Apparatus for in situ heating and vitrification

An apparatus for decontaminating ground areas where toxic chemicals are buried includes a plurality of spaced electrodes located in the ground and to which a voltage is applied for bringing about current flow. Power delivered to the ground volatilizes the chemicals that are then collected and directed to a gas treatment system. A preferred form of the invention employs high voltage arc discharge between the electrodes for heating a ground region to relatively high temperatures at relatively low power levels. Electrodes according to the present invention are provided with preferentially active lower portions between which current flows for the purpose of soil heating or for soil melting and vitrification. Promoting current flow below ground level avoids predominantly superficial treatment and increases electrode life.

BACKGROUND OF THE INVENTION 
A substantial number o ground contaminated areas exist, especially as the 
result of industrial disposal, which either threaten populated areas or 
which cannot be used for conventional purposes. Temporary storage methods 
and or/soil heating techniques have been proposed for treating 
contaminated soils containing dioxins, PCB'S, hydrocarbons and the like. 
Soil heating can drive off volatile substances but some methods of heating 
the soil, e.g. radio frequency heating, can be expensive or incapable of 
heating to the desired depth for removing large quantities of 
contaminants. In Brouns et al U.S. Pat. No. 4,376,598, in situ 
vitrification of soil is described wherein sufficient electrical energy is 
applied via electrodes in the ground for converting the soil itself to a 
conductive, i.e., liquid, state which is then allowed to harden into a 
vitrified mass. According to the latter method, non-volatile contaminant 
substances are stabilized as vitrified material, and volatile materials 
are driven off or pyrolyzed. However, electrical power requirements in 
melting the soil can be substantial. 
For the purpose of carrying out complete in-situ vitrification of the soil, 
or in heating of the soil to temperatures for driving off contaminants, 
pairs of metal electrodes can be driven into the ground and connected to a 
source of power. Electrical discharge or current flow then tends to take 
place at the surface, whereby a relatively large area is liquefied or 
treated near ground level, but lower regions are less effectively 
penetrated. Lower melted soil resistance encountered near the surface 
during in-situ vitrification is believed to promote spreading of the 
treatment area adjacent the surface as compared with treatment to a 
greater depth. Subjecting the electrodes to high operating temperatures 
and corrosive environments for extended periods of time reduces electrode 
life. The electrodes can deteriorate rapidly as a result of emphasizing 
surface treatment because of oxidation as well as because of diffusion of 
materials into the electrodes, recrystallization of the electrode 
material, and/or metal reduction and pitting. 
SUMMARY OF THE INVENTION 
In accordance with an aspect of the present invention, electrodes are 
provided which position the electrical discharge or current flow in the 
ground at a level which is substantially adjacent the surface, but which 
avoids predominantly superficial treatment and electrode deterioration. In 
accordance with one embodiment, an electrode insulating sleeve is provided 
in such manner that electrical discharge takes place, at least initially, 
below the ground level. In other embodiments, electric current is switched 
between portions of an electrode, or meltable fuse-like portions force the 
current flow downwardly. In yet another embodiment, a selective connecting 
member is slidable within an electrode. 
Apparatus according to one aspect of the present invention is used to heat 
a region of ground containing volatilizable material to a temperature 
below its melting temperature by applying a voltage to a pair of 
electrodes spanning the region for causing a current flow therebetween. In 
accordance with this embodiment, a voltage between electrodes is applied 
in a range of 100-2,000 kilovolts DC for heating the region by 
intermittent DC arcing. A high voltage impulse generator is preferably 
employed which causes direct current discharges between electrodes, 
separated by short time periods to permit any ionized gases to recombine. 
This system enables the delivery of effective power to the ground at 
reasonable power levels for heating the ground to the required temperature 
for volatilizing undesired material. 
In accordance with another aspect of the present invention, a plurality of 
electrodes are inserted in the ground, and a power supply is switched 
between various pairs of electrodes. According to a further aspect of the 
invention, a negative pressure is maintained with respect to the treated 
region of ground by means of a hood over the ground surface being treated 
or hollow electrodes through which the volatilized material is withdrawn 
by an induced draft or vacuum source. 
It is therefore an object of the present invention to provide an improved 
apparatus for detoxifying sites containing hazardous volatilizable 
materials. 
