Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a method and apparatus for improved
collection of particles which exhibit a relative potential, such as for example
cationic or anionic resins, particulate or fiber suspensions, and in particular
to a method and apparatus for recovering anionic tall oil soap particles from
black liquor produced from wood pulping processes.
In the wood pulping industry, tall oil soap in the black liquor ob-
tained from pulp digesters via the pulp washers and/or the evaporators is typi-
cally recovered by skimming off the tall oil soap particles which float on the
surface of the liquor as a scum. However, additional or residual tall oil soap
remains dispersed in fine particles within the skimmed black liquor and is
usually lost when that liquor is burned to recover soda ~alues.
The matter of residual tall oil in skimmed liquor has long been a
matter of concern for the pulp industry. A residual of .7%, based on black
liquor solids (also commonly referred to as BLS), has historically been establ-
ished as an acceptable value. Aside from a smaller dissolved fraction, this
residual is a result of minute soap particles which remain suspended in the
black liquor. Because of their small size, the rate ~hat these soap particles
rise through the liquor is so slow that, under normal retention conditions in
skimming tanks, the particles never make it to the top to be skimmed off and
thus are lost when the liquor is burned. Since the rate of ascent of these
particles is approxi~ately proportionate to the square of their radius, in
accordance
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with Stokes Law9 it would be advantageous if these minute
particles could be agglomerated into larger particles so
that retention time in the skimming tanks could be reduced
and so that the total amount of remaining colloidal suspen-
sion can be reduced substantially below that conventionally
attainable. Thus, it is an object of this invention to
agglomerate these minute particles.
It is another object of the present invention to
recover additional tall oil soap from the black liquor.
Another object of the present invention is to recover
tall oil soap from the black liquor of pulping processes by
a relatively sim~le and inexpensive process and apparatus~
A still further object of the present invention is
to cause agglomeration of tall oil soap particles in black
liquor so that the particles will float to the surface of
the liquor for re val by skimming or other processes.
A still further object of the present invention is
to improve collec~ion of colloidal or otherwise suspended
particles in a liquid that exhibit a colloidal charge or
20 ~ zeta potential, in the liquid.
The use of electricity to achieve a reduction in
tall oil residuals in black liquor has been attempted in
the past, for example by Drew as disclosed in U. S. Patent
No. 3,356,603. In bhat system a corona discharge is pro-
duced above the surface of the black liquor. HoweverJ the
voltage required to produce an electric field that ~ould to
any significant degree alter the rate of migration of soap
particles, even one foot beneath the liquor's surface t iS
manyfold the arcing potential of the required field, Thus
before the desired high field gradient is achieved, arcing
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would be induced which, for all practical purposes, reduces the instantaneous
field to zero.
Drew also suggested putting the black liquor in a tank in direct
contact with two electrodes, apparently after first treating the black liquor
in a corona discharge step. However, the voltage applied is less than that
necessary to induce the decomposition of water with the result that there is
essentially no response whatsoever in reducing the residual tall oil content
of the black liquor.
In accordance with one aspect of the invention~ there is provided
a system for removing suspended materials from a liquid comprising means for
providing a flow path for said liquid, said flow path including upstream and
downstream zones; first and second electrical conductors respectively located
in said upstream and downstream zones and insulated from each other and from
adjacent parts of said system; and means for applying opposite electrical
potentials to said first and second conductors, wherein particles in said
liquid having a relative potential opposite to the potential of said first
conductors will migrate toward the first conductor as the liquid flows through
said upstream zone and then agglomerate with others of said particles as the
liquid carries said particles along said path.
Another aspect of the invention provides the process of removing
particles from a liquid comprising, the steps of, flowing a stream of liquid
along a flow path through an upstream zone and a downstream zone~ electrically
insulating said upstream and downstream zones; contacting said liquid in each
of said zones with the surface of an electrical conductor, maintaining the
contact surface in one of said zones at a relatively fixed positive potential~
maintaining the contact surface at the other of said zones at a relatively
fixed negative potential, and superimposing a pulsating positive and negative
potential on said zones and said other contact surfaces, respectively.
