JP7494106B2 - Molten salt electrolysis apparatus and method for producing metallic magnesium - Google Patents

Description

This invention relates to a molten salt electrolysis device that includes an electrolytic cell and electrodes including an anode and a cathode, in which the cathode has a cylindrical portion that surrounds the anode, and to a method for producing metallic magnesium.

In molten salt electrolysis, a molten salt bath is generally formed by storing a specific amount of molten salt in the internal space surrounded by the peripheral and bottom walls of an electrolytic cell. When carrying out molten salt electrolysis, a voltage is applied between an anode and a cathode immersed in the molten salt bath to electrolyze the molten salt. This type of molten salt electrolysis is used, for example, in the electrolysis of magnesium chloride to produce metallic magnesium from magnesium chloride, which is produced as a by-product in the production of titanium sponge by the Kroll process.

For example, in the electrolysis of magnesium chloride using a molten salt electrolysis device in which the internal space of the electrolytic cell is divided into a recovery chamber and an electrolysis chamber by a partition, the molten salt containing magnesium chloride in the molten salt bath flows from the recovery chamber to the electrolysis chamber in the electrolysis cell, where the magnesium chloride is electrolyzed to produce metallic magnesium and chlorine. The metallic magnesium produced in the electrolysis chamber is further circulated to the recovery chamber in the electrolysis cell, where it floats to the surface of the molten salt bath due to the density difference with the molten salt, and is then collected. The chlorine is discharged to the outside of the electrolysis cell through a gas exhaust passage or the like that may be provided in the electrolysis cell. The metallic magnesium obtained in this way can be used to produce titanium sponge by the Kroll process.

In such a molten salt electrolysis device, the electrodes include a plate-shaped or columnar anode and a cathode having a cylindrical portion surrounding the anode, as described in Patent Document 1, for example. The electrodes may further include one or more cylindrical bipolar electrodes disposed between the anode and the cathode. In addition to the cylindrical portion, the cathode has an extension portion that extends from a part of the cylindrical portion, penetrates the peripheral wall, and protrudes to the outside of the electrolytic cell, and this extension portion is connected to a power source outside the electrolytic cell.

In more detail, Patent Document 1 discloses a molten salt electrolytic cell having a metal recovery chamber and an electrolysis chamber, and two or more electrolysis cell units in the electrolysis chamber, the electrolysis cell unit including a cathode having a prismatic space, a prismatic anode, and at least one rectangular cylindrical bipolar electrode, the bipolar electrode being disposed in the inner space of the cathode, and the anode being disposed in the inner space of the bipolar electrode, each flat surface forming the outer side of the rectangular cylinder of the bipolar electrode closest to the cathode faces at least a portion of the flat surface forming the prismatic space of the cathode, each flat surface forming the inner side of the rectangular cylinder of the bipolar electrode closest to the anode faces at least a portion of the flat surface forming the rectangular cylinder of the anode, and at least one surface of the cathode becomes one surface of the cathode of another electrolysis cell unit.

International Publication No. 2017/018441

However, when electrolysis is continued using the above-mentioned molten salt electrolysis device, the surrounding walls made of firebricks or the like that are erected around the bottom wall undergo thermal expansion, and the components of the molten salt bath penetrate into the walls, causing the bricks to deteriorate. These phenomena are particularly noticeable in the rear wall that is located near the electrode. As a result, the surrounding walls, including the rear wall, may gradually expand in a manner that points the cathode upward.

In this case, the cathode may be displaced upwards due to the expansion of the rear wall, etc., at a portion of the cylindrical portion that is located away from the rear wall, relative to the extension portion that penetrates the peripheral wall of the electrolytic cell and is fixed outside the electrolytic cell. When the cathode is displaced in this way, the portion of the cylindrical portion that is adjacent to the peripheral wall of the rear wall, etc., approaches the bipolar or anode placed inside, particularly at the lower end side in the depth direction of the electrolytic cell. This causes the portion of the cylindrical portion of the cathode to come into contact with the bipolar or anode, causing a short circuit between the cathode and the bipolar or anode.

The object of this invention is to provide a molten salt electrolysis device and a method for producing metallic magnesium that can suppress the occurrence of short circuits between electrodes.

The molten salt electrolysis device of this invention comprises an electrolytic cell having a peripheral wall and a bottom wall that define an internal space, and electrodes including an anode and a cathode that are disposed in the internal space of the electrolytic cell, the cathode having a cylindrical portion that surrounds the anode and an extension portion that extends from a part of the cylindrical portion, penetrates the peripheral wall, and protrudes to the outside of the electrolytic cell, and the portion of the cylindrical portion that is adjacent to the peripheral wall is composed of a movable cathode member on the peripheral wall side that can be displaced toward and away from the peripheral wall.

