	The following article appears in the journal JOM, 51 (10) (1999), pp. 14–16.

Waste Vitrification: Overview

Using the Centrifugal Method for the Plasma-Arc Vitrification of Waste

R.K. Womack

TABLE OF CONTENTS

INTRODUCTION THE PACT SYSTEM High-Temperature Plasma Material Feeders Negative-Pressure Operation Centrifugal Treatment Slag Durability and Collection CURRENT PROJECTS References

Plasma-arc centrifugal treatment vitrification technology has advanced from the first experiments in 1985 to occupy a niche in the waste-treatment market. The centrifugal action, the force of the plasma gas, and the water-cooled walls work together to generate a durable, homogenous, vitrified waste form coupled with the safe confinement of the hazardous feeds and high organic removal efficiency. This technology has recently been applied to treat wastes completely while achieving maximum volume reduction.

INTRODUCTION

The plasma-arc centrifugal treatment (PACT) system for waste vitrification was developed from an initial test with 11.5 kg of rubber gloves, cloth, glass, metal, and cesium acetate in 1985.¹ The test was conducted in an inert-gas atmosphere and yielded volume reductions of 20:1, but with a great deal of carbon. The subsequent development of a rotating system with oxidant introduction resulted in patent status² in 1988. Since then, 11 systems have been produced for both laboratory and production use.

Waste streams that have shown substantial benefit from the PACT process are low-level nuclear waste (LLW), paints, pharmaceutical sludges, pyrotechnics, military chemical agents, blast media, and solvents. Common features among these wastes are the presence of heavy metals and often heterogeneous mixtures of organic materials, soils, metals, and water.

The PACT system meets all of the U.S. Environmental Protection Agency's (EPA)'s requirements for air emissions through high-temperature treatment and system design. Pilot-scale tests were performed for the EPA SITE Superfund Program on wastes from Silverbow Creek and the Montana Pole plant in Butte, Montana, in July 1991. The achieved DREs for hexachlorobenzene, an added organic spike, were 99.9984%, 99.9991%, and 99.9999% for tests one, two, and three, respectively.³ The EPA limit is 99.99%.⁴ The numbers represent detection limits (no measurable amounts were detected).

In 1996, 31 tests on nine different waste streams were conducted for the U.S. Army Environmental Command, Aberdeen Proving Grounds, Maryland. The principal heavy-metal spikes were barium, chromium, and lead.⁵ Table I shows the leach rates from some of the tests and how they compare to the EPA regulatory limits. Leachate concentrations of hazardous organics were also examined; all were below detection limits.

Table I. A Comparison of Leach Rates to EPA Regulatory Limits*
Waste	Barium (mg/l)	Chromium (mg/l)	Lead (mg/l)
Medical Ash (Test 1R)	0.169	0.151	0.313
Open Burning Soil (Test 8R)	0.274	0.151	0.795
Walnut Shell Blast Media (Test 14)	0.0825	0.0714	2.79
Longhorn Sludge (Test 12)	2.460	0.0799	1.180
Regulatory Limits	100.0	5.0	5.0
* From U.S. Army tests, contract no. DAA21-93-C-0046.

THE PACT SYSTEM

The PACT system (Figure 1) was designed to produce a homogenous final waste form, safely treating waste at high temperatures with high destruction and removal efficiencies (DRE) for toxic components. In the system, wastes are fed into a tub rotating at 10–40 rpm and melted by a plasma arc, forming a molten pool of metals and oxides. The slag cools to form a glass-like, leach-resistant slag, while organics are evaporated, treated, cleaned up, and released.

The estimated 6,000°C temperatures generated by plasma and the centrifugal action allow one PACT system to treat a variety of heterogeneous wastes with different melting points. It also means that PACT systems can treat their own fly ash plus filtration media, minimizing secondary wastes. Volume reductions for LLW stored waste (including containers) can range from 7:1 for drums containing mostly metals to 270:1 for primarily organic wastes. The centrifugal action of PACT also produces homogeneity, which adds to strength and long-term durability.

High-Temperature Plasma

Figure 1. The PACT system.

Figure 2. Slag generated in the PACT process meets all applicable EPA disposal requirements, and ranks well in long-term durability (PCT) tests.

