Anode, molten salt electrolysis device, and method for producing metal

Abstract

To provide an anode that can reduce the effects of oxidative burn-off of an anode for use in molten salt electrolysis.SOLUTION: An anode 130 has an anode body 132 used in an electrolytic cell 110 in which molten salt bath Bf is stored, a cover member 134 that covers part of the anode body 132, and a cavity 136 between the anode body 132 and the cover member 134. The distance between the outer surface 132a of the anode body 132 and the inner surface 134b of the cover member 134 is 0.5 to 20 mm.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] The present invention relates to an anode, a molten salt electrolysis apparatus, and a method for producing a metal. [Background technology]

[0002] Titanium metal ingots and the like are industrially produced using titanium sponge produced by the Kroll process. The titanium sponge production process including the Kroll process can be broadly divided into four steps: a chlorination step, a reduction and separation step, a crushing step, and an electrolysis step. The electrolysis step, which is one of these steps, is a step in which magnesium chloride, a by-product of the reduction and separation step in which titanium sponge is produced by reducing titanium tetrachloride with metallic magnesium, is decomposed by molten salt electrolysis to obtain metallic magnesium.

[0003] Regarding the molten salt electrolysis, for example, Patent Document 1 describes "a molten salt electrolytic cell comprising an electrolysis chamber in which electrolysis of a molten salt is performed in a molten salt bath and a metal recovery chamber communicating with the electrolysis chamber, wherein a molten metal reservoir for storing metal in the metal recovery chamber is provided with a first wall on at least a portion thereof, and a molten salt solidified layer is formed on at least a portion of the inner surface of the first wall." By using such a molten salt electrolytic cell, it is possible to effectively prevent impurities from being mixed into the molten metal obtained by electrolysis of a metal chloride. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-021134 Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, for example, molten salt electrolysis for producing metallic magnesium is generally carried out under harsh conditions, such as using a molten salt bath at a high temperature of about 650 to 700 ° C. and generating chlorine gas as a reaction product. In addition, the inside of the molten salt electrolysis device is maintained at a negative pressure with respect to the external environment so that chlorine generated in the molten salt electrolysis does not leak to the outside. Therefore, since outside air may enter the electrolytic cell through a small clearance between the anode and the upper cover, the clearance is usually sealed. However, the anode is large and is often made of graphite. The large graphite anode may be manufactured through a high-temperature heat treatment, which causes the anode to contain small holes. Therefore, even if the clearance is appropriately sealed, the inside of the molten salt electrolysis device is maintained at a negative pressure, so that outside air may enter the inside of the molten salt electrolysis device through the inside of the anode rather than through the seal part. In the space above the surface of the molten salt bath in the molten salt electrolysis device, the temperature is high near the bath surface and low near the top lid, and there is a temperature gradient in the height direction. The oxygen in the outside air passing through the inside of the anode reacts with graphite more quickly at high temperatures, and the anode may be oxidized and consumed. An anode that has been reduced in thickness to a certain extent due to oxidative consumption may break under its own weight.

[0006] Therefore, in one embodiment, an object of the present invention is to provide an anode that can reduce the influence of oxidative wear of an anode used in molten salt electrolysis. [Means for solving the problem]

[0007] That is, in one aspect, the present invention provides an anode comprising: an anode body for use in an electrolytic cell in which a molten salt bath is stored; a cover member covering a part of the anode body; and a gap present between the anode body and the cover member, wherein the distance between the outer surface of the anode body and the inner surface of the cover member is within the range of 0.5 to 20 mm.

[0008] In one embodiment of the anode according to the present invention, the anode has an open end of the gap at a position lower than the height position of the bath surface of the molten salt bath.

[0009] In one embodiment of the anode according to the present invention, the anode body has at least one step, and the cover member covers at least a part of the anode body at a position higher than a flat surface of the step.

[0010] In one embodiment of the anode according to the present invention, the cover member contains one or more selected from silicon nitride, silicon carbide, aluminum oxide, silicon oxide, magnesium oxide, and calcium oxide.

[0011] In one embodiment of the anode according to the present invention, the anode body is made of graphite.

[0012] In another aspect, the present invention provides a molten salt electrolysis apparatus comprising any one of the anodes described above.

[0013] In another aspect, the present invention is a method for producing a metal using the above-mentioned molten salt electrolysis apparatus, the method including an electrolysis step of performing electrolysis of a metal chloride contained in the molten salt bath.

[0014] In one embodiment of the method for producing a metal according to the present invention, the method further includes a solidified layer forming step of forming a molten salt solidified layer in the gap at a position higher than the bath surface of the molten salt bath.

