HV Discharges to Material Fragmentation

8/9/2019 HV Discharges to Material Fragmentation

1/12

IEEE Transactions onDielectrics and Electrical Insulation

Vol. 7 No. 5 October2000

625

Application of Pulsed HV Discharges to

Material Fragmentation and Recycling

H.

Bluhm,

W.

rey,

H.

Gie se, P.

Hoppe, C.

SchultheiO, R. StraOner

Forschungszenttum Karlsruhe

Institut

f u r

Hochleistungsimpuls- und Mikrowellentechnik

Karlsruhe, Germany

AB ST

R

ACT

The physical basis of electric impulse fragmentation a nd its applications to the recycling of

composite materials are reviewed. The method is based on the initiation of a pulsed electric

discharge inside the solid dielectric material. With pulse amplitudes of 300 ky material

layers of 2 cm can be punctured. Specific energy deposition, of 100 Jlcm at a GW power

level, leads to pressure bu ildup of

5

10 Pa in the discharge channel. Pressure waves and ra-

dially propagating cracks are launched into the solid body, which can lead to the separa tion of

inclusions from the matrix

or

to detachment at material boundaries. To induce the discharge in

the solid dielectric it

must

be immersed in a dielectric liquid with higher breakdown strength.

Most applications use water, which has excellent breakdown strength at fast ramp rates and,

due to its high permittivity, leads to field concentration in the solid dielectric. Electric impulse

fragmentation

is

a clean physical method without any environmental burden and therefore

well suited f or recycling applications. In this paper we consider applications in the fields of

demolition debris, incineration ashes, contaminated surface layers, electric appliances, glass,

and e lastoplastic materials. Finally, the economy and the scaling of the technique to large ma-

terial throughput are discussed.

1

INTRODUCTION

ECYCLINC of waste materials is only reasonable if cer tain economic

R nd ecological criteria are met. Economically it is advan tageous if

the sum

of

earnings from the secondary raw materials and costs from

depositing, in case of non-recycling, is higher than the recycling costs.

Ecologically it is rational if the environmenta l alleviation by use of sec-

ondary raw materials is larger than the difference between environ-

menta l charges for recycling and depositing. Recycling of products like

concrete, electronic devices, electronic scrap, cables, electric appliances,

etc. requires separation into the constituents. In conventional recycling

plants this is achieved by shredding the materials into small pieces and

extracting the different components. Multiple steps are in general nec-

essary to obtain pure materials. To recover the material components

with their original quality in general is not possible with mechanical

methods. Fragmentation of composites by initiating a pulsed

HV

dis-

charge inside the solid material in som e cases offers an effective method

to separate the material into its components without degrading their

quality

The destruction of solid material through pulsed electric discharges

sometimes called 'electrodynamic fragmentation' originally has been

investigated since the early sixties in the former Soviet Union, mainly

at the Polytechnical University of Tomsk [I].The principal goal of this

development was to apply it to the disintegration of rocks to obtain a

higher yield of precious minerals and crystals, while conserving their

original size and shape. However, the method was a lso applied in

drilling of wel ls and for the destruction of reinforced concrete plates.

Unfortunately, much of the original literature is not easily accessible.

Electric impulse fragmentation is a clean physical method without any

environmental burden and therefore certainly meets the ecological cri-

teria for recycling. For which kind of applications it can also become

economically attractive is still under investigation. In this paper we re-

view the physical basis of electric impulse fragmentation and its appli-

cations to the recycling of materials. In Section 2 we describe the most

important phenomena and the energy balance. Section 3 discusses the

basis for the selectivity of the dest ruction. In Section

4

we present a

typical pulse generator. Some applications are discussed in Section

5.

Finally some thoughts on scale-up and economy are given in Section 6.

2

ENERGY BALANCE OF

A

'buried' pulsed electric discharge in a solid dielectric, depositing

an energy of 10 to

100

J/cm within -

o

5 ps heats the spark channel

to

temperatures >IO4

K

and creates a pressure of

l o 9

o 10 Pa. The

spark channel, initially only

10

to

50

pm wide, expands and launches

a pressure wave into the surrounding solid material, which can lead to

DISCHARGES IN SOLIDS

1070-9878101 3.00 000 IEEE


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626

Bluhm et al.: Application ofHV Discharges to Material Fragmentation and Recycling

its disintegration. The energy balance equation for the spark channel

can be written as

where p is the pressure in the channel, and

E =

Ei -

El

is the sum of

the internal energy E, of the channel products and the losses

El

due to

leakage at the channel ends, radiation and heat conduction. W is the

energy provided by the discharge.

pdV + dE = dW

(1)

35

a 25

e

1

2o

15

1

5

0

0 1 2

3

Disiancs from spa channel

[mm]

Figure

1.

Time dependent calculated pressure wave profiles origlnat-

ing from a discharge channel in

a

Plexiglas body

[Z].

HV

Electrode

\+&

Figure 2 Schematic of a setup to induce an electric discharge through

a

solid dielectric

material.

To determine the channel expansion and the pressure field around

the spark, one has to add the momentum equation and the equation

of mass conservation, and solve the system with the appropriate equa-

tion of state both for the channel products a nd the solid ,

A

complete

numerical simulation is very difficult and for heterogeneous composite

materials probably impossible. However, for initial guidance,

B

rather

simple hydrodynamic model using boundary conditions for the radius

of the expanding spark channel, derived from experimental observa-

tions, has been adop ted in the literature [2] Also losses from the chan-

nel are neglected and an equation of state of the form

is used [3],where s the effective ratio of specific heat at constant pres-

sure and constant volume. Results from this kind of analysis for spark

channels in PlexiglasTM amples are shown in Figure

1[2,3].

