Electrode, molten salt electrolysis apparatus, and method for producing metallic magnesium

Abstract

To provide an electrode that can appropriately maintain the anode-cathode distance in a molten salt electrolysis of magnesium chloride.SOLUTION: An electrode 10 comprises: a bipolar body 11 made of graphite including a first electrolytic surface 14 and a second electrolytic surface 15 facing the opposite direction to the direction of the first electrolytic surface 14; at least one through hole 12 formed in the bipolar body 11 for penetrating from the first electrolytic surface 14 through the second electrolytic surface 15; and a spacer 13 for keeping an anode-cathode distance from another electrode to be arranged adjacent to the electrode 10. The spacer 13 includes: a through-hole part 16 extending through in the through hole 12; a first fixed part 17 arranged at one end side of the through-hole part 16 and on the first electrolytic surface 14; and a second fixed part 18, which is arranged at the other end side opposite to the above one and on the second electrolytic surface 15, for sandwiching the bipolar body 11 to fix in cooperation with first fixed part 17 in the thickness direction. The through-hole part 16 and the first fixed part 17 are made of ceramics, and the second fixed part 18 is made of a steel.SELECTED DRAWING: Figure 1B

Description

Technical Field

[0001] The present invention relates to an electrode, a molten salt electrolysis apparatus, and a method for producing metallic magnesium.

Background Art

[0002] Ingots of metallic titanium, etc. are industrially produced using sponge titanium produced by the Kroll process. And the sponge titanium production process including this Kroll process can be roughly classified into four processes: a chlorination process, a reduction separation process, a crushing process, and an electrolysis process. Among these processes, in the reduction separation process, titanium tetrachloride is reduced with metallic magnesium to produce sponge titanium. And the electrolysis process is a process of decomposing magnesium chloride, which is a by-product of the reduction separation process, by molten salt electrolysis to obtain metallic magnesium.

[0003] In the electrolytic cell of a molten salt electrolysis apparatus used for molten salt electrolysis, an anode, a bipolar electrode, and a cathode are arranged side by side. At this time, the anode, the bipolar electrode, and the cathode may be placed on a refractory pedestal such as a brick in the electrolytic cell.

[0004] In the molten salt electrolysis of magnesium chloride, the electrolytic cell may be operated for a long time in a corrosive, high-temperature, and flowing environment of the molten salt bath. During that time, various stresses act on the bipolar electrode, which may cause displacement or shape change of the bipolar electrode. This leads to a decrease in current efficiency and the occurrence of short circuits. For example, due to the displacement of the bipolar electrode, the inter-pole distance is not properly maintained between the bipolar electrodes, between the bipolar electrode and the anode, or between the bipolar electrode and the cathode, resulting in a decrease in current efficiency and a decrease in the productivity of metallic magnesium by electrolysis of magnesium chloride. To prevent this, various techniques are known.

[0005] For example, Patent Document 1 describes "an electrolytic cell for molten magnesium chloride, wherein the electrode intervals between the anode and the cathode or bipolar electrode, or between the electrodes of each of the anode and the cathode or the bipolar electrode are maintained at a predetermined interval by a spacer embedded in the anode, the cathode or the bipolar electrode, or the furnace wall, and the spacer is made of silicon nitride ceramic." The spacer is "provided with indentations on the surfaces of the anode and the bipolar electrode to a depth of about half the thickness of the bipolar electrode, inserted into and fixed in these indentations, and configured to maintain the electrode intervals between the anode 7, the cathode 8, and the bipolar electrodes 12 and 13 respectively." (See FIG. 2 of Patent Document 1).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Referring to Patent Document 1, after inserting and installing a spacer made of alumina, which is an insulating ceramic, into the recessed holes provided on the surface of a graphite bipolar electrode, electrolysis of magnesium chloride was carried out. In Patent Document 1, a spacer is installed, but since the amount of chlorine gas generated during electrolysis is large and the floating speed is also fast, the bipolar electrode is prone to vibration. In this case, during electrolysis, mainly due to the flow of the molten salt bath caused by the rising of chlorine gas, the spacer sways, and the bipolar electrode with the spacer embedded therein may chip or wear, and the spacer embedded in the bipolar electrode may fall off. As a result, the position of the bipolar electrode is displaced and the inter-pole distance cannot be maintained, leading to a decrease in the electrolysis efficiency of magnesium chloride.

[0008] Therefore, in one embodiment, the present invention aims to provide an electrode capable of appropriately maintaining the interelectrode distance in the molten salt electrolysis of magnesium chloride. Further, by arranging such an electrode as a bipolar electrode in an electrolytic cell, an object of the present invention is to provide a molten salt electrolysis apparatus capable of stably obtaining metallic magnesium in the molten salt electrolysis of magnesium chloride and a method for producing metallic magnesium.

Means for Solving the Problems

[0009] That is, in one aspect, the present invention is an electrode used as a bipolar electrode immersed in a molten salt bath in the molten salt electrolysis of magnesium chloride, including a graphite bipolar electrode body including a first electrolysis surface and a second electrolysis surface facing the side opposite to the first electrolysis surface, at least one through-hole formed in the bipolar electrode body and penetrating from the first electrolysis surface to the second electrolysis surface, and a spacer for securing an interelectrode distance between the electrode and another electrode disposed adjacent thereto. The spacer includes a through portion extending through the through-hole, a first fixing portion provided on the first electrolysis surface at one end side of the through portion, and a second fixing portion provided on the second electrolysis surface at the other end side opposite to the one end and sandwiching and fixing the bipolar electrode body in its thickness direction between the second fixing portion and the first fixing portion. The through portion and the first fixing portion are made of ceramic, and the second fixing portion is made of steel.

[0010] In one embodiment of the electrode according to the present invention, the ratio (d / T) of the thickness (T) of the bipolar electrode body to the distance (d) from the first electrolysis surface to the tip of the first fixing portion in the thickness direction of the bipolar electrode body is in the range of 5% or more and 15% or less.

[0011] Further, in another aspect, the present invention is a molten salt electrolysis apparatus including an electrolytic cell, an anode, a bipolar electrode, and a cathode disposed in the electrolytic cell, wherein at least one of the bipolar electrodes is any one of the above electrodes.

