CA2123410C - Heating to detoxify solid earthen material having contaminants - Google Patents

Abstract

The present invention includes a method of treating solid earthen material having volatile, semi-volatile, and non-volatile contaminants. Six electrodes are inserted into a region of earthen material to be treated in a substantially equilateral hexagonal arrangement. Six phases of voltages are applied to corresponding electrodes.
The voltages are adjusted within a first range of voltages to create multiple current paths between pairs of the electrodes. The current paths are evenly distributed throughout the region defined by the electrodes and therefore uniformly heat the region. The region of earthen material is heated to a temperature sufficient to substantially remove volatile and semi-volatile contaminants by promoting microbial action. This temperature is less than a melting temperature of the earthen material.

Description

WO 93/~D9888 PCT/L592/0'~764 HEATING TO DETOXIFtf SOLID EARTHEN i~AATERI~L
HAYING CONTAP~INANTS..
FIELD OF THE INVENTION
This invention relates to a method for treating solid earthen material. This invention also_relates to a method for measuring moisture content and resistivity of solid earthen material.
BACKGROUND OF THE INZIENTION
The disposal of contaminated material has become an increasingly significant problem. Today, contaminated material, such as ~:ndustrial arid nuclear waste, is buried underground in specially designed storage containers.
'These dontaminants buried in the ground typically contain ve~lati7.e, semi°vol~tile, and non-volatile organic contami-nants. Unfortunately; burying the contaminants does not render hem innocuous to the environment. The storage containers can leale he contaminants into the soil, thereby polluting the soil. The contaminants may also pass into ~~unde~ground water tables end contaminate the water. supply f~r populated. regi~ns. Ground contaminants can also result from surface sp~.lla that seep into the soil.
One approach to -detoxifying organic-contaminated SOils was proposed by Buelt et al. in U.S. Fat. No.
4,957,393, assigned to~Ba-ttelle Memorial Institute. Buelt et al. proposed inserting a matrix of electrodes into a c~ntamina°ted soil region and applying very high do voltage 3~ ~~ single phase ac voltage to the electrodes. The voltages created current ,paths between the electrodes which effec-tively heated the contaminated soil to temperatures ranging from 100°C to 1200°C:

WO 93109888 PGT/LJS92f09764 The present invention arose out of a need to provide a more energy efficient and less costly system for treating contaminated soilsv The present invention provides --a method for treating contaminated soil which enables uniform heating of the soil at relatively low energy costs.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
FIG. 1 is a top plan view of a hexagonal electrode arrangement capable of supporting six phases of voltages in accordance with an aspect of the present invention.
FIG. 2 illustrates a formation of current paths created between six electrodes which are hexagon~lly arrayed v FIG: 3 is a diagrammatical illustration ~f a system for treating solid earthen material in accordance with an aspect ~f the present invention. FIG. 3: also' illustrates one~embodiment for venting contaminants from solid earthen material. . ~.. ,, F-IG: -4 . : . is a top , . plan , view illustrating an arrangement of twelve electrodes in accordance with another aspect of the present invention.
. FIG: 5 is a top plan view illustrating an arrangement of twelve electrodes in accordance with yet another aspect of the present invention.
FTG. 6 diagrammatically il3ustrates an initial step to chemically altering contam~.nants disposed in solid a earthen material.
FIG. 7.diagrammatically illustrates a formation and expansion of a dry region formed at a tame subsequent to the step shown-by FIG. 6.

W~ 93/0988 PCT/US92/09764 21~3~1~
__ 3 _ FIG. 8 diagrammatically illustrates a region of earthen material at a time subsequent to the step shown by FIG. 7.

FIG. 9 is a tpp plan view of the electrodes illus-trating the formation of dry regions in accordance with an aspect of the present invention.

FIG. lO diagrammatically illustrates an alternative embodiment for venting contaminants from solid earthen ma~eri~al FIG: 11 diagrammatically illustrates a system for treating solid earthen material in accordance with another aspect of the present invention. FIG. 11 also illustrates recycling off gas into a region of solid earthen material for further treatment.

FIG. 12' diagrammatically illustrates an apparatus for treating off gas in accordance with another aspect of the present invention.' FIG. 13 diagrammatically illustrates a process for forming vitrescent soil fragments in accordance with 20' ; another aspect of 'the present invention.

DETAINED 13ESCRIPTIC3N OF THE PREFERRED EMEOpIMENTS

According t~'one aspect of ;the present invention, a method of treating solid earthen material comprises the steps of (a) inserting a plur~liicy of electrodes into a region of solid earthen material to be treated; and (b) applying- at least six phases of voltages to corresponding ones of the el:ectro~es to create current paths between pairs of the electrodes and through the ' region of material:

''Solid earthen material" as used in this disclosure means fragmental material composing part of the surface of the globe. The term "solid earthen material" includes _ ._~ . _ .. _._ __ ~._ f , ,.. . _ . ... . ,.
r ,. .,.;...,x.,....., .,~. . ,.:~':.~. T..:.._.:~ -, : .:..:..,:-. .. .::~:
~::~'~.' , . ..,. .::....
., ~ . .. -,:. .. ..

~~ g3/~gggg PCTlLTS92/Og764 fine, densely packed particles having moisture interspersed between the particles, ground, dirt, sand, soil, sludge, .
slurry, mud, shale, in situ material, and material which has been extracted and removed from their, native location, etc.
The above method may further comprise:
(a) arranging the plurality of electrodes in a geometric configuration having diametrically opposing pairs of electrodes, the pairs defining opposing first and second electrodes; end (b) applying first and second phases of voltages to the respective first and second electrodes of respective pairs of electrodes, the first and second phases of volaages having voltage amplitudes which are substantially equal, the first and second phase. being approximately 280°
out of phase with respect to one another.
The six electrodes can be positioned in the solid 'earthen'material.and the six phases of voltages applied to the electrodes in a manner effective to produce a substantially constant voltage-to-distance ratio for all current: paths between electrode pairs. The voltage-to distance ratio for a given pair of electrodes is computed by dividing the voltage measured between the given pair of -electrodes .by.the~ distance between... the given pair of .electrodes:
According to another. aspect of the present invention., a method of -treating solid earthen material comprises of the steps of:
(a) inserting a plurality of electrodes into a regi~n of solid earthen material to be treated;
(b) applying at least six phases of voltages to corresponding ones of the electrodes- to create current paths between pairs of the electrodes and through the °
region of material; and WO 93/09888 ~ P(T/US92/09764 .s (c) heating the region of material to a temperature below a melting temperature of the solid .

earthen material, with, a temperature ranging from approximately 0C to 100C being preferred., ~..~

According to another aspect of the present invention, a method of treating solid earthen material comprises the steps ofs (a) inserting a plurality of peripheral electrodes into solid earthen material, the peripheral electrodes being arranged in a selected geometric perimeter Which defines an internal region of material to be treated;

(b) inserting at least one electrically neutral electrode in the region of material to be treated; and ,(c) applying multiple phases of voltages r to corresponding ones of the peripheral e3.ectrodes to create current: paths 'between (1) pairs of the peripheral elec-trodes, and (2) the peripheral electrodes and the neutral ,electrode, the current paths passing through and substan-tially uniformly heating the- region of material.

