Note: Descriptions are shown in the official language in which they were submitted.
31 257~
DEWATERING PROCESS
FIELD OF THE INVENTION
This invention relates to a method of concentrating
the finely divided solids in colloidal suspensions,
5 sludges or fine particle slurries in water by a combina-
tion of an electrical field, and an acoustical (sonic or
ultrasonic) field and removing the water therefrom. Such
sludges and slurries occur, for example, in coal washery
slimes, ore processing, transportation of coal by pipeline
10 and in production of ceramics. The method requires less
energy for the combination of electrical field and acoustical
field to remove a unit of water from a suspension than
the energy required if only an electrical field or an
acoustical (sonic or ultrasonic) field were used separately
15 or in sequence to remove that unit of water from the
e~uivalent slurry.
The method is of utility in removing water fxom the
above mentioned examples and from those described
below.
BACKGROUND OF THE INVENTION
Conventional methods of separating water from suspen-
sions, sludges and slurries to improve their solid concen-
tration include mechanical dewatering methods such as
25 centrifugation, vacuum filtration, vibration screening;
thermal drying; ponding;ultrasonic dewatering; and use of
anelectricalfield (electrophoretic or electrosmotic
dewatering. Some materials mixed with water, however, are
not amenable to the usual dewatering methods due to block-
30 age of the filter by fine particles so that the rate offiltration slows down significantly. In addition dewater-
ing of colloidal and small particle suspensions is diffi-
cult with conventional solid-liquid separation $echniques
such as vacuum filtration or centrifugation.
~;25 ~'~4~
Such suspensions and slurries can also be thermally dried;
but, the energy consumption is very high and it crea~es
problems with dust. In addition natural settling by
ponding requires large land areas for lagoons and the
5 like.
Because of the high cost of energy there has been a
renewed effort to reappraise all of the above methods and
reduce the cost associated therewith. The use of an
electrical field or acoustical field to separate water
10 from suspended particles has been studied as further
discussed below.
When colloidal particles such as finely divided clay
are suspended in water they become charged and when subjected
to an electrical field they will migrate towards one or
15 another of the electrodes. The charge will depend on the
type of suspension or slurry, e.g. certain types of clay
become positively charged while many coals become negatively
charged, thus the proper electrode polarity will need to be
chosen to cause migration of particles away from the water
20 permeable electrode and toward the impermeable electrode.
The application of an electrical field can agglomerate
particles by neutralizing charges, dehydrate solids by elec-
troosmosis, or cause the particles to migrate as noted
above. The electrically charged collecting plates will
25 sequester all migrating particles such as negatively or
positively charged particles but do not collect the
isoelectric particles. For example, in proteins, the
charges originate from the ionization of (C00 ) and NH3+)
ions. The net charge on protein will depend on the number
30 of these groups; the disassociation constant, pH, tempera-
ture, etc. However, it is an empirical observation that
most colloidal protein suspensions are usually negatively
charged under normal conditions when in water.
Another phenomenon present is that of electroosmosis.
35 Electroosmosis is the transport of the liquid medium along-
side a surface that is electrically charged but stationary.
~257~4~
The movement of the liquid medium is in the direction of the
electrode with the same sign of charge as the immovable sur~
face. Thus, as the slurry particles become more densely
packed and immovable, the water between the particles will be
5 subject to electroosmotic forces. If the particles are nega-
tively charged,the flow of water will be toward the negative
electrode. Since this electrode will also be permeable
to water,the dewatering process will be further improved.
Illustrative of the use of an electrical field in the
10 dewatering of coal washery slimes is the article by Neville
C. Lockhart,Sedimentation and Electro-osmotlc Dewatering
of Coal Washery Slimes, Fuel, Vol. 60, October, pp. 919-923.
The use of ultrasonic energy separately to dewater
coal is known, as illustrated in the articles by H.V.
