Patent Number: 045281297
Section: summary

This invention pertains to processing dry or aqueous radioactive wastes and uranium mill tailings and any accompanying toxic and hazardous waste materials into a substantially stabilized, insoluble, impermeable, encapsulated and solidified form suitable for safe and ecologically-acceptable disposal. The present invention is a significant advancement and improvement in the field of the disposal of radioactive wastes and uranium mill tailings. Other inventions by the present inventor in the specifically non-related and generally collateral fields of oil sump and sewage disposal are described in U.S. Pat. No. 4,038,240, granted June 7, 1977; U.S. Pat. No. 4,079,003, granted Mar. 14, 1978; and U.S. Pat. No. 4,184,958, granted Jan. 22, 1980. A considerable volume of uranium tailings have been and are being produced from mining and milling of uranium ores. The uranium mill tailings, which may be stockpiled or impounded in liquid-covered retention ponds, are of considerable concern from a safety and ecological standpoint. Radon gas RN-222, during the mining and milling process, will continually be produced and emanated from the mill tailings unless radium-226 and thorium-230 is removed from the uranium ore. Because of the long half lives of the radioactive elements, radon gas will be produced for a considerable period of time. For this reason, it is preferred that the uranium tailings typically be submerged in liquid in retention ponds. The liquid cover on the pond inhibits or prevents the emission of radon gas. Direct exposure to radon gas poses an immediate health threat, generally in the form of cancer. Tailings retention ponds are a source of potential ground water contamination, and are exposed to contact by humans and wildlife. The liquid in the pond is either highly acidic or highly basic, depending upon the type of recovery process used in milling the uranium ore. In addition, the tailings liquid includes amounts of aluminum, ammonia, arsenic, calcium, carbonate, cadmium, chloride, copper, fluoride, iron, lead, manganese, mercury, molybdenum, selenium, sodium, sulfate, vanadium, zinc, natural uranium, radium-226, thorium-230, polonium-210 and bismuth-210, and total dissolved solids, including uranium ore (U.sub.3 O.sub.8). Many of the materials accompanying the radioactive elements are hazardous and ecologically undesirable. These hazardous materials may leach from the retention pond into the ground water supply and are a health risk to humans and wildlife. The principal objectives for disposing of uranium tailings have been to attempt to protect the ground water against contamination, to control the atmospheric emission of radon gas, and to isolate the tailings from man's environment, permanently, or for long time periods. However, there is no universally followed or successful practice for disposing of the uranium tailings. Indeed, population centers have grown up around or on top of exposed stockpiles of tailings, and the ground water has exhibited some radioactive contamination in some locations. Past practices for disposing of uranium tailings have included burying the tailings in deep trenches or in abandoned mine shafts. The ecological impact of such practices is uncertain. Attempts have been made to neutralize the acidic liquid being discharged into the tailings retention ponds to a pH which is not substantially acidic or basic by adding milk of lime and other forms of alkaline reagents. This type of neutralization could not be maintained, partially because of feed-mix variables and the intrusion of fresh water into the ponds from natural sources. Barium chloride processes have also been considered, but such processes are not effective in removing radium. An ion exchange process employing an organic resin specially compounded to collect ions from a solution of the uranium tailings has also been considered, but the success of this process depended on the freedom of the solution from excessive solids, which was impractical and uneconomical to achieve. Other partial techniques involve removing water from the tailings by solar evaporation, thermal evaporation or filtration. This procedure only addresses the liquid fraction of the overall disposal problem and does not address the disposal problem associated with the remaining solid materials. Attempts have been made to encapsulate large quantities of uranium tailings to prevent leaching of the wastes into the ground water and to resist the diffusion and emission of radon gasses into the atmosphere. Prior encapsulation techniques involved constructing a clay or synthetic liner or clay covering of large quantity of tailings. Asphalt, asphalt and neoprene emulsions, resinous adhesives, elastomeric polymers, mixtures of wax and tar and pitch, and other chemical compounds have been tried as coatings or liners. These materials have, however, proved ineffective because of cracking and deterioration caused by the highly acidic or basic nature of the tailings, the weather and natural expansion and contraction. Cracked, deteriorated covering and expanded liners have allowed the diffusion of radon gasses into the atmosphere and the intrusion of natural water and seepage of tailings solutions into the ground water. Excessive costs were required to maintain or replace the liners or coverings. None of the prior techniques for disposing of radioactive wastes and uranium tailings have succeeded in controlling and terminating the persistent emission of radon gas into the atmosphere, and in protecting the ground water from potential contamination. SUMMARY The present invention possesses the capabilities, among others, of substantially or totally encapsulating the radioactive wastes and accompanying chemical contaminants present in uranium mill tailings in a solid mass of rock-like material which is substantially impermeable to the emission of radon gas therefrom and which is substantially insoluble to the leaching or seepage of the radiological and hazardous organic chemical wastes into the ground water. The characteristics of the manufactured rock-like material allow it to be disposed of relatively safely and without substantial known ecologically damaging effects. The manufactured rock-like material need not be covered with liners, sealers, or the like, since the product inherently possesses the required characteristics for substantially isolating the radioactive wastes from the environment. As a further result of the practice of the present invention, it is possible to eliminate tailings retention ponds and stockpiles of tailings and return the land to safe ecological utilization. According to its broad aspects, the process of the present invention comprises absorbing soluble radioactive wastes in an absorbing agent, preferably clay, and thereafter cementing the absorbed wastes and absorbing agent into a final matrix product having a pH of at least 8.0 and which is substantially impermeable. Cementing is achieved by use of lime, and the lime also renders the soluble radioactive wastes insoluble by pH neutralization or adjustment. Any non-neutralized contaminants are nonetheless captively held in the final matrix product. The final matrix product changes to a limestone in which the wastes are encapsulated by contact with carbon dioxide. The final matrix product achieves a significant reduction in the emission of radioactive gasses and the leaching out of soluble contaminants, and the resistance to both leaching and emission are improved in the limestone. The final matrix product is in a state for acceptable disposal. A more complete and thorough understanding of the invention can be obtained by reference to the detailed description of its preferred embodiment and the accompanying drawing.