In the molten salt electrolysis device of the present invention, it is preferable that the peripheral wall includes a rear wall adjacent to the electrode, and the movable negative electrode member on the peripheral wall side is a portion of the cylindrical portion located adjacent to the rear wall, and is a movable negative electrode member on the rear wall side that can move toward and away from the rear wall.

In the molten salt electrolysis device of the present invention, it is preferable that the electrolytic cell has a partition wall in the internal space that defines an electrolytic chamber in which the electrodes are placed, the cathode is made of steel, and the portion of the cylindrical portion adjacent to the partition wall is composed of a movable cathode member on the partition wall side that can be displaced toward and away from the partition wall.

In the molten salt electrolysis device of the present invention, it is preferable that the cylindrical portion includes a pair of fixed cathode members each extending in a direction perpendicular or inclined to the movable cathode member and positioned opposite each other across the anode, at least one of the fixed cathode members is continuous with the extension portion, and the movable cathode member is arranged so as to be electrically connectable to each of the fixed cathode members.

In this case, it is preferable that a groove portion is formed on the surface of the fixed negative electrode member facing the movable negative electrode member, the groove portion having a groove width wider than the width of the end portion of the movable negative electrode member, the end portion of the movable negative electrode member being inserted into the groove portion, and the movable negative electrode member is configured to be able to move toward and away from the fixed negative electrode member along the groove width direction.

In this case, it is also preferable that a stopper member that fixes the movable negative electrode member at the position where the approaching or separating displacement is made is removably disposed between the inner surface of the groove portion of the fixed negative electrode member and the surface near the end of the movable negative electrode member.

In addition, it is preferable that a groove-shaped recess through which the stopper member passes is formed on at least one of the surfaces of the movable negative electrode member near the end that contacts the stopper member.

In the molten salt electrolysis apparatus of the present invention, it is preferable that the peripheral wall of the electrolytic cell or the cylindrical portion of the cathode has a mounting surface that supports the movable cathode member on the peripheral wall side from below.

In the molten salt electrolysis apparatus of the present invention, the electrodes may further include one or more cylindrical bipolar electrodes disposed between the anode and the cylindrical portion of the cathode.

The method for producing metallic magnesium of the present invention involves producing metallic magnesium from magnesium chloride using any of the above-mentioned molten salt electrolysis devices.

The molten salt electrolysis device of this invention can prevent short circuits from occurring between the electrodes.

1 is a cross-sectional view taken along the depth direction of an electrolytic cell, showing a molten salt electrolysis apparatus according to one embodiment of the present invention. 2 is a cross-sectional view taken along line II-II in FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2, showing the manner in which the cathode is displaced and deformed as the rear wall expands upward. 4 is a cross-sectional view showing the positional relationship between the cathode and the bipolar electrode when the cathode in FIG. 3 is displaced and deformed. FIG. 1 , showing the displacement of a movable cathode member of a cylindrical portion of a cathode provided in the molten salt electrolysis apparatus of FIG. 1 . 3 is a partially enlarged cross-sectional view of FIG. 2 , showing a connection mode between a movable cathode member and a fixed cathode member of a cylindrical portion of a cathode provided in the molten salt electrolysis apparatus of FIG. 1 , and another example of the connection mode. 6(a) is a cross-sectional view taken along line VI-VI in FIG. FIG. 4 is a cross-sectional view similar to FIG. 3 , showing a cathode and a rear wall of a molten salt electrolysis apparatus according to another embodiment.

Hereinafter, an embodiment of the present invention will be described in detail.
As shown in FIG. 1 , a molten salt electrolysis apparatus 1 according to one embodiment of the present invention includes an electrolytic cell 2 having a peripheral wall 2b and a bottom wall 2c which define an internal space 2a, and electrodes 3 which are disposed in the internal space 2a of the electrolytic cell 2 and include an anode 3a and a cathode 3b.

Here, the electrolytic cell 2, which is made of _firebricks such as _Al2O3 or other suitable material, has a bottom wall 2c and a peripheral wall 2b standing around the entire periphery of the bottom wall 2c. The molten salt electrolysis apparatus 1 further includes a lid member 4 for covering an upper opening of the electrolytic cell 2. Inside the electrolytic cell 2, an internal space 2a is defined which is surrounded by the bottom wall 2c, the peripheral wall 2b and the lid member 4. To perform molten salt electrolysis, molten salt is stored in the internal space 2a to form a molten salt bath.

