U.S. patent number RE35,715 [Application Number 08/359,039] was granted by the patent office on 1998-01-13 for in-situ remediation and vitrification of contaminated soils, deposits and buried materials.
Invention is credited to Salvador L. Camacho, Louis J. Circeo, Jr..
United States Patent |
RE35,715 |
Circeo, Jr. , et
al. |
* January 13, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
In-situ remediation and vitrification of contaminated soils,
deposits and buried materials
Abstract
A method is disclosed in which a plasma arc torch is used to
vitrify and remediate a site containing contaminated soils,
resulting from a hazardous material deposit or spill, or
contaminated buried objects. The contaminated earthen material or
subterranean deposit is pyrolyzed, melted or solidified by the
plasma torch which is energized at the bottom of a cased, vertical
borehole, and then gradually raised to the surface. An array of
boreholes, appropriately spaced, will remediate an entire mass of
contaminated material. Similarly, burled objects such as metal
drums containing contaminants and underground storage tanks may be
selectively remediated at their specific buried depth. Similar use
is made of the plasma torch in a second embodiment with the
additional step of processing at selected underground locations in
the borehole array to create a sealed horizontal layer, vertical
cutoff walls or a sealed basin as a barrier against further
leaching of contaminants into surrounding soil and groundwater.
Gaseous by-products of the pyrolysis process are collected, treated
and processed, as appropriate.
Inventors: |
Circeo, Jr.; Louis J. (Atlanta,
GA), Camacho; Salvador L. (Raleigh, NC) |
[*] Notice: |
The portion of the term of this patent
subsequent to January 26, 2010 has been disclaimed. |
Family
ID: |
25482229 |
Appl.
No.: |
08/359,039 |
Filed: |
December 19, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
944890 |
Sep 9, 1992 |
05276253 |
Jan 4, 1994 |
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Current U.S.
Class: |
588/253;
405/128.65; 405/131; 405/269; 405/271; 588/260 |
Current CPC
Class: |
B09B
1/00 (20130101); B09C 1/067 (20130101); C03B
5/005 (20130101); C03B 5/025 (20130101); E02D
31/00 (20130101); B09C 2101/00 (20130101) |
Current International
Class: |
B09B
1/00 (20060101); B09C 1/00 (20060101); B09C
1/06 (20060101); C03B 5/00 (20060101); C03B
5/02 (20060101); E02D 31/00 (20060101); A62D
003/00 (); E02D 019/14 (); B09B 001/00 () |
Field of
Search: |
;405/128,129,131,130,258,263,269,271 ;588/252,253,260,249 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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914715 |
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Mar 1982 |
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SU |
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958590 |
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Sep 1982 |
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SU |
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977570 |
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Nov 1982 |
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SU |
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90124715 |
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Mar 1992 |
|
SU |
|
Other References
Technical Paper entitled: "An Innovative Process For The In-Situ
Waste Destruction And Vitrification Of Buried Hazardous And Mixed
Wastes". .
Examiner's Statement of Reasons for Allowance from the official
file of the '795 patent. .
Official Action dated Apr. 2, 1992 from the prosecution history of
the '797 patent. .
Letter sent by Reissue Applicant's patent counsel to the co-authors
of the Spectrum '94 technical paper. .
Response to letter sent by Reissue Applicants' patent counsel on
behalf of co-author J. K. Wittle. .
Declaration of Salvador L. Camacho..
|
Primary Examiner: Taylor; Dennis L.
Attorney, Agent or Firm: Olive & Olive
Claims
What is claimed is:
1. A method for in-situ remediation and vitrification of hazardous
waste contaminated soil comprising .Iadd.the steps of.Iaddend.:
(a) forming a vertical borehole of a size sufficient to accommodate
a plasma arc torch.Iadd., said vertical borehole .Iaddend.extending
from an upper surface of the soil to a lower end at a predetermined
depth in said hazardous waste contaminated soil;
.Iadd.(b) inserting a casing into said formed borehole for
preventing sidewall collapse; .Iaddend.