[0012] In one embodiment of the molten salt electrolysis apparatus according to the present invention, the anode, at least one of the bipolar electrodes, and the cathode are arranged in this order in the electrolytic cell, and the first fixing portion of the bipolar electrode is located on the cathode side.

[0013] In one embodiment of the molten salt electrolysis apparatus according to the present invention, in the arrangement direction of the anode, the bipolar electrode, and the cathode, the distance from the first electrolytic surface to the tip of the first fixing portion of the bipolar electrode is within a range of 50% or more and 90% or less of the interpolar distance from the first electrolytic surface to the cathode or bipolar electrode adjacent to the bipolar electrode.

[0014] Further, in another aspect, the present invention is a method for producing magnesium metal, which includes an electrolysis step of electrolyzing magnesium chloride to produce magnesium metal using any one of the above molten salt electrolysis apparatuses.

Advantages of the Invention

[0015] According to one embodiment of the present invention, the interpolar distance can be appropriately maintained in the molten salt electrolysis of magnesium chloride. Further, according to a further embodiment of the present invention, by arranging such an electrode as a bipolar electrode in the electrolytic cell, magnesium metal can be stably obtained in the molten salt electrolysis of magnesium chloride.

Brief Description of the Drawings

[0016]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 2A

Figure 2B

Figure 2C

Figure 3A

Figure 3B

Figure 4

Embodiments for Carrying Out the Invention

[0017] The present invention is not limited to the embodiments described below, and components can be modified and embodied without departing from the gist thereof. Also, various inventions can be formed by appropriately combining a plurality of components disclosed in each embodiment. For example, an invention may be formed by deleting some components from all the components shown in the embodiment. Note that in the drawings, there are also members schematically shown to assist in understanding the embodiments included in the invention, and the illustrated sizes, positional relationships, etc. may not necessarily be accurate. Furthermore, in this specification, "upward" means the direction from the bottom wall 123 side of the electrolytic cell 120 toward the upper lid 130 side as indicated by the arrow in FIGS. 2A, 2B, 2C, 3A, and 4, and "downward" means the direction from the upper lid 130 side toward the bottom wall 123 side of the electrolytic cell 120. Also, in this specification, "molten metallic magnesium" means molten metallic magnesium obtained by electrolytic decomposition of magnesium chloride. The molten salt separately supplied from the outside in molten salt electrolysis can have its composition appropriately adjusted. For example, it can be magnesium chloride, or it can have the same composition as the molten salt bath.

[0018] [1. Electrodes] The electrode 10 shown in FIGS. 1A and 1B is used as a bipolar electrode immersed in a molten salt bath in the molten salt electrolysis of magnesium chloride, and includes a bipolar body 11, through holes 12, and spacers 13.

[0019] Conventionally, as described above, an electrolytic cell as shown in FIG. 4 has been known. During electrolysis, the spacers 313a and 313b embedded in the bipolar electrodes 310a and 310b may fall off, causing the arrangement of the bipolar electrodes to shift, resulting in a decrease in current efficiency or short circuit. Also, even before the spacers 313a and 313b fall off, their arrangement may tilt, making it impossible to maintain the interpolar distance. In addition, the recessed holes provided with the spacers 313a and 313b may become deeper than necessary due to the swinging of the spacers 313a and 313b, etc., and the spacers 313a and 313b may retract inside the bipolar electrodes, making it impossible to maintain the interpolar distance. This problem is considered to occur due to the following factors. The material of the bipolar electrodes 310a and 310b is graphite, and the material of the spacers 313a and 313b is ceramic. Generally, ceramic is harder than graphite. When loads such as the rise of chlorine gas and the flow of the molten salt bath containing magnesium chloride accompanying it are applied to the spacers 313a and 313b, as the high-hardness spacers 313a and 313b swing inside the bipolar electrodes 310a and 310b in which they are embedded, the contact portions around the recessed holes in the bipolar electrodes 310a and 310b where the spacers 313a and 313b are embedded are gradually worn away. When the wear of the contact portions with the spacers 313a and 313b becomes severe, chips are generated in the recessed holes, the spacers 313a and 313b fall off, and the interpolar distance between the bipolar electrodes 310a and 310b adjacent to the bipolar electrodes 310a and 310b in which the spacers 313a and 313b were embedded cannot be appropriately maintained, and the bipolar electrodes 310a and 310b may tilt. This tilt of the bipolar electrodes 310a and 310b may occur not only due to the simple fall of the spacers 313a and 313b, but also due to the spacers 313a and 313b being pinched between the poles. In such a case, on the electrolytic surface of the tilted bipolar electrodes 310a and 310b, the distribution of voltage and current varies, and the amount of magnesium chloride generated by electrolysis changes. That is, it is considered that the balance of the electrolysis of magnesium chloride is lost.

[0020] Therefore, as a result of intensive studies by the present inventors, in order to make it difficult for a ceramic spacer to swing within a graphite bipolar electrode against stresses such as the rise of chlorine gas and the flow of the molten salt bath, a through-hole penetrating the bipolar electrode body is formed, and a part of the spacer is inserted into the through-hole to attach the spacer. At this time, by sandwiching the bipolar electrode body from both sides of the first electrolytic surface side and the second electrolytic surface side with the first fixing portion and the second fixing portion of the spacer, the spacer is firmly fixed. Therefore, even if a part of the through-hole is damaged, the dropout of the spacer is suppressed, and the inter-pole distance can be stably maintained.

[0021] Furthermore, in a molten salt electrolysis apparatus in which an anode, a bipolar electrode, and a cathode are arranged in this order in an electrolytic cell, among the first fixing portion and the second fixing portion of the spacer, it is advantageous that the first fixing portion located on the cathode side (the side where chlorine gas is generated in the bipolar electrode) is made of ceramic, and the second fixing portion located on the anode side (the side where metallic magnesium is generated in the bipolar electrode) is made of steel. In this way, even if the through-hole expands slightly due to the swinging of the spacer caused by the rise of chlorine gas or the flow of the molten salt bath, etc., and a gap is formed, the molten salt bath penetrates into the gap between the bipolar electrode body and the spacer on the cathode side, and metallic magnesium penetrates into the gap on the anode side, and the gap is likely to be filled on either side. In particular, the steel second fixing portion located on the anode side is considered to be able to form an alloy with its iron component and the metallic magnesium that penetrates into the gap, and further firmly fixes the spacer. As a result, during the electrolysis of magnesium chloride, the dropout of the spacer is more reliably suppressed, and the inter-pole distance can be stably maintained over a long period. Hereinafter, preferred embodiments will be described with reference to the drawings.

