Electrolytic reduction apparatus and electrolytic reduction method

Abstract

To provide an electrolytic reduction apparatus and an electrolytic reduction method, in which reduction efficiency can be improved by suppressing side reactions compared to the conventional ones, thereby reducing measures for by-products.SOLUTION: An electrolytic reduction apparatus comprises: a cathode 3 accommodating a metallic oxide 5 as a reduction target; an anode 2; a molten salt 4; a direct-current power supply 6; and a measurement electrode 10 immersed in the molten salt 4. The electrolytic reduction apparatus is a device for reducing the metallic oxide 5 by energizing between the cathode 3 and the anode 2 through the molten salt 4 using the direct-current power supply 6. The apparatus also calculates product concentration in the molten salt 4 from a current value when a specific electric potential is applied to the measurement electrode 10, and controls the applied potential to between the cathode 3 and the anode 2 on the basis of the calculated product concentration.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrolytic reduction apparatus and method in which a cathode and an anode in contact with a metal oxide to be reduced are immersed in a molten salt and a current is passed between the cathode and the anode to reduce the metal oxide. [Background technology]

[0002] Patent Document 1 describes an electrolytic reduction device including a cathode that holds an oxide to be reduced, an anode, molten salt in which the oxide and the anode are immersed, a container that contains the molten salt, and a DC power source that applies a current to the cathode and the anode to reduce the oxide, in which the molten salt is a lithium chloride-potassium chloride eutectic salt. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-166094 A Summary of the Invention [Problem to be solved by the invention]

[0004] Spent fuel generated from nuclear power plants contains uranium and transuranic elements, as well as fission products such as alkali metals, alkaline earth metals, and rare earth elements. Uranium and plutonium can be reused as fuel through a reprocessing process that separates and removes transuranic elements other than plutonium and fission products.

[0005] When spent oxide fuel or mixed oxide fuel (MOX fuel) is reused as metallic fuel, it is necessary to reduce the oxide to a metal, and electrolytic reduction is known as a reduction method.

[0006] In a conventional electrolytic reduction method, an electrolytic cell containing a cathode, an anode, and a molten salt is used, a metal oxide is held on the cathode side, and the metal oxide is reduced to a metal by receiving electrons from the cathode, and the molten salt contains cations of a metal (hereinafter referred to as a reducing metal) that can chemically reduce the metal oxide, and the electrolytic cell is operated at a potential higher than the potential at which the cations can receive electrons from the cathode and be reduced to deposit as a metal on the cathode, whereby the metal oxide is chemically reduced by the reducing metal deposited on the cathode. In Patent Document 1, a basket-type cathode is used as the cathode that holds the metal oxide to be reduced.

[0007] Spent fuel generated from nuclear power plants contains uranium and transuranic elements as well as fission products such as alkali metals, alkaline earth metals, and rare earth elements. Of these, uranium and plutonium can be reused as fuel after going through a reprocessing process that separates and removes transuranic elements other than plutonium and fission products.

[0008] When spent oxide fuel or mixed oxide fuel (MOX fuel) is reused as metallic fuel, it is necessary to reduce the oxide to a metal, and electrolytic reduction is known as a reduction method.

[0009] In Patent Document 1, a basket-type cathode is used as the cathode that holds the metal oxide to be reduced. In conventional methods, Li2O and other by-products are generated by electrolytic reduction. The by-product Li2O promotes the occurrence of side reactions, increasing the amount of by-products generated. This increases the amount of by-products, which poses a problem in that additional equipment and operations are required to remove the by-products.

[0010] An object of the present invention is to provide an electrolytic reduction device and an electrolytic reduction method that can improve reduction efficiency by suppressing side reactions compared to conventional methods and reducing the need to deal with by-products. [Means for solving the problem]

[0011] The present invention includes a number of means for solving the above-mentioned problems. One example is an electrolytic reduction device that includes a cathode containing a metal oxide to be reduced, an anode, a molten salt, a power source, and a measurement electrode immersed in the molten salt, and reduces the metal oxide by passing a current between the cathode and the anode via the molten salt from the power source, the device determining a product concentration in the molten salt from a current value when a specific potential is applied to the measurement electrode, and controlling the potential applied between the cathode and the anode based on the determined product concentration. Effect of the Invention