[0015] In one embodiment of the method for producing a metal according to the present invention, the metal chloride electrolyzed is magnesium chloride. Effect of the Invention

[0016] According to one embodiment of the present invention, the influence of oxidation consumption of an anode used in molten salt electrolysis can be reduced. [Brief description of the drawings]

[0017] [Figure 1A] 1 is a schematic cross-sectional view for explaining an internal structure of an embodiment of a molten salt electrolysis apparatus according to the present invention. FIG. [Figure 1B] FIG. 1B is a schematic end view taken along line XX in FIG. 1A. [Figure 1C] FIG. 1B is a schematic end view taken along line YY in FIG. 1A. [Figure 1D] FIG. 2 is a schematic end view for explaining the formation of a molten salt solidified layer in an electrolysis chamber of an embodiment of a molten salt electrolysis device according to the present invention. [Figure 1E] FIG. 2 is a schematic perspective view showing a modified example of the anode in FIG. 1B. [Figure 2A] FIG. 4 is a schematic cross-sectional view for explaining the internal structure of another embodiment of a molten salt electrolysis apparatus according to the present invention. [Figure 2B] FIG. 2B is a schematic end view taken along line ZZ in FIG. 2A. [Figure 2C] FIG. 11 is a schematic end view for explaining the formation of a molten salt solidified layer in an electrolysis chamber of another embodiment of a molten salt electrolysis device according to the present invention. [Figure 2D] FIG. 2B is a schematic end view illustrating a variation of the anode of FIG. 2A. [Figure 2E] FIG. 2B is a schematic end view illustrating a variation of the anode of FIG. 2A. [Diagram 3] FIG. 4 is a schematic cross-sectional view for explaining the internal structure of another embodiment of a molten salt electrolysis apparatus according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The present invention is not limited to the embodiments described below, and the components can be modified without departing from the scope of the present invention. Various inventions can be formed by appropriate combinations of the components disclosed in the embodiments. For example, an invention may be formed by deleting some components from all the components shown in the embodiments. In the drawings, some components are shown as outlines to aid in understanding the embodiments included in the invention, and the illustrated sizes, positional relationships, etc. may not be accurate. Further, in this specification, "upward" means a direction from the bottom wall 112 side of the electrolytic cell 110 to the top lid 120 side, as shown by the arrows in, for example, Figures 1A, 1B, 1D, 2A, 2B, 2C, 2D, 2E, and 3, and "downward" means a direction from the top lid 120 side to the bottom wall 112 side of the electrolytic cell 110. Further, in this specification, "molten metal" means a metal in a molten state obtained by electrolytic decomposition of a metal chloride. Further, in this specification, "molten salt solidified layer" means a layer in which the molten salt has become solid, and in the description of the embodiment described later, the molten salt solidified layer is formed in the gap between the anode body and the cover member. Note that, as described later, the molten salt solidified layer may be formed using a molten salt bath in the electrolytic cell 110, or may be formed using a molten salt separately supplied from the outside. The molten salt separately supplied from the outside may have the same composition as the molten salt bath.

[0019] [1. Molten salt electrolysis equipment] The molten salt electrolysis apparatus 100 shown in FIG. 1A includes an electrolytic cell 110, an upper cover 120, an anode 130, a cathode 140, an electrolysis chamber 150, and a metal recovery chamber 160.

[0020] (electrolytic cell) The electrolytic cell 110 has a container shape with an opening formed on the upper side, and is made of, for example, firebricks such as aluminum oxide or other suitable material. The electrolytic cell 110 is composed of a bottom wall 112 and two pairs of side walls 114 connected to the bottom wall and extending upward. The electrolytic cell 110 stores a molten salt bath Bf made of a molten salt containing a metal chloride supplied therein. The electrolytic cell 110 is divided into an electrolysis chamber 150 and a metal recovery chamber 160 by the presence of a first partition wall 116 and a second partition wall 117. The first partition wall 116 and the second partition wall 117 are arranged to send the metal generated by electrolysis of the molten salt in the electrolysis chamber 150 to the metal recovery chamber 160 and to send the molten salt from the metal recovery chamber 160 to the electrolysis chamber 150, thereby circulating the molten salt bath. That is, the molten salt electrolysis apparatus 100 can ensure the flow of the molten salt bath indicated by arrow A (flow from the electrolysis chamber 150 to the metal recovery chamber 160) by forming the flow port 115 between the first partition wall 116 and the second partition wall 117. In addition, a passage through which the molten salt bath can flow is also formed on the lower surface side of the second partition wall 117, so that the flow indicated by arrow B (flow from the metal recovery chamber 160 to the electrolysis chamber 150) can be ensured.

[0021] (molten salt) Hereinafter, an example will be described in which the molten salt contains magnesium chloride (MgCl2). In this case, magnesium chloride (Mg) is generated as a molten metal by electrolysis, and chlorine (Cl2) gas is generated as a gas. In addition to the above-mentioned magnesium chloride (MgCl2), the molten salt may contain sodium chloride (NaCl), calcium chloride (CaCl2), potassium chloride (KCl) and / or calcium fluoride (CaF2) as a supporting salt. It is preferable to use a component used as a supporting salt that has a higher electrolysis voltage than magnesium chloride. Metallic magnesium can be used for the reduction of titanium tetrachloride in the Kroll process for producing metallic titanium, and chlorine gas can be used for the chlorination of titanium ore. As the raw material for this electrolysis, magnesium chloride that is by-produced in the Kroll process can be used.