The availability of energy at high temperatures is much greater for electric arcs than for combustion energy. Only about 23% of the theoretical heat of combustion is available above 1,200°C when combusting methane with air at 140% of stoichiometric oxygen. Disassociation of nitrogen, air, and like gases at readily achievable temperatures of about 6,000°C results in more than 91% of available energy above 1,200°C.⁶

A transferred plasma arc is chosen for the system, principally because energy is transferred into the waste material to be heated more efficiently than with a nontransferred arc. A further advantage of plasma is that temperature can be increased at full system capacity without affecting the system off-gas volume.

Material Feeders

Material feeding is important to minimize handling and to control the feed rate into the centrifuge. Two feed-rate examples conducted in July at Retech Services, Inc. are shown in Table II. Loose material feeders are usually either the auger or Archimedes type and perform this function for both loose inorganic and organic wastes, dry or moist. Liquids and slurries are fed with an adjustable metering system.

Feeders for unopened 200 liter drums minimize pretreatment handling and are designed for either vertical or horizontal loading directly into the system. Horizontal drums are remotely cut open as they rotate inside the confines of the system. Vertically loaded drums are usually the heaviest (500 kg or more) and are batch melted. With limited characterization of drum contents, the operator can determine the initial power and feed rates.

Negative-Pressure Operation

The PACT system guards against fugitive emissions in three ways. The negative primary chamber operating pressure is maintained at about –50 mbar by induced draft fans. All PACT chambers are sealed, with elastomers held at near room temperature by water-cooled walls, and are designed to perform without leaking at a one atmosphere positive gauge pressure.

Centrifugal Treatment

Solidified slag closely resembles igneous rock in appearance and strength and offers a final waste form that is durable and insoluble in water (Figure 2).⁷ A homogenous slag is very important to slag-sampling reliability, predicting leachability, long-term durability, and strength.

Table II. PACT-8 Feed Rates in July 1999 at Retech
Waste	Feed Rate (kg/h)	kW	Feeder
Latex Paint	450	750	Liquid
Cement Block, Metal, Insulation	975	1,200	Archimedes

The force exerted by the plasma gas on the surface at the arc-termination point (repeated 15–50 times per minute) mixes the slag effectively. One method of measuring mixing effectiveness is carbon content, which is very low for PACT (0.009%).⁸ Data presented in cobalt/cesium tests show macroscopic chemistry variation at one sigma limits of 1–3% of abundance.⁹ Microscopically, much of the material crystallizes, forming tiny crystallites in a glassy matrix.¹⁰

The centrifugal mixing that produces uniform chemistry is also important to slag durability. Leach-test data show that high-melting slags made by plasma are very leach-resistant.

a

b
Figure 3. Results of seven-day product-consistency tests for (a) silicon and (b) sodium, designed to evaluate the long-term durability of the slag matrix.
Another centrifugal advantage is that waste spreads out on the hot rotating slag and melts faster as a thin-film layer (no piling). Feed rates in nonrotating hearth tests at Retech, with the same torch model, were less than half the rate for the cement-block surrogate waste in Table II. The centrifugal action also reduces refractory erosion from the high-heat flux at the arc termination.

Slag Durability and Collection

The most commonly used method of testing leachability is the EPA toxic characteristic leachate procedure, which uses mild acid to leach crushed slag for 24 hours at room temperature. The U.S. Department of Energy also developed a test designed to predict long-term durability–the product consistency test (PCT). The PCT test uses water at 90°C for a week or a month to assess the durability of the slag matrix. Figure 3 illustrates the results of 25 tests. Shown on the right are four, low-melting-point (1,150°C) borosilicate slags; next to them are crucible-melted slags (1,500°C) and 17 slags melted by the PACT process (1,400°C to 1,600°C). Clearly, the high-melting-point slags exhibit better resistance to leaching.

Slag is poured, by reducing the centrifuge rpms, through an axial throat into a mold within a collection chamber. Slag-mold volumes for the same-size PACT system can vary from one project to another, depending on customer requirements. For example, PACT-8 slag molds (the number refers to nominal centrifuge inside diameter in feet) have been as small as 0.15 m³ and as large as 0.6 m³. A safety mold is in position below the centrifuge at all times, except when pouring. Any material that might pass through the central pour orifice untreated is collected and fed back into the PACT system. The safety mold also acts as a container for the centrifuge contents in case of power failure.