In one embodiment of the present invention, tall oil soap particles
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from black liquor are agglomerated in an apparatus including an elongated
conduit providing a flow path for the black liquor and having upstream and down
stream zones. First and second electrical conductors are respectively located
in the upstream and downstream æones, and a positive electrical potential is
applied to the first conductor while a negative potential is applied to the se-
cond conductor. In the upstream æone ahead of the first electrode, means are
provided to introduce air into the stream and to agitate the stream vigorously
by a beater.
This invention takes advantage of the "charge-like" colloidal
nature of the tall oil soap particles, commonly referred to as æeta potential,
and reduces the repulsive forces of electric origin between the suspended par-
ticles, or even reverses the polarity of some of the particles, to produce at-
tractive forces such that the minute colloidal particles agglomerate into larger
particles which will have a much greater rate of ascent in the skimming tank.
By the arrangement of the apparatus of the present invention the anionic tall
oil soap particles in the liquor are attracted
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towards the first conductor as liquid flows through the
upstream zone and some of the particles are stripped of
a portion of their negative anions so that they exhibit
a positive charge and thus attract other, unstripped
particles to themselves. The particles then pass with
the liquor to the downstream zone wherein any excess
positively charged particles are attracted to the nega-
tively charged second conductor to induce further agglom-
eration while any~ excess positive charge is neutralized.
0 The flow of black liquor through openings in the second
conductor continually washes the agglomera~ed particles
off of the conductor, and the liquor and:particles flow
into a skimming tank where the agglomerated particles float
to the surface of the tank for removal, e. g. by skimming,
or sink to the bottom and are decanted as in the case of
denser agglomerates. The conductors are charged with
relatively fixed potentials, but it has been found-that
substantially improved agglomeration will occur if, in
addition to the D. C. base fleld a pulsating potential is
superimposed ~hereon.
This process is also adaptable to other liquids
and particles, particularly particles which exhibit a
relative cationic or anionic potential. When particles
'~ having a cationic potential, as opposed to the anionic
l~ 25 potential of tall oil, are to be removed the polarity of
the first and second conductors is reversed.
The above, and other objects, features and advan-
tages of this invention will be apparent in the following
detailed description of an illustrative embodiment thereof
which is to be read in connection with the accompanyin~
drawings wherein:
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Figure 1 is a perspective view of a system constructed
in accordance with the present invention;
Figure 2 is an enlarged sectional view of a por~ion
of the flow path of the system of Figure l;
Figure 3 is a sectional view takeD along line 3-3
of Figure 2;
Figure 4 is a somewhat schematic diagram of a con-
trol system used in conjunction with the system of Figure l;
Figure 5 is a diagrammatic view of a typical anionic
colloid;
Figure 6 is a side view, similar to Figure 4~ and
with parts broken away for clarity in illustration, of another
embodiment of the present invention;
- Figure 7 is a diagram illustrating the electrical
potentials applied to the conductors of the device shown in
Figure 6; and
- Figure 8 is a chart plotting residual tall oil
against constant current value applied to the conductors used
in the apparatus of the present invention.
Referring now to the drawings in detail, and initial-
ly to Figure 1 thereof, a system 10 for improved collecti~n of
particles, and in particular tall oil soap particles s~ch as
exist in the black liquor obtained from wood pulping processes,
is illustrated. As seen therein, the black liquor is supplied
from the pulp washers or evaporators (not shown) to a conduit
12 which defines a ~low path from the evaporators or pulp
washers to a collection tank 1~ or the like. Conduit 12 is
.
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formed of a plurality of pipe sections, including a pair o~
pipe sections 16, 18, which are formed of an electrically
nonconductive material that is resistant to high temperatures.
Such materials can take a variety of forms, and it has been
~ound that pipe sections formed of synthetic materials sold
under the trademarks "Kynar" or "FRP" are satisfactory, as
are most fiberglass materials.
Pipe section 18 divides conduit 12,into upstream
and downstream zones through which the black liquor ~lows.
