Metal magnesium production method

Abstract

To provide a metal magnesium production method in which a high current efficiency can be realized by appropriately managing a bath surface level of a molten bath in a recovery chamber.SOLUTION: Provided is a metal magnesium production method for producing metal magnesium by subjecting magnesium chloride in a molten salt bath to molten salt electrolysis by using an electrolytic tank having an electrolytic chamber and a recovery chamber, and electrodes including anodes, bipoles, and cathodes arranged in the electrolytic chamber. At least one of the bipoles has a thick width section with an electrolytic-reaction generating surface and a thin width section arranged on the upper side of the thick width section, and maintains a height position of a bath surface of a molten bath in the recovery chamber within a range of 15 cm or more and 42 cm or less from an opening of the electrolytic tank in a depth direction in the molten salt electrolysis.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing metallic magnesium.

Background Art

[0002] Ingots of metallic titanium, etc. are industrially produced using sponge titanium produced by a method based on 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 produced in the chlorination process is reduced with metallic magnesium to obtain a sponge titanium mass, and this sponge titanium mass is crushed to obtain sponge titanium. The electrolysis process is a process of obtaining metallic magnesium by subjecting magnesium chloride, which is a by-product of the reduction process, to molten salt electrolysis. As technologies related to the molten salt electrolysis in the electrolysis process, for example, those described in Patent Documents 1 to 2 are available.

[0003] Patent Document 1 discloses an invention in which, in the molten salt electrolysis of magnesium chloride (MgCl2) using a multipolar electrolytic cell, the position of the upper end of the electrode in the electrolysis chamber and the bath surface of the molten salt bath is controlled, and the bath surface position is controlled using a tank for controlling the bath surface level (see paragraphs 0009, 0024, etc. of Patent Document 1). That is, the invention of Patent Document 1 aims to improve the current efficiency by precisely controlling the temperature of the molten salt bath and its bath surface level (see paragraph 0003, etc. of Patent Document 1). Incidentally, it is also considered that the invention of Patent Document 1 suppresses current leakage in order to improve the current efficiency.

[0004] Further, Patent Document 2 discloses an invention in which, in the molten salt electrolysis of magnesium chloride using a multipolar type electrolytic cell, the bath surface level of the molten salt bath in the electrolysis chamber among the inside of the electrolytic cell is managed (see paragraph 0007 of Patent Document 2). That is, the invention of Patent Document 2 aims to improve the current efficiency by suppressing current leakage that does not contribute to the generation of metallic magnesium based on the swelling of the molten salt bath in the molten salt electrolysis (see paragraphs 0008 to 0013, etc. of Patent Document 2).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, during molten salt electrolysis, the current efficiency may decrease. One of the factors for such a decrease is, in addition to current leakage as in Patent Documents 1 and 2 above, what is called re-reaction is known. Re-reaction refers to the reaction between metallic magnesium generated by molten salt electrolysis of magnesium chloride and chlorine gas to produce magnesium chloride. The reason for such a phenomenon is considered that the re-reaction is promoted when the molten salt bath is at a high temperature or when metallic magnesium and chlorine gas are likely to come into contact with each other.

[0007] As described above, Patent Documents 1 and 2 manage the bath surface level of the electrolysis chamber of the molten salt bath to be constant to suppress the occurrence of current leakage, thereby improving the current efficiency. That is, Patent Documents 1 and 2 do not disclose or suggest an invention for improving the current efficiency by focusing on suppressing re-reaction. Therefore, from the viewpoint of improving the current efficiency, it can be said that there is still room for improvement with respect to Patent Documents 1 and 2 which are known technologies. In addition, since chlorine gas that affects the human body is released on the bath surface of the electrolysis chamber, it is assumed that it is difficult to confirm the bath surface position of the electrolysis chamber from the lid side of the electrolytic cell or the work load is large, and it can be said that there is room for improvement in this regard as well.

[0008] Therefore, an object of the present invention in one embodiment is to provide a method for producing metallic magnesium capable of realizing high current efficiency by appropriately managing the bath surface level of the molten bath in the recovery chamber.