In this embodiment, the electrolytic cell 2 has a partition wall 5 provided in the internal space 2a parallel to the peripheral wall 2b through which the extension portion 13b of the cathode 3b described below penetrates, and the partition wall 5 divides the internal space 2a into an electrolysis chamber 2f located on the right side of FIG. 1 where electrolysis is performed, and a recovery chamber 2e into which the molten metal, which is the product of electrolysis in the electrolysis chamber 2f, flows and accumulates on the upper side due to the density difference with the molten salt. In the illustrated example, the partition wall 5 is disposed close to the lid member 4. As a result, a molten salt circulation path 5a that enables the movement of the molten salt bath from the recovery chamber 2e to the electrolysis chamber 2f is formed between the partition wall 5 and the bottom wall 2c of the electrolytic cell 2. In addition, the molten metal flow path 5b provided in the partition wall 5 enables the molten metal to flow from the electrolysis chamber 2f to the recovery chamber 2e. The partition wall 5 can be appropriately modified in terms of its shape, number, and other configurations as long as it is possible to provide the molten salt circulation path 5a and the molten metal flow path 5b.

The portion of the peripheral wall 2b around the internal space 2a that is located on the opposite side of the partition wall 5 across the electrolytic chamber 2f and adjacent to the electrode 3 is referred to as the rear wall 2d. Here, the main focus is on the rear wall 2d among the peripheral walls 2b, and the rear wall 2d is sometimes referred to as the peripheral wall 2b. In addition to the rear wall 2d that faces the partition wall 5 on the electrolytic chamber 2f side, the peripheral wall 2b may also include a front wall 2h (see FIG. 1) that faces the partition wall 5 on the recovery chamber 2e side, and a side wall 2g (see FIG. 2) that extends in the front-rear direction to connect the rear wall 2d, the partition wall 5, and the front wall 2h at their ends. The front-rear direction refers to the direction in which the rear wall 2d, the partition wall 5, and the front wall 2h are lined up (the left-right direction in FIGS. 1 and 2), with the recovery chamber 2e side being the front side and the electrolytic chamber 2f side being the rear side.

Here, the electrode 3 includes a plate-like or column-like anode 3a that extends through the cover member 4 and into and out of the electrolytic cell 2, and a cathode 3b having a cylindrical portion 13a that surrounds the anode 3a and is spaced apart from the anode 3a, as can be seen from the cross-sectional view shown in FIG. 2. The cylindrical portion 13a of the cathode 3b can be a square cylindrical shape, such as a rectangular cylindrical shape, in which both the inner and outer contour shapes in a plan view are rectangular or other quadrangular shapes, as in the example shown in the figure. The cathode 3b further has an extension portion 13b that extends outward from a part of the cylindrical portion 13a, and this extension portion 13b is arranged so as to penetrate the rear wall 2d, which is a part of the peripheral wall 2b, and protrude to the outside of the electrolytic cell 2. The electrode 3 is connected to a power source (not shown) at the points of the extension portion 13b of the cathode 3b and the anode 3a that are located outside the electrolytic cell 2.

The electrode 3 can perform electrolysis of the molten salt in the molten salt bath as long as it has at least an anode 3a and a cathode 3b. On the other hand, from the viewpoint of improving the efficiency of producing metallic magnesium by electrolysis, it is preferable to further have one or more, for example two, cylindrical bipolar electrodes 3c that are not connected to a power source and are polarized by applying a voltage between the anode 3a and the cathode 3b, as shown in Figures 1 and 2. However, such a cylindrical bipolar electrode 3c is not essential. The electrode 3 including the anode 3a and the cathode 3b, and in some cases the cylindrical bipolar electrode 3c, can be a single set, but multiple sets can be arranged side by side as shown in Figure 2. In this case, one fixed cathode member 23b may be used as a component of each of the cylindrical portions 13a of the two adjacent cathodes 3b. Details of the cylindrical portion 13a and the fixed cathode member 23b will be described later.

In addition, although not shown, the molten salt electrolysis device 1 may further include a temperature adjustment tube or the like arranged in the recovery chamber 2e or the like as a heat exchanger for adjusting the temperature of the molten salt bath.

In such a molten salt electrolysis apparatus 1, the cylindrical portion 13a and the extension portion 13b of the cathode 3b are arranged to surround, for example, the inner surface of the rear wall 2d of the peripheral wall 2b, so that the rear wall 2d is less likely to come into contact with the molten salt circulating in the recovery chamber 2e and the electrolysis chamber 2f in the molten salt bath. As a result, in this type of molten salt electrolysis apparatus 1, compared with an apparatus in which both plate-shaped anodes and cathodes are arranged side by side, the elution of aluminum from the rear wall 2d made of _Al2O3 or the _like into the molten salt is suppressed, and metallic magnesium with a low aluminum content is obtained. Metallic magnesium with a low aluminum content is useful for producing high-purity sponge titanium.