The peripheral electrodes can be arranged at vertices of a substantia2ly equilateral polygon having at least six sides. They neural electrode would then preferably be ''positioned in . a substantially diametric vcenter of=the regioW.of material. ' 25. ' 'The neutral electrode may be farmed with a passage therethrough which communicates with the solid earthen 'material and a location external to the solid earthen material. In this manner, gases from the region of material may be removed through the passage in the neutral 3Q electrode:

According to yet another aspect of the present invention, a method for treating solid earthen material comprises the steps of inserting first, second, third, fourth, fifth, 35 and sixth electrodes into solid earthen material;

. . . . . . . - f 9--. ,:_ ; . ( ,: ~ -- , , . .. . . .: : , _ ,, ..
.. . ....... ..:. ..:,:... ..... ....... ._. :... _..::.,.. ...,.
.. . ;,.. .

WO 93J09888 PCl'/U~92109764 's (b) applying a first phase of voltage to the first electrode;
(c) applying a second phase of voltage to the second electrode.; , .._.

(d) applying a third phase of voltage to the third electrode;

(e} applying a fourth ghase of voltage to the fourth electrode;

(f) applying a fifth phase of vopage to the fifjth l~ electrode;

' (g} applying a sixth phase of voltage to the sixth electrode;

(h) the voltages applied to the first, second, third; fourth, fifth; and sixth electrodes being sufficient 15 to create current paths between pairs of the electrodes through the material, the current paths heating the material.

~p~e first through sixth electrodes can be arranged at vertices of a substantially equilateral hexagon.

'20 According to another aspect of the present invention, a method for treating solid earthen material having volatile, semi-volatile; and non-volatile contaminants comprises the steps of:

(aj inserting multiple electrodes -into,, solid 25 earthen material;_ (b) applying multiple phases of voltages to corre-spond.ing ones of the electrodes;

(c) adjusting the voltages within a first selected range of voltages to ;heat the material to a tezngerature 30 sufficient to substantially remove volatile and semi-volatile contaminants from the material; and (d) increasing the voltages through a second selected range of voltages effective to create a corona front which decomposes the non-volatile contaminants.

W~ 93109888 PCT/U592l09764 The first selected range of voltages is preferably less than the second selected range of voltages. Moreover, the temperature sufficient to substantially remove volatile and semi-volatile contaminants is less than a melting temperature of the solid earthen material.

According to yet another aspect of the present invention, a method for treating solid earthen material having volatile, semi-volatile, and non-volatile contami-nants comprises the steps of:

(a) inserting multiple electrodes into solid earthen material, the electrodes defining a region of material to be treaded;

(b) applying multiple phases of voltages to corre-sponding ones of the electrodes;

(c) adjusting the voltages within a first selected range of voltages to heat the material to a temperature sufficient to substantially remove volatile and semi-volatile contaminants from the region of material;

(d) creating dry regions of material' around individual electrodes 'as the material is heated, the dry regions having a' periphery which defines a boundary between the dry; regions of material and earthen material exterior to the' dry-regions.

Ce) increasing the voltages to a second range of voltages to create a corona at the boundary between the dry 'regions of material and earthen material exterior to the dry regions;

(f) moving the, boundary of the dry regions radially ~utward from the individual electrodes hrough the region of material, the corona being moved with the boundary of the' dry regions; and (g) decomposing the non-volatile contaminants as the corona-carrying boundary passes over 'the non-volatile contaminants.

._ ~ . a ... . : , .. ._ , .. .: .,.:.. . ..y" ....._ . ... ..... ~.. : :. - -.:....... .... . ..: :....
...

WO 93/0988 PCTiL:~S92/09764 ,....
~1234~0 _8_ According to yet another aspect of the present invention, a method for treating solid earthen material .
having volatile, semi-volatile, and non-volatile contami-pants in the presence of microbial organisms comprises the steps of (a} inserting a plurality of electrodes into solid earthen material, the electrodes being arranged in a selected geometric perimeter defining a region of material to be treated;
(bj applying at least six phases of voltages to corresponding ones of the electrodes;
(c} adjusting he voltages within a first selected range of voltages to heat the material substantially uniformly throughout the region to a temperature sufficient to promote activity of microbial organisms of feeding on volatile, semi--volatile, and non-volatile contaminants from the region of material; and (d) controlling the voltages for maintaining the temperature of the material for a time sufficient for the microbial organisms to substantially remove the volatile, semi-volatile, and non-volatile contaminants from the region of material.' Various' F aspects of the invention are more fully described by: reference~::ao.~.the accompanying figures.
Specifically, FIG. a illustrates an electrode arrangement and voltage configuration suitable for treating solid earthen material in accordance with the present invention.
Six electrodes (20-25) are inserted into a solid earthen material 30 to be treated. Electrodes 20-25 may be formed of aluminum, carbon steel, or anyather type of conductive material. Electrodes 20-25 are preferably cylindrical and can be hollow'or solid.
Six phases of ac voltages are applied to corresponding electrodes 20-25. Preferably, each electrode has a voltage phase which is 50° out of phase with the voltages of adjacent electrodes. For instance, electrode 20 has a voltage phase 60° apart from the voltage phase applied to electrode 21. To further illustrate the different phases of voltages applied to electrodes 20-25, exemplary wave forms of different phases are shown adjacent respective electrodes 20-25. Electrode 20 has a voltage Vsin(.omega.t); electrode 21 has a voltage Vsin(.omega.t+60); electrode 22 has a voltage Vsin(.omega.t+120); electrode 23 has a voltage Vsin(.omega.t+180); electrode 24 has a voltage Vsin(.omega.t+240); and electrode 25 has a voltage Vsin(.omega.t+300).

Electrodes 20-25 are preferably arranged in a geometric configuration in which pairs of electrodes are diametrically opposite one another. Electrodes 20, 21, and 22 are diametrically opposite electrodes 23, 24, and 25, respectively. Electrodes 20-25 are most preferably positioned at vertices of a substantially equilateral hexagon as shown.

The voltages applied to diametrically opposing pairs of electrodes preferably have voltage amplitudes which are substantially equal, as indicated by the voltage amplitude V. Additionally, the voltages applied to diametrically opposing electrodes are preferably approximately 180° out of phase with respect to one another. For example, the voltage applied to electrode 20 is 180° out of phase with the voltage applied to electrode 23. Similarly, the voltage applied to electrode 21 is approximately 180°
out of phase with the voltage applied to electrode 24.

The six phases of voltages applied to electrodes 20-25 may be applied at any frequency. The voltages are preferably applied at power frequencies less than 500 Hz, and most preferably at a frequency of approximately 60 Hz.
In this manner, power voltages carried by standard transmission lines may be used to drive the six phases of voltages applied to electrodes 20-25. Using a 60 Hz, six-phase voltage provides a significant advantage in that the current supplied to electrodes 20-25 is in phase with the driving voltages from power lines. This results in a significant reduction in transformer., losses.

Another ,aspect of the present invention includes positioning an electrically neutral electrode 34 centrally in a region 32 of solid earthen material 30. Region 32 is internal to a perimeter defined by electrodes 20-25.

Neutral electrode 34' is preferably positioned in a substantially diametric center of the hexagon formed by electrodes 20-25. When the multiple phases of voltages (preferably six phases) are applied to electrodes 20-25, current paths between peripheral electrodes 20-25 and neutral electrode 34 are created, as represented by current paths 36, 38, and 40. Additionally, current paths are created between-pairs of peripheral electrodes 20-25, as represented'by current path 42 between a pair of elec-trodes, 21 and 22, and current path 44, between a pait of peripheral electrodes 22 and 23.