Means for Solving the Problems

[0009] As a result of intensive studies, the present inventor has found that at least one of the bipolar electrodes has a thick-width portion having an electrolytic reaction generating surface and a thin-width portion disposed above the thick-width portion, and by maintaining the height position of the molten bath surface in the recovery chamber within a range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell in the depth direction in molten salt electrolysis, it is possible to appropriately manage the molten bath surface level in the recovery chamber and achieve high current efficiency. The present invention has been completed based on the above findings and is exemplified below. [1] A method for producing metallic magnesium by using an electrolytic cell having an electrolytic chamber and a recovery chamber, and an electrode including an anode, a bipolar electrode, and a cathode disposed in the electrolytic chamber, and subjecting magnesium chloride in a molten salt bath to molten salt electrolysis to produce metallic magnesium, at least one of the bipolar electrodes has a thick-width portion having an electrolytic reaction generating surface and a thin-width portion disposed above the thick-width portion, the method for producing metallic magnesium, including maintaining the height position of the molten bath surface in the recovery chamber within a range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell in the depth direction in the molten salt electrolysis. [2] The method for producing metallic magnesium according to [1], wherein at least a part of the thin-width portion of the bipolar electrode is exposed on the molten salt bath surface in the electrolytic chamber while maintaining the height position of the bath surface in the recovery chamber within a range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell. [3] The method for producing metallic magnesium according to [1] or [2], wherein the bipolar electrode has a constant thickness of the thin-width portion in the thickness direction and has a step between the thick-width portion and the thin-width portion.

Advantages of the Invention

[0010] According to an embodiment of the present invention, it is possible to provide a method for producing metallic magnesium capable of appropriately managing the molten bath surface level in the recovery chamber and achieving high current efficiency.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

[0012] 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 some 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. Also, members shown integrally in the drawings may be formed by combining appropriate parts. Furthermore, in this specification, "upward" means a direction from the bottom wall 114 side of the electrolytic cell 110 toward the lid body 120 side, as indicated by an arrow in FIGS. 1 to 3, for example, and "downward" means a direction from the lid body 120 side toward the bottom wall 114 side of the electrolytic cell 110. Also, in this specification, "molten metallic magnesium" means molten metallic magnesium obtained by subjecting magnesium chloride to molten salt electrolysis.

[0013] [Method for Producing Metallic Magnesium] In one embodiment, the method for producing metallic magnesium according to the present invention uses an electrolytic cell having an electrolysis chamber and a recovery chamber of a molten salt electrolysis apparatus, and electrodes including an anode, a bipolar electrode, and a cathode disposed in the electrolysis chamber, and electrolyzes magnesium chloride in a molten salt bath to produce metallic magnesium. Further, from the viewpoint of achieving high current efficiency, at least one of the bipolar electrodes has a thick portion having an electrolysis reaction generation surface and a thin portion disposed above the thick portion, and in the molten salt electrolysis, maintaining the height position of the bath surface S1 of the molten bath MB in the recovery chamber within a predetermined range is included. As illustrated in FIG. 2, the thick portions 152 and 156 mean those having a thickness thicker than that of the thin portions 153 and 157.

[0014] Typically, in the operation of a molten salt electrolysis apparatus, an air-cooled pipe (heat exchanger) for adjusting the temperature of the molten bath, which penetrates a lid for closing the opening of the electrolytic cell and is immersed in the molten bath of the recovery chamber of the electrolytic cell, is used. Conventionally, when the bath surface level of the molten salt bath decreases during the operation of the molten salt electrolysis apparatus, the operating rate of the air-cooled pipe tends to increase. In this case, the air-cooled pipe is controlled to start operation to lower the bath temperature when the bath temperature becomes equal to or higher than a specific temperature, and to stop operation to raise the bath temperature when the bath temperature becomes lower than the specific temperature. That is, in a state where the bath surface level has decreased, the temperature of the molten bath is high, the calorific value of the entire electrolytic cell increases, and a re-reaction is likely to occur, and it is considered that the current efficiency decreases due to the re-reaction (decrease in the productivity of metallic magnesium).

[0015] On the other hand, a decrease in the height position of the bath surface of the molten salt bath means a decrease in the magnesium chloride content in the molten salt bath (a decrease in concentration due to the progress of molten salt electrolysis), and thereby the solubility of metallic magnesium in the molten salt bath decreases. Therefore, when the height position of the bath surface of the molten salt bath decreases, it is also considered that the re-reaction between metallic magnesium and chlorine gas is less likely to occur.

[0016] In such a situation, as a result of various studies on the relationship between re-reaction and current efficiency, the inventor of the present invention has come to focus on the gas lift pump phenomenon. The gas lift pump phenomenon means, for example, a phenomenon in which, in a liquid stored in a tank having a cylindrical liquid lift pipe, the liquid outside the liquid lift pipe is pumped into the liquid lift pipe so that the pressure of the gas-liquid mixture with an apparent specific gravity in the liquid lift pipe and the pressure of the liquid outside the liquid lift pipe are balanced.