[0022] (Top lid) The top lid 120 serves to insulate the electrolytic cell 110 from the outside because the molten salt bath Bf is at a high temperature. In addition, by disposing the top lid 120, the electrolytic cell 110 becomes a closed space, and the inside of the electrolytic cell 110 is made negative pressure with respect to the outside in order to prevent leakage of chlorine gas generated from the anode body 132 during molten salt electrolysis. The material of the upper lid 120 is not particularly limited, but from the viewpoint of preventing a short circuit occurring between the upper lid 120 and the anode body 132 during molten salt electrolysis, the lid back surface 121 side of the upper lid 120 on the molten salt bath Bf side may be made of an insulating material, and a ceramic material may be disposed on the lid back surface 121 side of the upper lid 120 on the molten salt bath Bf side, or a castable refractory material may be applied. The method for providing this castable refractory material may be any known method, and for example, the castable refractory may be applied to the lid back surface 121 side by dry spraying, wet spraying, or the like.

[0023] The upper cover 120 may be provided with a supply port 122, a first gas recovery port 124, a second gas recovery port 126, and a supply / discharge port 128. There may be one or more of each of these ports. Of these, the supply port 122 is for supplying molten salt to a gap 136 between the anode body 132 and the cover member 134. The supply port 122 is provided in a region in which the electrolysis chamber 150 is located, that is, in a region located between the anode body 132 and the cover member 134. From the viewpoint of forming the closed space described above, the supply port 122 is usually filled with powder or granules of the components that constitute the molten salt bath. When supplying molten salt from the supply port 122, the filler is temporarily removed, and after the molten salt is supplied, the supply port 122 can be filled again with the powder or granules. The first gas recovery port 124 is used to recover chlorine gas produced by electrolysis in the electrolysis chamber 150. The first gas recovery port 124 is provided in the region where the electrolysis chamber 150 is located. Further, the second gas recovery port 126 may recover gas generated by electrolysis in the electrolysis chamber 150. The second gas recovery port 126 is provided in an area where the metal recovery chamber 160 is located. The second gas recovery port 126 may be used to recover the remaining gas generated by electrolysis that has not been recovered in the first gas recovery port 124 and has flowed into the metal recovery chamber 160. The supply and discharge port 128 is used to recover the molten metal produced by electrolysis in the electrolysis chamber 150 and to supply the molten salt to the electrolysis cell 110. The supply and discharge port 128 is provided in an area where the metal recovery chamber 160 is located.

[0024] (Electrolysis chamber) In the electrolysis chamber 150, the molten salt is electrolyzed to generate molten metal. For example, in the electrolysis of magnesium chloride, chlorine gas is generated in addition to molten magnesium metal. Inside the electrolysis chamber 150, an anode body 132 and a cathode 140 are arranged, each having an electrolysis surface that is approximately parallel to the depth direction of the molten salt bath (the vertical direction in FIG. 1A).

[0025] The anode 130 has an anode body 132 that is placed in the electrolytic cell 110 in which the molten salt bath Bf is stored and is used for electrolysis, and a cover member 134 that covers a portion of the anode body 132. As shown in Figures 1A and 1B, the anode body 132 is inserted through the upper lid 120 and extends downward, and is arranged such that a portion of it is immersed in the molten salt bath Bf. The cover member 134 is also inserted through the upper lid 120 and extends downward, and a portion of it is immersed in the molten salt bath Bf. However, in order to avoid blocking the gap between the cathode 140, which is inserted into the side wall 114 of the electrolytic cell 110 and extends toward the first partition wall 116 and the second partition wall 117, and the anode body 132, the cover member 134 is preferably arranged so that the lower end surface 134a of the cover member 134 is lower than the bath surface S of the molten salt bath Bf and higher than the upper end surface 140a of the cathode 140. If there is a very small clearance between the anode body 132 and the top lid 120 and between the cover member 134 and the top lid 120, outside air may enter the electrolytic cell 110 due to the negative pressure inside the electrolytic cell 110 during molten salt electrolysis. Therefore, from the viewpoint of forming a closed space inside the electrolytic cell 110, a sealing member may be provided in the clearance, and for example, the sealing member may be formed by filling powder or granules of the components that make up the molten salt bath.