[0022] (Bipolar electrode body) The bipolar electrode body 11 includes a first electrolytic surface 14 and a second electrolytic surface 15 facing the side opposite to the first electrolytic surface 14. The material of the bipolar electrode body 11 is graphite. The bipolar electrode body 11 may be manufactured from a single plate, but in consideration of workability, it may also be manufactured by combining a plurality of plates. Further, the shape of the bipolar electrode body 11 is not limited, and examples include a plate shape, a square tube shape (see FIG. 3B), and a cylindrical shape.

[0023] (Through-hole) The through-hole 12 is formed so as to penetrate from the first electrolytic surface 14 to the second electrolytic surface 15 within the bipolar body 11. At least one through-hole 12 is formed within the bipolar body 11. The number of through-holes 12 can be appropriately adjusted according to the number of spacers 13 provided in the bipolar body 11. When there are a plurality of through-holes 12 within the bipolar body 11, the plurality of through-holes 12 may be provided at a predetermined interval from each other so that the first fixing portions 17 and the adjacent second fixing portions 18 do not contact each other between the spacers 13 attached to each through-hole 12. Further, a plurality of through-holes 12 may be arranged, for example, regularly or irregularly within the bipolar body 11. As an example of a regular arrangement, as shown in FIG. 1C, the adjacent through-holes 12 may be arranged so as to have a substantially equal separation distance, or as shown in FIG. 1D, each through-hole 12 may be arranged in a staggered pattern. Note that the cross-sectional shape of the through-hole 12 in the direction orthogonal to the thickness direction of the bipolar body 11 depends on the cross-sectional shape of the through portion 16 of the spacer 13 described later, and examples thereof include a polygonal shape, a perfect circle, an ellipse, or other circular shapes.

[0024] (Spacer) The spacer 13 ensures the inter-pole distance between the electrode 10 and another electrode disposed adjacent thereto. The first fixing portion 17 of the spacer 13, which is made of ceramic, mainly contributes to the proper maintenance of the inter-pole distance. Here, the "inter-pole distance" means the distance between the electrode and the bipolar or the cathode when the bipolar or the cathode is disposed as another electrode adjacent to the electrode. The spacer 13 includes a through portion 16 extending through the through hole 12, a first fixing portion 17 provided on the first electrolytic surface 14 at one end side of the through portion 16, and a second fixing portion 18 provided on the second electrolytic surface 15 at the other end side opposite to the one end, and sandwiching and fixing the bipolar body 11 in its thickness direction between the first fixing portion 17. Thereby, even when the flow of chlorine gas or molten salt bath is received by the spacer 13 and the bipolar body 11 during the electrolysis of magnesium chloride, it is possible to preferably suppress the spacer 13 from coming off the bipolar body 11. Since the spacer 13 is firmly fixed to the bipolar body 11 in this way, the required inter-pole distance can be maintained over a long period by the spacer 13. Note that since the anode and the cathode extend to the outside of the electrolytic cell and are arranged, they are easier to fix than the bipolar. On the other hand, since the bipolar electrode is arranged inside the electrolytic cell, its arrangement is more likely to fluctuate compared to the anode and the cathode. The spacer 13 is for suppressing the arrangement fluctuation of the bipolar and appropriately maintaining the inter-pole distance.

[0025] (Through portion, first fixing portion) The through portion 16 and the first fixing portion 17 are made of ceramic from the viewpoint of avoiding the occurrence of short circuits and having a hardness that can withstand the flow of the molten salt bath containing magnesium chloride. Specific examples of the ceramic include aluminum oxide, silicon nitride, and silicon carbide. In addition, examples of the shape of the first fixing portion 17 include a columnar shape and a prismatic shape. When the shape of the first fixing portion 17 is columnar, the molten salt of the molten salt bath flows smoothly around it and it is difficult to inhibit the bath flow, so it is possible to reduce the load on the first fixing portion 17. For this reason, the fall of the spacer 13 from the bipolar body 11 is preferably suppressed, and it is easy to maintain a constant inter-pole distance. Further, for example, when the shape of the first fixing portion 17 is columnar and the shape of the through hole 12 is columnar, the diameter of the first fixing portion 17 is preferably larger than the diameter of the through hole 12. Thereby, not only is the bipolar body 11 sandwiched and fixed by the second fixing portion 18, but also the intrusion of the molten salt into the through hole 12 is suppressed during the electrolysis of magnesium chloride, so that the fall of the spacer 13 from the bipolar body 11 is preferably suppressed. Further, the shape of the through portion 16 may be appropriately adjusted according to the shape of the through hole 12. For example, when the cross-sectional shape of the through hole 12 is circular, the cross-sectional shape of the through portion 16 can also be circular. From the viewpoint of suppressing the fall of the spacer 13 from the bipolar body 11, it is preferable that the diameter of the through portion 16 is substantially the same as the diameter of the through hole 12 so that the through portion 16 is frictionally engaged in the through hole 12. A clearance may be provided between the through hole 12 and the through portion 16, but the clearance is preferably, for example, 1 mm or less from the viewpoint of facilitating the attachment operation of the spacer 13 to the bipolar body 11 and properly fixing the spacer 13 to the bipolar body 11. Since graphite is a smooth material, the spacer 13 can be attached to the bipolar body 11 even if there is almost no clearance. Note that the through portion 16 and the first fixing portion 17 may be integrally formed by molding or the like, or the through portion 16 and the first fixing portion 17 may be configured to be assemblable.