The current paths created between pairs of '20~ peripheral' electrodes 20-25 (i.e., current paths 42 and 44) and between electrodes 20~~5, and' neutral electrode 34 (i.e:, current paths 36, 38; and 40) pass through and substantially uniformly heat region 32 of solid earthen material 30: : Region 32 is preferably heated , to a temperature below a melting temgerature of solid earthen material 30. Most :preferably, region 32:is heated to a temperature ranging from approximately 0C to 100C. When used to enhance biodegradation by action of microbial organisms, region 32 is heated to a temperature ranging from about 20C to about 40C and preferably about 30C.

With the hexagonal arrangement, multiple phases of voltages can be-applied to peripheral electrodes 20-25 in a manner effective to produce a substantially constant voltage-to-distance ratio for all current paths between peripheral electrodes 20-25 (i.e:, current paths 42 and 44) ;.. ~-: - .,, - . ~ :;. :: ~,. ._. °.:: ,~..-, _. .... , ,,~ ,:,. -~.,:. , ., . ....: . . . ,. .., . . .. .. . _ . .,....

a." .. , ... . . .. , !'VO 93/09888 PCT/US92/097fiA

and between peripheral electrodes 20-25 and neutral electrode 34 (i.e.p current paths 36, 38, and 40). The .

voltage-to-distance ratio between any given pair of electrodes (for example, current path 44 between a pair of peripheral electrodes 22 and 23, or current path 38 between a pair of peripheral electrodes 21 and neutral electrode 34) is computed by dividing the voltage measured between the given pair of electrodes and the distance by the given pair of electrodes.

FIG. 2 shows an alternative current path formation created between pairs of electrodes 20-25 when neutral electrode 34 is removed from region 32. Fifteen current paths 1--15 are formed through region 32 when six phases of voltages are applied to electrodes 20-25. Preferably, a current path is formed between each electrode .and every other electrode in the hexagon. Current paths 1-15 are substantially evenly distributed throughout region 32, resulting in a substantially uniform heating of region 32.

The number of current paths created between pairs of electrodes depends: upon the number of phases of voltages applied to the electrodes. The following equation defines a minimumnumber of current paths between pairs of electrodes:

~. .,, ~ No. of > ~urr~nt paths. _ [Qn(Qn 1) ~ /2 25' where Qn equals the total number of applied phases of voltages. In the preferred embodiment, six phases of voltages are applied to electrodes 20-25. Using the above equation, applying six phases of voltages results in a minimum number of fifteen current paths (-i . e. , [6(6-1)]/2 = 15).

Arranging six electrodes as vertices of a substan-tially equilateral hexagon and app2ying six phases of voltages to the corresponding electrodes has several significant advantages. First,' current paths created laetween electrodes 20-25 substantially uniformly heat dvo 93io~sss Pcr~~.Ts9zso~~sa ,,. _ ~1~3~1~

region 32 during the treatment of solid earthen material 30. Uniform heating is achieved without the addition of _ foreign conductive medium, such as salt water. Uniform heating assists in complete removal of" volatile and semi-volatile contaminants contained in solid earthen material 30.
Another significant advantage of the present inven-tion is that relatively low voltages may be applied to the earthen material to effectuate appropriate heating. Due'to the hexagonal arrangement, many of the current paths are in parallel: In FIG: 2, parallel current paths include paths 7 and 11; paths 8 (and Z3 ; paths l0 and 12 ; paths 1, 4 , and 9; paths 3, 6, and 14; and paths 2, 5, and 15. As a result, the resistances of the paths are also in parallel.
Parallel resistances a~educe the total resistance of earthen material within region 3~. Because total resistance is decreased, the voltages applied to electrodes 20-25 may be reduced. This results in a significant reduction of equipment and power costs for treating solid earthen 20' materia3 30.
Another advantage of the above described aspect of the: invention is that the current supplied to the elec-trodes is in phase with.the power supplied to~_the,treatment site by utility companies. Unity power factar (wherein power factor. is the cosine of the phase angle between voltage and currentj. is therefore achieved. Unity power factor is the most 'efficient use of power.
FIG. 3 diagrammatically illustrates a system 45 for treating earthen material in accordance with the present invention. Electrodes 20-25 are inserted into region 32 of solzd earthen material 30 to be treated in an arrangement described above with reference to FIG. 1. ~nly four peripheral electrodes 20, 21, 22 and 23, and neutral electrode 34; are shown for purposes of clarity.
Peripheral electrodes 20-23 and neutral electrode 34 are W~ 93/09888 PCT/LJS92/09764 inserted into solid earthen material 30 in a substantially parallel relation. Electrodes 21-23 and 34 are inserted to a depth sufficient to ensure that most contaminants 46 lie within region 32 and above distal ends of electrodes 20-25, as represented by distal ends 50, 52, and 54.
System 45 includes a six-phase ac generator 56 and an off-gas treatment facility 62. Six-phase ac generator 56 is coupled to peripheral electrodes 20-25. Six conductors 58 electrically connect generator 56 to respective peripheral electrodes 20-25. A different phase of voltage is applied- to individual conductors 58. A
voltage having phase ~1 is applied to electrode 20. A
voltage having phase '~2 is applied to electrode 21.
Similarly, v~ltages having phases c~3-ø6 are applied to corresponding electrodes 22-25. As discussed above, phases ø~1-~6 are preferably 60° apart from each other.
Off-gas treatment facility 62 is connected in fluid c,o~unication to neutral electrode 34 through a conduit 60.
Neutral electrode 34 is preferably formed with a hollow passage 64 extending axially' therethrough. Passage 64 communicates with solid earthen material 30 and a location external to solid earthen: material 30 at end 66 of neutral electrode 34. Neutral elects~de 34 can be perforated to ~faci~.itate ' f luid communication between passage 64 , and solid earthen' material 30. As the voltages are applied to electrodes 20-25 t4' heat region 32, volatile and semi-volatile contaminants are removed from solid earthen material 30 via passage 64. The contaminants are pulled through conduit 60 0 off-gas treatment facility 62.
Preferably, off-gas treatment facility 62 has a vacuum to draw the contaminants from solid ~ earthen material 30 through neutral electrode 34 and Conduit 60. An advantage to ~mpioying an electrically neutral electrode 34 is that conduit 60 to' off-gas treatment facility 62 may be connected to electrode 34 without any threat of being i~0 93/d?9888 PCT/US92/09764 -~14 -electrically shocked. Off-°gas treatment facility 62 chemically treats the volatile and semi-volatile contain- -inants to render the contaminants innocuous. Alternately, the gases can b~ processed for commercial purposes.
FIG. 30 shows an alternative embodiment for venting or removing volatile and semi-volatile contaminants from solid earthen material 30 during the heating process. An off--gas hood 68 is positioned over and encircling electrodes 20-25. Off-gas hood 68 contacts a surface ?0 of solid earthen material 30 to minimize the escape of contain-inants into the atmosphere. Off-gas hood 68 collects gases during the heating process and transfers the gases to an off--gas treatment facility, . Again, the off-gas treatment facility advantageously' has a vacuum system to facilitate I5 removal of the gases. In this embodiment, a neutral electrode vent is not ut~:lized and is thus omitted from the figure.
FIGS. 4 and 5 demonstrate alternative electrode arrangements employing' more than six electrodes in actor dance with an' aspect of the present invention. FIG. 4 shows twelve peripheral electrodes, ?0-8I, are arranged in a star-like shaped polygon. Preferably, the polygon defined by electrodes 70-81 is substantially, equilateral, 'ras is shown.- FIG. S~shows twelve electrodes 84-95-arranged at vertices of an equilateral polygon having twelve sides.
The vertices also lie yin a circumference of a circle.
Other numbers of elects~des may Iae employed instead of six or twelve electrodes. Additionally, the present invention is described as using six-phase ac voltages, although other multiple phases of voltages may be employed with the six or more electrode arrangement.
FIGS. 6-9 diagrammatically demonstrate a method for treating solid earthen material according to an aspect of the present invention. Solid earthen material 30 has volatile, semi-volatile, and non-volatile contaminants !~~ 93/09888 PGT/US92lfl9764 provided therein. The non-volatile contaminants are referenced generally by numeral 100. Multiple electrodes are inserted into solid earthen material 30. Preferably, six peripheral electrodes and one neutral_.__electrode are positioned in an arrangement shown in FIG. 1, but only electrodes 20 and 23 are shown for explanatory purposes.

