Method for producing metallic magnesium

Abstract

To provide a method for producing metallic magnesium capable of performing molten salt electrolysis at a relatively low tank voltage.SOLUTION: A method for producing metallic magnesium includes an electrolysis step in which magnesium chloride in a molten salt bath is subjected to molten salt electrolysis in an electrolytic cell of a molten salt electrolysis apparatus to produce metallic magnesium. The molten salt bath contains magnesium chloride, sodium chloride, calcium chloride, and calcium fluoride by a total of 95 mass% or more, a content of sodium chloride is 51 mass% or more and 56 mass% or less, and a content of calcium fluoride is 3 mass% or less. The electrolytic step includes supplying sodium chloride per ton of molten salt bath per month at a rate within a range of 1 kg or more and 5 kg or less to maintain a ratio (NaCl / CaCl2 ratio) on a mass basis of sodium chloride to calcium chloride in the molten salt bath of 1.8 or more and 2.2 or less.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Titanium metal ingots and the like are industrially produced using titanium sponge produced by a method based on the Kroll process. The titanium sponge production process including the Kroll process can be broadly divided into four steps: a chlorination step, a reduction step, a crushing step, and an electrolysis step. Of these steps, in the reduction step, titanium tetrachloride is reduced with metallic magnesium to obtain a titanium sponge mass. In the electrolysis step, magnesium chloride, a by-product of the reduction step, is electrolyzed in a molten salt to obtain metallic magnesium. Technologies relating to molten salt electrolysis in the electrolysis step include those described in Patent Document 1, for example.

[0003] In a bipolar type molten salt electrolysis device that may be used in the electrolysis process, an anode, a bipolar electrode, and a cathode may be arranged in this order in the molten salt bath in the electrolysis cell from the viewpoint of production efficiency. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 4-214889 [Non-patent literature]

[0005] [Non-Patent Document 1] I. Karakaya and WT Thompson, “A THERMODYNAMIC STUDY OF THE SYSTEM MgCl2-NaCl-CaCl2”, Canadian Metallurgical Quarterly., 1986, Vol.25, No.4, p.307-317 [Non-Patent Document 2] T. Matsushima, and 2 others, “Density and Electrical Conductivity of Fused Magnesium Electrolyte,(I) Typical Magnesium Electrolyte with Additives of NaCl, KCl, and CaCl2”, Transactions of the Japan Institute of Metals, 1967, Vol.8, p.45-49 [Non-Patent Document 3] GOKHAN DEMIRCI, “ELECTROLYTIC MAGNESIUM PRODUCTION USING COAXIAL ELECTRODES”, A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, in molten salt electrolysis, it is believed that if a voltage equivalent to the decomposition voltage of magnesium chloride is applied to an electrolytic cell, the magnesium chloride will decompose to produce metallic magnesium.

[0007] However, in molten salt electrolysis, a cell voltage higher than the decomposition voltage is usually applied to the electrolytic cell. The reason for this is that the cell voltage is affected by not only the decomposition voltage, but also activation overvoltage, the bath resistance of the molten salt bath that contains supporting salts other than magnesium chloride, electrode resistance, etc. Since power is current multiplied by voltage (current x voltage), if the cell voltage in molten salt electrolysis can be kept low, metallic magnesium can be produced with less power consumption.

[0008] Therefore, an object of one embodiment of the present invention is to provide a method for producing metallic magnesium that can perform molten salt electrolysis at a relatively low cell voltage. [Means for solving the problem]

[0009] As a result of intensive research, the present inventors have found that, when a molten salt bath has a desired composition in a method for producing metallic magnesium, an electrolysis step of producing metallic magnesium by molten salt electrolysis of magnesium chloride in the molten salt bath includes supplying sodium chloride at a rate within a range of 1 kg or more and 5 kg or less per ton of molten salt bath per month, and maintaining a mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) of 1.8 or more and 2.2 or less, thereby making it possible to carry out molten salt electrolysis at a relatively low cell voltage. The present invention has been completed based on the above findings, and is exemplified below. [1] A method for producing metallic magnesium, comprising an electrolysis step of producing metallic magnesium by molten-salt electrolysis of magnesium chloride in a molten-salt bath in an electrolysis tank of a molten-salt electrolysis device, the molten salt bath contains magnesium chloride, sodium chloride, calcium chloride, and calcium fluoride in a total amount of 95% by mass or more, the sodium chloride content being 51% by mass or more and 56% by mass or less, and the calcium fluoride content being 3% by mass or less, The electrolysis step includes supplying sodium chloride at a rate within the range of 1 kg or more and 5 kg or less per ton of molten salt bath per month, and maintaining a mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) of 1.8 or more and 2.2 or less. [2] The method for producing metallic magnesium according to [1], wherein the content of the magnesium chloride in the molten salt bath is 15 mass% or more and 23 mass% or less. [3] The method for producing metallic magnesium according to [1] or [2], wherein solid sodium chloride is supplied in the electrolysis step. Effect of the Invention

