Patent Number: 046817050
Section: description

DETAILED DESCRIPTION As set forth earlier herein, prior to the present invention, it was believed that substantially all of the radioactivity present in waste lubricating oil was present in the aqueous portions of the oil/water emulsions which formed during the operation of certain typical types of machinery. Accordingly, it was similarly believed that the problem of decontaminating waste lubricating oil could be addressed simply by separating essentially pure oil from emulsions containing radioactive water. This has been found to be somewhat unsuccessful, however, as even after such separation techniques the resulting oil always has a significant enough radioactive contamination to require disposal as a radioactive low level waste. In contrast to all of these prior assumptions and methods, the present invention has developed under the more proper discovery that radioactivity present in the mixtures of water and waste lubricating oil formed as byproducts of the operation of a nuclear power plant has an organic component. This organic component tends to be substantially associated with the oil rather than with the aqueous portion of the mixture and thus is not removed when oil and water are separated regardless of the sophistication or effectiveness of the particular separation technique. Accordingly, the present invention is a method which has been developed for attacking and then removing the radioactive component present in the organic material as well as that present in the inorganic material in waste lubricating oil. The present invention is additionally novel in that contaminated oil is mixed with another material in order to accomplish the decontamination process. In almost all prior processes, it has been believed by those most familiar with the problem that physical separation techniques were required to decontaminate such oil and that any further mixing of such materials was an especially unattractive and unsatisfactory method of decontamination. Although the invention can be broadly applied to the decontamination of water-immiscible organic liquids, the decontamination of mixtures of water and water-immiscible organic liquids and the decontamination of mixtures of either of the above with particulate material, the embodiment described herein will refer to the decontamination of mixtures of water and waste lubricating oil produced during the normal operations of nuclear powered electric generating plants. According to the present invention, when any particulate matter is present in the waste oil, a first step in decontamination comprises separating the mixture of radioactively contaminated particulate material and a mixture of radioactively contaminated water and radioactively contaminated lubricating oil into an liquid mixture portion and a particulate portion. Such separation can be carried out by conventional means in a straightforward manner, such as filtration. The resulting particulate material, even if contaminated, can be disposed of as solid waste without the practical and regulatory problems set out earlier with respect to liquid waste. The thus separated liquid mixture is then separated into a waste water fraction carrying metal radionuclides and an organic liquid fraction carrying metal radionuclides. In one preferred embodiment of the invention, the separation is accomplished in a centrifugal separator, one commercial embodiment of which is an Alfa Laval 103B centrifuge, or an equivalent product. In order to enhance the separation of the waste water fraction from the lubricating oil fraction, the mixture may additionally be heated, preferably to a temperature of about 180.degree. F. Heating the mixture lowers the viscosity of the oil, improving its flow characteristics and enhancing the separation process. As in the case of the particulate material initially separated, the waste water fraction may be decontaminated by conventional methods such as selective precipitation or extraction which result in decontaminated water and low level solid waste which can be disposed of more conveniently than low level liquid waste. It will be apparent to those familiar with the problem and the industry that the use of existing commercial technology and materials in the present invention makes the invention attractive and useful for current application on a wide scale. In preferred embodiments of the invention, the separated contaminated oil fraction can be filtered at this stage to further remove any fine particulate material suspended therein which may not have been carried off in the separation from the waste water fraction. The contaminated oil fraction is then mixed with a sufficient amount of an aqueous solution of a water-soluble chelating agent in a manner and for a time period sufficient for water-soluble metal-chelate complexes to form between the chelating agent and substantially all of the metal radionuclides carried by the organic liquid fraction. Appropriate chelating agents are those organic molecules which contain enough functional groups, two or more, located in such positions on the molecule that when the functional groups attach to the metal ion with which they complex, they surround the metal ion and form a ring. In a preferred embodiment of the invention, one appropriate chelating agent is ethylenediamine tetraacetic acid (EDTA). EDTA forms stable, water-soluble complexes with many metal ions, can potentially bond to metal ions at as many as six different sites, and tends to result in the formation of five-membered chelate rings. Because the preferred embodiment of the invention relates to the decontamination of waste lubricating oil from a nuclear powered electric generating plant, the radionuclide which is characteristically most difficult to remove is Cobalt 60 (Co-60). EDTA is an especially attractive chelating agent for removing Cobalt 60, but the invention is equally applicable to situations in which other radionuclides need to be removed from either waste lubricating oil or some other water-immiscible organic liquid. In such circumstances, several other chelating agents may be appropriate. Typical chelating agents suitable for other radionuclides include the following: nitrilotriacetic acid, alpha-nitrosobeta-naphathol, 1,2-cyclohexanedionedioxime, disodium dihydrogen EDTA, ethylenediaminetetraacetate dihydrate, tetrasodiumethylenetiamine tetracetate, N-hydroxyethylethylenediaminetriacetic acid, and 4-(2-pyridylazo)-resorcinol. In another embodiment of the invention, the aqueous solution containing the chelating agent may also include a water soluble inorganic precipitating agent. When the aqueous solution contains an inorganic precipitating agent, insoluble