Method for manufacturing a bipolar and method for manufacturing metallic magnesium

Abstract

To provide a bipolar electrode capable of reducing the through-current generated in electrolysis of magnesium chloride.SOLUTION: A method for manufacturing a bipolar electrode for use in a molten salt electrolysis apparatus for electrolyzing magnesium chloride comprises: an impregnation step in which through-holes of an electrode provided with a porous graphite body having pores including the through-holes are impregnated with molten salt components and water and / or oxygen; and a heating step of heating of the electrode after the impregnation step.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a bipolar electrode and a method for manufacturing metallic magnesium.

Background Art

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

[0003] In a molten salt electrolysis apparatus used for electrolyzing magnesium chloride in the electrolysis process, an anode, a bipolar electrode, and a cathode may be arranged in this order in an electrolytic cell, and the anode, the bipolar electrode, and the cathode may be immersed in a molten salt bath in the electrolytic cell. Using a bipolar electrode is desirable from the viewpoint of improving production efficiency.

[0004] In the above-described molten salt electrolysis apparatus, since the anode, the bipolar electrode, and the cathode are immersed and arranged in the molten salt bath stored in the electrolytic cell, there are cases where it is inevitable that current flows through a plurality of paths in the molten salt bath. More specifically, in one path, the current from the anode flows through the bipolar electrode to the cathode, and electrolysis also occurs at the bipolar electrode. Since electrolysis is performed at the intended location, this can be called a normal current. On the other hand, in another path, the current from the anode penetrates through the bipolar electrode by passing through the molten salt bath that has entered the through-holes of the pores of the bipolar electrode made of, for example, a porous material, and directly flows to the cathode, so that electrolysis does not occur at the bipolar electrode, and electrolysis occurs only between the anode and the cathode. In this case, electrolysis is not performed at the intended location, and the current passing through the bipolar electrode can be called a through-current. Also, when a through-current occurs, electrolysis does not occur at the bipolar electrode, so the amount of metallic magnesium produced per unit of power decreases according to the amount of the generated through-current. The following techniques have been proposed as methods for reducing the through-current.

[0005] For example, Patent Document 1 discloses a molten salt electrolytic cell that "has at least one bipolar electrode made of graphite with a pore volume of 0.12 mL / g or less" in order to "reduce the through-ion current (so-called through-current) and improve efficiency." [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-70924 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, in the electrolysis of magnesium chloride contained in a molten salt bath, metallic magnesium is formed on one surface of the bipolar electrode (the anode side), and chlorine gas is generated on the other surface of the bipolar electrode (the cathode side). Since metals are easily corroded by this chlorine, bipolar electrodes are generally made of graphite to avoid this.

[0008] Graphite bipolars may be porous due to their manufacturing process, such as sintering. These pores are usually irregular and mesh-like (hereinafter simply referred to as mesh-like), and therefore, the pores of the porous material may include through-holes that extend internally from one surface to the other. When such a graphite bipolar is immersed in a molten salt bath, the molten salt constituting the molten salt bath flows in through the openings of the through-holes on the surface of the porous material and penetrates into the interior of the through-holes. This is thought to cause the generation of through-currents during electrolysis, as described in Patent Document 1.

[0009] Patent Document 1 describes the use of a bipolar electrode made of graphite with a relatively small pore volume in the electrolytic chamber of a molten salt electrolytic cell. However, graphite with a small pore volume is expensive, which presents a problem. Therefore, there has been a desire to suppress through-current by methods other than reducing the pore volume of the graphite bipolar electrode.

[0010] Therefore, according to one embodiment of the present invention, the objective is to provide a bipolar electrode capable of reducing the through-current generated in the electrolysis of magnesium chloride. [Means for solving the problem]

[0011] The present inventors conducted diligent studies to solve the above problems and found that it is effective to include an impregnation step in which a porous graphite body having holes including through holes contains components of a molten salt and water and / or oxygen in the through holes, and a heating step in which the electrode is heated after the impregnation step, and thus created the invention exemplified below. [1] A method for manufacturing a bipolar electrode used in a molten salt electrolytic apparatus for the electrolysis of magnesium chloride, An impregnation step in which a porous body made of graphite having holes including through holes is made to contain components of a molten salt and water and / or oxygen in the through holes of the electrode, A method for manufacturing a bipolar, comprising a heating step of heating the electrode after the impregnation step. [2] A method for manufacturing a bipolar electrode according to [1], further comprising a drying step of vacuum drying the electrode between the impregnation step and the heating step. [3] A method for producing metallic magnesium using a molten salt electrolytic apparatus comprising an electrolytic cell, an anode disposed in the electrolytic cell, a bipolar manufactured by the method for producing a bipolar described in [1] or [2], and a cathode, A method for producing metallic magnesium, comprising an electrolytic step of producing metallic magnesium by electrolyzing the aforementioned magnesium chloride. [Effects of the Invention]