[0010] According to one embodiment of the present invention, it is possible to provide a method for producing metallic magnesium in which molten salt electrolysis can be carried out at a relatively low cell voltage. [Brief description of the drawings]

[0011] [Figure 1] 1 is a front view showing an example of an internal structure of a molten salt electrolysis apparatus according to the present invention; [Diagram 2] FIG. 2 is an end view taken along line XX in FIG. [Diagram 3] 1 is a graph showing an estimated relationship between the mass ratio of sodium chloride to calcium chloride (NaCl / CaCl2 ratio) in a molten salt bath and the activity of magnesium chloride (bath temperature 660°C, magnesium chloride content 20 mass%). [Figure 4] 1 is a graph showing an estimated relationship between the mass ratio of sodium chloride to calcium chloride in a molten salt bath (NaCl / CaCl2 ratio) and the decomposition voltage (bath temperature: 660° C., magnesium chloride content: 20% by mass). [Diagram 5] 1 is a graph showing an estimated relationship between the mass ratio of sodium chloride to calcium chloride (NaCl / CaCl2 ratio) in a molten salt bath and electrical conductivity (bath temperature 660°C, magnesium chloride content 20 mass%). [Figure 6] FIG. 4 is a front view showing a schematic diagram of another example of the internal structure of a molten salt electrolysis apparatus according to the present invention. [Figure 7] FIG. 7 is an end view taken along line YY in FIG. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention is not limited to the embodiments described below, and the components can be modified without departing from the gist of the invention. Various inventions can be formed by appropriately combining the components disclosed in each embodiment. For example, an invention may be formed by deleting some components from all the components shown in the embodiments. In the drawings, some components are shown diagrammatically to aid in understanding the embodiments included in the invention, and the illustrated sizes, positional relationships, etc. may not be accurate. Furthermore, in this specification, "upward" means a direction from the bottom wall 113 side of the electrolytic cell 110 toward the top lid 130 side, for example, as shown by the arrows in Figures 1 to 2 and 6, and "downward" means a direction from the top lid 130 side toward the bottom wall 113 side of the electrolytic cell 110. Furthermore, in this specification, "molten metallic magnesium" means metallic magnesium in a molten state obtained by molten salt electrolysis of magnesium chloride.

[0013] [Method of manufacturing magnesium metal] In one embodiment, a method for producing metallic magnesium according to the present invention includes an electrolysis step of producing metallic magnesium by molten salt electrolysis of magnesium chloride in a molten salt bath stored in an electrolytic cell of a molten salt electrolysis apparatus.

[0014] For example, in the molten salt electrolysis apparatus 100 shown in Fig. 1 and Fig. 2, an anode 121, a first bipolar electrode 122, a second bipolar electrode 123, and a cathode 124 are arranged in this order in the molten salt bath Bf in the electrolysis cell 110, and in many cases, the anode and the cathode are arranged alternately. Considering that the decomposition voltage of magnesium chloride is about 2.75 V, the decomposition voltage in a configuration in which two bipolar electrodes 122, 123 are arranged between the anode 121 and the cathode 124 (n = 3 (anode-first bipolar electrode, first bipolar electrode-second bipolar electrode, second bipolar electrode-cathode)) is calculated to be about 8.25 V (= 2.75 V x 3). However, in the molten salt electrolysis, the actual value of the cell voltage of the conventional molten salt electrolysis apparatus 100 is about 10 to 11 V.

[0015] Here, when the mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) is increased, the decomposition voltage, which has the greatest effect on the cell voltage, tends to increase. According to this tendency, it would seem that the cell voltage would increase when the mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) is increased. However, unexpectedly, the cell voltage may decrease as a result. The tendency of the cell voltage is described in detail below.

[0016] First, the electromotive force E of an electrochemical cell is theoretically expressed by the Nernst equation. In the electrolysis of magnesium chloride, the Nernst equation is expressed as the following formula 1.