[0017] As a result of studying the above relationship using simulation, the inventor of the present invention obtained the finding that the flow rate of the molten salt bath in the inter-electrode portion of the electrode varies depending on the bath surface level. When the flow path of the molten salt bath between the electrodes is pseudo-considered as a liquid lift pipe, if the height position of the bath surface becomes higher, the liquid lift pipe in the vertical direction becomes longer and the gas lift pump phenomenon intensifies, and as a result, it is considered that the flow rate of the molten salt bath becomes faster.

[0018] That is, in the inter-electrode portion of the electrode, magnesium metal and chlorine gas are generated by molten salt electrolysis, and since these exist in a narrow space, re-reaction is likely to occur. However, when the molten salt passes quickly through this inter-electrode portion, both the magnesium metal and chlorine gas generated by molten salt electrolysis will pass quickly through the inter-electrode portion. As a result, it is considered that re-reaction is less likely to occur, and as a result, a decrease in current efficiency is suppressed. After passing through the inter-electrode portion of the electrode, the chlorine gas continues to rise and finally escapes to the upper side of the molten salt bath, and the molten salt bath flows from the electrolysis chamber toward the recovery chamber together with magnesium metal, so it is considered that magnesium metal and chlorine gas are well separated and less likely to re-react. Thus, based on the above findings, further studies were repeated, and the present invention was completed. However, the present invention is not limited to using the above findings.

[0019] (Height position of the bath surface of the molten bath) In one embodiment, in molten salt electrolysis, the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 is maintained within a range of 15 cm or more and 42 cm or less in the depth direction (vertical direction) from the opening 111 of the electrolytic cell (in the case shown in FIG. 1, the position of the back surface 121 of the lid of the lid body 120 coincides with the position of the opening 111) (see FIG. 1). Since the opening 111 is used as a reference in the depth direction, "15 cm" corresponds to the upper limit of the height of the bath surface, and "42 cm" corresponds to the lower limit of the bath surface height. By setting it within this range, the bath surface S1 of the molten bath MB is appropriately maintained at a high level, and due to the gas lift pump phenomenon, the molten salt bath passes through the space between the electrodes relatively quickly, so that the re-reaction between metallic magnesium and chlorine gas can be suppressed. As a result, it is considered that metallic magnesium can be produced efficiently. However, when the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 is less than 15 cm from the opening 111 in the depth direction, the bath surface S1 gets too close to the lid body 120, and for example, when the electrolytic cell 110 shakes due to an earthquake, there is a risk that the molten bath may leak out of the electrolytic cell 110. That is, the upper limit side of the height position of the bath surface S1 of the molten bath MB is set from the viewpoint of operation safety. Also, when the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 becomes lower than 42 cm from the opening of the electrolytic cell in the depth direction, the advantages of the gas lift pump phenomenon cannot be fully obtained, and there is a risk that the production efficiency of metallic magnesium may decrease. Note that the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 can be confirmed by an appropriate method. For example, it can be confirmed using a back pressure type level gauge. In one embodiment, by using a back pressure type level gauge, even when chlorine gas is continuously generated during molten salt electrolysis, the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 can be accurately confirmed without opening the lid body. The gas lift pump phenomenon is considered to affect the passing speed of metallic magnesium and chlorine gas between electrodes, and thus it is also considered that the bath surface height in the electrolysis chamber may be checked. However, in the electrolysis chamber, molten salt electrolysis continues, chlorine gas is generated and separated, and the bath surface is disturbed. Therefore, although it is possible to confirm whether the upper end of the bipolar electrode is located above the liquid surface of the molten salt bath MSB, it is difficult to continuously grasp the bath surface height in the electrolysis chamber. On the other hand, the inventor has found that if the molten salt electrolysis is carried out by controlling the bath surface height of the molten bath in the recovery chamber, as a result, metallic magnesium can be efficiently produced in the electrolysis chamber by utilizing the gas lift pump phenomenon.

