JP3005267B2 - Method and apparatus for removing ruthenium from aqueous solution - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for removing ruthenium in a radioactive waste liquid by means including anodic oxidation, and particularly to ruthenium from an acidic solution having a nitric acid concentration of 1 N or more. Suitable for removal.

[Prior Art] Conventionally, a technique for converting ruthenium into an insoluble form and removing it from a radioactive waste liquid by means including anodic oxidation has been discussed in JP-A-1-138496. That is, an electrochemical cell is formed by immersing a pair of electrodes in a solution containing ruthenium, in which ruthenium is electrochemically oxidized and then electrochemically reduced in the cell to make ruthenium insoluble. It is a method of inverting and removing. This method is based on the principle that ruthenium in a solution is oxidized at the anode and converted to a higher oxide ruthenium tetroxide, and the ruthenium tetroxide is reduced at the cathode and converted to ruthenium dioxide which is in an insoluble form. The radioactive waste liquid after the removal of ruthenium is solidified and reduced in volume by means such as evaporation to become waste.However, the above method does not require the addition of a chemical substance for removing ruthenium and increases the amount of exhaust after the solidification and volume reduction. It is especially good at not having any.

[Problems to be Solved by the Invention] In the above-mentioned conventional technique, a voltage is applied to an electrode immersed in an aqueous solution containing ruthenium to oxidize ruthenium on an anode and reduce an oxidation product of ruthenium on a cathode to reduce an oxidation product of ruthenium. And converting the ruthenium compound to an insoluble form. However, the pH range of the solution to which the above-mentioned technology can be applied is said to be 3 to 12, and the nitric acid concentration is 1N or higher, which is a low pH value represented by high-level waste liquid for nuclear fuel reprocessing.
It is not suitable for removing ruthenium from acidic wastewater.

However, as a method for removing ruthenium without increasing the amount of waste after solidification and volume reduction, an electrochemical method is optimal. Therefore, an object of the present invention is to provide a method and an apparatus suitable for removing ruthenium from a solution having a nitric acid concentration of 1 N or more by an electrochemical method.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides ruthenium tetroxide by self-decomposition by providing a structure for retaining the product on an anode or a photoexcited semiconductor structure for generating ruthenium tetroxide. It is converted to insoluble form.

[Function] Ruthenium tetroxide generated by the anode or the photoexcited semiconductor structure has a property of being converted into insoluble ruthenium dioxide by self-decomposition. On the other hand, ruthenium in the initial form (which is in the form of nitrosylruthenium in the solution as a normal nuclear fuel reprocessing waste liquid) remains in the solution, and the ruthenium tetroxide reacts with the ruthenium in the initial form. Thus, a third dissolved form of the ruthenium compound (its form is unidentified, but the color is reddish brown) is produced. The rate at which ruthenium tetroxide is converted to the insoluble form of ruthenium dioxide is proportional to the square of the ruthenium tetroxide concentration, while the rate at which ruthenium tetroxide is converted to the third dissolved form is proportional to the first power of the ruthenium tetroxide concentration. I do. Thus, the higher the concentration of ruthenium tetroxide, the higher the probability that ruthenium tetroxide will be converted to an insoluble form. On the other hand, when the acid concentration of the solution becomes 0.01N or more (corresponding to pH 2 or less), the higher the acid concentration, the lower the conversion rate of ruthenium tetroxide to the insoluble form and the reverse the conversion rate to the third dissolved form. Be faster. However, the concentration of ruthenium tetroxide generated at the anode decreases because it moves away from the vicinity of the anode due to diffusion.
Only conversion to the dissolved form takes place. Conventional technology is pH
The reason why it is not suitable for an acidic solution of 2 or less may be that ruthenium tetroxide produced at the anode is converted to dissolved ruthenium due to a decrease in concentration before reaching the cathode.

In the present invention, instead of utilizing the conversion of ruthenium tetroxide to an insoluble form by cathodic reduction, ruthenium in a solution is insoluble by utilizing the property that ruthenium tetroxide is converted to ruthenium dioxide which is an insoluble form by self-decomposition. Convert to shape. In the present invention, ruthenium in a solution is oxidized and converted to ruthenium tetroxide by an anode or a photoexcited semiconductor structure. Ruthenium tetroxide stays in the vicinity of the anode or the photoexcited semiconductor structure by making the anode or the photoexcited semiconductor structure into a tubular or wool-like or knitted structure,
Suppress the concentration decrease due to diffusion. Due to this suppression, the concentration of ruthenium tetroxide in the vicinity increases, and conversion of ruthenium tetroxide to an insoluble form occurs. Thus, ruthenium can be removed from solution by converting it to an insoluble form.