[0026] As shown in FIG. 1C, a gap 136 exists between the anode body 132 and the cover member 134. Since a part of the anode body 132 and a part of the cover member 134 are both immersed in the molten salt bath Bf, one open end 136a of the gap 136 is located at a position lower than the height position of the bath surface S of the molten salt bath Bf. A distance D1 between the outer surface 132a of the anode body 132 and the inner surface 134b of the cover member 134 is within a range of 0.5 to 20 mm. With this configuration, for example, in the gap 136, the molten salt of the molten salt bath Bf can rise to a position higher than the bath surface S of the molten salt bath Bf in the electrolytic cell 110 by capillary action, and when the capillary action cannot be utilized, the molten salt can be supplied from the supply port 122, and the molten salt bath can be supplied to a high position of the gap 136 by intentionally supplying magnesium chloride in excess to the molten salt bath, for example. The temperature of the molten salt in the gap 136 decreases toward the lid back surface 121. The molten salt located at the upper part of the gap 136 is cooled and solidified, and a molten salt solidified layer 138 is formed in the gap 136 as shown in FIG. 1D. Since the molten salt solidified layer 138 acts as a plug, it is possible to prevent oxygen from entering from the outside even if the molten salt bath is not filled below the molten salt solidified layer 138. As shown in the figure, the opening end 136b of the gap 136 is preferably provided at a position higher than the bath surface S of the molten salt bath Bf, so that the molten salt solidified layer 138 can be formed at a higher position and the anode body 132 located in the gas phase part of the electrolytic cell can be prevented from being thinned. Typically, during molten salt electrolysis, the inside of the electrolytic cell is divided into a liquid phase consisting of the molten salt bath and a gas phase containing gases such as chlorine generated from the anode body. As mentioned above, outside air can flow into the gas phase. Therefore, in the past, there was a problem that the anode was thinned due to the influence of oxygen in the flowing outside air, and the anode could break and fall during the operation of molten salt electrolysis. This thinning of the anode often occurred at a position higher than the bath surface S, and furthermore, thinning of the anode often occurred near the back surface 121 of the lid. In contrast, in the present invention, in the gap 136 between the anode body 132 and the cover member 134, the molten salt bath Bf enters a position higher than the bath surface S of the molten salt bath Bf, and further, the molten salt solidified layer 138 is formed, thereby reducing the portion of the anode body 132 exposed to the gas phase in the electrolytic cell. Even if the anode body 132 has minute holes formed by high-temperature heating or the like during the preparation of the graphite anode body 132, the molten salt solidified layer 138 blocks the minute holes, thereby reducing the amount of outside air passing through the inside of the anode body 132. Furthermore, since the molten salt solidified layer 138 is formed by cooling and solidifying the molten salt, even if outside air passes through the inside of the anode body, it passes mainly on the low-temperature side, so the reaction between the anode body and oxygen is suppressed. As a result of the above, the reduction in thickness of the anode body 132 is suppressed, and the effect of oxidation wear of the anode body can be reduced. Furthermore, the effect of oxidation wear of the anode body can be reduced by the following operation. That is, if a thinned portion occurs in the anode body 132, the anode body can be reinforced by forming a molten salt solidified layer 138 in the thinned portion using an appropriate method, such as moving the bath surface S of the molten salt bath Bf up and down or supplying molten salt from the outside to the gap 136. This makes the anode body 132 appear thicker and the thinned portion is protected by the molten salt solidified layer 138, thereby preventing the anode body 132 from breaking. Furthermore, when the other open end 136b of the gap 136 is disposed at a position higher than the height position of the bath surface S of the molten salt bath Bf, the molten salt may be supplied from the open end 136b toward the gap 136 using the supply port 122 of the upper lid 120. This makes the molten salt in the gap 136 higher than the bath surface S of the molten salt bath Bf in the electrolytic cell 110, and allows a molten salt solidified layer 138 to be formed there. The distance D1 has a lower limit of, for example, 0.5 mm or more, and a maximum limit of, for example, 1.0 mm or more, and an upper limit of, for example, 20 mm or less, and a maximum limit of, for example, 5 mm or less.

[0027] The shape of the anode body 132 is not particularly limited, and examples thereof include a plate shape, a column shape, and a rectangular column shape. The shape of the cover member 134 may be appropriately changed according to the shape of the anode body 132, and may be, for example, a plate shape, a rectangular tube shape, a cylindrical shape, and the like. An example of a plate-shaped cover member is the anode 130 shown in FIG. 1E. The anode 130 includes an anode body 132 and a plate-shaped cover member 135 on an outer surface 132a of the anode body 132. When each of the anode body 132 and the cover member 135 is partially immersed in a molten salt bath, the molten salt bath enters the gap between the anode body 132 and the cover member 135, and a molten salt solidified layer 138 is formed in the gap at a position higher than the bath surface of the molten salt bath as described above. The molten salt bath enters the minute holes of the anode body, and the molten salt solidified layer 138 is formed, thereby blocking the minute holes. The method of arranging the cover member 135 is not particularly limited, and the cover member 135 may be arranged by extending its upper end side and connecting it to the upper lid 120 as in the embodiment shown in FIG. 1A, or the cover member 135 may be arranged by connecting it to the anode body 132 using a connecting tool such as a bolt made of an insulating material, or the cover member 135 may be arranged by placing a part of the cover member 135 on a part of the anode body 132, etc. From the viewpoint of efficiency in producing the molten metal, the molten salt electrolysis apparatus 100 may include a plurality of anodes, cathodes, and bipolar electrodes.

[0028] Next, another embodiment will be described below with reference to Figures 2A to E and Figure 3. Note that the configurations of the above-described embodiment can be appropriately applied, and therefore repeated description will be omitted.

