Patent Number: 044328940
Section: summary

This invention relates to a process for the treatment of radioactive liquid wastes containing detergents, and particularly of laundry drainage containing detergents used for cleaning contaminated working clothes. At atomic power stations, etc. the radioactive laundry drainage containing detergents resulting from washing of contaminated working clothes usually amounts to several thousand cubic meters a year. These radioactive liquid wastes contain surface active agents such as sodium dodecylbenzenesulphonate and alkylphenol polyoxyethylene ether, which are ingredients of detergents. In general, radioactive liquid wastes are subjected to a final treatment after concentration to about 1/500 their original volume by evaporation or reverse osmosis. It is well-known, however, that radioactive liquid waste containing a surface active agent, if concentrated in its original condition, generates foams to prevent its concentration. Addition to the liquid waste of anti-foams such as powdered activated carbon (see Japanese Laid-open Patent Application No. 124,800/1976) and used ion exchange resin (see Japanese Laid-open Patent Application No. 101,100/1979), which adsorb the surface active agent, have hitherto been proposed as a means to prevent the foaming. On the other hand, some of the present inventors recently developed a process of converting the radioactive liquid waste which has been concentrated and made smaller in volume into powder by the use of a thin film evaporator and thereafter pelletizing the powdered waste for the convenience of storing it. The thin film evaporator is an apparatus having wiping blades within; the radioactive liquid waste is brought into contact with the heated inner wall of the apparatus in the form of a thin film, part of the waste is evaporated while it goes down along the wall and the remains are recovered in the form of powder. The following defects have been found for the first time as a result of the studies made by the present inventors. The process has a defect that the radioactive liquid waste fed to and concentrated in the evaporator tends to adhere to the blades, the recovered powdery material shows a high moisture content and at times the waste comes out of the apparatus without having been converted to powder, as was discovered later by the inventors. We then experimented on the effect of various kinds of adsorbent, which were added in accordance with the concentration of the detergent in the waste to counteract the surface active agent contained in the detergent, upon the powder-forming capacity of said evaporator, and found that although the capacity was clearly improved by the addition of adsorbent, the amount of the recovered powdery material was increased embarrassingly. This is a very serious problem, for in the treatment of radioative liquid waste it is most highly desirable to minimize the volume of the finished product in powder or pellet form. The object of the present invention is to produce a powdery material of low moisture content out of radioactive liquid waste containing detergents by adding a minimum of adsorbent to the waste. The invention is characterized in that an adsorbent is added in a quantity which corresponds to the chemical oxygen demand (COD) concentration of the waste when radioactive liquid waste containing detergents is to be converted to powder. It is based on our discovery that the COD concentration of the waste is the good measure of the total amount of the substances which hinder powder formation and are therefore to be removed by adsorbents. In other words, it has been found that not only detergents or surface active agents but also various auxiliaries included in the detergents for the purposes of pH adjustment and uniform distribution of surface active agent and impurities in the waste due to stains, etc. on clothing affect powder formation adversely. Accordingly, an optimum amount for additives can never be attained by merely determining the concentration or quantity of detergents or surface active agents in the waste and then estimating, on the basis thereof, the amount of adsorbents to be added for the purpose of restraining the foaming due to the presence of surface active agents, and consequently the additive amount is found to have been either so small as to increase the moisture content of the resultant powder or so large as to increase the amount thereof embarrassingly. On the other hand, the necessary and sufficient amount of adsorbents to be added for which the presence of auxiliaries, stains, etc. has been taken into account can be estimated on the basis of COD concentration of the waste, for the COD concentration sensitively corresponds to the types of surface active agents and auxiliaries, degree of deterioration of said agents due to stains, etc. on clothing, and other factors. Examples of suitable adsorbents are powdered active carbon, molecular sieve, silica gel, alumina, and other powdered adsorbents having a large surface area. The impurities in the radioactive waste are adsorbed by addition of a necessary quantity of the adsorbent. More particularly, the surface active agents that cause foaming are adsorbed all but entirely and the foaming ceases to occur in the course of powder formation, and consequently the moisture content of the resultant powder decreases remarkably. The invention is also characterized in that the radioactive waste converted to powder by the above process is further pelletized. The powder to be pelletized contains the adsorbent incorporated in powder form, the adsorbent particles having adsorbed a moderate amount of moisture coming from the waste on their porous surfaces. These porous surfaces help combine all the powder particles into tight pellets. For this reason the resultant pellets exhibit a breaking strength about 1.5 to 2 times as high as that of pellets containing no adsorbent. The pellets should be made as strong as possible lest they should cause breakage and scatter radioactive substances during their transportation. It is therefore an advantage of the present invention that the resultant pellets exhibit a higher breaking strength as well as a lower moisture content. It is a third characteristic of the invention that a thin film evaporator provided with wiping blades is employed for the formation of powder. Abrasion of the wiping blades poses a serious problem for the thin film evaporator, for the blades tend to come in contact with the inner wall of the evaporator during their rotation at high speed. In the present invention the powdered adsorbent added to the liquid waste serves as a buffer between the inner wall and the wiping blades to retard the rate of abrasion of the blades remarkably. It is a fourth characteristic of the invention that the liquid waste of a fixed COD concentration can be used for powder formation by adjusting the concentration of the radioactive waste to be converted to powder instead of varying the amount of the adsorbent to be added according to the COD concentration of the radioactive waste. In the invention the amount of adsorbent to be added is unchanged, since the COD concentration is fixed beforehand.