JP2019214773A - Molten salt electrolysis method, and method for producing metal magnesium - Google Patents

Abstract

To provide a molten salt electrolysis method, and a method for producing a metal magnesium, capable of refilling a molten salt bath with magnesium chloride without causing the increase of temperature of the molten salt bath, the magnesium chloride being consumed with the continuity of electrolysis.SOLUTION: This invention relates to a molten salt electrolysis method for electrolyzing magnesium chloride based on energization to an electrode 3 in a molten salt bath of a molten salt comprising the magnesium chloride and a supporting electrolyte to obtain a metal magnesium through the electrolysis, including: a bath-extracting step of extracting a part Ps of the molten salt comprising the magnesium chloride and the supporting electrolyte from the molten salt bath, when refilling the molten salt bath with the magnesium chloride; a mixture-producing step of mixing a molten salt P extracted in the bath-extracting step with a refilling magnesium chloride Rs to obtain a mixture Ms of the molten salt Ps and the refilling magnesium chloride Rs; and a mixture-supplying step of supplying the mixture Ms to the molten salt bath at temperature equal to or lower than the melting point of the magnesium chloride.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a molten salt electrolysis method for obtaining magnesium metal by electrolyzing magnesium chloride contained therein in a molten salt bath based on energization of an electrode, and to a method for producing metal magnesium. Another object of the present invention is to propose a technique capable of effectively replenishing a molten salt bath with magnesium chloride consumed as electrolysis continues.

  For example, in the production of titanium metal by the Kroll method, magnesium chloride produced as a by-product may be decomposed into magnesium metal and chlorine gas by electrolysis using a molten salt electrolytic cell. In this case, the metallic magnesium and the chlorine gas are reused for reduction of titanium tetrachloride and chlorination of titanium ore, respectively.

  In this type of electrolysis, a molten salt containing magnesium chloride and a supporting salt is generally stored in an electrolytic tank or the like partitioned into a storage chamber and an electrolytic chamber by a partition wall to form a molten salt bath. In this molten salt bath, the molten salt inside the electrolytic cell flows from the storage chamber to the electrolytic chamber, where magnesium chloride is decomposed into magnesium metal and chlorine based on the power supply to the electrodes. The metallic magnesium generated in the electrolytic chamber is further circulated to the storage chamber inside the electrolytic cell, and is recovered after floating on the liquid surface of the molten salt bath due to a difference in density with the molten salt. Note that chlorine is discharged to the outside of the electrolytic cell via a gas discharge passage or the like provided in the electrolytic cell.

  In such molten salt electrolysis, when magnesium chloride is electrolyzed and metallic magnesium is produced, the amount of magnesium chloride in the molten salt bath decreases accordingly. Therefore, in order to continuously carry out the molten salt electrolysis, it is necessary to supply magnesium chloride consumed in the electrolysis to the molten salt bath.

Here, the molten salt bath that also contains sodium chloride or the like as a supporting salt is maintained at a relatively low temperature such that the metallic magnesium does not solidify. If magnesium chloride in a molten state having a high melting point of about 714 ° C. is supplied and supplied alone to such a molten salt bath, there is a problem that the temperature of the molten salt bath temporarily rises during the replenishment. .
This increase in the temperature of the molten salt bath causes a reaction in which the electrolyzed metallic magnesium and chlorine return to magnesium chloride, thereby reducing the current efficiency. Further, in this case, there is a possibility that the bricks and the like constituting the outer wall of the electrolytic cell may be thermally damaged. Further, there is a concern that the molten salt whose temperature has risen may pass through a gap in an outer wall of a brick or the like to cause leakage of the molten salt bath.