A first conductor 20 is located within a pipe section 22,
between insulator sections 16, 18, in the upstream zone while
a second conductor 24 is located in a pipe section 26 in the
downstream zone. These conductors may take a variety of forms,
such as wire mesh grids, spaced graphite sheets, or simply a
series of individual wires spaced within their associated pipe
sections. However it has been found that a suitable conductor
is formed from conventional packing material used in a variety
of different applications. This packing material is ilLustra-
ted in Figures 2 and 3, and consists of a pl~rality of layers
of corrugated conductlve sheet material. The ad,~acent layers
are positioned at an angle to each other and secured together
at the intersection of the apices of their corrugations. In
this manner a series of individual flow paths are ~ormed through
the packing body or conductor. One such packing material is
the Koch-Sulzer packing and is available from the Koch Engin-
eering Company.
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Opposite electrical potentials are connected to each`
of the conductors 20, 24, as illustrated in Figure 1, i~ any
conventional manner. In the illustrative embodiment of the
invention wherein tall oil so~p particles are to be removed
from the black liquor, the first conductor Z0 is connected
to a positive potential while ~he second conductor 24 is
connected to a negative potent;aL. The source of the poten-
tial may be a rectiier, battery, generator, or constant cur-
rent source, as schematically indicated at 25 in the drawing.
A constant current source is preferred. Due to process varia-
tions, the conductivity of the liquor varies somewhat. A can-
stant current source (commercially available)~adjusts the vol-
tage up or down automatiacally to maintain a proper current
density.
The tall oil soap particles to be removed by the
apparatus are colloidal in nature and, as with nearly all
colloidal particles in-a solution containing free ions, have
a tendency to attract either positive or negative ions present
in the solution to their surface. Most such colloids are neg-
atively "charged", tha~ is they have ions held on their sur-
faces by relatively weak hydrogen bonding or "ield bonds", as
shown diagrammatically in Figure 5. The surface ions, depend-
ing on their nature, impart a marked tendency for the particles
~5 in suspension to migrate toward either a positive or negative
field. The qualitative measurement of this tendency is xeferred
to as "Zeta" potential. The greater these charges, the ~reater
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are the particles' inter-repulsion and therefore the greater the
"Brownian" stability; and the smaller the zeta potential the
less repulsion and accordingly the greater agglomeration and
flocculation. In the case of most of the constituents of sapon-
ified tall oil suspended in "black liquor" the particles are
surrounded by negative ions and have a tendency to migrate
toward a positive potential. As with all charged particles,
the colloids in black liquor will migrate to the oppositely
charged electrode at a transfer velocity proportionate to the
magnitude of the charge and the applied field.
The embodiment of the invention in Figure 1 uses these
phenomena advantageously by using the conductors 20, 24 to pro-
duce a field gradient whose potential exceeds the potentail re-
quired to induce the decomposition of water; for example, between
20 and 150 volts. The field gradient is applied in line such
that the direction of fluid flow is in direct opposition to the
ionic mass transfer induced by the applied field. That is, in
the region between the upstream and downstream zones, the fluid
moves through conduit 12 in a direction opposite to the direction
in which the colloid particles would tend to migrate~ i.e. between ;
the conductors under the influence of the applied field. This re-
duces the current density which would otherwise be required to
induce electrolytic polarizations and increases the relative colloid
concentration between the conductor. For example~ as shown in
- Figure 1, as the black liquor flows through the first conductor 20,
the tall oil soap particles suspended in the l~quor are attached ~;
toward the conductor surfaces because these particles have a natural
anionic tendency, that is, they tend to migrate toward a positive
potential. However, as the particles enter and pass
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through openings in the conductor 20, at'least some of the
particles become; in e~fect, positively charged. That is,
the positive voltage applied to conductors 20 strips a
portion of the negative ions from the soap colloids from
the outer layer of the colloid (see Fig. 5) thus producing
an affinity for negative ions, i. e. producing "holes".
Other minute soap colloids, from which anions have not been
stripped or have been less completely stripped, remain
surrounded by more of their normal complement of uegative
ions. The particles that still retain all or nearly all of
their anions share these anions with the holes. In this way
a multitude of soap colloids are allocated and bound to one
another to form larger particles and ultimately globules.