It is another object of the present invention to provide an improved 
electrode for vitrifying or detoxifying sites containing hazardous 
materials while avoiding entirely superficial treatment. 
It is another object of the present invention to provide an improved 
electrode for vitrifying or detoxifying sites containing hazardous 
materials wherein treatment can be directed to a given level. 
The subject matter of the present invention is particularly pointed out and 
distinctly claimed in the concluding portion of this specification. 
However, both the organization and method of operation, together with 
further advantages and objects thereof, may best be understood by 
reference to the following description taken in connection with 
accompanying drawings wherein like reference characters refer to like 
elements.

DETAILED DESCRIPTION 
Referring to the drawings, and particularly to FIGS. 1 and 2 illustrating 
soil heating apparatus according to the invention, a plurality of 
substantially vertically disposed electrodes 10-25 are slideably supported 
via insulating feedthroughs 26 from the roof of portable hood 23. Hood 28 
is movable with respect to the ground and may be placed over a region 
containing hazardous material which is to be removed. The hood is also 
equipped with off-gas outlet 30 connected with the top interior of the 
hood which suitably leads to a gas treating, recovery, and/or destruction 
system. 
The electrodes 10-25 are either driven into the ground or the ground is 
predrilled for their reception at locations within the underground area 
containing hazardous materials. The electrodes are connected to a power 
system that suitably builds an increasing charge amongst the electrodes 
until arcing discharge occurs. At the point of electrical discharge, heat 
is generated in the soil for raising its temperature. 
The electrodes are suitably connected to a power supply in the manner 
illustrated in FIG. 6 such that, for example, even numbered electrodes are 
connected or connectable to the positive side of the source and the odd 
numbered electrodes are connected or connectable to the negative side. The 
electrodes are evenly spaced, for example, in a 4.times.4 array as shown, 
so that each positive electrode is equally spaced from at least a pair of 
negative electrodes. Switching means 32 is employed for cyclically 
energizing positive electrodes 10, 12, 14, 16, 18, 20, 22 and 24 from the 
positive power supply terminal while the negative electrodes remain 
connected to the negative power supply terminal. Therefore, at least pairs 
of electrodes are sequentially actuated for initiating conduction in the 
ground between such pairs. Alternative switching means are clearly 
possible, i.e., a switching means similar to means 32 may be interposed 
between the negative power supply terminals and the respective odd 
numbered electrodes. The power system should be capable of delivering a 
voltage between 100 and 2,000 kilovolts to provide intermittent electrical 
discharges for heating the soil to temperatures above 200.degree. C. High 
voltage impulse generators can be used and are commercially available. 
The power supply utilized in FIG. 6 is an impulse generator represented by 
direct current source 30 and a capacitor bank 34 connected across the 
terminals of DC source 30. The supply is capable of delivering a high DC 
voltage in the range of 100-2,000 kilovolts. When the capacitor bank 
charges to a predetermined level, a discharge takes place in the ground 
between a pair of electrodes, e.g. between electrode 12 and one or more of 
electrodes 11, 14, and 15 for the switch position shown. After substantial 
discharge of capacitor bank 34, the capacitor bank recharges from source 
30 until the next discharge takes place between the same electrodes, or 
other electrodes if the position of switch 32 has been changed. 
Generally, the position of switch 32 is maintained for directing sequential 
discharges between a pair or pairs of electrodes until such electrodes 
reach a predetermined temperature level after which switch 32 is moved to 
the next position. Thus, switch 32 is suitably actuated by a timing 
mechanism (not shown) so that a given positive electrode will support, for 
example, ten arc discharges before the next positive electrode in sequence 
is selected. Typically a period of one second occurs between discharges 
which allows for gas recombination. The discharge voltage for the circuit 
of FIG. 6 is primarily dependent upon the spacing of the electrodes, as 
well as to some degree the type of soil therebetween. 
Although applicable to all soil types, the heating system according to the 
present invention is most economically employed in regions of dry, sandy 
soil. As capacitor bank 34 charges, a voltage will be reached for which a 
discharge will be initiated between selected electrodes. Clearly the 
circuit can be modified, if desired, to insert additional switching means 
between capacitor bank 34 and the electrode array such that discharge 
between electrodes is initiated at a selected voltage level, preferably 
between 100 kv and 2,000 kv. 