On the other hand, during molten salt electrolysis, the peripheral wall 2b, particularly the rear wall 2d, exposed to the high temperature of the electrolytic cell 2, expands due to thermal expansion, etc., so as to orient the cathode 3b upward as shown by the outlined arrow in FIG. 3. Accordingly, in the cathode 3b in which the extension portion 13b is inserted into the peripheral wall 2b, more specifically the rear wall 2d, the extension portion 13b is connected to a power source and fixed outside the electrolytic cell 2, while the cylindrical portion 13a is not restricted from displacing upward, so that the unfixed portion of the cathode 3b is lifted upward by the rear wall 2d expanding upward. As a result, the cathode 3b is displaced obliquely, as shown by the dashed line in the figure, especially the portion of the cylindrical portion 13a located away from the rear wall 2d (the portion located adjacent to the partition wall 5) is lifted upward.

When the cathode 3b is displaced as described above, the portion of the cathode 3b adjacent to the rear wall 2d approaches, or even comes into contact with, the inner cylindrical bipolar electrode 3c at the lower end of the electrolytic cell 2 in the depth direction, as shown in FIG. 4, causing a short circuit. In a molten salt electrolysis device in which the electrodes do not include a cylindrical bipolar electrode, the above displacement of the cathode may cause that portion of the cathode to come into contact with the inner anode. When a short circuit occurs, it may be necessary to stop operation of the molten salt electrolysis device 1 and dismantle it.

To address this problem, in this embodiment, the portion of the cylindrical portion 13a of the cathode 3b adjacent to the rear wall 2d of the peripheral wall 2b is configured with a movable cathode member 23a (also referred to as the "rear wall 2d side movable cathode member 23a") in the shape of a plate on the peripheral wall 2b side, which can be moved toward and away from the rear wall 2d of the peripheral wall 2b, as shown in FIG. 5. With this, when the cylindrical portion 13a of the cathode 3b is lifted obliquely as described above due to the upward expansion of the rear wall 2d, the movable cathode member 23a of the cylindrical portion 13a is moved to move toward the rear wall 2d (i.e., in the direction away from the cylindrical bipolar electrode 3c), thereby suppressing the occurrence of a short circuit between the cathode 3b and the cylindrical bipolar electrode 3c. Since a short circuit can be avoided only by the displacement of the movable cathode member 23a, the workload is significantly reduced compared to the case where the molten salt electrolysis device 1 is shut down and dismantled. In addition, the continuous operating time of the molten salt electrolysis device 1 can be extended.

When there is no risk of contact between the movable cathode member 23a on the rear wall 2d side and the cylindrical bipolar electrode 3c, such as at the beginning of molten salt electrolysis, the movable cathode member 23a is positioned close to the cylindrical bipolar electrode 3c in a position displaced away from the rear wall 2d, shortening the inter-electrode distance and suppressing the increase in voltage, thereby suppressing increases in electricity costs.

In the illustrated embodiment, the cylindrical portion 13a of the cathode 3b includes a pair of plate-like or other fixed cathode members 23b that sandwich the movable cathode member 23a on the rear wall 2d side, each extending perpendicularly or inclined to the movable cathode member 23a, and facing each other across the anode 3a. At least one of the fixed cathode members 23b, in the illustrated example, is formed so as to be continuous with the extension portion 13b that penetrates the rear wall 2d and protrudes to the outside of the electrolytic cell 2. The number of extension portions 13b may be determined appropriately based on the configuration of the molten salt electrolysis device 1. Therefore, depending on the situation, all of the fixed cathode members 23b may be continuous with the extension portion 13b.

In addition, when the cathode 3b is made of carbon steel, stainless steel, or other steel, the portion of the cylindrical portion 13a of the cathode 3b adjacent to the partition wall 5 is preferably configured with a movable cathode member 23c, such as a plate-like member, on the partition wall 5 side, which can be displaced toward and away from the partition wall 5, as shown in FIG. 5, similar to the portion adjacent to the rear wall 2d of the peripheral wall 2b described above. As the molten salt electrolysis continues, the fixed cathode member 23b in the cylindrical portion 13a of the steel cathode 3b is deformed by thermal expansion, expanding in the front-rear direction, as shown by the black arrow in FIG. 3. Due to this deformation of the fixed cathode member 23b, the portion of the cylindrical portion 13a adjacent to the partition wall 5 is located away from the cylindrical bipolar electrode 3c on the inside, as shown in FIG. 4. This may lead to an increase in the inter-electrode distance, which may result in an increase in voltage and therefore an increase in power costs. In contrast, if the portion of the cylindrical portion 13a adjacent to the partition wall 5 is configured with a movable negative electrode member 23c on the partition wall 5 side, when the fixed negative electrode member 23b expands, the movable negative electrode member 23c can be moved in a direction away from the partition wall 5 (i.e., in a direction approaching the cylindrical bipolar electrode 3c), thereby suppressing the increase in voltage due to the increase in the interelectrode distance.