15 Faixbanks et al., Acoustic Drying of Coal, IEEE Trans. on
Sonics and Ultrasonics. Vol. S-J-14, No. 4, (October 1967),
pp. 175-177; Acoustic Drying of Vltrafine Coals, Ultrasonics,
Vol. 8, No. 3 (July 1970), pp. 165-167.
Sonic or ultrasonic energy is a form of mechanical
20 vibratory energy. Sonic or ultrasonic energy propagates
as waves through all material media including solids,
liquids and gases at characteristic velocities. The wave
velocity is a function of the elastic and the inertial
properties of the medium.
During the propagation of these waves in a medium very
high inertial and elastic forces are generated locally due
to the high frequency of these waves. The amplitude of
particle motion in the medium due to the sonic and ultra-
sonic waves range from a few micro inches to 5 milliinches
30 (0.005 inch) (0.127 mm) depending on the power level. The
peak acceleration developed in the medium due to an ultra-
sonic wave at 20,000 Hertz and an amplitude of 0.001 inch
(0.0254 mm) is as hiah as 40,000 G(1.5 x 106 inch/sec2)
(3.810 x 107 mm/sec2). One G(9.807 x 103 mm/sec2) is the
35 acceleration due to gravity. The forces generated due to
these levels of acceleration are very high.
~2578~6
- 4 -
For any medium these high inertia forces generated
due to the sonic or ultrasonic waves can cause material
failure, disruption and separation. The sonic or ultra-
sonic impedances of different materials, Pspecially in
5 solid and liquid phases are different by factors of 3 to
8. If the medium is a mixture of different phases of two
or more types of materials such as water and coal, etc.,
the inertia and elastic forces between them are likely to
be even higher. These high inertial and elastic forces
10 are likely to breaX the surface tension and promote
separation of liquid from solids.
In liquids a high level of sonic and ultrasonic energy
is also known to cause cavitation, a phenomenon of micro
bubble formation due to degassing and change of phase to
15 vapors. In the presence of solid particulate matter, the
level of cavitation is higher. The micro bubbles a-e
formed on the surface of the solids and assist in the
separation of the solid and liquid due to the formation
of gas liquid surfaces with much lower surface energy
20 compared to solid liquid surface. Cavitation also generates
high local shock waves and in some cases charged free
radicals. Shock waves and free radicals are likely to
accelerate liquid solid separation.
High oscillatory forces are developed in a medium due
25 to the application of ultrasonic energy. These high
oscillatory forces between the solid media and water in a
mixture and ultrasonic cavitation are believed to be the
major mechanisms of sonic and ultrasonic dewatering.
Degassing, decrease of viscosity and decrease of surface
30 tension due to ultrasonic vibration are other possible
mechanisms.
Ultrasonic energy is also partially absorbed by the
medium and is converted to heat. Internal heat generation
and the consequent temperature rise will further decrease
35 the viscosity and the surface tension of the fluid and
facilitate its removal. Local temperature rise is also
likely to increase the cavitation activity and accelerate
~25;7~346
-- 5 --
the rate of fluid removal. Therefore internal heating due
to the partial absorption of the ultrasonic energy has
added benefits to acrelerate fluid removal as in aqueous
systems.
In U.S. Patents 3,864,249 and 4,028,232 to Wallis,
there are found teachings of the use of acoustical pressure
wa~es and coupling them to a separation screen to facili-
tate separation of a liquid from material to be dried.
The use of an electrical field or acoustical energy
10 separately requires a substantial amount of energy.