Multiple phases of voltages are applied to the electrodes, and preferably in a manner described above with reference to FIGS. 1 and 2. Neutral electrode 34 is connected to an off-gas treatment facility as explained above with reference to ~'TG. 2.

Multiple phases of voltages are applied to peripheral electrodes 20-25 to begin heating earthen material 30 (FIG. 6)The appropriate voltage level depends upon many factors inclu3ing the mineral type and moisture content of earthen material 30, the diameter of electrodes 20-25, the distance between electrodes 20-25, and -the length of the electrodes. Voltages increase with increasing distance between electrodes 20-25.

Additionally, voltages decrease with increasing water 'content, elects~de diameter, and electrode length. The voltages are adjusted within a first range of voltages to heat solid-earthen material 30 to a temperature sufficient ~'to - satbstantially remove volatile and . : semi-volatile contaminants from the material. A typical starting voltage for a field-scale operation .is 'approximately 1000 to 2004 V(ac) : Typical power, input to earthen material 30 is approximately 1000 W/m3:

An advantage to the six-electrode arrangement. and application of multigle phases of voltages is that no foreign'medium between electrodes is necessary to maintain conduction during all phases of drying. In many prior art methods, conductive material, such as salt water (brine) or NaOH, must be present in the region to maintain conduction during drying. However, salt water and NaOH are also con-ewo 9~ro~sss pcrrus92ro9~sa ., .. . - 16 -taminants. The present invention eliminates the necessity of adding a foreign contaminant simply to maintain conduction.

Solid -earthen material 30 is heated.,._at an approxi-mutely constant rate, such as 1C per hour. As the earthen material 30 heats, it becomes more conductive. The applied voltages are therefore lowered to maintain a constant rate of heating. Eventually, the voltages are reduced to approximately half of the starting value.

When a target temperature is reached, as for bio-remediation, voltages are varied as needed to maintain a steady state temperature of the earthen material 30.

When it is desired to remediate the earthen material by drying and electrical discharge, heating ' continues until the moisture contained in the material 30 , begins to boil: Boiling first occurs around peripheral electrodes 20-25. Next, the moisture inward of the peripheral electrodes near' neutral electrode 34 begins to boil. Additionally; some moisture-outside of the perimeter 20. defined by peripheral electrodes 20-25 will begin to boil.

Moisture near surface 70 of earthen material 30 begins to boil before moisture near the distal ends of electrodes 20--25 farther beneath the surface. : The moisture is last to :boil~-hear=- the- bottom of electrodes 20-25 because the earthen- material below the distal ends of electrodes 20-25 acts as a heat sink to remove heat'from the bottom portion of rEgxon 32.

The boiling moisture or- water forms steam which effectively strips volatile and semi-volatile organic compounds from earthen material- 30. Some non-volatile organic contaminants will a3so be removed with the steam. .

The steam is removed through passage 64 provided in neutral electrode 34 -and transferred to an aff-gas treatment facil-ity: Alternatively; the escaping steam can be captured by off-gas hood 68 as shown in FIG. ~Ø

..... . . ;.. ,. -.: -, .: :_ -. :. , .~ . .. .., _ ,: :. .: : _ -: . . , , .,; _ ; . ."... - - - : .. , .., WO 93/09888 Pf'T/LJS92/09764

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As water and steam are removed from region 32, earthen material 30 begins to .dry. The temperature of region 32 during thus drying process is less than the melting temperature of solid earthen material 30. Earthen material 30 is not vitrified. Preferably, the temperature of region 32 is approximately 0°C to 100°C.
The resistivity of solid earthen material 30 depends largely upon the moisture content thereof. As region 32 dries, resistivity increases. Therefore, the applied voltages are increased at some point to maintain an approximately constant heating rate. Yet, the power applied to earthen material 30 is preferably maintained at approximately he same level throughout the drying process.
When the moisture content falls below 10 wt% (and, more specifically, between 4 and 7 wt%), voltages applied to the peripheral electrodes must be increased to a second range of voltages. The second range of voltages is greater than the first selected-range of voltages. Preferably, the second range of voltages is approximately 2000 to ' 6000 V (ac) .
Dry regions 102-10? (FIGS. 7 and 9): begin to form 'around respective electrodes 20-25 asregion 32 dries and the applied~voltages are increased. Dr~:regions 102-107 shave: perimeters .which define.respectiVe boundaries 110-115 between dry regions 102107, respectively, ,and earthen materiel exterior to 'the dry regions: Boundaries 110-115 are wet-dry interfaces between dry regions .102-107 and "wet" regions exterior to the dry regions. Boundaries 110-115 are very narrow,'and have a thickness of approx imately less than one inch.
When the voltages are increased to the second ranges of voltages, a corona discharge is created at boundaries 110-11.5. A corona discharge provides an intense oxidizing environment which produces electrons, molecular ions, radicals, ion radicals, ozone, peroxides, and WO 93/~9~~8 PCT/US92/09764 ~123~10 ultraviolet light. Corona can be used to oxidize many organic materials, such as town gas (a complex mixture consisting of benzene, toluene,. benzo-A-pyrene (BAP), xylene, and naphthalene), trichloroethylene, and carbon tetrachloride. Corona also oxidizes metal such as lead, gold, zinc, arsenic, chromium, uranium, plutonium, and cadmium: Additi~nally; corona can oxidize radioactive waste such as radioactive salts including radioactive nitrate.

Earthen .material 30 is very dry at this time and acts essentially like a dielectric. This dielectric enables corona to be sustained. The corona causes the formation of a highly chemically reactive plasma. The corona or plasma is at an energy level sufficient to '15 chemically alter the non-volatile contaminants remaining in region 32 of solid earthen material 30.

The efficiency of the plasma reaction depends upon he electron energy released by the corona. The electrons released by in situ Corona are a magnitude more energetic ~0 than those released by convention conductive (metal) :electrodes, This is because 'the an situ corona occurs on nonconductive (dielectric) earthen particles, which require much higher field gradients to cause gas to emit electrons.

Field emissions on :dielectric (i.e:;vnonconductive) earthen 25- particles are called auto-electronic emissions, different than what is commonly meant by 'corona".