The upper end of the movable cathode member 23a on the rear wall 2d side in the depth direction of the electrolytic cell 2 may be located slightly lower than the bath surface of the molten salt bath as shown in the figure, but it is preferable to arrange it so that it is located higher than the bath surface. This is because it makes it easier to move the movable cathode member 23a on the rear wall 2d side. Even if the upper end of the movable cathode member 23a is located lower than the bath surface, it can be operated by temporarily removing the molten salt bath. On the other hand, it is preferable to arrange the upper end of the movable cathode member 23a on the partition wall 5 side at a position slightly lower than the bath surface so as not to excessively impede the flow of metallic magnesium through the molten metal flow path 5b.

It is preferable that the movable negative electrode members 23a and 23c on the rear wall 2d side and the partition wall 5 side are both positioned so that they can be electrically connected to the fixed negative electrode member 23b, respectively.

More specifically, for example, as shown in Fig. 6(a), a groove 24 is formed on the surface of the fixed negative electrode member 23b facing the movable negative electrode member 23a, the groove 24 being recessed from the surface and extending in the depth direction of the electrolytic cell 2 (the front-to-back direction of the paper in Fig. 6). The groove 24 has a groove width Wg wider than the width Wm of the end of the movable negative electrode member 23a. The movable negative electrode member 23a is disposed with the end of the movable negative electrode member 23a inserted into the groove 24 of the fixed negative electrode member 23b. This makes it possible to displace the movable negative electrode member 23a toward or away from the rear wall 2d along the groove 24 (the left-to-right direction in Fig. 6).
In this case, the movable range of the movable negative electrode member 23a is a region between the position where it is closest to the rear wall 2d and the position where it is farthest from the rear wall 2d in the groove portion 24. When the contact between the movable negative electrode member 23a and the fixed negative electrode member 23b is ensured in this movable range, the movable negative electrode member 23a is electrically connected to the fixed negative electrode member 23b in the movable range. This is also true in the examples of Figs. 6(b) and 6(c) described later. Note that an insulating film may be intentionally provided on the surface of the electrode 3. Even if an insulating film is provided on the surface of the electrode 3, the electrode 3 itself covered with the insulating film is electrically conductive. Therefore, even if an insulating film is provided on a part of the cathode 3b, the electrical connection with the fixed negative electrode member 23b in the movable range of the movable negative electrode member 23a can be ensured.

Here, as shown in FIG. 6(a), a stopper member 25 can be removably sandwiched between the inner surface of the groove 24 of the fixed negative electrode member 23b and the surface near the end of the movable negative electrode member 23a. The stopper member 25 fixes the movable negative electrode member 23a at the position where it is displaced toward or away from the fixed negative electrode member 23a, and can be rod-shaped, plate-shaped, or wedge-shaped with a tapered tip. Even if such a stopper member is not provided, it is possible to appropriately contact and connect the movable negative electrode member and the fixed negative electrode member by setting the gap between the end of the movable negative electrode member and the groove to a certain narrowness, due to the thermal expansion of the negative electrode during operation, the action of a magnetic field, etc.

The material of the stopper member 25 may be an insulator, but may also be a conductor. The conductive stopper member 25 is preferably shaped to match as closely as possible between the inner surface of the groove 24 of the fixed cathode member 23b and the surface of the movable cathode member 23a, so that it is approximately equipotential with the cathode 3b when a voltage is applied between the electrodes 3. In addition, when the stopper member 25 is made of a conductor, as in the illustrated example, the entire stopper member 25 is made to fit within the groove 24, thereby suppressing current concentration (edge effect) caused by protruding from the groove 24. Specifically, the stopper member 25 can be made of a metal such as steel, similar to the cathode 3b, carbon, or ceramics including brick.

To facilitate inserting and positioning the stopper member 25 between the inner surface of the groove portion 24 of the fixed negative electrode member 23b and the surface of the movable negative electrode member 23a, it is preferable to form a groove-shaped recess 26 through which the stopper member 25 passes on the surface of the movable negative electrode member 23a near the end that comes into contact with the stopper member 25. In FIG. 6(a), the groove-shaped recess 26 is formed only on one surface near the end of the movable negative electrode member 23a.

In contrast, in FIG. 6(b), a groove-shaped recess 126 is formed on each of the opposite surfaces near the end of the movable negative electrode member 123a. When the groove-shaped recess 126 is formed on each surface of the movable negative electrode member 123a, it becomes easier to position the stopper member 25 when fixing the movable negative electrode member 123a at each of the positions where it is displaced toward and away from the peripheral wall 2b. The structure other than the groove-shaped recess 126 is almost the same as that shown in FIG. 6(a).