metal precipitates may form between the precipitating agent and some of the metal radionuclides which are not affected by the chelating agent. In such cases, the metal precipitates will separate from the organic liquid fraction along with the aqueous solution after which the metal precipitates may in turn be separated from the aqueous solution before the aqueous solution is otherwise decontaminated. The invention thus provides a method whereby substantially all of the metal radionuclides may be removed from water immiscible organic liquids which contain some radionuclides which are more likely to complex with the chelating agent and which also contain other radionuclides which are less likely to form chelates but which will in turn be likely to precipitate out as insoluble salts of the inorganic precipitating agent. Typical appropriate inorganic precipitating agents include the following: NH.sub.4 OH, H.sub.2 S, (NH.sub.4).sub.2 S, (NH.sub.4).sub.2 HPO.sub.4, H.sub.2 SO.sub.4, H.sub.2 PtCl.sub.6, H.sub.2 C.sub.2 O.sub.4, (NH.sub.4).sub.2 MoO.sub.4, HCl, AgNO.sub.3, (NH.sub.4).sub.2 CO.sub.3, NH.sub.4 SCN, NaHCO.sub.3, HNO.sub.3, H.sub.5 IO.sub.6, NaCl, Pb(NO.sub.3).sub.2, BaCl.sub.2, NgCl.sub.2, and NH.sub.4 Cl. It is to be understood that the aforementioned chelating agents and inorganic precipitating agents are illustrative of the types of materials known by those familiar with this art and that while some examples have been given, the invention is not limited to these particular examples. As in the case of the initial separation of the mixture into an oil fraction and waste water fraction, the method of mixing the oil fraction with the chelating agent can give more satisfactory results. According to the present invention, it has been found that the mixing of the oil fraction with the aqueous solution of chelating agent can be best accomplished in a centrifuge similar to that used for separation, with the separation function of the centrifuge being switched over to a mixing function. Additionally, the mixing may be enhanced by heating the mixture during the mixing process. Additionally, as is known to those familiar with chelating agents, the acidity of a solution affects the chelating capability of the chelating agent. In the embodiment preferred for the removal of Co-60 with EDTA, best results are obtained when the mixture is kept at a pH greater than 7 with most preferable results being obtained with a pH of about 10.5. For the removal of other metals with EDTA, Co-60 with other chelating agents or other metals with other chelating agents, the optimum pH can be determined based on the chemical characteristics of both the particular chelating agent and those of the particular radionuclide to be removed. One further preferable characteristic of both the solution and the chelating agent is that the chelating agent selected be one which has a greater solubility in water than in the water immiscible organic liquid with which it is mixed. The relative solubility of EDTA in water and in oil is appropriate in this regard. In the embodiment of the invention preferred for removing Co-60 from waste lubricating oil, an EDTA concentration of 0.125 Molar (M) and a solution to oil ratio of 8% have been found to be most satisfactory. As presently best understood, of the radionuclides set out earlier which exist in the original waste mixtures of water and lubricating oil to be decontaminated, Co-60 generally dominates the total radioactivity of the oil fraction. Because waste lubricating oil, even when separated from an emulsion of waste water, always contains a residual measure of radioactivity, it is believed that the Co-60 forms organo-metallic complexes and other oil-soluble compositions which are not removed by any of the prior known techniques. According to the present invention, because EDTA is soluble both in water and in the oil component, Co-60 can be preferably encouraged to form EDTA complexes during the mixing process rather than forming or remaining a part of other organo-metallic compounds. When mixing is completed, the greater affinity of EDTA for water than for oil causes the EDTA to be preferably removed with the water and to carry with it the radioactive Co-60 originally picked up from the lubricating oil. Accordingly, after mixing the next step in the present invention comprises separating the aqueous solution of the chelating agent from the lubricating oil. This also can be accomplished by centrifugal separation and the aqueous fraction which results can be disposed of in a conventional manner. If so desired, the oil fraction may be filtered one more time to remove any further fine particulate material which may have been introduced and not removed to this point. The decontaminated lubricating oil, or other organic liquid, may then be disposed of in a conventional manner, preferably by burning. There are, of course, regulatory limits as to the amount of radioactivity that may be introduced into the atmosphere by the combustion of low level radioactive waste. One advantage of the present invention is that the method of decontamination set forth herein brings the contamination level of the oil to such a low level that combustion of the oil does not result in the release of any significant or prohibited amount of radioactivity into the atmosphere. To further reduce even these insignificant amounts, the waste oil decontaminated by the present method can be mixed with larger amounts of conventional fuel oil and burned in a conventional fuel boiler. It is to be understood that burning of waste lubricating oil decontaminated in the present manner is not the only conventional method of disposal and the scope of the invention or the claims is not to be considered as limited thereto. It will be further understood that while the preferred embodiment of the present invention has been described with regard to the decontamination of waste lubricating oil produced at nuclear powered electric generating plants, the invention is equally applicable to the decontamination of a number of radioactively contaminated water immiscible organic liquids. As in the case of waste lubricating oil, these liquids may be found alone, in mixtures, in water emulsions, in mixtures of water emulsions and particulate material or with particulate material alone. All of these various situations may be addressed by the method of the present invention and are to be considered within the scope of the description and the claims herein. Additionally, the foregoing embodiments are to be considered illustrative, rather than restrictive of the invention and those modifications which come within the meaning and range of equivalent of the claims are to be included therein.