Stabilization/Solidification—Vitrification
Description
Vitrification
uses heat to melt and then solidify harmful chemicals in a solid mass
of
glasslike material. It can be applied both in-situ (in-situ vitrification or ISV)
and above ground in a treatment unit
(ex-situ). These are described
below.
ISV
and Planar-ISV
ISV uses
electric power to create the heat needed to melt soil. Electrodes are
inserted
in the polluted area and an electric current is passed between them,
melting
the soil between them. ISV uses extremely high temperatures (1,600 to
2,000 ¡C
or 2,900 to 3,650 ¡F). Melting starts near the ground surface and
moves down.
As the soil melts, the electrodes sink further into the ground causing
deeper
soil to melt. When the power is turned off, the melted soil cools and
vitrifies, which means it turns into a solid block of glass-like
material. The
electrodes become part of the block. This causes the ground surface in
the area
to sink slightly. To level it, the sunken area is filled with clean
soil. Any
harmful chemicals that remain underground become trapped in the
vitrified
block, which is left in place. When the soil hardens, it forms a
glass-like
substance that retards migration of compounds encapsulated in the glass.
The
vitrification product is a chemically stable, leach-resistant, glass
and
crystalline material similar to obsidian or basalt rock. ISV destroys
or volatizes most organic pollutants by pyrolysis (i.e., application of heat
without
oxygen). A vacuum hood is often placed over the treated area to collect
off-gases, which are treated
before release. Radionuclides and heavy
metals
are retained within the molten
soil. The conventional method of top-down melting in ISV typically
results in
substantial over-melting of the remediation area. Planar-ISV involves
starting
the melting process in specific areas of the subsurface. Consequently,
the
melting process can be focused directly on the region requiring
treatment, and
it can attain greater melt depths.
Ex-situ
vitrification
Ex-situ vitrification is much
like ISV,
except that it is done inside a chamber. Heating devices include plasma
torches
and electric arc furnaces. With plasma torch technology, waste is fed
into a
rotating hearth; the waste and molten material are held against the
side by
centrifugal force. During the rotation, the waste moves through plasma
generated by a stationary torch. To remove the molten material from the
furnace, the hearthÕs rotation slows and the slag flows through
a bottom
opening. Effluent gases are generally kept in a separate container
where high
temperatures combust/oxidize the contents. The arc furnace contains
carbon
electrodes, cooled sidewalls, a continuous feed system, and an off-gas
treatment system. In this process, waste is fed into a chamber where it
is
heated to temperatures greater than 1500¡C. The melt exits the
vitrification
unit and cools to form a glassy solid that immobilizes inorganics. The
Department of Energy has developed a Transportable Vitrification System
(TVS)
for treating mixed
waste
(radioactive and toxic) so the process can be used at several sites.
Limitations
and Concerns
With
most Stabilization/Solidification processes, there is potential for a
substantial increase in waste volume.
Concerns
include the durability of the waste form. Glass waste forms, as
compared to a
grouted or cemented waste form, are expected to be more stable over
longer
periods due to the corrosion resistance of glass. However,
de-vitrification of
glass can occur over time periods involving thousands of years. While
the heat
used to melt the soil can destroy some of the harmful chemicals, it may
cause
others to evaporate. The evaporated chemicals must be captured and
treated.
Complete
characterization of the candidate waste stream is essential, before
initiating
either in-situ
or ex-situ
vitrification, to determine what glass forms are already present in the
waste
and what additional glass stabilizers and fluxes need to be added.
Onsite
analytical services with quick turnaround are required to determine
glass
product characteristics and to validate melter performance.
There
are specific limitations to ISV and ex-situ vitrification. These are
described
below.
ISV:
ISV
cannot be used with buried pipes or drums and rubble exceeding 20% by
weight.
Heating
the soil may cause the subsurface migration of contaminants into clean areas.
ISV
cannot be used where there are large accumulations of flammable or
explosive
materials.
The
solidified material may hinder future site use.
ISV
rapidly volatilizes some organic compounds and volatile radionuclides,
including Cs-137, Sr-90, and tritium. Control of these off-gases, as
well as
the high voltage used, present potential health and safety risks.
ISV
reduces the volume and mobility of radionuclides, but it does not
reduce their
radioactivity. Therefore, protective barriers that limit exposure to
radioactive emissions may still be required at some sites.
ISV is
most effective for near-surface contamination, although new approaches
may
increase treatment depths to 10 meters.
Ex-situ
Vitrification:
Use,
storage, or disposal of the vitrified slag is required. Vitrification
does not
reduce the wasteÕs radioactivity. Vitrified wastes containing
radioactive
contamination must be stored in facilities that protect the public from
radiation exposure.
Debris
greater than 60 mm in diameter typically must be removed prior to
processing.
Excavation
of radioactively contaminated soils could cause radiation exposure to
workers
from fugitive gas and dust emissions, and it may increase the risk to
nearby
populations.
There is
potential for the accumulation of volatile radionuclides in the melter
off-gas
system.
Heat
loads associated with cesium loading in the glass waste form should be
assessed
for any applicable limits or relationships to disposal site
requirements.
If
plutonium is being vitrified, provisions need to be made for preventing
its
theft, as it can be re-extracted from the glass.
Applicability
Ex-situ vitrification and ISV
can destroy
or remove organics and immobilize most inorganics in contaminated
soils,
sludges, or other earthen materials. Vitrification applies to a broad
range of
solid media (e.g.,
debris, soil, etc.). The process has been tested on a broad range of volatile organic compounds (VOCs) and semi-volatile
organic compounds (SVOCs), other organics
including dioxins and polychlorinated
biphenyls (PCBs),
and on most priority pollutant metals and radionuclides.
Technology
Development Status
ISV
is a proven, commercially available technology. An electrical
distribution
system, off-gas treatment system, and process components are developed.
Ex-situ
vitrification technologies
generally involve applying existing technologies (e.g., metals processing) to
new purposes.
Demonstrations and studies at several sites (including Oak Ridge, TN
and
Washington, DC) indicate that ex-situ vitrification
technologies can be implemented without
significant difficulties. The TVS technology is now available to users
at DOE
sites.
Web
Links
http://clu-in.org/download/citizens/vitrification.pdf
http://www.frtr.gov/matrix2/section4/4-8.html
Other
Resources and Demonstrations
See
descriptions of Stabilization/Solidification—Chemical and Stabilization/Solidification—Physical.
See http://www.osti.gov/bridge/servlets/purl/610197-wQKYkr/webviewable/ for a description of
vitrification
projects initiated by teh Energy DepartmentÕs Savannah River
Site. Also see http://www.osti.gov/bridge/servlets/purl/10159052-bABKfq/native/ for a paper on the
vitrification of
hazardous wastes.
See
http://www.osti.gov/bridge/servlets/purl/899770-b42JCN/ for a description of the
demonstration
of a bulk vitrification system at Hanford, where 53 million gallons of
radioactive liquid are planned to be vitrified.
See
http://www.ieer.org/sdafiles/vol_5/5-4/deararj.html for a description of
vitrification
and some of its problems. See www.ieer.org for numerous articles on
vitrifying
fissile materials.