[0020] In yet another embodiment, during the molten salt electrolysis, it is preferable that at least a part of the narrow-width portions 153 and 157 of the bipolar electrodes 151 and 155 is exposed on the bath surface S2 of the molten salt bath MSB (see Fig. 2). That is, at least the narrowest part between the electrodes is immersed in the molten salt bath MSB, and on top of that, a part of the narrow-width portions 153 and 157 of the bipolar electrodes is also immersed in the molten salt bath MSB. Note that on the premise that the operator and the equipment are fully prepared, it is possible to open a part of the lid 120 on the electrolysis chamber 130 side for a short time to check whether the narrow-width portion is exposed on the bath surface S2 of the molten salt bath MSB. In this embodiment, all the bipolar electrodes have narrow-width portions, but it is considered that the gas lift pump phenomenon can be utilized if at least one of the bipolar electrodes has a narrow-width portion. Since the thick-width portion 152 of the bipolar electrode 151 is immersed in the molten salt bath MSB, a wide area of the electrolysis reaction generation surface where magnesium chloride is molten salt electrolyzed can be ensured. On top of that, the generated molten metal magnesium and chlorine gas are quickly moved upward along with the flow of the molten salt bath MSB due to the gas lift pump phenomenon, and it is considered that the re-reaction between the molten metal magnesium and chlorine gas is less likely to occur. Also at this time, electrolysis of magnesium chloride is carried out between the electrolysis reaction generation surface 131a of the anode 131 and the electrolysis reaction generation surface 152a on the anode 131 side of the thick-width portion 152, but almost no electrolysis of magnesium chloride occurs between the electrolysis reaction generation surface 131a of the anode 131 and the surface 153a on the anode 131 side of the narrow-width portion 153 (see Fig. 2). The reason for this is that the shortest distance between the electrolysis reaction generation surface 131a of the anode 131 and the surface 153a of the narrow-width portion 153 is longer than the shortest distance between the electrolysis reaction generation surface 131a of the anode 131 and the electrolysis reaction generation surface 152a of the thick-width portion 152. Also, the resistance of the molten salt bath increases as the distance between the electrodes increases, and the presence of chlorine gas is also cited. In addition, the volume of the region between the electrolysis reaction generation surface 131a of the anode 131 and the surface 153a of the narrow-width portion 153 is larger than the volume of the region between the electrolysis reaction generation surface 131a of the anode 131 and the electrolysis reaction generation surface 152a of the thick-width portion 152. The molten metal magnesium and chlorine gas that have passed between the electrolysis reaction generation surface 131a of the anode 131 and the electrolysis reaction generation surface 152a of the thick-width portion 152 are likely to separate, and this is also considered to contribute to the suppression of the re-reaction.

[0021] (Bipolar) The bipolar electrodes 151, 155, 251, and 255 may have a constant thickness in the thickness direction in the thin-width portions 153, 157, 253, and 257, and have a step between the thick-width portions 152, 156, 252, 256 and the thin-width portions 153, 157, 253, 257 (see FIGS. 2, 3, and 4). The shape of the thin-width portion of the bipolar electrode is not particularly limited, and examples include a surrounding shape, a plate shape, a square tube shape, a cylindrical shape, etc. In order to promote the separation of chlorine gas and metallic magnesium, the thin-width object is preferably provided on the cathode side. Usually, the entire cathode is immersed in the molten salt bath, while the anode extends to the outside of the lid. Therefore, if the thin-width portion is provided on the cathode side, it is considered that the gas lift pump phenomenon can be utilized without any special processing of the anode or cathode. Here, when the bipolar electrode is cylindrical or surrounding, the thin-width portion is not provided only on the partition wall side described later, but has a thin-width portion above the thick-width portion in other portions, so it corresponds to the bipolar electrode having a thin-width portion. The reason for not providing the thin-width portion on the partition wall side is to facilitate the flow of the molten salt bath into the recovery chamber. Note that the step means a portion where the thickness size switches between the thin-width portion and the thick-width portion of the bipolar electrode in a cross section cut in the vertical direction. Also, the thickness direction means the arrangement direction of the anode, the bipolar electrode, and the cathode. Here, examples of the shape of the thick-width portion of the bipolar electrode include a plate shape, a square tube shape, a cylindrical shape, a conical shape having a hollow along the axis, a truncated pyramid shape having a hollow along the axis (the thick-width increases downward), a truncated conical shape having a hollow along the axis (the diameter increases downward), etc. As shown in FIGS. 2 and 4, the shape of the thick-width portions 152, 156, 252, 256 of the bipolar electrodes 151, 155, 251, 255 may be a plate shape or a square tube shape.