It is an object of this invention to remedy the above
drawbacks by reducing the energy requirements for dewater-
ing, by incxeasing the rate of dewatering over present
methods and by lowering the final moisture content of the
15 product. The inventor has discovered that a concurrent use
of an acoustical field (sonic or ultrasonic), and an
electrical field (electrophoresis/electroosmosis) gives
unexpectedly improved results over an acoustical field
acting alone or an electrical field acting alone in
that:
1. this combination of concurrent use requires
less energy than using either of the two alone;
2. this combination ofconcurrentuse dewaters at
a faster rate than using either of the two alone; and
3. this combination of concurrent use gives a
lower final moisture content to the product than using
either of these two alone or using both in sequence.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the present invention there is
30 provided a method for separating water from solids in aqueous
suspensions of solids e.g. coal slurries and other materials
as further described below. The method comprises concurrently
subjecting the suspension to an electrical field and an
acoustical field (sonic or ultrasonic) so as to separate water
35 bound to solid particles and cause a migration of the particles
and water so as to form a region depleted of particles. ~ater
~257846
-- 6 --
is then withdrawn from the region depleted of particles,
such as by mechanical means. The energy required for an
incremental amount of separation is less with the combina-
tion of an electrical field and an acoustical field than
5 for either of the two separately or in sequence.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates, in semischematic form, a useful
10 apparatus for the practice of the i~vention.
Pigure 2 presents a graph showing plotting of percent
moisture in the samples versus time for the different
methods.
Figure 3 presents a graph showing plotting of
15 decrease in power used during application of an electrical
field.
Figure 4 presents a graph showing the percent mois-
ture in the samples versus energy consumed.
20 DESCRIPTION OF THE INVENTION
The drawing shows a dewatering zone or a chamber 10
bounded by electrodes 11 and 12 connected to an external
supply 200 of direct current connected by electrical leads
111 and 112. Electrode 11 is water permeable with openings
25 13. A filter 14 may be employed if the electrode 11 does
not have sufficiently small openings 13 to retain the
particles in the suspension. An acoustical generating means
15 produces an acoustical field having sonic or ultrasonic
waves (not illustrated) that are transferred throughout the
30 mass of the suspension in the dewatering zone.
In operation, the suspension to be treated is moved
into the dewatering zone 10 and subjected to the acoustical
field (sonic or ultrasonic) of such a frequency and amplitude
as to cause separation of water from the particles in the
suspension. Concurrently, a D.C. voltage is applied between
the electrodes 11 and 12 to cause migration of charged
particles in the aqueous suspension away from the water
~Z5~7846
-- 7 --
permeable electrode ll and toward the water impermeable
electrode 12 and movemen~ of water toward the permeable
electrode.
Sonic or ultrasonic waves prevent packing of the
5 suspended particles and facilitate flow of water toward
the water permeable cathode. As the slurry passes through
the dewatering zone the water is remo~ed through the water
permeable electrode ll and the dewatered suspension emerges
at the other end of the zone lO. Dewatering through the
10 water permeable electrode ll may be increased by augmenta-
tion means 16 such as by reducing the pressure outside the
permeable electrode, by increasing the pressure within the
zone or by a combination of the abo~e. The process of the
invention is operable without these augmentation means.
15 It is preferred, however, that augmentation means be used.
The means used for reducing the pressure at the outer
surface of the permeable electrode or for increasing the
pressure within the dewatering zone are those conventionally
used for pressure reduction or increase for augmenting
20 filtration.
The dewatering zone 10 may be bounded by two walls
(not shown) that connect between the two electrodes ll
and 12 so as to contain the slurry within the zone lO.
The walls are insulated from the electrode by insulating
25 materials not shown or the walls may be of an electrically
nonconductive material.
In this embodiment the electrical field and acoustical
waves are applied concurrently while the suspension flows
through the dewatering zone 10.
In other embodiments parameters for the electrical
field and acoustical energy may be changed to remove
additional water from the suspension by flowing it through
additional apparatus of the kind illustrated in Figure l.
In still other embodiments the suspension may be
35 pretreated or posttreated by acoustical energy or an
electrical field separately before being subjected to the
concurrent method of the invention.
~2S,7~46
-- 8 --
The frequency of operation of the sonic/ultrasonic
generator may be about 5,000 to 100,000 Hertz, but
preferably about 20,000 to 40,000 Hertz so as to minimize
the effects o~ audible noise on the work environment and
5 to keep the efficiency at a high level. The amplitude of
the sonic and ultrasonic waves can be any amplitude
sufficient to separate water bound to particles but is
preferably in the range of about 0.002 mm to 0.01 mm.