[0012] According to the present invention, by suppressing side reactions compared to the conventional art, it is possible to reduce the amount of by-products to be dealt with and improve the reduction efficiency. Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a configuration diagram of an electrolytic reduction device according to a first embodiment. [Diagram 2] FIG. 2 is an operation flow diagram of the electrolytic reduction apparatus of the first embodiment. [Diagram 3] FIG. 13 is a diagram showing an example of the polarization curve measurement results of a side reaction. [Figure 4] FIG. 11 is a diagram showing an example of a calibration curve to be plotted. [Diagram 5] FIG. 4 is a diagram showing a flow of determining electrolysis conditions. [Figure 6] FIG. 13 is a diagram showing an example of the amount of by-products produced. [Figure 7] FIG. 1 is a configuration diagram of an electrolytic reduction device according to a second embodiment. [Figure 8] FIG. 11 is a configuration diagram of an electrolytic reduction device according to a third embodiment. [Figure 9] FIG. 11 is a configuration diagram of an electrolytic reduction device according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the electrolytic reduction apparatus and electrolytic reduction method of the present invention will be described with reference to the drawings. In the drawings used in this specification, the same or similar reference numerals are used to designate the same or corresponding components, and repeated description of these components may be omitted.

[0015] <Example 1> A first embodiment of the electrolytic reduction apparatus and electrolytic reduction method of the present invention will be described with reference to FIGS. 1 to 6. FIG.

[0016] First, the overall configuration of the electrolytic reduction device will be described with reference to Fig. 1. Fig. 1 is a diagram showing the configuration of the electrolytic reduction device of the present embodiment 1.

[0017] The electrolytic reduction device 1 shown in FIG. 1 includes an anode 2, a cathode 3 containing a metal oxide 5 to be reduced, a molten salt 4, a DC power source 6, a reaction vessel 7, a reference electrode 8, a temperature control mechanism 9, a measurement electrode 10 to be immersed in the molten salt 4, and the like.

[0018] This electrolytic reduction device 1 is a device for reducing a metal oxide 5 held by the cathode 3 or held by a holder electrically connected to the cathode 3 by passing a current between the cathode 3 and the anode 2 via the molten salt 4 using a DC power source 6. More specifically, by applying a potential difference between the anode 2 and the cathode 3 to the molten salt 4, the metal oxide 5 in electrical contact with the cathode 3 donates electrons from the cathode 3, thereby reducing the metal oxide 5 in a solid state to the target metal. In addition, cations contained in the molten salt 4 are precipitated on the cathode 3 as a reducing metal, and by coming into contact with the metal oxide 5, the metal oxide 5 is reduced to the target metal. At that time, the reducing metal becomes an oxide, which is a by-product.

[0019] In the electrolytic reduction device 1 of the present invention, the product concentration in the molten salt 4 is determined from the current value when a specific potential is applied between the measurement electrode 10 and the anode 2, and the potential applied between the cathode 3 and the anode 2 is controlled based on the determined product concentration. The details will be described later.

[0020] When the molten salt 4 is LiCl, the reducing metal is metallic Li, and the by-product is Li2O. The potential applied during energization is set to be equal to or higher than the side reaction occurrence potential measured between the reference electrode 8 and the measurement electrode 10.

[0021] The potential at which the side reaction occurs is measured based on the operation flow diagram of FIG. 2, and the electrolysis start condition of the metal oxide 5 is determined.

[0022] In the operation flow diagram of FIG. 2, first, a polarization curve measurement of a side reaction is acquired in advance for each by-product concentration (S21), and the polarization curve measurement result as shown in FIG. 3 is obtained.

[0023] Then, the potential at which a side reaction occurs for each by-product concentration is identified from the obtained polarization curve measurement. In addition, a calibration curve of the by-product concentration and current density at an arbitrary potential is obtained from the obtained polarization curve as shown in Figure 4 (S22).

[0024] Thereafter, the conditions of the electrolysis voltage to be applied between the anode 2 and the cathode 3 are set based on the obtained calibration curve (S23). For example, when the product concentration determined based on the calibration curve is equal to or higher than a predetermined standard, the applied potential for electrolysis is set to be changed to a potential that suppresses the occurrence of side reactions.

[0025] It is desirable to obtain the calibration curve in the above-mentioned step S22 each time the specifications, such as the type and size of the anode 2 and cathode 3, their installation positions in the reaction vessel 7, the type of molten salt 4, and the installation position and type of the measurement electrode 10, are changed.

[0026] The calibration curve created in step S22 is used during electrolytic reduction based on the electrolysis condition determination flow shown in FIG.

[0027] Specifically, as shown in Fig. 5, after starting electrolysis of metal oxide 5 (S51), at an arbitrary time, a potential of the same value as that used when creating the calibration curve in Fig. 4 is applied to measurement electrode 10, the current density at that time is read, and the by-product (here, Li2O) concentration is calculated from the current density using the calibration curve (S52). Then, it is determined whether the calculated by-product concentration has increased (S53), and if it is determined that the concentration has increased, the applied potential during electrolysis is increased (S54).