[0017]

number

[0018] In the electrolysis of magnesium chloride, "a" in the above formula 1 Mg " and "P Cl2 " is basically treated as 1. In order to calculate the electromotive force E using the above formula 1, the activity of magnesium chloride "a MgCl2 "You need to know.

[0019] Therefore, using the above formula 1, assuming that the decomposition voltage when electrolyzing is 2.75 V at a molten salt bath temperature of 660°C and the magnesium chloride content in the molten salt bath is 20% by mass, the activity of magnesium chloride is calculated to be 0.00758. This value is taken as the reference value for the NaCl / CaCl2 ratio of 1.65, and the relationship between the activity of magnesium chloride and the NaCl / CaCl2 ratio when electrolyzing is performed at a molten salt bath temperature of 660°C and the magnesium chloride content in the molten salt bath is 20% by mass was estimated by multiplying the value read from the ternary phase diagram (activity of magnesium chloride in ternary chloride diameter (800°C)) in Figure 11c of Non-Patent Document 1 by a constant. The estimation results are shown in Figure 3 and the following formula 2, respectively.

[0020]

number

[0021] As shown in Figure 3, the activity of magnesium chloride tends to decrease with an increase in the NaCl / CaCl2 ratio. Here, the relationship between the NaCl / CaCl2 ratio and the decomposition voltage was calculated using the estimated activity of magnesium chloride and the above equation 2. The results are shown in Figure 4.

[0022] As shown in Fig. 4, the decomposition voltage tends to increase gradually with an increase in the NaCl / CaCl2 ratio. From this tendency, it is expected that the bath voltage will increase with an increase in the NaCl / CaCl2 ratio. However, according to the knowledge of the present inventor, an increase in the NaCl / CaCl2 ratio simultaneously improves the electrical conductivity (reduces the bath resistance), and the effect of improving the electrical conductivity exceeds the effect of increasing the decomposition voltage, resulting in a decrease in the bath voltage. The improvement in electrical conductivity will be described in detail below.

[0023] First, Non-Patent Document 2 shows that when molten salt electrolysis of magnesium chloride is performed at a bath temperature of 660°C and a magnesium chloride content of 20 mass%, the electrical conductivity is 193.4 S / m when the NaCl / CaCl2 ratio is 1.67. Also, Non-Patent Document 3 shows the relationship between the calcium chloride content in the molten salt bath and electrical conductivity, and it can be understood that when the magnesium chloride content is 20 mass% and the sodium chloride and other supporting salts are equal in mass, the relationship of the following formula 3 holds.

[0024]

number

[0025] Here, the slope of the above formula 3 was fixed, and the relationship between the NaCl / CaCl2 ratio and the electrical conductivity was estimated when molten salt electrolysis of magnesium chloride was performed at a bath temperature of 660°C and a magnesium chloride content of 20 mass% based on the contents disclosed in Non-Patent Document 2. The estimation results are shown in Figure 5 and the following formula 4.

[0026]

number

[0027] From the above estimation results, the inventors estimated that the electrical conductivity increases with an increase in the NaCl / CaCl2 ratio. Furthermore, they estimated that the effect of increasing the electrical conductivity exceeds the effect of increasing the decomposition voltage, resulting in a decrease in the cell voltage. The inventor confirmed that the cell voltage decreases by appropriately increasing the NaCl / CaCl2 ratio, and after further investigation, completed the present invention. Note that the above assumption is merely a reference for completing the present invention, and therefore the present invention is not limited to using the above assumption. A preferred embodiment of the present invention will now be described.

[0028] (Bath composition) When the entire molten salt bath is taken as 100 mass%, the molten salt bath Bf contains 95 mass% or more of magnesium chloride (MgCl2), sodium chloride (NaCl), calcium chloride (CaCl2), and calcium fluoride (CaF2). Of course, the molten salt bath Bf may be formed from magnesium chloride, sodium chloride, calcium chloride, and calcium fluoride, but may also contain a small amount of other metal chloride components, such as potassium chloride (KCl).

[0029] In addition, when the entire molten salt bath is taken as 100 mass%, the sodium chloride content of the molten salt bath is 51 mass% or more and 56 mass% or less. Within the above range, the cell voltage is lowered, and the power consumption in the electrolysis of magnesium chloride can be reduced. If the sodium chloride content in the molten salt bath becomes too high, the NaCl / CaCl2 ratio may become too high as a result, and the liquid phase temperature of the molten salt bath Bf may become higher than the melting point of metallic magnesium. In this case, the molten salt electrolysis may be performed at a temperature significantly higher than the melting point of metallic magnesium, which may cause a re-reaction between metallic magnesium and chlorine gas, resulting in problems such as a decrease in current efficiency. The upper limit of the sodium chloride content is, for example, 55% by mass or less, or 54% by mass or less.