[0012] According to one embodiment of the present invention, the through-current generated during the electrolysis of magnesium chloride can be reduced. [Brief explanation of the drawing]

[0013] [Figure 1A] This is a schematic front view showing an example of the internal structure of a molten salt electrolytic apparatus according to the present invention. [Figure 1B] Figure 1A is an end view of XX. [Figure 2A] This is a schematic front view showing another example of the internal structure of a molten salt electrolytic apparatus according to the present invention. [Figure 2B] Figure 2A is an end view at YY. [Modes for carrying out the invention]

[0014] The present invention is not limited to the embodiments described below, and its components can be modified and implemented without departing from its essence. Furthermore, various inventions can be formed by appropriate combinations of the multiple components disclosed in each embodiment. For example, an invention may be formed by deleting some components from all the components shown in an embodiment. In addition, some components shown in the drawings are schematic to aid in understanding the embodiments included in the invention, and the sizes and positional relationships shown 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. 1A, 1B, and 2A for example, 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 magnesium" means molten metallic magnesium obtained, for example, by electrolytic decomposition of magnesium chloride. The composition of the molten salt separately supplied from the outside in the electrolysis can be adjusted as appropriate. For example, it can be magnesium chloride, or it can have the same composition as the molten salt bath. Also, "unused product" means a product that has not been used for electrolysis of the molten salt and has not undergone the production process of the ceramics.

[0015] [1. Method for manufacturing bipolar electrode] The method for manufacturing a bipolar electrode according to the present invention is a method for manufacturing a bipolar electrode used in a molten salt electrolysis apparatus for electrolyzing magnesium chloride, and includes an impregnation step and a heating step. It may further include a drying step.

[0016] In order to reduce the through-current generated in the electrolysis of magnesium chloride described above, the inventor has intensively studied methods other than minimizing the pore volume of the graphite bipolar electrode.

[0017] As a result, the present inventors have surprisingly obtained the finding that it is useful to recycle a graphite electrode that has been immersed in a molten salt bath by use in electrolysis or the like, as a bipolar electrode. Usually, due to reasons such as the oxidation and consumption of a graphite anode by contact with oxygen and the prevention of leakage of chlorine gas to the outside, air or the like is substantially not mixed into the electrolytic cell of a molten salt electrolysis apparatus, and the contact of a graphite electrode such as an anode with air is suppressed. Further, since the graphite electrode is a porous body, problems occur when the molten salt bath penetrates into the pores of the porous body. That is, the molten salt contains highly deliquescent magnesium chloride, and it is difficult to remove water because it forms a hydrate of magnesium chloride. Since water is an oxygen source, there is a risk of damaging the graphite electrode, an increase in the amount of impurities in the metallic magnesium produced by electrolysis, or a decrease in production efficiency, etc. In view of this, there is a demand to avoid the mixing of water into the molten salt electrolytic cell. Therefore, in the molten salt electrolysis of magnesium chloride, hitherto, it has been considered that a graphite electrode that has not been used is advantageous.

[0018] On the other hand, after completion of the electrolysis of magnesium chloride using an unused electrode, the electrolytic cell of the molten salt electrolysis apparatus was disassembled, the electrode recovered from the electrolytic cell was heated after being exposed to air and then dried, and when electrolysis of the molten salt was performed again using this as a bipolar electrode, an unexpected result that the cell efficiency was improved was obtained. It has been empirically found that even if the bipolar electrode, which was an unused product, is used for a long time while being immersed in the molten salt bath in the same electrolytic cell without disassembling the electrolytic cell, an improvement in the cell efficiency cannot be obtained.

[0019] The inventors have conducted thorough research on this matter and concluded that the improvement in the tank efficiency is achieved when the molten salt, such as a metal halide, that penetrates into the through-hole of the electrode during the electrolysis process absorbs moisture from the atmosphere upon exposure, and then, upon subsequent heating, transforms into a predetermined metal oxide, metal nitride, and / or metal oxynitride ceramic inside the through-hole. This ceramic is a solid and tends to have a relatively high resistivity. Furthermore, the ceramic has a relatively high melting point and remains solid in a molten salt bath at a temperature much lower than its melting point. Therefore, when electrolysis is performed by immersing a bipolar electrode containing the ceramic inside the through-hole in a molten salt bath, the molten salt bath can hardly pass through the inside of the through-hole, and the current is also interrupted. As a result, the presence of the ceramic can reduce the through-current during electrolysis. Note that graphite itself can be polarized by current and therefore can function as a bipolar electrode.

[0020] Therefore, by forming ceramics within the bipolar electrode, the through-current can be reduced. Based on these findings, the present invention was completed.

[0021] <Impregnation step> The impregnation step involves impregnating the through-holes of an electrode, which is a porous graphite body having pores including through-holes, with components of the molten salt and water and / or oxygen. This allows ceramics to be formed in at least a portion of the interior of the through-holes that open to the surface of the porous body when the heating step described later is subsequently performed.