The applied voltage may be any voltage that will
10 cause a sufficient electrical field and a current to flow
so as to cause a migration of the charged particles and
water so as to improve the dewatering process by reducing
total energy requirements over the above mentioned methods
used separately.
The method may be used with aqueous coal slurries
such as those produced when coal is shipped by pipeline;
coal washery slimes when coal is separated ~rom contaminants
after mining, suspensions produced in ore processing such
as il~menite ore processing slurries or sludges or h~ematite
20 ore processing slurries or sludges; clay suspensions
produced in production of ceramics; fermentation and sewage
sludges; and protein hydrolysates. It is believed
that the method of the invention is generally applicable
to the above listed sludges and slurries.
Dewatering may be further augmented by addition of
small quantities of surface modifiers such as detergents or
surfactants. Examples of these surface modifiPrs are
polyacryl amide gels, polystyrene sulfonates and the like.
The preferred amount is capable of being readily determined
30 by those skilled in the dewatering art. These surfactants
are to be added prior to the concurrent application of the
acoustical and electrical fields. The types of surfactants
could be nonionic, anionic or cationic types. The surfactants
because of their hydrophobic nature attach to the coal
35 surface and release the water.
~;7
Certain types of materials, when mixed with water,
e.g. certain clays and coals, do not exhibit sufficient
conductivity to allow for proper current flow during
application of the electrical field. In this situation
5 the conductivity of the mixture must be enhanced by the
addition of a salt or conductive enhancer e.g. NaC1, KCl,
KCl, Na2SO4, NaOH, KoH, NH4Cl. The same effect can be
achieved by changing the pH of the solution appropriately.
This will insure proper current flow and migration of the
10 particles.
The method may be used in the dewatering of any size
particles mixed with water. The preferred particle size
range is from about 10 micrometers to 100 micrometers.
~ ccording to the method of the present invention it
15 is important that the sonic and ultrasonic waves and the
electrical field be applied to the mass of the mixture
so as to penetrate throughout the mixture itself. If the
sonic and ultrasonic waves do not penetrate throughout the
mixture to be dewatered, an efficient separation of water
20 from the solids will not take place.
While the foregoing discussion and the following
examples use a vacuum in combination with ultrasonic and
electrical field dewatering it is to be understood that
use of a vacuum is not mandatory and that other means 16
25 may be used to augment the dewatering process of the
invention. Further, the following examples serve to
illustrate and not limit the present invention. Unless
otherwise indicated, all percentages are by weight.
Reference to a vacuum herein is in the colloquial
30 sense only and is used to refer to the application of a
reduced pressure (below atmospheric) to the samples.
Coal
A coal slurry with coal particles having a maximum
35 Tyler mesh size of -200 was prepared and is that used for
the tests of Examples 1 through 4.
~2~ 7~346
- 10 -
The slurry was preparecl using a dry coal to water
ratio of 1:1 resulting in a slurry of fift~ percent (50~)
water by weight. About 0.1a NaCl was added to all samples.
The particles were mixed with the water with a magnetic
5stlrrer until a homogeneous suspension resulted.
EXAMPLE 1
This comparative example illustrates how much water
is removed by usual vacuum filtration.
Table I Dewatering by vacuum Only
Time Vacuum
Sample (min) (cm. of Hg) % H?o
1 1 51 q4.46
2 4 38 (25" at 3 min) 42.0
3 7 38 (13" at 6 min) 39.0
4 10 38 (13" at Z min) 39.06
Sample size 35g.
This data is plotted for comparison in Fi~ure 2.