[0030] Furthermore, when the entire molten salt bath is taken as 100 mass%, the molten salt bath contains 3 mass% or less of calcium fluoride. If the molten salt bath contains even a small amount of calcium fluoride, the oxidation of the molten salt bath Bf can be suppressed. It is considered that the oxidation produces magnesium oxide (MgO). It is considered that magnesium oxide in the molten salt bath inhibits the aggregation of metallic magnesium droplets, which makes it difficult for fine metallic magnesium droplets to float and become difficult to recover. Furthermore, it is considered that the metallic magnesium droplets that remain fine and have a large specific surface area in contact with the molten salt bath increase their dissolution rate in the molten salt bath and increase the amount of re-reaction, which may result in a decrease in current efficiency. Therefore, it is advantageous to set the content of calcium fluoride in the molten salt bath within the above range from the viewpoint of improving current efficiency. The content of calcium fluoride may be, for example, 2 mass% or less. The lower limit of the content of calcium fluoride is, for example, 1 mass% or more. However, if the calcium fluoride content in the molten salt bath exceeds 3 mass%, the fluoride ions (F - ) may cause corrosion of bricks and other materials.

[0031] Incidentally, by containing a certain amount of calcium chloride, the density of the molten salt bath increases, and the metallic magnesium produced by molten salt electrolysis becomes more likely to float in the metal recovery chamber.

[0032] (Sodium chloride supply) In one embodiment, during operation of the molten salt electrolysis apparatus, sodium chloride is supplied to the molten salt bath at a rate within a range of 1 kg or more and 5 kg or less per ton of the molten salt bath per month, and the mass-based ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) is maintained at 1.8 or more and 2.2 or less. For example, when the temperature of the molten salt bath is operated within a range of 651°C or more and 670°C or less, sodium chloride continues to evaporate, although slightly. That is, if the operation is continued, sodium chloride evaporates, and the mass-based ratio of sodium chloride and calcium chloride in the molten salt bath changes, and as a result, the ratio may become lower than the predetermined range. If sodium chloride is not appropriately supplied to the molten salt bath, the balance of the desired composition of the molten salt bath is lost, and the calcium chloride content becomes excessively high. As a result, there is a problem that the cell voltage becomes high. Therefore, the molten salt electrolysis is continued while supplying sodium chloride continuously or intermittently at the above-mentioned rate. If the NaCl / CaCl2 ratio becomes excessively high, the current efficiency may decrease as described above. The NaCl / CaCl2 ratio is preferably 1.9 or more as a lower limit. The NaCl / CaCl2 ratio is preferably 2.1% or less as an upper limit. The amount of sodium chloride supplied per ton of molten salt bath per month can be, for example, in the range of 1 kg or more and 4 kg or less, or, for example, in the range of 1 kg or more and 3 kg or less. The total amount of the molten salt bath in the electrolytic cell 110 can be calculated by multiplying the volume of the molten salt bath Bf in the electrolytic cell 110 by the density of the molten salt bath. In addition, when the content of magnesium chloride in the molten salt bath Bf is maintained within a certain range and molten salt electrolysis is continued, the total amount of the molten salt bath introduced into the electrolytic cell 110 can be grasped at the start of operation.

[0033] The means for supplying sodium chloride to the molten salt bath is not particularly limited. For example, solid sodium chloride may be supplied. This is because the amount of sodium chloride supplied is small relative to the amount of the molten salt bath. The sodium chloride supplied in solid form melts in the molten salt bath. In this case, it is not necessary to supply the sodium chloride as a molten material to the molten salt bath as in the case of supplying magnesium chloride described later, so that the supply of solid sodium chloride can be carried out by a simple operation. Sodium chloride may be supplied to the metal recovery chamber. Since chlorine gas continues to be generated in the electrolysis chamber, it is preferable to supply sodium chloride to the metal recovery chamber from the viewpoint of operational safety. The timing of supplying sodium chloride may be, for example, directly to the molten salt bath immediately after recovering the molten magnesium metal. At this timing, the bath surface of the metal recovery chamber is the molten salt, and the desired amount of sodium chloride is supplied to the molten salt bath, making it easy to maintain the desired composition balance of the molten salt bath.

