Patent Number: 056132395
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawing, an electrical conduction tank 1 contains a solution composed of a chelating solution and/or an organic acid containing radioactive metal ions. Suitable chelating agents for preparation of a chelating solution include ethylenediamine tetraacetic acid, ethylenenitrilodiamine triacetic acid, hydroxyethylethylenediamine tetraacetic acid and the like. Suitable organic acids include citric acid, oxalic acid, acetic acid and the like. The chelating solution and organic acid may be used singly or in combination and contain radioactivity-laden metal ions such as Fe ions, Mn ions, Co ions and other metal ions. Denoted by numeral 9 is an alkaline agent-feeding device from which a NaOH or KOH solution is supplied. An alkaline agent used here is a hydroxide of an alkali metal or of an alkaline earth metal. Numeral 10 is a stirrer, numeral 11 is a pump, and numeral 12 is a filtration device, which is, as an example, a cartridge filter in this embodiment. A filtrate tank is denoted by numeral 13. An electrolytic device 3 is composed of an electrolyzer 14 and a plurality of electrodes 15 designed to flow direct current. An air pump 16 of an electromagnetic type is provided to feed air which removes or blows off various gases. A tank 17 is provided to store a solution electrolyzed in the electrolyzation device 3. A filtration device 4 communicates with the electrolyzer 14 and filters metal hydroxides which are little soluble in water coming out of the electrolyzer 14, to discharge the metal hydroxides as a filter cake and to convey the resultant filtrate to the tank 17. An ultraviolet irradiation device 6 has an elongate tank 18 to which an ultraviolet lamp 19 is secured. The filtrate comes into the device 6 from an inlet 20 and comes out from an inlet 21. A pump 22 is arranged to feed air and provided with an air inlet 23 and an air outlet 24. The air thus fed generates ozone upon irradiation with ultraviolet rays, to promote oxidation of the filtrate. Numeral 25 is a tank. A separation device 5, which is as an example of a reverse osmosis type, is provided to separate organic matter and water resulting from decomposition in the ultraviolet irradiation device 6. The organic matter so decomposed is near mineral, and the water so separated may be recycled as clean water or discharged. The separation device 5 may also be of an ion-exchange resin type. The operation of the apparatus which is constructed as described above will be described hereunder. Firstly, an organic solution composed of a chelating solution and/or an organic acid is applied to radioactive metals laden with radioactivity, to form an organic metal solution. In this embodiment, a mixture of a chelating solution and an organic acid is used. For example, a mixture of ethylenediamine tetraacetic acid and citric acid, each being a 1% aqueous solution, is used. Contained in the metal solution are metal ions such as Fe ions, Mn ions and the like, and here explanation will be made as to Fe ions as an example. The metal solution contained in the electrical conduction tank 1 is applied with a NaOH solution as an example from the alkaline agent-feeding device 9, and stirred by the stirrer 10. For example, the ratio of NaOH to a metal solution is 4 g/liter. Then iron hydroxides are formed in the metal solution and precipitated in colloid. The same kind of action occurs as to metals other than iron, too. Being a typical coprecipitant, iron helps precipitation of other radioactive metal ions. The metal solution is thereafter filtered in the filtration device 2 so that the iron hydroxides are separated and removed, while the resulting filtrate is fed into the electrolytic device 3 where the filtrate is electrolyzed. Efficient electrolyzation is made because the filtrate treated with an alkaline agent has markedly high electrical conductivity. With respect to electrical conductivity of the metal solution treated with an alkaline agent, as an example, the following are the current densities as measured. before addition of an alkaline agent: 0. 8 A/dm2 PA1 after addition of an alkaline agent: 10 A/dm2 Most of Fe ions are lost, and the chelating effect of the chelating solution is lost, presumably due to hydrolysis. For instance, using disodium ethylenediamine tetraacetic acid, C.sub.10 H.sub.14 N.sub.2 O.sub.8 Na.sub.2 +16H.sub.2 O.fwdarw.10CO.sub.2 +2NO+2NaOH+22H.sub.2. CO.sub.2 and NaOH are formed by anode oxidation and H.sub.2 is formed at a cathode side. The organic acid is also decomposed by electrolysis to form CO.sub.2, H.sub.2 and the like. Prior to electrolysis, most of the metal ions are removed in the treatment in the electrical conduction tank 1 and the filtration device 12 so that radioactive metals left to be deposited on the electrodes 15 of the electrolytic device 3 can be reduced greatly. Much deposition on the electrodes 15 is responsible for poor electrolysis efficiency. Moreover, radioactivity