(.[.b.]..Iadd.c.Iaddend.) lowering a plasma arc torch together with
connected utilities into said .[.formed borehole.]. .Iadd.casing
.Iaddend.and suspending the plasma arc torch at a location above
and proximate said borehole lower end;
(.[.c.]..Iadd.d.Iaddend.) utilizing the connected .[.plasma
torch.]. utilities to operate said torch .Iadd.to convert
electrical energy to heat so as .Iaddend.to create a plasma arc of
sufficient temperature to pyrolyze and melt .[.substantially in the
absence of combustion.]. a portion of said contaminated soil
located proximate said plasma arc;
(.[.D.]..Iadd.e.Iaddend.) deenergizing the plasma arc torch;
(.[.e.]..Iadd.f.Iaddend.) removing the plasma arc torch from the
borehole; and
(.[.f.]..Iadd.g.Iaddend.) allowing said molten soil to cool and
solidify thereby to produce a vitrified column of hard, inert
residue substantially less in volume than the contaminated soil
from which it was produced.
2. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 1 in which
said borehole is formed to a depth greater than the depth of said
contaminated soil.
3. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 1
.[.including the step of inserting a heat destructible casing into
said formed borehole prior to lowering said torch therein.].
.Iadd.further including the step of forming said casing of a heat
destructible material.Iaddend..
4. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 3 further
comprising gradually raising said torch in said borehole so as to
melt sequentially higher portions of contaminated soil.
5. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 3 in which
said plasma arc torch operates to melt a portion of contaminated
soil only at the lowest depth to which the borehole is formed and
is then removed from said borehole, and such process is repeated in
each of the plurality of boreholes so as to form a coalesced and
non-porous horizontal sealant layer to prevent further vertical
leaching of contaminants to surrounding soil and aquifers.
6. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 1 wherein
said borehole is formed as an initial hole and further comprising
lowering, operating, and raising said torch so as to determine from
said initial hole the effective diameter of said decontaminated and
vitrified soil and thereby to determine the distance required
between said initial hole and a remaining plurality of holes in
order to completely vitrify and remediate an area containing said
hazardous waste contaminated soil.
7. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 6 further
comprising forming a plurality of boreholes each at a distance from
adjacent boreholes so that the peripheral diameters of solidified
materials as determined from the initial hole will coalesce and
form a solidified mass throughout the area containing said
hazardous waste contaminated soil.
8. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 7 further
including the step of positioning said plurality of boreholes in
such relative proximity so that the heat transmitted from said
plasma torch through said contaminated soil beyond the portion
being melted creates peripheral zones of solidified brick-like
material and deplasticized material and the outer deplasticized
material of each hole is coalesced with the respective
deplasticized material formed around adjacent boreholes.
9. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 1 including
the step of operating said plasma torch in a non-transferred mode.
.[.10. A method for in situ remediation and vitrification of
hazardous waste contaminated soil wherein the contamination extends
to only a shallow depth below the surface of the soil,
comprising:
(a) suspending a plasma arc torch together with connected utilities
above said contaminated soil;
(b) utilizing the connected plasma torch utilities to operate said
torch to create a plasma arc of sufficient temperature to melt
substantially in the absence of combustion a portion of said
contaminated soil located proximate said plasma arc;
(c) deenergizing the plasma arc torch; and
(d) allowing said molten mass to cool and solidify thereby to
produce a volume of vitrified soil substantially less in volume
than the contaminated soil from which it was produced..]..[.11. The
method for in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 10, further comprising moving
said plasma arc torch to additional adjacent locations above said
contaminated soil and remediating and vitrifying said additional
adjacent locations of contaminated soil..]..[.12. The method for
in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 11 further
comprising operating said torch in a transferred arc mode..].13.
The method for in-situ remediation and vitrification of hazardous
waste contaminated soil as claimed in claim 1 wherein said borehole
is formed to a depth substantially equal to the depth of said
hazardous waste
contaminated soil. 14. The method for in-situ remediation and
vitrification of hazardous waste contaminated soil as claimed in
claim 1 wherein said hazardous waste contaminated soil
.[.comprises.]. .Iadd.includes .Iaddend.volatile compounds and
.Iadd.further .Iaddend.including .Iadd.the steps of
.Iaddend.forming throughout the contaminated soil area a plurality
of boreholes each at a distance from adjacent boreholes and
volatizing said volatile compounds in each of said
boreholes. 15. The method for in-situ remediation and vitrification
of hazardous waste contaminated soil as claimed in claim 1 further
comprising forming a plurality of boreholes each at a distance from
adjacent boreholes so that the peripheral diameters of solidified
materials as determined from the initial hole will coalesce and
form a solidified mass throughout the area containing said
hazardous waste contaminated soil.