[0034] (Magnesium chloride supply) Furthermore, from the viewpoint of realizing high current efficiency, the content of magnesium chloride in the molten salt bath in the electrolytic cell is preferably 15% by mass or more and 23% by mass or less. The lower limit of the content of magnesium chloride is, for example, 16% by mass or more. The upper limit of the content of sodium chloride is, for example, 21% by mass or less, or 19% by mass or less. If the content of magnesium chloride is too low, the efficiency of electrolysis decreases, and if the content of magnesium chloride is too high, the generated metallic magnesium and chlorine gas are likely to come into contact with each other, promoting re-reaction, which may result in a decrease in current efficiency. If the molten salt electrolysis of magnesium chloride is continued, the content of magnesium chloride in the molten salt bath decreases, so that molten magnesium chloride is replenished in the molten salt bath at appropriate timing. If this magnesium chloride is supplied in a batch manner and a relatively large amount of magnesium chloride is replenished to the molten salt bath each time, the content of magnesium chloride in the molten salt bath may change significantly during operation. In this case, if the content can be kept within the above content range, high current efficiency can be realized. If magnesium chloride is supplied continuously or semi-continuously, the above-mentioned magnesium chloride content can be achieved relatively easily. The supply means of magnesium chloride is not particularly limited, and may be, for example, a known means. As an example of the supply means, if a magnesium chloride supply device described in FIG. 1 of JP 2022-11141 A is used, the pressure in the storage tank can be adjusted by the pressure regulator to intermittently supply magnesium chloride to the molten salt bath in the electrolytic cell.

[0035] (Temperature of molten salt bath) The temperature of the molten salt bath is appropriately adjusted depending on the composition of the molten salt bath, but from the viewpoint of suppressing re-reaction between the molten metallic magnesium produced by molten salt electrolysis and chlorine gas, it is preferably set to, for example, 670° C. or less, for example, 665° C. or less, or for example, 660° C. or less. From the viewpoint of avoiding solidification of metallic magnesium, the temperature of the molten salt bath is preferably, for example, 651° C. or more, for example, 653° C. or more, or for example, 655° C. or more. When molten magnesium chloride is replenished to the molten salt bath in a batchwise manner, the temperature of the molten salt bath rises transiently, and therefore the transient temperature rise of the molten salt bath due to the replenishment of magnesium chloride is not considered as the temperature of the molten salt bath.

[0036] (Example of a molten salt electrolysis device) The molten salt electrolysis apparatus 100 shown in Fig. 1 and Fig. 2 includes an electrolytic cell 110 and an upper cover 130. The electrolytic cell 110 is partitioned into an electrolytic chamber 120 and a metal recovery chamber 140 by the presence of a first partition wall 111 and a second partition wall 112 described later. Furthermore, an anode 121, a first bipolar electrode 122, a second bipolar electrode 123, and a cathode 124 are arranged in this order in the electrolytic chamber 120 of the electrolytic cell 110, and the anode 121, the first bipolar electrode 122, the second bipolar electrode 123, and the cathode 124 are immersed in the molten salt bath Bf stored in the electrolytic cell 110. The direction in which the anode 121, the bipolar electrodes 122 and 123, and the cathode 124 are arranged in a direction perpendicular to the up-down direction (horizontal direction in this example) is referred to as the arrangement direction. In the arrangement direction, anodes 121 and cathodes 124 are often arranged alternately in multiples. In the illustrated electrolysis chamber 120, two bipolar electrodes 122 and 123 are arranged between the anode 121 and the cathode 124, but it is sufficient to arrange at least one bipolar electrode. Depending on the configuration of the electrolysis cell 110, there may be cases where no bipolar electrode is provided.

[0037] (electrolytic cell) The electrolytic cell 110 has a container shape with an opening formed on the upper side, and is made of, for example, firebricks mainly containing aluminum oxide or other suitable material. The electrolytic cell 110 is composed of a bottom wall 113 and two pairs of side walls 114 connected to the bottom wall 113 and extending upward. The electrolytic cell 110 stores a molten salt bath Bf inside. The metallic magnesium produced by the molten salt electrolysis of magnesium chloride in the electrolytic chamber 120 is sent to the metal recovery chamber 140 by the circulation of the molten salt bath, and the molten salt is sent from the metal recovery chamber 140 to the electrolytic chamber 120 to circulate the molten salt bath. Here, the molten salt electrolysis apparatus 100 has a flow port 115 formed between the first partition wall 111 and the second partition wall 112, thereby ensuring the flow of the molten salt bath (flow from the electrolytic chamber 120 to the metal recovery chamber 140) indicated by the arrow A. Also, a passage through which the molten salt bath can flow is formed on the lower surface side of the second partition wall 112, so that the flow of the arrow B (flow from the metal recovery chamber 140 to the electrolysis chamber 120) can be ensured.