[0022] From the viewpoint of utilizing the gas lift pump phenomenon, it is preferable that the upper end surfaces 153b, 157b, 253b, 257b of the thin-width portions 153, 157, 253, 257 of the bipolar electrodes 151, 155, 251, 255 are exposed above the bath surface S2 during the implementation of the molten salt electrolysis. On the other hand, although the upper end surface is separated from the opening 111 of the electrolytic cell 110, it may be within a range of, for example, 3 cm or more and 8 cm or less from the opening 111 of the electrolytic cell 110 in the depth direction.

[0023] (Current supply amount) In molten salt electrolysis, the current supply amount to the electrolytic cell 110 is appropriately adjusted, and for example, it can be carried out at a constant current. In addition, in molten salt electrolysis, it is also possible to carry out by alternately combining constant currents with different current amounts (for example, a 5A period and an 8A period).

[0024] (Supply of molten salt) When continuing molten salt electrolysis, magnesium chloride is replenished to the molten salt bath MSB in the electrolytic cell 110 at an appropriate timing. At this time, only magnesium chloride may be replenished, or it may be supplied as a mixture of magnesium chloride and a supporting salt. Magnesium chloride is usually supplied from the supply and discharge port 124 of the lid 120 to the recovery chamber 140 and is not supplied to the electrolysis chamber 130. The supply form to the molten bath MB in the recovery chamber 140 is not particularly limited, and it may be supplied continuously or in a batch manner. When a large amount of magnesium chloride is supplied at one time in a batch manner, the position of the bath surface S1 of the molten bath MB rapidly increases after the supply of magnesium chloride. From the viewpoint of production efficiency, it is preferable that the bath surface S1 of the molten bath MB after the supply is maintained within a range of 15 cm or more and 42 cm or less from the opening 111 of the electrolytic cell 110 in the depth direction.

[0025] (Recovery) After the molten metal magnesium flows into the recovery chamber 140, the molten metal magnesium with a specific gravity smaller than that of the molten salt floats to a shallow part of the recovery chamber 140 and accumulates there. The molten metal magnesium floating in the recovery chamber 140 can be recovered by inserting a recovery pipe or the like through the supply and discharge port 124. The recovered chamber 140 may contain the stored metallic magnesium and the molten salt bath, but usually, metallic magnesium does not float and accumulate in the electrolysis chamber 130. Therefore, the recovery chamber is called the molten bath MB, and the electrolysis chamber is called the molten salt bath MSB.

[0026] (Example of molten salt electrolysis apparatus) The molten salt electrolysis apparatus 100 shown in FIGS. 1 and 2 includes an electrolytic cell 110 and a lid 120. The electrolytic cell 110 is partitioned into an electrolysis chamber 130 and a recovery chamber 140 due to the presence of a first partition wall 112 and a second partition wall 113. Further, in the electrolysis chamber 130 of the electrolytic cell 110, an anode 131, a first bipolar electrode 151 and a second bipolar electrode 155, and a cathode 135 are arranged in this order, and the anode 131, the first bipolar electrode 151, the second bipolar electrode 155, and the cathode 135 are at least partially immersed in a molten salt bath MSB stored in the electrolysis chamber 130. The direction in which the anode 131, the bipolar electrodes 151, 155, and the cathode 135 are arranged in a direction orthogonal to the vertical direction (the horizontal direction in this example) is referred to as the arrangement direction. For example, when the shape of the electrode is plate-shaped, a plurality of anodes 131 and cathodes 135 are often arranged alternately in the arrangement direction. In the illustrated electrolysis chamber 130, two bipolar electrodes 151, 155 are arranged between the anode 131 and the cathode 135, but at least one bipolar electrode may be arranged.

[0027] (Electrolytic cell) The electrolytic cell 110 has a container shape with an opening 111 formed on the upper side, and is made of, for example, refractory bricks mainly containing aluminum oxide and other suitable materials. As shown in FIGS. 1 to 4, usually, the height of the opening 111 is constant in the depth direction. The electrolytic cell 110 is composed of a bottom wall 114 and two pairs of side walls 115 connected to the bottom wall 114 and extending upward. In the electrolysis chamber 130 of this electrolytic cell 110, a molten salt bath MSB is stored inside. The metallic magnesium generated by the molten salt electrolysis of magnesium chloride in the electrolysis chamber 130 is sent to the recovery chamber 140 by the circulation of the molten salt bath MSB, and the molten salt is sent from the recovery chamber 140 to the electrolysis chamber 130 to circulate the molten salt bath MSB. Here, the molten salt electrolysis apparatus 100 forms a circulation port 116 between the first partition wall 112 and the second partition wall 113, thereby ensuring the flow of the molten salt bath MSB shown by arrow A (the flow from the electrolysis chamber 130 to the recovery chamber 140). Also, a passage through which the molten bath MB can flow is formed on the lower surface side of the second partition wall 113, ensuring the flow shown by arrow B (the flow from the recovery chamber 140 to the electrolysis chamber 130).