The method for in-situ remediation and vitrification of hazardous
waste contaminated soil as claimed in claim 1 wherein said
contaminated soil .[.comprises having.]. .Iadd.includes
.Iaddend.buried metal objects and including during the step of
melting said soil the .Iadd.further
.Iaddend.step of melting said metal objects in situ. 17. The method
for in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 1 wherein said contaminated
soil .[.comprises having.]. .Iadd.includes .Iaddend.buried metal
objects containing volatile products and including during the step
of melting said soil the .[.step.]. .Iadd.further steps .Iaddend.of
melting said metal objects in situ and
volatilizing .[.the.]. said volatile products contained therein.
18. The method for in-situ remediation and vitrification of
hazardous waste contaminated soil as claimed in claim 1 in which
said plasma arc torch operates to melt a portion of contaminated
soil only at the lowest depth to which the borehole is formed and
is then removed from said borehole, and such process is repeated in
each of .[.the.]. .Iadd.a .Iaddend.plurality of boreholes so as to
form a coalesced and non-porous horizontal sealant layer to prevent
further vertical leaching of
contaminants to surrounding solid and aquifers. .Iadd.19. A method
for in-situ remediation and vitrification of hazardous waste
contaminated soil comprising the steps of:
(a) forming a vertical borehole of a size sufficient to accommodate
a plasma arc torch and extending from an upper surface of the soil
to a lower end at a predetermined depth in said hazardous waste
contaminated soil;
(b) lowering a plasma arc torch together with connected utilities
into said formed borehole and suspending the plasma arc torch at a
location above and proximate said borehole lower end;
(c) utilizing the connected utilities to operate said torch to
convert electrical energy to heat so as to create a plasma arc
flame of sufficient temperature to pyrolyze and melt a portion of
said contaminated soil located proximate said plasma arc flame;
(d) gradually raising said torch in said borehole so as to melt
sequentially higher portions of said contaminated soil;
(e) deenergizing the plasma arc torch;
(f) removing the plasma arc torch from the borehole; and
(g) allowing said molten soil to cool and solidify thereby to
produce a vitrified column of hard, inert residue substantially
less in volume than the contaminated soil from which it was
produced. .Iaddend..Iadd.20. The method for in-situ remediation and
vitrification of hazardous waste contaminated soil as claimed in
claim 19 in which said borehole is formed to a depth equal to or
greater than the depth of said contaminated soil.
.Iaddend..Iadd.21. The method for in-situ remediation and
vitrification of hazardous waste contaminated soil as claimed in
claim 19 including the step of inserting a heat destructible casing
into said formed borehole prior to lowering said torch therein.
.Iaddend..Iadd.22. The method for in-situ remediation and
vitrification of hazardous waste contaminated soil as claimed in
claim 19 wherein said borehole is formed as an initial hole and
further comprising the steps of lowering, operating, and raising
said torch for determining from said initial hole the effective
diameter of said decontaminated and vitrified soil and thereby
determining the distance required between said initial hole and a
plurality of additional holes in order to completely vitrify and
remediate an area containing said
hazardous waste contaminated soil. .Iaddend..Iadd.23. The method
for in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 19 further comprising the
step of forming a plurality of boreholes each at a distance from
adjacent boreholes so that the peripheral diameters of solidified
materials as determined from the initial hole will coalesce and
form a solidified mass throughout the area containing said
hazardous waste contaminated soil. .Iaddend..Iadd.24. The method
for in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 19 including the step of
operating said plasma torch in a non-transferred mode.
.Iaddend..Iadd.25. The method for in-situ remediation and
vitrification of hazardous waste contaminated soil as claimed in
claim 19 in which said plasma arc torch operates to melt a portion
of contaminated soil only at the lowest depth to which the borehole
is formed and then deactivating and removing said plasma arc torch
from said borehole, and repeating such steps in each of a plurality
of boreholes so as to form a coalesced and non-porous horizontal
sealant layer to prevent further downward leaching of contaminants
to surrounding soil and aquifers. .Iaddend..Iadd.26. The method for
in-situ remediation and vitrification of hazardous waste
contaminated soil as claimed in claim 22 further comprising forming
a plurality of boreholes each at a distance from adjacent boreholes
so that the peripheral diameters of solidified materials as
determined from the initial hole will coalesce and form a
solidified mass throughout the area containing said hazardous waste
contaminated soil. .Iaddend..Iadd.27. The method for in-situ
remediation and vitrification of hazardous waste contaminated soil
as claimed in claim 26 further including the step of positioning
said plurality of boreholes in such relative proximity so that the
heat transmitted from said plasma torch through said contaminated
soil beyond the portion being melted creates peripheral zones of
solidified brick-like material and deplasticized material and the
deplasticized material of each hole is coalesced with the
respective deplasticized material formed around adjacent boreholes.
