JP5522455B2 - Sodium production method and sodium production apparatus - Google Patents

Description

  The present invention relates to a sodium production method and a sodium production apparatus, and more particularly to a sodium production method and a sodium production apparatus for producing purified sodium by electrolytic purification of impurity-containing sodium.

  The sodium-sulfur battery is a secondary battery that uses sulfur for the positive electrode, sodium for the negative electrode, and β-alumina for the electrolyte. Sodium-sulfur batteries are attracting attention as power storage batteries, and many are manufactured. Accordingly, it is expected that a large amount of used sodium-sulfur batteries will be generated in the future, so that it is required to develop a technology for reusing useful resources such as sodium contained in the used sodium-sulfur batteries.

  The sodium recovered from the used sodium-sulfur battery may be contaminated with impurities such as iron, sulfur and calcium. Therefore, in order to reuse the sodium recovered from the used sodium sulfur battery, it is necessary to remove these impurities.

  In recent years, an electrolytic purification method using an electrolytic solution has been proposed as a method for purifying high-purity sodium from impurity-containing sodium (see, for example, Patent Documents 1 and 2).

In Patent Document 1, electrolysis is performed using an impurity-containing sodium as an anode, a molten salt composed of an aluminum halide (AlCl ₃ ) and an alkali metal halide (NaCl or KCl) as an electrolyte, and high-purity sodium is obtained. A method for deposition on the cathode is described. Patent Document 2 discloses that high-purity sodium is obtained by electrolysis using an impurity-containing sodium as an anode and a solution composed of a carbonate-based organic solvent (ethylene carbonate or propylene carbonate) and a sodium salt (NaPF ₆ or the like) as an electrolyte. Describes a method of depositing on the cathode.

  In the methods described in Patent Documents 1 and 2, at the anode, only sodium contained in the impurity-containing sodium becomes sodium ions and is eluted into the electrolytic solution, and many other impurities remain in the impurity-containing sodium. On the other hand, at the cathode, only sodium (sodium ion) contained in the electrolytic solution is deposited on the surface of the cathode. As a result, high-purity sodium can be produced from the impurity-containing sodium.

  On the other hand, although it is not a technology for purifying sodium, Patent Document 3 discloses a sodium salt of bis (trifluoromethanesulfonyl) imide (hereinafter abbreviated as “TFSI”) and an alkali metal salt of TFSI (excluding sodium salt). A technique for electrodepositing sodium using an ionic liquid composed of the above mixture as an electrolyte is described.

JP 2008-285728 A JP 2010-13673 A International Publication No. 2006/101141 Pamphlet

  However, the methods for purifying sodium described in Patent Documents 1 and 2 may not have sufficient operability, durability or safety of the electrolytic solution.

In the method for purifying sodium described in Patent Document 1, if electrolytic purification is performed for a long time, the aluminum halide (AlCl ₃ ) gradually evaporates, and the composition of the electrolytic solution may change. In such a case, it is necessary to add the evaporated aluminum halide. However, since molten sodium is present on the entire surface of the electrolytic cell, it may be difficult to supply the aluminum halide. It was. Thus, the sodium purification method described in Patent Document 1 has room for further improvement in operability.

  On the other hand, in the method for purifying sodium described in Patent Document 2, if electrolytic purification is performed for a long time, sodium and an organic solvent may react. Thus, when sodium and an organic solvent react, there exists a possibility that electrolyte solution may become unable to function as electrolyte solution. In the method for purifying sodium described in Patent Document 2, when high-purity sodium is to be recovered in a liquid state, the melting point of sodium and the flash point of the organic solvent are close to each other, and there is a risk of ignition. Thus, the sodium purification method described in Patent Document 2 has room for further improvement in the durability and safety of the electrolytic solution.

  The present invention has been made in view of the above points, and is a method and a manufacturing apparatus for manufacturing purified sodium from impurity-containing sodium by electrolytic purification, which is excellent in operability, durability of electrolyte, and safety. It aims at providing the manufacturing method of this, and a sodium manufacturing apparatus.

  The present inventor has found that the above problem can be solved by performing electrolytic purification using an ionic liquid composed of a mixture of a sodium salt of TFSI anion (NaTFSI) and a nonmetallic salt of TFSI anion as an electrolytic solution. The present invention has been completed.