[0022] The following embodiments are exemplified in the impregnation step. While the ceramic formation mechanism is not entirely clear, it will be explained based on plausible theories. However, the formation mechanism is not limited to this explanation. In a first embodiment, the components of the molten salt are first incorporated into the porous body of the graphite electrode, and then water and / or oxygen are incorporated into the porous body of the graphite. For example, after the electrolysis of the molten salt is completed using a molten salt electrolytic apparatus, the electrolytic cell of the molten salt electrolytic apparatus is dismantled, and the graphite bielectrode is recovered from the electrolytic cell. While the electrode is immersed in the molten salt bath during this electrolysis, the molten salt flows in through the openings of through holes that open to the surface of the porous body of the electrode, and the molten salt penetrates into many parts inside the through holes. Other forms in which the molten salt flows into the inside of the through holes include applying the molten salt to the graphite electrode, or heating and melting a solid component of the molten salt (e.g., metal halide) placed on the graphite electrode. It is preferable that the molten salt bath contains chloride as its main component. It is preferable that the molten salt bath contains magnesium chloride, but it may also consist of magnesium chloride. Here, the chloride content of the molten salt bath is preferably 95% by mass or more. Furthermore, the graphite electrodes recovered from the electrolytic cell may be either anodes or cathodes. Even if they are anodes or cathodes, they can be handled in the same way as bipolar electrodes. Next, the electrodes are placed in a moisture-containing atmosphere, such as air, to allow water and oxygen to be absorbed. In this case, nitrogen may also be present. The components of the molten salt, including magnesium chloride, impregnated into the through-holes are susceptible to hygroscopicity, so they absorb moisture from the air. In addition to placement in an air atmosphere, the components of the molten salt can also absorb moisture by contact with water, such as by spraying water vapor, washing with water, or immersing in water. When the electrodes are brought into contact with water, it is desirable to dry them to remove excess water after hydrates of the components in the molten salt bath, such as magnesium chloride, have been formed, before the heat treatment described below. For example, vacuum drying can be used here. To obtain ceramics from the components contained in the hygroscopic molten salt, it is preferable to adjust the amount of water used as appropriate. For example, as in the examples described later, once the molten salt has entered the through-holes of a porous graphite electrode, components from the hygroscopic molten salt may remain in the through-holes even if the electrode is briefly washed with water or immersed in water. On the other hand, supplying more water than the appropriate amount may cause the components in the molten salt to dissolve in the water and be removed. Therefore, the amount of water should be adjusted as appropriate in light of the shape and properties of the electrode.

[0023] A second embodiment involves first impregnating the porous body of a graphite electrode with water and / or oxygen, and then impregnating the porous body of the electrode with components of molten salt. For example, an unused electrode can be immersed in water to impregnate the inside of the electrode's through-holes with water. Then, when the water-impregnated electrode is immersed in a molten salt bath, the molten salt components of the molten salt bath are impregnated into the electrode. That is, by impregnating the through-holes of the electrode with water and then immersing it in a molten salt bath, the through-holes of the electrode are impregnated with components of molten salt and water and / or oxygen. When immersing the electrode in the molten salt bath, a heating step, which will be described later, may also be performed at that time. Furthermore, after impregnating the through-holes of the electrode with water, before immersing it in the molten salt bath, some of the water may be removed, for example, by performing atmospheric pressure drying. The temperature and time of atmospheric pressure drying can be adjusted as appropriate.

[0024] The difference between the second and first embodiments lies in the order in which the components are impregnated into the porous body of the graphite electrode. Therefore, the form shown in the first embodiment can also be applied to the second embodiment. For example, in the second embodiment, an unused electrode can be immersed in water, and then molten salt can be applied to the graphite electrode. Another example is that in the second embodiment, an unused graphite electrode can be immersed in water, and then a solid such as powder containing the same components as those in the molten salt (e.g., metal halides) can be placed on the graphite bipolar electrode.

[0025] In other words, one embodiment may include (A) and (B) below in any order. (A) Immerse the graphite electrode in a molten salt bath containing molten salt. (B) Contacting a graphite electrode with water and / or contacting a graphite electrode with oxygen.

[0026] The shape of the electrode used as a bipolar after manufacturing depends on the shapes of the anode and cathode that can be placed in the molten salt electrolytic apparatus in which the bipolar is used, but examples include plate-shaped, cylindrical, and rectangular tube-shaped electrodes. Furthermore, the thickness of the bipolar electrode can be appropriately determined in consideration of the electrolytic cell configuration, the magnesium metal production schedule, etc. From the viewpoint of improving cell efficiency, the bipolar electrode thickness should be, for example, 20 mm or more at the lower end and 40 mm or more at the upper end. From the viewpoint of ease of installation in the electrolytic cell and manufacturing cost, the bipolar electrode thickness should be, for example, 100 mm or less and 80 mm or less at the upper end.

