George G. Wicks and Rebecca L. Schulz
Westinghouse Savannah River Company
Aiken SC 29808

David E. Clark
University of Florida
Gainesville, FL 32611-6400

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Large quantities of hazardous wastes are being generated from a multitude of processes and products in today's society and this inventory is growing at an alarming rate. Microwave energy represents a unique heating source with the potential of providing a highly flexible technology for a) minimizing generation of selected future wastes, b) reducing existing wastes and immobilizing hazardous components and c) reclaiming or recycling reusable and sometimes valuable components located in waste products. Some of the many wastes under study using microwave treatments include remediation of discarded electronic circuitry and reclamation of the precious metals within, incinerator ashes, medical and infectious wastes, industrial wastes/ sludges, rubber products including tires, asbestos, groundwaters, volatile organic compounds (VOC's), shipboard wastes, contaminated soils and sediments, and radioactive wastes and sludges (high, low and intermediate level wastes, transuranic and mixed wastes).

A team from the Westinghouse Savannah River Technology Center (WSRC- a DOE Laboratory), and the University of Florida (UF- academia), has been active for about a decade in development of microwave technology for specialized waste management applications. This interaction has resulted in the development of unique equipment and uses of microwave energy for a variety of important applications for remediation of hazardous and radioactive wastes. Discussed below are results of this unique technology for processing of A. Electronic circuitry and components, B. Medical wastes, C. Discarded tires and D. Transuranic radioactive wastes.

Microwave energy interacts with matter is a way different than all other thermal treatment processes. As a result of these unique features, the advantages of using microwave energy for treating a vast array of hazardous wastes can include many potential advantages. The advantages ultimately realized will depend both on the type and characteristics of the wastes to be treated. Following are some of these important features:

• Significant waste volume reduction
• Rapid heating
• High temperature capabilities
• Selective heating
• Enhanced chemical reactivity
• Ability to treat wastes in-situ
• Treatment or immobilization of hazardous components to meet regulatory requirements for storage, transportation or disposal
• Rapid and flexible process that can also be made remote
• Ease of control
• Process equipment availability, compactness, cost, maintainability
• Portability of equipment and process
• Improved safety, including reductions in personnel exposure of potentially hazardous chemicals or materials (ALARA) for processing and disposition
• Energy savings
• Cleaner energy source compared to some more conventional systems
• Overall cost effectiveness/ savings

A new technology has been developed by WSRC and UF and applied to variety of waste management applications, including disposition of electronic circuitry. This technology consists of a tandem microwave unit that makes use of direct and hybrid microwave energy, which is designed to not only treat primary waste, but also simultaneously, off-gases, when necessary.

In Figure 1, a laboratory scale tandem microwave system is shown which consists of two interconnected microwave chambers. First, the lower chamber is the processing chamber that is used to ash or vitrify various solid and liquid waste streams, including electronic components and circuit boards. This unit can be "hybridized" to effectively reach the high temperatures necessary for vitrification, by the use of a susceptor shroud which allows processing of wastes types normally not receptive to microwave heating. This makes this system unique compared to commercially available microwave units. While the waste is being reduced in volume and transformed in this chamber into glassy and metal products, the off-gases produced pass into the upper microwave unit which contains a special series of filters and bedded material. The material is then heated by microwaves which decompose the off-gases produced. Gas chromatography has shown that the amount of hazardous components found in the off-gases are significantly reduced by the microwave treatment.

In today’s society, there exists many millions of circuit boards that must be disposed of every year from a vast array of discarded consumer products as well as a large and diverse inventory of DOE and DOD equipment. At present, these materials are often disposed of in landfills throughout our nation. This results in a number of concerns. First, since many of these landfills are at or near capacity levels, the addition of tons of obsolete electronic components hastens the need for new landfills. Next, there are a variety of hazardous materials that may be contained within electronic components which can leach from the waste in landfills and migrate into groundwater, which can result in undesirable environmental consequences. Finally, because electronic products are simply thrown away, natural and valuable resources in the wastes, including precious metals such as gold, are discarded and cannot be reused or recycled. A new strategy is needed to treat the large and increasing volumes of electronic waste, by reducing its volume, immobilizing hazardous components into leach resistant forms, and also reclaiming and recycling the processed products, including valuable materials. This would provide not only an environmentally sound strategy, but can also result in a significant potential return on investment.