This initial experiment was performed with vacuum
only using a typical water faucet aspirator. From Figure 2
20it can be c~served that the solids concentration achieved
after the first ten minutes levels off with little change
thereafter. This is due to the formation of a cake which
retards the rate of filtration. Also, the solids concen-
tration obtained is rather low due to the limiting proper-
25ties exhibited by the particles. The water held by surfacetension forces, capillary forces, etc. is still being
retained. The water removed is mostly bulk water. From
past experience the results from 50g samples would not
deviate much from the results of the 35g sam~les. Thus
30the above results are valid for comparison purposes.
EXAMPLE 2
This comparative example illustrates the use of an
electrical field~ (electrophoresis and electroosmosis)(E)
35and a vacuum. The slurry was the same as that pxepared
earlier. Sample size was 35g for samples 1, 2, 3 and
50g for samples 4, 5, 6.
;7~6
TABLE Il
E
E E Energy
Ini~i~l Fnergy Corrected Water
Vacuum Temp. Ti~e Power Wa~t Watt Content
Sample (cm. of Hg) C (min? Watts Hsu~s Hours 7. H2O
1 38 -- 0.7 12.50.141 0.0705 38.5
2 38 -- 1.4 12.50.283 0.1415 35.6
3 38 -- ~.9 12.50.567 0.283 36
4 38 -~ 5 7~ -- 1.365 34.8
10 5* 38100 4 120 -- 2.127 27
6* 38100 4 220 -- 3.201 28
* Samples not used in Figures 2 and 4 du~ to water loss from
high te~perature.
This example performed with an electrical field and
15 a vacuum only gave improved performance over a vacuum alone.
This is shown in Figure 2. Using only a vacuum as in
Example 1 the moisture content after two minutes was only
about forty-three percent (43~). In the presence of an
electrical field under similar conditions the moisture
20 concentration was reduced to about thirty-five percent
(35%). The time and power levels for samples 1, 2, and 3
have been corrected to those for a 50g sample by a direct
algebraic ration to allow direct comparison with 50g samples
from other tests.
It was noted that the power used dropped rapidly as
the dewatering process continued. This is shown in
Figure 3 for samples 4, 5, and 6. Thus for samples
1, 2, and 3 a first approximation that assumed that the
power level was reduced linearly during the time the
30 test was used. For samples 4, 5, and 6 the energy
required was obtained by assuming a straight line between
the data points and calculating the area under each curve.
At higher current throughput (1-2 amp) for longer
periods of time, the electrodes became hot. Therefore the
35 potential between electrodes was correspondingly decreased
to allow smaller amounts of current flow~
~5~7~46
- 12 -
EXAMPLE 3
Example 3 was a comparative example performed using
only ultrasonics (U~ and a vacuum. The slurry was the same
as that prepared earlier. Sample size is 50g. Frequency
5 was 20,000 Hertz.
TABLE III
V
U Energy Molsture
Vacuum Temp. Ti~e PQwer Watt Content
~ample (cm. f H~) C ~ ? Watts Hours ~ H~0
lO 1 64 ~5 l 1602.67 29.0
2 30 36 6 1~016.0 24.0
3 ~ 10 16026.7 22.0
4 S3 56 l 2804.67 26.7
-- 68 6 28028.0 19.3
15 6* 66 9610 28046.7 5.2
~ 61 42 1 ~80 8.0 24.8
8 58 74 3 48024.0 20.5
9* 56 91 6 48048.0 17.3
*Samples not used due to high temperatures.
By using ultrasonics and a vacuum, filtration rates
were much higher than either vacuum alone or vacuum and
an electrical field. As explained previously, dewatering
in the presence of ultrasonic energy occurs mainly due to
cavitation phenomena. At two minutes the solids concen-
25 tration achieved is about twenty-three percent (23%) as
compared to about thirty-five percent (35~) in the presence
of the electrical field of Example 2. However it should be
mentioned that there was generation of heat from the horn
at longer experimental duration. In order to reduce this
30 effect, an external cooling coil was used. The temperature
of the slurry was constantly monitored by means of a
thermocouple.