[0027] The pore volume of the electrode is not particularly limited, but a lower limit is, for example, 0.13 mL / g or more. As disclosed in Patent Document 1, a porous graphite material with a pore volume of 0.12 mL / g or less can effectively suppress through-current, so the effects of the invention become more pronounced when the pore volume is 0.13 mL / g or more. However, the above treatment may also be applied to a porous graphite material with a pore volume of 0.12 mL / g or less. The method for measuring the pore volume can be the method described in the examples below or the method disclosed in Patent Document 1.

[0028] <Drying step> In the drying step, the electrodes obtained in the impregnation step are vacuum-dried. This drying step is preferably performed when a large amount of water was supplied in the impregnation step. More specifically, when the bipolar electrodes are brought into contact with water, it is desirable to dry them to remove excess water after hydrates of components contained in the molten salt bath, such as magnesium chloride, have been formed, and before the heat treatment described below.

[0029] <Heating step> The heating step, performed after the impregnation step or drying step described above, involves heat-treating the electrode containing components of the molten salt, as well as water and oxygen, inside the through-hole at a heating maintenance temperature of, for example, 500°C or higher. Examples of heat treatment include heating in a furnace or immersing the electrode in a molten salt bath. It is believed that ceramics are formed through this heat treatment.

[0030] The heating step forms ceramics inside at least a portion of the through-holes that open to the surface of the porous body of the graphite bipolar. The bipolar obtained by this manufacturing method is similar to conventional bipolars in that it is polarized by the application of voltage to the electrodes (anode and cathode), but it differs from conventional bipolars in that ceramics are present inside at least a portion of the through-holes. Furthermore, the ceramics have a melting point higher than the temperature of the molten salt bath used in the electrolysis of the molten salt, and a higher resistivity than the molten salt. The ceramics may also contain metal oxides having metal and oxygen derived from the components of the molten salt, metal nitrides having the metal and nitrogen, and / or metal oxynitrides having the metal, oxygen, and nitrogen. Examples of metals derived from the components of the molten salt include Na, Mg, K, and Ca.

[0031] [2. Molten Salt Electrolysis Equipment] Next, we will describe an example of a molten salt electrolytic apparatus equipped with an anode, a bipolar manufactured by the bipolar manufacturing method described above, and a cathode.

[0032] The molten salt electrolytic apparatus 100 shown in Figure 1A comprises an electrolytic cell 120 and a top lid 130. The electrolytic cell 120 is divided into an electrolytic chamber 140 and a metal recovery chamber 150 by the presence of a first partition wall 121 and a second partition wall 122, which will be described later. Furthermore, as shown in Figure 1B, the electrolytic chamber 140 of the electrolytic cell 120 has an anode 142, a first bipolar electrode 110 and a second bipolar electrode 115, and a cathode 144 arranged in this order, and these anode 142, first bipolar electrode 110, second bipolar electrode 115 and cathode 144 are immersed in the molten salt bath Bf stored in the electrolytic cell 120. The direction in which the anode 142, first bipolar electrode 110, second bipolar electrode 115 and cathode 144 are arranged in a direction perpendicular to the vertical direction is called the arrangement direction. In the illustrated electrolytic chamber 140, two bipolar electrodes, a first bipolar electrode 110 and a second bipolar electrode 115, are arranged as bipolar electrodes, but it is sufficient to have at least one bipolar electrode. At least one of the arranged bipolar electrodes may be the aforementioned type, or all of the bipolar electrodes may be the aforementioned type. Since the through-current is thought to flow from the anode to the cathode through the molten salt bath present in the holes of the bipolar electrodes, the through-current can be effectively reduced if there is at least one bipolar electrode with ceramics. Furthermore, the through-current can be further reduced by increasing the number of bipolar electrodes with ceramics. The molten salt electrolytic apparatus 100 may have multiple anodes and / or cathodes. In this case, the number of bipolar electrodes will increase in proportion to the number of anodes and cathodes.

[0033] (electrolytic cell) The electrolytic cell 120 is a container-shaped structure with an opening formed on its upper side, and is made of, for example, mainly refractory bricks such as aluminum oxide or other suitable materials. The electrolytic cell 120 shown in Figure 1A consists 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, consisting of molten salt containing magnesium chloride, is stored inside this electrolytic cell 120. In the electrolysis chamber 140, a first partition wall 121 and a second partition wall 122 are arranged to circulate the molten salt bath Bf by sending metallic magnesium produced by the electrolysis of magnesium chloride to the metal recovery chamber 150, and by sending molten salt from the metal recovery chamber 150 to the electrolysis chamber 140. Here, the molten salt electrolytic device 100 can ensure the flow of the molten salt bath (flow from the electrolysis chamber 140 to the metal recovery chamber 150) shown by arrow A by forming a flow port 125 between the first partition wall 121 and the second partition wall 122. Furthermore, a passage allowing the molten salt bath to flow is also formed on the lower side of the second partition wall 122, ensuring the flow indicated by arrow B (flow from the metal recovery chamber 150 to the electrolysis chamber 140).