[Embodiment] An embodiment of the present invention will be described with reference to FIG.

An electrolytic cell 1 is filled with a nitric acid solution 2 containing ruthenium. Reference numeral 3 denotes an anode, which is inserted into a cylindrical glass tube 4 open at both ends. The anode 3 and the glass tube 4 are immersed in the solution 2. Reference numeral 5 denotes a cathode, which is immersed in the solution 2. Reference numeral 6 denotes a DC power supply for electrolysis, which supplies a current between the anode 3 and the cathode 5. The solution 2 in this embodiment is an aqueous solution containing 1 mol / liter of nitric acid and 1 mmol / liter of nitrosyl ruthenium trinitrate. The anode 3 has a diameter of 0.5 mm and a length of 50
The cathode 5 is a platinum plate 20 mm wide, 50 mm long and 0.5 mm thick. The inner diameter of the glass tube 4 is 6 mm.
When a current of 200 mA was supplied between the positive and negative electrodes by the DC power supply 6, the color of the solution in the glass tube 4 gradually changed from brown to yellow, and after about 15 minutes, black particles began to form in the glass tube 4. Was. The change in color of this solution indicates that ruthenium nitrosyl (brown) has changed to ruthenium tetroxide (yellow). The black particles were ruthenium dioxide.

For comparison, when the same test was carried out with the glass tube 4 removed, the solution in the range of about 5 mm in diameter in the vicinity of the anode 3 turned pale yellow in about 15 minutes, but turned reddish brown further outside. When the current supply was continued as it was, the red-brown portion gradually spread outward, while the pale yellow portion near the electrode 3 showed no change. The current supply was continued for 8 hours, but no black particles were formed. The light yellow color of the solution near the anode 3 indicates that ruthenium tetroxide is generated, but the color is lighter than when the glass tube 4 is present because ruthenium tetroxide is lost from the vicinity of the anode due to diffusion. It is. The reddish brown on the outside is because ruthenium tetroxide reacts with nitrosylruthenium and changes to the third dissolved form. The reason that no black particles are formed is that the concentration of ruthenium tetroxide is lower than that in the case of the glass tube 4 and the rate of change to the third dissolved form is lower than the rate of formation of black particles (ruthenium dioxide) by self-decomposition of ruthenium tetroxide. Is larger.

That is, the glass tube 4 in this embodiment has the effect of retaining ruthenium tetroxide generated at the anode near the anode and promoting the conversion of ruthenium tetroxide to ruthenium dioxide. According to this embodiment, it is possible to convert ruthenium in a nitric acid solution to an insoluble form. The glass tube 4 does not need to be made of glass and may be made of ceramic or metal. Further, the shape of the glass tube 4 does not need to be a cylindrical tube, and may be any structure as long as the anode product stays therein. For example, it may be a polygonal tube or a small chamber partitioned by a plurality of flat plates or curved plates. The compartment may be open to allow the solution to flow laterally.

Another embodiment of the present invention will be described with reference to FIG. 2, FIG. 3, and FIG. In FIG. 2, reference numeral 7 denotes a tubular electrode, the detailed structure of which is shown in FIG. In the present embodiment, a tubular electrode 7 is used in place of the anode 3 and the glass tube 4 in the embodiment of FIG. That is, in the embodiment of FIG. 1, the anode and the structure (glass tube) for retaining the anode product are independent, but in the present embodiment, the tubular electrode 7 has a structure also serving as the structure. . As shown in FIG. 3, the tubular electrode 7 is provided with a platinum plating 11 on the inner surface of a glass tube 10 having both ends open, and a platinum plated portion.
11 is used as an anode. FIG. 4 shows a modification of this embodiment. Reference numeral 8 in FIG. 4 denotes a disk-shaped rotor which is rotated by a magnet by a rotation motor 9. The solution flows from the upper part to the lower part of the tubular electrode 7 by the rotation of the rotor 8. The black particles are discharged from the tubular electrode 7 by the flow of the solution, and at the same time, the solution in the tubular electrode 7 is renewed.

According to this embodiment, it is possible to convert ruthenium in a nitric acid solution to an insoluble form. The glass tube 10 does not need to be made of glass, and may be an insulator. Further, the shape of the glass tube 10 does not need to be a cylindrical tube, and may be any structure as long as the anode product stays therein. For example, it may be a polygonal tube or a small chamber partitioned by a plurality of flat plates or curved plates. The compartment may be open to allow the solution to flow laterally.