Following is a summary of results obtained from laboratory processing studies which are provided in more detail elsewhere (1-12)

• A wide array of electronic components were successfully treated by a relatively simple and flexible, one-step, hybrid-heated microwave process.
• Significant volume reductions were achieved in all studies.
• In this process, circuitry was completely converted into two waste form products; a glass and a metal form, both of which can be reused or recycled.
• The glass form retained hazardous components and met important environmental leaching standards. The glass could also be fabricated into other products and in most cases, the glass could be produced without the need of any additives.
• The metal form produced was subsequently processed to effectively reclaim precious and valuable metals, including gold and silver. The separation of glass and metal waste products was very effective and efficient.
• A new "tandem microwave" system successfully handled both primary as well as secondary wastes in an integrated unit, i.e., the off-gases produced were simultaneous treated along with the electronic circuitry.
• One of the many unique features of this system is the ability to make treatment of potentially hazardous materials significantly safer compared to many other thermal treatment processes. This is a result of being able to remove or "remote" the source of microwaves, as well as the electronics and controls, from the main microwave processing chambers, which then contain virtually no moving parts or maintenance items. This allows operating and maintenance personnel to be removed from the proximity of the processing units during operation. This system is particularly suited to treatment of proprietary and/or very hazardous or radioactive materials, as well as potential explosive or other dangerous media. It also works equally well with less hazardous materials.
• The entire hybrid microwave process and specialized equipment (now being patented) can also be mocked-up to a larger scale and made mobile, if desired.

The tandem microwave unit successfully treated not only a vast array of electronic components and circuitry, and converted this discarded material into two new recyclable products, but also successfully treated the off-gases produced. Several studies involving microwave treatment of emissions from processing of electronic circuitry are summarized in Table I. Note that emissions of key components were generally reduced by factors of 10 to 1000x. While only laboratory scale studies have been performed to date, a preliminary assessment shows that the overall concept and process can be mocked-up to a much larger scale.

In the United States, 400 million pounds of medical wastes are generated every year, including infectious waste that poses a potential threat to the health and wellbeing of the population. There are two main means generally used for the treatment of these materials; destruction by incineration and disposal in landfills. Both of these methods have important economical, environmental and health related-concerns or limitations.

The tandem microwave system provides a unique means for treating both the primary and secondary medical waste, and provides a system that is compact and can be made portable. It has been used successfully, on a laboratory scale, for destruction of a variety of mixed simulated infectious medical wastes, including plastics, clothing, sharps and other metal components, etc. This technology has the potential of a) disinfecting b) sterilizing and c) destroying discarded medical waste products.

There are approximately 250 million tires discarded annually in the U.S. and there already exists well over 3 billion used tires currently being stockpiled. This can result in a variety of problems along with important health and environmental concerns. For example, tires in landfills do not biodegrade readily and are buoyant, which can cause them to rise to the surface at landfills and breach landfill covers. Stockpiled tires can also catch fire easily as well as result in soil, water and air pollution. In addition, they can also become breeding grounds for mosquitoes and vermin.

The WSRC/UF team has been working on an alternate approach to recycling used tires in a CRADA with a major tire manufacturer. The team has produced a new type of "crumb rubber" that can be combined with "new rubber" to produce high-quality tires, having 25% or more recycled rubber than obtained previously. In this process, microwave energy is used to selectively break S-S and S-C bonds in the rubber compound, while leaving the backbone of the rubber (C-C bonds) relatively intact, thus de-vulcanizing, but not de-polymerizing, the rubber to be recycled. This also produces activated surfaces that when combined with new rubber, yields a composite with excellent properties.

Over 40 different types of materials and mixes of materials have been studied thus far in the tandem microwave system, for destruction and remediation of radioactive transuranic (TRU) wastes. Volume reductions of 60-90% were generally achieved and main organic constituents, sources for gas generation during storage, were eliminated by this process. The products produced were either ashes or vitrified materials, depending on the application.

The WSRC/UF tandem microwave waste treatment system provides important advantages for treatment of a wide range of hazardous materials, and provides an important contribution to our existing arsenal of waste management and remediation technologies.

