage during electrolysis is preferably less than 6.3V, more preferably 6.0V or less, and even more preferably 5.0V or less, based on the Mg electrodeposition potential. From the viewpoint of enhancing the protective effect of the oxide film on the electrode, the lower limit of the voltage during electrolysis is preferably 1.9V or more, more preferably 2.5V or more, and even more preferably 3.7V or more.

[0033] The oxide film is presumed to be formed by the following reaction on the surface of, for example, MoSi2. The following reaction formula is for the case where MgO is added to the molten salt bath, and the oxide ions are derived from MgO. Also, the following reaction formula is a presumption and does not limit the reaction mechanism of the present invention. MoSi2 + 4O 2- →Mo 3+ + 2SiO2 + 11e - SiO2 + MgO → MgSiO3 Mg + 3O 2- + Si → MgSiO3 + 4e - When the oxide film is formed by the above reaction formula, the oxide film contains MgSiO3 and SiO2. As shown in the above reaction formula, since the oxide film is obtained by oxidizing MoSi2, it inevitably contains silicon and oxygen.

[0034] The oxide film is preferably thin and dense. Specifically, the thickness of the oxide film is preferably 0.01 - 4.0 μm, more preferably 0.5 - 2.0 μm, and even more preferably 0.1 - 1.0 μm. The thickness of the oxide film is preferable for sufficiently protecting the electrode and improving the conductivity of the present electrode.

[0035] The oxide film is preferably formed on the surface of the present electrode. In this specification, "the oxide film is formed on the surface" means that the surface of the present electrode in the molten salt bath is covered by the oxide film. By covering the surface of the present electrode with the oxide film, elution of molybdenum ions can be suppressed and the durability of the present electrode can be improved.

[0036] The oxide film may contain elements other than oxygen and silicon. Examples of elements that may be included in the oxide film include Mg, calcium (Ca), lithium (Li), potassium (K), sodium (Na), strontium (Sr), gallium (Ga), germanium (Ge), and aluminum (Al). It is preferable that the element included in the oxide film is Mg in order to further improve the strength of the oxide film and the purity of the deposited metallic Mg.

[0037] The oxide film preferably contains MgSiO3 and / or SiO2. The inclusion of MgSiO3 in the oxide film further improves the durability of the electrode. SiO2 is produced by the oxidation of MoSi2 and is inevitably generated when forming an oxide film on the surface of the electrode.

[0038] [2. Methods for producing magnesium] A method for producing metallic Mg according to one embodiment of the present invention (hereinafter also referred to as "this production method") includes a step of electrolyzing a magnesium salt by molten salt electrolysis using this electrode. By electrolyzing the Mg salt using this electrode, the electrode is not consumed and impurities originating from the electrode do not dissolve into the molten salt. Therefore, electrolysis can be carried out more efficiently than when conventional electrodes are used, and the purity of the resulting metallic Mg can be improved.

[0039] In this production method, it is preferable to use this electrode as an anode. By using this electrode as an anode, electrolysis can be carried out for a longer period of time, and the purity of the resulting metallic Mg can be improved.

[0040] In this production method, the cathode is not particularly limited as long as it is an electrode that can be used for the production of metallic Mg. Examples of cathode materials include those commonly used as cathodes in molten salt electrolysis, such as copper, platinum, and gold.

[0041] In the molten salt electrolysis method, water is not used as a solvent. That is, the molten salt electrolysis method can be carried out, for example, by using this electrode as the anode and a copper plate as the cathode, and performing an electrolytic reaction in a molten salt bath containing molten Mg salt at a temperature above the melting point of Mg salt. In this case, metallic Mg is deposited on the cathode.

[0042] The molten Mg salt is not particularly limited and may be, for example, chloride, sulfate, acetate, oxide, magnesium carbonate, other salts, or combinations thereof. From the viewpoint of being readily available, easily reduced by molten salt electrolysis, and efficiently obtaining metallic Mg, the molten Mg salt is preferably chloride (MgCl2).