As shown in FIG. 6(c), the inner side of the opening of the groove 224 provided in the fixed negative electrode member 223b has a protrusion 227 that protrudes from the inner side in a direction that narrows the groove width at that point. In this case, the stopper member 225 is held by the protrusion 227 by arranging it in a position recessed from the protrusion 227 in the groove 224. Otherwise, FIG. 6(c) is substantially similar to that shown in FIG. 6(a) and (b), except that the groove-shaped recesses 26, 126 of the movable negative electrode members 23a, 123a formed in the manner shown in FIG. 6(a) and (b) are not provided.

In FIG. 6, only one end of the movable negative electrode members 23a, 123a, and 223a on the peripheral wall 2b side is shown, but the other ends and each end of the movable negative electrode member 23c on the partition wall 5 side can also be structured substantially the same as the one end described above.

The groove 24 provided on the surface of the movable cathode member 23a side of the fixed cathode member 23b on the peripheral wall 2b side of the cylindrical part 13a of the cathode 3b may terminate at the lower end side of the fixed cathode member 23b in the depth direction of the electrolytic cell 2 (up and down direction in FIG. 7) as shown in FIG. 7. In this case, the lower end surface of the groove 24 becomes the mounting surface 28, and the movable cathode member 23a can be placed on the mounting surface 28. In other words, it can be said that the cylindrical part 13a of the cathode 3b has a mounting surface 28 that supports the movable cathode member 23a on the peripheral wall 2b side from below. The movable cathode member 23c on the partition wall 5 side may also be placed on a mounting surface (not shown) that may be provided on the fixed cathode member 23b of the cylindrical part 13a.

Alternatively, as in another embodiment shown in FIG. 8, the movable cathode member 323a on the peripheral wall 302b side can be placed on a mounting surface 328 provided by protruding the lower end of the rear wall 302d toward the electrolysis chamber 302f side. In this case, the peripheral wall 302b of the electrolysis cell has a mounting surface 328 that supports the movable cathode member 323a on the peripheral wall 302b side from below.

When molten salt electrolysis is performed in a molten salt bath containing magnesium chloride using the molten salt electrolysis device 1 as described above, the magnesium chloride contained in the molten salt bath can be a by-product of the Kroll process. The molten salt constituting the molten salt bath may contain a supporting salt in addition to magnesium chloride (MgCl ₂ ). The supporting salt means an electrolyte that lowers the crystallization temperature and reduces the viscosity when mixed with magnesium chloride. Specifically, the supporting salt may be at least one selected from the group consisting of sodium chloride (NaCl), calcium chloride (CaCl ₂ ), potassium chloride (KCl), magnesium fluoride (MgF ₂ ), and calcium fluoride (CaF ₂ ). The crystallization temperature refers to the temperature at which a phenomenon called crystallization occurs, in which one electrolyte component begins to precipitate as a solid, when a molten salt consisting of two or more electrolytes is cooled from a liquid state. If there is only one type of molten salt, the entire salt becomes solid at the freezing point when the temperature is lowered from a liquid state, so the crystallization temperature corresponds to the freezing point, i.e., the melting point. In order to preferentially decompose magnesium chloride by electrolysis, it is common to use an electrolyte having a higher decomposition voltage than magnesium chloride as the supporting electrolyte.

In this molten salt electrolysis, magnesium chloride is electrolyzed in the electrolytic chamber 2f, and based on the reaction _MgCl2 →Mg + _Cl2 , metallic magnesium (Mg) as molten metal is produced by a reduction reaction on the surface of the cylindrical portion 13a of the cathode 3b, while chlorine ( _Cl2 ) gas is generated by an oxidation reaction on the surface of the anode 3a.

More specifically, due to convection in the molten salt bath, as shown in FIG. 1, the molten salt flows from the recovery chamber 2e through the molten salt circulation path 5a on the bottom wall 2c side to the electrolysis chamber 2f. In the electrolysis chamber 2f, the magnesium chloride in the molten salt is electrolyzed to generate metallic magnesium. This metallic magnesium then flows into the recovery chamber 2e through the molten metal flow path 5b on the bath surface side of the partition wall 5. Thereafter, the metallic magnesium, which has a smaller specific gravity than the molten salt, floats to a shallow portion of the recovery chamber 2e and accumulates there. The metallic magnesium that floats in the recovery chamber 2e can be recovered by a pump or the like (not shown). This allows metallic magnesium to be produced from magnesium chloride, and chlorine gas is also obtained.

The metallic magnesium obtained by molten salt electrolysis can be used to reduce titanium tetrachloride in the Kroll process for producing metallic titanium, and the chlorine gas can be used to chlorinate titanium ore.

Next, we have created a prototype of the molten salt electrolysis device of this invention and confirmed its effects, which will be described below. However, the description here is merely for illustrative purposes and is not intended to be limiting.