[0028] (molten salt) By electrolyzing the molten salt of 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 contains sodium chloride (NaCl), calcium chloride (CaCl2), calcium fluoride (CaF2), etc. as supporting salts. Note that the above-mentioned supporting salts may contain other components such as potassium chloride (KCl). The supporting salt usually has a higher decomposition 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 a raw material for molten salt electrolysis, that produced as a by-product in the Kroll process can be used.

[0029] (lid) Since the molten salt bath MSB is at a high temperature, the lid 120 serves as heat insulation against the outside of the electrolytic cell 110. Also, the lid 120 is arranged to make the electrolytic cell 110 a closed space, and the inside of the electrolytic cell 110 is made negative with respect to the outside in order to prevent the leakage of chlorine gas generated from the anode 131, bipolar electrodes 151, 155 during molten salt electrolysis. Also, the material of the lid 120 is not particularly limited, but from the viewpoint of preventing a short circuit occurring between the lid 120 and the anode 131 during molten salt electrolysis, it is sufficient that the back surface 121 side of the lid, which is on the molten salt bath MSB side of the lid 120, is made of an insulating material. Also, a ceramic material may be arranged on the back surface 121 side of the lid 120 on the molten salt bath MSB side, or a castable refractory may be applied. The method of providing this castable refractory may be a known method. For example, the castable refractory may be applied to the back surface 121 side by dry spraying or wet spraying.

[0030] The lid 120 may be provided with a first gas recovery port 122, a second gas recovery port 123, and a supply / discharge port 124. Each of these ports may be one or a plurality. The first gas recovery port 122 is used to recover the chlorine gas generated by the electrolysis of magnesium chloride in the electrolysis chamber 130. The first gas recovery port 122 is provided in the region where the electrolysis chamber 130 is located. The second gas recovery port 123 is provided in the region where the recovery chamber 140 is located. The second gas recovery port 123 may be used to recover the remaining gas that has flowed into the recovery chamber 140 without being recovered by the first gas recovery port 122 among the gases generated by electrolysis. In addition, the supply and discharge port 124 is used for recovering the molten metal magnesium generated by the electrolysis of magnesium chloride in the electrolysis chamber 130, supplying magnesium chloride into the electrolytic cell 110, etc. The supply and discharge port 124 is provided in the region where the recovery chamber 140 is located.

[0031] (Electrolysis chamber) In the electrolysis chamber 130, magnesium chloride is subjected to molten salt electrolysis to generate molten metal magnesium and chlorine gas by the molten salt electrolysis. In one embodiment, in the electrolysis chamber 130, the electrolysis reaction generating surfaces of the anode 131, the first bipolar electrode 151, the second bipolar electrode 155, and the cathode 135 are arranged to be substantially parallel to the depth direction (the vertical direction in FIG. 2) of the molten salt bath MSB.

[0032] The anode 131 is inserted through the lid 120 and extends downward, and is arranged such that a part of it is immersed in the molten salt bath MSB. The shape of the anode 131 is not particularly limited, and examples include a plate shape, a cylindrical shape, and a prismatic shape. From the viewpoint of the production efficiency of molten metal magnesium, the molten salt electrolysis apparatus 100 may be provided with a plurality of anodes 131 and cathodes 135 respectively. The material of the anode 131 is not particularly limited, and examples include graphite. The material of the cathode 135 is not particularly limited, and examples include graphite, carbon steel, etc. The anode 131 and the cathode 135 are connected to a power source via a bus bar, a conductive wire, or the like.

[0033] The first bipolar electrode 151 and the second bipolar electrode 155 may be respectively arranged on pedestals 117 and 118 made of, for example, refractory bricks.

[0034] As shown in FIG. 1, the cathode 135 has an extension portion 136 extending outward, and this extension portion 136 is arranged so as to penetrate the side wall 115 and protrude outside the electrolytic cell 110. The shape of the cathode 135 may be plate-shaped, but can be appropriately changed in consideration of the shape of the anode 131 and the like, and may be square tubular, cylindrical, or the like. Even in this case, the cathode 135 has the extension portion 136.