  Regarding the temperature adjustment of the molten salt bath, Patent Document 1 discloses that “a combustion burner and a cooling air introduction passage are provided in an inlet duct of a heat exchanger having one or more conduits immersed in the molten salt bath, When the bath temperature falls below a predetermined value, the combustion burner is operated, the combustion gas is fed into the conduit to heat the bath, and when the bath temperature rises above the predetermined value, the operation of the burner is stopped. A method for adjusting the bath temperature of a molten salt electrolytic cell, characterized in that cooling air is fed into each of the conduits through a passage provided on the outer periphery of the burner to carry away the heat of the bath.

  Patent Document 2 focuses on "a method for producing magnesium chloride to be used for magnesium electrolysis", and "extracts an electrolytic bath salt having a reduced concentration of magnesium chloride from an electrolytic cell for producing magnesium by electrolyzing magnesium chloride and electrolyzing the same. Magnesium oxide or / and magnesium carbonate in the form of solid powder are suspended in the bath salt, and a gas containing chlorine is passed through the suspension to react the magnesium oxide with chlorine to produce magnesium chloride. The method for producing magnesium, comprising returning the electrolytic bath salt having an increased value to the electrolytic cell and performing electrolysis. According to this, "chlorination and electrolysis can be repeatedly performed by circulating an electrolytic bath salt. Therefore, it is not necessary to take out magnesium chloride as a solid outside the system. There is no problem associated with the absorption of magnesium chloride, and the required amount of magnesium chloride can be produced in accordance with the amount of electrolysis, that is, the amount of magnesium produced, and its quality is stable. "

JP-A-4-214889 JP-A-2-243789

  However, even by adjusting the temperature of the molten salt bath by starting and stopping the combustion burner as described in Patent Document 1, when the high-temperature magnesium chloride is supplied to the molten salt bath as described above, at least after the replenishment, It is inevitable that the temperature of the molten salt bath rises for a certain period of time. Therefore, there is room for improvement in temperature control during magnesium chloride replenishment.

  In the proposed technique of Patent Document 2, “an electrolytic bath salt having a reduced magnesium chloride concentration is extracted from an electrolytic bath, and solid oxide magnesium oxide and / or magnesium carbonate is suspended in the electrolytic bath salt. A gas containing chlorine is passed through to react magnesium oxide with chlorine to produce magnesium chloride. " However, as described in Example 1, the temperature of the chlorination reaction for producing such magnesium chloride is set to be extremely higher than the temperature of the molten salt bath. Therefore, when the magnesium chloride thus produced is supplied to the molten salt bath, the temperature of the molten salt bath rises significantly. Therefore, the problem of bath temperature rise cannot be solved even by Patent Document 2. Further, here, magnesium oxide and the like may be mixed into the molten salt bath, which may hinder stable electrolysis.

  It is an object of the present invention to solve such a problem, and an object of the present invention is to suppress the temperature rise of a molten salt bath and to reduce the magnesium chloride consumed with the continuation of electrolysis by molten salt. It is an object of the present invention to provide a molten salt electrolysis method that can be supplied to a bath and a method for producing metallic magnesium.

  The molten salt electrolysis method of the present invention electrolyzes the above-mentioned magnesium chloride in a molten salt bath of a molten salt containing magnesium chloride and a supporting salt, based on power supply to an electrode, and obtains metallic magnesium by the electrolysis. In replenishing the molten salt bath with magnesium chloride, a bath extraction step of extracting a part of the molten salt containing the magnesium chloride and the supporting salt from the molten salt bath, and a supply of the molten salt extracted in the bath extraction step. Mixing with magnesium chloride to form a mixture of the molten salt and replenishing magnesium chloride, and a mixture supplying step of supplying the mixture to the molten salt bath at a temperature equal to or lower than the melting point of magnesium chloride. .

Here, in the molten salt electrolysis method of the present invention, when the mixture is supplied to the molten salt bath in the mixture supplying step, the temperature of the mixture is preferably 645 ° C to 700 ° C.
Further, here, in the molten salt electrolysis method of the present invention, when the mixture is supplied to the molten salt bath in the mixture supplying step, the temperature of the mixture is not more than ± 10 ° C. from the temperature of the molten salt bath. The temperature is preferably set to
In the molten salt electrolysis method of the present invention, the temperature of the molten salt bath can be maintained at 651 ° C. or higher.