1. Wicks, G.G., Schulz, R.L. and Clark, D.E., "Microwave Technology for Waste Management Applications Including Disposition of Electronic Circuitry," presented at the 100th Annual Meeting of the American Ceramic Society, Cinn. OH, and published, in Environmental Issues and Waste Management Technologies in the Ceramic and Nuclear Industries IV, J.C. Marra and G.T. Chandler, eds., Vol. 93, pp. 89-96 (1999).
2. Wicks, G.G., "Nuclear Waste Glasses", Treatise on Materials Science and Technology, Glass IV, M. Tomozawa and R.H. Doremus, eds., Vol. 26, pp. 57-118 Academic Press, Inc. (1985).
3. Oda, S.J., "Microwave Remediation of Hazardous Waste: A Review", Microwave Processing of Materials III, (R.L. Beatty, W.H. Sutton and M.F. Iskander, eds.), Materials Research Society, Vol. 269, pp. 453-464 (1992).
4. Dauerman, L., Windgasse, G., Zhu, N. and He, Y., "Microwave Treatment of Hazardous Wastes: Physical Chemical Mechanisms", Microwave Processing of Materials III, (R.L. Beatty, W.H. Sutton and M.F. Iskander, eds.), Materials Research Society, Vol. 269, pp.465-469 (1992).
5. Krause, R.T. and Helt, J.E., "Applications of Microwave Radiation in Environmental Remediation Technologies", Microwaves: Theory and Application in Materials Processing II, Ceramic Transactions (D.E. Clark, W.R. Tinga and J.R. Laia, eds.), American Ceramic Society, Vol. 36, pp. 53-59 (1993).
6. Wicks, G.G., Clark, D.E., Schulz, R.L. and Folz, D.C., "Microwave Technology for Waste Management Applications Including Disposition of Electronic Circuitry", Microwaves: Theory & Application in Materials Processing III, Ceramic Transactions, (D.E. Clark, D.C. Folz, S.J. Oda and R. Silberglitt, eds.), Vol. 59, pp. 79-89 (1995).
7. Schulz, R.L., Fathi, Z., Clark, D.E., and Wicks G.G., "Microwave Processing of Simulated Nuclear Waste Glass", presented at the Symposium on Microwaves: Theory and Application in Materials Processing, April 28- May 2, 1991, Cinn. OH, Ceramic Transactions, Nuclear Waste Management IV, (G.G. Wicks, D.F. Bickford and L.R. Bunnell, eds.), Vol. 23, pp. 779- 786 (1991).
8. Schulz, R.L., Clark, D.E., Hutcheon, R.M. and Wicks, G.G., "Microwave Processing of Simulated Nuclear Waste Glass II", Microwaves: Theory and Application in Materials Processing II, Ceramic Transactions (D.E. Clark, W.R. Tinga and J.R. Laia, eds.), American Ceramic Society, Vol. 36, pp. 89-97 (1993).
9. Schulz, R.L., Folz, D.C., Clark, D.E. and Wicks, G.G., "Microwave Destruction/ Vitrification of Electronic Components", Microwaves: Theory and Application in Materials Processing II, Ceramic Transactions (D.E. Clark, W.R. Tinga and J.R. Laia, eds.), American Ceramic Society, Vol. 36, pp. 81-88 (1993).
10. Schulz, R.L., Folz, D.C., Clark, D.E., and Wicks, G.G., "Microwave Energy for Waste Remediation Applications", Microwave Processing of Materials IV, (M.F. Iskander, ed.), MRS Symposium Proceedings, Vol. 347, pp. 401-406 (1994).
11. Schulz, R.L., Folz, D.C., Clark, D.E., Schmidt, C.J. and Wicks, G.G., "Microwave Treatment of Emissions from the Destruction of Electronic Circuitry", Microwaves: Theory & Application in Materials Processing III, Ceramic Transactions, D.E. Clark, D.C. Folz, S.J. Oda and R. Silberglitt, eds., Vol. 59, pp. 107-114 (1995).
12. Schulz, R.L., Folz, D.C., Clark, D.E., Schmidt, C.J. and Wicks, G.G., "Microwave Waste Treatment System", poster presentation at the First World Congress on Microwave Processing, Lake Buena Vista, FL, Jan. 5-9 (1997).