［２］前記イオン液体電解液は、前記ＮａＴＦＳＩおよび前記テトラエチルアンモニウムＴＦＳＩからなり；前記イオン液体電解液中の、前記ＮａＴＦＳＩと前記テトラエチルアンモニウムＴＦＳＩとのモル比は、１：４〜１：９の範囲内である、［１］に記載のナトリウムの製造方法。
［３］前記イオン液体電解液は、前記ＮａＴＦＳＩおよび前記テトラブチルアンモニウムＴＦＳＩからなり；前記イオン液体電解液中の、前記ＮａＴＦＳＩと前記テトラブチルアンモニウムＴＦＳＩとのモル比は、２：３〜１：９の範囲内である、［１］に記載のナトリウムの製造方法。
［４］前記イオン液体電解液の融点は、７０〜１５０℃の範囲内である、［１］〜［３］のいずれか一項に記載のナトリウムの製造方法。
［５］前記不純物含有ナトリウムは、ナトリウム硫黄電池から得られたものである、［１］〜［４］のいずれか一項に記載のナトリウムの製造方法。 That is, the first of the present invention relates to the following method for producing sodium.
[1] A method for producing purified sodium by electrolytic purification of impurity-containing sodium: depositing sodium contained in the impurity-containing sodium on the cathode using the impurity-containing sodium as an anode and an ionic liquid electrolyte as an electrolyte The ionic liquid electrolyte is NaTFSI, which is a sodium salt of a TFSI anion of the following chemical formula (1), and tetraethylammonium TFSI, which is a tetraethylammonium salt of the TFSI anion, or a tetrabutylammonium salt of the TFSI anion. A method for producing sodium, comprising certain tetrabutylammonium TFSI or a mixture thereof.

[2] The ionic liquid electrolyte is composed of the NaTFSI and the tetraethylammonium TFSI; the molar ratio of the NaTFSI and the tetraethylammonium TFSI in the ionic liquid electrolyte is in the range of 1: 4 to 1: 9. The method for producing sodium according to [1], wherein
[3] The ionic liquid electrolyte comprises the NaTFSI and the tetrabutylammonium TFSI; the molar ratio of the NaTFSI and the tetrabutylammonium TFSI in the ionic liquid electrolyte is 2: 3 to 1: 9. The method for producing sodium according to [1], which falls within the range of
[4] The method for producing sodium according to any one of [1] to [3], wherein the melting point of the ionic liquid electrolytic solution is in a range of 70 to 150 ° C.
[5] The method for producing sodium according to any one of [1] to [4], wherein the impurity-containing sodium is obtained from a sodium sulfur battery.

The second of the present invention relates to the following sodium production apparatus.
[6] A sodium production apparatus for producing purified sodium by electrolytic purification of impurity-containing sodium: NaTFSI which is a sodium salt of a TFSI anion of the following chemical formula (1) and tetraethylammonium which is a tetraethylammonium salt of the TFSI anion An ionic liquid electrolyte composed of TFSI or tetrabutylammonium TFSI which is a tetrabutylammonium salt of the TFSI anion, or a mixture thereof; an electrolytic cell containing the ionic liquid electrolyte; an anode containing the impurity-containing sodium; A cathode; and a power supply means for applying a voltage between the anode and the cathode; a pair of electrical conductivity electrically connecting the power supply means and the anode and between the power supply means and the cathode A member; the ionic liquid electrolyte and the impure Heating means for heating the contained sodium; and a voltage is applied between the anode and the cathode in a state where the anode and the cathode are in contact with the ionic liquid electrolyte, and the contained sodium is contained in the impurity-containing sodium. A sodium production device that deposits sodium on the cathode.

  According to the present invention, purified sodium can be produced from impurity-containing sodium more easily and safely than conventional techniques.

Sectional drawing which shows the structure of the sodium manufacturing apparatus which concerns on one embodiment of this invention The schematic diagram which shows a mode that sodium is manufactured using the sodium manufacturing apparatus of FIG. Sectional drawing which shows the structure of the sodium manufacturing apparatus which concerns on another embodiment of this invention. The schematic diagram which shows a mode that sodium is manufactured using the sodium manufacturing apparatus of FIG. Sectional drawing which shows the structure of the sodium manufacturing apparatus used in the Example The graph which shows the result of the fluorescent X-ray elemental analysis of the black residue obtained by experiment of the comparative example

1. About the method for producing sodium of the present invention The method for producing sodium of the present invention is a method for producing purified sodium by electrolytic purification of impurity-containing sodium, using the impurity-containing sodium as an anode and an ionic liquid electrolyte as an electrolyte. Performing decomposition and depositing sodium contained in the impurity-containing sodium on the cathode.

  In the present specification, “impurity-containing sodium” means a composition containing sodium as a main component and containing impurities other than sodium, such as iron, sulfur, and calcium. Further, “purified sodium” means a composition containing sodium as a main component and having a lower impurity content than the above-mentioned impurity-containing sodium.