  In the molten salt electrolysis method of the present invention, it is preferable that the mixture in the mixture producing step contains 10% to 95% by mass of magnesium chloride and 5% to 90% by mass of a supporting salt.

  Further, in the molten salt electrolysis method of the present invention, the supporting salt may include at least one selected from the group consisting of sodium chloride, calcium chloride, potassium chloride, magnesium fluoride, and calcium fluoride.

  Further, in the molten salt electrolysis method of the present invention, the mass ratio of the replenishing magnesium chloride to be mixed with the molten salt in the mixture generation step with respect to the molten salt extracted from the molten salt bath in the bath extraction step is 1.0 to 10%. .0 is preferable.

  The method for producing metallic magnesium of the present invention is to produce metallic magnesium from a molten salt by using any one of the above-described molten salt electrolysis methods.

  According to the present invention, a part of the molten salt is withdrawn from the molten salt bath, and a mixture obtained by mixing the molten salt with supplementary magnesium chloride is supplied to the molten salt bath at a temperature equal to or lower than the melting point of magnesium chloride. This makes it possible to effectively suppress a rise in the temperature of the molten salt bath when magnesium chloride is supplied to the molten salt bath.

It is a longitudinal section showing an example of a molten salt electrolysis tank which can perform a molten salt electrolysis method of one embodiment of the present invention. FIG. 2 is a sectional view taken along line II-II in FIG. 1.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The molten salt electrolytic cell 1 exemplified in the longitudinal sectional view of FIG. 1 has a container shape mainly made of a refractory brick such as Al ₂ O ₃ or another suitable material, and contains magnesium chloride and a supporting salt stored therein. And an electrolytic bath 2 for electrolyzing magnesium chloride in the molten salt with a molten salt bath comprising a molten salt containing: Further, the molten salt electrolytic cell 1 includes an electrode 3 including an anode 3a and a cathode 3b having portions arranged in the electrolytic cell 2 in parallel with the depth direction of the molten salt bath. Although not shown, the molten salt electrolytic cell 1 may be further provided with a temperature adjusting tube or the like as a heat exchanger that is disposed in the storage chamber 2b or the like and adjusts the temperature in the electrolytic cell 2.

In the molten salt electrolysis performed using the molten salt electrolyzer 1, as shown in FIG. 1, metallic magnesium (Mg) is produced as a molten metal and chlorine (Cl ₂ ) is produced as a gas by electrolysis of magnesium chloride. Occurs. Metal magnesium produced by molten salt electrolysis can be used for reduction of titanium tetrachloride in the Kroll method for producing metal titanium, and chlorine gas can be used for chlorination of titanium ore in the method. As the magnesium chloride used as the raw material for the electrolysis, it is possible to use those produced by the chlor method as a by-product.

The molten salt contains a supporting salt in addition to the above-mentioned magnesium chloride (MgCl ₂ ). This supporting salt means an electrolyte which lowers the crystallization temperature and lowers the viscosity when dissolved in magnesium chloride and mixed with magnesium chloride. Specifically, the supporting salt is at least 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 ). It can be a kind. The crystallization temperature refers to a temperature at which a phenomenon occurs in which a certain kind of electrolyte component starts to precipitate as a solid when a molten salt composed of two or more kinds of electrolytes is cooled from a liquid state. When only one type of molten salt is used, when the temperature is lowered from the liquid state, the whole becomes solid at the freezing point, so that crystallization temperature = freezing point = melting point. In order to preferentially decompose magnesium chloride by electrolysis, it is general to use an electrolyte having a higher decomposition voltage than magnesium chloride as a supporting salt.