~Z~'7846
-- 13 -
EXAMPLE 4
This example was performed using a combination of
electrical field effects (electrophoresls/electroosmosis)
(E~, ultrasonics (U) and a YaCuum. ~he slurry was the
5 samP as that prepared earlier. Sample size is 50g except
as noted in Table IV.
3.z578~6
-- 14 -
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~,2~7~6
- 16 -
The results of the co~ination of both electrical
field ~nd ultrasonics are shown in Table IV and Figures
2 and 4. The moisture concentration achieved in two
minutes W35 about eighteen F)ercent (18%) as opposed to
5 about twenty-three percent (23~) by ultrasonics or about
thirty-five percent (35%) by use of an electrical field.
At comparable pswer inputs the same relationships were
observed. For example the combination of electrical
field and ultrasonics at 112.5 watts gave a liquid
10 concentration of about fifteen ~ercent (15) while that for
electrophoresis alone at 120 watts and ultrasonics at
160 watts was much higher. At comparable power levels
he rate of dewatering is also much higher.
Figure 4 shows the variation of percent moisture
15 content of the slurry or suspension versus energy consumed.
The energy values for an electrical field alone and in
combination have been corrected to reflect the reduction
in energy used as the dewatering process proceeds. This
correction has been made using a straight line approxima-
20 tion. The actual and corrected values are shown in theprevious tables.
To achieve similar amounts of dewatering in the sam-
ples the combination of an electrical field and ultrasonics
gave much lower values than either method alone. For
25 example at four watt hours, the percent moisture content
in the presence of an electrical field is about thirty-
five ~ercent (35%) while in the presence of ultrasonics
it is about twenty-six percent (26%). The combination of
both techniques concurrently; however, gives a much lower
30 moisture content of about nineteen percent (19%).
These tables and figures show that the rate of
dewatering, power consumption and ultimate solids content
are superior for the combination of the techniques than
the individual techniques.
7~
- 17 -
The data further indicates that optimum separation is
obtained when the energy input from the electrical field
and acoustical field are approximately equal. Thus samples
15, 16, 17, 18, and 19 which have a much larger proportion
5 of their enexgy input as ultrasonic energy initially give
data similar to samples using only ultrasonics but gradually
decrease to separation levels of the combination. Other
samples, such as l through 13, 20, and 21, where power
levels for the two effects are more nearly equal give
10 better results. Once ~nowing the teaching and advantages
of the invention a person skilled in th~ art can readily
determine ~he optimum power ratios to be used.
Vacuum figur~s in Example l, samples 2, 3,and 4,
Example 2, samples l through 6 and Example 4, sample 8 were
15 lower than those for the remainder of the data. This is
not believed to affect the results of the experimentsin the
the higher vacuum, above that used in the above cited
samples will not give an increased dewatering rate or a
final lower moisture content.
Temperatures were measured for some of the samples
with a thermocouple.
Certain data points in the tables were not used in the
graphs. These points exhibited higher temperatures in the
slurry. It is believed their presentation would give
25 unreliable or erroneous results for moisture content in
that higher temperatures would cause water loss by
vaporization. While a moderate temper~ture increase
benefits the dewatering process as discussed earlier,
excessive temperatures are generally avoided in that it is
30 difficult to clearly demonstrate the significant differences
of the process of utilizing a smaller amount of energy in
dewatering coal with a combination of an electrical and
ultrasonic field. Generally a temperature above 90C is
deemed excessive.
~,257~6
- 18 -
Ceramic Slurry
EXAMPLE 5
~ ceramic slurry was used for Example 5 with
5 the results listed in Table V below. The ceramic
slurry consisted of very fine clay particles in the
order of 50-75 um in size.
The ceramic slurry is further characterized
by having an initial solids content of 27%. Sample
10 sizes were approximately lOOg. The voltage during
electrophoresis was 50 volts for Samples 3-5 and
25 volts for Samples 6-8. Fxequency of the ultrasonic
energy applied was 20,000 Hertz.