The particles'then move with the black liquor through the
conductors and the insulator section 18 into the second
conductor 24. In addition, the particles that have not been
stripped are repelled by the second conductor 24 and
attracted by the positive conductor 20 so that thPy tend to
move against the flow of black liquor in the region between
conductors 20 and 24. In so doing these particles encounter
the more completely stripped particles and agglomerate with
them to orm larger globuIes. As the size of each globule
increases, the hydraulic flow of black liquor provides
increasing pressure and the more neutralized c~ar~e reduces
the electric field orce so that the globules get carried
through the second conductor 24 by the black liquor.
When the particles enter the second conductor 24,
any particles still having a net e~fective positive charge
move toward the layers of the negatively charged conductor.',
, 30 Movement of the particles toward the layer walls of
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conductor 24 causes the particles to contact one another,
coalesce and agglomerate. This agglomeration is further
enhanced by the fact that any positive charge remaining
on the particLes is "neu~ralized" by the negative potential
of the second conductor. As the agglomerated particle
size increases, the particles, adhering to the conductor
walls ? are swept and cleaned from the walls by the flow
of black liquor through the conductor. Thus the black
liquor and agglomerated particles pass from the conduit
12 into tank 14.
Tank 14 may simply be a settling and skimming tank,
in which the black liquor resides for a predetermined period
of time to allow the agglomerated tall oil soap particles
to float to the surface of the liquor wherein they are
skimmed off, as for example by mechanically or pneumatically
pushing the curdy tall oil soap on the top-of the liquor
through an opening or discharge orifice 30 formed adjacent
the upper edge of the tank.
In a presently preferred embodiment additîonal
concentration and accordingly agglomeration of tall oil
soap particles within the black liquor contained in tank
14 is achiev~d by providing a third conductor 32 within
the tank. This conductor, as illustrated in Figure 1, is
formed from a plurality of electrically connected vert-
ically extending rods or wires 34 which are electrically
insulated from tank 14 in any convenient manner. For
example, wires 34 can be mounted in a frame 36 formed of
an insulating material, such as ~iberglass or high tempera-
ture PVC plastic.
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Where tall oil soap is being removed from the
liquid flowing through the apparatus, conductor 32 is
connected ~o a positive potential source. The tank
itself is electrically connected to ground or to t'ne
same negative potential source which is connected to the
second conductor 24. By this arrangement, when the
neutralized tall oil particles enter the tank with the
black liquor, they once again tend to migrate toward the
positively charged rods 34 and locally concentrate and
accordingly further agglomerate, because of their natural
relative anionic potential. As the particles migrate
toward the rods, due to increased local concentration, they
merge with one another to form larger globules of tall oil
soap. The larger the globules become the greater their
buoyancy is, and they will ascend to the surface of the
liquor in the tank more readily.
In order to further improve the agglomeration of
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the tall oil particles in the black liquor, air is intro-
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duced into the flow of black liquor upstream o first
conductor 20. The air is introduced from a source (not
shown) through a pressure regulator hO and air flow meter
42 connected in any convenient manner to a nozzle h2
contained within conduit 12. Preferably a static mixer
consisting of a series of rîgid vanes is placed downstream
~25 of the air supply but upstream of first conductor 20. The
air and static mi~er produce turbulence and small~air
bubbles in the black Iiquor which appear to enhance the
effectiveness of the charging girds or conductors. ~t is
believed that this improved effectiveness is the result
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of an increase in the effective charge transfer surface
(the minute air bubbles) and the surface exposure of the
liquid to the conductors. In addition the minute air
bubbles become entrapped ln the agglomerated soap part-
icles and thus increase the buoyancy of the globules and
their rate of ascent in tank 14. For this same reason
and those previously mentioned, it is necessary that the
relative potential applied to the various conductors
within the system be greater than the decomposition poten-
tial of water (i. e. greater than approximately 1.5 volts)
and typically about 23 volts for black liquor, so that
the flotation of the particles is enhanced by the small
quantity of hydrogen and oxygen bubbles resul~ing rom
electrolysis of the wa~er in the black liquor. These
bubbles also become entrapped in the tall oil soap glob-
ules to improve the buoyancy and increase the rate of
ascent of the globules in the tank 14.