  Here, the illustrated electrolytic cell 2 further includes a partition wall 4 disposed substantially along the depth direction as shown in FIG. Due to the partition walls 4, the inside of the electrolytic cell 2 is provided on the right side in FIG. 1 with the electrolytic chamber 2a in which the electrode 3 is disposed, and on the left side, metal magnesium obtained by electrolysis in the electrolytic chamber 2a. The metal magnesium flows into and is partitioned into a storage chamber 2b that accumulates on the upper side due to a density difference from the molten salt. Specifically, the partition 4 is arranged close to a lid member (not shown) for covering the upper opening of the electrolytic cell 2. Thus, a molten salt circulation path 4a is formed between the storage chamber 2b and the electrolysis chamber 2a so that the molten salt bath can move from the storage chamber 2b to the electrolysis chamber 2a. Further, the molten metal flow path 4b provided above the partition wall 4 allows the molten metal to flow from the electrolytic chamber 2a to the storage chamber 2b.

Here, the electrode 3 disposed in the electrolytic chamber 2a has at least an anode 3a and a cathode 3b connected to a power supply (not shown). The anode 3a and the cathode 3b generate a gas such as chlorine by an oxidation reaction on the surface of the anode 3a based on a reaction of MgCl ₂ → Mg + Cl ₂ , and a metal which is a molten metal by a reduction reaction on the surface of the cathode 3b. Generates magnesium.

  As long as the electrode 3 has at least the anode 3a and the cathode 3b, the electrolysis of magnesium chloride in the molten salt can be performed. On the other hand, from the viewpoint of improving the generation efficiency of electrolysis and the like, as can be seen from FIG. 2, the voltage between the anode 3a and the cathode 3b is not connected to the power supply between the anode 3a and the cathode 3b. It is preferable to further include one or more, for example, two bipolar electrodes 3c which are polarized by the application of. However, such a bipolar electrode 3c is not always necessary.

  In the molten salt electrolysis method using the molten salt electrolyzer 1 described above, the convection of the molten salt bath caused the fluid to flow from the storage chamber 2b to the electrolysis chamber 2a via the molten salt circulation path 4a at the bottom as shown in FIG. Magnesium chloride in the molten salt is electrolyzed. Thereby, metallic magnesium is generated in a molten state in the electrolytic chamber 2a. The metallic magnesium flows into the storage chamber 2b through the molten metal flow path 4b on the bath surface side of the partition wall 4. Thereafter, the metallic magnesium having a small specific gravity with respect to the molten salt floats on a shallow portion of the storage chamber 2b and accumulates therein. The metallic magnesium floating in the storage chamber 2b can be collected by a pump or the like (not shown). Therefore, according to this, metallic magnesium can be produced from a molten salt.

  When magnesium chloride is electrolyzed in the molten salt electrolytic cell 1 in this manner, magnesium chloride in the molten salt bath is consumed and reduced with the execution of the electrolysis. Therefore, continuing the electrolysis requires that the molten salt bath be replenished with magnesium chloride, for example, periodically.

  Here, conventionally, magnesium chloride having a melting point of about 714 ° C. has been melted at about 750 ° C., and only the magnesium chloride has been supplied to the molten salt bath for replenishment. If only magnesium chloride is replenished in this way, there is a problem that the temperature of the molten salt bath, which is maintained at about 660 ° C. at which the metallic magnesium does not solidify during operation, greatly increases due to the replenishment of the high-temperature magnesium chloride described above. Was. In this case, as described above, a reaction occurs in which metallic magnesium and chlorine generated by electrolysis return to magnesium chloride due to a rise in the temperature of the molten salt bath. The re-electrolysis of magnesium chloride consumes electricity, so that the current efficiency decreases. Also, the outer wall of the electrolytic cell 2 may contain solidified molten salt in gaps such as refractory bricks. However, the rise in temperature of the molten salt bath dissolves the solidified molten salt on the outer wall of the electrolytic cell. This may cause leakage of the molten salt bath from the electrolytic cell 2. In addition, the outer wall of the electrolytic cell 2 may be thermally damaged.

