Patent Number: 054141974
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method which includes the steps of incorporating the waste into melted asphalt or other polymer and forming the waste-polymer blend into aggregate pellets for concrete. The pellets (used as aggregate) are coated with powdered siliceous or carbonate material to improve bonding between the pellets and the cementitious matrix. The coated pellets are mixed into a cementitious matrix to form a concrete. This concrete with polymer aggregate is cast into wasteforms for storage or burial. It is desirable to produce a wasteform with the polymer-waste composite in the center of the concrete monolith. To accomplish this, the mold can be placed on a turntable and spun, or otherwise exposed to a centrifugal force, to force the matrix material into a continuous layer on the outside of the wasteform. The polymer-waste pellets typically have a specific gravity near 1.5 while that of the cementitious matrix is greater than 2.0. This difference in density makes it possible to separate the materials by spinning. The wasteform of the present invention is safer and more durable than either of the earlier wasteforms. The new wasteforms can be cast as discrete, self-supporting units of a convenient size and shape that can be stored, transported, and retrieved as required by regulations. This technology does not require that the wasteform be permanently cast in a trench or vault. The waste can be cast as blocks with all exterior surfaces consisting of uncontaminated, normal-strength concrete. No containers or crates should be required to maintain the shape of the wasteform. According to the present invention, the problem of holding soluble salts in a porous medium such as concrete is overcome. The leaching characteristics of the composite are controlled by the polymer encapsulation, not by the cementitious matrix. The major problem with cement-based systems has been the interaction of the wastes with the cement which weakens the resulting solid. By initially isolating the waste, especially the soluble salt, in a polymer, chemical interaction between the cement and the wastes cannot occur. Typically, polymer wasteforms will contain 40-60% waste by mass. It would be useful to have an outer layer of uncontaminated material around the waste to improve waste isolation and, in the case of radioactive wastes, to provide radiation shielding. According to the present invention, the polymer aggregate is disposed in a flowable cementitious mixture and a shielding layer is formed by modifying the wasteform casting process. Because the polymer-waste pellets used as aggregate are much less dense than the unhardened mortar mixture, if the mold is spun during the casting process the heavier mortar mixture will be forced to the outside. When the concrete then hardens, the polymer with the waste enclosed will be concentrated in the center of the concrete mass and a layer of clean, uncontaminated concrete will be formed on the outside of the wasteform. This monolith with the waste concentrated in the center is a wasteform that is safer to handle and less likely to release waste through diffusion. The present inventors know of no other waste disposal technique that involves mixing materials having different densities and separating the materials by spinning. Some asphalt wasteforms have been shown to expand and form surface cracks if they are free to swell in contact with water. The concrete shell can provide confining pressure to prevent the asphalt from expanding and can limit the exposure of the asphalt surface to water. Cement-based wasteforms have been favored in the past because of the relatively high pH maintained in the matrix. The high pH reduces the solubility of many metal salts that are common in wastes and a problem to the environment. For example, cadmium and lead are typically more soluble in acid than alkaline aqueous systems. It has not been possible to develop this type of chemical barrier with asphalt alone. Thus, the present invention provides both the chemical barrier from the high pH and the improved containment of a polymer wasteform. Many materials that are plastic in nature may be used for making pellets. Polyethylene, asphalt, elemental sulfur, and other organic and inorganic materials that are thermoplastic may be used. The waste to be contained and isolated usually begins in the form of a slurry or a solution. The waste is first dried to remove free liquid. The result of this drying step is a dry waste salt. Preferably, a spray evaporator dryer is used which produces a hot, dry salt. The salt product is then mixed or kneaded into the thermosetting polymer after the polymer has been heated to form a liquid or a plastic solid. The resultant mixture is then formed into pellets. Pellets can be formed by making long strips of the waste salt-polymer mixture and cutting the strips into pellet-sized forms. An extruder may be used to form the strips and the strips may be cut as they exit the extruder. Of course, other methods of forming pellets, particularly spherical pellets, can alternatively be used. The pellets can be of various sizes and shapes. For a wasteform monolith weighing approximately 75 pounds, the ideal pellet is roughly equidimensional and is between 0.5 to 2.0 inches in size. Spherical pellets for this size of monolith should have an average diameter between 0.5 and 2.0 inches. This last-mentioned range is a good size for pellets to be used in a monolith which has a size in the range of a 6-inch cube to a 12-inch cube. A monolith size of about a 10-inch cube may be preferred in order to achieve a size which could be handled conveniently. The preferable shape for a monolith would be a cube or a cylinder. The maximum size of a pellet should typically be no larger than one-third of the smallest dimension of the monolith. The pellets are then rolled in or otherwise coated with a fine granular or powdered inorganic compound, such as sand, to improve bonding of the pellet to the cementitious matrix. The pellets are coated with the granular or powdered material while the thermosetting material is in a plastic or semi-solid condition. Heating and tumbling causes adhesion of the grains of powder to the exterior of the pellets. The pellets are then cooled, removed from the powder, and incorporated in the cementitious matrix. Any cementitious or pozzolanic material, fumed silica, ground limestone, fly ash, ground clay, portland cement, sand or ground slag may be used to coat the pellets. Portland cement is a preferred coating substance. The coated pellets are then mixed with a cementitious matrix such as mortar and the mixture is allowed to harden. Alternatively, the mixture may be spun before hardening to force the heavier cementitious matrix to the outside of the monolith. Tests were done with 2-inch diameter by 4-inch long cylinders prepared according to a method of the present invention using a simulated salt waste mixed in asphalt. The tests showed that because there was no loss of strength due to the reactions between the cementitious matrix and the waste it was possible to develop cylinders with unconfined compressive strengths of over 1000 psi using a simple mortar design with Type I-II portland cement. Spinning the samples moved the fresh mortar mixture to the outside of the wasteform. The sample products had a smooth, dust-free exterior surface with no exposed asphalt aggregate. In the above examples salt loadings were below 10% by mass. In a typical concrete, coarse aggregate would form approximately one-half of the volume of a concrete mixture. Assuming a mortar consisting of sand and cement is used as a matrix, replacing the coarse aggregate with pellets of asphalt containing 60% salt would result in a wasteform that contained approximately 20% by mass salt. The wasteforms prepared according to the method of the present invention can have salt loadings comparable to or higher than those wasteforms prepared by mixing waste salt directly into a cementitious matrix. According to the present invention, even salts containing large amounts of chelating agents that would normally require very low salt loading levels (below 1% by mass) can be added into a wasteform without changing the cement content in the matrix. Although the present invention has been described in connection with preferred embodiments, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention defined in the appended claims.