[0034] (molten salt) Electrolysis of magnesium chloride produces metallic magnesium (Mg) as a molten metal and chlorine gas (Cl2) as a gas. In addition to magnesium chloride (MgCl2), the molten salt may contain sodium chloride (NaCl), calcium chloride (CaCl2), potassium chloride (KCl), magnesium fluoride (MgF2), and / or calcium fluoride (CaF2) as supporting salts. It is preferable to use a supporting salt that is electrolyzed at a higher voltage than magnesium chloride. Metallic magnesium can be used for the reduction of titanium tetrachloride in the Chlor process for producing metallic titanium, and chlorine gas can be used for the chlorination of titanium ore. The magnesium chloride used as a raw material for this electrolysis can be the magnesium chloride produced as a by-product of the Chlor process.

[0035] (Top lid) The top cover 130 serves as insulation for the electrolytic cell 120 from the outside, given the high temperature of the molten salt bath Bf. Furthermore, by placing the top cover 130, the electrolytic cell 120 is enclosed, creating a negative pressure inside the electrolytic cell 120 relative to the outside to prevent leakage of chlorine gas generated from the anode 141 during the electrolysis of magnesium chloride.

[0036] The top cover 130 may be provided with a first gas recovery port 132, a second gas recovery port 133, and a gas supply / discharge port 134. Each of these ports may be present individually or in multiple locations.

[0037] (Electrolysis chamber) In the electrolysis chamber 140, magnesium chloride is electrolyzed to produce molten metallic magnesium and chlorine gas. As shown in Figure 1B, in the electrolysis chamber 140 of the electrolytic cell 120, the anode 142, the first bipolar 110 and the second bipolar 115, and the cathode 144 may be immersed in the molten salt bath Bf in that order. At this time, within the electrolysis chamber 140, the electrolytic surfaces of the anode 142, the first bipolar 110 and the second bipolar 115, and the cathode 144 are arranged to be approximately parallel to the depth direction (vertical direction in Figure 1B) of the molten salt bath Bf.

[0038] The anode 141 is inserted through the top cover 130 and extends downward, and is positioned so that a portion of it is immersed in the molten salt bath Bf. The shape of the anode 142 is not particularly limited and can be plate-shaped, cylindrical, or prismatic. From the viewpoint of the efficiency of producing metallic magnesium by electrolysis, the molten salt electrolytic apparatus 100 may be equipped with multiple anodes 142 and cathodes 144. The material of the anode 142 is not particularly limited, but graphite is one example. When electrolysis of magnesium chloride is performed using a graphite anode, after the electrolysis is complete, the through-hole of the anode 142 is filled with a molten salt bath containing magnesium chloride. By supplying water, vacuum drying, and heat treatment to the anode 142 as described above, ceramics containing, for example, magnesium oxide are formed in the through-hole of the anode 142. From the viewpoint of reducing through-current, this anode can be used as a dual electrode to perform electrolysis of magnesium chloride again. In other words, the used anode can be recycled as a dual electrode.

[0039] The first dual pole 110 and the second dual pole 115 may be placed on, for example, a base made of firebrick (not shown).

[0040] The anode 142 and cathode 144 are connected to a power source via a busbar, conductive wires, etc. (not shown). In the electrolysis of magnesium chloride, magnesium chloride is decomposed into chlorine and metallic magnesium at the anode 142 and cathode 144 based on a predetermined reaction, such as the one shown in chemical formula (1) below. MgCl2→Mg+Cl2...Chemical formula (1)

[0041] As shown in Figure 1A, the cathode 144 has an outwardly extending portion 144a, and this extended portion 144a is positioned to penetrate the side wall 124 and protrude to the outside of the electrolytic cell 120. The shape of the cathode 144 may be plate-shaped, but can be appropriately changed considering the shape of the anode 142, etc., and may be rectangular tube-shaped or cylindrical, etc. Even in this case, the cathode 144 has the aforementioned extended portion 144a. For example, in the molten salt electrolytic apparatus 200 shown in Figures 2A and 2B, the anode 242, first bipolar electrode 210, second bipolar electrode 215, and cathode 244 are arranged in the order of anode 242, first bipolar electrode 210, second bipolar electrode 215, and cathode 244, along the arrangement direction from the anode 242 (in the illustrated configuration, in the direction away from the anode 242). More specifically, a rectangular tubular first bipolar electrode 210 is arranged around the anode 242 at a distance from the anode 242, a rectangular tubular second bipolar electrode 215 is arranged around the first bipolar electrode 210 at a distance from the first bipolar electrode 210, and a rectangular tubular cathode 244 is arranged around the second bipolar electrode 215 at a distance from the second bipolar electrode 215. The cathode 244 further has an extension portion 244a that extends outward from a part of the rectangular tubular shape, and this extension portion 244a is arranged to penetrate the side wall 124 and protrude to the outside of the electrolytic cell 120. The materials of the cathodes 144 and 244 are not particularly limited, but examples include graphite and carbon steel. If graphite cathodes 144 and 244 are used, they can be recycled as dual electrodes in the molten salt electrolytic devices 100 and 200, similar to the anodes mentioned above. Furthermore, molten metallic magnesium can be produced from the electrolytic surface 145 of the cathode 144 by the electrolysis of magnesium chloride.