For the FIG. 6 circuit as illustrated, if moisture is present in the soil 
to any great extent, a steady current can first pass through water in the 
soil, driving off water vapor by resistance heating. As the soil begins to 
develop non-conductive dry spots, the voltage across the capacitor bank 
increases further and repetitive arcing through the soil begins. The 
charge-discharge cycle then continues to impart energy to the soil, 
heating the soil and driving off the volatiles. Once the soil adjacent the 
electrodes is dried, the arcing will usually provide higher voltage and 
higher power input to the soil than the resistive heating. 
Although steel or aluminum rods can be used as electrodes in the system for 
heating the soil, a preferred electrode for the above-described soil 
heating apparatus is illustrated in longitudinal cross section at 10 in 
FIG. 3. The electrode is cylindrical, having an inner axial electrode 
portion 36 suitably formed of carbon steel or aluminum and provided with 
an enlarged cylindrical tip 38 at its lower extremity. The inner axial 
portion 36 is covered by an insulating sleeve 40 formed of a high voltage 
insulating material such as preformed mica. Disposed over insulting sleeve 
40 is a further metal sleeve 42, suitably carbon steel or aluminum, having 
the same outside diameter as electrode tip 38 but separated from tip 38 by 
radial flange 44 of insulating sleeve 40, the last mentioned flange also 
having the same outside diameter as tip 38. The metal sleeve 42 may be 
partially or fully withdrawn after the electrode assembly 10 is driven or 
inserted into the ground in the case of the soil heating system to 
eliminate the possibility of electric arcing between the electrode tip 38 
and metal sleeve 42. 
Central portion 36 extends a distance outwardly above sleeves 40 and 42 for 
receiving electrical connection 46 which may lead to switching means 32 in 
FIG. 6. Electrical connection 46, as well as the protruding part of 
electrode portion 36, are suitably covered by high voltage shrink plastic 
insulation (not shown) rated at 100 kv or greater. An example is 
shrink-fit Okanite material. The voltage required to arc through dry soil 
is found to be greater than that required for arcing through air and it is 
therefore necessary to provide electrical insulation above the soil to 
prevent unwanted arcing. Alternatively, or in addition, pairs of arcing 
electrodes may be disposed in angular relation to one another rather than 
vertically as depicted in FIG. 2. For instance, the lower tips of 
electrodes 13 and 14 may be angled closer to one another with the upper 
portions farther apart. 
Insulating the upper part of the electrode provides a means for 
concentrating electrical arcing at a given level below the ground into 
which the electrode is driven. At the same time, sleeve 42 and flange 44 
suitably have the same outer diameter as tip 38 to facilitate driving or 
insertion of the electrode into the ground. Assuming it is desired to 
initiate electrical discharge at a fairly low ground level, followed by 
raising the level of discharge so as to sweep through a given ground 
region, electrodes of the type illustrated in FIG. 3 may be gradually or 
intermittently raised after performing desired heating at different 
levels. The power supply of FIG. 6 may be periodically deactivated and the 
capacitor bank discharged, after which the electrodes are raised manually 
from the top of hood 28 by sliding the same upwardly through insulators 
26. After adjusting the levels of various electrodes to a higher level, 
arcing operation can be resumed. Alternatively, each electrode is suitably 
supplied with means for raising the same. Referring to FIG. 3, a hydraulic 
cylinder 48 which is mounted to the frame of hood 28 (by means not shown) 
is provided with an actuating rod 50 pivotally engaging a bracket 52 
secured to the outer metal sleeve 42 locked to an electrode. The hydraulic 
cylinder 48 is periodically or continuously actuated to gradually move the 
electrode assembly upwardly. 
Another type of electrode is illustrated in FIGS. 4 and 5. This type of 
electrode as illustrated in longitudinal cross-section in FIG. 5 is 
similar in construction to the FIG. 3 electrode, and primed reference 
numerals are employed to refer to corresponding elements. however, this 
electrode is provided with an axial passage 54 extending the whole length 
thereof for communicating with a header 56 by way of insulating tube 59. 