[0029] 2A and 2B includes an anode body 232 having at least one step downward on its outer surface, and a cover member 234. The step is formed by connecting an outer surface 232a at the upper part of the anode body 232 and an outer surface 232b at the lower part of the anode body 232 via a flat surface 231. The anode body 232 has one step in the illustrated example, but may have two or more steps. The cover member 234 covers at least a part of the anode body 232 at a position higher than the flat surface 231 of the step of the anode body 232. A gap is provided between the anode body 232 and the cover member 234, and a distance D1 between the outer surface 232a of the anode body 232 and the inner surface 234b of the cover member 234 is within a range of 0.5 to 20 mm. Here, not only the gap is provided between the anode body 232 and the cover member 234, but also a gap may be provided between the flat surface 231 of the step of the anode body 232 and the cover member 234, and if the gap is located below the bath surface S of the molten salt bath Bf, the molten salt can enter the gap between the anode body 232 and the cover member 234 from an opening end 236a formed by the flat surface 231 of the step of the anode body 232 and the lower end surface 234a of the cover member 234. As long as the ingress of the molten salt bath is ensured in this way, the arrangement and shape of the opening end are not particularly limited. For example, a plurality of through holes 237 may be arranged on the lower end surface side of the cover member 234 to allow the molten salt bath to ingress into the gap. The embodiment shown in Figs. 2A to 2C includes the above-mentioned opening end 236a and through hole 237, but only one of them may be provided. Furthermore, although not shown, the anode 230 may have a U-shaped or inverted concave cutout portion on the lower end surface 234a of the cover member 234, and the through hole may be formed by contacting the cover member 234 with the flat surface 231. If the through hole 237 is located at a position lower than the height position of the bath surface S, the opening end of the gap is located below the bath surface S of the molten salt bath Bf, so that the molten salt bath Bf can ingress into the gap. As described above, the distance D2 between the flat surface 231 of the step of the anode body 232 and the lower end surface 234a of the cover member 234 is not particularly limited as long as the molten salt can enter the gap, but as an example, it is preferably within a range of 0.5 to 20 mm. When a through hole is provided from the opening end to the gap, the distance D2 means the longest line length that can be taken within the opening end. As described above, as the bath surface S of the molten salt bath Bf in the gap becomes higher, the temperature of the molten salt bath Bf at that portion becomes lower, and a part of the molten salt bath Bf on the upper lid 120 side is cooled and solidified, and as shown in FIG. 2C, a molten salt solidified layer 138 is formed in the gap. In addition, when the other opening end 236b of the gap is positioned at a position higher than the height position of the bath surface S of the molten salt bath Bf, a molten salt solidified layer 138 may be formed in the gap by opening the supply port 122 of the upper lid 120 and supplying molten salt from the opening end 236b toward the gap. As shown in Fig. 2D, a gap is provided between the anode body 232 and the cover member 234, but a gap need not be provided between the flat surface 231 of the step of the anode body 232 and the cover member 234. That is, the flat surface 231 of the step of the anode body 232 is in contact with the lower end surface 234a of the cover member 234. In this case, the molten salt bath can enter the gap from the through hole 237. The upper end of the cover member 234 is inserted into the upper lid 120, but the upper end of the cover member does not need to be inserted into the upper lid 120 as in the anode 230 shown in Fig. 2E. Although not shown, as shown in FIG. 2D , no gap may be provided between the flat surface 231 of the step of the anode body 232 and the cover member 234, and no through-hole 237 may be provided in the cover member 234. The upper open end 236b of the gap is located at a position higher than the height position of the bath surface S of the molten salt bath Bf, and the supply port of the top lid can be opened to supply molten salt from the open end 236b toward the gap. As a result, the molten salt in the gap becomes higher than the bath surface S of the molten salt bath Bf in the electrolytic cell 110, and a part of the molten salt bath Bf on the top lid 120 side is cooled and solidified, forming a molten salt solidified layer 138 in the gap.

[0030] The cover members 134, 234 may be made of bricks or the like, and preferably contain at least one selected from ceramics such as silicon nitride, silicon carbide, aluminum oxide, silicon oxide, magnesium oxide, calcium oxide, and steel. From the viewpoint of insulation, it is more preferable that the cover members 134, 234 are made of any one of silicon nitride, silicon carbide, aluminum oxide, silicon oxide, magnesium oxide, and calcium oxide, and among these, it is particularly preferable that the cover members 134, 234 are made of silicon nitride or silicon carbide from the viewpoint of heat resistance and corrosion resistance. When the cover members 134, 234 are made of steel, it is preferable that the cover members 134, 234 are provided with an insulator such as a castable refractory material in order to ensure insulation between the cover members 134, 234 and the cover members 120.

[0031] The anode body 132, 232 and the cathode 140 are connected to a power source via conductive wires. In the anode body 132, 232 and the cathode 140, a chlorine-containing gas is generated by an oxidation reaction on the electrolytic surface of the anode body 132, 232 immersed in the molten salt bath Bf, based on a predetermined reaction such as the following chemical formula (1), and a molten metal such as metallic magnesium is generated by a reduction reaction on the electrolytic surface of the cathode 140 immersed in the molten salt bath Bf. MgCl2→Mg+Cl2...Chemical formula (1) The material of the cathode 140 is not particularly limited, but examples include iron, graphite, etc. The shape of the cathode 140 can be appropriately changed in consideration of the shape of the anode main bodies 132, 232.