  Further, in the present specification, a solution in which a salt made of an inorganic material is dissolved is referred to as a “molten salt”, and a solution in which a salt containing an organic material is dissolved is referred to as an “ionic liquid”. Therefore, in the present specification, any salt in which a salt containing an organic substance is dissolved is referred to as an “ionic liquid” regardless of whether the melting point exceeds 100 ° C.

  When electrolysis is performed using impurity-containing sodium as an anode and an ionic liquid electrolyte as an electrolyte as described above, sodium contained in the impurity-containing sodium becomes sodium ions and elutes into the ionic liquid electrolyte at the anode. The ionization tendency of sodium with respect to an ionic liquid electrolyte described later is larger than the ionization tendency of impurities such as iron, sulfur and calcium. Therefore, only sodium is converted into sodium ions and eluted into the ionic liquid electrolyte, and impurities such as iron, sulfur and calcium remain in the impurity-containing sodium. Further, since the insulator does not dissolve in the ionic liquid electrolyte, the insulator such as ceramics remains in the impurity-containing sodium.

  On the other hand, at the cathode, sodium ions contained in the ionic liquid electrolyte are deposited as sodium on the surface of the cathode. In the electrolytic purification method of sodium, when calcium ions are contained in the electrolytic solution, it is particularly problematic that calcium is precipitated together with sodium. In the method for producing sodium of the present invention, by using an ionic liquid electrolyte described later, the difference in precipitation potential between sodium and calcium can be set to 300 mV or more. Can be prevented. The cathode is not particularly limited as long as sodium can be deposited on the surface thereof. For example, purified sodium or a conductive member can be used.

  As described above, in the method for producing sodium of the present invention, sodium is eluted from the impurity-containing sodium into the ionic liquid electrolytic solution, and sodium contained in the ionic liquid electrolytic solution is precipitated on the surface of the cathode, so that Only sodium can be removed.

[About ionic liquid electrolyte]
The method for producing sodium of the present invention uses an ionic liquid comprising a mixture of 1) a sodium salt of TFSI anion (NaTFSI) and 2) a non-metallic salt of TFSI anion as an electrolytic solution for electrolytic purification. One feature.

Here, the “TFSI anion” means a bis (trifluoromethanesulfonyl) imide (TFSI) anion represented by the following chemical formula (1). The anion of the following chemical formula (1) is considered to be an amide rather than an imide based on the IUPAC nomenclature, but in this specification, “TFSI (bis (trifluoromethanesulfonyl) imide”, which is widely used as a common name, is used. ) ".

  First, the electrolytic solution used in the method for producing sodium of the present invention contains sodium bis (trifluoromethanesulfonyl) imide (NaTFSI) which is a sodium salt of a TFSI anion. Since the TFSI anion does not react with sodium, the durability of the electrolytic solution can be improved by using NaTFSI ionic liquid as the electrolytic solution. Moreover, since NaTFSI has a low vapor pressure and is nonflammable, there is no need for additional input of NaTFSI and danger of ignition. Therefore, operability and safety can be improved by using NaTFSI ionic liquid as the electrolyte.

  As a preliminary experiment, the present inventor performed electrolytic purification of sodium at 230 ° C., which is the melting point of NaTFSI, using an electrolytic solution composed only of NaTFSI ionic liquid. As a result, good results were obtained for the durability and safety of the electrolyte, but a black film was observed to form on the surface of the purified sodium deposited on the cathode, and the purity of the purified sodium decreased. (See the comparative example below). This is presumably because part of the TFSI anion was thermally decomposed due to electrolytic purification at a high temperature (230 ° C.), and the sulfur generated by the thermal decomposition reacted with the purified sodium deposited on the cathode. It was.

  Therefore, from the viewpoint of preventing the purity of this purified sodium from being lowered, the ionic liquid electrolyte used in the method for producing sodium of the present invention further contains a non-metallic salt of a TFSI anion in addition to the NaTFSI described above. By adding a non-metallic salt of TFSI anion, the melting point of the electrolytic solution can be lowered, and the TFSI anion can be prevented from thermally decomposing and reacting with purified sodium during electrolytic purification.