[0042] During the electrolysis of magnesium chloride, the inter-electrode distance between the anode 142 and the first bipolar 110, the inter-electrode distance between the first bipolar 110 and the second bipolar 115, and the inter-electrode distance between the second bipolar 115 and the cathode 144 can be adjusted as appropriate. These inter-electrode distances are, for example, 5 mm or more and 20 mm or less. The inter-electrode distances between the anode and the bipolar, between the bipolar and bipolar, and between the cathode and bipolar may be the same or different from each other.

[0043] (Metal recycling room) In the metal recovery chamber 150, molten metallic magnesium produced by electrolysis in the electrolysis chamber 140 is recovered. The metal recovery chamber 150 is in communication with the electrolysis chamber 140 and may have a heat exchanger (not shown). In another embodiment, the molten salt electrolytic apparatus may further include a heat exchange chamber (not shown) with a heat exchanger (not shown), in addition to the electrolysis chamber and metal recovery chamber arranged laterally. For example, molten metallic magnesium can be recovered using the inlet and outlet of the metal recovery chamber, and molten salt such as molten magnesium chloride can be replenished to the heat exchange chamber. As a result, molten metallic magnesium can be stored more stably in the metal recovery chamber. In addition to these, the heat exchange chamber may further include a stirrer (not shown) for stirring the molten salt bath.

[0044] [3. Method for producing metallic magnesium] In one embodiment of the present invention for the production of metallic magnesium, a molten salt electrolytic apparatus is used, comprising an electrolytic cell, an anode disposed in the electrolytic cell, a bipolar manufactured by the bipolar manufacturing method described above, and a cathode, and the method includes an electrolytic step of producing metallic magnesium by electrolysis of magnesium chloride contained in a molten salt bath. Hereinafter, a preferred embodiment of the electrolytic step will be described using the molten salt electrolytic apparatus 100 shown in Figures 1A and 1B as an example.

[0045] <Electrolysis process> In the electrolysis process, the magnesium chloride contained in the molten salt bath Bf is electrolyzed. The molten salt bath Bf flows from the electrolysis chamber 140 through the flow port 125 to the metal recovery chamber 150 as shown by arrow A in Figure 1A, and then flows from the metal recovery chamber 150 through the underside of the second partition wall 122 to the electrolysis chamber 140 as shown by arrow B in Figure 1A. In the electrolysis chamber 140, the magnesium chloride in the molten salt bath Bf is electrolyzed to produce molten metallic magnesium. This molten metallic magnesium then flows into the metal recovery chamber 150 due to the flow of the molten salt bath Bf. Subsequently, the molten metallic magnesium, which has a lower specific gravity relative to the molten salt, floats to the shallow part of the metal recovery chamber 150 and accumulates there. The molten metallic magnesium that floats in the metal recovery chamber 150 can be recovered by inserting a recovery pipe or the like through the supply / discharge port 134. In one embodiment, during the electrolysis of molten salt, the aforementioned bipolar is immersed, but because the melting point of the ceramics formed in the through-holes of the bipolar is higher than the bath temperature of the molten salt bath, the ceramics, which have a higher electrical resistivity than the molten salt bath, remain solid. As a result, the through-current to the bipolar can be reduced, thereby improving the efficiency of the tank. [Examples]

[0046] The present invention will be specifically described based on examples and comparative examples. The following examples and comparative examples are merely specific examples to facilitate understanding of the technical content of the present invention, and the technical scope of the present invention is not limited by these examples.

[0047] [Example 1] [Manufacturing of ceramic-formed bipolars] First, a first molten salt electrolytic apparatus 100 was installed, having the configuration shown in Figures 1A and 1B, except that it used only one bipolar electrode 110. In this molten salt electrolytic apparatus 100, the bottom wall 123, side walls 124, and partition walls 121 and 122 of the electrolytic cell 120 were made of brick. Each electrode was arranged under the following conditions. The pore volume of each electrode was measured using the method described later. <Conditions for each electrode> (1) Anode 142 Quantity: 1 (unused) Shape: Plate-like Size: 20mm thick Material: Graphite Pore ​​volume: 0.13 mL / g (2) Cathode 144 Quantity: 1 (unused) Shape: Plate-like Size: 20mm thick Material: Graphite Pore ​​volume: 0.13 mL / g (3) Bipolarity 110 Quantity: 1 (unused) Shape: Plate-like Size: 15mm thick Material: Graphite Pore ​​volume: 0.13 mL / g (4) Arrangement of each electrode The distance between each electrode was set to 10 mm. The area of ​​the electrolytic surface of each electrode was assumed to be the same.