[0026] (Second fixing portion) The second fixing portion 18 is made of steel. In order to prevent the first fixing portion 17 and the through portion 16 from falling off from the bipolar body 11, the second fixing portion 18 is used in engagement with the through portion 16. Thereby, since the spacer 13 is firmly fixed to the bipolar body 11, the fall of the spacer 13 from the bipolar body 11 is preferably suppressed. Although the mechanism of action is not necessarily clear, the advantages of using the second fixing portion 18 made of steel are considered as follows. For example, in a molten salt electrolysis apparatus (see FIGS. 2A and 2B), when magnesium chloride is electrolyzed, metallic magnesium generated from the second electrolytic surface 115a side between the anode 141 and the second electrolytic surface 115a of the bipolar body 111a enters between the second fixing portion 118a and the second electrolytic surface 115a and is alloyed with the iron component in the second fixing portion 118a. Further, due to the formation of the alloy, the gaps between the second fixing portion 118a and the through portion 116a and the second electrolytic surface 115a and the through hole 112a are filled, and as a result, it is considered that the spacer 113a and the bipolar body 111a are more firmly joined. Specific examples of the steel include, for example, carbon steel and stainless steel, and carbon steel is preferable from the viewpoint of suppressing the mixing of impurities into the produced metallic magnesium. In addition, the method of attaching the second fixing portion 18 to the through portion 16 is not particularly limited. For example, a concave hole is formed on the surface of the tip of the through portion 16 and a through hole is formed in the second fixing portion 18, and a steel connecting pin is passed through the through hole and inserted into the concave hole. Another method is to form a spiral groove (male thread portion) on the outer surface of the tip of the through portion 16, form a concave hole on the surface of the second fixing portion 18 on the side of the second electrolytic surface 15, and form a spiral groove (female thread portion) on the inner surface of the concave hole, and then screw the male thread portion and the female thread portion together. In addition, examples of the shape of the second fixing portion 18 include a pin shape, a ring shape, a cylindrical shape, and a prismatic shape. When the shape of the second fixing portion 18 is cylindrical, the flow of the molten salt bath becomes smooth, and it is possible to reduce the load on the second fixing portion 18. Therefore, the fall of the spacer 13 from the bipolar body 11 is well suppressed, and it is easy to maintain a constant inter-pole distance. Further, for example, when the shape of the second fixing portion 18 is cylindrical and the shape of the through hole 12 is cylindrical, it is preferable that the diameter of the second fixing portion 18 is larger than the diameter of the through hole 12. Thereby, the bipolar body 11 is more reliably sandwiched and fixed between the second fixing portion 18 and the first fixing portion 17, and the fall of the spacer 13 from the bipolar body 11 is well suppressed. Note that the thickness of the second fixing portion 18 (the distance from the second electrolytic surface 15 to the tip of the second fixing portion 18 in the thickness direction of the bipolar body 11) may be large enough to sandwich the bipolar body 11 so that the spacer 13 does not fall. The thickness of the second fixing portion 18 may be, for example, in the range of 0.3 mm to 2.0 mm so that the steel second fixing portion 18, which may have conductivity, does not adversely affect the electrolysis of magnesium chloride.

[0027] From the viewpoint of stabilizing the inter-pole distance in the molten salt electrolysis of magnesium chloride by suppressing the dropout of the spacer, as shown in FIG. 1B, the ratio (d / T) of the thickness (T) of the bipolar body 11 to the distance (d) from the first electrolytic surface 14 to the tip of the first fixing portion 17 in the thickness direction of the bipolar body 11 is preferably in the range of 5% or more and 15% or less. More specifically, the distance (d) can be determined in consideration of the inter-pole distance, and is, for example, in the range of 3 mm to 10 mm.

[0028] [2. Molten Salt Electrolysis Apparatus] The molten salt electrolysis apparatus 100 shown in Fig. 2A includes an electrolytic cell 120 and an upper lid 130. The electrolytic cell 120 is partitioned into an electrolysis chamber 140 and a metal recovery chamber 150 due to the presence of a first partition wall 121 and a second partition wall 122 described later. Further, as shown in Fig. 2B, in the electrolysis chamber 140 of the electrolytic cell 120, an anode 141, a first bipolar electrode 110a and a second bipolar electrode 110b, and a cathode 142 are arranged in this order. The anode 141, the first bipolar electrode 110a, the second bipolar electrode 110b, and the cathode 142 are arranged by being immersed in a molten salt bath Bf stored in the electrolytic cell 120. In a direction orthogonal to the vertical direction, the direction in which the anode 141, the bipolar electrodes 110a and 110b, and the cathode 142 are arranged is referred to as the arrangement direction. In the illustrated electrolysis chamber 140, two bipolar electrodes 110a and 110b are arranged, but at least one bipolar electrode may be arranged. Note that at least one of the first bipolar electrode 110a and the second bipolar electrode 110b may be the electrode 10 described above.

[0029] (Electrolytic Cell) The electrolytic cell 120 has a container shape with an opening formed on the upper side, and is made of, for example, mainly refractory bricks such as aluminum oxide and other suitable materials. The electrolytic cell 120 shown in Fig. 2A is composed of a bottom wall 123 and two pairs of side walls 124 connected to the bottom wall 123 and extending upward. A molten salt bath Bf composed of a molten salt containing a metal chloride supplied therein is stored in this electrolytic cell 120. In order to send the metallic magnesium generated by electrolysis of magnesium chloride in the electrolysis chamber 140 to the metal recovery chamber 150 and send the molten salt from the metal recovery chamber 150 to the electrolysis chamber 140 to circulate the molten salt bath, the first partition wall 121 and the second partition wall 122 are arranged. Here, the molten salt electrolysis apparatus 100 forms a circulation port 125 between the first partition wall 121 and the second partition wall 122, thereby ensuring the flow of the molten salt bath shown by arrow A (flow from the electrolysis chamber 140 to the metal recovery chamber 150). Also, a passage through which the molten salt bath can flow is formed on the lower surface side of the second partition wall 122, and the flow shown by arrow B (flow from the metal recovery chamber 150 to the electrolysis chamber 140) can be ensured.

[0030] (molten salt) By electrolyzing magnesium chloride, metallic magnesium (Mg) is produced as a molten metal, and chlorine gas (Cl2) 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), etc. as supporting salts. As the component used as the supporting salt, it is preferable to use a substance with a higher electrolysis voltage than magnesium chloride. Metallic magnesium can be used for the reduction of titanium tetrachloride in the Kroll process for producing titanium metal, and chlorine gas can be used for the chlorination of titanium ore, respectively. As the magnesium chloride used as the raw material for this electrolysis, that produced as a by-product in the Kroll process can be used.

