(Plasma Arc Vitrification)
Plasma arc vitrification occurs in a plasma arc centrifugal treatment (PACT) system, where heat from a transferred plasma arc torch creates a molten bath that detoxifies the feed material (see figure below). Solids are melted into the molten bath while organics are evaporated and destroyed. Metallic feed material can either form a separate liquid phase underneath the metal oxide slag layer or can be oxidized and become part of the slag layer.
Waste material is fed into a sealed centrifuge, where a plasma torch heats solids to approximately 3,200 °F and gas headspace to a minimum of 1,800 °F. Organic material is evaporated and destroyed. Off-gases travel through a gas-slag separation chamber to a secondary chamber, where the temperature is maintained at over 2,000 °F for at least 2 seconds. The off-gases then flow through an off-gas treatment system.
Inorganic material is reduced to a molten phase that is uniformly heated and mixed by the centrifuge and the plasma arc. Material can be added in-process to control slag quality. When the centrifuge slows, the molten material is discharged as a homogeneous, nonleachable, glassy slag into a mold or drum in the slag collection chamber. When cooled, the resulting product is a nonleachable, glassy residue which meets toxicity characteristic leaching procedure (TCLP) criteria.
The off-gas treatment system removes particulates, acid gases, and volatilized metals. Off-gas monitoring verifies that all applicable environmental regulations are met. The design of the off-gas treatment system depends on the waste material.
The entire system is hermetically sealed and operated below atmospheric pressure to prevent leakage of process gases. Pressure relief valves connected to a closed surge tank provide relief if gas pressures in the system exceed safe levels. Vented gas is held in the tank, then recycled through the PACT system.
The technology can process organic and inorganic solid and liquid wastes. It is most appropriate for mixed, transuranic, and chemical plant wastes; soil containing both heavy metals and organics; incinerator ash; and munitions, sludge, and hospital waste.
Waste may be loose (shredded or flotation process) or contained in 55-gallon drums. It can be in almost any physical form: liquid, sludge, metal, rock, or sand. Volatile metals in the waste, such as mercury, are recovered by the off-gas treatment system.
The PACT-6 System, formerly PCF-6, was demonstrated under the SITE Program in July 1991 at the Component Development and Integration Facility of the U.S. Department of Energy in Butte, Montana. During the demonstration, about 4,000 pounds of waste was processed. The waste consisted of heavy metal-bearing soil from Silver Bow Creek Superfund site spiked with 28,000 parts per million (ppm) of zinc oxide, 1,000 ppm of hexachlorobenzene, and a 90-to-10 weight ratio of No. 2 diesel oil. All feed and effluent streams were sampled. The Demonstration Bulletin (EPA/540/M5-91/007), Applications Analysis Report (EPA/540/A5-91/007), and Technology Evaluation Report (EPA/540/5-91/007b) are available from EPA.
During subsequent testing at the Component Development and Integration Facility,
the PACT-6 system achieved the following results:
Two PACT-2 systems are in use in Europe, while three PACT-8 systems are under construction for European and domestic nuclear and commercial applications. Two PACT-1 bench-scale systems are also in domestic use for nuclear and shipboard testing.
EPA PROJECT MANAGER:
Laurel Staley
U.S. EPA
National Risk Management Research Laboratory
26 West Martin Luther King Drive
Cincinnati, OH 45268
513-569-7863
Fax: 513-569-7620
TECHNOLOGY DEVELOPER CONTACTS:
Ronald Womack or Leroy Leland
Retech, M4 Environmental Management Inc.
P.O. Box 997
100 Henry Station Road
Ukiah, CA 95482
707-462-6522
Fax: 707-462-4103