Environmental Research &Technology Division

Technology Development/Applications Group

Encapsulating Fly-Ash-Contaminated Mixed Waste with Sulfur Polymer As much as 2.5 times more incinerator fly ash can be solidified in sulfur polymer cement than in hydraulic cement. This latest process for encapsulating contaminated mixed waste minimizes the potential for releasing toxic materials to the environment. Introduction The U.S. Department of Energy (DOE) is a major generator of both hazardous and mixed (hazardous/radioactive) wastes because of its defense and research activities. Many of the wastes are identified as "problem" wastes because they are difficult to encapsulate using conventional technologies, or they produce waste forms of poor quality that do not successfully retain hazardous constituents during disposal. These phenomena usually relate to the chemical and physical properties of the wastes and how they interact with the binder materials. Incinerating the radioactively contaminated, combustible materials at DOE sites naturally reduces the volume of waste, but at the same time leaves ash residues containing large quantities of hazardous elements. These toxic metals meet the U.S. Environmental Protection Agency's (EPA's) definitions for characteristic toxic waste. Such mixed wastes must either be disposed of at a licensed mixed-waste disposal facility or treated to immobilize hazardous constituents before petitioning EPA to delist it as a hazardous waste. If delisting is approved, the waste may be disposed of at a low-level waste disposal site. Concept Description Sulfur polymer cement is a thermoplastic material developed by the U.S. Bureau of Mines in 1972 to use by-product or waste sulfur for the construction applications. The sulfur cement is easily melted at 120°C to form a low viscosity liquid, which can be adapted for use in a number of different waste processes. Brookhaven National Laboratory (BNL) has developed an encapsulation process that solidifies incinerator fly ash in sulfur polymer cement. As shown in the figure below, the incinerator ash mixtures are processed in a dual-action mixer. Pre-melting the binder and slowly adding the incinerator ash waste eliminate the tendency for the mixture to clump. The process simultaneously heats the waste and binder with several additives to form an homogenous mixture. This mixture is transferred to a drum or reusable mold where it cools to a solid monolithic waste form for diposal. Additives are used to enhance waste form durability and to immobilize soluble toxic metals contained in the waste. Sulfur concrete compressive and tensile strengths twice those of comparable portland concretes can be achieved using this porcess. Full strength is attained in several hours rather than weeks. Economic and Market Potential Unlike the hyraulic cement processes that rely on chemical reactions for setting and solidifying, the sulfur polymer cement encapsulation process does not require a chemical reaction. Rather, it is compatible with a wide range of waste types and can accommodate higher waste loadings ( more waste per durm than conventional systems). Further, as much as 2.5 times more incinerator fly ash can be solidified in sulfur polymer cement than in hydraulic cement. These improvements in waste loading reduce the number of waste drums processed for storage, transport, and disposal, thus reducing overall costs. Sulfur polymer cement encapsulation can be used to improve waste treatment at DOE facilities, nuclear power plants, chemical and manufacturing facilities that generate hazardous wastes, environmental restoration of Superfund sites, municipal solid waste incinerators, and waste-to-energy plants. Key Experimental Results Waste form performance test criteria established by the U.S. Nuclear Regulatory Commission for licensing commercial radioactive waste forms and by the EPA for hazardous waste were applied in the testing. These criteria included mechanical strength, water immersion, freeze-thaw cycling, biodegradation, radiation stability, and leachability of both radioactive and toxic constituents. In each of these tests, waste forms performed well above the minimum standards set by the regulatory agencies. Sulfur polymer cement waste forms also demonstrated excellent durability and performance under anticipated storage, transport, and disposal conditions.

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Last Modified: November 12, 2009
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One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

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