[0031] (upper lid) Since the molten salt bath Bf is at a high temperature, the upper lid 130 serves as heat insulation against the outside of the electrolytic cell 120. Further, the upper lid 130 is arranged to make the electrolytic cell 120 a closed space, and the inside of the electrolytic cell 120 is made negative pressure with respect to the outside in order to prevent leakage of chlorine gas generated from the anode 141 during molten salt electrolysis. Also, the material of the upper lid 130 is not particularly limited, but from the viewpoint of preventing a short circuit occurring between the upper lid 130 and the anode during molten salt electrolysis, it is sufficient that the back surface 131 side of the lid on the molten salt bath Bf side of the upper lid 130 is an insulating material, and a ceramic material may be arranged on the back surface 131 side of the lid on the molten salt bath Bf side of the upper lid 130, and a castable refractory may also be constructed. The method of providing this castable refractory may be a known method. For example, the castable refractory may be constructed on the back surface 131 side by dry spraying, wet spraying, or the like.

[0032] The upper lid 130 may be provided with a first gas recovery port 132, a second gas recovery port 133, and a supply / discharge port 134. Each of these ports may be one or a plurality. The first gas recovery port 132 is used to recover the chlorine gas generated by the electrolysis of magnesium chloride in the electrolysis chamber 140. The first gas recovery port 132 is provided in the region where the electrolysis chamber 140 is located. Also, in the second gas recovery port 133, the gas generated by the electrolysis of magnesium chloride in the electrolysis chamber 140 may be recovered. The second gas recovery port 133 is provided in the region where the metal recovery chamber 150 is located. The second gas recovery port 133 may be used to recover the remaining gas that has flowed into the metal recovery chamber 150 without being recovered by the first gas recovery port 132 among the gases generated by electrolysis. Further, the supply / discharge port 134 is used for recovering the molten magnesium metal generated by the electrolysis of magnesium chloride in the electrolysis chamber 140 and for supplying the molten salt into the electrolytic cell 120. The supply / discharge port 134 is provided in the region where the metal recovery chamber 150 is located.

[0033] (Electrolysis chamber) In the electrolysis chamber 140, magnesium chloride is electrolyzed to generate molten magnesium metal and chlorine gas by the electrolysis. As shown in FIG. 2B, in the electrolysis chamber 140 of the electrolytic cell 120, the anode 141, the first bipolar electrode 110a and the second bipolar electrode 110b, and the cathode 142 may be immersed and arranged in the direction of arrangement in the stored molten salt bath Bf. At this time, in the electrolysis chamber 140, the electrolytic surfaces of the anode 141, the first bipolar electrode 110a, the second bipolar electrode 110b, and the cathode 142 are arranged so as to be substantially parallel to the depth direction of the molten salt bath Bf (the vertical direction in FIG. 2A).

[0034] The anode 141 is inserted through the upper lid 130 and extends downward, and is arranged so that a part of it is immersed in the molten salt bath Bf. The shape of the anode 141 is not particularly limited, and examples include a plate shape, a columnar shape, and a prismatic shape. From the viewpoint of the production efficiency of molten magnesium metal, the molten salt electrolysis apparatus 100 may include a plurality of anodes 141 and cathodes 142, respectively. The material of the anode 141 is not particularly limited, and examples include graphite.

[0035] The first bipolar electrode 110a and the second bipolar electrode 110b may be respectively arranged on pedestals 126a, 126b made of refractory bricks. In the arrangement example of the spacers 113a, 113b shown in FIG. 2B, the first fixing portion 117a of the first bipolar electrode 110a and the second fixing portion 118b of the second bipolar electrode 110b are arranged so as to face each other in the arrangement direction at the same height in the vertical direction. Also, as another arrangement example of the spacers 113a, 113b shown in FIG. 2C, from the viewpoint of more surely avoiding contact between the spacers, in the arrangement direction, the first fixing portion 117a of the first bipolar electrode 110a and the second fixing portion 118b of the second bipolar electrode 110b may be arranged so as not to face each other and be displaced in the vertical direction. That is, the first fixing portion 117a of the first bipolar electrode 110a and the second fixing portion 118b of the second bipolar electrode 110b are arranged so that the region where the first fixing portion 117a exists on the first electrolytic surface 114a of the first bipolar electrode 110a and the region where the second fixing portion 118b exists on the second electrolytic surface 115b of the second bipolar electrode 110b do not overlap in the arrangement direction.

[0036] In the arrangement direction, the distances from the first electrolytic surfaces 114a, 114b of the bipolar electrodes 110a, 110b to the tips of the first fixing portions 117a, 117b are preferably 50% or more and 90% or less of the inter-pole distance from the first electrolytic surfaces 114a, 114b to the cathode 142 adjacent to the bipolar electrode 110a or the inter-pole distance to the bipolar electrode 110b. More specifically, in the arrangement direction, the distance d1 from the first electrolytic surface 114a of the first bipolar electrode 110a to the tip of the first fixing portion 117a is preferably 50% or more and 90% or less of the inter-pole distance L1 from the first electrolytic surface 114a of the first bipolar electrode 110a to the second bipolar electrode 110b adjacent to the first bipolar electrode 110a. Also, in the arrangement direction, the distance d2 from the first electrolytic surface 114b of the second bipolar electrode 110b to the tip of the first fixing portion 117b is preferably 50% or more and 90% or less of the inter-pole distance L2 from the first electrolytic surface 114b of the second bipolar electrode 110b to the cathode 142 adjacent to the second bipolar electrode 110b. By being within the above ranges respectively, during the molten salt electrolysis of magnesium chloride, it is easy to appropriately maintain the inter-pole distance L1 between the first bipolar electrode 110a and the second bipolar electrode 110b, and the inter-pole distance L2 between the second bipolar electrode 110b and the cathode 142.