[0028] On the other hand, if there is no change in the current density, or if the current density decreases due to a change in the applied potential, the electrolysis conditions are maintained and electrolysis is continued (No in S53).

[0029] After a certain period of time has elapsed, judgment is made again using the judgment flow of the electrolysis conditions in Fig. 5. It is desirable to obtain the application of the measurement potential every 0.5 to 1 hour.

[0030] In the above-mentioned flow of FIG. 5, it is desirable to temporarily suspend the electrolytic reduction at the timing when a specific potential is applied to the measuring electrode 10 in step S52.

[0031] By evaluating the increase in by-product concentration in real time during electrolytic reduction from the relationship between the current density measured by the measurement electrode and the calibration curve, it becomes possible to perform electrolysis at a potential that suppresses the occurrence of side reactions. As a result, as shown in Figure 6, the amount of by-products generated can be suppressed compared to conventional methods.

[0032] In this embodiment, the metal oxide 5 is UO2, which is a spent oxide fuel from a nuclear power plant, but is not limited thereto and may be, for example, a mixed oxide fuel or a mineral.

[0033] The molten salt 4 may be any one of an alkali metal halide, an alkaline earth metal halide, and an ionic liquid, for example, LiCl, but is not limited thereto, and may be any one of an alkali metal halide, an alkaline earth metal halide, and an ionic liquid.

[0034] Furthermore, an inorganic salt or an ionic liquid containing one or more of Li, K, Ca, and Mg may be added as a source of a reducing metal to the molten salt 4. The source of the reducing metal dissolves in the molten salt 4 and is reduced to a zero-valent reducing metal at the cathode, and the reducing metal can reduce the metal oxide 5 to a metal.

[0035] The reaction vessel 7 is a vessel that contains the molten salt 4, and is, for example, a glass vessel. Note that the reaction vessel 7 is not limited to a glass vessel, and a vessel made of stainless steel, ceramic, or the like may also be used.

[0036] Either the cathode 3 or the anode 2 may be made of a material containing one or more of Pt, Ni, La, Zr, and Bi. For example, the anode 2 is made of Pt, but an electrode made of a material containing one or more of Ni, La, Zr, Bi, and Fe other than Pt may be used. The cathode 3 is made of Ti, but is not limited to this and may be an electrode made of a material containing one of Pt, Ni, La, Zr, Bi, Ti, and Fe.

[0037] The material used for the measuring electrode 10 is selected to be the same as that for the cathode 3. The measuring electrode 10 is suitably structured to have a small surface area, such as a wire.

[0038] The arrangement of the cathode 3, metal oxide 5, and measuring electrode 10 is not limited to the form shown in FIG. 1, and the same effect can be obtained in the axial direction centered on the cathode 3.

[0039] The procedure for electrolytically reducing the metal oxide 5 using the above-mentioned electrolytic reduction device 1 will be described below.

[0040] LiCl is placed in a reaction vessel 7, and the molten salt 4 is heated to 650°C by a temperature control mechanism 9. At this time, Li2O may be added as an oxygen carrier to the molten salt 4. In addition, an anode 2, a cathode 3 holding UO2, a reference electrode 8, and a measurement electrode 10 are immersed in the molten salt 4.

[0041] After this state is reached, a DC power source 6 is connected to the anode 2 and the cathode 3 to apply a voltage. The potential applied at this time is the potential determined in the operation flow of FIG.

[0042] By applying a voltage, it is reduced to metallic lithium, which is a reducing metal, on the surface of the cathode 3. This metallic lithium, which is a reducing metal, reduces UO2 in contact with the surface of the cathode 3 to U.

[0043] The metal oxide 5 to be reduced is the spent oxide fuel and mixed oxide fuel products from nuclear power plants. In other words, the uranium oxide and plutonium oxide contained in the spent oxide fuel and mixed oxide fuel products are the targets of reduction. This makes it possible to convert the oxide fuel from the light water reactor fuel cycle into metal fuel.

[0044] Although spent fuel oxide from a nuclear power plant is used as the metal oxide 5 to be reduced, the present invention is not limited to this and may be, for example, mixed fuel oxide or minerals.

[0045] Next, the effects of this embodiment will be described.