.Iaddend..Iadd.28. A method for in-situ remediation and
vitrification of hazardous waste contaminated soil comprising the
steps of:
(a) forming a vertical borehole of a size sufficient to accommodate
a plasma arc torch and extending from an upper surface of the soil
to a lower end at a predetermined depth in said hazardous waste
contaminated soil;
(b) lowering a plasma arc torch together with connected utilities
into said formed borehole and suspending the plasma arc torch at a
location above and proximate said borehole lower end;
(c) utilizing the connected utilities to operate said torch to
convert electrical energy to heat so as to create a plasma arc
flame of sufficient temperature to pyrolyze and melt a portion of
said contaminated soil located proximate said plasma arc flame;
(d) deenergizing the plasma arc torch;
(e) removing the plasma arc torch from the borehole;
(f) allowing said molten soil to cool and solidify thereby to
produce a vitrified column of hard, inert residue substantially
less in volume than the contaminated soil from which it was
produced; and
(g) forming a plurality of additional boreholes each at a distance
from adjacent boreholes so that the peripheral diameters of
solidified materials as determined from the initial hole will
coalesce and form a solidified mass throughout the area containing
said hazardous waste
contaminated soil. .Iaddend..Iadd.29. The method for in-situ
remediation and vitrification of hazardous waste contaminated soil
as claimed in claim 28 in which said plasma arc torch operates to
melt a portion of contaminated soil only at the lowest depth to
which the borehole is formed and said plasma arc torch is then
deactivated and removed from said borehole, and such process is
repeated in each of said plurality of boreholes so as to form a
coalesced and non-porous horizontal sealant layer to prevent
further downward leaching of contaminants to surrounding soil and
aquifers. .Iaddend.
Description
FIELD OF INVENTION
The invention disclosed relates to the field of remediation and
vitrification of soils containing hazardous materials at or beneath
the surface of the earth.
REFERENCE TO RELATED APPLICATION
Two related copending applications provide useful background for
the present application. One is U.S. patent application Ser. No.
07/827,384 filed Jan. 29, 1992 for "IN-SITU SOIL STABILIZATION
METHOD AND APPARATUS", and the second is the application entitled
"IN-SITU LANDFILL PYROLYSIS, REMEDIATION AND VITRIFICATION" filed
Aug. 17, 1992.
BACKGROUND OF THE INVENTION
A serious contemporary problem is the accidental spilling or
intentional discarding of toxic or environmentally hazardous
materials comprising both organic and inorganic contaminants. These
injurious materials may be liquid or solid, and may be at the
ground surface or buried. When such a situation occurs, the land
affected can be permanently destroyed for productive use, or at
least until an expensive clean-up process is accomplished. In
addition to the effect on the soil, such contaminating wastes often
find their way into streams, rivers and municipal water supplies,
causing untold damage and potential sickness. Such hazardous waste
materials include, by way of example, petroleum products,
chemicals, mine tailings, sludges, and low-level radioactive
materials whether exposed or contained in metal drums or
underground storage tanks.
The sort of clean up operations which have been done in the past
have involved excavating the contaminated soils or objects, and
transporting them to a treatment facility or to a safe storage
site. Alternatively, means of remediating contaminated soil in-situ
have been outlined in U.S. Pat. No. 4,376,598 to Brouns et al., for
"In Situ Vitrification of Soil". The '598 patent teaches the
insertion of two or more conductive graphite electrodes into the
soil, providing a conductive electrical path on the surface of the
soil, and generating a current through the electrical path to heat
the surrounding soil to its melting temperature. Once a melted soil
stream has been established between electrodes, the molten soil
provides the conductive path. However, this method has several
drawbacks; e.g., soil melting must begin at the ground surface and
proceed downward, it cannot operate at deep depths or at selective
depths, the continuity of the soil electrical path is not reliable,
and restarting the current flow once the melt has moved below
ground surface level is quite difficult. In addition, since
processing temperatures are below 2000.degree. C., soil additives
may be necessary to lower the melting point of particular soils,
and the method does not work well if there is too much metal or
moisture in the soil.