[0038] (molten salt) Molten salt electrolysis of magnesium chloride produces metallic magnesium (Mg) as a molten metal, and produces chlorine gas (Cl2) as a gas. The molten salt contains sodium chloride (NaCl), calcium chloride (CaCl2), and calcium fluoride (CaF2) as supporting salts, in addition to the above-mentioned magnesium chloride (MgCl2). The above-mentioned supporting salt may contain other components, such as potassium chloride (KCl), in small amounts. The supporting salts usually have a higher decomposition voltage than magnesium chloride. Metallic magnesium can be used for the reduction of titanium tetrachloride in the Kroll process for producing metallic titanium, and chlorine gas can be used for the chlorination of titanium ore. Magnesium chloride, which is a by-product of the Kroll process, can be used as a raw material for molten salt electrolysis.

[0039] (Top lid) The top lid 130 serves to insulate the electrolytic cell 110 from the outside because the molten salt bath Bf is at a high temperature. In addition, by disposing the top lid 130, the electrolytic cell 110 becomes a closed space, and the inside of the electrolytic cell 110 is made negative pressure with respect to the outside in order to prevent leakage of chlorine gas generated from the anode 121 during molten salt electrolysis. The material of the upper lid 130 is not particularly limited, but from the viewpoint of preventing a short circuit occurring between the upper lid 130 and the anode during molten salt electrolysis, the lid back surface 131 side of the upper lid 130 on the molten salt bath Bf side may be made of an insulating material, and a ceramic material may be disposed on the lid back surface 131 side of the upper lid 130 on the molten salt bath Bf side, or a castable refractory material may be applied. The method of providing this castable refractory material may be a known method, and for example, the castable refractory may be applied to the lid back surface 131 side by dry spraying, wet spraying, or the like.

[0040] The top cover 130 may be provided with a first gas recovery port 132, a second gas recovery port 133, and a supply / discharge port 134. There may be one or more of each of these ports. The first gas recovery port 132 is used to recover chlorine gas produced by electrolysis of magnesium chloride in the electrolysis chamber 120. The first gas recovery port 132 is provided in the region where the electrolysis chamber 120 is located. The second gas recovery port 133 is provided in an area where the metal recovery chamber 140 is located. The second gas recovery port 133 may be used to recover the remaining gas generated by electrolysis that has not been recovered in the first gas recovery port 132 and has flowed into the metal recovery chamber 140. The supply / discharge port 134 is used to recover molten metallic magnesium produced by electrolysis of magnesium chloride in the electrolysis chamber 120, and to supply magnesium chloride and sodium chloride to the electrolysis cell 110. The supply / discharge port 134 is provided in an area where the metal recovery chamber 140 is located.

[0041] (Electrolysis chamber) In the electrolysis chamber 120, magnesium chloride is subjected to molten salt electrolysis, and molten magnesium metal and chlorine gas are generated by the molten salt electrolysis. In the electrolysis chamber 120, the electrolysis surfaces of the anode 121, the first bipolar electrode 122, the second bipolar electrode 123, and the cathode 124 are arranged so as to be approximately parallel to the depth direction of the molten salt bath Bf (the vertical direction in FIG. 2).

[0042] The anode 121 is inserted through the upper lid 130, extends downward, and is disposed so that a part of the anode 121 is immersed in the molten salt bath Bf. The shape of the anode 121 is not particularly limited, and examples of the shape include a plate shape, a cylindrical shape, and a rectangular column shape. From the viewpoint of the production efficiency of the molten metallic magnesium, the molten salt electrolysis apparatus 100 may include a plurality of anodes 121 and cathodes 124. The material of the anode 121 is not particularly limited, but examples thereof include graphite. The material of the cathode 124 is not particularly limited, but examples thereof include graphite and carbon steel. The material of the bipolar electrode is not particularly limited, but may be, for example, graphite. The anode 121 and the cathode 124 are connected to a power source via a bus bar, a conductive wire, or the like.

[0043] The first bi-pole 122 and the second bi-pole 123 may be disposed on pedestals 116, 117, respectively, made of, for example, firebrick.