Electrodes 13', 14', 21 and 22' in FIG. 4 may be successively negative and 
positive electrodes and are connected to power supply means by separate 
conductors (not shown). However, a negative pressure, i.e. vacuum, can be 
applied to header 56 from conduit 60 for drawing hazardous material from 
the ground as it is volatilized by electric heating. The conduit 60 can 
extend to a plant for generating the negative pressure and treating, 
recovering, or destroying the gaseous material removed from the ground. 
Alternatively, selected ones of the hollow electrodes may be connected to 
a source of stripping air, while other hollow electrodes may serve as 
means for removing stripping air from the soil being treated. 
It will be appreciated the array of hollow electrodes illustrated in side 
view in FIG. 4 is desirably extended to a 4.times.4 array as illustrated 
in FIGS. 1 and 6, with similar connections being made thereto. Such array 
may or may not be provided with a covering hood 28, inasmuch as gaseous 
substance can be withdrawn by means of conduit 60 rather than conduits 30. 
However, the electrodes of FIGS. 1 and 2 may also be made hollow, i.e., to 
have the cross-section of FIG. 5, being provided with venting means 62 in 
FIG. 2 underneath hood 28 whereby the gaseous effluent is withdrawn from 
below the surface of the ground via the electrodes and into hood 28 so as 
to be withdrawn through conduits 30 in combination with gasses emitted 
directly upwardly through the ground surface. 
Intermittent DC potential applied to the electrodes passes a series of 
electrical discharges between the electrodes inserted in the contaminated 
soil such that energy dissipated by the discharges heats the soil and 
volatilizes or destroys organic wastes in the soil. In general, the soil 
temperature should be raised to at least 150.degree. C. above the boiling 
point of an organic contaminant to achieve greater than 99% removal 
efficiency. This means that for removal of light organics, a temperature 
of about 200.degree. C. should be achieved, and for heavy organics the 
soil should be heated to about 500.degree. C. or greater. Therefore, a 
range between 200.degree. C. and 600.degree. C. is preferred in order to 
attain good efficiency on the one hand without requiring excessive power 
on the other. However, it is clear some removal can take place below and 
above this range. The total duration of time required by the discharge 
regime to heat the soil to the requisite temperature sufficiently for 
decontamination will depend upon the individual soil content as well as on 
the material buried therein. Soil temperature is readily measured by 
conventional means and the process may be continued until the soil region 
is substantially out-gassed with respect to the contaminant. 
Higher soil temperatures which assure destruction of hazardous chemicals 
are an option. Accordingly, the ground may be heated to a temperature for 
substantially destroying the contaminant chemicals by pyrolysis, followed 
by combustion of the pyrolysis products when these products reach the 
surface. In this case, a higher ground temperature than 600.degree. C. is 
preferred, although many materials will begin to pyrolyze at 300.degree. 
C. Thus, a range of 300.degree. C. to 1200.degree. C. is suitable for some 
degree of destruction of the offending materials in the ground. For 
achieving combustion when the pyrolysis products reach the surface, the 
hood 28, as illustrated in FIGS. 1 and 2, may be employed, and an 
additional inlet (not shown) for combustion gas is suitably provided, with 
the combustion products being removed via conduits 30. 
As another alternative, the ground may be heated to the preferred 
temperature range, i.e., between 200.degree. C. and 600.degree. C., with 
destruction or other treatment taking place at an above ground location to 
which the offending substances are conveyed via conduits 30 and FIGS. 1 
and 2 or conduit 60 in FIG. 4. 
In a test for the removal of 2-chlorophenol test chemical, a removal 
efficiency of 95 wt. % was achieved in a run time of 4.2 hours, with an 
average power expenditure of 115 watts. The maximum soil temperature was 
304.degree. C. in sandy soil. Successful tests have also been conducted 
for test deposits of trichloroethene and hexachlorobenzene. 
The effluent is suitably conveyed by conduits 30 in FIGS. 1 and 2, or 60 in 
FIG. 4 to a gas treatment, recovery, or destruction system. By way of 
example, a treating or cleaning system is depicted in FIG. 7 where the off 
gas is received at 64 either from the hood of FIGS. 1 and 2 or the header 
of FIG. 4. In the case of off gas received at very high temperatures, for 
example in the instance of combustion within hood 28, a cooler 66 is 
employed and comprises a finned air-to-glycol heat exchanger. This cooler 
can be by-passed by opening valve 70 and closing valve 68. 