  As the non-metallic salt of the TFSI anion, tetraalkylammonium TFSI which is a tetraalkylammonium salt of the TFSI anion is preferable. The non-metal salt of the TFSI anion may react with sodium or may not be mixed with NaTFSI depending on the type, but tetraalkylammonium TFSI does not react with sodium and can be mixed uniformly with NaTFSI. Examples of tetraalkylammonium TFSI are tetraethylammonium salts of TFSI anions, tetraethylammonium bis (trifluoromethanesulfonyl) imide (hereinafter abbreviated as “tetraethylammonium TFSI”), and tetrabutylammonium salts of TFSI anions. And tetrabutylammonium bis (trifluoromethanesulfonyl) imide (hereinafter abbreviated as “tetrabutylammonium TFSI”). These compounds may be used alone or in combination of two types.

  By adding these tetraalkylammonium TFSI to an electrolytic solution consisting only of NaTFSI ionic liquid, the melting point of the electrolytic solution can be lowered to about 70 to 150 ° C. without reducing the durability and safety of the electrolytic solution. it can. Although it is difficult to clearly specify the melting point of these tetraalkylammonium TFSIs, tetraethylammonium TFSI is surely liquid at about 60 ° C. and tetrabutylammonium TFSI is definitely liquid at about 40 ° C.

  In addition, when performing electrolytic purification of sodium using an electrolytic solution composed only of NaTFSI ionic liquid, the difference in deposition potential between sodium ions and calcium ions is only 30 mV, so when calcium ions are present in the electrolytic solution, There was a possibility that calcium was precipitated together with sodium. On the other hand, the difference in the precipitation potential between sodium ions and calcium ions can be increased to 300 mV by adding the non-metal salt of the TFSI anion to an electrolyte solution composed only of NaTFSI ionic liquid. Therefore, even when calcium ions are present in the electrolytic solution, only sodium can be deposited easily and reliably.

  It should be noted that the melting point of the electrolyte can be lowered even if a metal salt of TFSI anion (excluding sodium salt) is added to the electrolyte composed only of NaTFSI ionic liquid (see Patent Document 3). However, when a metal salt of TFSI anion is added, metals other than sodium are precipitated together with sodium, and the purity of purified sodium may be reduced. In order to purify high-purity sodium, it is preferable that the electrolytic solution does not contain a metal element other than sodium. Therefore, in the method for producing sodium of the present invention, it is preferable that the ionic liquid electrolyte does not contain a metal salt of TFSI anion (except for a sodium salt) but contains a non-metal salt of TFSI anion (tetraalkylammonium TFSI).

  The molar ratio of 1) NaTFSI and 2) non-metal salt of TFSI anion (tetraalkylammonium TFSI) can sufficiently reduce the melting point of the ionic liquid, and each ionic liquid can be mixed without separation. If it is in, it will not specifically limit. Specifically, when the ionic liquid electrolyte is composed of NaTFSI and tetraethylammonium TFSI, the molar ratio of NaTFSI to tetraethylammonium TFSI is preferably in the range of 1: 4 to 1: 9. Moreover, when the ionic liquid electrolyte is composed of NaTFSI and tetrabutylammonium TFSI, the molar ratio of NaTFSI and tetrabutylammonium TFSI is preferably in the range of 2: 3 to 1: 9.

  By setting the molar ratio within the above range, the melting point of the ionic liquid electrolyte can be lowered to about 70 to 150 ° C., and the difference in precipitation potential between sodium ions and calcium ions is increased to about 0.3V. Can do. When the ratio of tetraethylammonium TFSI or tetrabutylammonium TFSI is smaller than this, the melting point of the ionic liquid electrolyte cannot be lowered sufficiently, and thermal decomposition of the TFSI anion cannot be prevented. On the other hand, when the ratio of tetraethylammonium TFSI or tetrabutylammonium TFSI is larger than this, it is separated without being mixed with the ionic liquid of NaTFSI.

  Particularly preferably, when the ionic liquid electrolyte is composed of NaTFSI and tetraethylammonium TFSI, the molar ratio of NaTFSI and tetraethylammonium TFSI is 1: 4, and when the ionic liquid electrolyte is composed of NaTFSI and tetrabutylammonium TFSI. The molar ratio of NaTFSI to tetrabutylammonium TFSI is 2: 3. This is because the concentration of sodium ions becomes higher and the current flows more easily.

  When using the ionic liquid electrolytic solution, the temperature of the electrolytic solution at the time of electrolytic purification is not particularly limited as long as it is equal to or higher than the melting point, but from the viewpoint of preventing current from flowing and preventing the reaction between purified sodium and the electrolytic solution. In the range of 120 to 170 ° C, about 160 ° C is particularly preferable. If it exceeds 180 ° C., purified sodium and the electrolytic solution may start to react.