[0044] 1, the cathode 124 has an extension portion 125 extending outward, and this extension portion 125 is disposed so as to penetrate the side wall 114 and protrude to the outside of the electrolytic cell 110. The shape of the cathode 124 may be a plate shape, but this can be appropriately changed in consideration of the shape of the anode 121, etc., and may be a square tube shape, a cylindrical shape, etc. Even in this case, the cathode 124 has the extension portion 125. 6 and 7, the anode 221, the first bipole 222, the second bipole 223, and the cathode 224 are arranged in this order from the anode 221 along the arrangement direction (in the direction away from the anode 221 in the illustrated embodiment). More specifically, the rectangular cylindrical first bipole 222 is arranged on the pedestal 216 so as to surround the periphery of the anode 221 and be spaced apart from the anode 221, the rectangular cylindrical second bipole 223 is arranged on the pedestal 217 so as to surround the periphery of the first bipole 222 and be spaced apart from the first bipole 222, and the rectangular cylindrical cathode 224 is arranged so as to surround the periphery of the second bipole 223 and be spaced apart from the second bipole 223. The cathode 224 further has an extension portion 225 extending outward from a part of the rectangular cylindrical shape, and this extension portion 225 is disposed so as to penetrate the side wall 114 and protrude to the outside of the electrolytic cell 110 .

[0045] The inter-electrode distances between the anode and bipolar electrode, between the cathode and bipolar electrode, and between the bipolar electrodes may be the same or different.

[0046] (Metal Recovery Room) In the metal recovery chamber 140, the molten metal produced by electrolysis in the electrolysis chamber 120 is recovered. The metal recovery chamber 140 is in communication with the electrolysis chamber 120 and may have a heat exchanger (not shown). The heat exchanger can adjust the temperature of the molten salt bath Bf in the electrolysis cell 110. The heat exchanger may be configured to include an inlet for flowing a fluid therein, an outlet for discharging the fluid, and a pipe connecting the inlet and the outlet. The pipe may be made of steel (carbon steel or stainless steel). In another embodiment, the molten salt electrolysis apparatus may further include a heat exchange chamber (not shown) having a heat exchanger (not shown) in addition to the electrolysis chamber and the metal recovery chamber arranged laterally. For example, the molten metal can be recovered using the supply and discharge port of the metal recovery chamber, and the molten salt such as molten magnesium chloride can be replenished to the heat exchange chamber. As a result, the molten metal can be stored more stably in the metal recovery chamber. In addition to these, the heat exchange chamber may further include a stirrer (not shown) for stirring the molten salt bath.

[0047] (collect) After the molten metallic magnesium flows into the metal recovery chamber, the molten metallic magnesium, which has a smaller specific gravity than the molten salt, floats up to and accumulates in a shallow portion of the metal recovery chamber 140. The molten metallic magnesium that floats up in the metal recovery chamber 140 can be recovered by inserting a recovery pipe or the like into the supply / discharge port 134. EXAMPLES

[0048] The present invention will be specifically described based on examples and comparative examples. The following examples and comparative examples are merely experimental examples for facilitating understanding of the technical content of the present invention, and the technical scope of the present invention is not limited by these examples.

[0049] [Example 1] (Preparation for operation of molten salt electrolytic cell) In Example 1, a molten salt electrolysis apparatus 100 having the configuration shown in Fig. 1 and Fig. 2 was used. In the molten salt electrolysis apparatus 100, the electrolytic cell 110, the first partition wall 111, the second partition wall 112, and the pedestals 116 and 117 were made of standard refractory materials (firebricks) containing aluminum oxide, the upper lid 130 was made of carbon steel, and a layer of insulating castable refractory material was applied to the lid back surface 131 of the upper lid 130. In addition, as electrodes, an anode 121 made of graphite and a cathode 124 made of carbon steel were arranged, and two bipolar electrodes 122 and 123 made of graphite were arranged between the anode 121 and the cathode 124 (i.e., n=3 (anode 121-first bipolar electrode 122, first bipolar electrode 122-second bipolar electrode 123, second bipolar electrode 123-cathode 124)). All the electrodes were plate-shaped and arranged so that the electrodes were spaced apart at equal intervals. Next, molten salt was introduced into the molten salt electrolysis apparatus 100, and the temperature of the molten salt was adjusted to within the range of 653°C or more and 670°C or less. The composition of the molten salt introduced was 20 mass% magnesium chloride-54 mass% sodium chloride-25 mass% calcium chloride-1 mass% calcium fluoride, as shown in Table 1. At this time, the mass-based ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) was 2.1.