The invention disclosed herein recognizes that there exists a
relatively new technology which may be employed in the remediation
of all contaminated soils or buried materials at any depth by the
process of pyrolysis, melting and vitrification of waste materials
using large quantities of very high temperature heat energy. The
basic tool used in this technology is the plasma arc torch. Plasma
arc torches can routinely operate at energy levels ranging from 100
kw to 10 MW. Plasma torches produce temperatures of 4000.degree. C.
to 7000.degree. C. in the range of 85-93% electric to heat energy
conversion efficiency. For purposes of comparison, the highest
temperature attainable by fuel combustion sources or the
aforementioned graphite electrode process is in the vicinity of
2700.degree. C.
A plasma arc torch operates by causing a high energy electric arc
to form a stream of plasma, or ionized gas, thus generating large
amounts of heat energy. There are many types of plasma torches.[.,
but all torches.].. .Iadd.A plasma torch can operate on AC or DC
power, using inert, reducing or oxidizing gas, and have metal or
graphite electrodes which may be solid or hollow. All plasma
torches .Iaddend.generally fall into one of two basic categories
according to the arc configuration relative to the torch
electrodes, i.e., transferred arc type and non-transferred arc
type. The arc of a transferred arc torch is formed by and jumps
from a single electrode on the torch, through the plasma gas, and
to an external electrode which is connected to the opposite
electrical terminal. The arc of a non-transferred arc torch is
formed by and jumps from one electrode on the torch along the
plasma gas and back to another electrode on the torch.
In the plasma arc torch, the heat energy produced is proportional
to the length of the arc, assuming the type of plasma gas and
applied electrical current both remain constant.
Since the present invention makes use of a plasma arc torch,
reference is next made to U.S. Pat. No. 4,067,390 granted to the
present inventors for "Apparatus And Method For The Recovery Of
Fuel Products From Subterranean Deposits Of Carbonaceous Matter
Using A Plasma Arc" which patent teaches the use or a plasma arc
torch to gasify or to liquify underground deposits of coal, oil,
oil shale, tar sands and other carbonaceous materials. The
teachings of the '390 patent are incorporated hereby by
reference.
It is a major objective of the invention to afford an efficient and
environmentally safe system for the in-situ remediation of soils
containing hazardous materials and other buried contaminants such
as metal drums or waste matter, underground storage tanks and other
toxic deposits.
It is a further objective of the invention to prevent the leaching
and spread of contaminated material to the surrounding soil medium
and to the subterranean water system.
Additional objectives of the invention will become apparent from
the disclosure which follows.
SUMMARY OF THE INVENTION
The invention disclosed utilizes plasma torch heating for the
remediation by in-situ pyrolysis and vitrification of hazardous
waste sites, spills or deposits buried at any depth below the
ground surface. A series of boreholes are formed throughout a
contaminated area in an array such that the perimeters of the
vitrified, solidified or remediated columns which are formed will
coalesce together. A plasma arc torch is inserted, energized and
moved within each borehole so as to form a column of material which
has been remediated by pyrolysis of the organic material in the
soil and vitrification of the surrounding contaminated inorganic
soils and buried materials. The gases generated in this process are
collected at the top of each borehole and treated as required. Upon
cooling, the molten mass solidifies into a dense, inert, vitrified
mass which effectively immobilizes and neutralizes any remaining
contaminants. Certain contaminants may be remediated within
solidified or devolatilized zones which extend beyond the vitrified
zone. In these situations, the borehole spacing would be
increased.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional elevation view of a typical hazardous spill
site showing the contaminated areas of soil radiating from the
initial spill location into the surrounding soil.
FIG. 2 is the view of FIG. 1 with an initial borehole formed into
the earth, with a plasma torch inserted in the borehole and with a
gas collector positioned above the borehole according to the
invention.
FIG. 3 is a top plan view of an area of a hazardous material spill
showing a typical pattern of boreholes drilled at an extended
spacing and the resultant coalesced columns of melted, solidified
and devolatilized soil treated to immobilize and remediate the
contamination.
FIG. 4A is a sectional elevation view of an underground site
containing buried drums of hazardous materials, with contaminated
material radiating into the surrounding soil.
FIG. 4B is a sectional elevation view of an underground site having
a subterranean liquid storage tank, with contaminated material
radiating from a leak in the storage tank.
FIG. 5 is a perspective illustration of leachate cutoff walls and a
containment basin established according to the present
invention.