[0048] (Measurement of pore volume) The pore volume of each electrode was measured using the mercury intrusion method with the Autopore III 9400 series manufactured by Shimadzu Corporation. In this measurement, a test piece cut from the graphite used for the bipolar electrode was placed in a measurement cell, the pressure inside the cell was reduced to 0.003 MPa, and then mercury was introduced and the pressure was increased to 421 MPa. The pore volume could be determined by measuring the volume of mercury intruded into the pores present in the test piece at mercury pressures from 0.015 MPa to 413 MPa.

[0049] [Ceramics Formation Process] (Impregnation step) Next, molten salt (composition: 50 mol% NaCl - 25 mol% CaCl2 - 25 mol% MgCl2) was added to the electrolytic cell 120. After adding the salt, the temperature of the molten salt bath Bf was heated and maintained within the range of 650°C to 700°C. This immersed each electrode in the molten salt bath Bf. Then, electrolysis of magnesium chloride was carried out for 3 hours by applying a voltage between the anode 142 and cathode 144 via a wire from the power supply.

[0050] (Drying step) After the electrolysis was complete, the bipolar electrode 110 was removed from the electrolytic cell 120 and immersed in water for 10 minutes. Then, the bipolar electrode 110 was removed and vacuum-dried at 80°C for 24 hours to remove water from the surface and inside of the bipolar electrode 110.

[0051] (Heating step) Next, a second molten salt electrolytic apparatus 100 was installed as a separate apparatus from the first molten salt electrolytic apparatus 100 described above. At this time, the configuration of the second molten salt electrolytic apparatus 100 was the same as that of the first molten salt electrolytic apparatus 100, except that a bipolar electrode that had been vacuum-dried was attached and molten salt (composition: 50% by mass NaCl - 30% by mass CaCl2 - 20% by mass MgCl2) was put into the electrolytic cell 120, and the number of bipolar electrodes used was one. Then, the bipolar electrode 110 was immersed in a molten salt bath Bf, which was maintained at a temperature within the range of 650°C to 700°C, for 60 minutes.

[0052] (Manufacturing of metallic magnesium) Electrolysis of magnesium chloride was performed for 5 hours by applying a voltage between the anode 142 and cathode 144 via a wire from a power source. The tank efficiency is shown in Table 1. Here, cell efficiency is an evaluation index specific to electrolytic cells with a bipolar arrangement between the anode and cathode. A higher cell efficiency means that a larger proportion of the bipolar arrangement contributes to the electrolysis of magnesium chloride, which in turn means less leakage current and higher productivity. The cell efficiency η is defined by Equation 1.

[0053]

number

[0054] [Comparative Example 1] In Comparative Example 1, metallic magnesium was produced by electrolysis of magnesium chloride for 5 hours, in the same manner as in Example 1, except that the ceramic formation treatment (a treatment in which ceramics are formed inside the through-holes opening on the surface of the porous body of the bipolar) was not performed, and an unused bipolar was used, and electrolysis of magnesium chloride was performed using a molten salt (composition: 50% by mass NaCl - 30% by mass CaCl2 - 20% by mass MgCl2). Table 1 shows the tank efficiency.

[0055] [Example 2] In Example 2, the electrolysis of magnesium chloride was performed for 5 hours in the same manner as in Example 1, except that the thickness of the unused bipolar electrode was set to 50 mm. The tank efficiency is shown in Table 1.

[0056] [Comparative Example 2] In Comparative Example 2, metallic magnesium was produced by electrolysis of magnesium chloride for 5 hours, in the same manner as in Example 2, except that unused bipolar electrodes were used without performing the ceramic formation treatment (a treatment to form ceramics inside the through-holes opening on the surface of the porous bipolar body). The tank efficiency is shown in Table 1.

[0057] [Example 3] First, the electrodes were immersed in water for 10 minutes. After removing the electrodes from the water, atmospheric pressure drying was performed at 50°C for 1 hour to remove some of the water. <Electrode conditions> Quantity: 1 (unused) Shape: Plate-like Size: 15mm thick Material: Graphite Pore ​​volume: 0.13 mL / g

[0058] Next, a first molten salt electrolytic apparatus 100 was set up, using one bielectrode, similar to Example 1. Then, the electrode, dried at atmospheric pressure, was immersed for 1 hour in molten salt (composition: 50 mol% NaCl - 25 mol% CaCl2 - 25 mol% MgCl2) maintained at a temperature within the range of 650°C to 700°C.