While

the pressure pulse propagates into the solid material, its amplitude de-

creases and its profile becomes triangularly shaped, which is important

for

the destruction of composite materials.

To initiate the discharge, the arrangement schematically drawn in

Figure 2is used:

A

capacitive energy supply delivers a fast rising volt-

age pulse

of

5500

kV to a rod electrode touching the solid which rests

on

a

grounded plate elertrode, A dlschargo through the solid will oc-

cur if its breakdown voltage is lower than the applied voltage and if

the breakdown strength of any other path outside the solid is higher. A

necessary condltlon for

this

is that the local electric field inside the solid

body exceeds the breakdown field while it does not in the dielectric liq-

uid. This always can be accomplished if the solid body is embedded

into a dielectric liquid with higher breakdown strength. A further pos-

sibility is to concentrate the electric

field

in the

solid

and to lower it

outside, This requires a liquid with much larger permittivity than that

of the solid. Finally the path length between the electrodes through the

liquid could be made much larger than that in the body, e.g. if the solid

body is spherically shaped, the shortest length outside the body is

TX

larger.

Figure 3, Dynamic breakdown strength

of

liquid, solid and

gaseous

dlelertrlcs

as a functioh

of the voltage ramp

rise

time. Below a criti-

cal

value

of

the voltage ramp time,

the

breakdown strength of water

becomes larger than that of

most

solld dielectrics

A suitable dielectric liquid is water, of which the breakdown

strength increases strongly if the risetime of the voltage pulse is re-

duced, This is schematically shown in Figure 3, where the breakdown

field strength of water i s compared to that of solid rock material a nd

transformer oil, as well as gas. It is seen that a t short voltage ramp rise-

times, the breakdown strength of water becomes higher than that of the

solid material. The effect was f irst discovered in the late fifties both at

Tomsk Polytechnical Universlty [l] nd at Aldermaston, where it was

utilized for the design of low impedance high power pulse forming

lines with water dielectric [ 4 ] . This can be understood by the streamer

mechanism of electric breakdown in liquids, where the streamer ve-

locity only weakly depends on the macroscopic electric field strength,

but is determined by the field at the tip of the gaseous filaments (for

positive streamers) or by charge buildup (for negative streamers) at

the head of the streamer [5]. Nevertheless it is difficult to unders tand

why the breakdown strength of liquids can become larger than that of

a solid material, since it is well known that the intrinsicbreakdown


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IEEE Transactions on D ielectrics and Electrical Insulation

Vol.

7

No. 5, October

2000

627

field strength (for thin samples and short periods of stress) of solid di-

electrics is higher. Recently it has been suggested and experimenta lly

demonstrated that the breakdown of gas filled pores inside the solid

may play an important role in this process

[ 6 ] .

Although transformer oil has

a

higher breakdown strength than wa-

ter, it is not appropriate for many technical applications, especially for

recycling. Water does not only have

a

good insulation streng th if it is

stressed for short periods of time, but also has a very high permittivity

and thus pushes the electric field lines into the solid material, which in

general has a much lower permittivity Of course this effect only occurs

if we have

a

layered distribution of liquid and solid materials between

the electrodes. However this is the most frequent situation in an oper-

ating discharge vessel filled with pieces of material to be fragmented.

Figure

4.

Electrode arrangementto achieve scrapingof a surface.This

requires that the

HV

breakdown strength between the electrodes

is

larger outside

the

body than inside.

Water also allows the realization of a configuration where both the

HV

and the grounded electrode contact the solid

at

the same side (Fig-

ure

4).

In this case the discharge can be carried through the solid and

blow off pieces from its surface. Scanning across the surface with this

pair of electrodes, one can remove material layers from large areas.

To assess the efficiency of materia l destruct ion by pulsed electric dis-

charges one has to consider the following steps: Charging of the capac-

itor certainly can be carried out with efficiencies

71 >

0.95.

For

most

industrial applications tap water with a conductivity 0.6 mS/cm is

chosen as the dielectric liquid. In many cases the conductivity rises

during the process because of salt release from the fragments. There-

fore electrolytic current losses occur before breakdown. To minimize

these losses, most of the electrode rod needs to be covered by a solid

insulating material. If the electrode tip does not contact the material,

the discharge may be delayed and the electrolytic losses can become

quite large before breakdown, depending on the water quality, and

the material filling factor in the interelec trode gap. Values

as

low as

0.2 to 0.4 have been measured

for

the pulse coupling efficiency

772

on

FRANKA-Stein (a semi-industrial prototype for concrete fragmentation,

see Section

5.1)

in positive polarity In negative polarity 112 fell to 4l.1.

However, by controlling the water quality and filling fraction, the losses

can be reduced to

10%

and

72

---t 0.9. We have found that a conduc-

tivity of 5

2

mS/cm can be tolerated in the process water.

Since breakdown is

a

stochastic process not every pulse will lead

to

a

discharge in the solid. The probability can vary over

a

wide ran e,

depending on the geometry, the dielectric properties of the material,

&e

electrolytic conductivity of the water in the process chamber,

etc.;

how-

ever in an optimized configuration

773

=

0.8

to

0.9.

Only a fraction

7 4

of the available electric energy is deposited in the discharge channel.

The rest is wasted in the generator, i.e. appears as ohmic and dielectric

losses. However, experience has shown that r 4 = 0.65 to 0.7 can be

achieved. Since the elec trodes do not always touch the solid, part of

the arc channel can arise in the surrounding liquid, where it is less ef-

fective. Therefore, another efficiency coefficient 5 must be introduced,

accounting for this effect ( r ) ~ 0.9).