From the gas cooler, the off gas is suitably split and directed into one of 
two wet scrubber systems that operate in parallel. One such system, 
indicated at 72, is shown in block fashion and the other parallel system 
will be described. Valve 74 leads to quench tower 76 feeding tandem nozzle 
scrubber 78 which in turn leads to vane separator 80. The tandem nozzle 
scrubber may comprise a tandem nozzle hydrosonic scrubber manufactured by 
Hydro-Sonic Systems, Dallas, Tex. The quencher reduces the gas temperature 
to about 66.degree. C., and supplies some scrubbing action to remove a 
portion of entrained particles. The primary functions of the tandem nozzle 
scrubber are to remove any remaining particles and condense remaining 
semivolatile components as well as to provide additional cooling of the 
off gas. The vane separator that follows is designed to remove all 
droplets greater than or equal to 12 .mu.m. 
A glycol scrub solution that is injected into the quencher and tandem 
nozzle scrubber from tank 82 is cooled through heat exchanger 84 before 
being returned to the process. After the scrub solution is returned to 
tank 82, it is circulated via pump 86 back to quencher 76 and scrubber 78. 
Following the scrubber system, the gas is cooled in condenser 88. The 
condenser and mist eliminator or vane separator 90 remove droplets greater 
than or equal to 12 .mu.m. Final decontamination of off-gas particulates 
is achieved in a two stage filter/absorber assembly following heating of 
the gas at 92. The first stage is composed of two parallel HEPA 
(high-efficiency particulate air) filters and charcoal absorber 94 feeding 
a single HEPA filter and charcoal absorber 96. 
The gaseous effluents are drawn through the off-gas system components by an 
induced draft system, the driving force being provided by a blower 98. 
This blower, which has substantial capacity, is employed to provide 
negative pressure within hood 28 or within header 56 and the hollow 
electrodes for aiding in removing gaseous products from the ground. After 
passing through the blower system, the off-gasses are exhausted to a stack 
which is indicated at 100. 
The system of FIG. 7 is somewhat conventional and it is understood it could 
be replaced by other gas treatment systems. It may be used alternatively 
in conjunction with a destruction system comprising controlled air 
incinerators coupled between the ground site being detoxified and the 
off-gas system of FIG. 7, particularly in the case where combustion within 
the hood 28 is not being carried out. Alternatively, ground chemicals may 
be recovered in a cryogenically cooled condenser or air exchange condenser 
prior to delivery to the off-gas system of FIG. 7. Various combinations of 
gas treating systems of this type can be employed. 
As an alternative embodiment, a continuous conduction system may be 
employed with the electrode configuration depicted in FIGS. 1 and 2, 
wherein an electrically conductive heavy oil is sprayed, inserted or 
injected into the ground for supporting conduction between the electrodes 
before power is applied. A higher current, lower voltage source of power 
is employed in such case. The voltage utilized is suitably between 1,000 
and 4,000 volts, for supporting a current in the ground between electrodes 
of between 1600 amps and 450 amps. The electrically conductive fluid is 
suitably sprayed on or inserted into the soil to be treated, so that it is 
absorbed evenly into the soil, and the electrodes are then inserted into 
the soil. The electrical conductivity of the fluid will allow sufficient 
current to pass among the electrodes to dissipate substantial heat in the 
soil. This method is suitable for heating volumes of soil to relatively 
low temperatures, e.g. less than 200.degree. C. The first described 
method, (i.e., utilizing high potential arc discharge, e.g. 100 to 2,000 
kilovolts), is preferred for several reasons. High temperatures can be 
more easily reached without the introduction of conductive materials and, 
moreover, the high potential arcing is less dependent upon soil types, 
i.e., less dependent upon the absorption of the conducting medium in the 
soil and the appropriate distribution of the conducting medium through the 
soil. 
As a further alternative embodiment, high potential intermittent arcing may 
be followed by more continuous arcing with a suitable power supply. Source 
30 may in such case take the form of an impulse source and a somewhat 
lower voltage parallel source capable of delivering greater and 
substantially continuous arcing current. 