Conventional "corona' formed on metal electrodes in .

less than 20 eV over a very small ambient air has an energy plasma volume, such as a few micron thick sheath. Contami-30 pants passing through the sheath interact directly with the corona (requires about 5 eV). Additionally, wet air pass-ing through the sheath forms oxidizing radicals that scavenge and react with some of the rest of the contaminant that-does not pass through the sheath. Approximately 10 eV

35 is required to produce one OH radical, and approximately WO 93/09888 PCTlUS92109764 ..., , six OH radicals are required to decompose typical organic contaminants. With these yields, good destruction effi-ciency only occurs on a very small~scale. The conversion of contaminants ,to innocuous CO/C02 can be ,a~. low as a few percent.

Auto-electron emissions on dielectric particles (such as earthen particles) emit electrons at energies of several eV: As a result, a very energetic plasma with good oxidant yields is formed (for example, 20-30 OH radicals per electron). Further, more oxidants per coulomb of electricity are produced:' The excess of oxidants causes a nearly 100% mineralization of contaminants. Direct destruction of contaminants by electron bombardment is also 'enhanced, and the plasma volume can directly contact contaminants in a much -larger (100 to 1000 micron) sheath.

In solid earthen material, the ionized sheath extends farther than inter-particle dimensions:

The formation of dry regions 102-107 and the forma-tion of corona at boundaries 110-115 occurs approximately '20 simultaneously. Dry regions 102-107 begin at respective :electrodes 20=25 and then move radially outward relative to individual electrodes as the voltages are increased. The 'corona~',is car~iedv.by wet-dry. interfaces or boundaries i10-115-: =Material exterior to dry regions 102-107, still contains some moisture content which conducts electricity through region 3 2, which is still exterior to dry regions '102-107: For example; the moisture content within dry ;( :regions 102-107 may be 0.5 wt%, and the moisture content in wet" regions exterior- to the dry regions' may be 4-7 wt%.

Therefore, heating ,and drying continues- throughout region 32.

Dry regions- 102-107 expand radially outward from respective electrodes 20-25 at a relatively slow rate. The slow movement is caused by the moisture content gradient across boundaries 110-115. The relatively higher moisture W~ 93/09888 PCT/L'S92/09764 ~~~3410 content outside dry regions 102-107 impedes expansion of the dry regions: The slow expansion is very advantageous. .

The slow growth of dry. regions 102.-107 enables the corona boundaries 110-115 to move slowly through non-volatile contaminants and sorbed or otherwise bounded contaminants.

As a result, the corona has sufficient time to decompose the contaminants.

Voltages are increased slowly beyond the second range of voltages to a third range of voltages. The third range of voltages can be extremely high, up to 100 kV . As the voltages are increased slowly through the third range of voltages; dry regions 102-107 continue to move radially outward from respective electrodes 20-25. The total power applied to earthen material 30 begins to drop, but the dry regions expand and the corona continues to spread.

Corona boundaries 1i0-115 encounter non-volatile 'contaminants and sorbed or otherwise bounded contaminants, referenced generally as 100, as the boundaries move through region 32 (FIG. 7). The corona produces oxidants and reductants that scavenge and react with the remaining contaminants 100. The reaction causes a decompositian of the non-volatilized contaminants 100. Contaminants 100 are often decomposed into volatile .fragments which may then be vremoved through passage 64 of:.- neutral -electrode 34 and 25' :treated'in the off-gas treatment facility.

Eventually, corona boundaries 110-7;15 pass substan-tially through region 32 (FIG: 8). As a result, substan-tially-all contaminants within region 32 a~e removed and/or :rendered innocuous. Water, humid air, or steam may then be added ~o earthen material 30 to restore the earthen material to its'natural state: -An advantage of the present invention is that .

region 32 is substantially uniformly heated due to the arrangement of peripheral electrodes 20-25 and the application of six phases of voltages. The uniform heating .. ...:.:.
,.. ..:;~...
.: : : :.;-s. .. .
:j .: ~~-:
,... . ,:~:;
, , .. .
, . .
r,:.,.;

~" , . , . .. "., . ., .., .. . ..
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, . .
,. ,... .,.....
. . l ..,, ; -. .::..
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.., r ..
.. . ..
, .. . ,.
_,. , WO 93/09888 PCTlUS92109764 is at a temperature which is less than a melting temperature of solid earthen material 30. Solid earthen material 30 is not vitrified during the process. As a result, substantially lower voltages may be_.used to treat the contaminated. material. Additionally, the voltages required to produce the effective heating are further reduced due to the hexagonal arrangement of electrodes 20-25 which: permits the formation of parallel resistances.

The' reduction in voltages results in a significant reduction in equipment: and process costs. Further, a reduction in voltages impraves safety.

The drying process removes most volatile and semi-volatile contaminants. The present invention includes formation of a corona' front which decomposes any non-' volatilized contaminants which remain after the dr~ing ;process. , FIG'. 11 is a diagrammatical illustration showing a system 300 for -treating earthen material: in accordance with 'another' aspect of the present invention. Six electrodes 20. :are inserted into~region 32 of solid earthen material 30 to be treated, preferably in a hexagonal arrangement described above With reference o FIG: 1. Only four peripheral electrodes -(301',.",302,'- 303, and 304, are shown in this view.

-Neutral' electrode r:'306 is positioned at , an approximately : diametric center. Peripheral electrodes 301304 and neutral electrode 306 are inserted into solid earthen ' material 30 in a substantially parallel relation.

Electrodes 301-304' and, 306 are inserted to a depth sufficient to ensure that most contaminants' 46 lie within regiori 32 and above distal ends of electrodes 301-304 and 306, as represented by distal ends 308, 310, and 312.

Preferably, peripheral electrodes 301-304 and neutral electrode 306. are substantially cylindrical:

Peripheral electrodes 301-304 and neutral electrode 306 each have a pa sage extending axially therethrough ____.._. : . ___ .. . . . ..... . . ., a.. ,., ,. ~..... ~ -~-.. ~,-. ~,.... . _ . ~,.. ._.. ... .. w~. , . .. .. ~ _ WO 93f09888 PCTfU592f09764 which communicates with solid earthen material 30 and a location exterior to solid earthen material 30. For example, peripheral electrodes 301 ,and 304 have respective passages 314 and 3200 Neutral electrode 306 has a passage 324. Peripheral electrodes 301-304 and neutral electrode 306 are perforated to permit fluid communication between earthen material 30 and the passages within the electrodes.
Peripheral electrodes 301 and 304 are perforated with multiple through-holes or apertures 318 and 322 to permit fluid communication between solid earthen material 30 and respective passages 314 and 320. Similarly, neutral electrode 306 is perforated with multiple apertures 326.
System 300 includes a six-phase ac generator 330 coupled to peripheral electrodes 301-304. Six conductors 33Z electrically connect generator 330 to respective per~:pheral electrodes 301-304 (and the two electrodes not shown) . ' A different phase of ~roltage is applied to individual conductors 332 in a manner described above with 'reference 'to FIG-. 3. the voltages are sufficient to effec-tuate drying and'the creation c~f corona.
An off-gas treatment facility 334 is connected in fluid communication to neutral electrode 306 through a conduit 336: Off-gas treatment facility 334 is preferably ~Y -:equipped with a"vacuum apparatus to'draw contaminated gases 25. from solid earthen material 30 through neutral electrode 306 and conduit~336.' System 300 has a.pump unit 340 coupled in fluid communication o peripheral' electrodes 301-304 through conduit 342. Conduit 342 is connected to peripheral electrodes 301-304 (and the two electrodes not shown).
Conduit 342 may be a singla conduit with blanches going to individual peripheral electrodes, or may be six distinct conduits coupled to corresponding ones of the peripheral electrodes.

if~ 93/09~~~ PCTI ~.~5921097b4 ~,,, . .