The procedure differed slightly from that in
15 Examples 1-4 in that acoustical energy was not applied
initially but only after the slurry had begun to
form a cake. Water was thus initially removed by
vacuum or a combination of vacuum and electrophoresis.
Acoustical energy was not applied until after cake
20 formation because it was noted that application of
acoustical energy did not improve the dewatering
rates until a cake had begun to form. This is because
the acoustical energy is not needed until unbound water
has been removed and/or the slurry particles begin
25 to clog the filter.
The figures given in Table V are those where
it was determined that the dewatering rate had reached
an asymptote and no further water could be removed.
Table V lists the solids content using a vacuum (V)
30 only (Sample l), a vacuum (V) and ultrasonics (U)
(Sample 2), a vacuum and electrophoresis (E) (Samples
3-6) and a combination of a vacuum (V), electrophoresis
(E) and acoustics (~) (Samples 7-8).
It is apparent from Table V that by the use
35 of a combination of electrophoresis and acoustics
~,2~78~6
19 -
a higher total solids content can be achieved. Further,
the same energy levels give a higher solids content
for the combination of electrophore~ic and acoustic
means over either alone. Finally, for incremental
water removal WR, each milliliter of water removed
for the combination required less energy than the
use of either electrophore~ic or acoustic means alone.
In fact, ultrasonics alone was detrimental in that
- it did not improve dewateri~g characteristics.
Samples 7 and 8 use a level of ultrasonlc energy
about double that for electrophoresis, it is to be
expected that this higher level of ultrasonic energy
would reduce the dewatering effectiveness over that
where the energy levels are approximately equal.
WR is calculated by dividing the additional water
removed, over that by vacuum alone, by the total
energy used. This method is applicable to other
materials with characteristics similar to ceramic
slurries.
Sewage and Antibiotic Sludges
EXAMPLE 6
~he materials used here represented a mixture
of sewage sludges and antibiotic sludges (fermentation
sludges). The antibiotic sludges are those typically
produced by fermentation processes in the pharmaceutical
industry. For this example, the antibiotic sludyes
had been mixed with typical sewage sludges. Initial
solids concentration in the sludges was 4% and sample
size was 50g. The frequency used for acoustical
energy was 20,000 Hertz. The voltage for electrophoresis
was SOV. Except for Sample 2 which was 25V. As
in Examp~e 5 ultrasonic energy was not applied until
the sludge had begun to form a cake.
~25q~46
- 20 -
In Examples 5-8 the pH of the materials was
monitored and was noted to increase slightly during
the dewatering process. The material was adjusted
to low~ neutral, and high pH at the start of the
5 dewatering process. From this it was determined that
best results were obtained when the initial pH was
about 7Ø
Table VI lists results for vacuum (V~ ISample
1) vacuum and electrophoresis (E) (Samples 2-3) and
lO the combination vacuum, electrophoresis (E) and ultra-
sonic (U) dewatering (Sample 4). The figures given
are those where it was determined that the dewatering
rate had reached an asymptote and no further water
could be removed.
In this example ultrasonic data is not listed
since no dewatering was obtained with a vacuum and
ultrasonics alone. The combination of vacuum, electropho-
resic, and ultrasonic dewatering means gave a higher
total solids content that either electrophoretic or
20 ultrasonic means,also the same energy levels would
give a higher solids content for the combination
of electrophoretic and acoustic means over either
alone. Finally, for incremental water removal WR,
each milliliter of water removed for the combination
25 required less energy than either electrophoretic
means or ultrasonic means alone.
This method is applicable to waste activated
sludges, anaerobic and secondary sludges as well
as microbial sludges obtained by fermentation processes.
Protein Hydrolysate
Example 7
A protein hydrolysate sludge (containing the
35 valuable product in the supernatant) was used for
~L2~ 6
- 21 -
Example 7 that is typical of products obtained from
digestion of for example soybean meal. The product
contains a complex mixture of proteins and polysaccharides.