The electrodes as illustrated in FIGS. 3 through 5, as used in a system for 
heating the soil, are effective in concentrating the electrical discharge, 
at least initially, below the soil surface to avoid superficial treatment. 
Electrodes of the same type may be employed for initially reaching levels 
below the ground in the higher temperature, in-situ vitrification process, 
especially when such electrodes are formed of materials hereinafter 
described which are adapted to withstand higher operating temperatures for 
at least a predetermined time period. 
Considering the process of in-situ vitrification generally, reference is 
first made to FIGS. 8-10 illustrating an in-situ vitrification 
installation. A plurality of substantially vertically disposed electrodes, 
110, 112, 114 and 116 are supported via insulating feedthroughs 118 from 
the roof of portable hood 120. Hood 120, which is typically about twelve 
to eighteen meters square by two meters high, is movable with respect to 
the ground and may be placed over a section of buried trench or the like 
122 containing waste materials. The hood is also equipped with a 
combustion air inlet system 124 and off-gas outlets 126 connected with the 
top interior of the hood. The off-gas outlets suitably lead to a gas 
treating or scrubbing system. 
The electrodes 110-116 are placed in the ground, with first electrodes 110 
and 112 on opposite sides of the trench being connected via conductors 130 
and 132 respectively to a first phase of current, while second electrodes 
114 and 116, also disposed on opposite sides of the trench, are connected 
by way of conductors 134 and 136 respectively to a second phase of 
current. The conductors 130-136 may be joined to the respective electrodes 
by connector clamps (not shown). Suitably, the apparatus is electrically 
supplied by way of a transportable power substation delivering three-phase 
power to transformers in a Scott connection for providing two-phase 
current to conductors 130-132 and 134-136 respectively. Adjustable means 
may be included for determining the desired voltage and current levels. A 
load voltage between approximately 4,000 and 400 volts is suitably 
supplied with a corresponding current capacity on each of two phases of 
between 450 and 4,000 amps. In tests, the final voltage is typically 700 
to 600 volts delivering a current between 2,000 and 3,000 amperes. 
In-situ vitrification is further illustrated schematically in FIG. 10. 
Electrodes 110 and 112 are disposed vertically on either side of or within 
a region of buried waste material represented by a large X. For starting 
the soil melting process, a horizontal layer of graphite or glass frit, 
which may be buried below the surface of the ground and over the waste 
material, can be placed between the two electrodes in contacting relation 
thereto. A voltage of a few hundred volts is applied between conductors 
130 and 132 causing conduction and an elevation in the temperature. A 
current-carrying liquid glass Pool 140 is established which progressively 
enlarges both laterally and vertically downwardly in a typical instance, 
engulfing the waste materials. The temperature of the pool is typically 
above 1200.degree. C. The waste materials are melted, pyrolyzed or 
dissolved in the molten soil mass. Metals within the mass may be dissolved 
or may eventually be found as solid portions at the lower boundary of the 
glass pool. When the glass pool reaches a desired depth, current flow is 
typically discontinued after which the pool forms a vitrified mass as a 
glass, a glass ceramic, or a partially devitrified glass, with crystals 
and glass dispersed within a solid matrix. 
Further electrodes according to the present invention which are 
particularly adapted for use in in-situ vitrification are illustrated in 
FIGS. 11-15. It is understood in each case that one or more pairs of such 
electrodes will be employed in spaced relation on either side of or within 
the area to be vitrified. Referring particularly to FIG. 11 showing such 
an electrode in vertical cross-section, the electrode 150 is cylindrical, 
including an inner axial conductive metal electrode portion 152 having an 
enlarged cylindrical tip 154 at its lower extremity. The inner axial 
portion 152 is covered by an insulating sleeve 156, while disposed over 
insulating sleeve 156 is a further cylindrical conductive metal sleeve 
158, the lower end of which has the same outside diameter as electrode tip 
154. However, metal sleeve 158 is separated from tip 154 by radial 
insulating flange 160 which is the same in outside diameter as tip 154. 