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Although the flow of black liquor through conduit
12 will carry agglomerated particles of tall oil soap with it,
it is possible that the conductor grids may at times ~ecome
clogged with the soap particles or pulp fiber, etc. For this
reason a flow control system is provided for backwashing the
conductors when necessary. As seen in Figure 1 flow conduit
12 is provided with pressure gauges 50, 52 at opposite ends of
the upstream and downstream zones. At this point, T-connectors
51, 53 are provided which respectively include drain and supply
pipes 54, 56 controlled by valves 58, 60. In the normal mode
of operation thes valves are closed. Conduit 12 is also pro-
vided with valves 62, 64 which are normally open. Finally a
bypass conduit 66 is provided controlled by a valve 68.
When the pressure differential recorded by gauges 50,
52 reaches a predetermined specific process limit, the operator
of the apparatus opens valve 68, closes valves 62,64 and then
opens valves 58 and 60. By opening valve 60 water under pres- -
sure from a source (not s'nown) enters conduit 12 and ~lows in
a reverse direction from the normal flow of the black liquor
20~ through conductors 24, 20 and out discharge pipe 54 through
valve 58. The black liquor which is continuously supplied from
the evaporators or washers simply bypasses the conductors
through conduit 66 and enters tank 14. A~ter the d~sired back-
washing time cycle, valves 58 and 60 are closed and valves 62,
64 are opened and valve 68 closed, in that order.
In anot'ner form of the invention illustrated in
Figure 4, the backwashing control is automated. As seen therein,
in lieu of pressure gauges 50, 52 pressure sensors are provided
at the juncture of conduit 12 with the T-fittings 51, 53. These
pressure sensors are of conventional well known construction and
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are connected to a conventional pressure controller which
monitors the pressure difference between the upstream and down-
stream sides of the conduit. When the differential pressure
controller detects the specified process limit differential
pressure it produces a signal directed to a time relay sequencer
70, also of conventional construction, which in turn controls
valves 58, 60, 62, 6~ and 68 to close valves 62, 64 and open
the remainder of the valves in proper sequence to allow for
backwashing. Then after a predetermined period of time,
sequencer 70 closes val~es 58 and 60, reopens valves 62 and
64 and then closes valve 6~. -
It has been found ~hat in addition to applying a
relatively constant potential to the conductors, it is advan-
tageous to apply a pulsating potential to the conductoss in
the presence of an existing s~able electric field, with the
result that the DC pulses are more selective as to the electro-
lysis of ~he attached ions in the colloids versus the decomposi-
tion of the water and other constituents within the liquor.
.
As a result there is assurance that the desired small but
steady percentage of soap colloids in the liquor will be
stripped of their ions, to produce holes, in order to induce
a greater amount of agglomeration of the colloids within the
liquor stream. An embodiment of the invention adapted for this
process is illustrated in Figure 6. This apparatus is similar
in construction to the apparatus shown in Figure 1, and only
the flow path upstream of the skimming tank is illustrated.
As seen in Figure 6, the apparatus 100 includes a
flow path 112 through the black liquor flows from a source
thereof (not shown) to the skimming tank. In this embodiment
the flow path or conduit 112 includes a first T connection 151
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formed of fiber-reinforced plastic or other suitable insulating
material. This T connection electrically insulates the upstream
end of a stainless steel tube 122, which contains grid conductor
120, which is formed in a manner similar to the conductive grid
20 previously described, in order to divide the flow path of
black liquor into a plurality of small streams flowing through
the passages provided in the grid. The black liquor flows from
the tube 122 to a second T member 118 formed, like the T member
151, of an electrically non-conductive or insulating material
such as for example fiber-reinforced plastic which serves to
electrically isolate the upstream tube 122 from the conductor 124,
which is located downstream of the T member 118. The latter
conductor is contained within another tube 126 formed of stainless
steel or the like. The black liquor flows from this conductor
through a third T member 153, formed of an electrically insulating
material, to the remainder of the conduit 112, for passage to the
skimming tank. Both the upstream and downstream extremities of the
conduit 112 are electrically connected to ground. The entire
assembly of the T members 151, 118, 153 and tubes 122, 126 is surrounded
by a fiberglass cover, or tube, 119, to electrically shield the
tubes 122, 126 and thus the conductors 120, 124. These conduc-
tors are connected by lines 120', 124', respectively, to a
junction box 125 which in turn is electrically connected to a
control panel 127. The control panel comprises the power supply
to the conductors 120, 124 and consists of a constant current
source, which applies the relatively stable DC base voltage to
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the conductors L20, 124 and, in accordance with an important
aspect of the embodiment of the invention in Figure 6, also
applies a pulsating voltage to each of these conductors.