[0046] The electrolytic reduction device 1 of the above-mentioned Example 1 of the present invention includes a cathode 3 accommodating a metal oxide 5 to be reduced, an anode 2, a molten salt 4, a DC power source 6, and a measurement electrode 10 immersed in the molten salt 4, and is a device for reducing the metal oxide 5 by passing a current between the cathode 3 and the anode 2 via the molten salt 4 from the DC power source 6. The device determines the product concentration in the molten salt 4 from the current value when a specific potential is applied to the measurement electrode 10, and controls the potential applied between the cathode 3 and the anode 2 based on the determined product concentration.

[0047] This makes it possible to estimate the concentration of (side reaction) products in the molten salt 4 by applying a measurement voltage to the measurement electrode 10, and therefore makes it possible to shift the electrolytic reduction potential to the high potential side at an appropriate timing so as to suppress the occurrence of side reactions based on the concentration, and since the amount of reducing metal contributing to reduction can be increased compared to the conventional method, the reduction efficiency can be improved. Furthermore, since the total amount of side reaction products is small, it is also possible to reduce the configuration and removal time for the side reaction products compared to the conventional method.

[0048] Furthermore, a current density that depends on the product concentration from a specific potential is obtained in advance, a calibration curve showing the relationship between the obtained product concentration and the current density is prepared, the product concentration is obtained based on the calibration curve, and the applied potential is changed to a potential that suppresses the occurrence of side reactions. As a result, the concentration of the by-reaction products in the molten salt 4 can be obtained with higher accuracy, and electrolytic reduction with less generation of by-reaction products can be realized.

[0049] Furthermore, by temporarily suspending the electrolytic reduction at the timing when the specific potential is applied to the measurement electrode 10, it is possible to prevent the voltage for electrolytic reduction from affecting the measurement of the concentration of the by-reaction product, thereby improving the accuracy of the measurement of the concentration of the by-reaction product.

[0050] <Example 2> Second Embodiment An electrolytic reduction apparatus and an electrolytic reduction method according to a second embodiment of the present invention will be described with reference to Fig. 7. Fig. 7 is a configuration diagram of the electrolytic reduction apparatus according to the second embodiment.

[0051] 7, the electrolytic reduction device 1A of the present embodiment uses a potentiostat 6A instead of a DC power supply 6. In this manner, by providing the potentiostat 6A, it is possible to quickly switch between application of a specific potential by the measuring electrode 10 and application of a voltage for electrolytic reduction, as in the case of the first embodiment.

[0052] In order to energize the measurement electrode 10, the electrolytic reduction device 1 described in Example 1 needs to be equipped with two or more DC power supplies 6, but by using the potentiostat 6A, it is possible to unify the power supplies into the potentiostat 6A. In addition, by using the measurement function of the potentiostat 6A, it is possible to quickly switch between measuring the current density using the measurement electrode 10 and energizing the cathode 3, thereby shortening the electrolysis time of the metal oxide 5.

[0053] The other configurations and operations are substantially the same as those of the electrolytic reduction device and electrolytic reduction method of the first embodiment described above, and the details are omitted here.

[0054] The electrolytic reduction apparatus and electrolytic reduction method according to the second embodiment of the present invention also provide substantially the same effects as those of the electrolytic reduction apparatus and electrolytic reduction method according to the first embodiment described above.

[0055] <Example 3> An electrolytic reduction apparatus and an electrolytic reduction method according to a third embodiment of the present invention will be described with reference to Fig. 8. Fig. 8 is a configuration diagram of the electrolytic reduction apparatus according to the third embodiment.

[0056] The electrolytic reduction device 1B of this embodiment shown in FIG.

[0057] For example, by providing a plurality of measuring electrodes 10 inside and outside the cathode 3, it is possible to obtain the distribution of the by-product concentration inside the reaction vessel 7. By comparing the by-product concentrations inside and outside the reaction vessel 7, the potential to be applied to the cathode 3 can be determined.

[0058] Specifically, when the by-product concentration inside the reaction vessel 7 is high, electrolysis is stopped and the molten salt 4 is allowed to diffuse, or a stirring mechanism for the molten salt 4, such as a stirring blade, is inserted into the electrolytic reduction device 1 to reduce the by-product concentration inside the reaction vessel 7, and then electrolysis is resumed. On the other hand, when the by-product concentration outside the reaction vessel 7 is high, the surface area of ​​the anode 2 is increased, for example by increasing the number of anodes 2. Alternatively, the by-product concentration outside the reaction vessel 7 is reduced by inserting a stirring mechanism for the molten salt 4, such as a stirring blade. When the by-product concentrations inside and outside the reaction vessel 7 are uniform, the occurrence of side reactions can be suppressed by continuing the electrolysis conditions.

[0059] In this embodiment, it is necessary to obtain a calibration curve for each measuring electrode 10. Furthermore, the power source is not limited to the potentiostat 6A, and a plurality of DC power sources 6 can also be used.