The hydrolysate suspension is characterized by an
5 initial suspended solids content of 15~. Sample
size is approximately 50g. The voltage during electropho-
resis is listed in Table VII. Frequency of the ultrasonic
energy applied was 20,000 Hertz. As in Exampie 5
ultrasonic energy was not applied until the hydrolysate
10 had begun to form a cake.
The figures listed in Table VII are those where
it had been determined that the dewatering rate had
reached an asymptote and no further water could be
removed. Table VII lists the solids content using
15 a vacuum (V) only (Samples l and 2)l a vacuum and
ultrasonics (U) (Sample 3), a vacuum and electrophoresis
(E) (Samples 4 and 5), and a vacuum and the combination
of electrophoresis (E) and ultrasonics (U) (Samples
6-8). CaCl2 was added to the suspension to improve
20 the dewatering characteristics but had no noticeable
effect. There is some evidence in the literature
that calcium ions bind to the proteins and might
release the water bound between the protein chains.
It is apparent from the data in Table VII that
25 by the use of a combination of electrophoresis and
acoustics a higher total solids content can be achieved.
Further, the same energy levels give a higher solids
content for the combination than either electrophoresis
or ultrasonics alone would. Finally, for incremental
30 water removal WR, each milliliter of water removed
for the combination required less energy than the
use of either electrophoretic or acoustic means alone.
In fact, the use of ultrasonic means alone was detrimental
~ the dewatering characteristics.
5'7~
- 22 -
Samples 6 and 7 that use much higher levels
of ultrasonic energy show reduced dewatering efficiency
over that of Sample 8 where the energy levels for
electrophoresis and ultrasonics are more equal.
It is noted that the efficiency of water removal
as shown by WR is very high for Sample 4 with a value
of 12.1 ml removed per watt hour. However, a level
of only 53.2% solids could be reached making the
use of electrophoresis alone not much better than
10 the use of a vacuum only. It is expected that the
combination of electrophoretic and ultrasonic means
would give still higher efficiencies when the material
is only dewatered by 2-3% as in Sample 4. Since
each further milliliter of aqueous product (protein)
15 removed requires more energy than the previous one,
the efficiency values will be lower for a solids
content of about 60% compared to a final solids
content of about 50~. ,
The greatest advantage of the invention, in
20 addition to higher efficiency, is the much higher
separation of solids from liquid resulting in a much
more valuable product or recovering more product
that had previously been lost.
This method is applicable to food and pharm ceutical
25 products where thermal techniques cannot be used
due to heat sensitivity and where valuable products
soluble in water are left in the c~ke. Efficient
recovery of the supernatant is possible by this technique.
The method is further applicable to materials with
30 charac~eristics similar to protein hydrolysates.
57
-- 23 -
TABLE V
CERAMIC Sl,URRY
E U Energy
Total Moisture WR
Vacuum (Watt (Watt ~Watt Percent ML
Sample (cm Hg) Hours) Hours) Hours) (Solids) Energy
1 38 ~ -- ---- 55 --__
2 38 ---- 4.17 4.17 54 -- -
3 38 5 ---- 5 56 0.18
4 38 4.17 ---- 4.17 57 Q.41
38 3.75 ---- 3.75 58 ~.68
6 38 3.17-___ 3.17 55.5 0.08
7 38 2.~33.33 5.57 64 1.24
8 38 1.673.33 5.00 60 0.82
TABLE VI
SEWAGE AND ANTIBIOTIC SLUDGE
E U Energy
Total Moisture WR
Vacuum (Watt (Watt (Watt Percent Ml
Sample (cm Hg) Hours) Hours) Hours) (Solids) ~nergy
1 38 - - ---~ - 25.7 ----
2 38 3.60 ---- 3.60 36 0.59
3 38 5.67 ---- 5.67 34 0.3~
4 38 5.5 5 10.5 45 0.31
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