FIG. 6 is a sectional elevation view of a site of a hazardous
liquid surface spill having leached into surrounding soil to a
small extent and a plasma torch suspended above the spill site.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention relates to the problem illustrated, by way of
example, in FIG. 1, showing a spill of liquid material, hazardous
waste 50, onto the upper surface of a section of earth. Over a
period of time, the hazardous waste 50 leaches along the surface
and into the ground 52, thereby creating contaminated area 54 which
extends for some distance in all directions from the original site
of the spill. Uncontaminated earth 52 beyond the contaminated area
54 is unaffected, although in sufficient time the leaching will
typically continue until the contaminated area 54 encompasses a
much larger area red could ultimately reach an underground aquifer
which would permit the hazardous waste 50 to migrate along its path
of flow. In other situations, the hazardous spill or deposit may be
a solid substance or a buried object, but its contaminating effect
is also subject to leaching and migration into the surrounding
earth and water with the aid of rain.
As shown in FIG. 2, a borehole 57 is formed by drilling through the
contaminated soil mass 54 to a point just below the maximum depth
of contamination and a thin heat-destructible metal casing (not
shown) is inserted into the hole. The metal casing serves to
prevent collapse of the sidewalls of borehole 57, facilitate the up
and down movement of plasma arc torch 56 within the borehole 57,
and to permit gases formed by the pyrolysis process to quickly
reach the surface for collection and treatment. Next, plasma torch
56 is lowered to a point near the bottom of borehole 57 and is
suspended by supply conduit 58 which contains electrical cables, a
plasma gas supply line and coolant water necessary for torch
operation as described above. The process of lowering and raising
plasma torch 56 is accomplished by means of appropriate lift
equipment (not shown).
The torch 56 is also provided with a protective heat shroud 60
adapted to shield the supply conduit 58 from being damaged by the
heat generated. In the treatment of shallow contaminated deposits,
this shroud would extend to the ground surface. When energized in
the non-transferred mode, as is preferred, plasma arc 62 is
established at the lowest point in borehole 57 and slowly raised to
the top of the hole to gasify, pyrolyze, and melt a column of the
surrounding earth and contaminants. Since the plasma torch is
capable of creating temperatures in the range of 4000.degree. C. to
7000.degree. C., the heat produced gasifies and pyrolyzes all
organic materials in the presence of steam in the soil immediately
surrounding the borehole. It is preferred according to the
subterranean embodiment disclosed to operate plasma torch 56 in the
non-transferred arc mode.
The gases are collected at the top of the borehole 57 for
treatment, as required, by standard gas treatment technology. A gas
collection hood 61 is positioned over borehole 57 to channel the
generated gas to a location for treatment. The inorganic materials
and the soil are melted and, when allowed to cool, become a
virtually unleachable and relatively nonporous, dense vitrified
mass. This process of gasification, pyrolysis, melting and cooling,
according to the invention, is done with an array of individual
boreholes creating a group of treated columns which will coalesce
together over the entire site (see FIG. 3) which contains the
hazardous material.
As depicted in FIG. 2, the heat energy from plasma torch 56
radiates in a generally spherical pattern with the greatest degree
of heat closest to plasma arc 62, and with the temperature
lessening with greater distance from the plasma arc 62. Therefore,
the area closest arc 62 is completely vitrified and the more
distant portions of earth 66, 68 are heated to a lesser degree as
will be described below.
Immediately beneath plasma arc 62, molten waste 64 generated at
temperatures above 1100.degree. C. accumulates in a pool. Beyond
the distance to which sufficient heat for soil melting and
vitrification has travelled, the curing zone 66 is subjected to
sufficient heat (typically 900.degree. C. or greater) to cure the
earthen material into a brick-like hardness. The third zone in
terms of distance from plasma arc 62 is a deplasticized zone 68
which is not cured, but has all liquid permanently removed
therefrom by exposure of 200.degree. C. or more and effectively
becomes a rigid, non-absorbent envelope of soil. The distance to
which heat will travel and effectively generate cured zone 66 and
deplasticized zone 68 depends upon the power levels generated by
plasma torch 56, the moisture content of the soil, and the nature
of the surrounding contaminated earth 54 and uncontaminated earth
52.
The spacing between adjacent boreholes 57 is based in part on the
nature of the contaminating materials and in part on the degree of
soil vitrification and solidification required to contain,
immobilize, and neutralize those materials.