The product of all these efficiencies leads to qt and the estimate

that a fraction

~ 0 4

f the stored electric energy can be released in the

useful part of the arc channel. This energy

W

splits into different forms

W =

A+

E, + El

A= [ p d V

(3)

where

A

is the mechanical work performed by the expanding channel

in the surrounding solid. If losses are neglected due to the fast pulsed

character of the process one can estimate the thermodynamic efficiency

1 =

A/W

= 1

Ei/W.

W

is obtained from current and voltage

measurements. Using Equation 2) for

E

wi t hy

=

1.1 o

1.2

and de-

riving V from experimental observations of the channel radius and

p

from the simplified hydrodynamic simulations mentioned above [l,

1

one estimates

7 = 0.1

to 0.2. This relatively small value results from

the large part of the internal energy that is spent for dissociation and

ioniza tion. Losses start to become important if the ratio of the channel

radius R, and the channel length

L,

> 0 1 Taking all efficiencies

into account leads to the conclusion that qt

= 4

o

8%

of the elec trical

energy is available for the destruction of the solid material. Part of this

energy is expended to defo rm the solid. If the main application is frag-

mentation one may consider only that fraction of energy beneficial that

is

used to create new surfaces. In this case the fragmentation efficiency

qf becomes

w s

7lf = A 7 t (4)

where

w

is the specific free surface energy, and S the area of the newly

created surface.

Using the assumption that most of the energy A is expended for

plastic deformation of the solid Epl and app lying the approximate re-

lation

111

=

9 w S l n

z )

(5)

where T~ is the yield strength

(3

to 300x10' N/m) and

G

the shear

modulus

(1

o

4 ~ 1 0 ~ ~ N / m * ) .

ne obtains

w S x

(0.013

---t

0.047)A;

i.e.

0.04 to 0.32 .

This value has to be compared with the corresponding value of me-

chanical fragmentation devices which is of the order of 0.002 to 1 , e-

pending on the degree of fragmentation. We can therefore conclude that

electric impulse fragmenta tion is energetically comparable , but not su-

perior, to mechanical fragmentation methods. Consequently one should

use

the electric method especially for those applications where its tech-

nological benefits become obvious. Among these the smaller width of


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628

Bluhm et

al.:

Application

of HV

Discharges to Material Fragmentation and Recycling

the grain size distribution curve, the relatively small amount of con-

tamination introduced by the process, and the low heat transferred to

the milled material on the average. However its main advantage is the

high degree of selec tivity

3

SELECTIVITY

OF

DESTRUCTION

Recycling of composite materia ls (e.g. concrete, fiber glass enforced

plastics, etc. or material composites (e.g. electrical apparatus, circuit

boards,

etc.)

requires separation into the basic components. Electric im-

pulse destruction can produce separation at material

or

grain bound-

aries via three effects.

I 4- HV

J

Discharge

Channel

_ \ ~ .

Weak Medium

Strong

Pressure Wave

Figure 5. Mechanisms by which components in a composite mate-

rial

can be

separated: Top:

Metallic

inclusions

or

inclusions with

high

permittivity can attract the discharge track. Middle:

A

compression

wave can be transformed

into

a tensile and shear wave by reflection

and refraction

at

an inclusion and separate

it

from the matrix.

Bot-

tom:

A crack propagating from the discharge channel into the solid

can branch around an inclusionif its mechanical properties are differ-

ent from that of the matrix.

At inclusions where the dielectric properties are very different from

that of the matrix, the electric field intensity can be magnified and at-

tract the discharge track to the inclusion, where it can continue

to

de-

velop along the boundary This is shown schematically nFigure

5

(top),

where a conducting sphere has been embedded in an insulating matrix.

In

this case separation of the inclusion from the matrix is caused directly

by the discharge channel.

A second more important effect starts from cracks created in the

immediate surrounding of the channel. As can be concluded from Fig-

ure 1, the pressure exerted by the expanding channel almost always

exceeds the tensile strength of m aterials and leads to the formation of

cracks. If c racks have been formed in contact with the spark channel,

channel products can penetrate into them and exert force on the crack

walls. The character, dynamics and intensity of the crack formation

is determined by the ra te of energy deposition in the channel and by

the properties of the material. Brittle materials show a large number

of cracks in a radial zone of 3 mm around the discharge channel,

created early in the discharge. During a later phase, a number of ra-

dially propagating cracks start to grow from this zone. The extension

and crack density around the channel correlates with the rate of energy

release

[l].

However, the number of cracks reaching the surface of the

probe depends more

on

the total energy released in the spark channel.

Consequently, one can conclude that to achieve comminution, a high

power of the pulse is required while the detachment of large fragments

is most effectively achieved with high pulse energies deposited over a

longer time interval.

For

the selectivity of fragmentation it is impor-

tant to realize that material inhomogeneities in general, and acoustic

inhomogeneities in particular, influence the propagation of cracks in a

composite material. The reason for this is the existence of increased me-

chanical stress at the boundary

of

an inclusion. Stress waves reflected

from inhomogeneities

or

inclusions can interact with the growing crack

before the inclusion is reached

[7-101.

If cracks hit the inclusion they

can branch, depending on the angle of incidence, as schematically ndi-

cated in Figure

5

(bottom), and separate the inclusion from the matrix.

A third effect leading to separation a t the interface of a n inclusion

and the sur rounding medium is connected with the action of an incident

compressive wave launched from the discharge channel [ll-131. This

is schematically shown in Figure 5(middle). Initially, and in the imme-

diate surrounding of the spark channel, the wave has the character of

a shock wave, while later it develops into a compression wave.