[0035] Also, for example, in the molten salt electrolysis apparatus 200 shown in FIGS. 3 and 4, along the arrangement direction from the anode 231 (in the direction away from the anode 231 in the illustrated embodiment), the anode 231, the first bipolar electrode 251, the second bipolar electrode 255, and the cathode 235 are arranged in this order. More specifically, a square tubular first bipolar electrode 251 is arranged on the pedestal 217 surrounding the anode 231 and spaced apart from the anode 231, a square tubular second bipolar electrode 255 is arranged on the pedestal 218 surrounding the first bipolar electrode 251 and spaced apart from the first bipolar electrode 251, and a square tubular cathode 235 is arranged surrounding the second bipolar electrode 255 and spaced apart from the second bipolar electrode 255. The cathode 235 further has an extension portion 236 extending outward from a part of the square tubular shape, and this extension portion 236 is arranged so as to penetrate the side wall 115 and protrude outside the electrolytic cell 110. At this time, from the viewpoint of realizing high current efficiency without inhibiting the flow of the molten salt bath MSB, the portions on the partition wall side of the first bipolar electrode 251 and the second bipolar electrode 255 have only the thick-width portions 252 and 256 (and do not have the thin-width portions 253 and 257).

[0036] (Recovery chamber) In the recovery chamber 140, the molten metal magnesium generated by electrolysis in the electrolysis chamber 130 is stored and appropriately recovered. The recovery chamber 140 communicates with the electrolysis chamber 130 and may have a heat exchanger (not shown). The heat exchanger can adjust the temperature of the molten bath MB in the recovery chamber 140. 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. The pipe may be made of steel (for example, carbon steel or stainless steel).

Example

[0037] 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.

[0038] [Example 1] (Operation Preparation of Molten Salt Electrolysis Device) In Example 1, a molten salt electrolysis apparatus 100 having the configuration shown in FIGS. 1 and 2 was used. The materials of the electrolytic cell 110, the first partition wall 112, the second partition wall 113, and the pedestals 117 and 118 are respectively shaped refractory materials (refractory bricks) containing aluminum oxide, and the material of the lid 120 is carbon steel. A layer of castable refractory material having insulating properties was applied to the back surface 121 of the lid 120. Further, as electrodes, a graphite anode 131 and a carbon steel cathode 135 were respectively arranged, and two bipolar electrodes 151 and 155 made of graphite were arranged between the anode 131 and the cathode 135 (that is, n = 3 (anode 131 - first bipolar electrode 151, first bipolar electrode 151 - second bipolar electrode 155, second bipolar electrode 155 - cathode 135)). At this time, the first and second bipolar electrodes 151 and 155 were provided with thick portions 152 and 156 having electrolytic reaction generating surfaces 152a and 156a and thin portions 153 and 157 arranged above the thick portions 152 and 156 and on the cathode side. Note that the anode 131 and the cathode 135 were plate-shaped. Further, the bipolar electrodes were arranged such that the upper end surfaces of the thin portions were at a position 5 cm from the opening (upper end of the side wall) of the electrolytic cell in the depth direction (note that during the implementation of the molten salt electrolysis described later, the upper end surfaces of the thin portions were exposed on the bath surface of the molten salt bath in the electrolysis chamber). The inter-pole distance was set as follows. <Inter-pole distance> The shortest distance between the anode 131 and the thick portion 152 of the first bipolar electrode 151 in the thickness direction: 7 mm The shortest distance between the anode 131 and the thin portion 153 of the first bipolar electrode 151 in the thickness direction: 52 mm The shortest distance between the thick portion 152 of the first bipolar electrode 151 and the thick portion 156 of the second bipolar electrode 155 in the thickness direction: 7 mm The shortest distance between the thin portion 153 of the first bipolar electrode 151 and the thin portion 157 of the second bipolar electrode 155 in the thickness direction: 52 mm The shortest distance between the thick portion 156 of the second bipolar electrode 155 and the cathode 135 in the thickness direction: 7 mm

[0039] Next, molten salt was introduced into the molten salt electrolysis apparatus 100, and the temperature of the molten bath MB was adjusted within the range of 651 °C or higher and 670 °C or lower.