[0043] The molten salt bath used in the electrolysis step preferably contains molten MgCl2 and oxide ions. In this specification, "molten MgCl2" refers to MgCl2 in a state where it has been heated above its melting point (714°C) and melted, i.e., Mg 2+ and Cl - This refers to a state containing ions. Since molten MgCl2 is in an ionized state, the molten salt bath can contain oxide ions.

[0044] As described above, the presence of oxide ions in the molten salt allows the oxidation reaction of MoSi2 to continue on the surface of the electrode even during electrolysis, making it possible for the oxide film to self-repair during electrolysis.

[0045] The concentration of soluble MgCl2 contained in the molten salt bath is preferably 5% by weight or more, more preferably 10% by weight or more, and even more preferably 20% by weight or more. By satisfying the above lower limit requirement for the concentration of MgCl2, a sufficient amount of metallic Mg can be obtained by electrolysis. A higher concentration of MgCl2 is preferable, but in reality, it may be 90% by weight or less.

[0046] The molten salt may contain chlorides other than molten MgCl2, to the extent that it does not hinder the objectives of the present invention. Examples of such chlorides include LiCl, NaCl, CaCl2, KCl, GaCl2, BaCl2, and SrCl2.

[0047] The chloride content is preferably 80% by weight or less, more preferably 60% by weight or less, even more preferably 20% by weight or less, and most preferably none at all, relative to the total amount of molten salt.

[0048] The molten salt may further contain oxides, to the extent that they do not hinder the objectives of the present invention. Examples of such oxides include MgO, CaO, and GaO. The content of the oxides is preferably 5% by weight or less, more preferably 3% by weight or less, and even more preferably 1% by weight or less, relative to the total weight of the molten salt.

[0049] The method for obtaining the molten salt bath is not particularly limited. The molten salt bath may be obtained, for example, by dehydrating a brine obtained by filtering seawater, or a high-concentration brine discharged in reverse osmosis treatment of seawater, and then adding the oxide ions. In this case, it is not necessary to perform a high degree of dehydration, and a small amount of water may remain in the molten salt.

[0050] The concentration of oxide ions in the molten salt is preferably 0.01 to 0.2% by weight, more preferably 0.05 to 0.2% by weight, and even more preferably 0.1 to 0.2% by weight. By satisfying the above lower limit requirement for the oxide ion concentration, the purity of the resulting metallic magnesium can be improved. The solubility of oxide ions in the molten salt is approximately 0.2 mass% when expressed in mass%.

[0051] The oxide ions are preferably derived from MgO.

[0052] The oxide ions in the molten salt may be originally present in the material used as the raw material for the molten salt, or they may originate from oxides added as needed. From the viewpoint of ease of adjusting the concentration, it is preferable that they originate from added oxides.

[0053] The oxide is not particularly limited as long as it does not affect the deposited metal Mg, and examples include MgO, CaO, and GaO. Among these, MgO is preferred because, as mentioned above, it can be easily obtained by dehydrating MgCl2. Furthermore, from the viewpoint of improving the purity of the obtained metal Mg and improving the durability of the self-repairing oxide film on the surface of the electrode, the oxide is preferably MgO.

[0054] In the electrolysis step described above, the anode voltage is preferably less than 6.3V, more preferably 6.0V or less, and even more preferably 5.0V or less, with respect to the Mg electrodeposition potential. By satisfying the above upper limit requirement for the anode voltage, the peeling of the oxide film on the electrode during electrolysis can be suppressed more effectively than when the above requirement is not met. The lower limit of the anode voltage is not particularly limited, but from the viewpoint of efficiently depositing metallic Mg, it may be, for example, 1.9V or more, and from the viewpoint of sufficiently passing through the surface of the electrode, it may be, for example, 3.7V or more.

[0055] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Examples]

[0056] An embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment.

[0057] [Manufacturing method for MoSi2 electrodes] A molten salt bath was prepared by adding 50 g of MgCl2-NaCl-CaCl2 (weight ratio 20:50:30) to a glove box filled with argon, and then adding 1% by weight of MgO.