It

has

been shown

[ll]

hat a compressive stress wave is converted into a ten-

sile wave after refraction and reflection inside the inclusion. At small

amplitude, separation occurs first at the shadow side if the inclusion

has a higher acoustic impedance. Complete separation over the entire

interface of the inclusion and the m atrix was observed at sufficiently

high wave pressures.

An important question is whether inclusions can be separated, but

remain unbroken, from the matrix. It is well known that a shock wave

arriving at a free surface

or

at a material interface with a strong jump in

acoustic impedance can lead to spallation [14].A strong advantage of

electric impulse destruction is that the energetic parameters of the pulse

power generator can be varied over a wide range, and adapted to the

physical, mechanical and acoustic properties of the composite. Because

of the complexity of com posite materials, this has to be determ ined

experimentally for each material.

4 THE FRAGMENTATION DEVICE

Since gas bubbles, created during the HV discharge, must disap-

pear from the discharge vessel before the next pulse can be applied,

the pulse frequency cannot be extended much above 10Hz. Therefore

a material throughput per discharge section of

500 kg/h,

which is


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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 N o .

5

October2000 629

necessary for many industrial processes

to

become economic, requires

an interelectrode distance of 5 cm. To break down a material bed

of this s ize electrically, a voltage pulse of -

00

kV is required. A fast

rising voltage pulse

of

this amplitude can be generated with the help of

a Marx generator [15]. This Marx generator should be able

to

produce

pulses with an energy content of 55 kJ and deliver a large fraction of

this energy to the discharge channel. This requires a low internal resis-

tance of the Marx, which is also necessary to obtain a fast rising voltage

ramp. High power, typically 5 5 GW

, s

needed if a large degree of

fragmentation is desired. Therefore, the Marx and its connections to

the discharge vessel also should have a low inductance.

Figure 7. Schematic

of

the wiring inside the Marx generator to mini-

mize the magnetic stray field in the surrounding. In the right part of

the Figure the calculated magnetic stray field

is

presented.

the Marx, careful shielding is required. It turns ou t that shielding of

the magnetic stray field is difficult to achieve with non-ferromagnetic

materials. On the other hand p-metal may become too expensive for

some applications. However, skillful internal wiring of the Marx can

lead to compensating currents and thus reduce the outside magnetic

field. As shown in Figure 7, the field decays rapidly with distance from

the Marx. Another source of electromagnetic interference and noise is

the spark in the discharge vessel. Therefore, in the setup of Figure 8the

discharge vessel has been surrounded completely by a Faraday cage

which at the same time serves as sound insulation.

In many cases the discharge vessel is built from PE, except for the

bottom part, to reduce electrolytic current losses. Since the process

water can become increasingly conductive during operation, the HV

electrode must be insulated anyway except for a small tip. In this case

it is acceptable to use a metallic vessel which then itself can shield the

electromagnetic noise. To relieve the electric field stress at the insula-

tor/meta l/water triole ooint. the tip at the end of the electrode has been

Figure 6. Photo of a 400 kV, 1.8 kJ

Marx

generator designed to oper-

ate

at 10 Hz with a component lifetime of 10' pulses.

The

generator

discharge period

is 2.5

ps.

I

shaped like a mushroom. Depending on the process, the bottom part

quirements is shown in Figure

6.

This generator, built for recycling of

metals from dross , consists of

7

stages with

two

capacitors of 72 nF per

and

current

wave shapes and the

derived

stage,

Its inductance, including the contributions from the lead and spark channel resistance are presented for different conditions in the

the HV electrode,has been measured as

7,7

pH, The internal resistance discharge vessel. For this series of measurements an interelectrode d is-

used as closing switches in the configuration, and has been measured

which has been measured with a fast resistive voltage divider, includes

to be

0.5

fi

for

the

present

design. With this small resistance,

80% the inductive voltage drop at the electrode and spark inductance. To

of the available energy

can

be deposited in the reaction chamber, Each calculate the spark resistanceRE, t must therefore be corrected to

(6)

capacitor is charged to

60

kV Thus a 1.8

kJ

oscillating pulse train with a

maximum amplitude of

>400

kV and a period of 2.5 psis achieved. De-

spite the large voltage a n ~igh

r@%

alif etim e of If the material (concrete) ills the entire gap, a rapid breakthrough is

>lo8

ulses has been rated

the capacitor vendor AtesYs. The switch achieved during the initial rise of the voltage pulse and the mean value

are

tungsten

with a

Profile o

a

of the spark resistance remains

>2 fi

It is observed that the spark

homogeneous burn of the electrode material. PE is used as the switch resistance periodically rises near the zero-crossing

of

the current, we

housing material.

attribute this to an inflow of cold material from the channel wall and to

The Marx itself is housed in a thick walled metallic tank visible be- heat losses dominating over heat production at this time, both leading

hind the generator in Figure 6.Transformer oil is used in the tank for to cooling of the channel plasma. If a large fraction of the discharge

HV insulation and as a cooling medium. The Marx pulse is transmit-

runs

through water, an ignition delay occurs and the vo ltage begins to

ted through a Plexiglas interface to the reaction chamber using a large drop due to electrolytic current losses. Correspondingly the maximum

diameter flexible metallic tube. attainable spark current and the energy deposited into the spark also

Important aspects of operation in an industrial environm ent are HV

are

safety, electromagnetic interference, and noise protection. HV safety Spark resistance RE, gnition delay, and the energy efficiency can

can be assured by standard regulations and will not be discussed here. be used

to

control the operation, determine the filling level, and the

However, to operate electronic devices safely in the environment of interelectrode distance.