In the embodiment of FIG. 1, ruthenium tetroxide generated at the anode 3 diffuses from the center of the tube 4 to the outside, so that the concentration of ruthenium tetroxide decreases as the distance from the electrode 3 increases. For this reason, ruthenium tetroxide is not converted to an insoluble form at a position away from the center. 2 and 3
When the anode is applied to the inner surface of the glass tube as in the embodiment shown in the figure, the substance generated at the anode diffuses toward the center of the tube, so that the concentration of ruthenium tetroxide does not decrease even if the anode is separated from the anode. Therefore, in this embodiment, the conversion efficiency of ruthenium tetroxide to the insoluble form is better than in the embodiment of FIG.

Instead of the tubular electrode 7 in this embodiment, a thread-like electrode [for example, a wire of an electrode metal (such as platinum), or
Glass fibers plated with an electrode metal] may be used as a stitch-like or wool-like structure.

Another embodiment of the present invention will be described with reference to FIG. This embodiment is an example in which the photoexcited semiconductor structure 14 and the light source 12 are used in place of the tubular electrodes, the cathode, and the DC power supply of the embodiments shown in FIGS. The structure of the photoexcited semiconductor structure 14 is shown in FIG. The photoexcited semiconductor 16 is applied to the inner surface of the glass tube 15 which has a scattering property by including bubbles 18. The glass tube 15 is a support structure for the photoexcited semiconductor 16. The outer surface of the glass tube 15 is a silver-plated surface 17. Reflector from light source 12
The light reflected by 13 and incident from the upper part of the glass tube 15 is scattered by the bubbles 18 in the glass tube 19 and changes its direction, partly to the surface coated with the photoexcited semiconductor 16, partly downward, and Department goes outward. The light directed outward changes its direction by reflection on the silver-plated surface 17 and goes to the surface coated with the photoexcited semiconductor 16 or downward. Most of the incident light is also a photoexcited semiconductor, since the light going downward is also scattered by the bubbles 18 below.
Irradiation is applied to the surface coated with 16. When the photoexcited semiconductor 16 receives light, it is polarized and acts as a fine anode, so that ruthenium in the solution is converted to ruthenium tetroxide. As in the previously described embodiment, ruthenium tetroxide is converted to an insoluble form within the diameter of the tubular photoexcited semiconductor structure 14. Optically pumped semiconductor
The material 16 may be any material that makes the anode side potential more positive by 2.0 volts or more than the standard hydrogen electrode potential when polarized by light irradiation. For example, titanium dioxide or silicon carbide can be used. The glass tube 15 does not need to be made of glass and may be made of a transparent material. Also bubbles
Scattering small particles may be used instead of 18, and reflective or refractive small particles may be used instead of scattering. Also glass tube
The shape of 15 does not need to be a cylindrical tube, and may be a structure in which the product stays. For example, it may be a polygonal tube or a small chamber partitioned by a plurality of flat plates or curved plates. The compartment may be open to allow the solution to flow laterally. It is necessary to irradiate light with a wavelength of 440 nanometers or less to polarize the photoexcited semiconductor. Although difficult, however, according to the present embodiment, it is possible to efficiently irradiate light to the photoexcited semiconductor 16 immersed in the nitric acid solution. In addition,
An embodiment using the following photoexcited semiconductor structure instead of the photoexcited semiconductor structure 14 is also possible. That is,
A thread-shaped or wool-shaped structure formed by applying a photoexcited semiconductor to the outer surface of a glass fiber is immersed in an acid solution, and a means for introducing light into the glass fiber from an end of the glass fiber is provided. . Since the introduced light leaks out of the glass fiber as it travels along the glass fiber, the photo-excited semiconductor applied to the outer surface is polarized and acts as a fine anode, and as in the above-described embodiment, the light-excited semiconductor is in a solution. Of ruthenium to ruthenium tetroxide.

[Effects of the Invention] According to the present invention, by retaining ruthenium tetroxide generated by oxidation in a solution, it is possible to convert the ruthenium to an insoluble form by self-decomposition. It can be converted to insoluble form and removed. In addition, the present invention provides a low pH (pH
2 or less) can be effectively applied to the removal of ruthenium in acidic waste liquid.