[0040] A voltage was applied between the anode 131 and the cathode 135 of the molten salt electrolysis apparatus 100 to initiate molten salt electrolysis. During the molten salt electrolysis, the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 was maintained within a range of 25 cm or more and 35 cm or less from the opening 111 of the electrolytic cell 110 in the depth direction as shown in Table 1, and magnesium chloride was replenished at an appropriate timing so that the magnesium chloride concentration in the molten salt bath could be maintained between 13 and 23 mass%. After 10 days had elapsed since the start of the molten salt electrolysis, the supply of current was stopped. During the molten salt electrolysis, the bath surface S1 of the molten bath MB in the recovery chamber 140 was confirmed using a back pressure type level gauge.

[0041] [Example 2, Comparative Example 1] In Example 2 and Comparative Example 1, unlike Example 1, molten salt electrolysis was carried out except that the height position of the bath surface S1 of the molten bath MB in the recovery chamber 140 was changed as shown in Table 1. In Comparative Example 1, magnesium chloride was to be supplied after the bath surface S1 reached within a range of 44 cm or more and 45 cm or less in the depth direction from the opening. Also, in Examples 1 to 2, the bath surface was kept higher because the amount of the molten salt bath charged into the electrolytic cell 110 was larger than that in Comparative Example 1.

[0042] [Evaluation] (Current Efficiency) In Examples 1 to 2 and Comparative Example 1, the current efficiency of the 10-day molten salt electrolysis was determined by the following formula 1. A=(M1 / M0)×100 A: Current efficiency [%] M1: Mass of metallic magnesium recovered from the electrolytic cell [g] M0: Mass of theoretically generated metallic magnesium [g] Furthermore, the differences between the current efficiency calculated in Examples 1 to 2 and the current efficiency calculated in Comparative Example 1 were calculated respectively. These results are shown in Table 1.

[0043]

Table 1

[0044] (Consideration Based on Examples) In Examples 1 to 2, compared with Comparative Example 1, the current efficiency could be improved by 1.0% or more. The current efficiency of Example 1 was higher than that of Example 2. This is considered to be due to maintaining a higher bath surface position in the recovery chamber 140. From the above, in Examples 1 to 2, in the molten salt electrolysis, the height position of the bath surface of the molten bath in the recovery chamber was maintained within the range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell in the depth direction, and the bath surface level of the molten bath in the recovery chamber was appropriately managed to achieve a high current efficiency. This is because the bath surface of the molten bath was appropriately maintained at a high level, and due to the gas lift pump phenomenon, the molten salt bath passed through between the electrodes relatively quickly, so the re-reaction between magnesium metal and chlorine gas could be suppressed. As a result, it is considered that magnesium metal could be efficiently produced.

Explanation of Signs

[0045] 100, 200 Molten salt electrolysis apparatus 110 Electrolytic cell 111 Opening 112 First partition wall 113 Second partition wall 114 Bottom wall 115 Side wall 116 Flow port 117, 118, 217, 218 Pedestal 120 Cover 121 Inner surface of the cover 122 First gas recovery port 123 Second gas recovery port 124 Supply and discharge port 130 Electrolysis chamber 131, 231 Anode 131a, 152a, 156a Electrolysis reaction generation surface 135, 235 Cathode 136, 236 Extension part 140 Recovery chamber 151, 251 First bipolar (bipolar) 152, 156, 252, 256 Thickness part 152b, 156b, 252b, 256b, 153b, 157b, 253b, 257b Upper end surface 155, 255 Second bipolar 153, 157, 253, 257 Narrow part A, B Arrows MSB Molten salt bath MB Molten bath S1, S2 Bath surface

Claims

1. A method for producing metallic magnesium by electrolyzing magnesium chloride in a molten salt bath using an electrolytic cell having an electrolysis chamber and a recovery chamber, and electrodes including an anode, a bipolar electrode, and a cathode disposed in the electrolysis chamber, comprising: at least one of the bipolar electrodes has a thick portion having an electrolysis reaction generating surface and a thin portion disposed above the thick portion; the method for producing metallic magnesium includes maintaining, in the molten salt electrolysis, the height position of the molten bath surface in the recovery chamber within a range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell in the depth direction.

2. The method for producing metallic magnesium according to claim 1, wherein at least a part of the thin portion of the bipolar electrode is exposed on the molten salt bath surface in the electrolysis chamber while maintaining the height position of the bath surface in the recovery chamber within a range of 15 cm or more and 42 cm or less from the opening of the electrolytic cell.

3. The method for producing metallic magnesium according to claim 1 or 2, wherein the bipolar electrode has a constant thickness of the thin portion in the thickness direction and has a step between the thick portion and the thin portion.

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