CURRENT PROJECTS

There are several production-sized systems on three continents in place or in progress. There are also five laboratory systems, two PACT-1 units used for laboratory studies, and three PACT-2 units and one PACT-6 that have been used primarily for pilot-scale treatability studies.

Figure 4. The top deck of the PACT-8 system assembled at Retech for acceptance testing prior to the proposed installation at the Norfolk Naval Base.
The following PACT-8 systems have been installed or are planned.

A system in Muttenz, Switzerland, processes pharmaceutical sludges in 200 liter drums and recovers copper and zinc from brass dross. It also serves as a demonstration unit for its owner, Moser-Glaser Company.
A system in Munster, Germany, is currently undergoing qualification trials. This system is designed to process the products from washing soils contaminated by military explosives and chemical warfare agents.
Zwilag, a Swiss organization that stores and treats nuclear waste, is installing a system to treat low- and medium-level beta and gamma production waste from Swiss nuclear power plants.
A system is planned for Norfolk Naval Base (Figure 4), subject to obtaining the necessary permits and completing an environmental impact statement. It is currently at Retech in the final testing phase and is designed to treat waste paint, solvents, oily rags, batteries, and other nonrecyclable waste.
The Retech system was originally slated to treat low-level transuranic waste, including metals, sludges, and soils, in a waste pit at the Idaho National Engineering and Environmental Laboratory.
A contract is underway with Toyo Engineering Company of Japan for the Tsuruga nuclear power plant. This system is currently scheduled for shipping in mid-year 2000. It is designed to feed 200 liter drums, both horizontally and vertically, and loose organic waste.

References
1. R.C. Eschenbach and K.D. Boomer, "Plasma Arc Stabilization of Hazardous Mixed Wastes" (Paper presented at the ANS Winter Meeting, Los Angeles, CA, 15–19 November 1987).
2. M.P. Schlienger, U.S. patent 4,770,109 (September 13, 1988); U.S. patent 5,005,494 (April 9, 1991); U.S. patent 5,136,137 (August 4, 1992).
3.Retech Plasma Centrifugal Furnace Application Analysis Report (Cincinnati, OH: EPA, Risk Reduction Engineering Laboratory, June, 1992), p. 9.
4. "U.S. Code of Federal Regulations," 40 CFR (07/01/90 Edition) Paragraph 264.343(c).
5. Plasma Arc Technology Evaluation, Scientific and Technical Report (City: NDCEE, 1997), Table 6A.
6. R.C. Eschenbach, "Plasma Arc Centrifugal Treatment (PACT) of Hazardous Waste" (Paper presented at the 8th Symposium on Plasma Science for Materials, Tokyo, Japan, 15–16 June 1995).
7. R.C. Eschenbach, X. Feng, and R.E. Einziger, "A Direct, Single Step Plasma Arc-Vitreous Ceramic Process for Stabilizing Spent Nuclear Fuels, Sludges, and Associated Wastes" (Paper presented at the Scientific Basis for Nuclear Waste Management XX, Boston, MA, 2–6 December 1996).
8. Y. Tsuji et al., "PACT Solidification Tests on Surrogates for Low Level Waste from Nuclear Power Plants," Proceedings of the 1994 Incineration Conference (City: Publisher, 1994), pp. 151–153.
9. Y. Nakayama et al., "Cobalt and Cesium Volatility Test in Plasma Arc Centrifugal Treatment," Proceedings of the 1995 Incineration Conference (City: Publisher, 1995), pp. 427–429.
10. R.C. Eschenbach and C.G. Whitworth, "Glassy Slag from Rotary Hearth Vitrification" (Paper presented at the American Ceramic Society Meeting, Cincinnati, Ohio, 30 April–4 May 1995).

R.K. Womack is business development manager for Retech Services, Inc., a Lockheed Martin Company.

For more information, contact R.K. Womack, Retech Services, Inc., P.O. Box 997, Ukiah, California 97482; (707) 467-1721; fax (707) 467-1638; e-mail ronald.k.womack@lmco.com.

Copyright held by The Minerals, Metals & Materials Society, 1999

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