As plasma arc torch 56 continues to generate heat energy, to melt
portion 64 and to cure and deplasticize portions 66 and 68
respectively, torch 56 is gradually raised by appropriate
mechanisms (not shown). The lifting mechanism for torch 56 is
optionally either manually or automatically controlled. Movement of
plasma arc torch 56 upwardly in borehole 57 melts and solidifies
increasingly higher sections of contaminated earth 54 into a column
until torch 56 reaches the top surface of the earth. The molten
material 64 occupies substantially less volume than the soil from
which it is formed due to the gasification of organic material,
elimination of the water in the soil and the densification caused
by the vitrification process. This will result in a significant
subsidence in the original ground surface surrounding the borehole
57 as shown in FIG. 2. Therefore, the result of the process is,
after removal of the torch 56 and cooling of the molten material, a
significantly reduced volume of hard, dense rock-like material in a
subsided column encompassing the former borehole location.
The basic nature of a plasma arc torch is that of generating heat
by electric arc as opposed to any sort of combustion. The process
employed, therefore, does not require air or oxygen. The amount of
plasma gas required to sustain an arc is on the order of 10 times
less than the amount of air which is required to combust a fuel and
generate similar amount of heat energy. Because of this lack of
oxygen, even if air were to be the chosen plasma gas, no combustion
of organic contaminants, such as petroleum spill products, is
possible.
To completely treat the area affected by the leaching into soil 54
of contaminant 50, an array of holes encompassing the area must be
formed. A typical array is that shown in FIG. 3 which generally
depicts a top plan view of the contaminated soil situation of FIGS.
1 and 2 after having been drilled and processed with plasma arc
torch 56 as discussed above. In the approximate center of FIG. 3 is
an illustrative surface hazardous waste spill 50 with contaminated
area 54 indicated within a dashed line. The spacing of the
boreholes 57 in the array of FIG. 3 is dependent upon the nature of
the soil and the type of contaminant which is present. According to
the method of the invention, an initial borehole 57 is drilled and
processed with a plasma arc torch 56 so as to determine the actual
maximum diameter which can be effectively decontaminated and
remediated. The placement of successive boreholes 57 is such that
sufficient overlapping of treated material occurs to remediate the
entire affected area. In a typical drilling and heat treatment site
with increased borehole spacing, cooled vitrified soil mass 70 is
shown surrounded by cured brick-like zone 72 and deplasticized zone
74. The final result of the processed array of boreholes 57 is that
at least all areas represented by deplasticized zone 74 are
coalesced together so that all contaminated earthen material 54 is
either gasifier, pyrolyzed, vitrified or otherwise immobilized.
This resultant remediated soil is no longer harmful and will
protect against further spread of the hazardous spill.
A relatively large spacing of boreholes 57 to coalesce the
deplasticized zones as described above would be appropriate to
treat and remediate soil contamination of simple organic volatile
compounds such as petroleum products. The pyrolysis process would
rapidly volatilize this contaminant out to the edge of the
deplasticized zone 68 of FIG. 2 (temperatures greater than
200.degree. C.). Medium borehole 57 spacing to coalesce the
brick-like zone 66 of treated material (temperatures greater than
900.degree. C.) should be considered for such contaminants as
organic solids, sludges, or non-organic compounds of low toxicity.
Finally, close spacing of boreholes 57 will be required to coalesce
the thermally treated columns of the vitrified zone 64, which has
been melted by heating to temperatures exceeding 1100.degree. C.
The contaminants required to be fully melted, vitrified and
immobilized would include highly hazardous/toxic inorganic
materials, such as heavy metals, low-level radioactive wastes, and
underground deposits of unknown contaminants such as that found in
unrecorded, buried metal drums 76 of waste (FIG. 4A). Over time,
such buried drums are prone to rusting and leakage, causing an area
of contaminated soil 54, as is illustrated.
A specific borehole spacing for a particular site remediation
program would be ultimately determined by analysis of the results
of an in-situ vitrification test in the medium to be
remediated.
The invention also includes subterranean remediation capabilities
unique to plasma arc torch processing of contaminated materials.
Principal among these unique capabilities are the ability to
operate at any depth underground, selective depth remediation of
buried contaminants, and the remediation of contaminants located
below the groundwater table. An example of selective remediation is
the insertion of the plasma torch 56 directly into an underground
storage tank 78 (FIG. 4B) which would readily remediate and
volatilize all the residual petroleum products in the tank. If
necessary, the plasma torch could be operated until the storage
tank itself is melted and any surrounding contaminated soil 54, as
would seep from break 55, is vitrified or otherwise remediated. The
accompanying ground surface subsidence into the cavity of the
storage tank would be backfilled with clean material.