In Example 1, molten salt electrolysis was performed using a molten salt electrolysis apparatus having a cylindrical cathode portion with movable cathode members on the peripheral wall side and the partition wall side as shown in Figures 1, 2 and 5. This molten salt electrolysis apparatus had five sets of electrodes, each of which was an anode, two cylindrical bipolar electrodes and a cathode.
In Example 2, molten salt electrolysis was performed using a molten salt electrolysis apparatus having the same structure as in Example 1, except that the movable cathode member on the partition wall side was integrally attached to the fixed cathode member so that only the movable cathode member on the partition wall side could not be moved.
In both Examples 1 and 2, the cylindrical portion of the cathode and the bipolar electrode had inner and outer contour shapes in a plan view that were rectangular with short sides adjacent to the rear wall and the partition wall, respectively. The cylindrical portion of the cathode had a short side (length of the movable cathode member) of 0.3 m, a long side (length of the fixed cathode member) of 1.0 m, and a height of 0.7 m along the depth direction of the electrolytic cell.

In Comparative Example 1, molten salt electrolysis was performed using the same molten salt electrolysis apparatus as in Example 1, except that five sets of electrodes, each of which was a set of a flat cathode, two bipolar electrodes, an anode, two bipolar electrodes, and a flat cathode, were arranged in this order. The length and height of the cathode in Comparative Example 1 were 1.0 m and 0.7 m, respectively.
In Comparative Example 2, molten salt electrolysis was performed using a molten salt electrolysis apparatus having the same structure as in Example 1, except that both the movable cathode members on the peripheral wall side and the partition wall side were made immovable.
In Comparative Example 3, molten salt electrolysis was performed using a molten salt electrolysis apparatus having the same structure as in Example 1, except that both the movable cathode members on the peripheral wall side and the partition wall side were made immovable and the portion of the cathode adjacent to the rear wall side was disposed 3 cm away from the bipolar electrode on the inner side.

In Examples 1 and 2, the distances between the cathode and bipolar electrode, between the bipolar electrode and the bipolar electrode, and between the bipolar electrode and the anode at the start of molten salt electrolysis were set to 1 cm. Thereafter, at least one of the movable cathode members was displaced during the molten salt electrolysis, so that the distance between the cathode and the bipolar electrode at that point changed. Note that this distance was measured in a direction perpendicular to the sides at the center of each side of the cylindrical part of the cathode or the bipolar electrode that is rectangular in plan view.
In Comparative Examples 1 and 2, the distances between the cathode and bipolar electrode, between the bipolar electrodes, and between the bipolar electrode and the anode were set to 1 cm.
In Comparative Example 3, the cathode adjacent to the rear wall was positioned 3 cm away from the bipolar electrode, so the distance between the cathode and the bipolar electrode at that location was 4 cm. The distances between the cathode and the bipolar electrode at other locations on the cathode, as well as the distances between the bipolar electrodes and between the bipolar electrodes and the anode, were the same as in Comparative Examples 1 and 2.

In all of the molten salt electrolysis devices, the peripheral wall, bottom wall and partition of the electrolytic cell were made of bricks with _{an Al2O3} content of 95% or more, the electrolysis chamber was 2 ^m3 and the recovery chamber was 1 ^m3 . The molten salt bath had a composition of 20%, 30%, 49% and 1% by mass of _MgCl2 , _CaCl2 , NaCl and _MgF2 , respectively, and was operated for a period of one year by passing a current at a current density of 0.48 A/ ^cm2 .

For each of Examples 1 and 2 and Comparative Examples 1 to 3, the occurrence of short circuits during molten salt electrolysis, the current efficiency, and the aluminum content in the metallic magnesium produced by molten salt electrolysis were confirmed. The results are shown in Table 1.

In Table 1, the current efficiency was calculated by the following formula, and the current efficiency of Comparative Example 1 was set to 100, and the current efficiency of Examples 1 and 2 and Comparative Examples 2 and 3 were shown as relative values to the current efficiency of Comparative Example 1.
Current efficiency=mass of metallic magnesium recovered from electrolytic cell/theoretical metallic magnesium production amount The theoretical metallic magnesium production amount is the theoretical production amount of metal obtained from Faraday's law, and is calculated by the following formula.
Theoretical metallic magnesium production amount=((current (A)×current application time (sec))/(number of charges on magnesium ion n×Faraday constant F))×(number of electrolysis times N)×atomic weight of magnesium

It can be seen from Table 1 that a short circuit occurred in Comparative Example 2, whereas in Examples 1 and 2, the movable negative electrode member on the peripheral wall side was displaced, so that a short circuit did not occur.
In Comparative Example 2, a short circuit occurred, and therefore the molten salt electrolysis apparatus could not be used from the time of the short circuit occurrence, and the lifespan was short. On the other hand, in Examples 1 and 2, a short circuit did not occur due to the movement of the movable cathode member on the partition wall side, and therefore the lifespan of the molten salt electrolysis apparatus was about twice as long as that of Comparative Example 2.

