Patent Number: 056493231
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a composition and process for disposing of radioactive, hazardous and mixed wastes, and, in particular, to encapsulation and stabilization of low-level radioactive, hazardous and mixed wastes. Industrial wastes which are not economical to recycle must be disposed of in the environment. Some of this waste material can be rendered harmless and then disposed of in a convenient manner. Other wastes, however, such as heavy metals, for example, mercury, lead, antimony, arsenic, and radioactive substances, cannot be rendered harmless. Consequently, the disposal of these materials into the environment must be made in a manner in which they are stabilized against dispersion into the environment. Large volumes of low-level radioactive wastes are routinely generated through the operation of defense-related and commercial nuclear facilities. As a result of its defense and research activities, the Department of Energy (DOE) generates not only low level radioactive waste, but also hazardous and mixed waste. Hazardous and mixed wastes at DOE facilities include a broad range of waste types, such as evaporator concentrates, salts, blowdown slurries, sludges, filter materials, ion exchange resins, and incinerator ash, that encompass diverse physical and chemical properties. Many of these wastes have been identified as problem wastes because they are difficult to encapsulate using conventional technologies and/or produce waste forms of poor quality that do not successfully retain hazardous constituents in the disposal environment. Due to reductions in volume resulting from incineration of contaminated combustible materials, remaining ash residues may contain sufficient quantities of hazardous elements, including heavy metals, that they meet the Environment Protection Agency (EPA) definitions for hazardous waste as well as DOE definitions for low-level radioactive waste. According to EPA's guidelines for delisting of hazardous and mixed wastes, before disposal at approved low-level waste disposal sites, such wastes must first be treated to immobilize the hazardous constituents. Attempts have been made in the past to render radioactive, hazardous and mixed wastes harmless by immobilizing the wastes against dispersion by ecological forces. In one method, the mixed wastes are sealed into metal or plastic containers which are then stored underground or in the ocean. In another method, the wastes are incorporated into a matrix of materials, such as inorganic cements and polymers while in their fluid or molten state followed by solidification. The high viscosity of molten plastics generally has limited the quantity of waste which can be loaded into the plastic matrix. Frequently, incorporation of wastes in a plastic mixture is limited by the inability of the matrix to isolate the waste from the environment. Highly loaded matrices having over 30 percent loadings have been unsatisfactory because of leaching. Thus, one disadvantage of known processes used to encapsulate waste is the tendency of waste to become mobilized. Other disadvantages of conventional hydraulic cement and other thermosetting polymer processes include low efficiency of waste encapsulation, a requirement to cure the matrix by adding chemicals and/or increasing the temperature, steps which result in increased operating costs. Accordingly, there is still a need in the art of waste disposal to provide a composition and method for encapsulating and stabilizing radioactive, hazardous and mixed wastes. It is, therefore, an object of the present invention to provide a composition and process for encapsulating and stabilizing radioactive, hazardous and mixed wastes in a multi-barrier process. Another object of this invention is to provide durable waste forms which can withstand mobilization by ecological forces. Yet, another object of the present invention is to develop an encapsulating process which has no curing requirements for solidification, thus providing significant cost savings. SUMMARY OF THE INVENTION The present invention, which addresses the needs of the prior art, provides a composition and a process for the disposal and immobilization of radioactive, hazardous and mixed wastes. More specifically, it has now been found that by heating and mixing in an extruder, substantially simultaneously, (i) dry waste powder including radioactive, hazardous and mixed wastes; PA1 (ii) a non-biodegradable thermoplastic polymer; and PA1 (iii) an anhydrous additive capable of forming a precipitate with the radioactive and/or toxic materials found in the dry waste powder; a molten homogenous waste matrix is obtained which is conveyed into a substantially pure or "clean" polyethylene container, wherein the molten matrix cools slowly to form a solid waste form which has an exterior layer of substantially pure or "clean" polyethylene. The anhydrous additives used to form insoluble precipitates with the radioactive and toxic materials include calcium hydroxide, sodium hydroxide, sodium sulfide, calcium oxide, magnesium oxide or a mixture thereof. Loss-in-weight feeders are used to provide the components of the molten waste mixture to the extruder in precise pre-determined ratios. Pretreating steps include drying and comminuting all waste to a dry waste powder having a particle size not greater than 3,000 microns. Waste particles which are less than 50 microns are pre-mixed with polyethylene binder by a high-speed kinetic batch mixer to form a homogenous waste mixture prior to charging into the extruder. As a result of the present invention a composition and process for disposal of radioactive, hazardous and mixed waste are provided, wherein radioactive and toxic wastes are stabilized by forming insoluble precipitates with chemical additives, all enclosed in a monolithic waste form having at least a second barrier to leaching of waste components into the environment. In contrast to hydraulic cement and thermosetting polymer processes, the present invention does not require chemical reactions for solidification, so that waste-binder interactions are minimized. The present invention allows a broader range of waste types to be encapsulated and results in better waste loading efficiencies, i.e., more waste per container. Preferred formulations prepared in accordance with the present invention can contain as much as 60% by weight, while still maintaining leachability of toxic metals below allowable EPA criteria. In contrast, maximum loadings using conventional Portland cement are limited to about 16% by weight ash. Other improvements which the present invention provides over the prior art will be identified as a result of the following description which sets forth the preferred embodiments of the present invention. The description is not in any way intended to limit the scope of the present invention, but rather only to provide a working example of the present preferred embodiments. The scope of the present invention will be pointed out in the appended claims.