The base line Of

a

Marx-generator

with these re-

of the vessel is built as a mesh, a grid, or as a closed half-sphere,

In

Figure the

of this Marx is largely determined by the resistance of the spark gaps,

tance Of 30mm has been chosen. The sipahown h~Figure 9~

u t )

L

i t )

RE =


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630

Bluhm et al.: Application of

H V

Discharges

t

Material Fragmentation and R ecycling

Material feed unit

HV processing unit

HV

Generation

Material classification

Figure

8. Complete setup of an industrial demonstration

facility

for

metal

recyclingwith screen box

and

sound insulation

5 APPLICATIONS

5.1 RECYCLING OF BUILDING

MATERIALS

Approximately 1m3 of concrete per inhabitant per year is used up

for building purposes in Germany. Similar quantities are consumed in

other industrialized countries. The raw materials gravel, sand and ce-

ment are completely taken from natural resources. On the other hand

30 million tons residual m asses of concrete, mortar, and brick are cre-

ated per year in Germany The rate of reutilization of these building

materials is quite low and restricted to secondary constructional opera-

tions like backfilling, noise protection dams, etc. Reutilization without

degradation requires an improved separation into sand, gravel, and ce-

ment. Crushing the material with multistage jaw breakers or impact

mills cannot separate its constituents and produces a large fraction of

dust and small particles.

Concrete is a composite heterogeneous material and therefore well

suited for separation into its original components by electrodynamic

fragmentation. Microcracks between the aggregates and the cement

matrix already exist in the unstressed concrete. Alternation of loads ex-

pedites the detachment of aggregates from the cement matrix. Since the

acoustic inhomogeneities are rather la rge in concrete, ideal conditions

exist for separation by pressure waves.

Also

the pressure impulse at

the discharge channel mainly creates tensile and shearing forces, con-

ditions at which the strength of concrete is low. Therefore cracks will

originate and spread from the channel.

Figure 10demonstrates that pre-broken concrete indeed can be sep-

arated com pletely into its components.

Figure

11

shows the achieved grading curves after different treat-

ment times, together with the initial grading curve of the natural

ag-

gregates for the specific concrete (according to DIN) [16]. It is striking

that no coarse fragments appear in the grading curve of the separated

concrete aggregates. Nearly all particles consist of single minerals. Un-

der a n optical microscope, the gravel fraction >2 mm) is apparently free

from contaminants and baked particles. Spherical particles are domi-

nant in the sand fraction (0.5nun) and seldom with cement

adhesions. The total fraction of cement in the aggregate part (gravel,

sand)

is

-1 .The recycled aggregates are not mechanically predam-

aged a nd fulfill the increased demands of the frost-dew resistance ac-

cording to DIN 52104. Concrete made from these recycled aggregates

has the same material strength as that from natural aggregates.

The radiography of the silt fraction

(


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IEEE Transactions on Dielec trics an d Electrical Insulatio n Vol. 7

No.

5 October 2000

631

w

200

1W

0

-1CQ

w

200

1W

0

16

12

8

La

203

1W

0

1W

16

12

8

4

2

0

2 3

4llSlO

......m~.

...

~ 2

[kVl

2w

CQ

0

lW

16

12

a

4

2

Figure 9. Voltage

u ( t )

nd current

i t)

wave shapes and the derived

spark channel resistance RE or different conditions in the interelec-

trode gap.

For

this series of mea surem ents an interelectrode distance

of 30 mm had

been

chosen. The voltage trace shown has not yet been

corrected for the inductive voltage drop.

of concrete were dest royed within 45

s

of operation at a repetition rate

of

4

to

5

Hz.

To demonstrate a larger throughput of

1000

kg/h the semi-industrial

prototype facility

F R A N K A 2

alias FRANKA-Stein shown in Figure

12

was built. The concrete lumps are transported to the p rocessing cham-

ber with the help of a vibrating conveyor. The material treatment time is

controlled by the c onveyor vibration speed and by gates at the entranc e

and exit ports

of

the chamber. The

Marx

consist s of

6

stages powered

from a 10kW charging unit. At

60

kV ch arging voltage, the Marx out-

put rises to the

350

kV pulse amp litude within 0.2 to

0.4 ps,

depending

Figure

10.

Pre-broken concrete piece before (right) and after tr eatment

in the discharge vessel of F R A NKA

0.

Sand and gravel are recovered

without degradation. The cement fraction (aside of the steel pieces)

can

be

baked to produce cement clinker.

0.2 015

2

4 8 16

Mesh-width [mm]

Figure 11. Gradi ng curves of fragmented concrete after different num

bers of pulses.

DIN

1045 is the original grading curve of the aggre-

gates.

For

comparison, the particle size distribution

after

heat treat-

ment is shown also.

Table 1. Productivity of concrete fragmentation a t

F RANKA

0.

Parameter Value

Unit

Productivity

160

kg/h

AV.electric

power

on the water quality So far

a

throughput of

280

kg/h could be realized.

The limitations result f rom the strong qu ality requirements for the sec-

ondary aggregates. A closed water reprocessing circuit has been ad ded

to

keep the w ater condu ctivity low. The specific energy consumption

is

similar to that achieved

for

the smaller facility FR ANK A-0. owever

the value of 20 kWh/t includes a contribu tion of 6 kWh/t from water

reprocessing.

5.2

TREATMENT OF INCINERATION

ASHES

Thermal treatment of municipal solid waste is an effective method

of waste d isposal, which becomes incr easingly important.

It

does not

only reduce the volume to be dumped but is a valuable source of en-

ergy and a resource for metals and mineral building materials. The


8/12

632

Bluhm et

al.:

Application of HV Discharges to Material Fragmentation and Recycling

Figure

12.

