Patent Number: 048083186
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a method for immobilizing radioactive wastes for long term storage. More specifically, the invention relates to a method for recovering cesium from solutions containing cesium together with other metal values and for immobilizing radioactive cesium in a highly stable, nonvolatile and insoluble product suitable for long-term storage. The principal long-term problem caused by nuclear reactor accidents is contamination of the environment with radioactive material as was evidenced by the Chernobyl nuclear reactor accident in 1986. Cesium is a particular problem in this respect because it is very volatile and can be carried in the upper atmosphere for long distances. Therefore, suitable methods of decontaminating the environment, i.e. water and soil, of cesium before its ingestion by animals or humans are highly desirable. Also desirable are methods for recovering radioactive cesium from the gastrointestinal tracts of contaminated animals. Certain cation exchange resins and various cation exchangers are available which are selective for the recovery of cesium from solution. These include clay minerals and zeolites, both naturally occurring and synthetic. Furthermore, naturally occuring mordenite zeolite has been mixed with animal feed to remove ingested cesium-137 from the gastrointestinal tracts of contaminated animals. Other natural zeolites include clinoptilolite, erionite and chabazite. In addition to recovering the ions, the radioactive ions must be immobilized in a form suitable for longterm storage so that they cannot be released back into the environment or leached from the storage medium into the surrounding soil or groundwater. Many methods and materials have been developed over the years for immobilizing various nuclear wastes, and especially cesium, for long-term storage. For example, U.S. Pat. No. 3,161,601 dated Dec. 19, 1964, and assigned to the common assignee, incorporated the radioactive cesium into a glass containing cesium oxide, alumina, phosphate and an additive such as lanthanum or zirconium. Another method was to incorporate the cesium-137 as cesium carbonate with spodumene or with a mixture of silica and kaolin at a temperature of at least 1000.degree. C. to form a synthetic pollucite. Another process mixed an inorganic zeolite containing radioactive cesium with about 20% additives, mainly iron and calcium oxides, which was melted at 1500.degree. C. and control cooled to form an iron-enriched basalt. U.S. Pat. No. 4,537,710, also assigned to the common assignee, describes a cation exchanger which is a modified tobermorite containing aluminum, that is selective for a small number of ions including cesium. The advantage of the modified tobermorite is that it is compatable with matrix materials such as concrete and, therefore, more resistant to leaching. Other, less satisfactory, methods involve incorporating the radioactive waste directly into a matrix material such as asphalt or concrete before emplacement for storage. Almost all of the materials or processes suitable or selective for the separation and recovery of cesium from contaminated water or radioactive waste streams require additional processing to immobilize the cesium, in order to prevent the radioactive ions from being leached or otherwise separated from the storage medium. For example, the cesium ions may be eluted from the ion exchanger and incorporated directly into the matrix material for storage. Preferably, the recovery material (ion exchanger) containing the radioactive ions, is itself incorporated into the storage medium, such as a glass or cement, for storage. Alternatively, the radioactive ions may be mixed with other inorganic materials and by applying heat and/or pressure, formed into a synthetic mineral which is satisfactory for storage. Thus, most processes require several steps to recover and prepare the cesium for storage. This increases the cost of preparing the radioactive ions for storage. Furthermore, some processes require high pressures and/or temperatures which in addition to increasing costs, increases the opportunities for the loss of radioactive material. Finally, many of the storage materials are not wholly suitable for long-term storage because leaching of the radioactive ions can occur. SUMMARY OF THE INVENTION It has been found that a modified phlogopite mica is very ion selective for cesium ions, even from solutions which also contain sodium and calcium ions. Furthermore, it has been found that the modified phlogopite mica will trap the cesium ions in such a manner that the phlogopite containing the cesium is suitable for emplacement for long-term storage, with little or no additional processing. The modified phlogopite mica of the invention is a phlogopite mica which has been hydrated and in which the potassium ions have been replaced by sodium ions. The invention is a process for the separation and recovery of cesium ions from a feed solution containing cesium ions, and which may contain other metal ions, by contacting the solution with the modified phlogopite which is a hydrated sodium phlogopite mica whereby the cesium ions are selectively taken up by the modified phlogopite while the other ions remain in the solution, and separating the modified phologopite containing the cesium ions from the feed solution thereby recovering the cesium ions. The invention is also a process for fixing radioactive cesium for long-term storage by contacting a solution containing radioactive cesium with the modified phlogopite which is a hydrated, sodium mica maintaining the contact until sufficient cesium is taken up by the modified phlogopite to reduce the c-axis spacing an amount sufficient to immobilize the cesium, thereby fixing the radioactive cesium ions for long-term storage. Alternatively, the cesium may be fixed by heating the modified phlogopite containing the cesium to a temperature sufficient and for a period of time sufficient to reduce the c-axis spacing thereby fixing the cesium ions in the modified phlogopite. Since the modified phlogopite of the invention is very selective for cesium ions, it is especially useful for the recovery of radioactive cesium ions which are present in radioactive waste solutions along with other metallic ions including sodium and calcium. The solutions may be either low level, intermediate or high level nuclear wastes. It is also useful for the recovery of cesium-137 from large volumes of water containing low levels of cesium such as nuclear reactor coolant systems which have become contaminated because of fuel element ruptures or from stream or water supplies which have become contaminated due to Cs.sup.137 fallout. Furthermore, because of its inertness and stability in an acidic environment, the modified phlogopite is suitable for ingestion by animals for the recovery and removal of ingested radioactive cesium fron the gastrointestinal tract, result from nuclear mishaps like the Chernobyl incident. It is therefore one object of the invention to provide a process for recovering cesium. It is another object of the invention to provide a process for recovery of cesium from solutions containing cesium together with other ions. It is a further object of the invention to provide a process for fixing cesium for the long-term storage. It is still another object of the invention to provide a one-step process for recovering and fixing cesium ions for long-term storage. Finally, it is the object of the invention to provide a process for recovering and immobilizing radioactive cesium ions for long-term storage which does not require conditions of high temperature or high pressure.