  In order to solve this problem, in this embodiment, when supplying magnesium chloride to the molten salt bath, instead of supplying magnesium chloride for replenishment alone to the molten salt bath, a part of the molten salt is supplied from the molten salt bath. Withdraw and supply a mixture of this and magnesium chloride for replenishment. As a result, the supporting salt contained in the molten salt in the mixture not only lowers the crystallization temperature of the mixture but also lowers its viscosity, so that the mixture is melted at a relatively low temperature below the melting point of magnesium chloride. It becomes possible to supply to a salt bath. Therefore, a large rise in the temperature of the molten salt bath during magnesium chloride replenishment can be suppressed.

  To describe this point in more detail, in this embodiment, in replenishing the molten salt bath with magnesium chloride, as shown in FIG. 1, a bath for extracting a part of the molten salt Ps from the molten salt bath. In the extraction step, for example, in the mixing tank 5, a part of the molten salt Ps extracted in the bath extraction step is mixed with the replenishing magnesium chloride Rs, and a mixture Ms of the part of the molten salt Ps and the replenishing magnesium chloride Rs is mixed. And a mixture supplying step of supplying the mixture Ms to the molten salt bath at a temperature equal to or lower than the melting point of magnesium chloride.

  In the bath extraction step, for example, a part of the molten salt Ps constituting the molten salt bath is mixed into the mixing tank 5 via the conduit 5a whose tip is located deeper than the metallic magnesium accumulated on the bath surface side of the storage chamber 2b. And a part of the molten salt Ps is extracted from the molten salt bath. In addition, as the mixing tank 5, for example, a siphon furnace that can be used for recovering metallic magnesium or the like can be used.

  Some of the molten salts Ps include substances contained in the molten salt bath, specifically, magnesium chloride, and sodium chloride, calcium chloride, potassium chloride, magnesium fluoride, and calcium fluoride as supporting salts. It is preferable that at least one selected from the group is included.

  Next, in the mixture generation step, a part of the molten salt Ps extracted from the molten salt bath in the above-mentioned bath extraction step is mixed with the replenishing magnesium chloride Rs separately supplied to the mixing tank 5 in the mixing tank 5. Thereby, a mixture Ms of a part of the molten salt Ps and the supplementary magnesium chloride Rs is generated.

  At this time, the mass of the replenishing magnesium chloride Rs mixed with some of the molten salts Ps is expressed as a ratio to the mass of some of the molten salts Ps extracted from the molten salt bath in the bath extraction step, and is 1.0 to 1.0. Preferably, it is set to 0. That is, the mixture Ms preferably contains a part of the molten salt Ps and the replenishing magnesium chloride Rs at this mass ratio. In addition, when it is assumed that the mass of a part of the molten salt Ps is 1.0 and the mass of the replenishing magnesium chloride Rs is 3.3, the mass ratio is 3.3. More preferably, the mass ratio is between 1.4 and 5.0.

  By setting the mass ratio of the replenishing magnesium chloride Rs in the mixture Ms to a part of the molten salt Ps to 10.0 or less, the crystallization temperature is effectively lowered, and the temperature of the mixture Ms can be lowered. In addition, a large rise in the temperature of the molten salt bath during replenishment can be further suppressed. On the other hand, by setting the mass ratio of the replenishing magnesium chloride Rs in the mixture Ms to a part of the molten salt Ps to 1.0 or more, it is possible to suppress an increase in the work amount of the bath extraction step and the mixture supply step. , Workability is improved. From this viewpoint, the ratio of the mass of the replenishing magnesium chloride Rs to the mass of a part of the molten salt Ps is preferably set to 1.0 to 10.0.