[0032] As long as the electrolysis chamber 150 has at least the anode body 132, 232 and the cathode 140, it can perform electrolysis of metal chlorides in a molten salt bath. However, the electrolysis chamber 150 may have a plurality of pairs of electrodes from the viewpoint of improving the production efficiency of electrolysis. In addition, from the viewpoint of improving the production efficiency of electrolysis, it is preferable that the electrodes further have one or more bipolar electrodes 142, 144 between the anode body 132, 232 and the cathode 140, which are polarized by applying a voltage between the anode body 132, 232 and the cathode 140, as shown in FIG. 1B and FIG. 2B. In this example, the number of bipolar electrodes 142, 144 is two, but the number of bipolar electrodes can be appropriately adjusted. However, such bipolar electrodes 142, 144 are not necessarily required. The inter-electrode distances between the anode body 132, 232 and the bipolar electrode 142, between the cathode 140 and the bipolar electrode 144, and between the bipolar electrodes 142 and 144 may be the same or different.

[0033] (Metal Recovery Room) The metal recovery chamber 160 recovers the molten metal produced by electrolysis in the electrolysis chamber 150. The metal recovery chamber 160 communicates with the electrolysis chamber 150 and may include a heat exchanger (not shown).

[0034] The heat exchanger can adjust the temperature of the molten salt bath Bf in the electrolytic cell 110. The heat exchanger may include an inlet for allowing a fluid to flow, an outlet for discharging the fluid, and a pipe connecting the inlet and the outlet. The pipe may be made of steel (carbon steel or stainless steel).

[0035] (heat exchange room) The molten salt electrolysis apparatus 300 shown in FIG. 3 includes an electrolytic cell 110, a top cover 120, an anode 130, a cathode 140, an electrolysis chamber 150, a metal recovery chamber 160, and a heat exchange chamber 370 having a heat exchanger (not shown). In the molten salt electrolysis apparatus 300, the electrolysis chamber 150, the metal recovery chamber 160, and the heat exchange chamber 370 are arranged in this order in the lateral direction in the figure. The top cover 120 has an inlet / outlet 328 in the area of ​​the heat exchange chamber 370. This allows the recovery of the molten metal to be performed using the inlet / outlet 128, and the replenishment of the molten salt such as molten magnesium chloride to be performed using the inlet / outlet 328. As a result, the molten metal can be more stably stored in the metal recovery chamber 160. In addition to these, the heat exchange chamber 370 may further include a stirrer (not shown) for stirring the molten salt bath Bf.

[0036] The electrolytic cell 110 is divided into an electrolysis chamber 150, a metal recovery chamber 160, and a heat exchange chamber 370 by a first partition wall 116, a second partition wall 317, and a third partition wall 318, respectively.

[0037] [2.Metal manufacturing methods] In one embodiment of the method for producing a metal according to the present invention, an electrolysis step for producing a metal is included using the above-mentioned molten salt electrolysis apparatus 100, 200, 300. In addition, a solidified layer formation step may be included at an appropriate timing such as before or during the electrolysis step. Hereinafter, preferred aspects of each step using the molten salt electrolysis apparatus 100 shown in Figs. 1A to 1D will be described.

[0038] <Solidified layer formation process> In the solidified layer forming step, a molten salt solidified layer 138 is formed at a position higher than the bath surface S of the molten salt bath Bf in the gap 136 between the anode body 132 and the cover member 134 by the method described above. The lower side of the molten salt solidified layer 138 can be filled with molten salt, but it is not necessary that the lower side of the molten salt solidified layer 138 is filled with molten salt. When molten salt electrolysis is performed for a long period of time, the bath surface S may move up and down, but even in such a case, the molten salt solidified layer 138 functions effectively. Note that the up and down movement of the bath surface S may cause outside air to enter the inside of the molten salt electrolysis device 100 through the inside of the anode, but the above-mentioned molten salt solidified layer 138 acts to suppress the entry of outside air. As described above, the formation of the molten salt solidified layer 138 can suppress the entry of outside air into the inside of the molten salt electrolysis device 100 through the inside of the anode body 132, and thus the effect of oxidation wear of the anode body 132 can be reduced. As a result, the life of the anode body 132 in molten salt electrolysis is extended.

[0039] <Electrolysis process> In the electrolysis step, electrolysis of metal chlorides contained in the molten salt bath Bf is carried out. As an example, molten magnesium chloride, which is a by-product obtained in the production of sponge titanium, is added to the molten salt bath Bf in the molten salt electrolysis apparatus 100, and the molten magnesium chloride is electrolyzed to produce metallic magnesium. The solidified layer formation step can be carried out at any timing, and may be carried out, for example, before the start of the electrolysis step or after the start of the electrolysis step.