A gas supply 344 is coupled in fluid communication to pump unit 340 through conduit 346. Gas supply 344 stores and supplies gases such as ,air, oxygen, hydrogen, and free electron gases (such as noble gases, and molecular nitrogen).
System 300 is different from the embodiment depicted in FTG. 3 in that gases may be injected into solid, earthen material 30, to help control the reaction occurring in region 32. 1~ desired gas is supplied by gas supply X44 through conduit 346 into pump unit 340. Pump unit 340 pumps the gas through conduit 342 into peripheral elec-trodes 301-304. The gas flows through the passages (i.a., 314 and 320) ~f peripheral electrodes and is forced out through the apertures (i.e., 318 and 322), as shown diagrammatically wit: arrows. The gases are then pulled through- region 32 to neutral electrode 306. The vacuum grovided in off-gas treatment facility 334 provides ufficient suction to pull the gases through region 32.
The gases are drawn through aperture 326 into passage 324 of neutral electrode 306, as shown with arrows. The gases are then removed through conduit 336 to off-gas treatment facility 334.
'Any number of gases may be injected into solid 'earthen material ~30._: ~v oxygen may be injected into region 32.
~5 o increase the oxygen content of the region. The excess oxygen helps optimise the rate of reaction as the corona boundary sweeps through region 32: Hydrogen may be added to' facilitate chemical'reduction: Chemical reduction is often desired to decompose highly chlorinated contaminants, such as carbon tetrachloride (CCl~):
Free-electron gases (such as noble gases and molec-ular nitrogen) may be injected intfl region 32 to help reduce .the voltages required to sustain corona discharge.
Free-electron gases displace free atoms of oxygen, which are electron attaching. Without the addition of free-WO 93!09888 PCTlL'S92/09764 ?....
2~1~3~~0 .

electron gases, a higher voltage is required to reduce a stronger field to overcome the tendency of electrons to attach to oxygen. Injecting free-electron gases into region 32 displaces some of the free atoms of oxygen, allowing for a reduction in field strength without d~.minishing corona: As a result, voltages can be reduced.

FIG. 11 also diagrammatically illustrates an alternative aspect of the present invention. System 300 may be'adapted with a vacuum unit 350 provided in a recyc~.e loop consisting of conduits 352 and 354. Conduit 352 would be'connected in fluid communication with passage 324 of 'neutral electrode 306: Vacuum unit 350 would provide sufficient suction .to remove contaminated gases from region 32. The contaminated -gases would then be returned to region 32 through conduit 354, pump unit 340, conduit 342, and peripheral electrodes 301-304. In this embodiment, off-gas.areatment facility 334 is not utilized.

According to this aspect of the present invention, the contaminated gases are recycled through the dry regions and through the corona boundaries discussed above with reference to FIGS. 6-9. The contaminated gases are decom-posed when 'passed through the corona discharge. Accord-ingly,'the off-gas produced while treating earthen material 'v30~ s effectively treated by recycling the, off gas back v through'region 32 of earthen material 30.

FIG: l2.is'a diagrammatic representation of an off-gas treatment apparatus 400 in accordance With another aspect of the present invention. Off-gas treatment appa-ratus 400 chemically treats contaminated gases( or. the like. For example,; off-gas treatment apparatus 400 may be used to treat gases shch as NOx, or gases produced during in situ vitrification processes, soil vapor extraction operations, or other process exhausts:

Apparatus 400 is preferably dimensioned at a scale to be portable. In this manner, apparatus 400 may be CVO 93109888 PCT/L'S92/09764 ~~.1v3~~.0 transported to vitrification or extraction sites, or other locatibn~ in need of off-gas treatment.
Off-gas treatment apparatus 400 has a container 402 which is preferably formed of a durable p~l~stic or other insulative material: Container 402 has a floor 412 and walls 403, and may be cylindrically shaped or have multiple substantially flat. walis. Container 402 is (filled with material 404, such as solid earthen material. Preferably, material 404 is sand.
Six peripheral electrodes are inserted into material 404. The peripheral electrodes are preferably arxanged at vertices of a substantially equilateral hexagon as shown in FIG: 1. Only four peripheral electrodes (406, 407, 408, and, 409) are shown in this illustration.
'Peripheral electrodes 406-409 are positioned adjacent to and spaced from wall 403 of container 402. A neutral electrode 4I0 is inserted into a center region of container 402. Peripheral electrodes 406-409 and neutral electrode 410 are inserted to a;depth such that distal ends of the electrodes do not contact floor 412 of container 402.
Electrodes' 406-409 and neutral electrode 410 are substantially hollow and thereby deffine passages axially therethrough. Electrodes 406, 409, and 4l0 have respective ''passages 414, 416, and 418.v Peripheral electrodes 406-409 ~5 and 'neutxal electrode 410 have through--holes or apertures formed therein topermit fluid communication between passages (414, 416, 418) and material 404.
Off-gas treatment apparatus 400 has a six-phase ac generator 420 coupled to peripheral electrodes 406-409.
Six-phase ac generator 420 generates six phases of voltages and applies the voltages via conductors 422 to correspond-ing electrodes 406-409 (and the two electrodes not shown).
The voltages applied to the peripheral electrodes are sufficient to create and sustain corona within material ~~23410 404. The formation of corona is discussed above with reference to FIGS. 6-9.
Apparatus 4OO also has a ,fluid-removing unit to remove fluid from the material 404. The."fluid-removing unit preferablylincludes a vacuum unit 424 coupled in fluid communication :via conduit 426 to neutral electrode 410.
Vacuum unit 424 applies sufficient suction to draw gases through material 404 into passage 416 of neutral electrode 410: Alternatively, the fluid-removing unit may comgrise' a vacuum attached to an off-gas hood positioned above material 404 in container 402, as described above with ref erence to FIG . 10 ':
A recycle unit 430 is connected in fluid communi cation with vacuum unit 424 through conduit 432. The recycle unit is also connected ir. fluid communication with peripheral electrodes 406-409, through conduits 434, 435, 436,'and 437, respectively. Recycle unit 430 receives the gas to be treated via conduit 440. An outflow conduit 442 is provided to exhaust gas which' has been treated and rendered innocuous.
Recycle unit 430 can include a pump for pumping gases through conduits 434-437 to peripheral electrodes 406-409. Recycie unit 430 may further a.nclude a heater to .heat . the = c~as - at a temperature suf f icient to :. maintain a vapor state- The gas is at a vaporized ~empera~ture when xemoved 'from material:404: It may be, desirable to maintain the 'gases in the vapor form and thus heating is required.
Alternately, it may be desirable to condense the vapors.
If condensation is desiced, recycle unit 430 may be equipped with a heat exchanger to cool the extracted vapors 'and condense the vapors. ' In operation, the gas to be treated f lows through conduit 440 to recycle. unit 430. Recycle unit 430 directs the gases to be treated through conduits 4~4-437 to respec-35' tine peripheral electrodes 406-409. The gas flows through i~0 93/0988 P~C TIUS92/09764 S ~,.w ,i _ . _ 27 _ respective passages in the peripheral electrodes and out the apertures (as shown by arrows). The gases are drawn through material 404 from peripheral electrodes 406-409 to neutral electrode 410 via the suction provided by vacuum unit 424: The gases enter passage 416 of neutral electrode 430 and are removed. through conduits 426 and 432 into recycle unit 430. The gases at this time may be very hot (such as 100°G):
If these gases are permitted to cool, the gases may condense to leave the dapor state. Recycle unit 430 may heat the gases at a temperature sufficient to maintain a vapor state if desired. The recycle unit 430 forces the gases back trirough conduits 434-437 to peripheral elec trodes 406-409 to continue the recycling and treating process:
The gases are' chemically treated as they move between peripheral electrodes 406-409 and neutral electrode 410: 'This is because six-phase ac generator 420 supplies 'six phases of voltages to peripheral electrodes 406-409 (and the two electrodes not shown) effective t~ produce a corona discharge within material 404. The contaminated gases are chemically altered or decomposed as they pass through the corona'discharge. Acc~rdingly, recycling the ~.:_ -:.gases through apparatus--400 decomposes the contaminants and renders them innocuous. When the'gas~s are efficiently 'decomposed, recycle unit 430 vents these gases through conduit 442.
In accordance, with an aspect of the present. inven tion, off-gas reatment apparatus 400 may be equipped. with wet air or steam supply '452. ~ Steam supply 452 stores arid supplies wet air or steam to recycle unit 430 via conduit 454: The steam can then be injected into material 404 through peripheral electrodes 406-409. Steam may be added 'to material 404 to effectuate an approximately constant. dry region and an approximately stationary corona boundary.