[0037] The anode 141 and the cathode 142 are connected to a power source via a bus bar, a conductive wire, or the like. In molten salt electrolysis, magnesium chloride is decomposed into chlorine and metallic magnesium at the anode 141 and the cathode 142 based on a predetermined reaction such as the following chemical formula (1). More specifically, in the molten salt bath Bf, chlorine gas is generated by an oxidation reaction at the electrolysis surface of the anode 141 facing the cathode 142 side, the first electrolysis surface 114a of the first bipolar electrode 110a, and the first electrolysis surface 114b of the second bipolar electrode 110b. Also, in the molten salt bath Bf, metallic magnesium is generated by a reduction reaction at the second electrolysis surface 115a of the first bipolar electrode 110a facing the anode 141 side, the second electrolysis surface 115b of the second bipolar electrode 110b, and the electrolysis surface of the cathode 142. MgCl2 → Mg + Cl2 ··· Chemical formula (1) The material of the cathode 142 is not particularly limited, and examples include graphite and carbon steel.

[0038] As shown in Fig. 2A, the cathode 142 has an extended portion 142a that extends outward, and this extended portion 142a is arranged to penetrate the side wall 124 and protrude outside the electrolytic cell 120. The shape of the cathode 142 may be plate-shaped, but it can be appropriately changed considering the shape of the anode 141 and the like, and it may be square tubular, cylindrical, or the like. Even in this case, the cathode 142 has the extended portion 142a. For example, in the molten salt electrolysis apparatus 200 shown in Figs. 3A and 3B, the anode 241, the first bipolar electrode 210a, the second bipolar electrode 210b, and the cathode 242 are arranged in this order along the arrangement direction from the anode 241 (in the direction away from the anode 241 in the illustrated embodiment). More specifically, a square tubular first bipolar electrode 210a is arranged on the pedestal 226a with a space from the anode 241 surrounding the anode 241, a square tubular second bipolar electrode 210b is arranged on the pedestal 226b with a space from the first bipolar electrode 210a surrounding the first bipolar electrode 210a, and a square tubular cathode 242 is arranged with a space from the second bipolar electrode 210b surrounding the second bipolar electrode 210b. The cathode 242 further has an extended portion 242a that extends outward from a part of the square tubular shape, and this extended portion 242a is arranged to penetrate the side wall 124 and protrude outside the electrolytic cell 120. At this time, in Fig. 3B, the first fixing portion 217a of the spacer 213a in the first bipolar electrode 210a and the second fixing portion 218b of the spacer 213b in the second bipolar electrode 210b are arranged to face each other, but they may also be arranged so that the first fixing portion 217a of the first bipolar electrode 210a and the second fixing portion 218b of the second bipolar electrode 210b do not face each other.

[0039] Note that the inter-pole distances between the anode and the bipolar electrode, between the cathode and the bipolar electrode, and between the bipolar electrodes may be the same as or different from each other.

[0040] (Metal recovery chamber) In the metal recovery chamber 150, the molten metal generated by electrolysis in the electrolysis chamber 140 is recovered. The metal recovery chamber 150 communicates with the electrolysis chamber 140 and may have a heat exchanger (not shown). The heat exchanger can adjust the temperature of the molten salt bath Bf in the electrolytic cell 120. The heat exchanger may be configured to include an inlet through which a fluid flows, an outlet through which the fluid is discharged, and a pipe connecting the inlet and the outlet. And the pipe may be made of steel (carbon steel or stainless steel). In addition, as another embodiment, the molten salt electrolysis apparatus may further include a heat exchange chamber (not shown) having a heat exchanger (not shown) arranged side by side with the electrolysis chamber and the metal recovery chamber in the horizontal direction. For example, the recovery of the molten metal is performed using the supply and discharge ports of the metal recovery chamber, and the replenishment of the molten salt such as molten magnesium chloride can be performed with respect to the heat exchange chamber. As a result, the molten metal can be stored more stably in the metal recovery chamber. In addition to these, the heat exchange chamber may further have a stirrer (not shown) for stirring the molten salt bath.

[0041] [3. Method for Producing Magnesium Metal] One embodiment of the method for producing magnesium metal according to the present invention includes an electrolysis step of electrolyzing magnesium chloride using the molten salt electrolysis apparatuses 100, 200, etc. described above to produce magnesium metal. Hereinafter, a preferred embodiment of each step will be described by taking the case of using the molten salt electrolysis apparatus 100 shown in FIGS. 2A and 2B as an example.

[0042] <Electrolysis Step> In the electrolysis process, electrolysis of magnesium chloride contained in the molten salt bath Bf is carried out. The molten salt bath Bf flows from the electrolysis chamber 140 through the flow port 125 into the metal recovery chamber 150 as shown by the arrow A in FIG. 2A, and flows from the metal recovery chamber 150 through the lower side of the second partition wall 122 into the electrolysis chamber 140 as shown by the arrow B in FIG. 2A. In the electrolysis chamber 140, magnesium chloride in the molten salt bath Bf is electrolyzed to produce molten magnesium metal. Then, this molten magnesium metal flows into the metal recovery chamber 150 by the flow of the molten salt bath Bf. Thereafter, the molten magnesium metal, which has a lower specific gravity than the molten salt, floats to a shallow portion of the metal recovery chamber 150 and accumulates there. The molten magnesium metal floating in the metal recovery chamber 150 can be recovered by inserting a recovery pipe or the like into the supply / discharge port 134.