[0040] The molten salt bath Bf flows from the electrolysis chamber 150 through the flow port 115 to the metal recovery chamber 160 as indicated by an arrow A in FIG. 1A, and then flows from the metal recovery chamber 160 through the underside of the second partition wall 117 to the electrolysis chamber 150 as indicated by an arrow B in FIG. 1A. In the electrolysis chamber 150, the metal chloride in the molten salt bath Bf is electrolyzed to generate molten metal. This molten metal then flows into the metal recovery chamber 160 due to the flow of the molten salt bath Bf. Thereafter, the molten metal, which has a smaller specific gravity than the molten salt, floats to a shallow portion of the metal recovery chamber 160 and accumulates there. The molten metal that floats in the metal recovery chamber 160 can be recovered by inserting a recovery pipe or the like through the supply and discharge port 128.

[0041] During the electrolysis process, cracks may occur in the molten salt solidified layer 138 formed in the solidified layer formation process. In this case, it is preferable to form the molten salt solidified layer 138 again. For example, molten salt is poured from the supply / discharge port 128 of the upper lid 120 to raise the height position of the bath surface S of the molten salt bath Bf to near the lid back surface 121, and the molten salt solidified layer is melted once. After melting, a recovery pipe or the like is inserted into the supply / discharge port 128, and a part of the molten salt bath Bf is slowly extracted from the pipe, and the bath surface S is lowered while the molten salt solidified layer 138 is formed. The formed molten salt solidified layer 138 is maintained even without contact with the molten salt bath, so that the molten salt solidified layer 138 suppresses oxygen from entering from the outside.

[0042] When molten salt electrolysis is performed using the molten salt electrolysis apparatus 100, 200, 300, the clearance between the anode body 132, 232 and the upper cover 120 may be filled with powder or the like of the components constituting the molten salt bath Bf to serve as a sealing member so as to prevent outside air from entering the molten salt electrolysis apparatus 100, 200, 300. During the long-term performance of molten salt electrolysis, the amount of the powder or the like that slides down into the electrolytic cell 110 may increase. In addition, the electrical resistance between the electrodes may suddenly increase. When these phenomena occur, it is suspected that the anode body 132 is thinning, and therefore it is preferable to form or re-form the molten salt solidified layer 138. EXAMPLES

[0043] The present invention will be specifically described based on examples and comparative examples. The following examples and comparative examples are merely experimental examples for facilitating understanding of the technical content of the present invention, and the technical scope of the present invention is not limited by these examples.

[0044] [Example 1] (Formation of a solidified molten salt layer) In Example 1, a molten salt electrolysis apparatus 100 having the configuration shown in Figures 1A to 1C was used. In the molten salt electrolysis apparatus 100, the electrolytic cell 110, the first partition wall 116, and the second partition wall 117 were made of standard refractory material (firebrick) containing aluminum oxide, the upper lid 120 was made of carbon steel, and an insulating castable refractory layer was applied to the lid back surface 121 of the upper lid 120. In addition, the anode body 132, the cathode 140, and the two bipolar electrodes 142 and 144 were all plate-shaped, and a rectangular cylindrical cover member 134 was placed on the outside of the plate-shaped anode body 132. The cover member 134 was made of firebrick (mullite). The width (thickness) of the anode body 132 was set to 200 mm, and the distance D1 between each outer side surface 132a of the anode body 132 and each inner surface 134b of the cover member 134 was adjusted to 3 mm. Next, molten salt was charged into the molten salt electrolysis apparatus 100, and the temperature of the molten salt was adjusted to 650 to 700°C. As a result, the distance in the height direction from the lower end surface 134a of the cover member 134 to the bath surface S was within a range of 30 mm to 150 mm. The composition of the molten salt was set to 5 to 25 mass% magnesium chloride, 28 to 36 mass% calcium chloride, and 47 to 59 mass% sodium chloride. The material of the anode body 132 was graphite, and the material of the cathode 140 was carbon steel. The material of the bipolar electrodes 142 and 144 was graphite.

[0045] The temperature of the molten salt bath Bf was maintained within a range of 650 to 700° C. By maintaining the temperature in this range, the molten salt bath Bf could enter the gap 136 from the open end 136a, and the bath surface S of the molten salt bath Bf in the gap 136 rose due to capillary action and was cooled as it approached the top lid 120. A part of the molten salt bath Bf on the top lid 120 side in the gap 136 solidified, and a molten salt solidified layer 138 was formed as shown in FIG. 1D.

[0046] (Production of metallic magnesium) The electrolysis process was carried out by supplying a current between the anode body 132 and the cathode 140 from a power source via a conductive wire. The electrolysis process was started after the formation of a solidified layer of molten salt. The supply of current was stopped 30 months after the start of electrolysis. After that, the anode body 132 of the molten salt electrolysis apparatus 100 was removed and checked, and the maximum depth of the thinning in the width direction of the anode body 132 was 20 mm. The results are shown in Table 1.

