In conventional ISV, operators insert electrodes into the soil with graphite starter paths laid in between. Passing high levels of electrical power through the starter paths generates resistance heat in the range of 1,600–2,000ºC and initiates melting of soil, which then becomes electrically conductive and sustains the resistance heating as power continues to be applied, causing the melt to progress down and out. A hood over the treatment zone draws off-gases through filters and scrubbers. Cooling leaves a leach-resistant, glasslike waste form that can be excavated or left in place. Sequential melts can extend the treated area laterally, but not downward: power requirements limit the technology to about 20 feet of melted depth.

ISV has been commercialized as GeoMelt™ by the Battelle-founded Geosafe Corporation (the sole licensed vendor of the technology) and applied at many sites in the United States, Japan, and Australia. Geosafe has processed more than 25,000 tons of material in over 300 melts, including a U.S. Environmental Protection Agency Superfund Innovative Technology Evaluation (SITE) Program demonstration. GeoMelt is the only vitrification process with a national Toxic Substances Control Act permit for treatment of soils and other solids contaminated with PCBs.

Current environmental restoration efforts are directed for the most part at cleaning up and stabilizing larger volumes of more accessible wastes, but eventually the more difficult wastes must also be treated. Although they make up only a fraction of the total volume, some of the most problematic wastes have been disposed of below the reach of conventional, surface-initiated ISV. Many sites also have high moisture, dissolved gases, and manmade or natural barriers that can present challenges for conventional (top-down) ISV. The sudden release of pressure from trapped bubbles has produced several “melt expulsion events.” Site screening and preparation, better process monitoring, and other improvements have been developed, but there is clearly room for further advances in subsurface vitrification technology. Three emerging ISV technologies are designed to work deeper below the surface and to avoid pressure problems by initiating the melt beside or beneath the zone to be treated and by providing an escape path for vapors.

GeoSafe’s planar melting

Geosafe has developed a new GeoMelt method based on conventional ISV and using essentially the same power system and off-gas controls. “Planar” melting uses starter paths injected through a series of closely spaced holes bored or driven alongside the zone to be treated—in planes on either side of the waste or above and below the waste—and the two melts are allowed to converge. Since planar melting is typically completed below the surface, this approach enables deeper zones to be treated without higher power requirements. In fact, the unmelted soil above the treatment zone acts as a thermal insulator, conserving energy at the melt depth and keeping surface equipment significantly cooler. Planar melting can be applied from the sides of buried tanks and is ideal for narrow treatment zones like trenches.

Planar melt ISV was successfully demonstrated on a 4,500-gallon tank 10 feet below the surface, using nonradioactive cesium as a surrogate for ¹³⁷Cs, one of the most problematic contaminants because of its volatility and high gamma radiation. Planar melts initiated along either side of the tank progressed quickly and coalesced once the 1,500 gallons of water in the 12-foot-long, 8-foot-diameter tank had evaporated. Gases generated by the melt escaped through risers to the off-gas hood. Analysis showed a cesium retention rate of 99.996 percent, and the technology has since been selected as the preferred remedy for the V-tanks at Idaho National Engineering and Environmental Laboratory’s Test Area North.

OST’s Subsurface Contaminants Focus Area is funding a two-part, large-scale demonstration hosted by Los Alamos National Laboratory’s Environmental Restoration Project to verify the potential for applying nontraditional ISV throughout the DOE complex. Geosafe’s planar melting is being tested on absorption beds that received contaminated waste water from a site laundry. The first demonstration—a “cold test” with surrogates for radionuclides—used starter paths injected below the surface in parallel, vertical planes between two pairs of electrodes. The dual melts coalesced to form a monolith roughly 24 feet square and 13 feet deep (10–23 feet below the surface) and weighing roughly 300 tons. Preliminary analysis indicated contaminant retention beyond detection levels, according to Geosafe project manager Brett Campbell. A second, “hot” demonstration is planned. For further information, contact James Hansen at Geosafe, (509) 375-0710, or see www.geomelt.com.

In situ plasma vitrification

Another developing alternative to top-down vitrification uses heat generated by plasma arcing, a technology developed in the ’60s by the space program to simulate vehicle re-entry temperatures and used for decades in the metal industry. In this approach, the heat source is lowered into a casing inserted into a bored hole or driven to the bottom of the intended treatment zone, enabling operation far below the surface and precise control of the location and extent of the treated region.

When direct current is applied to electrodes, ionized gas (“plasma”) resists the arc between the electrodes, creating a “flame” that quickly melts the casing and nearby material to create a dense, molten mass at the bottom of an expanding cavity. As the heat source is slowly withdrawn up the casing, additional material sloughs off the walls and mixes with molten material below, creating a column of vitrified matter that solidifies as it cools. Air or gas is metered in to replenish the plasma and control the arc. Off-gases from the melt zone are withdrawn through the annulus between the casing and the electrode for cleanup and release. High operating temperatures and the ability to add oxidants and glass-formers enable plasma technologies to work in high-silica soil and other hard-to-treat materials.

Carrying the torch

Georgia Tech’s Lou Circeo and Salvatore Camacho of Plasma Technology Corporation are using a plasma torch developed by Camacho that has two copper electrodes at the end of the tube capable of generating 4,000–7,000ºC. This torch has been tested in full-scale melts in the laboratory, and the Savannah River Site field-tested a 1-MW torch in 1996. Three melts in uncontaminated SRS soil produced vitrified columns 2–3.5 feet in diameter and weighing 600–1,100 pounds. Two of the columns were in contact but did not fuse together. Some of the power delivered to the torch is drawn off by a water cooling system that protects the sacrificial electrodes. Funded in part by DOE, developers are working to prolong electrode life and improve the integrity of the cooling system in a project planned to culminate in field-scale demonstrations. Circeo can be contacted at (404) 894-2070 or lou.circeo@gtri.gatech.edu.

Montec’s concentric graphite electrode

Montec Associates, Inc. of Butte, Montana, is developing a patented concentric electrode to perform bottom-up ISV. The plasma that resists arcing between a hollow, outer graphite cylinder and a central graphite rod projects a 2,000–3,000ºC plume of heat. The robust graphite electrodes do not require cooling, so a high portion of the expended power goes into the melting process.

Using a 500-kW power source in its laboratory, Montec has performed over 30 pilot-scale melts in uncontaminated soils of various compositions and moisture content, yielding monoliths up to a meter in diameter and weighing over a ton. Montec is teaming with Oak Ridge National Laboratory under a Phase I Small Business Technology Transfer Program grant from DOE to demonstrate the technology’s ability to remediate mixed buried wastes deemed problematic for conventional ISV, such as volatile materials and containers of liquids. For more information, contact Montec’s Larry Farrar, (406) 494-5555, lcfarrar@aol.com.

What next?

Geosafe’s planar melting ISV is closest to being ready for deployment, but with further development, the plasma technologies may be better suited for certain applications, particularly “hot spot” treatment. It will also take time and further field-testing to determine relative cost-effectiveness, but it seems likely that one or more new approaches to in situ vitrification have potential to play a significant role in future subsurface and tank remediation.