[0050] (Production of metallic magnesium) Molten salt electrolysis was carried out by supplying a current between the anode and the cathode from a power source via a conductive wire. As shown in Table 1, during the electrolysis, magnesium chloride was supplied once every 3 to 6 hours, and the magnesium chloride content in the molten salt bath in the electrolytic cell was maintained at 15 mass% or more and 21 mass% or less. In addition, sodium chloride was supplied at a rate of 2 kg per ton of molten salt bath over a one-month period, so that the mass-based ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) was maintained at 2.1. The sodium chloride was supplied to the molten salt bath in solid form. Then, 36 months after the start of the molten salt electrolysis, the supply of electric current was stopped.

[0051] [evaluation] (cell voltage) The cell voltage was checked during operation of the molten salt electrolysis apparatus 100. The cell voltage was calculated as a relative value of the cell voltage in Example 1, with the cell voltage in Comparative Example 1 described below being taken as 100% (reference). The results are shown in Table 2.

[0052] [Comparative Example 1] In Comparative Example 1, as shown in Table 1, the composition of the molten salt was 20 mass% magnesium chloride-49 mass% sodium chloride-30 mass% calcium chloride-1 mass% calcium fluoride, and the mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) was 1.6, but molten salt electrolysis was performed for 36 months in the same manner as in Example 1. Then, the cell voltage was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0053] [Comparative Example 2] In Comparative Example 2, as shown in Table 1, molten salt electrolysis was carried out for 36 months in the same manner as in Example 1, except that sodium chloride was not supplied until the 10th month after the start of operation. That is, in Comparative Example 2, sodium chloride was supplied from the 11th month after the start of operation onwards in the same manner as in Example 1. Then, the cell voltage was evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0054] [Table 1]

[0055] [Table 2]

[0056] (Considerations based on examples) In Example 1, unlike Comparative Examples 1 and 2, the molten salt bath in the molten salt electrolysis was made to have the desired composition, sodium chloride was supplied at a rate within the range of 1 kg or more and 5 kg or less per ton of molten salt bath over a one-month period, and the mass-based ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl2 ratio) was maintained at 1.8 or more and 2.2 or less, making it possible to perform molten salt electrolysis at a relatively low cell voltage. Assuming that 186 tons of metallic magnesium is produced per month, a 1% decrease in cell voltage can be said to have a saving effect of approximately 20 MWh in terms of electricity. Since this figure is for one electrolytic cell, it is expected that this effect will increase if multiple electrolytic cells are installed and operated simultaneously. In addition, since this figure is for one month of electrolytic cell operation, it is expected that this effect will increase if the operation period of the electrolytic cell is extended. [Explanation of symbols]

[0057] 100, 200 Molten salt electrolysis equipment 110 Electrolytic cell 111 First Bulkhead 112 Second bulkhead 113 Bottom wall 114 Side wall 115 Distribution port 116, 117, 216, 217 Pedestal 120 Electrolysis chamber 121, 221 Anode 122, 222 First double pole (double pole) 123, 223 Second bipolar (bipolar) 124, 224 cathode 125, 225 extension part 130 Top lid 131 Back of the lid 132 First gas recovery port 133 Second gas recovery port 134 Supply / discharge port 140 Metal Recovery Room A, B arrows Bf molten salt bath

Claims

1. A method for producing metallic magnesium, comprising an electrolysis step of producing metallic magnesium by molten-salt electrolysis of magnesium chloride in a molten-salt bath in an electrolysis tank of a molten-salt electrolysis device, the molten salt bath contains magnesium chloride, sodium chloride, calcium chloride, and calcium fluoride in a total amount of 95% by mass or more, the sodium chloride content being 51% by mass or more and 56% by mass or less, and the calcium fluoride content being 3% by mass or less, The electrolysis step is performed by supplying sodium chloride at a rate of 1 kg or more and 5 kg or less per ton of the molten salt bath per month, and adjusting the mass ratio of sodium chloride to calcium chloride in the molten salt bath (NaCl / CaCl 2 maintaining a ratio of 1.8 or more to 2.2 or less.

2. 2. The method for producing metallic magnesium according to claim 1, wherein the content of the magnesium chloride in the molten salt bath is 15 mass % or more and 23 mass % or less.

3. The method for producing metallic magnesium according to claim 1 or 2, wherein solid sodium chloride is supplied in the electrolysis step.

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