If the contaminant situation is one in which leaching has
progressed no more than a small distance beneath the surface, e.g.
50 cm or less, it is not typically necessary to drill a hole.
Thorough pyrolysis and vitrification can be accomplished with the
plasma arc torch suspended a selected distance H above the ground
surface, e.g., approximately 30 centimeters for a 1 MW plasma torch
and radiating heat downwardly (see FIG. 6). In this surface
application, the transferred arc torch mode would be the preferred
remediation configuration. In the transferred arc mode, a second
electrode (not shown) is driven into the ground in the vicinity of
the spill to complete the electric circuit. This process is
repeated at appropriately spaced locations until the entire spill
is remediated. Whether operating above or below ground level, it is
practical in many situations to utilize several torches
simultaneously and reduce the total time required to remediate the
hazardous deposit.
In certain situations, when a spilled or buried contaminant covers
a very large area, is leaching rapidly or imminently threatening a
water supply, it may be desirable to employ a second embodiment of
the invention as described below. Correction of such a condition
requires the first priority to be the creation of a barrier against
further spreading of the seeping, contaminating leachate.
The initial steps of forming a borehole 57 to a depth shown in FIG.
2, inserting a thin metal casing (not shown), inserting and
energizing a plasma arc torch 56 remain as described above.
However, in order to treat an area as quickly as possible according
to the second embodiment, plasma torch 56 is deenergized after
pyrolyzing, melting and solidifying sections 64, 66 and 68
surrounding flame 62; torch 56 is then removed from borehole 57
without gradually being lifted and melting vertically up to the
ground surface. Torch 56 is subsequently lowered into each of the
other boreholes 57 illustrated in the array of FIG. 3 to be
energized at or near the same depth so as to create a coalesced
horizontal sealant layer 82 (FIG. 5) below the depth of the
leaching contaminant which layer acts as a non-porous sealant to
protect against further vertical leaching. After creation of this
sealant layer 82 through treatment of the deepest portions of all
holes 57, it may be desirable to reintroduce the plasma torch into
each of the peripheral boreholes 57 so as to continue to pyrolyze,
melt, and vitrify the soil from the level of the established
horizontal sealant layer 82 up to the ground surface, thereby
establishing one or more substantially vertical sealant cutoff
walls of coalesced columns 80 from the horizontal containment layer
up to the ground surface. The result is essentially a vitrified,
impermeable set of cutoff walls or a complete sealed basin to
prevent further leaching into surrounding soil 52. Following the
creation of this barrier, the plasma torch could be reintroduced
into the existing boreholes as a lower priority to remediate the
entire volume of contaminated material.
As mentioned earlier, even if contaminants have reached the ground
water table and are migrating out of the original contaminated
area, plasma torch remediation still presents a viable solution.
Plasma torches have been shown to work well underwater and at
selective locations and depths. Therefore, once located, the
contaminants migrating within the aquifer or underground stream
could be readily pyrolyzed and vitrified along with the surrounding
media and immobilized from further migration. In the case of any
leached contaminant, it is essential to quickly determine the
extent of spread of the contamination and to position boreholes 57
accordingly.
The plasma torch soil remediation process of the invention, because
of the very high temperature created, will act to volatilize and
pyrolyze the organic contaminants and generate gaseous by-products.
The off-gases are collected by means of a hood 61 (FIG. 2) and
scrubbed or chemically cleaned, as is known. Any residual carbon in
the soil, together with the inorganic contaminants, would be melted
into a slag which, when cooled, results in a high density, inert
vitrified mass, which is not subject to leaching.
In summary, the process of the invention disclosed herein provides
a highly reliable, efficient and environmentally safe method of
remediating problems of soil contamination by intentional or
accidental waste spills and deposits on or below the surface of the
earth. The plasma arc torch method will pyrolyze, melt and vitrify
any contaminated soil or rock material deposit or object and
produce gaseous by-products according to the material being
processed. Organic and inorganic materials, including heavy metals,
can be processed and remediated at any depth or location
underground, leaving an environmentally safe and chemically inert
residue containing immobilized residual contaminants. Testing of
the invention process in accordance with standards of the United
States Environmental Protection Agency has demonstrated hazardous
chemical levels in the inert residue lower than one-tenth the
permissible concentration for leachability.
It is understood that the specific embodiments are used herein as
examples and are not to be construed as limitations on the scope
and principles of the invention.
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