[Voltage applied between anode and cathode]
In the method for producing sodium of the present invention, electrolysis is performed by applying a DC voltage between the anode (impurity-containing sodium) and the cathode. At this time, the voltage applied between the electrodes is not particularly limited as long as it is a voltage higher than an overvoltage necessary for the reaction and only sodium is dissolved in the ionic liquid electrolyte as sodium ions at the anode. If the voltage applied between the electrodes is too large, impurities such as iron, sulfur and calcium may elute into the electrolyte at the anode. The potential of the cathode on which sodium is deposited may be appropriately set according to the temperature of the electrolytic solution, the material of the conductive member, and the like. For example, it is −3.5 V or less, preferably −3 V or less with respect to the platinum reference electrode. That's fine.

[How to monitor the elution of impurities]
In the method for producing sodium of the present invention, if most of the sodium contained in the impurity-containing sodium is eluted in the ionic liquid electrolyte, the impurities remaining in the impurity-containing sodium may also be eluted in the electrolyte. There is. If impurities such as iron, sulfur and calcium are eluted in the electrolyte, these impurities will be deposited on the surface of the cathode before sodium, so that the purity of sodium will be reduced. . Therefore, in the present invention, in order to monitor the elution of impurities from the impurity-containing sodium, it is preferable to perform electrolytic purification by constant current electrolysis or constant potential (voltage) electrolysis.

  For example, when purifying sodium by constant current electrolysis, the elution of impurities at the anode can be monitored by monitoring the voltage between the anode and the cathode when a current is applied at a constant current density. it can. That is, while only the sodium is eluted from the impurity-containing sodium, the voltage shows a substantially constant value, but after the impurities start to elute, the voltage rapidly increases.

  Similarly, when purifying sodium by controlled potential electrolysis, the elution of impurities at the anode should be monitored by monitoring the current value when the voltage between the anode and cathode is maintained at a constant value. Can do. That is, while only sodium is eluted from the impurity-containing sodium, the current shows a substantially constant value, but after the impurity starts to elute, the current value rapidly decreases.

  Thus, the presence or absence of impurity elution can be confirmed by monitoring the presence or absence of a sudden change in voltage or current. In order to purify high-purity sodium, it is sufficient to further provide the anode with impurity-containing sodium as a raw material before the impurities are eluted into the ionic liquid electrolyte (that is, before a sudden change in voltage or current occurs). . By doing in this way, since elution of impurities can be prevented, only sodium can be deposited on the cathode.

2. About the sodium manufacturing apparatus of this invention The sodium manufacturing method of this invention should just be implemented by carrying out the electrolytic purification of impurity containing sodium, for example using the apparatus shown below.

  FIG. 1 is a schematic view (sectional view) showing an example of the sodium production apparatus of the present invention. In this example, impurity-containing sodium is used as an anode, and purified sodium is used as a cathode.

  In FIG. 1, the sodium production apparatus 100 includes an electrolytic bath 110 that contains an ionic liquid electrolytic solution 170, a partition wall 120, a heating unit 130, a first conductive member 140, a second conductive member 150, and a power source 160. When using the sodium purification apparatus 100, as shown in FIG. 1, the impurity-containing sodium 180 is disposed so as to come into contact with the first conductive member 140 and the ionic liquid electrolyte 170, and the purified sodium 190 is secondly added. The conductive member 150 and the ionic liquid electrolytic solution 170 are arranged in contact with each other.

  The electrolytic cell 110 is an insulating container that stores the ionic liquid electrolytic solution 170. As long as the material of the electrolytic cell 110 is heat resistant enough to withstand the temperature during electrolysis of the ionic liquid electrolytic solution 170, the impurity-containing sodium 180, and the purified sodium 190, and does not react with sodium and the ionic liquid electrolytic solution 170, There is no particular limitation. For example, as the electrolytic cell 110, a container made of polytetrafluoroethylene (PTFE), a container made of alumina ceramics, or the like can be used.

  The partition wall 120 is an insulating member that isolates the impurity-containing sodium 180 and the purified sodium 190 when the impurity-containing sodium 180 and the purified sodium 190 are provided in the electrolytic cell 110. The partition wall 120 is made of a material that has heat resistance that can withstand the temperature during electrolysis of the ionic liquid electrolyte 170, the impurity-containing sodium 180, and the purified sodium 190, and that does not react with sodium and the ionic liquid electrolyte 170. It is not limited. For example, as the partition wall 120, a member made of polytetrafluoroethylene (PTFE), a member made of alumina ceramics, or the like can be used.