~ 12 3 410 F~-T/~-'S92/097~4 ' - as -That is, corona will remain relatively close to peripheral electrodes 406-409 and not move radially outward through material 404 as described above with reference to FIGS. 6-9. A moving corona front is notlas important in this embodiment because non-volatilized contaminants are buried in material 4~4. A stationary corona boundary is therefore sufficient to decompose the gases being passed therethrough.
Steam may- also be supplied, if desired, by g'as sugply 344 in system 300 shown in FIG. 11.
The embodiments described above with respect to FIGS. 11 and 12 were described as having all peripheral electrodes formed with passages. Alternatively, only one or two peripheral electrodes may be provided with passages and conduits for recycling gases back through the region of material. Additionally, another independent electrode may be added to in~ect,gases into the regi~n to be treated. In this manner, gases would be pumped only to this additional electrode and not to'the peripheral electrodes.
The conduits described in the present invention may be formed of any material suitable for passing gases. For example, the conduits may be formed of rubber hoses, metal pipe, or any other tube means: . ..
According ~:o another aspect- of the-. v present 'invention, a -method for reating s~lid earthen material having volatile, semi-volatile, and non-volatile contaminants comprises the_steps of:
(a) inserting, multiple electrodes, into solid earthen material, the electrodes defining a region of material to be treated;
(b) applying multiple phases of voltages to corre- -sponding ones of the electrodes;
(c) adjusting the voltages within a first selected range of voltages to heat the material to a temperature ~V~ 93/09888 PCT/US92/09?fi4 .",, .' 29 _ sufficient to substantially remove volatile and semi volatile contaminants from the region of material;
(d) creating dry regions of material around individual electrodes as the material is ,heated, the dry regions having a periphery which defines a boundary between the dry regions of material and earthen material exterior to the dry regions;
(e) increasing the voltage through a second selected range of voltages e~fective to form. vitrescent earthen material fragments within the dry regions of material;
(f} moving the boundary of the dry regions radially outward from he individual electrodes through the region of material; and ~5 (g} decomposing the non-volatile contaminants as the boundary passes over the non-volatile contaminants.
6Ohhen the uoltages are increased through the second rangy df vo3tages (i.e., 2000 to 6000 V(ac}), formation of vitrescent fragments can be formeda The fragments are partially or completely melted, vitrified, sintered, or ~taporized, and recQndensed to form a highly branched or dendratic mineral'structure (similar to, but more highly -branched than, fulgur~.tes). The fragments have a dendritic appearance and somet~.mes have a higher density thin the density ~f the surrounding earthen material. The fragments arm often hollow or contain bubbles. The dendritic frag~
meets can be retrievable intact from the earthen material 30:
According to another aspect of the present invention, a method for producing vitrescent soil fragments in a soil region comprises the steps ofa (a) placing two conductive elements into a soil region, the conductive elements being spaced apart a selec-ted distance;

CVO 93/098~~ PCTlUS92/09764 ~~234~.0 . ~v (b) applying a voltage across the conductive ele-ments, the voltage being sufficient to form vitrescent soil fragments within the soil region;
(c9 reducing the voltage to a level__ sufficient to solidify the vitrescent soil fragments; and (d) collecting the vitrescent soil fragments after the vitrescent soil fragments have solidified.
SIG. 13 illustrates a method for producing vitrescent soil 'fragments in accordance with an aspect of the present invention. At least two conductive elements or electrodes 202 'and 204 are placed in a soil region 200.
Soil 200 is preferably very dry, such as dune sand. The electrodes 202 and 204 are spaced apart a distance °'d" . A
voltage source 206 is coupled to apply a 'voltage across the electrodes 202 and 204. The voltage applied is sufficient o form vitrescent soil fragments within soil region 200.
After formation of the soil fragments, the voltage applied to electrodes 202 and 2~4 is reduced to a level sufficient to result in solidification of the'~oil fragments. The resulting soil fragments can have a density higher than the density of the soil 200. The soil fragments may then be 'collected after the fragments have solidified.
:The voltage . applied. to electrodes ~ . 202 and 204 varies.;considerably. Remarkably; he voltage appears o be.
independent of the distance "d" between electrodes 202 and 204, and the mineral and moisture content of soil 200.
When the electrodes are ;spaced a distance of 6 to 18 inches, voltages ranging, from 1000 volts 'to 30,000 volts may be applied t~ produce the soil fragments:
~,toltage source 206 may be a do or ac power supply.
Voltage source 206 may be a pulsed paw~r supply which outputs pulses of voltages. These pulses electrically jolt the soil 200 which can facilitate formation of the soil fragments:

WO 93/09888 PCT/L~S92/09764 More than two electrodes may be used to form the vitrescent soil fragments. In accordance with another aspect of the present invention, six electrodes may be inserted into soil 200 in an arrangement shown in FIG. 1.
S Six phases of voltages may then be applied to corresponding electrodes at a level effective to form vitrescent soil fragments within soil 200. The voltages may then be reduced so that the fragments can solidify and be collected:
According to another aspect of the present invention, a method for measuring resistivity and moisture content of solid earthen material comprises the steps of:
(a) inserting six electrodes into a region of solid earthen material;
~5 (b) applying six phases of voltages to corresponding ones ~f he electrodes to create current paths between pairs of the electrodes;
(c) adjusting the voltages to a level sufficient to form ,an electric ffield within the region of material without substantially ,altering resi tivity and moisture content of the region;
(d) monitoring'the voltages; .
(a) monitoring the current .:passing between the pairs:of the electrodes; and.
(f) computing resistivity and moisture content of the .material based upon the current and the voltages ia~~r~itored . .
Accurate measurements of moisture content and resistivity of soils or other earth materials are important for many reasons. Farmers are interested in moisture dontent of their soil'. Geologists and geophysicists are interested in the resistivity of soil to help characterize the subterranean geological formations. Characterizing subterranean formations are important for science and for commercial activities such as oil exploration. Accurate 't'V~ 9314988 PGTlLJS92/097f~4 ~~~~4~0 soil resistivity measurements are also important for power engineers when attempting to ground power lines.
Present techniques for measuring resistivity and moisture content include placing two to four electrodes into the soil approximately 50 to 100 feet apart. Voltage is then applied across the electrodes. The current and voltage are monitored to determine moisture content and resistivity. Unfortunately, current does not usually flow directly among the electrodes. Current tends to fldw 1~ towards the mantel of the earth. Accordingly, the current may f low from one electrode toward the earth mantel and then back towards the other electrode. Current also tends to flow along undergraund bodies of water or any other path of leash resistance. The current may therefore flow in a direction tangential to the path directly bet~,reen the elec°
trodes: The longer current path results in an inaccurate resistivity and moisture content measurement.
Another problem with present techniques is that measurements as a function of depth are uncertain. The 0 electrodes are typically inserted to a predetermined depth, such as 8 to 10 feet, 9~or each and every measurement in an attempt o standardize measurements.
An appro~chv°~o solving this problem is to position the electrodes: closer together. However, this procedure has a drawback i:n that only one type of soil may be mea°
stared. Placing~the electrodes farther apart is desirable because current must pass .through many different types of soil. A more accurate measurement can therefore be obtained through this increased distance.
According to the present invention, six electrodes may be inserted into a solid earthen material region in a hexagonal arrangement shown in FIG. 1. Six phases of volt-ages are applied to the corresponding electrodes to create current paths between pairs of electrodes. The voltages applied to the electrodes are very low. The voltages are WO 93/09888 . PCT/US92/09764 ...., ~~~J~~~

sufficient, however, to form an electric field within the region of solid earthen material to be measured. The electric field is at a sufficiently-low energy level which does not substantially alter resistivity__-and moisture content of the region. The voltage and current are then monitored using known amperage and voltage meters. From this, resistivity and moisture content can be computed manually or with specially designed circuitry.
An advantage of employing six electrodes arranged g0 ~,n a substantially equilateral hexagon is that the electric field is substantially constrained within the hexagon. The shape of the electric field is thus readily discernable.
Therefore, accurate measurements of resistivity and moisture content may be obtained as a function of depth.
~.5 Conventional techniques are unable to compute accurately resis~ivity and moisture content as a function of depth.
Middle electrode 34, although preferably neutral, can have a voltage applied thereto. Applying a voltage to electrode 34 would unbalance the uniform heating of region 20 32 within peripheral electrodes 20-25. For example, heat ing may be unbalanced so that a hot region is formed close t~ middle electrode ~ 4 In compliance with the statute, the invention has laeen described in language more or less specifis as to 25 str~xctural and methodical features. It is to be under stood, however,. that the invention is nod linvited to the ~padific features shown and described, since the means herein disclosed comprise preferred forms of putting the intention into effect. The invention is, therefore, 30 claimed in any of its forms or modifications within the proper scope of the appended claims appropriately inter-prated in accordance with the doctrine of equivalents.

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for treating solid earthen material comprising the steps of:
(a) inserting a plurality of electrodes into a region of solid earthen material to be treated; and (b) applying at least six phases of voltages to the ones of electrodes to create current paths between the electrodes and through the region of material.

2. A method according to claim 1, further comprising the steps of:
arranging the plurality of electrodes in a geometric configuration having at least one pair of diametrically opposing electrodes, the pair defining opposing first and second electrodes; and applying first and second phases of voltages to the respective first and second electrodes.

3. A method according to claim 2, wherein the applied first and second phases of voltages have substantially equal voltage amplitudes and are out of phase by approximately 180°
with respect to one another.

4. A method according to any one of claims 1 to 3, wherein resistivity and moisture content of the earthen material are computed based upon monitored currents and voltages.

5. A method according to any one of claims 1 to 4 wherein the six phases of voltages are applied to the first to sixth electrodes arranged in a sequential order in an approximately hexagonal geometric configuration.

6. A method according to any one of claims 1 to 5, wherein the region of material is heated to a temperature below a melting temperature of the solid earthen material.

7. A method according to any one of claims 1 to 6, further comprising the step of inserting at least one electrically neutral elect rode in the region of material to be treated.

8. A method according to any one of claims 1 to 7 wherein the applied voltages are adjusted to a value within a first selected voltage, to heat the material to a temperature for substantially removing volatile and/or semi-volatile contaminants of the solid earthen material in the region of the material to be treated.

9. A method according to claim 8, wherein the first selected voltage is between about 1000 to about 2000 V.

10. A method according to claim 8 or 9 further comprising the subsequent step of creating dry regions of material around individual electrodes as the material is heated.

11. A method according to claim 8, 9 or 10 further comprising the subsequent step of increasing the applied voltages to a value within a second selected range of voltages between about 2000 and about 6000 V.

12. A method according to claim 11, wherein the second selected range of voltages is suitable to create a corona at the boundary between the dry regions of material and the earthen material outside the dry regions.

13. A method according to claim 11 or claim 12, wherein the second selected range of voltages is suitable to form vitrescent earthen material fragments within the dry regions.

14. A method according to claim 12 or 13, further comprising the subsequent step of moving the boundary of the dry regions radially outward from the electrodes through the region of earthen material.

15. A method according to claim 14, further comprising the subsequent step of decomposing non-volatile contaminants of the earthen material as the boundary of the dry regions passes over the non-volatile contaminants.

16. A method according to claim 14 or 15, wherein the step of moving the boundary comprises increasing the voltages to a value within a third selected range of voltages between about 6000 V and about 100 KV.

17. A method according to any one of claims 1 to 16, further comprising the steps of:
controlling the voltage and a current across the electrodes and creating branched paths of current in the soil region, the voltage and current being sufficient to vitrify the region within the branched paths and forming vitrescent soil fragments within the soil region that exhibit a branched or dendritic structure; and reducing the voltage to a level sufficient to solidify the vitrescent soil fragments.

18. A method according to claim 17, further comprising the step of collecting the vitrescent soil fragments after the vitrescent soil fragments have solidified.

19. A method according to any one of claims 1 to 18, wherein voltages are applied to the electrodes at power frequencies.

20. A method according to any one of claims 1 to 19 wherein applying voltages heats the solid earthen material to a temperature below a melting temperature of the solid earthen material.

21. A method according to any one of claims 1 to 20, wherein the plurality of electrodes are inserted at vertices of a substantially equilateral polygon.

22. A method for producing vitrescent soil fragments in a soil region comprising the steps of:
(a) placing at least six conductive elements into a soil region, the conductive elements being spaced apart a selected distance;
(b) applying at least six phases of voltages of about 1000 V to 30000 V, across the conductive elements to form vitrescent soil fragments within the soil region;
(c) reducing the voltages to a level sufficient to solidify the vitrescent soil fragments; and (d) collecting the vitrescent soil fragments after the vitrescent soil fragments have solidified.