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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
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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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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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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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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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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
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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.
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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.
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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
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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
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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
w s
eceived on 19 une
2000.