of the electrodes 15 is kept low because just a small extent of radioactive metal ions are still remaining. Thus troublesome post-treatment can be minimized. According to the invention, although a chelating solution and/or an organic acid containing radioactive ions are generally little conductive and difficult to be electrolyzed, they can be easily electrolyzed by increasing electrical conductivity by addition of an alkaline agent. This ensures easy collection of radioactive metal ions contained in such solutions. In the above electrolytic treatment, in addition to the decomposition through the anode oxidation, a material convertible by electrolysis into highly oxidative matter, such as NaOH for example, may also be used to take advantage of its high oxidative action. As a result of electrolysis, radioactive metal ions are converted into little-water-soluble hydroxides in the electrolyzer 14. The filtrate in the electrolyzer 14 is filtered in the filtration device 4 where the hydroxides are separated and removed. The separated filtrate is allowed to enter the ultraviolet irradiation device 6 from the lower inlet 20 and to be discharged out of the upper outlet 21 while being supplied with air bubbles. Oxygen in the bubbles generates ozone upon exposure to ultraviolet rays, and ozone acts to decompose any remaining organic matter, promoting decomposition of such organic matter by ultraviolet rays. The organic matter decomposed by ultraviolet irradiation is near mineral and sent to the separation device 5 that is of a reverse osmosis type for example. In the device 5, the irradiated filtrate put inside a translucent membrane (not shown) is brought in pressurized condition so that water alone iscaused to pass through the membrane outside in clean water and thus separated from the decomposed organic matter. The separation device 5 may be an ion-exchange resin type. The solution containing the matter decomposed in the ultraviolet irradiation device 6 is sent to the ion-exchange resin device, where the decomposed matter is absorbed by the ion-exchange resin, while water free from the decomposed matter is separated as cleanwater. The water may be discharged or recycled to the first stage of the apparatus. Being constructed as described above, the present invention enables the following available: It is available to electrolyze a solution composed of a chelating solution and/or an organic acid containing a radioactive metal ions, which is little soluble in water and difficult to be electrolyzed, by increasing electrical conductivity thereof by adding an alkaline agent, and to collect without difficulty radioactive metals contained in the solution. It is also available in collecting radioactive metals by way of dissolution to decrease mass volume of the radioactive matter required to be stored in safety quite significantly because there is no need to store radioactive chelating solutions and/or organic acids. Therefore, it is available to realize a quite high extent of reduction of storage space for radioactive matter. It is also available to make cementing treatment of the chelating solution and the organic solution after electrolysis because carboxyl groups therein have been decomposed. It is available to collect radioactive metal ions dissolved in the chelating solution and/or the organic acid, as hydroxides easily, because inherent activities of such solutions are lost by decomposition by electrolysis. Prior to electrolysis, the radioactive metal ions in the chelating solution and/or the organic acid have already been considerably reduced, through convertion into little-water-soluble matter by addition of an alkaline agent and filtration thereafter. Therefore loading on the electrodes can be minimized, metals to deposit on the electrodes can be minimized, lowering of electrolytic efficiency can be prevented, and further, necessity to remove radioactive metals, which is dangerous to handle, from the electrodes can be minimized. The alkaline agent which is added prior to electrolysis in order to decrease radioactive metal ions through conversion into matter poorly soluble in water and filtration thereafter serves to increase electrical conductivity of the electrolytic solution, and therefore realizes quite effective electrolysis. This may be well understood from the fact that electrical conductivity of the chelating solution and/or the organic acid is low before the treatment. Furthermore, it is available in the invention to collect most of radioactive metal ions through formation of matter poorly soluble in water by alkaline treatment combined with the electrolytic treatment. Further, it is available in the invention to collect almost all of radioactive metal ions by additional ultraviolet irradiation treatment and separation step. Furthermore, it is available in the invention to collect radioactive metal ions almost completely because an additional separation device is that by use of reverse osmosis or ion-exchange resin.