The content of magnesium chloride in the mixture Ms produced in the mixture production step is preferably 10% by mass to 95% by mass, more preferably 60% by mass to 90% by mass. Further, the content of the supporting salt in the mixture Ms is preferably from 5% by mass to 90% by mass, more preferably from 10% by mass to 100% by mass, on the assumption that the sum of the content of the supporting salt and the content of magnesium chloride is 100% by mass or less. 40 mass%.
By including the supporting salt to some extent in the mixture Ms, the temperature of the mixture Ms can be lowered by lowering the crystallization temperature, and the large rise in the temperature of the molten salt bath during replenishment can be effectively suppressed. it can. In addition, by including magnesium chloride in the above range, it is possible to suppress a decrease in workability due to an increase in the amount of work in the bath extraction step and the mixture supply step.

  Then, as a mixture supply step, the mixture Ms obtained as described above is supplied to the molten salt bath in the electrolytic cell 2. At this time, the temperature of the mixture Ms at the time of supply is equal to or lower than the melting point of magnesium chloride. In this embodiment, the supply to the molten salt bath is a mixture Ms in which a part of the molten salt Ps and the replenishing magnesium chloride Rs are mixed, so that the action of the supporting salt contained in the part of the molten salt Ps is achieved. As a result, the crystallization temperature and the viscosity of the mixture Ms decrease, so that the fluidity is ensured even when the temperature of the mixture Ms is equal to or lower than the melting point of magnesium chloride. Therefore, the mixture Ms can be fed to the molten salt bath at a temperature below the melting point of the magnesium chloride, for example at a temperature close to the temperature of the relatively low temperature of the molten salt bath. As a result, an increase in the temperature of the molten salt bath when replenishing magnesium chloride can be effectively suppressed.

  Further, the mixture Ms is less likely to be clogged in the conduit 5a due to its lower viscosity than the single magnesium chloride, and since the temperature is low, the electric power required for storage in the mixing tank 5 or the like can be reduced. Damage to the mixing tank 5 is also suppressed, and handling is good.

  Incidentally, the molten salt bath is often operated at a temperature of about 651 ° C. or higher, generally 655 ° C. or higher, so as to prevent solidification of the metal magnesium generated by electrolysis. On the other hand, the temperature of the molten salt bath may be 680 ° C. or lower during operation from the viewpoint of suppressing the reaction of returning metallic magnesium and chlorine to magnesium chloride and reducing the load on the electrolytic cell 2. The molten salt bath may have a crystallization temperature of about 450 ° C. by including a supporting salt.

  Furthermore, in the mixture supply step, the temperature at the time of supplying the mixture Ms to the molten salt bath may be a temperature at which the difference from the temperature of the molten salt bath (temperature during operation) is within ± 10 ° C. It is suitable. By lowering the temperature of the mixture Ms at the time of supplying the molten salt bath to a temperature higher by 10 ° C. than the temperature of the molten salt bath, a decrease in current efficiency caused by a rise in the temperature of the molten salt bath is more effectively suppressed. can do. Further, by setting the temperature of the mixture Ms at the time of supply to the molten salt bath to be at least 10 ° C. lower than the temperature of the molten salt bath, solidification of the metallic magnesium can be suppressed. When the amount of the mixture Ms is small relative to the molten salt bath or when the operating temperature is relatively high, such solidification of the metallic magnesium may have no effect. For such a reason, it is preferable that the temperature of the mixture Ms supplied to the molten salt bath is a temperature at which the difference from the temperature of the molten salt bath is within ± 10 ° C.

  Specifically, the temperature of the mixture Ms when supplying the mixture Ms to the molten salt bath is preferably 645 ° C to 700 ° C, more preferably 650 ° C to 680 ° C. In addition, even if the temperature of the mixture Ms may be lower than the temperature of the molten salt bath, the temperature of the molten salt bath can be easily maintained in an appropriate range if the temperature difference is small. This is because the temperature of the molten salt bath is usually controlled. Further, in a preferred embodiment, the amount of the mixture Ms is too small compared to the amount of the molten salt bath, so that even when the mixture Ms is supplied to the molten salt bath, the temperature of the molten salt bath is easily maintained appropriately.