[0059] After the immersion was completed, the electrodes were removed, and a second molten salt electrolytic apparatus 100 was assembled in the same manner as in Example 1, using one bipolar electrode. At this time, the configuration of the second molten salt electrolytic apparatus 100 was the same as that of the first molten salt electrolytic apparatus 100, except that an electrode with ceramics formed on it was attached as a bipolar electrode 110, and molten salt (composition: 50% by mass NaCl - 30% by mass CaCl2 - 20% by mass MgCl2) was added to the electrolytic cell 120. The bipolar electrode 110 was immersed in a molten salt bath Bf, where the bath temperature was maintained within the range of 650°C to 700°C. Then, electrolysis of magnesium chloride was carried out for 5 hours by applying a voltage between the anode 142 and the cathode 144 via a wire from the power supply. The cell efficiency is shown in Table 1.

[0060] [Table 1]

[0061] [Confirmation of dual electrodes after electrolysis] After the electrolysis of magnesium chloride in Examples 1 to 3, the molten salt bath stored in the electrolytic cell of the molten salt electrolytic apparatus was cooled, the solidified bath was removed by known means, and the electrolytic cell was dismantled. The dismantled bipolar was taken out, cut along the height direction, and the cross-section of the bipolar was visually observed. As a result of the observation, a white powder was confirmed in part of the inside of the through-hole. The porous body of the graphite bipolar has a mesh-like pore structure, and the white powder was confirmed at a certain depth in the thickness direction. Furthermore, the white powder remained even when exposed to water. That is, the above powder is presumed to be a ceramic, as it is generated by heating the molten salt bath components containing water and is sparingly soluble in water.

[0062] [Discussion based on examples] In Examples 1 to 3, it was confirmed that, from the viewpoint of reducing through-current, it is useful to include an impregnation step in which the through-holes of a porous graphite body having through-holes contain components of a molten salt and water and / or oxygen, and a heating step in which the electrode is heated after the impregnation step. In Example 2, compared to Example 1, the tank efficiency was improved by increasing the thickness of the bipolar electrode. That is, the through-current is also affected by the thickness of the bipolar electrode. Therefore, the effect of reducing through-current can be verified by comparing an example using bipolar electrodes of the same thickness with a comparative example. Furthermore, in Example 3, since the tank efficiency was equivalent to that of Example 1, it was confirmed that the through-current can be effectively reduced by first impregnating the porous body of the graphite electrode with water and / or oxygen, and then impregnating the porous body of the electrode with components of a molten salt. On the other hand, in Comparative Examples 1 and 2, the tank efficiency was reduced compared to Examples 1 and 2 because the impregnation and heating steps were not performed. [Explanation of symbols]

[0063] 100, 200 Molten Salt Electrolytic Devices 110, 210 First bipolar (bipolar) 115, 215 Second bipolar (bipolar) 120 Electrolytic cell 121 First bulkhead (bulkhead) 122 Second bulkhead (bulkhead) 123 Bottom wall 124 Side wall 125 Distribution port 130 Top lid 132 First gas recovery port 133 Second gas recovery port 134 Supply / discharge port 140 Electrolysis chamber 142, 242 anode 144, 244 cathode 145 Electrolytic surface 150 Metal Recycling Room

Claims

1. A method for manufacturing a bipolar electrode used in a molten salt electrolytic apparatus for the electrolysis of magnesium chloride, An impregnation step in which a porous body made of graphite having holes including through holes is made to contain components of a molten salt and water and / or oxygen in the through holes of the electrode, The process includes, after the impregnation step, a heating step of heating the electrode, A method for manufacturing a bipolar, wherein the impregnation step is carried out by performing either (A) or (B) below. (A) The components of the molten salt are first introduced into the through-hole, and then the water and / or oxygen are introduced into the through-hole. (B) In order to first introduce the water and / or oxygen into the through hole, at least one selected from the group consisting of water vapor spraying, water washing, and water immersion is performed, and then the components of the molten salt are introduced into the through hole.

2. The impregnation step includes impregnating the through hole with water, The method for manufacturing a bipolar electrode according to claim 1, further comprising a drying step of vacuum drying the electrode between the impregnation step and the heating step.

3. The method for producing a bipolar pole according to claim 1, wherein the chloride content in the molten salt in the impregnation step is 95% by mass or more.

4. The method for producing a bipolar electrode according to claim 1, wherein the molten salt in the impregnation step includes magnesium chloride.

5. A method for producing metallic magnesium using a molten salt electrolytic apparatus comprising an electrolytic cell, an anode disposed in the electrolytic cell, a bipolar manufactured by the method for producing a bipolar according to any one of claims 1 to 4, and a cathode, A method for producing metallic magnesium, comprising an electrolytic step of producing metallic magnesium by electrolyzing the aforementioned magnesium chloride.

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