Figure 7 illustrates the power supply to the conduc-
tor. As seen therein, each of the conductors has a base vol-
tage applied to it, which is relatively steady and is determined
by the constant current source. As previously mentioned this
base voltage consists of DC voltage of approximately plus or
minus 20 volts to plus or minus 150 volts relative to ground,
~ which may be considered to be at a potential of 0 volts. The
piping and components upstream of the T 151 and downstream of
the T 153 is at ground potential. This means that the ~ystem
has three electric fields: conductor 120 to ground, conductor
124 to ground, and conductor 120 to conductor 124. The base
voltage, as controlled by the constant current power supply,
maintains a relatively constant current density of for example
1-3 amps per cross-sectional square inch, assuming a ten gallon
per minute flow per cross-sectional square inch of bLack liquor
through the conduit 112, regardless of the fluctuations in
fluid conductivity. In addition to this output, the control
power supply unit may be capable of producing a variable fre-
quency pulse train of variable magnitude having peak voltages
~ of,for example, between 100 and 280 volt9, applied to the res-
¦ pective conductors or grids to make the positive conductor
¦ 25 more positive and the negative conductor more negative, res-
; pectively, for the duration of each pulse. The positive and
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negative pulses may be applied to the respective positively
and negatively based conductors 120, 124 simultaneously or
alternately. For black liquor, it has been found that approxi-
mately 120 pulses per second at plus or minus 280 volts res-
pectively relative to ground and having a duty cycle of about10% to 20% produces satisfactory results. The power supply
unit, used with the present invention to supply the stable
DC base voltage and the pulsating positive and negative volt-
ages to the respective conductor 120, 124 can take any con-
venient form, as would appear to those skilled in the art,however one such apparatus can be for example a power supply
unit identified as Model 60544 sold by Research Inc. of
Minneapolis, Minnesota.
In order to lncrease the relative percentage of
colloids being affected, a high degree of turbulence is intro-
duced into the system of this embodiment of the invention via
a violent in-line agitator, in a manner similar to that of the
previously described embodiment. In this case however the agi-
tator or beater 146 consists of a rotating beater element mounted
within the conduit 112 and driven by a motor 147 at a relatively high
rate of speed of rotation, for example 2000 RPM. The beater
element may resemble a single-hoop egg beater. In order to
further increase the turbulence in the li~uor flowing through
the conduit, and to increase the overall surface area exposed
to the grid, while at the same time enhancing flotation, air
is introduced into the conduit upstream of the beater 146
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through an air supply conduct 142 or the like. The air is
supplied at the rate of approximately ten standard cubic
feet per hour per 100 gpm flow through the conduit.
The agitation of the liquid and the air bubbles
produced therein as a result of the introduction of air and
the electrolysis of the water, aids in cleaning agglomerate
film off the surface o~ the conductors or grids. If this
film were not continuously mechanically cleaned from the
conductors in this manner, and were left in a sta~tic situa-
tion, it would inhibit additional electrical transfer
between the colloids, and defeat any tendency or agglomera-
tion~ Therefore the agitation of the liquid and the intro-
duc~ion of air thereto provides synergistic effect, in that
it not only increases the electrical transfer between the
conductors in the colloids, but also keeps the conductors
,
clean.