[Brief description of the drawings]

1 is a view showing one embodiment of the present invention, FIG. 2 is a view showing another embodiment of the present invention, FIG. 3 is an enlarged sectional view of the tubular electrode in FIG. 2, and FIG. The figure which shows the modification of FIG.
The figure is a view showing still another embodiment, and FIG. 6 is an enlarged sectional view of the photoexcited semiconductor structure in FIG. DESCRIPTION OF SYMBOLS 1 ... Electrolysis tank, 2 ... Nitric acid solution 3 ... Anode, 4 ... Glass tube 5 ... Cathode, 6 ... DC power supply 8 ... Rotor, 9 ... Rotor motor 10 ... Glass tube, 11 Platinum-plated surface 12 Light source 13 Reflecting mirror 15 Glass tube 16 Photoexcited semiconductor 17 Reflective silver-plated surface 18 Bubbles 19 Light beam

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masanori Takahashi 1168 Moriyama-cho, Hitachi City, Ibaraki Pref. Energy Laboratory, Hitachi, Ltd. (56) References JP-A-62-172298 (JP, A) JP-A-63- 42499 (JP, A) JP-A-63-243232 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21F 9/06 G21F 9/10

Claims (10)

(57) [Claims] 1. A method for converting ruthenium in an aqueous solution to insoluble ruthenium and removing it from the solution, wherein the ruthenium in the aqueous solution is converted to ruthenium tetroxide by anodic oxidation, and the ruthenium tetroxide is converted to the vicinity of the anode. Wherein the ruthenium tetroxide is converted to insoluble ruthenium by self-decomposition by allowing the ruthenium to stay in the aqueous solution. 2. A linear or plate-like anode and a tubular structure are installed in an aqueous solution containing ruthenium so as to cover the anode, and ruthenium tetroxide produced by oxidation at the anode is converted into the tubular structure. An apparatus for removing ruthenium in an aqueous solution, wherein the apparatus is configured to be retained in a substance and converted into insoluble ruthenium by self-decomposition. 3. An anode structure in which a part or all of the inner surface of a tubular structure is used as an anode surface in a ruthenium-containing aqueous solution, and ruthenium tetroxide generated by oxidation on the anode surface is produced. An apparatus for removing ruthenium from an aqueous solution, wherein the apparatus is configured to be retained inside a tubular structure and converted into insoluble ruthenium by self-decomposition. 4. An anode structure in which a part or all of a stitch-like or wool-like structure is used as an anode surface in an aqueous solution containing ruthenium, and ruthenium tetroxide produced by oxidation on the anode surface A device for removing ruthenium from an aqueous solution, wherein the ruthenium is retained in gaps of the structure and converted into insoluble ruthenium by self-decomposition. 5. A light-excited semiconductor structure comprising a transparent material and a part of or the entire inner surface of a light-scattering tubular structure is made of a light-excited semiconductor, placed in an aqueous solution containing ruthenium. A light irradiating means for irradiating the photoexcited semiconductor structure with light; ruthenium tetroxide oxidized and generated by the photoexcited semiconductor by the light irradiation is retained inside the tubular structure to form an insoluble form by self-decomposition; An apparatus for removing ruthenium from an aqueous solution, wherein the apparatus is configured to convert the ruthenium into ruthenium. 6. The apparatus for removing ruthenium in an aqueous solution according to claim 5, further comprising a light reflecting layer on the outer surface of the tubular structure. 7. The light scattering property inside the tubular structure is provided by mixing bubbles, light scattering particles, or light reflecting or light refracting particles in a transparent material. A device for removing ruthenium in an aqueous solution as described above. 8. An optically-pumped semiconductor structure in which a part or all of the surface of an optically-transmitting fiber in which light propagating inside leaks out of the surface one by one is provided with a photo-excited semiconductor and is formed into a stitch or wool shape. It is provided in an aqueous solution containing ruthenium, and is provided with a means for introducing light from the end of the optical transmission fiber into the inside thereof, and the ruthenium tetroxide generated by being oxidized by the photoexcited semiconductor by the introduction of the light is produced. An apparatus for removing ruthenium from an aqueous solution, wherein the apparatus is configured to be retained in a gap between the photoexcited semiconductor structures and converted into insoluble ruthenium by self-decomposition. 9. The apparatus for removing ruthenium from an aqueous solution according to claim 2, 3, 5, or 7, wherein the partitioning plate defining a small chamber is used instead of the tubular structure. 10. A mechanism for forcing an aqueous solution to flow through each of said structures.
9. The apparatus for removing ruthenium from an aqueous solution according to any one of claims 8 to 8.