  The heating unit 130 heats the ionic liquid electrolyte 170, the impurity-containing sodium 180, and the purified sodium 190 to a temperature at which electrolysis can be performed. For example, the heating unit 130 is a heater disposed on the lower surface and / or the side surface of the electrolytic cell 110. What is necessary is just to heat the temperature which the heating part 130 heats, for example so that the ionic liquid electrolyte solution 170 may become about 120-170 degreeC (preferably 160 degreeC). At this temperature, the impurity-containing sodium 180 and the purified sodium 190 are also melted into a liquid state.

  The first conductive member 140 is electrically connected to the power supply 160 and is a conductive member that applies a positive voltage to the impurity-containing sodium 180 (anode). Similarly, the second conductive member 150 is electrically connected to the power source 160 and is a conductive member that applies a negative voltage to the purified sodium 190 (cathode). The materials of the first conductive member 140 and the second conductive member 150 have heat resistance that can withstand the temperature during electrolysis of the ionic liquid electrolytic solution 170, the impurity-containing sodium 180, and the purified sodium 190, and sodium and There is no particular limitation as long as it does not react with the ionic liquid electrolyte 170. For example, as the first conductive member 140 or the second conductive member 150, a member made of a refractory metal such as tungsten or molybdenum or glassy carbon can be used. The first conductive member 140 and the second conductive member 150 are electrically connected to the impurity-containing sodium 180 or the purified sodium 190 even if the amount of the impurity-containing sodium 180 or the purified sodium 190 changes with the progress of electrolysis. It is preferable that they are arranged so that the respective connections can be maintained.

  The power supply 160 applies a DC voltage between the electrodes using the impurity-containing sodium 180 as an anode and the purified sodium 190 as a cathode. What is necessary is just to set suitably the voltage applied between the impurity containing sodium 180 and the refinement | purification sodium 190 according to the temperature of the ionic liquid electrolyte 170, the material of the electroconductive members 140 and 150, etc. FIG.

  FIG. 2 is a schematic diagram showing a state in which sodium is purified using the sodium purification apparatus 100. In FIG. 2, the heating unit 130 and the power source 160 are not shown.

  As shown in FIG. 2A, when a DC voltage is applied between the electrodes using the impurity-containing sodium 180 as the anode and the purified sodium 190 as the cathode, the sodium contained in the impurity-containing sodium 180 is sodium ions at the anode (impurity-containing sodium 180). It elutes into the ionic liquid electrolyte 170, and at the cathode, sodium ions contained in the ionic liquid electrolyte 170 are deposited on the surface of the purified sodium 190 as sodium. Since this reaction continues as long as a DC voltage is applied between the electrodes, as shown in FIG. 2B, the impurity-containing sodium 180 decreases with the passage of time, and the purified sodium 190 increases with the passage of time. Therefore, the purified sodium 190 can be produced from the impurity-containing sodium 180 by providing the impurity-containing sodium 180 on the anode side and collecting the purified sodium 190 deposited on the cathode side. The method for recovering the purified sodium 190 is not particularly limited. For example, as shown in FIG. 2B, the height of the side wall on the cathode side of the electrolytic cell 110 is partially reduced, and the purified sodium 190 flows out of the electrolytic cell 110. You can make it.

  FIG. 3 is a schematic view (cross-sectional view) showing another example of the sodium purification apparatus of the present invention. In this example, the impurity-containing sodium is used as an anode, and the conductive member is used as a cathode. In FIG. 3, the same components as those in the sodium purification apparatus in FIG.

  In FIG. 3, the sodium purification apparatus 200 includes an electrolytic bath 110 that contains an ionic liquid electrolytic solution 170, a partition wall 120, a heating unit 130, a first conductive member 140, a second conductive member 210, and a power source 160. When using the sodium purification apparatus 200, as shown in FIG. 3, the impurity-containing sodium 180 is disposed so as to be in contact with the first conductive member 140 and the ionic liquid electrolyte 170.

  The second conductive member 210 is a conductive member that is electrically connected to the power source 160 and that also functions as a cathode. The material of the second conductive member 210 can be the same as that of the above-described conductive member. The second conductive member 210 is preferably immersed in the ionic liquid electrolytic solution 170.

  FIG. 4 is a schematic diagram showing how sodium is purified using the sodium purification apparatus 200. In FIG. 4, the heating unit 130 and the power source 160 are not shown.

