Idaho National Engineering Laboratory
Description
Arc melter vitrification is a high-temperature extractive metallurgy based technology capable of melting soil and metals, pyrolizing or oxidizing residual organics, and melting structural metals from melted slag (silica and metal oxides), and partitioning transuranic (TRU) waste into the slag phase. Depending upon cooling rate and composition, the slag will consist of a glassy phase and several crystalline phases. Suitable composition and heat treatment will partition the TRU into a highly nonleachable, very durable crystalline phase with a probable geologic lifetime. High vapor pressure metals will probably evaporate from the melted phases and be collected in downstream collectors for further treatment. A process technology to recycle or immobilize the collected high vapor pressure metals (HVPM) will need to be developed. Immobilization in a low-temperature final waste form material, such as borosilicate glass, is a possibility.
Vitrification can produce a safe and highly durable final waste material for permanent disposal. Waste volumes can be substantially reduced (up to 65%, depending on waste composition). Selected clean metals may be separable for recycling. High-temperature electric melter technology requires smaller volume off-gas systems, and can facilitate processing of a wide range of materials with less presorting, sizing, and separating. The base arc melter technology has been used for smelting of similar heterogeneous ores for many years, providing a related applications experience base from which to proceed. The glass/ceramic final waste form will likely be suitable for disposal anywhere and may not need to be disposed at a deep geologic repository, such as the Waste Isolation Pilot Plant (WIPP). Further, it would produce a much safer and more compact material to ship.
Technical Performance Data
The glass/ceramic (crystalline rock) final waste form product will be the most technically effective waste form for long-term encapsulation of TRU. Hazardous organics and materials capable of hydrogen generation will be eliminated from the final waste form material. Buried wastes can be processed essentially as received, along with contaminated soils, with minimal separations, additives, characterization, and pretreatment. However, feed systems will not handle all wastes. Volume of waste and contaminated soils will be reduced. Selected metals may be separable for reuse.
Projected Performance
Arc furnace applications for pyrometallurgical processing of heterogeneous materials (ores) are a proven industrial base technology, the operation of which is routinely accepted by the public. Proceeding to design and develop an arc smelter system for mixed waste processing from a strong existing technology experience base should provide a better-performing and more publicly acceptable process technology. The cost of developing and implementing a melting process for the encapsulation of TRU will be a small fraction of the total cost of retrieval and processing. Extensive engineering development will be required for the implementation of a reliable, effective process. A pilot facility will require 3 to 5 years to examine and solve such problems as sealing, remote maintenance, process optimization, fume handling, and encapsulation issues.
A well-designed melting process will incorporate all the TRUs and selected metals in a durable slag and eliminate organics. All TRU elements are expected to be dissolved and retained in the glass/ceramic material formed upon cooling of the molten slag. Volume will be reduced substantially by vitrification (two to five times, depending upon waste soil mix). Selected clean structural metals, such as, iron, may be reduced and tapped. All organics will be pyrolized or combusted, and combustion will be completed in a secondary chamber, possibly in the off-gas system. Fume from the high-temperature melter will require processing by the air pollution control system. Depending upon waste composition, fume may contain acid gases, chlorides, heavy metals, and sulfides. Collected residues from the Air Pollution Control System will require further processing for final disposal.
Buried waste materials and soils are expected to be processable with a minimum of separation, sizing, and precharacterization using a soils buffering approach to ensure a high-integrity final waste form material.
Existing arc melter technology has not been designed for radioactive service; therefore, modifications will be necessary for radiation contamination control.
Cost. The cost to develop and build a system that will process 5 ton/h is estimated at $50-100M.
Waste Applicability
Arc melter vitrification is applicable to the treatment and immobilization of TRU wastes, toxic metals, hazardous organics in buried wastes, heterogeneous mixed wastes, and soils.
Status
Arc melter technology exists but requires extensive engineering development for alpha-controlled processing and optimization.
The American Society of Mechanical Engineers (ASME) and the U.S. Bureau of Mines (USBOM) had been pursuing the evaluation and demonstration of commercial arc melter furnace technology to the vitrification of municipal incinerator waste ashes. Upon becoming aware of their work in 1992, the present technical task plan to evaluate and demonstrate application of this technology to materials typical of buried mixed wastes was developed. A test series on commercial incinerator waste ash was completed in late 1992 by the USBOM/ASME, showing generally favorable results and identifying some problems to be considered. These results were factored into the test plan for the baseline series of tests for buried waste representative materials in April/May 1993. In parallel with this effort on a three-phase AC melter system, Electropyrolisis Inc., in cooperation with Pacific Northwest Laboratory and Massachusetts Institute of Technology, is evaluating a direct current (DC) arc melter system of somewhat different proposed configuration. Integration of these projects is in progress.
Regulatory Considerations
The melter system is part of an overall treatment system. All output materials from the process system will either be clean and suitable for reuse or release to the environment (meeting all environmental standards) or will be in a material form suitable for permanent long-term disposal.
High-temperature processing carries with it inherent hazards requiring thorough safety design and analyses. Alpha contamination control and hazardous materials control require further extensive safety considerations typical of any radioactive materials processing facility. Remote operation will protect workers. Potential for pressurization transients, e.g., steam explosion, or combustibles explosion, requires thorough treatment.
Potential Commercial Applications
Potential commercial applications for the arc melter furnace technology include vitrification of municipal incinerator waste ashes.
Baseline Technology
There is no accepted baseline technology for treatment of mixed TRU contaminated buried wastes. Preliminary systems design studies for buried wastes identified and compared the most likely treatment options. The highest-ranked process options were those which involved ex situ vitrification in a high-temperature melter. Repackaging of untreated buried waste for disposal storage at WIPP (present baseline plan for stored TRU wastes) is not a probable option. Various arc melter furnace configurations are possible. The baseline configuration for arc melters is standard commercially available three-phase graphite electrode furnace technology. This pilot test system is representative of that baseline electric arc furnace configuration. A number of other similar thermal treatment systems are also under development.
Intellectual Property Rights
Patents: None.
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References
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