Example

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

[0044] [Example 1] (Operation Preparation of Molten Salt Electrolysis Cell) In Example 1, a molten salt electrolysis apparatus 100 having the configuration shown in FIGS. 2A and 2B was used. The materials of the electrolysis cell 120, the first partition wall 121, the second partition wall 122, and the pedestals 126a and 126b are respectively standard refractory materials (refractory bricks) containing aluminum oxide, the material of the upper lid 130 is carbon steel, and a layer of castable refractory material having insulating properties was applied to the back surface 131 of the lid of the upper lid 130. Also, the anode 141, the cathode 142, and the two bipolar electrodes 110a and 110b are all in plate shape. Using each spacer 113a and 113b, the bipolar electrode bodies 111a and 111b were respectively sandwiched and fixed in the thickness direction between the first fixing portions 117a and 117b and the second fixing portions 118a and 118b. Next, molten salt was introduced into the molten salt electrolysis apparatus 100, and the temperature of the molten salt was adjusted to 650 to 700°C. The composition of the molten salt was a ternary bath of magnesium chloride - calcium chloride - sodium chloride, and magnesium chloride was in the range of 10 to 25% by mass. Also, the material of the anode 141 was graphite, and the material of the cathode 142 was carbon steel. The material of each of the bipolar main bodies 111a and 111b of the first bipolar 110a and the second bipolar 110b was graphite. As shown in Table 1, for the spacers 113a and 113b, the first fixing parts 117a and 117b and the through parts 116a and 116b were made of aluminum oxide, and these were of an integral structure. The second fixing parts 118a and 118b were made of carbon steel. Also, although not shown in the figure, recessed holes were formed on the surfaces of the tip parts of the through parts 116a and 116b, through holes were formed in the second fixing parts 118a and 118b, and a steel connecting pin was passed through the through holes and inserted into the recessed holes. Note that the electrolysis conditions are shown below. Regarding the bipolar electrodes under the conditions shown below, the inter - electrode distance is shown based on the bipolar main body. The thickness of the bipolar electrode is the thickness of the bipolar main body. (Electrolysis conditions) Inter - electrode distance between the anode 141 and the first bipolar 110a: 10 mm Inter - electrode distance between the first bipolar 110a and the second bipolar 110b: 10 mm Inter - electrode distance between the second bipolar 110b and the cathode 142: 10 mm Thickness of the first bipolar 110a and the second bipolar 110b: 80 mm Sizes of the first fixing parts 117a and 117b of the first bipolar 110a and the second bipolar 110b: φ40 mm × 9 mm t Sizes of the second fixing parts 118a and 118b of the first bipolar 110a and the second bipolar 110b: φ40 mm × 0.5 mm t Number of spacers 113a in the first bipolar 110a: 8 (Arrangement: 3, 2, 3 (in the vertical direction)) Number of spacers 113b in the second bipolar 110b: 8 (Arrangement: 3, 2, 3 (in the vertical direction)) Cross - sectional shape of the through holes 112a and 112b: Circular Inner diameter of the through holes 112a and 112b: 31 mm Outer diameter of the through parts 116a and 116b: 30 mm

[0045] (Production of metallic magnesium) An electrolysis process was carried out by supplying an electric current between the anode 141 and the cathode 142 through a conductive wire from a power source. After 24 months had elapsed since the start of electrolysis, the supply of the electric current was stopped.

[0046] [Evaluation] (Presence or absence of short circuit) During the operation of the molten salt electrolysis apparatus 100, when a sudden voltage drop was confirmed, it was judged that a short circuit had occurred. The results are shown in Table 2.

[0047] (Observation after operation) After the operation of the molten salt electrolysis apparatus 100 was completed, the molten salt electrolysis apparatus 100 was disassembled, and the presence or absence of the spacer falling off or being displaced was confirmed. The results are shown in Table 2.

[0048] (Current efficiency) First, the current efficiency was calculated based on the following formula (1). A[%]=(M1 / M0)×100 A: Current efficiency M1: Mass of metallic magnesium recovered from the electrolytic cell M0: Mass of theoretically generated metallic magnesium Next, when the current efficiency of Comparative Example 1 described later was set to 100%, the relative values of the current efficiency of each example were calculated.

[0049] [Example 2] In Example 2, as shown in Table 1, using the molten salt electrolysis apparatus 200 shown in FIGS. 3A and 3B, except for using the rectangular tube-shaped first bipolar electrode 210a, second bipolar electrode 210b, and cathode 242, the operation of the molten salt electrolysis apparatus 200 was carried out for 24 months in the same manner as in Example 1. The electrolysis conditions are shown below. Regarding the bipolar electrode under the conditions shown below, the inter-pole distance is shown based on the bipolar electrode body. The thickness of the bipolar electrode is the thickness of the bipolar electrode body. (Electrolysis conditions) Inter-pole distance between the anode 241 and the first bipolar electrode 210a: 10 mm Inter-pole distance between the first bipolar electrode 210a and the second bipolar electrode 210b: 10 mm Inter-pole distance between the second bipolar plate 210b and the cathode 242: 10 mm Thickness of the first bipolar plate 210a and the second bipolar plate 210b: 70 mm Sizes of the first fixing parts 217a and 217b of the first bipolar plate 210a and the second bipolar plate 210b: φ40 mm × 9 mm t Sizes of the second fixing parts 218a and 218b of the first bipolar plate 210a and the second bipolar plate 210b: φ40 mm × 0.5 mm t Spacers 213a in the first bipolar plate 210a: 30 pieces (arrangement: 10 pieces, 10 pieces, 10 pieces (in the vertical direction)) Spacers 213b in the second bipolar plate 210b: 30 pieces (arrangement: 10 pieces, 10 pieces, 10 pieces (in the vertical direction)) Materials of the first fixing parts 217a and 217b and the through parts 216a and 216b of the spacers 213a and 213b: Aluminum oxide Materials of the second fixing parts 218a and 218b of the spacers 213a and 213b: Carbon steel Cross-sectional shapes of the through holes 212a and 212b: Circular Inner diameters of the through holes 212a and 212b: 31 mm Outer diameters of the through parts 216a and 216b: 30 mm

[0050] And, similar to Example 1, evaluation was carried out. The results are shown in Table 2.

[0051] [Comparative Example 1] In Comparative Example 1, as shown in FIG. 4, recessed holes were formed in the first electrolytic surfaces 314a and 314b of the bipolar plate bodies 311a and 311b, and cylindrical spacers 313a and 313b made of aluminum oxide were embedded in the recessed holes. The operation of the molten salt electrolysis apparatus was carried out for 24 months in the same manner as in Example 1. A rapid decrease in voltage was confirmed at the 15th month from the start of operation, but it was unclear which inter-pole short circuit occurred. The electrolysis conditions are shown below. (Electrolysis conditions) Inter-pole distance between the anode 141 and the first bipolar plate 310a: 10 mm Inter-pole distance between the first bipolar plate 310a and the second bipolar plate 310b: 10 mm Inter-pole distance between the second bipolar plate 310b and the cathode 142: 10 mm Thickness of the first bipolar electrode 310a and the second bipolar electrode 310b: 80 mm Protrusion formed by embedding the spacer in the recessed hole: φ40 mm × 9 mm t Spacers 313a in the first bipolar electrode 210a: 8 pieces (arrangement: 3 pieces, 2 pieces, 3 pieces (in the vertical direction)) Spacers 313b in the second bipolar electrode 210b: 8 pieces (arrangement: 3 pieces, 2 pieces, 3 pieces (in the vertical direction)) Material of each spacer 313a, 313b: Aluminum oxide

[0052] Note that, similar to Example 1, the evaluation was carried out. The results are shown in Table 2.