  As shown in FIG. 4A, when a DC voltage is applied between the electrodes using the impurity-containing sodium 180 as an anode and the second conductive member 210 as a cathode, sodium contained in the impurity-containing sodium 180 becomes sodium ions at the anode. The sodium ion contained in the ionic liquid electrolytic solution 170 is deposited as sodium on the surface of the second conductive member 210 at the cathode. Since this reaction continues as long as a DC voltage is applied between the electrodes, as shown in FIG. 4B, the impurity-containing sodium 180 decreases with the passage of time, and the purified sodium 190 increases with the passage of time. When a certain amount of purified sodium 190 is deposited on the surface of the second conductive member 210 (cathode), the deposited purified sodium 190 floats on the ionic liquid electrolyte 170. Therefore, the purified sodium 190 can be produced from the impurity-containing sodium 180 by providing the impurity-containing sodium 180 on the anode side and recovering the purified sodium 190 floating on the cathode side.

  As described above, the sodium production method and sodium production apparatus according to the present invention comprises an ionic liquid composed of a mixture of NaTFSI and a nonmetallic salt of TFSI anion (tetraalkylammonium TFSI), which is excellent in operability, durability and safety. Since it is used as an electrolyte, purified sodium can be produced from impurity-containing sodium more easily and safely than conventional techniques.

  For example, the sodium production method and sodium production apparatus of the present invention are suitable for purifying sodium from sodium containing impurities such as iron, sulfur and calcium collected from a used sodium sulfur battery.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail with reference to an Example, this invention is not limited by these Examples.

[Example]
HTFSI (99%; Kanto Chemical Co., Inc.) was dissolved in water to prepare a 0.1 mol / dm ³ aqueous HTFSI solution. Similarly, sodium hydroxide (99.99%) was dissolved in water to prepare a 0.1 mol / dm ³ sodium hydroxide aqueous solution. NaTFSI was synthesized by mixing the same amount of these aqueous solutions and removing water after neutralization.

  The obtained NaTFSI and tetraethylammonium TFSI (99.0%; Kishida Chemical Co., Ltd.) were placed in a glass beaker, and the mixture was heated to prepare an ionic liquid electrolyte solution composed of NaTFSI and tetraethylammonium TFSI. The molar ratio of NaTFSI to tetraethylammonium TFSI was 1: 4 or 1: 9.

  Similarly, NaTFSI and tetrabutylammonium TFSI (Tokyo Chemical Industry Co., Ltd.) were put in another glass beaker, and the mixture was heated to prepare an ionic liquid electrolytic solution composed of NaTFSI and tetrabutylammonium TFSI. The molar ratio of NaTFSI to tetrabutylammonium TFSI was 2: 3 or 1: 9.

  As shown in FIG. 5, the anode 330, the cathode 340, and the reference electrode 350 were immersed in an ionic liquid electrolytic solution 320 in a glass beaker (electrolyzer) 310. Metal sodium (99%; Aldrich) was used for the anode 330. Metal sodium was cut out from a block-like lump using a cutter knife, was sandwiched between clips, and immediately immersed in the electrolyte. This metallic sodium is known to contain a small amount of calcium. Glassy carbon was used for the cathode 340. A platinum wire was used for the reference electrode 350. The anode 330, the cathode 340, and the reference electrode 350 are connected to the DC power supply device 370. In addition, the temperature of the electrolytic solution 320 was maintained at 120 ° C., 140 ° C., or 160 ° C. using the heater 360.

  When electrolysis was performed for 2 hours at a constant current of 20 mA using the electrolysis cell having the above configuration, it was confirmed that liquid sodium was deposited on the cathode 340.

  Impurity concentration was measured using an ICP emission spectrometer for sodium before purification as an anode and purified sodium deposited on the cathode. Since it is known that the metallic sodium used as the anode contains a slight amount of calcium, in this example, quantitative analysis was performed on calcium. As the sample, about 100 mg of sodium (sodium before purification or sodium after purification) dissolved in 500 mL of water was used. When the concentration of calcium in sodium was calculated from the result of quantitative analysis, the sodium before purification contained 540 ppm of calcium. On the other hand, the purified sodium contained 40 to 380 ppm of calcium.

Table 1 shows the calcium concentration in the sodium after purification and the removal rate of calcium in the sodium after purification relative to the sodium before purification.

  From the results in Table 1, it can be seen that by the method of the present invention, 30 to 93% of calcium was removed from the raw material sodium metal to produce higher purity sodium.

[Comparative example]
Only NaTFSI obtained by the same procedure as in the example was placed in a glass beaker and heated to 230 ° C. to prepare an ionic liquid electrolytic solution consisting only of NaTFSI.

  Electrolysis was performed for 2 hours at a constant current of 20 mA using the same electrolytic cell as in the example (see FIG. 5). At this time, the temperature of the electrolytic solution was maintained at 230 ° C. using a heater. As a result, it was confirmed that liquid sodium was deposited on the cathode. It was also confirmed that a thin black film was formed on the surface of the precipitated sodium.