By supplying magnesium chloride to the molten salt bath through the above-described bath extraction step, mixture generation step, and mixture supply step, a rise in the temperature of the molten salt bath can be effectively suppressed.
In addition, in replenishing magnesium chloride while suppressing the rise in temperature of the molten salt bath, even if a series of steps of a bath extraction step, a mixture generation step, and a mixture supply step is performed once, the required supply amount of magnesium chloride is reduced. If not reached, the series of steps can be repeated a plurality of times.

  In addition, after the bath extraction step and the mixture generation step are sequentially performed, the mixture Ms may be supplied a plurality of times little by little in the mixture supply step. That is, the mixture Ms obtained in the mixture generation step does not necessarily need to be supplied once to the molten salt bath in the mixture supply step, and may be supplied in small portions.

  In addition, the mixture Ms obtained in the mixture generation step may be partially used, and may be diluted and used at the time of the next replenishment. For example, after the bath extraction step and the mixture generation step, about half of the mixture Ms is left in the mixing tank 5 in the mixture supply step. At this time, the mixture Ms in the mixing tank 5 contains a supporting salt. Thereafter, for example, at the time of replenishment at a later date, magnesium chloride is charged into the mixing tank 5, and a mixture generation step is performed in which the mixture Ms is added with magnesium chloride. In this case, the concentration of the supporting salt in the mixture decreases, but if the crystallization temperature is low to some extent, a mixture supplying step of supplying the supporting salt to the molten salt bath can be performed. Thereafter, such a mixture generation step and a mixture supply step can be repeated. When the concentration of the supporting salt is too low, a bath extraction step may be performed, and a molten salt may be added thereto.

  Next, the molten salt electrolysis method of the present invention was experimentally performed, and its effect was confirmed. However, the description here is for the purpose of illustration only, and is not intended to be limiting.

In a molten salt electrolytic cell having the configuration shown in FIG. 1, magnesium chloride was electrolyzed under the following conditions. The molten salt electrolyzer used was an electrolyzer having an inner wall made of a brick having an Al ₂ O ₃ content of 95% or more, and having an electrolysis chamber of 2 m ³ and a storage chamber of 1 m ³ . In this electrolytic chamber, an anode and a cathode made of graphite and two bipolar electrodes were arranged in an electrode structure of an enclosure type electrode. The number N of times of electrolysis was N = 3. Regarding the bath composition and mass of the molten salt bath, 2900 kg of molten salt composed of MgCl ₂ , CaCl ₂ , NaCl, and MgF _{2 at a} mass ratio of 20%, 30%, 49%, and 1%, respectively, is maintained at the target of the molten salt bath. The operation was performed at a temperature of 660 ° C. at a current density of 0.48 A / cm ² for one week. The theoretical metal magnesium production is 21.8 kg / h and the theoretical magnesium chloride consumption is 85.4 kg / h.

In such electrolysis, Comparative Example 1 supplied magnesium chloride at a temperature of 750 ° C. in an amount consumed by electrolysis every two hours. In Comparative Example 2 and Examples 1 to 4, the amount of magnesium chloride consumed in the electrolysis every 2 hours was mixed with the molten salt extracted from the electrolytic cell, and this mixture was supplied. Table 1 shows the conditions of each electrolysis and the results of the current efficiency in Comparative Examples 1 and 2 and Examples 1 to 4. This current efficiency was calculated by the following equation. The “current efficiency” in Table 1 is defined assuming that the current efficiency of Comparative Example 1 is 100 and the current efficiency of Comparative Example 2 and Examples 1-4 is Comparative Example 1. Is shown as a relative value with respect to the current efficiency. In addition, the value of each example is an average value in a period of one week.
Current efficiency = mass of metallic magnesium recovered from the electrolytic cell / theoretical metallic magnesium production amount The theoretical metallic magnesium production amount is a theoretical production amount of metal obtained from Faraday's law, and is calculated by the following formula.
Theoretical metallic magnesium production = ((current (A) x conduction time (sec)) / (number of charges of magnesium ion n x Faraday constant F)) x (number of times of electrolysis N) x atomic weight of magnesium