[0058] Using a polished MoSi2 rod for heating (purchased from Motoyama Co., Ltd.) as the anode and a copper plate as the cathode, constant potential electrolysis was performed in an electric furnace with an observation window (voltage 3.1V (based on Mg electrodeposition potential), under an argon atmosphere, at approximately 720°C for 4 hours). After the electrolysis was completed, a MoSi2 electrode with an oxide film covering its surface was obtained.

[0059] [Test 1: Cyclic voltammogram of MoSi2 electrode] During the constant potential electrolysis described above, the cyclic voltammogram of the MoSi2 electrode was measured using HAL3001A (manufactured by Hokuto Denko). The scan speed was 100 mVs. -1 The following was done. Silver and silver chloride were used as reference electrodes. The measurement results are shown in Figure 1.

[0060] In Figure 1, the vertical axis represents current density, and the horizontal axis represents potential. As shown in Figure 1, the current density at the anode, MoSi2, increased sharply and then decreased sharply. From this, it can be seen that after molybdenum ions were briefly eluted from MoSi2, an oxide film was formed on the surface, and a passivation phenomenon occurred.

[0061] [Test 2. Analysis of the MoSi2 electrode surface] The cross-sections of the MoSi2 electrodes obtained in the above-mentioned [Method for Manufacturing MoSi2 Electrodes] were subjected to scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) measurements to confirm the elemental distribution and analyze the present compounds. A JEOL JSX-1000S energy-dispersive X-ray fluorescence analyzer (XRF) was used as the measurement device. The electrodes manufactured in the above-mentioned [Method for Manufacturing MoSi2 Electrodes] were cut to obtain sections with circular cross-sections. The obtained sections were sealed in resin and used as samples.

[0062] Figure 2 shows the results of measuring the cross-section of the MoSi2 electrode using SEM-EDX. Figure 2 shows the results of measuring the distribution of different elements in the same cross-section of the same sample. The upper left of Figure 2 shows the elemental distribution of MoSi2, the upper right shows Si, the lower left shows O, and the lower right shows Mg. In each image, the right side is the surface side, and the left side is the electrode substrate (MoSi2) side. In each image, darker colors indicate a low elemental distribution, and lighter colors indicate a high elemental distribution.

[0063] Figure 2 shows that a large amount of MoSi2, the substrate, is present inside the MoSi2 electrode, while Si, O, and Mg are mainly present on the surface of the MoSi2 electrode. Therefore, it is shown that an oxide film containing Si, O, and Mg is formed on the surface of the MoSi2 electrode.

[0064] Figure 3 shows the results of measuring the cross-section of the MoSi2 electrode by XRD. From Figure 3, it can be seen that MoSi2, MgSiO3, and SiO2 are present in the cross-section of the MoSi2 electrode. The results shown in Figures 2 and 3 indicate that the oxide film on the surface of the MoSi2 electrode contains MgSiO3 and SiO2.

[0065] [Test 3. Measurement of weight change of MoSi2 electrode] Using the aforementioned MoSi2 electrode as the anode, the copper plate as the cathode, and the molten salt bath prepared in the [Method for Manufacturing a MoSi2 Electrode], metallic Mg was deposited by molten salt electrolysis. During electrolysis, the anode voltages were set to 1.9V, 3.1V, and 6.3V, respectively, and the weight of each anode was measured at regular intervals. The results are shown in Figure 4.

[0066] In Figure 4, the weight of the anode is shown as a relative value, with the initial value set to 0. From Figure 4, it can be seen that when the anode potential is 1.9V and 3.1V, no change in the weight of the anode is observed. Furthermore, when the anode potential is 6.3V, the weight of the anode decreases over time, and a weight decrease of about 5g was observed after 4 hours. This is presumed to be due to the peeling of the oxide film on the surface of the MoSi2 electrode. Therefore, it was shown that when using a MoSi2 electrode for the deposition of metallic Mg by molten salt electrolysis, a voltage of less than 6.3V is preferable.