As can be seen from Table 1, when a cylindrical cathode and bipolar electrode were used, the current efficiency was improved and the aluminum content in the metallic magnesium obtained by molten salt electrolysis was reduced, compared with Comparative Example 1, in which a plate-shaped cathode and bipolar electrode were used. In Comparative Example 3, the portion of the cathode adjacent to the rear wall side was always located away from the bipolar electrode on the inside, and therefore, a current efficiency as high as that of Examples 1 and 2 was not obtained, as compared with Examples 1 and 2.

REFERENCE SIGNS LIST 1 Molten salt electrolysis device 2 Electrolytic cell 2a Internal space 2b, 302b Peripheral wall 2c Bottom wall 2d, 302d Rear wall 2e Recovery chamber 2f Electrolysis chamber 3 Electrode 3a Anode 3b, 303b Cathode 3c Cylindrical bipolar electrode 4 Lid member 5 Partition wall 5a Molten salt circulation path 5b Molten metal flow path 13a, 313a Cylindrical portion of cathode 13b, 313b Extension portion of cathode 23a, 123a, 223a, 323a Movable cathode member on peripheral wall side 23b, 123b, 223b, 323b Fixed cathode member 23c, 323c Movable cathode member on partition wall side 24, 124, 224 Groove portion 25, 125, 225 Stopper member 26, 126 Groove-shaped recess 227 Protrusion 28, 328 Mounting surface Wg Groove width of groove Wm Width of end of movable negative electrode member

Claims (10)

A molten salt electrolysis apparatus comprising: an electrolytic cell having a peripheral wall and a bottom wall that define an internal space; and electrodes including an anode and a cathode that are disposed in the internal space of the electrolytic cell,
the cathode has a cylindrical portion surrounding the anode and an extension portion extending from a part of the cylindrical portion, penetrating the peripheral wall, and protruding to the outside of the electrolytic cell;
The molten salt electrolysis apparatus comprises a movable cathode member on the side of the cylindrical portion, the portion of the cylindrical portion being adjacent to the peripheral wall and capable of moving toward and away from the peripheral wall. the peripheral wall includes a rear wall adjacent the electrode;
2. The molten salt electrolysis apparatus according to claim 1, wherein the movable cathode member on the peripheral wall side is a portion of the cylindrical portion located adjacent to the rear wall, and is a movable cathode member on the rear wall side that is movable toward and away from the rear wall. the electrolytic cell has a partition wall defining an electrolytic chamber in which the electrodes are disposed in the internal space;
the cathode is made of steel;
3. The molten salt electrolysis apparatus according to claim 1, wherein a portion of the cylindrical portion adjacent to the partition wall is constituted by a movable cathode member on the partition wall side which is movable toward and away from the partition wall. the cylindrical portion includes a pair of stationary cathode members each extending in a direction perpendicular or inclined to the movable cathode member and positioned opposite each other across the anode,
At least one of the stationary cathode members is continuous with the extension portion;
4. The molten salt electrolysis apparatus according to claim 1, wherein the movable cathode member is arranged so as to be electrically connectable to each of the fixed cathode members. a groove portion is formed in a surface of the fixed negative electrode member facing the movable negative electrode member, the groove portion being recessed from the surface and extending in a depth direction of the electrolytic cell,
the groove portion has a groove width greater than a width of an end portion of the movable negative electrode member,
5. The molten salt electrolysis apparatus according to claim 4, wherein the movable cathode member is arranged with its end inserted into the groove and configured to be capable of moving toward and away from the groove in a groove width direction. The molten salt electrolysis device according to claim 5, wherein a stopper member that fixes the movable cathode member at the position where the approaching or separating displacement is made is removably disposed between the inner surface of the groove of the fixed cathode member and the surface near the end of the movable cathode member. The molten salt electrolysis device according to claim 6, wherein a groove-shaped recess for passing the stopper member is formed on at least one of the surfaces of the movable cathode member near the end portion that contacts the stopper member. The molten salt electrolysis device according to any one of claims 1 to 7, wherein the peripheral wall of the electrolytic cell or the cylindrical portion of the cathode has a mounting surface that supports the movable cathode member on the peripheral wall side from below. The molten salt electrolysis apparatus according to any one of claims 1 to 8, wherein the electrodes further include one or more cylindrical bipolar electrodes arranged between the anode and the cylindrical portion of the cathode. A method for producing metallic magnesium, which produces metallic magnesium from magnesium chloride using the molten salt electrolysis apparatus according to any one of claims 1 to 9.