Metal sleeve 158 toward the upper portion thereof is smaller in diameter 
than the aforementioned 154 and is covered by a further insulating sleeve 
162 having a lower end which terminates in a radial flange 164 of the same 
outside diameter as the rest of the electrode, i.e., the same outside 
diameter as tip 154. In surrounding relation to insulating sleeve 162, 
above flange 164, a cylindrical conductive metal covering sleeve 166 is 
received. 
The various metal sleeves 152, 158 and 166 are concentric and all suitably 
terminate at an upper end 168 above soil level 170, the electrode 50 being 
supported by an insulating feedthrough 18 (FIG. 9). The sleeves 152, 158 
and 156 are connected to fixed terminals of a selection switch 72 having a 
movable terminal coupled to a power supply. 
By first operating switch 172 so that its movable terminal contacts 
terminal 174, current from the power supply is delivered to electrode tip 
54 in preference to the other electrode portions for the purpose of 
initially directing current flow through the ground below ground level 
170. It is assumed that another electrode or other electrodes employed 
simultaneously in circuit with electrode 50 are similarly constructed and 
arranged so that a lower portion is preferentially operative. 
After melting the soil in the area between the similar electrodes, switch 
172 (and a corresponding switch on a cooperating electrode) can be moved 
to the middle contact whereby current will then flow from electrode sleeve 
portion 158, and finally, after desired soil melting at this level, the 
switch arm is positioned at the top terminal such that soil at the surface 
is melted between corresponding electrode sleeve portions 166. Although 
the upper portion of sleeve 166 is exposed to the air, nevertheless it 
will not have been connected to a source of power while the lower portions 
of the electrode were selected and therefore its integrity is preserved 
for a longer time period. As will be seen, the electrode of FIGS. 11 and 
12 is similar to the electrode of FIG. 3 but has additional switchable 
sections for gradually moving conduction closer to the surface. 
Another electrode according to the present invention suitable for in-situ 
vitrification is illustrated at 190 in FIG. 13. This electrode is similar 
in construction and operation to the electrode as shown in FIGS. 11 and 12 
and a similar switching means can be connected thereto. A first inner 
axial electrode portion 176 takes the form of a cylindrical conductive 
metal rod covered over approximately two-thirds of its length, starting 
from upper end 178, with an insulating sleeve 180, the latter being 
received within a cylindrical conductive metal sleeve 182 coextensive with 
sleeve 180. Adhered in surrounding relation to metal electrode sleeve 182 
is a further insulating sleeve 184 which is coextensive with electrode 
portion 182 starting from the top thereof and extending over about 
two-thirds of the length of electrode portion 182. An additional 
cylindrical conducting electrode sleeve 186 covers insulating sleeve 184 
for completing the concentric configuration. Electrode 190 of FIG. 13 is 
operable in substantially the same way as electrode 150 depicts in FIGS. 
11 and 12 but is somewhat simpler to manufacture. However, it will be 
appreciated electrode 150 is easier to drive into the ground. 
A further electrode according to the present invention is illustrated at 
192 in FIG. 14. This cylindrical electrode includes a movable inner axial 
portion 194 in the form of a conductive metal rod provided with an 
enlarged cylindrical tip 196 at its lower extremity. Above tip 196, axial 
portion 194 is covered with a cylindrical insulating sleeve 198 having the 
same outside diameter as the aforementioned lower tip 196. The structure 
including electrode portion 194 and sleeve 198 is movable as a unit 
upwardly and downwardly as indicated by arrow 200 closely within 
cylindrical conductive metal sleeve 202 which is divided into an upper 
section 202, a middle section 202'' and a lower section 202' by 
intermediate insulating disks 204 respectively adhered to cylindrical 
metal sections of sleeve 202 immediately thereabove and therebelow. 
As illustrated in FIG. 14, the electrode portion 194 is connectable to a 
power supply via lead 204 such that for the position illustrated power is 
coupled to lower cylindrical metal sleeve portion 202' inasmuch as 
enlarged tip 196 makes contact therewith. However, if rod 194 is upraised 
until tip 196 resides entirely between the insulating disks 204, then the 
power supply will be connected solely to electrode portion 202''. 