The system illustrated in Figure 6 also includes
an automatic backwash control system, which is similar to
the automatic backwash arrangement illustrated and described
.
with respect to the embodiment of Figure 1 thereof. The
T 153 has a short pipe 156 connected to a valve 60 to
control the flow of wash water into what is normally the
. .
output end of the flow path 112. The T 118 has a short
pipe 172 connected through a valve 174 to a pipe 176 through
which wash water can leave the system after having cleansed
the conductor 124. The T 151 has a pipe 54 connected to
a valve 5~ through which wash water can also leave the
system after having cleansed the conductor 120~
In order not to impede the continuance of the
process, after having opened valve 68 and closed valves
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62 and 69, in that order, the valves 60 and 174 are prefer-
ably opened first and the valve 58 is kept closed so that
wash water entering the pipe 156 and passing through the
conductor 124 will emerge directly through the pipe 172,
the valve 184, and thepipe 176. After the conductor 124
has been cleansed by this reverse 10w of the wash water,
the valve 174 may be closed and the valve 58 opened to
allow the wash water to 10w in the reverse direction
through openings in the conductor 120 as well as the
conductor 124. During the washing process, the valve 68
in the by-pass conduit 66 is kept open to allow the blac~
liquor to flow around the electrical components in the
flow path 112.
It has been found that optimum stripping of tall
oil soap colloidal particles occurs when about 16% of the
particles have anions s~ripped from them by the electric
fields. The remaining approximately 84V/o are not afected
- and thus are oppositely charged. It is thought t~at
several of the latter particles can agglomerate with each
stripped particle. If the voltage, current, and flow-rate ,
conditions are such that anions are stripped rom too
many particles, there apparently are not enough unaected
particles to agglomerate with them, and the ef~lciency o
agglomeration drops o under such conditions. Since it
is the agglomeration oE individual colloidal particles
- into clumps that acilitates their movement through the
liquid in the tank 14 Gf Figure 1, the voltage conditions
that avor optimum stripping are most desirable.
It may be diicult to maintain optimum voltage
conditions to strip anions from a selected percentage of
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109Z~54
,
the particles that pass through the openings in the conduc-
tors 120, 124. If so, some of the incoming black liquor
may be diverted through the pipe 66 by opening the valve
68 enough to allow particles having the original va~ue zeta
potential to by-pass the electrical stripping fields but to
mingle with the excessively stripped colloidal particles as
they enter the tank 14. This has been found to produce
good agglomeration efficiency. Another alternative is to
connect the pipe 176 to the source of incoming black liquor
to inject colloidal particles directly into the region o
greatest numbers of stripped particles.
EXAMPLE 1
In a black li~uor supply at 165F. having 27.4%
of black liquor solids, and a conductivity of 146,~00 ~
mhos, and an initial tall oil availab;lity value of 1,78%
of black liquor solids, was passed through the apparatus
of Figure 6, at a flow rate of 250 gallons per minute and
subjected to a base voltage of 23 volts at the conductors
120, 124, and a current density of 1.3 amps RMS per cross-
sectional square inch, and to a superimposed pulsating
voltag~ having a maximum value of 150 volts. When the
liquor was subjected in the apparatus to the field includ-
ing the base voltage and the pulsating potential, as well
as the agitator 146 and air supplied through the conduit~
142, the residual tall oil value was .36%, when the liquor
was subjected only to the air and the agitator, the resid-
ual tall oil value was .67%; when the liquor was subjected
to only ~he agitator and the electric field, the residual
tall oil value was .48%; and when the liquor was subjected
to the potential field only, the residual tall oil value
was .51%.
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" ' , , ' , ' ' , " ' ' ' , " ~ ' ' ' . . . ' " ' ' . '' ' . '
. ' , , ' ,., , ' ' ' . :, '
~o~zo~ ~
EXAMPLE 2
A second sample of black liquor at 159F., having
24.8% black liquor solids content and a conductivity of
137,000 ~ mhos, and an initial tall oil availability of
1.84% of black liquor solids, was subjected to the same
conditions as in the prior example. When the liquor was
subjected to the air, agitator and the potential field,
the residual tall oil value was .39%; when subjected only
to the air in the agitator the residual tall oil value was
.66%; when subjected to only the agitator and the full
potential field (DC base and pulsation), the residual tall
oil value was .53% and when subjected to the potential
field only, the tall oil residual vaLue was .56%.