[0047] [Example 2] In Example 2, the molten salt electrolysis apparatus 100 was operated in the same manner as in Example 1, except that the cover member 134 was changed to one made of silicon nitride as shown in Table 1. After 30 months had passed since the start of electrolysis, the supply of current was stopped. Thereafter, the anode body 132 of the molten salt electrolysis apparatus 100 was removed and checked, and the maximum depth of the reduced metal in the width direction of the anode body 132 was 1 mm or less. The results are shown in Table 1.

[0048] [Comparative Example 1] In Comparative Example 1, the molten salt electrolysis apparatus was operated in the same manner as in Example 1, except that no cover member was used, as shown in Table 1. After 30 months had passed since the start of electrolysis, the supply of current was stopped. After that, the anode body of the molten salt electrolysis apparatus was removed and checked, and the maximum thickness reduction depth in the width direction of the anode body 132 inserted into the upper lid was 50 mm. The results are shown in Table 1.

[0049] [Table 1]

[0050] (Considerations based on examples) In Examples 1 and 2, the degree of thinning in the width direction of the anode body was smaller than in Comparative Example 1, and therefore the effect of oxidative wear of the anode body could be reduced. That is, in Examples 1 and 2, it can be said that it is useful to cover a part of the anode body with a cover member and set the distance between the outer surface of the anode body and the inner surface of the cover member within the range of 0.5 to 20 mm. Moreover, in Example 2, the degree of thinning in the width direction of the anode body was considerably smaller than in Example 1. The reason for this is believed to be that silicon nitride has better heat conductivity than mullite and is more likely to form a molten salt solidified layer. In addition, the silicon nitride cover member has a longer life than the mullite cover member, and is less likely to crack. Therefore, it is believed that the fact that cracks in the molten salt solidified layer due to cracks in the cover member are less likely to occur also contributed to the good results. [Explanation of symbols]

[0051] 100, 200, 300 Molten salt electrolysis equipment 110 Electrolytic cell 112 Bottom wall 114 Side wall 115 Distribution port 116 First Bulkhead 117, 317 Second bulkhead 120 Top lid 121 Back of the lid 122 Supply port 124 First gas recovery port 126 Second gas recovery port 128, 328 Supply / discharge port 130, 230 anode 132, 232 Anode body 132a, 232a, 232b outer surface 134, 135, 234 Cover member 134a, 234a Bottom end surface 134b, 234b Inside 136 Cavity 136a, 136b, 236a, 236b open end 138 Molten Salt Solidification Layer 140 Cathode 140a Top surface 142, 144 Bipolar electrodes 150 Electrolysis chamber 160 Metal Recovery Room 231 Flat surface 237 Through hole 318 Third Bulkhead 370 Heat exchange room A, B arrows Bf molten salt bath S bath surface

Claims

1. An anode for use in the electrolysis of magnesium chloride, comprising: an anode body for use in an electrolytic cell in which a molten salt bath is stored; A cover member for covering a portion of the anode body; a gap portion that is present between the anode body and the cover member and that is used to form a solidified molten salt layer; An anode, wherein the distance between the outer surface of the anode body and the inner surface of the cover member is within a range of 0.5 to 20 mm.

2. The anode according to claim 1 , wherein the anode has an open end of the gap at a position lower than a height position of a bath surface of the molten salt bath.

3. The anode body has at least one step, The anode according to claim 1 , wherein the cover member covers at least a portion of the anode body at a position higher than a flat surface of the step.

4. 4. The anode according to claim 1, wherein the cover member comprises at least one selected from the group consisting of silicon nitride, silicon carbide, aluminum oxide, silicon oxide, magnesium oxide, and calcium oxide.

5. The anode according to any one of claims 1 to 4, wherein the anode body is made of graphite.

6. A molten salt electrolysis apparatus comprising the anode according to any one of claims 1 to 5.

7. A method for producing a metal using a molten salt electrolysis apparatus including an anode having an anode body used in an electrolytic cell in which a molten salt bath is stored, a cover member covering a part of the anode body, and a void portion existing between the anode body and the cover member, an electrolysis step for performing electrolysis of metal chlorides contained in the molten salt bath; and forming a solidified layer of the molten salt in the gap at a position higher than the surface of the molten salt bath, A method for producing a metal, wherein the distance between the outer surface of the anode body and the inner surface of the cover member is within a range of 0.5 to 20 mm.

8. 8. The method of claim 7, wherein the metal chloride to be electrolyzed is magnesium chloride.

9. A method for producing a metal as described in claim 7 or 8, wherein the anode has an opening end of the gap at a position lower than the height position of the bath surface of the molten salt bath.

10. The anode body has at least one step, The method for producing a metal according to any one of claims 7 to 9, wherein the cover member covers at least a portion of the anode body at a position higher than the flat surface of the step.

11. A method for producing a metal described in any one of claims 7 to 10, wherein the cover member contains one or more selected from silicon nitride, silicon carbide, aluminum oxide, silicon oxide, magnesium oxide and calcium oxide.

12. A method for producing a metal as described in any one of claims 7 to 11, wherein the anode body is made of graphite.

Images