  Quantitative analysis was performed on the unpurified sodium used as the anode and the purified sodium deposited on the cathode using an ICP emission spectrometer. The sample of sodium before purification was obtained by dissolving 44 mg of sodium in 500 mL of water. As the sample of sodium after purification, 40 mg of sodium (including black film) was dissolved in 500 mL of water and filtered. In this comparative example, sodium and sulfur were quantitatively analyzed, and the amounts of sodium and sulfur in sodium were calculated from the results of the quantitative analysis. As a result, sodium before purification contained 43 mg of sodium and 0.0 mg of sulfur. On the other hand, the purified sodium contained 18 mg sodium and 18 mg sulfur.

  Moreover, the black residue obtained when the aqueous solution of sodium after purification was filtered was placed on an iron substrate and analyzed by fluorescent X-ray elemental analysis. As a result, as shown in FIG. 6, sulfur was detected. In the graph of FIG. 6, an iron peak is observed, which is due to the iron substrate.

  As described above, when electrolytic purification is performed at 230 ° C. using an ionic liquid electrolytic solution consisting only of NaTFSI, a black film containing sulfur is formed on the surface of the purified sodium, and the purity of the purified sodium is reduced. It was. This black film is considered to be a compound of sulfur and sodium, which is purified by the reaction of the TFSI anion in the electrolytic solution with the active metal and decomposition, and the resulting sulfur reacts with purified sodium.

  According to the present invention, sodium can be produced (purified) from impurity-containing sodium by using low-temperature equipment and an inexpensive electrolyte without generating chlorine gas and with low energy consumption. For example, the sodium production method and sodium production apparatus of the present invention are useful for recycling sodium recovered from a used sodium sulfur battery.

DESCRIPTION OF SYMBOLS 100,200 Sodium production apparatus 110 Electrolysis tank 120 Partition 130 Heating part 140 1st electroconductive member 150,210 2nd electroconductive member 160 Power supply 170,320 Ionic liquid electrolyte 180 Impurity containing sodium 190 Purified sodium 310 Glass beaker 330 Anode 340 Cathode 350 Reference electrode 360 Heater 370 DC power supply

Claims (6)

A method for producing purified sodium by electrolytically purifying impurity-containing sodium,
Using the impurity-containing sodium as an anode and an ionic liquid electrolyte as an electrolyte, and depositing sodium contained in the impurity-containing sodium on a cathode,
The ionic liquid electrolyte includes NaTFSI which is a sodium salt of a TFSI anion of the following chemical formula (1), tetraethylammonium TFSI which is a tetraethylammonium salt of the TFSI anion, or tetrabutylammonium TFSI which is a tetrabutylammonium salt of the TFSI anion. Or consisting of these,
Method for producing sodium.

The ionic liquid electrolyte comprises the NaTFSI and the tetraethylammonium TFSI,
2. The method for producing sodium according to claim 1, wherein a molar ratio of the NaTFSI and the tetraethylammonium TFSI in the ionic liquid electrolytic solution is within a range of 1: 4 to 1: 9. The ionic liquid electrolyte comprises the NaTFSI and the tetrabutylammonium TFSI,
2. The method for producing sodium according to claim 1, wherein a molar ratio of NaTFSI and tetrabutylammonium TFSI in the ionic liquid electrolyte is in a range of 2: 3 to 1: 9.   The method for producing sodium according to claim 1, wherein the ionic liquid electrolyte has a melting point in the range of 70 to 150 ° C.   The method for producing sodium according to claim 1, wherein the impurity-containing sodium is obtained from a sodium sulfur battery. A sodium production apparatus for producing purified sodium by electrolytic purification of impurity-containing sodium,
NaTFSI, which is a sodium salt of a TFSI anion of the following chemical formula (1), and tetraethylammonium TFSI, which is a tetraethylammonium salt of the TFSI anion, or tetrabutylammonium TFSI, which is a tetrabutylammonium salt of the TFSI anion, or a mixture thereof. An ionic liquid electrolyte,
An electrolytic cell containing the ionic liquid electrolytic solution;
An anode containing the impurity-containing sodium;
A cathode,
Power supply means for applying a voltage between the anode and the cathode;
A pair of conductive members that electrically connect between the power supply means and the anode and between the power supply means and the cathode;
Heating means for heating the ionic liquid electrolyte and the impurity-containing sodium;
Have
A voltage is applied between the anode and the cathode in a state where the anode and the cathode are in contact with the ionic liquid electrolytic solution, and sodium contained in the impurity-containing sodium is deposited on the cathode.
Sodium production equipment.

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