[0067] Here, we will estimate the case where a carbon electrode is used as the anode instead of the MoSi2 electrode, and the molten salt electrolysis method is performed under the same conditions. The concentration of MgCl2 in the molten salt bath is 20% by weight, and the solubility of MgO in the molten salt bath is generally about 0.2% by weight. Therefore, the reaction equation when electrolysis is performed using a carbon electrode and all of the raw materials, MgCl2 and MgO, are consumed is as follows.

[0068] 49MgCl2+MgO+0.5C→50Mg+49Cl2+0.5CO2 Therefore, in industrial production, it is predicted that approximately 5 kg of carbon will be consumed when 1 ton of Mg is produced using a carbon electrode as the anode.

[0069] [Test 4. Purity measurement of metallic Mg] The molten salt bath used in Experiment 3 contains MgO as a source of oxide ions. As mentioned above, this MgO causes the carbon electrode to wear down when molten salt electrolysis is performed with the carbon electrode as the anode. Therefore, in this case, MgO can be considered an impurity in the molten salt bath.

[0070] On the other hand, if MgO causes wear on the carbon electrode but does not affect the purity of the deposited Mg, then it is considered that the purity of the deposited Mg will not be affected by MgO even if molten salt electrolysis is performed using an electrode according to one embodiment of the present invention instead of a carbon electrode.

[0071] Therefore, in this example, we investigated the effect of MgO in the molten salt bath on the purity of the precipitated Mg when a carbon electrode is used as the anode.

[0072] A Mo wire with its surface area restricted by a BN sleeve was used as the cathode, and the Mo wire was covered with a BN partition (with an open bottom) to collect the electrodeposited or floating Mg metal. A graphite rod was used as the anode, and an Ag / AgCl electrode with an HB diaphragm was used as the reference electrode.

[0073] Molten salt electrolysis (potential-constant electrolysis) was performed using the same molten salt bath as in Test 3, except that the amount of MgO added was changed to 0 mol%, 0.1 mol%, and 1 mol%, respectively. The overpotential during electrolysis was set to 0.10V, 0.15V, or 0.20V. Here, the 0.10V overpotential test was performed twice for 0 mol% MgO, once for 0.1 mol%, and twice for 1 mol%. The 0.15V overpotential test was performed once for each of the 0 mol%, 0.1 mol%, and 1 mol% MgO addition amounts. Furthermore, the 0.20V overpotential test was performed five times for 0 mol%, three times for 0.1 mol%, and five times for 1 mol% MgO addition. The purity of the deposited metallic Mg was measured by X-ray fluorescence analysis using the XRF method described above. The results are shown in Figure 5.

[0074] As shown in Figure 5, all of the precipitated metals were pure metallic Mg. Therefore, it is considered that the effect of MgO in the molten salt bath on the purity of the precipitated Mg is not significant.

[0075] From these results, it is considered that even when using the electrode according to one embodiment of the present invention, MgO does not affect the purity of the deposited Mg. Therefore, it is considered that by using the electrode, electrode wear can be suppressed and high-purity metallic Mg can be obtained. [Industrial applicability]

[0076] The present invention can be used as a method for producing insoluble electrodes usable in molten salt electrolysis and for producing metallic magnesium.

Claims

1. MoSi ２ An electrode whose main component is silicon dioxide, with an oxide film containing oxygen and silicon formed on its surface.

2. The electrode of claim 1 , wherein the oxide film comprises magnesium.

3. The oxide film is MgSiO ３ and / or SiO ２ 3. The electrode of claim 1 or 2, comprising:

4. A method for producing metallic magnesium, comprising a step of electrolyzing a magnesium salt by molten salt electrolysis using the electrode according to any one of claims 1 to 3.

5. The molten salt bath used in the molten salt electrolysis method is molten MgCl ２ and oxide ions.

6. 6. The method for producing magnesium metal according to claim 5, wherein the oxide ions are derived from MgO.

7. 7. The method for producing metallic magnesium according to claim 4, wherein in the electrolysis step, the voltage of the anode is less than 6.3 V.

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