Similarly, if the central rod is raised even farther so that tip 196 is 
entirely above both disks 204, the electrical connection will be to the 
upper part of metal electrode sleeve 202. As will be seen, a selective 
connection can thus be made with different levels of sleeve 202, e.g. 
starting at a lower level whereby electrical current is directed through a 
lower region in the ground, after which the central rod is raised to- 
deliver electrical current to regions closer to the surface. A gradual 
change can be made between regions by positioning tip 196 so that it 
bridges, for example, between electrode sections 202' and 202''. 
The electrode of FIG. 14 is suitably mounted with the upper portion of 
electrode sleeve 202 received within feedthrough 118 in FIG. 9, with the 
upper end of rod 194 and its accompanying insulating sleeve 198 extending 
above the same feedthrough 118. The entire length of electrode portion 194 
and tip 196 may therefore be somewhat longer than the entire length of 
sleeve 202. 
A further in-situ vitrification electrode embodiment is illustrated in FIG. 
15 wherein a cylindrical electrode 210 includes an inner axial electrode 
portion 212 comprising a conductive metal rod provided with an enlarged 
cylindrical tip 214 at its lower extremity. The inner axial portion above 
tip 214 is covered by a cylindrical insulating sleeve 216 which is in turn 
received within an upper cylindrical conductive metal electrode portion 
218 as well as within a central cylindrical conductive metal electrode 
portion 220 and a lower cylindrical conductive metal electrode portion 222 
all adhered to sleeve 216 and having the same outside diameter as tip 214. 
Lower cylindrical electrode portion 222 is separated from lower tip 214 by 
an annular metal disk 224 which is the same in outside diameter as tip 
214. The inner rod 212 and the cylindrical electrode portion 222 are 
electrically connected via disk 224. Furthermore, electrode portions 220 
and 222 are separated and electrically connected by a similar annular 
metal disk 226, while the same type of metal disk 228 separates electrode 
portions 218 and 220. The combination of cylindrical electrode portions 
218, 220 and 222 together with annular metal disks 224, 226 and 228 are 
coextensive with the axial electrode portion 212 having its upper end 
connected to a power supply by means of lead 230. 
The operation of electrode 210 in FIG. 15 is generally the reverse of the 
operation described in connection with the electrodes of FIGS. 11-14 in 
that conduction through the ground between a pair of electrodes 210 will 
at first more likely occur nearer the surface of the ground, or at least 
not preferentially at a distance from ground surface 170. This will be 
especially true if a layer of graphite and glass frit just below the 
ground surface is used for starting conduction. Annular metal disks 228 
are formed of a conductive metal having a lower melting point than 
electrode portions 218, 220 and 222 and each disk is adapted to melt when 
the ground therearound reaches a given temperature. Assuming the ground 
near surface 170 melts first and melting proceeds downwardly toward 
annular metal sleeve 228, the annular metal disk 228 will melt whereby 
current cannot then reach annular electrode portion 218 but will be 
concentrated toward lower annular electrode portions 220 and 222. When the 
extent of melting reaches a lower level, annular metal disk 226 will also 
melt such that conduction will takes place via sleeve portion 222 and tip 
214. Subsequent melting of annular disk 224 will, of course, concentrate 
conduction in the region of tip 214. Although conduction may thus start 
toward the upper part of the electrode, it should be apparent that, during 
the course of operation, current will become concentrated farther 
downwardly whereby upper portions of the electrode will be protected and 
solely superficial treatment is avoided. 
In the case of the electrodes illustrated in FIGS. 3-5 and 11-15 used for 
in-situ vitrification purposes, the conductive metal portions are formed 
of conductive metal adapted to withstand the higher temperatures involved, 
e.g. such conductive metal portions are suitably molybdenum or graphite, 
with the exception of annular metal disks 224, 226 and 228 of FIG. 15 
which may be formed of copper. The insulating material in the electrodes 
as used for in-situ vitrification is a suitable refractory material, e.g. 
a high temperature ceramic comprising or containing alumina. While the 
electrodes of FIGS. 11-15 have their primary utility in connection with 
in-situ vitrification, and are described in connection therewith, the same 
electrode structures can also be used for the in-situ heating process if 
desired. 
While several embodiments of the present invention have been shown and 
described, it will be apparent to those skilled in the art that many 
changes and modifications may be made without departing from the invention 
in its broader aspects. The appended claims are therefore intended to 
cover all such changes and modifications as fall within the true spirit 
and scope of the invention.