EXAMPLE 3
A third sample of black liquor at 164F. having
25.7% black liquor solids content, a conductivity of 129,000
~ mhos, and an initial tall oil availability of 2.75/~ oE
black liquor solids, was subjected to the same conditions
as in the prior examples. When the liquor was subjected
to the air, agitator, and the full field, including pulsa-
tion, the residual tall oil value was .38% of black liquor
solids; when subjected to all of the above except the pulsa-
tion of the potential field, the tall oil residual value
was .51~
From these results, it is clear that the cambina-
tion of a constant base field and the sporadic, more intense
high voltage field, is more effective than any of the compon-
ents alone and that each of the other components also
contribute to the overall improved performance of the
system. The specific values of the base voltage and the
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sporadic pulsating high voltage field, are emperically determined according
to the liquor or material being subjected to the process of the invention.
Figure 8 represents a graph showing the variation in the residual tall oil
contents of black liquor after skimming while using the electro-skimmer at
various current density set points. Also indicated on the graph is the base
line residual tall oil content of the black liquor after skimming without
the use of ehe electro-skimmer. In both cases it should be noted that the
average incoming tall oil availability was 2.11% of the BLS. The electro- -
skimmer set points represent the current supplied to the conductors 120, 124
in a pipe having a diameter of 5 3/4 inches. From this chart is is seen that
the optimum current supply applied to the pipe, is between 26 and 31 RMS
amps.
It has been found that by the method and apparatus of the present
invention the amount of tall oil soap recovered from black liquor is as much
as 10% to 30% greater as compared to simple conventional skimming techniques
(depending on liquor composition and other factors). In view of the current ;
price of tall oil soap this increase represents a substantial economic gain
by the use of the invention. For example, tall oil produced in the south~
eastern part of the United States currently sells for about $165 per ton.
About 3000 pounds of black liquor solids of which a portion is tall oil soap
(typically 1 - 8 %) are produced per ton of processed pulp. Prior to this
invention approximately 0.7% of the total black liquor solids which are tall
oil soap was lost, but, as shown, in Figure 8, the present invention permits
this loss to be reduced to 0.4% or even less. Thus~ there is a saving
of 0.3% or even more. Multiplying 0.3%, or .003, times 3000
.~, ,
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:.: . , . . : .
i~392054
pounds shows that 9 pounds of tall oil are saved per ton
of pulp processed. A paper mill producing 1000 tons of
pulp per day--a reasonable quanti~.y--would produce an
extra 9000 pounds, or ~.5 tons, of tall oil using ~his
invention. Paper mills operate practically continuously,
so, on the basis of 360 days per year, such a mill would
produce over 1600 extra tons of tall oil per year. At
$165 per ton, that is over $264,000 per year from tall
oil that, but for this invention, would be lost.
It has been found that for treatment o~ black
liquor according to the present invention the liquor
should preferably be at a ~emperature between 135~F. and
1~0F. in order to obtain the optimum additional agglom-
eration of tall oil soap particles, as well as the alkali
concentration of the black liquor, at 30% BLS, should be
between .08 to .22% as Na20 for optimum performance.
As previously mentioned, although the preferred
embodiment of the present invention is directed towards
the recovery of tall oil soap from black liquor, the
process of the invention is also suitable for use in the
recovery of other colloidal materials and/or particulate
or ibrous suspensions in liquids which exhibit a relative
potential or colloidal c'narge, e. g. cationic or anionic
resins or various organic and inorganic floc. More specif-
ically the process may be used to improve alum or monocal-
cium phosphate efficiencies in removing color Erom water
or sugar juices, and induce fiber flocculation and zeta
potential control for pulp and paper processing. The use
of the pulsating potential in the embodiment of Figure 8
has the further advantage of reducing degradation of the
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0921J~
conductors. The pulsating potential places a demand on the fluid to conduct
at the same rate as the conductors, At the higher voltage the ions in the
liquid have a greater resistivity and as a result an increased concentration
of cations and anions occurs at the conductors since they cannot migrate as
fast as the relative demand. The cations and anions at these higher concen-
trations then act as extensions of the conductors and prerent their electro-
lytic decomposition or degradation. Accordingly this also reduces the
: . .: . . .
tendency for colloidal coating build up on the conductors themselves. ;
Although an illustrative embodiment of the present invention has
been described herein with reference to the accompanying drawings, it is to
be understood that the invention is not limited to that precise embodiment7
and that rarious changes and modifications may be effected therein by one
skilled in the art without departing from the scope or spirit of this
invention.
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