View

of

FRANKA-Stein,

a

semi-industrial prototype to

demonstrate concrete recycling with complete separation of the ag-

gregates at throughput of , lt/h. The Marx-generator is housed in

the

top cylindrical case.

utilization of ashes as an aggregate for the production of concrete re-

quires the separation of metal and the immobilization of heavy metals.

Heavy metals can be extracted from fresh ashes in contact with water.

An unavoidable content of anhydrous lime is responsible for the high

pH value >U)n ashes. Thus the elution of lead ions in water is in-

creased by a factor of

1000

over that in a pH-neutral solution. Therefore,

fresh ashes must be stored for

a

period of at least three month before

utilization as a building material. During this time absorption of CO2

from the atmosphere reduces the pH value.

We have found that under-water electrodynamic fragmentation

UWEDF )

to separate the metal from the ashes also reduces the pH value.

This is attributed to the production of free OH radicals created in the

discharge channel and by the shock wave launched from it. Subsequent

measurements of heavy metal elution show spectacular reductions,

so

that in principle storage can be replaced by an on line treatment with

UWEDF

directly after the furnace.

To demonstrate this process in an industrial environment, the fa-

cility

FRANKA 1

was built and installed at

a

municipal incineration

plant.

FRANKA

1 (shown in Figure 13before shipment to the incin-

eration plant) can treat 2 tons of ashes per hour. FRANKA 1 operates

with

7

Hz at a mean power of

10

kW

and produces voltage pulses of

350 kV amplitude. Figure 14shows the values of heavy metal elution

from the ashes before and after treatment with the

FRANKA 1

acility

For comparison the threshold values of the German LAGA

22

regula-

tion for reutilization

as

building material are shown also.

A problem of this process is the strong enrichment of salts in the

process water, leading to increasing electrolytic losses. An important

part of the process is therefore desalting of the process water.

Figure

13.

F R A NKA 1 before shipment to an incineration plant.

F RANKA 1 s used

to

separate metallic components from the ashes and

to

immobilizeheavy metals in

the

ashes.

Figure 14. Comparison of heavy metal elution from treated and un-

treated fresh ashes. Also shown are

the

values of the German regula-

tion LAGA

22.

5.3

REMOVAL OF SURFACE

LAYERS

The configuration shown in Figure

4

can be used to remove surface

layers contaminated by hazardous chemicals or radionuclides. Labora-

tory experiments have been carried out at Textron [17], Tomsk [l], nd

at our own laboratory, where a device was built that can be moved in

all three dimensions above

a 2x 3

m2 large water filled basin [16].

While we have tried

so

far only single pairs of electrodes, long par-

allel strips of electrodes have been used at Textron. It was expected that

in this configuration breakdowns would travel randomly along the elec-

trode gap and that removal of concrete at one location would increase

the breakdown strength there and transfer the discharge to another po-

sition. By this kind of self-regulation, a uniform depth across the scab-

ble path was predicted. The prototype system consisted of a 120 kV

Marx delivering

a 0.8

to

2

kJ pulse a t

a

repetition rate of 5 to

40

Hz. The

average scabble speed was 5 to

20

cm/min, and thus a factor of 10

larger than tha t of

a

low voltage

(20

to 30 kV electro-hydraulic scabble

system based on water breakdown, although the energy consumption

of the latter w as a factor of

4x

larger. The specific energy consump-

tion of the HV system was 500 to 1000J/cm3 . Trials were also conducted

at a US Dept. of Energy (DOE) test site where a uranium contaminated

concrete layer was removed from the plant

floor.

According

to

the ex-

perience gained in these experiments, the technique can be used to de-

contaminate large floor areas. The radioactive products released during

the process are contained either in the concrete rubble or in the water

as fine suspended particles. These components must be removed from

the water by filtering or evaporation of the water. Recontamination of

the concrete surface laid open cannot be avoided completely, but can

be kept small by fast recycling

of

the water.


9/12

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 7 N o .5 October 2000

633

Figure

15.

View of removed surface

layer

from

a

block of reinforced

concrete.

Here

the two-electrode system

shown

in

the

upper left cor-

ner of the photo was used.

As for the case of concrete decomposition the conductivi ty of the

process water rises with time and needs to be controlled. This is espe-

cially important

for

the parallel strip electrodes used in the experiments

at Textron. We therefore preferred rod elec trodes that can be enveloped

entirely by an insulator except for

a

small section

at

the tip. Also the

pulse amplitude should be raised to >400 kV. At 350 kV the specific en-

ergy consumption was as low

as

70 J/cm3 in our experiments.

A

result

from these trials

is

shown in

Figure

15, where the concrete had been

removed up

to

the first grid

of

reinforcement steel. Even

lower

spe-

cific energy consumption has been found by Kurez

et

d. They used

a

powerful

420

kV, 19 kJ per pulse

Marx

generator a nd

a

comb-like elec-

trode system to destroy reinforced concrete plates. The electrodes were

connected sequentially to the generator. With this system they found

values between

7

and 30 J/cm3, depending on the number of reinforce-

ment grids.

5.4 RECYCLING OF OTHER

PRODUCTS

Besides building materials, numerous other composite materials

have been explored. We can divide these into two groups: Material

composites containing metallic and dielectric components like electric

appliances, spark plugs, circuit boards,

etc.;

and brittle homogeneous

materials like glass, silicon, coloring pigments, minerals

etc.

Easy separation of metallic and nonmetallic components can be

achieved directly for small electric appliances as shown in Figure 16.