  As shown in Table 1, in Examples 1 to 4 in which the mixture of magnesium chloride and the molten salt was supplied, the mixture could be supplied at a low temperature, whereby the temperature of the molten salt bath after supplying the mixture was low. It can be seen that the current efficiency has improved. On the other hand, in Comparative Example 1, since only magnesium chloride was supplied, the supply temperature had to be relatively high, and as a result, the temperature of the molten salt bath at the time of supply increased, and the current efficiency was improved. I couldn't do that. In Comparative Example 2, although the mixture was supplied, the current efficiency was lower than that of Comparative Example 1 because the mixture was supplied at a temperature higher than the melting point of magnesium chloride.

Comparing Example 1 with Examples 2 and 4, Examples 2 and 4 show that the temperature of the mixture was set to a temperature at which the difference from the temperature of the molten salt bath was within ± 10 ° C. It is considered that the current efficiency was improved as compared with the first embodiment.
Comparing Example 2 with Example 3, Example 2 in which the ratio of the mass of the replenishing magnesium chloride to the mass of the molten salt extracted from the molten salt bath was increased, even if the same amount of magnesium chloride was supplied. It can be seen that since the amount of the mixture can be reduced, the temperature rise of the molten salt bath can be suppressed, and the current efficiency has been improved.

DESCRIPTION OF SYMBOLS 1 Molten salt electrolyzer 2 Electrolyzer 2a Electrolyzer 2b Storage chamber 3 Electrode 3a Anode 3b Cathode 3c Bipolar electrode 4 Partition 4a Molten salt circulation path 4b Molten metal flow path 5 Mixing tank 5a Pipe Ps Some molten salt Rs Replenishment chloride Magnesium Ms mixture

Claims (8)

In a molten salt bath for a molten salt containing a molten salt containing magnesium chloride and a supporting salt, the magnesium chloride is electrolyzed on the basis of an electric current to an electrode to obtain metal magnesium by the electrolysis. In replenishing magnesium chloride,
From the molten salt bath, a bath extraction step of extracting a part of the molten salt containing magnesium chloride and a supporting salt, and mixing the molten salt extracted in the bath extraction step with magnesium chloride for replenishment, the molten salt and magnesium chloride for replenishment And a mixture supply step of supplying the mixture to the molten salt bath at a temperature equal to or lower than the melting point of magnesium chloride.   The molten salt electrolysis method according to claim 1, wherein the temperature of the mixture when the mixture is supplied to the molten salt bath in the mixture supplying step is 645C to 700C.   The melting according to claim 1 or 2, wherein the temperature of the mixture when supplying the mixture to the molten salt bath in the mixture supplying step is a temperature at which a difference from the temperature of the molten salt bath is within ± 10 ° C. Salt electrolysis method.   The molten salt electrolysis method according to any one of claims 1 to 3, wherein the temperature of the molten salt bath is maintained at 651 ° C or higher.   The molten salt electrolysis method according to any one of claims 1 to 4, wherein the mixture in the mixture forming step contains 10% to 95% by mass of magnesium chloride and 5% to 90% by mass of a supporting salt.   The molten salt electrolysis method according to any one of claims 1 to 5, wherein the supporting salt includes at least one selected from the group consisting of sodium chloride, calcium chloride, potassium chloride, magnesium fluoride, and calcium fluoride.   The mass ratio of magnesium chloride for replenishment to be mixed with the molten salt in the mixture generation step with respect to the molten salt extracted from the molten salt bath in the bath extraction step is set to 1.0 to 10.0. The molten salt electrolysis method according to claim 1.   A method for producing metallic magnesium, comprising producing metallic magnesium from a molten salt by using the molten salt electrolysis method according to any one of claims 1 to 7.

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