[0053] [Comparative Example 2] In Comparative Example 2, except that through holes were not formed in the bipolar electrode body and no spacers were used, the operation of the molten salt electrolysis device was carried out for 12 months in the same manner as in Example 1. A sharp drop in voltage was confirmed at the 6th month from the start of operation, but it was unclear which poles were short-circuited.

[0054] Note that, similar to Example 1, the evaluation was carried out. The results are shown in Table 2.

[0055]

Table 1

[0056]

Table 2

[0057] (Consideration based on the example) In Examples 1 to 2, compared with Comparative Examples 1 to 2, during the operation of the molten salt electrolysis cell, the electrode spacer includes a ceramic through-hole extending through the through-hole, a ceramic first fixing portion provided on the first electrolysis surface at one end side of the through-hole, and a second electrolysis surface provided on the other end side opposite to the one end, and a steel second fixing portion that sandwiches and fixes the bipolar body in its thickness direction between the first fixing portion, and no displacement of the bipolar electrode was confirmed. The reason for not confirming the displacement of the bipolar electrode is that since steel is prone to wetting with metallic magnesium, the metallic magnesium generated on the cathode side of the bipolar electrode is covered in layers on the surface of the second fixing portion and alloyed, and the melting point of the alloy is higher than that of metallic magnesium, so it is presumed that the spacer was firmly fixed due to solidification. That is, in Examples 1 to 2, it was confirmed that the interelectrode distance could be appropriately maintained in the molten salt electrolysis of magnesium chloride. Further, in Examples 1 to 2, by arranging the electrodes as bipolar electrodes in the electrolysis cell, metallic magnesium could be stably obtained in the molten salt electrolysis of magnesium chloride. On the other hand, in Comparative Example 1, during the operation of the molten salt electrolysis cell, the spacer was shaken mainly due to the flow of the molten salt bath caused by the rise of chlorine gas, etc., and the bipolar electrode in which the spacer was embedded was chipped or worn due to the shaking, and it is presumed that the spacer embedded in the bipolar electrode fell off. In addition, in Comparative Example 2, it is presumed that the displacement of the bipolar electrode occurred during the operation of the molten salt electrolysis cell because the spacer was not installed.

Explanation of Signs

[0058] 10 Electrode 11 Bipolar body 12 Through-hole 13 Spacer 14 First electrolysis surface 15 Second electrolysis surface 16 Through-hole portion 17 First fixing portion 18 Second fixing portion 100, 200 Molten salt electrolysis device 110a, 210a, 310a First bipolar electrode (bipolar electrode) 110b, 210b, 310b Second bipolar electrode (bipolar electrode) 111a, 111b, 211a, 211b, 311a, 311b bipolar bodies 112a, 112b, 212a, 212b through holes 113a, 113b, 213a, 213b, 313a, 313b spacers 114a, 114b, 214a, 214b, 314a, 314b first electrolytic surfaces 115a, 115b, 215a, 215b second electrolytic surfaces 116a, 116b, 216a, 216b through portions 117a, 117b, 217a, 217b first fixing portions 118a, 118b, 218a, 218b second fixing portions 120 electrolytic cell 121 first partition wall 122 second partition wall 123 bottom wall 124 side wall 125 flow port 126a, 126b, 226a, 226b pedestals 130 upper lid 131 inner surface of the lid 132 first gas recovery port 133 second gas recovery port 134 supply and discharge port 140 electrolytic chamber 141, 241 anodes 142, 242 cathodes 142a, 242a extension portions 150 metal recovery chamber A, B arrows Bf molten salt bath

Claims

1. An electrode used as a bipolar electrode immersed in a molten salt bath in the electrolysis of molten magnesium chloride, comprising: a graphite bipolar electrode body including a first electrolysis surface and a second electrolysis surface facing the side opposite to the first electrolysis surface; at least one through-hole formed in the bipolar electrode body and penetrating from the first electrolysis surface to the second electrolysis surface; a spacer for ensuring an inter-pole distance between the electrode and another electrode disposed adjacent thereto; the spacer includes a through portion extending through the through-hole, a first fixing portion provided on the first electrolysis surface at one end side of the through portion, and a second fixing portion provided on the second electrolysis surface at the other end side opposite to the one end, and sandwiching and fixing the bipolar electrode body in its thickness direction between the second fixing portion and the first fixing portion; the through portion and the first fixing portion are made of ceramic; the second fixing portion is made of steel, the electrode.

2. The electrode according to claim 1, wherein a ratio (d / T) of the thickness (T) of the bipolar electrode body to the distance (d) from the first electrolysis surface to the tip of the first fixing portion in the thickness direction of the bipolar electrode body is in the range of 5% or more and 15% or less.

3. A molten salt electrolysis apparatus comprising an electrolytic cell, an anode, a bipolar electrode, and a cathode disposed in the electrolytic cell; wherein at least one of the bipolar electrodes is the electrode according to claim 1 or 2.

4. In the electrolytic cell, the anode, at least one of the bipolar electrodes, and the cathode are arranged and disposed in this order; the molten salt electrolysis apparatus according to claim 3, wherein the first fixing portion of the bipolar electrode is located on the cathode side.

5. In the arrangement direction of the anode, the bipolar electrode, and the cathode, the distance from the first electrolysis surface to the tip of the first fixing portion of the bipolar electrode is in the range of 50% or more and 90% or less of the inter-pole distance from the first electrolysis surface to the cathode or bipolar electrode adjacent to the bipolar electrode.

6. A method for producing metallic magnesium, comprising an electrolysis step of electrolyzing magnesium chloride using the molten salt electrolysis apparatus according to any one of claims 3 to 5 to produce metallic magnesium.

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