Large electric appliances need coarse crushing before processing. Un-

like in conventional recycling, the metallic components can be retrieved

easily as complete parts, enabling much easier recovery of precious

metals. It is assumed that the discharge occurs at material interfaces

and thus detaches the bonding between components.

The interest of using e lectrodynamic fragmentation to mill or de-

stroy homogeneous materials is based on the observation that relatively

small amounts of contamination are introduced by the process and tha t

more favorab le grain size distributions, without a large fraction of fines

can be achieved,e.g. contamination poor m illing of borosilicateglass for

bioteshnici msti ated

Figure 16. Pulsed electric discharges

can

be used to separate metallic

and dielectric components in electric appliances (a). The result of

a

razor treatment after a few pulses (b),

The

glass

particles suspended

in

the process water were sucked off

continuously through a filter of suitable pore size. Iron contam ination

from electrode burn was further reduced through magnetic separators

at the entrance to the sedimentation pit. Compared to the conventional

milling process not only the amount

of

contamination was reduced,

but also the grain size distribution was much smaller and thus the use-

ful yield was increased. Another advantage

is

that a relatively small

fraction of the material came into contact with the hot channel prod-

ucts and became molten. Thus the porous structure on the surface of

the borosilicate grains, an important feature for biological applications,

was preserved for

a

larger fraction of particles than with conventional

milling.

5.5 RECYCLING

OF

ELASTOPLASTIC MATERIALS

Electrodynamic fragmentation turned out to be less successful for

elastic

or

impact resistant materials. In an attempt to improve the per-

formance of the method in this field of application, investigations were

launched at Tomsk Polytechnical University [NI, in which elastic ma-

terials (in particular rubber) were immersed into liquid nitrogen (LNz)

to increase their brittleness.

However, the success of this approach does not only depend on the

increased brittleness of the material but also on the dielectric properties

of LN2.

A

real drawback of LN2 as a dielectric liquid is its small permit-

tivity of E,

=

1.454. Thus field intensification inside a solid material

immersed in the liquid cannot be expected. On the contrary, since many

solid materials have

a

higher permittivity,

a

weak field enhancement in

the liquid may occur.


10/12

634

Bluhm et

al.:

Application ofHV Discharg es to Material Fragmentation and R ecycling

Sample Thickness [mm]

Figure 17. Dependence of the breakdown voltage in LN2 on sample

thickness and for different ramp rates. Also shown is the breakdown

voltage

of rubber at

cryogenic temperatures.

1181.

Although a considerable amount of information is found in the liter-

ature on the breakdown characteristics of LN2, very little is of relevance

for the dynamic stress situation typical for electrodynamic fragmenta-

tion. A summary of the results obtained from experiments conducted

at Tomsk is shown in Figure

17

[lS]. Here the change of breakdown

voltage has been depicted as a function of sam ple thickness in the case

of

LN2

for three different pulse rise times:

200

ns,

1

ps and a 5 ms sine

wave, an d for two different rubber samples: car tire rubber and vacuum

seal rubber. The risetimes always refer to a pulse amplitude of 250 kV

and thus can be expressed

as

ramp rates of 1.25 MV/ps, 250 k V / p and

50 V/ ps. It was observed that the breakdown behav ior of LN2 differs

significantly from that of other dielectric liquids, like transformer oil,

glycerol, ethanol, or water (at normal conditions).

Only for LN2 layer thicknesses (gap widths)

120

mm the breakdown

voltage rises linearly The increase becomes much slower,

>20 mm.

This

behavior probably is connected with

a

change of the breakdown m ech-

anism from an area to a volume effect. The probability for the appear-

ance of bubbles grows if the s tressed volume increases. Such bubbles

promote the formation of streamers inside the volume and reduce the

macroscopic breakdown field strength. The formation of bubbles in

the LN2 bath is of particular relevance since the temperature of liquid

gases under atmospheric pressure stabilizes close to the boiling point.

In such liquids, even the minute heat production inferred by prebreak-

down curren ts can lead to the appearance of bubble chains, along which

premature breakdown can occur, before a discharge path through the

solid material has been established.

It can be concluded also from Figure 17that the variation of break-

down strength with pulse rise time is much less pronounced than for

water. While in water, the breakdown strength increases by a factor of

10when passing from a pulse rise time of

1

ps to 200

ns,

the correspond-

ing increase in LN2 is merely 25%. On the other hand the investigated

rubber materials showed a linear increase of breakdown voltage with

sample thickness.

From these results, the authors of I181 concluded that to induce the

discharge in a solid material like rubber with greater probability than

in LN2, one shou ld apply pulses with risetimes of

(0.2 to

0 . 5 ) ~ 1 0 - ~

and pulse amplitudes of 200 kV for samples to

30 mm

thick.

Another reason for weakening of the electric breakdown strength

in LN2 can be the accumulation of ice crystals within the liquid, origi-

nating from frozen out air humidity if the surface is exposed to normal

atmospheric air. Of course these problems could be reduced signifi-

cantly by pressurizing the LN2 and by minimizing the con tact of its

surface with atmospheric air.

The removal of bubbles from the LN2 bath also can limit severely the

achievable repetition rate. If typically 125

J

of energy

are

deposited

in

the discharge channel and converted into heat,

-0.5 1of N2

gas must

be removed from the bath between shots. Although the viscosity of

LN2 is relatively small, O . 2 l ~ l O - ~as at 77K, as opposed to lop3Pa

s

for water at

20T,

which facilitates bubble movement to the surface

under buoyancy force, corresponding experiments 119,201have shown

that N2

gas

bubbles in LN2 submitted

to

buoyancy force only, move

upward with

a

speed of only 0.2 m/s . Given

a

depth of the LN2 bath

of 20 cm, the pulse rate will be limited to


11/12

IEEE Transactions on D ielectrics and Electrical Insu lation Vol.7 N o .5 October 2000 635

Figure 19. Details

of

the LN2 discharge vessel of F R A NKA

3.

number of components was treated:

1. Components from motor vehicles containing

rubber or

other plastic

2. Optical fiber wave guides,

3.

