11    IN SITU VITRIFICATION APPLICATIONS
TO HAZARDOUS WASTES
Stephen C. Liikala, Chemical Engineer
MAECORP INCORPORATED
Homewood, IL 60430
INTRODUCTION
In Situ Vitrification (ISV) utilizes electrical energy to melt contaminated soils into a durable glass
form. An electric current is passed between electrodes placed in the soil which creates temperatures
high enough to melt the soil and produce a molten mass. The vitreous zone pyrolyzes organic
compounds and encapsulates the inorganic compounds. Process gases emitted from the melt are
collected and routed through a process gas treatment system. When power to the electrodes is shut
off, the molten mass cools into a glass form that resembles natural obsidian. Subsidence of the
vitrification zone, due to soil densification, is then covered with clean backfill.
PROCESS DESCRIPTION
In Situ Vitrification (ISV) is a hazardous waste remediation process which converts contaminated
soils into a durable glass form through the use of electricity. An array of four molybdenum electrodes
are inserted into the contaminated soil and an electrical current is passed between the electrodes. To
establish a start up conductive path between the electrodes, a small trench (approximately 2 inches in
diameter) is formed in a diagonal "x pattern" and the trench is packed with large flaked graphite
powder and glass frit. Power to the electrodes is started and the graphite begins to heat up the
adjacent soils creating a conductive molten zone. Convective currents within the melt distribute the
wastes evenly throughout the molten mass,1 and the melt depth is determined by the length of the
electrodes and their separation. The process gases emitted from the melt are collected by covering the
melt with an "off gas hood" and routed through a vapor treatment system.
The ISV system currently being designed for applications to hazardous waste sites is mounted on
two semi trailers for easy transport from site to site. The first trailer (the support trailer) houses a 3750
kVA transformer and an ethylene glycol cooling system. The second trailer, or the off gas trailer, is
divided with half of the trailer housing the process gas treatment equipment and the other half
containing the distributed process control system. Other major components of the ISV system include
the off gas hood, and portable diesel generators, if desired, for operation in remote areas.
The off gas treatment system cools, scrubs, and filters the gaseous effluents from the hood.2 The
gases are first passed through a quencher for cooling and removing particulates. A tandem nozzle
scrubber then removes remaining particulates and condenses semi volatile components. A vane separator, or mist eliminator, then removes the remaining liquid droplets within the gas. A caustic scrub
solution is used to quench and scrub the gases and is contained in a scrub solution tank. The scrub
solution is recirculated through a single pass heat exchanger to remove the heat. The scrubbed gases
are then heated to remove all moisture prior to filtering. Filtering is accomplished by passing the gases
through high efficiency particulate (HEPA) filters, followed by a final polish with vapor phase
activated carbon. The gases are drawn through the system by a blower assembly and exhausted out of
the top of the off gas trailer. The melt is monitored and controlled by a distributed process control
system.
The electrical system consists of a 3750 kVA transformer with a scott-tee transformer used to
convert three phase line power into two phase power for delivery to the electrodes. The transformer is
equipped with multiple voltage taps for maximum power efficiency. Voltages are achievable from
4160 to 310 V at an amperage from 450 to 4000 A per electrode pair.2 As the melt grows in size, the
resistance between the electrodes decreases according to the following power equation:'
P = I2 x R (1)
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Primed in U.S.A.
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