[0060] The other configurations and operations are substantially the same as those of the electrolytic reduction device and electrolytic reduction method of the first embodiment described above, and the details are omitted here.

[0061] The electrolytic reduction apparatus and electrolytic reduction method according to the third embodiment of the present invention also provide substantially the same effects as those of the electrolytic reduction apparatus and electrolytic reduction method according to the first embodiment described above.

[0062] <Example 4> An electrolytic reduction apparatus and an electrolytic reduction method according to a fourth embodiment of the present invention will be described with reference to Fig. 9. Fig. 9 is a configuration diagram of the electrolytic reduction apparatus according to the fourth embodiment.

[0063] The electrolytic reduction device 1C of this embodiment shown in FIG. 9 includes a plurality of cathodes 3, anodes 2, and measuring electrodes 10.

[0064] In this embodiment, as in the case of the third embodiment, it is necessary to obtain a calibration curve for each measuring electrode 10. In addition, the power source is not limited to the potentiostat 6A, and a plurality of DC power sources 6 can be used.

[0065] The other configurations and operations are substantially the same as those of the electrolytic reduction device and electrolytic reduction method of the first embodiment described above, and the details are omitted here.

[0066] The electrolytic reduction apparatus and electrolytic reduction method of the fourth embodiment of the present invention also provide substantially the same effects as those of the electrolytic reduction apparatus and electrolytic reduction method of the first embodiment described above.

[0067] Furthermore, by providing a plurality of cathodes 3, anodes 2, and measuring electrodes 10, a large amount of metal oxides 5 can be reduced while maintaining the same effects as in the first embodiment.

[0068] <Other> The present invention is not limited to the above-mentioned embodiment, but includes various modified examples. The above-mentioned embodiment has been described in detail to explain the present invention in an easily understandable manner, and the present invention is not necessarily limited to the embodiment having all of the described configurations.

[0069] It is also possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace part of the configuration of each embodiment with the configuration of another embodiment. [Explanation of symbols]

[0070] 1, 1A, 1B, 1C...Electrolytic reduction device 2...Anode 3...Cathode 4. Molten salt 5...Metal oxide 6…DC power supply 6A…Potentiostat 7. Reaction vessel 8...Reference electrode 9…Temperature control mechanism 10...Measurement electrode

Claims

1. a cathode containing a metal oxide to be reduced; An anode; Molten salt; Power supply, a measurement electrode that is immersed in the molten salt; An electrolytic reduction device that reduces the metal oxide by passing a current between the cathode and the anode via the molten salt using the power source, A product concentration in the molten salt is obtained from a current value when a specific potential is applied to the measurement electrode, and a potential applied between the cathode and the anode is controlled based on the obtained product concentration. Electrolytic reduction device.

2. The electrolytic reduction apparatus according to claim 1 , a current density dependent on the product concentration from the specific potential is obtained in advance, and a calibration curve showing the relationship between the product concentration and the current density is prepared; determining the product concentration based on the calibration curve; The applied potential is changed to a potential that suppresses the occurrence of side reactions. Electrolytic reduction device.

3. The electrolytic reduction apparatus according to claim 1 , The molten salt is composed of any one of an alkali metal halide, an alkaline earth metal halide, and an ionic liquid. Electrolytic reduction device.

4. The electrolytic reduction apparatus according to claim 1 , The material of either the cathode or the anode is made of a material containing at least one of Pt, Ni, La, Zr, and Bi. Electrolytic reduction device.

5. The electrolytic reduction apparatus according to claim 1 , A potentiostat is used as the power source. Electrolytic reduction device.

6. The electrolytic reduction apparatus according to claim 1 , The measuring electrodes are provided in plurality. Electrolytic reduction device.

7. The electrolytic reduction apparatus according to claim 1 , The cathode, the anode, and the measuring electrode are each provided in a plurality of portions. Electrolytic reduction device.

8. The electrolytic reduction apparatus according to claim 1 , At the timing when the specific potential is applied to the measurement electrode, the electrolytic reduction is temporarily stopped. Electrolytic reduction device.

9. A cathode and an anode holding a metal oxide to be reduced are immersed in a molten salt; An electrolytic reduction method for reducing the metal oxide by passing a current between the cathode and the anode from a power source, comprising: A measuring electrode is immersed in the molten salt, and a specific potential is applied between the measuring electrode and the anode; determining a product concentration in the molten salt from the current value when the current is applied; The potential applied between the cathode and the anode is controlled based on the determined product concentration. Electrolytic reduction method.

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