Printed circuit boards,

4.

Laminated plastics, and

5 . Pressure tubing, vacuum seals,etc.

Gross separation of metal-rubber and metal-plastic composites was

easily achieved. However, small traces of rubber remained on the

metallic surface and could not be removed even by prolonged treat-

ment. Also little craters appeared on the metallic surface at arc foot

points.

The separation of components from thin sheets of material, like

printed circu it boards, could not be achieved efficiently In this con-

figuration the channel products can escape rapidly from the channel

and prevent the buildup of any significant pressure.

Also

it was found that many organic materials were decomposed

at the high temperatures in the discharge channel and, during cooling,

lead to new uncontrollable substances. Although the quantities were

small and may be tolerable in some processes, they are certainly unac-

ceptable in the treatment of pharmaceu tical and food products. There-

fore, and because of the appreciable costs of LN2, we have been unable

so

far to identify an industrial application that justifies electrodynamic

fragmentation or milling under LN2.

parts,

6 SCALING AND ECONOMIC

CONSIDERATIONS

The economy of any recycling technique is determined by the ma-

chine price, the specific energy consumption, operating and mainte-

nance costs and by the number and quality of personal to run the fa-

cility. Most industrial applications need large throughput to become

economical and therefore considerable extrapolations from present lab-

oratory type electrodynamic fragmentation devices to industrial size

facilities are necessary, leading to big uncertainties in cost estimates.

It is obvious that the quantity of material that can be treated per

arc channel is limited and cannot be increased much above that of an

optimized laboratory device. The repetition rate must stay

5 15

Hz to

remove the gas bubbles between pulses. Also it does not seem reason-

able to raise the pulse amplitude appreciably above 500 kV, since the

expenditure for insulation may become prohibitive. The pulse ampli-

tude determines the possible length of the discharge channel and thus

the accessible volume in the treated material. Other parameters that

have a n influence on productivity are the power of the pulse and its

energy, Augmenting the energy per pulse is however counterac ted by

a

reduction of the discharge channel resistance leading to smaller effi-

ciencies. Nevertheless, raising the power m ay lead to

a

certain gain in

productivity , especially if milling is the main task of the device. The

achievable increment in productivity will of course depend

on

the spe-

cific product. A systematic study on the disintegration of gran ite sam-

ples has been carried out in [21] where an optimum set of parameters

in an energy-field plane was derived. However, the term 'disintegra-

tion'

was

not specified and therefore it is difficult to relate these data to

specific productivity Considering concrete fragmentation for complete

separation of the aggregates, we expect that a factor of 3 to

5

increase

in throughpu t over that of our

FRANKA

facility can be achieved for an

optimized discharge channel,

.e. a

throughput of

1000 to

3000 kg/h

of completely separated concrete may be produced per channel with an

average power of 30 kW and a pulse energy of 2 kJ operating at 15 Hz.

Thus to realize an industrial facility with

l o 5

kg/h,

100

arcs operating

simultaneously are required. Every arc needs

a

certain process space

so that it does not interfere with its neighbors. Either parallel or serial

arrangements of the active arc zones are conceivable.

To produce 100 arcs simultaneously does not necessarily mean tha t

all components need to be multiplied by this number, e.g. the Marx

generator can

run

at a higher frequency and distribute its pulses alter-

nately to different discharge sections. Also a capacitor charging unit of

sufficient power can supply several units in parallel. Since the price of

a power supply does not increase proportional to the power there is

a

large saving potential in the scale-up of a facility

Component wear and lifetime are further importan t economic fac-

tors. The components with the largest wear are the

HV

electrodes in

contact with the material to be fragmented and the switch electrodes.

We have found that the material

loss

from the steel electrode used for

concrete fragmentation amounted to 10 pg per shot. Consequently an

electrode with

1

cm cross section consumes

z

cm of its length per

week of full operation

(8

h working day). Therefore, provisions must be

made to adjust the electrode, and replacement becomes necessary from

time to time. However the wear is sufficiently small to be economically


12/12

636

Bluhm et al.: App lication of

H V

Discharges to Ma terial Fragmentation and R ecycling

irrelevan t. Concerning the lifetime of the insulator enveloping the steel

rod, insufficient experience exists for operation under industrial condi-

tions. Damage of the insulator can occur through material fragments

impacting on its surface. Laboratory experience shows that it is most

important to relieve the triple point at the electrode tip.

Of more concern than the erosion of the operating electrode is the

wear of the switch electrodes. We have measured a loss of 3

g

after lo6

pulses from spherica l steel electrodes. Using CuW with Borda profiles

and sufficiently large diameters should lead to acceptably small wear

at the power densities involved. Never theless, adaptation of the switch

gas pressure will be routinely necessary and the electrode gap distances

probably must be readjuste d at maintenance intervals of the order of ca.

a week.

Capacito r lifetime has been guaranteed for > l o 8pulses even for the

conditions of large vo ltage reversal occurring in the electric discharges

and is presently not considered to be an economic limitation.

Depending on the process, a strong liberation of salts can occur and

increase the conductivity

of

the process water. At a conductivity level

>1500 pS/cm, efficient operation becomes imposs ible and the w ater

must either be replaced or conditioned, which can become a n important

factor for cost effectiveness.

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Manuscript

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2000.

HV Discharges to Material Fragmentation

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